硬X射线微分相衬成像的噪声特性分析

量子噪声引起光强随机波动是影响X射线相衬图像质量的一个重要因素.提出了一种针对硬X射线微分相衬成像噪声特性的分析方法,通过数值模拟获得了量子噪声对成像结果的作用规律.模拟结果表明,折射像和散射像的噪声均方差与叠栅条纹对比度成反比关系,叠栅条纹对比度越大,统计所得的均方差值越小.在保证X射线探测效率的前提下,提高叠栅条纹对比度将有利于获得高质量的相衬图像,并为有效减少X射线曝光剂量提供条件.     

Potential of propagation‐based synchrotron X‐ray phase‐contrast computed tomography for cardiac tissue engineering

Hydrogel‐based cardiac tissue engineering offers great promise for myocardial infarction repair. The ability to visualize engineered systems in vivo in animal models is desired to monitor the performance of cardiac constructs. However, due to the low density and weak X‐ray attenuation of hydrogels, conventional radiography and micro‐computed tomography are unable to visualize the hydrogel cardiac constructs upon their implantation, thus limiting their use in animal systems. This paper presents a study on the optimization of synchrotron X‐ray propagation‐based phase‐contrast imaging computed tomography (PCI‐CT) for three‐dimensional (3D) visualization and assessment of the hydrogel cardiac patches. First, alginate hydrogel was 3D‐printed into cardiac patches, with the pores filled by fibrin. The hydrogel patches were then surgically implanted on rat hearts. A week after surgery, the hearts including patches were excised and embedded in a soft‐tissue‐mimicking gel for imaging by using PCI‐CT at an X‐ray energy of 25 keV. During imaging, the sample‐to‐detector distances, CT‐scan time and the region of interest (ROI) were varied and examined for their effects on both imaging quality and radiation dose. The results showed that phase‐retrieved PCI‐CT images provided edge‐enhancement fringes at a sample‐to‐detector distance of 147 cm that enabled visualization of anatomical and microstructural features of the myocardium and the implanted patch in the tissue‐mimicking gel. For visualization of these features, PCI‐CT offered a significantly higher performance than the dual absorption‐phase and clinical magnetic resonance (3 T) imaging techniques. Furthermore, by reducing the total CT‐scan time and ROI, PCI‐CT was examined for lowering the effective dose, meanwhile without much loss of imaging quality. In effect, the higher soft tissue contrast and low‐dose potential of PCI‐CT has been used along with an acceptable overall animal dose to achieve the high spatial resolution needed for cardiac implant visualization. As a result, PCI‐CT at the identified imaging parameters offers great potential for 3D assessment of microstructural features of hydrogel cardiac patches.     

微焦点X射线源类同轴相衬成像

微焦点X射线源相衬成像是利用微焦点X射线源透过样品后携带的相位信息对样品内部结构成像. 通过获取相面上的强度信息利用相位复原方法可以得到物体内部的相位信息. 获取了不同放大倍数下的蚂蚁图像, 通过强度传播方程(Transport of Intensity Equation, TIE)方法得到了蚂蚁的相衬图像, 采用 边缘可见度(Edge Visibility)方法和熵法比较了不同放大倍数下的成像效果以及复原前后图像质量.     

Experiment on the object-image distance parameter in the X-ray phase contrast imaging system

For the X-ray phase contrast imaging system, in addition to the hardware-related parameters, the object-image distance is an important parameter that affects the imaging results. Based on the Fresnel diffraction theory, the X-ray phase contrast formation mechanism is analyzed to find the theoretical relationship between the object-image distance parameter and the image contrast. Through the experimental simulation on a ball model, images with edge enhancement effect and image contrast data were obtained with regard to different object-image distance conditions. The analysis and experimental results show that, clear image information about the edges of the sample structures can be got if the object-image distance parameter is set suitably.     

