THYROID UPTAKE MEASUREMENT USING IODINE-125 AND IODINE-131

Due to the low energy (27.4 kev) of the photons emitted by 125I, the external measurement of thyroid activity requires a significant correction for tissue absorption. This correction can be determined by administering simultaneously a low dose of 131I. A geometry-independent coefficient, R131, is proposed, which is the ratio between the count rates measured over the patient's neck at 30 kev and at 364 kev. For each patient, the exact correction for the absorption both of the 364 kev radiation from 131I and the 27.4 kev radiation from 125I can then be found, and the use of a phantom becomes unnecessary. The method can be used immediately for 131I, but for 125I it still requires some technical improvements. Good agreement was obtained for uptake measurements of 131I and 125I in 13 double-tracer experiments.