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 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 useful in diffusion imaging where it is thought to reflect fibre density.
- FA is calculated from the eigenvalues (λ1,λ2,λ3) of the diffusion tensor:
- FA is calculated from the eigenvalues (λ1,λ2,λ3) of the diffusion tensor:
with the trace 
Mean Diffusivity
Mean Diffusivity (MD) is a scalar value that gives an overall estimation of the diffusivity.
MD=(λ1 + λ2 + λ3)/3.