Arterial spin labeling (ASL), also known as arterial spin tagging, is a magnetic resonance imaging method for measuring perfusion using water already in the subject as tracers by setting and tracking magnetic spin. Perfusion MRI is perfusion scanning by the use of a particular MRI sequence. ASL specifically refers to magnetic labeling of arterial blood below the imaging slab, without the need of gadolinium contrast, instead inferring perfusion from a drop in signal observed in the imaging slice arising from inflowing spins (outside the imaging slice) having been selectively saturated. A number of ASL schemes are possible, the simplest being flow alternating inversion recovery (FAIR) which requires two acquisitions of identical parameters with the exception of the out-of-slice saturation; the difference in the two images is theoretically only from inflowing spins, and may be considered a 'perfusion map'. The technique was developed by John Detre, Alan P. Koretsky and coworkers in 1992.
Arterial spin labelling takes advantage of the water molecules circulating with the brain, and using a radiofrequency pulse, tracks the blood water as it circulates throughout the body. After a certain delay, a 'label' image is captured. A 'control' image is also acquired before the labelling of the blood water. A subtraction technique (simple subtraction, or surround subtraction) gives a measurement of perfusion. In order to increase SNR, a collection of control and label images can be averaged. There are also other specifications that can increase SNR, like the amount of head coils (eight or more is recommended), or a stronger field strength (3 T is standard, but 1.5 T is satisfactory).
In cerebral infarction, the penumbra has decreased perfusion. Besides acute and chronic neurovascular diseases, the value of ASL has been demonstrated in brain tumors, epilepsy and neurodegenerative disease, such as Alzheimer’s disease, frontotemporal dementia and Parkinson disease.
ASL is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error like other MRI techniques.
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