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Respiratory motion is one source of degradation in PET images. Respiration leads to reduced contrast and quantitative accuracy in terms of recovered activity concentration and functional volumes, as a result of the associated image blurring. Consequently, the motion of lungs hindering the localization, detection, and the quantification of tracer uptake in lung lesions.Our long term goal is to study the impact of respiratory motion and evaluate the degradation on lesion detection in PET images. For that purpose, we used Monte Carlo simulations to produce realistic simulated data for the FDG radiotracer using the dynamic respiratory feature available in the 4D Mouse Whole-Body (MOBY) phantom including a lung lesion model. lates the Geant4 libraries to achieve a modular, versatile and scripted simulation toolkit adapted to emission and transmission tomography. The use of GATE facilitates the description of different components necessary for the accurate modelling of a PET or a SPECT system, starting from the geometry up to the creation of a processing chain for the detected events. It allows describing time-dependent phenomena such as detector or patient movements, source decay kinetics and dead time for coincidence acquisitions including delay coincidences measurement. 1) Modelling the microPET® FOCUS 2202) The MOBY phantom description within GATE We modelled the microPET® FOCUS 220 system within the GATE platform. The system is suitable for acquiring high resolution images of small animal as rodents (mice and rats) and primates (macaque and small baboon). Its axial Field of View (FOV) is 7.6 cm and has a diameter of 26.0 cm. The FOCUS system has a volume resolution of 2.5 JlL and an absolute sensitivity of 3.4 %, both measured at the center of the FOV [2]. The complete validation results of the microPET® FOCUS 220 simulation system using GATE can be found in [3].MATERIAL AND METHODS II. The GATE platform A.The MOBY mouse phantom combines the realism of a voxelized phantom, with the flexibility of a mathematical phantom, based on non-uniform rational B-splines (NURBS). We applied a resampling on the default MOBY matrix to reduce the voxel number to 40x40xl24 voxels with a voxel unit size of 0.5xO.5xO.5 mm 3• This allowed to significantly reduce the computational time resulting from the particle tracking inside the simulated volume and took into account the spatial resolution of the scanner.The MOBY phantom includes 4D models of the mouse's cardiac and respiratory motions. The MOBY respiratory motion was set up to be dependent on two time varying parameters: the change in the height of the diaphragm and the amount of chest expansion. We manipulated these parameters to produce a "stress breathing" condition, in order to reproduce the respiratory motion of a mouse during a typical PET examination: the diaphragm height was set to 6.0 mm and the expansion of the chest was set to 4.2 mm. GATE is a generic Monte Carlo simulation platform dedicated A spherical lung lesion was implemented in the middle region to nuclear med...