Polarization effects on scatter normalization of block detectors in PET

Annihilation photons are produced in mutually orthogonal polarization states, resulting in correlated angular distributions if both photons Compton scatter. This may impact the total scatter distribution, but may also impact detector normalization values for scattered coincidences. Single scatters within the source object are a significant fraction or the total scatter distribution. Further, 53 (62)% of first interactions within BGO (LSO) will be Compton scatter for 511 keV photons. 30 (39)% of unpolarized photons interacting in a 4×8×30 mm BGO (LSO) crystal will exit an individual crystal element prior to photoelectric absorption, possibly resulting in lost or mis-coded events. Thus, angular correlations associated with polarization effects may affect the efficiency response of the individual crystal elements of a block structure. Relative crystal efficiencies for axial and transverse polarization conditions were measured using a GE Advance BGO block detector. Results show a systematic difference in polarized crystal efficiencies within the block structure. The data impose an upper limit of 4% on the crystal sensitivity variations, for the BGO block tested, between incident photons with axial and transverse polarization states. The SimSET simulation software has been modified to include polarization. Investigations with measured and simulated data are ongoing