COUNT RATE PERFORMANCES OF TTV03 - THE CEA-LETI TIME-OF-FLIGHT POSITRON EMISSION TOMOGRAPH

We have investigated the effect of varying the lower energy threshold of the BaF2 detectors on the rate of true events and on the fractions of scattered and random coincidences for the CEA-LETI TTV03 TOF-PET system. The behavior of this tomograph at high count rates has also been studied with both instrumental and clinical experiments with bolus of (H2O)-O-15. We found that the optimal energy threshold lies around 150 keV. At this level, for a direct plane, the machine sensitivity is 7900 unscattered trues/s/mu-Ci/cc. In the entire field of view, the scatter fraction is 18%, and the random to true ratio is 20%/mu-Ci/cc. At 1-mu-Ci/cc the noise equivalent counts are 7670 events/s. High count rate experiments demonstrate a 9-mu-s/event front-end deadtime, and a 385K events/s maximum coincidence rate recorded on the acquisition disk, which corresponds to 15 mCi in the field of view. A simple deadtime correction based on the single rate has been derived that predicts a 13% event loss (correction factor = 1.19) at the maximum recorded count rate. The 24 mCi myocardial blood flow study shows a 164K events/s count rate (correction factor = 1.12), with a flow value almost insensitive to the deadtime correction. During the 80 mCi cerebral blood flow study, count rate reached 100K events/s, and the same experiment performed on a high resolution BGO brain machine gave only a 30% increase in signal. These results demonstrate that TTV03 is particularly suitable for (H2O)-O-15 flow studies.