An effective method to verify line and point spread functions measured in computed tomography
An effective method to verify line and point spread functions measured in computed tomography
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DOI：
10.1118/1.2214168
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发表时间：
2006-08-01
期刊：
影响因子：
3.8
通讯作者：
Nishizawa, Kanae
中科院分区：
文献类型：
--
作者：
Ohkubo, Masaki;Wada, Sinichi;Nishizawa, Kanae
本研究描述了一种验证计算机断层扫描 (CT) 中测量的线扩散函数 (LSF) 和点扩散函数 (PSF) 的有效方法。已知假定的目标函数的 CT 图像可以使用基于线性成像系统中的空间分辨率模型的 LSF 或 PSF 进行计算。因此，LSF和PSF的有效性可以通过将计算图像与扫描对应于目标函数的模型获得的图像进行比较来确认。计算图像和测量图像之间的差异将取决于计算中使用的 LSF 和 PSF 的准确性。首先，我们在扫描仪中测量 LSF，并从 LSF 导出扫描平面中的二维 PSF。其次，我们扫描了包含平行于患者身体长轴（z 方向）的均匀圆柱形物体的体模。这种体模的测量图像根据扫描平面中的空间分辨率来表征，而不取决于z方向上的空间分辨率。第三，通过将作为空间函数的真实物体与 PSF 进行二维卷积来计算图像。将计算图像与测量图像进行比较的结果是，发现了良好的一致性，并通过图像减法得到了证明。作为定量评估图像整体差异的标准，我们定义了计算图像和测量图像之间差异的归一化标准偏差（SD）。对于三种类型的图像重建核和各种直径的圆柱形物体，这些归一化的 SD 小于 5.0%（范围从 1.3% 到 4.8%），表明 PSF 和 LSF 的高精度导致了成功的测量。此外，我们还利用不恰当的方式获得了另一个LSF，并计算了如上的图像。这次，计算图像与测量图像不一致。归一化的 SD 为 6.0% 或更高（范围为 6.0% 至 13.8%），表明 PSF 和 LSF 不准确。我们可以验证三种类型的重建内核的 LSF 和 PSF，并证明源自经过验证的 LSF 和不准确的 LSF 的调制传递函数 (MTF) 之间的差异。我们的技术需要一个适合临床扫描的简单体模，并且不需要包含一些金属或特定精细结构的特定体模，而这在以前用于测量空间分辨率的方法中是必需的。因此，模型的扫描图像将是可靠的且质量良好，并且可以直接用作验证的可靠参考图像。当获得 LSF、PSF 或 MTF 值时，建议使用我们的方法进行验证。此外，当开发出另一种测量 LSF 和 PSF 的方法时，可以使用我们的技术对其进行验证，如 Boone [Med.物理。 28, 356-360 (2001)]并用于本文。 (C) 2006 年美国医学物理学家协会。
This study describes an effective method for verifying line spread function (LSF) and point spread function (PSF) measured in computed tomography (CT). The CT image of an assumed object function is known to be calculable using LSF or PSF based on a model for the spatial resolution in a linear imaging system. Therefore, the validities of LSF and PSF would be confirmed by comparing the computed images with the images obtained by scanning phantoms corresponding to the object function. Differences between computed and measured images will depend on the accuracy of the LSF and PSF used in the calculations. First, we measured LSF in our scanner, and derived the two-dimensional PSF in the scan plane from the LSF. Second, we scanned the phantom including uniform cylindrical objects parallel to the long axis of a patient's body (z direction). Measured images of such a phantom were characterized according to the spatial resolution in the scan plane, and did not depend on the spatial resolution in the z direction. Third, images were calculated by two-dimensionally convolving the true object as a function of space with the PSF. As a result of comparing computed images with measured ones, good agreement was found and was demonstrated by image subtraction. As a criterion for evaluating quantitatively the overall differences of images, we defined the normalized standard deviation (SD) in the differences between computed and measured images. These normalized SDs were less than 5.0% (ranging from 1.3% to 4.8%) for three types of image reconstruction kernels and for various diameters of cylindrical objects, indicating the high accuracy of PSF and LSF that resulted in successful measurements. Further, we also obtained another LSF utilizing an inappropriate manner, and calculated the images as above. This time, the computed images did not agree with the measured ones. The normalized SDs were 6.0% or more (ranging from 6.0% to 13.8%), indicating the inaccuracy of the PSF and LSF. We could verify LSFs and PSFs for three types of reconstruction kernels, and demonstrated differences between modulation transfer functions (MTFs) derived from validated LSFs and inaccurate LSFs. Our technique requires a simple phantom that is suitable for clinical scanning, and does not require a particular phantom containing some metals or specific fine structures, as required in methods previously used for measurements of spatial resolution. Therefore, the scanned image of the phantom will be reliable and of good quality, and this is used directly as a confident reference image for the verification. When one obtains LSF, PSF or MTF values, verification using our method is recommended. Further, when another method for the measurement of LSF and PSF is developed, it could be validated using our technique, as illustrated in the method proposed by Boone [Med. Phys. 28, 356-360 (2001)] and used in this paper. (C) 2006 American Association of Physicists in Medicine.