MRI Sequences

Gradient echo sequences with steady state residual transverse magnetization

出版元 Denis Hoa

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.

 

Steady-state gradient echo

Type of sequence Philips Siemens GE Hitachi Toshiba
Steady state GE FFE FISP MPGR, GRE TRSG FE

 

Steady-state gradient echo

 

 

In « standard » steady state gradient echo sequences:

  • phase encoding is cancelled at the end of each repetition
  • only the echo corresponding to the free induction signal (FID) is recorded

 

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.

 

Hahn echo

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:

  1. Magnetiztion flip after the first 90°RF excitation pulse
  2. Dephased transverse magnetization
  3. Magnetization flip after the second 90° RF excitation pulse
  4. Only the remaining transverse component is taken into account for the Hahn echo
  5. Partial rephasing occurs
  6. in the le transverse plane
  7. Maximum rephasing corresponds to the peak of the Hahn echo
  8. Dephasing then continues in the transverse plane

 

 

Magnetization

 

Transverse magnetization