The fraction of detected scattered radiation in transmission measurements with a single photon transmission (SPT) source was investigated by means of Monte Carlo (MC) techniques. The scatter fraction (SF) was determined in dependence of different energy thresholds and the use of interplane septa. The simulations were validated with measurements performed at the whole-body 3D PET scanner ECAT EXACT 3D (CTV/Siemens, Knoxville, TN), which uses a SPT source. The comparison of the results from the simulations and the measurements shows good agreement. The transmission of a water-filled cylinder (0=20 cm) gave values of the SF of about 27% at a lower level discriminator (LLD) value of 500 keV in the center of the projection. A reduction to 17% was achieved by an increase of the LLD to 600 keV; a relative decrease of 37%. But a corresponding loss of counts by a factor of 1.5 was observed. Furthermore, simulations of the ECAT EXACT HR/sup +/ have been done, a whole-body PET scanner which can be operated in 2D and 3D mode, but not in the SPT mode. At a value of the LLD of 500 keV, the simulations showed a decrease of the SF in the 2D mode of 31% relative to the 3D mode for the transmission of the water-filled cylinder.
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