Simulation of measurements of uptake of 123I-iodide in the thyroid of fetal chimpanzees

The quantitative assessment of the uptake of ${}^{123}{\rm I}\text{-iodide}\ ({}^{123}{\rm I}{}^{-})$ in the fetal thyroid in vivo was simulated in phantom measurements. First, the relationship between the depth of the fetal thyroid phantom and the two-peak ratio, the ratio of the counts in the γ-ray and the X-ray energy windows of the registered spectrum, was determined. Subsequently, the attenuation of the γ-ray signal in relationship to the depth of the fetal thyroid phantom was determined. Finally, the relationship between the two-peak ratio and the attenuation of the γ-ray signal was deduced. For a reliable correlation, the signal recorded from the radioactivity surrounding the fetal thyroid phantom has to be subtracted from that obtained with the fetal thyroid phantom present. A correction curve was generated to be applied to the in vivo measurements. It is concluded that with this method determination of uptake of ${}^{123}{\rm I}{}^{-}$ in the fetal thyroid is feasible.