Electromagnetic Tracking in Medicine—A Review of Technology, Validation, and Applications
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paper_title: Electromagnetic tracking in the clinical environment

paper_content:

When choosing an electromagnetic tracking system (EMTS) for image-guided procedures several factors must be taken into consideration. Among others these include the system's refresh rate, the number of sensors that need to be tracked, the size of the navigated region, the system interaction with the environment, whether the sensors can be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. To date, the only factors that have been studied extensively are the accuracy and the susceptibility of EMTSs to distortions caused by ferromagnetic materials. In this paper the authors shift the focus from analysis of system accuracy and stability to the broader set of factors influencing the utility of EMTS in the clinical environment. The authors provide an analysis based on all of the factors specified above, as assessed in three clinical environments. They evaluate two commercial tracking systems, the Aurora system from Northern Digital Inc., and the 3D Guidance system with three different field generators from Ascension Technology Corp. The authors show that these systems are applicable to specific procedures and specific environments, but that currently, no single system configuration provides a comprehensive solution across procedures and environments.
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paper_title: Computer-aided navigation in neurosurgery

paper_content:

The article comprises three main parts: a historical review on navigation, the mathematical basics for calculation and the clinical applications of navigation devices. Main historical steps are described from the first idea till the realisation of the frame-based and frameless navigation devices including robots. In particular the idea of robots can be traced back to the Iliad of Homer, the first testimony of European literature over 2500 years ago. In the second part the mathematical calculation of the mapping between the navigation and the image space is demonstrated, including different registration modalities and error estimations. The error of the navigation has to be divided into the technical error of the device calculating its own position in space, the registration error due to inaccuracies in the calculation of the transformation matrix between the navigation and the image space, and the application error caused additionally by anatomical shift of the brain structures during operation. In the third part the main clinical fields of application in modern neurosurgery are demonstrated, such as localisation of small intracranial lesions, skull-base surgery, intracerebral biopsies, intracranial endoscopy, functional neurosurgery and spinal navigation. At the end of the article some possible objections to navigation-aided surgery are discussed.
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paper_title: Image-guided interventions : technology and applications

paper_content:

Overview and History of Image-Guided Interventions.- Tracking Devices.- Visualization in Image-Guided Interventions.- Augmented Reality.- Software.- Rigid Registration.- Nonrigid Registration.- Model-Based Image Segmentation for Image-Guided Interventions.- Imaging Modalities.- MRI-Guided FUS and its Clinical Applications.- Neurosurgical Applications.- Computer-Assisted Orthopedic Surgery.- Thoracoabdominal Interventions.- Real-Time Interactive MRI for Guiding Cardiovascular Surgical Interventions.- Three-Dimensional Ultrasound Guidance and Robot Assistance for Prostate Brachytherapy.- Radiosurgery.- Radiation Oncology.- Assessment of Image-Guided Interventions.
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paper_title: Optimisation and evaluation of an electromagnetic tracking device for high-accuracy three-dimensional ultrasound imaging of the carotid arteries.

paper_content:

Electromagnetic tracking devices provide a flexible, low cost solution for three-dimensional ultrasound (3-D US) imaging. They are, however, susceptible to interference. A commercial device (Ascension pcBIRD) was evaluated to assess the accuracy in locating the scan probe as part of a digital, freehand 3-D US imaging system aimed at vascular applications. The device was optimised by selecting a measurement rate and filter setting that minimised the mean deviation in repeated position and orientation measurements. Experimental evaluation of accuracy indicated that, overall, absolute errors were small: the RMS absolute error was 0.2 mm (range: -0.7 to 0.5 mm) for positional measurements over translations up to 90 mm, and 0.2 degrees (range: -0.8 to 0.9 degrees ) for rotational measurements up to 30 degrees. In the case of position measurements, the absolute errors were influenced by the location of the scanner relative to the scan volume. We conclude that the device tested provides an accuracy sufficient for use within a freehand 3-D US system for carotid artery imaging.
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paper_title: Intraventricular catheter placement by electromagnetic navigation safely applied in a paediatric major head injury patient

paper_content:

INTRODUCTION ::: In the management of severe head injuries, the use of intraventricular catheters for intracranial pressure (ICP) monitoring and the option of cerebrospinal fluid drainage is gold standard. In children and adolescents, the insertion of a cannula in a compressed ventricle in case of elevated intracranial pressure is difficult; therefore, a pressure sensor is placed more often intraparenchymal as an alternative option. ::: ::: ::: DISCUSSION ::: In cases of persistent elevated ICP despite maximal brain pressure management, the use of an intraventricular monitoring device with the possibility of cerebrospinal fluid drainage is favourable. We present the method of intracranial catheter placement by means of an electromagnetic navigation technique.
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paper_title: Volumetric characterization of the Aurora magnetic tracker system for image-guided transorbital endoscopic procedures.

paper_content:

In some medical procedures, it is difficult or impossible to maintain a line of sight for a guidance system. For such applications, people have begun to use electromagnetic trackers. Before a localizer can be effectively used for an image-guided procedure, a characterization of the localizer is required. The purpose of this work is to perform a volumetric characterization of the fiducial localization error (FLE) in the working volume of the Aurora magnetic tracker by sampling the magnetic field using a tomographic grid. Since the Aurora magnetic tracker will be used for image-guided transorbital procedures we chose a working volume that was close to the average size of the human head. A Plexiglass grid phantom was constructed and used for the characterization of the Aurora magnetic tracker. A volumetric map of the magnetic space was performed by moving the flat Plexiglass phantom up in increments of 38.4 mm from 9.6 mm to 201.6 mm. The relative spatial and the random FLE were then calculated. Since the target of our endoscopic guidance is the orbital space behind the optic nerve, the maximum distance between the field generator and the sensor was calculated depending on the placement of the field generator from the skull. For the different field generator placements we found the average random FLE to be less than 0.06 mm for the 6D probe and 0.2 mm for the 5D probe. We also observed an average relative spatial FLE of less than 0.7 mm for the 6D probe and 1.3 mm for the 5D probe. We observed that the error increased as the distance between the field generator and the sensor increased. We also observed a minimum error occurring between 48 mm and 86 mm from the base of the tracker.
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paper_title: Dynamic Response of Electromagnetic Spatial Displacement Trackers

paper_content:

Overall system latency-the elapsed time from input human motion until the immediate consequences of that input are available in the display-is one of the most frequently cited shortcoming of current virtual environment VE technology. Given that spatial displacement trackers are employed to monitor head and hand position and orientation in many VE applications, the dynamic response intrinsic to these devices is an unavoidable contributor to overall system latency. In this paper, we describe a testbed and method for measurement of tracker dynamic response that use a motorized rotary swing arm to sinusoidally displace the VE sensor at a number of frequencies spanning the bandwidth of volitional human movement. During the tests, actual swing arm angle and VE sensor reports are collected and time stamped. By calibrating the time stamping technique, the tracker's internal transduction and processing time are separated from data transfer and host computer software execution latencies. We have used this test-bed to examine several VE sensors-most recently to compare latency, gain, and noise characteristics of two commercially available electromagnetic trackers: Ascension Technology Corp.'s Flock of BirdsTM and Polhemus Inc.'s FastrakTM.
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paper_title: Advanced superconducting gradiometer/Magnetometer arrays and a novel signal processing technique

paper_content:

Recent developments in superconducting magnetic gradiometer technology have led to the construction of advanced ultrasensitive gradiometer/magnetometer arrays. Details of construction techniques and data showing operational capabilities are presented. The most recent of the gradiometer/magnetometer arrays simultaneously measures five independent spatial gradients of the magnetic field and three vector components of the magnetic field. The measured signals from this array are subjected to a novel signal processing technique which provides detailed information about the magnetic signal source.
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paper_title: Accuracy of electromagnetic tracking with a prototype field generator in an interventional OR setting.

paper_content:

PURPOSE ::: The authors have studied the accuracy and robustness of a prototype electromagnetic window field generator (WFG) in an interventional radiology suite with a robotic C-arm. The overall purpose is the development of guidance systems combining real-time imaging with tracking of flexible instruments for bronchoscopy, laparoscopic ultrasound, endoluminal surgery, endovascular therapy, and spinal surgery. ::: ::: ::: METHODS ::: The WFG has a torus shape, which facilitates x-ray imaging through its centre. The authors compared the performance of the WFG to that of a standard field generator (SFG) under the influence of the C-arm. Both accuracy and robustness measurements were performed with the C-arm in different positions and poses. ::: ::: ::: RESULTS ::: The system was deemed robust for both field generators, but the accuracy was notably influenced as the C-arm was moved into the electromagnetic field. The SFG provided a smaller root-mean-square position error but was more influenced by the C-arm than the WFG. The WFG also produced smaller maximum and variance of the error. ::: ::: ::: CONCLUSIONS ::: Electromagnetic (EM) tracking with the new WFG during C-arm based fluoroscopy guidance seems to be a step forward, and with a correction scheme implemented it should be feasible.
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paper_title: Design and application of an assessment protocol for electromagnetic tracking systems.

paper_content:

This paper defines a simple protocol for competitive and quantified evaluation of electromagnetic tracking systems such as the NDI Aurora (A) and Ascension microBIRD with dipole transmitter (B). It establishes new methods and a new phantom design which assesses the reproducibility and allows comparability with different tracking systems in a consistent environment. A machined base plate was designed and manufactured in which a 50 mm grid of holes was precisely drilled for position measurements. In the center a circle of 32 equispaced holes enables the accurate measurement of rotation. The sensors can be clamped in a small mount which fits into pairs of grid holes on the base plate. Relative positional/orientational errors are found by subtracting the known distances/ rotations between the machined locations from the differences of the mean observed positions/ rotation. To measure the influence of metallic objects we inserted rods made of steel (SST 303, SST 416), aluminum, and bronze into the sensitive volume between sensor and emitter. We calculated the fiducial registration error and fiducial location error with a standard stylus calibration for both tracking systems and assessed two different methods of stylus calibration. The positional jitter amounted to 0.14 mm(A) and 0.08 mm(B). A relative positional error of 0.96 mm +/- 0.68 mm, range -0.06 mm; 2.23 mm(A) and 1.14 mm +/- 0.78 mm, range -3.72 mm; 1.57 mm(B) for a given distance of 50 mm was found. The relative rotation error was found to be 0.51 degrees (A)/0.04 degrees (B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2 mm(A)/ > or = 100 mm(B) occurs when the rod is close to the sensor(20 mm). While (B) is more sensitive with respect to metallic objects, (A) is less accurate concerning orientation measurements. (B) showed a systematic error when distances are calculated.
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paper_title: Method for estimating dynamic EM tracking accuracy of surgical navigation tools

paper_content:

Optical tracking systems have been used for several years in image guided medical procedures. Vendors often state static accuracies of a single retro-reflective sphere or LED. Expensive coordinate measurement machines (CMM) are used to validate the positional accuracy over the specified working volume. Users are interested in the dynamic accuracy of their tools. The configuration of individual sensors into a unique tool, the calibration of the tool tip, and the motion of the tool contribute additional errors. Electromagnetic (EM) tracking systems are considered an enabling technology for many image guided procedures because they are not limited by line-of-sight restrictions, take minimum space in the operating room, and the sensors can be very small. It is often difficult to quantify the accuracy of EM trackers because they can be affected by field distortion from certain metal objects. Many high-accuracy measurement devices can affect the EM measurements being validated. EM Tracker accuracy tends to vary over the working volume and orientation of the sensors. We present several simple methods for estimating the dynamic accuracy of EM tracked tools. We discuss the characteristics of the EM Tracker used in the GE Healthcare family of surgical navigation systems. Results for other tracking systems are included.
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paper_title: Magnetic Position and Orientation Tracking System

paper_content:

Three-axis generation and sensing of quasi-static magneticdipole fields provide information sufficient to determine both the position and orientation of the sensor relative to the source. Linear rotation transformations based upon the previous measurements are applied to both the source excitation and sensor output vectors, yielding quantities that are linearly propotional to small changes in the position and orientation. Changes are separated using linear combinations of sensor output vectors, transformed to the desired coordinate frame, and used to update the previous measurements. Practical considerations for a head-tracking application are discussed.
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paper_title: SPASYN-an electromagnetic relative position and orientation tracking system

paper_content:

Two relatively remote independent body coordinate frames are related in both position and orientation (six degrees of freedom) using precise electromagnetic field measurements. Antenna triads are fixed in each body frame. Variously polarized excitations in one body are correlated with signals detected in the remote body. Near-field and far-field processing strategies are presented with applications.
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paper_title: Image-Guided Endoscopic Surgery: Results of Accuracy and Performance in a Multicenter Clinical Study Using an Electromagnetic Tracking System

paper_content:

Image-guided surgery has recently been described in the literature as a useful technology for improved functional endoscopic sinus surgery localization. Image-guided surgery yields accurate knowledge of the surgical field boundaries, allowing safer and more thorough sinus surgery. We have previously reviewed our initial experience with The InstaTrak System. This article presents a multicenter clinical study (n=55) that assesses the system's capability for localizing structures in critical surgical sites. The purpose of this paper is to present quantitative data on accuracy and performance. We describe several new advances including an automated registration technique that eliminates the redundant computed tomography scan, compensation for head movement, and the ability to use interchangeable instruments.
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paper_title: Application of electromagnetic navigation in surgical treatment of intracranial tumors: analysis of 12 cases.

paper_content:

OBJECTIVE ::: To explore the application and characteristics of electromagnetic navigation in neurosurgical operation. ::: ::: ::: METHODS ::: Neurosurgical operations with the assistance of electromagnetic navigation were performed in 12 patients with intracranial tumors. ::: ::: ::: RESULTS ::: Total removal of the tumor was achieved in 8 cases, subtotal removal in 3 and removal of the majority of the tumor in 1 case. The error in the navigation averaged 1.9+/-0.9 mm and the time consumed by preoperative preparation was 19+/-2 min with the exception in 1 case. ::: ::: ::: CONCLUSION ::: In comparison with optic navigation, electromagnetic navigation offers better convenience and absence of signal blockage, and with a head frame, automatic registration can be achieved.
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paper_title: A Paired-Orientation Alignment Problem in a Hybrid Tracking System for Computer Assisted Surgery

paper_content:

Coordinate Alignment (CA) is an important problem in hybrid tracking systems involving two or more tracking devices. CA typically associates the measurements from two or more tracking systems with respect to distinct base frames and makes them comparable in the same aligned coordinate system. In this article, we discuss a sub-problem, Paired-Orientation Alignment (POA), in the category of CA. This sub-problem occurs during the development of an integrated electromagnetic and inertial attitude (orientation) tracking system, where only the orientation information is acquired from the two tracking devices. The problem is modeled as a matrix equation YC = D with constraints, which can be solved as a least-squares problem using quaternions. A closed-form analytical solution is given by the pseudo-inverse matrix. This method is specifically for registering the paired-orientation measurements between two coordinate systems, without using position information. The algorithm is illustrated by simulations and proof-of-concept tracking experiments.
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paper_title: Study on an experimental AC electromagnetic tracking system

paper_content:

3D tracking system is one of the key devices to realize the sense of immersion and human computer interaction in a virtual or augmented reality system. This paper presents the design of an experimental AC electromagnetic 3D tracking system that is based on AC magnetic field transmitting and sensing. The proposed system is composed of 3-axis orthogonal magnetic sensor, 3-axis orthogonal magnetic transmitter, 2-axis accelerometers, data acquisition and processing system etc. After obtaining the orientation of the receiver by measuring earth magnetic and gravity field with the DC output of magnetic sensors and accelerometers, the position can be calculated from the received AC magnetic field generated by the magnetic transmitter. The design of the experimental system on the basis of theoretical analysis is presented in detail and the results of the actual experiment prove the feasibility of the proposed system.
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paper_title: Evaluation of dynamic electromagnetic tracking deviation

paper_content:

Electromagnetic tracking systems (EMTS's) are widely used in clinical applications. Many reports have evaluated their static behavior and errors caused by metallic objects were examined. Although there exist some publications concerning the dynamic behavior of EMTS's the measurement protocols are either difficult to reproduce with respect of the movement path or only accomplished at high technical effort. Because dynamic behavior is of major interest with respect to clinical applications we established a simple but effective modal measurement easy to repeat at other laboratories. We built a simple pendulum where the sensor of our EMTS (Aurora, NDI, CA) could be mounted. The pendulum was mounted on a special bearing to guarantee that the pendulum path is planar. This assumption was tested before starting the measurements. All relevant parameters defining the pendulum motion such as rotation center and length are determined by static measurement at satisfactory accuracy. Then position and orientation data were gathered over a time period of 8 seconds and timestamps were recorded. Data analysis provided a positioning error and an overall error combining both position and orientation. All errors were calculated by means of the well know equations concerning pendulum movement. Additionally, latency - the elapsed time from input motion until the immediate consequences of that input are available - was calculated using well-known equations for mechanical pendulums for different velocities. We repeated the measurements with different metal objects (rods made of stainless steel type 303 and 416) between field generator and pendulum. ::: We found a root mean square error ( e RMS ) of 1.02mm with respect to the distance of the sensor position to the fit plane (maximum error e max = 2.31mm, minimum error e min = -2.36mm). The eRMS for positional error amounted to 1.32mm while the overall error was 3.24 mm. The latency at a pendulum angle of 0° (vertical) was 7.8ms.
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paper_title: Towards image guided robotic surgery: multi-arm tracking through hybrid localization

paper_content:

Objective ::: Use of the robotic assisted surgery has been increasing in recent years, due both the continuous increase in the number of applications and the clinical benefits that surgical robots can provide. Currently robotic assisted surgery relies on endoscopic video for navigation, providing only surface visualization, thus limiting subsurface vision. To be able to visualize and identify subsurface information, techniques in image-guidance can be used. As part of designing an image guidance system, all arms of the robot need to be co-localized in a common coordinate system.
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paper_title: An improved calibration framework for electromagnetic tracking devices

paper_content:

Electromagnetic trackers have many favorable characteristics but are notorious for their sensitivity to magnetic field distortions resulting from metal and electronic equipment in the environment. We categorize existing tracker calibration methods and present an improved technique for reducing the static position and orientation errors that are inherent to these devices. A quaternion-based formulation provides a simple and fast computational framework for representing orientation errors. Our experimental apparatus consists of a 6-DOF mobile platform and an optical position measurement system, allowing the collection of full-pose data at nearly arbitrary orientations of the receiver. A polynomial correction technique is applied and evaluated using a Polhemus Fastrak resulting in a substantial improvement of tracking accuracy. Finally, we apply advanced visualization algorithms to give new insight into the nature of the magnetic distortion field.
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paper_title: Dynamic Response of Electromagnetic Spatial Displacement Trackers

paper_content:

Overall system latency-the elapsed time from input human motion until the immediate consequences of that input are available in the display-is one of the most frequently cited shortcoming of current virtual environment VE technology. Given that spatial displacement trackers are employed to monitor head and hand position and orientation in many VE applications, the dynamic response intrinsic to these devices is an unavoidable contributor to overall system latency. In this paper, we describe a testbed and method for measurement of tracker dynamic response that use a motorized rotary swing arm to sinusoidally displace the VE sensor at a number of frequencies spanning the bandwidth of volitional human movement. During the tests, actual swing arm angle and VE sensor reports are collected and time stamped. By calibrating the time stamping technique, the tracker's internal transduction and processing time are separated from data transfer and host computer software execution latencies. We have used this test-bed to examine several VE sensors-most recently to compare latency, gain, and noise characteristics of two commercially available electromagnetic trackers: Ascension Technology Corp.'s Flock of BirdsTM and Polhemus Inc.'s FastrakTM.
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paper_title: Investigation of Attitude Tracking Using an Integrated Inertial and Magnetic Navigation System for Hand-Held Surgical Instruments

paper_content:

Due to the need for accurate navigation in minimally invasive surgery, many methods have been introduced to the operating room for tracking the position and orientation of instruments. This paper considers the subproblem of using integrated inertial and magnetic sensing to track the attitude (orientation) of surgical instruments. In this scenario, it is usually assumed that the sensor is quasi-static and the surrounding magnetic field is steady. For practical hand-held surgical instruments, perturbations exist due to intended and unintended (e.g., tremor) motion and due to distortion of the surrounding magnetic field. We consider the problem of estimating the gravity and magnetic field in the inertial sensor frame with small perturbations. The dynamics of the gravity and magnetic field is studied under perturbations, their relationships to gyroscope measurements are analyzed, and Kalman filters (KFs) are formulated to reduce these perturbations. The estimated gravity and magnetic values (outputs of the KFs) are subsequently used in an extended KF for attitude estimation. In this filter, the prediction model is given by the system dynamics, formulated using quaternions, and the observation model is given by vector analysis of the estimated gravity and magnetic field. Experiments are performed to validate the algorithms under clinically realistic motions. The complete system demonstrates an improvement in the accuracy of the attitude estimate in the presence of small perturbations, and satisfies the specified accuracy requirement of 1°.
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paper_title: A review of RFID localization: Applications and techniques

paper_content:

Indoor localization has been actively researched recently due to security and safety as well as service matters. Previous research and development for indoor localization includes infrared, wireless LAN and ultrasonic. However, these technologies suffer either from the limited accuracy or lacking of the infrastructure. Radio Frequency Identification (RFID) is very attractive because of reasonable system price, and reader reliability. The RFID localization can be categorized into tag and reader localizations. In this paper, major localization techniques for both tag and reader localizations are reviewed to provide the readers state of the art of the indoor localization algorithms. The advantage and disadvantage of each technique for particular applications were also discussed.
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paper_title: A new system to perform continuous target tracking for radiation and surgery using non-ionizing alternating current electromagnetics

paper_content:

Abstract A new technology based on alternating current (AC) nonionizing electromagnetic fields has been developed to enable precise localization and continuous tracking of mobile soft tissue targets or tumors during radiation therapy and surgery. The technology utilizes miniature, permanently implanted, wireless transponders (Beacon™ Transponders) and a novel array to enable objective localization and continuous tracking of targets in three-dimensional space. The characteristics of this system include use of safe, nonionizing electromagnetic fields; negligible tissue interaction, delivery of inherently objective three-dimensional data, continuous operation during external beam radiation therapy or surgery. Feasibility testing has shown the system was capable of locating and continuously tracking transponders with submillimeter accuracy at tracking rates of up to 10 Hz and was unaffected by operational linear accelerator. Preclinical animal studies have shown the transponders to be suitable for permanent implantation and early human clinical studies have demonstrated the feasibility of inserting transponders into the prostate. This system will enable accurate initial patient setup and provide real-time target tracking demanded by today's highly conformal radiation techniques and has significant potential as a surgical navigation platform.
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paper_title: Advanced superconducting gradiometer/Magnetometer arrays and a novel signal processing technique

paper_content:

Recent developments in superconducting magnetic gradiometer technology have led to the construction of advanced ultrasensitive gradiometer/magnetometer arrays. Details of construction techniques and data showing operational capabilities are presented. The most recent of the gradiometer/magnetometer arrays simultaneously measures five independent spatial gradients of the magnetic field and three vector components of the magnetic field. The measured signals from this array are subjected to a novel signal processing technique which provides detailed information about the magnetic signal source.
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paper_title: Review on Patents about Magnetic Localisation Systems for in vivo Catheterizations

paper_content:

Abstract: in vivo Catheterizations are usually performed by physicians using X-Ray fluoroscopic guide and contrast-media. The X-Ray exposure both of the patient and of the operators can induce collateral effects. The present review describes the status of the art on recent patents about magnetic position/orientation indicators capable to drive the probe during in-vivo medical diagnostic or interventional procedures. They are based on the magnetic field produced by sources and revealed by sensors. Possible solutions are: the modulated magnetic field produced by a set of coils positioned externally to the patient is measured by sensors installed on the intra-body probe; the magnetic field produced by a thin permanent magnet installed on the intra-body probe is measured by magnetic field sensors positioned outside the patient body. In either cases, position and orientation of the probe are calculated in real time: this allows the elimination of repetitive X-Ray scans used to monitor the probe. The aim of the proposed systems is to drive the catheter inside the patient vascular tree with a reduction of the X-Ray exposure both of the patient and of the personnel involved in the intervention. The present paper intends also to highlight advantages/disadvantages of the presented solutions.
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paper_title: Accuracy of a wireless localization system for radiotherapy.

paper_content:

PURPOSE ::: A system has been developed for patient positioning based on real-time localization of implanted electromagnetic transponders (beacons). This study demonstrated the accuracy of the system before clinical trials. ::: ::: ::: METHODS AND MATERIALS ::: We describe the overall system. The localization component consists of beacons and a source array. A rigid phantom was constructed to place the beacons at known offsets from a localization array. Tests were performed at distances of 80 and 270 mm from the array and at positions in the array plane of up to 8 cm offset. Tests were performed in air and saline to assess the effect of tissue conductivity and with multiple transponders to evaluate crosstalk. Tracking was tested using a dynamic phantom creating a circular path at varying speeds. ::: ::: ::: RESULTS ::: Submillimeter accuracy was maintained throughout all experiments. Precision was greater proximal to the source plane (sigmax = 0.006 mm, sigmay = 0.01 mm, sigmaz = 0.006 mm), but continued to be submillimeter at the end of the designed tracking range at 270 mm from the array (sigmax = 0.27 mm, sigmay = 0.36 mm, sigmaz = 0.48 mm). The introduction of saline and the use of multiple beacons did not affect accuracy. Submillimeter accuracy was maintained using the dynamic phantom at speeds of up to 3 cm/s. ::: ::: ::: CONCLUSION ::: This system has demonstrated the accuracy needed for localization and monitoring of position during treatment.
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paper_title: Method for estimating dynamic EM tracking accuracy of surgical navigation tools

paper_content:

Optical tracking systems have been used for several years in image guided medical procedures. Vendors often state static accuracies of a single retro-reflective sphere or LED. Expensive coordinate measurement machines (CMM) are used to validate the positional accuracy over the specified working volume. Users are interested in the dynamic accuracy of their tools. The configuration of individual sensors into a unique tool, the calibration of the tool tip, and the motion of the tool contribute additional errors. Electromagnetic (EM) tracking systems are considered an enabling technology for many image guided procedures because they are not limited by line-of-sight restrictions, take minimum space in the operating room, and the sensors can be very small. It is often difficult to quantify the accuracy of EM trackers because they can be affected by field distortion from certain metal objects. Many high-accuracy measurement devices can affect the EM measurements being validated. EM Tracker accuracy tends to vary over the working volume and orientation of the sensors. We present several simple methods for estimating the dynamic accuracy of EM tracked tools. We discuss the characteristics of the EM Tracker used in the GE Healthcare family of surgical navigation systems. Results for other tracking systems are included.
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paper_title: Accurate localization of RFID tags using phase difference

paper_content:

Due to their light weight, low power, and practically unlimited identification capacity, radio frequency identification (RFID) tags and associated devices offer distinctive advantages and are widely recognized for their promising potential in context-aware computing; by tagging objects with RFID tags, the environment can be sensed in a cost- and energy-efficient means. However, a prerequisite to fully realizing the potential is accurate localization of RFID tags, which will enable and enhance a wide range of applications. In this paper we show how to exploit the phase difference between two or more receiving antennas to compute accurate localization. Phase difference based localization has better accuracy, robustness and sensitivity when integrated with other measurements compared to the currently popular technique of localization using received signal strength. Using a software-defined radio setup, we show experimental results that support accurate localization of RFID tags and activity recognition based on phase difference.
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paper_title: Magnetic Position and Orientation Tracking System

paper_content:

Three-axis generation and sensing of quasi-static magneticdipole fields provide information sufficient to determine both the position and orientation of the sensor relative to the source. Linear rotation transformations based upon the previous measurements are applied to both the source excitation and sensor output vectors, yielding quantities that are linearly propotional to small changes in the position and orientation. Changes are separated using linear combinations of sensor output vectors, transformed to the desired coordinate frame, and used to update the previous measurements. Practical considerations for a head-tracking application are discussed.
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paper_title: Phase difference based RFID navigation for medical applications

paper_content:

RFID localization is a promising new field of work that is eagerly awaited for many different types of applications. For use in a medical context, special requirements and limitations must be taken into account, especially regarding accuracy, reliability and operating range. In this paper we present an experimental setup for a medical navigation system based on RFID. For this we applied a machine learning algorithm, namely support vector regression, to phase difference data gathered from multiple RFID receivers. The performance was tested on six datasets of different shape and placement within the volume spanned by the receivers. In addition, two grid based training sets of different size were considered for the regression. Our results show that it is possible to reach an accuracy of tag localization that is sufficient for some medical applications. Although we could not reach an overall accuracy of less than one millimeter in our experiments so far, the deviation was limited to two millimeters in most cases and the general results indicate that application of RFID localization even to highly critical applications, e. g., for brain surgery, will be possible soon.
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paper_title: SPASYN-an electromagnetic relative position and orientation tracking system

paper_content:

Two relatively remote independent body coordinate frames are related in both position and orientation (six degrees of freedom) using precise electromagnetic field measurements. Antenna triads are fixed in each body frame. Variously polarized excitations in one body are correlated with signals detected in the remote body. Near-field and far-field processing strategies are presented with applications.
---
paper_title: Image-guided interventions : technology and applications

paper_content:

Overview and History of Image-Guided Interventions.- Tracking Devices.- Visualization in Image-Guided Interventions.- Augmented Reality.- Software.- Rigid Registration.- Nonrigid Registration.- Model-Based Image Segmentation for Image-Guided Interventions.- Imaging Modalities.- MRI-Guided FUS and its Clinical Applications.- Neurosurgical Applications.- Computer-Assisted Orthopedic Surgery.- Thoracoabdominal Interventions.- Real-Time Interactive MRI for Guiding Cardiovascular Surgical Interventions.- Three-Dimensional Ultrasound Guidance and Robot Assistance for Prostate Brachytherapy.- Radiosurgery.- Radiation Oncology.- Assessment of Image-Guided Interventions.
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paper_title: 3D magnetic tracking of a single subminiature coil with a large 2D-array of uniaxial transmitters

paper_content:

A novel system and method for magnetic tracking of a single subminiature coil is described. The novelty of the method consists in employing a large, 8 /spl times/ 8 array of coplanar transmitting coils. This allows us to always keep the receiving coil not far from the wide, flat transmitting array, to increase the signal-to-noise ratio, and to decrease the retransmitted interference. The whole transmitting array, 64 coils, is sequentially activated only at the initiation stage to compute the initial position of the receiving coil. The redundancy in the transmitters number provides fast and unambiguous convergence of the optimization algorithm. At the following tracking stages, a small (8 coils) transmitting subarray is activated. The relatively small subarray size allows us to keep a high update rate and resolution of tracking. For a 50-Hz update rate, the tracking resolution is not worse than 0.25 mm, 0.2/spl deg/ rms at a 200-mm height above the transmitting array's center. This resolution corresponds to an /spl sim/1-mm, 0.6/spl deg/ tracking accuracy. The novelty of the method consists as well in optimizing the transmitting coils' geometry to substantially (down to 0.5 mm) reduce the systematic error caused by the inaccuracy of the dipole field approximation.
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paper_title: A new system to perform continuous target tracking for radiation and surgery using non-ionizing alternating current electromagnetics

paper_content:

Abstract A new technology based on alternating current (AC) nonionizing electromagnetic fields has been developed to enable precise localization and continuous tracking of mobile soft tissue targets or tumors during radiation therapy and surgery. The technology utilizes miniature, permanently implanted, wireless transponders (Beacon™ Transponders) and a novel array to enable objective localization and continuous tracking of targets in three-dimensional space. The characteristics of this system include use of safe, nonionizing electromagnetic fields; negligible tissue interaction, delivery of inherently objective three-dimensional data, continuous operation during external beam radiation therapy or surgery. Feasibility testing has shown the system was capable of locating and continuously tracking transponders with submillimeter accuracy at tracking rates of up to 10 Hz and was unaffected by operational linear accelerator. Preclinical animal studies have shown the transponders to be suitable for permanent implantation and early human clinical studies have demonstrated the feasibility of inserting transponders into the prostate. This system will enable accurate initial patient setup and provide real-time target tracking demanded by today's highly conformal radiation techniques and has significant potential as a surgical navigation platform.
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paper_title: An Electromagnetic Tracking Method Using Rotating Orthogonal Coils

paper_content:

In this paper, an electromagnetic tracking method that uses two rotating orthogonal coils is proposed. Two cross-shaped coils can rotate together and be driven in sequence to generate a magnetic field. One of the rotating orthogonal coils is used to track the position of the three-axis sensor and the other is to help solve the orientation of the sensor. As rotation can provide a geometrical relationship between the magnetic source and the sensor, the method does not require the generated magnetic field to imitate the ideal dipole in calculation. A fast noniterative algorithm and simple 1-D mapping along the axis of one coil are used to realize six degree-of-freedom (6DOF) tracking. Thus, the complexity of the magnetic tracking method and the corresponding system can be greatly reduced. Simulation results show that the method has good tracking accuracy and speed, which can be further improved by using an adaptive step-size searching strategy. So, it is concluded that the method can be used as an effective target monitoring method in many application domains such as minimally invasive therapy.
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paper_title: Accuracy of a wireless localization system for radiotherapy.

paper_content:

