Ritschl Ludwig, Fahrig Rebecca, Knaup Michael, Maier Joscha, Kachelrieß Marc
Ziehm Imaging, Nürnberg 90451, Germany.
Radiological Science Laboratory, Stanford University, 1201 Welch Road Palo Alto, Stanford, California 94304.
Med Phys. 2015 Jan;42(1):469-78. doi: 10.1118/1.4903261.
Scattered radiation is one of the major problems facing image quality in flat detector cone-beam computed tomography (CBCT). Previously, a new scatter estimation and correction method using primary beam modulation has been proposed. The original image processing technique used a frequency-domain-based analysis, which proved to be sensitive to the accuracy of the modulator pattern both spatially and in amplitude as well as to the frequency of the modulation pattern. In addition, it cannot account for penumbra effects that occur, for example, due to the finite focal spot size and the scatter estimate can be degraded by high-frequency components of the primary image.
In this paper, the authors present a new way to estimate the scatter using primary modulation. It is less sensitive to modulator nonidealities and most importantly can handle arbitrary modulator shapes and changes in modulator attenuation. The main idea is that the scatter estimation can be expressed as an optimization problem, which yields a separation of the scatter and the primary image. The method is evaluated using simulated and experimental CBCT data. The scattering properties of the modulator itself are analyzed using a Monte Carlo simulation.
All reconstructions show strong improvements of image quality. To quantify the results, all images are compared to reference images (ideal simulations and collimated scans).
The proposed modulator-based scatter reduction algorithm may open the field of flat detector-based imaging to become a quantitative modality. This may have significant impact on C-arm imaging and on image-guided radiation therapy.
散射辐射是平板探测器锥形束计算机断层扫描（CBCT）中图像质量面临的主要问题之一。此前，已提出一种使用主束调制的新型散射估计与校正方法。原始图像处理技术采用基于频域的分析，结果证明其在空间和幅度上对调制器图案的精度以及调制图案的频率都很敏感。此外，它无法考虑例如由于有限焦点尺寸而产生的半影效应，并且散射估计可能会因原始图像的高频成分而降低。
在本文中，作者提出了一种使用主调制来估计散射的新方法。它对调制器的非理想性不太敏感，最重要的是能够处理任意调制器形状和调制器衰减的变化。主要思想是散射估计可以表示为一个优化问题，从而实现散射与原始图像的分离。使用模拟和实验CBCT数据对该方法进行评估。使用蒙特卡罗模拟分析调制器本身的散射特性。
所有重建结果均显示图像质量有显著改善。为了量化结果，将所有图像与参考图像（理想模拟和准直扫描）进行比较。
所提出的基于调制器的散射减少算法可能会使基于平板探测器的成像领域成为一种定量模式。这可能会对C形臂成像和图像引导放射治疗产生重大影响。
