from diffusion imaging data, the most frequently reported metrics are general diffusivity, a measure of how quickly water diffuses in any direction, and fractional anisotropy (FA), which refers to the proportion of water molecules within a certain brain region that are diffusing in the same direction. Factors that increase FA include organization of tissue into tightly packed unidirectional structures and increased myelination; thus structures such as the corpus callosum and interior capsule tend to have high fractional anisotropy values. Mean diffusivity tends to be decreased in the presence of these same factors. Tissues such as the cortex will have lower FA levels despite their highly organized states because tissue elements are going in different directions, a phenomenon termed fiber crossing (see paper by Schmithorst, this issue).
