PNAS：我国X射线相位衬度成像研究获重大突破 医疗CT技术有望实现新飞跃

中国科学技术大学研究员吴自玉领导的北京同步辐射装置和合肥国家同步辐射实验室联合成像科研小组,在X射线相位衬度成像研究领域取得重大突破,其研究成果克服了医学X射线CT技术应用X射线相位衬度成像方法的障碍,为形成更加快速、灵敏度更高、更安全的X射线相位CT技术奠定了基础。专家预测,这项新技术的诞生。将催生新型X射线相位CT产业。     

硬X射线相位衬度成像的实验研究

介绍了硬X射线(类同轴)相位衬度成像的工作原理及其实验研究结果。X射线波长为0．08860nm,样品为未经任何处理的飞蛾，记录介质为X射线胶片。胶片经处理以后，用光学显微镜读出，可以看出样品的许多细节,尤其在折射率突变处。而同样条件下基于吸收衬度机制的硬X射线吸收成像，由于是弱吸收样品．没有观察到任何图像。     

衍射增强成像折射衬度的应用

折射衬度是衍射增强成像(diffraction enhanced imaging,DEI)中的一种重要衬度,在弱吸收物质的成像中,折射衬度远超过吸收衬度.折射衬度应用的关键是提取出样品的折射信息,折射信息由相应的折射图像表示.根据生物样品和材料样品的特点,研究和比较了两种折射信息提取方法.利用样品的折射图像,对聚苯乙烯样品进行了简单的定量分析,比较了正常肝组织和肝血管瘤组织.实验结果表明,折射衬度清楚地描述了肝组织的微细结构和血管瘤损伤,精确地显示了无法在传统X射线成像技术中获得的聚苯乙烯样品的清晰结构.     

一维高斯分布物体X射线同轴相衬成像参数优化

采用图像衬度、信噪比、分辨率和探测器抽样数作为X射线同轴相衬成像质量的综合评价标准,针对具有广泛应用意义的一维高斯型分布特征的物体,建立了最大化衬度优化和信噪比优化两种参数优化方法的优化流程.通过数值模拟的方式分别对亚微焦点源、激光驱动微焦点源及同步辐射源3种不同X射线源下的成像系统相关参数进行了优化.结果表明,两种优化方法各具优势,可根据不同的相衬成像需求选择相应的优化方法.同步辐射源同轴相衬成像系统采用信噪比优化方法较为合适,而亚微焦点源和激光驱动微焦点源则需根据不同的需求来选择优化方法,必要时还可对优化方法进行一定的修正.研究为不同条件下有着不同要求的优质X射线同轴相衬成像实验提供了保证.     

相衬技术在现代光学中的应用

相衬技术由于能解决相位到振幅的转换而备受关注。随着近几年对其更进一步的研究,发现相衬技术在图像合成,三元相控阵列照明的重建、光镊技术以及X射线相位衬度成像技术中有着广泛的应用,本文对相衬技术的原理及其在现代光学中的应用做简要介绍。     

快速高能X射线光栅相衬成像

X射线光栅相衬成像对弱吸收物质成像能够获得较高的图像衬度,然而使用高分辨探测器,成像时间长。此外,受光栅工艺限制,成像能量通常在30 keV左右。文中基于投影成像原理,大大放宽了对光栅工艺的要求,提高了成像能量。同时,利用医用CT球管以及医用探测器,基于周步进扫描模式,实现了快速相衬CT成像。在国家同步辐射实验室搭建的成像系统上,完成了80 kV管电压（等效能量约48 keV）180 mA管电流,物体80 s曝光的二维和三维成像实验。针对实验结果,进一步探讨了提高密度分辨率的方法和途径。     

激光衍射仪检验JEM-200CX点分辨率及测定Cs和△f

用光学衍射环状花样检验透射电镜点分辨率,要比从图象直接测量间距更为正确,厂商和用户都正在推广这种检验法。在高分辨率成像领域内,象衬度主要是相位衬度。电子波穿越很薄的“弱相位”物体时,物体结构产生的散射效应将只引起电子波微小的相位差(衬度)。为了在最终的象内把相位衬度转化为探测元件能感受的     

