Radionuclide imaging, including positron emission tomography (PET) and single-photon emission computed tomography (SPECT), is a type of medical imaging that produces images by detecting radiation after a radioactive material is administered by vein. Radionuclide imaging of the heart can be helpful in determining the cause of chest pain.
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 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 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.
For evaluating blood flow through heart muscle, 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 medication, 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. (Alternately, the stress and resting scans are done on different days.) 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 type of radionuclide imaging, is primarily used with tracers that can help evaluate blood flow to the heart muscle. It can produce a series of computer-enhanced cross-sectional images or a 3-dimensional image can also be produced. SPECT is often used with stress testing.
Positron emission tomography (PET) is another type of radionuclide imaging. In PET, a substance necessary for heart cell function (such as oxygen or sugar) is labeled with a radioactive substance (radionuclide) that gives off positrons (electrons with a positive charge). The labeled nutrient is injected into a vein and reaches the heart in a few minutes. A sensor detects the positrons and uses them to create a picture of the body part being studied.
PET is usually combined with computed tomography (PET-CT) to determine how much blood is reaching different parts of the heart muscle and how different parts of the heart muscle process (metabolize) various substances. For example, when labeled sugar is injected, doctors can determine which parts of the heart muscle have an inadequate blood supply because those parts use more sugar than normal. CT generates cross-sectional slices of these areas and then combines them to produce detailed three-dimensional images. PET-CT scans produce clearer images than SPECT and can also be used for stress testing. However, the procedure is expensive and not as widely available as SPECT.
Radionuclide imaging can be also be used to evaluate for heart muscle damaged in prior heart attacks, the extent of diseases such as amyloidosis that infiltrate the heart muscle, and overall heart function.
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.
