Tominaga Chiaki, Azumi Hiroki, Goto Mitsunori, Taura Masaaki, Homma Noriyasu, Mori Issei
Health Sciences, Graduate School of Medicine, Tohoku University, Seiryo-machi 2-1, Aoba-ku, Sendai, 980-8575, Japan.
Sendai Medical Center, Sendai, 983-8520, Japan.
Radiol Phys Technol. 2018 Jun;11(2):125-137. doi: 10.1007/s12194-018-0443-8. Epub 2018 Feb 23.
Edge methods are predominantly used for modulation transfer function (MTF) measurements in computed tomography (CT) images reconstructed using iterative methods. However, edge methods employ a relatively large and distinct test object, which is intended to simulate relatively large and distinct clinical organs. If one wants to assess the image quality of a small low-contrast object that is visually indistinct against a noisy background, a small and indistinct test object is desired. Another concern is that information related to the signal amount is discarded during MTF measurements. Choosing a weak impulse as the ultimately small test object, we have developed a tilted-wire method, which is a type of point spread function (PSF) method compatible with extremely low contrast-to-noise ratio (CNR) conditions. The signal amount is measured as the PSF volume. We used two commercial CT systems to evaluate the measurement accuracy of the tilted-wire method. When ensemble-averaged images are used, one can measure the MTF even when the wire is indiscernible from noise. The measurement error under such conditions is a few percent for both the MTF and signal amount. We also applied the tilted-wire method to two hybrid iterative reconstruction methods, namely AIDR-3D and ASiR. The results show that the MTF of ASiR is completely CNR-dependent, but that of AIDR-3D is noise-dependent. The signal amount obtained with ASiR is unchanged from that obtained through filtered back-projection (FBP). The signal amount obtained with AIDR-3D is less than that obtained through FBP, depending on the noise level.
边缘方法主要用于在使用迭代方法重建的计算机断层扫描（CT）图像中进行调制传递函数（MTF）测量。然而，边缘方法使用的是相对较大且明显的测试物体，旨在模拟相对较大且明显的临床器官。如果想要评估一个在噪声背景下视觉上不明显的小低对比度物体的图像质量，就需要一个小且不明显的测试物体。另一个问题是，在MTF测量过程中与信号量相关的信息会被丢弃。我们选择一个弱脉冲作为最终的小测试物体，开发了一种倾斜线方法，这是一种与极低对比度噪声比（CNR）条件兼容的点扩散函数（PSF）方法。信号量作为PSF体积进行测量。我们使用两个商用CT系统来评估倾斜线方法的测量准确性。当使用总体平均图像时，即使线与噪声难以区分，也可以测量MTF。在这种情况下，MTF和信号量的测量误差均为百分之几。我们还将倾斜线方法应用于两种混合迭代重建方法，即AIDR - 3D和ASiR。结果表明，ASiR的MTF完全依赖于CNR，而AIDR - 3D的MTF依赖于噪声。通过ASiR获得的信号量与通过滤波反投影（FBP）获得的信号量没有变化。通过AIDR - 3D获得的信号量小于通过FBP获得的信号量，这取决于噪声水平。
