1. A PET imaging system comprising: (a) one or more detectors for detecting gamma rays emitting from an object to be imaged; and (b) a processing unit coupled to said one or more detectors, said processing unit comprising: i). a means for assigning time stamps to each gamma ray detection event; ii). a means for determining coincidence pairs from said time stamps; iii). a means for determining a mean emitting point and a time of flight probability function for each coincidence pair; iv). a means for determining a detector point spread function based on the mean emitting point and the time of flight probability function for each coincidence pair; v). a means for determining a probability volume based on the time of flight probability function and the detector response function for each coincidence pair; and vi). a means for using the probability volume for each coincidence pair in a reconstruction algorithm to produce an image.