The Digital Scintillation Detector

PET provides the function and metabolical information of tissues by measuring the distribution of radioactive tracer in vivo. Normally, PET is mainly consisted of scanner sys-tem, data acquiring (DAQ) system and imaging reconstruction system. The scanner system which realizes by arranging scintillation detectors in a ring is used to captured Gamma pho-tons and generate corresponding scintillation pulses. The DAQ system is used to analyze the pulse, pick up the information and filter out the coincidence events. The distribution of the radioactive tracer is then inverted by the imaging reconstruction system according the coincidence events. In a modern PET, the DAQ system is implemented by using mixed-signal circuit. The information of a Gamma photon is picked up by a dedicated analog electronics and then digitized for further analyzing. It’s known that the analog electronics need to be specifically designed according the characteristic of the scintillation pulse and al-so is impossible to implement modern sophisticated signal processing method which might improve the information picking up accuracy. Further more, it’s also hard to process the baseline shifting or pile-up scintillation pulse. Recently, as the developing of digital signal processing method and technology, it becomes interesting to directly digitize the event pulse and pick up the information by digitally analyzing the resulted samples. Unlike the analog signal, the bandwidth and noise of a circuit won’t deteriorate the digital signal during the transmitting and processing. The dedicated analog electronics will also be replaced by digi-tal algorithm. The accuracy of the piked up information will be improved as the developing of sophisticated signal processing method. And the calibration is also becoming easy.It’s attractive to building a digital scintillation detector which directly outputs digital scintillation pulse by combining digitizing circuit with scintillation detectors. It allows one to tap the power of modern digital electronics and processing algorithms to lower system cost and shorten the development/upgrade cycle. It can also lead to simplified the DAQ electronics and system architecture of a PET. Digitizing the scintillation pulse precisely is the key of building such detector. Due to the sampling rate limitation and high power dissipation, it’s unsatisfactory to sample the pulse directly by using ADCs. As an alternative solution, we have previously proposed the multi-voltage threshold (MVT) method that takes samples of a pulse with respect to a set of pre-defined reference voltages. The digitizer based on this method is easy to be implemented which only requires a few voltage comparators and time to digital converters (TDCs). In this dissertation, we building such a detector by using the MVT method. The key issue, system architecture and application is studied.Firstly, we find the noise at the decay tail of a scintillation pulse will deteriorate the precise of the obtained digital pulse and picked up energy information when using current MVT sampling method. Although the noise could be smoothed by using a low-pass fil-ter, the accuracy of picked up time information is deteriorated. Hence, we proposed and investigated the enhanced MVT sampling method. Results show that the enhanced MVT sampling method obtained the digital event pulse more precise and improve the accuracy of the extracted information. The energy resolution is improved from16.9%@511keV to13.0%@511keV without deteriorating the timing resolution.Secondly, we implemented such a MVT digitizer according the enhanced MVT sam-pling method. Considering the variations of the comparators and TDCs inside the digitizer, a calibration method which improving the sampling precise is proposed. By connecting the calibrated enhanced MVT digitizer to a pair of LYSO/SiPM detectors, we measured a13.9%@511keV energy resolution and a438ps timing resolution.Thirdly, we proposed a digital scintillation detector architecture which is consisted of detector unit, digitizer unit and communicator unit. The event pulse from detector unit is directly connected to the digitizer unit and converted to digital event pulse. The commu-nicator unit is then send the digital pulse to a following digital signal processing platform which might be implemented by a PC. Having such a digital scintillation detector, one only need to develop software in the digital signal processing platform when developing a PET system. Developing a PET system become much easier and more low-cost. In this thesis we have built such a digital scintillation detector, the timing and energy resolution is measured to be525ps and15.1%@511keV.Lastly, we proposed a digital PET system architecture. PET system in this architec-ture has a simple structure, a high performance and is also easy to be realized. Using the digital scintillation detector, We quickly build3different PET system. The performance of one of the system which is dedicated for clinic brain imaging is investigated. A543ps timing resolution, a16.3%@511keV energy resolution and a2.5mm spatial resolution at the center of FOV is measured. We also successfully obtained the imaging of derezon phantom with the system. The results show the benefits of building PET system using the digital architecture and digital scintillation detector.