Anatomy of the facial and vestibulocochlear nerves in the internal auditory canal.

PURPOSE: To define the anatomy of the facial and vestibulocochlear nerves in the internal auditory canal on parasagittal CT scans of cadaveric specimens and to compare this anatomy with findings on in vivo T2-weighted two-dimensional fast spin-echo and three-dimensional turbo spin-echo MR images. METHODS: Thirty-eight formalin-fixed cadaveric temporal bones were examined with 1-mm-thick contiguous parasagittal CT sections to determine the anatomy of the nerves in the internal auditory canal. Ten specimens underwent limited dissection. Fourteen canals in 12 patients were examined with T2-weighted two-dimensional fast spin-echo oblique parasagittal MR imaging and 12 canals in 8 patients were examined with T2-weighted three-dimensional turbo spin-echo MR imaging. The anatomy depicted on MR images was compared with the cadaveric anatomy. RESULTS: On cadaveric specimens, the facial nerve coursed superior and anterior to the vestibulocochlear nerve as a tubular structure throughout the length of the canal. The vestibulocochlear nerve entered the canal as a tubular structure but became crescent shaped in cross section in the middle portion of the canal and separated into individual nerves only in the most lateral portion of the canal. The anatomy of the nerves differed among the specimens. Similar anatomy was demonstrated by MR imaging. CONCLUSION: The ability to define the nerves in the internal auditory canal in the parasagittal plane may provide greater sensitivity and specificity in identifying abnormalities of this anatomic structure.