Determining the position of a radioactive source in a PET system. Detecting a scatter coincidence event characterized by a full-energy photon detected at a first detector and partial-energy photon at a second detector. Measuring the arrival time difference between the partial energy photon and the full energy photon. Measuring the energy of the partial-energy photon. Determining a scattering point as a function of the position of the first detector, the position of the second detector, the energy of the partial-energy photon, the energy of an unscattered photon, the mass of a scattering electron, and the speed of light. Determining the position of a radioactive PET source along a line between the scatter point and the first detector as a function of the distance between scatter point and the first detector, the distance between scatter point and the second detector, and the measured time difference.