Investigation of the scatter contribution in single photon transmission measurements by means of Monte Carlo simulations

The fraction of detected scattered radiation in transmission measurements w ith a single photon transmission (SPT) source of Cesium-137 was investigate d by means of Monte Carlo (MC) techniques. The scatter contamination was de termined for different energy thresholds and the use of interplane septa. T he simulations were validated with measurements performed at the whole-body 3D PET scanner ECAT EXACT 3D (CTI/Siemens, Knoxville, TN), which uses a SP T source. The comparison of the results from the simulations and the measur ements shows good agreement. Transmission through a water-filled cylinder ( phi = 20 cm) gave values of the scatter fraction 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 r elative decrease of 37%. But a corresponding loss of counts by a factor of 1.5 was observed. Furthermore, simulations of the ECAT EXACT HR+ have been performed, a whole-body PET scanner which can be operated in 2D and 3D mode , but has no SPT mode yet. At a value of the LLD of 500 keV, the simulation s showed a decrease of the SF in the 2D mode of 45% relative to the 3D mode for the transmission of the water-filled cylinder.