Center for Advanced Radiotherapy Technologies and Department of Radiation Medicine and Applied Sciences, University of California, San Diego, La Jolla, CA 92037-0843, USA.
Phys Med Biol. 2012 Apr 7;57(7):2063-80. doi: 10.1088/0031-9155/57/7/2063.
While compressed sensing (CS)-based algorithms have been developed for the low-dose cone beam CT (CBCT) reconstruction, a clear understanding of the relationship between the image quality and imaging dose at low-dose levels is needed. In this paper, we qualitatively investigate this subject in a comprehensive manner with extensive experimental and simulation studies. The basic idea is to plot both the image quality and imaging dose together as functions of the number of projections and mAs per projection over the whole clinically relevant range. On this basis, a clear understanding of the tradeoff between the image quality and imaging dose can be achieved and optimal low-dose CBCT scan protocols can be developed to maximize the dose reduction while minimizing the image quality loss for various imaging tasks in image-guided radiation therapy (IGRT). Main findings of this work include (1) under the CS-based reconstruction framework, image quality has little degradation over a large range of dose variation. Image quality degradation becomes evident when the imaging dose (approximated with the x-ray tube load) is decreased below 100 total mAs. An imaging dose lower than 40 total mAs leads to a dramatic image degradation, and thus should be used cautiously. Optimal low-dose CBCT scan protocols likely fall in the dose range of 40-100 total mAs, depending on the specific IGRT applications. (2) Among different scan protocols at a constant low-dose level, the super sparse-view reconstruction with the projection number less than 50 is the most challenging case, even with strong regularization. Better image quality can be acquired with low mAs protocols. (3) The optimal scan protocol is the combination of a medium number of projections and a medium level of mAs/view. This is more evident when the dose is around 72.8 total mAs or below and when the ROI is a low-contrast or high-resolution object. Based on our results, the optimal number of projections is around 90 to 120. (4) The clinically acceptable lowest imaging dose level is task dependent. In our study, 72.8 mAs is a safe dose level for visualizing low-contrast objects, while 12.2 total mAs is sufficient for detecting high-contrast objects of diameter greater than 3 mm.
虽然已经开发出基于压缩感知（CS）的算法用于低剂量锥形束 CT（CBCT）重建，但仍需要清楚地了解低剂量水平下图像质量和成像剂量之间的关系。在本文中，我们通过广泛的实验和模拟研究全面地定性研究了这个问题。基本思想是将图像质量和成像剂量一起绘制为投影数量和每个投影的 mAs 函数，范围涵盖整个临床相关范围。在此基础上，可以清楚地了解图像质量和成像剂量之间的权衡，并且可以开发最佳的低剂量 CBCT 扫描协议，以在各种图像引导放射治疗（IGRT）的成像任务中最大限度地减少剂量减少，同时最小化图像质量损失。这项工作的主要发现包括：（1）在基于 CS 的重建框架下，在很大的剂量变化范围内，图像质量几乎没有下降。当成像剂量（用 X 射线管负载近似）降低到低于 100 总 mAs 以下时，图像质量的下降变得明显。当成像剂量低于 40 总 mAs 时，图像会急剧恶化，因此应谨慎使用。最佳的低剂量 CBCT 扫描协议可能落在 40-100 总 mAs 的剂量范围内，具体取决于特定的 IGRT 应用。（2）在恒定低剂量水平的不同扫描协议中，投影数小于 50 的超稀疏视图重建是最具挑战性的情况，即使使用强正则化。使用低 mAs 协议可以获得更好的图像质量。（3）最佳扫描协议是中等数量的投影和中等水平的 mAs/视图的组合。当剂量约为 72.8 总 mAs 或更低，并且 ROI 为低对比度或高分辨率物体时，这种情况更为明显。基于我们的结果，最佳的投影数量约为 90 到 120。（4）临床可接受的最低成像剂量水平取决于任务。在我们的研究中，72.8 mAs 是可视化低对比度物体的安全剂量水平，而 12.2 总 mAs 足以检测直径大于 3 毫米的高对比度物体。
