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The performance of a fast OSEM reconstruction method with 3D collimator beam modeling and optional attenuation correction ("Flash-3D"), developed by Siemens is investigated using various phantoms, and acquisition parameters. Data spectrum's cylinder phantom with rod and spheres inserts are acquired with varying protocols by systematically changing image quality relevant parameters, such as orbit, or collimator. They are then reconstructed with different methods and varying reconstruction parameters. The rod phantom is acquired with its long axis parallel and perpendicular to the rotation axis. A comparison between the 3D, 2D, 1D iterative methods, with and without attenuation correction, and FBP is done using qualitative and semiquantitative measures of image quality. The attenuation correction is done using CT-derived mu-maps. The contrast of the hot rods in parallel alignment is for OSEM-3D AC 17% larger than for OSEM-2D AC for the 3 largest rod sections. In this configuration where each slice shows the same structure the difference between 3D and 2D is minor. For the perpendicular alignment the contrast of the OSEM-3D AC reconstruction is 118% larger than from OSEM-2D AC. OSEM-2D AC shows almost identical contrast values as FBP, which is mainly due to the distortions of the rods, due to the 2D beam modeling. OSEM-3D AC comes closest to the expected hot contrast of 3.7 for the hot-cold sphere phantom. Future work will further develop the analysis tools for the phantoms and investigate the affect of varying image quality relevant acquisition and reconstruction parameters.