In contrast to spin echo (SE), in gradient echo (GRE) there may be residual transverse
magnetisation at the end of each cycle remaining for the next cycle. This residual
magnetisation reaches a steady state value after a few cycles. This residual steady state
magnetisation (MSS) is added to the transverse magnetisation created by the next α RF
pulse and thus increases the length of the vector in the x-y plane. This then yields more T2* weighting. In other words, tissues with a longer T2 have a longer MSS than do tissues with shorter T2. To preserve this steady state component, a “rewinder gradient” is applied in the phase-encoding direction at the end of the cycle to reverse the effects of the
phase-encoding gradient applied at the beginning of the cycle.
