Assessment Of Scatter In Two PET Scanners With And Without Interplane Septa

Removal of interplane septa increases the sensitivity of multi-ring positron tomographs at the cost of an increase in the scatter fraction (SF) and randoms. We have Bssessed SF both experimentally and theoretically for two commercial tomographs: CTI 931/08-12 and CTI 953B with and without interplane septa. Monte Carlo simulations were undertaken using the GEANT package from CERN. SFs were calculated for various source geometries as a function of energy discrimination. For a line source at the center of a 21.6 cm diameter scattering cylinder (6.4 liters) in the 931/08-12 with a 250 keV threshold, the measured (calculated) SF in the whole field of view (FOV) are 15% (14%) for the sum of all direct and cross planes and 45% (46%) for the sum of all 64 planes, with and without septa, respectively. At 350 keV, the corresponding values are 12% (11%) and 35% (37%). respectively. For the 953B with the same source geometry and a threshold set at 380 keV, the measured (calculated) SF in a 40 cm FOV, is 14 % (15%) and 46% (46%). for the sum of all planes up to the third coincidence order (i.e. 100 sinograms or axial lines of response (LOR'S)). When varying the energy discrimination threshold. the agreement is not as good and may be due to the sharp threshold model used in the simulation.