Research On Monolithic Integration Technology Of PET Imaging Front-end Readout Circuits

Positron Emission Tomography（PET）is one of the most advanced technologies in the modern biomedical imaging fields,which can be used to observe the metabolism of living organisms in a molecular level with a noninvasive way,hence is widely adopted in the clinical diagnoses and biomedical researches.A PET imaging system consists of radiation detectors,correlated front-end readout circuits as well as the image reconstruction modules,of which front-ended readout circuits are employed to amplify and digitalize the weak electrical signals from the detectors.With the need of high spatial resolution and massive pixels of PET imaging detectors,the readout channels are increasing as a result.Therefore,PET front-end readout circuits should to be low noise,high precision and speed,high reliability and small volume,low power consumption and low cost.Hence the monolithic integration of the multichannel front-end readout circuits is a trend for PET imaging technology,instead of discrete elements on printed board.In detector fields recently,the Cd Zn Te as a new semiconductor radiation detector,has the advantages of high detection efficiency as well as low working temperature,and has been widely exploit in the PET imaging system.In this thesis,the Application-Specific Integrated Circuit（ASIC）chips for the front-end readout of PET imaging system dedicated for Cd Zn Te detector are researched and designed.The main research works are as follows:1.Critical circuit blocks of integrated PET imaging front-end readout chip are researched.In order to improve the linearity of the front-end readout circuit,a filter/shaper circuit with improved linearity,high-precision peaking detection and voltage capture circuit,is proposed and designed.Furthermore,to realize low noise readout,the noise characteristics of multichannel front-end readout circuit are studied,including optimization of circuit structure and component parameters.2.For PET imaging system used for clinical diagnosis,a prototype 16-channel front-end readout chip is designed,implemented and tested.The chip consists of both the energy and time measurement channels.In the energy measurement channel,a novel shaper circuit with small shaping time drift,a high precision peak voltage detection and hold circuit are designed to improve readout linearity.The electrical test results show:the input energy range is from 2.4f C to 22.5 f C,the conversion gain is 87 m V/f C,the nonlinearity of the conversion gain is less than 0.4%,the input equivalent noise charge is 109.7 e^-,the power consumption of single channel is about 4.0 m W,the inconsistency between channels is less than 0.3%,and the crosstalk is less than 2.0%.Additionally system test with the Cd Zn Te detector also shows a good linearity:the energy resolution for ²⁴¹Am radiation source is 5%（2.975 ke V@FWHM）,and the time walk is less than 4.0 ns.Compared with similar research results,the chip has the features of high-linearity,low-noise and low-power consumption.3.Aimed at small animal PET/CT imaging system,a 16-channel front-end readout chip based on ToT（Time-over-Threshold）readout strategy is designed,fabricated and tested.The front-end readout circuit based on the ToT readout strategy with its advantages of simple structure,small area and low power consumption,is especially suitable for the small animal PET imaging system which is with small pixel area,high spatial resolution and multi-channels.In this thesis,the structure of the front-end readout circuit based on the ToT readout strategy is proposed and analyzed.The test results show that,the input energy ranges from 3.5 f C to 56 f C,the conversion gain is 0.84 ns/f C,the nonlinearity of the conversion gain is less than 2.0%,the input equivalent noise charge is 211.4 e^-,the power consumption of single channel is about 1.82m W,the inconsistency between channels is less than 1.6%,and the crosstalk is less than 1.5%.Compared with similar researches,the chip has the characteristics of wide input energy range,good linearity,low noise and low power consumption.The main innovations in this thesis are as follows:1.To overcome the issues of degradation of gain linearity of front-end readout circuit caused by shaping time drift,a novel shaper circuit with high-Z circuit is proposed.Compared with the existed technologies,the shaping time drift is decreased by 74%on average,hence the gain nonlinearity error is restrained by 89%,which improves the linearity and energy resolution of PET imaging system.2.For accurate detection of shaper peaking signal,a high precision peaking detection and capture circuit is proposed.By exploiting the hysteresis comparator and programmable delay chain,the circuit can not only obtain the accurate peak voltage,but also eliminate the influences of noise and burr voltages.Compared with the existing technologies,the average detection error of the peak voltage is reduced by 93%,which benefits the peak detection accuracy of the energy channel and the overall gain linearity of the front-end readout circuit.3.A low-noise and high-linearity front-end readout circuit structure is proposed for the ToT front-end readout strategy,in which a shaper is used instead of the traditional integrator to improve the signal-to-noise（SNR）ratio and gain linearity,and a zero-pole cancellation circuit is added to eliminate the long tail of the shaper’s output signal.Compared with the existing technologies,the equivalent noise charge is reduced about 20%,which are beneficial to improve the overall energy resolution.And eliminating the tail of the shaper’s output signal also boost the count rate of PET imaging system.Benefit to the above innovative technologies,both multichannel front-end readout chips for Cd Zn Te detectors are designed and implemented with commercial 0.35μm CMOS process,which are used for clinical diagnosis and small animal PET imaging systems,respectively.The front-end readout chips designed in this thesis are featured with the characteristics of high gain linearity,low noise and low power consumption,and their performances meet the requirements of PET imaging system.The research results of this thesis have certain theoretical significance for the front-end readout microelectronics of the detector’s weak signal,and have important engineering practical value for the developments of PET imaging front-end readout chips.