In radionuclide imaging Radionuclide Scanning In radionuclide scanning, radionuclides are used to produce images. A radionuclide is a radioactive form of an element, which means it is an unstable atom that becomes more stable by releasing... read more , a tiny amount of a radioactive substance (radionuclide), called a tracer, is injected into a vein. The amount of radiation the person receives from the radionuclide is tiny. The tracer emits gamma rays, which are detected by a gamma camera. A computer analyzes this information and constructs an image to show the different amounts of tracer taken up by tissues.
Radionuclide imaging of the heart is particularly useful in the diagnosis of chest pain Chest Pain Chest pain is a very common complaint. Pain may be sharp or dull, although some people with a chest disorder describe their sensation as discomfort, tightness, pressure, gas, burning, or aching... read more when the cause is unknown. If the coronary arteries are narrowed, radionuclide imaging is used to learn how the narrowing is affecting the heart's blood supply and function. Radionuclide imaging is also used to assess improvement in blood supply to the heart muscle after bypass surgery Coronary Artery Bypass Grafting Coronary artery disease is a condition in which the blood supply to the heart muscle is partially or completely blocked. The heart muscle needs a constant supply of oxygen-rich blood. The coronary... read more or similar procedures and may be used to help determine a person's prognosis after a heart attack.
Different tracers are used depending on which disorder is suspected. For evaluating blood flow through heart muscle, the tracers typically used are technetium-99m sestamibi or thallium-201, and images are obtained after the person has an exercise stress test Stress Testing Stressing the heart (by exercise or by use of stimulant drugs to make the heart beat faster and more forcibly) can help identify coronary artery disease. In coronary artery disease, blood flow... read more . The amount of tracer absorbed by the heart muscle cells depends on the blood flow. At peak exercise, an area of heart muscle that has an inadequate blood supply (ischemia) absorbs less tracer—and produces a fainter image—than neighboring muscle with a normal supply. In people unable to exercise, an intravenous injection of a drug, such as dipyridamole, or adenosine, may be used to simulate the effects of exercise on blood flow.
After the person rests for a few hours, a second scan is done, and the resulting image is compared with that obtained during exercise. Doctors can then distinguish areas of the heart where inadequate blood flow is reversible (usually caused by narrowing of the coronary arteries) from areas where it is irreversible (usually caused by scarring due to a previous heart attack).
Single-photon emission computed tomography (SPECT), a specialized type of radionuclide imaging, can produce a series of computer-enhanced cross-sectional images. A three-dimensional image can also be produced. SPECT provides more information about function, blood flow, and abnormalities than does conventional radionuclide imaging but exposes the person to more radiation than conventional radionuclide imaging.
Multiple gated acquisition (MUGA), a specialized type of radionuclide imaging, can provide information on the pumping function of the heart. It is not often used because similar information can be obtained from echocardiography Echocardiography and Other Ultrasound Procedures Ultrasonography uses high-frequency (ultrasound) waves bounced off internal structures to produce a moving image. It uses no x-rays. Ultrasonography of the heart (echocardiography) is one of... read more , which does not expose the person to radiation.
The radioactive material used in radionuclide imaging remains in the body for a few days, so people may trigger radiation alarms in airports for a few days after the procedure. People should obtain notes from their doctor if they have had a radionuclide imaging test and intend to travel via airplane or across international borders via car, train, or boat, as security agents will likely ask for proof.