1. A method of attenuation correction of emission image data in combined PET/CT examinations, the method comprising: acquiring emission image data having attenuation therein; averaging a plurality of phases of 4D-CT images so that temporal resolution of an average CT image approximates one breathing cycle, wherein the temporal resolution of the average CT image approximates the temporal resolution of the emission image data, the emission image data having a temporal resolution of an average of multiple breathing cycles; and using the average CT image to correct the attenuation in the emission image data.