数字闪烁探测器

发布时间：2018-07-20 08:55

【摘要】：PET通过检测摄入活体内放射性示踪剂的分布状态对机体内各器官的代谢水平、生化反应、功能活动和灌注进行定量、动态地评估。按信号流顺序,PET被分为探测器系统、数据获得系统、图像重建系统以及图像显示与分析系统。探测器系统通常由多个闪烁探测器排布为环形结构的方式构成,用于捕获放射性示踪剂衰变后产生的Gamma粒子并形成相应的闪烁脉冲。数据获得系统通过对闪烁脉冲的分析、处理获取粒子的能量沉积信息并形成符合数据。图像重建系统则利用符合数据反演出放射性示踪剂在机体内的分布状态并通过后续的图像显示与分析系统展现出来。现代PET系统中,数据获得系统通常采用模数混合的架构。模拟电路被用来对闪烁脉冲进行处理,粒子的能量沉积大小、时间以及位置等信息转化为相应的模拟电压量或触发信号量后再由后续的电路进行数字化获取以及符合甄别。众所周知,模拟电路对闪烁脉冲的幅值大小、脉冲持续时间、频率范围等特性较为敏感,数据获得系统需要依据闪烁探测器的特性进行针对性设计;模拟电路难以实现复杂的脉冲分析、处理方法,粒子能量信息多由一些简单的线性方法及电路提取,性能优异但算法复杂的脉冲处理方法无法应用；模拟电路系统难以应对闪烁探测器输出脉冲的基线漂移、事件堆叠等情况,限制了系统的稳定性和性能的进一步提升。近年来,随着数字信号处理技术和方法的发展,将闪烁脉冲直接数字化,利用软件算法替代传统模拟电路提取粒子能量沉积信息的方式极具吸引力。数字化后的闪烁脉冲在传输、处理等过程中不再因模拟电路的信号带宽、噪声干扰等因素影响信号质量、恶化粒子能量沉积信息提取精度,这有利于大型、复杂系统的设计和开发：粒子能量沉积信息提取的准确性也将因高性能的数字信号处理技术和方法的应用得以提高：此外,数字闪烁脉冲下基线漂移和事件堆叠等情况可在信号处理和分析的过程中得到补偿和校正,系统的性能和稳定性将因此而进一步提高。将数字化电路与闪烁探测器相结合,直接输出数字闪烁脉冲的数字闪烁探测器极具发展前景。数字闪烁探测器的实现将简化PET系统的结构,降低系统的工程开发难度；快速发展的数字信号处理技术与方法也将得以应用于闪烁脉冲的分析与处理,加速PET系统的发展。闪烁脉冲的准确数字化是数字闪烁探测器实现的关键。受采样率和功耗的限制,一直以来利用ADC直接获取数字闪烁脉冲的方式存在工程实现困难、硬件成本昂贵、数字脉冲欠采样等诸多问题。在对粒子能量沉积时间信息提取精度有较高要求的TOF PET等系统中,ADC难以直接应用。由我课题组提出的多阈值电压采样(Multi-Voltage Threshold,以下简称MVT)方法则通过对闪烁脉冲过阈值电压的时间点进行采样的方式完成闪烁脉冲的数字化。相应的采样电路仅需少量比较器和时间数字转换器便可实现,具有工程开发难度小、硬件成本低等特点。在此背景下,本文以MVT方法为基础围绕数字闪烁探测器的关键技术、系统架构以及应用展开研究工作。首先,通过研究指出闪烁脉冲下降沿中不可避免的噪声影响现有MVT采样方法获取的数字闪烁脉冲精度和后续的粒子能量沉积大小信息的提取精度。这一噪声虽然可以通过低通滤波器滤除,但粒子能量沉积时间信息的提取精度却因此而恶化。针对这一问题,论文提出了精确MVT采样方法并与现有MVT采样方法下得到的数字闪烁脉冲提取的粒子能量沉积精度进行了对比研究。精确MVT采样方法实现了噪声干扰下数字闪烁脉冲的精确数字化,在不恶化粒子能量沉积时间信息提取精度的前提下有效提高了能量沉积大信息的提取精度。对应的能量分辨率由原先的16.9%@511keV优化到13.0%@511keV。其次,设计并实现了精确MVT采样电路。解决了传统TDC因死时间而无法应用于精确MVT采样电路的问题。提出了采用LVDS接受器实现阈值比较器的方法,不仅提高了MVT采样电路的集成度、降低了系统功耗,还提高了脉冲过阈值电压的时间信息获取精度。提出了精确MVT采样电路校正方法,解决了MVT采样电路中不同通道间的阈值电压和时间响应不一致带来的数字化精度下降的问题。将实现的精确MVT采样电路应用于一对LYSO/SiPM探测器输出脉冲的数字化,最终获得了13.9%@511keV的能量分辨率和438ps的时间分辨率。再次,提出了数字闪烁探测器架构,按信号流的顺序划分为探测器单元、数字化单元以及接口单元。数字闪烁脉冲通过接口单元传输出来,粒子能量沉积信息则由后续数字脉冲分析、处理平台通过分析数字闪烁脉冲的方式提取。利用该架构下的数字闪烁探测器进行PET等系统的开发,仅需在计算机等系统实现的数字脉冲分析、处理平台中开发相应的算法和程序即可完成。数字闪烁探测器的实现,降低了PET等系统的开发难度。论文以该架构和精确MVT采样电路为基础完成了数字闪烁探测器的实现,时间分辨率为525ps、能量分辨率为15.1%@511keV。最后,提出了数字PET系统架构,该架构下的数字PET成像系统具有系统架构简洁,工程实现简单,系统成像性能优异等特点。为针对应用,适应性的构建PET成像系统提供了技术基础。采用该系统架构和数字闪烁探测器,本文针对具体应用需求完成了三种具有不同成像视野的数字PET系统的设计与实现。对其中针对临床脑部成像的PET系统进行了初步的性能评估和假体成像。该系统的空间分辨率为2.5mmm,时间、能量分辨率分别为543ps和16.3%@511keV。与单个数字PET探测器的性能相比,系统性能未发生显著恶化。这一些结果初步揭示了数字PET探测器及数字化架构下PET成像系统的优势。
