Gao Hewei, Zhang Tao, Bennett N Robert, Wang Adam S
Department of Engineering Physics, Tsinghua University, Beijing, 100084, China.
Key Laboratory of Particle & Radiation Imaging (Tsinghua University), Ministry of Education, Beijing, 100084, China.
Med Phys. 2021 Apr;48(4):1557-1570. doi: 10.1002/mp.14704. Epub 2021 Feb 16.
Modulation of the x-ray source in computed tomography (CT) by a designated filter to achieve a desired distribution of photon flux has been greatly advanced in recent years. In this work, we present a densely sampled spectral modulation (DSSM) as a promising low-cost solution to quantitative CT imaging in the presence of scatter. By leveraging a special stationary filter (namely a spectral modulator) and a flying focal spot, DSSM features a strong correlation in the scatter distributions across focal spot positions and sees no substantial projection sparsity or misalignment in data sampling, making it possible to simultaneously correct for scatter and spectral effects in a unified framework.
The concept of DSSM is first introduced, followed by an analysis of the design and benefits of using the stationary spectral modulator with a flying focal spot (SMFFS) that dramatically changes the data sampling and its associated data processing. With an assumption that the scatter distributions across focal spot positions have strong correlation, a scatter estimation and spectral correction algorithm from DSSM is then developed, where a dual-energy modulator along with two flying focal spot positions is of interest. Finally, a phantom study on a tabletop cone-beam CT system is conducted to understand the feasibility of DSSM by SMFFS, using a copper modulator and by moving the x-ray tube position in the X direction to mimic the flying focal spot.
Based on our analytical analysis of the DSSM by SMFFS, the misalignment of low- and high-energy projection rays can be reduced by a factor of more than 10 when compared with a stationary modulator only. With respect to modulator design, metal materials such as copper, molybdenum, silver, and tin could be good candidates in terms of energy separation at a given attenuation of photon flux. Physical experiments using a Catphan phantom as well as an anthropomorphic chest phantom demonstrate the effectiveness of DSSM by SMFFS with much better CT number accuracy and less image artifacts. The root mean squared error was reduced from 297.9 to 6.5 Hounsfield units (HU) for the Catphan phantom and from 409.3 to 39.2 HU for the chest phantom.
The concept of DSSM using a SMFFS is proposed. Phantom results on its scatter estimation and spectral correction performance validate our main ideas and key assumptions, demonstrating its potential and feasibility for quantitative CT imaging.
近年来，通过指定滤波器对计算机断层扫描（CT）中的X射线源进行调制以实现所需的光子通量分布取得了很大进展。在这项工作中，我们提出了一种密集采样光谱调制（DSSM）方法，作为在存在散射情况下进行定量CT成像的一种很有前景的低成本解决方案。通过利用一种特殊的固定滤波器（即光谱调制器）和飞行焦点，DSSM在焦点位置的散射分布之间具有很强的相关性，并且在数据采样中不存在明显的投影稀疏性或错位，从而有可能在一个统一的框架中同时校正散射和光谱效应。
首先介绍DSSM的概念，然后分析使用具有飞行焦点的固定光谱调制器（SMFFS）的设计和优点，这会显著改变数据采样及其相关的数据处理。假设焦点位置的散射分布具有很强的相关性，然后开发了一种来自DSSM的散射估计和光谱校正算法，其中感兴趣的是双能调制器以及两个飞行焦点位置。最后，在桌面锥形束CT系统上进行了体模研究，以了解通过SMFFS实现DSSM的可行性，使用铜调制器并在X方向上移动X射线管位置以模拟飞行焦点。
基于我们对SMFFS的DSSM的分析，与仅使用固定调制器相比，低能和高能投影射线的错位可减少10倍以上。关于调制器设计，在给定光子通量衰减的能量分离方面，铜、钼、银和锡等金属材料可能是很好的选择。使用Catphan体模以及人体胸部体模进行的物理实验证明了SMFFS的DSSM的有效性，具有更好的CT值准确性和更少的图像伪影。对于Catphan体模，均方根误差从297.9降至6.5亨氏单位（HU），对于胸部体模，从409.3降至39.2 HU。
提出了使用SMFFS的DSSM概念。体模关于其散射估计和光谱校正性能的结果验证了我们的主要思想和关键假设，证明了其在定量CT成像中的潜力和可行性。
