Vestibular Implant Imaging

Abstract

Analogous to hearing restoration via cochlear implants, vestibular function could be restored via vestibular implants that electrically stimulate vestibular nerve branches to encode head motion. This study presents the technical feasibility and first imaging results of CT for vestibular implants in 8 participants of the first-in-human Multichannel Vestibular Implant Early Feasibility Study. Imaging characteristics of 8 participants (3 men, 5 women; median age, 59.5 years; range, 51-66 years) implanted with a Multichannel Vestibular Implant System who underwent a postimplantation multislice CT (n = 2) or flat panel CT (n = 6) are reported. The device comprises 9 platinum electrodes inserted into the ampullae of the 3 semicircular canals and 1 reference electrode inserted in the common crus. Electrode insertion site, positions, length and angle of insertion, and number of artifacts were assessed. Individual electrode contacts were barely discernible in the 2 participants imaged using multislice CT. Electrode and osseous structures were detectable but blurred so that only 12 of the 18 stimulating electrode contacts could be individually identified. Flat panel CT could identify all 10 electrode contacts in all 6 participants. The median reference electrode insertion depth angle was 9° (range, -57.5° to 45°), and the median reference electrode insertion length was 42 mm (range, -21-66 mm). Flat panel CT of vestibular implants produces higher-resolution images with fewer artifacts than multidetector row CT, allowing visualization of individual electrode contacts and quantification of their locations relative to vestibular semicircular canals and ampullae. As multichannel vestibular implant imaging improves, so will our understanding of the relationship between electrode placement and vestibular performance.

© 2021 by American Journal of Neuroradiology.

Figures

FIG 1.

A, The MVI stimulator comprises 3 fixation magnets, an inductive coil link, electrical current stimulator circuitry, a stimulation electrode array, a stimulation reference electrode, and a recording reference electrode. The electrode array includes a 3-electrode shank for the posterior canal (B, E3–E5), a forked subarray with 2 shanks for the horizontal (C, E6–E8) and anterior (C, E9–E11) canals, and a stimulation reference electrode (D). eCAP indicates electrically evoked compound action potential. Reprinted with permission from Labyrinth Devices, LLC, 2019.

FIG 2.

Method for generating MSCT (A and B) and FPCT (C and D) MPR. Two planes are generated. The first plane is approximately tangential to the thin segments of the superior and horizontal SCCs at their junctions with their ampullae and includes the 6 electrode contacts of the forked array inserted into the superior and horizontal ampullae. The second plane is in the posterior plane of the SCC and includes the 3 electrode contacts of the linear array implanted in the posterior canal and the tip of braided platinum/iridium wire inserted into the common crus. Section thickness was set to 2 mm to include all electrode contacts on 1 image for both planes. Window width and contrast level were adjusted as needed to optimize the visibility of electrode contacts. A 3D representation of the vestibular lumen and vestibular nerve is added in transparency (E and F) to help visualize the anatomy.

FIG 3.

Subject: Participant 7. Method for calculating the angle (θ) of the angular insertion depth of the common crus reference electrode. CC indicates common crus of the implanted labyrinth; PE, posterior electrode array; PSCC, posterior semicircular canal; SSCC, superior semicircular canal; RE, reference electrode; Sup, superior; Lat, lateral.

FIG 4.

MSCT (A–D) and FPCT (E–P) multiplanar reconstructions for all participants. A and B, Participant 1 MSCT. C and D, Participant 2 MSCT. E and F, Participant 3 FPCT. G and H, Participant 4 FPCT. I and J, Participant 5 FPCT. K and L, Participant 6 FPCT, M and N, Participant 7 FPCT. O and P, Participant 8 FPCT. In every panel, the top of the image is superior and the left edge of the image is anteromedial. Sep indicates September; Feb, February; Oct, October; Nov, November; Jan, January.

FIG 5.

A, Lateral scout view showing electrode arrays visible through the external auditory canals (yellow arrowhead) and cephalic edges of the infraorbital rims (yellow diamond). Those palpable landmarks define the Reid plane (yellow line) and the plane of horizontal canals (red line). The plane of the horizontal semicircular canals, the standard “axial” plane for temporal bone CT reconstructions, is at a ∼20° pitch from the Reid plane. By supporting the head on a firm wedge to pitch the head forward from supine (and flexing the neck until the Reid plane is pitched ∼20° nose toward chest from Earth vertical), one can minimize scatter artifacts from the stimulator cannister to the inner ear by keeping them on opposite sides of a separation plane perpendicular to the gantry rotation axis (B).