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

Following administration of 2.5-5.0 mCi f 9mTc]pertechnetate, several hundred microcuries may be present in the necks of hyperthyroid patients after 6 hr. Coincidence summing of Tc-99m photons may disturb 1-131 uptake measurements in such patients if an oral diagnostic dose of 1-131 is administered immediately after completion of a f 9mTc]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 1-131 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 1-131 countrate by approximately 28%.