Thallium Heart Scan

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Thallium Heart Scan

Definition

A thallium heart scan is a diagnostic test that uses a special perfusion-scanning camera and a small amount of thallium-201, a radioactive substance, injected into the bloodstream to produce 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 reduced blood supply as a result of damage from a previous myocardial infarction (heart attack) or blocked coronary arteries. While exercise testing has long been a standard examination in the diagnosis of coronary artery disease, in some instances, the thallium scan may provide more sensitive and more specific information. In other words, the test may be better able to detect a problem and to differentiate one condition from another. For example, a thallium heart scan may more accurately detect ischemic heart disease. A thallium scan is most likely to aid diagnosis in cases where 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 coronary blood flow following 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 radioisotope thallium 201 is injected into the patient's bloodstream through an IV (intravenous) line. The patient then lies down under a gamma scintillation camera, which generates photographs from the gamma rays emitted by the thallium.

Thallium attaches 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 accumulates in the cells of the heart muscle. Since the thallium can only reach those areas of the heart with an adequate blood supply, it can help to detect perfusion defects. In patients with perfusion defects, no thallium will show up in poorly perfused areas of the heart. Instead, these areas show up as "cold spots" on the thallium scan. The patient is then given a second injection of thallium. Several hours later, the gamma scintillation camera takes more pictures in order to obtain an image of the heart when the patient is at rest.

Cold spots that appear at rest as well as during exercise often indicate an area of previously damaged heart tissue or scars that have resulted from a prior myocardial infarction. 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 cold spot indicates ischemia resulting from a blockage in the coronary arteries. In ischemia, the heart temporarily does not get enough blood flow. Patients with perfusion defects, especially perfusion defects that appear only during exercise, have the greatest risk of future cardiac events such as myocardial infarctions.

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, in place of thallium, a 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.

When patients are unable to exercise because of another medical condition, such as arthritis or lung disease, they may be given a pharmacological thallium test instead of an exercise thallium stress test. In the pharmacological test, a drug is administered to mimic the effects of exercise on the heart such as dipyridamole (Persantine), which dilates the coronary arteries, or dobutamine, which increases blood flow through the heart muscle.

Preparation

Patients should be instructed not to 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 be advised 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. Patient education preceding a thallium scan may be performed by a nurse or cardiovascular laboratory technician.

Aftercare

In most cases, another set of scans may be needed (one in conjunction with exercise, one at rest), and the patient may be given special instructions regarding eating and test preparation. In most cases, patients are free to return to their normal daily activities.

Complications

Radioisotopes such as thallium 201 should not be administered during pregnancy because they may be harmful to the fetus.

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.

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.

Health care team roles

A thallium scan is generally ordered by a primary care physician or cardiologist and is performed by a trained technician. All healthcare providers performing or monitoring cardiac tests should be prepared to provide emergency medical intervention, such as defibrillation. The exam is interpreted by a radiologist, cardiologist, or nuclear medicine physician.

Patient education

Patients must be well prepared for a thallium scan. They should not only know the purpose of the test, but also signs and symptoms that indicate the test should be stopped. Physicians, nurses, and ECG technicians can ensure patient safety by encouraging them to immediately communicate discomfort at any time during the scan.

KEY TERMS

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.

Resources

BOOKS

The Faculty Members at the Yale University School of Medicine. "Myocardial Perfusion Scan." In The Patient's Guide to Medical Tests. Ed. by Barry L. Zaret, et al. Boston: Houghton Mifflin Company, 1997, 99-101.

Thelan, Lynne A., et al. Critical Care Nursing Diagnosis and Management. St. Louis, MO: Mosby, 1998, pp. 438.