Characterization of single and multiple scatter from matter and activity distributions outside the FOV in 3-D PET

Operation of PET in 3-D mode, without internal septa results in increased sensitivity but at the cost of increased scatter fraction. Significant amounts of scattered events in 3-D PET either originate from activity outside the FOV (OFOV) or scatter from matter outside the FOV, complicating scatter correction methods. We performed 3-D PET simulations with scanner geometry based on the GE Advance with several phantom geometries designed to characterize all scatter with particular attention to that from outside the FOV. Scatter from OFOV matter was deduced by performing simulations with matter and activity within the FOV only (short phantom). Studies were repeated with phantoms with identical characteristics inside the FOV but with additional OFOV material (long phantom) but no activity outside the FOV. The incremental effect of OFOV activity was then determined by repeating the long phantom studies with additional activity. Scatter was also classified as single or multiple. In summary, the data allow extraction of yields from unscattered and all types of scattered events. The effect of source position, both within and external to the FOV, on scatter was determined by performing a series of simulations with point sources and techniques similar to those described above. The effect of energy discriminator setting (with BGO detectors) was also studied and data sets were generated for five settings from 300 to 425 keV. Using a Zubal phantom with the lungs and heart in the FOV and the abdomen outside the FOV and with a 300 keV discriminator, we find a total scatter fraction of 57%. Of the total scatter, 7.4% comes from OFOV matter, and 24% from OFOV activity. If the discriminator is increased to 425 keV the scatter fraction drops to 36%. OFOV scatter, particularly from OFOV activity, becomes relatively less important with the OFOV matter contribution dropping to 5.3% and OFOV activity contribution to 17%. Additionally, the spatial distributions of scatter from external matter and external activity were found to be different.