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In 1945, Kety and Schmidt1 devised a method to measure cerebral blood flow using a diffusible indicator, nitrous oxide. In 1949, Kety2 substituted 24Na and described the first practical method to measure blood flow using a radionuclide tracer. In the following years, some groups3–11 elaborated further on Kety’s work and developed methods for measuring myocardial blood flow by injection of diffusible tracers into a coronary artery or into the myocardium itself. The disappearance, or “washout,” of radioactivity was measured by an external radiation-detection system placed over the myocardium. Kety had shown that the rate of washout of the diffusible indicator from tissue to blood was a function of tissue blood flow.1,2,12