康普顿成像技术中后向散射效应甄别及抑制

Compton imaging technology use the Compton scattering effect of the gamma-rays within the sensitive volume of the detector to give the location or profile of radioactive gamma source according to the Compton scattering formula. Compton imaging is very promising in arms control verification, public security, astrophysics and nuclear medicine diagnostics. Because of the extremely short time interval between the continuous reactions of gamma photon within system, it is almost impossible to distinguish the sequential order of these reactions, usually resulting that all the events, including the backward-scattering event, are regarded as the forward scattering events. Therefore, the backward-scattering events are wrongly reconstructed to degrade the imaging performance. This phenomenon is known as "Backward-scattering effect". For any Compton imager the backward-scattering effect is unavoidable. Currently, this phenomenon has been widely recognized by international research communities, however, there are only a few studies involving the "backward-scattering effect", indicating the lack of studies through systematic methodology focusing on the "backward-scattering effect". This project, entitled of "discrimination and suppression of the backward-scattering effect in Compton imaging”, aims to understand the rules of this effect changing with the Compton imager configuration, to clarify the mechanism of Compton imager performance degradation resulted from this effect, and to develop a versatile method for discrimination and suppression of the backward-scattering effect, based on which the performance of Compton imager will be effectively improved. Research conclusions of this project must be instructive and significant for the imager design, image reconstruction, background suppression, and the signal to noise ratio (SNR) enhancement, provide a theoretical basis and data supports for practical application of Compton imaging technology.

康普顿成像技术是利用放射源的伽马射线在探测系统内发生康普顿作用，根据康普顿散射公式逆推放射源位置或形状信息的成像技术，在军控核查、公共安全、天体物理和核医学等领域具广阔的应用前景。由于伽马光子在成像系统中的两次作用时间间隔极短，难以区分两次作用发生的先后顺序，通常只能把全部成像事件都视作所期望的前向散射事件，导致后向散射事件被错误地利用于图像重建引起成像性能降低，该现象称为“后向散射效应”。目前国际上仅有少量研究工作涉及后向散射效应，尚缺乏以后向散射效应为主体的系统性研究。本课题结合数值模拟与实验验证开展研究工作，旨在掌握后向散射效应随成像系统结构的变化规律，阐明后向散射效应影响系统成像性能的作用机理，建立一种后向散射效应甄别及抑制方法，有效地提升系统的成像性能。本课题的研究结论对于成像系统设计、图像重建、本底抑制和信噪比提升具有指导意义，为康普顿成像技术的实际应用提供理论依据和数据支撑。

康普顿成像技术在核安全、核医学和天体物理等领域有着广阔的应用前景。该技术依赖于有效的本底抑制才能保证其灵敏度，然而后向散射效应的存在会导致本底增加、信噪比降低、成像精度变差。因此，后向散射效应的甄别及抑制对于提升康普顿相机的成像性能具有至关重要的作用。本研究通过Geant4蒙特卡洛仿真软件构建了一套由双层碲锌镉像素阵列探测器组成的康普顿相机模型，结合数值模拟、理论分析及实验验证，系统性地研究了后向散射效应随成像系统和测量对象的变化规律，阐明了后向散射效应影响康普顿成像精度的作用机理，改进出一种适用于低计数条件下的后向散射事件理论计算甄别方法，并提出了一种针对准点源成像的后向散射事件有效利用重建算法，从而实现了对后向散射效应的甄别及抑制，使得康普顿相机的成像性能得以有效改善。. 研究结果表明，高Z材料前探测器有助于增加成像效率，但同时也会导致严重的后向散射效应。放射源能量越高、偏离探测器中心轴线的距离越远，后向散射效应越严重。经过长时间测量，后向散射的能量沉积有望表现出集中分布特性，据此可通过能量阈值法对后向散射事件进行甄别。然而，在快速识别场景或低计数条件下，该特性却难以被凸显。本研究依据放射源与探测器之间的几何布局，通过理论计算预先推导出后向散射的能量集中分布范围，从而实现了低计数条件下的后向散射事件有效甄别，其甄别效率通常可超过99%。此外，本研究针对常用的准点源成像应用场景，提出了一种新颖的康普顿成像算法，称为准点源自适应康普顿成像算法（SCIA）。该算法通过先验信息和测量数据的反复迭代，能够显著改善康普顿相机的成像精度，实现了对单个或多个点源的准确识别和精确定位。通过Geant4模拟和实验验证，全面对比和评估了SCIA算法和传统康普顿成像算法的成像性能，验证了该算法在多种场景下的适用性。