Diffusible tracers pass the blood-brain-barrier and are taken up by the brain parenchyma. The concentration of the tracer in the brain parenchyma is therefore correlated with the amount of blood flow to the brain parenchyma. With knowledge of the tracer concentration in the arterial blood supply, absolute CBF values can be calculated. Perfusion imaging techniques that use diffusible tracers include Xenon-CT, Single Photon Emission CT (SPECT) and positron emission tomography (PET). PET is generally viewed as the gold-standard perfusion imaging technique, but it is used little in clinical practice due to its high cost, limited availability, and the poor resolution of PET perfusion images.
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