ABSTRACT
Diffusion magnetic resonance imaging (MRI) measures the random molecular motion (diffusion) of water in biological tissue. Tissues with an oriented structure, such as the fibers of muscle or the axons of nervous tissue, produce measurable diffusion anisotropy: a pattern is imposed on the diffusing molecules by the shape of the tissue. One such tissue is shown in Figure 13.1, an image of some major white matter fiber tracts in a sagittal view of the human brain. Fiber tracts are bundles of neurons whose membranes hinder water diffusion [5]. The orientation of cells in neural tissue is thus reflected in MRI measurements of water diffusion, which vary with direction. This is shown schematically in Figure 13.2.
