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In this study we investigate the feasibility of performing small animal SPECT imaging on a clinical SPECT scanner with a dedicate pinhole collimator. Analytical formulae are used to calculate spatial resolution and absolute sensitivity given a pinhole collimator design scheme. Two optimal pinhole designs are proposed in terms of achieving the best trade-off between spatial resolution and absolute sensitivity. One consists of one single pinhole aperture and the other one has 7 pinholes. In the 7-pinhole design scheme, the pinholes are arranged in the way that each pinhole only covers part of the region of the object to maximize the sensitivity in the central slice of field of view (FOV) and to make use of the entire detection area without much overlapping. Monte Carlo simulation studies confirm the predicted spatial resolution values from analytical formulae. Imaging reconstruction results of a simulated ultra-micro hotrod phantom demonstrate that the 0.5 mm hot rods are clearly identified with the 7-pinhole collimator. Despite that the intrinsic resolution of the SPECT detector is only 3.55 mm, its large detection area allows large magnification and good spatial resolution. We conclude that high-resolution animal SPECT imaging can be performed through the proposed approach with relatively low cost for clinical SPECT users comparing to purchasing a dedicated small animal SPECT.