An adaptive method for triggering, event validation and pulse pile-up recovery in PET

PETBox4 is a high sensitivity PET system dedicated to preclinical imaging of mice. Due to its high sensitivity, highly multiplexed detectors, and the long decay constant of BGO, PETBox4 reaches count rate limitations at lower activities than most other preclinical tomographs. Here, we are investigating methodologies to improve the triggering, validation and pulse pile-up recovery for PETbox4. For pulse pile-up detection, a rising edge detection method with a paralyzable model is applied, by continuously taking the derivative of the incoming signal waveform. Since integration of an event is terminated upon the detection of a new incoming trigger, a number of events are integrated for durations between 300 ns and 800 ns, less than the 800 ns allowed when there is no pile-up. From these, the first event of each pile-up are recovered by an integration time based energy recovery LUT (look up table). Shorter integration than 300ns is not allowed and in those cases, all pile-up events are rejected. To minimize the losses of good events at low activity levels, the threshold for the detection of pile-up has been implemented as an activity adaptive threshold based on the rising edge triggering rate. Pile-up recovery was validated by extracting the flood histogram and energy spectrum of recovered events. To evaluate the adaptive threshold method, the peak system sensitivity was measured. Compared to at least a 29% sensitivity loss with a constant threshold method, the activity adaptive threshold method maintains the sensitivity close to the GATE (Geant4 Application for Tomographic Emission) simulation result. A phantom study was performed with the measured parameters, evaluating image uniformity and prompt rates. The new adaptive threshold based pile-up rejection and recovery achieves optimized rejection performance with increasing activity and maintains image uniformity at low activity levels. Detected prompt rates increase by about 50% at 40μCi due to pile-up recove- y.