Using radioactive isotopes, such as 201 TI or 99 mTc sestamibi, to gauge the blood supply and viability of the regions or walls of the heart. MPI frequently is used to assess patients with coronary artery disease, often in conjunction with exercise tolerance tests. A patient with a coronary artery that is almost totally blocked, for example, may take up only a small quantity of radioisotope during exercise but much more of the tracer after several hours of rest. By contrast, heart muscle that is fed by a completely blocked artery will take up no radioisotope either during or after exercise.