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Objective:
The trigeminal nerve (TN) has 3 branches providing motor and sensory innervation to the face: the ophthalmic (V1), the maxillary (V2) and the mandibular (V3) nerves. Each branch has smaller distal nerves (n.), some of which can be challenging to image with conventional MRI sequences. Magnetic resonance neurography refers to dedicated sequences aimed at enhancing nerve visualization, useful for diagnosing pathology, defining extent of disease, and surgical planning. 3D T1-weighted sequences such as magnetization prepared rapid-acquisition gradient-echo (MPRAGE) are often used to study cranial nerves, including the TN. 3D short tau inversion – turbo spin echo sequence for extraforaminal cranial nerve imaging (CRANI) is a novel sequence which uniformly suppresses signal generated from fat, muscle and blood enabling excellent visualization of smaller distal cranial nerves. The goal of this study was to assess the performance of 3D CRANI and 3D MPRAGE in the visualization of distal TN branches, and to assess corresponding interobserver reliabilities across a spectrum of experience levels.

Materials and Methods:
Brain MR exams including 3D CRANI and 3D MPRAGE without and with contrast from 7 normal healthy subjects (aged 18-26) were evaluated.  Axial source images and post-processed multiplanar reformats were evaluated to identify continuous distal TN branch trajectories. Three readers (first-year radiology resident, neuroradiology fellow, and neuroradiology attending) independently graded selected distal TN branches as follows: V1 – frontal, nasociliary and lacrimal nerves; V2 - infraorbital, zygomatic, and greater palatine nerves; V3 - masseteric, buccal, auriculotemporal, lingual and inferior alveolar nerves. Grades (0-4) were assigned according to a modified scale described by Fuji et al. Scores for each distal nerve were averaged from all readers, and interobserver variability (kappa (k) values) were calculated (poor, 0.2; fair, 0.2 to 0.4; moderate, 0.4 to 0.6; good, 0.6 to 0.8; and excellent, 0.8 to 1.0).

Results:
All branches of the V1 and V2 segments were best visualized on MPRAGE without contrast. All branches of the V3 segment were best visualized on CRANI without contrast. Highest average grade and k-value were as follows: V1 – frontal n. (4.0, k-value 1.0); V2 – infraorbital n. (3.9, k-value 1.0); V3 – inferior alveolar n. (4.0, k- value 1.0). Average k-values for V1, V2 and V3 segments were 0.76, 0.79 and 0.84, respectively. Several distal nerves were equally well-observed on CRANI and MPRAGE.

Conclusion:
3D MPRAGE demonstrated superior visualization of all examined distal branches of V1 and V2, while 3D CRANI demonstrated superior visualization of distal V3 branches. Overall, 3D CRANI demonstrated superior average interobserver agreement when compared to 3D MPRAGE. These results aid in development of imaging protocols, and aid image interpretation when assessing distal TN branch nerves and pathology.