Thallium and sestamibi stress tests; MIBI stress test
Thallium and sestamibi stress tests are nuclear imaging methods that provide a view of the blood flow into the heart muscle. The thallium and sestamibi tests are also called “MIBI stress test” and “myocardial perfusion scintigraphy”, and are used to evaluate how well your heart is perfused (supplied with blood) at rest as compared with activity.
During these tests, heart images can be obtained because the patient receives an injection of a substance that is labeled with a radioactive marker or radiotracer to make it visible in the bloodstream. These substances are also called radiopharmaceuticals, and include thallium-201 and technetium-99m MIBI or sestamibi.
In comparison to the standard treadmill stress test, thallium and sestamibi stress tests are more accurate and provide additional information.
How the test is performed
In a special room or lab at a medical center, you will be instructed to exercise as hard as you can on a treadmill or bicycle. If your doctor considers that exercise is not safe for you, or that you may be unable to exercise enough because of orthopedic problems, then you will be given an intravenous medication that will challenge your heart as if you were exercising.
When you reach your maximum level of exercise, a nurse will inject in your vein a small amount of a radioactive substance (radiotracer), either thallium or sestamibi. The radiotracer will travel in the bloodstream and, through the coronary arteries, will enter into the heart muscle as you complete your exercise session.
After you finish exercising, you will lie down on a special table under a bulky camera called a gamma camera. The gamma camera can scan your heart and detect the radiotracer in it. The distribution of the radiotracer in your heart will be processed by a computer to create pictures of your heart. The first pictures are made shortly after the exercise test, to show the circulation of blood to your heart during exercise. This is the part considered “a stress test” and is the most challenging for your heart.
Then you will need to lie quietly for 2-3 hours, and at that point the scanner will make another series of pictures of your heart. These images will show the circulation of blood through your heart muscle at rest.
If your doctor has indicated that your test should be performed without exertion, then at the beginning of the test you will not exercise, but instead will receive an intravenous medication, a vasodilator (usually dipyridamole or adenosine). This medication will selectively dilate (widen) the coronary arteries as long as they are normal; arteries with blockages will receive less blood flow and will be less dilated, allowing less blood flow into the heart muscle. After this initial medication, you will receive the injection with the radiotracer. The test done using a vasodilator can potentially show a defect in the same way as the test with exercise does.
How to prepare for the test
You will be instructed to wear comfortable clothes and shoes with non-skid soles. You will probably be asked not to eat or drink anything after midnight, except for a few sips of water if you need to take medicines.
For one entire day prior to the test, you will be required to abstain from caffeine and certain medications. Caffeine is in food and beverages such as all regular and decaffeinated coffees or teas, chocolate products, many sodas, and certain pain relievers. These are just a few examples. Please read the labels of your food and beverages carefully, because caffeine consumed before the test may invalidate the test results.
Medications that you may need to stop taking before the test include some Asthma medicines and medicines for chest pain (angina); check with your doctor. If you take insulin to control your blood sugar, ask your physician if or how much insulin you should take the day of the test.
The entire nuclear imaging test may last about 4 hours, so you will need to prepare accordingly. At some point during your stay for the test you will likely be given a long break and be allowed to have lunch or a snack at a nearby cafeteria. During the test you will not be sedated and therefore once the test is completed you will be able to drive yourself home.
How the test will feel
If you will not be exercising but will be receiving a vasodilator injection, you may feel a sting as the medication goes into the IV, which may be followed by a feeling of warmth. Some patients also develop a headache or nausea.
If any of the symptoms listed above appear during your test, let the lab personnel know immediately.
Why the test is performed
The thallium and sestamibi stress tests are indicated when your doctor needs to evaluate for example:
- how well your heart responds to exercise
- the cause of your chest pains
- the degree of blockage in your coronary arteries
- what to expect after you have had a Heart attack
- how well a heart procedure done to improve blood flow in your coronary arteries is working
When a normal amount of the radiotracer arrives into all areas of the heart, the heart images are obtained are normal. The heart images at peak exercise are compared to the heart images at rest. If during both exercise and rest all images are normal, then your blood flow through the coronary arteries is considered to be normal.
What abnormal results mean
In your heart pictures, an area may lack the radiotracer and thus show a spot of a different color, called a “defect”. Defects represent poor uptake of the radiotracer by the heart because of reduced blood flow.
When a defect occurs at peak exercise and not at rest, the most likely cause is a significant blockage of a coronary artery. When a defect is observed both at rest and with exertion, that indicates that previous damage from a Heart attack has occurred and that the heart muscle has a scar.
What the risks are
Nuclear imaging stress tests are very safe. The estimated risk of complications are only about 1 in 1,000 to 1 in 2,000 cases. These complications involve rare skin Rashes, large fluctuations in blood pressure, arrhythmias, and difficulty breathing or asthma-like reactions. These, and any additional risks that may apply specifically to you, will be explained to you in advance by your doctor and by the health care provider performing the test.
Radiation exposure to radiotracers can be a concern for the nuclear lab staff, but not for patients undergoing an occasional nuclear imaging test.
Sometimes a thallium or sestamibi test produces heart image that looks like a defect, but it’s a pseudo-defect that gives a “false positive” test result (i.e., a falsely abnormal test). This is due to interference with the heart image from non-heart tissues such as the diaphragm (diaphragm attenuation), and, in women, the breast tissue (“breast attenuation”).
When non-heart tissue attenuation is clearly the cause of an abnormal image, nothing additional needs to be done. If an attenuated image instead is thought to coincide with a true defect or area of ischemia (reduced blood flow), then your doctor may recommend additional testing such as a stress echocardiogram or a Cardiac catheterization.
by Dave R. Roger, M.D.