Characterization of scatter factors in thyroid studies using a pinhole collimator by Monte Carlo Simulation.

Abstract

To study the scatter factors during I-131 thyroid scintigraphic studies with a pinhole collimator (5mm hole) was developed a Monte Carlo (MC) simulation using GAMOS code. First, to check the accuracy of the Monte Carlo model, simulated and measured data using a thyroid phantom were compared. The accuracy of the Monte Carlo model was verified by the good agreement between measured and simulated energy spectra and the maximum discrepancies of 2% in the counts/sec/MBq. Next, simulations to investigate scatter were performed for different tissue thickness between the thyroid and collimator (5-15mm). The image’s scatter contribution was significant in the 5mm pinhole, being between 27-40%. On the basis of the separated scatter from direct count included in window energy spectra, a preliminary evaluation of multiple window energy correction methods was performed. For the simulated thyroid geometry with pinhole, the reduce inferior double energy window methods (15% on 364keV photopeak window) provides a reasonable correction for scatter. This study is the first approach; we recommend including real thyroid geometry with different thyroid depth-thickness and mass.