The purpose of this study is to investigate the effectiveness of respiratory gating in myocardial SPECT using different numbers of gates where each gate contains an equal magnitude of heart and diaphragm motion. The 4D NURBS-based Cardiac-Torso (NCAT) phantom was used to generate 96 3D phantoms equally spaced over a complete respiratory cycle modeling the activity distribution from a typical Tc-99m Sestamibl study with the maximum movement of the diaphragm and heart set at 4 cm (heavy breathing). The 96 time frames were grouped to simulate various gating schemes (4, 6, 8, and 10 gates) in which each gate contained an equal magnitude of respiratory motion (1/3, 1/4, 1/5, and 1/6 the total motion respectively). Projection data, including effects of attenuation, collimator-detector response and scatter, from each respiratory gate and each gating scheme were generated and reconstructed using the OS-EM algorithm with correction for attenuation using gated attenuation maps. Bull's-eye polar plots were generated from the reconstructed images for each gate. Two regions-of-interest were placed over the lateral and inferior walls, and their average intensity ratio was determined. A ratio that deviates from 1 indicates a non-uniformity artifact caused by RM. Our results indicate that the RM artifacts are less prominent in gates near end-expiration and more prominent near end-inspiration. The artifacts are reduced the most when going from the ungated case to the gated case. We conclude that respiratory gating is an effective way to reduce RM artifacts. Effective implementation of respiratory gating to further improve quantitative myocardial SPECT requires optimization of the gating scheme based on the amount of respiratory motion of the heart during each gate and the placement of the gates within the respiratory cycle. © 2006 IEEE.