Berit Verbist

Wyger Brink

Since cochlear implantation (CI) has become a well-established tool for auditory rehabilitation in patients with sensorineural hearing loss (SNHL), there has been a growing interest in the detailed imaging assessment of the inner ear. At first, CI was restricted to adults who were postlingually deafened, but growing expertise and the development of specific implant designs have led to the expansion of indications, including children with severely malformed inner ears. In very severe malformations, or auditory nerve dysplasia, auditory brain stem implants can be considered.¹ Hence, careful assessment of the shape and patency of the cochleovestibular system is needed for patient selection and counseling on outcome, operative planning, selection, and further development of electrode designs.

In our study, we explored the benefits of an increased signal-to-noise ratio of MRI at ultra-high magnetic field strength (7T) in patients with SNHL. Comparison with 3T images showed improved visualization of delicate and small-sized inner ear structures and in the internal auditory canal. The use of high-permittivity dielectric pads² with gender-specific geometric designs, optimized for the inner ear, was essential to obtain high-quality images in this challenging imaging region. Without these pads, severe inhomogeneities in the transmit radiofrequency (B1+) field generally induce nonuniform tissue contrast and signal intensity, rendering nondiagnostic images. These B1+ inhomogeneities result from wave interference effects due to the shortened wavelength of the radiofrequency field in tissue at a high-static magnetic field. The pads correct this B1+ distribution without increasing the specific absorption rate and, thanks to their thin, compact design, enable practical application within the tight-fitting head coils typically used at 7T.

Following this study, we have further developed and applied high-permittivity dielectric pads in other MR imaging applications such as abdominal, musculoskeletal, cardiac, fetal, and neuro applications both at 3T and 7T at our hospital,^3-5 as well as in other collaborating institutes.^6,7

Even though the pads offer a practical means to improve image quality, the application-specific nature of the pad design still impedes broader clinical impact. Our recent research has therefore focused on the development of fast computer simulation tools for automatic pad design.^8,9

By reformulating the mathematical equations that describe the radiofrequency field and applying model compression techniques, we were able to accelerate such simulations more than 1,000-fold.⁹ 

This fast formulation of the problem enabled optimization techniques to automatically generate the optimal pad size, composition, and placement for 3T and 7T applications using a user-friendly graphic interface. A stand-alone version of this software has recently been disseminated to the MR imaging community.¹⁰

The improved representation of inner ear structures at 7T continues to be a subject of our ongoing research in patients with CI. We are studying the influence of variability in size and shape on surgical and audiologic outcomes¹¹ and plan to achieve individualized postoperative assessments by registering postoperative CT with 7T MRI. In addition, we have started to investigate the role of 7T in patients with vertigo and Ménière disease. The advent of robust MR imaging techniques at higher spatial resolutions is anticipated to offer superior anatomic information, enhancing our understanding of both congenital and acquired inner ear diseases.

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References

1. Vesseur A, Free R, Snels C, et al. Hearing restoration in cochlear nerve deficiency: the choice between cochlear implant or auditory brainstem implant, a meta-analysis. Otol Neurotol 2018;39:428–37, 10.1097/MAO.0000000000001727.
2. Haines K, Smith NB, Webb AG. New high dielectric constant materials for tailoring the B1+ distribution at high magnetic fields. J Magn Res 2010;203:323–27, 10.1016/j.jmr.2010.01.003.
3. Brink WM, van den Brink JS, Webb AG, et al. The effect of high-permittivity pads on specific absorption rate in radiofrequency-shimmed dual-transmit cardiovascular magnetic resonance at 3T. J Cardiovasc Magn Reson 2015;17:82, 10.1186/s12968-015-0188-z.
4. Brink WM, Versluis MJ, Peeters JM, et al. Passive radiofrequency shimming in the thighs at 3 Tesla using high permittivity materials and body coil receive uniformity correction. Magn Reson Med 2016;76:1951–56, 10.1002/mrm.26070.
5. O'Reilly TP, Webb AG, Brink WM. Practical improvements in the design of high permittivity pads for dielectric shimming in neuroimaging at 7T. J Magn Reson 2016;270:108–14, 10.1016/j.jmr.2016.07.003.
6. Manoliu A, Spinner G, Wyss M, et al. Magnetic resonance imaging of the temporomandibular joint at 7.0 T using high-permittivity dielectric pads: a feasibility study. Invest Radiol 2015;50:843–49, 10.1097/RLI.0000000000000196.
7. Kuhn FP, Spinner G, Del Grande F, et al. MR imaging of the temporomandibular joint: comparison between acquisitions at 7.0 T using dielectric pads and 3.0 T. Dentomaxillofac Radiol 2017;46: 20160280, 10.1259/dmfr.20160280.
8. van Gemert JH, Brink WM, Webb AG, et al. An efficient methodology for the analysis of dielectric shimming materials in magnetic resonance imaging. IEEE Trans Med Im 2017;36: 666–73, 10.1109/TMI.2016.2624507.
9. van Gemert JH, Brink WM, Webb AG, et al. High-permittivity pad design for dielectric shimming in magnetic resonance imaging using projection based model reduction and a nonlinear optimization scheme. IEEE Trans Med Im 2018;37:1035–44, 10.1109/TMI.2018.2791179.
10. van Gemert J, Brink WM, Webb AG, et al. High‐permittivity pad design tool for 7T neuroimaging and 3T body imaging. Magn Reson Med 2019;81:3370–78, 10.1002/mrm.27629.
11. van der Jagt AM, Kalkman RK, Jeroen J, et al. Variations in cochlear duct shape revealed on clinical CT images with an automatic tracing method. Sci Rep 2017;7:17566, 10.1038/s41598-017-16126-6.