A method of evaluating inter-detector scattering in block detectors for positron emission tomography

In order to evaluate the performance and localization capabilities of a BGO block detector, used in PET, the effects of inter-crystal scattering must be assessed and quantified. The spatial response of an eight by four block detector made up of a 55.6 mm wide, 12.9 mm high and 30 mm thick individual detector crystals to a collimated line source of 5511 keV annihilation photons was examined. Each column and row of the block was collimated in turn by lead. To isolate some of the resolution components of the block detector system a β− emitter, 90Sr source, was used. Absorption of the maximum energy 2.27 MeV electrons with no gammas, will produce a light output factor of 4.5 times larger than the 511 keV annihilation photons, reducing the statistical uncertainty in the event positioning. The values of FWHM and FWTM of an outer column are 11.2 and 27 mm for annihilation photons and 9 and 16 mm for β− particles, respectively. The ratio of FWTM to FWHM for the annihilation photons and β− particles for the outer column are 2.4 and 1.8 and for the inner column 2.1 and 1.8, respectively. Therefore, with β− particles crosstalk and scattering are reduced and the difference gives a measure of the magnitude of these factors found in the annihilation photon measurements. The method shown offers a simple check for estimation of the extent of interdetector scattering.