ABSTRACT
An important technical difference between the various methods for perfusion imaging relates to whether the tracer used can be considered as diffusible or non-diffusible. The physiological principles and mathematical models underlying these two approaches are distinct. The diffusible tracer technique relies upon rapid passage of the tracer across the blood-brain barrier (BBB) into the cerebral tissue, whereas non-diffusible tracers remain in the intravascular compartment. Contrast agents can be considered as non-diffusible during their first passage through the circulation, particularly if the BBB is intact. Although comparative studies often utilize
Kenneth A Miles and Max Wintermark
both diffusible and non-diffusible techniques, there are recognized situations for which diffusible and non-diffusible tracers will give quite different results. Diffusible tracers need sufficient opportunity to pass from the vascular space into the cerebral tissue. Such opportunities may not exist for large and medium-sized arteries and for arteriovenous malformations and shunts. Furthermore, diffusible tracers must be retained within cerebral tissue. For some tracers, the mechanism of retention can be impaired by disease. For example, brain tumors may not retain the radiopharmaceutical used for cerebral perfusion SPECT. Because diffusible tracers rely upon perfusion of intact cerebral tissue, such tracers are often said to depict ‘nutrient’ flow. On the other hand, non-diffusible tracers will depict flow in all blood vessels but larger vessels can be excluded during image processing.4 Microspheres, which provide the ‘gold standard’ reference method for validating perfusion measurements in animals, represent a non-diffusible tracer that is trapped in arterioles and therefore do not depict arteries and arteriovenous connections. It is not the intention here to enter the debate into the relative merits of diffusible and non-diffusible tracers. Suffice it to say that one should be cognizant of the differences and clear as to which technique is being used in any particular situation.
