# Why *fractional* anisotropy?

Does anyone know why the FA (fractional anisotropy) measure in diffusion tensor imaging is 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, but 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 at 10:01
Makes sense! :) –  wildetudor Aug 5 at 12:47