基于光栅相衬成像的螺旋CT算法研究
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摘要
在硬X射线波段,由于轻元素物质对X射线相位的改变是其对X射线吸收幅度改变的1000-100000倍,所以相位衬度成像有可能获得比吸收衬度成像高得多的密度分辨率。迄今为止,已经发展出四种X射线相位衬度成像方法,分别为晶体干涉仪成像法、相位传播成像法、衍射增强成像法、光栅干涉仪成像法。前三种方法虽各有所长,也均能获得较好的成像结果,但都有其自身局限性,所以上述三种成像方法均不具备广泛推广应用的潜力。而基于Lau效应的光栅干涉仪成像法能利用非相干X射线进行成像,向人们展示了光栅干涉仪成像法对X射线光束的单色性和相干性要求均不高,不仅可以使用平行度较高的同步辐射X射线光束,而且可以使用非相干的普通X射线光源。因此,基于光栅Lau效应X射线相位衬度成像方法具有推广到医学临床、工业和安全检查的广阔发展前景。
而现在国际上通用的基于“phase-stepping”的相位信息提取方法过于复杂,必须将分析光栅至少移动五步,分别曝光五次(简称五步曝光),才能获得并分离出样品的相位信息。这种方法使得相位衬度CT数据采集过程繁琐耗时,不利于光栅相位衬度成像方法的推广应用。基于光栅“位移曲线”理论的“一步曝光”方法的提出,为光栅干涉仪成像在CT领域的推广提供了手段。这种方法大幅度降低了样品的曝光剂量,而且为相位衬度CT进一步简化和推广应用奠定了基础。
本文在“一步曝光”方法的基础上,将螺旋CT的概念引入光栅相位衬度成像研究领域,发展具有广阔应用前景的X射线相位衬度螺旋CT方法。根据平行束投影数据和扇束投影数据等价的原理,本文给出了一种光栅相位衬度螺旋CT投影数据的采集方法,利用插值方法求出每根射线的共轭射线,并根据一对共轭射线吸收相同、折射角相反的性质,提取出样品的相位衬度投影数据,再利用滤波反投影算法,给出一种扇形束螺旋相位CT重建方法,并重建出样品的相位项。
Phase-contrast imaging is a state-of-the-art non-destructive detecting technology, it can provide high sensitivity, with which the internal structure of light elements can be observed, because the change of phase-shift is 1000 to 100000 times more than that of absorbing attenuation in hard X-ray wave band. Until now, the scientists have developed four Phase-contrast imaging methods, they are interferometer method, in-line method, diffraction enhanced imaging method and grating imaging method. The first three methods can not be used in the application area because of their own flaws, but grating imaging based on the Law of Lau can successfully use the incoherence source, which brought us into a new epoch of incoherence source phase contrast imaging. It has the potential to be utilized in many fields such as medicine, biology and material science etc.
The conventional phase-contrast information extraction method, which is known as "phase stepping", is too complex, it needs to move the analyzer grating at least five times to expose in one projection angle, so this method will cost much longer time in the projection-acquisition process, which make grating imaging can hardly be used for human detection. The propose of "one-time exposure in one projection angle" solves the problem perfectly by decreasing the expose dose and simplifying the experimental complexity. In this thesis, by analyzing the feature of equivalence principle between parallel data and fan-beam data, a new phase-contrast CT method which is based on "one-time exposure in one projection angle" is developed by introducing the helical scanning into the field of phase-contrast imaging. First, every conjugation X-ray in the helical scanning is calculated and all the X-ray is put in the same plane by interpolation, then according to the property that the absorption of the conjugation X-ray is the same while the refraction angle of it is contrary, the phase-contrast information is extracted. At last the phase-contrast part is reconstructed by using filtered-back projection method. Computer simulations and performance analysis demonstrate the efficacy of the proposed method.
