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Pinhole SPECT allows to image physiological functions in small animals with high spatial resolution. In case of cardiac imaging with higher resolution, the resolution can be improved by positioning a collimator close to an object, but truncation arises from radioisotope in the surrounding organs. This study was intended to evaluate the effect of truncation in quantitative cardiac imaging with small field-of-view pinhole SPECT by a computer simulation. Assuming a rat, a numerical phantom consists of cylinder with 10-mm diameter and 10-mm height, and cylinder with 36-mm diameter and 70-mm height as the cardiac region and the surrounding region (Background). The counts of the cardiac region and the background were 10 and 1, 2, 4 or 8, respectively. In this simulation, the field-of-view of reconstruction was defined as a sphere with 30-mm diameter. The projection data with truncation were reconstructed using pinhole 3D-OSEM algorithm. The counts on the reconstructed images were compared with that on original image. The counts on the cardiac regions in reconstructed images were overestimated 14.5%, 17.6%, 20.5% and 23.2% for the background counts of 1, 2, 4 and 8, respectively. These results suggest that the effect of truncation needs to be considered in quantitative cardiac imaging with pinhole SPECT.