PURPOSE ::: A system has been developed for patient positioning based on real-time localization of implanted electromagnetic transponders (beacons). This study demonstrated the accuracy of the system before clinical trials. ::: ::: ::: METHODS AND MATERIALS ::: We describe the overall system. The localization component consists of beacons and a source array. A rigid phantom was constructed to place the beacons at known offsets from a localization array. Tests were performed at distances of 80 and 270 mm from the array and at positions in the array plane of up to 8 cm offset. Tests were performed in air and saline to assess the effect of tissue conductivity and with multiple transponders to evaluate crosstalk. Tracking was tested using a dynamic phantom creating a circular path at varying speeds. ::: ::: ::: RESULTS ::: Submillimeter accuracy was maintained throughout all experiments. Precision was greater proximal to the source plane (sigmax = 0.006 mm, sigmay = 0.01 mm, sigmaz = 0.006 mm), but continued to be submillimeter at the end of the designed tracking range at 270 mm from the array (sigmax = 0.27 mm, sigmay = 0.36 mm, sigmaz = 0.48 mm). The introduction of saline and the use of multiple beacons did not affect accuracy. Submillimeter accuracy was maintained using the dynamic phantom at speeds of up to 3 cm/s. ::: ::: ::: CONCLUSION ::: This system has demonstrated the accuracy needed for localization and monitoring of position during treatment.
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paper_title: Standardized assessment of new electromagnetic field generators in an interventional radiology setting.

paper_content:

PURPOSE ::: Two of the main challenges associated with electromagnetic (EM) tracking in computer-assisted interventions (CAIs) are (1) the compensation of systematic distance errors arising from the influence of metal near the field generator (FG) or the tracked sensor and (2) the optimized setup of the FG to maximize tracking accuracy in the area of interest. Recently, two new FGs addressing these issues were proposed for the well-established Aurora(®) tracking system [Northern Digital, Inc. (NDI), Waterloo, Canada]: the Tabletop 50-70 FG, a planar transmitter with a built-in shield that compensates for metal distortions emanating from treatment tables, and the prototypical Compact FG 7-10, a mobile generator designed to be attached to mobile imaging devices. The purpose of this paper was to assess the accuracy and precision of these new FGs in an interventional radiology setting. ::: ::: ::: METHODS ::: A standardized assessment protocol, which uses a precisely machined base plate to measure relative error in position and orientation, was applied to the two new FGs as well as to the well-established standard Aurora(®) Planar FG. The experiments were performed in two different settings: a reference laboratory environment and a computed tomography (CT) scanning room. In each setting, the protocol was applied to three different poses of the measurement plate within the tracking volume of the three FGs. ::: ::: ::: RESULTS ::: The two new FGs provided higher precision and accuracy within their respective measurement volumes as well as higher robustness with respect to the CT scanner compared to the established FG. Considering all possible 5 cm distances on the grid, the error of the Planar FG was increased by a factor of 5.94 in the clinical environment (4.4 mm) in comparison to the error in the laboratory environment (0.8 mm). In contrast, the mean values for the two new FGs were all below 1 mm with an increase in the error by factors of only 2.94 (Reference: 0.3 mm; CT: 0.9 mm) and 1.04 (both: 0.5 mm) in the case of the Tabletop FG and the Compact FG, respectively. ::: ::: ::: CONCLUSIONS ::: Due to their high accuracy and robustness, the Tabletop FG and the Compact FG could eliminate the need for compensation of EM field distortions in certain CT-guided interventions.
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paper_title: A survey of electromagnetic position tracker calibration techniques

paper_content:

This paper is a comprehensive survey of various techniques used to calibrate electromagnetic position tracking systems. A common framework is established to present the calibration problem as the interpolation problem in 3D. All the known calibration techniques are classified into local and global methods and grouped according to their mathematical models. Both the location error and the orientation error correction techniques are surveyed. Data acquisition devices and methods as well as publicly available software implementations are reviewed, too.
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paper_title: Evaluation of dynamic electromagnetic tracking deviation

paper_content:

Electromagnetic tracking systems (EMTS's) are widely used in clinical applications. Many reports have evaluated their static behavior and errors caused by metallic objects were examined. Although there exist some publications concerning the dynamic behavior of EMTS's the measurement protocols are either difficult to reproduce with respect of the movement path or only accomplished at high technical effort. Because dynamic behavior is of major interest with respect to clinical applications we established a simple but effective modal measurement easy to repeat at other laboratories. We built a simple pendulum where the sensor of our EMTS (Aurora, NDI, CA) could be mounted. The pendulum was mounted on a special bearing to guarantee that the pendulum path is planar. This assumption was tested before starting the measurements. All relevant parameters defining the pendulum motion such as rotation center and length are determined by static measurement at satisfactory accuracy. Then position and orientation data were gathered over a time period of 8 seconds and timestamps were recorded. Data analysis provided a positioning error and an overall error combining both position and orientation. All errors were calculated by means of the well know equations concerning pendulum movement. Additionally, latency - the elapsed time from input motion until the immediate consequences of that input are available - was calculated using well-known equations for mechanical pendulums for different velocities. We repeated the measurements with different metal objects (rods made of stainless steel type 303 and 416) between field generator and pendulum. ::: We found a root mean square error ( e RMS ) of 1.02mm with respect to the distance of the sensor position to the fit plane (maximum error e max = 2.31mm, minimum error e min = -2.36mm). The eRMS for positional error amounted to 1.32mm while the overall error was 3.24 mm. The latency at a pendulum angle of 0° (vertical) was 7.8ms.
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paper_title: Review on Patents about Magnetic Localisation Systems for in vivo Catheterizations

paper_content:

Abstract: in vivo Catheterizations are usually performed by physicians using X-Ray fluoroscopic guide and contrast-media. The X-Ray exposure both of the patient and of the operators can induce collateral effects. The present review describes the status of the art on recent patents about magnetic position/orientation indicators capable to drive the probe during in-vivo medical diagnostic or interventional procedures. They are based on the magnetic field produced by sources and revealed by sensors. Possible solutions are: the modulated magnetic field produced by a set of coils positioned externally to the patient is measured by sensors installed on the intra-body probe; the magnetic field produced by a thin permanent magnet installed on the intra-body probe is measured by magnetic field sensors positioned outside the patient body. In either cases, position and orientation of the probe are calculated in real time: this allows the elimination of repetitive X-Ray scans used to monitor the probe. The aim of the proposed systems is to drive the catheter inside the patient vascular tree with a reduction of the X-Ray exposure both of the patient and of the personnel involved in the intervention. The present paper intends also to highlight advantages/disadvantages of the presented solutions.
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paper_title: The Effects of Metals and Interfering Fields on Electromagnetic Trackers

paper_content:

The operation of six degree-of-freedom electromagnetic trackers is based on the spatial properties of the electromagnetic fields generated by three small coils. Anything in the environment that causes these fields to be distorted will result in measurement noise and/or errors. An experimental investigation was undertaken to measure the effect of external fields present in a typical working environment (namely mains and computer monitor fields) and the presence of metals (25-mm cubes of various types of metals, a large steel bar, and a large steel sheet). A theoretical model is proposed to explain the observations. Two devices were used in this investigation: a Polhemus Fastrak and an Ascension Flock of Birds.
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paper_title: Magnetic Position and Orientation Tracking System

paper_content:

Three-axis generation and sensing of quasi-static magneticdipole fields provide information sufficient to determine both the position and orientation of the sensor relative to the source. Linear rotation transformations based upon the previous measurements are applied to both the source excitation and sensor output vectors, yielding quantities that are linearly propotional to small changes in the position and orientation. Changes are separated using linear combinations of sensor output vectors, transformed to the desired coordinate frame, and used to update the previous measurements. Practical considerations for a head-tracking application are discussed.
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paper_title: SPASYN-an electromagnetic relative position and orientation tracking system

paper_content:

Two relatively remote independent body coordinate frames are related in both position and orientation (six degrees of freedom) using precise electromagnetic field measurements. Antenna triads are fixed in each body frame. Variously polarized excitations in one body are correlated with signals detected in the remote body. Near-field and far-field processing strategies are presented with applications.
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paper_title: Electromagnetic tracking in the clinical environment

paper_content:

When choosing an electromagnetic tracking system (EMTS) for image-guided procedures several factors must be taken into consideration. Among others these include the system's refresh rate, the number of sensors that need to be tracked, the size of the navigated region, the system interaction with the environment, whether the sensors can be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. To date, the only factors that have been studied extensively are the accuracy and the susceptibility of EMTSs to distortions caused by ferromagnetic materials. In this paper the authors shift the focus from analysis of system accuracy and stability to the broader set of factors influencing the utility of EMTS in the clinical environment. The authors provide an analysis based on all of the factors specified above, as assessed in three clinical environments. They evaluate two commercial tracking systems, the Aurora system from Northern Digital Inc., and the 3D Guidance system with three different field generators from Ascension Technology Corp. The authors show that these systems are applicable to specific procedures and specific environments, but that currently, no single system configuration provides a comprehensive solution across procedures and environments.
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paper_title: Immersive 3DUI on one dollar a day

paper_content:

A convergence between consumer electronics and virtual reality is occurring. We present an immersive head-mounted-display-based, wearable 3D user interface that is inexpensive (less than $900 USD), robust (sourceless tracking), and portable (lightweight and untethered). While the current display has known deficiencies, the user tracking quality is within the constraints of many existing applications, while the portability and cost offers opportunities for innovative applications that are not currently feasible.
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paper_title: Method for estimating dynamic EM tracking accuracy of surgical navigation tools

paper_content:

Optical tracking systems have been used for several years in image guided medical procedures. Vendors often state static accuracies of a single retro-reflective sphere or LED. Expensive coordinate measurement machines (CMM) are used to validate the positional accuracy over the specified working volume. Users are interested in the dynamic accuracy of their tools. The configuration of individual sensors into a unique tool, the calibration of the tool tip, and the motion of the tool contribute additional errors. Electromagnetic (EM) tracking systems are considered an enabling technology for many image guided procedures because they are not limited by line-of-sight restrictions, take minimum space in the operating room, and the sensors can be very small. It is often difficult to quantify the accuracy of EM trackers because they can be affected by field distortion from certain metal objects. Many high-accuracy measurement devices can affect the EM measurements being validated. EM Tracker accuracy tends to vary over the working volume and orientation of the sensors. We present several simple methods for estimating the dynamic accuracy of EM tracked tools. We discuss the characteristics of the EM Tracker used in the GE Healthcare family of surgical navigation systems. Results for other tracking systems are included.
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paper_title: Image-guided interventions : technology and applications

paper_content:

Overview and History of Image-Guided Interventions.- Tracking Devices.- Visualization in Image-Guided Interventions.- Augmented Reality.- Software.- Rigid Registration.- Nonrigid Registration.- Model-Based Image Segmentation for Image-Guided Interventions.- Imaging Modalities.- MRI-Guided FUS and its Clinical Applications.- Neurosurgical Applications.- Computer-Assisted Orthopedic Surgery.- Thoracoabdominal Interventions.- Real-Time Interactive MRI for Guiding Cardiovascular Surgical Interventions.- Three-Dimensional Ultrasound Guidance and Robot Assistance for Prostate Brachytherapy.- Radiosurgery.- Radiation Oncology.- Assessment of Image-Guided Interventions.
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paper_title: Towards unified electromagnetic tracking system assessment-static errors

paper_content:

Recent advances in Image-Guided Surgery allows physicians to incorporate up-to-date, high quality patient data in the surgical decision making, and sometimes to directly perform operations based on pre- or intra-operatively acquired patient images. Electromagnetic tracking is the fastest growing area within, where the position and orientation of tiny sensors can be determined with sub-millimeter accuracy in the field created by a generator. One of the major barriers to the wider spread of electromagnetic tracking solutions is their susceptibility to ferromagnetic materials and external electromagnetic sources. The research community has long been engaged with the topic to find engineering solutions to increase measurement reliability and accuracy. This article gives an overview of related experiments, and presents our recommendation towards a robust method to collect representative data about electromagnetic trackers.
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paper_title: The reliability and accuracy of an electromagnetic motion analysis system when used conjointly with an accelerometer

paper_content:

The effect of an accelerometer driven electronic postural monitor (Spineangel®) placed within the electromagnetic measurement field of the Polhemus Fastrak™ is unknown. This study assessed the reliability and accuracy of Fastrak™ linear and angular measurements, when the Spineangel® was placed close to the sensor(s) and transmitter. Bland Altman plots and intraclass correlation coefficient (2,1) were used to determine protocol reproducibility and measurement consistency. Excellent reliability was found for linear and angular measurements (0.96, 95% CI: 0.90–0.99; and 1.00, 95% CI: 1.00–1.00, respectively) with the inclusion of Spineangel®; similar results were found, without the inclusion of Spineangel®, for linear and angular measurements, (0.96, 95% CI: 0.89–0.99; and 1.00, 95% CI: 1.00–1.00, respectively). The greatest linear discrepancies between the two test conditions were found to be less than 3.5 mm, while the greatest angular discrepancies were below 3.5°. As the effect on accuracy was minimal, t...
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paper_title: Electromagnetic tracking for US-guided interventions: standardized assessment of a new compact field generator

paper_content:

PURPOSE ::: One of the main challenges related to electromagnetic tracking in the clinical setting is a placement of the field generator (FG) that optimizes the reliability and accuracy of sensor localization. Recently, a new mobile FG for the NDI Aurora(®) tracking system has been presented. This Compact FG is the first FG that can be attached directly to an ultrasound (US) probe. The purpose of this study was to assess the precision and accuracy of the Compact FG in the presence of nearby mounted US probes. ::: ::: ::: MATERIALS AND METHODS ::: Six different US probes were mounted onto the Compact FG by means of a custom-designed mounting adapter. To assess precision and accuracy of the Compact FG, we employed a standardized assessment protocol. Utilizing a specifically manufactured plate, we measured positional data on three levels of distances from the FG as well as rotational data. ::: ::: ::: RESULTS ::: While some probes had negligible influence on tracking accuracy two probes increased the mean distance error up to 1.5 mm compared with a reference measurement of 0.5 mm. The jitter error consistently stayed below 0.2 mm in all cases. The mean relative error in orientation was found to be smaller than 3°. ::: ::: ::: CONCLUSION ::: Attachment of an US probe to the Compact FG does not have a critical influence on tracking accuracy in most cases. Clinical benefit of this promising mobile FG must be shown in future studies.
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paper_title: Evaluation of dynamic electromagnetic tracking deviation

paper_content:

Electromagnetic tracking systems (EMTS's) are widely used in clinical applications. Many reports have evaluated their static behavior and errors caused by metallic objects were examined. Although there exist some publications concerning the dynamic behavior of EMTS's the measurement protocols are either difficult to reproduce with respect of the movement path or only accomplished at high technical effort. Because dynamic behavior is of major interest with respect to clinical applications we established a simple but effective modal measurement easy to repeat at other laboratories. We built a simple pendulum where the sensor of our EMTS (Aurora, NDI, CA) could be mounted. The pendulum was mounted on a special bearing to guarantee that the pendulum path is planar. This assumption was tested before starting the measurements. All relevant parameters defining the pendulum motion such as rotation center and length are determined by static measurement at satisfactory accuracy. Then position and orientation data were gathered over a time period of 8 seconds and timestamps were recorded. Data analysis provided a positioning error and an overall error combining both position and orientation. All errors were calculated by means of the well know equations concerning pendulum movement. Additionally, latency - the elapsed time from input motion until the immediate consequences of that input are available - was calculated using well-known equations for mechanical pendulums for different velocities. We repeated the measurements with different metal objects (rods made of stainless steel type 303 and 416) between field generator and pendulum. ::: We found a root mean square error ( e RMS ) of 1.02mm with respect to the distance of the sensor position to the fit plane (maximum error e max = 2.31mm, minimum error e min = -2.36mm). The eRMS for positional error amounted to 1.32mm while the overall error was 3.24 mm. The latency at a pendulum angle of 0° (vertical) was 7.8ms.
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paper_title: Electromagnetic tracking in the clinical environment

paper_content:

When choosing an electromagnetic tracking system (EMTS) for image-guided procedures several factors must be taken into consideration. Among others these include the system's refresh rate, the number of sensors that need to be tracked, the size of the navigated region, the system interaction with the environment, whether the sensors can be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. To date, the only factors that have been studied extensively are the accuracy and the susceptibility of EMTSs to distortions caused by ferromagnetic materials. In this paper the authors shift the focus from analysis of system accuracy and stability to the broader set of factors influencing the utility of EMTS in the clinical environment. The authors provide an analysis based on all of the factors specified above, as assessed in three clinical environments. They evaluate two commercial tracking systems, the Aurora system from Northern Digital Inc., and the 3D Guidance system with three different field generators from Ascension Technology Corp. The authors show that these systems are applicable to specific procedures and specific environments, but that currently, no single system configuration provides a comprehensive solution across procedures and environments.
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paper_title: THE EFFECT OF TRANSPONDER MOTION ON THE ACCURACY OF THE CALYPSO ELECTROMAGNETIC LOCALIZATION SYSTEM

paper_content:

Purpose: To determine position and velocity-dependent effects in the overall accuracy of the Calypso Electromagnetic localization system, under conditions that emulate transponder motion during normal free breathing. Methods and Materials: Three localization transponders were mounted on a remote-controlled turntable that could move the transponders along a circular trajectory at speeds up to 3 cm/s. A stationary calibration established the coordinates of multiple points on each transponder's circular path. Position measurements taken while the transponders were in motion at a constant speed were then compared with the stationary coordinates. Results: No statistically significant changes in the transponder positions in (x,y,z) were detected when the transponders were in motion. Conclusions: The accuracy of the localization system is unaffected by transponder motion.
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paper_title: Design and application of an assessment protocol for electromagnetic tracking systems.

paper_content:

This paper defines a simple protocol for competitive and quantified evaluation of electromagnetic tracking systems such as the NDI Aurora (A) and Ascension microBIRD with dipole transmitter (B). It establishes new methods and a new phantom design which assesses the reproducibility and allows comparability with different tracking systems in a consistent environment. A machined base plate was designed and manufactured in which a 50 mm grid of holes was precisely drilled for position measurements. In the center a circle of 32 equispaced holes enables the accurate measurement of rotation. The sensors can be clamped in a small mount which fits into pairs of grid holes on the base plate. Relative positional/orientational errors are found by subtracting the known distances/ rotations between the machined locations from the differences of the mean observed positions/ rotation. To measure the influence of metallic objects we inserted rods made of steel (SST 303, SST 416), aluminum, and bronze into the sensitive volume between sensor and emitter. We calculated the fiducial registration error and fiducial location error with a standard stylus calibration for both tracking systems and assessed two different methods of stylus calibration. The positional jitter amounted to 0.14 mm(A) and 0.08 mm(B). A relative positional error of 0.96 mm +/- 0.68 mm, range -0.06 mm; 2.23 mm(A) and 1.14 mm +/- 0.78 mm, range -3.72 mm; 1.57 mm(B) for a given distance of 50 mm was found. The relative rotation error was found to be 0.51 degrees (A)/0.04 degrees (B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2 mm(A)/ > or = 100 mm(B) occurs when the rod is close to the sensor(20 mm). While (B) is more sensitive with respect to metallic objects, (A) is less accurate concerning orientation measurements. (B) showed a systematic error when distances are calculated.
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paper_title: A hardware and software protocol for the evaluation of electromagnetic tracker accuracy in the clinical environment: a multi-center study

paper_content:

This paper proposes an assessment protocol that incorporates both hardware and analysis methods for evaluation of electromagnetic tracker accuracy in different clinical environments. The susceptibility of electromagnetic tracker measurement accuracy is both highly dependent on nearby ferromagnetic interference sources and non-isotropic. These inherent limitations combined with the various hardware components and assessment techniques used within different studies makes the direct comparison of measurement accuracy between studies difficult. This paper presents a multicenter study to evaluate electromagnetic devices in different clinical environments using a common hardware phantom and assessment techniques so that results are directly comparable. Measurement accuracy has been shown to be in the range of 0.79-6.67mm within a 180mm 3 sub-volume of the Aurora measurement space in five different clinical environments.
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paper_title: An Evaluation of the Aurora System as a Flesh-Point Tracking Tool for Speech Production Research

paper_content:

Purpose Northern Digital Instruments (NDI; Waterloo, Ontario, Canada) manufactures a commercially available magnetometer device called Aurora that features real-time display of sensor position trac...
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paper_title: Standardized assessment of new electromagnetic field generators in an interventional radiology setting.

paper_content:

PURPOSE ::: Two of the main challenges associated with electromagnetic (EM) tracking in computer-assisted interventions (CAIs) are (1) the compensation of systematic distance errors arising from the influence of metal near the field generator (FG) or the tracked sensor and (2) the optimized setup of the FG to maximize tracking accuracy in the area of interest. Recently, two new FGs addressing these issues were proposed for the well-established Aurora(®) tracking system [Northern Digital, Inc. (NDI), Waterloo, Canada]: the Tabletop 50-70 FG, a planar transmitter with a built-in shield that compensates for metal distortions emanating from treatment tables, and the prototypical Compact FG 7-10, a mobile generator designed to be attached to mobile imaging devices. The purpose of this paper was to assess the accuracy and precision of these new FGs in an interventional radiology setting. ::: ::: ::: METHODS ::: A standardized assessment protocol, which uses a precisely machined base plate to measure relative error in position and orientation, was applied to the two new FGs as well as to the well-established standard Aurora(®) Planar FG. The experiments were performed in two different settings: a reference laboratory environment and a computed tomography (CT) scanning room. In each setting, the protocol was applied to three different poses of the measurement plate within the tracking volume of the three FGs. ::: ::: ::: RESULTS ::: The two new FGs provided higher precision and accuracy within their respective measurement volumes as well as higher robustness with respect to the CT scanner compared to the established FG. Considering all possible 5 cm distances on the grid, the error of the Planar FG was increased by a factor of 5.94 in the clinical environment (4.4 mm) in comparison to the error in the laboratory environment (0.8 mm). In contrast, the mean values for the two new FGs were all below 1 mm with an increase in the error by factors of only 2.94 (Reference: 0.3 mm; CT: 0.9 mm) and 1.04 (both: 0.5 mm) in the case of the Tabletop FG and the Compact FG, respectively. ::: ::: ::: CONCLUSIONS ::: Due to their high accuracy and robustness, the Tabletop FG and the Compact FG could eliminate the need for compensation of EM field distortions in certain CT-guided interventions.
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paper_title: An improved calibration framework for electromagnetic tracking devices

paper_content:

Electromagnetic trackers have many favorable characteristics but are notorious for their sensitivity to magnetic field distortions resulting from metal and electronic equipment in the environment. We categorize existing tracker calibration methods and present an improved technique for reducing the static position and orientation errors that are inherent to these devices. A quaternion-based formulation provides a simple and fast computational framework for representing orientation errors. Our experimental apparatus consists of a 6-DOF mobile platform and an optical position measurement system, allowing the collection of full-pose data at nearly arbitrary orientations of the receiver. A polynomial correction technique is applied and evaluated using a Polhemus Fastrak resulting in a substantial improvement of tracking accuracy. Finally, we apply advanced visualization algorithms to give new insight into the nature of the magnetic distortion field.
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paper_title: Towards unified electromagnetic tracking system assessment-static errors

paper_content:

Recent advances in Image-Guided Surgery allows physicians to incorporate up-to-date, high quality patient data in the surgical decision making, and sometimes to directly perform operations based on pre- or intra-operatively acquired patient images. Electromagnetic tracking is the fastest growing area within, where the position and orientation of tiny sensors can be determined with sub-millimeter accuracy in the field created by a generator. One of the major barriers to the wider spread of electromagnetic tracking solutions is their susceptibility to ferromagnetic materials and external electromagnetic sources. The research community has long been engaged with the topic to find engineering solutions to increase measurement reliability and accuracy. This article gives an overview of related experiments, and presents our recommendation towards a robust method to collect representative data about electromagnetic trackers.
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paper_title: Stability of miniature electromagnetic tracking systems

paper_content:

This study aims at a comparative evaluation of two recently introduced electromagnetic tracking systems under reproducible simulated operating-room (OR) conditions: the recently launched Medtronic StealthStation™ Treon-EM™ and the NDI Aurora™. We investigate if and to what extent these systems provide improved performance and stability in the presence of surgical instruments as possible sources of distortions compared with earlier reports on electromagnetic tracking technology. To investigate possible distortions under pseudo-realistic OR conditions, a large Langenbeck hook, a dental drill with its handle and an ultrasonic (US) scanhead are fixed on a special measurement rack at variable distances from the navigation sensor. The position measurements made by the Treon-EM™ were least affected by the presence of the instruments. The lengths of the mean deviation vectors were 0.21 mm for the Langenbeck hook, 0.23 mm for the drill with handle and 0.56 mm for the US scanhead. The Aurora™ was influenced by the three sources of distortion to a higher degree. A mean deviation vector of 1.44 mm length was observed in the vicinity of the Langenbeck hook, 0.53 mm length with the drill and 2.37 mm due to the US scanhead. The maximum of the root mean squared error (RMSE) for all coordinates in the presence of the Langenbeck hook was 0.3 mm for the Treon™ and 2.1 mm for the Aurora™; the drill caused a maximum RMSE of 0.2 mm with the Treon™ and 1.2 mm with the Aurora™. In the presence of the US scanhead, the maximum RMSE was 1.4 mm for the Treon™ and 5.1 mm for the Aurora™. The new generation of electromagnetic tracking systems has significantly improved compared to common systems that were available in the middle of the 1990s and has reached a high level of technical development. We conclude that, in general, both systems are suitable for routine clinical application.
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paper_title: Optimisation and evaluation of an electromagnetic tracking device for high-accuracy three-dimensional ultrasound imaging of the carotid arteries.

paper_content:

Electromagnetic tracking devices provide a flexible, low cost solution for three-dimensional ultrasound (3-D US) imaging. They are, however, susceptible to interference. A commercial device (Ascension pcBIRD) was evaluated to assess the accuracy in locating the scan probe as part of a digital, freehand 3-D US imaging system aimed at vascular applications. The device was optimised by selecting a measurement rate and filter setting that minimised the mean deviation in repeated position and orientation measurements. Experimental evaluation of accuracy indicated that, overall, absolute errors were small: the RMS absolute error was 0.2 mm (range: -0.7 to 0.5 mm) for positional measurements over translations up to 90 mm, and 0.2 degrees (range: -0.8 to 0.9 degrees ) for rotational measurements up to 30 degrees. In the case of position measurements, the absolute errors were influenced by the location of the scanner relative to the scan volume. We conclude that the device tested provides an accuracy sufficient for use within a freehand 3-D US system for carotid artery imaging.
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paper_title: Electromagnetic Tracking for Thermal Ablation and Biopsy Guidance: Clinical Evaluation of Spatial Accuracy

paper_content:

Purpose To evaluate the spatial accuracy of electromagnetic needle tracking and demonstrate the feasibility of ultrasonography (US)–computed tomography (CT) fusion during CT- and US-guided biopsy and radiofrequency ablation procedures. Materials and Methods The authors performed a 20-patient clinical trial to investigate electromagnetic needle tracking during interventional procedures. The study was approved by the institutional investigational review board, and written informed consent was obtained from all patients. Needles were positioned by using CT and US guidance. A commercial electromagnetic tracking device was used in combination with prototype internally tracked needles and custom software to record needle positions relative to previously obtained CT scans. Position tracking data were acquired to evaluate the tracking error, defined as the difference between tracked needle position and reference standard needle position on verification CT scans. Registration between tracking space and image space was obtained by using reference markers attached to the skin ("fiducials"), and different registration methods were compared. The US transducer was tracked to demonstrate the potential use of real-time US-CT fusion for imaging guidance. Results One patient was excluded from analysis because he was unable to follow breathing instructions during the acquisition of CT scans. Nineteen of the 20 patients were evaluable, demonstrating a basic tracking error of 5.8 mm ± 2.6, which improved to 3.5 mm ± 1.9 with use of nonrigid registrations that used previous internal needle positions as additional fiducials. Fusion of tracked US with CT was successful. Patient motion and distortion of the tracking system by the CT table and gantry were identified as sources of error. Conclusions The demonstrated spatial tracking accuracy is sufficient to display clinically relevant preprocedural imaging information during needle-based procedures. Virtual needles displayed within preprocedural images may be helpful for clandestine targets such as arterial phase enhancing liver lesions or during thermal ablations when obscuring gas is released. Electromagnetic tracking may help improve imaging guidance for interventional procedures and warrants further investigation, especially for procedures in which the outcomes are dependent on accuracy.
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paper_title: An evaluation of interference of inflatable penile prostheses with electromagnetic localization and tracking system

paper_content:

Purpose: ::: The Calypso system is stated by the manufacturer to be contraindicated for cases where the patient has been implanted with a penile prosthesis. This is due to concern for potential metal interference-related reduction of spatial localization and tracking accuracy. Here we quantify the localization and tracking accuracy of the Calypso system in the presence of inflatable penile prosthesis devices from three most widely used models which account for, essentially, 100% of implants in North America. ::: ::: Methods: ::: Phantom studies were first performed to quantify the interference of Calypso localization and tracking accuracy from both varying metal (steel) masses, and from the penile prosthetic devices themselves. The interference of varying steel masses was studied as a function of two factors: (a) the mass and (b) the location of steel material. The Calypso daily quality assurance (QA) phantom with three implanted Beacon® transponders was used to measure any aliasing of position that might occur due to metal interference. After confirming the safety of use in phantom, we implanted Calypso Beacon® transponders in one patient with a previously implanted AMS Model 700 inflatable penile prosthetic device. For each of the 42 delivered treatment fractions, redundant stereotactic ultrasound (US) image guidance was performed to ensure good agreement between US and Calypso guidance. ::: ::: Results: ::: We observed that a steel mass of less than 18 g did not cause any detectable positional aliasing for the Calypso tracking function. The mass of metal material measured to exist in the three penile prosthetic devices studied here (MP35N alloy) was approximately 1 g for each. No positional aliasing was observed for the three prosthetic devices in phantom, and good agreement between redundant US and Calypso was also observed in patient. ::: ::: Conclusions: ::: Both phantom and patient evaluations with the penile prosthetic devices showed no measurable interference with the Calypso system, thus indicating that accurate Calypso-based alignments can be performed in the presence of current industry standard inflatable penile prosthetic devices.
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paper_title: The reliability and accuracy of an electromagnetic motion analysis system when used conjointly with an accelerometer

paper_content:

The effect of an accelerometer driven electronic postural monitor (Spineangel®) placed within the electromagnetic measurement field of the Polhemus Fastrak™ is unknown. This study assessed the reliability and accuracy of Fastrak™ linear and angular measurements, when the Spineangel® was placed close to the sensor(s) and transmitter. Bland Altman plots and intraclass correlation coefficient (2,1) were used to determine protocol reproducibility and measurement consistency. Excellent reliability was found for linear and angular measurements (0.96, 95% CI: 0.90–0.99; and 1.00, 95% CI: 1.00–1.00, respectively) with the inclusion of Spineangel®; similar results were found, without the inclusion of Spineangel®, for linear and angular measurements, (0.96, 95% CI: 0.89–0.99; and 1.00, 95% CI: 1.00–1.00, respectively). The greatest linear discrepancies between the two test conditions were found to be less than 3.5 mm, while the greatest angular discrepancies were below 3.5°. As the effect on accuracy was minimal, t...
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paper_title: Electromagnetic tracking for US-guided interventions: standardized assessment of a new compact field generator

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PURPOSE ::: One of the main challenges related to electromagnetic tracking in the clinical setting is a placement of the field generator (FG) that optimizes the reliability and accuracy of sensor localization. Recently, a new mobile FG for the NDI Aurora(®) tracking system has been presented. This Compact FG is the first FG that can be attached directly to an ultrasound (US) probe. The purpose of this study was to assess the precision and accuracy of the Compact FG in the presence of nearby mounted US probes. ::: ::: ::: MATERIALS AND METHODS ::: Six different US probes were mounted onto the Compact FG by means of a custom-designed mounting adapter. To assess precision and accuracy of the Compact FG, we employed a standardized assessment protocol. Utilizing a specifically manufactured plate, we measured positional data on three levels of distances from the FG as well as rotational data. ::: ::: ::: RESULTS ::: While some probes had negligible influence on tracking accuracy two probes increased the mean distance error up to 1.5 mm compared with a reference measurement of 0.5 mm. The jitter error consistently stayed below 0.2 mm in all cases. The mean relative error in orientation was found to be smaller than 3°. ::: ::: ::: CONCLUSION ::: Attachment of an US probe to the Compact FG does not have a critical influence on tracking accuracy in most cases. Clinical benefit of this promising mobile FG must be shown in future studies.
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paper_title: Technical accuracy of optical and the electromagnetic tracking systems

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Thousands of operations are annually guided with computer assisted surgery (CAS) technologies. As the use of these devices is rapidly increasing, the reliability of the devices becomes ever more critical. The problem of accuracy assessment of the devices has thus become relevant. During the past five years, over 200 hazardous situations have been documented in the MAUDE database during operations using these devices in the field of neurosurgery alone. Had the accuracy of these devices been periodically assessed pre-operatively, many of them might have been prevented. The technical accuracy of a commercial navigator enabling the use of both optical (OTS) and electromagnetic (EMTS) tracking systems was assessed in the hospital setting using accuracy assessment tools and methods developed by the authors of this paper. The technical accuracy was obtained by comparing the positions of the navigated tool tip with the phantom accuracy assessment points. Each assessment contained a total of 51 points and a region of surgical interest (ROSI) volume of 120x120x100 mm roughly mimicking the size of the human head. The error analysis provided a comprehensive understanding of the trend of accuracy of the surgical navigator modalities. This study showed that the technical accuracies of OTS and EMTS over the pre-determined ROSI were nearly equal. However, the placement of the particular modality hardware needs to be optimized for the surgical procedure. New applications of EMTS, which does not require rigid immobilization of the surgical area, are suggested.
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paper_title: ELECTROMAGNETIC TRACKING SYSTEM FOR NEUROVASCULAR INTERVENTIONS

