A thallium scan is a method of examining the heart in order to get information about the blood supply to the heart muscle.
Special cameras take a series of pictures of the heart during the scan.
A radioactive substance, called a tracer, is injected into the bloodstream. The tracer attaches to certain cells and makes them visible to the special camera. The tracer attaches to the muscle cells of the heart so the imaging camera can take pictures of the heart muscles. If an area of the heart does not have a good blood supply, then the cells in that area will not receive as much tracer. This will show up as a darker area on the picture taken by the camera.