62 Arterial Spin Labeling (ASL)

The past decade has seen the emergence of arterial spin labeling (ASL) perfusion MR and its development as a noninvasive method for the evaluation of cerebral blood flow-without the need for intravenous contrast media injection. ASL uses magnetically labeled arterial blood water as an endogenous tracer for measuring blood flow. This magnetic labeling is performed using radiofrequency pulses, which alter the magnetization of the blood flowing into the brain. The extent of this alteration is determined by comparing the ASL image with a reference (control) image obtained without blood water labeling. Quantitative regional maps of cerebral blood flow can be obtained in units of mL 100 g^-1 min^-1 if the kinetic parameters of perfusion are known. Several ASL methods have been proposed, but in the following only two methods will be described in detail: continuous ASL (CASL) and pulsed ASL (PASL).

CASL.

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