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We have developed and evaluated a realistic simulation dataset for simultaneous cardiac and respiratory gated ECT/CT using the 4D NURBS-based cardiac-torso (NCAT) phantom and Monte Carlo simulation methods. To include both cardiac and respiratory motions simultaneously, we generated 3D NCAT phantoms with different combinations of respiratory and cardiac gated time frames. The respiratory motion of the body excluding the heart was modeled by a total of 24 separate 3D NCAT phantoms over a respiratory cycle, and the beating heart was simulated separately with 48 frames per cardiac cycle for each of the 24 respiratory phases. Then, an almost noise-free projection dataset for each 3D NCAT phantom was generated using a long Monte Carlo simulation to simulate 99 mTc sestamibi myocardial perfusion (MP) SPECT projections with different gating schemes and relative phases. To evaluate the dataset, we performed simulation studies of gated MP SPECT that demonstrates the effect of the motions. The separate projection datasets were combined and grouped into different gating schemes including no gating, 6 respiratory-gates only, 8 cardiac-gates only, and some combinations of them. Each projection was reconstructed using the 3D OS-EM without and with attenuation correction using the averaged and respiratory gated attenuation maps. The image artifacts of the reconstructed images obtained from the different projection datasets were compared. The result demonstrated that respiratory motion generated artifactual decreases in both the anterior and inferior regions of the cardiac gated and non-gated MP polar map. While respiratory gating reduces the artifact significantly, attenuation correction with the gated attenuation map further reduced it. We conclude that the simulation dataset provides a powerful tool in the study of the effects of cardiac and respiratory motions, and development of cardiac and respiratory gating schemes and motion correction methods for improved ECT/CT imaging.