A gradientecho is generated by using a pair of bipolar gradient pulses. The gradient field is negatively pulsed, causing the spins of the xy-magnetization to dephase. A secondgradient pulse is applied with the opposite polarity. During the pulsing, the spins that dephased begin to rephase and generate a gradientecho.
Spoiling can be accomplished by RF or a gradient.
The incoherent RF spoiled type of a gradient echo sequence use a continuous shifting of the RF pulse to spoil the residual transverse magnetization. The phase of the RF excitation and receiver channel are varied pseudo randomly with each excitation cycle to prevent the xy magnetization from achieving steady state. T2* does not dominate image contrast, so T1 and PD weighting is practical.
This method is effective and can be used to achieve a shorter TR, due to a lack of additional gradients. Spoiling eliminates the effect of the remaining xy-magnetization and
leads to steady state longitudinal magnetization. These sequences can be used for breath hold, dynamic imaging and in cine and volume acquisitions.
(SPGR) The SPGR pulse sequence is similar to the spoiled GRASS sequence. The spoiled gradient recalled (SPGR)
acquisition in steady state uses semi-random changes in
the phase of the radio frequency (RF) pulses to produce a spatially independent phase shift.