A method and system for determining a PET image of the scan volume based on one or more PET sub-images is provided. The method may include determining a scan volume of a subject supported by a scan table； dividing the scan volume into one or more scan regions； for each scan region of the one or more scan regions, determining whether there is a physiological motion in the scan region； generating, based on a result of the determination, a PET sub-image of the scan region based on first PET data of the scan region acquired in a first mode or based, at least in part, on second PET data of the scan region acquired in a second mode； and generating a PET image of the scan volume based on one or more PET sub-images.
