A Thallium Scan is less common than other tests for Heart Disease.  A small amount of radioactive substance (thallium) is injected into your blood, usually while you are exercising on a treadmill (exercise ECG).  If you find it difficult to exercise on a treadmill then you may be given a drug which increases the blood flow to your heart.  A camera is then used to take pictures of the heart to show how the blood is flowing to the heart and how well the heart is pumping.