Variation of 3γγ-to-2γγ ratio from 18F in haematological components measured using the GAMMASPHERE

The potential of incorporating three-quanta annihilation into positron emission tomography (PET) to detect local tissue chemistry was investigated using the GAMMASPHERE facility, which features a spherical array of 110 Compton-suppressed high-purity germanium (HPGe) detectors. Fluoro-2-deoxyglucose (FDG) containing F18 was introduced into 11 samples of haemolysed blood, serum, cell concentrate and whole blood, some of which had been either oxygenated or deoxygenated. The relative three-quanta yield was estimated from the reduced counts in the full-energy photopeak at 511 keV. Compton-suppression produced larger effects in the calculations than time-gating, suggesting its importance for this method of estimation. The relative three-quanta yields were found to vary as much as 11% between the samples. This level of sensitivity to different biological samples commends three-quanta annihilation for molecular imaging.