All arterial spin labeling (ASL) pulse sequences consist of two components: 1) a preparation module to magnetically label/tag flowing blood, and 2) a readout module to generate paired images of the target tissue under "control" and "tagged" conditions. ASL methods may be classified as to how each of these modules is constructed.
3D methods take slightly longer than 2D methods to perform, but are easier to set up. They offer higher signal-to-noise and better spatial resolution, as well as a fixed labeling delay across all slices. At present 3D techniques are not compatible with fMRI studies as perfusion changes over short time intervals cannot be obtained. Background suppression is also generally required that reduces perfusion signal and may confound calculation of absolute blood flows.
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A fourth ASL method, still largely confined to research centers, is known as VSASL (Velocity-Selective Arterial Spin Labeling). Unlike the other ASL methods, VSASL tags blood based on its velocity, not spatial location. Crusher gradients are used to saturate spins flowing faster than a chosen cutoff velocity, thus allowing labeling of slower blood flows. Because labeling is not spatially selective, VSASL is much less sensitive to long transit delay times than other ASL methods. Disadvantages include lower SNR and difficulties in selecting appropriate cutoff velocities.
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What is PASL and how does it differ from CASL?
What is pCASL and how does it differ from CASL and PASL?