Myocardial perfusion is a type of imaging test also known as a nuclear stress test. This test measures how well blood flows through the heart muscle. Additionally, it shows how well the heart is pumping. In the case of a heart attack, it may be used to discover areas of the damaged heart muscle. Exercise and rest are both acceptable times for this test. A radioactive tracer is used in myocardial perfusion scans to detect a small amount of radioactive substance. Bloodstream carries the tracer, which is absorbed by heart muscle in healthy individuals. The areas that absorb tracer appear different on the scan than those that do not. In areas with poor blood flow or damaged tissue, the tracer cannot be absorbed. In some cases, the damaged areas are referred to as cold spots or defects. When the heart muscle is stressed, a stress myocardial perfusion scan measures blood flow. Exercise usually stresses the heart. However, if you are unable to exercise, you may be able to stress your heart by taking a medication that increases your heart rate or dilates your blood vessels. Using a special camera, a radioactive tracer is injected outside the body and detected using radioactive energy. During stress and later at rest, the camera takes images of the heart and a comparison is made between the two sets of images.