Evaluation of high resolution MRI for preoperative screening for cochlear implantation

The success of a cochlear implant is dependent on a functioning auditory nerve. An accurate non-invasive method for screening cochlear implant patients to help determine viability of the auditory nerve would allow physicians to better predict the success of the operation. In this study we measured the size of the auditory nerve relative to the size of the juxtaposed facial nerve and correlated these measurements with audiologic test results. The study involved 15 patients, and three normal volunteers. Non-invasive high-resolution bilateral MRI images were acquired from both 1.5T and 3T scanners. The images were reformatted to obtain an anatomically referenced oblique plane perpendicular to the auditory nerve. The cross-sectional areas of the auditory and facial nerves were determined in this plane. Assessment of the data is encouraging. The ratios of auditory to facial nerve size in the control subjects are close to the expected value of 1.0. Patient data ratios range from 0.73 to 1.3, with numbers significantly less than 1.0 suggesting auditory nerve atrophy. The acoustic nerve area correlated to audiologic test findings, particularly (R² = 0.68) to the count of words understood from a list of 100 words. These preliminary analyses suggest that a threshold of size may be determined to differentiate functional from nonfunctional auditory nerves.