Diffusion-weighted imaging

Diffusion-weighted imaging is a specific MRI modality that produces in vivo magnetic resonances images of biological tissues weighted with the local characteristics of water diffusion.

More precisely, given a spatial direction and a chosen amount of time during which water molecules are left free to diffuse, a sophisticated MRI scanner produces a T2 (3D) image attenuated according to the intensity of the diffusion. The more attenuated the image is at a given position, the more diffusion there is locally. Interestingly, this image intensity varies whenever the spatial direction or the diffusion gradient is changed. As a consequence, simple models have been proposed to account for such changes, such as the diffusion tensor model.

Diffusion-weighted images are very useful to diagnose vascular strokes in the brain, to study the diseases of the white matter or to (try to) infer the connectivity of the brain (i.e. tractography; try to see which part of the cortex is connected to another one, and so on).

See also

• Diffusion MRI

Diffusion-weighted MRI (DW-MRI), is a technique used for relating image intensities to the relative mobility of endogenous tissue water molecules. Two equal and opposite large magnetic field gradients are applied before the data is acquired. In the absence of any motion of the water molecules, the magnetic spins will be de-phased by the first gradient, and then completely re-phased by the second gradient. However, because the actual motion of the water molecules is that of a random walk, the second gradient will not completely re-phase the spins. The signal intensity will therefore be exponentially attenuated proportional to the mean diffusion length. Areas with relatively high mean diffusion length will appear dark on the MRI images relative to areas with low mean diffusion length. source; http://www.irc.cchmc.org/about_diffusion_mri.htm