Nuclear tracers are radioactive elements that are injected into the bloodstream with an intravenous catheter, and enter heart cells in proportion to the blood flow reaching those cells. Thus, with more blood flow to the heart, greater amounts of radioactive tracer enter the heart cells. These tracers have not been found to have side effects in clinically used dosages. The tracers give off radioactive particles called photons that can be detected and measured by a special camera. The camera reconstructs an image of the heart based on the number of photons received. Initially the tracers are injected, and imaging starts when the patient is in a resting state. The patient then undergoes stress testing, either with exercise or pharmacologic stress. During the testing, a second tracer injection is performed and another set of images is taken. The two sets of images are analyzed and compared by a physician trained in the technique. Each portion of the left ventricle of the heart is evaluated. The cardiac blood supply is deemed to be normal if the heart muscle takes up maximum amounts of tracer. A reduced amount of tracer may be taken up in a specific heart segment on the stress images but be normal on the rest images. This situation indicates a significant heart artery blockage. In the resting state, blood flow may be adequate to meet the needs of the heart even with a partial blockage, but in the stress situation that artery is incapable of sufficiently increasing blood flow. Thus, relatively less tracer is delivered to the heart muscle supplied by that artery, compared to heart segments with less or greater blood flow. This is reflected as a "defect" in the heart pictures. Lastly, a defect may be present on both the rest and stress images. This suggests a "myocardial infarction" or heart attack, whereby heart muscle is irreversibly damaged and the dead cells cannot take up the tracer.