Quantitative imaging of slow coherent motion by stimulated echoes with suppression of stationary water signal

Abstract

A method exploiting stimulated echoes is proposed to quantitatively measure or image very slow coherent flows. The pulse sequence consists of four 90° y hard pulses and two identical velocity-encoding gradient pulses applied after the first and the third RF pulses. The first three RF pulses produce a stimulated echo which is dephased from the x axis of the rotating frame by an angle proportional to the mean velocity over the considered volume. The fourth pulse, applied at the time of occurrence of the stimulated echo, rejects the x component of the transverse magnetization by turning it back to the z axis in order to eliminate any contribution to the signal from stationary spins. A maximum dynamic range is then available for velocity measurements. Phase cycling can remove the effects of imperfect 90° pulses or, for an imaging experiment, of a nonuniform distribution of the phase shifts caused by the gradient pulses over the voxel size. To obtain an image, the velocity-weighted signal is phase-encoded and partly refocused by a slice-selective 180° pulse. It is shown that a quantitative velocity map unaffected by diffusion effects can be directly obtained from a normal 2D FT phase or absolute-mode reconstructed image. Velocities as low as 10 −2 mm s −1 can be measured or imaged.