High spatial resolution nerve-specific DTI protocol outperforms whole-brain DTI protocol for imaging the trigeminal nerve in healthy individuals

Diffusion tensor imaging (DTI) can provide markers of axonal micro-structure of the trigeminal nerve (cranial nerve five [CNV]), which may be affected in trigeminal neuralgia (TN) and other disorders. Previous attempts to image CNV have used low spatial resolution DTI protocols designed for whole-brain acquisition that are susceptible to errors from partial volume effects, particularly with adjacent cerebrospinal fluid (CSF). The purpose of this study was to develop a nerve-specific DTI protocol in healthy subjects that provides more accurate CNV tractography and diffusion quantification than whole-brain protocols. Four DTI protocols were compared in five healthy individuals (age 22-45 years, three males) on a 3 T Siemens Prisma MRI scanner: two newly developed nerve-specific high resolution (1.2 x 1.2 x 1.2 = 1.7 mm3 ) DTI protocols without (3.5 minutes) and with CSF suppression (fluid-attenuated inversion recovery [FLAIR]; 7.5 minutes) with limited slice-coverage, and two typical whole-brain protocols with either isotropic (2 x 2 x 2 = 8 mm3 ) or thicker slice anisotropic (1.9 x 1.9 x 3 = 10.8 mm3 ) voxels. Deterministic tractography was used to identify the CNV and quantify bilateral fractional anisotropy (FA), and mean (MD), axial (AD) and radial diffusivity (RD). CNV volume was determined by manual tracing on T1-weighted images. High spatial resolution nerve-specific protocols yielded better delineation of CNV, with less distortions and blurring, and markedly different diffusion parameters (42% higher FA, 35% lower MD, 27% lower RD and 43% lower AD) compared with the two lower resolution whole-brain protocols. The anisotropic whole-brain protocol showed a positive correlation between CNV FA and volume. The high resolution nerve-specific protocol with FLAIR yielded additional reductions in CNV AD and MD with a value of 1.0 x 10-3 mm2 /s, approaching that expected for healthy young adult white matter. In conclusion, high resolution nerve-specific DTI with FLAIR enhances the identification of CNV and provides more accurate quantification of diffusion compared with lower resolution whole-brain approaches.