Secondary Logo

Journal Logo

In Vivo Inner Ear Imaging at 7 T

van der Jagt, Annerie M. A. M.D.; Brink, Wyger M.; Webb, Andrew Ph.D.; Frijns, Johan H. M. M.D., Ph.D.; Verbist, Berit M. M.D., Ph.D.

doi: 10.1097/MAO.0000000000000826
Letters to the Editor
Free

Department of Otorhinolaryngology, Leiden University Medical Center, Leiden, The Netherlands M.A.van_der_Jagt@lumc.nl

Department of Radiology, Leiden University Medical Center, Leiden, The Netherlands

Department of Otorhinolaryngology and Leiden Institute for Brain and Cognition, Leiden University Medical Center, Leiden, The Netherlands

Department of Radiology, Leiden University Medical Center, Leiden and Department of Radiology, Radboud University Medical Center, Nijmegen, The Netherlands

To the Editor:

We read with interest the article entitled “In Vivo Imaging of the Inner Ear at 7 T MRI: Image Evaluation and Comparison with 3 T” from Van Egmond et al. (1).

As referenced by the authors, we have previously reported on the development of an imaging protocol at 7 T for inner ear imaging using high-permittivity dielectric pads. Our research group has studied the importance of geometry, positioning, and contents of these pads to overcome B1 field inhomogeneities present at high field (2,3). Specifically, because of the intrinsic asymmetry of the B1 field, different geometries were needed on either side of the head, and a sex-specific design was needed to account for anatomic differences between males and females (2). Neither of these aspects was discussed in the study of Van Egmond et al. (1). Therefore, we are interested in the design details of the unilaterally applied dielectric pad. Furthermore, we like to emphasize that it is the elliptical shape of the head that leads to the typical B1 inhomogeneities at the location of the inner ear (4) and not the air-bone-fluid interfaces, as mentioned by Van Egmond et al. (1).

In our studies, we successfully applied our protocol in a patient population (n = 13) with detailed evaluation of 24 anatomic structures imaged at 3 T and 7 T, as published in the American Journal of Neuroradiology (5). We found a significant improvement of visualization of 11 anatomic structures scanned at 7 T compared with 3 T. Seven of these structures were also evaluated by Van Egmond et al. (1), but a fair comparison with our results cannot be made because no statistical tests were performed and the resolution between the 7-T protocols differs notably.

One detail of the study by Van Egmond et al. (1) that drew our attention is the marginal increase in in-plane resolution with the transition from 3- to 7-T images. Besides a reduction in slice thickness from 1.0 to 0.5 mm, the in-plane resolution was only increased from 0.5 × 0.6 mm2 to 0.5 × 0.5 mm2. It is not clear whether this marginal increase in resolution has any potential in yielding a more detailed depiction of the inner ear microstructures, as suggested by the authors in the introduction. Indeed, the acquisition time was more than tripled (from 4.32 to 15 min). Especially from a clinical perspective, it can be questioned whether this time cost is justified for a minor gain in diagnostic value, as confirmed also by the authors; no additional anatomic information was obtained in the 7-T images. In contrast, in our study, the acquisition time was also increased, albeit by a much smaller fraction, from 6 to 10 hours; however, this was motivated by an eightfold increase in resolution (0.6–0.3 mm3) and a significant improvement in the visualization of small anatomic structures of the inner ear (5). The choice of imaging resolution in the current study therefore needs more explanation by the authors to justify their approach.

Annerie M. A. van der Jagt, M.D.

Department of Otorhinolaryngology

Leiden University Medical Center

Leiden, The Netherlands

M.A.van_der_Jagt@lumc.nl

Wyger M. Brink

Andrew Webb, Ph.D.

Department of Radiology

Leiden University Medical Center

Leiden, The Netherlands

Johan H. M. Frijns, M.D., Ph.D.

Department of Otorhinolaryngology

and Leiden Institute for Brain and Cognition

Leiden University Medical Center

Leiden, The Netherlands

Berit M. Verbist, M.D., Ph.D.

Department of Radiology

Leiden University Medical Center

Leiden and Department of Radiology

Radboud University Medical Center

Nijmegen, The Netherlands

The authors disclose no conflicts of interest.

This research was financially supported by Advanced Bionics.

Back to Top | Article Outline

REFERENCES

1. Van Egmond SL, Visser F, Pameijer FA, Grolman W. In vivo imaging of the inner ear at 7 T MRI: image evaluation and comparison with 3 T. Otol Neurotol 2015; 36: 687–93.
2. Brink WM, van der Jagt AM, Versluis MJ, Verbist BM, Webb AG. High-permittivity dielectric pads improve high spatial resolution magnetic resonance imaging of the inner ear at 7 T. Invest Radiol 2014; 49: 271–7.
3. Teeuwisse WM, Brink WM, Webb AG. Quantitative assessment of the effects of high-permittivity pads in 7 Tesla MRI of the brain. Magn Reson Med 2012; 67: 1285–93.
4. Sled JG, Pike GB. Standing-wave and RF penetration artifacts caused by elliptic geometry: an electrodynamic analysis of MRI. IEEE Trans Med Imaging 1998; 17: 653–62.
5. Van der Jagt MA, Brink WM, Versluis MJ, et al. Visualization of human inner ear anatomy with high-resolution MR imaging at 7 T: initial clinical assessment. AJNR Am J Neuroradiol 2015; 36: 378–83.
Copyright © 2015 by Otology & Neurotology, Inc. Image copyright © 2010 Wolters Kluwer Health/Anatomical Chart Company