Figure 3.

Differential tensor behaviors in white matter. Cell membranes are thought to be the main restricting boundary to water mobility in white matter. As such, the ratio of axial to radial diffusivities, commonly described by a metric known as “fractional anisotropy” (FA), can reflect the coherence (sometimes thus the integrity) of packed axons in white matter. FA approaches one in well-organized tracts, where the diffusion ellipsoid is elongated along the principal tract orientation; and tends to zero, in less coherent environments—i.e., in heterogeneous areas of many crossing fibers, and in gray matter or cerebrospinal fluid—, where the ellipsoid resembles a sphere. In the full complexity of white matter microstructure, however, the ellipsoid can take a wider range of geometries from that of a rugby ball to that of surfboard or a dish—with the degree of sphericity and planarity dictated primarily—but not only—by axon packing density, degree of myelination and/or the geometrical arrangement of crossing, kissing and/or splaying fiber populations.