Fractional Anisotropy, FA
If full tensor data acquisition is applied, the Fractional Anisotropy is calculated from the eigenvalues l1, l2, l3 of the diffusion tensor:
where l is the mean diffusivity (trace/3).
FA takes on values between 0 (perfectly isotropic diffusion) and 1 (the hypothetical case of an infinite cylinder) and is thus directly comparable between subjects. We here multiply FA by 1000.Definition of FA can be found in Basser P.J. and Pierpaoli C.: "Microstructural and physiological features of tissues elucidated by quantitative-diffusion-tensor MRI.",JMR B 111, 209-219 (1996). ( Medline citation)
Example FA map: