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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4$\begingroup$ 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. $\endgroup$– leaAug 5 '14 at 10:01
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$\begingroup$ Makes sense! :) $\endgroup$– z8080Aug 5 '14 at 12:47
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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
answered May 5 '15 at 18:10