During a SPECT scan, the patient is injected with a small amount of radioactive tracer. This tracer remains in the bloodstream and can be tracked as blood travels through an organ (e.g., perfuses the organ). As the tracer travels, gamma cameras (which are rotating around the patient’s body) take pictures to monitor its progress. The cameras record the distribution, position and rate of flow of the tracer through the heart. Computer graphics are used to create three-dimensional images of blood flow through the heart that can be projected onto a computer monitor for the physician to study.

Any marked change in the absorption rate from established “normal values” could be a sign of heart disease. If the heart absorbs abnormally high or low levels of the tracer element, the pictures produced will be either very bright (a “hot spot”) or very dark (a “cold spot”), respectively. Either extreme is a clear sign of abnormal function, damage or disease of the heart. Depending on which organ of the body is being imaged, physicians may use one of several different chemicals as the tracer. These chemicals are called radiopharmaceuticals, which are proteins or organic molecules with radionuclides attached. Some radiopharmaceuticals contain proteins that will collect in the heart muscle, while others are more useful for studies of other organs such as the lungs or the brain.

The two most common radiopharmaceuticals used for cardiac SPECT imaging today are thallium-201, and technetium-99m. A relative newcomer is technetium Tc99m sestamibi, which has been very successful in obtaining vivid images of the female heart. Women’s thicker breast tissue and smaller heart makes it more difficult to obtain good diagnostic images of the heart, and as a result, women’s heart problems are often misdiagnosed. Recent studies have suggested that this tracer is more effective than thallium when used in cardiac SPECT imaging in women.

Still other chemicals are being studied, including several forms of technetium-99m such as technetium-99m tetrofosmin. This chemical was shown to be roughly as effective as sestamibi. Another new chemical, technetium-99 NOET, seems to have the imaging capability of regular technetium-99 but with better redistribution qualities.