CPMG - CPMG static pseudo steady state

There is a fundamental difference between the steady state of gradient echo sequences and CPMG sequences. As shown in the gradient echo part the steady state consists of a certain elliptical distribution of magnetization. For CPMG the steady state echo amplitude is zero, since after some time the transverse magnetization has decayed to zero by T2 relaxation. It is thus more interesting to analyze the CPMG steady state without relaxation (which is for most applications a good approximation, since the echo train length is shorter than T2). The CPMG steady state created without relaxation (but with constant refocusing flip angles) is called pseudo steady state. However, for the pseudo steady state only echo amplitudes are constant, single magnetization vectors do not reach the steady state (they never reach the steady state for constant refocusing flip angles, except for 180°).

A real pseudo steady state can be achieved with a certain distribution of magnetization. For this so-called static pseudo steady state both echo amplitudes as well as each single magnetization vector are identical from echo to echo. The motion of magnetization from one echo to the next is shown below. The start configuration was the static pseudo steady state for 60°.

CPMG approach to static pseudo steady state

There exist different strategies to reach the static pseudo steady state (obviously not with constant refocusing flip angles). One possibility to approach the static pseudo steady state for 60° is shown below using varying refocusing flip angles.

Last updated: Montag, 18.04.2011