CPMG - One refocusing pulse

After excitation the magnetization is dephased by the first gradient lobe placed between excitation and refocusing pulse. The refocusing pulse turns the dephased disk of magnetization by a certain angle. The second gradient lobe produces a refocusing of the transverse magnetization, and a spin echo is formed. Dephasing produced by other mechanisms such as chemical shift will also be refocused.
Below is the motion of magnetization during excitation, dephasing, refocusing, and dephasing (initial M = (0,0,1), T1 = 20TR, T2=10TR, α=90°-TE-180°-TE-echo-TE)

However, this was the trivial case. Everything gets much more interesting and realistic for refocusing flip angles that are below 180° (which is always the case at the edges of the slice profile, or to reduce SAR). Combination of a 90° excitation pulse with a 100° refocusing pulse generates the eight-ball echo. This is the combination of a conventional spin echo with some additional (sinusoidal) distribution of longitudinal magnetization. Below is the motion of magnetization during excitation, dephasing, refocusing, and dephasing (initial M = (0,0,1), T1 = 20TR, T2=10TR, α=90°-TE-100°-TE-echo-TE)

Last updated: Monday, 18.04.2011