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It is possible by external monitoring alone to measure blood flow per unit volume through any vascular bed accessible to external monitoring of the radioactive tracer if the tracer input function or if the quantity of tracer entering the system is known. The methods proposed exploit the fact that the mean transit time through the system is the flow per unit volume of distribution of tracer. The equations are free of assumptions that require solutions as exponentials or any other specified frequency function of transit times. If the input function is known, measurement of the concentration of tracer in blood leaving the part sensed by the external detector, combined with external detection of radioactivity, can lead to measurement of absolute blood flow.
Kenneth L. Zierler (Thu,) studied this question.