Diffusion-weighted imaging is another refinement of the spin echo sequence. It can provide information about the organization of white matter tracts in the brain that cannot be obtained by other MRI methods.
The basic idea is that protons move within and between cells by random motion. Typically, a proton may travel around 20 pm in 100 ms by this Brownian motion or diffusion. The rate of diffusion will be greatest for protons that are moving freely through the cerebrospinal fluid and less for protons constrained by physical barriers such as myelinated cell membranes. The rate of diffusion affects the spin-spin relaxation time, with rapidly diffusing protons tending to relax more quickly. To acquire images that are weighted by differences in diffusion, two extra gradients are briefly applied during a spin echo sequence. (1)
White matter is generally hyperintense in diffusion-weighted imaging because closely packed axonal tracts provide the greatest barrier to the free diffusion of water in the brain. Furthermore, it is possible to deduce from diffusion-weighted imaging data how compactly organized the white matter is, and even to estimate in what direction the fibre tracts are oriented (.Plate . . .8). This information is of considerable interest to psychiatry, since the pathology of many psychiatric disorders may involve the axonal connections between multiple cortical areas.(2)
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