Freehand 3D Ultrasound Calibration: A Review
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paper_title: Ultrasound-based navigation system incorporating preoperative planning for liver surgery

paper_content:

Abstract An overview of the current status of the development of our navigation system for resection of liver tumours is given. The system is based on intraoperative three-dimensional (3D) ultrasound volumes for image-guided navigation of surgical instruments. It is also possible to perform a virtual 3D planning of the tumour resection and to non-rigidly register these preoperative models to the intraoperative ultrasound volume.
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paper_title: Registration of freehand 3D ultrasound and magnetic resonance liver images

paper_content:

Abstract We present a method to register a preoperative MR volume to a sparse set of intraoperative ultrasound slices. Our aim is to allow the transfer of information from preoperative modalities to intraoperative ultrasound images to aid needle placement during thermal ablation of liver metastases. The spatial relationship between ultrasound slices is obtained by tracking the probe using a Polaris optical tracking system. Images are acquired at maximum exhalation and we assume the validity of the rigid body transformation. An initial registration is carried out by picking a single corresponding point in both modalities. Our strategy is to interpret both sets of images in an automated pre-processing step to produce evidence or probabilities of corresponding structure as a pixel or voxel map. The registration algorithm converts the intensity values of the MR and ultrasound images into vessel probability values. The registration is then carried out between the vessel probability images. Results are compared to a “bronze standard” registration which is calculated using a manual point/line picking algorithm and verified using visual inspection. Results show that our starting estimate is within a root mean square target registration error (calculated over the whole liver) of 15.4 mm to the “bronze standard” and this is improved to 3.6 mm after running the intensity-based algorithm.
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paper_title: Three-dimensional ultrasound imaging.

paper_content:

The objective of this article is to provide scientists, engineers and clinicians with an up-to-date overview on the current state of development in the area of three-dimensional ultrasound (3-DUS) and to serve as a reference for individuals who wish to learn more about 3-DUS imaging. The sections will review the state of the art with respect to 3-DUS imaging, methods of data acquisition, analysis and display approaches. Clinical sections summarize patient research study results to date with discussion of applications by organ system. The basic algorithms and approaches to visualization of 3-D and 4-D ultrasound data are reviewed, including issues related to interactivity and user interfaces. The implications of recent developments for future ultrasound imaging/visualization systems are considered. Ultimately, an improved understanding of ultrasound data offered by 3-DUS may make it easier for primary care physicians to understand complex patient anatomy. Tertiary care physicians specializing in ultrasound can further enhance the quality of patient care by using high-speed networks to review volume ultrasound data at specialization centers. Access to volume data and expertise at specialization centers affords more sophisticated analysis and review, further augmenting patient diagnosis and treatment.
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paper_title: Three-Dimensional Ultrasound in Gynecology: Current Status and Future Perspectives

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A two piece nozzle insert for use in a steelmaking ladle comprising a nozzle segment and a lock ring segment. The nozzle segment includes a prefired refractory block suited for insertion into the discharge bore of a ladle well block, and the lock ring segment includes a fastening means for removeably attaching the lock ring segment within the discharge bore of a ladle mounting plate. The nozzle segment and lock ring segment include cooperating tapered surfaces to provide a slip plane for compressing and extruding bonding material from between mortar joints of the two piece nozzle insert.
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paper_title: 3D ultrasound guided breast biopsy system

paper_content:

Abstract Stereotactic X-ray mammography (SM) and ultrasound (US) guidance are commonly used techniques for breast biopsy. While SM provides 3D targeting information and US provides real-time guidance, both techniques have limitations. SM is a long and uncomfortable procedure and the US guided procedure is inherently 2D, requiring a skilled physician for both safety and accuracy. We have developed a 3D US-guided biopsy system to integrate with SM. The dual modality breast biopsy system combines the advantages of both approaches with 3D US and SM targeting, near real-time 3D and real-time 2D US guidance, breast stabilisation and a confined needle trajectory. Our goal is to be able to biopsy a larger percentage of suspicious masses using ultrasound, by clarifying ambiguous structures with mammographic imaging. Using breast phantoms, we have shown that our ultrasound guided biopsy system was capable of targeting artificial lesions that were 3.2 mm in diameter, with a 96% success rate. Through this study, we also demonstrated that our system was equivalent to current clinical practice, for an in vitro biopsy task. Metal beads in known relative positions allowed us to determine the geometry of the SM system, so that stereotactic mammography could be registered to 3D US images. The target registration error was found to be 1.6 mm. This error was dominated by positioning error in the vertical direction (perpendicular to the film surface). As an adjunct to SM, we propose that 3D US could provide more complete imaging information for target identification and real-time monitoring of needle insertion, as well as providing a means for rapid confirmation of biopsy success.
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paper_title: 3D and 4D ultrasound in fetal cardiac scanning: a new look at the fetal heart

paper_content:

Over the last decade we have been witness to a burgeoning literature on three-dimensional (3D) and four-dimensional (4D) ultrasound-based studies of the fetal cardiovascular system. Recent advances in the technology of 3D/4D ultrasound systems allow almost real-time 3D/4D fetal heart scans. It appears that 3D/4D ultrasound in fetal echocardiography may make a significant contribution to interdisciplinary management team consultation, health delivery systems, parental counseling, and professional training. Our aim is to review the state of the art in 3D/4D fetal echocardiography through the literature and index cases of normal and anomalous fetal hearts. Copyright  2007 ISUOG. Published by John Wiley & Sons, Ltd.
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paper_title: Ultrasound-guided extracranial radiosurgery: technique and application.

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PURPOSE ::: Stereotactic radiosurgery is an effective treatment modality for many intracranial lesions, but target mobility limits its utility for extracranial applications. We have developed a new technique for extracranial radiosurgery based on optically guided three-dimensional ultrasound (3DUS). The 3DUS system provides the ability to image the target volume and critical structures in real time and determine any misregistration of the target volume with the linear accelerator. In this paper, we describe the system and its initial clinical application in the treatment of localized metastatic disease. ::: ::: ::: METHODS AND MATERIALS ::: The extracranial stereotactic system consists of an ultrasound unit that is optically tracked and registered with the linear accelerator coordinate system. After an initial patient positioning based on computed tomographic (CT) simulation, stereotactic ultrasound images are acquired and correlated with the CT-based treatment plan to determine any soft-tissue shifts between the time of the planning CT and the actual treatment. Optical tracking is used to correct any patient offsets that are revealed by the real-time imaging. ::: ::: ::: RESULTS ::: Preclinical testing revealed that the ultrasound-based stereotactic navigation system is accurate to within 1.5 mm in comparison with an absolute coordinate phantom. Between March 2001 and March 2002, the system was used to deliver extracranial radiosurgery to 17 metastatic lesions in 16 patients. Treatments were delivered in 1 or 2 fractions, with an average fractional dose of 16 Gy (range 12.5-24 Gy) delivered to the 80% isodose surface. Before each fraction, the target misalignment from isocenter was determined using the 3DUS system and the misalignments averaged over all patients were anteroposterior = 4.8 mm, lateral = 3.6 mm, axial = 2.1 mm, and average total 3D displacement = 7.4 mm (range = 0-21.0 mm). After correcting patient misalignment, each plan was delivered as planned using 6-11 noncoplanar fields. No acute complications were reported. ::: ::: ::: CONCLUSIONS ::: A system for high-precision radiosurgical treatment of metastatic tumors has been developed, tested, and applied clinically. Optical tracking of the ultrasound probe provides real-time tracking of the patient anatomy and allows computation of the target displacement before treatment delivery. The patient treatments reported here suggest the feasibility and safety of the technique.
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paper_title: 3D ultrasound imaging of the carotid arteries.

paper_content:

Although ultrasonography is an important cost-effective imaging modality, technical improvements are needed before its full potential is realized for accurate and reproducible monitoring of carotid disease and plaque burden. 2D viewing of 3D anatomy, using conventional ultrasonography limits our ability to quantify and visualize carotid disease and is partly responsible for the reported variability in diagnosis and monitoring of disease progression. Efforts of investigators have focused on overcoming these deficiencies by developing 3D ultrasound imaging techniques that are capable of acquiring B-mode, color Doppler and power Doppler images of the carotid arteries using existing conventional ultrasound systems, reconstructing the information into 3D images, and then allowing interactive viewing of the 3D images on inexpensive desktop computers. In addition, the availability of 3D ultrasound images of the carotid arteries has allowed the development of techniques to quantify plaque volume and surface morphology as well as allowing registration with other 3D imaging modalities. This paper describes 3D ultrasound imaging techniques used to image the carotid arteries and summarizes some of the developments aimed at quantifying plaque volume and morphology.
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paper_title: Three-dimensional ultrasound imaging

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Ultrasound is an inexpensive and widely used imaging modality for the diagnosis and staging of a number of diseases; nevertheless, technical improvements are needed before its full potential is realized. We believe that 2-D viewing of the 3-D anatomy, using conventional ultrasound procedures, limits our ability to quantify, diagnose and stage a number of diseases because: conventional ultrasound images are 2-D, multiple images must be integrated in the diagnostician's mind to develop a 3-D impression of the anatomy leading to a time-consuming process with increased operator variability; the patient's anatomy or orientation sometimes restricts the image angle, resulting in the optimal image plane necessary for diagnosis being unavailable; and, it is difficult to localize the conventional 2-D image plane and reproduce it at a later time, making it suboptimal for monitoring of therapy. Our efforts have focused on overcoming these deficiencies by developing 3-D ultrasound imaging techniques that are capable of acquiring B-mode, color Doppler and power Doppler images from existing ultrasound instruments, reconstructing the information in 3-D, and then allowing interactive viewing of 3-D ultrasound images on inexpensive desktop computers.
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paper_title: Beam calibration without a phantom for creating a 3-D freehand ultrasound system

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Abstract To create a freehand three-dimensional (3-D) ultrasound (US) system for image-guided surgical procedures, an US beam calibration process must be performed. The calibration method presented in this work does not use a phantom to define in 3-D space the pixel locations in the beam. Rather, the described method is based on the spatial relationship between an optically tracked pointer and a similarly tracked US transducer. The pointer tip was placed into the US beam, and US images, physical coordinates of the pointer and the transducer location were simultaneously recorded. US image coordinates of the pointer were mapped to the physical points using two different registration methods. Two sensitivity studies were performed to determine the location and number of points needed to calibrate the beam accurately. Results showed that the beam is most efficiently calibrated with approximately 20 points collected from throughout the beam. This method of beam calibration proved to be highly accurate, yielding registration errors of approximately 0.4 mm. (E-mail: diane.muratore@vanderbilt.edu)
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paper_title: Performance of a miniature magnetic position sensor for three-dimensional ultrasound imaging

paper_content:

A miniature magnetic position sensor used for three-dimensional ultrasound imaging was tested for precision and accuracy in vitro. The sensor alone was able to locate points with root-mean-square (rms) uncertainty of 1.7 mm and accuracy of 0.05 ± 0.62 mm over its specified operating range of 50 cm. With an ultrasound imaging system, a point was located from arbitrary viewing windows with 2.4-mm rms uncertainty and 0.06 ± 0.68 mm accuracy. If viewing windows were limited to those representative of a typical ultrasound examination, the system could achieve rms uncertainty in point location of < 1 mm. Performance was not affected by operation of the imaging system when the sensor was mounted on an ultrasound scanhead. Sensitivity to metals in the operating environment was also measured.
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paper_title: Accurate fiducial location for freehand 3D ultrasound calibration

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The most intuitive way to perform freehand 3D ultrasound calibration is to locate fiducials in the scan plane directly with a pointer. The main problem of using this approach is the difficulty of aligning the tip of the pointer with the scan plane. The thick beam width makes the tip of the pointer visible in the B-scan even if the tip is not exactly in the elevational centre of the scan plane. Furthermore, reliable automatic segmentation of isolated points in ultrasonic imaging systems is difficult. As a result, it is usual to rely on manual segmentation, making calibration a lengthy process. In this paper, we present a novel phantom that solves the alignment problem and simplifies the segmentation process, while maintaining the other advantages of using a pointer.
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paper_title: Self-calibrating 3D-ultrasound-based bone registration for minimally invasive orthopedic surgery

paper_content:

Intraoperative freehand three-dimensional (3-D) ultrasound (3D-US) has been proposed as a noninvasive method for registering bones to a preoperative computed tomography image or computer-generated bone model during computer-aided orthopedic surgery (CAOS). In this technique, an US probe is tracked by a 3-D position sensor and acts as a percutaneous device for localizing the bone surface. However, variations in the acoustic properties of soft tissue, such as the average speed of sound, can introduce significant errors in the bone depth estimated from US images, which limits registration accuracy. We describe a new self-calibrating approach to US-based bone registration that addresses this problem, and demonstrate its application within a standard registration scheme. Using realistic US image data acquired from 6 femurs and 3 pelves of intact human cadavers, and accurate Gold Standard registration transformations calculated using bone-implanted fiducial markers, we show that self-calibrating registration is significantly more accurate than a standard method, yielding an average root mean squared target registration error of 1.6 mm. We conclude that self-calibrating registration results in significant improvements in registration accuracy for CAOS applications over conventional approaches where calibration parameters of the 3D-US system remain fixed to values determined using a preoperative phantom-based calibration.
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paper_title: Frameless stereotactic ultrasonography: method and applications.

paper_content:

In stereotactic neurosurgery, computed tomography (CT) and magnetic resonance (MR) images are registered in a coordinate system defined with respect to the skull. By intraoperatively tracking the coordinate position of a surgical instrument, various displays can be formed which show the position of the instrument in the MR and/or CT images. However, the accuracy of this display varies because intracranial structures may shift or warp from their position prior to surgery. Ultrasonic imaging systems provide real-time images of the brain, but structures in these images are difficult to interpret because the images are based on ultrasonic echoes. A method has been developed for the real-time registration of these images. With this registration, software continuously updates a corresponding image constructed from the set of MR and/or CT images used for guidance. By developing this second view of the structures in the ultrasound image, the surgeon can easily interpret the ultrasound image, and it becomes possible to determine the extent of the intra-operative structure shift between the two images.
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paper_title: Automatic calibration of a robotized 3D ultrasound imaging system by visual servoing

paper_content:

Three-dimensional free-hand ultrasound imaging consists of capturing a set of ultrasound images with a 2D ultrasound system and their respective locations in order to position them in a 3D reference frame. Usually the clinician performs the acquisition manually through the use of an optical or magnetic localization system attached to the ultrasound probe. To assist the clinician, we propose to use a robotic system to automatically move the ultrasound probe and measure its position. As for manual 3D ultrasound imaging, it is crucial to know precisely the spatial calibration parameters of the ultrasound system in order to perform accurate 3D imaging. Therefore, we propose to automate the spatial calibration procedure. A robotic task is developed to automatically position the ultrasound image on the intersection point of a cross-wire phantom used for spatial calibration. To perform this task, a new visual servoing technique based on 2D ultrasound images is used to control automatically the motion of the ultrasound probe held by a medical robot
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paper_title: 3D ultrasonic image feature localization based on magnetic scanhead tracking: in vitro calibration and validation.

paper_content:

The basis of a three-dimensional (3D) ultrasound imaging system was constructed from a commercially available magnetometer-based position and orientation measurement (POM) device, a standard B-Mode ultrasound instrument and a personal computer. To evaluate the system's performance, a novel method was devised using an iterative, least-squares technique to simultaneously determine the system's calibration parameters and measure its precision in locating points in three-dimensional space. When tested separately, the POM system located single points with a root mean squared (RMS) uncertainty of from 1.4 mm to 3.2 mm over the 610 mm working radius of the system. When combined with the ultrasound instrument, the RMS uncertainty in locating point targets varied from 2.1 mm to 3.5 mm. These results establish the lower limits of variability to be expected from this system when locating fiducial anatomical landmarks for repeated examinations of the same region of the body, and when making numerical 3D reconstructions from multiple planar images.
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paper_title: Self-calibrating 3D-ultrasound-based bone registration for minimally invasive orthopedic surgery

paper_content:

Intraoperative freehand three-dimensional (3-D) ultrasound (3D-US) has been proposed as a noninvasive method for registering bones to a preoperative computed tomography image or computer-generated bone model during computer-aided orthopedic surgery (CAOS). In this technique, an US probe is tracked by a 3-D position sensor and acts as a percutaneous device for localizing the bone surface. However, variations in the acoustic properties of soft tissue, such as the average speed of sound, can introduce significant errors in the bone depth estimated from US images, which limits registration accuracy. We describe a new self-calibrating approach to US-based bone registration that addresses this problem, and demonstrate its application within a standard registration scheme. Using realistic US image data acquired from 6 femurs and 3 pelves of intact human cadavers, and accurate Gold Standard registration transformations calculated using bone-implanted fiducial markers, we show that self-calibrating registration is significantly more accurate than a standard method, yielding an average root mean squared target registration error of 1.6 mm. We conclude that self-calibrating registration results in significant improvements in registration accuracy for CAOS applications over conventional approaches where calibration parameters of the 3D-US system remain fixed to values determined using a preoperative phantom-based calibration.
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paper_title: Frameless stereotactic ultrasonography: method and applications.

paper_content:

In stereotactic neurosurgery, computed tomography (CT) and magnetic resonance (MR) images are registered in a coordinate system defined with respect to the skull. By intraoperatively tracking the coordinate position of a surgical instrument, various displays can be formed which show the position of the instrument in the MR and/or CT images. However, the accuracy of this display varies because intracranial structures may shift or warp from their position prior to surgery. Ultrasonic imaging systems provide real-time images of the brain, but structures in these images are difficult to interpret because the images are based on ultrasonic echoes. A method has been developed for the real-time registration of these images. With this registration, software continuously updates a corresponding image constructed from the set of MR and/or CT images used for guidance. By developing this second view of the structures in the ultrasound image, the surgeon can easily interpret the ultrasound image, and it becomes possible to determine the extent of the intra-operative structure shift between the two images.
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paper_title: Development of a portable 3D ultrasound imaging system for musculoskeletal tissues.

paper_content:

3D ultrasound is a promising imaging modality for clinical diagnosis and treatment monitoring. Its cost is relatively low in comparison with CT and MRI, no intensive training and radiation protection is required for its operation, and its hardware is movable and can potentially be portable. In this study, we developed a portable freehand 3D ultrasound imaging system for the assessment of musculoskeletal body parts. A portable ultrasound scanner was used to obtain real-time B-mode ultrasound images of musculoskeletal tissues and an electromagnetic spatial sensor was fixed on the ultrasound probe to acquire the position and orientation of the images. The images were digitized with a video digitization device and displayed with its orientation and position synchronized in real-time with the data obtained by the spatial sensor. A program was developed for volume reconstruction, visualization, segmentation and measurement using Visual C++ and Visualization toolkits (VTK) software. A 2D Gaussian filter and a Median filter were implemented to improve the quality of the B-scan images collected by the portable ultrasound scanner. An improved distance-weighted grid-mapping algorithm was proposed for volume reconstruction. Temporal calibrations were conducted to correct the delay between the collections of images and spatial data. Spatial calibrations were performed using a cross-wire phantom. The system accuracy was validated by one cylinder and two cuboid phantoms made of silicone. The average errors for distance measurement in three orthogonal directions in comparison with micrometer measurement were 0.06+/-0.39, -0.27+/-0.27, and 0.33+/-0.39 mm, respectively. The average error for volume measurement was -0.18%+/-5.44% for the three phantoms. The system has been successfully used to obtain the volume images of a fetus phantom, the fingers and forearms of human subjects. For a typical volume with 126 x 103 x 109 voxels, the 3D image could be reconstructed from 258 B-scans (640 x 480 pixels) within one minute using a portable PC with Pentium IV 2.4 GHz CPU and 512 MB memories. It is believed that such a portable volume imaging system will have many applications in the assessment of musculoskeletal tissues because of its easy accessibility.
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paper_title: System for quantitative three-dimensional echocardiography of the left ventricle based on a magnetic-field position and orientation sensing system

paper_content:

Accurate measurement of left-ventricular (LV) volume and function are important to monitor disease progression and assess prognosis in patients with heart disease. Existing methods of three-dimensional (3-D) imaging of the heart using ultrasound have shown the potential of this modality, but each suffers from inherent restrictions which limit its applicability to the full range of clinical situations. The authors have developed a technique for image acquisition using a magnetic-field system to track the 3-D echocardiographic imaging planes and 3-D image analysis software including the piecewise smooth subdivision method for surface reconstruction. The technique offers several advantages over existing methods of 3-D echocardiography. The results of validation using in vitro LV's show that the technique allows accurate measurement of LV volume and anatomically accurate 3-D reconstruction of LV shape and is, therefore, suitable for analysis of regional as well as global function.
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paper_title: Interactive 3D registration of ultrasound and magnetic resonance images based on a magnetic position sensor

paper_content:

The use of stereotactic systems has been one of the main approaches for image-based guidance of the surgical tool within the brain. The main limitation of stereotactic systems is that they are based on preoperative images that might become outdated and invalid during the course of surgery. Ultrasound (US) is considered the most practical and cost-effective intraoperative imaging modality, but US images inherently have a low signal-to-noise ratio. Integrating intraoperative US with stereotactic systems has recently been attempted. In this paper, we present a new system for interactively registering two-dimensional US and three-dimensional magnetic resonance (MR) images. This registration is based on tracking the US probe with a DC magnetic position sensor. We have performed an extensive analysis of the errors of our system by using a custom-built phantom. The registration error between the MR and the position sensor space was found to have a mean value of 1.78 mm and a standard deviation of 0.18 mm. The registration error between US and MR space was dependent on the distance of the target point from the US probe face. For a 3.5-MHz phased one-dimensional array transducer and a depth of 6 cm, the mean value of the registration error was 2.00 mm and the standard deviation was 0.75 mm. The registered MR images were reconstructed using either zeroth-order or first-order interpolation.
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paper_title: Automatic calibration of a robotized 3D ultrasound imaging system by visual servoing

paper_content:

Three-dimensional free-hand ultrasound imaging consists of capturing a set of ultrasound images with a 2D ultrasound system and their respective locations in order to position them in a 3D reference frame. Usually the clinician performs the acquisition manually through the use of an optical or magnetic localization system attached to the ultrasound probe. To assist the clinician, we propose to use a robotic system to automatically move the ultrasound probe and measure its position. As for manual 3D ultrasound imaging, it is crucial to know precisely the spatial calibration parameters of the ultrasound system in order to perform accurate 3D imaging. Therefore, we propose to automate the spatial calibration procedure. A robotic task is developed to automatically position the ultrasound image on the intersection point of a cross-wire phantom used for spatial calibration. To perform this task, a new visual servoing technique based on 2D ultrasound images is used to control automatically the motion of the ultrasound probe held by a medical robot
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paper_title: Performance of a miniature magnetic position sensor for three-dimensional ultrasound imaging

paper_content:

A miniature magnetic position sensor used for three-dimensional ultrasound imaging was tested for precision and accuracy in vitro. The sensor alone was able to locate points with root-mean-square (rms) uncertainty of 1.7 mm and accuracy of 0.05 ± 0.62 mm over its specified operating range of 50 cm. With an ultrasound imaging system, a point was located from arbitrary viewing windows with 2.4-mm rms uncertainty and 0.06 ± 0.68 mm accuracy. If viewing windows were limited to those representative of a typical ultrasound examination, the system could achieve rms uncertainty in point location of < 1 mm. Performance was not affected by operation of the imaging system when the sensor was mounted on an ultrasound scanhead. Sensitivity to metals in the operating environment was also measured.
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paper_title: The Levenberg-Marquardt algorithm: Implementation and theory

paper_content:

A hammer is provided with a housing accommodating a barrel which is axially movable relative to the housing. The barrel has two portions of different diameter. One portion adjacent to the handle is of a smaller diameter, and the other portion is of a greater diameter. These portions form therebetween a shoulder having the end wall facing the handle. An annular space is provided between the outer surface of the barrel and the inner surface of the housing, and mounted in this annular space on both the first and second portions of the barrel are sealing bushings which are in sealing engagement with the housing so that annular spaces are provided between the outer periphery of the bushings and the inner surface of the housing, and a spring member is mounted between the bushings. The invention is aimed at lowering the vibration level at the hammer handle and at reducing the compressed air leakage.
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paper_title: 3D ultrasonic image feature localization based on magnetic scanhead tracking: in vitro calibration and validation.

paper_content:

The basis of a three-dimensional (3D) ultrasound imaging system was constructed from a commercially available magnetometer-based position and orientation measurement (POM) device, a standard B-Mode ultrasound instrument and a personal computer. To evaluate the system's performance, a novel method was devised using an iterative, least-squares technique to simultaneously determine the system's calibration parameters and measure its precision in locating points in three-dimensional space. When tested separately, the POM system located single points with a root mean squared (RMS) uncertainty of from 1.4 mm to 3.2 mm over the 610 mm working radius of the system. When combined with the ultrasound instrument, the RMS uncertainty in locating point targets varied from 2.1 mm to 3.5 mm. These results establish the lower limits of variability to be expected from this system when locating fiducial anatomical landmarks for repeated examinations of the same region of the body, and when making numerical 3D reconstructions from multiple planar images.
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paper_title: Three-dimensional freehand ultrasound : Image reconstruction and volume analysis

paper_content:

A system is described that rapidly produces a regular 3-dimensional (3-D) data block suitable for processing by conventional image analysis and volume measurement software. The system uses electromagnetic spatial location of 2-dimensional (2-D) freehand-scanned ultrasound B-mode images, custom-built signal-condi- tioning hardware, UNIX-based computer processing and an efficient 3-D reconstruction algorithm. Utilisation of images from multiple angles of insonation, "compounding," reduces speckle contrast, improves structure coherence within the reconstructed grey-scale image and enhances the ability to detect structure boundaries and to segment and quantify features. Volume measurements using a series of water-filled latex and cylindrical foam rubber phantoms with volumes down to 0.7 mL show that a high degree of accuracy, precision and reproduc- ibility can be obtained. Extension of the technique to handle in vivo data sets by allowing physiological criteria to be taken into account in selecting the images used for reconstruction is also illustrated. © 1997 World Federation for Ultrasound in Medicine & Biology.
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paper_title: Reconstruction and visualization of irregularly sampled three- and four-dimensional ultrasound data for cerebrovascular applications

paper_content:

Abstract Although recent studies have demonstrated the potential value of compounded data for improvement in signal-to–noise ratio and speckle contrast for three-dimensional (3-D) ultrasonography, clinical applications are lacking. We investigated the potential of six degrees-of–freedom (6-DOF) scanhead position and orientation measurement (POM) devices for registration of in vivo multiplanar, irregularly sampled ultrasound (US) images to a regular 3-D volume space. The results demonstrate that accurate spatial and temporal registration of four-dimensional (4-D) US data can be achieved using a 6-DOF scanhead tracking system. For reconstruction of arbitrary, irregularly sampled US data, we introduce a technique based upon a weighted, ellipsoid Gaussian convolution kernel. Volume renderings of 3-D and 4-D compounded in vivo US data are presented. The results, although restricted to the field of cerebrovascular disease, will be of value to other applications of 3-D sonography, particularly those in which compounding of data through irregular sampling may provide superior information on tissue or vessel structure.
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paper_title: Kronecker products and matrix calculus in system theory

paper_content:

The paper begins with a review of the algebras related to Kronecker products. These algebras have several applications in system theory including the analysis of stochastic steady state. The calculus of matrix valued functions of matrices is reviewed in the second part of the paper. This calculus is then used to develop an interesting new method for the identifiication of parameters of lnear time-invariant system models.
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paper_title: Error analysis for a free-hand three-dimensional ultrasound system for neuronavigation

paper_content:

Image-guided neurosurgery that is directed by a preoperative imaging study, such as magnetic resonance (MR) imaging or computerized tomography (CT) scanning, can be very accurate provided no significant changes occur during surgery. A variety of factors known to affect brain tissue movement are not reflected in the preoperative images used for guidance. To update the information on which neuronavigation is based, the authors propose the use of three-dimensional (3-D) ultrasound images in conjunction with a finite-element computational model of the deformation of the brain. The 3-D ultrasound system will provide real-time information on the displacement of deep structures to guide the mathematical model. This paper has two goals: first, to present an outline of steps necessary to compute the location of a feature appearing in an ultrasound image in an arbitrary coordinate system; and second, to present an extensive evaluation of this system's accuracy. The authors have found that by using a stylus rigidly coupled to the 3-D tracker's sensor, they were able to locate a point with an overall error of 1.36 ± 1.67 mm (based on 39 points). When coupling the tracker to an ultrasound scanhead, they found that they could locate features appearing on ultrasound images with an error of 2.96 ± 1.85 mm (total 58 features). They also found that when registering a skull phantom to coordinates that were defined by MR imaging or CT scanning, they could do so with an error of 0.86 ± 0.61 mm (based on 20 coordinates). Based on their previous finding of brain shifts on the order of 1 cm during surgery, the accuracy of their system warrants its use in updating neuronavigation imaging data.
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paper_title: Using a 3D Position Sensor for Registration of SPECT and US Images of the Kidney

paper_content:

The correlation of functional images of the kidney with anatomical images is important to improve diagnosis and treatment. We propose a method to register SPECT (Single Photon Emission Computed Tomography) and US (ultrasound) images of the kidney that consists in tracking the US probe in the space thanks to an optical three-dimensional position sensor. This method has been experimented on three patients and has provided very promising results.
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paper_title: Rapid calibration method for registration and 3D tracking of ultrasound images using spatial localizer

paper_content:

Conventional freehand 3D ultrasound (US) is a complex process, involving calibration, scanning, processing, volume reconstruction, and visualization. Prior to calibration, a position sensor is attached to the probe for tagging each image with its position and orientation in space; then calibration process is performed to determine the spatial transformation of the scan plan with respect to the position sensor. Finding this transformation matrix is a critical, but often underrated task in US-guided surgery. The purpose of this study is to enhance previously known calibration methods by introducing a novel calibration fixture and process. The proposed phantom is inexpensive, easy to construct, easy to scan, while yielding more data points per image than previously known designs. The processing phase is semi-automated, allowing for fast processing of a massive amount of data, which in turn increases accuracy by reducing human errors.
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paper_title: Rapid, easy and reliable calibration for freehand 3D ultrasound

paper_content:

This paper presents improvements to the plane-based technique for calibrating freehand 3D ultrasound systems. The improvements are designed to make it easier for inexperienced users to perform plane-based calibration and to know that they have got a reliable result. In particular, we enable the calibration to be performed using water at room temperature while producing a result that is valid for average soft tissue and we show how it is possible to provide feedback on the reliability of the calibration using a metric based on the curvature of the calibration criterion function. We present comprehensive results showing that these innovations improve the precision of the calibration and offer useful feedback to the user.
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paper_title: Robot Hand-Eye Calibration Using Structure-from-Motion

paper_content:

In this paper, we propose a new flexible method for hand-eye calibration. The vast majority of existing hand-eye calibration techniques require a calibration rig that is used in conjunction with camera pose estimation methods. Instead, we combine structure-from-motion with known robot motions, and we show that the solution can be obtained in linear form. The latter solves for both the hand-eye parameters and the unknown scale factor inherent with structure-from-motion methods. The algebraic analysis that is made possible with such a linear formulation allows investigation of not only the well-known case of general screw motions but also of such singular motions as pure translations, pure rotations, and planar motions. In essence, the robot-mounted camera looks to an unknown rigid layout, tracks points over an image sequence, and estimates the camera-to-robot relationship. Such a self-calibration process is relevant for unmanned vehicles, robots working in remote places, and so forth. We conduct a large nu...
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paper_title: Calibration Method for Determining the Physical Location of the Ultrasound Image Plane

paper_content:

This paper describes a calibration method for determining the physical location of the ultrasound (US) image plane relative to a rigidly attached 3D position sensor. A calibrated US probe can measure the 3D spatial location of anatomic structures relative to a global coordinate system. The calibration is performed by aiming the US probe at a calibration target containing a known point (1 mm diameter sphere) in physical space. This point is repeatedly collected at various locations in the US image plane to produce the calibration dataset. An idealized model of the collection process is used to eliminate outliers from the calibration dataset and also to examine the theoretical accuracy limits of this method. The results demonstrate accurate and robust calibration of the 3D spatial relationship between the US image plane and the 3D position sensor.
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paper_title: Least-squares estimation of transformation parameters between two point patterns

paper_content:

In many applications of computer vision, the following problem is encountered. Two point patterns (sets of points) (x/sub i/) and (x/sub i/); i=1, 2, . . ., n are given in m-dimensional space, and the similarity transformation parameters (rotation, translation, and scaling) that give the least mean squared error between these point patterns are needed. Recently, K.S. Arun et al. (1987) and B.K.P. Horn et al. (1987) presented a solution of this problem. Their solution, however, sometimes fails to give a correct rotation matrix and gives a reflection instead when the data is severely corrupted. The proposed theorem is a strict solution of the problem, and it always gives the correct transformation parameters even when the data is corrupted. >
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paper_title: Least-Squares Fitting of Two 3-D Point Sets

paper_content:

Two point sets {pi} and {p'i}; i = 1, 2,..., N are related by p'i = Rpi + T + Ni, where R is a rotation matrix, T a translation vector, and Ni a noise vector. Given {pi} and {p'i}, we present an algorithm for finding the least-squares solution of R and T, which is based on the singular value decomposition (SVD) of a 3 × 3 matrix. This new algorithm is compared to two earlier algorithms with respect to computer time requirements.
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paper_title: Using a 3D Position Sensor for Registration of SPECT and US Images of the Kidney

paper_content:

The correlation of functional images of the kidney with anatomical images is important to improve diagnosis and treatment. We propose a method to register SPECT (Single Photon Emission Computed Tomography) and US (ultrasound) images of the kidney that consists in tracking the US probe in the space thanks to an optical three-dimensional position sensor. This method has been experimented on three patients and has provided very promising results.
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paper_title: Augmented reality visualization of ultrasound images: system description, calibration, and features

paper_content:

We developed a system for augmented reality visualization of ultrasound images. This system is based on our earlier "augmented workspace" setup. The user wears a custom video-see-through head-mounted display (HMD). Two color video cameras attached to the HMD provide a stereo view of the scene. A third head-mounted video camera is added for tracking. A set of markers is attached to the ultrasound transducer; a set of stationary markers can be positioned above the workspace. The system runs at the full 30 Hz video frame rate with a latency of about 0.1 sec, generating a stable augmentation with no apparent jitter visible in the composite images. Three SGI Visual Workstations provide the computing power for the system. The authors describe the details of the system, its calibration with a configuration of optical markers partially immersed in a water bath, and some system features like spatiotemporal freezing and 3D target localization that promise to be helpful for practical applications.
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paper_title: Rapid calibration for 3-D freehand ultrasound.

paper_content:

3-D freehand ultrasound is a new imaging technique that is rapidly finding clinical applications. A position-sensing device is attached to a conventional ultrasound probe so that, as B-scans are acquired, they can be labelled with their relative positions and orientations. This allows a 3-D data set to be constructed from the B-scans. A key requirement of all freehand imaging systems is calibration; that is, determining the position and orientation of the B-scan with respect to the position sensor. This is typically a lengthy and tedious process that may need repeating every time a sensor is mounted on a probe. This paper describes a new calibration technique that takes only a few minutes to perform and produces results that compare favourably (in terms of both accuracy and precision) with previously published alternatives.
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paper_title: Temporal calibration of freehand three-dimensional ultrasound using image alignment

paper_content:

Abstract All freehand 3-D ultrasound systems have some latency between the acquisition of an image and its associated position. Previously, estimation of latency has been made by tracking a phantom in a sequence of images and correlating its motion to that recorded by the position sensor. However, tracking-based temporal calibration uses the assumption that latency is constant between scans. This paper presents a new method for temporal calibration that avoids this assumption. Temporal calibration is performed on the scan data by finding the latency at which the best alignment of the 2-D images within the reconstructed volume occurs. The mean voxel intensity variance is used as a global measure of the quality of alignment within the volume and is minimized with respect to latency for each scan. The new method is compared with previous methods using an ultrasound phantom. Finally, integration of temporal calibration with existing spatial calibration methods is discussed. (E-mail: gooding@robots.ox.ac.uk )
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paper_title: A new calibration method in 3D ultrasonic imaging system

paper_content:

The authors' three-dimensional (3D) ultrasonic imaging system is constructed from a B-Mode ultrasound scanner, a magnetometer-based position and orientation measurement system (Flock of Birds), and a personal computer. To accurately determine the relationship between the magnetic receiver and the image plane, the authors propose a new calibration method which determines the system calibration parameters by measuring a triangular pyramid phantom. The new method not only reduces the effect of the thickness and divergence of the ultrasound beam, but also improves the efficiency of the algorithm.
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paper_title: Beam calibration without a phantom for creating a 3-D freehand ultrasound system

paper_content:

Abstract To create a freehand three-dimensional (3-D) ultrasound (US) system for image-guided surgical procedures, an US beam calibration process must be performed. The calibration method presented in this work does not use a phantom to define in 3-D space the pixel locations in the beam. Rather, the described method is based on the spatial relationship between an optically tracked pointer and a similarly tracked US transducer. The pointer tip was placed into the US beam, and US images, physical coordinates of the pointer and the transducer location were simultaneously recorded. US image coordinates of the pointer were mapped to the physical points using two different registration methods. Two sensitivity studies were performed to determine the location and number of points needed to calibrate the beam accurately. Results showed that the beam is most efficiently calibrated with approximately 20 points collected from throughout the beam. This method of beam calibration proved to be highly accurate, yielding registration errors of approximately 0.4 mm. (E-mail: diane.muratore@vanderbilt.edu)
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paper_title: A novel phantom-less spatial and temporal ultrasound calibration method

paper_content:

This paper introduces a novel method for ultrasound calibration for both spatial and temporal parameters. The main advantage of this method is that it does not require a phantom, which is usually expensive to fabricate. Furthermore, the method does not require extensive image processing. For spatial calibration, we solve an optimization problem established by a set of equations that relate the orientations of a line (i.e., calibration pointer) to the intersection points appearing in the ultrasound image. The line orientation is provided through calibration of both ends of the calibration pointer. Temporal calibration is achieved by processing of the captured pointer orientations and the corresponding image positions of intersection along with the timing information. The effectiveness of the unified method for both spatial and temporal calibration is apparent from the quality of the 3D reconstructions of a known object.
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paper_title: Ultrasound Guided Surgery: Image Processing and Navigation

paper_content:

The need for spectrally efficient transmission on mobile and wireless channels is prevalent. A promising scheme for such transmission is adaptive coded modulation. In this thesis, techniques for as ...
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paper_title: Random sample consensus: a paradigm for model fitting with applications to image analysis and automated cartography

paper_content:

A new paradigm, Random Sample Consensus (RANSAC), for fitting a model to experimental data is introduced. RANSAC is capable of interpreting/smoothing data containing a significant percentage of gross errors, and is thus ideally suited for applications in automated image analysis where interpretation is based on the data provided by error-prone feature detectors. A major portion of this paper describes the application of RANSAC to the Location Determination Problem (LDP): Given an image depicting a set of landmarks with known locations, determine that point in space from which the image was obtained. In response to a RANSAC requirement, new results are derived on the minimum number of landmarks needed to obtain a solution, and algorithms are presented for computing these minimum-landmark solutions in closed form. These results provide the basis for an automatic system that can solve the LDP under difficult viewing
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paper_title: A visual probe localization and calibration system for cost-effective computer-aided 3D ultrasound

paper_content:

The 3D ultrasound systems produce much better reproductions than 2D ultrasound, but their prohibitively high cost deprives many less affluent organization this benefit. This paper proposes using the conventional 2D ultrasound equipment readily available in most hospitals, along with a single conventional digital camera, to construct 3D ultrasound images. The proposed system applies computer vision to extract position information of the ultrasound probe while the scanning takes place. The probe, calibrated in order to calculate the offset of the ultrasound scan from the position of the marker attached to it, is used to scan a number of geometrical objects. Using the proposed system, the 3D volumes of the objects were successfully reconstructed. The system was tested in clinical situations where human body parts were scanned. The results presented, and confirmed by medical staff, are very encouraging for cost-effective implementation of computer-aided 3D ultrasound using a simple setup with 2D ultrasound equipment and a conventional digital camera.
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paper_title: Rapid calibration for 3-D freehand ultrasound.

paper_content:

3-D freehand ultrasound is a new imaging technique that is rapidly finding clinical applications. A position-sensing device is attached to a conventional ultrasound probe so that, as B-scans are acquired, they can be labelled with their relative positions and orientations. This allows a 3-D data set to be constructed from the B-scans. A key requirement of all freehand imaging systems is calibration; that is, determining the position and orientation of the B-scan with respect to the position sensor. This is typically a lengthy and tedious process that may need repeating every time a sensor is mounted on a probe. This paper describes a new calibration technique that takes only a few minutes to perform and produces results that compare favourably (in terms of both accuracy and precision) with previously published alternatives.
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paper_title: Wavelet-based edge detection in ultrasound images.

paper_content:

We introduce a new wavelet-based method for edge detection in ultrasound (US) images. Each beam that is analyzed is first transformed into the wavelet domain using the continuous wavelet transform (CWT). Because the CWT preserves both scale and time information, it is possible to separate the signal into a number of scales. The edge is localized by first determining the scale at which the power spectrum, based on the wavelet transform, has its maximum value. Next, at this scale we find the position of the peak for the squared CWT. This method does not depend on any threshold, after the range of scales have been determined. We suggest a range of scales for US images in general. Sample edge detections are demonstrated in US images of straight and jagged edges of simple structures submerged in water bath, and of an abdominal aorta aneurysm phantom.
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paper_title: Rapid calibration method for registration and 3D tracking of ultrasound images using spatial localizer

paper_content:

Conventional freehand 3D ultrasound (US) is a complex process, involving calibration, scanning, processing, volume reconstruction, and visualization. Prior to calibration, a position sensor is attached to the probe for tagging each image with its position and orientation in space; then calibration process is performed to determine the spatial transformation of the scan plan with respect to the position sensor. Finding this transformation matrix is a critical, but often underrated task in US-guided surgery. The purpose of this study is to enhance previously known calibration methods by introducing a novel calibration fixture and process. The proposed phantom is inexpensive, easy to construct, easy to scan, while yielding more data points per image than previously known designs. The processing phase is semi-automated, allowing for fast processing of a massive amount of data, which in turn increases accuracy by reducing human errors.
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paper_title: Rapid, easy and reliable calibration for freehand 3D ultrasound

paper_content:

This paper presents improvements to the plane-based technique for calibrating freehand 3D ultrasound systems. The improvements are designed to make it easier for inexperienced users to perform plane-based calibration and to know that they have got a reliable result. In particular, we enable the calibration to be performed using water at room temperature while producing a result that is valid for average soft tissue and we show how it is possible to provide feedback on the reliability of the calibration using a metric based on the curvature of the calibration criterion function. We present comprehensive results showing that these innovations improve the precision of the calibration and offer useful feedback to the user.
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paper_title: A phantom with reduced complexity for spatial 3-D ultrasound calibration.

paper_content:

Abstract The design of a new phantom for 3-D ultrasound calibration is presented. The phantom provides a viable alternative to existing phantoms that are significantly more complex and require high precision fabrication. The phantom, referred to as a “plane-of-wires” phantom, consists of two wires mounted at the same fixed height above the bottom of a water tank. Data collection for calibration involved rotating and translating the phantom so that the wires remained in a single plane parallel to the tank bottom. The mean reconstruction accuracy of the plane-of-wires calibration is 0.66 mm at a mean depth of 12.3 mm, with a precision of 1.23 mm at the same mean depth. The calibration was used to determine the volume of a cube with known volume with an error of 2.51%. The calibration performance achieved is comparable with that of existing approaches. E-mail: ( jh7fj@virginia.edu )
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paper_title: Confhusius: a robust and fully automatic calibration method for 3D freehand ultrasound.

paper_content:

This paper describes a new robust and fully automatic method for calibration of three-dimensional (3D) freehand ultrasound called Confhusius (CalibratiON for FreeHand UltraSound Imaging USage). 3D Freehand ultrasound consists in mounting a position sensor on a standard probe. The echographic B-scans can be localized in 3D and compounded into a volume. However, especially for quantitative use, this process dramatically requires a calibration procedure that determines its accuracy and usefulness. Calibration aims at determining the transformation (translations, rotations, scaling) between the coordinates system of the echographic images and the coordinate system of the localization system. To calibrate, we acquire images of a phantom whose 3D geometrical properties are known. We propose a robust and fully automatic calibration method based on the Hough transform and robust estimators. Experiments have been done with synthetic and real sequences, and this calibration method is shown to be easy to perform, accurate, automatic and fast enough for clinical use.
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paper_title: Dynamic three-dimensional freehand echocardiography using raw digital ultrasound data.

paper_content:

In this paper, we present a new method for simple acquisition of dynamic three-dimensional (3-D) ultrasound data. We used a magnetic position sensor device attached to the ultrasound probe for spatial location of the probe, which was slowly tilted in the transthoracic scanning position. The 3-D data were recorded in 10-20 s, and the analysis was performed on an external PC within 2 min after transferring the raw digital ultrasound data directly from the scanner. The spatial and temporal resolutions of the reconstruction were evaluated, and were superior to video-based 3-D systems. Examples of volume reconstructions with better than 7 ms temporal resolution are given. The raw data with Doppler measurements were used to reconstruct both blood and tissue velocity volumes. The velocity estimates were available for optimal visualization and for quantitative analysis. The freehand data reconstruction accuracy was tested by volume estimation of balloon phantoms, giving high correlation with true volumes. Results show in vivo 3-D reconstruction and visualization of mitral and aortic valve morphology and blood flow, and myocardial tissue velocity. We conclude that it was possible to construct multimodality 3-D data in a limited region of the human heart within one respiration cycle, with reconstruction errors smaller than the resolution of the original ultrasound beam, and with a temporal resolution of up to 150 frames per second.
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paper_title: Image guidance of breast cancer surgery using 3-D ultrasound images and augmented reality visualization

paper_content:

This paper describes augmented reality visualization for the guidance of breast-conservative cancer surgery using ultrasonic images acquired in the operating room just before surgical resection. By combining an optical three-dimensional (3-D) position sensor, the position and orientation of each ultrasonic cross section are precisely measured to reconstruct geometrically accurate 3-D tumor models from the acquired ultrasonic images. Similarly, the 3-D position and orientation of a video camera are obtained to integrate video and ultrasonic images in a geometrically accurate manner. Superimposing the 3-D tumor models onto live video images of the patient's breast enables the surgeon to perceive the exact 3-D position of the tumor, including irregular cancer invasions which cannot be perceived by touch, as if it were visible through the breast skin. Using the resultant visualization, the surgeon can determine the region for surgical resection in a more objective and accurate manner, thereby minimizing the risk of a relapse and maximizing breast conservation. The system was shown to be effective in experiments using phantom and clinical data.
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paper_title: A real-time freehand 3D ultrasound system for image-guided surgery

paper_content:

Current freehand 3D ultrasound techniques separate the scanning or acquisition step from the visualization step. The process leads to a single image volume dataset that can be rendered for viewing later. While satisfactory for diagnostic purposes, the method is not useful for surgical guidance where the anatomy must be visualized in real time. The Image Guidance Laboratories are currently developing a freehand 3D ultrasound system that will allow real-time updates to the scanned volume data as well as the capability to simultaneously view cross-sections through the volume and a volume-rendered perspective view. The equipment used is not unlike other freehand 3D ultrasound systems: an optical tracking system for locating the position and orientation of the ultrasound probe, a video frame grabber for capturing ultrasound frames, and a high-performance computer for performing real-time volume updates and volume rendering. The system incorporates novel methods for inserting new frames into, and removing expired frames from, the volume dataset in real time. This paper reports on current work in progress, and focuses on methods unique to achieving real-time 3D visualization using freehand 3D ultrasound.
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paper_title: An efficient calibration method for freehand 3-D ultrasound imaging systems.

paper_content:

A phantom has been developed to quickly calibrate a freehand 3-D ultrasound (US) imaging system. Calibration defines the spatial relationship between the US image plane and an external tracking device attached to the scanhead. The phantom consists of a planar array of strings and beads, and a set of out-of-plane strings that guide the user to proper scanhead orientation for imaging. When an US image plane is coincident with the plane defined by the strings, the calibration parameters are calculated by matching of homologous points in the image and phantom. The resulting precision and accuracy of the 3-D imaging system are similar to those achieved with a more complex calibration procedure. The 3-D reconstruction performance of the calibrated system is demonstrated with a magnetic tracking system, but the method could be applied to other tracking devices.
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paper_title: Probe calibration for freehand 3-D ultrasound.

paper_content:

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.
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paper_title: A robust and automatic method for evaluating accuracy in 3-D ultrasound-based navigation

paper_content:

Abstract We present a robust and automatic method for evaluating the 3-D navigation accuracy in ultrasound (US) based image-guided systems. The method is based on a precisely built and accurately measured phantom with several wire crosses and an automatic 3-D template matching by correlation algorithm. We investigated the accuracy and robustness of the algorithm and also addressed optimization of algorithm parameters. Finally, we applied the method to an extensive data set from an in-house US-based navigation system. To evaluate the algorithm, eight skilled observers identified the same wire crosses manually and the average over all observers constitutes our reference data set. We found no significant differences between the automatic and the manual procedures; the average distance between the point sets for one particular volume (27 point pairs) was 0.27 ± 0.17 mm. Furthermore, the spread of the automatically determined points compared with the reference set was lower than the spread for any individual operator. This indicates that the automatic algorithm is more accurate than manual determination of the wire-cross locations, in addition to being faster and nonsubjective. In the application example, we used a set of 35 3-D US scans of the phantom under various acquisition configurations. The US frequency was 6.7 MHz and the average target depth was 6 cm. The accuracy, represented by the mean distance between automatically-determined wire-cross locations and physically measured locations, was found to be 1.34 ± 0.62 mm. (E-mail: thomasl@sintef.no)
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paper_title: A fast calibration method for 3-D tracking of ultrasound images using a spatial localizer

paper_content:

Abstract We have developed a fast calibration method for computing the position and orientation of 2-D ultrasound (US) images in 3-D space where a position sensor is mounted on the US probe. This calibration is required in the fields of 3-D ultrasound and registration of ultrasound with other imaging modalities. Most of the existing calibration methods require a complex and tedious experimental procedure. Our method is simple and it is based on a custom-built phantom. Thirty N-fiducials (markers in the shape of the letter “N”) embedded in the phantom provide the basis for our calibration procedure. We calibrated a 3.5-MHz sector phased-array probe with a magnetic position sensor, and we studied the accuracy and precision of our method. A typical calibration procedure requires approximately 2 min. We conclude that we can achieve accurate and precise calibration using a single US image, provided that a large number (approximately ten) of N-fiducials are captured within the US image, enabling a representative sampling of the imaging plane. (E-mail: ykim@u.washington.edu)
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paper_title: Integrated MR and ultrasound imaging for improved image guidance in neurosurgery

paper_content:

We present a surgical guidance system that incorporates preoperative image information (e.g. MRI or CT) and intraoperative ultrasound imaging to detect brain tissue deformation during image guided neurosurgery. Many interactive IGNS implementations involve using pre-operative image information (e.g. MRI or CT) as a guide to the surgeons throughout the procedure. Tissue movement during a procedure can be a significant source of error in these systems. By incorporating intraoperative imaging, the target volume can be scanned at any time, and mapped into the pre- operative image space. The surgeon can use this information to assess the accuracy of the guidance system at any time during the procedure. In addition, the system can be used to provide updated information of the progress of this procedure (e.g. extent of lesion removal). Validation results using a deformable multimodality imaging phantom are presented as well as initial examples of the system used in surgery.
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paper_title: Intraoperative ultrasound for guidance and tissue shift correction in image-guided neurosurgery

paper_content:

We present a surgical guidance system that incorporates pre-operative image information (e.g., MRI) with intraoperative ultrasound (US) imaging to detect and correct for brain tissue deformation during image-guided neurosurgery (IGNS). Many interactive IGNS implementations employ pre-operative images as a guide to the surgeons throughout the procedure. However, when a craniotomy is involved, tissue movement during a procedure can be a significant source of error in these systems. By incorporating intraoperative US imaging, the target volume can be scanned at any time, and two-dimensional US images may be compared directly to the corresponding slice from the pre-operative image. Homologous points may be mapped from the intraoperative to the pre-operative image space with an accuracy of better than 2 mm, enabling the surgeon to use this information to assess the accuracy of the guidance system along with the progress of the procedure (e.g., extent of lesion removal) at any time during the operation. Anatomical features may be identified on both the pre-operative and intraoperative images and used to generate a deformation map, which can be used to warp the pre-operative image to match the intraoperative US image. System validation is achieved using a deformable multi-modality imaging phantom, and preliminary clinical results are presented.
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paper_title: Ultrasound Probe Tracking for Real-Time Ultrasound/MRI Overlay and Visualization of Brain Shift

paper_content:

Stereotactic techniques are prevalent in neurosurgery. A fundamental assumption of stereotaxis is that the brain is a rigid body. It has been demonstrated, however, that following a craniotomy the brain tissue will shift by 10 mm on average. We are investigating intra-operative ultrasound, using an optical tracking system to record the position and orientation of the ultrasound probe, as a method of measuring and correcting for brain shift. We have determined that the accuracy to which ultrasound image coordinates can be tracked (including the errors involved in calibration) is better than 0.5 mm within the ultrasound image plane, and better than 2 mm perpendicular to the plane. We apply two visualization methods to compare the ultrasound and the pre-operative MRI: the first is real-time overlay of the ultrasound with the co-planar MR slice, and the second is the real-time texture mapping of the ultrasound video into a 3D view with the MRI. Our technique is demonstrated on a poly vinyl alcohol cryogel phantom.
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paper_title: Automated 3D freehand ultrasound calibration with real-time accuracy control

paper_content:

3D ultrasound (US) is an emerging new imaging technology that appeals to more and more applications in intraoperative guidance of computer-assisted surgery. In a freehand US imaging system, US probe calibration is typically required to construct a 3D image of the patient’s anatomy from a set of 2D US images. Most of the current calibration techniques concern primarily with the precision and accuracy. However, for computer-assisted surgeries that may require a calibration task inside the operating room (OR), many other important aspects have to be considered besides accuracy. In this paper, we propose a novel system for automated calibration that is optimized for the OR usage with real-time feedback and control of the calibration accuracy. We have also designed a novel N-wire phantom, with greatly reduced complexity to facilitate mass production without compromising the accuracy and robustness.
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paper_title: Surface extraction with a three-dimensional freehand ultrasound system.

paper_content:

This paper presents a system for acquiring three-dimensional ultrasound data and extracting surfaces of the examined structures. The acquisition is performed freehand with a PC-based two dimensional ultrasound machine and an optical tracker. The extraction of surfaces from ultrasound data are normally inhibited by speckle, shadowing and gaps in the acquired data. A new method is developed that extracts a surface directly from the irregularly spaced, noisy freehand ultrasound data. The freehand data are first interpolated with radial basis functions and then a mesh is formed along an isosurface of the functional interpolation. The ability of radial basis functions to smooth speckle and interpolate across gaps is demonstrated on a series of experiments with phantoms and human tissue in a water bath. The geometry of the extracted surface matches the external measurements with an average difference ranging from 0.8 to 2.9 mm. These differences are within the range of errors from calibration, resolution and landmark localization. The experiments also show the ability to create continuous and realistic surfaces from scans that require multiple sweeps over a structure.
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paper_title: Probe calibration for freehand 3-D ultrasound.

paper_content:

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.
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paper_title: An Image Registration Approach to Automated Calibration for Freehand 3D Ultrasound

paper_content:

This paper describes an image registration approach to calibration for freehand three-dimensional (3D) ultrasound. If a conventional ultrasound probe is tracked using a position sensor, and the relation between this sensor and the two-dimensional (2D) image plane is known, the resulting set of B-scans may be correctly compounded into an image volume. Calibration is the process of determining the transformation (rotation, translation, and optionally image scaling) that maps B-mode image slice coordinates to points in the coordinate system of the tracking sensor mounted on the ultrasound probe. A set of 2D ultrasound images of a calibration phantom is obtained using a tracked ultrasound probe. Calibration is performed by searching for the calibration parameters that maximise the similarity between a model of the calibration phantom, which can be an image volume or a geometrical model, and the ultrasound images transformed into the coordinate space of the phantom. Validation of this calibration method is performed using a gelatin phantom. Measures of the calibration reproducibility, reconstruction precision and reconstruction accuracy are presented for this technique, and compared to those obtained using a conventional cross-wire phantom. Registration-based calibration is shown to be a rapid and accurate method of automatic calibration for freehand 3D ultrasound.
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paper_title: An overlap invariant entropy measure of 3D medical image alignment

paper_content:

Abstract This paper is concerned with the development of entropy-based registration criteria for automated 3D multi-modality medical image alignment. In this application where misalignment can be large with respect to the imaged field of view, invariance to overlap statistics is an important consideration. Current entropy measures are reviewed and a normalised measure is proposed which is simply the ratio of the sum of the marginal entropies and the joint entropy. The effect of changing overlap on current entropy measures and this normalised measure are compared using a simple image model and experiments on clinical image data. Results indicate that the normalised entropy measure provides significantly improved behaviour over a range of imaged fields of view.
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paper_title: Mutual-information-based registration of medical images: a survey

paper_content:

An overview is presented of the medical image processing literature on mutual-information-based registration. The aim of the survey is threefold: an introduction for those new to the field, an overview for those working in the field, and a reference for those searching for literature on a specific application. Methods are classified according to the different aspects of mutual-information-based registration. The main division is in aspects of the methodology and of the application. The part on methodology describes choices made on facets such as preprocessing of images, gray value interpolation, optimization, adaptations to the mutual information measure, and different types of geometrical transformations. The part on applications is a reference of the literature available on different modalities, on interpatient registration and on different anatomical objects. Comparison studies including mutual information are also considered. The paper starts with a description of entropy and mutual information and it closes with a discussion on past achievements and some future challenges.
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paper_title: Tracking a 3-D ultrasound probe with constantly visible fiducials

paper_content:

Abstract A new method is presented for tracking a 3-D ultrasound (US) probe in space. The key is to use small fiducials that create point features in the US volume near the probe face and which are connected rigidly to the tracker on the probe. In this way, fiducials appear in every US volume and are used to calculate the position and orientation of the volume with respect to a fixed base. The main novelty is the elimination of a separate calibration device because calibration can be considered to be performed on every volume. After acquisition, the user simply marks the locations of the fiducials in the volume and solves a single equation to convert volume coordinates to the base. The point accuracy of the new method is slightly lower than those of conventional methods mainly because of the suboptimal appearance of the fiducials near the top of the image and little data averaging, but ease of use is improved. (E-mail: rohling@ece.ubc.ca )
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paper_title: Comparison of calibration methods for spatial tracking of a 3-D ultrasound probe.

paper_content:

Real-time 3-D ultrasound (US) is a new-generation US system that uses a dedicated probe to create volume data sets instead of standard 2-D cross-sectional images. For applications in image-guided surgery and radiation therapy, a position tracker is added to the probe so that the volumes can be located in space. Calibration plays a critical role in determining the overall accuracy of an US volume-tracking system. In this paper, three calibration methods are developed specifically for 3-D probes. The three methods are based on a IXI-shaped wire phantom, a cube phantom and a stylus. The performance of each method was evaluated in terms of calibration reproducibility, point accuracy and reconstruction accuracy by distance measurement. The mean errors in the reproducibility tests were 1.50 mm (IXI-wire), 1.16 mm (cube) and 5.13 mm (stylus). The root mean square errors of the point accuracy measure were 2.15 mm (IXI-wire), 4.91 mm (cube) and 2.36 mm (stylus). The root mean square errors of the reconstruction accuracy by distance measure were 1.52 mm (IXI-wire), 1.59 mm (cube) and 1.85 mm (stylus). Overall, the IXI-wire phantom achieved the best results.
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paper_title: A technique for noninvasive respiratory gated radiation treatment system based on a real time 3D ultrasound image correlation: A phantom study

paper_content:

This paper proposes a new respiratory gated radiation treatment system that allows real-time tumor localization while avoiding invasive operation to a patient. The proposed system employs a three-dimensional (3D) ultrasound device, a 3D digital localizer, and a real-time image processing system. At the planning time, CT and 3D ultrasound reference data are simultaneously acquired under a breath-hold condition. At the treatment time, ultrasound data on three orthogonal planes are acquired and transferred to the image processing system on a real-time basis. Subsequently, normalized image correlation indices using the reference and the real-time ultrasound data are calculated for the three orthogonal planes after performing real-time coordinate transform using the 3D digital localizer attached to an ultrasound probe. Prior to the system execution, the coordinate transform matrices are partially calculated using an ultrasoundcalibration phantom and the 3D digital localizer. A trigger pulse to a linac can be generated when the normalized image correlation index exceeds a predetermined threshold level. Experiments have been carried out using a moving-target phantom that simulates a patient respiratory motion. We have observed that the variation of the calculated real-time correlation index synchronizes with the periodical motion of the moving-target, suggesting that real-time localization for a moving tumor is feasible with the proposed system.
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paper_title: A method for the calibration of 3D ultrasound transducers

paper_content:

Background: Three-dimensional (3D) ultrasound has a great potential in medical diagnostics. However, there are also some limitations of 3D ultrasound, e.g., in some situations morphology cannot be imaged accurately due to acoustical shadows. Acquiring 3D datasets from multiple positions can overcome some of these limitations. Prior to that a calibration of the ultrasound probe is necessary. Most calibration methods descibed rely on two-dimensional data. We describe a calibration method that uses 3D data. Methods: We have developed a 3D calibration method based on single-point cross-wire calibration using registration techniques for automatic detection of cross centers. For the calibration a cross consisting of three orthogonal wires is imaged. A model-to-image registration method is used to determine the cross center. Results: Due to the use of 3D data less acquisitions and no special protocols are necessary. The influence of noise is reduced. By means of the registration method ::: the time-consuming steps of image plane alignment and manual cross ::: center determination becomes dispensable. Conclusion: A 3D ::: calibration method for ultrasound transducers is described. The ::: calibration method is the base to extend state-of-the-art 3D ::: ultrasound devices, i.e., to acquire multiple 3D, either morphological or functional (Doppler), datasets.
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paper_title: Calibration of three-dimensional ultrasound images for image-guided radiation therapy.