[Abstract]:PET quantified the metabolic levels, biochemical reactions, functional activities and perfusion of various organs in the body by detecting the distribution of radioactive tracers in the living body. According to the sequence of signal flow, PET was divided into detector system, data acquisition system, image reconstruction system and image display and analysis system. It is often made up of multiple scintillation detectors as a ring structure. It is used to capture Gamma particles produced by radioactive tracer decay and form corresponding scintillation pulses. The data acquisition system can obtain the energy deposition information of the particles by analysis of the scintillation pulse and form the consistent data. The distribution of radioactive tracers in the body is shown by the data. In the modern PET system, the data acquisition system usually uses a modular mixed structure. Analog circuits are used to process scintillation pulses, the size, time and position of the particle size, time and position. It is well known that the analog circuit is more sensitive to the magnitude of the scintillation pulse, the duration of the pulse, the frequency range and so on. The data acquisition system needs to be designed according to the characteristics of the scintillation detector. It is difficult to realize complex pulse analysis, processing method, the processing method, the particle energy information is extracted by some simple linear methods and circuits. The performance is excellent, but the algorithm complex pulse processing method can not be applied. The analog circuit system is difficult to cope with the base line drift of the output pulse of the scintillation detector, the event stacking and so on, which limits the system. In recent years, with the development of digital signal processing technology and methods, it is very attractive to digitize scintillation pulses directly and use software algorithms instead of traditional analog circuits to extract particle energy deposition information. The digital scintillation pulse is no longer due to analog electricity in the process of transmission and processing. The signal bandwidth, noise interference and other factors affect the quality of the signal and deteriorate the precision of particle energy deposition information extraction. This is beneficial to the design and development of large and complex systems. The accuracy of particle energy deposition information extraction will also be improved by the application of high performance digital signal processing and square methods: in addition, digital scintillation pulses will be improved. The conditions of lower baseline drift and event stacking can be compensated and corrected in the process of signal processing and analysis, and the performance and stability of the system will be further improved.
