Myocardial Perfusion Imaging (MPI) is a diagnostic procedure used to show how well your heart muscle is being supplied (perfused) with blood at rest and under stress. It shows areas of the heart that have reduced blood supply due to narrowing of one or more coronary arteries. A small amount of radioactive tracer is used to show the heart muscle. Areas of the heart receiving adequate blood flow are yellow on a monitor. Those with reduced flow are blue.