Document Type : Original Article
Department of Physics, Faculty of Science, University of Guilan, Rasht, Iran
Radiation Applications Research School, Nuclear Science and Technology Research Institute, Tehran, Iran
Introduction: 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.
Methods: 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.
Results: 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.
Conclusion: Compared to pinhole, the lofthole offers superior performance in terms of scattering and penetration for both low- and high-energy SPECT imaging.