Digital scintillation detectors, which combine digital circuits with scintillation detectors, are very promising for digital scintillation detectors with direct output of digital scintillation pulses. The realization of digital scintillation detectors will simplify the structure of the PET system and reduce the difficulty of the development of the system. The rapid development of digital signal processing techniques and methods will also be applied to the scintillation pulse. Analysis and processing to accelerate the development of the PET system. The key to the realization of the digital scintillation detector is the accurate digitization of the scintillation pulse. Under the restriction of the sampling rate and the power consumption, there are many problems such as the difficulty of engineering realization, the expensive hardware cost, the undersampling of the digital pulse and so on. In the case of the particle energy, the method of obtaining the digital scintillation pulse by the sampling rate and the power consumption is limited. In the system of TOF PET, such as high precision of extracting time information, ADC is difficult to apply directly. The method of multi threshold voltage sampling (Multi-Voltage Threshold, hereinafter referred to as hereinafter referred to as MVT) proposed by our group is to digitize the scintillation pulse by sampling the time point of the threshold voltage of the scintillation pulse. The sampling circuit only needs a small amount of comparator and time digital converter. It has the characteristics of little difficulty in engineering development and low cost of hardware. Under this background, this paper is based on the key technology, system architecture and application of digital scintillation detector based on MVT method.
First, the precision of the digital scintillation pulse obtained by the existing MVT sampling method and the extraction precision of the subsequent particle energy deposition information obtained by the unavoidable noise in the falling edge of the scintillation pulse are studied. Although the noise can be filtered through a low pass filter, the extraction precision of the particle energy deposition time information is therefore the result of this. In order to solve this problem, an accurate MVT sampling method is proposed and compared with the particle energy deposition precision obtained from the digital scintillation pulse obtained under the existing MVT sampling method. The precise MVT sampling method realizes the exact digital character of the digital scintillation pulse under the noise interference, and does not deteriorate the energy deposition time letter of the particle. Under the premise of accuracy of extraction, the extraction accuracy of large energy information is effectively improved. The corresponding energy resolution is optimized from original 16.9%@511keV to 13.0%@511keV..
Secondly, an accurate MVT sampling circuit is designed and implemented. The problem that the traditional TDC can not be applied to the precise MVT sampling circuit because of the dead time is solved. A method of using the LVDS receiver to realize the threshold comparator is proposed, which not only improves the integration degree of the MVT sampling circuit, reduces the power consumption of the system, but also improves the time information of the pulse over threshold voltage. The accurate MVT sampling circuit correction method is proposed to solve the problem of the digital precision decline caused by different threshold voltage and time response between different channels in the MVT sampling circuit. The accurate MVT sampling circuit is applied to the digitization of the output pulse of a pair of LYSO/SiPM detectors, and the 13.9%@511keV is finally obtained. The resolution of energy and the time resolution of 438ps.
Thirdly, the architecture of digital scintillation detector is proposed, which is divided into detector unit, digital unit and interface unit according to the sequence of signal flow. The digital scintillation pulse is transmitted through the interface unit. The particle energy deposition information is analyzed by the subsequent digital pulse, and the processing platform is extracted through the analysis of the digital scintillation pulse. The development of the digital scintillation detector for PET, such as the digital pulse analysis and the development of the corresponding algorithms and programs in the processing platform. The realization of the digital scintillation detector reduces the difficulty of the development of PET and other systems. This paper is based on the architecture and the accurate MVT sampling circuit. The implementation of digital scintillation detector has a time resolution of 525ps and an energy resolution of 15.1%@511keV..
Finally, the architecture of digital PET system is proposed. The digital PET imaging system under this architecture has the characteristics of simple system architecture, simple engineering implementation and excellent imaging performance. It provides a technical basis for the application of the adaptive construction of PET imaging system. The system architecture and digital scintillation detectors are adopted. This paper is aimed at the specific application requirements. The design and implementation of three digital PET systems with different imaging fields are completed. A preliminary performance evaluation and prosthesis imaging of the PET system for clinical brain imaging are performed. The spatial resolution of the system is 2.5mmm, time, and energy resolution is compared to the performance of 543ps and 16.3%@ 511keV., respectively, with a single digital PET detector. The performance of the system has not significantly deteriorated. These results preliminarily reveal the advantages of the digital PET detector and the PET imaging system under the digital architecture.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：R817

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