The physical properties of iodine-123 are such that it has been considered an ideal radionuclide for thyroid diagnostic use (17-18). Similarities to technetium-99m in decay characteristics plus the biological behavior of todine indicate that it should be superior to technetium. The only drawback has been lack of availability due to limited production capabilities of most cyclotrons. Hopefully this limitation can be overcome in the near future. PHYSICAL ASPECTS The decay characteristics of technetium-99m and iodine-123 are rather similar. Both possess principal gamma photons of intermediate energy and minimal amounts of beta-type emission. gamma photon in 1.3% abundance. Iodine-123 also possesses a 530 kev This is insufficient to cause any problems in imaging or quantitative studies. Both nuclides can be efficiently used with the gamma camera and low energy collimators. The principal gamma energy of fLodine-123 (159 keV) is somewhat higher than the 140 keV gamma of technetium-99m. This results in some advantage to the use of fodine-123 as can be seen in Figure 1. There is a better separation of the photopeak from the Compton scatter and the level of scattered radiation relative to the height of the photopeak is more | favorable for fodine-123. | The short physical half-life of six hours for technetium-99m is appropriate to the studies performed with it. It presents no problem so far as availability is concerned because of its production by decay of molybdenum-99. The physical half-life of iodine-123, while nearly ideal for the usual studies of thyroid physiology, presents problems with supply. Shelf-life ordinarily can be no more than one day, and even this is not possible if there is any serious contamination with longer- Lived 50017399