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Heart disease is a leading cause of mortality. Currently, PET is the gold standard for determining the viability of heart tissue. CT is gaining popularity as a transmission (Tx) source for attenuation correction (AC), but due to the differences in scan duration between the emission and Tx scans, breathing motion can cause mis-registration leading to errors in the final image. We explored this problem in the context of a large animal model chosen for its similarities to human physiology. This study investigates the effects of CT based AC (CTAC) and the use of 4D CT for respiratory gated AC, in an in vivo canine model of cardiac PET. Five 22 kg dogs were sedated, ventilated, and injected with 250 MBq of FDG. 4D CT Tx, gated PET, and 68Ge Tx (GeAC) scans were acquired. Both the 4D CT and the gated PET scans were broken into 15 frames of 0.5 seconds in length. The data were reconstructed using OSEM, applying five different AC maps: Ge, end expiration CT, end inspiration CT, average CT, and phase-matched CT. Ungated PET images were then created by summing the reconstructed gates together. Tendencies were present for end-expiration CTAC to cause over corrections on the anterior side of the heart, while the same was true for end-inspiration CTAC on the inferior side of the heart. On average we found regional differences of up to 3%, though in individual cases these differences were up to 20% on the anterior side and -10% on the inferior side of the heart. Comparing the average CTAC to the phase-matched CT corrected image showed both data sets to be nearly identical. Finally, when the average CTAC was compared to the GeAC image, differences were found along the lateral side of the heart. On average these percentage differences were up to 4%, however in individual cases they were up to 20%. Overall the observed tendencies indicate that there are respiratory motion artifacts, which can be large in individual cases.