纯相位物体的鬼成像

鬼成像因为其具有的非定域特性和突破衍射极限的高分辨率引起了人们的广泛关注。研究了光源的强度波动对纯相位物体的鬼成像的影响。用理论分析和仿真实验证明了光源的强度波动可以影响鬼成像中纯相位物体的信息:光源的强度波动越大,可以从鬼像中得到的物体信息就越多。同时,发现了随着光源强度波动的增大,不同的采样样本得到的纯相位物体的鬼成像中物体信息的差别越大。     

相干衍射成像研究进展:叠层扫描相干衍射成像和相干调制成像

相干衍射成像技术是一种无透镜计算成像技术。该技术通过迭代算法求解相位问题,直接从衍射强度数据重构物体的振幅和相位图像信息,可获得由照明光源波长和数据记录有效数值孔径决定的衍射极限分辨率。由于相干衍射成像技术不依赖于高质量光学成像元件的使用,因而适合用于深紫外、X射线以及电子束等诸多很难制作高性能成像器件的辐射源。过去20年来新型光源（冷致电子枪、第三代和第四代同步辐射光源、自由电子激光器）、单粒子计数高灵敏度、宽动态范围面阵检测器的迅猛发展,也极大地促进了相干衍射成像技术的发展。目前相干衍射成像在材料科学和生物学等热门学科方向上已经显示出了相较传统方法的独特优势,在部分应用上已经逐渐成为主流技术。文中概略回顾相干衍射成像的发展历史。重点介绍叠层扫描相干衍射成像和相干调制成像这两种快速发展的技术。     

4f相位相干成像技术中相位光阑的改进

4f相位相干成像技术是测量三阶光学非线性的新方法.其原理是利用一个相位光阑将放置在4f系统频谱面上的非线性样品中产生的非线性相移调制成像平面上的振幅变化,从而测量样品的非线性折射率的大小和符号.常用的带圆形相位物体的相位光阑灵敏度随相位物体尺寸减小而增加,但是在相位物体很小的情况下制作难度很大,通常达不到最佳的灵敏度.通过将相位光阑中原来处于光阑中心的圆形相位物体改为半圆形相位物体,使得系统可以处于最佳灵敏度,相位光阑在制作工艺上的难度大大降低.     

硬X射线相位相衬成像研究进展

自从20世纪90年代中叶以来,硬X射线相位相衬成像技术因其对弱吸收材料的极高灵敏度引起了国内外广泛的关注,医学及生物成像是相位相衬成像的主要目标.详细论述了各种相位相衬成像技术的研究进展情况.     

板壳状微电子器件的数字层析成像检测方法

 传统工业CT(industrial computed tomography, ICT)成像方法受扫描原理和探测器、射线源等硬件条件的限制,难以对芯片、印刷电路板等板、壳状微电子器件实施有效的数字层析成像检测. 为此,讨论了一种基于锥束射线倾斜扫描和代数重建技术（Algebraic Reconstruction Technique, ART）的薄板层析成像（Computed Laminography, CL）方法,研究了其基于投影凸集理论的投影预处理方法及基于不同区域相似性的重建图像非局部平均降噪后处理方法,建立了基于非晶硅面阵探测器的实验系统,并完成了CPU芯片和印刷电路板的CL成像. 实验结果证明了该方法的正确性.     

三结构各向同性包覆燃料颗粒相衬CT图像的噪声和伪像抑制方法初探

对三结构各向同性包覆燃料颗粒相衬CT图像的噪声和伪像抑制进行了初步探索,提出了三种方法分别抑制由探测器随机噪声、非线性漂移以及灵敏度非均匀性所导致的图像噪声或伪像.对抑制噪声或伪像前后的图像质量进行了主观评价以及定量分析,结果表明,本文提出的三种方法在有效抑制上述三种类型噪声或伪像的同时仍然能够保证重建图像中不同结构间边缘的清晰度.     