基于相位一致性和Hough变换的多源图像配准方法

由于红外图像与可见光图像对比度不同,常用基于梯度幅值的特征匹配方法难以正确配准。在分析红外图像与可见光图像成像机制的基础上,提出了一种结合相位一致性边缘检测与Hough变换的多源图像配准新方法。该算法首先采用高通滤波和平台直方图均衡方法对红外图像进行预处理以提高红外图像的对比度,再利用具有图像对比度不变性的相位一致性边缘检测法提取两幅图像的边缘,结合Hough变换选取图像空间中最长的线作为特征,采用改进相位相关法作为相似性度量,在对数极坐标域下计算出两幅图像的几何变形参数。仿真实验结果表明,该方法能够以较高查准率实现红外与可见光图像自动配准,并具有较强的鲁棒性。     

Elemental x-ray imaging using Zernike phase contras

We develop an element-specific x-ray microscopy method by using Zernike phase contrast imaging near absorption edges, where a real part of refractive index changes abruptly. In this method two phase contrast images are subtracted to obtain the target element: one is at the absorption edge of the target element and the other is near the absorption edge. The x-ray exposure required by this method is expected to be significantly lower than that of conventional absorption-based x-ray elemental imaging methods. Numerical calculations confirm the advantages of this highly efficient imaging method.     

At-wavelength inspection of sub-40 nm defects in extreme ultraviolet lithography mask blank by photoemission electron microscopy

A new at-wavelength inspection technology to probe nanoscale defects buried underneath Mo/Si multilayers on an extreme ultraviolet (EUV) lithography mask blank has been implemented using EUV photoemission electron microscopy (EUV-PEEM). EUV-PEEM images of programmed defect structures of various lateral and vertical sizes recorded at an ~13.5 nm wavelength show that 35 nm wide and 4 nm high buried line defects are clearly detectable. The imaging technique proves to be sensitive to small phase jumps, enhancing the edge visibility of the phase defects, which is explained in terms of a standing wave enhanced image contrast at resonant EUV illumination.     

Elemental x-ray imaging using Zernike phase contrast

We develop an element-specific x-ray microscopy method by using Zernike phase contrast imaging near absorption edges, where a real part of refractive index changes abruptly. In this method two phase contrast images are subtracted to obtain the target element: one is at the absorption edge of the target element and the other is near the absorption edge. The x-ray exposure required by this method is expected to be significantly lower than that of conventional absorption-based x-ray elemental imaging methods. Numerical calculations confirm the advantages of this highly efficient imaging method.     

Novel Ghost Imaging Method for a Pure Phase Object

We propose a novel ghost imaging scheme which is especially served to a pure phase object. A spatially incoherent beam is mixed with a coherent beam of the same frequency field by a beamsplitter. Then we perform the ghost imaging scheme using the mixed beam. Our theoretical result shows that this approach is capable of reconstructing a pure phase object in joint-intensity measurement. The visibility of the images is also analysed for two pure phase objects, an optical wedge and a phase grating.     

天文图像多帧盲反卷积收敛性的增强方法

天文图像多帧盲反卷积的收敛性受到初始目标、约束条件和光子噪声等因素的影响.提出了用实际光学成像系统参数确定频率带宽有限约束的方法.用Knox-Thompson 方法重构初始目标相位形成盲反卷积算法的初始目标函数.研究了一种新颖的有效减小光子噪声、边缘效应和振铃现象的方法.根据最大似然估计理论，用期望最大化的优化方法建立了改进的严格约束多帧盲反卷积算法.模拟图像和实际天文图像的复原结果表明，所建立的多帧盲反卷积，可以有效克服大气湍流和减小光子噪声，改善天文观察图像的分辨率，并部分消除光学系统衍射效应对恢复图像的影响. 关键词： 大气光学 天文观测 图像处理和恢复     

Using synchrotron radiation inline phase‐contrast imaging computed tomography to visualize three‐dimensional printed hybrid constructs for cartilage tissue engineering

