X射线荧光CT成像中荧光产额、退激时间、散射、偏振等关键物理问题计算与分析

Calculation and analysis of key physical problems:Fluorescence yield,deexcitation time,scattering and polarization in X-ray fluorescence CT imaging

X射线荧光CT(X-ray fluorescence computed tomography,XFCT)是一种使用X射线荧光(X-ray fluorescence,XRF)实现功能性成像的新技术,在生物医学成像中表现出较大潜力.但是,X射线穿过生物体的同时还会产生大量康普顿散射光子,对XRF信号的采集形成很强的背景噪声;因此,如何有效消除康普顿散射噪声对于提高XFCT成像质量至关重要.本文研究总结了XFCT成像过程中涉及的物理过程,包括:荧光的产额、退激发时间、荧光发射角分布、荧光偏振态、康普顿散射角分布与散射光偏振态,并通过研究荧光与散射光物理性质的差异寻找去除康普顿散射噪声的方法.经过物理过程推导和分析计算,发现:1)高原子序数元素的K层荧光退激发时间极短,在现有探测器的时间分辨率条件下,无法分辨散射光与荧光;2)K层发射荧光的角分布各向同性,康普顿散射角分布在与入射光偏振方向附近取得最小值,而且入射光线偏振度越高,散射光的微分截面越小,偏振光源将有利于减少康普顿散射噪声;3)K层荧光线偏振度为零,而康普顿散射光子在一些散射方向上具有一定线偏振度,因此偏振态的差异可能用于降低康普顿散射噪声.

X-ray fluorescence computed tomography(XFCT)is a molecular imaging technique with great potential applications in biomedical imaging,in which used is the primary X-ray to excite element probes with high atomic number inside samples or tissues for functional imaging.However,owing to the limitation of molecular sensitivity and spatial resolution,the XFCT has not been widely used in the molecular imaging.A large number of Compton scattering photons are produced as the broadband primary X-ray passes through the samples or tissues,forming a strong noise background in the collected XRF signal,which is a major cause of limited molecular sensitivity.Therefore,eliminating the Compton scattering noise is very important for improving molecular sensitivity.In this paper,we summarize the main physical processes involved in the imaging process of XFCT,including the angle distribution and polarization state of the fluorescence and Compton scattering photons,fluorescence yield and deexcitation time of K-shell vacancy.The above physical processes are the main limitations of the imaging quality of XFCT.Through the derivation and analysis of physical processes,we explore the possibility of using these physical effects to reduce the Compton scattering noise and draw some conclusions below.1)The deexcitation time of K-shell vacancy of the element with high atomic number is very short,consequently the scattered light and fluorescence cannot be distinguished between each other under the time resolution condition of the existing detector.2)The angular distribution of the K-shell fluorescence emission is isotropic,and the differential cross section of Compton scattering reaches a minimum value near the polarization direction of the incident light of which the minimum decreases as the linear polarization degree of the incident light increases.Therefore,the polarized light source is beneficial to reducing the Compton scattering noise.3)The linear degree of polarization of K-shell fluorescence is zero,while the Compton scattering photons possess a certain linear degree of polarization in some scattering directions,so the difference between polarization states may be helpful in reducing the Compton scattering noise.