Non-CPMG Fast Spin Echo with Full Signal

Abstract

The standard Fast Spin Echo sequence used in MR imaging relies on the CPMG condition. A consequence of this condition is that only one component of the transverse magnetization can be measured. To counter this, some phase modulation schemes (XY, MLEV . .) for the pulse train have been proposed, but they are useful only over a very restricted range, close to π, of the refocusing pulse rotation angle. Some other solutions not relying on phase modulation have also been suggested, but they destroy one half the available signal. Revisiting the phase modulation approach, J. Murdoch (“Second SMR Scientific Meeting,” p. 1145, 1994) suggested that a quadratic phase modulation could generate a train of classical echoes. We show here that indeed a quadratic phase modulation has a very suitable property: after an adequate change of frame, the dynamic of the system composed of all the protons situated in one pixel can be seen as stationary. If the parameter of the quadratic phase modulation is well chosen, it is then possible to put the dynamic system in a combination of two suitable states and obtain a signal identical to the signal of a classical spin echo, at least for nutation of the refocusing pulse higher than, approximately, two radians.