Perfusion is a physiological term that refers to the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue.
Two main categories of functional magnetic resonance imaging (MRI) techniques can be used to measure tissue perfusion in vivo. The first is based on the use of injected contrast agent that changes the magnetic susceptibility of blood and thereby the MR signal which is repeatedly measured during bolus passage. The other category is based on arterial spin labeling (ASL) where arterial blood is magnetically tagged before it enters into the tissue of interest and the amount of labeling is measured and compared to a control recording obtained without spin labeling.