Abstract：
Purpose. Fast kV-switching (FKS) and dual-layer flat-panel detector (DL-FPD) technologies have been actively studied as promising dual-energy spectral imaging solutions for FPD-based cone-beam computed tomography (CT). However, cone-beam CT (CBCT) spectral imaging is known to face challenges in obtaining accurate and robust material discrimination performance. That is because the energy separation by either FKS or DL-FPD, alone, is still limited, along with apparently unpaired signal levels in the effective low- and high-energy projections in real applications, not to mention the x-ray scatter in cone-beam scan which will make the material decomposition almost impossible if no correction is applied. To further improve CBCT spectral imaging capability, this work aims to promote a source-detector joint multi-energy spectral imaging solution which takes advantages of both FKS and DL-FPD, and to conduct a feasibility study on the first tabletop CBCT system with the joint spectral imaging capability developed.Methods. For CBCT, development of multi-energy spectral imaging can be jointly realized by using an x-ray source with a generator whose kilo-voltages can alternate in tens of Hertz (i.e. FKS), and a DL-FPD whose top- and bottom-layer projections corresponds to different effective energy levels. Thanks to the complimentary characteristics inherent in FKS and DL-FPD, the overall energy separation will be significantly better when compared with FKS or DL-FPD alone, and the x-ray photon detection efficiency will be also improved when compared with FKS alone. In this work, a noise performance analysis using the Cramér-Rao lower bound (CRLB) method is conducted. The CRLB for basis material after a projection-domain material decomposition is derived, followed by a set of numerical calculations of CRLBs, for the FKS, the DL-FPD and the joint solution, respectively. To compensate for the slightly angular mismatch between low- and high- projections in FKS, a dual-domain projection completion scheme is implemented. Afterwards material decomposition from the complete projection data is carried out by using the maximum-likelihood method, followed by reconstruction of basis material and virtual monochromatic images (VMI). In this work, the first FKS and DL-FPD jointly enabled multi-energy tabletop CBCT system, to the best of our knowledge, has been developed in our laboratory. To evaluate its spectral imaging performance, a set of physics experiments are conducted, where the multi-energy and head phantoms are scanned using the 80/105/130 kVp switching pairs and projection data are collected using a prototype DL-FPD, whose both top and bottom layer of panels are composed of 550μm of cesium iodine (CsI) scintillators with no intermediate metal filter in-between.Results. The numerical simulations show that the joint spectral imaging solution can lead to a significant improvement in energy separation and lower noise levels in most of material decomposition cases. The physics experiments confirmed the feasibility and superiority of the joint spectral imaging, whose CNRs in the selected regions of interest of the multi-energy phantom were boosted by an average improvement of 21.9%, 20.4% for water basis images and 32.8%, 62.8% for iodine images when compared with that of the FKS and DL-FPD, respectively. For the head phantom case, the joint spectral imaging can effectively reduce the streaking artifacts as well, and the standard deviation in the selected regions of interest are reduced by an average decrement of 19.5% and 8.1% for VMI when compared with that of the FKS and DL-FPD, respectively.Conclusions. A feasibility study of the joint spectral imaging solution for CBCT by utilizing both the FKS and DL-FPD was conducted, with the first tabletop CBCT system having such a capability being developed, which exhibits improved CNR and is more effective in avoiding streaking artifacts as expected.
摘要：
目的：快速kV开关（FKS）和双层平板探测器（DL-FPD）技术已被积极研究，作为基于FPD的锥形束计算机断层扫描（CBCT）的双能量解决方案。然而,已知由于有限的能量分离,CBCT光谱成像在获得准确且稳健的材料辨别性能方面面临挑战。进一步提高CBCT能谱成像能力,这项工作旨在促进源-探测器联合光谱成像解决方案，它同时利用FKS和DL-FPD，并对第一个具有联合光谱成像能力的桌面CBCT系统进行可行性研究。＆＃xD;方法：使用Cramér-Rao下界（CRLB）方法进行了噪声性能分析。推导了投影域材料分解后基础材料的CRLB，其次是一组CRLB的数值计算，对于FKS来说,DL-FPD和联合解决方案，分别。在这项工作中,第一个FKS和DL-FPD联合启用了多能量桌面CBCT系统，据我们所知,是在我们实验室开发的。为了评估其光谱成像性能，进行了一系列物理实验，其中使用80/105/130kVp开关对扫描多能量和头部体模，并使用原型DL-FPD收集投影数据。为了补偿FKS中的低能量和高能量投影之间的轻微角度不匹配，实现了双域投影完成方案。然后使用最大似然法进行材料分解，其次是基础材料和虚拟单色图像的重建。＆＃xD;结果：数值模拟表明，联合解决方案可以导致能量分离和更低的噪声水平的显着改善。物理实验证实了联合光谱成像的可行性和优越性,其多能体模的CNR平均提高了21.9%，水的20.4%和32.8%，与扇形束和锥形束实验中的FKS和DL-FPD相比，碘占62.8％，分别。＆＃xD;结论：通过利用FKS和DL-FPD进行了CBCT联合光谱成像的可行性研究，随着第一个具有这种能力的桌面CBCT系统的开发，这表现出改进的光谱成像性能的预期。
