10.14639/0392-100X-N2434
Objective. To assess artifact size and MRI visibility when applying the “Orthopedic-Metal Artifact Reduction” (O-MAR) algorithm for cochlear implant (CI) scanning. Methods. Two volunteers were submitted to 1.5 T MRI with an Ultra 3D CI receiver stimulator placed on their head. Four angular CI orientations were adopted: 90, 120, 135 and 160 degrees. Volunteers were scanned in each condition using T1w and T2w TSE sequences, as well as O-MAR sequences, in both axial and coronal planes. Quantitative comparisons were made of signal void and penumbra extent. Additionally, qualitative evaluations of global image quality, MRI readability with respect to 12 anatomical structures and visibility through the penumbra were undertaken. Results. After application of the O-MAR protocol, the radius of the signal void reduced from 50.76 mm to 45.43 mm and from 49.22 mm to 40.15 mm on T1w and T2w TSE axial sequences, respectively (p &lt; 0.05). Qualitatively, sequences acquired with O-MAR produced better outcomes in terms of image quality and anatomical depiction. Despite the area of the penumbra being increased for the O-MAR protocol, visibility through penumbra was improved. Conclusions. Application of O-MAR may provide a complementary strategy to those already in use to obtain diagnostically useful MRI examinations in the presence of a CI, especially in case of skull base diseases requiring MRI monitoring.
text/xml
1827-675X
2434
en
Marco Benazzo
http://creativecommons.org/licenses/by-nc-nd/4.0
ACTA Otorhinolaryngologica Italica
O-MAR
The usefulness of the O-MAR algorithm in MRI skull base assessment to manage cochlear implant-related artifacts
Text.Serial.Journal
https://www.actaitalica.it/
43
Skull base surgery
2023-07-24
2022-12-16
2023-07-25
2023-07-25
273-282
1
Marco Benazzo
2023/07/24
10.14639/0392-100X-N2434
273
Department of Clinical, Surgical, Diagnostic and Pediatric Sciences, University of Pavia, Pavia, Italy; Department of Otorhinolaryngology, University of Pavia, Fondazione IRCCS Policlinico San Matteo, Pavia, Italy
1827-675X
O-MAR
en
282
The usefulness of the O-MAR algorithm in MRI skull base assessment to manage cochlear implant-related artifacts
https://www.actaitalica.it/article/download/2434/1016
43
ACTA OTORHINOLARYNGOLOGICA ITALICA 2023;43:273-282; doi: 10.14639/0392-100X-N2434




Skull base surgery


The usefulness of the O-MAR algorithm in MRI skull
base assessment to manage cochlear
implant-related artifacts
L’utilità dell’algoritmo O-MAR nello studio del basicranio in risonanza magnetica
per la riduzione degli artefatti da impianto cocleare
Pietro Canzi1,2, Elena Carlotto2, Anna Simoncelli3, Elvis Lafe3, Andrea Scribante1,4, Domenico Minervini2, Matteo Nardo2,
Stefano Malpede2, Luisa Chiapparini3, Marco Benazzo1,2
1
 Department of Clinical, Surgical, Diagnostic and Pediatric Sciences, University of Pavia, Pavia, Italy; 2 Department
of Otorhinolaryngology, University of Pavia, Fondazione IRCCS Policlinico San Matteo, Pavia, Italy; 3 Department
of Diagnostic Radiology and Interventional Radiology and Neuroradiology, University of Pavia, Fondazione IRCCS Policlinico
San Matteo, Pavia, Italy; 4 Unit of Orthodontics and Pediatric Dentistry, Section of Dentistry, Department of Clinical, Surgical,
Diagnostic and Pediatric Sciences, University of Pavia, Pavia, Italy


