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ABSTRACT 27 While it is well recognized that pulmonary deposition of inhaled particles is lowered in 28 microgravity (µG) compared to 1G, the absence of sedimentation causes fine particles to 29 penetrate deeper in the lung in µG. Using quantitative magnetic resonance imaging 30 (MRI), we determined the effect of gravity on peripheral deposition of fine particles. 31 Aerosolized 0.95 µm-diameter ferric oxide particles were delivered to spontaneously 32 breathing rats placed in plethysmographic chambers both in µG aboard the NASA 33 Microgravity Research Aircraft and on the ground (1G). Following exposure, lungs were 34 perfusion-fixed, fluid-filled, and imaged in a 3T MR scanner. The MR signal decay rate, 35 R2* was measured in each voxel of the left lung from which particle deposition (DEP) 36 was determined based on a calibration curve. Regional deposition was assessed by 37 comparing DEP …