类同轴X线相衬成像中空间相干性

Spatial coherence in the in-line X-ray phase-contrast imaging

目的:在临床应用中,X线光源是有限尺寸的多色光源。本文假定微焦点X光源是一个强度分布均匀的圆形准单色光源。基于基尔霍夫衍射理论,且结合空间相干性,再通过傅里叶变换,最终计算得到吸收衰减函数和相位衰减函数,分别代表对吸收效应和相位效应的影响,本文就空间相干性的两个参数:源半径和最佳成像距离在X线成像中对相衬成像的影响进行计算分析。方法:通过MATLAB计算得到了不同X射线源半径(a)和源-探测器距离(Source-Detector Distance,SDD)情况下的相衬效应曲线,由此分析不同的a和SDD对应的最佳成像距离。结果:(1)SDD对于相衬效应所产生的影响:当a取一定值时,SDD数值越大,其对相衬效应产生的影响越明显;(2)a对于相衬效应的影响:SDD取一定值,a越小,其相衬成像效果越好;但当a<5μm,a继续减小,其对相衬效应产生的影响几乎无变化。因此,当a<5μm时这种影响可以忽略。结论:a取0.5μm^20μm时相衬成像效果较好;若确定了a的值,适当增大SDD,既保证了相衬成像的效果,同时成像最佳位置也较容易确定。

Objective In clinical application, X-ray source is a polychromatic source with a finite size. The microfocus X-ray source is assumed as a circular monochromatic source with uniform intensity distribution in this paper. Based on Kirchhoff diffraction theory, combined with spatial coherence, and then processed by Fourier transform, absorption attenuation function and phase attenuation function can be calculated. The two functions respectively show the influence on absorption effect and phase effect. In the X-ray imaging, the influences of two parameters of the spatial coherence, source radius （a） and source- detector distance （SDD）, on the phase-contrast imaging are calculated and analyzed in this paper. Methods Phase-contrast effect curves of different a and different SDD were calculated by MATLAB to analyze the best imaging distance of different a and different SDD. Results When the a was a certain value, the influence of SDD on phase effect was getting obvious with the higher SDD. When SDD took a certain value, the phase effect was better with the smaller a. However, when the value of a was less than 5 lam, the phase effect showed no differences. So the influence on phase effect was ignored when a was less than 5/am. Conclusion The phase-contrast imaging effect is satisfactory when a is between 0.5 lam and 20 lam. When a is a certain value, SDD can be increased properly to guarantee the phase-contrast effect and more easily determine the optimal imaging position.