A positron-decay triggered transmission source for positron emission tomography

Attenuation of gamma rays is the most important correction required to provide artefact-free quantitative images in positron emission tomography (PET). We have tested a new technique, which allows multiple sources to be used for transmission scans. Each point source is surrounded by plastic scintillator, and the energy the positron must lose before annihilation is collected in the plastic scintillator coupled to a miniature photo-multiplier tube (PMT). The fast trigger from the PMT signals that positron decay has occurred, and arms a circuit to anticipate the arrival of one of the annihilation gamma rays in one of the scanner's conventional detectors. When this occurs, a line of response joining the current source position and detector position is identified. This technique has been tested using four Ge-68 sources imbedded in 5 mm diameter plastic cylinders coupled to miniature PMTs on the "ANIPET" small animal PET scanner at the Montreal Neurological Institute. Cross-talk (one plastic scintillator recording a gamma ray from any adjacent source instead of a positron from its own source) was 0.5%, and the count rate in one channel was 198 kcps implying a positron detection efficiency of 44%. Finally, preliminary images were obtained of the feasibility of using positron-decay as a transmission source.