A quantitative parameter derived from diffusion tensor imaging that measures the degree of directional preference of water molecule diffusion within a tissue. Fractional anisotropy (FA) values range from 0 (isotropic diffusion) to 1 (highly directional diffusion) and provide information about tissue microstructure, fiber organization, and structural integrity. This measurement can be applied to various biological tissues including brain white matter, skeletal muscle, kidney, and other organs. The trait is mean fractional anisotropy measurement (EFO_0008399, an Experimental Factor Ontology id). Also known as: fractional anisotropy.