Gang Grace J, Zbijewski Wojciech, Webster Stayman J, Siewerdsen Jeffrey H
Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21205, USA.
Med Phys. 2012 Aug;39(8):5145-56. doi: 10.1118/1.4736420.
Dual-energy computed tomography and dual-energy cone-beam computed tomography (DE-CBCT) are promising modalities for applications ranging from vascular to breast, renal, hepatic, and musculoskeletal imaging. Accordingly, the optimization of imaging techniques for such applications would benefit significantly from a general theoretical description of image quality that properly incorporates factors of acquisition, reconstruction, and tissue decomposition in DE tomography. This work reports a cascaded systems analysis model that includes the Poisson statistics of x rays (quantum noise), detector model (flat-panel detectors), anatomical background, image reconstruction (filtered backprojection), DE decomposition (weighted subtraction), and simple observer models to yield a task-based framework for DE technique optimization.
The theoretical framework extends previous modeling of DE projection radiography and CBCT. Signal and noise transfer characteristics are propagated through physical and mathematical stages of image formation and reconstruction. Dual-energy decomposition was modeled according to weighted subtraction of low- and high-energy images to yield the 3D DE noise-power spectrum (NPS) and noise-equivalent quanta (NEQ), which, in combination with observer models and the imaging task, yields the dual-energy detectability index (d(')). Model calculations were validated with NPS and NEQ measurements from an experimental imaging bench simulating the geometry of a dedicated musculoskeletal extremities scanner. Imaging techniques, including kVp pair and dose allocation, were optimized using d(') as an objective function for three example imaging tasks: (1) kidney stone discrimination; (2) iodine vs bone in a uniform, soft-tissue background; and (3) soft tissue tumor detection on power-law anatomical background.
Theoretical calculations of DE NPS and NEQ demonstrated good agreement with experimental measurements over a broad range of imaging conditions. Optimization results suggest a lower fraction of total dose imparted by the low-energy acquisition, a finding consistent with previous literature. The selection of optimal kVp pair reveals the combined effect of both quantum noise and contrast in the kidney stone discrimination and soft-tissue tumor detection tasks, whereas the K-edge effect of iodine was the dominant factor in determining kVp pairs in the iodine vs bone task. The soft-tissue tumor task illustrated the benefit of dual-energy imaging in eliminating anatomical background noise and improving detectability beyond that achievable by single-energy scans.
This work established a task-based theoretical framework that is predictive of DE image quality. The model can be utilized in optimizing a broad range of parameters in image acquisition, reconstruction, and decomposition, providing a useful tool for maximizing DE-CBCT image quality and reducing dose.
双能计算机断层扫描和双能锥束计算机断层扫描（DE-CBCT）是很有前景的成像方式，应用范围涵盖从血管到乳腺、肾脏、肝脏以及肌肉骨骼成像等领域。因此，针对此类应用的成像技术优化将显著受益于对图像质量的一般理论描述，该描述应恰当地纳入DE断层扫描中采集、重建和组织分解等因素。本文报道了一种级联系统分析模型，该模型包括X射线的泊松统计（量子噪声）、探测器模型（平板探测器）、解剖学背景、图像重建（滤波反投影）、DE分解（加权减法）以及简单观察者模型，以生成一个基于任务的DE技术优化框架。
该理论框架扩展了先前对DE投影射线照相和CBCT的建模。信号和噪声传递特性在图像形成和重建的物理及数学阶段进行传播。根据低能和高能图像的加权减法对双能分解进行建模，以生成三维DE噪声功率谱（NPS）和噪声等效量子（NEQ），将其与观察者模型和成像任务相结合，得出双能可探测性指数（d(')）。通过模拟专用肌肉骨骼四肢扫描仪几何结构的实验成像平台进行的NPS和NEQ测量，对模型计算进行了验证。以d(')作为目标函数，针对三个示例成像任务优化了成像技术，包括kVp对和剂量分配：（1）肾结石鉴别；（2）在均匀软组织背景中区分碘与骨骼；（3）在幂律解剖学背景上检测软组织肿瘤。
在广泛的成像条件下，DE NPS和NEQ的理论计算与实验测量结果显示出良好的一致性。优化结果表明，低能采集所施加的总剂量占比更低，这一发现与先前文献一致。最佳kVp对的选择揭示了量子噪声和对比度在肾结石鉴别和软组织肿瘤检测任务中的综合作用，而在碘与骨骼任务中，碘的K边效应是决定kVp对的主要因素。软组织肿瘤任务说明了双能成像在消除解剖学背景噪声以及提高可探测性方面的优势，这是单能扫描无法实现的。
本文建立了一个基于任务的理论框架，该框架可预测DE图像质量。该模型可用于优化图像采集、重建和分解中的各种参数，为最大化DE-CBCT图像质量和降低剂量提供了一个有用的工具。
