Center for Devices and Radiological Health, U.S. Food and Drug Administration, Silver Spring, MD 20993, USA.
Med Phys. 2012 Jun;39(6):3214-28. doi: 10.1118/1.4718572.
Quality assurance in computed tomography (CT) is commonly performed with the Fourier-based modulation transfer function (MTF) and the noise variance, while more recently the noise power spectrum (NPS) has increased in popularity. The Fourier-based methods make assumptions such as shift-invariance and cyclostationarity. These assumptions are violated in real clinical systems and consequently are expected to result in systematic errors. A spatial approach, based on the object transfer matrix (T) and the covariance matrix (K) theory, does not require these assumptions and can provide a more general description of the imaging system. In this paper, the authors present an experimental methodology and data treatment for quality assessment of a lab cone-beam CT system by comparing the spatial with the Fourier approach in 2D reconstructed slices.
In order to have control over all experimental parameters and image reconstruction, a bench-top flat-panel-based cone-beam CT scanner and a cylindrical water-filled poly(methyl methacrylate) (PMMA) phantom were used for the noise measurements. An aluminum foil inserted in the water phantom enabled the estimation of the line response function (LRF) with a limited number of assumptions. The authors evaluated the spatial blur, the noise and the signal-to-noise ratio (SNR) using the spatial approach as well as the Fourier-based approach. In order to evaluate the degree of noise nonstationarity of their cone-beam CT system, the authors evaluated both the local and global CT noise properties and compared them using both approaches.
For the laboratory cone-beam CT, the location-dependent noise evaluation showed that in addition to the noise variance, the NPS and covariance eigenvector symmetry depend on the location in the image. The estimated signal transfer was similar for both approaches. Unlike the Fourier approach which uses the same exponential wave function basis for both MTF and NPS, the eigenvectors of T and K were significantly different.
By using the eigenvectors of the noise and object transfer to characterize the system, the spatial approach provides additional information to the Fourier approach and is therefore an important tool for a thorough understanding of a CT system.
计算机断层扫描（CT）的质量保证通常采用基于傅里叶的调制传递函数（MTF）和噪声方差，而最近噪声功率谱（NPS）越来越受欢迎。基于傅里叶的方法假设了平移不变性和循环平稳性。这些假设在实际临床系统中被违反，因此预计会导致系统误差。一种基于对象传递矩阵（T）和协方差矩阵（K）理论的空间方法不要求这些假设，可以更全面地描述成像系统。本文作者提出了一种实验方法和数据处理方法，通过在 2D 重建切片中比较空间方法和傅里叶方法，来评估实验室锥形束 CT 系统的质量。
为了控制所有实验参数和图像重建，使用基于桌面的平板锥形束 CT 扫描仪和圆柱形充水聚甲基丙烯酸甲酯（PMMA）体模进行噪声测量。在水模体中插入铝箔，使作者能够在假设数量有限的情况下估计线响应函数（LRF）。作者使用空间方法和基于傅里叶的方法评估了空间模糊度、噪声和信噪比（SNR）。为了评估其锥形束 CT 系统的噪声非平稳程度，作者评估了局部和全局 CT 噪声特性，并使用两种方法进行了比较。
对于实验室锥形束 CT，位置相关的噪声评估表明，除了噪声方差外，NPS 和协方差特征向量的对称性还取决于图像中的位置。两种方法的估计信号传递相似。与基于傅里叶的方法不同，后者使用相同的指数波函数基来表示 MTF 和 NPS，T 和 K 的特征向量差异显著。
通过使用噪声和对象传递的特征向量来描述系统，空间方法为傅里叶方法提供了额外的信息，因此是全面理解 CT 系统的重要工具。