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Electromagnetic tracking systems (EMTS) are widely used in a computer-assisted surgery, motion capture, kinematic studies and military applications. The main application in the medical domain represents minimal invasive surgery. For this purpose, the EMTS systems are integrated into the surgery device, i.e. needle, endoscope or catheter supporting tracking of this device within the human body without X-ray radiation. However, in neurovascular interventions this technique hardly could be applied so far. This is mainly due to a too large size of the receiver coils which have to be integrated into the microcatheter. In this paper we present such a microcatheter for neurovascular interventions featured with a 5 DOF sensor enabling tracking of the catheter tip by a commercial EMTS system and first evaluation of its accuracy.
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paper_title: Evaluation of dynamic electromagnetic tracking deviation

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Electromagnetic tracking systems (EMTS's) are widely used in clinical applications. Many reports have evaluated their static behavior and errors caused by metallic objects were examined. Although there exist some publications concerning the dynamic behavior of EMTS's the measurement protocols are either difficult to reproduce with respect of the movement path or only accomplished at high technical effort. Because dynamic behavior is of major interest with respect to clinical applications we established a simple but effective modal measurement easy to repeat at other laboratories. We built a simple pendulum where the sensor of our EMTS (Aurora, NDI, CA) could be mounted. The pendulum was mounted on a special bearing to guarantee that the pendulum path is planar. This assumption was tested before starting the measurements. All relevant parameters defining the pendulum motion such as rotation center and length are determined by static measurement at satisfactory accuracy. Then position and orientation data were gathered over a time period of 8 seconds and timestamps were recorded. Data analysis provided a positioning error and an overall error combining both position and orientation. All errors were calculated by means of the well know equations concerning pendulum movement. Additionally, latency - the elapsed time from input motion until the immediate consequences of that input are available - was calculated using well-known equations for mechanical pendulums for different velocities. We repeated the measurements with different metal objects (rods made of stainless steel type 303 and 416) between field generator and pendulum. ::: We found a root mean square error ( e RMS ) of 1.02mm with respect to the distance of the sensor position to the fit plane (maximum error e max = 2.31mm, minimum error e min = -2.36mm). The eRMS for positional error amounted to 1.32mm while the overall error was 3.24 mm. The latency at a pendulum angle of 0° (vertical) was 7.8ms.
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paper_title: Evaluation of a new electromagnetic tracking system using a standardized assessment protocol

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This note uses a published protocol to evaluate a newly released 6 degrees of freedom electromagnetic tracking system (Aurora, Northern Digital Inc.). A practice for performance monitoring over time is also proposed. The protocol uses a machined base plate to measure relative error in position and orientation as well as the influence of metallic objects in the operating volume. Positional jitter (ERMS) was found to be 0.17 mm ± 0.19 mm. A relative positional error of 0.25 mm ± 0.22 mm at 50 mm offsets and 0.97 mm ± 1.01 mm at 300 mm offsets was found. The mean of the relative rotation error was found to be 0.20° ± 0.14° with respect to the axial and 0.91° ± 0.68° for the longitudinal rotation. The most significant distortion caused by metallic objects is caused by 400-series stainless steel. A 9.4 mm maximum error occurred when the rod was closest to the emitter, 10 mm away. The improvement compared to older generations of the Aurora with respect to accuracy is substantial.
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paper_title: Electromagnetic tracking in the clinical environment

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When choosing an electromagnetic tracking system (EMTS) for image-guided procedures several factors must be taken into consideration. Among others these include the system's refresh rate, the number of sensors that need to be tracked, the size of the navigated region, the system interaction with the environment, whether the sensors can be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. To date, the only factors that have been studied extensively are the accuracy and the susceptibility of EMTSs to distortions caused by ferromagnetic materials. In this paper the authors shift the focus from analysis of system accuracy and stability to the broader set of factors influencing the utility of EMTS in the clinical environment. The authors provide an analysis based on all of the factors specified above, as assessed in three clinical environments. They evaluate two commercial tracking systems, the Aurora system from Northern Digital Inc., and the 3D Guidance system with three different field generators from Ascension Technology Corp. The authors show that these systems are applicable to specific procedures and specific environments, but that currently, no single system configuration provides a comprehensive solution across procedures and environments.
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paper_title: Quality assurance for clinical implementation of an electromagnetic tracking system

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The Calypso Medical 4D localization system utilizes alternating current electromagnetics for accurate, real-time tumor tracking. A quality assurance program to clinically implement this system is described here. Testing of the continuous electromagnetic tracking system (Calypso Medical Technologies, Seattle, WA) was performed using an in-house developed four-dimensional stage and a quality assurance fixture containing three radiofrequency transponders at independently measured locations. The following tests were performed to validate the Calypso system: (a) Localization and tracking accuracy, (b) system reproducibility, (c) measurement of the latency of the tracking system, and (d) measurement of transmission through the Calypso table overlay and the electromagnetic array. The translational and rotational localization accuracies were found to be within 0.01 cm and 1.0 degree, respectively. The reproducibility was within 0.1 cm. The average system latency was measured to be within 303 ms. The attenuation by the Calypso overlay was measured to be 1.0% for both 6 and 18 MV photons. The attenuations by the Calypso array were measured to be 2% and 1.5% for 6 and 18 MV photons, respectively. For oblique angles, the transmission was measured to be 3% for 6 MV, while it was 2% for 18 MV photons. A quality assurance process has been developed for the clinical implementation of an electromagnetic tracking system in radiation therapy.
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paper_title: Accuracy assessment protocols for electromagnetic tracking systems

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Electromagnetic tracking systems have found increasing use in medical applications during the last few years. As with most non-trivial spatial measurement systems, the complex determination of positions and orientations from their underlying raw sensor measurements results in complicated, non-uniform error distributions over the specified measurement volume. This makes it difficult to unambiguously determine accuracy and performance assessments that allow users to judge the suitability of these systems for their particular needs. Various assessment protocols generally emphasize different measurement aspects that typically arise in clinical use. This can easily lead to inconclusive or even contradictory conclusions. We examine some of the major issues involved and discuss three useful calibration protocols. The measurement accuracy of a system can be described in terms of its 'trueness' and its 'precision'. Often, the two are strongly coupled and cannot be easily determined independently. We present a method that allows the two to be disentangled, so that the resultant trueness properly represents the systematic, non-reducible part of the measurement error, and the resultant precision (or repeatability) represents only the statistical, reducible part. Although the discussion is given largely within the context of electromagnetic tracking systems, many of the results are applicable to measurement systems in general.
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paper_title: A framework for calibration of electromagnetic surgical navigation system

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In this paper, we present a framework of calibrating an electromagnetic tracker (Northern Digital's Aurora) using an accurate optical tracker (the Optotrak system, also from Northern Digital). First, registration methods for these two navigation systems are developed. Sub millimeter accuracy registration is achieved for both cases. We also address the latency between the two different trackers. The registration accuracy for dynamic acquired data is greatly improved after we compensate for the tracker latency. In our calibration approach, we sample the measurement field of the Aurora and compute the position and orientation error using the Optotrak measurements and previously computed registration results as "ground truth". Then we approximate the error field using Bernstein polynomials. Another comparative technique we use is to decompose the error space using KD tree, and then approximate each atomic cell with local interpolation. Experimental results show significant improvement in tracking accuracy for both position and orientation. Finally we discuss our future directions.
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paper_title: Assesment of metallic distortions of an electromagnetic tracking system

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Electromagnetic tracking systems are affected by the presence of metal or more general conductive objects. In this paper results of two protocols are presented, which will access the amount of distortions caused by certain type of metals. One of the main application areas of electromagnetic tracking systems is the medical field. Therefore this paper concentrates on types of metals, which are common in a medical environment, like typical tool and implant materials and OR table steel. Results are obtained and compared for the first generation of Aurora systems (Aurora 1), released in September 2003 and for the new Aurora system (Aurora 2), which was released in September 2005.
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paper_title: Accuracy of electromagnetic tracking with a prototype field generator in an interventional OR setting.

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PURPOSE ::: The authors have studied the accuracy and robustness of a prototype electromagnetic window field generator (WFG) in an interventional radiology suite with a robotic C-arm. The overall purpose is the development of guidance systems combining real-time imaging with tracking of flexible instruments for bronchoscopy, laparoscopic ultrasound, endoluminal surgery, endovascular therapy, and spinal surgery. ::: ::: ::: METHODS ::: The WFG has a torus shape, which facilitates x-ray imaging through its centre. The authors compared the performance of the WFG to that of a standard field generator (SFG) under the influence of the C-arm. Both accuracy and robustness measurements were performed with the C-arm in different positions and poses. ::: ::: ::: RESULTS ::: The system was deemed robust for both field generators, but the accuracy was notably influenced as the C-arm was moved into the electromagnetic field. The SFG provided a smaller root-mean-square position error but was more influenced by the C-arm than the WFG. The WFG also produced smaller maximum and variance of the error. ::: ::: ::: CONCLUSIONS ::: Electromagnetic (EM) tracking with the new WFG during C-arm based fluoroscopy guidance seems to be a step forward, and with a correction scheme implemented it should be feasible.
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paper_title: THE EFFECT OF TRANSPONDER MOTION ON THE ACCURACY OF THE CALYPSO ELECTROMAGNETIC LOCALIZATION SYSTEM

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Purpose: To determine position and velocity-dependent effects in the overall accuracy of the Calypso Electromagnetic localization system, under conditions that emulate transponder motion during normal free breathing. Methods and Materials: Three localization transponders were mounted on a remote-controlled turntable that could move the transponders along a circular trajectory at speeds up to 3 cm/s. A stationary calibration established the coordinates of multiple points on each transponder's circular path. Position measurements taken while the transponders were in motion at a constant speed were then compared with the stationary coordinates. Results: No statistically significant changes in the transponder positions in (x,y,z) were detected when the transponders were in motion. Conclusions: The accuracy of the localization system is unaffected by transponder motion.
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paper_title: Accuracy of navigation: a comparative study of infrared optical and electromagnetic navigation.

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We evaluated the accuracy of navigation systems for measuring the mechanical axis in patients undergoing total knee arthroplasty and in the synthetic bone model. Infrared optical and electromagnetic navigation systems were compared. Both systems were found to be accurate and reproducible in an experimental environment. However, the accuracies of both systems were affected by erroneous registration, and the optical system was found to be more reproducible. In clinical situations, the mean difference was 1.23 degrees, and difference greater than 3 degrees occurred in 15% of clinical trials. These discordances may have been due to ambiguous anatomic landmarks causing registration errors and the possibility of electromagnetic signal interference in the operating room.
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paper_title: Volumetric characterization of the Aurora magnetic tracker system for image-guided transorbital endoscopic procedures.

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In some medical procedures, it is difficult or impossible to maintain a line of sight for a guidance system. For such applications, people have begun to use electromagnetic trackers. Before a localizer can be effectively used for an image-guided procedure, a characterization of the localizer is required. The purpose of this work is to perform a volumetric characterization of the fiducial localization error (FLE) in the working volume of the Aurora magnetic tracker by sampling the magnetic field using a tomographic grid. Since the Aurora magnetic tracker will be used for image-guided transorbital procedures we chose a working volume that was close to the average size of the human head. A Plexiglass grid phantom was constructed and used for the characterization of the Aurora magnetic tracker. A volumetric map of the magnetic space was performed by moving the flat Plexiglass phantom up in increments of 38.4 mm from 9.6 mm to 201.6 mm. The relative spatial and the random FLE were then calculated. Since the target of our endoscopic guidance is the orbital space behind the optic nerve, the maximum distance between the field generator and the sensor was calculated depending on the placement of the field generator from the skull. For the different field generator placements we found the average random FLE to be less than 0.06 mm for the 6D probe and 0.2 mm for the 5D probe. We also observed an average relative spatial FLE of less than 0.7 mm for the 6D probe and 1.3 mm for the 5D probe. We observed that the error increased as the distance between the field generator and the sensor increased. We also observed a minimum error occurring between 48 mm and 86 mm from the base of the tracker.
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paper_title: Design and application of an assessment protocol for electromagnetic tracking systems.

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This paper defines a simple protocol for competitive and quantified evaluation of electromagnetic tracking systems such as the NDI Aurora (A) and Ascension microBIRD with dipole transmitter (B). It establishes new methods and a new phantom design which assesses the reproducibility and allows comparability with different tracking systems in a consistent environment. A machined base plate was designed and manufactured in which a 50 mm grid of holes was precisely drilled for position measurements. In the center a circle of 32 equispaced holes enables the accurate measurement of rotation. The sensors can be clamped in a small mount which fits into pairs of grid holes on the base plate. Relative positional/orientational errors are found by subtracting the known distances/ rotations between the machined locations from the differences of the mean observed positions/ rotation. To measure the influence of metallic objects we inserted rods made of steel (SST 303, SST 416), aluminum, and bronze into the sensitive volume between sensor and emitter. We calculated the fiducial registration error and fiducial location error with a standard stylus calibration for both tracking systems and assessed two different methods of stylus calibration. The positional jitter amounted to 0.14 mm(A) and 0.08 mm(B). A relative positional error of 0.96 mm +/- 0.68 mm, range -0.06 mm; 2.23 mm(A) and 1.14 mm +/- 0.78 mm, range -3.72 mm; 1.57 mm(B) for a given distance of 50 mm was found. The relative rotation error was found to be 0.51 degrees (A)/0.04 degrees (B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2 mm(A)/ > or = 100 mm(B) occurs when the rod is close to the sensor(20 mm). While (B) is more sensitive with respect to metallic objects, (A) is less accurate concerning orientation measurements. (B) showed a systematic error when distances are calculated.
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paper_title: Systematic distortions in magnetic position digitizers

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W. Birkfellner, F. Watzinger, F. Wanschitz, G. Enislidis, C. Kollmann, D. Rafolt, R. Nowotny, R. Ewers, and H.Bergmann Citation: Medical Physics 25, 2242 (1998); doi: 10.1118/1.598425 View online: http://dx.doi.org/10.1118/1.598425 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/25/11?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Organ motion tracking in USgFUS - A feasibility study using sonoelastography AIP Conf. Proc. 1503, 135 (2012); 10.1063/1.4769931 Integration and evaluation of a needle-positioning robot with volumetric microcomputed tomography imageguidance for small animal stereotactic interventions Med. Phys. 37, 1647 (2010); 10.1118/1.3312520 Magnetoacoustic tomographic imaging of electrical impedance with magnetic induction Appl. Phys. Lett. 91, 083903 (2007); 10.1063/1.2772763 Magnetoacoustic tomography with magnetic induction for imaging electrical impedance of biological tissue J. Appl. Phys. 99, 066112 (2006); 10.1063/1.2186371 Evaluation of a miniature electromagnetic position tracker Med. Phys. 29, 2205 (2002); 10.1118/1.1508377
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paper_title: Accuracy of a wireless localization system for radiotherapy.

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PURPOSE ::: A system has been developed for patient positioning based on real-time localization of implanted electromagnetic transponders (beacons). This study demonstrated the accuracy of the system before clinical trials. ::: ::: ::: METHODS AND MATERIALS ::: We describe the overall system. The localization component consists of beacons and a source array. A rigid phantom was constructed to place the beacons at known offsets from a localization array. Tests were performed at distances of 80 and 270 mm from the array and at positions in the array plane of up to 8 cm offset. Tests were performed in air and saline to assess the effect of tissue conductivity and with multiple transponders to evaluate crosstalk. Tracking was tested using a dynamic phantom creating a circular path at varying speeds. ::: ::: ::: RESULTS ::: Submillimeter accuracy was maintained throughout all experiments. Precision was greater proximal to the source plane (sigmax = 0.006 mm, sigmay = 0.01 mm, sigmaz = 0.006 mm), but continued to be submillimeter at the end of the designed tracking range at 270 mm from the array (sigmax = 0.27 mm, sigmay = 0.36 mm, sigmaz = 0.48 mm). The introduction of saline and the use of multiple beacons did not affect accuracy. Submillimeter accuracy was maintained using the dynamic phantom at speeds of up to 3 cm/s. ::: ::: ::: CONCLUSION ::: This system has demonstrated the accuracy needed for localization and monitoring of position during treatment.
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paper_title: Method for estimating dynamic EM tracking accuracy of surgical navigation tools

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Optical tracking systems have been used for several years in image guided medical procedures. Vendors often state static accuracies of a single retro-reflective sphere or LED. Expensive coordinate measurement machines (CMM) are used to validate the positional accuracy over the specified working volume. Users are interested in the dynamic accuracy of their tools. The configuration of individual sensors into a unique tool, the calibration of the tool tip, and the motion of the tool contribute additional errors. Electromagnetic (EM) tracking systems are considered an enabling technology for many image guided procedures because they are not limited by line-of-sight restrictions, take minimum space in the operating room, and the sensors can be very small. It is often difficult to quantify the accuracy of EM trackers because they can be affected by field distortion from certain metal objects. Many high-accuracy measurement devices can affect the EM measurements being validated. EM Tracker accuracy tends to vary over the working volume and orientation of the sensors. We present several simple methods for estimating the dynamic accuracy of EM tracked tools. We discuss the characteristics of the EM Tracker used in the GE Healthcare family of surgical navigation systems. Results for other tracking systems are included.
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paper_title: An electromagnetic “Tracker-in-Table” configuration for X-ray fluoroscopy and cone-beam CT-guided surgery

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PURPOSE ::: A novel electromagnetic tracking configuration was characterized and implemented for image-guided surgery incorporating C-arm fluoroscopy and/or cone-beam CT (CBCT). The tracker employed a field generator (FG) with an open rectangular aperture and a frame enclosure with two essentially hollow sides, yielding a design that presents little or no X-ray attenuation across the C-arm orbit. The "Window" FG (WFG) was characterized in comparison with a conventional "Aurora" FG (AFG), and a configuration in which the WFG was incorporated directly into the operating table was investigated in preclinical phantom studies. ::: ::: ::: METHOD ::: The geometric accuracy and field of view (FOV) of the WFG and AFG were evaluated in terms of target registration error (TRE) using an acrylic phantom on an (electromagnetic compatible) experimental bench. The WFG design was incorporated in a prototype operating table featuring a carbon fiber top beneath, which the FG could be translated for positioning under the patient. The X-ray compatibility was evaluated using a prototype mobile C-arm for fluoroscopy and CBCT in an anthropomorphic chest phantom. The susceptibility to EM field distortion associated with surgical tools (e.g., spine screws) and the C-arm itself was investigated in terms of TRE, and calibration methods were tested to provide robust image-world registration with minimal perturbation from the rotational C-arm. ::: ::: ::: RESULTS ::: The WFG demonstrated mean TRE of 1.28 ± 0.79 mm compared to 1.13 ± 0.72 mm for the AFG, with no statistically significant difference between the two (p = 0.32 and n = 250). The WFG exhibited a deeper field of view by ~10 cm providing an equivalent degree of geometric accuracy to a depth of z ~55 cm, compared to z ~45 cm for the AFG. Although the presence of a small number of spine screws did not degrade tracker accuracy, the mobile C-arm perturbed the electromagnetic field sufficiently to degrade TRE; however, a calibration method was identified to mitigate the effect. Specifically, the average calibration between posterior-anterior and lateral orientations of the C-arm was found to yield fairly robust registration for any C-arm pose with only a slight reduction in geometric accuracy (1.43 ± 0.31 mm in comparison with 1.28 ± 0.79 mm, p = 0.05). The WFG demonstrated reasonable X-ray compatibility, although the initial design of the window frame included suboptimal material and shape of the side bars that caused a level of streak artifacts in CBCT reconstructions. The streak artifacts were of sufficient magnitude to degrade soft-tissue visibility in CBCT but were negligible in the context of high-contrast imaging tasks (e.g., bone visualization). ::: ::: ::: CONCLUSION ::: The open frame of the WFG offers a potentially valuable configuration for electromagnetic trackers in image-guided surgery applications that are based on X-ray fluoroscopy and/or CBCT. The geometric accuracy and FOV are comparable to the conventional AFG and offers increased depth (z-direction) FOV. Incorporation directly within the operating table offers a streamlined implementation in which the tracker is in place but "invisible," potentially simplifying tableside logistics, avoidance of the sterile field, and compatibility with X-ray imaging.
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paper_title: A hardware and software protocol for the evaluation of electromagnetic tracker accuracy in the clinical environment: a multi-center study

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This paper proposes an assessment protocol that incorporates both hardware and analysis methods for evaluation of electromagnetic tracker accuracy in different clinical environments. The susceptibility of electromagnetic tracker measurement accuracy is both highly dependent on nearby ferromagnetic interference sources and non-isotropic. These inherent limitations combined with the various hardware components and assessment techniques used within different studies makes the direct comparison of measurement accuracy between studies difficult. This paper presents a multicenter study to evaluate electromagnetic devices in different clinical environments using a common hardware phantom and assessment techniques so that results are directly comparable. Measurement accuracy has been shown to be in the range of 0.79-6.67mm within a 180mm 3 sub-volume of the Aurora measurement space in five different clinical environments.
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paper_title: EFFECT OF 3D ULTRASOUND PROBES ON THE ACCURACY OF ELECTROMAGNETIC TRACKING SYSTEMS

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Abstract In the last few years, 3D ultrasound probes have became readily available. New fields of image-guided surgery applications are opened by attaching small electromagnetic position sensors to 3D ultrasound probes. However, nothing is known about the distortions caused by 3D ultrasound probes regarding electromagnetic sensors. Several trials were performed to investigate error-proneness of state-of-the-art electromagnetic tracking systems when used in combination with 3D ultrasound probes. It was found that 3D ultrasound probes do distort electromagnetic sensors more than 2D probes do. When attaching electromagnetic sensors to 3D probes, maximum errors of 5 mm up to 119 mm occur. The distortion strongly depends on the electromagnetic technology as well on the probe technology used. Thus, for 3D ultrasound-guided applications using electromagnetic tracking technology, the interference of ultrasound probes and electromagnetic sensors have to be checked carefully. (E-mail: mark.hastenteufel@gmx.de )
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paper_title: An Evaluation of the Aurora System as a Flesh-Point Tracking Tool for Speech Production Research

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Purpose Northern Digital Instruments (NDI; Waterloo, Ontario, Canada) manufactures a commercially available magnetometer device called Aurora that features real-time display of sensor position trac...
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paper_title: Method for evaluating compatibility of commercial electromagnetic (EM) microsensor tracking systems with surgical and imaging tables

paper_content:

Electromagnetic (EM) tracking systems have been successfully used for Surgical Navigation in ENT, cranial, and spine ::: applications for several years. Catheter sized micro EM sensors have also been used in tightly controlled cardiac ::: mapping and pulmonary applications. EM systems have the benefit over optical navigation systems of not requiring a ::: line-of-sight between devices. Ferrous metals or conductive materials that are transient within the EM working volume ::: may impact tracking performance. Effective methods for detecting and reporting EM field distortions are generally well ::: known. Distortion compensation can be achieved for objects that have a static spatial relationship to a tracking sensor. ::: New commercially available micro EM tracking systems offer opportunities for expanded image-guided navigation ::: procedures. It is important to know and understand how well these systems perform with different surgical tables and ::: ancillary equipment. By their design and intended use, micro EM sensors will be located at the distal tip of tracked ::: devices and therefore be in closer proximity to the tables. ::: Our goal was to define a simple and portable process that could be used to estimate the EM tracker accuracy, and to ::: vet a large number of popular general surgery and imaging tables that are used in the United States and abroad.
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paper_title: Accuracy of an electromagnetic tracking device: a study of the optimal range and metal interference.

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The positional and rotational accuracy of a direct-current magnetic tracking device commonly used in biomechanical investigations was evaluated. The effect of different metals was also studied to determine the possibility of interference induced by experimental test fixtures or orthopaedic implants within the working field. Positional and rotational data were evaluated for accuracy and resolution by comparing the device output to known motions as derived from a calibrated grid board or materials testing machine. The effect of different metals was evaluated by placing cylindrical metal samples at set locations throughout the working field and comparing the device readings before and after introducing each metal sample. Positional testing revealed an optimal operational range with the transmitter and receiver separation between 22.5 and 64.0 cm. Within this range the mean positional error was found to be 1.8 percent of the step size, and resolution was determined to be 0.25 mm. The mean rotational error over a 1-20 degree range was found to be 1.6% of the rotational increment with a rotational resolution of 0.1 degrees. Of the metal alloys tested only mild steel produced significant interference, which was maximum when the sample was placed adjacent to the receiver. At this location the mild steel induced a positional difference of 5.26 cm and an angular difference of 9.75 degrees. The device was found to be insensitive to commonly used orthopaedic alloys. In this study, the electromagnetic tracking device was found to have positional and rotational errors of less than 2 percent, when utilized within its optimal operating range. This accuracy combined with its insensitivity to orthopaedic alloys should make it suitable for a variety of musculoskeletal research investigations.
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paper_title: The Effects of Metals and Interfering Fields on Electromagnetic Trackers

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The operation of six degree-of-freedom electromagnetic trackers is based on the spatial properties of the electromagnetic fields generated by three small coils. Anything in the environment that causes these fields to be distorted will result in measurement noise and/or errors. An experimental investigation was undertaken to measure the effect of external fields present in a typical working environment (namely mains and computer monitor fields) and the presence of metals (25-mm cubes of various types of metals, a large steel bar, and a large steel sheet). A theoretical model is proposed to explain the observations. Two devices were used in this investigation: a Polhemus Fastrak and an Ascension Flock of Birds.
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paper_title: Standardized assessment of new electromagnetic field generators in an interventional radiology setting.

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PURPOSE ::: Two of the main challenges associated with electromagnetic (EM) tracking in computer-assisted interventions (CAIs) are (1) the compensation of systematic distance errors arising from the influence of metal near the field generator (FG) or the tracked sensor and (2) the optimized setup of the FG to maximize tracking accuracy in the area of interest. Recently, two new FGs addressing these issues were proposed for the well-established Aurora(®) tracking system [Northern Digital, Inc. (NDI), Waterloo, Canada]: the Tabletop 50-70 FG, a planar transmitter with a built-in shield that compensates for metal distortions emanating from treatment tables, and the prototypical Compact FG 7-10, a mobile generator designed to be attached to mobile imaging devices. The purpose of this paper was to assess the accuracy and precision of these new FGs in an interventional radiology setting. ::: ::: ::: METHODS ::: A standardized assessment protocol, which uses a precisely machined base plate to measure relative error in position and orientation, was applied to the two new FGs as well as to the well-established standard Aurora(®) Planar FG. The experiments were performed in two different settings: a reference laboratory environment and a computed tomography (CT) scanning room. In each setting, the protocol was applied to three different poses of the measurement plate within the tracking volume of the three FGs. ::: ::: ::: RESULTS ::: The two new FGs provided higher precision and accuracy within their respective measurement volumes as well as higher robustness with respect to the CT scanner compared to the established FG. Considering all possible 5 cm distances on the grid, the error of the Planar FG was increased by a factor of 5.94 in the clinical environment (4.4 mm) in comparison to the error in the laboratory environment (0.8 mm). In contrast, the mean values for the two new FGs were all below 1 mm with an increase in the error by factors of only 2.94 (Reference: 0.3 mm; CT: 0.9 mm) and 1.04 (both: 0.5 mm) in the case of the Tabletop FG and the Compact FG, respectively. ::: ::: ::: CONCLUSIONS ::: Due to their high accuracy and robustness, the Tabletop FG and the Compact FG could eliminate the need for compensation of EM field distortions in certain CT-guided interventions.
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paper_title: A novel technique for analysis of accuracy of magnetic tracking systems used in image guided surgery

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With the increased use and development of image-guided surgical applications, there is a need for methods of ::: analysis of the accuracy and precision of the components which compose these systems. One primary component ::: of an image-guided surgery system is the position tracking system which allows for the localization of a tool within ::: the surgical field and provides information which is translated back to the images. Previously much work has been ::: done in characterizing these systems for spatial accuracy and precision. Much of this previous work examines ::: single tracking systems or modalities. We have devised a method which allows for the characterization of a novel ::: tracking system independent of modality and location. We describe the development of a phantom system which ::: allows for rapid design and creation of surfaces with different geometries. We have also demonstrated a method of ::: analysis of the data generated by this phantom system, and used it to compare Biosense-Webster's CartoXP TM , ::: and Northern Digital's Aurora TM magnetic trackers. We have determined that the accuracy and precision of the ::: CartoXP was best, followed closely by the Aurora's dome volume, then the Aurora's cube volume. The mean ::: accuracy for all systems was better than 3mm and decays with distance from the field generator.
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paper_title: A direction space interpolation technique for calibration of electromagnetic surgical navigation systems

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A new generation of electromagnetic (EM) navigation systems with extremely compact sensors have great potential for clinical applications requiring that surgical devices be tracked within a patient’s body. However, electro-magnetic field distortions limit the accuracy of such devices. Further, the errors may be sensitive both to position and orientation of EM sensors within the field. This paper presents a computationally efficient method for in-situ 5 DOF calibration of the basic sensors of a typical EM system (Northern Digital’s Aurora), and presents preliminary results demonstrating an improvement of approximately 2.6 : 1 positional accuracy and 1.6 : 1 for orientation even when the sensors are moved through arbitrary orientation changes. This work represents one step in a larger effort to understand the field distortions associated with these systems and to develop effective and predictable calibration and registration strategies for their use in stereotactic image-guided interventions.
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paper_title: Quantification of AC electromagnetic tracking system accuracy in a CT scanner environment

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The purpose of this study was to quantify the effects of a computed tomography (CT) scanner environment on the ::: positional accuracy of an AC electromagnetic tracking system, the second generation NDI Aurora. A three-axis ::: positioning robot was used to move an electromagnetically tracked needle above the CT table throughout a 30cm by ::: 30cm axial plane sampled in 2.5cm steps. The corresponding position data was captured from the Aurora and was ::: registered to the positioning system data using a rigid body transformation minimizing the least squares L2-norm. Data ::: was sampled at varying distances from the CT gantry (three feet, two feet, and one foot) and with the CT table in a ::: nominal position and lowered by 10cm. A coordinate system was defined with the x axis normal to the CT table and the ::: origin at the center of the CT table, and the z axis spanning the table in the lateral direction with the origin at the center ::: of the CT table. In this coordinate system, the positional relationships of each sampled point, the CT table, and the ::: Aurora field generator are clearly defined. This allows error maps to be displayed in accurate spatial relationship to the ::: CT scanner as well as to a representative patient anatomy. By quantifying the distortions in relation to the position of CT ::: scanner components and the Aurora field generator, the optimal working field of view and recommended guidelines for ::: operation can be determined such that targeting inside human anatomy can be done with reasonable expectations of ::: desired performance.
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paper_title: Dynamic Response of Electromagnetic Spatial Displacement Trackers

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Overall system latency-the elapsed time from input human motion until the immediate consequences of that input are available in the display-is one of the most frequently cited shortcoming of current virtual environment VE technology. Given that spatial displacement trackers are employed to monitor head and hand position and orientation in many VE applications, the dynamic response intrinsic to these devices is an unavoidable contributor to overall system latency. In this paper, we describe a testbed and method for measurement of tracker dynamic response that use a motorized rotary swing arm to sinusoidally displace the VE sensor at a number of frequencies spanning the bandwidth of volitional human movement. During the tests, actual swing arm angle and VE sensor reports are collected and time stamped. By calibrating the time stamping technique, the tracker's internal transduction and processing time are separated from data transfer and host computer software execution latencies. We have used this test-bed to examine several VE sensors-most recently to compare latency, gain, and noise characteristics of two commercially available electromagnetic trackers: Ascension Technology Corp.'s Flock of BirdsTM and Polhemus Inc.'s FastrakTM.
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paper_title: Accuracy assessment for navigated maxillo-facial surgery using an electromagnetic tracking device

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Abstract Purpose To evaluate the accuracy and the usability of an electromagnetic tracking device in maxillo-facial surgery through testing on a phantom skull under operating room (OR) conditions. Material and methods A standard plastic skull phantom was equipped with a custom made model of the maxilla and with target markers and dental brackets. Imaging was performed with a computed tomography (CT) scanner. The extent and robustness of the electromagnetic tracking system’s target registration error (TRE) was evaluated under various conditions. Results For each measurement a total of 243 registrations were performed with 5 point registration and 4374 registrations with 6 point registration. The average target registration error for the 5 point registration under OR conditions was 2.1 mm (SD 0.86) and 1.03 (SD 0.53) for the 6 point registration. Metallic instruments applied to the skull increased the TRE significantly in both registration methods. Conclusion The electromagnetic tracking device showed a high accuracy and performed stable in both registration methods. Electromagnetic interference due to metallic instruments was significant but the extent of TRE was still acceptable in comparison to optical navigation devices. A benefit of EM tracking is the absence of line-of-sight hindrance. The test setting simulating OR conditions has proven suitable for further studies.
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paper_title: Electromagnetic Tracking for Thermal Ablation and Biopsy Guidance: Clinical Evaluation of Spatial Accuracy

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Purpose To evaluate the spatial accuracy of electromagnetic needle tracking and demonstrate the feasibility of ultrasonography (US)–computed tomography (CT) fusion during CT- and US-guided biopsy and radiofrequency ablation procedures. Materials and Methods The authors performed a 20-patient clinical trial to investigate electromagnetic needle tracking during interventional procedures. The study was approved by the institutional investigational review board, and written informed consent was obtained from all patients. Needles were positioned by using CT and US guidance. A commercial electromagnetic tracking device was used in combination with prototype internally tracked needles and custom software to record needle positions relative to previously obtained CT scans. Position tracking data were acquired to evaluate the tracking error, defined as the difference between tracked needle position and reference standard needle position on verification CT scans. Registration between tracking space and image space was obtained by using reference markers attached to the skin ("fiducials"), and different registration methods were compared. The US transducer was tracked to demonstrate the potential use of real-time US-CT fusion for imaging guidance. Results One patient was excluded from analysis because he was unable to follow breathing instructions during the acquisition of CT scans. Nineteen of the 20 patients were evaluable, demonstrating a basic tracking error of 5.8 mm ± 2.6, which improved to 3.5 mm ± 1.9 with use of nonrigid registrations that used previous internal needle positions as additional fiducials. Fusion of tracked US with CT was successful. Patient motion and distortion of the tracking system by the CT table and gantry were identified as sources of error. Conclusions The demonstrated spatial tracking accuracy is sufficient to display clinically relevant preprocedural imaging information during needle-based procedures. Virtual needles displayed within preprocedural images may be helpful for clandestine targets such as arterial phase enhancing liver lesions or during thermal ablations when obscuring gas is released. Electromagnetic tracking may help improve imaging guidance for interventional procedures and warrants further investigation, especially for procedures in which the outcomes are dependent on accuracy.
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paper_title: ELECTROMAGNETIC TRACKING SYSTEM FOR NEUROVASCULAR INTERVENTIONS

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Electromagnetic tracking systems (EMTS) are widely used in a computer-assisted surgery, motion capture, kinematic studies and military applications. The main application in the medical domain represents minimal invasive surgery. For this purpose, the EMTS systems are integrated into the surgery device, i.e. needle, endoscope or catheter supporting tracking of this device within the human body without X-ray radiation. However, in neurovascular interventions this technique hardly could be applied so far. This is mainly due to a too large size of the receiver coils which have to be integrated into the microcatheter. In this paper we present such a microcatheter for neurovascular interventions featured with a 5 DOF sensor enabling tracking of the catheter tip by a commercial EMTS system and first evaluation of its accuracy.
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paper_title: Accuracy of navigation: a comparative study of infrared optical and electromagnetic navigation.