SUMMARY
Objective. To assess artifact size and MRI visibility when applying the “Orthopedic-Metal
Artifact Reduction” (O-MAR) algorithm for cochlear implant (CI) scanning.
Methods. Two volunteers were submitted to 1.5 T MRI with an Ultra 3D CI receiver stimu-
lator placed on their head. Four angular CI orientations were adopted: 90, 120, 135 and 160
degrees. Volunteers were scanned in each condition using T1w and T2w TSE sequences,
as well as O-MAR sequences, in both axial and coronal planes. Quantitative comparisons
were made of signal void and penumbra extent. Additionally, qualitative evaluations of
global image quality, MRI readability with respect to 12 anatomical structures and visibility
through the penumbra were undertaken.
Results. After application of the O-MAR protocol, the radius of the signal void reduced
from 50.76 mm to 45.43 mm and from 49.22 mm to 40.15 mm on T1w and T2w TSE                         Received: December 16, 2022
                                                                                                   Accepted: February 2, 2023
axial sequences, respectively (p < 0.05). Qualitatively, sequences acquired with O-MAR
produced better outcomes in terms of image quality and anatomical depiction. Despite the
area of the penumbra being increased for the O-MAR protocol, visibility through penumbra           Correspondence
                                                                                                   Elena Carlotto
was improved.
                                                                                                   Department of Otorhinolaryngology, University
Conclusions. Application of O-MAR may provide a complementary strategy to those al-                of Pavia, Fondazione IRCCS Policlinico
ready in use to obtain diagnostically useful MRI examinations in the presence of a CI,             San Matteo, viale Camillo Golgi 19, 27100 Pavia,
especially in case of skull base diseases requiring MRI monitoring.                                Italy
                                                                                                   E-mail: e.carlotto.ec@gmail.com
KEY WORDS: cochlear implants, magnetic resonance imaging, artifacts, metal artifact
reduction sequences, O-MAR
                                                                                                     How to cite this article: Canzi P, Carlotto E,
                                                                                                     Simoncelli A, et al. The usefulness of the O-
RIASSUNTO                                                                                            MAR algorithm in MRI skull base assessment to
Obiettivo. Indagare gli effetti dell’applicazione dell’algoritmo “Orthopedic-Metal Artifact          manage cochlear implant-related artifacts. Acta
Reduction” (O-MAR) sulla qualità delle immagini e sulle dimensioni dell’artefatto indotto            Otorhinolaryngol Ital 2023;43:273-282. https://
dall’interazione tra impianto cocleare (IC) e RM.                                                    doi.org/10.14639/0392-100X-N2434
Metodi. Due volontari sono stati sottoposti a simulazioni in RM 1.5 T con il ricevitore sti-
molatore di un IC HiRes™ Ultra 3D posizionato in 4 angolazioni (90°, 120°, 135° e 160°).           © Società Italiana di Otorinolaringoiatria
Ciascun volontario è stato sottoposto a scansioni con sequenze tradizionali e O-MAR T1             e Chirurgia Cervico-Facciale
e T2, in proiezioni assiali e coronali. Sono state calcolate le differenze in dimensione di
artefatto e penombra tra sequenze non O-MAR e O-MAR. Sono stati indagati i vantaggi                               OPEN ACCESS
dell’applicazione di O-MAR in termini di qualità delle immagini considerando 12 strutture          This is an open access article distributed in accordance with
anatomiche rispetto ai 4 posizionamenti dell’IC.                                                   the CC-BY-NC-ND (Creative Commons Attribution-Non-
Risultati. Con l’applicazione di O-MAR il raggio dell’artefatto si è ridotto rispettivamente da    Commercial-NoDerivatives 4.0 International) license. The
                                                                                                   article can be used by giving appropriate credit and mentio-
50,76 mm a 45,43 mm nelle sequenze T1 e da 49,22 mm a 40,15 mm nelle T2 TSA assiali                ning the license, but only for non-commercial purposes and
(p < 0,05). È emerso un significativo miglioramento della qualità globale delle immagini, della    only in the original version. For further information: https://
visualizzazione di specifiche strutture intracraniche e della visibilità attraverso la penombra.   creativecommons.org/licenses/by-nc-nd/4.0/deed.en




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P. Canzi et al.




Conclusioni. L’algoritmo O-MAR può essere adottato in associazione ad altre strategie di riduzione degli artefatti per le indagini RM nei
portatori di IC, in particolare in caso di patologie del basicranio che richiedano monitoraggio in RM.
PAROLE CHIAVE: impianti cocleari, risonanza magnetica, artefatti, sequenze di riduzione dell’artefatto metallico, O-MAR



