Feasibility Study Of MVT Digitizing Method For Acquiring Position Spectrum

Positron emission tomography(PET)is a nuclear medicine imaging instrument that observes metabolic processes in organism.It can detect subtle changes in molecular biology level before the anatomical structure changes,so it is an important technique for the prediction,diagnosis,and treatment of some diseases.PET is used in areas such as oncology,neuroimaging,and so on.Modern PET systems tend to use digital PET detectors for its strong anti-interference capability and good maintainability.And the digital signal processing technology can be used to extract relevant information.PET systems built on the digital architecture have great prospects for development and application.The frequency of scintillation pulses in PET detectors are very high,and it is unacceptable to directly sample the pulses by analog-to-digital converters(ADCs)because of undersampling and high cost.The multi-voltage threshold(MVT)digitizing method has been proved to be capable of sampling scintillation pulses.Some digital PET detectors have been developed based on the MVT digitizing method,and they can achieve a good time resolution and energy resolution,as well as high count rate.In current clinical PET systems,most of the PET detectors are based on block design or quadrant sharing design.These PET detectors capture gamma photon and generate 4 scintillation pulses,and the position of the gamma photon was determined by the energy of these pulses.Generally,we characterize the space resolution of these PET detectors with position spectrum.In this dissertation,we analyzed the feasibility of MVT digitizing method for acquiring position spectrum,proposed a solution,and built an experiment platform for performance evaluation.The outputs of discretized positioning circuit(DPC)were similar to the outputs of block-designed PET detector,and it was selected as the basic platform for the study.We found that peaks of scintillation pulses generated by DPC have a wide range,and it is impossible to acquire a complete and clear position spectrum by directly sampling the pulses with MVT digitizing method.To solve the problem,the 4 outputs of DAQ were added to the adjacent output(except for the diagonal)to generate 4 new outputs,and now the position spectrum can be acquired as long as 3 outputs could be sampled.Meanwhile,we proposed a dynamic-threshold MVT scheme which can increase the span of voltage threshold.Finally,we designed a readout circuit and data acquisition card(DAQ),and build the experimental platform to verify the performance.The scintillation detector we used was a 6×6 LYSO/SiPM array with a pixel size of 4.2mm×4.2mm.The positions of 36 channels can be clearly distinguished,and energy resolution was 11%@662keV.The results showed that the MVT digitizing method can acquire a clear position spectrum.