Research On The Digitization Methods For The PET Scintillation Pulse Based On Linear Photon Detectors

Positron emission tomography (PET) is a non-invasive functional imaging instrument.PET is able to evaluate the metabolic activity, the biochemical reaction, the biological activeand the perfusion of a tissue dynamically and quantitatively. PET plays a significant rolein the clinical diagnosis of oncology, cardiovascular and nervous system, and also in theresearches of pathology, pharmacology and medicine development of related conditions.To improve PET image quality, the ability of high count rate and the performance ofhigh resolution are the aims of the development of PET instrument. Conventional data ac-quisition (DAQ) system of PET instrument is implemented based on analog technology, themaximum count rate of PET system is limited because of enormous analog integration cir-cuit in the data acquisition process. Otherwise, the anti-interference ability of analog circuitis weak, the analog circuit is easily interfered by environmental factors, such as tempera-ture. Besides, due to the enormous channels in a PET DAQ system, the PET system basedon analog implementation is difficult to keep high performance. The implementation of PETDAQ system based on digital technology is a hot issue in the PET instrument development.However, the scintillation pulses come from PET detectors are quite rapid. For the scintil-lation pulse come from a mainstream PET detector based on LSO/LYSO crystal, the risetime is about2～10ns, and the decay time constant is about40～50ns. It requires a veryhigh sampling rate of ADC to digitize the scintillation pulse directly. That makes it is verydifficult to develop a digital PET DAQ system based on ADCs.The thesis investigates the digital sampling method and digital processing method toobtain the timing, energy and position information of scintillation pulses for sampling thepulses from a PET detector directly. The data acquisition methods based on conventionalADC digitizer are studied comprehensively and systematically, and the influences of somefactors on obtaining timing, energy and position information are evaluated in the thesis, suchas sampling rate, ADC work mode, digital integration region.The thesis also studies the digital processing method based multi-voltage threshold(MVT) sampling method digitizing PET scintillation pulses directly, to obtain the timinginformation, energy information and position information from the PET scintillation pulses. the influences of count of thresholds, setting of voltage threshold on obtaining timing, energyand position information are evaluated in the thesis. According to the characteristics of theMVT sampling procedure, the improved processing method for energy determination, whichis implemented by correcting the error caused by the noises on the decay tail of scintillationpulses, is proposed in the thesis. The improved processing method based on MVT samplingfor position determination, which is implemented by setting some extra thresholds with ultralow voltage, is also proposed. These improved methods acquire the PET scintillation pulsesinformation more precise.The thesis proposes a novel digital processing method named DiSER to detect pileupsand to reconstruct each single-event pulse in a pileup according to the scintillation pulsemodel. The information of each single-event, such as timing, energy, position, can beobtained from the reconstructed pulse. The experiment results show that the DiSER methodcan recover timing information and energy information of each single-event in pileups witha count rate up to7MCPS, and artifact-free imaging with a count rate up to11MCPS.Therefore, the DiSER method can significantly increase the coincidence count rate in veryhigh count rate cases.