Why “fractional” anisotropy?

Why is Fractional Anisotropy (FA), a measure in diffusion tensor imaging, called "fractional"? I'd have thought "local", i.e. voxel-wise, would have been the correct way to qualify the anisotropy that is calculated from the tensor model, so why fractional?

• I am not completely fresh on the subject, but from what I recall when studying the subject, FA measurements are meant to represent not an absolute signal, but a radio between axial and radial diffusion. – lea Aug 5 '14 at 10:01
• Makes sense! :) – z8080 Aug 5 '14 at 12:47