Introduction                                                          male volunteers who provided written informed consent.
                                                                      The investigation was performed in the radiology depart-
Historically, access to magnetic resonance imaging (MRI)
                                                                      ment of our tertiary otologic referral centre after obtain-
has been limited in cochlear implant (CI) recipients due to
                                                                      ing approval from our institutional review board and the
the interaction of the external magnetic field with the CI
                                                                      medical ethics committee. An Advanced Bionics (AB)
magnet and the metallic components of implants. In the last
                                                                      HiRes™ Ultra 3D CIs with Slim J electrode array was
decades, advances in CI technology have improved MRI
                                                                      supplied for research purposes. For this investigation, a
safety and have overcome complications such as magnet
                                                                      thin single-layer medical gauze was tied to the volun-
displacement, demagnetisation and patient discomfort 1.
                                                                      teers’ head. The implant package, with its magnet system
The novelties in MRI usability in CI recipients have led
                                                                      in place, was then placed onto the head of each volunteer
to the broadening of CI selection criteria to patients with
                                                                      and held in place by a medical patch. No bandage or poly-
skull base and intracranial pathologies which require MRI
                                                                      ethene block were used to restrain the implant package.
monitoring. The current literature has demonstrated grow-
                                                                      The CI was evaluated for different positions, each defined
ing interest about CI rehabilitation in skull base tumours,
                                                                      by a nasion-outer ear canal angle. Four angular orienta-
such as vestibular schwannoma (both sporadic and in NF2
                                                                      tions were adopted: 90, 120, 135 and 160 degrees. The
patients), where the aim to rehabilitate hearing needs to
                                                                      distance from the centre of the magnet to the outer ear
take into consideration the issue of artifacts in MRI follow-
                                                                      canal was 9 cm. For each of the four CI angular positions,
up 2. In this respect, the presence of a CI may reduce the
                                                                      both volunteers were scanned with the different MRI se-
diagnostic power of MRI in detecting disease evolution or
                                                                      quences.
recurrences. Image artifacts occur in the vicinity of the CI’s
metallic components due to their magnetic susceptibility
                                                                      Imaging study protocol
leading to their magnetisation when exposed to the MR en-
vironment. In these conditions, the devices exert their own           All imaging was performed using the Ingenia™ (Philips
magnetic fields, distorting the external magnetic field. This         Medical Systems, Best, Netherlands) 1.5 T MRI scanner.
results in signal void areas and distortion signals, which            The examination protocol was chosen in accordance with
cause concerns especially when attempting visualisation of            our institutional protocol (without gadolinium) for the inves-
near-by brain structures 3. Among the strategies employed             tigation of brain pathologies. The MRI assessment involved
in the endeavour to improve MRI image quality, tailored               planar T1 and T2 weighted (w) turbo spin echo sequences
positioning of the CI in relation to the anatomic site to be          (TSE) with and without the application of O-MAR 6. De-
explored has been described 3. In addition to this, the use of        tailed MRI scanning parameters are reported in Table I.
metal artifact reduction sequences (MARS) has been pro-
posed. MARS is a general term, which refers to techniques             Imaging analyses
intended to reduce the size and intensity of artifacts gener-         In line with the recent study of Amin et al. 7, we analysed
ated around metal components 4. Dedicated MR protocols                two specific features of the artefact: the signal void and the
have been developed especially in the orthopaedic field,              ‘‘penumbra.’’ The signal void was identified as the com-
with the aim of improving the image quality for patients              plete obscuration of image, whereas the penumbra was
with metallic implants 5. The application of these advanced           defined as the area of partial visibility. The evaluation in-
techniques in CI recipients represents an emerging research           cluded both quantitative and qualitative analyses.
topic, in which any potential benefit needs to be investi-
gated. The current article aims to evaluate the effect of the         Quantitative analysis
Philips’ “Orthopedic-Metal Artifact Reduction” (O-MAR)                The signal void and penumbra sizes were measured for
algorithm on cochlear implant MRI artifact in terms of ar-            each sequence in the axial plane, using the IntelliSpace™
tifact size and MRI visibility.                                       Portal (Philips Medical Systems, Best, Netherlands) certi-
                                                                      fied reporting station. The maximum signal void radius was
                                                                      calculated by matching a circle to the visible edges of the
Materials and methods                                                 complete obscuration within the brain. This was accord-
We undertook a preclinical study on two healthy adult                 ing to the method previously described by Canzi et al. 8.



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Table I. MRI scanning parameters in different sequences of acquisition.
 Sequence                                            Repetition time                   Echo time         Slice thickness       Field of view     Acquisition time
                                                           (ms)                          (ms)                 (mm)                 (mm2)              (min)
 Axial T1w TSE                                                     474                        14               3                29.4 x 19             2.02
 Axial T1w TSE                                                     465                        14               3                29.4 x 19             2.02
 O-MAR
 Coronal T1w TSE                                                   579                        14               3                29.4 x 19             3.10
 Coronal T1w TSE                                                   526                        14               3                29.4 x 19             3.10
 O-MAR
 Axial T2w TSE                                                    4000                        95               3                27.9 x 18             2.06
 Axial T2w TSE                                                    4000                        95               3                27.9 x 18             2.06
 O-MAR
 Coronal T2w TSE                                                  4000                        95               3                27.9 x 18             3.20
 Coronal T2w TSE                                                  4000                        95               3                27.9 x 18             3.20
 O-MAR
TSE: turbo spin echo sequences; ms: milliseconds; mm: milimetre; w: weighted; min: minute.




For measurement of the penumbra, a circle was matched                                        c. How does the anatomical visibility change when O-
to the visible edges of the partial visibility area. The radius                                 MAR is used for each CI position?
of the signal void circle was subtracted from the radius of                                  d. Does the use of O-MAR change the global visibility
the penumbra circle to calculate the extent, or width, of the                                   through the penumbra?
penumbra. The assessment of signal void radius and pe-                                       For assessment of penumbra, we adopted the scoring pro-
numbra widths were repeated for each CI position in both                                     tocol introduced by Amin et al. 7: 1) severe obscuration; 2)
of the volunteers. Mean sizes and standard deviations (SD)                                   moderate obscuration; 3) mild obscuration; 4) no obscura-
of the signal void and the penumbra were calculated.                                         tion. This evaluation was repeated for each sequence and
                                                                                             CI position in both the volunteers comparing traditional
Qualitative analysis                                                                         and O-MAR sequences.
The MRI scans were submitted to two experienced neu-
roradiologists and one experienced otoneurosurgeon who                                       Statistical analysis
independently evaluated them. A 4-point scale (0 = com-                                      Statistical analysis was performed with R software (R version
pletely unusable, 1 = visible but not suitable for diagnos-                                  3.1.3, R Development Core Team, R Foundation for Statis-
tic purposes due to artifact, 2 = obscured by artifact but                                   tical Computing, Wien, Austria). Descriptive statistics, in-
adequate for diagnostic purposes, 3 = high-quality view                                      cluding the mean, standard deviation, median, minimum and
of the anatomic structure) was adopted to describe the                                       maximum values, were calculated for all measures. The Sha-
following ipsilateral brain structures, with respect to the                                  piro-Wilk test was applied to assess for data normality. The
CI side: hypophysis, cochlea, semi-circular canals, ves-                                     Analysis of Variance (ANOVA) test was used to determine
tibulum, internal auditory canal, cerebellopontine angle,                                    whether significant differences existed among the measures.
brainstem, anterior part of the cerebellum, posterior part                                   The Tukey test was used as a post hoc measure. Significance
of the cerebellum, middle cerebellar pedunculus, cerebel-                                    for all statistical tests was predetermined at p < 0.05. Inter-
lar vermis and the occipital lobe. When unpaired median                                      rater reliability was calculated with Fleiss’ kappa.
structures were described (e.g., hypophysis, brainstem,
cerebellar vermis), the ipsilateral side of each structure
was evaluated. The assessment was repeated for each
                                                                                             Results
MRI sequence across the two volunteers. Finally, MRI                                         Quantitative analysis
findings were analysed in order to investigate the follow-                                   Round-shaped artifacts were found in both traditional and
ing questions:                                                                               O-MAR sequences acquired for axial planes. Employ-
a. Does use of the O-MAR protocol improve the global im-                                     ment of the O-MAR sequence significantly reduced the
   age quality?                                                                              extent of the signal void area (Fig. 1). The maximum sig-
b. What are the benefits of the O-MAR protocol for each                                      nal void radius was 50.76 mm (SD 2.33) and 49.22 mm
   MRI sequence?                                                                             (SD 7.38) on T1w and T2w TSE axial sequences, respec-



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P. Canzi et al.




                                                                    A                                                              B

Figure 1. Assessment of signal voids. (A) Axial T1w TSE sequence without O-MAR, (B) with O-MAR.



tively. After application of the O-MAR protocol, the ra-                     of the penumbra increased significantly from 8.73 mm
dius of the signal void reduced significantly to 45.43 mm                    (SD 5.3) to 11.43 mm (SD 4.79) after application of O-
(SD 2.18) and 40.15 mm (SD 4.86) on T1w and T2w TSE                          MAR (p < 0.05).
axial sequences, respectively (p < 0.05). On the other
hand, the size of the penumbra increased after activation                    Qualitative analysis
of the O-MAR protocol (Fig. 2). The penumbra width was                       a. Does the use of the O-MAR protocol improve the global
10.52 mm (SD 3.24) and 13.3 mm (SD 3.77) on T1w TSE                             image quality?
sequences acquired without and with O-MAR, respec-                              When global image quality with and without application
tively (p = 0.05). For planar T2w TSE sequences, the size                       of the O-MAR protocol was compared, O-MAR sequenc-




                                                                     A                                                             B

Figure 2. Assessment of penumbra.(A) Axial T1w TSE sequence without O-MAR. (B) with O-MAR; blue arrow: width of penumbra.



