Nuclear imaging, or scanning, techniques produce highly accurate pic- tures of your heart and its function by introducing a small, safe amount

of radiation into your body. Trace amounts of radioactive material, called radionuclides, are injected into your bloodstream. The radionuclides tag your red blood cells and circulate with them into your heart and heart muscle. A specialized gamma camera, which reacts to the radiation by emitting light, constructs an image that is displayed on a monitor. Your doctor can study both your heart muscle and blood ?ow. Nuclear imaging techniques are often done in combination with a stress test and an injection of thallium. The nuclear isotope ?ows in the blood and may not appear on areas of the heart where there is a decreased blood supply; this is called a cold spot, or a perfusion (?ow) defect.

Thallium Stress Test

A thallium stress test shows how well blood is ?owing to the heart mus- cle during exercise. It can show any decreased blood ?ow to speci?c parts of the heart due to blockage of a coronary artery, the aftereffects of a heart attack, the effectiveness of procedures done to open coronary arteries, and other causes of chest pain. This test is also called a perfu- sion scan (perfusion is the ?ow of blood through a speci?c organ or tis- sue) or an isotope stress test (thallium and technetium are the two most common isotopes, or radioactive substances, used for these tests).
As with other nuclear scanning techniques, thallium or another trac- ing substance injected into your bloodstream travels to your heart and enters heart muscle cells. The images produced by the scan show how much blood is getting to different parts of your heart. If the supply appears reduced to a certain area, it indicates a coronary artery block- age. If no blood is getting to some tissue, it is probably dead (scar) tis- sue from a previous heart attack. The ?ow is compared at rest and after medication-induced stress. The test should not be performed during pregnancy.

What to Expect

You prepare for a thallium stress test like any other stress test. Do not eat or drink anything for 3 or 4 hours before the procedure, and wear clothing and shoes that will be comfortable during a treadmill exercise test—for example, athletic shoes or running shoes. Ask your doctor about whether you should take your usual medications before the test, particularly if you have diabetes and are taking insulin. For people who cannot exercise, this test can also be performed after medication is injected to simulate the effects of exercise. Ask how long the test will take; it may take several hours.
At the time of the test, a technician will apply electrodes to your chest and back that are attached to an ECG (electrocardiography) machine. Your heart rate and blood pressure will be measured, and then you will get on a treadmill. You will continue on the treadmill at a grad- ually increasing pace, until you are at or near your maximum level of exercise. You will be injected with the thallium (or other tracing mate- rial). Your heart rhythm is monitored continuously, and your blood pressure is checked periodically. You will then lie down on a table with a gamma camera over it, and the technician will take images of your heart while it is still working hard. You may need to hold a position for several minutes with your arm raised over your head.
After the exercise portion of the test is over, you can leave the of?ce or laboratory for 3 or 4 hours. You may get something to eat and drink, as long as it does not contain caffeine or chocolate. When you return, you will lie down on the table under the gamma camera for images of your heart at rest. The thallium has moved through your body and can now be seen. It is important to lie still during this part of the test, which may last from 10 to 20 minutes. Some people ?nd it challenging to lie in one position on a hard table, but there is no actual pain. When the test is complete, you can return to your usual activities and eat or drink anything you like.
Some laboratories choose to do the resting scan ?rst and then the exercise scan. This test should not be performed during pregnancy.

What the Results Mean

You will probably get the full results in a few days. Generally, the results of a thallium stress test are as follows:
• If your results are normal during both exercise and rest, the blood flow through your coronary arteries to your heart muscle is adequate.
• If the blood ?ow is normal during rest but not during exercise (which your doctor may call a perfusion defect), then your heart is not getting enough blood when it is working harder. An artery is probably blocked.
• If your blood ?ow is reduced during rest and worsens during exer- cise, a portion of your heart is undersupplied at all times.
• If no thallium is present at all in some of your heart muscle both during and after exercise (the so-called ?xed effect), you have prob- ably had a heart attack and some tissue is dead; it is now scar tissue.