Coincidences originating from a single photon: An unrecognized and potentially significant source of scatter in small animal PET?

Reported scatter fractions measured for small animal PET systems are commonly much higher than one would expect based on the size of the object being imaged. The large scatter fractions are generally attributed to ´gantry scatter´ and are made worse by the common use of wide energy windows to increase sensitivity. In this paper we propose the hypothesis that a significant number of the detected scattered events in animal PET systems operating with wide energy windows actually originate from the scattering of a photon in a detector and the subsequent detection of this scattered photon in another detector. These coincidences originating from a single photon form a scatter profile that is distinct from the normal low frequency profile associated with scatter in PET due to the attenuation of the scattered photons in the object being imaged. This leads to the sinogram profile of these single photon coincidences resembling a transmission sinogram. Line source emission data, for a line source in air and in a 50 mm diameter rat sized phantom, is simulated using GATE for the geometry of a Siemens Inveon PET system. The magnitude of single photon scatter coincidences is estimated by simulating a single photon emission source in place of the positron emission source. The results suggest that for an energy window of 150-850 keV 53% of detected scattered events for the line source in air originate from single photon scatter events. For the line source in the rat phantom, the results suggest that 13% of the detected scattered events originate from single photon scatter events. Our preliminary data suggest that coincidences from a single photon should be considered as a significant source of scatter in small animal PET systems that either operate with a wide energy window or have poor energy resolution.