In modern PET/CT systems, the CT provides a fast and relatively noise-free attenuation map and improving lesions localization and the possibility of accurate quantitative analysis. In CARDIAC imaging, there is a strong attenuation gradient along the myocardial free wall, with muscle next to the air of the lung space and heart has movement and located in the place that move due to breathing cause misalignment increases in this area. If MISALIGNMENTS occur along these boundaries, the attenuation correction factors are potentially inaccurate, causing as much as a 60% error in the PET tracer emission image in the critical regions of diagnostic interest. Artifacts caused by misalignment are particularly disconcerting in CARDIAC imaging because they can present themselves as perfusion abnormalities or erroneous information on myocardial viability. In This paper the accuracy of some CT protocols such as gated, normal (a high-pitch helical CT), slow ct, low-temporalresolution helical CT, time-averaged CT (ACT), ultra low dose CT that normally used for attenuationcorrection in PET/CT were compared, moreover the image quality and dose that induced to patient from each protocols. Acquiring a slow CT improved registration between the transmission and emission. Potential for a heightened radiation dose delivered by the slow CT was compensated by doubling the default noise index and increasing the slice thickness to 5 mm. In the low-dose average cine CT, Further reduction in dose is possible by lowering the upper threshold of the auto-mA settings or modulation of the CT tube current based on anatomy. ACT protocols consist of multiple images acquired sequentially (also referred to as cine or axial) along the bed length over the span of one or more respiratory cycles. 2% average increase in ACT-PET rest reconstruction values compared the HCT-PET rest reconstruction values was slightly higher than the bias calculated. Contrary to the HCT protocol, the ACT protocol provides more flexibility in addressing artifacts such as varying the respiratory phases used to create the time-averaged CT to suppress respiratory artifacts. In addition, photon starvation can be addressed by optimizing the acquisition parameters, such as increasing the tube voltage and current in patients with high BMI values. Ultra-low-dose CT’s shorter duration and the lower radiation and revealed no severe shift of the myocardium between the CT-based transmission and the emission in the patients.