背景:直接转换X射线光子计数探测器(PCD)在未来的医学成像设备(例如计算机断层扫描(CT)扫描仪)中起着至关重要的作用。PCD有望在几个方面改进当前的CT技术,如决议,剂量利用,和光谱性能。然而,它们不容易改善对物体散射的处理,CT技术中图像伪影的主要来源之一。
目的:我们探索了一种潜在的方法,用于使用X射线成像系统中使用的相同PCD阵列获得原位物体散射估计,比如计算机断层扫描。使用PCD的这种意想不到的益处具有通过向图像重建中使用的散射估计和校正算法提供更好的输入来改善图像质量的潜力。
方法:在CT扫描仪中,用于拒绝源自扫描对象的散射信号的主要方法依赖于将抗散射栅格(ASG)放置在靠近检测器平面的位置。当转换到使用PCD而不是CT中的能量积分检测器时,情况仍然如此。然而,PCD和ASG的组合打开了使用PCD的一些独特属性的可能性,即,获得非常低的噪声和符合性计数器,除了衰减数据,到达每个检测器元件的散射信号的同时和瞬时估计。当在ASG和检测器表面之间引入小气隙时,具有大角度分布的散射辐射具有更大的概率产生可以被符合计数器检测到的电荷共享事件。在这项工作中,我们证明了这种方法在桌面实验中的可行性,使用缺乏巧合计数能力的PCD检测器,相反,我们使用分裂电荷事件的光谱特征作为巧合计数的代理。为此,我们首先使用相同的实验设置演示ASG未对准的频谱影响。此外,我们从狭窄的水柱中进行了单独的桌面散射实验,这证明了PCD的低噪声功能的另一种潜在用途。
结果:我们在PCD探测器像素阵列上测量并量化了光谱响应对ASG对准的高灵敏度,我们发现,当ASG对准不良时,60keV光子的能量配准不良的可能性可以增加3倍。然后,我们利用这些结果来估算宽范围的散射与初级(SPR)比率的重合计数的预期增加,并从系统的几何建模中找到与预期的良好匹配。其中预期的巧合增加是SPR的数量级。最后,低噪声探测器还允许我们测量与水的相干分子形状因子相关的真实空间散射信号,揭示具有能量相关分布的环形散射信号,该分布通过计算很好地捕获。
结论:PCD探测器的出现及其在商用CT扫描仪中的应用为以意想不到的方式利用PCD探测器开辟了新的令人兴奋的可能性。在这个概念验证研究中,我们展示了如何共享电荷,光谱信息退化效应,可以改为用于获得原位散射估计。我们还证明了PCD能够使用负担得起的台式设备进行极其灵敏的测量,以进行通常为同步加速器设施保留的调查。
BACKGROUND: Direct conversion x-ray Photon Counting Detectors (PCD) are posed to play a vital role in future medical imaging devices such as Computed Tomography (CT) scanners. PCD are expected to improve current CT technology on several fronts, such as resolution, dose utilization, and spectral performance. However, they are not readily expected to improve the handling of object scatter, one of the major sources of image artifacts in CT technology.
OBJECTIVE: We explore a potential method for obtaining in-situ object scatter estimation using the same PCD array used in the x-ray imaging system, such as in computed tomography. This unexpected benefit of using PCD has the potential to improve the image quality by providing better input into the scatter estimation and correction algorithms used in image reconstruction.
METHODS: In CT scanners the primary method for rejecting scatter signal originating from the scanned object relies on placing anti-scatter grids (ASG) close to the detector plane. This remains the case when transitioning to using PCD instead of energy integration detectors in CT. However, the combination of PCD and ASG opens a possibility to use some of the unique properties of PCD, namely, very low noise and coincidence counters to obtain, in addition to the attenuation data, a simultaneous and instantaneous estimate of the scatter signal reaching every detector element. When a small air gap is introduced between the ASG and the detector surface, the scatter radiation with large angular distribution has a greater probability of producing charge sharing events that can be detected by a coincidence counter. In this work we demonstrate the feasibility of such an approach in a tabletop experiment using PCD detector that lacks coincidence counting capability, instead we use the spectral signature of split charge events as proxy to coincidence counting. For this purpose, we first demonstrate the spectral impact of ASG misalignment using the same experimental setup. In addition, we perform a separate tabletop scattering experiment from a narrow column of water that demonstrates another potential use of the low noise capabilities of PCDs.
RESULTS: We measured and quantified the high sensitivity of the spectral response to ASG alignment on the PCD detector pixel array, we found that the probability of energy misregistration of 60 keV photons can increase by up to a factor of 3 when the ASG is poorly aligned. We then leveraged these results to obtain an estimate on the expected increase in coincidence counts for a wide range of scatter-to-primary (SPR) ratio and find a good match with expectations from a geometric modeling of the system, where the expected increase in coincidences was of the order of the SPR. Finally, the low noise detector also allowed us to measure the real space scatter signal associated with the coherent molecular form factor of water, revealing the ring-shaped scatter signal with an energy dependent distribution that was well captured by calculation.
CONCLUSIONS: The advent of PCD detectors and their imminent use in commercial CT scanners opens new and exciting possibilities for utilizing PCD detectors in unexpected ways. In this proof-of-concept study, we showed how charge sharing, a spectral information degrading effect, can instead be used to obtain in-situ scatter estimation. We also demonstrated the PCD ability to perform extremely sensitive measurements using affordable benchtop setup for investigations normally reserved for synchrotron facilities.