COMPARATIVE EVALUATION OF 1237 ann 99™e FOR THYROID STUDIES H.L. Atkins, A.N. Ansari, P.R. Bradley-Moore, R. Lambrecht and A. Wolf Technetium-99m as pertechnetate is now widely used for imaging the thyroid. It is used to a somewhat lesser extent for assessment of the functional status of the thyroid. Technetium can be so utilized because of its being “trapped” by the thyroid. In addition to being trapped, iodine is organically bound within the thyroid. Radioiodine has been used to study both trapping and binding functions of the thyroid. LIodine-131 has been the isotope of iodine most widely used for these applications but is far from ideal for several reasons. Recently iodine-123 has received wide attention because it combines some of the favorable physical attributes of technetium-99m with the biological characteristics of iodine. Both technetium-9%and Lodine-123 possess short physical half-lives, intermediate range gamma photon energies and virtual absence of beta-like emissions. These features allow large amounts of radioactivity to be administered without excessive radiation dose to the patient. Both technetium-99m and iodine-123 can be utilized efficiently with the gamma camera. detected. The intermediate gamma energy ts readily collimated and The slightly higher energy of fodine-123 is advantageous in that separation of photopeak from Compton scatter is more readily accomplished than with technetium-99m. Theoretically this should provide slightly better resolution for 123] over I9mp¢ | 5007392 |!