Diffusion Weighted Imaging
In DWI contrast is created based on how far water molecules can randomly disperse due to Brownian motion. This tells us about the structure of tissues at the cellular level.
Application of a strong gradient causes dephasing of spins. Application of an identical gradient opposite to the first reverses that dephasing, except where motion along the gradient direction has occurred. This causes signal loss, and an image acquired with these strong gradients turned on prior to data acquisition is a diffusion weighted image.
In apparently stationary tissue, diffusion of water molecules along the direction of the diffusion gradients causes signal loss, seen on a diffusion weighted image. Where diffusion in tissue is restricted (e.g. in tumours), there is less signal loss, creating contrast to surrounding anatomy.