Gradient Echo Sequences - The steady state

In the steady state the characteristic distribution of the magnetization does not change from one TR to the next. In other words, application of an excitation pulse and dephasing period results in the same magnetization as before. The evolution of the steady state magnetization between one excitation pulse and dephasing period is shown below. Flip angle was 50°, and T1=20TR, T2=10TR.

The steady state magnetization before and after the excitation pulse is distributed on a tilted ellipse (or on a circle if T1=T2). The complex sum of the transverse part of this distribution corresponds to the signal intensity of an S+ (FLASH, FAST, GRASS, FISP,…) sequence if the echo is acquired after the excitation pulse; or to the signal of an S- (CE-FAST, PSIF, T2-FFE, …) sequence if the echo is measured just before the excitation pulse. With more gradients it is also possible to acquire higher order echoes.
Last updated: Monday, 18.04.2011