A π pulse at time ½ TE is used to generate an echo at time TE. The phase change from two independent signal sources 1 and 2 are shown in (b). The echo occurs when all signal sources are once again in phase (a). Since the phase orientations are effectively inverted at the time of the pulse π, T2* effects are undone. The echo amplitude is proportional to since the intrinsic dephasing cannot be recovered (c).