18F-FDG positron autoradiography with a particle counting silicon pixel detector

We report on tests of a room-temperature particle counting silicon pixel detector of the Medipix2 series as the detector unit of a positron autoradiography (AR) system, for samples labelled with F-18-FDG radiopharmaceutical used in PET studies. The silicon detector (1.98 cm(2) sensitive area, 300 mu m thick) has high intrinsic resolution (55 mu m pitch) and works by counting all hits in a pixel above a certain energy threshold. The present work extends the detector characterization with F-18-FDG of a previous paper. We analysed the system's linearity, dynamic range, sensitivity, background count rate, noise, and its imaging performance on biological samples. Tests have been performed in the laboratory with F-18-FDG drops (37-37 000 Bq initial activity) and ex vivo in a rat injected with 88.8 MBq of F-18-FDG. Particles interacting in the detector volume produced a hit in a cluster of pixels whose mean size was 4.3 pixels/event at 11 keV threshold and 2.2 pixels/event at 37 keV threshold. Results show a sensitivity for beta(+) of 0.377 cps Bq(-1), a dynamic range of at least five orders of magnitude and a lower detection limit of 0.0015 Bq mm(-2). Real-time F-18-FDG positron AR images have been obtained in 500-1000 s exposure time of thin (10-20 mu m) slices of a rat brain and compared with 20 h film autoradiography of adjacent slices. The analysis of the image contrast and signal-to-noise ratio in a rat brain slice indicated that Poisson noise-limited imaging can be approached in short (e. g. 100 s) exposures, with similar to 100 Bq slice activity, and that the silicon pixel detector produced a higher image quality than film-based AR.