A comparison of methods to calculate the energy spectrum of a PET system operating in coincidence mode

PET system energy response is normally characterized with the system operating in singles detection mode. However, there are many imaging situations in which the energy spectrum of coincidence events can be different that that measured in singles mode. In this work we develop a simple method to measure the energy spectrum of coincidence events in a PET system operating in coincidence mode. The method uses a linear combination of two sets of coincident count measurements, one made by stepping the lower level discriminator (LLD) and the other made by stepping the upper level discriminator (ULD). Monte Carlo simulations were used to generate coincident count data for the geometry of a Siemens microPET R4 that included a model of the intrinsic radioactivity present in the scintillator crystals. The results indicate that the energy spectrum calculation method we propose here accurately recovers the true energy spectrum of the coincident events including the beta^- emission spectrum characteristic of the ¹⁷⁶Lu present in the LSO scintillator crystals. In addition, the 511 keV photopeak location and FWHM energy resolution were measured using the calculation technique. This spectrum calculation technique can be used for any tomograph that can independently vary the upper and lower discriminator levels.