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Fig. 1A —Diffusion of water molecules. Restricted diffusion: cellularity and intact cell membranes. Drawing represents 1 voxel of tissue evaluated by diffusion-weighted imaging (DWI) containing cells and blood vessel. Note water molecules (black circles with arrows) within extracellular space, intracellular space, and intravascular space, all of which contribute to measured MR signal. In this highly cellular environment, water diffusion is restricted because of reduced extracellular space and by cell membranes, which act as barrier to water movement.