[Experimental determination of the lower energy discriminator level of positron emission tomographs]

Using the continuous energy spectra of Compton scattered photons we measured the lower energy discriminator level of a positron emission tomograph (PET).

In PET scans, coincident photons with an energy between the 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 scattered photons, providing a continuous energy spectrum for the direct measurement of the energy response of the detectors. For our measurements we used an activated Cu-64 point source (phi = 1 mm) which was positioned in an aluminium sphere (phi = 2 cm).

The measured LLD values for a whole-body PET-scanner ECAT EXACT HR+ (CTI/Siemens) were systematically lower than the nominal values (327 keV instead of 350 keV) and confirm the results of C. Watson, recently found for line sources.

This leads to an increased number of detected low energy photons (mainly scattered photons) and has to be taken into account within the scatter correction.