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?

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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! :) –  wildetudor Aug 5 '14 at 12:47

Like lea's comment indicated, FA is called "fractional" anisotropy simply because it's the degree of anisotropic diffusion, i.e., a ratio (Soares, Marques, Alves and Sousa, 2013).

Fractional Anisotropy is a normalized measure of the fraction of the tensor's magnitude due to anisotropic diffusion, corresponding to the degree of anisotropic diffusion or directionality and ranges from 0 (isotropic diffusion) to 1 (anisotropic diffusion).

References

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