Figure 3.

Timeline for encoding the diffusion of the water molecules in MRI.

Just after being excited, the position of each molecule is tagged based on the phase of its spins, by applying gradient #1 (parametrized by its strength G and duration δ). During a time Δ, the spins (illustrated by small circles) move (due to Brownian motion or convection). The longer the time Δ is, the further the molecules can move. A second and identical gradient is applied to quantify this displacement through a signal loss in the MR response.