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The success of cochlear implants is contingent on a functioning auditory nerve. An accurate non-invasive method of screening cochlear implant candidates for quantitative measurement of auditory nerve viability would allow physicians to better determine the likelihood for success of the procedure. Previous studies have indicated a relationship between auditory nerve diameters and their functionality. In order to investigate this finding, we made morphological measurements of the auditory and facial nerves and correlated these measurements with audiologic test results. In addition, we developed a technique to segment a portion of the nerves with minimal user interaction. The study included 11 cochlear implant candidates. Non-invasive high-resolution bilateral MR images were acquired from 3T and 1.5T scanners using either CISS or Fast Spin Echo sequences. The images were processed with an anisotropic diffusion filter to enhance the edges of the nerves. Segmentation involved morphological processing of the original filtered image to produce a related binary image, which was then subtracted from the original image at a suitable threshold to isolate the auditory and facial nerves from other structures in the internal auditory canal. The volumes of the auditory nerve and the facial nerve were computed from the five best continuously segmented saggital slices, and the corresponding ratios were then determined. Preliminary analysis of the segmentation process suggests that this method is most effective on images acquired using the CISS sequence. Correlation of the measurements of the subjects to the findings of collaborating audiologists was carried out. Preliminary results suggest there is a threshold of ratios below which a value is indicative of a degenerated nerve, and consequently a higher risk for unsuccessful cochlear implant. A semi-automated segmentation technique was developed which allows effective segmentation of multiple slices of MRI data. Since the segmentation and measurement processes require little user interaction, the results are significantly reproducible. In addition to this, volume measurements increase accuracy. This technique requires less than ten minutes for completion of one case by an experienced operator. This is a promising technique that should allow accurate, reproducible and rapid segmentation of the auditory and facial nerves for volume measurements in assessment of nerve viability. The information provided by these measurements can assist physicians in determination before the procedure of the efficacy of a cochlear implant.
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