Performance of the PETRRA positron camera: reduction of scatter and randoms

The PETRRA positron camera consists of two 60×40 cm2 detectors mounted on a rotating gantry. The detectors each contain 1 cm thick BaF2 crystals interfaced to a MWPC filled with tetrakis dimethylamino ethylene (TMAE) vapour. PETRRA acquires data in 3D only and images can contain high levels of scatter and random coincidences. Scatter comes from the patient and the detector support structure whereas randoms rates depend on the detector count rates and coincidence timing resolution. The camera has little energy resolution but low energy scattered photons produce a smaller range of pulses and can be discriminated against using signal thresholding. Scatter can also be reduced by shielding the camera from radioactivity outside of the field of view and by minimising the amount of scattering material in the camera itself. We conclude that the most effective way of reducing scatter and randoms count rates is to minimise the support structures in the detectors and shield the detectors from out-of-field activity. Energy thresholding does reduce scatter and randoms but severely reduces the numbers of true events.