In-Vivo Nerve Tractography for Microstructural Neural Changes after Trigeminal Radiosurgery

Abstract

Focal radiosurgery is a common treatment option for medically refractory trigeminal neuralgia (TN). Given the paucity of investigational tools to assess the trigeminal nerve changes, assessment of treatment effectiveness is primarily clinical. Diffusion tensor imaging (DTI) provides information on white matter microstructure. We explored the feasibility of trigeminal nerve tractography and assessment of DTI parameters to study microstructural changes after treatment. We hypothesized that trigeminal tractography provides more information than 2D-MR imaging, allowing detection of unique, focal changes in the target area after radiosurgery. 10 medically refractory TN patients (8 males, 2 male, average age 67 years) underwent treatment with Linac radiosurgery using stereotactic cones, 6DOF image guidance & FFF beams, 85 Gy/100% isodose line underwent 3 MR trigeminal nerve tractography before and sequentially at 4mon, 8mo and 12mo after treatment. Multi-tensor tractography was used to extract DTI-derived metrics-axial (AD), radial (RD), mean diffusivity (MD), and fractional anisotropy (FA)-from the trigeminal nerve cisternal segment (Retrogasserian region near pars triangularis through pontine segment of trigeminal nerves) defined as region-of-interest. Areas outside target and the contralateral nerve served as controls. Trigeminal tractography accurately detected the radiosurgical target. At 3months post radiosurgery resulted in 55% drop in FA values at the target with no significant change in FA outside the target, demonstrating highly focal changes after treatment. RD but not AD changed markedly, suggesting that radiosurgery primarily affects myelin. Tractography was more sensitive than conventional gadolinium-enhanced post-treatment MR, since FA changes were detected regardless of trigeminal nerve enhancement. In subjects with long term follow-up, recovery of FA/RD correlated with pain recurrence. DTI parameters accurately detect the effects of focal radiosurgery on the trigeminal nerve, serving as an in vivo imaging tool to study TN.