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We evaluated the accuracy of navigation systems for measuring the mechanical axis in patients undergoing total knee arthroplasty and in the synthetic bone model. Infrared optical and electromagnetic navigation systems were compared. Both systems were found to be accurate and reproducible in an experimental environment. However, the accuracies of both systems were affected by erroneous registration, and the optical system was found to be more reproducible. In clinical situations, the mean difference was 1.23 degrees, and difference greater than 3 degrees occurred in 15% of clinical trials. These discordances may have been due to ambiguous anatomic landmarks causing registration errors and the possibility of electromagnetic signal interference in the operating room.
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paper_title: Navigation with Electromagnetic Tracking for Interventional Radiology Procedures: A Feasibility Study

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PURPOSE ::: To assess the feasibility of the use of preprocedural imaging for guide wire, catheter, and needle navigation with electromagnetic tracking in phantom and animal models. ::: ::: ::: MATERIALS AND METHODS ::: An image-guided intervention software system was developed based on open-source software components. Catheters, needles, and guide wires were constructed with small position and orientation sensors in the tips. A tetrahedral-shaped weak electromagnetic field generator was placed in proximity to an abdominal vascular phantom or three pigs on the angiography table. Preprocedural computed tomographic (CT) images of the phantom or pig were loaded into custom-developed tracking, registration, navigation, and rendering software. Devices were manipulated within the phantom or pig with guidance from the previously acquired CT scan and simultaneous real-time angiography. Navigation within positron emission tomography (PET) and magnetic resonance (MR) volumetric datasets was also performed. External and endovascular fiducials were used for registration in the phantom, and registration error and tracking error were estimated. ::: ::: ::: RESULTS ::: The CT scan position of the devices within phantoms and pigs was accurately determined during angiography and biopsy procedures, with manageable error for some applications. Preprocedural CT depicted the anatomy in the region of the devices with real-time position updating and minimal registration error and tracking error (<5 mm). PET can also be used with this system to guide percutaneous biopsies to the most metabolically active region of a tumor. ::: ::: ::: CONCLUSIONS ::: Previously acquired CT, MR, or PET data can be accurately codisplayed during procedures with reconstructed imaging based on the position and orientation of catheters, guide wires, or needles. Multimodality interventions are feasible by allowing the real-time updated display of previously acquired functional or morphologic imaging during angiography, biopsy, and ablation.
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paper_title: Accuracy assessment for navigated maxillo-facial surgery using an electromagnetic tracking device

paper_content:

Abstract Purpose To evaluate the accuracy and the usability of an electromagnetic tracking device in maxillo-facial surgery through testing on a phantom skull under operating room (OR) conditions. Material and methods A standard plastic skull phantom was equipped with a custom made model of the maxilla and with target markers and dental brackets. Imaging was performed with a computed tomography (CT) scanner. The extent and robustness of the electromagnetic tracking system’s target registration error (TRE) was evaluated under various conditions. Results For each measurement a total of 243 registrations were performed with 5 point registration and 4374 registrations with 6 point registration. The average target registration error for the 5 point registration under OR conditions was 2.1 mm (SD 0.86) and 1.03 (SD 0.53) for the 6 point registration. Metallic instruments applied to the skull increased the TRE significantly in both registration methods. Conclusion The electromagnetic tracking device showed a high accuracy and performed stable in both registration methods. Electromagnetic interference due to metallic instruments was significant but the extent of TRE was still acceptable in comparison to optical navigation devices. A benefit of EM tracking is the absence of line-of-sight hindrance. The test setting simulating OR conditions has proven suitable for further studies.
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paper_title: How Does Electromagnetic Navigation Stack Up Against Infrared Navigation in Minimally Invasive Total Knee Arthroplasties

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Abstract Forty-six primary total knee arthroplasties were performed using either an electromagnetic (EM) or infrared (IR) navigation system. In this IRB-approved study, patients were evaluated clinically and for accuracy using spiral computed tomographic imaging and 36-in standing radiographs. Although EM navigation was subject to metal interference, it was not as drastic as line-of-sight interference with IR navigation. Mechanical alignment was ideal in 92.9% of EM and 90.0% of IR cases based on spiral computed tomographic imaging and 100% of EM and 95% of IR cases based on x-ray. Individual measurements of component varus/valgus and sagittal measurements showed EM to be equivalent to IR, with both systems producing subdegree accuracy in 95% of the readings.
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paper_title: Technical accuracy of optical and the electromagnetic tracking systems

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Thousands of operations are annually guided with computer assisted surgery (CAS) technologies. As the use of these devices is rapidly increasing, the reliability of the devices becomes ever more critical. The problem of accuracy assessment of the devices has thus become relevant. During the past five years, over 200 hazardous situations have been documented in the MAUDE database during operations using these devices in the field of neurosurgery alone. Had the accuracy of these devices been periodically assessed pre-operatively, many of them might have been prevented. The technical accuracy of a commercial navigator enabling the use of both optical (OTS) and electromagnetic (EMTS) tracking systems was assessed in the hospital setting using accuracy assessment tools and methods developed by the authors of this paper. The technical accuracy was obtained by comparing the positions of the navigated tool tip with the phantom accuracy assessment points. Each assessment contained a total of 51 points and a region of surgical interest (ROSI) volume of 120x120x100 mm roughly mimicking the size of the human head. The error analysis provided a comprehensive understanding of the trend of accuracy of the surgical navigator modalities. This study showed that the technical accuracies of OTS and EMTS over the pre-determined ROSI were nearly equal. However, the placement of the particular modality hardware needs to be optimized for the surgical procedure. New applications of EMTS, which does not require rigid immobilization of the surgical area, are suggested.
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paper_title: Electromagnetic navigation bronchoscopy: A descriptive analysis

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Electromagnetic navigation bronchoscopy (ENB) is an exciting new bronchoscopic technique that promises accurate navigation to peripheral pulmonary target lesions, using technology similar to a car global positioning system (GPS) unit. Potential uses for ENB include biopsy of peripheral lung lesions, pleural dye marking of nodules for surgical wedge resection, placement of fiducial markers for stereotactic radiotherapy, and therapeutic insertion of brachytherapy catheters into malignant tissue. This article will describe the ENB procedure, review the published literature, compare ENB to existing biopsy techniques, and outline the challenges for widespread implementation of this new technology.
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paper_title: Evaluation of dynamic electromagnetic tracking deviation

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Electromagnetic tracking systems (EMTS's) are widely used in clinical applications. Many reports have evaluated their static behavior and errors caused by metallic objects were examined. Although there exist some publications concerning the dynamic behavior of EMTS's the measurement protocols are either difficult to reproduce with respect of the movement path or only accomplished at high technical effort. Because dynamic behavior is of major interest with respect to clinical applications we established a simple but effective modal measurement easy to repeat at other laboratories. We built a simple pendulum where the sensor of our EMTS (Aurora, NDI, CA) could be mounted. The pendulum was mounted on a special bearing to guarantee that the pendulum path is planar. This assumption was tested before starting the measurements. All relevant parameters defining the pendulum motion such as rotation center and length are determined by static measurement at satisfactory accuracy. Then position and orientation data were gathered over a time period of 8 seconds and timestamps were recorded. Data analysis provided a positioning error and an overall error combining both position and orientation. All errors were calculated by means of the well know equations concerning pendulum movement. Additionally, latency - the elapsed time from input motion until the immediate consequences of that input are available - was calculated using well-known equations for mechanical pendulums for different velocities. We repeated the measurements with different metal objects (rods made of stainless steel type 303 and 416) between field generator and pendulum. ::: We found a root mean square error ( e RMS ) of 1.02mm with respect to the distance of the sensor position to the fit plane (maximum error e max = 2.31mm, minimum error e min = -2.36mm). The eRMS for positional error amounted to 1.32mm while the overall error was 3.24 mm. The latency at a pendulum angle of 0° (vertical) was 7.8ms.
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paper_title: Navigation Systems for Ablation

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Navigation systems, devices, and intraprocedural software are changing the way interventional oncology is practiced. Before the development of precision navigation tools integrated with imaging systems, thermal ablation of hard-to-image lesions was highly dependent on operator experience, spatial skills, and estimation of positron emission tomography–avid or arterial-phase targets. Numerous navigation systems for ablation bring the opportunity for standardization and accuracy that extends the operator's ability to use imaging feedback during procedures. In this report, existing systems and techniques are reviewed and specific clinical applications for ablation are discussed to better define how these novel technologies address specific clinical needs and fit into clinical practice.
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paper_title: Electromagnetic tracking in the clinical environment

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When choosing an electromagnetic tracking system (EMTS) for image-guided procedures several factors must be taken into consideration. Among others these include the system's refresh rate, the number of sensors that need to be tracked, the size of the navigated region, the system interaction with the environment, whether the sensors can be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. To date, the only factors that have been studied extensively are the accuracy and the susceptibility of EMTSs to distortions caused by ferromagnetic materials. In this paper the authors shift the focus from analysis of system accuracy and stability to the broader set of factors influencing the utility of EMTS in the clinical environment. The authors provide an analysis based on all of the factors specified above, as assessed in three clinical environments. They evaluate two commercial tracking systems, the Aurora system from Northern Digital Inc., and the 3D Guidance system with three different field generators from Ascension Technology Corp. The authors show that these systems are applicable to specific procedures and specific environments, but that currently, no single system configuration provides a comprehensive solution across procedures and environments.
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paper_title: Immersive 3DUI on one dollar a day

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A convergence between consumer electronics and virtual reality is occurring. We present an immersive head-mounted-display-based, wearable 3D user interface that is inexpensive (less than $900 USD), robust (sourceless tracking), and portable (lightweight and untethered). While the current display has known deficiencies, the user tracking quality is within the constraints of many existing applications, while the portability and cost offers opportunities for innovative applications that are not currently feasible.
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paper_title: Model for defining and reporting reference-based validation protocols in medical image processing

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Objectives Image processing tools are often embedded in larger systems. Validation of image processing methods is important because the performance of such methods can have an impact on the performance of the larger systems and consequently on decisions and actions based on the use of these systems. Most validation studies compare the direct or indirect results of a method with a reference that is assumed to be very close or equal to the correct solution. In this paper, we propose a model for defining and reporting reference-based validation protocols in medical image processing.
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paper_title: THE EFFECT OF TRANSPONDER MOTION ON THE ACCURACY OF THE CALYPSO ELECTROMAGNETIC LOCALIZATION SYSTEM

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Purpose: To determine position and velocity-dependent effects in the overall accuracy of the Calypso Electromagnetic localization system, under conditions that emulate transponder motion during normal free breathing. Methods and Materials: Three localization transponders were mounted on a remote-controlled turntable that could move the transponders along a circular trajectory at speeds up to 3 cm/s. A stationary calibration established the coordinates of multiple points on each transponder's circular path. Position measurements taken while the transponders were in motion at a constant speed were then compared with the stationary coordinates. Results: No statistically significant changes in the transponder positions in (x,y,z) were detected when the transponders were in motion. Conclusions: The accuracy of the localization system is unaffected by transponder motion.
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paper_title: Accuracy of navigation: a comparative study of infrared optical and electromagnetic navigation.

paper_content:

We evaluated the accuracy of navigation systems for measuring the mechanical axis in patients undergoing total knee arthroplasty and in the synthetic bone model. Infrared optical and electromagnetic navigation systems were compared. Both systems were found to be accurate and reproducible in an experimental environment. However, the accuracies of both systems were affected by erroneous registration, and the optical system was found to be more reproducible. In clinical situations, the mean difference was 1.23 degrees, and difference greater than 3 degrees occurred in 15% of clinical trials. These discordances may have been due to ambiguous anatomic landmarks causing registration errors and the possibility of electromagnetic signal interference in the operating room.
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paper_title: Design and application of an assessment protocol for electromagnetic tracking systems.

paper_content:

This paper defines a simple protocol for competitive and quantified evaluation of electromagnetic tracking systems such as the NDI Aurora (A) and Ascension microBIRD with dipole transmitter (B). It establishes new methods and a new phantom design which assesses the reproducibility and allows comparability with different tracking systems in a consistent environment. A machined base plate was designed and manufactured in which a 50 mm grid of holes was precisely drilled for position measurements. In the center a circle of 32 equispaced holes enables the accurate measurement of rotation. The sensors can be clamped in a small mount which fits into pairs of grid holes on the base plate. Relative positional/orientational errors are found by subtracting the known distances/ rotations between the machined locations from the differences of the mean observed positions/ rotation. To measure the influence of metallic objects we inserted rods made of steel (SST 303, SST 416), aluminum, and bronze into the sensitive volume between sensor and emitter. We calculated the fiducial registration error and fiducial location error with a standard stylus calibration for both tracking systems and assessed two different methods of stylus calibration. The positional jitter amounted to 0.14 mm(A) and 0.08 mm(B). A relative positional error of 0.96 mm +/- 0.68 mm, range -0.06 mm; 2.23 mm(A) and 1.14 mm +/- 0.78 mm, range -3.72 mm; 1.57 mm(B) for a given distance of 50 mm was found. The relative rotation error was found to be 0.51 degrees (A)/0.04 degrees (B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2 mm(A)/ > or = 100 mm(B) occurs when the rod is close to the sensor(20 mm). While (B) is more sensitive with respect to metallic objects, (A) is less accurate concerning orientation measurements. (B) showed a systematic error when distances are calculated.
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paper_title: Electromagnetic Servoing—A New Tracking Paradigm

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Electromagnetic (EM) tracking is highly relevant for many computer assisted interventions. This is in particular due to the fact that the scientific community has not yet developed a general solution for tracking of flexible instruments within the human body. Electromagnetic tracking solutions are highly attractive for minimally invasive procedures, since they do not require line of sight. However, a major problem with EM tracking solutions is that they do not provide uniform accuracy throughout the tracking volume and the desired, highest accuracy is often only achieved close to the center of tracking volume. In this paper, we present a solution to the tracking problem, by mounting an EM field generator onto a robot arm. Proposing a new tracking paradigm, we take advantage of the electromagnetic tracking to detect the sensor within a specific sub-volume, with known and optimal accuracy. We then use the more accurate and robust robot positioning for obtaining uniform accuracy throughout the tracking volume. Such an EM servoing methodology guarantees optimal and uniform accuracy, by allowing us to always keep the tracked sensor close to the center of the tracking volume. In this paper, both dynamic accuracy and accuracy distribution within the tracking volume are evaluated using optical tracking as ground truth. In repeated evaluations, the proposed method was able to reduce the overall error from 6.64±7.86 mm to a significantly improved accuracy of 3.83±6.43 mm. In addition, the combined system provides a larger tracking volume, which is only limited by the reach of the robot and not the much smaller tracking volume defined by the magnetic field generator.
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paper_title: Accuracy of a wireless localization system for radiotherapy.

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PURPOSE ::: A system has been developed for patient positioning based on real-time localization of implanted electromagnetic transponders (beacons). This study demonstrated the accuracy of the system before clinical trials. ::: ::: ::: METHODS AND MATERIALS ::: We describe the overall system. The localization component consists of beacons and a source array. A rigid phantom was constructed to place the beacons at known offsets from a localization array. Tests were performed at distances of 80 and 270 mm from the array and at positions in the array plane of up to 8 cm offset. Tests were performed in air and saline to assess the effect of tissue conductivity and with multiple transponders to evaluate crosstalk. Tracking was tested using a dynamic phantom creating a circular path at varying speeds. ::: ::: ::: RESULTS ::: Submillimeter accuracy was maintained throughout all experiments. Precision was greater proximal to the source plane (sigmax = 0.006 mm, sigmay = 0.01 mm, sigmaz = 0.006 mm), but continued to be submillimeter at the end of the designed tracking range at 270 mm from the array (sigmax = 0.27 mm, sigmay = 0.36 mm, sigmaz = 0.48 mm). The introduction of saline and the use of multiple beacons did not affect accuracy. Submillimeter accuracy was maintained using the dynamic phantom at speeds of up to 3 cm/s. ::: ::: ::: CONCLUSION ::: This system has demonstrated the accuracy needed for localization and monitoring of position during treatment.
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paper_title: Computer-aided navigation in neurosurgery

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The article comprises three main parts: a historical review on navigation, the mathematical basics for calculation and the clinical applications of navigation devices. Main historical steps are described from the first idea till the realisation of the frame-based and frameless navigation devices including robots. In particular the idea of robots can be traced back to the Iliad of Homer, the first testimony of European literature over 2500 years ago. In the second part the mathematical calculation of the mapping between the navigation and the image space is demonstrated, including different registration modalities and error estimations. The error of the navigation has to be divided into the technical error of the device calculating its own position in space, the registration error due to inaccuracies in the calculation of the transformation matrix between the navigation and the image space, and the application error caused additionally by anatomical shift of the brain structures during operation. In the third part the main clinical fields of application in modern neurosurgery are demonstrated, such as localisation of small intracranial lesions, skull-base surgery, intracerebral biopsies, intracranial endoscopy, functional neurosurgery and spinal navigation. At the end of the article some possible objections to navigation-aided surgery are discussed.
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paper_title: Image-guided neurosurgery with 3-dimensional multimodal imaging data on a stereoscopic monitor.

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BACKGROUND ::: In the past 2 decades, intraoperative navigation technology has changed preoperative and intraoperative strategies and methodology tremendously. ::: ::: ::: OBJECTIVE ::: To report our first experiences with a stereoscopic navigation system based on multimodality-derived, patient-specific 3-dimensional (3-D) information displayed on a stereoscopic monitor and controlled by a virtual user interface. ::: ::: ::: METHODS ::: For the planning of each case, a 3-D multimodality model was created on the Dextroscope. The 3-D model was transferred to a console in the operating room that was running Dextroscope-compatible software and included a stereoscopic LCD (liquid crystal display) monitor (DexVue). Surgery was carried out with a standard frameless navigation system (VectorVision, BrainLAB) that was linked to DexVue. Making use of the navigational space coordinates provided by the VectorVision system during surgery, we coregistered the patient's 3-D model with the actual patient in the operating room. The 3-D model could then be displayed as seen along the axis of a handheld probe or the microscope view. The DexVue data were viewed with polarizing glasses and operated via a 3-D interface controlled by a cordless mouse containing inertial sensors. The navigational value of DexVue was evaluated postoperatively with a questionnaire. A total of 39 evaluations of 21 procedures were available. ::: ::: ::: RESULTS ::: In all 21 cases, the connection of VectorVision with DexVue worked reliably, and consistent spatial concordance of the navigational information was displayed on both systems. The questionnaires showed that in all cases the stereoscopic 3-D data were preferred for navigation. In 38 of 39 evaluations, the spatial orientation provided by the DexVue system was regarded as an improvement. In no case was there worsened spatial orientation. ::: ::: ::: CONCLUSION ::: We consider navigating primarily with stereoscopic, 3-D multimodality data an improvement over navigating with image planes, and we believe that this technology enables a more intuitive intraoperative interpretation of the displayed navigational information and hence an easier surgical implementation of the preoperative plan.
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paper_title: A comparison of optical and electromagnetic computer-assisted navigation systems for fluoroscopic targeting.

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Objectives: Freehand targeting using fluoroscopic guidance is routine for placement of interlocking screws associated with intramedullary nailing and for insertion of screws for reconstruction of pelvic and acetabular injuries. New technologies that use fluoroscopy with the assistance of computer guidance have the potential to improve accuracy and reduce radiation exposure to patient and surgeon. We sought to compare 2 fluoroscopic navigation tracking technologies, optical and electromagnetic versus standard freehand fluoroscopic targeting in a standardized model. Intervention: Three experienced orthopaedic trauma surgeons placed 3.2-mm guide pins through test foam blocks that simulate cancellous bone. The entry site for each pin was within a circular (18-mm) entry zone. On the opposite surface of the test block (130-mm across), the target was a 1-mm-diameter radioopaque spherical ball marker. Each surgeon placed 10 pins using freehand targeting (control group) navigation using Medtronic iON StealthStation (Optical A), navigation using BrainLAB VectorVision (Optical B), or navigation using GE Medical Systems InstaTrak 3500 system (EM). Outcome Measurements: Data were collected for accuracy (the distance from the exit site of the guidewire to the target spherical ball marker), fluoroscopy time (seconds), and total number of individual fluoroscopy images taken. Results: The 2 optical systems and the electromagnetic system provided significantly improved accuracy compared to freehand technique. The average distance from the target was significantly (3.5 times) greater for controls (7.1 mm) than for each of the navigated systems (Optical A = 2.1 mm, Optical B = 1.9 mm EM = 2.4 mm; P 0.05). The ability to place guidewires in a 5-mm safe zone surrounding the target sphere was also significantly improved with the optical systems and the EM system (99% of wires in the safe zone) compared to controls (47% in the safe zone) (P 0.05). Fluoroscopy time (seconds) and number of fluoroscopy images were similar among the three navigated groups (P > 0.05). Each of these parameters was significantly less when using the computer-guided systems than for freehand-unguided insertion (P < 0.01). Conclusions: Both optical and electromagnetic computer-assisted guidance systems have the potential to improve accuracy and reduce radiation use for freehand fluoroscopic targeting in orthopaedic surgery.
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paper_title: Standardized assessment of new electromagnetic field generators in an interventional radiology setting.

paper_content:

PURPOSE ::: Two of the main challenges associated with electromagnetic (EM) tracking in computer-assisted interventions (CAIs) are (1) the compensation of systematic distance errors arising from the influence of metal near the field generator (FG) or the tracked sensor and (2) the optimized setup of the FG to maximize tracking accuracy in the area of interest. Recently, two new FGs addressing these issues were proposed for the well-established Aurora(®) tracking system [Northern Digital, Inc. (NDI), Waterloo, Canada]: the Tabletop 50-70 FG, a planar transmitter with a built-in shield that compensates for metal distortions emanating from treatment tables, and the prototypical Compact FG 7-10, a mobile generator designed to be attached to mobile imaging devices. The purpose of this paper was to assess the accuracy and precision of these new FGs in an interventional radiology setting. ::: ::: ::: METHODS ::: A standardized assessment protocol, which uses a precisely machined base plate to measure relative error in position and orientation, was applied to the two new FGs as well as to the well-established standard Aurora(®) Planar FG. The experiments were performed in two different settings: a reference laboratory environment and a computed tomography (CT) scanning room. In each setting, the protocol was applied to three different poses of the measurement plate within the tracking volume of the three FGs. ::: ::: ::: RESULTS ::: The two new FGs provided higher precision and accuracy within their respective measurement volumes as well as higher robustness with respect to the CT scanner compared to the established FG. Considering all possible 5 cm distances on the grid, the error of the Planar FG was increased by a factor of 5.94 in the clinical environment (4.4 mm) in comparison to the error in the laboratory environment (0.8 mm). In contrast, the mean values for the two new FGs were all below 1 mm with an increase in the error by factors of only 2.94 (Reference: 0.3 mm; CT: 0.9 mm) and 1.04 (both: 0.5 mm) in the case of the Tabletop FG and the Compact FG, respectively. ::: ::: ::: CONCLUSIONS ::: Due to their high accuracy and robustness, the Tabletop FG and the Compact FG could eliminate the need for compensation of EM field distortions in certain CT-guided interventions.
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paper_title: A buyer's guide to electromagnetic tracking systems for clinical applications

paper_content:

When choosing an Electromagnetic Tracking System (EMTS) for image-guided procedures, it is desirable for the system to be usable for different procedures and environments. Several factors influence this choice. To date, the only factors that have been studied extensively, are the accuracy and the susceptibility of electromagnetic tracking systems to distortions caused by ferromagnetic materials. In this paper we provide a holistic overview of the factors that should be taken into account when choosing an EMTS. These factors include: the system’s refresh rate, the number of sensors that need to be tracked, the size of the navigated region, system interaction with the environment, can the sensors be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. We evaluate the Aurora EMTS (Northern Digital Inc., Waterloo, Ontario, Canada) and the 3D Guidance EMTS with the flat-panel and the short-range field generators (Ascension Technology Corp., Burlington, Vermont, USA) in three clinical environments. We show that these systems are applicable to specific procedures or in specific environments, but that, no single system is currently optimal for all environments and procedures we evaluated.
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paper_title: Effect of metal and sampling rate on accuracy of Flock of Birds electromagnetic tracking system.

paper_content:

Electromagnetic tracking devices are used in many biomechanics applications. Previous studies have shown that metal located within the working field of direct current electromagnetic tracking devices produces significant errors. However, the effect of sampling rate on the errors produced in a metallic environment has never been studied. In this study, the accuracy of Ascension Technologies' Flock of Birds was evaluated at sampling rates of 20, 60, 100, and 140 Hz, in the presence of both aluminum and steel. Aluminum interference caused an increase in measurement error as the sampling rate increased. Conversely, steel interference caused a decrease in measurement error as the sampling rate increased. We concluded that the accuracy of the Flock of Birds tracking system can be optimized in the presence of metal by careful choice in sampling rate.
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paper_title: Publication bias in clinical research

paper_content:

In a retrospective survey, 487 research projects approved by the Central Oxford Research Ethics Committee between 1984 and 1987, were studied for evidence of publication bias. As of May, 1990, 285 of the studies had been analysed by the investigators, and 52% of these had been published. Studies with statistically significant results were more likely to be published than those finding no difference between the study groups (adjusted odds ratio [OR] 2.32; 95% confidence interval [Cl] 1.25-4.28). Studies with significant results were also more likely to lead to a greater number of publications and presentations and to be published in journals with a high citation impact factor. An increased likelihood of publication was also associated with a high rating by the investigator of the importance of the study results, and with increasing sample size. The tendency towards publication bias was greater with observational and laboratory-based experimental studies (OR = 3.79; 95% Cl = 1.47-9.76) than with randomised clinical trials (OR = 0.84; 95% Cl = 0.34-2.09). We have confirmed the presence of publication bias in a cohort of clinical research studies. These findings suggest that conclusions based only on a review of published data should be interpreted cautiously, especially for observational studies. Improved strategies are needed to identify the results of unpublished as well as published studies.
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paper_title: Image-guided interventions : technology and applications

paper_content:

Overview and History of Image-Guided Interventions.- Tracking Devices.- Visualization in Image-Guided Interventions.- Augmented Reality.- Software.- Rigid Registration.- Nonrigid Registration.- Model-Based Image Segmentation for Image-Guided Interventions.- Imaging Modalities.- MRI-Guided FUS and its Clinical Applications.- Neurosurgical Applications.- Computer-Assisted Orthopedic Surgery.- Thoracoabdominal Interventions.- Real-Time Interactive MRI for Guiding Cardiovascular Surgical Interventions.- Three-Dimensional Ultrasound Guidance and Robot Assistance for Prostate Brachytherapy.- Radiosurgery.- Radiation Oncology.- Assessment of Image-Guided Interventions.
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paper_title: Successful placement of postpyloric enteral tubes using electromagnetic guidance in critically ill children*

paper_content:

OBJECTIVES ::: Initiation of postpyloric feeding is often delayed by difficulties in placement of enteral tubes. We evaluated the effectiveness of bedside postpyloric enteral tube (PET) placement using an electromagnetic (EM)-guided device. We hypothesized that: 1) EM-guided placement of PETs would be successful more often than standard blind placement with a shorter total time to successful placement and 2) the EM-guided technique would have similar overall costs to the standard technique. ::: ::: ::: DESIGN ::: Prospective cohort trial with serial control groups in a pediatric intensive care unit at a tertiary care children's hospital. ::: ::: ::: INTERVENTIONS ::: We collected data on a cohort of consecutive pediatric intensive care unit patients who underwent PET placement by standard blind technique followed by a cohort who underwent EM-guided placement. The primary outcome measure was successful placement determined by abdominal radiography. ::: ::: ::: MEASUREMENTS AND MAIN RESULTS ::: One hundred seven patients were evaluated in the trial: 57 in the standard group and 50 in the EM-guided group. Demographic data, percent intubated, and admission diagnosis were similar in both groups. Forty-one of 50 patients (82%) in the EM-guided group had successful placement compared with 22 of 57 in the standard group (38%) (p < 0.0001). The average time to successful placement was 1.7 vs. 21 hours in the EM-guided group and standard group, respectively (p < 0.0001). Children in the EM-guided group received fewer radiographs (p = 0.007) and were given more prokinetic drugs (p = 0.045). There were no episodes of pneumothorax in either group. After controlling for prokinetic drug use, EM-guided placement was more likely to result in successful placement than the standard blind technique (odds ratio 6.4, 95% confidence interval 2.5-16.3). An annual placement rate of 250 PETs by EM guidance, based on our institution's current utilization rates, is associated with a cost savings of $55.46 per PET placed. ::: ::: ::: CONCLUSION ::: EM guidance is an efficient and cost-effective method of bedside PET placement.
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paper_title: Navigated targeting of liver lesions: pitfalls of electromagnetic tracking

paper_content:

One of the major challenges related to percutaneous radiofrequency ablation (RFA) of liver tumors is the exact placement of the instrument within the lesion. Previous studies have shown the benefit of computer-assisted needle insertion based on optical tracking of both the instrument and internal fiducials used for registration. However, the concept has not been accepted for clinical use. This may in part be attributed to the line-of-sight constraint imposed by optical tracking systems which results in the use of needles thick enough to avoid bending. Electromagnetic (EM) tracking systems allow the localization of medical instruments without line-of-sight requirements, but are known to be less robust to the influence of metallic and/or ferromagnetic objects. In this paper, we apply a previously introduced fiducial-based system for navigated needle insertion with an EM tracking system and assess the overall targeting error using a static phantom in two different settings: in a non-metallic environment (REF) and on a CT stretcher (CT). While accurate needle insertion could be achieved in the reference environment (REF: 2.6±0.7 mm), targeting errors dropped drastically in the presence of metal (CT: 10.4±6.1 mm). For accurate and robust computer-assisted needle insertion, EM field distortions should thus either be avoided by assuring a suitable environment or by using methods for shielding or error compensation.
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paper_title: Calibration of electromagnetic tracking devices

paper_content:

Electromagnetic tracking devices are often used to track location and orientation of a user in a virtual reality environment. Their precision, however, is not always high enough because of the dependence of the system on the local electromagnetic field which can be altered easily by many external factors. The purpose of this article is to give an overview of the calibration techniques used to improve the precision of the electromagnetic tracking devices and to present a new method that compensates both the position and orientation errors. It is shown numerically that significant improvements in the precision of the detected position and orientation can be achieved with a small number of calibration measurements to be taken. Unresolved problems and research topics related to the proposed method are discussed.
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paper_title: Algorithm for calibration of the electromagnetic tracking system

paper_content:

Electromagnetic tracking systems (EMTS) are increasingly used in computer assisted surgery systems, motion capture units and military systems. The main application in the medical domain represents the support of minimally invasive surgeries. However, those electromagnetic tracking systems suffer from low accuracy and systematic errors caused by conductive and ferromagnetic media. In this paper we present a method for the calibration of electromagnetic tracking systems, which could be adopted in the development of novel electromagnetic tracking devices. The proposed method was evaluated on an experimental setup of a tracking system.
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paper_title: A New Calibration Procedure for Magnetic Tracking Systems

paper_content:

