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The aim of this study is to design an optimized pinhole collimator using Monte Carlo simulation for the development of an ultra high-resolution SPECT using a position sensitive photo-multiplier tube. Simulations using Monte Carlo N-Particle Transport code, version 4c were performed to model the pinhole SPECT system. The simulation geometries consist of a cone-shaped pinhole collimator with tungsten aperture and a NaI(Tl) scintillation crystal measuring 6 mm in thickness and 120 mm in diameter. Spatial resolution, sensitivity, edge penetration, and scatter fraction were simulated by changing the pinhole diameter and channel height. The optimal ranges of pinhole diameter and channel height were determined from tradeoff curves of resolution and sensitivity and from penetration and scatter fraction. Tradeoff curves allowed us to determine the optimal range of pinhole diameter to be from 1 mm to 1.5 mm for the system configured in this study. The penetration and scatter fraction curve indicated that the channeled aperture was preferable over knife-edge. The optimal range of channel height was from 0.3 to 0.6 mm. The results demonstrate that the pinhole collimator designed in this study could be utilized to perform ultra high-resolution small animal imaging.