INVESTIGATION OF DEADTIME CHARACTERISTICS FOR SIMULTANEOUS EMISSION-TRANSMISSION DATA-ACQUISITION IN PET

Modern PET systems generally utilize rotating rod sources and sinogram windowing for transmission scan and are also usually capable of simul taneous emission-transmission data acquisition. The count rate capabil ity of a PET system using EGO is primarily limited by the deadtime of the block detectors. This deadtime is directly related to the singles event rate of the system. In a simultaneous emission-transmission acqu isition, the moving rod sources generate a spatially varying singles d istribution which rotates synchronously with the rods. The emission ac tivities tend to create a nearly uniform singles distribution which is static or varies slowly depending on the half life of the radioisotop e. In this study, the system deadtime is characterized for different s ources of activity in a simultaneous acquisition. Typically, there are 60 to 150 MBq Ge-68 in each of the three rods causing a 26% peak-to-p eak variation in the instantaneous detector deadtime. The time-average d deadtime in a transmission sinogram has a variation of 30% peak-to-p eak. Rod activities create a 4 to 10% peak-to-peak variation in the ti me-averaged deadtime in the emission sinogram of a simultaneous scan. The current deadtime correction method results in a 10% error in a sim ultaneously acquired emission sinogram with 60 MBq in each rod and 74 MBq in a plane source. A spatially variant deadtime correction techniq ue was developed for simultaneous scans. The error was reduced to 4% f or the same study.