Figure 2a.

(a) Schematic illustrates the effect of a diffusion-weighted sequence on water molecules (solid circles) within highly cellular tissue or a restricted environment. The diffusion-weighted sequence is fundamentally a T2-weighted sequence with the application of a dephasing gradient (diffusion sensitizing gradient) prior to the 180° RF pulse, followed by a symmetric rephasing gradient after the 180° pulse. Water molecules within a restricted environment do not move long distances and acquire phase shifts during the application of the first gradient that are cancelled out by phase shifts acquired during the second (opposing) gradient. As a result, no net loss in signal intensity occurs (aside from normal T2 decay). (b) Schematic illustrates the effect of a diffusion-weighted sequence on water molecules (solid circles) within tissue with low cellularity or a less restricted environment. Water molecules within a less restricted environment can move long distances. Such highly mobile molecules acquire phase information from the first gradient, but because of their motion, their signal does not completely rephase with the second gradient, resulting in a net loss in signal intensity in addition to normal T2 decay. SE = spin-echo.