Flow-induced phase effects and compensation technique for slice-selective pulses

Flowing spins experience a time-varying frequency during the application of slice-selective radiofrequency (RF) pulses. As a result the flowing spins accumulate phase relative to stationary spins as the spins are rotated toward the transverse plane. Using finite difference techniques to solve the Bloch equations for flowing spins, the phase of the transverse magnetization after a slice-selective pulse was evaluated for varying flow velocities and for tip angles which ranged between 0 degrees and 180 degrees. The following results were obtained for constant slice thickness: (1) For a fixed tip angle, the phase varies nonlinearly with velocity. (2) For a fixed velocity, the phase varies nonlinearly with tip angle. For small tip angles the nonlinearity in the variation as a function of velocity is very small but increases for tip angles greater than 90 degrees and becomes especially severe near 180 degrees. A method is proposed to desensitize the phase of flowing spins during the application of slice-selective pulses. The compensation scheme depends upon tip angle but is virtually independent of flow velocity. The technique was tested and verified with clinical images.