Synchrotron radiation inline phase‐contrast imaging combined with computed tomography (SR‐inline‐PCI‐CT) offers great potential for non‐invasive characterization and three‐dimensional visualization of fine features in weakly absorbing materials and tissues. For cartilage tissue engineering, the biomaterials and any associated cartilage extracellular matrix (ECM) that is secreted over time are difficult to image using conventional absorption‐based imaging techniques. For example, three‐dimensional printed polycaprolactone (PCL)/alginate/cell hybrid constructs have low, but different, refractive indices and thicknesses. This paper presents a study on the optimization and utilization of inline‐PCI‐CT for visualizing the components of three‐dimensional printed PCL/alginate/cell hybrid constructs for cartilage tissue engineering. First, histological analysis using Alcian blue staining and immunofluorescent staining assessed the secretion of sulfated glycosaminoglycan (GAGs) and collagen type II (Col2) in the cell‐laden hybrid constructs over time. Second, optimization of inline PCI‐CT was performed by investigating three sample‐to‐detector distances (SDD): 0.25, 1 and 3 m. Then, the optimal SDD was utilized to visualize structural changes in the constructs over a 42‐day culture period. The results showed that there was progressive secretion of cartilage‐specific ECM by ATDC5 cells in the hybrid constructs over time. An SDD of 3 m provided edge‐enhancement fringes that enabled simultaneous visualization of all components of hybrid constructs in aqueous solution. Structural changes that might reflect formation of ECM also were evident in SR‐inline‐PCI‐CT images. Summarily, SR‐inline‐PCI‐CT images captured at the optimized SDD enables visualization of the different components in hybrid cartilage constructs over a 42‐day culture period.     

Optimization of the in-line X-ray phase-contrast imaging setup considering edge-contrast enhancement and spatial resolution

Employing the approximation theory based on refraction and the definition of the total point-spread-function of the imaging system, the variation in the edge contrast of simple model samples is discussed with different source-to-sample and sample-to-detector distances, which actually means different spatial resolutions of the imaging system. The experiments were carried out with the Beamline 4W1A imaging setup at the Beijing Synchrotron Radiation Facility for simple model and insect samples. The results show that to obtain clear phase-contrast images of biologic tissues for the X-ray in-line imaging setup, with determined parameters such as the size of the X-ray source, the pixel size of the detector and the fixed source-to-sample distance, there is a range of optimized sample-to-detector distances. The analysis method discussed in this article can be helpful in optimizing the setup of X-ray in-line phase-contrast imaging.     

基于统计光学的正负热光非定域成像

本文, 基于经典统计光学, 建立符合热光特性的统计模型, 通过数值模拟证明了吴令安和Meyers提出的图像重构算法, 并进行了定性的理论分析. 在关联成像获得的数据样本中, 根据桶探测器的光强涨落进行分组, 分别以某个阈值作为下限和上限, 再将分组后的独立样本和相应的面探测器信号进行强度关联, 则可以得到物体的正像或负像. 然而, 不经过关联运算, 直接对分组后的面探测器信号进行算数平均也可以得到物体的正像或负像, 同时成像的对比度得到较大提高. 这种分组对应的非定域成像进一步说明强度涨落在热光成像中的重要性. 最后以字符掩膜版作为成像物体, 分别运用关联成像和分组对应正负成像算法重构物体的图像, 实验结果证明这种新的正负算法可以提高非定域成像的对比度. 关键词： 统计光学 热光 关联成像 正负非定域成像     

基于高光谱的番茄叶片斑潜蝇虫害检测

番茄植株在生长过程中受病虫害的侵染,将导致番茄减产和种植户的经济效益降低,该研究用高光谱技术结合化学计量学方法,实现了番茄叶片斑潜蝇虫害的快速识别。搭建了简易的高光谱成像系统,包括光源单元、高光谱图像采集单元和数据处理单元,用该系统获取番茄叶片的高光谱图像,对高光谱图像进行校准,并从每一幅图像中提取光谱信息。分别采用了光谱角匹配(SAM)分析方法和光谱红边参数判别分析(DA)方法识别番茄叶片斑潜蝇虫害。在SAM分析中,对高光谱数据进行了归一化预处理,以消除多余信息,增加样品之间的差异。比较了以不同番茄叶片样品的反射光谱作为测试光谱时,虫害识别效果的差异,当以受到斑潜蝇侵染的番茄叶片的平均反射光谱作为测试光谱时,虫害识别的正确率较高,达到96.5%。在光谱红边参数判别分析中,从光谱数据中提取了红边位置、红边振幅、最小振幅、红边面积、红谷位置和红边振幅/最小振幅6组红边信息,利用判别分析方法建立番茄叶片斑潜蝇虫害的判别模型,比较了距离判别、Fisher判别、Bayes判别分析方法的判别效果,使用距离判别分析建模的判别正确率最低,判别正确率为88.0%,使用Fisher判别分析建模的效果最佳,判别正确率为96.0%。研究结果表明,采用高光谱技术识别番茄叶片斑潜蝇虫害具有可行性。     

