Department of Radiation Physics, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, Gothenburg SE-413 45, Sweden.
Section of Diagnostic Imaging and Functional Medicine, NU Hospital Group, Trollhättan SE-461 85, Sweden.
Radiat Prot Dosimetry. 2021 Oct 12;195(3-4):416-425. doi: 10.1093/rpd/ncab058.
Denoising reconstruction techniques can introduce nonlinear properties into computed tomography (CT) systems. These nonlinear algorithms introduce distortion which affects the assessment of the resolution of the system. The purpose of the present study was to decouple and investigate amplitude modulation and waveform distortion in reconstruction algorithms in CT. The methodology developed by Wells, J. R. and Dobbins, J. T. III [Frequency response and distortion properties of nonlinear image processing algorithms and the importance of imaging context. Med. Phys. 40, 091906 (2013)] was adapted to CT reconstruction algorithms. The CT simulating program ASTRA Toolbox© for MATLAB™ was used for the reconstruction of the sinusoidal wave functions. Filtered back projection and the simultaneous iterative reconstruction technique were investigated with simple nonlinear mechanisms: a median filter and a non-negative constraint, respectively. The native reconstruction algorithms were not free from nonlinear waveform distortion, however, none of the metrics showed any dependence on the contrast-to-noise ratio (CNR). Furthermore, the algorithms including nonlinear mechanisms showed a clear and specific CNR dependence, indicating the necessity for distortion analysis in nonlinear CT reconstruction.
去噪重建技术可以为计算机断层扫描（CT）系统引入非线性特性。这些非线性算法会引入失真，从而影响系统分辨率的评估。本研究的目的是分离和研究 CT 重建算法中的幅度调制和波形失真。由 Wells，J. R. 和 Dobbins，J. T. III [非线性图像处理算法的频率响应和失真特性以及成像上下文的重要性。医学物理学 40, 091906 (2013)] 开发的方法适用于 CT 重建算法。使用用于 MATLAB™ 的 ASTRA Toolbox© CT 模拟程序对正弦波函数进行重建。研究了滤波反投影和同时迭代重建技术，分别采用了中值滤波器和非负约束等简单的非线性机制。原始重建算法不受非线性波形失真的影响，但没有一个指标显示出与对比度噪声比（CNR）的任何依赖关系。此外，包括非线性机制的算法显示出明显且特定的 CNR 依赖性，表明在非线性 CT 重建中需要进行失真分析。
