Fig. 2.

Gray-matter seeding from two pairs of nearby positions, demonstrating that the precise positioning of gray matter seeds impacts the success of tracking into the deep white matter. (A) FSL track line density derived from four different seeds. Seeds 1 and 2 are less than 2 mm apart in the dorsal motor region, and seeds 3 and 4 are less than 1 mm apart in the IPS. In each case, 10,000 probabilistic track lines were seeded from a single surface coordinate within the middle thickness of the gray matter. The color map indicates the percentage of the track lines that reached a given voxel. Despite the physical proximity and, in the case of seeds 1 and 2, the functional and cytoarchitectural similarity of the seed regions, the track lines emanating from the four example seed locations varied markedly in their white matter penetration. Track lines emanating from seeds 1, 3, and 4 became trapped in local fiber systems and failed to enter the deep white matter. Probability tracks were overlaid on MTR maps. (B) Tracks from each seed in A, shown as the dominant orientation in each voxel reached by probabilistic tracking, overlaid on FA maps. (C) The 3D maps of the entire brain show the tracking of distance projections for each of the seeds.