A nuclear stress test measures blood flow to your heart muscle at rest and during stress. It is performed similar to a routine exercise stress test but provides images in addition to electrocardiograms. During a nuclear stress test, a radioactive substance is injected into your bloodstream. This substance mixes with your blood and travels to your heart. A special scanner which detects the radioactive material in your heart creates images of your heart muscle. Inadequate blood flow to any part of your heart will show up as a light spot on the images because not as much of the radioactive substance is getting there.