276
                                                                                                                MRI-induced artifacts by CI: usefulness of O-MAR




   es resulted in significantly better qualitative outcomes:                             and with O-MAR, respectively, (p < 0.0001). For T2w
   mean image quality score was 2.64 (SD 0.93) with O-                                   sequences, global mean scores were 2.51 (SD 0.88)
   MAR vs 2.44 (SD 0.76) without O-MAR, (p < 0.0001).                                    without O-MAR and 2.70 (SD 0.70) with O-MAR,
b. What are the benefits of the O-MAR protocol according                                 (p < 0.0001).
   to each MRI sequence?                                                                 The sequences acquired in the coronal plane with O-
   When planar T1w and T2w TSE sequences were glob-                                      MAR had better quality compared to those acquired in
   ally evaluated, application of the O-MAR protocol sig-                                the axial plane. This applied to both T1w and T2w ac-
   nificantly improved visibility for both T1w and T2w.                                  quisitions (Fig. 3).
   In the case of T1w sequences, the mean image quality                                  In addition, the greatest improvement of image for O-
   scores were 2.37 (SD 0.98) and 2.57 (SD 0.80) without                                 MAR over traditional sequences was reported for acqui-




  A                                                                                        B


  C                                                                                        D




Figure 3. Image quality. (A) mean sequence-specific scores and confidence intervals referred to T1w TSE images (dark bar refers to traditional sequences, light bar to
O-MAR sequences, the mean score referring to O-MAR sequences is surrounded by a rectangle). (B) mean sequence-specific scores and confidence intervals referred
to T2w TSE images (dark bar refers to traditional sequences, light bar to O-MAR sequences, the mean score referring to O-MAR sequences is surrounded by a rectan-
gle). (C) axial T1w TSE sequences without (c1) and with O-MAR (c2) and coronal T1w TSE sequences without (c3) and with O-MAR (c4). (D) axial T2w TSE sequences
without (d1) and with O-MAR (d2) and coronal T2w TSE sequences without (d3) and with O-MAR (d4).



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P. Canzi et al.




Table II. Comparison of image quality scores between sequences acquired without and with O-MAR.
 Sequence                                                      T1w TSE        T2w TSE          Axial          Coronal         Axial       Coronal
                                                                                             T1w TSE          T1w TSE       T2w TSE       T2w TSE
 Without O-MAR                                                  2.37           2.51           2.36          2.38           2.53            2.50
 With O-MAR                                                     2.57           2.70           2.52          2.62           2.68            2.71
 Mean difference                                                0.20           0.19           0.16          0.24           0.15            0.21
 P-value                                                      < 0.0001       < 0.0001       < 0.0001      < 0.0001       < 0.0001        < 0.0001
w: weighted; TSE: Turbo Spin Echo Sequences.




   sitions in coronal planes. Table II shows the difference                     in the coronal plane with the O-MAR returned the best
   in image quality score for sequences acquired with and                       quality outcome in both T1w and T2w acquisitions:
   without O-MAR.                                                               mean 2.36 (SD 0.68) for T1w sequences and mean 2.61
c. How does anatomical visibility change with the use of                        (SD 0.55) for T2w. The greatest advantages for penum-
   O-MAR for each CI position?                                                  bra visibility with O-MAR were found in coronal acqui-
   When each anatomical structure was analysed for the                          sitions compared to axial scans in both T1w and T2w
   four different CI positions, visibility was significantly                    sequences. Table III reports the difference in penumbra
   modified (p < 0.05) when the O-MAR protocol was used                         visibility scores in sequences acquired with and with-
   in the following cases:                                                      out O-MAR. Inter-rater reliability agreement ranged
   • 90°: occipital lobe, semicircular canals, anterior part                    from 0.71 to 0.74 across all image evaluations and was
      of cerebellum, posterior part of cerebellum (Fig. 4a);                    consistent with a “high positive correlation” among the
   • 120°: occipital lobe, semicircular canals, vestibulum,                     three investigators.
      anterior part of cerebellum, posterior part of cerebel-
      lum, cerebellar vermis (Fig. 4b);                                     Discussion
   • 135°: occipital lobe, semicircular canals, anterior part
      of cerebellum, posterior part of cerebellum, cerebellar               A deterioration of MRI legibility in the presence of mag-
      vermis (Fig. 4c);                                                     netic and metallic CI components occurs due to a combi-
   • 160°: occipital lobe, semicircular canals, vestibulum,                 nation of signal voids, image distortion, and signal inho-
      brainstem, anterior part of cerebellum, cerebellar ver-               mogeneity 9. Despite the recent introduction of adaptive
      mis, middle cerebellar peduncle (Fig. 4d).                            CI magnet systems, designed to reduce the limits of MR
   Inter-rater reliability agreement ranged from 0.75 to                    compatibility, the artifact size generated in presence of
   0.92 across all image evaluations and was consistent                     different magnetic systems has been found to be simi-
   with a “high positive correlation” among the three in-                   lar 10. Several strategies are commonly used to reduce the
   vestigators.                                                             severity of artifacts 8,11-15, including MRI algorithm ma-
d. Does the use of O-MAR change the global visibility                       nipulation 8,14,15. Simple concessions, aiming to improve
   through the penumbra?                                                    image quality, have been traditionally adopted in MR
   The use of O-MAR significantly improved visibility                       acquisition techniques. These include: reduction of field
   through the penumbra (p < 0.0001), producing mean                        strength, decreasing slice thickness, using fast imaging
   scores of 1.29 (SD 0.51) and 2.17 (SD 0.74) without and                  sequencies, adoption of fast spin echo sequences instead
   with O-MAR, respectively. Planar sequences acquired                      of gradient echo sequences, increasing the frequency en-


