to measure tissue perfusion in vivo. The first is based on the use of an 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 labelling (ASL), where arterial blood is magnetically tagged before it enters into the tissue being examined and the amount of labelling that is measured and compared to a control recording obtained without spin labelling. CT Brain perfusion (more correctly transit times) can be estimated with contrast-enhanced computed tomography. Thermal diffusion Perfusion can be determined by measuring the total thermal diffusion and then separating it into thermal conductivity and perfusion components. rCBF is usually measured continuously in time. It is necessary to stop the measurement periodically to cool down and reassess the thermal conductivity. See also Pressure ulcer — continuous external pressure impairs perfusion References External links Cardiac Surgery Portal Perfusion Education Portal Respiratory physiology Cardiovascular physiology Underwater diving physiology