Definition of Thallium Stress Test
This is a scintillation scan (gamma camera or photoscan) involving the use of thallium-201, a radioisotope.

The radioisotope is administered intravenously and localizes, like calcium, in the myocardium (heart muscle).

A scintillation camera produces an image of the distribution of the radioisotope. Areas of inadequate perfusion (blood flow) appear as "cold spots," thus pinpointing sites of myocardium (heart muscle) at risk.

Description of Thallium Stress Test
Various nuclear scans have been developed to provide your doctor with specific types of information. Nuclear scans can show the size of the heart chambers, how effectively the heart pumps (ventricular function), how well the coronary arteries supply the heart with blood (myocardial perfusion), whether there is scarring of the heart muscle from previous heart attacks, and the status of the lung circulation.

Additional information can often be obtained from nuclear scanning by performing it in combination with an exercise stress test. When performed in combination with exercise or another form of stress, nuclear scanning tests can provide vital information about the flow of blood through the coronary arteries.

Radionuclide ventriculography can show this when the heart is scanned before and during increasing stages of exercise. The pumping functions of all parts of the ventricle are compared at each level of exercise. A normal heart will contract more vigorously during exertion. A heart in which one or more regions beat poorer than expected during exercise is likely to be receiving inadequate oxygen-rich blood to keep up with the muscle's increased need for energy. When the scan shows a region with poor function, the doctor can conclude that the coronary artery supplying that portion of the heart muscle is at least partially blocked.

Another very useful nuclear scanning procedure for evaluating the coronary blood flow is perfusion scanning. This is usually performed in conjunction with exercise. Unlike the radionuclide tracer used in radionuclide ventriculography, the tracer used for perfusion scanning does not remain in the bloodstream. Rather, it enters the cells of the myocardium (heart muscle) after it is carried there through the coronary arteries. During the scanning, an image is made of the myocardium itself, not of the blood in the ventricle. If a portion of the myocardium receives less blood than the rest of the heart, it will receive less tracer and show up on the scan as a lighter area (perfusion defect).

The number of cases of heart disease detected with thallium scans is about 20 percent greater than it would be with exercise testing alone. A common tracer used for perfusion scans is radioactive thallium; the term "thallium scan" is therefore often used synonymously with "perfusion scan."

In experienced laboratories, stress perfusion scans are positive in 75 to 90 percent of patients with anatomically significant coronary disease and in 20 to 30 percent of those without it (false positive).
 

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