Table III. Comparison of penumbra visibility scores between sequences acquired without and with O-MAR.
 Sequence                                               T1w TSE       T2w TSE           Axial       Coronal         Axial             Coronal
                                                                                     T1w TSE        T1w TSE       T2w TSE             T2w TSE
 Without O-MAR                                         1.15           1.43           1.14           1.17            1.30                1.56
 With O-MAR                                            2.10           2.26           1.83           2.36            1.92                2.61
 Mean difference                                       0.95           0.83           0.69           1.19            0.62                1.05
 P-value                                             < 0.0001       < 0.0001       < 0.0001       < 0.0001        < 0.0001            < 0.0001
w: weighted; TSE: Turbo Spin Echo Sequences.




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                                                                                                              MRI-induced artifacts by CI: usefulness of O-MAR




   A                                                                                 B




   C                                                                                 D

Figure 4. Visual assessment of intracranial structures for: (A) 90° CI positioning. (B) 120° CI positioning. (C) 135° CI positioning. (D) 160° CI positioning. Dark
bars refer to traditional sequences, light bars to O-MAR sequences. Image quality was evaluated merging data from both T1w and T2w sequences.



coding bandwidth or orienting the long axis of the met-                             models, matching a standard T1w sequence with a gado-
al along the frequency encoding direction 4,16. In recent                           linium and SEMAC-VAT sequence. They demonstrated
years, reduction of metallic artefacts has been attempted                           that the application of SEMAC-VAT improved the visibil-
with the introduction of dedicated MRI protocols. MARS                              ity of the ipsilateral hemisphere 7. In contrast, Canzi et al.
have been developed and applied predominantly in the                                conducted an investigation on head specimens, reporting
orthopaedic and neurosurgical fields, demonstrating their                           the improvement of global image quality score in planar
clinical superiority for the depiction of periprosthetic tis-                       axial T1w and T2w TSE sequences with the application
sues 17-19 compared to conventional imaging sequences.                              of the O-MAR protocol 8. However, the visibility scores
On the other hand, to the best of our knowledge, the ap-                            referred to each anatomic structure were not analytically
plication of these advanced techniques for artifact reduc-                          compared between traditional and O-MAR sequences.
tion in MRI with CI have been reported only twice in                                In addition, only traditional surgical CI positioning was
the literature 7,8. This is probably due to new CI magnet                           evaluated.
systems that have overcome the historical CI-MR incom-                              In the current study, the impact of O-MAR on MR image
patibility only recently. Amin et al. conducted a clinical                          quality was investigated according to different sequences
study on 8 CI recipients implanted with different device                            and acquisition planes, considering 12 intracranial struc-



