Accurate velocity mapping with FAcE.

Spin dislocation between the slice selection, phase encoding, and frequency encoding is a source of image distortions. Two strategies can be pursued to improve the appearance of moving spins in an image. Either the sequence is made equally sensitive to velocity-dependent dislocation artifacts for all spatial directions or the sensitivity is reduced with a shorter echo time. The first approach increases the dislocation for the phase-encoding direction and is therefore not useful if velocity maps with minimal distortion are the goal. FAcE (FID acquired echoes) is a sequence with separate sampling of the left and right k-space half-planes that allows for very short echo times. It was applied for velocity mapping of flow in the slice select direction. Special attention was paid to a compact design of the velocity-encoding select gradient to achieve short echo times even with high velocity sensitivity. Artifacts introduced by in-plane motion were studied for FAcE and conventional gradient-echo sequences, both in phantom experiments and simulation. FAcE allows for very short echo times with inherent motion compensation of the frequency-encoding gradient. Thus, both motion-related dislocation artifacts and signal voids due to coherence loss in regions with irregular flow are minimal.