Tehran University of Medical Sciences Frontiers in Biomedical Technologies 2345-5837 6 4 2019 12 30 181 186 Mahak Osouli Alamdari Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran Pardis Ghafarian Chronic Respiratory Diseases Research Center, National Research Institute of Tuberculosis and Lung Diseases, Shahid Beheshti University of Medical Sciences, Tehran, Iran Parham Geramifar Research Center for Nuclear Medicine, Shariati Hospital, Tehran University of Medical Sciences, Tehran, Iran Mohammad Reza Ay Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran. 2019 11 19 2019 12 14 Purpose: Positron Emission Tomography (PET) imaging is a nuclear medicine imaging technique based on the recording of two photons as coincidence created by positron annihilation. Materials and Methods: PET coincidence events include true and unwanted coincidences (random, scattered, multiple coincidences). We modeled the Discovery 690 (D-690) PET scanner using the GATE simulation tool and estimated the effect of the diameter of the scattering medium out of the Axial Field of View (AFOV) on the random coincidence rates. Results: The validation results indicated that the average difference between simulated and measured data for sensitivity and scatter fraction tests are 5% and 3%, respectively. Moreover, the results revealed that the increasing diameter of the scattering medium out of the AFOV has a direct effect on the random coincidence rates within the Field of View (FOV). Conclusion: The study concluded that the presence of a scattering medium near the FOV increases the rate of random coincidences. https://fbt.tums.ac.ir/index.php/fbt/article/view/237 https://fbt.tums.ac.ir/index.php/fbt/article/download/237/143