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Definition A thallium heart scan is a test using a special camera and a small amount of radioactive substance injected into the bloodstream to make an image of the blood flow to the heart. Purpose A thallium heart scan is used to evaluate the blood supply to the heart muscle. It can identify areas of the heart that may have a poor blood supply as a result of damage from a previous heart attack or blocked coronary arteries. While exercise testing has long been a standard examination in the diagnosis of coronary artery disease, in some cases, the thallium scan may be more sensitive and more specific in the information it provides. In other words, the test may be better able to detect a problem and to differentiate one condition from another. A thallium heart scan may more accurately detect ischemic heart disease. This type of scan is most likely to be helpful in cases in which the exercise test is inconclusive, the patient cannot exercise adequately, or a quantitative evaluation of blood flow is required. In addition to evaluating coronary artery disease, thallium scanning can help to evaluate blood flow following treatment of clogged arteries with coronary artery bypass graft surgery or angioplasty. Precautions Radioisotopes such as thallium 201 should not be administered during pregnancy because they may be harmful to the fetus. Description The thallium scan is performed in conjunction with an exercise stress test. At the end of the stress test (once the patient has reached the highest level of exercise he or she can comfortably achieve), a small amount of the harmless radioisotope thallium 201 is injected into the patient's bloodstream through an intravenous (IV) line. The patient then lies down under a special camera called a gamma scintillation camera, which makes photographs from the gamma rays emitted by the thallium. The thallium attaches itself to the red blood cells and is carried throughout the body in the bloodstream. It enters the heart muscle by way of the coronary arteries and collects in the cells of the heart muscle that come into contact with the blood. Since the thallium can reach only those areas of the heart with an adequate blood supply, no thallium will show up in poorly perfused areas of the heart (perfusion defects). These areas show up as "cold spots" on the thallium scan. The patient may then be given a second injection of thallium. Several hours later, the gamma scintillation camera takes more pictures in order to get an image of the heart when the patient is at rest. Cold spots that appear at rest as well as during exercise often indicate areas where the heart tissue has been damaged (for example, as a result of a prior heart attack). Sometimes perfusion is adequate during rest but cold spots appear during exercise, when the heart has to work harder and has a greater demand for blood. This can indicate some blockage in the coronary arteries, producing a condition called ischemia. In ischemia, the heart temporarily does not get enough blood flow. People with perfusion defects, especially perfusion defects that appear only during exercise, have the greatest risk of such future cardiac events as heart attacks. In recent years, there have been improvements in heart scanning. Many centers now use a single photon emission computed tomographic (SPECT) camera, which provides a clearer image. Some centers also use a type of radioactive chemical called sestamibi. Sestamibi is used along with a radioactive compound called technetium. While thallium may still be better for some uses, such as providing a better image of the heart muscle itself, sestamibi may produce clearer images in overweight patients and is more useful in assessing how well the heart pumps blood. If the patient is unable to exercise because of another medical condition, such as arthritis, he or she may be given a drug to mimic the effects of exercise on the heart. Some of these drugs include dipyridamole (Persantine), which dilates the coronary arteries; and dobutamine, which increases blood flow through the heart muscle. Preparation Patients should not drink alcoholic or caffeinated beverages, smoke tobacco, or ingest other nicotine products for 24 hours before the test. These substances can affect test results. Patients should also not eat anything for at least three hours before the test. They may also be instructed to stop taking certain medications during the test that may interfere with test results. Aftercare In some cases, another set of scans may be needed, and the patient may be given special instructions regarding eating and test preparation. Otherwise, the patient is free to return to his or her normal daily activities. Risks Radioisotopes such as thallium 201 should not be administered during pregnancy because they may be harmful to the fetus. Normal results A normal thallium scan shows healthy blood flow through the coronary arteries and normal perfusion of the heart muscle, without cold spots, both at rest and during exercise. Abnormal results Cold spots on the scan, where no thallium shows up, indicate areas of the heart that are not getting an adequate supply of blood. Cold spots appearing both at rest and during exercise may indicate areas where the heart tissue has been damaged. However, "reversible" cold spots appearing only during exercise usually indicate some blockage of the coronary arteries. - Angioplasty
- The reconstruction of damaged blood vessels.
- Coronary bypass surgery
- Surgery in which a section of blood vessel is used to bypass a blocked coronary artery and restore an adequate blood supply to the heart muscle.
- Perfusion
- The passage of fluid (such as blood) through a specific organ or area of the body (such as the heart).
- Radioisotope
- A radioactive form of a chemical element, which is used in medicine for therapeutic or diagnostic purposes.
For Your Information Books - "Myocardial Perfusion Scan." In The Patient's Guide to Medical Tests, ed. Barry L. Zaret, et al. Boston: Houghton Mifflin, 1997.
Periodicals- Christian, T. F., et al. "Exercise Tomographic Thallium-201 Imaging in Patients with Severe Coronary Artery Disease and Normal Electrocardiograms." Annals of Internal Medicine 121 (1 Dec. 1994): 825-832.
- Rivitz, S. Mitchell, and Salvatore A. DeLuca. "Perfusion Imaging in Ischemic Heart Disease." American Family Physician 48 (1 Nov. 1993): 1071- 1078.
- Simon, Harvey B., ed. "Cardiac Stress Testing: New Variations on an Old Theme." Harvard Men's Health Watch 1, no. 8 (Mar. 1997): 1-4.
- Wiley Jr., John P. "Phenomena, Comment and Notes." Smithsonian (Oct. 1995): 28-31.
- Zabel, M. K., and R. M. Califf. "The Value of Exercise Thallium Imaging." Annals of Internal Medicine 121 (1994): 891-893.
Organizations- American Heart Association. 7320 Greenville Ave. Dallas, TX 75231. (214) 373-630 or (800) 242-8721. inquire@heart.org. http://www.americanheart.org
- National Heart, Lung and Blood Institute. P.O. Box 30105, Bethesda, MD 20824-0105. (301) 251-1222. http://www.nhlbi.nih.gov
Source: Gale Encyclopedia of Medicine, Published December, 2002 by the Gale Group
The Essay Author is Robert Scott Dinsmoor.
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