Diffusion tensor imaging used to guide ECoG strip placement. (A), (B), (C) depict a screen capture from the StealthViz neuronavigational system showing the localization of the origin of the “hyperdirect pathway” as targeted during surgery. (D) Sagittal view of a DTI image used to guide the positioning of the ECoG strip during surgery. Deterministic fiber tracking was first performed using a commercial software package (StealthViz, Medtronic Navigation, Louisville, CO). A trapezoidal region of interest was defined initially at 12 mM lateral to the midline, 4 mM below the anterior commissure–posterior commissure (AC–PC) plane, and 3 mM behind the midpoint of the AC–PC line. This ROI was adjusted to fit the STN as identified by typical signal characteristics on T2 CUBE imaging. From this starting ROI, fibers were automatically traced using a fractional anisotropy threshold of 0.2, a minimal fiber length of 30 mM, and a curvature threshold of 50°. The resulting tractographically-defined “hyperdirect pathway” was displayed in three dimensions, overlaid on the co-registered T2 and SPGR images. This surface was exported along with the T2 CUBE dataset for use during surgery (A,B,C) to localize the cortical origin of the HDP.