Relationships Between Smooth- and Small-Phase Conditions in X-Ray Phase-Contrast Imaging

We analyze some relationships between the small- and smooth-phase conditions in propagation-based X-ray phase-contrast imaging. Although these conditions are generally well-understood, our analysis yields the identification of physical conditions under which they are mathematically equivalent. We also demonstrate that the smooth-phase condition depends not only on the imaging system resolution and object-to-detector distance, but also on the topography of the wavefield phase function.      

Hybrid System for Simultaneous Fluorescence and X-Ray Computed Tomography

A hybrid imaging system for simultaneous fluorescence tomography and X-ray computed tomography (XCT) of small animals has been developed and presented. The system capitalizes on the imaging power of a 360 $^{circ}$-projection free-space fluorescence tomography system, implemented within a microcomputed tomography scanner. Image acquisition is based on techniques that automatically adjust a series of imaging parameters to offer a high dynamic range dataset. Image segmentation further allows the incorporation of structural priors in the optical reconstruction problem to improve the imaging performance. The functional system characteristics are showcased, and images from a brain imaging study are shown, which are reconstructed using XCT-derived priors into the optical forward problem.      

Investigation of X-ray fluorescence computed tomography (XFCT) and K-edge imaging

This work provides a comprehensive Monte Carlo study of X-ray fluorescence computed tomography (XFCT) and K-edge imaging system, including the system design, the influence of various imaging components, the sensitivity and resolution under various conditions. We modified the widely used EGSnrc/DOSXYZnrc code to simulate XFCT images of two acrylic phantoms loaded with various concentrations of gold nanoparticles and Cisplatin for a number of XFCT geometries. In particular, reconstructed signal as a function of the width of the detector ring, its angular coverage and energy resolution were studied. We found that XFCT imaging sensitivity of the modeled systems consisting of a conventional X-ray tube and a full 2-cm-wide energy-resolving detector ring was 0.061% and 0.042% for gold nanoparticles and Cisplatin, respectively, for a dose of ～ 10 cGy. Contrast-to-noise ratio (CNR) of XFCT images of the simulated acrylic phantoms was higher than that of transmission K-edge images for contrast concentrations below 0.4%.     

Phase-based multidimensional volume registration

We present a method for accurate image registration and motion compensation in multidimensional signals, such as two-dimensional (2-D) X-ray images and three-dimensional (3-D) computed tomography/magnetic resonance imaging volumes. The method is based on phase from quadrature filters, which makes it robust to noise and temporal intensity variations. The method is equally applicable to signals of two, three or higher number of dimensions. We use parametric models, e.g., affine models, finite elements or local affine models with global regularization. Experimental results show high accuracy for 2-D and 3-D motion compensation.     

Towards cardiac C-arm computed tomography

Cardiac interventional procedures would benefit tremendously from sophisticated three-dimensional image guidance. Such procedures are typically performed with C-arm angiography systems, and tomographic imaging is currently available only by using preprocedural computed tomography (CT) or magnetic resonance imaging (MRI) scans. Recent developments in C-arm CT (Angiographic CT) allow three-dimensional (3-D) imaging of low contrast details with angiography imaging systems for noncardiac applications. We propose a new approach for cardiac imaging that takes advantage of this improved contrast resolution and is based on intravenous contrast injection. The method is an analogue to multisegment reconstruction in cardiac CT adapted to the much slower rotational speed of C-arm CT. Motion of the heart is considered in the reconstruction process by retrospective electrocardiogram (ECG)-gating, using only projections acquired at a similar heart phase. A series of N almost identical rotational acquisitions is performed at different heart phases to obtain a complete data set at a minimum temporal resolution of 1/N of the heart cycle time. First results in simulation, using an experimental phantom, and in preclinical in vivo studies showed that excellent image quality can be achieved.