Effect of Technetium-99m on Iodine-131 Thyroid Uptake Measurements

Abstract

Following administration of 2.5–5.0 mCi [^99mTc]pertechnetate, several hundred microcuries may be present in the necks of hyperthyroid patients after 6 hr. Coincidence summing of Tc-99m photons may disturb I-131 uptake measurements in such patients if an oral diagnostic dose of I-131 is administered immediately after completion of a [^99mTc]pertechnetate scintigram. Using a lower discriminator level of 300 keV, false increases of 10%–50% may occur at 6 hr. The 24-hr uptake is also affected if an I-131 predose measurement is performed after administration of the Tc-99m dose. The authors have shown that these errors may be prevented by using a 1-mm lead filter in front of the scintillation detector. The attenuation of Tc-99m photons by the filter effectively eliminates summation pulses while it reduces the I-131 countrate by approximately 28%.