paper_content:

A new technique of patient positioning for radiotherapy/radiosurgery of extracranial tumours using three-dimensional (3D) ultrasound images has been developed. The ultrasound probe position is tracked within the treatment room via infrared light emitting diodes (IRLEDs) attached to the probe. In order to retrieve the corresponding room position of the ultrasound image, we developed an initial ultrasound probe calibration technique for both 2D and 3D ultrasound systems. This technique is based on knowledge of points in both room and image coordinates. We first tested the performance of three algorithms in retrieving geometrical transformations using synthetic data with different noise levels. Closed form solution algorithms (singular value decomposition and Horn's quaternion algorithms) were shown to outperform the Hooke and Jeeves iterative algorithm in both speed and accuracy. Furthermore, these simulations show that for a random noise level of 2.5, 5, 7.5 and 10 mm, the number of points required for a transformation accuracy better than 1 mm is 25, 100, 200 and 500 points respectively. Finally, we verified the tracking accuracy of this system using a specially designed ultrasound phantom. Since ultrasound images have a high noise level, we designed an ultrasound phantom that provides a large number of points for the calibration. This tissue equivalent phantom is made of nylon wires, and its room position is optically tracked using IRLEDs. By obtaining multiple images through the nylon wires, the calibration technique uses an average of 300 points for 3D ultrasound volumes and 200 for 2D ultrasound images, and its stability is very good for both rotation (standard deviation: 0.4 degrees) and translation (standard deviation: 0.3 mm) transformations. After this initial calibration procedure, the position of any voxel in the ultrasound image volume can be determined in world space, thereby allowing real-time image guidance of therapeutic procedures. Finally, the overall tracking accuracy of our 3D ultrasound image-guided positioning system was measured to be on average 0.2 mm, 0.9 mm and 0.6 mm for the AP, lateral and axial directions respectively.
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paper_title: An Image Registration Approach to Automated Calibration for Freehand 3D Ultrasound

paper_content:

This paper describes an image registration approach to calibration for freehand three-dimensional (3D) ultrasound. If a conventional ultrasound probe is tracked using a position sensor, and the relation between this sensor and the two-dimensional (2D) image plane is known, the resulting set of B-scans may be correctly compounded into an image volume. Calibration is the process of determining the transformation (rotation, translation, and optionally image scaling) that maps B-mode image slice coordinates to points in the coordinate system of the tracking sensor mounted on the ultrasound probe. A set of 2D ultrasound images of a calibration phantom is obtained using a tracked ultrasound probe. Calibration is performed by searching for the calibration parameters that maximise the similarity between a model of the calibration phantom, which can be an image volume or a geometrical model, and the ultrasound images transformed into the coordinate space of the phantom. Validation of this calibration method is performed using a gelatin phantom. Measures of the calibration reproducibility, reconstruction precision and reconstruction accuracy are presented for this technique, and compared to those obtained using a conventional cross-wire phantom. Registration-based calibration is shown to be a rapid and accurate method of automatic calibration for freehand 3D ultrasound.
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paper_title: Beam calibration without a phantom for creating a 3-D freehand ultrasound system

paper_content:

Abstract To create a freehand three-dimensional (3-D) ultrasound (US) system for image-guided surgical procedures, an US beam calibration process must be performed. The calibration method presented in this work does not use a phantom to define in 3-D space the pixel locations in the beam. Rather, the described method is based on the spatial relationship between an optically tracked pointer and a similarly tracked US transducer. The pointer tip was placed into the US beam, and US images, physical coordinates of the pointer and the transducer location were simultaneously recorded. US image coordinates of the pointer were mapped to the physical points using two different registration methods. Two sensitivity studies were performed to determine the location and number of points needed to calibrate the beam accurately. Results showed that the beam is most efficiently calibrated with approximately 20 points collected from throughout the beam. This method of beam calibration proved to be highly accurate, yielding registration errors of approximately 0.4 mm. (E-mail: diane.muratore@vanderbilt.edu)
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paper_title: A fast calibration method for 3-D tracking of ultrasound images using a spatial localizer

paper_content:

Abstract We have developed a fast calibration method for computing the position and orientation of 2-D ultrasound (US) images in 3-D space where a position sensor is mounted on the US probe. This calibration is required in the fields of 3-D ultrasound and registration of ultrasound with other imaging modalities. Most of the existing calibration methods require a complex and tedious experimental procedure. Our method is simple and it is based on a custom-built phantom. Thirty N-fiducials (markers in the shape of the letter “N”) embedded in the phantom provide the basis for our calibration procedure. We calibrated a 3.5-MHz sector phased-array probe with a magnetic position sensor, and we studied the accuracy and precision of our method. A typical calibration procedure requires approximately 2 min. We conclude that we can achieve accurate and precise calibration using a single US image, provided that a large number (approximately ten) of N-fiducials are captured within the US image, enabling a representative sampling of the imaging plane. (E-mail: ykim@u.washington.edu)
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paper_title: Rapid calibration for 3-D freehand ultrasound.

paper_content:

3-D freehand ultrasound is a new imaging technique that is rapidly finding clinical applications. A position-sensing device is attached to a conventional ultrasound probe so that, as B-scans are acquired, they can be labelled with their relative positions and orientations. This allows a 3-D data set to be constructed from the B-scans. A key requirement of all freehand imaging systems is calibration; that is, determining the position and orientation of the B-scan with respect to the position sensor. This is typically a lengthy and tedious process that may need repeating every time a sensor is mounted on a probe. This paper describes a new calibration technique that takes only a few minutes to perform and produces results that compare favourably (in terms of both accuracy and precision) with previously published alternatives.
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paper_title: Performance of a miniature magnetic position sensor for three-dimensional ultrasound imaging

paper_content:

A miniature magnetic position sensor used for three-dimensional ultrasound imaging was tested for precision and accuracy in vitro. The sensor alone was able to locate points with root-mean-square (rms) uncertainty of 1.7 mm and accuracy of 0.05 ± 0.62 mm over its specified operating range of 50 cm. With an ultrasound imaging system, a point was located from arbitrary viewing windows with 2.4-mm rms uncertainty and 0.06 ± 0.68 mm accuracy. If viewing windows were limited to those representative of a typical ultrasound examination, the system could achieve rms uncertainty in point location of < 1 mm. Performance was not affected by operation of the imaging system when the sensor was mounted on an ultrasound scanhead. Sensitivity to metals in the operating environment was also measured.
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paper_title: A mechanical instrument for 3D ultrasound probe calibration.

paper_content:

Abstract We present a novel technique for 3D ultrasound probe calibration. The principle of operation is that the beam is aligned with a set of coplanar wires strung across a rigid frame. The probe and frame are mounted on a precision-manufactured mechanical instrument which allows adjustment and measurement of their relative pose. Semi-automatic image processing facilitates alignment of the beam and wires to within a tolerance of around 0.2 mm, despite the considerable beam thickness. The calibration process requires just a single view and relatively little user expertise. In a series of experiments with different ultrasound probes, we demonstrate the technique’s high accuracy and precision. The latter is partly due to the elimination of the position sensor, a significant source of measurement noise, from the end-user calibration process. (E-mail: ahg@eng.cam.ac.uk )
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paper_title: Rapid calibration method for registration and 3D tracking of ultrasound images using spatial localizer

paper_content:

Conventional freehand 3D ultrasound (US) is a complex process, involving calibration, scanning, processing, volume reconstruction, and visualization. Prior to calibration, a position sensor is attached to the probe for tagging each image with its position and orientation in space; then calibration process is performed to determine the spatial transformation of the scan plan with respect to the position sensor. Finding this transformation matrix is a critical, but often underrated task in US-guided surgery. The purpose of this study is to enhance previously known calibration methods by introducing a novel calibration fixture and process. The proposed phantom is inexpensive, easy to construct, easy to scan, while yielding more data points per image than previously known designs. The processing phase is semi-automated, allowing for fast processing of a massive amount of data, which in turn increases accuracy by reducing human errors.
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paper_title: Rapid, easy and reliable calibration for freehand 3D ultrasound

paper_content:

This paper presents improvements to the plane-based technique for calibrating freehand 3D ultrasound systems. The improvements are designed to make it easier for inexperienced users to perform plane-based calibration and to know that they have got a reliable result. In particular, we enable the calibration to be performed using water at room temperature while producing a result that is valid for average soft tissue and we show how it is possible to provide feedback on the reliability of the calibration using a metric based on the curvature of the calibration criterion function. We present comprehensive results showing that these innovations improve the precision of the calibration and offer useful feedback to the user.
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paper_title: A phantom with reduced complexity for spatial 3-D ultrasound calibration.

paper_content:

Abstract The design of a new phantom for 3-D ultrasound calibration is presented. The phantom provides a viable alternative to existing phantoms that are significantly more complex and require high precision fabrication. The phantom, referred to as a “plane-of-wires” phantom, consists of two wires mounted at the same fixed height above the bottom of a water tank. Data collection for calibration involved rotating and translating the phantom so that the wires remained in a single plane parallel to the tank bottom. The mean reconstruction accuracy of the plane-of-wires calibration is 0.66 mm at a mean depth of 12.3 mm, with a precision of 1.23 mm at the same mean depth. The calibration was used to determine the volume of a cube with known volume with an error of 2.51%. The calibration performance achieved is comparable with that of existing approaches. E-mail: ( jh7fj@virginia.edu )
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paper_title: 3D ultrasonic image feature localization based on magnetic scanhead tracking: in vitro calibration and validation.

paper_content:

The basis of a three-dimensional (3D) ultrasound imaging system was constructed from a commercially available magnetometer-based position and orientation measurement (POM) device, a standard B-Mode ultrasound instrument and a personal computer. To evaluate the system's performance, a novel method was devised using an iterative, least-squares technique to simultaneously determine the system's calibration parameters and measure its precision in locating points in three-dimensional space. When tested separately, the POM system located single points with a root mean squared (RMS) uncertainty of from 1.4 mm to 3.2 mm over the 610 mm working radius of the system. When combined with the ultrasound instrument, the RMS uncertainty in locating point targets varied from 2.1 mm to 3.5 mm. These results establish the lower limits of variability to be expected from this system when locating fiducial anatomical landmarks for repeated examinations of the same region of the body, and when making numerical 3D reconstructions from multiple planar images.
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paper_title: Confhusius: a robust and fully automatic calibration method for 3D freehand ultrasound.

paper_content:

This paper describes a new robust and fully automatic method for calibration of three-dimensional (3D) freehand ultrasound called Confhusius (CalibratiON for FreeHand UltraSound Imaging USage). 3D Freehand ultrasound consists in mounting a position sensor on a standard probe. The echographic B-scans can be localized in 3D and compounded into a volume. However, especially for quantitative use, this process dramatically requires a calibration procedure that determines its accuracy and usefulness. Calibration aims at determining the transformation (translations, rotations, scaling) between the coordinates system of the echographic images and the coordinate system of the localization system. To calibrate, we acquire images of a phantom whose 3D geometrical properties are known. We propose a robust and fully automatic calibration method based on the Hough transform and robust estimators. Experiments have been done with synthetic and real sequences, and this calibration method is shown to be easy to perform, accurate, automatic and fast enough for clinical use.
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paper_title: An efficient calibration method for freehand 3-D ultrasound imaging systems.

paper_content:

A phantom has been developed to quickly calibrate a freehand 3-D ultrasound (US) imaging system. Calibration defines the spatial relationship between the US image plane and an external tracking device attached to the scanhead. The phantom consists of a planar array of strings and beads, and a set of out-of-plane strings that guide the user to proper scanhead orientation for imaging. When an US image plane is coincident with the plane defined by the strings, the calibration parameters are calculated by matching of homologous points in the image and phantom. The resulting precision and accuracy of the 3-D imaging system are similar to those achieved with a more complex calibration procedure. The 3-D reconstruction performance of the calibrated system is demonstrated with a magnetic tracking system, but the method could be applied to other tracking devices.
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paper_title: Calibration Method for Determining the Physical Location of the Ultrasound Image Plane

paper_content:

