Flow sensitivity and coherence in steady-state free spin precession.

Steady-state free precession (SSFP) of nuclear spins in the presence of a magnetic field gradient is known to be very sensitive to flow. We present a theoretical and experimental study of flow sensitivity in a regime where the spacing of the radio-frequency pulses is extremely short compared with the free induction decay time and the relaxation times. Under these rather drastic conditions, a truly continuous wave free precession (CWFP) regime is established, in which, unlike standard SSFP, a large degree of coherence is preserved. This leads to a quite different flow sensitivity, which is significant even when very small magnetic field gradients are present. The unspoiled coherence is predicted to cause different flow effects which we confirmed experimentally. Tailored flow sensitivity can be achieved by adjusting the frequency offset from resonance, which plays a dominant role in the CWFP regime.