In this study, we suggest a new approach for the calibration of magnetic tracking systems that allows us to calibrate the entire system in a single setting. The suggested approach is based on solving a system of equations involving all the system parameters. These parameters include: 1) the magnetic positions of the transmitting coils; 2) their magnetic moments; 3) the magnetic position of the sensor; 4) its sensitivity; and 5) the gain of the sensor output amplifier. We choose a set of parameters that define the origin, orientation, and scale of the reference coordinate system and consider them as constants in the above system of equations. Another set of constants is the sensor output measured at a number of arbitrary positions. The unknowns in the above equations are all the other system parameters. To define the origin and orientation of the reference coordinate system, we first relate it to a physical object, e.g., to the transmitter housing. We then use special supports to align the sensor with the edges of the transmitter housing and measure the sensor output at a number of aligned positions. To define the scale of the reference coordinate system, we measure the distance between two arbitrary sensor locations with a precise instrument (a caliper). This is the only parameter that should be calibrated with the help of an external measurement tool. To illustrate the efficiency of the new approach, we applied the calibration procedure to a magnetic tracking system employing 64 transmitting coils. We have measured the systematic tracking errors before and after applying the calibration. The systematic tracking errors were reduced by an order of magnitude due to applying the new calibration procedure.
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paper_title: An improved calibration framework for electromagnetic tracking devices

paper_content:

Electromagnetic trackers have many favorable characteristics but are notorious for their sensitivity to magnetic field distortions resulting from metal and electronic equipment in the environment. We categorize existing tracker calibration methods and present an improved technique for reducing the static position and orientation errors that are inherent to these devices. A quaternion-based formulation provides a simple and fast computational framework for representing orientation errors. Our experimental apparatus consists of a 6-DOF mobile platform and an optical position measurement system, allowing the collection of full-pose data at nearly arbitrary orientations of the receiver. A polynomial correction technique is applied and evaluated using a Polhemus Fastrak resulting in a substantial improvement of tracking accuracy. Finally, we apply advanced visualization algorithms to give new insight into the nature of the magnetic distortion field.
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paper_title: Electromagnetic tracking in the clinical environment

paper_content:

When choosing an electromagnetic tracking system (EMTS) for image-guided procedures several factors must be taken into consideration. Among others these include the system's refresh rate, the number of sensors that need to be tracked, the size of the navigated region, the system interaction with the environment, whether the sensors can be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. To date, the only factors that have been studied extensively are the accuracy and the susceptibility of EMTSs to distortions caused by ferromagnetic materials. In this paper the authors shift the focus from analysis of system accuracy and stability to the broader set of factors influencing the utility of EMTS in the clinical environment. The authors provide an analysis based on all of the factors specified above, as assessed in three clinical environments. They evaluate two commercial tracking systems, the Aurora system from Northern Digital Inc., and the 3D Guidance system with three different field generators from Ascension Technology Corp. The authors show that these systems are applicable to specific procedures and specific environments, but that currently, no single system configuration provides a comprehensive solution across procedures and environments.
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paper_title: Standardized assessment of new electromagnetic field generators in an interventional radiology setting.

paper_content:

PURPOSE ::: Two of the main challenges associated with electromagnetic (EM) tracking in computer-assisted interventions (CAIs) are (1) the compensation of systematic distance errors arising from the influence of metal near the field generator (FG) or the tracked sensor and (2) the optimized setup of the FG to maximize tracking accuracy in the area of interest. Recently, two new FGs addressing these issues were proposed for the well-established Aurora(®) tracking system [Northern Digital, Inc. (NDI), Waterloo, Canada]: the Tabletop 50-70 FG, a planar transmitter with a built-in shield that compensates for metal distortions emanating from treatment tables, and the prototypical Compact FG 7-10, a mobile generator designed to be attached to mobile imaging devices. The purpose of this paper was to assess the accuracy and precision of these new FGs in an interventional radiology setting. ::: ::: ::: METHODS ::: A standardized assessment protocol, which uses a precisely machined base plate to measure relative error in position and orientation, was applied to the two new FGs as well as to the well-established standard Aurora(®) Planar FG. The experiments were performed in two different settings: a reference laboratory environment and a computed tomography (CT) scanning room. In each setting, the protocol was applied to three different poses of the measurement plate within the tracking volume of the three FGs. ::: ::: ::: RESULTS ::: The two new FGs provided higher precision and accuracy within their respective measurement volumes as well as higher robustness with respect to the CT scanner compared to the established FG. Considering all possible 5 cm distances on the grid, the error of the Planar FG was increased by a factor of 5.94 in the clinical environment (4.4 mm) in comparison to the error in the laboratory environment (0.8 mm). In contrast, the mean values for the two new FGs were all below 1 mm with an increase in the error by factors of only 2.94 (Reference: 0.3 mm; CT: 0.9 mm) and 1.04 (both: 0.5 mm) in the case of the Tabletop FG and the Compact FG, respectively. ::: ::: ::: CONCLUSIONS ::: Due to their high accuracy and robustness, the Tabletop FG and the Compact FG could eliminate the need for compensation of EM field distortions in certain CT-guided interventions.
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paper_title: Electromagnetic navigation bronchoscopy

paper_content:

In 2009, lung cancer was estimated to be the second most common form of cancer diagnosed in men, after prostate, and the second, after breast cancer, in women. It is estimated that it caused 159,390 deaths more than breast, colon and prostate cancers combined. While age-adjusted death rates for this cancer have been declining since 2000, they remain high.
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paper_title: A survey of electromagnetic position tracker calibration techniques

paper_content:

This paper is a comprehensive survey of various techniques used to calibrate electromagnetic position tracking systems. A common framework is established to present the calibration problem as the interpolation problem in 3D. All the known calibration techniques are classified into local and global methods and grouped according to their mathematical models. Both the location error and the orientation error correction techniques are surveyed. Data acquisition devices and methods as well as publicly available software implementations are reviewed, too.
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paper_title: Interpolation Volume Calibration: A Multisensor Calibration Technique for Electromagnetic Trackers

paper_content:

AC electromagnetic trackers are well suited for head tracking but are adversely affected by conductive and ferromagnetic materials. Tracking performance can be improved by mapping the tracking volume to produce coefficients that correct position and orientation (PnO) measurements caused by stationary distorting materials. The mapping process is expensive and time consuming, requiring complicated high-precision equipment to provide registration of the measurements to the source reference frame. In this study, we develop a new approach to mapping that provides registration of mapping measurements without precision equipment. Our method, i.e., the interpolation volume calibration system, uses two simple fixtures, each with multiple sensors in a rigid geometry, to determine sensor PnO in a distorted environment without mechanical measurements or other tracking technologies. We test our method in a distorted tracking environment, constructing a lookup table of the magnetic field that is used as the basis for distortion compensation. The new method compares favorably with the traditional approach providing a significant reduction in cost and effort.
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paper_title: Systematic distortions in magnetic position digitizers

paper_content:

W. Birkfellner, F. Watzinger, F. Wanschitz, G. Enislidis, C. Kollmann, D. Rafolt, R. Nowotny, R. Ewers, and H.Bergmann Citation: Medical Physics 25, 2242 (1998); doi: 10.1118/1.598425 View online: http://dx.doi.org/10.1118/1.598425 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/25/11?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Organ motion tracking in USgFUS - A feasibility study using sonoelastography AIP Conf. Proc. 1503, 135 (2012); 10.1063/1.4769931 Integration and evaluation of a needle-positioning robot with volumetric microcomputed tomography imageguidance for small animal stereotactic interventions Med. Phys. 37, 1647 (2010); 10.1118/1.3312520 Magnetoacoustic tomographic imaging of electrical impedance with magnetic induction Appl. Phys. Lett. 91, 083903 (2007); 10.1063/1.2772763 Magnetoacoustic tomography with magnetic induction for imaging electrical impedance of biological tissue J. Appl. Phys. 99, 066112 (2006); 10.1063/1.2186371 Evaluation of a miniature electromagnetic position tracker Med. Phys. 29, 2205 (2002); 10.1118/1.1508377
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paper_title: Magneto-Optical Tracking of Flexible Laparoscopic Ultrasound: Model-Based Online Detection and Correction of Magnetic Tracking Errors

paper_content:

Electromagnetic tracking is currently one of the most promising means of localizing flexible endoscopic instruments such as flexible laparoscopic ultrasound transducers. However, electromagnetic tracking is also susceptible to interference from ferromagnetic material, which distorts the magnetic field and leads to tracking errors. This paper presents new methods for real-time online detection and reduction of dynamic electromagnetic tracking errors when localizing a flexible laparoscopic ultrasound transducer. We use a hybrid tracking setup to combine optical tracking of the transducer shaft and electromagnetic tracking of the flexible transducer tip. A novel approach of modeling the poses of the transducer tip in relation to the transducer shaft allows us to reliably detect and significantly reduce electromagnetic tracking errors. For detecting errors of more than 5 mm, we achieved a sensitivity and specificity of 91% and 93%, respectively. Initial 3-D rms error of 6.91 mm were reduced to 3.15 mm.
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paper_title: Electromagnetic tracking method and apparatus for compensation of metal artifacts using modular arrays of reference sensors

paper_content:

An electromagnetic tracking method includes generating an electromagnetic field (14) in a region of interest (16). The electromagnetic field is subject to distortion in response to a presence of metalartifacts proximate the electromagnetic field. An array of reference sensors (30,50,102,104,110) having a predefined known configuration are disposed proximate the region of interest. A first set oflocations of the array of reference sensors is determined with respect to the electromagnetic field generator (12) in response to an excitation of one or more of the reference sensors via the electromagnetic field. A second mechanism (28), other than the electromagnetic field, determines a first portion of a second set of locations of at least one or more sensors of the array of reference sensorswith respect to the second mechanism, the second mechanism being in a known spatial relationship with the electromagnetic field generator. A remainder portion of the second set of locations of the reference sensors of the array of reference sensors is determined in response to (i) the first portion of the second set of locations determined using the second mechanism and (ii) the predefined known configuration of the array of reference sensors. The method further includes compensating for metal distortion of the electromagnetic field in the region of interest as a function of the first and second sets of reference sensor locations of the array of reference sensors.
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paper_title: New approaches to online estimation of electromagnetic tracking errors for laparoscopic ultrasonography

paper_content:

In abdominal surgery, a laparoscopic ultrasound transducer is commonly used to detect lesions such as metastases. The determination and visualization of the position and orientation of its flexible tip in relation to the patient or other surgical instruments can be a great support for surgeons using the transducer intraoperatively. This difficult subject has recently received attention from the scientific community. Electromagnetic tracking systems can be applied to track the flexible tip; however, current limitations of electromagnetic tracking include its accuracy and sensibility, i.e., the magnetic field can be distorted by ferromagnetic material. This paper presents two novel methods for estimation of electromagnetic tracking error. Based on optical tracking of the laparoscope, as well as on magneto-optic and visual tracking of the transducer, these methods automatically detect in 85% of all cases whether tracking is erroneous or not, and reduce tracking errors by up to 2.5 mm.
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paper_title: An improved calibration framework for electromagnetic tracking devices

paper_content:

Electromagnetic trackers have many favorable characteristics but are notorious for their sensitivity to magnetic field distortions resulting from metal and electronic equipment in the environment. We categorize existing tracker calibration methods and present an improved technique for reducing the static position and orientation errors that are inherent to these devices. A quaternion-based formulation provides a simple and fast computational framework for representing orientation errors. Our experimental apparatus consists of a 6-DOF mobile platform and an optical position measurement system, allowing the collection of full-pose data at nearly arbitrary orientations of the receiver. A polynomial correction technique is applied and evaluated using a Polhemus Fastrak resulting in a substantial improvement of tracking accuracy. Finally, we apply advanced visualization algorithms to give new insight into the nature of the magnetic distortion field.
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paper_title: A direction space interpolation technique for calibration of electromagnetic surgical navigation systems

paper_content:

A new generation of electromagnetic (EM) navigation systems with extremely compact sensors have great potential for clinical applications requiring that surgical devices be tracked within a patient’s body. However, electro-magnetic field distortions limit the accuracy of such devices. Further, the errors may be sensitive both to position and orientation of EM sensors within the field. This paper presents a computationally efficient method for in-situ 5 DOF calibration of the basic sensors of a typical EM system (Northern Digital’s Aurora), and presents preliminary results demonstrating an improvement of approximately 2.6 : 1 positional accuracy and 1.6 : 1 for orientation even when the sensors are moved through arbitrary orientation changes. This work represents one step in a larger effort to understand the field distortions associated with these systems and to develop effective and predictable calibration and registration strategies for their use in stereotactic image-guided interventions.
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paper_title: Intraoperative Magnetic Tracker Calibration Using a Magneto-Optic Hybrid Tracker for 3-D Ultrasound-Based Navigation in Laparoscopic Surgery

paper_content:

This paper describes a ultrasound (3-D US) system that aims to achieve augmented reality (AR) visualization during laparoscopic surgery, especially for the liver. To acquire 3-D US data of the liver, the tip of a laparoscopic ultrasound probe is tracked inside the abdominal cavity using a magnetic tracker. The accuracy of magnetic trackers, however, is greatly affected by magnetic field distortion that results from the close proximity of metal objects and electronic equipment, which is usually unavoidable in the operating room. In this paper, we describe a calibration method for intraoperative magnetic distortion that can be applied to laparoscopic 3-D US data acquisition; we evaluate the accuracy and feasibility of the method by in vitro and in vivo experiments. Although calibration data can be acquired freehand using a magneto-optic hybrid tracker, there are two problems associated with this method--error caused by the time delay between measurements of the optical and magnetic trackers, and instability of the calibration accuracy that results from the uniformity and density of calibration data. A temporal calibration procedure is developed to estimate the time delay, which is then integrated into the calibration, and a distortion model is formulated by zeroth-degree to fourth-degree polynomial fitting to the calibration data. In the in vivo experiment using a pig, the positional error caused by magnetic distortion was reduced from 44.1 to 2.9 mm. The standard deviation of corrected target positions was less than 1.0 mm. Freehand acquisition of calibration data was performed smoothly using a magneto-optic hybrid sampling tool through a trocar under guidance by realtime 3-D monitoring of the tool trajectory; data acquisition time was less than 2 min. The present study suggests that our proposed method could correct for magnetic field distortion inside the patient's abdomen during a laparoscopic procedure within a clinically permissible period of time, as well as enabling an accurate 3-D US reconstruction to be obtained that can be superimposed onto live endoscopic images.
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paper_title: Electromagnetic navigation bronchoscopy

paper_content:

In 2009, lung cancer was estimated to be the second most common form of cancer diagnosed in men, after prostate, and the second, after breast cancer, in women. It is estimated that it caused 159,390 deaths more than breast, colon and prostate cancers combined. While age-adjusted death rates for this cancer have been declining since 2000, they remain high.
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paper_title: Model for defining and reporting reference-based validation protocols in medical image processing

paper_content:

Objectives Image processing tools are often embedded in larger systems. Validation of image processing methods is important because the performance of such methods can have an impact on the performance of the larger systems and consequently on decisions and actions based on the use of these systems. Most validation studies compare the direct or indirect results of a method with a reference that is assumed to be very close or equal to the correct solution. In this paper, we propose a model for defining and reporting reference-based validation protocols in medical image processing.
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paper_title: Fused video and ultrasound images for minimally invasive partial nephrectomy: a phantom study

paper_content:

The shift to minimally invasive abdominal surgery has increased reliance on image guidance during surgical procedures. However, these images are most often presented independently, increasing the cognitive workload for the surgeon and potentially increasing procedure time. When warm ischemia of an organ is involved, time is an important factor to consider. To address these limitations, we present a more intuitive visualization that combines images in a common augmented reality environment. In this paper, we assess surgeon performance under the guidance of the conventional visualization system and our fusion system using a phantom study that mimics the tumour resection of partial nephrectomy. The RMS error between the fused images was 2.43mm, which is sufficient for our purposes. A faster planning time for the resection was achieved using our fusion visualization system. This result is a positive step towards decreasing risks associated with long procedure times in minimally invasive abdominal interventions.
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paper_title: A Navigation Platform for Guidance of Beating Heart Transapical Mitral Valve Repair

paper_content:

Traditional surgical approaches for repairing diseased mitral valves (MVs) have relied on placing the patient on cardiopulmonary bypass (on pump), stopping the heart and accessing the arrested heart directly. However, because this approach has the potential for adverse neurological, vascular, and immunological sequelae, less invasive beating heart alternatives are desirable. Emerging beating heart techniques have been developed to offer high-risk patients MV repair using ultrasound guidance alone without stopping the heart. This paper describes the first porcine trials of the NeoChord DS1000 (Minnetonka, MN), employed to attach neochordae to a MV leaflet using the traditional ultrasound-guided protocol augmented by dynamic virtual geometric models. The distance errors of the tracked tool tip from the intended midline trajectory (5.2 ± 2.4 mm versus 16.8 ± 10.9 mm, p = 0.003), navigation times (16.7 ± 8.0 s versus 92.0 ± 84.5 s, p = 0.004), and total path lengths (225.2 ± 120.3 mm versus 1128.9 ± 931.1 mm, p = 0.003) were significantly shorter in the augmented ultrasound compared to navigation with ultrasound alone,1 indicating a substantial improvement in the safety and simplicity of the procedure.
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paper_title: CT-guided percutaneous lung biopsy: comparison of conventional CT fluoroscopy to CT fluoroscopy with electromagnetic navigation system in 60 consecutive patients.

paper_content:

PURPOSE ::: To determine if use of an electromagnetic navigation system (EMN) decreases radiation dose and procedure time of CT fluoroscopy guided lung biopsy in lesions smaller than 2.5 cm. ::: ::: ::: MATERIALS/METHODS ::: 86 consecutive patients with small lung masses (<2.5 cm) were approached. 60 consented and were randomized to undergo biopsy with CT fluoroscopy (CTF) (34 patients) or EMN (26 patients). Technical failure required conversion to CTF in 8/26 EMN patients; 18 patients completed biopsy with EMN. Numerous biopsy parameters were compared as described below. ::: ::: ::: RESULTS ::: Average fluoroscopy time using CTF was 28.2s compared to 35.0 s for EMN (p=0.1). Average radiation dose was 117 mGy using CTF and 123 mGy for EMN (p=0.7). Average number of needle repositions was 3.7 for CTF and 4.4 for EMN (p=0.4). Average procedure time was 15 min for CTF and 20 min for EMN (p=0.01). There were 7 pneumothoracesin the CTF group and 6 pneumothoraces in the EMN group (p=0.7). One pneumothorax in the CTF group and 3 pneumothoraces in the EMN group required chest tube placement (p=0.1). One pneumothorax patient in each group required hospital admission. Diagnostic specimens were obtained in 31/34 patients in the CTF group and 22/26 patients in the EMN group (p=0.4). ::: ::: ::: CONCLUSIONS ::: EMN was not statistically different than CTF for fluoroscopy time, radiation dose, number of needle repositions, incidence of pneumothorax, need for chest tube, or diagnostic yield. Procedure time was increased with EMN.
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paper_title: How Does Electromagnetic Navigation Stack Up Against Infrared Navigation in Minimally Invasive Total Knee Arthroplasties

paper_content:

Abstract Forty-six primary total knee arthroplasties were performed using either an electromagnetic (EM) or infrared (IR) navigation system. In this IRB-approved study, patients were evaluated clinically and for accuracy using spiral computed tomographic imaging and 36-in standing radiographs. Although EM navigation was subject to metal interference, it was not as drastic as line-of-sight interference with IR navigation. Mechanical alignment was ideal in 92.9% of EM and 90.0% of IR cases based on spiral computed tomographic imaging and 100% of EM and 95% of IR cases based on x-ray. Individual measurements of component varus/valgus and sagittal measurements showed EM to be equivalent to IR, with both systems producing subdegree accuracy in 95% of the readings.
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paper_title: Intraventricular catheter placement by electromagnetic navigation safely applied in a paediatric major head injury patient

paper_content:

INTRODUCTION ::: In the management of severe head injuries, the use of intraventricular catheters for intracranial pressure (ICP) monitoring and the option of cerebrospinal fluid drainage is gold standard. In children and adolescents, the insertion of a cannula in a compressed ventricle in case of elevated intracranial pressure is difficult; therefore, a pressure sensor is placed more often intraparenchymal as an alternative option. ::: ::: ::: DISCUSSION ::: In cases of persistent elevated ICP despite maximal brain pressure management, the use of an intraventricular monitoring device with the possibility of cerebrospinal fluid drainage is favourable. We present the method of intracranial catheter placement by means of an electromagnetic navigation technique.
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paper_title: A new method for three-dimensional laparoscopic ultrasound model reconstruction

paper_content:

Background ::: Laparoscopic ultrasound is an important modality in the staging of gastrointestinal tumors. Correct staging depends on good spatial understanding of the regional tumor infiltration. Three-dimensional (3D) models may facilitate the evaluation of tumor infiltration. The aim of the study was to perform a volumetric test and a clinical feasibility test of a new 3D method using standard laparoscopic ultrasound equipment.
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paper_title: The reliability and accuracy of an electromagnetic motion analysis system when used conjointly with an accelerometer

paper_content:

The effect of an accelerometer driven electronic postural monitor (Spineangel®) placed within the electromagnetic measurement field of the Polhemus Fastrak™ is unknown. This study assessed the reliability and accuracy of Fastrak™ linear and angular measurements, when the Spineangel® was placed close to the sensor(s) and transmitter. Bland Altman plots and intraclass correlation coefficient (2,1) were used to determine protocol reproducibility and measurement consistency. Excellent reliability was found for linear and angular measurements (0.96, 95% CI: 0.90–0.99; and 1.00, 95% CI: 1.00–1.00, respectively) with the inclusion of Spineangel®; similar results were found, without the inclusion of Spineangel®, for linear and angular measurements, (0.96, 95% CI: 0.89–0.99; and 1.00, 95% CI: 1.00–1.00, respectively). The greatest linear discrepancies between the two test conditions were found to be less than 3.5 mm, while the greatest angular discrepancies were below 3.5°. As the effect on accuracy was minimal, t...
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paper_title: Intracranial Image-Guided Neurosurgery: Experience with a new Electromagnetic Navigation System

paper_content:

Summary.Summary.Background: The aim of image-guided neurosurgery is to accurately project computed tomography (CT) or magnetic resonance imaging (MRI) data into the operative field for defining anatomical landmarks, pathological structures and tumour margins. To achieve this end, different image-guided and computer-assisted, so-called “neuronavigation” systems have been developed in order to offer the neurosurgeon precise spatial information.Method: The present study reports on the experience gained with a prototype of the NEN-NeuroGuardTM neuronavigation system (Nicolet Biomedical, Madison, WI, USA). It utilises a pulsed DC electromagnetic field for determining the location in space of surgical instruments to which miniaturised sensors are attached. The system was evaluated in respect to its usefulness, ease of integration into standard neurosurgical procedures, reliability and accuracy.Findings: The NEN-system was used with success in 24 intracranial procedures for lesions including both gliomas and cerebral metastases. It allowed real-time display of surgical manoeuvres on pre-operative CT or MR images without a stereotactic frame or a robotic arm. The mean registration error associated with MRI was 1.3 mm (RMS error) and 1.5 mm (RMS error) with CT-data. The average intra-operative target-localising error was 3.2 mm (± 1.5 mm SD). Thus, the equipment was of great help in planning and performing skin incisions and craniotomies as well as in reaching deep-seated lesions with a minimum of trauma.Interpretation: The NEN-NeuroGuardTM system is a very user-friendly and reliable tool for image-guided neurosurgery. It does not have the limitations of a conventional stereotactic frame. Due to its electromagnetic technology it avoids the “line-of-sight” problem often met by optical navigation systems since its sensors remain active even when situated deep inside the skull or hidden, for example, by drapes or by the surgical microscope.
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paper_title: Electromagnetic navigation bronchoscopy: A descriptive analysis

paper_content:

Electromagnetic navigation bronchoscopy (ENB) is an exciting new bronchoscopic technique that promises accurate navigation to peripheral pulmonary target lesions, using technology similar to a car global positioning system (GPS) unit. Potential uses for ENB include biopsy of peripheral lung lesions, pleural dye marking of nodules for surgical wedge resection, placement of fiducial markers for stereotactic radiotherapy, and therapeutic insertion of brachytherapy catheters into malignant tissue. This article will describe the ENB procedure, review the published literature, compare ENB to existing biopsy techniques, and outline the challenges for widespread implementation of this new technology.
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paper_title: Magnetic Assisted Navigation in Electrophysiology and Cardiac Resynchronisation: A Review

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Magnetic assisted navigation is a new innovation that may prove useful in catheter ablation of cardiac arrhythmias and cardiac resynchronization therapy. The ability to steer extremely floppy catheters and guidewires may allow for these to be positioned safely in previously inaccessible areas of the heart. The integration of other new technology, such as image integration and electroanatomic mapping systems, should advance our abilities further. Although studies have shown the technology to be feasible, with the advantage to the physician of decreased radiation exposure, studies need to be performed to show additional benefit over standard techniques.
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paper_title: Navigation Systems for Ablation

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Navigation systems, devices, and intraprocedural software are changing the way interventional oncology is practiced. Before the development of precision navigation tools integrated with imaging systems, thermal ablation of hard-to-image lesions was highly dependent on operator experience, spatial skills, and estimation of positron emission tomography–avid or arterial-phase targets. Numerous navigation systems for ablation bring the opportunity for standardization and accuracy that extends the operator's ability to use imaging feedback during procedures. In this report, existing systems and techniques are reviewed and specific clinical applications for ablation are discussed to better define how these novel technologies address specific clinical needs and fit into clinical practice.
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paper_title: Evaluation of magnetic scope navigation in screening endoscopic examination of colorectal cancer

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BackgroundColorectal cancer is the most common cancer in Europe. Early diagnosis and treatment gives the patient a chance for complete recovery. Screening colonoscopies in the symptom-free patients are currently performed on a wide scale. The examinations are performed under local anesthesia which does not eliminate all discomfort and pain related to the examination. The aim of this study was to evaluate magnetic scope navigation in screening endoscopic examinations performed to detect early-stage colorectal cancer.MethodsThe study group consisted of 200 patients, aged 40–65 years, who were free from colon cancer symptoms. All patients underwent complete colonoscopy under local anesthesia. The equipment could be fitted with the scope that allows three-dimensional observation of instrument localization in the bowel. The examination was performed by three experienced endoscopists, each of whom performed over 5,000 colonoscopies. The patients were randomized to two groups: those whose equipment did not have 3D navigation (group I) and those whose equipment did have 3D navigation (group II). Each group consisted of 100 cases matched by gender, age, and BMI. The authors compared the duration of introducing instrument to cecum, the pulse rate before the examination and at the time the instrument reached the cecum, and subjective pain evaluation by the patient on the visual analog scale.ResultsGroup I consisted of 54 women and 46 men with a mean age of 54.6 years and mean BMI of 27.8 kg/m2, and group II had 58 women and 42 men, mean age of 55.1 years and mean BMI of 26.4 kg/m2. The average time it took for the instrument to reach the cecum was 216s in group I and 181s in group II (P < 0.05). Pain measured on the 10-point VAS scale was 2.44 in group I and 1.85 in group II (P < 0.05). The results showed a significantly shorter time for the instrument to reach the cecum in group II and significantly lower pain intensity during the examination was reported by the group II patients. No significant differences were found in the pulse measurements between the groups (P = 0.5).Conclusions3D navigation during colonoscopy decreases the time for the instrument to reach the cecum and lowers pain intensity subjectively reported by the patients. The use of 3D and the possibility to observe instrument localization and maneuvers brings more comfort to the patients.
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paper_title: Comparison of transabdominal ultrasound and electromagnetic transponders for prostate localization

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The aim of this study is to compare two methodologies of prostate localization in a large cohort of patients. Daily prostate localization using B-mode ultrasound has been performed at the Nebraska Medical Center since 2000. More recently, a technology using electromagnetic transponders implanted within the prostate was introduced into our clinic (Calypso®). With each technology, patients were localized initially using skin marks. Localization error distributions were determined from offsets between the initial setup positions and those determined by ultrasound or Calypso. Ultrasound localization data was summarized from 16619 imaging sessions spanning 7 years; Calypso localization data consists of 1524 fractions in 41 prostate patients treated in the course of a clinical trial at five institutions and 640 localizations from the first 16 patients treated with our clinical system. Ultrasound and Calypso patients treated between March and September 2007 at the Nebraska Medical Center were analyzed and compared, allowing a single institutional comparison of the two technologies. In this group of patients, the isocenter determined by ultrasound-based localization is on average 5.3 mm posterior to that determined by Calypso, while the systematic and random errors and PTV margins calculated from the ultrasound localizations were 3 - 4 times smaller than those calculated from the Calypso localizations. Our study finds that there are systematic differences between Calypso and ultrasound for prostate localization. Normal 0 false false false MicrosoftInternetExplorer4
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paper_title: Electromagnetically navigated laparoscopic ultrasound.

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A three-dimensional (3D) representation of laparoscopic ultrasound examinations could be helpful in diagnostic and therapeutic laparoscopy, but has not yet been realised with flexible laparoscopic ultrasound probes. Therefore, an electromagnetic navigation system was integrated into the tip of a conventional laparoscopic ultrasound probe. Navigated 3D laparoscopic ultrasound was compared with the imaging data of 3D navigated transcutaneous ultrasound and 3D computed tomography (CT) scan. The 3D CT scan served as the "gold standard". Clinical applicability in standardized operating room (OR) settings, imaging quality, diagnostic potential, and accuracy in volumetric assessment of various well-defined hepatic lesions were analyzed. Navigated 3D laparoscopic ultrasound facilitates exact definition of tumor location and margins. As compared with the "gold standard" of the 3D CT scans, 3D laparoscopic ultrasound has a tendency to underestimate the volume of the region of interest (ROI) (Delta3.1%). A comparison of 3D laparoscopy and transcutaneous 3D ultrasonography demonstrated clearly that the former is more accurate for volumetric assessment of the ROI and facilitates a more detailed display of the lesions. 3D laparoscopic ultrasound imaging with a navigated probe is technically feasible. The technique facilitates detailed ultrasound evaluation of laparoscopic procedures that involve visual, in-depth, and volumetric perception of complex liver pathologies. Navigated 3D laparoscopic ultrasound may have the potential to promote the practical role of laparoscopic ultrasonography, and become a valuable tool for local ablative therapy. In this article, our clinical experiences with a certified prototype of a 3D laparoscopic ultrasound probe, as well as its in vitro and in vivo evaluation, is reported.
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paper_title: A novel technique for tailoring frontal osteoplastic flaps using the ENT magnetic navigation system.

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CONCLUSION ::: The ENT magnetic navigation system is potentially useful and offers the most accurate technique for harvesting frontal osteoplastic flaps. It represents a valid tool in the wide range of instruments available to rhinologists. ::: ::: ::: OBJECTIVE ::: Precise delineation of the boundaries of the frontal sinus is a crucial step when harvesting a frontal osteoplastic flap. We present a novel technique using the ENT magnetic navigation system. ::: ::: ::: METHODS ::: Nineteen patients affected by different pathologies involving the frontal sinus underwent an osteoplastic flap procedure using the ENT magnetic navigation system between January 2009 and April 2011. ::: ::: ::: RESULTS ::: The ENT magnetic navigation system was found to be a safe and accurate tool for delineating the frontal sinus boundaries. No intraoperative complications occurred during the osteoplastic procedures.
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paper_title: Computer-aided navigation in neurosurgery

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The article comprises three main parts: a historical review on navigation, the mathematical basics for calculation and the clinical applications of navigation devices. Main historical steps are described from the first idea till the realisation of the frame-based and frameless navigation devices including robots. In particular the idea of robots can be traced back to the Iliad of Homer, the first testimony of European literature over 2500 years ago. In the second part the mathematical calculation of the mapping between the navigation and the image space is demonstrated, including different registration modalities and error estimations. The error of the navigation has to be divided into the technical error of the device calculating its own position in space, the registration error due to inaccuracies in the calculation of the transformation matrix between the navigation and the image space, and the application error caused additionally by anatomical shift of the brain structures during operation. In the third part the main clinical fields of application in modern neurosurgery are demonstrated, such as localisation of small intracranial lesions, skull-base surgery, intracerebral biopsies, intracranial endoscopy, functional neurosurgery and spinal navigation. At the end of the article some possible objections to navigation-aided surgery are discussed.
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paper_title: Next generation distal locking for intramedullary nails using an electromagnetic X-ray-radiation-free real-time navigation system.

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BACKGROUND ::: Distal locking marks one challenging step during intramedullary nailing that can lead to an increased irradiation and prolonged operation times. The aim of this study was to evaluate the reliability and efficacy of an X-ray-radiation-free real-time navigation system for distal locking procedures. ::: ::: ::: METHODS ::: A prospective randomized cadaver study with 50 standard free-hand fluoroscopic-guided and 50 electromagnetic-guided distal locking procedures was performed. All procedures were timed using a stopwatch. Intraoperative fluoroscopy exposure time and absorbed radiation dose (mGy) readings were documented. All tibial nails were locked with two mediolateral and one anteroposterior screw. Successful distal locking was accomplished once correct placement of all three screws was confirmed. ::: ::: ::: RESULTS ::: Successful distal locking was achieved in 98 cases. No complications were encountered using the electromagnetic navigation system. Eight complications arose during free-hand fluoroscopic distal locking. Undetected secondary drill slippage on the ipsilateral cortex accounted for most problems followed by undetected intradrilling misdirection causing a fissural fracture of the contralateral cortex while screw insertion in one case. Compared with the free-hand fluoroscopic technique, electromagnetically navigated distal locking provides a median time benefit of 244 seconds without using ionizing radiation. ::: ::: ::: CONCLUSION ::: Compared with the standard free-hand fluoroscopic technique, the electromagnetic guidance system used in this study showed high reliability and was associated with less complications, took significantly less time, and used no radiation exposure for distal locking procedures. ::: ::: ::: LEVEL OF EVIDENCE ::: Therapeutic study, level II.
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paper_title: Image-guided neurosurgery with 3-dimensional multimodal imaging data on a stereoscopic monitor.

