MRI Distortion Correction Produces Geometrically Relevant Shifts in Cisternal Trigeminal Nerve Localization: Implications for Gamma Knife Radiosurgery.

OBJECTIVE: Gamma Knife radiosurgery (GKRS) for trigeminal neuralgia (TN) requires millimeter-scale accuracy because a single 4-mm collimator targets the cisternal trigeminal nerve (CTN). MRI is essential for CTN visualization but is vulnerable to geometric distortion, particularly near the skull base. This study evaluated whether CT-based MRI distortion correction produces geometrically relevant changes in CTN localization. METHODS: Forty adult patients with normal intracranial anatomy who underwent cranial MRI and CT were retrospectively analyzed. Bilateral CTN segmentations were performed on uncorrected and distortion-corrected MRI by two independent raters. Segmentation masks were exported to 3D Slicer to extract center-of-mass (CoM) coordinates. The primary endpoint was patient-level 3D Euclidean CoM shift between corrected and uncorrected MRI. Dice Similarity Coefficient (DSC), axis-specific displacement, threshold positivity, inter-rater reliability, and Bland-Altman agreement were also assessed. RESULTS: The mean patient-level 3D shift was 1.12 ± 0.34 mm. All patients had at least one side-rater observation exceeding 0.5 mm, and 28 of 40 patients (70.0%) had at least one observation exceeding 1.0 mm. Mean DSC was 0.84 ± 0.03, indicating high but incomplete volumetric overlap. Displacement was anisotropic, with absolute magnitudes ordered as |ΔY| > |ΔX| > |ΔZ|. Inter-rater reliability was excellent, with ICC(A,1) values ranging from 0.927 to 0.988. CONCLUSION: CT-based MRI distortion correction produces measurable, anatomically structured shifts in CTN localization. These shifts are non-negligible relative to the 4-mm GKRS collimator. Their clinical consequences remain unknown; dosimetric and outcome studies in treated cohorts are required to determine whether correction alters nerve dose coverage or clinical results.