Thallium Scan for Coronary Artery Diseases

Exercise stress testing has been the standard examination in the diagnosis of coronary artery diseases. During exercise, myocardial blood flow increases to meet the increase in the oxygen demand. The drawbacks are that exercise test is inconclusive. When quantitative evaluation of blood flow is required and while the patient cannot exercise adequately the exercise test will not be accurate.

Thallium heart scan uses a special perfusion scanning camera, and then a small dose of thallium 201, a radioactive substance is injected into the blood stream. The thallium scan provides more specific information and is able to detect a problem and to compare one condition from another. Thallium attaches itself to the red blood cells and is carried throughout the body in the blood stream and it enters the heart muscles. Thallium can reach these areas of the heart which has adequate supply of blood, i.e., no thallium will show up in poorly perfused areas of the heart. These poorly perfused areas of the heart where no thallium will show up are called cold spots. Often cold spots indicate previously damaged heart tissue or scars that must have resulted from a prior myocardial infraction. This cold spots indicates ischemia resulting from blockage in the coronary arteries. In ischemia, the heart does not get enough blood flow. Patients with perfusion defects which appear only during exercise have greater risk of future cardiac events such as myocardial infraction.

The latest diagnostic test uses a single photo emission computed tomographic (SPECT) camera which produces a clearer image and it gives 3 dimensional information. The camera rotates around the left chest. The image obtained is reconstructed in the computer. This gives a 3 plane through the left ventricle horizontal, long and short axis slices. In the place of thallium a chemical called sestamibi is used. Sestamibi is used along with a radioactive compound called technotrium providing better image of the heart muscles. Sestamibi helps to view images better in overweight patients. It is useful in assessing how well the heart pumps blood. The advantage that SPECT has is it removes overlying activity. It makes vessel territory identification much easier.

When patients are unable to exercise because of other medical complication such as, arthritis or lung diseases they may be given a pharmacological thallium test instead of an exercise thallium stress test. This form of testing is also used in patients with left bundle branch block; this shows false positive defects after exercise testing. Intravenous dipyridamole is used. Dipyridamole is a coronary vasodilator, it increases the myocardial blood flow higher than exercise.