Accurate velocity mapping with FAcE |
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Institution: | 1. Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, USA;2. The Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., Bethesda, MD, USA;3. National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, Bethesda, MD, USA |
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Abstract: | 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. |
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