Thallium scan of Heart
The concept of introducing a radioactive substance into the body for diagnostic purposes has been in practice for more than three decades. Radioactive iodine is being used for diagnosis and treatment of various thyroid gland diseases. The isotope scan is also used for assessing kidney function and for diagnosis of spread of different cancers in body. The isotope emits radioactive energy scintillates (lights up) in a gamma camera. Radioactive particles like thallium and MIBI (methyl isonitryl butyl isonitryl) scan are frequently used for the diagnosis of coronary artery disease. Thallium scan and MIBI scan are almost like a conventional stress test. The test is also carried out like a conventional exercise test. The patient is exercised (walking fast or running of a treadmill) until near fatigue or development of chest pain. The isotope is then injected through intravenous route in one hand vein. Then the test is terminated. The patient then lies down while a gamma camera is focused on the heart. This camera then takes multiples picture of the heart at different angles. Thallium and MIBI is preferentially taken up by viable myocardial (heart muscle) cells in the presence of normal coronary flow with out any block. Areas of the heart which are supplied by stenos ed or blocked coronary artery can not take thallium or MIBI normally and shows impaired uptake of the isotope. This defect documented by gamma camera is called perfusion defect. In this test perfusion at rest and at peak exercise are also compared to document reversibility of blood flow. Thallium scan is superior to exercise ECG test in the diagnosis of coronary artery disease. The accuracy of isotope scan has been shown to be around 95% in the detection of significant blockage in coronary artery as opposed to 75-85% in the case of the conventional exercise ECG test. Exercise ECG can detect coronary artery block but can not accurately detect the territory of muscle involved and how much muscle is in threat. The added advantage of isotope scan over a conventional exercise ECG test is the ability to localize the blocked coronary arteries. The pump function of the heart can also be assessed by isotope scanning. This heart function test is called MUGA scan. Coronary angiogram is a invasive test which carries definite risk so difficult to perform repeatedly for the evaluation of continued patency of coronary arteries after balloon angioplasty and by-pass operation. But isotope scan can be done repeatedly on the same patient in evaluating continued patency of coronary arteries after balloon angioplasty and by-pass surgery. That’s why nuclear cardiology has established itself as a most useful non-invasive diagnostic modality. These procedures do not require any special preparation on the part of the patient. It is a safe procedure and the isotopes used exhibit very low radioactivity.
