An adenosine thallium stress test provides information on the blood supply to an individual's heart, according to MedicineNet. During the procedure, cameras take pictures of the heart for doctors to evaluate.
The first part of the test involves injecting radioactive thallium into the bloodstream, notes MedicineNet. The radioactive thallium serves as a tracer, making the cells that it attaches to visible to the camera. Areas of the heart not receiving enough blood do not receive the same amount of the tracer as the rest of the heart. As a result, these areas appear darker in the photographs produced by the camera.