Establishing normal diameter range of the cochlear and facial nerves with 3D-CISS at 3T

We explored normal diameter values of the cochlear (CN) and facial (FN) nerves on 3-dimensional constructive interference in steady state (3D-CISS) using a 3-tesla magnetic resonance (MR) imaging scanner.

A total of 142 patients underwent MR imaging of the inner ear at 3T. We retrospectively analyzed data from 172 ears. All ears had normal hearing and no history of ipsilateral facial palsy. Parasagittal images of 0.5-mm thickness perpendicular to the internal auditory canal were reconstructed from 0.4-mm thick axial 3D-CISS images for all ears. Two independent radiologists measured the long diameter (LD), short diameter (SD), and cross-sectional area (CSA) of each CN and FN. For each measurement, we calculated the mean value from the 2 observers and evaluated interobserver agreement. We also evaluated the correlation between nerve diameter and sex and age.

Both observers could perform measurements in 157 CN from 172 ears (91%) and 165 FN from 172 ears (96%). For the CN, the mean LD was 1.35±0.16 mm, the mean SD was 0.99±0.18 mm, and the mean CSA was 1.07±0.30 mm2; for the FN, the mean values were 1.18±0.17 mm (LD), 0.87±0.16 mm (SD), and 0.83±0.27 mm2 (CSA). Interobserver reliability was good for each measurement (r=0.569 to 0.691, P<0.01). The diameters of the 2 nerves tended to be larger in men, but only the CSA of the FN was significantly larger in men (P<0.05). Age did not affect nerve diameter.

We present normal values of CN and FN diameters on 3D-CISS at 3T. Our results warrant further study to clarify the pathophysiological state of the CN or FN using 3D-CISS.