In recent years, more studies have begun to appear using imaging methods that aim to quantify brain tissue characteristics directly rather than by measuring volumes of gray and white matter in anatomic structures. One of these is magnetization transfer imaging, discussed above. Another is diffusion tensor imaging (DTI), which provides information about the properties of diffusion of water through different regions of the brain(Mori & Zhang, 2006). If unconstrained, water molecules will randomly diffuse in all directions, whereas water molecules interacting with tissue components such as cell membranes or large molecules will be more likely to diffuse in particular directions. In DTI, a tensor can be calculated for each voxel which represents the overall distribution of the diffusion of the water molecules in that area (Le Bihan et al., 2001). Although many potential measures of water diffusion can be obtained 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
