Navid Zeraatkar, Mohammad R. Ay, Member, IEEE, Saeed Sarkar, Parham Geramifar, and Arman Rahmim Member, IEEE Abstract– As the dimensions of crystal elements in modern PET systems is getting smaller, the fraction of events undergoing inter-crystal scattering (ICS) and penetration increases. This has deteriorative impact on the spatial resolution of images. However, with the utilization of statistical image reconstruction methods in PET, compensation for ICS and penetration impacts is feasible by accurately modeling their effects in the projection space. We aim to perform such a work for the GE Discovery RX (DRX) PET/CT system. In this work, we investigated ICS and penetration in the DRX for acquiring a quantitative view of their respective contributions. We analyzed the events in the form of coincidences instead of single photons. Also as a novelty, we discriminated between the origins of the event mispositiong. For this, we applied the GATE (Geant4 Application for Tomographic Emission) Monte Carlo toolkit and used our previously validated model of DRX. Numerous points in different positions were analyzed inside the field of view (FOV) of DRX. Finally, using geometrical symmetries of the DRX and interpolation, the fraction of ICS and penetration could be determined for all possible positions inside the FOV. The results revealed that the notable variations in quantitative behavior of ICS and penetration occurred with varying radial positions; the fraction of ICS/penetration-induced mispositioned coincidences out of true coincidences ranged from 28.7% at the center to 57.8% at the edge of the transaxial FOV. This comprehensive quantification of ICS and penetration not only provides a deeper understanding of their respective contributions, but is also aimed to be utilized in refining the This work was supported by the Research Center for Science and Technology in Medicine under grant No. 88.256. Navid Zeraatkar is with the Research Center for Science and Technology in Medicine, Tehran University of Medical Sciences, Tehran, Iran (e-mail: navid_zeraatkar@yahoo.com). Mohammad Reza Ay is with the Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran and Research Center for Science and Technology in Medicine, Tehran, Iran, and Research Institute for Nuclear Medicine, Tehran University of Medical Sciences, Tehran, Iran (e-mail: mohammadreza_ay@tums.ac.ir). Saeed Sarkar is with the Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran and Research Center for Science and Technology in Medicine, Tehran, Iran. Parham Geramifar is with the Faculty of Physics and Nuclear Engineering, Amir Kabir University of Technology and Research Center for Science and Technology in Medicine, Tehran, Iran, and Research Institute for Nuclear Medicine, Tehran University of Medical Sciences, Tehran, Iran. Arman Rahmim is with the Department of Radiology, Johns Hopkins Medical Institutions, Baltimore, Maryland, USA (e-mail: arahmim1@jhmi.edu). system matrix in the image reconstruction task to achieve resolution modeling in the scanner. I. INTRODUCTION hile there is continuing demand for higher resolution in PET systems, technological improvements are still challenged by the presence of inter-crystal scatter (ICS) and inter-crystal penetration phenomena in detectors [1]. Penetration (exhibiting itself in the form of the so-called parallax effect) occurs when a photon passes through a crystal on which it is incident with no interaction and is instead detected in another crystal. The penetration effect is therefore probable only for photons entering the crystal at non- perpendicular angles and intensifies as the photon energy increases and/or the attenuation coefficient of the detector material decreases. Hence, the probability of a 511-keV photon in a PET system for penetrating through the incident crystal with no interaction is considerable. In the other hand, ICS can be present even for photons incident perpendicularly to the crystal and happens when a photon escapes from the crystal it initially entered after one or more Compton interactions and hence deposits some of its energy in one or more crystals other than the initial crystal. Fig. 1 depicts an example of a photon experiencing ICS and penetration in a typical detector block. Both ICS and penetration have deteriorative impact on spatial resolution of PET scanners because they cause some photons to be detected in crystals not corresponding to the position of the annihilation from which the photon emitted. The consequence of this photon mispositioning is that the coincidences in which at least one of the two photons mispositioned due to ICS or penetration are likely registered in incorrect line of responses (LORs). In addition, the current trend of PET detection system implementation that leans toward fabrication of smaller crystals for reaching better intrinsic spatial resolution worsens the impact of ICS and penetration due to the higher probability of the photon escape from the prime crystal before interaction or after Compton interaction. Nonetheless, following the introduction and utility of statistical image reconstruction methods in PET, and the so- called system matrix, it has been shown that it is possible to Quantitative Investigation of Inter-Crystal Scatter and Penetration in the GE Discovery RX PET/CT Scanner using Monte Carlo Simulations W 2403 978-1-4244-9105-6/10/$26.00 ©2010 IEEE