In this common type of scan, the radioactive isotope thallium is injected into your vein at rest and at the end of an exercise or pharmacologic stress test. Images are taken after each injection that show how the blood is flowing through your the arteries in your heart. If the arteries have blockages, there is not enough blood flowing through them during stress, resulting in an abnormality on the scan.
This test can also identify regions of damage or scarring of the heart muscle due to previous heart attack. scarring of the heart muscle due to blocked coronary arteries or a previous heart attack.
The S. Mark Taper Foundation Imaging Center has more information about thallium scans, including how to prepare and what to expect.