Flow suppression in rapid FLASH NMR images.

Rapid FLASH (fast low angle shot) NMR images are very sensitive to flow phenomena. In particular, a steady reflow of unsaturated spins from outside the imaging plane results in high image intensities which depend on the flip angle, the repetition time, and the flow velocity. Here we describe a technique that suppresses these signals by saturating remote spins prior to entering the imaging plane, e.g., by intercalating the FLASH sequence with slice-selective 1-2-1 pulse packages. Experiments have been carried out on phantoms and human extremities. Flow artifacts in the phase-encoding direction of the images are strongly reduced. Vascular structures may be easily delineated using difference images obtained with and without flow suppression.