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The goal of this paper was to evaluate the effects of collimator penetration and scatter on myocardial SPECT image quality. We chose two designs: a LEHR collimator for GE Millennium VG with a longer bore and thicker septa, and a LEHR collimator for Siemens E.CAM with a shorter bore and thinner septa. These two collimators have similar resolution properties, but very different penetration fractions. In particular, the Siemens collimator has higher detection efficiency. We used Monte Carlo (MC) simulation to simulate projection data from the three-dimensional (3-D) NCAT phantom. For each collimator, we generated three sets of projection data: the first one included only the geometric components of the collimator response, the second one included both the geometric and penetration components, and the third one included geometric, penetration and collimator scatter components. The resulting projections were reconstructed with the OSEM algorithm including attenuation and geometric response compensation. For each collimator and reconstruction, we computed the defect contrast in a short-axis slice. We found very small differences in defect contrast between the two collimators with and without penetration and collimator-scattered photons. Since the collimator with higher penetration had greater detection efficiency and showed little loss in defect contrast, a collimator with higher penetration fraction may be acceptable for use in Tc-99 m myocardial perfusion imaging.