Mr. Sun gets all the cool commenters:

The idea is that in the presence of dense, fibrous tissue such as major white matter tracts in the brain, water diffusion will be restricted in the direction perpendicular to the tract but not in the direction parallel to the tract. So, by looking at the shape of water diffusion, we can infer the shape of the underlying tissue. The process of mapping the white matter architecture of the brain is called tractography, and it’s way, way cool: Tractography.