Experimental determination of the lower energy discriminator level of a positron emission tomograph

Aim: Using the continuous energy spectra of Compton scattered photons we me asured the lower energy discriminator level of a positron emission tomograp h (PET). Methods: In PET scans, coincident photons with an energy between t he lower and upper level of the energy discriminator (LLD/ULD) are acquired . Usually, the energy response of a detector is determined by measurements using Various radiation sources with different energies. But this method is limited to the availability of the sources with the desired energies. The procedure described in this paper uses the energy spectrum from Compton sca ttered photons, providing a continuous energy spectrum for the direct measu rement of the energy response of the detectors. For our measurements we use d an activated Cu-64 point source (empty set = 1 mm) which was positioned i n an aluminium sphere (empty set = 2 cm). Results: The measured LLD Values for a whole-body PET-scanner ECAT EXACT HR+ (CTI/Siemens) were systematical ly lower than the nominal values (327 keV instead of 350 keV) and confirm t he results of C. Watson, recently found for line sources. Conclusion: This leads to an increased number of detected low energy photons (mainly scatter ed photons) and has to be taken into account within the scatter correction.