This paper describes a calibration method for determining the physical location of the ultrasound (US) image plane relative to a rigidly attached 3D position sensor. A calibrated US probe can measure the 3D spatial location of anatomic structures relative to a global coordinate system. The calibration is performed by aiming the US probe at a calibration target containing a known point (1 mm diameter sphere) in physical space. This point is repeatedly collected at various locations in the US image plane to produce the calibration dataset. An idealized model of the collection process is used to eliminate outliers from the calibration dataset and also to examine the theoretical accuracy limits of this method. The results demonstrate accurate and robust calibration of the 3D spatial relationship between the US image plane and the 3D position sensor.
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paper_title: Probe calibration for freehand 3-D ultrasound.

paper_content:

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.
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paper_title: Reconstruction and visualization of irregularly sampled three- and four-dimensional ultrasound data for cerebrovascular applications

paper_content:

Abstract Although recent studies have demonstrated the potential value of compounded data for improvement in signal-to–noise ratio and speckle contrast for three-dimensional (3-D) ultrasonography, clinical applications are lacking. We investigated the potential of six degrees-of–freedom (6-DOF) scanhead position and orientation measurement (POM) devices for registration of in vivo multiplanar, irregularly sampled ultrasound (US) images to a regular 3-D volume space. The results demonstrate that accurate spatial and temporal registration of four-dimensional (4-D) US data can be achieved using a 6-DOF scanhead tracking system. For reconstruction of arbitrary, irregularly sampled US data, we introduce a technique based upon a weighted, ellipsoid Gaussian convolution kernel. Volume renderings of 3-D and 4-D compounded in vivo US data are presented. The results, although restricted to the field of cerebrovascular disease, will be of value to other applications of 3-D sonography, particularly those in which compounding of data through irregular sampling may provide superior information on tissue or vessel structure.
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paper_title: Correction of probe pressure artifacts in freehand 3D ultrasound

paper_content:

We present an algorithm which combines non-rigid image-based registration and conventional position sensing to correct probe-pressure-induced registration errors in freehand three-dimensional (3D) ultrasound volumes. The local accuracy of image-based registration enables the accurate freehand acquisition of high resolution (>15 MHz) 3D ultrasound data, opening the way for 3D musculoskeletal examinations. External position sensor readings guarantee the large-scale positional accuracy of the data. Pressure correction is shown to dramatically increase the perceived quality of extended-field-of-view data sets and reslices through volumetric data sets, while quantitative comparisons of multiple in vivo volumes demonstrate the superior precision of the corrected data.
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paper_title: An Image Registration Approach to Automated Calibration for Freehand 3D Ultrasound

paper_content:

This paper describes an image registration approach to calibration for freehand three-dimensional (3D) ultrasound. If a conventional ultrasound probe is tracked using a position sensor, and the relation between this sensor and the two-dimensional (2D) image plane is known, the resulting set of B-scans may be correctly compounded into an image volume. Calibration is the process of determining the transformation (rotation, translation, and optionally image scaling) that maps B-mode image slice coordinates to points in the coordinate system of the tracking sensor mounted on the ultrasound probe. A set of 2D ultrasound images of a calibration phantom is obtained using a tracked ultrasound probe. Calibration is performed by searching for the calibration parameters that maximise the similarity between a model of the calibration phantom, which can be an image volume or a geometrical model, and the ultrasound images transformed into the coordinate space of the phantom. Validation of this calibration method is performed using a gelatin phantom. Measures of the calibration reproducibility, reconstruction precision and reconstruction accuracy are presented for this technique, and compared to those obtained using a conventional cross-wire phantom. Registration-based calibration is shown to be a rapid and accurate method of automatic calibration for freehand 3D ultrasound.
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paper_title: A visual probe localization and calibration system for cost-effective computer-aided 3D ultrasound

paper_content:

The 3D ultrasound systems produce much better reproductions than 2D ultrasound, but their prohibitively high cost deprives many less affluent organization this benefit. This paper proposes using the conventional 2D ultrasound equipment readily available in most hospitals, along with a single conventional digital camera, to construct 3D ultrasound images. The proposed system applies computer vision to extract position information of the ultrasound probe while the scanning takes place. The probe, calibrated in order to calculate the offset of the ultrasound scan from the position of the marker attached to it, is used to scan a number of geometrical objects. Using the proposed system, the 3D volumes of the objects were successfully reconstructed. The system was tested in clinical situations where human body parts were scanned. The results presented, and confirmed by medical staff, are very encouraging for cost-effective implementation of computer-aided 3D ultrasound using a simple setup with 2D ultrasound equipment and a conventional digital camera.
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paper_title: A mechanical instrument for 3D ultrasound probe calibration.

paper_content:

Abstract We present a novel technique for 3D ultrasound probe calibration. The principle of operation is that the beam is aligned with a set of coplanar wires strung across a rigid frame. The probe and frame are mounted on a precision-manufactured mechanical instrument which allows adjustment and measurement of their relative pose. Semi-automatic image processing facilitates alignment of the beam and wires to within a tolerance of around 0.2 mm, despite the considerable beam thickness. The calibration process requires just a single view and relatively little user expertise. In a series of experiments with different ultrasound probes, we demonstrate the technique’s high accuracy and precision. The latter is partly due to the elimination of the position sensor, a significant source of measurement noise, from the end-user calibration process. (E-mail: ahg@eng.cam.ac.uk )
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paper_title: Accuracy evaluation of a 3D ultrasound-based neuronavigation system.

paper_content:

We have investigated the 3D navigation accuracy of a frameless ultrasound-based neuronavigation system (SonoWand) for surgical planning and intraoperative image guidance. In addition, we present a detailed description and review of the error sources associated with surgical neuronavigation based on preoperative MRI data and intraoperative ultrasound. A phantom with 27 precisely defined points was scanned with ultrasound by various translation and tilt movements of the ultrasound probe (180 3D scans in total), and the 27 image points in each volume were located using an automatic detection algorithm. These locations were compared to the physically measured locations of the same 27 points. The accuracy of the neuronavigation system and the effect of varying acquisition conditions were found through a thorough statistical analysis of the differences between the two point sets. The accuracy was found to be 1.40 +/- 0.45 mm (arithmetic mean) for the ultrasound-based neuronavigation system in our laboratory setting. Improper probe calibration was the major contributor to this figure. Based on our extensive data set and thorough evaluation, the accuracy found in the laboratory setting is expected to be close to the overall clinical accuracy for ultrasound-based neuronavigation. Our analysis indicates that the overall clinical accuracy may be as low as 2 mm when using intraoperative imaging to compensate for brain shift.
---
paper_title: An efficient calibration method for freehand 3-D ultrasound imaging systems.

paper_content:

A phantom has been developed to quickly calibrate a freehand 3-D ultrasound (US) imaging system. Calibration defines the spatial relationship between the US image plane and an external tracking device attached to the scanhead. The phantom consists of a planar array of strings and beads, and a set of out-of-plane strings that guide the user to proper scanhead orientation for imaging. When an US image plane is coincident with the plane defined by the strings, the calibration parameters are calculated by matching of homologous points in the image and phantom. The resulting precision and accuracy of the 3-D imaging system are similar to those achieved with a more complex calibration procedure. The 3-D reconstruction performance of the calibrated system is demonstrated with a magnetic tracking system, but the method could be applied to other tracking devices.
---
paper_title: Probe calibration for freehand 3-D ultrasound.

paper_content:

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.
---
paper_title: Reconstruction and visualization of irregularly sampled three- and four-dimensional ultrasound data for cerebrovascular applications

paper_content:

Abstract Although recent studies have demonstrated the potential value of compounded data for improvement in signal-to–noise ratio and speckle contrast for three-dimensional (3-D) ultrasonography, clinical applications are lacking. We investigated the potential of six degrees-of–freedom (6-DOF) scanhead position and orientation measurement (POM) devices for registration of in vivo multiplanar, irregularly sampled ultrasound (US) images to a regular 3-D volume space. The results demonstrate that accurate spatial and temporal registration of four-dimensional (4-D) US data can be achieved using a 6-DOF scanhead tracking system. For reconstruction of arbitrary, irregularly sampled US data, we introduce a technique based upon a weighted, ellipsoid Gaussian convolution kernel. Volume renderings of 3-D and 4-D compounded in vivo US data are presented. The results, although restricted to the field of cerebrovascular disease, will be of value to other applications of 3-D sonography, particularly those in which compounding of data through irregular sampling may provide superior information on tissue or vessel structure.
---
paper_title: An Image Registration Approach to Automated Calibration for Freehand 3D Ultrasound

paper_content:

This paper describes an image registration approach to calibration for freehand three-dimensional (3D) ultrasound. If a conventional ultrasound probe is tracked using a position sensor, and the relation between this sensor and the two-dimensional (2D) image plane is known, the resulting set of B-scans may be correctly compounded into an image volume. Calibration is the process of determining the transformation (rotation, translation, and optionally image scaling) that maps B-mode image slice coordinates to points in the coordinate system of the tracking sensor mounted on the ultrasound probe. A set of 2D ultrasound images of a calibration phantom is obtained using a tracked ultrasound probe. Calibration is performed by searching for the calibration parameters that maximise the similarity between a model of the calibration phantom, which can be an image volume or a geometrical model, and the ultrasound images transformed into the coordinate space of the phantom. Validation of this calibration method is performed using a gelatin phantom. Measures of the calibration reproducibility, reconstruction precision and reconstruction accuracy are presented for this technique, and compared to those obtained using a conventional cross-wire phantom. Registration-based calibration is shown to be a rapid and accurate method of automatic calibration for freehand 3D ultrasound.
---
paper_title: Beam calibration without a phantom for creating a 3-D freehand ultrasound system

paper_content:

Abstract To create a freehand three-dimensional (3-D) ultrasound (US) system for image-guided surgical procedures, an US beam calibration process must be performed. The calibration method presented in this work does not use a phantom to define in 3-D space the pixel locations in the beam. Rather, the described method is based on the spatial relationship between an optically tracked pointer and a similarly tracked US transducer. The pointer tip was placed into the US beam, and US images, physical coordinates of the pointer and the transducer location were simultaneously recorded. US image coordinates of the pointer were mapped to the physical points using two different registration methods. Two sensitivity studies were performed to determine the location and number of points needed to calibrate the beam accurately. Results showed that the beam is most efficiently calibrated with approximately 20 points collected from throughout the beam. This method of beam calibration proved to be highly accurate, yielding registration errors of approximately 0.4 mm. (E-mail: diane.muratore@vanderbilt.edu)
---
paper_title: A fast calibration method for 3-D tracking of ultrasound images using a spatial localizer

paper_content:

Abstract We have developed a fast calibration method for computing the position and orientation of 2-D ultrasound (US) images in 3-D space where a position sensor is mounted on the US probe. This calibration is required in the fields of 3-D ultrasound and registration of ultrasound with other imaging modalities. Most of the existing calibration methods require a complex and tedious experimental procedure. Our method is simple and it is based on a custom-built phantom. Thirty N-fiducials (markers in the shape of the letter “N”) embedded in the phantom provide the basis for our calibration procedure. We calibrated a 3.5-MHz sector phased-array probe with a magnetic position sensor, and we studied the accuracy and precision of our method. A typical calibration procedure requires approximately 2 min. We conclude that we can achieve accurate and precise calibration using a single US image, provided that a large number (approximately ten) of N-fiducials are captured within the US image, enabling a representative sampling of the imaging plane. (E-mail: ykim@u.washington.edu)
---
paper_title: Automated 3D freehand ultrasound calibration with real-time accuracy control

paper_content:

3D ultrasound (US) is an emerging new imaging technology that appeals to more and more applications in intraoperative guidance of computer-assisted surgery. In a freehand US imaging system, US probe calibration is typically required to construct a 3D image of the patient’s anatomy from a set of 2D US images. Most of the current calibration techniques concern primarily with the precision and accuracy. However, for computer-assisted surgeries that may require a calibration task inside the operating room (OR), many other important aspects have to be considered besides accuracy. In this paper, we propose a novel system for automated calibration that is optimized for the OR usage with real-time feedback and control of the calibration accuracy. We have also designed a novel N-wire phantom, with greatly reduced complexity to facilitate mass production without compromising the accuracy and robustness.
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paper_title: A new calibration method in 3D ultrasonic imaging system

paper_content:

The authors' three-dimensional (3D) ultrasonic imaging system is constructed from a B-Mode ultrasound scanner, a magnetometer-based position and orientation measurement system (Flock of Birds), and a personal computer. To accurately determine the relationship between the magnetic receiver and the image plane, the authors propose a new calibration method which determines the system calibration parameters by measuring a triangular pyramid phantom. The new method not only reduces the effect of the thickness and divergence of the ultrasound beam, but also improves the efficiency of the algorithm.
---
paper_title: Probe calibration for freehand 3-D ultrasound.

