FIG. 1.

Signal evolution (amplitude and phase) of two different magnetizations after an inversion recovery pulse. The two magnetizations pointing to the z direction at equilibrium (a), will both be inverted after an inversion pulse (b). Due to the different T₁ relaxation rates, after a certain delay (TI), opposite polarities will result (c). If an image acquisition is performed at this time, using, for example, a 90° excitation, both of the magnetizations will be flipped to the x–y plane. But due to the opposite Mz polarities before excitation, they will present a π phase difference in the transverse plane (d).