Mapping Peripheral Nerve Fibers With Diffusion MRI Tractography To Guide Peripheral Nerve Sheath Tumor Resections

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PURPOSE: Magnetic Resonance Imaging (MRI) is an important, routine, non-invasive tool for the pre-surgical assessment of peripheral nerve sheath tumors (PNSTs). However, relying solely on conventional MRI (i.e., T1- and T2-weighted images) or MR neurography poses a significant challenge in delineating the path of nerve fibers relative to the tumor and in classifying PNST subtypes. In this study, we leveraged diffusion MRI techniques, specifically, constrained spherical deconvolution-based probabilistic fiber tractography and diffusion tensor imaging (DTI) metrics, to better delineate the path of nerve fibers in relation to the PNST and to classify tumor subtypes into schwannomas and neurofibromas. METHODS: We studied 16 patients with a PNST (6 males and 10 females, mean age +/- SD: 44.06 +/- 16.45 years) in a broad range of anatomical locations, including the brachial plexus (n=5), lower limb (n=5), forearm (n=3), lower spine (n=2), and neck (n=1). Each patient underwent a preoperative 3T MRI session to acquire T1-/T2-weighted anatomical and DTI scans. The data were processed using the MRtrix3 software package. Probabilistic fiber tractography was performed on the surrounding, affected nerve and DTI metrics (fractional anisotropy and mean diffusivity) were extracted from the tumor core. Differences in DTI metrics between schwannomas and neurofibromas were assessed using independent-samples t-test. RESULTS: We found that constrained spherical deconvolution-based probabilistic fiber tractography successfully reconstructed the course of peripheral nerve fibers in relation to the PNST in all cases. Importantly, schwannomas demonstrated nerve fiber tracts that were more pronounced and ran alongside the tumor (n=9, example case shown in Figure A), whereas neurofibromas demonstrated considerably more envelopment of normal nerve fibers within the tumor (n=7, example case shown in Figure B). Furthermore, we found mean diffusivity to be a robust, objective metric in differentiating PNST subtypes, where schwannomas demonstrated significantly lower mean diffusivity compared to neurofibromas (p 0.05). CONCLUSION: Our diffusion MRI acquisition protocol and analysis pipeline support the direct clinical utility of this imaging modality in helping clinicians anticipate the technical aspects of intraoperative resection and inform preoperative discussions with patients. The incorporation of DTI as part of routine MRI and MR neurography may be a crucial component of standard of care for the assessment and management of PNSTs.

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