Visualization of Cranial Nerves in the Orbit Using Fat-Suppressed 3D FLAIR MR Imaging.

BACKGROUND AND PURPOSE Visualization of cranial nerves III-VI (CN III-VI) within the orbit has traditionally been challenging on conventional MRI, even with advanced imaging techniques. Detailed nerve anatomy has largely been inferred from anatomical studies, while most previous MR imaging focused on proximal segments such as the cisternal or cavernous portions of CN III-VI. This study evaluated whether fat-suppressed three-dimensional (3D) FLAIR imaging obtained as part of brain MRI can depict these nerves.

MATERIALS AND METHODS This retrospective, IRB-approved study included 46 subjects (92 orbits) without orbital disease who underwent brain MRI, including fat-suppressed 3D FLAIR imaging. Thirteen orbital structures were evaluated: the superior and inferior divisions and 5 orbital branches of the oculomotor nerve (CN III); trochlear nerve (CN IV); abducens nerve (CN VI); 3 branches of the ophthalmic nerve (CN V1) - frontal, lacrimal, nasociliary nerves; and the ciliary ganglion. Three independent readers scored each structure as 1 if identifiable or 0 if not, and detection rates and interobserver agreement were quantified using observed agreement and Gwet's AC1.

RESULTS The inferior and superior divisions of the oculomotor nerve and their branches (medial, inferior, and superior rectus branches), the abducens nerve, and the frontal nerve of V1 were identified in 100% of orbits, with observed agreement ranging from 83.7-98.9% and AC1 ranging from .87-.99. The trochlear nerve, ciliary ganglion, and lacrimal nerve were detected in 96.7-98.9% of orbits, with observed agreement of 78.3-81.5% and AC1 of .82-.84. Smaller or variable branches, including the inferior oblique and levator palpebrae superioris branches of CN III and the nasociliary nerve, showed detectability of 87.0-95.7%, with observed agreement of 17.4-38.0% and AC1 of -.08-.13.

CONCLUSIONS Fat-suppressed 3D FLAIR imaging acquired as part of brain MRI enables visualization of orbital nerves of CN III-VI and the ciliary ganglion without specialized sequences. This practical MR neurography approach may support future studies of orbital nerve anatomy and subtle abnormalities.