In steady state gradient echo sequences, residual transverse magnetization is conserved. This will participate in the signal and the contrast and vary according to the type of sequence.
By maintaining residual transverse magnetization, excitation pulses will produce new echos (Hahn echos, stimulated echos) in addition to the gradient echo that depends on the free precession signal (FID).
There are several variants in the family of steady state gradient echo sequences, according to the type of echo recorded (which determines contrast) and how the gradients are adjusted.
Two RF pulses at the same excitation angle produce a Hahn echo (partial spin echo), whose amplitude depends on T2. With two 90° pulses, we obtain :
At least three RF pulses at the same excitation angle produc a stimulated echo whose amplitude depends on both T1 and T2. With three 90° pulses we obtain :
|Type of sequence||Philips||Siemens||GE||Hitachi||Toshiba|
|Steady state GE||FFE||FISP||MPGR, GRE||TRSG||FE|
In « standard » steady state gradient echo sequences:
It is necessary to cancel phase encoding with a rewinder gradient to avoid the next echo being altered by a different phase encoding. Hahn and stimulated echos are not recorded (thanks to the lengthening of the readout gradient). These gradient adjustments at the end of repetition are needed to avoid band artifacts.
For short TR (less than T2) and fairly large flip angles (40° – 90°), the contrast of this type of sequence varies according to T2/T1 ratio.