Radionuclide Imaging of the Heart
In radionuclide imaging, a tiny amount of a radioactive substance (radionuclide), called a tracer, is injected into a vein. The amount of radiation the person receives is tiny—less than that produced by most x-rays. 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 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 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-99 sestamibi or thallium-201, and images are obtained after the person has an exercise stress test. 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, dobutamine, 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).
If a heart attack may have occurred very recently, technetium-99m is used instead of thallium-201. With technetium, damage due to a heart attack can be detected after 12 to 24 hours and for up to about 1 week. Unlike thallium, which accumulates primarily in normal tissue, technetium accumulates primarily in abnormal tissue. However, because technetium also accumulates in bone, the ribs somewhat obscure the image of the heart.
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.
Radionuclide imaging exposes people to less radiation than comparable x-rays. However, the radioactive material does remain 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.