7.1.
Rapid Gradient-Echo Imaging
115
180~ 90~
/
900•
i~
c~
N-times
.... ~[
r.f.
Gx(readout)
Gy(phase)
F\
....
F~
........
Gz (slice)
............... A
A
Figure 7.3. Magnetization-prepared snapshot FLASH sequence for acquiring T2-weighted images. imaging of the prepared magnetization is frequently referred to as MPRAGE (magnetization prepared rapid acquisition gradient-echo).
Refocused Gradient-Echo Imaging The idea of this approach, frequently referred to as FISP (fast imaging with steady-state precession) or SSFP (steady state free precession), is to create conditions under which both longitudinal and transverse magnetization are in equilibrium [14, 15]. The problem of accumulated phase changing from excitation to excitation due to the varying phase-encoding gradient is solved by applying a gradient (sometimes called rewinding gradient) with reverse polarity in the end of each repetition period. In principle, only refocusing along the phase-encoding direction is necessary because the slice-select and frequency-encoding gradients do not vary from cycle to cycle. However, in order to null the accumulated phase rewinding gradients can be applied along all axes. Hinshow [16] has shown that in the case when the phase ofr.f, pulses alternates from cycle to cycle, the steady-state magnetization can be relatively large even in the case of extremely short TR. Neglecting magnetic