Wave Union TDC Implementation And Integration On MVT Digital Scintillation Detector

Recently,MVT(Multi-Voltage Threshold)method for digitizing scintillation pulse has been widely applied in PET(Positron Emission Tomography)backend readout electronics.MVT is an all-digital architecture that recovers the scintillation pulse waveform by setting several threshold voltages in advance and using a TDC(Time-to-Digital Converter)to record the time point at which the scintillation pulse crosses the preset voltage.As the timing performances of the photoelectric converter and the scintillation crystal progress,the enhancement on timing measurement accuracy of TDC integrated on the MVT digitizer can effectively improves the coincidence timing resolution of the PET detector,the better the coincidence timing resolution is,the higher the SNR(Signal-to-Noise Ratio)of TOF-PET imaging.As the FPGA implementation of the sub-gate delay TDC,Wave Union TDC splits the nonlinearity of the delay cell into new bins,making the timing resolution better than the average cell delay.In order to meet the timing resolution requirements of TOF-PET imaging,Wave Union method is selected for optimizing the TDC design and some improvements have been made as follows: The sum-out and carry-out logics of the LUTs are modified to solve the significant differential nonlinearity of Cyclone IV FPGA delay cells;the arrival times of rising and falling edge of the pulse are measured by connecting the wave union delay line output to two encoders to optimize the resource consumption and input delay correction of multi-channel TDCs;Combine Wave Union TDC and MVT scintillation pulse digital sampling logic to improve the scintillation pulse digitization unit.The Wave Union TDC was first integrated into an MVT digitizer(Altera EP4CE115F29I7)using discrete voltage comparators to test the average timing resolution,Differential nonlinearity(DNL)and Integral nonlinearity(INL)of the TDC.Two single-strip scintillation detectors were connected to the digitizer to form a pair of digital scintillation detectors and the coincidence timing resolution,energy resolution,and energy linearity data were obtained.A scalable multi-channel clock distribution system and two FPGA firmware were implemented for the 72-channel FPGA-Only MVT digitizer and a small coincidence system was set up.By upgrading the FPGA firmware,the coincidence timing resolution enhancement by the 72-channel integration of Wave Union TDC were evaluated.Finally,the research and discussion focused on the timing resolution improvement of multi-channel MVT digital scintillation detector were proposed.