Fig. 2.

(a) Diffusion MRI allows for quantifying, for each imaging voxel, the orientation distribution of the water diffusion (termed spin distribution function, SDF) to reveal the underlying structural characteristics of axonal fiber bundles in a color-coded surface (red-blue-green indicates the orientation at the x-y-z axis, respectively). The protruding points of the SDFs indicate the orientation of fiber bundles. (b) The color sticks represent the peak orientations on SDFs. The coronal view shows that SDF can resolve crossing fibers at central semiovale, a white matter region where the corpus callosum crosses vertical passing fibers. The SDFs averaged from a total 842 subjects provide orientations of the local axonal connections. The information can be used to drive a fiber tracking algorithm to delineate white matter connections. (c) The SDF template of the human brain averaged from 842 diffusion MRI scans (termed the HCP-842 template) shows structural characteristics of the human brain. The magnitude map of the HCP-842 template reveals structures such as hippocampus (HIP), thalamus (TH), red nucleus (RN), and substantia nigra (SN), which are consistent with the histology image from BigBrain slides (d). (e) The orientation map of the HCP-842 template allows for delineating the complicated structures, such as the clamping structure between the anterior commissures (AC) and the pre-commissural (PreC) and post-commissural (PostC) branches of the fornix. The structural characteristics are also illustrated by the SDFs of the HCP-842 template in (f).