Positron Emission Tomography system is a functional medical imaging technique providing 3D images of the living processes inside the body relying on isotopes. These systems are used to study animal models of human disease. It has been found that significant benefits in spatial resolution, sensitivity, image quality, and quantification were achievable using preclinical systems. The physics of PET systems is based on the detection in coincidence of the two 511-keV rays, produced by an electron-positron annihilation, and emitted in opposite directions, as dictated by the conservation of energy and momentum. The radioactive nuclei used as sources of emission of positrons for PET systems are mainly 11C, 13N, 15O, and 18F, which are produced in cyclotrons and decay with half-lives of 20.3 min, 9.97 min, 124 sec, and 110 min, respectively. These radio-nuclides can be incorporated in a wide variety of radiopharmaceuticals that are inhaled or injected, leading to a medical diagnosis based on images obtained from a PET system. Traditionally, Monte Carlo codes (MC) are powerful tools for the research and development of new scanners and advanced image reconstruction algorithms. Among the benefits from extensive use of MC simulations we can highlight the design of new PET scanners, the development and assessment of image reconstruction algorithms and other applications, such as the Effect of inter-crystal scatter on estimation methods for random coincidences and image reconstruction algorithms assessment. Simulations make it feasible not only to refine the design parameters of PET scanners, but they also help to identify obstacles concerning count rate, resolution, sensitivity, etc. There are different Monte Carlo codes that simulate the transport of radiation through matter, such as FLUKA, GEANT4, MCNP, EGS4 and PENELOPE. All of them have been widely employed in the field of nuclear imaging and internal dosimetry. In this work we present and validate a new PET-dedicated Monte Carlo tool. This tools is based on FLUKA and implemented on FLAIR the GUI of FLUKA. FLUKA is a multi-purpose particle physics code for calculations of particle transport and interaction with matter. It is used in a wide range of applications in high energy experimental physics and engineering including accelerator driven systems, shielding, detector or target design, neutrino physics, dosimetry, activation and medical physics etc. this tools is an easy-to-use application which allows comprehensive simulations of PET systems within FLUKA. The developed tools include a PET scanner geometry builder and a dedicated scoring routine for coincident event determination. The geometry builder allows the efficient construction of PET scanners with nearly arbitrary parameters. We also present recent medically-oriented developments for Flair, which allows to import DICOM files and convert them into FLUKA voxel geometry or into a density map of radioactive isotopes, which could be employed as source in a convenient way. The coincidence events from the scoring can be saved in standard output formats, including list mode and binary sinogram. Such coincidence events can be further 2D- or 3D-reconstructed using Filtered back-projection (FBP) or Maximum Likelihood Expectation Maximization (MLEM) algorithms. In the MLEM method, the user can specify the size of the voxel as well as the size of the reconstructed image. Another source of flexibility is the possibility of adding new functionalities: a user can write a Python, C++ and FORTRAN routine and add it to Flair. This paper describes the PET tools and shows the results of extensive validations and comparisons of simulations against real measurements from commercial acquisition systems.


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