User:Alpheratzstar/Arterial Spin Labeling

Introduction
Arterial Spin Labelling (ASL), also known as arterial spin tagging, is an Magnetic Resonance Imaging technique capable of measuring cerebral blood flow (CBF, also known as cerebral perfusion) in vivo. ASL can providing cerebral perfusion maps, without requiring the administration of a contrast agent or the use of ionising radiation, as it uses magnetically-labelled endogenous blood water as a freely-diffusible tracer. It was first proposed in 1992 (Detre et al., 1992; Williams et al., 1992) and has since benefited from a number of modifications aimed at improving its robustness. This has lead to the appearance of a wide number of ASL implementations accompanied by a matching number of acronyms.

The Arterial Spin Labelling Experiment
The main steps of a standard ASL experiment are summarised in. After placing the subject in the scanner, an inversion (or 180°) pulse is applied to invert the longtidunal magnetization of part of the neck (the inversion slab, in green). This step aims to invert (i.e. label) the magnetization of arterial blood downstream from the brain. A time TI later, when the labelled blood is expected to have reached the brain, an excitation pulse is applied to the area of the brain where CBF is to be measured (the imaging slice, in blue). Some of the signal in the acquired image will have come from the inverted blood that has reached the imaging slice (this is typically 1% of the total signal), but the overwhelming majority will have come from static brain tissue. This image is called the labelled (or tag) image. To remove the contribution from static tissue, an identical image (the control image) is acquired in the absence of the blood labelling step. To mimic magnetization transfer (MT) effects of the blood labelling pulse, however, the control image acquisition needs to be preceded by a pulse with a similar RF deposition to the one used to invert blood.