표준계수 측정 시 기하학적 요인이 방사성 요오드 갑상선 섭취율에 미치는 영향

The Effect of Geometric Factors When Measuring Standard Count for Radioactive Iodine Thyroid Uptake Rate

Objectives: It is certain that Radioactive iodine thyroid uptake(RAIU) rate should be measured with the standard counts considering the thyroid gland depth in enlarged thyroid patients for the variation from geometric factors. The purpose of this paper is to consider the effects of geometric factors according to detector to source distance and the effective thyroid depth on RAIU rate with experiment test. Materials and Methods: I-131 370 kBq (10 μCi) point source was measured by Captus-3000 thyroid uptake system (Capintec, NJ, USA) with a change Detector-Source Distance from 20 cm to 30 cm at an interval of 1 cm. And we changed the Neck phantom surface-Source Depth in the phantom with 1 cm, 2 cm, 5 cm using the neck phantom in order to reproduce the effective thyroid depth. Results: Every experimental group follows power curve as inverse square curve (R2≧0.915). The average count rates in the case not using a phantom and the every case applied the effective thyroid depth using a phantom was not identical each other. There was significant fluctuations upon the effective thyroid depths applied the effective thyroid depth above 1 cm in 364.4 keV±10% energy ROI (p<0.01). There was not significant difference between the count rates of 1 cm and 2 cm in 364.4 keV±20% and 637.1 keV±6.2% (p=0.354, p=0.397). In assumed RAIU rate from regression equation, 364.4 keV±20% was lower difference than 364.4 keV±10% as 6.42% and 5.09% per 1 cm. Every change of count rate upon depth appears decreased line on Linear Regression, but the case of 284.3 keV±10% increased only. And also, The graphs of coefficient of variation upon depth increased as straight line on every experimental group. Conclusion: The result appears that application of 364.4 keV±20% energy ROI is more suitable for reducing error from the effective thyroid depth. And also, we can estimate the error of 20 cm should be highly reduced than 30 cm for Inverse Square Law. Therefore, If there is not information of the thyroid depth, it is considered that the error from thyroid depth can reduce through set up energy ROIs for 364.4 keV±20%, and increase Detector-Source Distances.