7T Structural, Vascular, and Diffusion Imaging of Trigeminal Neuralgia: Preliminary Results in Patients

Ultra-high field MRI scanners, such as those operating at 7 Tesla, offer higher signal-to-noise ratio and enhanced contrast that leads to imaging with unprecedented anatomical and physiological detail in vivo. 7T-MRI is capable of visualizing anatomy, tissue microstructure, and vasculature in the skull base, and may provide improved diagnostic imaging for trigeminal neuralgia (TN), a condition resulting in neuropathic pain. In this study, we report imaging of TN with high-resolution 7T structural-MRI, vascular MRI, and diffusion-weighted MRI (dMRI). Here, we evaluated a multimodal 7T-MRI protocol for visualization of the nerve and surrounding vessels in six patients and three controls and performed quantitative analysis of structural nerve characteristics.

Six TN patients aged 23 (F), 31 (M), 38 (M), 45 (M), 48 (F), and 66 (F) and three controls aged 22 (F), 54 (F), and 55 (M) were scanned under an approved IRB protocol.

Subjects underwent the 7T-MRI protocol listed in Table 1 and high angular-resolved, simultaneous multislice dMRI (1.05 mm isotropic resolution, 68 directions). We reduced artifacts in the brainstem region by performing localized B0 shimming. dMRI images were corrected for distortions and registered to T1-weighted images.

Processing: Fiber orientation distributions for tractography were obtained from the corrected diffusion-weighted images by spherical deconvolution. Tractography for visualization of the nerve, shown in Fig. 1, was performed using the iFOD2 algorithm implemented in MRTRIX3 using a seed placed in the nerve.

The cross-sectional area (CSA) of the nerve was measured by drawing regions of interest (ROIs) in Osirix, using high-resolution T1-weighted coronal images. CSA was measured for both left and right trigeminal nerves, with two ROIs drawn for each nerve: the first at the point where the nerve emerges from the brainstem pons and the second at the point that the nerve enters Meckel's cave. The two ROI CSA values were averaged to obtain mean CSA of the nerve. An asymmetry index (AI) was calculated by subtracting right from left and dividing by the average CSA. A group comparison of AI values was performed.

7T-MRI data was used in addition to clinical standard of care imaging for treatment planning. 7T multimodal imaging consistently provided improved visualization of the trigeminal nerve emerging from the brainstem compared with clinical standard of care. This included identification of encroaching vessels in cases of possible NVC as well as compression of the nerve due to other abnormalities such as an epidermoid lesion.

The structural imaging quantitative metrics performed are shown in Table 2. Qualitatively, asymmetry was observed in the trigeminal nerves of patients. We found initial quantitative differences, with an average AI of 0.441 for patients compared with 0.144 for controls. A t-test was performed, indicating a trend toward significance (p ∼ 0.07) and an effect size of ∼0.50.

7T-MRI provided increased visualization of trigeminal nerve integrity and proximity to adjacent vessels. In general, greater asymmetry in nerve CSA was found in patients compared with controls with a high effect size. Ongoing work is focused on validating these findings in a larger sample. 7T-MRI could provide a powerful tool to enhance our understanding of TN pathophysiology and improved diagnosis and treatment of TN.