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Radionuclide imaging is a technique that allows physicians to obtain very clear images of various parts of the body, such as the heart. The scans can evaluate organs, tissues, veins and arteries in a patient’s body. Nuclear imaging technology is a valuable tool for detecting chemical changes that may indicate cancer. Certain tests can also evaluate how blood flows to tissues and organs.
To obtain these images, tiny amounts of radioactive materials (called “tracers”) are introduced into the patient’s body. The tracers emit a certain type of energy called gamma rays, which are detected by special devices.
There are several types of radionuclide imaging tests that may be used for heart disease. The tests are performed in similar fashion but with different technology and for different reasons. These tests include:
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Positron emission tomography (PET). PET imaging or PET scan is a diagnostic test that uses photomultiplier-scintillator detectors to detect the gamma rays. PET scan is most commonly used to detect cancer and evaluate treatment. The scan can be performed on the entire body and can determine if and where cancer has spread in the body.
PET scans can also be used to evaluate blood flow to the heart and to detect signs of coronary heart disease. The scan can help determine if areas of the heart with decreased function have been damaged from a heart attack (myocardial infarction). PET scans are often used in conjunction with other tests to help determine if a patient would benefit from procedures, such as angioplasty or coronary artery bypass graft (CABG).
To conduct a PET scan, a radioactive substance that is attached to a compound, most commonly glucose (sugar) is injected intravenously (I.V.). It takes 30 to 90 minutes for the compound to travel through the body and accumulate in the tissues. After that time period, the scanning begins, which can take up to an hour. The PET scanner will obtain images of the area(s) being studied. The status of the tissue or organ will determine how the compound appears on the image. For example, cancerous tissue will attract more of the substance and appear as “hot spots” on the images.
When a PET scan is used to evaluate the heart, they may be conducted before and after a stress test. This procedure can help evaluate blood flow to the heart in different situations.
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Single photon emission computed tomography (SPECT). This type of nuclear scan is used primarily to evaluate blood flow to tissues and organs in the body. It uses radioactive tracer along with computed assisted tomography (CAT) scan to obtain images. Unlike a PET scan, the radioactive compound remains in the blood stream rather than being absorbed by tissues. For that reason, this test only obtains images of areas of blood flow in the body.
SPECT scans are commonly used to view how blood flows in the veins and arteries to and from organs, such as the brain and heart. It is helpful to detect areas that have been deprived of blood as with a stroke.
Patients with heart disease may undergo a cardiac SPECT also known as myocardial perfusion imaging. This scan uses computer technology to assess blood flow through the heart and to detect areas of abnormal heart function. It can help detect blockages in the coronary arteries and determine whether a person has suffered a heart attack. In addition, it can help assess a patient’s condition after angioplasty or bypass surgery.
Imaging is typically performed while the patient is at rest, and after exercise or medically-induced stress. The severity of the abnormalities noted during the test are considered a predictor of future cardiac death. Patients with a normal study have a very low probability of cardiac death (less than one percent annually) in the few years following the SPECT study.
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Multiple gated acquisition scan (MUGA). Also known as radionuclide angiography (RNA), this test helps evaluate heart function. It measures the amount of blood pumped out of the ventricles with each heartbeat (ejection fraction). In this test, the tracer substance attaches itself to red blood cells. A scanner obtains images of the blood cells as they travel through the heart and a computer calculates the ejection fraction.
For most people, the tracer used in these scans is harmless and is flushed from the body after the test. Patients may be encouraged to drink increased amounts of liquids and to urinate frequently to help flush the tracer from their system. For most people, the amount of radiation received during radionuclide imaging is considered safe. It is normally less than the amount received during a chest x-ray. However, there are a number of people who should avoid radionuclide imaging. These include women who are pregnant or breastfeeding. Other patients who should avoid this type of test include those with the following conditions:
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Inflammation of the heart muscle (myocarditis)
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Recent myocarditis (lung) infection
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Coarctation of the aorta
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Severe narrowing (stenosis) of the aortic valve
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Severe heart failure
A patient’s primary physician or cardiologist can determine which scan is best for his or her condition. In some cases, patients may need to receive more than one scan or additional tests following the scan.
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