Brain's white matter. Close-up of an area of the brain imaged using tract density imaging and 3D diffusion tensor imaging (DTI), a magnetic resonance imaging (MRI) technique. This frontal view of the centrum semiovale (white matter area) shows crossing areas between the corpus callosum (red), the corticospinal tract (blue) and the superior longitudinal fasciculus (green). The fibres are locally coloured red-green-blue if they are orientated in x-y-z alignment (left-right, posterior-anterior, inferior-superior). Diffusion tensor imaging (tractography) measures the direction of water diffusion, which in the brain reveals the orientation of nerve fibres.