而现在国际上通用的基于“phase-stepping”的相位信息提取方法过于复杂,必须将分析光栅至少移动五步,分别曝光五次(简称五步曝光),才能获得并分离出样品的相位信息。这种方法使得相位衬度CT数据采集过程繁琐耗时,不利于光栅相位衬度成像方法的推广应用。基于光栅“位移曲线”理论的“一步曝光”方法的提出,为光栅干涉仪成像在CT领域的推广提供了手段。这种方法大幅度降低了样品的曝光剂量,而且为相位衬度CT进一步简化和推广应用奠定了基础。
本文在“一步曝光”方法的基础上,将螺旋CT的概念引入光栅相位衬度成像研究领域,发展具有广阔应用前景的X射线相位衬度螺旋CT方法。根据平行束投影数据和扇束投影数据等价的原理,本文给出了一种光栅相位衬度螺旋CT投影数据的采集方法,利用插值方法求出每根射线的共轭射线,并根据一对共轭射线吸收相同、折射角相反的性质,提取出样品的相位衬度投影数据,再利用滤波反投影算法,给出一种扇形束螺旋相位CT重建方法,并重建出样品的相位项。
Phase-contrast imaging is a state-of-the-art non-destructive detecting technology, it can provide high sensitivity, with which the internal structure of light elements can be observed, because the change of phase-shift is 1000 to 100000 times more than that of absorbing attenuation in hard X-ray wave band. Until now, the scientists have developed four Phase-contrast imaging methods, they are interferometer method, in-line method, diffraction enhanced imaging method and grating imaging method. The first three methods can not be used in the application area because of their own flaws, but grating imaging based on the Law of Lau can successfully use the incoherence source, which brought us into a new epoch of incoherence source phase contrast imaging. It has the potential to be utilized in many fields such as medicine, biology and material science etc.
The conventional phase-contrast information extraction method, which is known as "phase stepping", is too complex, it needs to move the analyzer grating at least five times to expose in one projection angle, so this method will cost much longer time in the projection-acquisition process, which make grating imaging can hardly be used for human detection. The propose of "one-time exposure in one projection angle" solves the problem perfectly by decreasing the expose dose and simplifying the experimental complexity. In this thesis, by analyzing the feature of equivalence principle between parallel data and fan-beam data, a new phase-contrast CT method which is based on "one-time exposure in one projection angle" is developed by introducing the helical scanning into the field of phase-contrast imaging. First, every conjugation X-ray in the helical scanning is calculated and all the X-ray is put in the same plane by interpolation, then according to the property that the absorption of the conjugation X-ray is the same while the refraction angle of it is contrary, the phase-contrast information is extracted. At last the phase-contrast part is reconstructed by using filtered-back projection method. Computer simulations and performance analysis demonstrate the efficacy of the proposed method.
引文
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[17] Mayo S C, Miller P R, Wilkins S W, et al. Quantitative X-ray projection microscopy: phase-contrast and multi-spectral imaging[J]. Journal of Microscopy, 2002, 207: 79-96
[18] Davis T J, Gao D, Gureyev T E, et al. Phase-contrast imaging of weakly absorbing materials using hard X-rays[J]. Nature, 1995, 373: 595-597
[19] Chapman D, Thomlinson W, Johnston R E, et al. Diffraction enhanced x-ray imaging[J]. Phys. Med. Boil. 1997,42: 2015-2025
[20]Chapman D,Pisano E,Thomlinson W,et al.Medical applications of diffraction enhanced imaging[J].Breast Disease 1998,10(3,4):197-207
[21]Dilmanian F A,Zhong Z,Ben B,et al.Computed tomography of x-ray index of refraction using the diffraction enhanced imaging method[J].Phys.Med.Biol.2000,45:933-946
[22]Oltulu O.A unified approach to x-ray absorption-refraction-exctinction contrast with diffraction-enhanced imaging[PHD dissertation],Illinois Institute of Technology,2003.
[23]孙怡,朱佩平,于健,陈欣,利用衍射增强成像方法三维重建吸收衰减系数、散射消光系数和折射率的原理和方法[J],光学学报,vol.27(4),pp.749-754,2007
[24]孙怡,朱佩平,刘明贺,于健,硬X射线衍射增强峰位成像CT的几何失真纠正方法[J],CT理论与应用研究,第15卷,第2期(2005)
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[26]Yi Sun,PeiPing Zhu,Jian Yu,Ying Hou,Computerized tomography based on X-ray refraction information[C],IEEE International Symposium on BIOMEDICAL IMAGING(ISBI),2006
[27]孙怡,朱佩平,于健,陈欣,利用衍射增强成像方法三维重建吸收衰减系数、散射消光系数和折射率的原理和方法[J],光学学报,vol.27(4),pp.749-754,2007
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