FIG 2. Schematic diagrams of the fast spin-echo sequence show the effects of section selection gradients on the formation of spin echoes and stimulated echoes.

A, Diagram shows the spin echoes produced with RF excitation by the 90° pulse and refocusing by the ß pulses. At even echoes, the section selection gradient shows a 1-(-2)-1 waveform (dotted-dashed-dotted areas, respectively). Hence, the spin echoes are have inherent motion compensation at even echoes.

B, Diagram shows stimulated echo formation from RF excitation by the 90° pulse and refocusing by the composite of two successive ß pulses. For stimulated echoes, the magnetization vector is stored in the longitudinal direction in between the two ß pulses and, thus, the gradient has no effect on the phase distribution. The resultant 1-(-1) waveform (dotted-dashed areas, respectively) shows no inherent motion compensation. Note that, with flip angle reduction of the ß pulses, as with the selection of the TRF pulse, the signals have a decreased contribution from the spin echo and increases the stimulated echo portion. Therefore, the noncompensated stimulated echo reduces the high-intensity CSF artifacts due to rapid inflow of CSF in the prepontine cisterns.