Appearance of normal cranial nerves on steady-state free precession MR images

Abstract

As radiologic imaging technology improves and more intricate details of the anatomy can be evaluated, images provide more precise diagnostic information and allow better localization of abnormalities. For example, standard T2-weighted magnetic resonance (MR) imaging sequences adequately depicted only the larger cranial nerves, whereas current steady-state free precession (SSFP) sequences are capable of depicting the cisternal segments of all 12 cranial nerves. SSFP sequences provide submillimetric spatial resolution and high contrast resolution between cerebrospinal fluid and solid structures, allowing the reconstruction of elegant multiplanar images that highlight the course of each nerve. These sequences have become a mainstay in the evaluation of the cerebellopontine angles and inner ear. Usually referred to by their trade names or acronyms (eg, constructive interference steady state, or CISS, and fast imaging employing steady-state acquisition, or FIESTA), SSFP sequences allow precise differentiation between branches of the facial and vestibulocochlear nerves, accurate detection of small masses in the cerebellopontine angles and internal auditory canals, and detailed evaluation of endolymph and perilymph within the inner ear. To take full advantage of these imaging sequences, radiologists must be familiar with the appearances of similar anatomic details of all 12 cranial nerves on SSFP MR images.