Study of the Compton scattering effect of soft tissue in PET imaging by Monte Carlo method

In PET imaging, one or both of two annihilation photons may change the direction before reaching the detector due to Compton scattering interaction in body. This article, a Monte Carlo simulation study, examined the effect of soft tissue on this error.In this work, the PET BiographTM 6 scanner, a simple geometry of soft tissue -including a sphere of soft tissue in center of PET ringwith various radii (from 0.5 to 30cm)- and two kinds of 511keV gamma source –point source and spherical source - were simulated by Monte Carlo MCNPX code to investigate scattering effect of soft tissue on PET imaging. Analysis of the results of the simulation showed that, the majority scattered photons fell within the energy window without much loss of energy. Soft tissue around the point source at a distance of 8 to 12cm from the source and soft tissue around the spherical source at a distance of 8cm from the center had the most scattering effect in PET imaging. The scattering effect of soft tissue around the point source was more than the spherical source. The scattering effect of the adjacent organs is more than the non-adjacent organs.For high thicknessof soft tissue (more than20cm of radius), the attenuation effect is as obvious as the scattering effect. According to the results of this study, the patient's body thickness -in the abdominal region-could be a more accurate alternative for the patient's weight for increasing the injected dose into obese patients.