Thallium and Technetium Heart Scans Overview

Several non-invasive tests are useful in the evaluation of coronary artery disease (CAD). Among the most useful are heart scans performed with either thallium or technetium.

Thallium-201 and technetium-99m sestamibi (Cardiolite) are two radioactive substances used in tests, called “nuclear perfusion studies,” that look for blockages in the coronary arteries. By injecting thallium or technetium into the bloodstream, usually during a cardiac stress test, an image of the heart can be made that shows how well blood is flowing to the various parts of the heart muscle. If a coronary artery is partially or completely blocked because of CAD, the muscle being supplied by the diseased artery will show up on the image as a dark spot—an area of reduced or absent blood flow.

What Are Thallium and Sestamibi?

Thallium and technetium are radioactive substances that have been used for many years in cardiac imaging studies. When injected into the bloodstream, these substances attach to certain kinds of cells, including heart muscle cells. A special imaging camera that detects radioactivity can then be used to make an image of the heart muscle that has gathered the thallium or technetium.

However, thallium and technetium attach only to the portions of heart muscle that have good blood flow. If one of the coronary arteries is blocked or partially blocked, relatively little radioactivity reaches the muscle supplied by that blocked artery.

How Nuclear Perfusion Studies Are Performed

During a stress test, either thallium or technetium is injected into a vein at the point of maximum exercise. The radioactive substance then distributes itself throughout the heart muscle, in proportion to the blood flow received by that muscle. Cardiac muscle receiving normal blood flow accumulates a larger amount of thallium/technetium than cardiac muscle that is obstructed by an atherosclerotic plaque.

When patients need stress testing but are unable to exercise, adenosine or a newer drug, regadenoson, is injected into a vein to simulate exercise. Adenosine causes blood flow to redistribute in the heart muscle in a manner similar to exercise—areas with a partial blockage receive relatively less blood flow for a few minutes after an adenosine injection.

An image of the heart will then be made by a camera that can "see" the radioactivity emitted by thallium, technetium, or another drug. From these pictures, any portions of the heart that are not receiving normal blood flow (because of blockage in the coronary arteries) can be identified as “dark spots.”

Benefits

Using thallium or technetium perfusion imaging greatly increases the accuracy of a stress test in diagnosing obstructive CAD. A normal thallium/technetium test is an excellent indication that there are no significant blockages in the coronary arteries. On the other hand, patients with abnormal perfusion scans are highly likely to have significant blockages.

Nuclear perfusion studies are used in three general circumstances. First, they are useful in patients who are suspected to have stable angina due to fixed blockages in the coronary arteries.

Second, these studies are used in patients who have been treated medically (that is, non-invasively) for unstable angina or non-ST-segment myocardial infarction (NSTEMI), and who have appeared to stabilize. If their thallium/technetium tests show no significant residual blockages, it is relatively safe to continue with medical therapy alone. Otherwise, they should be considered for angioplasty and stenting, or for bypass surgery.

Third, these studies are used to assess the viability of the heart muscle beyond a severe blockage in a coronary artery. If the heart muscle “lights up” to any extent with thallium/technetium, then it is still partially viable—and stenting or bypassing the artery can be expected to improve the function of the heart. Otherwise, a revascularization procedure would not be expected to provide many benefits.

Risks

These noninvasive studies are quite safe. Their only drawback is that a small amount of radiation is used. The level of radiation the patient receives is thought to produce a very small risk of harm, if any, and for appropriately selected patients the potential for benefit far outweighs this small risk.