Count Rate Performance and Dead Time in Singles Transmission Scanning for the microPET Focus 220 Scanner

Count rate loss in singles transmission scans due to dead time can decrease the accuracy of attenuation correction if not corrected and adds noise to the data. The aim of this study was to investigate count rate performance and dead time in singles transmission scanning on the microPET Focus 220 animal scanner, and to determine the optimal range of activity for accurate attenuation correction. A point source filled with ^99mTc was used due to the similarity of its photon energy (140 keV) to that of ⁵⁷Co (122 keV) which was previously found to provide accurate attenuation coefficients. Transmission scans were performed with the isotope decaying and using wide (21%) and narrow (4%) energy windows to evaluate different components of dead time. We found that count rate losses occur at the detector level and in the front end electronics for transmission sources with activities higher than 80 MBq, independent of energy window width. Further losses and mispositioning artifacts emerged due to multiplexing at the detector head interface for activities higher than 150 MBq for the wide energy window. Substantial count rate losses during transfer of the list mode data from the microPET to the PC were also observed for this condition. The bias and noise in attenuation correction factors and reconstructed attenuation coefficients suggest that a useful activity range is 10 to 150 MBq for ⁵⁷Co singles transmission studies.