Count Rate Performance of the MicroPET Focus 220 Animal Scanner in Singles Transmission Scanning Mode

Count rate loss in singles transmission scans due to dead time can decrease the accuracy of attenuation correction. In addition, for very low transmission source activities measured attenuation may be affected by noise and background radiation emitted by radioactive lutetium-176 in the scintillator material. The aims of this study were to investigate count rate performance in singles transmission scanning for the microPET Focus 220 animal scanner, to examine the effect of energy window width on noise, scatter and dead time, and to determine the optimal range of activity for accurate attenuation correction. Transmission measurements were performed with a decaying ^99mTc point source and using wide (21%) and narrow (4%) energy windows. We observed count rate losses at the detector level and in the front end electronics during energy discrimination. Further losses 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 list mode data from the microPET to the PC were also observed for this condition. Lutetium-176 background caused a significant deviation of measured attenuation for activities lower than 10 MBq for both energy windows. No significant bias due to scatter was introduced when using the wide energy window. We conclude that accurate attenuation data can be obtained with single photon transmission source activity in the range 10 to 150 MBq without requiring deadtime correction.