Post injection transmission scanning in a volume imaging PET camera

Quantitative, low noise, measured attenuation correction has been established for the PENN PET 240H Volume Imaging Camera. This is achieved, without septa, using a narrow energy (450-570 keV) and sinogram position (2 cm wide mask) gate to minimize scatter contamination. Twelve minute transmission acquisitions with a 0.5 mCi rod source are adequate for this purpose. Post injection transmission scans (with emission activity in the FOV) suffer from emission contamination simulating transmitted gamma ray flux. This emission contamination may be measured by performing a transmission acquisition with normal transmission energy and position gating but without a transmission source. This contamination is then subtracted from the measured post injection transmission scan. Emission activity within the FOV adds to detector deadtime, whereas only a small fraction of events are accepted into the transmission position gate, resulting in a net loss of scan statistics when compared to pre-injection transmission scanning. Removal of emission contamination and compensation for this excess deadtime results in corrected attenuation coefficients close to pre-injection values. For residual activity levels typical of FDG whole-body cancer and cardiac studies the post injection measurements are within 4% of pre-injection values