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BACKGROUND ::: In the past 2 decades, intraoperative navigation technology has changed preoperative and intraoperative strategies and methodology tremendously. ::: ::: ::: OBJECTIVE ::: To report our first experiences with a stereoscopic navigation system based on multimodality-derived, patient-specific 3-dimensional (3-D) information displayed on a stereoscopic monitor and controlled by a virtual user interface. ::: ::: ::: METHODS ::: For the planning of each case, a 3-D multimodality model was created on the Dextroscope. The 3-D model was transferred to a console in the operating room that was running Dextroscope-compatible software and included a stereoscopic LCD (liquid crystal display) monitor (DexVue). Surgery was carried out with a standard frameless navigation system (VectorVision, BrainLAB) that was linked to DexVue. Making use of the navigational space coordinates provided by the VectorVision system during surgery, we coregistered the patient's 3-D model with the actual patient in the operating room. The 3-D model could then be displayed as seen along the axis of a handheld probe or the microscope view. The DexVue data were viewed with polarizing glasses and operated via a 3-D interface controlled by a cordless mouse containing inertial sensors. The navigational value of DexVue was evaluated postoperatively with a questionnaire. A total of 39 evaluations of 21 procedures were available. ::: ::: ::: RESULTS ::: In all 21 cases, the connection of VectorVision with DexVue worked reliably, and consistent spatial concordance of the navigational information was displayed on both systems. The questionnaires showed that in all cases the stereoscopic 3-D data were preferred for navigation. In 38 of 39 evaluations, the spatial orientation provided by the DexVue system was regarded as an improvement. In no case was there worsened spatial orientation. ::: ::: ::: CONCLUSION ::: We consider navigating primarily with stereoscopic, 3-D multimodality data an improvement over navigating with image planes, and we believe that this technology enables a more intuitive intraoperative interpretation of the displayed navigational information and hence an easier surgical implementation of the preoperative plan.
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paper_title: [Magnetic-field-based navigation system for ultrasound-guided interventions].

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PURPOSE ::: To describe a new magnetic field-based navigation system for ultrasound-guided interventional procedures. ::: ::: ::: METHODS ::: The navigation system supports biopsies either in plane or out of plane. To evaluate the accuracy of the system, targets in four standard silicone phantoms were punctured. Six cystic and six solid lesions were present in every phantom. The success of the puncture was controlled by aspiration of the content of the lesions, which was coloured. Furthermore, liver lesions of different sizes (2-6 mm) and depths (3-6 cm) were produced by injecting a viscous fluid and punctured in three swine carcasses. Functions of the gallbladder and nephrostomies were carried out as well. ::: ::: ::: RESULTS ::: All 48 targets were successfully punctured using the in plane and out of plane modus in the phantom. In the carcasses all 16 lesions were reached in plane. In the out of plane modus two 6 cm deep lesions were not reached. All other 14 biopsies were successful. Nephrostomy of the not dilated renal pelvis and punction of the gallbladder were successfully carried out. ::: ::: ::: CONCLUSIONS ::: The described navigation system is a promising tool for fast and safe performance of ultrasound-guided interventional procedures.
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paper_title: Placement of Intraventricular Catheters Using Flexible Electromagnetic Navigation and a Dynamic Reference Frame: A New Technique

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Background: Catheterization of narrow ventricles may prove difficult resulting in misplacement or inefficient trials with potential damage to brain tissue. Material and Met
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paper_title: Freehand placement of depth electrodes using electromagnetic frameless stereotactic guidance.

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The presurgical evaluation of patients with epilepsy often requires an intracranial study in which both subdural grid electrodes and depth electrodes are needed. Performing a craniotomy for grid placement with a stereotactic frame in place can be problematic, especially in young children, leading some surgeons to consider frameless stereotaxy for such surgery. The authors report on the use of a system that uses electromagnetic impulses to track the tip of the depth electrode. Ten pediatric patients with medically refractory focal lobar epilepsy required placement of both subdural grid and intraparenchymal depth electrodes to map seizure onset. Presurgical frameless stereotaxic targeting was performed using a commercially available electromagnetic image-guided system. Freehand depth electrode placement was then performed with intraoperative guidance using an electromagnetic system that provided imaging of the tip of the electrode, something that has not been possible using visually or sonically based systems. Accuracy of placement of depth electrodes within the deep structures of interest was confirmed postoperatively using CT and CT/MR imaging fusion. Depth electrodes were appropriately placed in all patients. Electromagnetic-tracking-based stereotactic targeting improves the accuracy of freehand placement of depth electrodes in patients with medically refractory epilepsy. The ability to track the electrode tip, rather than the electrode tail, is a major feature that enhances accuracy. Additional advantages of electromagnetic frameless guidance are discussed.
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paper_title: Electromagnetic navigation improves minimally invasive robot-assisted lung brachytherapy.

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Objective: Recent advances in minimally invasive thoracic surgery have renewed an interest in the role of interstitial brachytherapy for lung cancer. Our previous work has demonstrated that a minimally invasive robot-assisted (MIRA) lung brachytherapy system produced results that were equal to or better than those obtained with standard video-assisted thoracic surgery (VATS) and comparable to results with open surgery. The purpose of this project was to evaluate the performance of an integrated system for MIRA lung brachytherapy that incorporated modified electromagnetic navigation and ultrasound image guidance with robotic assistance.Methods: The experimental test-bed consisted of a VATS box, ZEUS® and AESOP® surgical robotic arms, a seed injector, an ultrasound machine, video monitors, a computer, and an endoscope. Our previous custom-designed electromagnetic navigational software and the robotic controller were modified and incorporated into the MIRA III system to become the next-generation MIRA IV. In...
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paper_title: An Evaluation of the Aurora System as a Flesh-Point Tracking Tool for Speech Production Research

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Purpose Northern Digital Instruments (NDI; Waterloo, Ontario, Canada) manufactures a commercially available magnetometer device called Aurora that features real-time display of sensor position trac...
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paper_title: Free-hand CT-based electromagnetically guided interventions: accuracy, efficiency and dose usage.

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The purpose of this paper was to evaluate computed tomography (CT) based electromagnetically tip-tracked (EMT) interventions in various clinical applications. An EMT system was utilized to perform percutaneous interventions based on CT datasets. Procedure times and spatial accuracy of needle placement were analyzed using logging data in combination with periprocedurally acquired CT control scans. Dose estimations in comparison to a set of standard CT-guided interventions were carried out. Reasons for non-completion of planned interventions were analyzed. Twenty-five procedures scheduled for EMT were analyzed, 23 of which were successfully completed using EMT. The average time for performing the procedure was 23.7 ± 17.2 min. Time for preparation was 5.8 ± 7.3 min while the interventional (skin-to-target) time was 2.7 ± 2.4 min. The average puncture length was 7.2 ± 2.5 cm. Spatial accuracy was 3.1 ± 2.1 mm. Non-completed procedures were due to patient movement and reference fixation problems. Radiation doses (dosis-length-product) were significantly lower (p = 0.012) for EMT-based interventions (732 ± 481 mGy x cm) in comparison to the control group of standard CT-guided interventions (1343 ± 1054 mGy x cm). Electromagnetic navigation can accurately guide percutaneous interventions in a variety of indications. Accuracy and time usage permit the routine use of the utilized system. Lower radiation exposure for EMT-based punctures provides a relevant potential for dose saving.
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paper_title: Treatment response assessment of radiofrequency ablation for hepatocellular carcinoma: usefulness of virtual CT sonography with magnetic navigation.

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PURPOSE ::: Virtual CT sonography using magnetic navigation provides cross sectional images of CT volume data corresponding to the angle of the transducer in the magnetic field in real-time. The purpose of this study was to clarify the value of this virtual CT sonography for treatment response of radiofrequency ablation for hepatocellular carcinoma. ::: ::: ::: PATIENTS AND METHODS ::: Sixty-one patients with 88 HCCs measuring 0.5-1.3 cm (mean±SD, 1.0±0.3 cm) were treated by radiofrequency ablation. For early treatment response, dynamic CT was performed 1-5 days (median, 2 days). We compared early treatment response between axial CT images and multi-angle CT images using virtual CT sonography. ::: ::: ::: RESULTS ::: Residual tumor stains on axial CT images and multi-angle CT images were detected in 11.4% (10/88) and 13.6% (12/88) after the first session of RFA, respectively (P=0.65). Two patients were diagnosed as showing hyperemia enhancement after the initial radiofrequency ablation on axial CT images and showed local tumor progression shortly because of unnoticed residual tumors. Only virtual CT sonography with magnetic navigation retrospectively showed the residual tumor as circular enhancement. In safety margin analysis, 10 patients were excluded because of residual tumors. The safety margin more than 5 mm by virtual CT sonographic images and transverse CT images were determined in 71.8% (56/78) and 82.1% (64/78), respectively (P=0.13). The safety margin should be overestimated on axial CT images in 8 nodules. ::: ::: ::: CONCLUSION ::: Virtual CT sonography with magnetic navigation was useful in evaluating the treatment response of radiofrequency ablation therapy for hepatocellular carcinoma.
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paper_title: Three-Dimensional Electromagnetic Navigation vs. Fluoroscopy for Endovascular Aneurysm Repair: A Prospective Feasibility Study in Patients

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PurposeTo evaluate the in vivo feasibility of a 3-dimensional (3D) electromagnetic (EM) navigation system with electromagnetically-tracked catheters in endovascular aneurysm repair (EVAR).MethodsThe pilot study included 17 patients undergoing EVAR with a bifurcated stent-graft. Ten patients were assigned to the control group, in which a standard EVAR procedure was used. The remaining 7 patients (intervention group) underwent an EVAR procedure during which a cone-beam computed tomography image was acquired after implantation of the main stent-graft. The 3D image was presented on the navigation screen. From the contralateral side, the tip of an electromagnetically-tracked catheter was visualized in the 3D image and positioned in front of the contralateral cuff in the main stent-graft. A guidewire was inserted through the catheter and blindly placed into the stent-graft. The placement of the guidewire was verified by fluoroscopy before the catheter was pushed over the guidewire. If the guidewire was incorrec...
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paper_title: Out-of-Plane Computed-Tomography-Guided Biopsy Using a Magnetic-Field-Based Navigation System

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The purpose of this article is to report our clinical experience with out-of-plane computed-tomography (CT)-guided biopsies using a magnetic-field-based navigation system. Between February 2002 and March 2003, 20 patients underwent CT-guided biopsy in which an adjunct magnetic-field-based navigation system was used to aid an out-of-plane biopsy approach. Eighteen patients had an underlying primary malignancy. All biopsies involved the use of a coaxial needle system in which an outer 18G guide needle was inserted to the lesion using the navigation system and an inner 22G needle was then used to obtain fine-needle aspirates. Complications and technical success were recorded. Target lesions were located in the adrenal gland (n = 7), liver (n = 6), pancreas (n = 3), lung (n = 2), retroperitoneal lymph node (n = 1), and pelvis (n = 1). The mean lesion size (maximum transverse diameter) was 26.5 mm (range: 8–70 mm) and the mean and median cranial–caudal distance, between the transaxial planes of the final needle tip location and the needle insertion site, was 40 mm (range: 18–90 mm). Needle tip positioning was successfully placed within the lesion in all 20 biopsies. A diagnosis of malignancy was obtained in 14 biopsies. Benign diagnoses were encountered in the remaining six biopsies and included a benign adrenal gland (n = 2), fibroelastic tissue (n = 1), hepocytes with steatosis (n = 2) and reactive hepatocytes (n = 1). No complications were encountered. A magnetic-field-based navigation system is an effective adjunct tool for accurate and safe biopsy of lesions that require an out-of-plane CT approach.
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paper_title: Magnetic navigation in ultrasound-guided interventional radiology procedures

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Aim To evaluate the usefulness of magnetic navigation in ultrasound (US)-guided interventional procedures. Materials and methods Thirty-seven patients who were scheduled for US-guided interventional procedures (20 liver cancer ablation procedures and 17 other procedures) were included. Magnetic navigation with three-dimensional (3D) computed tomography (CT), magnetic resonance imaging (MRI), 3D US, and position-marking magnetic navigation were used for guidance. The influence on clinical outcome was also evaluated. Results Magnetic navigation facilitated applicator placement in 15 of 20 ablation procedures for liver cancer in which multiple ablations were performed; enhanced guidance in two small liver cancers invisible on conventional US but visible at CT or MRI; and depicted the residual viable tumour after transcatheter arterial chemoembolization for liver cancer in one procedure. In four of 17 other interventional procedures, position-marking magnetic navigation increased the visualization of the needle tip. Magnetic navigation was beneficial in 11 (55%) of 20 ablation procedures; increased confidence but did not change management in five (25%); added some information but did not change management in two (10%); and made no change in two (10%). In the other 17 interventional procedures, the corresponding numbers were 1 (5.9%), 2 (11.7%), 7 (41.2%), and 7 (41.2%), respectively (p = 0.002). Conclusion Magnetic navigation in US-guided interventional procedure provides solutions in some difficult cases in which conventional US guidance is not suitable. It is especially useful in complicated interventional procedures such as ablation for liver cancer.
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paper_title: Image-Guided Endoscopic Surgery: Results of Accuracy and Performance in a Multicenter Clinical Study Using an Electromagnetic Tracking System

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Image-guided surgery has recently been described in the literature as a useful technology for improved functional endoscopic sinus surgery localization. Image-guided surgery yields accurate knowledge of the surgical field boundaries, allowing safer and more thorough sinus surgery. We have previously reviewed our initial experience with The InstaTrak System. This article presents a multicenter clinical study (n=55) that assesses the system's capability for localizing structures in critical surgical sites. The purpose of this paper is to present quantitative data on accuracy and performance. We describe several new advances including an automated registration technique that eliminates the redundant computed tomography scan, compensation for head movement, and the ability to use interchangeable instruments.
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paper_title: Real-time FDG PET Guidance during Biopsies and Radiofrequency Ablation Using Multimodality Fusion with Electromagnetic Navigation

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Combined electromagnetic device tracking and CT/US/fluorine 18 fluorodeoxyglucose (FDG) PET fusion allows successful biopsy and ablation of lesions that either demonstrate heterogeneous FDG uptake or are not well seen or are totally inapparent at conventional diagnostic imaging.
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paper_title: Fusion of MRI and sonography image for breast cancer evaluation using real-time virtual sonography with magnetic navigation: first experience.

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OBJECTIVE ::: We recently developed a real-time virtual sonography (RVS) system that enables simultaneous display of both sonography and magnetic resonance imaging (MRI) cutaway images of the same site in real time. The aim of this study was to evaluate the role of RVS in the management of enhancing lesions visualized with MRI. ::: ::: ::: METHODS ::: Between June 2006 and April 2007, 65 patients underwent MRI for staging of known breast cancer at our hospital. All patients were examined using mammography, sonography, MRI and RVS before surgical resection. Results were correlated with histopathologic findings. MRI was obtained on a 1.5 T imager, with the patient in the supine position using a flexible body surface coil. Detection rate was determined for index tumors and incidental enhancing lesions (IELs), with or without RVS. ::: ::: ::: RESULTS ::: Overall sensitivity for detecting index tumors was 85% (55/65) for mammography, 91% (59/65) for sonography, 97% (63/65) for MRI and 98% (64/65) for RVS. Notably, in one instance in which the cancer was not seen on MRI, RVS detected it with the supplementation of sonography. IELs were found in 26% (17/65) of the patients. Of 23 IELs that were detected by MRI, 30% (7/23) of IELs could be identified on repeated sonography alone, but 83% (19/23) of them were identified using the RVS system (P = 0.001). The RVS system was able to correctly project enhanced MRI information onto a body surface, as we checked sonography form images. ::: ::: ::: CONCLUSIONS ::: Our results suggest that the RVS system can identify enhancing breast lesions with excellent accuracy.
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paper_title: Electromagnetic navigation platform for endovascular surgery: how to develop sensorized catheters and guidewires

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Background ::: ::: Endovascular procedures are nowadays limited by difficulties arising from the use of 2D images and are associated with dangerous X-ray exposure and the injection of nephrotoxic contrast medium. ::: ::: ::: ::: Methods ::: ::: An electromagnetic navigator is proposed to guide endovascular procedures with reduced radiation dose and contrast medium injection. Five DOF electromagnetic sensors are calibrated and used to track in real time the positions and orientation of endovascular catheters and guidewires, while intraoperative 3D rotational angiography is used to acquire 3D models of patient anatomy. A preliminary prototype is developed to prove the feasibility of the system using an anthropomorphic phantom. ::: ::: ::: ::: Results ::: ::: The spatial accuracy of the system was evaluated during 70 targeting trials obtaining an overall accuracy of 1.2 ± 0.3 mm; system usability was positively evaluated by three surgeons. ::: ::: ::: ::: Conclusions ::: ::: The strategy proposed to sensorize endovascular instruments paves the way for the development of surgical strategies with reduced radiation dose and contrast medium injection. Further in vitro, animal and clinical experiments are necessary for complete surgical validation. Copyright © 2012 John Wiley & Sons, Ltd.
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paper_title: Assessment of the accuracy of infrared and electromagnetic navigation using an industrial robot: Which factors are influencing the accuracy of navigation?

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Our objectives were to detect factors that influence the accuracy of surgical navigation (magnitude of deformity, plane of deformity, position of the navigation bases) and compare the accuracy of infrared with electromagnetic navigation. Human cadaveric femora were used. A robot connected with a computer moved one of the bony fragments in a desired direction. The bases of the infrared navigation (BrainLab) and the receivers of the electromagnetic device (Fastrak-Pohlemus) were attached to the proximal and distal parts of the bone. For the first part of the study, deformities were classified in eight groups (e.g., 0 to 5°). For the second part, the bases were initially placed near the osteotomy and then far away. The mean absolute differences between both navigation system measurements and the robotic angles were significantly affected by the magnitude of angulation with better accuracy for smaller angulations (p < 0.001). The accuracy of infrared navigation was significantly better in the frontal and sagittal plane. Changing the position of the navigation bases near and far away from the deformity apex had no significant effect on the accuracy of infrared navigation; however, it influenced the accuracy of electromagnetic navigation in the frontal plane (p < 0.001). In conclusion, the use of infrared navigation systems for corrections of small angulation-deformities in the frontal or sagittal plane provides the most accurate results, irrespectively from the positioning of the navigation bases. © 2011 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29: 1476–1483, 2011
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paper_title: Upgrade of an optical navigation system with a permanent electromagnetic position control: a first step towards "navigated control" for liver surgery.

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INTRODUCTION ::: The main problems of navigation in liver surgery are organ movement and deformation. With a combination of direct optical and indirect electromagnetic tracking technology, visualisation and positional control of surgical instruments within three-dimensional ultrasound data and registration of organ movements can be realised simultaneously. ::: ::: ::: METHODS ::: Surgical instruments for liver resection were localised with an infrared-based navigation system (Polaris). Movements of the organ itself were registered using an electromagnetic navigation system (Aurora). The combination of these two navigation techniques and a new surgical navigation procedure focussed on a circumscribed critical dissection area were applied for the first time in liver resections. ::: ::: ::: RESULTS ::: This new technique was effectively implemented. The position of the surgical instrument was localised continuously. Repeated position control with observation of the navigation screen was not necessary. During surgical resection, a sonic warning signal was activated when the surgical instrument entered a "no touch" area--an area of reduced safety margin. ::: ::: ::: CONCLUSION ::: Optical tracking of surgical instruments and simultaneous electromagnetic registration of organ position is feasible in liver resection.
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paper_title: Accuracy considerations in image-guided cardiac interventions: experience and lessons learned

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MOTIVATION ::: Medical imaging and its application in interventional guidance has revolutionized the development of minimally invasive surgical procedures leading to reduced patient trauma, fewer risks, and shorter recovery times. However, a frequently posed question with regard to an image guidance system is "how accurate is it?" On one hand, the accuracy challenge can be posed in terms of the tolerable clinical error associated with the procedure; on the other hand, accuracy is bound by the limitations of the system's components, including modeling, patient registration, and surgical instrument tracking, all of which ultimately impact the overall targeting capabilities of the system. ::: ::: ::: METHODS ::: While these processes are not unique to any interventional specialty, this paper discusses them in the context of two different cardiac image guidance platforms: a model-enhanced ultrasound platform for intracardiac interventions and a prototype system for advanced visualization in image-guided cardiac ablation therapy. ::: ::: ::: RESULTS ::: Pre-operative modeling techniques involving manual, semi-automatic and registration-based segmentation are discussed. The performance and limitations of clinically feasible approaches for patient registration evaluated both in the laboratory and in the operating room are presented. Our experience with two different magnetic tracking systems for instrument and ultrasound transducer localization is reported. Ultimately, the overall accuracy of the systems is discussed based on both in vitro and preliminary in vivo experience. ::: ::: ::: CONCLUSION ::: While clinical accuracy is specific to a particular patient and procedure and vastly dependent on the surgeon's experience, the system's engineering limitations are critical to determine whether the clinical requirements can be met.
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paper_title: Successful placement of postpyloric enteral tubes using electromagnetic guidance in critically ill children*

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OBJECTIVES ::: Initiation of postpyloric feeding is often delayed by difficulties in placement of enteral tubes. We evaluated the effectiveness of bedside postpyloric enteral tube (PET) placement using an electromagnetic (EM)-guided device. We hypothesized that: 1) EM-guided placement of PETs would be successful more often than standard blind placement with a shorter total time to successful placement and 2) the EM-guided technique would have similar overall costs to the standard technique. ::: ::: ::: DESIGN ::: Prospective cohort trial with serial control groups in a pediatric intensive care unit at a tertiary care children's hospital. ::: ::: ::: INTERVENTIONS ::: We collected data on a cohort of consecutive pediatric intensive care unit patients who underwent PET placement by standard blind technique followed by a cohort who underwent EM-guided placement. The primary outcome measure was successful placement determined by abdominal radiography. ::: ::: ::: MEASUREMENTS AND MAIN RESULTS ::: One hundred seven patients were evaluated in the trial: 57 in the standard group and 50 in the EM-guided group. Demographic data, percent intubated, and admission diagnosis were similar in both groups. Forty-one of 50 patients (82%) in the EM-guided group had successful placement compared with 22 of 57 in the standard group (38%) (p < 0.0001). The average time to successful placement was 1.7 vs. 21 hours in the EM-guided group and standard group, respectively (p < 0.0001). Children in the EM-guided group received fewer radiographs (p = 0.007) and were given more prokinetic drugs (p = 0.045). There were no episodes of pneumothorax in either group. After controlling for prokinetic drug use, EM-guided placement was more likely to result in successful placement than the standard blind technique (odds ratio 6.4, 95% confidence interval 2.5-16.3). An annual placement rate of 250 PETs by EM guidance, based on our institution's current utilization rates, is associated with a cost savings of $55.46 per PET placed. ::: ::: ::: CONCLUSION ::: EM guidance is an efficient and cost-effective method of bedside PET placement.
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paper_title: Application of electromagnetic navigation in surgical treatment of intracranial tumors: analysis of 12 cases.

paper_content:

OBJECTIVE ::: To explore the application and characteristics of electromagnetic navigation in neurosurgical operation. ::: ::: ::: METHODS ::: Neurosurgical operations with the assistance of electromagnetic navigation were performed in 12 patients with intracranial tumors. ::: ::: ::: RESULTS ::: Total removal of the tumor was achieved in 8 cases, subtotal removal in 3 and removal of the majority of the tumor in 1 case. The error in the navigation averaged 1.9+/-0.9 mm and the time consumed by preoperative preparation was 19+/-2 min with the exception in 1 case. ::: ::: ::: CONCLUSION ::: In comparison with optic navigation, electromagnetic navigation offers better convenience and absence of signal blockage, and with a head frame, automatic registration can be achieved.
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paper_title: A Navigation Platform for Guidance of Beating Heart Transapical Mitral Valve Repair

paper_content:

Traditional surgical approaches for repairing diseased mitral valves (MVs) have relied on placing the patient on cardiopulmonary bypass (on pump), stopping the heart and accessing the arrested heart directly. However, because this approach has the potential for adverse neurological, vascular, and immunological sequelae, less invasive beating heart alternatives are desirable. Emerging beating heart techniques have been developed to offer high-risk patients MV repair using ultrasound guidance alone without stopping the heart. This paper describes the first porcine trials of the NeoChord DS1000 (Minnetonka, MN), employed to attach neochordae to a MV leaflet using the traditional ultrasound-guided protocol augmented by dynamic virtual geometric models. The distance errors of the tracked tool tip from the intended midline trajectory (5.2 ± 2.4 mm versus 16.8 ± 10.9 mm, p = 0.003), navigation times (16.7 ± 8.0 s versus 92.0 ± 84.5 s, p = 0.004), and total path lengths (225.2 ± 120.3 mm versus 1128.9 ± 931.1 mm, p = 0.003) were significantly shorter in the augmented ultrasound compared to navigation with ultrasound alone,1 indicating a substantial improvement in the safety and simplicity of the procedure.
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paper_title: Implementation of an electromagnetic tracking system for accurate intrahepatic puncture needle guidance: accuracy results in an in vitro model.

paper_content:

RATIONALE AND OBJECTIVES ::: Electromagnetic tracking potentially may be used to guide percutaneous needle-based interventional procedures. The accuracy of electromagnetic guided-needle puncture procedures has not been specifically characterized. This article reports the functional accuracy of a needle guidance system featuring real-time tracking of respiratory-related target motion. ::: ::: ::: MATERIALS AND METHODS ::: A needle puncture algorithm based on a "free-hand" needle puncture technique for percutaneous intrahepatic portocaval systemic shunt was employed. Preoperatively obtained computed tomographic images were displayed on a graphical user interface and registered with the electromagnetically tracked needle position. The system and procedure was tested on an abdominal torso phantom containing a liver model mounted on a motor-driven platform to simulate respiratory excursion. The liver model featured two hollow tubes to simulate intrahepatic vessels. Registration and respiratory motion tracking was performed using four skin fiducials and a needle fiducial within the liver. Success rates for 15 attempts at simultaneous puncture of the two "vessels" of different luminal diameters guided by the electromagnetic tracking system were recorded. ::: ::: ::: RESULTS ::: Successful "vessel" puncture occurred in 0%, 33%, and 53% of attempts for 3-, 5-, and 7-mm diameter "vessels," respectively. Using a two-dimensional accuracy prediction analysis, predicted accuracy exceeded actual puncture accuracy by 25%-35% for all vessel diameters. Accuracy outcome improved when depth-only errors were omitted from the analysis. ::: ::: ::: CONCLUSIONS ::: Actual puncture success rate approximates predicted rates for target vessels 5 mm in diameter or greater when depth errors are excluded. Greater accuracy for smaller diameter vessels would be desirable for implementation in a broader range of clinical applications.
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paper_title: How Does Electromagnetic Navigation Stack Up Against Infrared Navigation in Minimally Invasive Total Knee Arthroplasties

paper_content:

Abstract Forty-six primary total knee arthroplasties were performed using either an electromagnetic (EM) or infrared (IR) navigation system. In this IRB-approved study, patients were evaluated clinically and for accuracy using spiral computed tomographic imaging and 36-in standing radiographs. Although EM navigation was subject to metal interference, it was not as drastic as line-of-sight interference with IR navigation. Mechanical alignment was ideal in 92.9% of EM and 90.0% of IR cases based on spiral computed tomographic imaging and 100% of EM and 95% of IR cases based on x-ray. Individual measurements of component varus/valgus and sagittal measurements showed EM to be equivalent to IR, with both systems producing subdegree accuracy in 95% of the readings.
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paper_title: Intracranial Image-Guided Neurosurgery: Experience with a new Electromagnetic Navigation System

paper_content:

Summary.Summary.Background: The aim of image-guided neurosurgery is to accurately project computed tomography (CT) or magnetic resonance imaging (MRI) data into the operative field for defining anatomical landmarks, pathological structures and tumour margins. To achieve this end, different image-guided and computer-assisted, so-called “neuronavigation” systems have been developed in order to offer the neurosurgeon precise spatial information.Method: The present study reports on the experience gained with a prototype of the NEN-NeuroGuardTM neuronavigation system (Nicolet Biomedical, Madison, WI, USA). It utilises a pulsed DC electromagnetic field for determining the location in space of surgical instruments to which miniaturised sensors are attached. The system was evaluated in respect to its usefulness, ease of integration into standard neurosurgical procedures, reliability and accuracy.Findings: The NEN-system was used with success in 24 intracranial procedures for lesions including both gliomas and cerebral metastases. It allowed real-time display of surgical manoeuvres on pre-operative CT or MR images without a stereotactic frame or a robotic arm. The mean registration error associated with MRI was 1.3 mm (RMS error) and 1.5 mm (RMS error) with CT-data. The average intra-operative target-localising error was 3.2 mm (± 1.5 mm SD). Thus, the equipment was of great help in planning and performing skin incisions and craniotomies as well as in reaching deep-seated lesions with a minimum of trauma.Interpretation: The NEN-NeuroGuardTM system is a very user-friendly and reliable tool for image-guided neurosurgery. It does not have the limitations of a conventional stereotactic frame. Due to its electromagnetic technology it avoids the “line-of-sight” problem often met by optical navigation systems since its sensors remain active even when situated deep inside the skull or hidden, for example, by drapes or by the surgical microscope.
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paper_title: Electromagnetic navigation bronchoscopy: A descriptive analysis

paper_content:

Electromagnetic navigation bronchoscopy (ENB) is an exciting new bronchoscopic technique that promises accurate navigation to peripheral pulmonary target lesions, using technology similar to a car global positioning system (GPS) unit. Potential uses for ENB include biopsy of peripheral lung lesions, pleural dye marking of nodules for surgical wedge resection, placement of fiducial markers for stereotactic radiotherapy, and therapeutic insertion of brachytherapy catheters into malignant tissue. This article will describe the ENB procedure, review the published literature, compare ENB to existing biopsy techniques, and outline the challenges for widespread implementation of this new technology.
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paper_title: Assessment of the ablated area after radiofrequency ablation by the spread of bubbles: comparison with virtual sonography with magnetic navigation.

paper_content:

BACKGROUND/AIMS ::: The purpose of this study was to investigate whether bubble images after radiofrequency ablation (RFA) can predict the ablated area. ::: ::: ::: METHODOLOGY ::: The spread of bubbles 5 minutes after RFA were compared with the unenhanced area of virtual sonography with magnetic navigation in two RFA methods: expandable needle and cool-tip needle. ::: ::: ::: RESULTS ::: Thirty-one hepatocellular carcinoma nodules were treated by RFA with either an expandable needle or cool-tip needle (n=14 and n=17, respectively) and examined. In the 14 nodules treated by expandable needle, bubble images (puncture direction; r=0.833, p=0.0002, perpendicular direction; r=0.803, p=0.0005) were closely correlated with the unenhanced area of virtual sonography. On the other hand, in 17 nodules treated by cool-tip needle, there was no correlation between the bubble images and virtual sonography (puncture direction; r=0.590, p=0.0127, perpendicular direction; r=0.342, p=0.180). ::: ::: ::: CONCLUSIONS ::: The observation of bubbles with the expandable needle can accurately predict the ablated area and is helpful for assessing local control of RFA.
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paper_title: Review on Patents about Magnetic Localisation Systems for in vivo Catheterizations

paper_content:

Abstract: in vivo Catheterizations are usually performed by physicians using X-Ray fluoroscopic guide and contrast-media. The X-Ray exposure both of the patient and of the operators can induce collateral effects. The present review describes the status of the art on recent patents about magnetic position/orientation indicators capable to drive the probe during in-vivo medical diagnostic or interventional procedures. They are based on the magnetic field produced by sources and revealed by sensors. Possible solutions are: the modulated magnetic field produced by a set of coils positioned externally to the patient is measured by sensors installed on the intra-body probe; the magnetic field produced by a thin permanent magnet installed on the intra-body probe is measured by magnetic field sensors positioned outside the patient body. In either cases, position and orientation of the probe are calculated in real time: this allows the elimination of repetitive X-Ray scans used to monitor the probe. The aim of the proposed systems is to drive the catheter inside the patient vascular tree with a reduction of the X-Ray exposure both of the patient and of the personnel involved in the intervention. The present paper intends also to highlight advantages/disadvantages of the presented solutions.
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paper_title: Evaluation of magnetic scope navigation in screening endoscopic examination of colorectal cancer

paper_content:

BackgroundColorectal cancer is the most common cancer in Europe. Early diagnosis and treatment gives the patient a chance for complete recovery. Screening colonoscopies in the symptom-free patients are currently performed on a wide scale. The examinations are performed under local anesthesia which does not eliminate all discomfort and pain related to the examination. The aim of this study was to evaluate magnetic scope navigation in screening endoscopic examinations performed to detect early-stage colorectal cancer.MethodsThe study group consisted of 200 patients, aged 40–65 years, who were free from colon cancer symptoms. All patients underwent complete colonoscopy under local anesthesia. The equipment could be fitted with the scope that allows three-dimensional observation of instrument localization in the bowel. The examination was performed by three experienced endoscopists, each of whom performed over 5,000 colonoscopies. The patients were randomized to two groups: those whose equipment did not have 3D navigation (group I) and those whose equipment did have 3D navigation (group II). Each group consisted of 100 cases matched by gender, age, and BMI. The authors compared the duration of introducing instrument to cecum, the pulse rate before the examination and at the time the instrument reached the cecum, and subjective pain evaluation by the patient on the visual analog scale.ResultsGroup I consisted of 54 women and 46 men with a mean age of 54.6 years and mean BMI of 27.8 kg/m2, and group II had 58 women and 42 men, mean age of 55.1 years and mean BMI of 26.4 kg/m2. The average time it took for the instrument to reach the cecum was 216s in group I and 181s in group II (P < 0.05). Pain measured on the 10-point VAS scale was 2.44 in group I and 1.85 in group II (P < 0.05). The results showed a significantly shorter time for the instrument to reach the cecum in group II and significantly lower pain intensity during the examination was reported by the group II patients. No significant differences were found in the pulse measurements between the groups (P = 0.5).Conclusions3D navigation during colonoscopy decreases the time for the instrument to reach the cecum and lowers pain intensity subjectively reported by the patients. The use of 3D and the possibility to observe instrument localization and maneuvers brings more comfort to the patients.
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paper_title: Electromagnetic Navigation to Aid Radiofrequency Ablation and Biopsy of Lung Tumors

paper_content:

