Navigated Transcranial Magnetic Stimulation for “Somatotopic” Tractography of the Corticospinal Tract

BACKGROUND: Diffusion tensor imaging tractography provides 3-dimensional reconstruction of principal white matter tracts, but its spatial accuracy has been questioned. Navigated transcranial magnetic stimulation (nTMS) enables somatotopic mapping of the motor cortex. OBJECTIVE: We used motor maps to reconstruct the corticospinal tract (CST) by integrating elements of its somatotopic organization. We analyzed the accuracy of this method compared with a standard technique and verified its reliability with intraoperative subcortical stimulation. METHODS: We prospectively collected data from patients who underwent surgery between January 2012 and October 2013 for lesions involving the CST. nTMS-based diffusion tensor imaging tractography was compared with a standard technique. The reliability and accuracy between the 2 techniques were analyzed by comparing the number of fibers, the concordance in size, and the location of the cortical end of the CST and the motor area. The accuracy of the technique was assessed by using direct subcortical stimulation. RESULTS: Twenty patients were enrolled in the study. nTMS-based tractography provided a detailed somatotopic reconstruction of the CST. This nTMS-based reconstruction resulted in a decreased number of fibers (305.1 ± 231.7 vs 1024 ± 193, P < .001) and a significantly greater overlap between the motor cortex and the cortical end-region of the CST compared with the standard technique (90.5 ± 8.8% vs 58.3 ± 16.6%, P < .001). Direct subcortical stimulation confirmed the CST location and the somatotopic reconstruction in all cases. CONCLUSION: These results suggest that nTMS-based tractography of the CST is more accurate and less operator dependent than the standard technique and provides a reliable anatomic and functional characterization of the motor pathway. ABBREVIATIONS: CST, corticospinal tract DSS, direct subcortical stimulation DTI, diffusion tensor imaging DWI, diffusion-weighted imaging FDI, first dorsal interosseous FLAIR, fluid attenuated inversion recovery MEN, mentalis MEP, motor evoked potential nTMS, navigated transcranial magnetic stimulation RMT, resting motor threshold ROI, region of interest TA, tibialis anterior TE, echo time TMS, transcranial magnetic stimulation TR, repetition time