paper_content:

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.
---
paper_title: An Image Registration Approach to Automated Calibration for Freehand 3D Ultrasound

paper_content:

This paper describes an image registration approach to calibration for freehand three-dimensional (3D) ultrasound. If a conventional ultrasound probe is tracked using a position sensor, and the relation between this sensor and the two-dimensional (2D) image plane is known, the resulting set of B-scans may be correctly compounded into an image volume. Calibration is the process of determining the transformation (rotation, translation, and optionally image scaling) that maps B-mode image slice coordinates to points in the coordinate system of the tracking sensor mounted on the ultrasound probe. A set of 2D ultrasound images of a calibration phantom is obtained using a tracked ultrasound probe. Calibration is performed by searching for the calibration parameters that maximise the similarity between a model of the calibration phantom, which can be an image volume or a geometrical model, and the ultrasound images transformed into the coordinate space of the phantom. Validation of this calibration method is performed using a gelatin phantom. Measures of the calibration reproducibility, reconstruction precision and reconstruction accuracy are presented for this technique, and compared to those obtained using a conventional cross-wire phantom. Registration-based calibration is shown to be a rapid and accurate method of automatic calibration for freehand 3D ultrasound.
---
paper_title: Automatic calibration of a robotized 3D ultrasound imaging system by visual servoing

paper_content:

Three-dimensional free-hand ultrasound imaging consists of capturing a set of ultrasound images with a 2D ultrasound system and their respective locations in order to position them in a 3D reference frame. Usually the clinician performs the acquisition manually through the use of an optical or magnetic localization system attached to the ultrasound probe. To assist the clinician, we propose to use a robotic system to automatically move the ultrasound probe and measure its position. As for manual 3D ultrasound imaging, it is crucial to know precisely the spatial calibration parameters of the ultrasound system in order to perform accurate 3D imaging. Therefore, we propose to automate the spatial calibration procedure. A robotic task is developed to automatically position the ultrasound image on the intersection point of a cross-wire phantom used for spatial calibration. To perform this task, a new visual servoing technique based on 2D ultrasound images is used to control automatically the motion of the ultrasound probe held by a medical robot
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paper_title: Rapid calibration for 3-D freehand ultrasound.

paper_content:

3-D freehand ultrasound is a new imaging technique that is rapidly finding clinical applications. A position-sensing device is attached to a conventional ultrasound probe so that, as B-scans are acquired, they can be labelled with their relative positions and orientations. This allows a 3-D data set to be constructed from the B-scans. A key requirement of all freehand imaging systems is calibration; that is, determining the position and orientation of the B-scan with respect to the position sensor. This is typically a lengthy and tedious process that may need repeating every time a sensor is mounted on a probe. This paper describes a new calibration technique that takes only a few minutes to perform and produces results that compare favourably (in terms of both accuracy and precision) with previously published alternatives.
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paper_title: Performance of a miniature magnetic position sensor for three-dimensional ultrasound imaging

paper_content:

A miniature magnetic position sensor used for three-dimensional ultrasound imaging was tested for precision and accuracy in vitro. The sensor alone was able to locate points with root-mean-square (rms) uncertainty of 1.7 mm and accuracy of 0.05 ± 0.62 mm over its specified operating range of 50 cm. With an ultrasound imaging system, a point was located from arbitrary viewing windows with 2.4-mm rms uncertainty and 0.06 ± 0.68 mm accuracy. If viewing windows were limited to those representative of a typical ultrasound examination, the system could achieve rms uncertainty in point location of < 1 mm. Performance was not affected by operation of the imaging system when the sensor was mounted on an ultrasound scanhead. Sensitivity to metals in the operating environment was also measured.
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paper_title: Rapid calibration method for registration and 3D tracking of ultrasound images using spatial localizer

paper_content:

Conventional freehand 3D ultrasound (US) is a complex process, involving calibration, scanning, processing, volume reconstruction, and visualization. Prior to calibration, a position sensor is attached to the probe for tagging each image with its position and orientation in space; then calibration process is performed to determine the spatial transformation of the scan plan with respect to the position sensor. Finding this transformation matrix is a critical, but often underrated task in US-guided surgery. The purpose of this study is to enhance previously known calibration methods by introducing a novel calibration fixture and process. The proposed phantom is inexpensive, easy to construct, easy to scan, while yielding more data points per image than previously known designs. The processing phase is semi-automated, allowing for fast processing of a massive amount of data, which in turn increases accuracy by reducing human errors.
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paper_title: Rapid, easy and reliable calibration for freehand 3D ultrasound

paper_content:

This paper presents improvements to the plane-based technique for calibrating freehand 3D ultrasound systems. The improvements are designed to make it easier for inexperienced users to perform plane-based calibration and to know that they have got a reliable result. In particular, we enable the calibration to be performed using water at room temperature while producing a result that is valid for average soft tissue and we show how it is possible to provide feedback on the reliability of the calibration using a metric based on the curvature of the calibration criterion function. We present comprehensive results showing that these innovations improve the precision of the calibration and offer useful feedback to the user.
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paper_title: A phantom with reduced complexity for spatial 3-D ultrasound calibration.

paper_content:

Abstract The design of a new phantom for 3-D ultrasound calibration is presented. The phantom provides a viable alternative to existing phantoms that are significantly more complex and require high precision fabrication. The phantom, referred to as a “plane-of-wires” phantom, consists of two wires mounted at the same fixed height above the bottom of a water tank. Data collection for calibration involved rotating and translating the phantom so that the wires remained in a single plane parallel to the tank bottom. The mean reconstruction accuracy of the plane-of-wires calibration is 0.66 mm at a mean depth of 12.3 mm, with a precision of 1.23 mm at the same mean depth. The calibration was used to determine the volume of a cube with known volume with an error of 2.51%. The calibration performance achieved is comparable with that of existing approaches. E-mail: ( jh7fj@virginia.edu )
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paper_title: An efficient calibration method for freehand 3-D ultrasound imaging systems.

paper_content:

A phantom has been developed to quickly calibrate a freehand 3-D ultrasound (US) imaging system. Calibration defines the spatial relationship between the US image plane and an external tracking device attached to the scanhead. The phantom consists of a planar array of strings and beads, and a set of out-of-plane strings that guide the user to proper scanhead orientation for imaging. When an US image plane is coincident with the plane defined by the strings, the calibration parameters are calculated by matching of homologous points in the image and phantom. The resulting precision and accuracy of the 3-D imaging system are similar to those achieved with a more complex calibration procedure. The 3-D reconstruction performance of the calibrated system is demonstrated with a magnetic tracking system, but the method could be applied to other tracking devices.
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paper_title: Probe calibration for freehand 3-D ultrasound.

paper_content:

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.
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paper_title: A phantom with reduced complexity for spatial 3-D ultrasound calibration.

paper_content:

Abstract The design of a new phantom for 3-D ultrasound calibration is presented. The phantom provides a viable alternative to existing phantoms that are significantly more complex and require high precision fabrication. The phantom, referred to as a “plane-of-wires” phantom, consists of two wires mounted at the same fixed height above the bottom of a water tank. Data collection for calibration involved rotating and translating the phantom so that the wires remained in a single plane parallel to the tank bottom. The mean reconstruction accuracy of the plane-of-wires calibration is 0.66 mm at a mean depth of 12.3 mm, with a precision of 1.23 mm at the same mean depth. The calibration was used to determine the volume of a cube with known volume with an error of 2.51%. The calibration performance achieved is comparable with that of existing approaches. E-mail: ( jh7fj@virginia.edu )
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paper_title: Confhusius: a robust and fully automatic calibration method for 3D freehand ultrasound.

paper_content:

This paper describes a new robust and fully automatic method for calibration of three-dimensional (3D) freehand ultrasound called Confhusius (CalibratiON for FreeHand UltraSound Imaging USage). 3D Freehand ultrasound consists in mounting a position sensor on a standard probe. The echographic B-scans can be localized in 3D and compounded into a volume. However, especially for quantitative use, this process dramatically requires a calibration procedure that determines its accuracy and usefulness. Calibration aims at determining the transformation (translations, rotations, scaling) between the coordinates system of the echographic images and the coordinate system of the localization system. To calibrate, we acquire images of a phantom whose 3D geometrical properties are known. We propose a robust and fully automatic calibration method based on the Hough transform and robust estimators. Experiments have been done with synthetic and real sequences, and this calibration method is shown to be easy to perform, accurate, automatic and fast enough for clinical use.
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paper_title: Probe calibration for freehand 3-D ultrasound.

paper_content:

Ultrasound (US) probe calibration establishes the rigid body transformation between the US image and a tracking device attached to the probe. This is an important requirement for correct 3-D reconstruction of freehand US images and, thus, for accurate surgical navigation based on US. In this study, we evaluated three methods for probe calibration, based on a single-point phantom, a wire-cross phantom requiring 2-D alignment and a wire phantom for freehand scanning. The processing of acquired data is fairly common to these methods and, to a great extent, based on automated procedures. The evaluation is based on quality measures in 2-D and 3-D reconstructed data. With each of the three methods, we calibrated a linear-array probe, a phased-array sector probe and an intraoperative probe. The freehand method performed best, with a 3-D navigation accuracy of 0.6 mm for one of the probes. This indicates that clinical accuracy in the order of 1 mm may be achieved in US-based surgical navigation.
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Title: Freehand 3D Ultrasound Calibration: A Review

Section 1: Introduction
Description 1: Introduce the concept of freehand 3D ultrasound imaging, its clinical applications, and the necessity of probe calibration. Provide a brief overview of the paper's structure.

Section 2: Probe Calibration
Description 2: Describe the general process of probe calibration and introduce the tools and methods used in modern probe calibration.

Section 3: Point Phantom
Description 3: Discuss the use of point phantoms in probe calibration and differentiate between methods with and without a stylus. 

Section 4: Point Phantom Without a Stylus
Description 4: Explain the method and principles of calibrating a probe using a point phantom without a stylus.

Section 5: Point Phantom With a Stylus
Description 5: Elaborate on the calibration process when a stylus is available and its advantages.

Section 6: Point Phantom Variants
Description 6: Examine different variants of point phantoms and their respective advantages and difficulties.

Section 7: Stylus
Description 7: Discuss the use of a stylus itself for probe calibration, including the challenges and alignment issues.

Section 8: Three-wire Phantom
Description 8: Describe the three-wire phantom calibration technique and its unique aspects.

Section 9: Plane Phantom
Description 9: Provide details about the plane phantom calibration technique, including different design complexities and segmentation methods.

Section 10: Two-plane Phantom
Description 10: Explain the concept of the two-plane phantom and its calibration process.

Section 11: Two-dimensional Alignment Phantom
Description 11: Discuss the calibration using two-dimensional alignment phantoms with various shapes and fiducial points.

Section 12: Z-phantom
Description 12: Detail the Z-phantom design and its calibration method that does not require alignment of the scan plane with the phantom.

Section 13: Mechanical Instrument
Description 13: Describe the use of mechanical instruments in calibration, including the separation of position sensor and scan plane calibrations.

Section 14: Image Registration
Description 14: Explain the image registration technique and how it can be used for probe calibration.

Section 15: 3D Probe Calibration
Description 15: Briefly mention the applicability of 2D probe calibration techniques to 3D probes and discuss any additional considerations.

Section 16: Calibration Quality Assessment
Description 16: Provide an overview of the methods used to assess calibration quality, focusing on precision and accuracy metrics.

Section 17: Precision
Description 17: Define precision and describe how reconstruction precision and calibration reproducibility are measured.

Section 18: Accuracy
Description 18: Discuss the challenges of measuring calibration accuracy and the importance of system accuracy for clinical use.

Section 19: Point Reconstruction Accuracy
Description 19: Explain the point reconstruction accuracy (PRA) measure and its significance for system accuracy.

Section 20: Distance Accuracy
Description 20: Describe the method of using distance measurements between objects to assess calibration accuracy.

Section 21: Volume Measurement
Description 21: Discuss how volume measurements can be used to assess the accuracy of probe calibration and the challenges involved.

Section 22: Comparison
Description 22: Compare the different calibration techniques and their respective strengths and weaknesses.

Section 23: Choosing a Phantom
Description 23: Provide guidance on selecting the most suitable phantom for probe calibration, considering various factors such as ease of use, accuracy, and calibration time.

Section 24: Conclusion
Description 24: Summarize the findings, discussing the advantages and disadvantages of different calibration techniques and listing the situations where specific phantoms may be preferred.