Purpose We evaluated an electromagnetic (EM) navigation system (Veran Medical Technologies Inc, St. Louis, MO) to determine its potential to reduce the number of skin punctures and instrument adjustments during computed tomographic-guided percutaneous ablation and biopsy of lung nodules. Description Ten patients undergoing lung percutaneous ablation were prospectively enrolled. The mean age was 70 years. Positioning of the needle device was verified with computed tomographic fluoroscopy prior to the execution of any biopsy or ablation. Each EM navigation-guided procedure was defined as an EM-intervention. Evaluation Nineteen EM interventions were performed. When an EM-guided biopsy was performed, the intervention was done immediately prior to ablation. For all 19 EM interventions, only one skin-puncture was required. The mean number of instrument adjustments required was 1.2 (range, 0 to 2). The mean time for each EM intervention was 5.2 minutes (range, 1 to 20 minutes). Pneumothorax occurred in 5 patients (50%). Only the number of instrument adjustments was significantly related to the pneumothorax rate ( p = 0.005). Conclusions The EM navigation is feasible and seems to be a useful aid for image-guided procedures. Early experience suggests a low number of skin-puncture and instrument adjustments using the EM navigation system. Instrument adjustments were a key factor in pneumothorax development.
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paper_title: [Ultrasound-guided interventions using magnetic field navigation. First experiences with Ultra-Guide 2000 under operative conditions].

paper_content:

Abstract Ultrasound-guided interventions are presently performed as free-hand-type procedures or using biopsy transducers. In this article we report on our experience with a new navigation-system for sonographically guided interventional procedures under OR-conditions. The US-Guide 2000 trade mark is an electromagnetic guidance system that assists physicians in ultrasound-guided interventional procedures. This system accommodates both in-plane and out-of-plane needle placement. We evaluated this system for the first time under OR-conditions. Overall, for 39 interventional procedures (23 thermoablations of malignant liver lesions, 16 diagnostic punctures) were performed. All targets were reached successfully without any complications. No interactions with other OR-devices were seen. The US-Guide 2000 trade mark as a virtual needle-guiding system allows a safe and accurate sonographically assisted intervention. The major advantage is the possibility of out-of-plane needle placement and the combination of flexibility of free-hand-type procedures with the accuracy of a biopsy transducer. This increases the safety of punctures especially when lesions are difficult to reach and/or are situated next to vulnerable structures. It also reduces the interventional trauma.
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paper_title: A novel technique for tailoring frontal osteoplastic flaps using the ENT magnetic navigation system.

paper_content:

CONCLUSION ::: The ENT magnetic navigation system is potentially useful and offers the most accurate technique for harvesting frontal osteoplastic flaps. It represents a valid tool in the wide range of instruments available to rhinologists. ::: ::: ::: OBJECTIVE ::: Precise delineation of the boundaries of the frontal sinus is a crucial step when harvesting a frontal osteoplastic flap. We present a novel technique using the ENT magnetic navigation system. ::: ::: ::: METHODS ::: Nineteen patients affected by different pathologies involving the frontal sinus underwent an osteoplastic flap procedure using the ENT magnetic navigation system between January 2009 and April 2011. ::: ::: ::: RESULTS ::: The ENT magnetic navigation system was found to be a safe and accurate tool for delineating the frontal sinus boundaries. No intraoperative complications occurred during the osteoplastic procedures.
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paper_title: Next generation distal locking for intramedullary nails using an electromagnetic X-ray-radiation-free real-time navigation system.

paper_content:

BACKGROUND ::: Distal locking marks one challenging step during intramedullary nailing that can lead to an increased irradiation and prolonged operation times. The aim of this study was to evaluate the reliability and efficacy of an X-ray-radiation-free real-time navigation system for distal locking procedures. ::: ::: ::: METHODS ::: A prospective randomized cadaver study with 50 standard free-hand fluoroscopic-guided and 50 electromagnetic-guided distal locking procedures was performed. All procedures were timed using a stopwatch. Intraoperative fluoroscopy exposure time and absorbed radiation dose (mGy) readings were documented. All tibial nails were locked with two mediolateral and one anteroposterior screw. Successful distal locking was accomplished once correct placement of all three screws was confirmed. ::: ::: ::: RESULTS ::: Successful distal locking was achieved in 98 cases. No complications were encountered using the electromagnetic navigation system. Eight complications arose during free-hand fluoroscopic distal locking. Undetected secondary drill slippage on the ipsilateral cortex accounted for most problems followed by undetected intradrilling misdirection causing a fissural fracture of the contralateral cortex while screw insertion in one case. Compared with the free-hand fluoroscopic technique, electromagnetically navigated distal locking provides a median time benefit of 244 seconds without using ionizing radiation. ::: ::: ::: CONCLUSION ::: Compared with the standard free-hand fluoroscopic technique, the electromagnetic guidance system used in this study showed high reliability and was associated with less complications, took significantly less time, and used no radiation exposure for distal locking procedures. ::: ::: ::: LEVEL OF EVIDENCE ::: Therapeutic study, level II.
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paper_title: Freehand placement of depth electrodes using electromagnetic frameless stereotactic guidance.

paper_content:

The presurgical evaluation of patients with epilepsy often requires an intracranial study in which both subdural grid electrodes and depth electrodes are needed. Performing a craniotomy for grid placement with a stereotactic frame in place can be problematic, especially in young children, leading some surgeons to consider frameless stereotaxy for such surgery. The authors report on the use of a system that uses electromagnetic impulses to track the tip of the depth electrode. Ten pediatric patients with medically refractory focal lobar epilepsy required placement of both subdural grid and intraparenchymal depth electrodes to map seizure onset. Presurgical frameless stereotaxic targeting was performed using a commercially available electromagnetic image-guided system. Freehand depth electrode placement was then performed with intraoperative guidance using an electromagnetic system that provided imaging of the tip of the electrode, something that has not been possible using visually or sonically based systems. Accuracy of placement of depth electrodes within the deep structures of interest was confirmed postoperatively using CT and CT/MR imaging fusion. Depth electrodes were appropriately placed in all patients. Electromagnetic-tracking-based stereotactic targeting improves the accuracy of freehand placement of depth electrodes in patients with medically refractory epilepsy. The ability to track the electrode tip, rather than the electrode tail, is a major feature that enhances accuracy. Additional advantages of electromagnetic frameless guidance are discussed.
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paper_title: Free-hand CT-based electromagnetically guided interventions: accuracy, efficiency and dose usage.

paper_content:

The purpose of this paper was to evaluate computed tomography (CT) based electromagnetically tip-tracked (EMT) interventions in various clinical applications. An EMT system was utilized to perform percutaneous interventions based on CT datasets. Procedure times and spatial accuracy of needle placement were analyzed using logging data in combination with periprocedurally acquired CT control scans. Dose estimations in comparison to a set of standard CT-guided interventions were carried out. Reasons for non-completion of planned interventions were analyzed. Twenty-five procedures scheduled for EMT were analyzed, 23 of which were successfully completed using EMT. The average time for performing the procedure was 23.7 ± 17.2 min. Time for preparation was 5.8 ± 7.3 min while the interventional (skin-to-target) time was 2.7 ± 2.4 min. The average puncture length was 7.2 ± 2.5 cm. Spatial accuracy was 3.1 ± 2.1 mm. Non-completed procedures were due to patient movement and reference fixation problems. Radiation doses (dosis-length-product) were significantly lower (p = 0.012) for EMT-based interventions (732 ± 481 mGy x cm) in comparison to the control group of standard CT-guided interventions (1343 ± 1054 mGy x cm). Electromagnetic navigation can accurately guide percutaneous interventions in a variety of indications. Accuracy and time usage permit the routine use of the utilized system. Lower radiation exposure for EMT-based punctures provides a relevant potential for dose saving.
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paper_title: Image guided surgery of paranasal sinuses and anterior skull base - Five years experience with the InstaTrak ® -System*

paper_content:

SUMMARY We report on our experience with navigational tools in paranasal sinus and anterior skull base surgery, especially with electromagnetic guidance systems. During the last five years we operated over 80 selected cases with the InstaTrak ® system from VTI (Lawrence, MS, USA). Applicability and user friendliness were explored. The InstaTrak ® 3500 employs a Sun ® Workstation and is a frameless and free-arm and navigation system. Two different suction devices, used as sensors (receivers), and one transmitter are interconnected to this workstation. The position of the tip of the aspirator is displayed as a pair of crosshairs on the screen in axial, coronal and sagittal planes of the patient’s CTscan on the computerscreen online. Our results showed high accuracy-level, usually better than one millimeter and a setup-time less than ten minutes, on average. No additional personnel is required in the OR. We believe that the system enhances efficacy in selected cases like revision surgery, tumor surgery or difficult anterior skull base surgery. However, one should consider that medicolegal responsibility stays always with the surgeon and not with any navigation system.
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paper_title: Electromagnetic navigation for percutaneous guide-wire insertion: Accuracy and efficiency compared to conventional fluoroscopic guidance

paper_content:

The combination of electromagnetic (EM) navigation with intraoperative fluoroscopic images has the potential to create the ideal environment for spinal surgical applications. This technology enhances standard intraoperative fluoroscopic information for localization of the pedicle entry point and trajectory and may be an effective alternative to other image-guided surgery (IGS) systems. This study was performed to assess the accuracy and time efficiency (placement and fluoroscopy) using EM navigation versus conventional fluoroscopy in the placement of pedicle guide-wires. ::: ::: Kirschner wire (K-wire) placement was performed in cadavers from T8 to S1 using EM navigation versus conventional fluoroscopy. Time for set-up, placement, and fluoroscopy was recorded. After insertion, the accuracy for each level was assessed for the presence and location of facet joint, pedicle, or vertebral cortical perforation using computed tomography imaging with multiplanar reconstructions. ::: ::: K-wire placements were 100% successful for both methods. Comparing EM-based IGS-assisted placement with the conventional fluoroscopy method showed a longer set-up time of 9.6 min versus 3.6 min, respectively. However, mean placement times of 6.3 min versus 9.7 min (P = 0.005) and mean fluoroscopy times of 11 s versus 48 s (P < 0.0001) were both shorter for the EM group. There were no significant differences in the proportion of pedicle, vertebral body, or facet joint breaches. A higher proportion of ideal trajectories was achieved in the EM group. Therefore, we have shown that an EM IGS system can assist the spine surgeon in minimally invasive pedicle screw insertion by providing high-accuracy K-wire placement with a significant reduction in fluoroscopy time.
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paper_title: Electromagnetic-guided postpyloric tube placement in children: Pilot study of its use as a rescue therapy

paper_content:

Summary Background & aims Postpyloric feeding is frequently indicated in clinical practice. Postpyloric enteral tube placement can be cumbersome, necessitating use of fluoroscopy and/or endoscopy. Guiding devices have been developed to facilitate bedside tube placement. We aimed to evaluate the feasibility and safety of postpyloric enteral tube placement in children using an electromagnetic-guided system as a rescue strategy in case blind tube insertion failed. Method In a prospective pilot study in 10 children postpyloric enteral tube placement using an electromagnetic-guided placement was attempted after blind placement failure. Results Postpyloric enteral tube placement was successful in 6 of the 10 included patients. No adverse events occurred and it was well tolerated. Conclusion This pilot study suggests that electromagnetic-guided placement as a rescue technique for postpyloric enteral tube placement can prevent the use of fluoroscopy and/or endoscopic placement in a substantial portion of patients.
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paper_title: Needle and catheter navigation using electromagnetic tracking for computer-assisted C-arm CT interventions

paper_content:

Integrated solutions for navigation systems with CT, MR or US systems become more and more popular for medical products. Such solutions improve the medical workflow, reduce hardware, space and costs requirements. The purpose of our project was to develop a new electromagnetic navigation system for interventional radiology which is integrated into C-arm CT systems. The application is focused on minimally invasive percutaneous interventions performed under local anaesthesia. Together with a vacuum-based patient immobilization device and newly developed navigation tools (needles, panels) we developed a safe and fully automatic navigation system. The radiologist can directly start with navigated interventions after loading images without any prior user interaction. The complete system is adapted to the requirements of the radiologist and to the clinical workflow. For evaluation of the navigation system we performed different phantom studies and achieved an average accuracy of better than 2.0 mm.
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paper_title: Magnetic navigation in ultrasound-guided interventional radiology procedures

paper_content:

Aim To evaluate the usefulness of magnetic navigation in ultrasound (US)-guided interventional procedures. Materials and methods Thirty-seven patients who were scheduled for US-guided interventional procedures (20 liver cancer ablation procedures and 17 other procedures) were included. Magnetic navigation with three-dimensional (3D) computed tomography (CT), magnetic resonance imaging (MRI), 3D US, and position-marking magnetic navigation were used for guidance. The influence on clinical outcome was also evaluated. Results Magnetic navigation facilitated applicator placement in 15 of 20 ablation procedures for liver cancer in which multiple ablations were performed; enhanced guidance in two small liver cancers invisible on conventional US but visible at CT or MRI; and depicted the residual viable tumour after transcatheter arterial chemoembolization for liver cancer in one procedure. In four of 17 other interventional procedures, position-marking magnetic navigation increased the visualization of the needle tip. Magnetic navigation was beneficial in 11 (55%) of 20 ablation procedures; increased confidence but did not change management in five (25%); added some information but did not change management in two (10%); and made no change in two (10%). In the other 17 interventional procedures, the corresponding numbers were 1 (5.9%), 2 (11.7%), 7 (41.2%), and 7 (41.2%), respectively (p = 0.002). Conclusion Magnetic navigation in US-guided interventional procedure provides solutions in some difficult cases in which conventional US guidance is not suitable. It is especially useful in complicated interventional procedures such as ablation for liver cancer.
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paper_title: A novel technique for analysis of accuracy of magnetic tracking systems used in image guided surgery

paper_content:

With the increased use and development of image-guided surgical applications, there is a need for methods of ::: analysis of the accuracy and precision of the components which compose these systems. One primary component ::: of an image-guided surgery system is the position tracking system which allows for the localization of a tool within ::: the surgical field and provides information which is translated back to the images. Previously much work has been ::: done in characterizing these systems for spatial accuracy and precision. Much of this previous work examines ::: single tracking systems or modalities. We have devised a method which allows for the characterization of a novel ::: tracking system independent of modality and location. We describe the development of a phantom system which ::: allows for rapid design and creation of surfaces with different geometries. We have also demonstrated a method of ::: analysis of the data generated by this phantom system, and used it to compare Biosense-Webster's CartoXP TM , ::: and Northern Digital's Aurora TM magnetic trackers. We have determined that the accuracy and precision of the ::: CartoXP was best, followed closely by the Aurora's dome volume, then the Aurora's cube volume. The mean ::: accuracy for all systems was better than 3mm and decays with distance from the field generator.
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paper_title: CT-guided percutaneous lung biopsy: comparison of conventional CT fluoroscopy to CT fluoroscopy with electromagnetic navigation system in 60 consecutive patients.

paper_content:

PURPOSE ::: To determine if use of an electromagnetic navigation system (EMN) decreases radiation dose and procedure time of CT fluoroscopy guided lung biopsy in lesions smaller than 2.5 cm. ::: ::: ::: MATERIALS/METHODS ::: 86 consecutive patients with small lung masses (<2.5 cm) were approached. 60 consented and were randomized to undergo biopsy with CT fluoroscopy (CTF) (34 patients) or EMN (26 patients). Technical failure required conversion to CTF in 8/26 EMN patients; 18 patients completed biopsy with EMN. Numerous biopsy parameters were compared as described below. ::: ::: ::: RESULTS ::: Average fluoroscopy time using CTF was 28.2s compared to 35.0 s for EMN (p=0.1). Average radiation dose was 117 mGy using CTF and 123 mGy for EMN (p=0.7). Average number of needle repositions was 3.7 for CTF and 4.4 for EMN (p=0.4). Average procedure time was 15 min for CTF and 20 min for EMN (p=0.01). There were 7 pneumothoracesin the CTF group and 6 pneumothoraces in the EMN group (p=0.7). One pneumothorax in the CTF group and 3 pneumothoraces in the EMN group required chest tube placement (p=0.1). One pneumothorax patient in each group required hospital admission. Diagnostic specimens were obtained in 31/34 patients in the CTF group and 22/26 patients in the EMN group (p=0.4). ::: ::: ::: CONCLUSIONS ::: EMN was not statistically different than CTF for fluoroscopy time, radiation dose, number of needle repositions, incidence of pneumothorax, need for chest tube, or diagnostic yield. Procedure time was increased with EMN.
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paper_title: How Does Electromagnetic Navigation Stack Up Against Infrared Navigation in Minimally Invasive Total Knee Arthroplasties

paper_content:

Abstract Forty-six primary total knee arthroplasties were performed using either an electromagnetic (EM) or infrared (IR) navigation system. In this IRB-approved study, patients were evaluated clinically and for accuracy using spiral computed tomographic imaging and 36-in standing radiographs. Although EM navigation was subject to metal interference, it was not as drastic as line-of-sight interference with IR navigation. Mechanical alignment was ideal in 92.9% of EM and 90.0% of IR cases based on spiral computed tomographic imaging and 100% of EM and 95% of IR cases based on x-ray. Individual measurements of component varus/valgus and sagittal measurements showed EM to be equivalent to IR, with both systems producing subdegree accuracy in 95% of the readings.
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paper_title: Intracranial Image-Guided Neurosurgery: Experience with a new Electromagnetic Navigation System

paper_content:

Summary.Summary.Background: The aim of image-guided neurosurgery is to accurately project computed tomography (CT) or magnetic resonance imaging (MRI) data into the operative field for defining anatomical landmarks, pathological structures and tumour margins. To achieve this end, different image-guided and computer-assisted, so-called “neuronavigation” systems have been developed in order to offer the neurosurgeon precise spatial information.Method: The present study reports on the experience gained with a prototype of the NEN-NeuroGuardTM neuronavigation system (Nicolet Biomedical, Madison, WI, USA). It utilises a pulsed DC electromagnetic field for determining the location in space of surgical instruments to which miniaturised sensors are attached. The system was evaluated in respect to its usefulness, ease of integration into standard neurosurgical procedures, reliability and accuracy.Findings: The NEN-system was used with success in 24 intracranial procedures for lesions including both gliomas and cerebral metastases. It allowed real-time display of surgical manoeuvres on pre-operative CT or MR images without a stereotactic frame or a robotic arm. The mean registration error associated with MRI was 1.3 mm (RMS error) and 1.5 mm (RMS error) with CT-data. The average intra-operative target-localising error was 3.2 mm (± 1.5 mm SD). Thus, the equipment was of great help in planning and performing skin incisions and craniotomies as well as in reaching deep-seated lesions with a minimum of trauma.Interpretation: The NEN-NeuroGuardTM system is a very user-friendly and reliable tool for image-guided neurosurgery. It does not have the limitations of a conventional stereotactic frame. Due to its electromagnetic technology it avoids the “line-of-sight” problem often met by optical navigation systems since its sensors remain active even when situated deep inside the skull or hidden, for example, by drapes or by the surgical microscope.
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paper_title: Electromagnetic navigation bronchoscopy: A descriptive analysis

paper_content:

Electromagnetic navigation bronchoscopy (ENB) is an exciting new bronchoscopic technique that promises accurate navigation to peripheral pulmonary target lesions, using technology similar to a car global positioning system (GPS) unit. Potential uses for ENB include biopsy of peripheral lung lesions, pleural dye marking of nodules for surgical wedge resection, placement of fiducial markers for stereotactic radiotherapy, and therapeutic insertion of brachytherapy catheters into malignant tissue. This article will describe the ENB procedure, review the published literature, compare ENB to existing biopsy techniques, and outline the challenges for widespread implementation of this new technology.
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paper_title: Assessment of the ablated area after radiofrequency ablation by the spread of bubbles: comparison with virtual sonography with magnetic navigation.

paper_content:

BACKGROUND/AIMS ::: The purpose of this study was to investigate whether bubble images after radiofrequency ablation (RFA) can predict the ablated area. ::: ::: ::: METHODOLOGY ::: The spread of bubbles 5 minutes after RFA were compared with the unenhanced area of virtual sonography with magnetic navigation in two RFA methods: expandable needle and cool-tip needle. ::: ::: ::: RESULTS ::: Thirty-one hepatocellular carcinoma nodules were treated by RFA with either an expandable needle or cool-tip needle (n=14 and n=17, respectively) and examined. In the 14 nodules treated by expandable needle, bubble images (puncture direction; r=0.833, p=0.0002, perpendicular direction; r=0.803, p=0.0005) were closely correlated with the unenhanced area of virtual sonography. On the other hand, in 17 nodules treated by cool-tip needle, there was no correlation between the bubble images and virtual sonography (puncture direction; r=0.590, p=0.0127, perpendicular direction; r=0.342, p=0.180). ::: ::: ::: CONCLUSIONS ::: The observation of bubbles with the expandable needle can accurately predict the ablated area and is helpful for assessing local control of RFA.
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paper_title: Navigation Systems for Ablation

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Navigation systems, devices, and intraprocedural software are changing the way interventional oncology is practiced. Before the development of precision navigation tools integrated with imaging systems, thermal ablation of hard-to-image lesions was highly dependent on operator experience, spatial skills, and estimation of positron emission tomography–avid or arterial-phase targets. Numerous navigation systems for ablation bring the opportunity for standardization and accuracy that extends the operator's ability to use imaging feedback during procedures. In this report, existing systems and techniques are reviewed and specific clinical applications for ablation are discussed to better define how these novel technologies address specific clinical needs and fit into clinical practice.
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paper_title: A comparison of blood loss in minimally invasive surgery with and without electromagnetic computer navigation in total knee arthroplasty.

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OBJECTIVE ::: To compare the blood loss after minimally invasive surgery total knee arthroplasty (MIS-TKA) between the procedures performed with and without electromagnetic computer navigation. ::: ::: ::: MATERIAL AND METHOD ::: Eighty patients were recruited for a cohort study of the minimally invasive surgery total knee arthroplasty (MIS-TKA) for the treatment of osteoarthritis. They were divided into two groups, 40 patients had a computer-assisted surgery procedure for the minimally invasive surgery total knee arthroplasty (CAS-MIS-TKA) and the other 40 patients had a conventional procedure for the minimally invasive surgery total knee arthroplasty (MIS-TKA). The surgery in both groups was carried out by a single surgeon at one institution using a uniform approach. The blood loss in each group was evaluated and analyzed for the statistical difference. ::: ::: ::: RESULTS ::: The result showed that the mean blood loss from the drainage of the CAS-MIS-TKA group (389.88 +/- 215.57 milliliters) was slightly lower than the MIS-TKA group (425.25 +/- 269.40 milliliters), which had no significant difference (p-value 0.519). Moreover, the whole blood loss in the CAS-MIS-TKA group (948.45 +/- 431.63 milliliters) was slightly lower than the MIS-TKA group (1075.32 +/- 419.02 milliliters). The difference was also not statistically significant. ::: ::: ::: CONCLUSION ::: Electromagnetic computer-assisted surgery did not reduce blood loss in the minimally invasive surgery total knee arthroplasty (MIS-TKA).
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paper_title: Reduction in patient-reported acute morbidity in prostate cancer patients treated with 81-Gy Intensity-modulated radiotherapy using reduced planning target volume margins and electromagnetic tracking: assessing the impact of margin reduction study.

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OBJECTIVE ::: To investigate whether patient-reported quality of life after high-dose external beam intensity-modulated radiotherapy for prostate cancer can be improved by decreasing planning target volume margins while using real-time tumor tracking. ::: ::: ::: METHODS ::: Study patients underwent radiotherapy with nominal 3-mm margins and electromagnetic real-time tracking. Morbidity was assessed before and at the end of radiotherapy using Expanded Prostate Cancer Index Composite (EPIC) questionnaires. Changes in scores were compared between the Assessing Impact of Margin Reduction (AIM) study cohort and the comparator Prostate Cancer Outcomes and Satisfaction with Treatment Quality Assessment (PROST-QA) cohort, treated with conventional margins. ::: ::: ::: RESULTS ::: The 64 patients in the prospective AIM study had generally less favorable clinical characteristics than the 153 comparator patients. Study patients had similar or slightly poorer pretreatment EPIC scores than comparator patients in bowel, urinary, and sexual domains. AIM patients receiving radiotherapy had less bowel morbidity than the comparator group as measured by changes in mean bowel and/or rectal domain EPIC scores from pretreatment to 2 months after start of treatment (-1.5 vs -16.0, P = .001). Using a change in EPIC score >0.5 baseline standard deviation as the measure of clinical relevance, AIM study patients experienced meaningful decline in only 1 health-related quality of life domain (urinary) whereas decline in 3 health-related quality of life domains (urinary, sexual, and bowel/rectal) was observed in the PROST-QA comparator cohort. ::: ::: ::: CONCLUSIONS ::: Prostate cancer patients treated with reduced margins and tumor tracking had less radiotherapy-related morbidity than their counterparts treated with conventional margins. Highly contoured intensity-modulated radiotherapy shows promise as a successful strategy for reducing morbidity in prostate cancer treatment.
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paper_title: The Cathlocator: a novel non-radiological method for the localization of enteral tubes.

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Safe placement of nasogastric tubes requires reliable positioning of the tip of the tube within the stomach. Radiology and aspiration are currently used to confirm tube position, but suffer from significant problems of cost and efficacy, respectively. We have developed a novel method to locate the position of a catheter tip within the body, using the detection of low energy electromagnetic field generated in a coil located in the catheter with an external hand-held unit (Cathlocator). In vitro, the unit detected the distance of the coil from the detector with an accuracy of 0.1 cm over a range of 4-12 cm. In vivo studies were performed in 11 healthy volunteers using a purpose-built manometric assembly that incorporated the signal generating coil in its tip. In all subjects the Cathlocator showed the position of the signal generating coil to be cranial to the xiphisternum when manometric and transmucosal potential difference criteria showed it to be located above the lower oesophageal sphincter. When the coil was within the stomach, the Cathlocator identified its position within the epigastric, umbilical and left hypochondrial regions of the abdomen. The distance of the coil from the surface was significantly greater when in the duodenum mean (+/- s.e.m. 7.6 +/- 0.3 cm; P < 0.001) and oesophagus (8.6 +/- 0.2 cm; P < 0.002) than the stomach (5.0 +/- 0.4 cm). In one subject studied twice there was a close correlation between the location and depth measured by the device on each occasion. The Cathlocator is a novel non-radiological device that has the potential to be useful in the placement of gastrointestinal catheters.
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paper_title: A novel technique for tailoring frontal osteoplastic flaps using the ENT magnetic navigation system.

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CONCLUSION ::: The ENT magnetic navigation system is potentially useful and offers the most accurate technique for harvesting frontal osteoplastic flaps. It represents a valid tool in the wide range of instruments available to rhinologists. ::: ::: ::: OBJECTIVE ::: Precise delineation of the boundaries of the frontal sinus is a crucial step when harvesting a frontal osteoplastic flap. We present a novel technique using the ENT magnetic navigation system. ::: ::: ::: METHODS ::: Nineteen patients affected by different pathologies involving the frontal sinus underwent an osteoplastic flap procedure using the ENT magnetic navigation system between January 2009 and April 2011. ::: ::: ::: RESULTS ::: The ENT magnetic navigation system was found to be a safe and accurate tool for delineating the frontal sinus boundaries. No intraoperative complications occurred during the osteoplastic procedures.
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paper_title: Computer-aided navigation in neurosurgery

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The article comprises three main parts: a historical review on navigation, the mathematical basics for calculation and the clinical applications of navigation devices. Main historical steps are described from the first idea till the realisation of the frame-based and frameless navigation devices including robots. In particular the idea of robots can be traced back to the Iliad of Homer, the first testimony of European literature over 2500 years ago. In the second part the mathematical calculation of the mapping between the navigation and the image space is demonstrated, including different registration modalities and error estimations. The error of the navigation has to be divided into the technical error of the device calculating its own position in space, the registration error due to inaccuracies in the calculation of the transformation matrix between the navigation and the image space, and the application error caused additionally by anatomical shift of the brain structures during operation. In the third part the main clinical fields of application in modern neurosurgery are demonstrated, such as localisation of small intracranial lesions, skull-base surgery, intracerebral biopsies, intracranial endoscopy, functional neurosurgery and spinal navigation. At the end of the article some possible objections to navigation-aided surgery are discussed.
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paper_title: Freehand placement of depth electrodes using electromagnetic frameless stereotactic guidance.

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The presurgical evaluation of patients with epilepsy often requires an intracranial study in which both subdural grid electrodes and depth electrodes are needed. Performing a craniotomy for grid placement with a stereotactic frame in place can be problematic, especially in young children, leading some surgeons to consider frameless stereotaxy for such surgery. The authors report on the use of a system that uses electromagnetic impulses to track the tip of the depth electrode. Ten pediatric patients with medically refractory focal lobar epilepsy required placement of both subdural grid and intraparenchymal depth electrodes to map seizure onset. Presurgical frameless stereotaxic targeting was performed using a commercially available electromagnetic image-guided system. Freehand depth electrode placement was then performed with intraoperative guidance using an electromagnetic system that provided imaging of the tip of the electrode, something that has not been possible using visually or sonically based systems. Accuracy of placement of depth electrodes within the deep structures of interest was confirmed postoperatively using CT and CT/MR imaging fusion. Depth electrodes were appropriately placed in all patients. Electromagnetic-tracking-based stereotactic targeting improves the accuracy of freehand placement of depth electrodes in patients with medically refractory epilepsy. The ability to track the electrode tip, rather than the electrode tail, is a major feature that enhances accuracy. Additional advantages of electromagnetic frameless guidance are discussed.
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paper_title: Fixation, registration, and image-guided navigation using a thermoplastic facial mask in electromagnetic navigation-guided radiofrequency thermocoagulation.

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OBJECTIVE ::: For fixation, registration, and image-guided navigation, the aim of this study was to evaluate a thermoplastic facial mask with plastic markers in achieving frameless stereotactic radiofrequency thermocoagulation (RFT). ::: ::: ::: STUDY DESIGN ::: A thermoplastic facial mask was remolded according to each subject's face. Six markers were placed on the surface and 6 inside. Series of 1.25-mm- and 2.5-mm-slice computerized tomography (CT) scans were made to provide radiologic data. During the phantom study, each plastic sphere inside was selected in turn as the target for frameless stereotaxy. The clinical Hartel puncture of the foramen ovale (FO) was imitated using an electromagnetic navigation system. Navigation-guided RFT was tried in 3 patients. ::: ::: ::: RESULTS ::: The mean location error was 1.29 mm (SD ± 0.39 mm). No significant difference (P > .05) was proven between 1.25-mm and 2.5-mm CT slice acquisition for the image datasets used. The FO punctures in clinical trials were successful and confirmed by CT. ::: ::: ::: CONCLUSIONS ::: Registration and fixation via a fiducial marker-based thermoplastic facial mask is accurate and feasible for use in navigation-guided RFT.
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paper_title: Image guided surgery of paranasal sinuses and anterior skull base - Five years experience with the InstaTrak ® -System*

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SUMMARY We report on our experience with navigational tools in paranasal sinus and anterior skull base surgery, especially with electromagnetic guidance systems. During the last five years we operated over 80 selected cases with the InstaTrak ® system from VTI (Lawrence, MS, USA). Applicability and user friendliness were explored. The InstaTrak ® 3500 employs a Sun ® Workstation and is a frameless and free-arm and navigation system. Two different suction devices, used as sensors (receivers), and one transmitter are interconnected to this workstation. The position of the tip of the aspirator is displayed as a pair of crosshairs on the screen in axial, coronal and sagittal planes of the patient’s CTscan on the computerscreen online. Our results showed high accuracy-level, usually better than one millimeter and a setup-time less than ten minutes, on average. No additional personnel is required in the OR. We believe that the system enhances efficacy in selected cases like revision surgery, tumor surgery or difficult anterior skull base surgery. However, one should consider that medicolegal responsibility stays always with the surgeon and not with any navigation system.
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paper_title: A comparison of image guidance systems for sinus surgery

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Objective: Intraoperative computed tomographic guidance systems are available which utilize either electromagnetic (radiofrequency) or optical (infrared) signals to localize instruments within the surgical field. The objective of this study was to compare the use of these two different image guidance technologies for sinus surgery. Study Design: Prospective cohort study. Methods: The electromagnetic-based InstaTrak system (n= 24) and the optical-based Stealth-Station (n = 49) were compared in a series of 73 consecutive sinus series which utilized image guidance technology. Results: Both the electromagnetic and optical systems provided anatomic localization to within 2 mm during surgery. Intraoperative reregistration was effective in correcting for any anatomic drift. There were no intraoperative complications. Mean operative times were 156.3 ± 8.9 minutes for the electromagnetic and 139.2 ± 17.7 minutes for the optical system (P <.05). The average intraoperative blood loss did not differ significantly between groups (electromagnetic, 190.6 ± 28.7 mL; optical, 172.4 ± 23.0 mL). Each system was noted to have limitations. The presence of metallic objects in the operative field interfered with functioning of the electromagnetic system, whereas the optical system required a clear line of sight to be maintained between the infrared camera and surgical handpiece. Both systems required specialized headsets to be worn by patients during surgery to monitor head position. The electromagnetic system also required these headsets to be worn during the preoperative computed tomography scan. Conclusion: Although these two image guidance systems both proved valuable for anatomic localization during sinus surgery, individual preferences can be based on distinct differences in their design and operation.
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paper_title: An electromagnetic navigation system for transbronchial interventions with a novel approach to respiratory motion compensation.

