Intensity of fractional anisotropy of human brain from diffusion MRI converted to 3D model.  19m 

 

From Wikipedia Fractional anisotropy article:

Fractional anisotropy (FA) is a scalar value between zero and one that describes the degree of anisotropy of a diffusion process. A value of zero means that diffusion is isotropic, i.e. it is unrestricted (or equally restricted) in all directions. A value of one means that diffusion occurs only along one axis and is fully restricted along all other directions. FA is a measure often used in diffusion imaging where it is thought to reflect fiber density, axonal diameter, and myelination in white matter. In most parts the axonal fibers go in all directions, and thus although they constrain the diffusion the FA is 0. In some regions, such as thecorpus callosum the fibers are aligned over a large enough scale (on the order of a mm) for their directions to mostly agree within the resolution element of a magnetic resonance image, and it is these regions that stand out in an FA image.