1. An imaging system, comprising: a Positron Emission Tomography (PET) scanner which generates event data including true coincident events and scatter events, the event data includes each end point of a line of response (LOR) and an energy of each end point; and one or more processors programmed to: generate a plurality of activity map and attenuation map pairs based on the true coincident events; and select an activity map and an attenuation map from the plurality of activity and attenuation map pairs based on the scattered events.