                                                                                                                                                             279
P. Canzi et al.




tures and in relation to 4 different angular CI positions.       We strongly believe the combination of customised CI
The results of Canzi et al. 8 in their ex-vivo study were        placement with MARS application would have significant
confirmed: a significant reduction in artifact size on both      practical implications in challenging cases. For example,
T1w and T2w TSE sequences was measured after activa-             specific CI positionings have been demonstrated to allow
tion of O-MAR protocol. Moreover, the current qualita-           the MRI postsurgical follow up after vestibular schwan-
tive findings appear to corroborate the encouraging data in      noma resection and cochlear implantation 2,13. In our opin-
the published literature, demonstrating superiority for ar-      ion the employment of O-MAR can represent an additional
tifact reduction techniques in the depiction of intracranial     tool for tumour follow-up. In accordance with Amin et al. 7,
structures over traditional sequences. To our knowledge,         we introduced the evaluation of penumbra visibility in or-
this is the first study in CI recipients in which the artifact   der to emphasise this aspect of the artifact. Our findings
reduction protocol has been applied for different planes         confirmed the trend already reported by Amin et al. 7, who
of acquisition. The scans acquired in the coronal plane          demonstrated that the application of MARS induced an in-
with the application of O-MAR showed the best quality            crease in penumbra size but also an improvement in vis-
outcome both for T1w and T2w acquisitions. This is in            ibility through it.
alignment with some clinical studies where traditional se-       On the other hand, a few remarks about the application of
quences reported better visibility in coronal plane views        the MARS in clinical practice deserve to be discussed. First
compared to axial ones 7,13,19. Our image quality analysis       of all, nowadays this technology is widely adopted espe-
concentrated on 12 intracranial structures, which were           cially in the orthopaedic and neurosurgical fields, although
chosen by summarising the data from earlier studies 7,8,         it may not be available in every clinical setting. Secondly,
concentrating on the posterior cranial fossa and on the an-      MARS are only applicable to certain sequences. For ex-
atomic structures of major otoneurosurgical interest. The        ample, the O-MAR protocol can be used with T1w, T2w,
improved visualisation of ipsilateral structures we found        PDw and short tau inversion recovery (STIR) contrast 6.
with the application of O-MAR is consistent with the data        However, this algorithm cannot be employed in the con-
of Amin et al. 7. However, the number of anatomic ar-            text of 3D fast spin echo, steady state free procession or
eas considered in the study of Amin et al. was limited:          diffusion weighted sequences, which are commonly used
internal auditory canal, cerebellopontine angle, cerebel-        in otological field. Third, some MARS require additional
lar hemisphere and brainstem. Additionally, our analysis         scan time, representing a limit also in consideration of
was provided by three independent observers with high            “conditional” MR safety recommendations made by cer-
positive correlation between them, whereas in the study          tain CI manufacturers 21. However, this does not apply for
of Amin et al. 7 the analysis had been conducted by con-         all the existing artifact reduction protocols. For example,
sensus.                                                          the O-MAR method does not increase scan time. Fourth,
As a further novelty, we examined the advantages of              the commercial costs of this technology have to be con-
MARS in visualisation of intracranial structures with ref-       sidered, although it should be noted that the application of
erence to different CI positioning. According with a re-         these algorithms is not limited to CI. Moreover, we might
cent study of Canzi et al. 20 focused on traditional MRI se-     speculate the application of MARS could prevent in select-
quences, four rotational CI orientations were investigated.      ed cases the additional costs of a surgical procedure for CI
That work demonstrated, in accordance with literature 13,        magnet removal.
that the surgical location of the magnet represents a key        Considering the present study, some limitations should
point for the visual assessment of intracranial structures.      be kept in mind. The investigations concerned only adult
Moreover, the present study points out that the use of ad-       males, and other skull sizes were not investigated. In chil-
vanced artifact reduction algorithms could make a further        dren, or women with smaller heads, a bigger artifact with
contribution to artifact reduction. More specifically, the       reduced visibility of the intracranial structures might be
occipital lobe, the semicircular canals and the anterior         expected. Furthermore, these results relate to the sole
part of cerebellum had their visibility significantly im-        magnetic field strength of 1.5T. In addition to this, our
proved for all CI orientations with the application of O-        analyses are limited to a single CI model and to only one
MAR. Interestingly, the highest number of structures for         MARS.
which visibility was improved with O-MAR was for the
160° orientation. This can be explained by the fact the
brain structures we analysed mainly belonged to the pos-
                                                                 Conclusions
terior cranial fossa, which had been reported to have the        Preliminary evidence about the adoption of metal-artifact
worse visibility at a 160° CI position 20.                       reduction algorithms when conducting MRI imaging with



280
                                                                                               MRI-induced artifacts by CI: usefulness of O-MAR




