The extent, location, and severity of stenotic atherosclerotic lesions in the coronary arteries are determined by intravenous injection into the circulating blood of a number of small, discrete positron-emitting particles and by tracking the particles in three dimensions in the region of the heart using suitable gamma detectors disposed external to the patient's chest. The resolution and counting rates of the gamma detectors permit the sequence of positions of each particle to be recorded as a function of time as each particle flows through a coronary vessel. Data analysis of these recordings of particle position provide information about the velocity of blood flow through the course of each coronary artery. This velocity information is used to determine the extent, severity, and location of stenotic lesions of the coronary arteries.