A thallium scan involves the injection of a very small amount of radioactive thallium into the vein of the patient undergoing the test. The amount used is so tiny that it does not produce a harmful amount of radiation for the patient. Although the radiation is not harmful for an adult, a pregnant woman must notify her doctor before undergoing this test to ensure the safety of the fetus. As the test proceeds, monitors are used to follow the radioactive thallium as it travels through the bloodstream.
During a thallium scan, a small amount of radioactive substance is used to create images of a particular area of the body. This test is typically used to examine the heart or brain and determine whether these areas are receiving enough blood. The thallium scan is often combined with an exercise test to determine how well the heart functions under stress.
The thallium scan is often combined with an exercise test. In this kind of test, the initial thallium injection is administered while the patient is walking on a treadmill. A further injection is given at another time, when the heart is at rest. By comparing the two sets of results, a doctor can determine if any areas of the heart are receiving an inadequate blood supply either at rest or after exercise.
At the beginning of the test, the patient begins by exercising on the treadmill for several minutes. His heart rate and blood pressure are monitored throughout. A minute or so before he stops exercising, the thallium is injected into a vein. During the remaining minute of the exercise portion of the test, the radioactive thallium travels through the bloodstream and reaches the heart. Once it enters the heart, the thallium accumulates in locations which have an adequate blood supply.
At the end of the exercise period, the patient is immediately scanned using an imaging camera, so that the location of the radioactive thallium can be pinpointed. Areas of the heart that have accumulated thallium are judged to have good access to the blood supply. Any areas of the heart that have less thallium, or none at all, have a limited blood supply.
The thallium scan therefore pinpoints areas of the heart that do not have an adequate supply of blood and oxygen. In people who have had a heart attack, it is common for certain areas of the heart to have suffered damage that reduces blood flow both at rest and during exercise. People who are at risk of a heart attack often show signs of reduced thallium accumulation only after exercise. The test therefore helps doctors pinpoint people who are at risk of heart attack and other cardiac issues.