Anisotropy

Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies homogeneity in all directions. It can be defined as a difference, when measured along different axes, in a material's physical property (absorbance, refractive index, density, etc.) An example of anisotropy is the light coming through a polarizing lens.

Diffusional MRI
Relative anistrophy is the ratio of anistropic to isotopic diffusion.