Investigation on deadtime characteristics for simultaneous emission-transmission data acquisition in PET

Modern PET systems utilize rotating rod sources and sinogram windowing for transmission scan and are capable of simultaneous emission-transmission data acquisition. Count rate capability of a PET system using EGO is mainly limited by deadtime of block detectors. Deadtime of a EGO block detector is directly associated with the rate of single events. In a simultaneous acquisition, moving rod sources generate a spatially varying singles distribution which rotates synchronously with rod motions. Emission activities create a nearly uniform singles distribution which is static or varies slowly depending on the half life of radioisotope. System deadtime is characterized for different sources of activity in a simultaneous acquisition. Typically, there are 60 to 150 MBq Ge-68 in each of the three rods causing 26% peak-to-peak variation in the instant detector deadtime. Time-averaged deadtime in transmission sinogram has a variation of 30% peak-to-peak. Rod activities create a 4 to 10% peak-to-peak variation in the time-averaged deadtime in emission sinogram of a simultaneous scan. The current deadtime correction method results a 10% error in simultaneously acquired emission sinogram with 60 MBq in each rod and 74 MBq in a uniform phantom. A spatially variant deadtime correction technique was developed. Error was reduced to 4% for the same study