Abstract: Particulate sources of signal which can be detected outside the body of a patient are introduced into the circulating blood and are tracked in three dimensions in the region of the heart using suitable detectors disposed about the patient's chest. This permits the sequence of positions of each particle to be recorded as a function of time as each particle flows through a coronary vessel. Data analyses of these recordings of particle positions 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.