A Monte Carlo study on quantification of scattering and edge penetration in a breast-dedicated SPECT scanner with lofthole collimation

Quantitative accuracy in SPECT is mainly affected by collimator penetration and scattering, particularly for high-energy imaging. Lofthole collimation offers superior performance in terms of penetration and scattering. In this research, the GATE Monte Carlo simulator was exploited to calculate edge penetration and scattering in the lofthole collimator using an in-air and in-phantom point source of Tc-99m and I-123. The performance of the lofthole was then compared to that of a pinhole. Both lofthole and pinhole collimators were assumed to have the same geometry including an aperture diameter of 3.04 mm and an opening angle of 75°. Furthermore, the angular distribution of the scattering and penetration were investigated for a multi-lofthole collimator. The results show that penetration, scattering, and sensitivity are all a function of the photon energy. The penetration and scattering of the pinhole are about 4% higher than that of the lofthole collimator, for Tc-99m SPECT. Compared to the Tc-99m, I-123 SPECT suffers from approximately 1.5- and 1.42-fold higher penetration and scatter fractions, respectively, for lofthole aperture. Moreover, the lofthole collimator presents a higher sensitivity compared with the pinhole (0.030 versus 0.023 for the Tc-99m SPECT). In addition, the findings exhibit a reduction in sensitivity by increasing the photon incidence angle. Both scattering and penetration fractions illustrate a decreasing trend across the angle of incidence. Compared to pinhole, the lofthole offers superior performance in terms of scattering and penetration for both low- and high-energy SPECT imaging.