a CI are encouraging. However, confirmation from further              2    Sudhoff H, Gehl HB, Scholtz LU, et al. MRI observation after in-
studies is mandatory to corroborate the existing data. In                  tralabyrinthine and vestibular schwannoma resection and cochlear
                                                                           implantation. Front Neurol 2020;11:759. https://doi.org/10.3389/
addition, it can be expected that MARS would be effec-                     fneur.2020.00759
tive not only with CI, but also in the presence of other              3    Canzi P, Luzzi S, Carlotto E, et al. Customized cochlear implant
neural implants. We believe these protocols may represent                  positioning in a patient with a low-grade glioma: towards the best
an additional strategy to be combined with others already                  MRI artifact management. Otol Neurotol 2022;43:E628-E634.
in use, especially with reference to MRI surveillance of                   https://doi.org/10.1097/MAO.0000000000003532
the main neurological diseases in neural implants recipi-             4    Jungmann PM, Agten CA, Pfirrmann CW, et al. Advances in MRI
ents. In our view, the application of MARS could make a                    around metal. J Magn Reson Imaging 2017;46:972-991. https://
                                                                           doi.org/10.1002/jmri.25708
difference in particular circumstances to obtain diagnosti-
                                                                      5    Khodarahmi I, Isaac A, Fishman EK, et al. Metal about the hip
cally useful information with respect to specific structures
                                                                           and artifact reduction techniques: from basic concepts to ad-
or pathologies in cerebral MRI.                                            vanced imaging. Semin Musculoskelet Radiol 2019;23:E68-E81.
                                                                           https://doi.org/ 10.1055/s-0039-1687898
Conf﻿lict of interest statement                                       6    Hey S, Hoogenraad D, Elanchezhian V, et al. Orthopedic metal
The authors state that this work has received funding from                 artifact reduction. Distortion correction in the presence of a ortho-
Advanced Bionics AG [Advanced Bionics, Stäfa, Switzer-                     pedic implant (https://philipsproductcontent.blob.core.windows.
                                                                           net/assets/20180315/747912f2871a472da8eba8a400e4a95d.pdf,
land] in the amount of 17,000€. Additionally, the authors
                                                                           accessed on 20/4/2022).
confirm that the funders had no role in our study design,             7    Amin N, Pai I, Touska P, et al. Utilization of SEMAC-VAT MRI
data acquisition, analysis and interpretation, and writing of              for improved visualization of posterior fossa structures in patients
the manuscript.                                                            with cochlear implants. Otol Neurotol 2021;42:E451-E458. htt-
This does not alter the authors’ adherence to Acta Otorhi-                 ps://doi.org/10.1097/MAO.0000000000003016
nolaryngologica Italica policies on sharing data and materials.       8    Canzi P, Aprile F, Simoncelli A, et al. MRI-induced artifact by a
                                                                           cochlear implant with a novel magnet system: an experimental
Funding                                                                    cadaver study. Eur Arch Otorhinolaryngol 2021;278:3753-3762.
                                                                           https://doi.org/10.1007/s00405-020-06464-z
The authors thank Advance Bionics AG (Stäfa, Switzer-                 9    Edmonson HA, Carlson ML, Patton AC, et al. MR imaging and
land) for loan of the cochlear implant and funding the                     cochlear implants with retained internal magnets: reducing arti-
study. Additionally, the authors confirm that the funders                  facts near highly inhomogeneous magnetic fields. Radiographics
had no role in their study design, data acquisition, analysis              2018;38:94‑106. https://doi.org/10.1148/rg.2018170135
and interpretation, and writing of the manuscript.                    10   Todt I, Guerkov R, Gehl HB, et al. Comparison of cochlear
                                                                           implant magnets and their MRI artifact size. Biomed Res Int
Author contributions                                                       2020;2020:5086291. https://doi.org/10.1155/2020/5086291
CP: conceptualisation; CP, CE, SA, LE, SA, MD, NM, MS:
                                                                      11   Wackym PA, Michel MA, Prost RW, et al. Effect of magnetic
                                                                           resonance imaging on internal magnet strength in Med-El Combi
material preparation, data collection and analysis; BM, CL:                40+ cochlear implants. Laryngoscope 2004;114:1355-1361. htt-
supervision; CP, CE: writing. All authors have read and                    ps://doi.org/10.1097/00005537-200408000-00007
agreed to the final version of the manuscript.                        12   Wagner F, Wimmer W, Leidolt L, et al. Significant artifact re-
                                                                           duction at 1.5T and 3T MRI by the use of a cochlear implant
Ethical consideration                                                      with removable magnet: an experimental human cadaver study.
This study was approved by the Institutional Ethics Com-                   PLoS One 2015;10:E0132483. https://doi.org/10.1371/journal.
                                                                           pone.0132483
mittee of the IRCCS Policlinico San Matteo, Pavia (IRB#               13   Todt I, Rademacher G, Mittmann P, et al. MRI artifacts and coch-
20210027855). The research was conducted ethically,
                                                                           lear implant positioning at 3 T in vivo. Otol Neurotol 2015;36:972-
with all study procedures being performed in accordance                    976. https://doi.org/10.1097/MAO.0000000000000720
with the requirements of the World Medical Association’s              14   Sharon JD, Northcutt BG, Aygun N, et al. Magnetic resonance im-
Declaration of Helsinki. Written informed consent was ob-                  aging at 1.5 tesla with a cochlear implant magnet in place: image
tained from each volunteer for study participation and data                quality and usability. Otol Neurotol 2016;37:1284-1290. https://
publication.                                                               doi.org/10.1097/MAO.0000000000001176
                                                                      15   Majdani O, Rau TS, Götz F, et al. Artifacts caused by cochlear im-
                                                                           plants with non-removable magnets in 3T MRI: phantom and ca-
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