Cardiolite is the brand name of a radioactive drug called technetium Tc99m sestamibi, which is not used for treatment but is a substance that is injected into the body during certain heart tests. These tests are used to determine whether there is any blockage in the coronary arteries that supply blood to the heart muscle and to assess how well the heart muscle is contracting. Cardiolite works by entering the coronary arteries and flowing with the blood to the heart muscle. It then accumulates in the heart muscle cells. Because it is radioactive, it can be seen with special scanning devices. If a coronary artery is blocked, the Cardiolite will not be seen in the muscle supplied by that artery. With Cardiolite tests, your doctor can tell if you have any areas of your heart where there is either reversible or irreversible decrease in blood flow. Cardiolite is a liquid that is injected into your vein by a trained healthcare professional. It is classified as a diagnostic drug or radiopharmaceutical (radioactive drug).