EVALUATION OF MRI COMPATIBILITY OF THE MODIFIED NUCLEUS MULTICHANNEL AUDITORY BRAIN-STEM AND COCHLEAR IMPLANTS

Magnetic resonance imaging (MRI) has been contraindicated for users of cochlear implants because of the internal magnet and other possible i nteractions. Users of the Nucleus Mini-22 Cochlear Implant (CI) or the experimental Multichannel Auditory Brainstem Implant (ABI) may have o ther disorders that are best diagnosed by MRI. The CI and ABI were mod ified by replacing the internal magnet and integrated circuit lid with nonmagnetic material. Tests were conducted in a 1.5-T MRI machine. Sa fety tests for force, heating, induced current, unintentional implant output, and implant damage were conducted by using various phantom mod els. Image distortion was evaluated in two subjects with implants. The maximum force measured was 2,818 dynes. There was <0.1 degrees C temp erature increase in the vicinity of the implant. The maximum induced c harge was greater than or equal to 667 times less than the minimum cha rge for auditory stimulation. There was no unintentional output during MRI scans and no change in implant function after 10 repeated scans. Image distortion consisted primarily of darkening and was worst in the axial plane, where it extended 1-2 cm medially and inferiorly from th e receiver/stimulator. Compatibility-test results were acceptable, wit h a large margin of safety. Image distortion is limited to darkening i n the immediate vicinity of the implant.