Arterial Spin Labelling (ASL)
The goal of Arterial Spin Labeling (ASL) methods is to image tissue perfusion. Perfusion is defined as the amount of blood delivered to capillary beds in a given tissue per unit time. Because the microvasculature is too small to be spatially resolved using current MRI techniques, perfusion cannot be measured by directly imaging blood flow using angiographic methods, and it is necessary to measure the net delivery of blood to each imaging voxel instead.
The general approach taken by ASL is to modify the magnetization of arterial blood, use this magnetically tagged blood as an endogenous tracer, and measure the delivery of the tracer to target tissues. The generic ASL measurement is as follows: inflowing arterial blood is tagged by inversion, and after a delay to allow for tagged blood to flow into the target tissue, an image is acquired. Because this image consists of signal from both tagged blood and static tissues in the imaging region, a second image, acquired without prior arterial tagging, is subtracted from the first image in order to isolate the signal from the tagged blood. For appropriate pulse sequence parameters, this difference image is directly proportional to the local tissue perfusion.
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