The present disclosure relates to systems and methods for determining a target activity map and a target attenuation map for producing a PET image. The systems may execute the methods to acquire, based on a PET system, a first dataset relating to coincidence events with TOF information, and a second dataset relating to single events. The systems may also execute the methods to determine a target activity map and a target attenuation map based on the first dataset and the second dataset through a plurality of iterations. The systems may further execute the methods to generate the PET image based on the target activity map and the target attenuation map.