An improved model for the energy resolution of multicrystal encoding detectors for PET

Recently, we have shown that the program DETECT provides a realistic platform to model the performances of scintillation counters with geometries as involved as that of the EXACT HR PLUS, a state-of-the-art position and energy encoding EGO detector manufactured by Siemens-CTI for positron emission tomography (PET). In this paper we present the results of new simulations of this block's performances, updated to include the nonproportionality of the scintillation response of EGO and the statistical noise from photoelectron amplification in photomultiplier tubes (PMT's). The updated model accounts for the crystal-dependent energy resolutions observed with 511-keV photons in the detector with excellent accuracy. Measured and simulated resolutions are shown to agree within -1 to 4%, with an average absolute difference of 1% over the entire block. The impact of the nonproportional scintillation response of EGO on the energy resolution is shown to be relatively small compared to that of the amplification noise in the four PMT's reading out the block.