Fig. 2.

A simplified pulsed gradient spin-echo sequence used in standard DW-MRI, showing only the gradient pairs (top), and a schematic representation of the magnetic spin orientation of an ensemble of protons (water molecules). In the situation of no diffusion, the rephasing gradient exactly cancels the phase difference induced by the dephasing gradient, and the net magnetization of the ensemble is large (strong signal). In the case of diffusion, illustrated as two water molecules switching location during the diffusion time Δ, the rephasing gradient does not eliminate the phase differences of the diffused molecules, and the resulting magnetization and signal is weaker