生物组织对X射线的折射率因子

The Refractive Index of Biological Tissue for X-ray

目的:传统X射线医学成像是利用X光透过物体时的吸收不同而成像,即只考虑了X光波的强度衰减信息,而忽略X光波的相位变化信息。为提高成像分辨率,利用X光波的相位信息,可以实现相衬成像新技术,以进一步区分不同软组织之间的差别。事实上,X光与物质相互作用时强度的衰减与相位的变化均与物体的折射率有关。本文旨在通过讨论X射线对生物组织的折射率的实部和虚部所包含的物理意义,分析比较了对于通常生物组织X射线通过时吸收与相位改变的大小。方法:基于X光属于电磁波这一性质,借助X光的电磁理论,从微观层次上考虑X光与物质相互作用的过程和细节。为简单起见,先分别考虑了n个原子中的电子和原子核对X光的散射效应,然后综合计及生物活细胞中k种不同组分的物质对X光的散射效应的总和,以得到接近实际的吸收项与相位项。结果:给出了不均匀介质中线性吸收系数和空间位置不同引起的相位改变量的表达式;导出了复折射率的表达式,得到了实数折射率和虚数折射率与原子散射因子的关系;分析比较了含有不同组份的生物组织的相位项与吸收项的大小。结论:物质对X光的吸收与相位变化与物质折射率的实部与虚部有关,根据导出的复折射率的结果,表明X射线与生物组织相互作用时,其相位改变项要远大于吸收项。一般情况下,X射线能量在10keV-100keV之间时,相位项δ大约是吸收项β的1000倍。因此,利用相衬成像技术,可以得到比传统X光吸收成像清晰度高得多的相位衬度图像。

Objective： The medical images of familiar obtained by the conventional X-rays imaging techniques is caused by the differences of the objects＇ absorption to X-rays, that is to say, only the intensity attenuation information have been considered, while the phase shift information have been overlooked during the course. In order to improve the image resolution, a new technology with the phase shift information named phase contrast imaging has been used, for further distinguish the differences between parenchyma. In fact, the attenuation intensity and the phase shift are related to the refraction of the samples. The physical meanings of the real and imaginary parts of the objects＇ refraction index have been discussed. The X-rays absorption and phase shift to the biological tissues have also been analyzed and compared in this paper. Methods： The transmission process of X-rays, which have the properties of electromagnetic wave, has been analyzed by the electromagnetic theory, and the process and detail of interaction between X-rays and materials also be considered in microcosmic. For simply, the scatters of the electrons in n atoms and the atomic nucleus to the X-rays are considered respectively, and then the results of k kinds of different elements in biological cells are synthesized, in this way, the absorption item and phase shift item which much closer to reality been gained. Results： The expression about linearity absorption coefficient and phase shift caused by different spatial position in irregular medium has been derived, and the complex refraction index expressions also be exported. Then the relationship between the refraction index＇s real and imaginary parts and atomic scattering factor has been derived. At last, the phase item and absorption item to biological tissue with different elements have been compared. Conclusion： For the reality that the refraction index＇s real and imaginary parts are related to the absorption and phase shift of the X-rays. The phase shift item is much larger than absorption item in the course of X-ray interaction with biological tissue, which has been illuminated according to the derived results. Generally speaking, phase item is larger than absorption item about 1000 times for 10 keV-100 keV X-rays. Therefore, the much more clear images can be gained using phase contrast imaging which is better than traditional X-rays imaging technique.