A New Method of Analyzing Edge Effect in Phase Contrast Imaging with Incoherent X-rays

A method has been devised to calculate the half-width of the phase contrast enhanced edge of an X-ray- image with incoherent X-rays. This method is based on changes in the wave fronts of X-rays after penetrating an object, and it takes into account the blur due to penumbra observed in practical radiographic imaging. The method closely predicted edge enhancement when images of a plastic fiber were produced with practical X-ray tubes (Cu, Mo, and W anodes with focus-diameters of 10,100, and 40 μ, respectively). Thus, the method shows promise in facilitating phase contrast X-ray imaging in both medical radiography and non-destructive inspection using incoherent X-rays from X-ray tubes with substantial focus-sizes.     

微焦点源X射线相衬成像技术

 相衬成像方法利用硬X射线对低密度弱吸收物质成像，可获得高衬度图像。用菲涅尔衍射理论分析了X射线图像的形成机理。在频域中根据光学传递函数，对物像距离、样品空间频率等对图像相位衬度的影响进行了分析。分辨率和衬度是决定图像可见度的两个依据，分辨率主要依赖于光源的空间相干性，空间相干性又决定于源点尺寸，而时间相干性（单色性）是一个不重要的影响因子。利用多色微焦点源实现了X射线相衬成像技术，获得了有价值的相衬图像，如低原子序数低密度泡沫材料的硬X射线相衬图像，与吸收衬度成像相比，其图像质量得到了很大提高，能观察到泡沫材料的细微结构，分辨率可达μm量级。     

能见距离测试条件及其对测试结果的影响

气象学的能见距离是在光电成像系统分析和设计中常用的大气性能评价参量。在能见距离的实际测试中,往往由于对测试条件的忽略或限制,使测量结果偏离能见距离。根据辐射度学原理,从理论上研究景物对比度在大气传输中的衰减,得到了在两种可能的条件下所测得的目标最大探测距离与能见距离的关系。利用这一关系可避免或减小测量偏差对成像系统分析设计的影响。     

双层塑料靶丸的X射线相衬成像

 为对微聚焦X射线相衬成像技术应用于双层以及多层塑料靶丸成像和特征分析进行可行性研究,基于类同轴X射线成像技术,综合考虑成像放大倍数、分辨率和衬度等因素,选择合适的实验参数,成功获得了较为清晰的双层塑料靶丸X射线相衬成像照片;采用数字图像处理技术的图像分割手段,如拉普拉斯高斯边缘检测法等对所成像中靶丸边界特征进行分析处理,获得了双层靶丸内层厚度为（10.5±0.6） μm,外层厚度为（9.2±0.7） μm,靶丸外径为（273.3±1.0） μm等参数。     

X-ray phase-sensitive microscope imaging with a grating interferometer: Theory and simulation

A general theoretical framework is presented to explain the formation of the phase signal in an x-ray microscope integrated with a grating interferometer, which simultaneously enables the high spatial resolution imaging and the improved image contrast. By using this theory, several key parameters of phase contrast imaging can be predicted, for instance, the fringe visibility and period, and the conversion condition from the differential phase imaging (DPI) to the phase difference imaging (PDI). Additionally, numerical simulations are performed with certain x-ray optical components and imaging geometry. Comparison with the available experimental measurement [Appl. Phys. Lett. 113 063105 (2018)] demonstrates the accuracy of this developed quantitative analysis method of x-ray phase-sensitive microscope imaging.