Description

Intensity of fractional anisotropy of human brain from diffusion MRI converted to 3D model. 19m From Wikipedia Fractional anisotropy article: Fractional anisotropy (FA) (http://en.wikipedia.org/wiki/Fractional_anisotropy) is a scalar (https://en.wikipedia.org/wiki/Scalar_(mathematics)) value between zero and one that describes the degree of anisotropy (https://en.wikipedia.org/wiki/Anisotropy) of a diffusion (https://en.wikipedia.org/wiki/Diffusion) process. A value of zero means that diffusion is isotropic, i.e. it is unrestricted (or equally restricted) in all directions. A value of one means that diffusion occurs only along one axis and is fully restricted along all other directions. FA is a measure often used in diffusion imaging (https://en.wikipedia.org/wiki/Diffusion_imaging) where it is thought to reflect fiber density (https://en.wikipedia.org/w/index.php?title=Fiber_density&action=edit&redlink=1) , axonal (https://en.wikipedia.org/wiki/Axon) diameter, and myelination (https://en.wikipedia.org/wiki/Myelination) in white matter (https://en.wikipedia.org/wiki/White_matter) . In most parts the axonal fibers go in all directions, and thus although they constrain the diffusion the FA is 0 . In some regions, such as the corpus callosum (https://en.wikipedia.org/wiki/Corpus_callosum) the fibers are aligned over a large enough scale (on the order of a mm) for their directions to mostly agree within the resolution element of a magnetic resonance image (https://en.wikipedia.org/wiki/Magnetic_resonance_image) , and it is these regions that stand out in an FA image.