A method may include: obtaining a 3D CT image of a scanning area of a subject； obtaining PET data of the scanning area of the subject； gating the PET data based on a plurality of motion phases； reconstructing a plurality of gated 3D PET images； registering the plurality of gated 3D PET images with a reference 3D PET image； determining a motion vector field corresponding to a gated 3D PET image of the plurality of gated 3D PET images based on the registration； determining a motion phase for each of the plurality of CT image layers； correcting, for each of the plurality of CT image layers, the CT image layer with respect to a reference motion phase； and reconstructing a gated PET image with respect to the reference motion phase based on the corrected CT image layers and the PET data.