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PURPOSE ::: Bronchoscopic interventions, such as transbronchial needle aspiration (TBNA), are commonly performed procedures to diagnose and stage lung cancer. However, due to the complex structure of the lung, one of the main challenges is to find the exact position to perform a biopsy and to actually hit the biopsy target (e.g., a lesion). Today, most interventions are accompanied by fluoroscopy to verify the position of the biopsy instrument, which means additional radiation exposure for the patient and the medical staff. Furthermore, the diagnostic yield of TBNA is particularly low for peripheral lesions. ::: ::: ::: METHODS ::: To overcome these problems the authors developed an image-guided, electromagnetic navigation system for transbronchial interventions. The system provides real time positioning information for the bronchoscope and a transbronchial biopsy instrument with only one preoperatively acquired computed tomography image. A twofold respiratory motion compensation method based on a particle filtering approach allows for guidance through the entire respiratory cycle. In order to evaluate our system, 18 transbronchial interventions were performed in seven ventilated swine lungs using a thorax phantom. ::: ::: ::: RESULTS ::: All tracked bronchoscope positions were corrected to the inside of the tracheobronchial tree and 80.2% matched the correct bronchus. During regular respiratory motion, the mean overall targeting error for bronchoscope tracking and TBNA needle tracking was with compensation on 10.4 ± 1.7 and 10.8 ± 3.0 mm, compared to 14.4 ± 1.9 and 13.3 ± 2.7 mm with compensation off. The mean fiducial registration error (FRE) was 4.2 ± 1.1 mm. ::: ::: ::: CONCLUSIONS ::: The navigation system with the proposed respiratory motion compensation method allows for real time guidance during bronchoscopic interventions, and thus could increase the diagnostic yield of transbronchial biopsy.
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paper_title: Electromagnetic-guided postpyloric tube placement in children: Pilot study of its use as a rescue therapy

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Summary Background & aims Postpyloric feeding is frequently indicated in clinical practice. Postpyloric enteral tube placement can be cumbersome, necessitating use of fluoroscopy and/or endoscopy. Guiding devices have been developed to facilitate bedside tube placement. We aimed to evaluate the feasibility and safety of postpyloric enteral tube placement in children using an electromagnetic-guided system as a rescue strategy in case blind tube insertion failed. Method In a prospective pilot study in 10 children postpyloric enteral tube placement using an electromagnetic-guided placement was attempted after blind placement failure. Results Postpyloric enteral tube placement was successful in 6 of the 10 included patients. No adverse events occurred and it was well tolerated. Conclusion This pilot study suggests that electromagnetic-guided placement as a rescue technique for postpyloric enteral tube placement can prevent the use of fluoroscopy and/or endoscopic placement in a substantial portion of patients.
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paper_title: Novel Magnetic Technology for Intraoperative Intracranial Frameless Navigation: In Vivo and in Vitro Results

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OBJECTIVE ::: To characterize the accuracy of the Magellan electromagnetic navigation system (Biosense Webster, Tirat HaCarmel, Israel) and to demonstrate the feasibility of its use in image-guided neurosurgical applications. ::: ::: ::: DESCRIPTION OF INSTRUMENTATION ::: The Magellan system was developed to provide real-time tracking of the distal tips of flexible catheters, steerable endoscopes, and other surgical instruments, using ultra-low electromagnetic fields and a novel miniature position sensor for image-correlated intraoperative navigation and mapping applications. ::: ::: ::: METHODS ::: An image registration procedure was performed, and static and qualitative accuracies were assessed in a series of phantom, animal, and human neurosurgical studies. ::: ::: ::: EXPERIENCE AND RESULTS ::: During the human study phase, an accuracy error of up to 5 mm was deemed acceptable. Results demonstrated that this degree of accuracy was maintained throughout all procedures. All anatomic landmarks were reached with precision and were accurately viewed on the display screen. Navigation that relied on the system was also successful. No interference with operating room equipment was noted. The accuracy of the system was maintained during regular surgical procedures, using standard surgical tools. ::: ::: ::: CONCLUSION ::: The system provides precise lesion localization without limiting the line of vision, the mobility of the surgeon, or the flexibility of instruments. Electromagnetic navigation promises new advances in neuronavigation and frameless stereotactic surgery.
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paper_title: Electromagnetic navigation in lung cancer: research update.

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Unfortunately, flexible bronchoscopy, the least invasive bronchoscopic procedure, is of limited value for obtaining tissue from lesions in the peripheral segments of the lung. Biopsy success is further compromised if the lesion is less than 3 cm in diameter. The main limitation of flexible bronchoscopy is the difficulty in reaching peripheral lesions with the accessory tools. In this paper, we will discuss a new bronchoscopic advance in the diagnosis and treatment of lung cancer. Once extended beyond the tip of the bronchoscope, these tools are difficult to guide to the desired location. Localizing the lesion under fluoroscopy is difficult, and alternative diagnostic guidance methods, such as computer tomography-guided bronchoscopy and endobronchial ultrasound, are more demanding. Therefore, new methods for navigation and localization are needed. One of these new technologies is electromagnetic navigation bronchoscopy. The aim of this special report is to provide an analysis of the published literature. A literature search was constructed and performed on PubMed to identify the literature from 2000 to 2008. The search words were 'electromagnetic navigation', 'coin lesion', 'solitary pulmonary nodule' and 'lung cancer'. We review a number of recent studies that utilize electromagnetic navigation and guidance, and analyze their performance characteristics for clinical applications of the technology. Electromagnetic navigation is likely to play an increasing and integral role in the diagnosis and staging of lung cancer in the near future. Electromagnetic registration may impact both the staging and diagnosis of peripheral lesions.
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paper_title: Image-Guided Endoscopic Surgery: Results of Accuracy and Performance in a Multicenter Clinical Study Using an Electromagnetic Tracking System

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Image-guided surgery has recently been described in the literature as a useful technology for improved functional endoscopic sinus surgery localization. Image-guided surgery yields accurate knowledge of the surgical field boundaries, allowing safer and more thorough sinus surgery. We have previously reviewed our initial experience with The InstaTrak System. This article presents a multicenter clinical study (n=55) that assesses the system's capability for localizing structures in critical surgical sites. The purpose of this paper is to present quantitative data on accuracy and performance. We describe several new advances including an automated registration technique that eliminates the redundant computed tomography scan, compensation for head movement, and the ability to use interchangeable instruments.
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paper_title: Preliminary experience with electromagnetic navigation system in TKA.

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Abstract Accuracy of implant positioning and precise reconstruction of leg alignment offers the best way to achieve good long-term results in total knee arthroplasty. Computer instrumentation was developed to improve the final position of the component and restore the mechanical axis. Current navigation systems use either optical or electromagnetic tracking. The advantage of the Electromagnetic (EM) navigation system is that no line-of-sight issues are present. However, special iron-free instruments are required. This report analyzes the postoperative radiological results of 32 knees treated using an EM system. All the measurements were recorded using software able to subtend angles automatically by five physicians, three radiologist and two orthopedic residents not involved with the surgery. Each radiograph was measured three times, in random order, and at delayed intervals. We found an ideal alignment for the mechanical axis (180 ± 3°) in 30 out of 32 cases, whereas all the patients achieved a value of 90° ± 3° for both femoral and tibial frontal component angles. An apparently over-corrected implant position for the sagittal femoral component was reported, with a mean value of 11.2° ± 3.6. The mean position of the tibial component was 90.6° ± 2.8; just four measurements were outside of the ± 3° of the desired value. EM is safe and there were no complications related to this system. An almost perfect correlation was found between the mechanical axis value of the EM navigation system (179.8° ± 1.8) and the median value of the all reviewers (180.3° ± 1.9) with a difference of 0.5°.
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paper_title: Fusion of MRI and sonography image for breast cancer evaluation using real-time virtual sonography with magnetic navigation: first experience.

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OBJECTIVE ::: We recently developed a real-time virtual sonography (RVS) system that enables simultaneous display of both sonography and magnetic resonance imaging (MRI) cutaway images of the same site in real time. The aim of this study was to evaluate the role of RVS in the management of enhancing lesions visualized with MRI. ::: ::: ::: METHODS ::: Between June 2006 and April 2007, 65 patients underwent MRI for staging of known breast cancer at our hospital. All patients were examined using mammography, sonography, MRI and RVS before surgical resection. Results were correlated with histopathologic findings. MRI was obtained on a 1.5 T imager, with the patient in the supine position using a flexible body surface coil. Detection rate was determined for index tumors and incidental enhancing lesions (IELs), with or without RVS. ::: ::: ::: RESULTS ::: Overall sensitivity for detecting index tumors was 85% (55/65) for mammography, 91% (59/65) for sonography, 97% (63/65) for MRI and 98% (64/65) for RVS. Notably, in one instance in which the cancer was not seen on MRI, RVS detected it with the supplementation of sonography. IELs were found in 26% (17/65) of the patients. Of 23 IELs that were detected by MRI, 30% (7/23) of IELs could be identified on repeated sonography alone, but 83% (19/23) of them were identified using the RVS system (P = 0.001). The RVS system was able to correctly project enhanced MRI information onto a body surface, as we checked sonography form images. ::: ::: ::: CONCLUSIONS ::: Our results suggest that the RVS system can identify enhancing breast lesions with excellent accuracy.
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paper_title: Electromagnetic navigation bronchoscopy

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In 2009, lung cancer was estimated to be the second most common form of cancer diagnosed in men, after prostate, and the second, after breast cancer, in women. It is estimated that it caused 159,390 deaths more than breast, colon and prostate cancers combined. While age-adjusted death rates for this cancer have been declining since 2000, they remain high.
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paper_title: Successful placement of postpyloric enteral tubes using electromagnetic guidance in critically ill children*

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OBJECTIVES ::: Initiation of postpyloric feeding is often delayed by difficulties in placement of enteral tubes. We evaluated the effectiveness of bedside postpyloric enteral tube (PET) placement using an electromagnetic (EM)-guided device. We hypothesized that: 1) EM-guided placement of PETs would be successful more often than standard blind placement with a shorter total time to successful placement and 2) the EM-guided technique would have similar overall costs to the standard technique. ::: ::: ::: DESIGN ::: Prospective cohort trial with serial control groups in a pediatric intensive care unit at a tertiary care children's hospital. ::: ::: ::: INTERVENTIONS ::: We collected data on a cohort of consecutive pediatric intensive care unit patients who underwent PET placement by standard blind technique followed by a cohort who underwent EM-guided placement. The primary outcome measure was successful placement determined by abdominal radiography. ::: ::: ::: MEASUREMENTS AND MAIN RESULTS ::: One hundred seven patients were evaluated in the trial: 57 in the standard group and 50 in the EM-guided group. Demographic data, percent intubated, and admission diagnosis were similar in both groups. Forty-one of 50 patients (82%) in the EM-guided group had successful placement compared with 22 of 57 in the standard group (38%) (p < 0.0001). The average time to successful placement was 1.7 vs. 21 hours in the EM-guided group and standard group, respectively (p < 0.0001). Children in the EM-guided group received fewer radiographs (p = 0.007) and were given more prokinetic drugs (p = 0.045). There were no episodes of pneumothorax in either group. After controlling for prokinetic drug use, EM-guided placement was more likely to result in successful placement than the standard blind technique (odds ratio 6.4, 95% confidence interval 2.5-16.3). An annual placement rate of 250 PETs by EM guidance, based on our institution's current utilization rates, is associated with a cost savings of $55.46 per PET placed. ::: ::: ::: CONCLUSION ::: EM guidance is an efficient and cost-effective method of bedside PET placement.
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paper_title: Magnetic versus manual catheter navigation for ablation of free wall accessory pathways in children.

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BACKGROUND ::: Transcatheter ablation of accessory pathway (AP)-mediated tachycardia is routinely performed in children. Little data exist regarding the use of magnetic navigation (MN) and its potential benefits for ablation of AP-mediated tachycardia in this population. ::: ::: ::: METHODS AND RESULTS ::: We performed a retrospective review of prospectively gathered data in children undergoing radiofrequency ablation at our institution since the installation of MN (Stereotaxis Inc, St. Louis, MO) in March 2009. The efficacy and safety between an MN-guided approach and standard manual techniques for mapping and ablation of AP-mediated tachycardia were compared. During the 26-month study period, 145 patients underwent radiofrequency ablation for AP-mediated tachycardia. Seventy-three patients were ablated with MN and 72 with a standard manual approach. There were no significant differences in demographic factors between the 2 groups with a mean cohort age of 13.1±4.0 years. Acute success rates were equivalent with 68 of 73 (93.2%) patients in the MN group being successfully ablated versus 68 of 72 (94.4%) patients in the manual group (P=0.889). During a median follow-up of 21.4 months, there were no recurrences in the MN group and 2 recurrences in the manual group (P=0.388). There were no differences in time to effect, number of lesions delivered, or average ablation power. There was also no difference in total procedure time, but fluoroscopy time was significantly reduced in the MN group at 14.0 (interquartile range, 3.8-23.9) minutes compared with the manual group at 28.1 (interquartile range, 15.3-47.3) minutes (P<0.001). There were no complications in either group. ::: ::: ::: CONCLUSIONS ::: MN is a safe and effective approach to ablate AP-mediated tachycardia in children.
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paper_title: Update on radiation-based therapies for prostate cancer.

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PURPOSE OF REVIEW ::: This overview summarizes recent developments in radiation-based therapy for prostate cancer. ::: ::: ::: RECENT FINDINGS ::: Radiation dose escalation continues to be validated as an effective strategy in prostate cancer. Adjuvant radiation therapy became the standard of care after long-term follow-up of the pivotal Southwest Oncology Group 8794 trial demonstrated an overall survival benefit in patients with pT3 disease or positive margin after prostatectomy. Strategies such as hypofractionation and stereotactic body radiation therapy are becoming more common but have yet to be validated in a large trial. New technologies such as Calypso 4D real-time tumor tracking and volumetric-modulated arc therapy promise to potentially increase cure rates and decrease toxicity due to increased accuracy of radiation delivery. ::: ::: ::: SUMMARY ::: Radiation therapy continues to play a prominent role in the management of prostate cancer. However, new strategies and technologies such as hypofractionation, stereotactic body radiation therapy, volumetric-modulated arc therapy, and Calypso tumor tracking must be prospectively validated.
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paper_title: A review of RFID localization: Applications and techniques

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Indoor localization has been actively researched recently due to security and safety as well as service matters. Previous research and development for indoor localization includes infrared, wireless LAN and ultrasonic. However, these technologies suffer either from the limited accuracy or lacking of the infrastructure. Radio Frequency Identification (RFID) is very attractive because of reasonable system price, and reader reliability. The RFID localization can be categorized into tag and reader localizations. In this paper, major localization techniques for both tag and reader localizations are reviewed to provide the readers state of the art of the indoor localization algorithms. The advantage and disadvantage of each technique for particular applications were also discussed.
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paper_title: A new system to perform continuous target tracking for radiation and surgery using non-ionizing alternating current electromagnetics

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Abstract A new technology based on alternating current (AC) nonionizing electromagnetic fields has been developed to enable precise localization and continuous tracking of mobile soft tissue targets or tumors during radiation therapy and surgery. The technology utilizes miniature, permanently implanted, wireless transponders (Beacon™ Transponders) and a novel array to enable objective localization and continuous tracking of targets in three-dimensional space. The characteristics of this system include use of safe, nonionizing electromagnetic fields; negligible tissue interaction, delivery of inherently objective three-dimensional data, continuous operation during external beam radiation therapy or surgery. Feasibility testing has shown the system was capable of locating and continuously tracking transponders with submillimeter accuracy at tracking rates of up to 10 Hz and was unaffected by operational linear accelerator. Preclinical animal studies have shown the transponders to be suitable for permanent implantation and early human clinical studies have demonstrated the feasibility of inserting transponders into the prostate. This system will enable accurate initial patient setup and provide real-time target tracking demanded by today's highly conformal radiation techniques and has significant potential as a surgical navigation platform.
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paper_title: Study on an experimental AC electromagnetic tracking system

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3D tracking system is one of the key devices to realize the sense of immersion and human computer interaction in a virtual or augmented reality system. This paper presents the design of an experimental AC electromagnetic 3D tracking system that is based on AC magnetic field transmitting and sensing. The proposed system is composed of 3-axis orthogonal magnetic sensor, 3-axis orthogonal magnetic transmitter, 2-axis accelerometers, data acquisition and processing system etc. After obtaining the orientation of the receiver by measuring earth magnetic and gravity field with the DC output of magnetic sensors and accelerometers, the position can be calculated from the received AC magnetic field generated by the magnetic transmitter. The design of the experimental system on the basis of theoretical analysis is presented in detail and the results of the actual experiment prove the feasibility of the proposed system.
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paper_title: In Vivo Validation of a Hybrid Tracking System for Navigation of an Ultrathin Bronchoscope Within Peripheral Airways

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Transbronchial biopsy of peripheral lung nodules is hindered by the inability to access lesions endoluminally due to the large diameter of conventional bronchoscopes. An ultrathin scanning fiber bronchoscope has recently been developed to advance image-guided biopsy several branching generations deeper into the peripheral airways. However, navigating a potentially complex 3-D path to the region of interest presents a challenge to the bronchoscopist. An accompanying guidance system has also been developed to track the bronchoscope through the airways, and display its position and intended path on a virtual display. Intraoperative localization of the bronchoscope was achieved by combining electromagnetic tracking (EMT) and image-based tracking (IBT). An error-state Kalman filter was used to model the disagreement between the two tracking sources. The positional tracking error was reduced from 14.22 and 14.92 mm by independent EMT and IBT, respectively, to 6.74 mm using the hybrid approach. Hybrid tracking of the scope orientation and respiratory motion compensation further improved tracking accuracy and stability, resulting in an average tracking error of 3.33 mm and 10.01°.
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paper_title: Review on Patents about Magnetic Localisation Systems for in vivo Catheterizations

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Abstract: in vivo Catheterizations are usually performed by physicians using X-Ray fluoroscopic guide and contrast-media. The X-Ray exposure both of the patient and of the operators can induce collateral effects. The present review describes the status of the art on recent patents about magnetic position/orientation indicators capable to drive the probe during in-vivo medical diagnostic or interventional procedures. They are based on the magnetic field produced by sources and revealed by sensors. Possible solutions are: the modulated magnetic field produced by a set of coils positioned externally to the patient is measured by sensors installed on the intra-body probe; the magnetic field produced by a thin permanent magnet installed on the intra-body probe is measured by magnetic field sensors positioned outside the patient body. In either cases, position and orientation of the probe are calculated in real time: this allows the elimination of repetitive X-Ray scans used to monitor the probe. The aim of the proposed systems is to drive the catheter inside the patient vascular tree with a reduction of the X-Ray exposure both of the patient and of the personnel involved in the intervention. The present paper intends also to highlight advantages/disadvantages of the presented solutions.
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paper_title: Passive tracking of catheters and guidewires by contrast‐enhanced MR fluoroscopy

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Passive MR tracking of catheters and guidewires is usually done by dynamically imaging a single thick slab, subtracting a baseline image, and combining the result with a previously acquired MR angiogram. In the in vitro and in vivo experiments reported here, it is demonstrated that this approach may be greatly simplified by using a suitable intravascular contrast agent. The proposed method, contrast-enhanced MR fluoroscopy, combines tracking and angiography into a single sequence and allows direct visualization of the magnetically prepared parts of catheters and guidewires with respect to the vasculature at a frame rate of about one image per 1.5 seconds. Contrast-enhanced MR fluoroscopy, although still limited in temporal resolution, thus obviates the need for subtraction and overlay techniques and eliminates the sensitivity of tracking to subject motion between acquisitions. Magn Reson Med 45:17–23, 2001. © 2001 Wiley-Liss, Inc.
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paper_title: Quality assurance for clinical implementation of an electromagnetic tracking system

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The Calypso Medical 4D localization system utilizes alternating current electromagnetics for accurate, real-time tumor tracking. A quality assurance program to clinically implement this system is described here. Testing of the continuous electromagnetic tracking system (Calypso Medical Technologies, Seattle, WA) was performed using an in-house developed four-dimensional stage and a quality assurance fixture containing three radiofrequency transponders at independently measured locations. The following tests were performed to validate the Calypso system: (a) Localization and tracking accuracy, (b) system reproducibility, (c) measurement of the latency of the tracking system, and (d) measurement of transmission through the Calypso table overlay and the electromagnetic array. The translational and rotational localization accuracies were found to be within 0.01 cm and 1.0 degree, respectively. The reproducibility was within 0.1 cm. The average system latency was measured to be within 303 ms. The attenuation by the Calypso overlay was measured to be 1.0% for both 6 and 18 MV photons. The attenuations by the Calypso array were measured to be 2% and 1.5% for 6 and 18 MV photons, respectively. For oblique angles, the transmission was measured to be 3% for 6 MV, while it was 2% for 18 MV photons. A quality assurance process has been developed for the clinical implementation of an electromagnetic tracking system in radiation therapy.
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paper_title: Electromagnetic Servoing—A New Tracking Paradigm

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Electromagnetic (EM) tracking is highly relevant for many computer assisted interventions. This is in particular due to the fact that the scientific community has not yet developed a general solution for tracking of flexible instruments within the human body. Electromagnetic tracking solutions are highly attractive for minimally invasive procedures, since they do not require line of sight. However, a major problem with EM tracking solutions is that they do not provide uniform accuracy throughout the tracking volume and the desired, highest accuracy is often only achieved close to the center of tracking volume. In this paper, we present a solution to the tracking problem, by mounting an EM field generator onto a robot arm. Proposing a new tracking paradigm, we take advantage of the electromagnetic tracking to detect the sensor within a specific sub-volume, with known and optimal accuracy. We then use the more accurate and robust robot positioning for obtaining uniform accuracy throughout the tracking volume. Such an EM servoing methodology guarantees optimal and uniform accuracy, by allowing us to always keep the tracked sensor close to the center of the tracking volume. In this paper, both dynamic accuracy and accuracy distribution within the tracking volume are evaluated using optical tracking as ground truth. In repeated evaluations, the proposed method was able to reduce the overall error from 6.64±7.86 mm to a significantly improved accuracy of 3.83±6.43 mm. In addition, the combined system provides a larger tracking volume, which is only limited by the reach of the robot and not the much smaller tracking volume defined by the magnetic field generator.
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paper_title: A Machine Learning Approach for Deformable Guide-Wire Tracking in Fluoroscopic Sequences

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Deformable guide-wire tracking in fluoroscopic sequences is a challenging task due to the low signal to noise ratio of the images and the apparent complex motion of the object of interest. Common tracking methods are based on data terms that do not differentiate well between medical tools and anatomic background such as ribs and vertebrae. A data term learned directly from fluoroscopic sequences would be more adapted to the image characteristics and could help to improve tracking. In this work, our contribution is to learn the relationship between features extracted from the original image and the tracking error. By randomly deforming a guide-wire model around its ground truth position in one single reference frame, we explore the space spanned by these features. Therefore, a guide-wire motion distribution model is learned to reduce the intrisic dimensionality of this feature space. Random deformations and the corresponding features can be then automatically generated. In a regression approach, the function mapping this space to the tracking error is learned. The resulting data term is integrated into a tracking framework based on a second-order MAP-MRF formulation which is optimized by QPBO moves yielding high-quality tracking results. Experiments conducted on two fluoroscopic sequences show that our approach is a promising alternative for deformable tracking of guide-wires.
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paper_title: New spatial localizer based on fiber optics with applications in 3D ultrasound imaging

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Spatial localizers provide a reference coordinate system and make the tracking of various objects in 3D space feasible. A number of different spatial localizers are currently available. Several factors that determine the suitability of a position sensor for a specific clinical application are accuracy, ease of use, and robustness of performance when used in a clinical environment. In this paper, we present a new and low-cost sensor with performance unaffected by a the materials present in the operating environment. This new spatial localizer consists of a flexible tape with a number of fiber optic sensor along its length. The main idea is that we can obtain the position and orientation of the end of the tape with respect to its base. The end and base of the tape are locations along its length determined by the physical location of the fiber optic sensors. Using this tape, we tracked an ultrasound probe and formed 3D US data sets. In order to validate the geometric accuracy of those 3D data sets, we measured known volumes of water-filled balloons. Our results indicate that we can measure volumes with accuracy between 2-16 percent. Given the fact that the sensor is under further development and refinement, we expect that this sensor could be an accurate, cost-effective and robust alternative in many medical applications, e.g., image-guided surgery and 3D ultrasound imaging.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
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paper_title: Hybrid bronchoscope tracking using a magnetic tracking sensor and image registration

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In this paper, we propose a hybrid method for tracking a bronchoscope that uses a combination of magnetic sensor tracking and image registration. The position of a magnetic sensor placed in the working channel of the bronchoscope is provided by a magnetic tracking system. Because of respiratory motion, the magnetic sensor provides only the approximate position and orientation of the bronchoscope in the coordinate system of a CT image acquired before the examination. The sensor position and orientation is used as the starting point for an intensity-based registration between real bronchoscopic video images and virtual bronchoscopic images generated from the CT image. The output transformation of the image registration process is the position and orientation of the bronchoscope in the CT image. We tested the proposed method using a bronchial phantom model. Virtual breathing motion was generated to simulate respiratory motion. The proposed hybrid method successfully tracked the bronchoscope at a rate of approximately 1 Hz.
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paper_title: Standardized assessment of new electromagnetic field generators in an interventional radiology setting.

paper_content:

PURPOSE ::: Two of the main challenges associated with electromagnetic (EM) tracking in computer-assisted interventions (CAIs) are (1) the compensation of systematic distance errors arising from the influence of metal near the field generator (FG) or the tracked sensor and (2) the optimized setup of the FG to maximize tracking accuracy in the area of interest. Recently, two new FGs addressing these issues were proposed for the well-established Aurora(®) tracking system [Northern Digital, Inc. (NDI), Waterloo, Canada]: the Tabletop 50-70 FG, a planar transmitter with a built-in shield that compensates for metal distortions emanating from treatment tables, and the prototypical Compact FG 7-10, a mobile generator designed to be attached to mobile imaging devices. The purpose of this paper was to assess the accuracy and precision of these new FGs in an interventional radiology setting. ::: ::: ::: METHODS ::: A standardized assessment protocol, which uses a precisely machined base plate to measure relative error in position and orientation, was applied to the two new FGs as well as to the well-established standard Aurora(®) Planar FG. The experiments were performed in two different settings: a reference laboratory environment and a computed tomography (CT) scanning room. In each setting, the protocol was applied to three different poses of the measurement plate within the tracking volume of the three FGs. ::: ::: ::: RESULTS ::: The two new FGs provided higher precision and accuracy within their respective measurement volumes as well as higher robustness with respect to the CT scanner compared to the established FG. Considering all possible 5 cm distances on the grid, the error of the Planar FG was increased by a factor of 5.94 in the clinical environment (4.4 mm) in comparison to the error in the laboratory environment (0.8 mm). In contrast, the mean values for the two new FGs were all below 1 mm with an increase in the error by factors of only 2.94 (Reference: 0.3 mm; CT: 0.9 mm) and 1.04 (both: 0.5 mm) in the case of the Tabletop FG and the Compact FG, respectively. ::: ::: ::: CONCLUSIONS ::: Due to their high accuracy and robustness, the Tabletop FG and the Compact FG could eliminate the need for compensation of EM field distortions in certain CT-guided interventions.
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paper_title: Upgrade of an optical navigation system with a permanent electromagnetic position control: a first step towards "navigated control" for liver surgery.

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INTRODUCTION ::: The main problems of navigation in liver surgery are organ movement and deformation. With a combination of direct optical and indirect electromagnetic tracking technology, visualisation and positional control of surgical instruments within three-dimensional ultrasound data and registration of organ movements can be realised simultaneously. ::: ::: ::: METHODS ::: Surgical instruments for liver resection were localised with an infrared-based navigation system (Polaris). Movements of the organ itself were registered using an electromagnetic navigation system (Aurora). The combination of these two navigation techniques and a new surgical navigation procedure focussed on a circumscribed critical dissection area were applied for the first time in liver resections. ::: ::: ::: RESULTS ::: This new technique was effectively implemented. The position of the surgical instrument was localised continuously. Repeated position control with observation of the navigation screen was not necessary. During surgical resection, a sonic warning signal was activated when the surgical instrument entered a "no touch" area--an area of reduced safety margin. ::: ::: ::: CONCLUSION ::: Optical tracking of surgical instruments and simultaneous electromagnetic registration of organ position is feasible in liver resection.
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paper_title: Image-guided interventions : technology and applications

paper_content:

Overview and History of Image-Guided Interventions.- Tracking Devices.- Visualization in Image-Guided Interventions.- Augmented Reality.- Software.- Rigid Registration.- Nonrigid Registration.- Model-Based Image Segmentation for Image-Guided Interventions.- Imaging Modalities.- MRI-Guided FUS and its Clinical Applications.- Neurosurgical Applications.- Computer-Assisted Orthopedic Surgery.- Thoracoabdominal Interventions.- Real-Time Interactive MRI for Guiding Cardiovascular Surgical Interventions.- Three-Dimensional Ultrasound Guidance and Robot Assistance for Prostate Brachytherapy.- Radiosurgery.- Radiation Oncology.- Assessment of Image-Guided Interventions.
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paper_title: Robust Intraoperative US Probe Tracking Using a Monocular Endoscopic Camera

paper_content:

In the context of minimally-invasive procedures involving both endoscopic video and ultrasound, we present a vision-based method to track the ultrasound probe using a standard monocular video laparoscopic instrument. This approach requires only cosmetic modification to the ultrasound probe and obviates the need for magnetic tracking of either instrument. We describe an Extended Kalman Filter framework that solves for both the feature correspondence and pose estimation, and is able to track a 3D pattern on the surface of the ultrasound probe in near real-time. The tracking capability is demonstrated by performing an ultrasound calibration of a visually-tracked ultrasound probe, using a standard endoscopic video camera. Ultrasound calibration resulted in a mean TRE of 2.3mm, and comparison with an external optical tracker demonstrated a mean FRE of 4.4mm between the two tracking systems.
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paper_title: A non-disruptive technology for robust 3d tool tracking for ultrasound-guided interventions

paper_content:

In the past decade ultrasound (US) has become the preferred modality for a number of interventional procedures, offering excellent soft tissue visualization. The main limitation however is limited visualization of surgical tools. A new method is proposed for robust 3D tracking and US image enhancement of surgical tools under US guidance. Small US sensors are mounted on existing surgical tools. As the imager emits acoustic energy, the electrical signal from the sensor is analyzed to reconstruct its 3D coordinates. These coordinates can then be used for 3D surgical navigation, similar to current day tracking systems. A system with realtime 3D tool tracking and image enhancement was implemented on a commercial ultrasound scanner and 3D probe. Extensive water tank experiments with a tracked 0.2mm sensor show robust performance in a wide range of imaging conditions and tool position/orientations. The 3D tracking accuracy was 0.36 ± 0.16mm throughout the imaging volume of 55° × 27° × 150mm. Additionally, the tool was successfully tracked inside a beating heart phantom. This paper proposes an image enhancement and tool tracking technology with sub-mm accuracy for US-guided interventions. The technology is non-disruptive, both in terms of existing clinical workflow and commercial considerations, showing promise for large scale clinical impact.
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Title: Electromagnetic Tracking in Medicine—A Review of Technology, Validation, and Applications

Section 1: Introduction
Description 1: Introduce the concept of electromagnetic (EM) tracking and its importance in modern patient care, especially in computer-assisted interventions (CAI).

Section 2: Methods of Literature Research
Description 2: Describe the methodology employed to systematically search and categorize relevant literature on EM tracking in medicine.

Section 3: Fundamentals
Description 3: Provide an overview of the physical principles, magnetic sensors, tracking techniques, field generators, sources of error, and commercial systems relevant to EM tracking.

Section 4: System Assessment
Description 4: Discuss various assessment protocols, study results, and the discussion around them to evaluate the accuracy and robustness of different EM tracking systems.

Section 5: Distortion Compensation
Description 5: Explain different approaches to optimize EM tracking systems by passive protection and active compensation to improve accuracy and robustness in clinical settings.

Section 6: Clinical Applications
Description 6: Highlight diverse medical applications of EM tracking, discuss commercial systems available, and review clinical evidence supporting their effectiveness.

Section 7: Summary and Outlook
Description 7: Summarize the findings from the literature review and offer outlooks on future developments and potential advancements in the field of EM tracking in medicine.

Section 8: Conclusion
Description 8: Conclude the paper by summarizing the usefulness of EM tracking in specific medical applications and emphasizing the importance of evaluation and risk analysis in clinical practice.