Thallium-201 as a myocardial imaging agent

The field of myocardial perfusion imaging has made many advances but still is in its infancy. The limitations in the technology at this time include limited instrument resolution of 6-9 mm, intrinsic at the energy of the mercury x-ray; significant Rayleigh scatter, which is particularly distrubing because this scatter cannot be removed by pulse-height analysis; and an effective half-life of thallium in the myocardium, which makes repeated imaging over a short period of time very difficult. Although hopes for the development of a technetium-labeled myocardial imaging tracer have burnt brightly, no new agents are presently in sight. Resolution with a technetium-labeled tracer would almost double that of thallium, and the dose that could be administered to the patient would increase by at least a factor of 4. The effective half-life of the tracer in the myocardium would permit multiple images to be obtained at least in the same day. Even with the limitations of the current techniques, however, myocardial perfusion imaging can make a real contribution to the care of the patients with heart disease. Thallium is now produced commercially and reasonably easily obtained. Extraction of thallium by the myocardium is probably somewhat, but not exactly, analogous to potassium. The tracer has major applications in defining shape and size of the heart, thickness of muscle, and especially myocardial ischemia and infarction. This review is aimed at providing a current perspective of these uses.