Arterial spin labeling (ASL) is a new magnetic resonance imaging (MRI) technique that measures blood perfusion or how much blood is being delivered into different areas of the brain. It does this by 'tagging' blood before it enters the brain, using a magnetic process to make blood appear darker in the MRI images. As blood supplies brain, it makes the areas it is supplying darker as well. With ASL, images are acquired before and after the tagging process, which provides a measure of blood flow in different regions of the brain. Higher blood flow is associated with higher brain activity in these areas. The picture shows an arterial spin labeling (ASL) perfusion scan from a patient with posterior cortical atrophy, with bright colours corresponding to brain regions with higher blood flow and darker colours indicating regions with reduced blood flow.