基于面阵CCD相机的高能X射线工业CT技术研究
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摘要
高能X射线工业CT能够对大型工业构件实施断层扫描，是国防、航空、航天及大型工业产品制造行业不可缺少的检测设备。就其扫描结构而言，工业CT可分为线阵探测器工业CT和面阵探测器工业CT两种。针对两种高能X射线工业CT系统的特点，根据我国工业检测的实际需要，本文选择基于转换屏的面阵探测器工业CT作为研究对象。
本文首先分析了高能X射线检测的特点，研究了空间分辨率、密度分辨率、射线强度和曝光时间之间的相互关系，设计了基于面阵CCD相机的高能X射线工业CT系统，确定了平行束、扇束和锥束滤波反投影重建算法在本系统中的使用条件，分析了成像系统中射线源焦点强度分布和CCD相机透镜引起的渐晕效应、CCD暗电流的尖峰噪声和转换屏内部的缺陷是引起投影数据不一致的主要原因，并提出了相应的校正方法。在此基础上，对三代坦克上使用的三种典型的大型构件：大口径炮弹、发动机叶轮和铸铝汽缸盖实施了多切片CT重建，获得了较好的重建图像。
其次，本文研究了投影获取及重建过程中，产生CT图像伪像的各个环节，如滤波函数的长度的选取、旋转中心偏移、成像系统渐晕、CCD暗电流、脉冲噪声、散射等，提出了相应的消除方法。
最后，研究了对尺寸大于本系统投影视域的物体进行CT扫描时，物体的投影被截断，以不完全的投影数据重建图像引起的误差。提出了利用先验知识，补充截断的投影数据，而后重建物体CT图像的方法。
通过本文的研究与实践，为基于转换屏的高能X射线工业CT工程化打下了基础。
The cross section of large industrial components can be reconstructed using high energy X-ray industrial computed tomograpgy (ICT). It is absolutely necessarily equipment in manufacturing industry, such as national defense, aeronautics and astronautics industry. According to the structure of detectors in the CT scanner, ICT can be divided into two types: the ICT based on linear-array detectors and the ICT based on area-array detectors. To satisfy industrial actually requirement in our country, the technology of high energy X-ray ICT based on area-array detectors is selected as research object in this paper.
Firstly, the characteristic of high energy X-ray detection and the relation among spatial resolution, density resolution, X-ray intensity and exposure time are analyzed. ICT system based on area-array CCD camera is designed in which the application range of the filtered back-projection reconstructed algorithm with parallel beam, fan beam or cone beam is considered. Causes to result in projection data nonuniformity include the vignetting effect resulted from the irradiation intensity distribution of the accelerator and the lens of CCD camera, the peak noise of CCD dark current and the inner defect of converter screen, and the corresponding correction methods are presented. Then multi-slice images of three typically large components used in the tank including large diameter cannonball, impeller and cylinder cover are reconstructed in the ICT system.
Secondly, every cause resulting in reconstructed image artifacts during obtaining projection and reconstructing image is studied, such as the length selection of the filter, rotation center deviation, vignette in the imaging system, CCD dark current, pulse noise and scatter in this paper. The corresponding methods to remove image artifact are given.
Finally, the reconstruction error using truncated projections is researched while the object which size is bigger than the projection field of view is scanned
in the ICT system. A reconstruction method complementing the truncated projection by means of prior knowledge of the object, then reconstructing CT image is proposed in this paper.
本文首先分析了高能X射线检测的特点，研究了空间分辨率、密度分辨率、射线强度和曝光时间之间的相互关系，设计了基于面阵CCD相机的高能X射线工业CT系统，确定了平行束、扇束和锥束滤波反投影重建算法在本系统中的使用条件，分析了成像系统中射线源焦点强度分布和CCD相机透镜引起的渐晕效应、CCD暗电流的尖峰噪声和转换屏内部的缺陷是引起投影数据不一致的主要原因，并提出了相应的校正方法。在此基础上，对三代坦克上使用的三种典型的大型构件：大口径炮弹、发动机叶轮和铸铝汽缸盖实施了多切片CT重建，获得了较好的重建图像。
其次，本文研究了投影获取及重建过程中，产生CT图像伪像的各个环节，如滤波函数的长度的选取、旋转中心偏移、成像系统渐晕、CCD暗电流、脉冲噪声、散射等，提出了相应的消除方法。
最后，研究了对尺寸大于本系统投影视域的物体进行CT扫描时，物体的投影被截断，以不完全的投影数据重建图像引起的误差。提出了利用先验知识，补充截断的投影数据，而后重建物体CT图像的方法。
通过本文的研究与实践，为基于转换屏的高能X射线工业CT工程化打下了基础。
The cross section of large industrial components can be reconstructed using high energy X-ray industrial computed tomograpgy (ICT). It is absolutely necessarily equipment in manufacturing industry, such as national defense, aeronautics and astronautics industry. According to the structure of detectors in the CT scanner, ICT can be divided into two types: the ICT based on linear-array detectors and the ICT based on area-array detectors. To satisfy industrial actually requirement in our country, the technology of high energy X-ray ICT based on area-array detectors is selected as research object in this paper.
Firstly, the characteristic of high energy X-ray detection and the relation among spatial resolution, density resolution, X-ray intensity and exposure time are analyzed. ICT system based on area-array CCD camera is designed in which the application range of the filtered back-projection reconstructed algorithm with parallel beam, fan beam or cone beam is considered. Causes to result in projection data nonuniformity include the vignetting effect resulted from the irradiation intensity distribution of the accelerator and the lens of CCD camera, the peak noise of CCD dark current and the inner defect of converter screen, and the corresponding correction methods are presented. Then multi-slice images of three typically large components used in the tank including large diameter cannonball, impeller and cylinder cover are reconstructed in the ICT system.
Secondly, every cause resulting in reconstructed image artifacts during obtaining projection and reconstructing image is studied, such as the length selection of the filter, rotation center deviation, vignette in the imaging system, CCD dark current, pulse noise and scatter in this paper. The corresponding methods to remove image artifact are given.
Finally, the reconstruction error using truncated projections is researched while the object which size is bigger than the projection field of view is scanned
in the ICT system. A reconstruction method complementing the truncated projection by means of prior knowledge of the object, then reconstructing CT image is proposed in this paper.
引文
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