X射线成像术(下)

该文的前半部分(本刊上一期)已扼要介绍了X射线成像的三种衬度机制及成像设备各主要组件的构造,下半部分将继续介绍应用各种衬度的不同的成像方法和一些实例。     

X射线成像术(上)

该文着重从衬度的形成讨论了X射线成像。介绍衬度生成的三种机制:吸收、位相变动和衍射。还介绍了成像设备的主要构成部件:X射线源(X射线发生器、直线加速器和同步辐射),各类探测器、像增强器与显示器,机械、控制与数据处理系统等;应用各种衬度形成的一些成像方法也作了简述,如利用吸收衬度的造影成像、数字减影和双色减影成像、计算机断层成像;利用相位衬度的干涉仪法、类同轴全息法和衍射增强法;利用衍射衬度的Lang透射法和Berg-Barrett反射法等,并用少量例子说明。     

New opportunities for 3D materials science of polycrystalline materials at the micrometre lengthscale by combined use of X-ray diffraction and X-ray imaging

Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials.     

Three-dimensional imaging of human stem cells using soft X-ray tomography

Three-dimensional imaging of human stem cells using transmission soft X-ray tomography (SXT) is presented for the first time. Major organelle types—nuclei, nucleoli, mitochondria, lysosomes and vesicles—were discriminated at approximately 50 nm spatial resolution without the use of contrast agents, on the basis of measured linear X-ray absorption coefficients and comparison of the size and shape of structures to transmission electron microscopy (TEM) images. In addition, SXT was used to visualize the distribution of a cell surface protein using gold-labelled antibody staining. We present the strengths of SXT, which include excellent spatial resolution (intermediate between that of TEM and light microscopy), the lack of the requirement for fixative or contrast agent that might perturb cellular morphology or produce imaging artefacts, and the ability to produce three-dimensional images of cells without microtome sectioning. Possible applications to studying the differentiation of human stem cells are discussed.     

Quantitative evaluation on 3D fetus morphology via X-ray grating based imaging technique

Synchrotron-based X-ray phase sensitive micro-tomography techniques enable to visualize detailed three-dimensional (3D) insight into nondestructive inner-structure of biomedical samples. Different phase sensitive mechanisms have been employed for discrimination of tissue's tiny density variations in biomedical research. We effectively visualized and analyzed the phase-contrast experimental results of X-ray grating-based imaging, based on grating interferometry with phase stepping, by using transgenic mouse fetus. We quantitatively measured and evaluated the contrast-to-noise ratio or the mass density resolution, spatial resolution, radiation dose, and figure of merit of X-ray grating-based imaging technique in biomedical research respectively. Moreover, the complex coherent degrees of light source were duly taken into account in the analysis of spatial resolution. In addition, the mass density distribution of soft biomedical specimens can be estimated using our presented method preliminarily. For most soft tissue and organ observation, this work provides explicit guidelines to help future synchrotron users obtain the quantitative image information, suitable for their specific biomedical research.     

台阶模糊边的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台阶模糊边分布特征的物体,提出了参数优化方法并建立了优化流程.通过数值模拟的方式分别对具有广泛代表性的亚微米焦点源、激光驱动微米焦点源及同步辐射源3种X射线源下成像系统的相关参数进行了优化.结果表明,基于优化方法的优化流程很好地完成了3种X射线源的同轴相衬成像参数优化.可见,这种优化方法具有广泛的应用意义,在根据具体情况对方法进行简单修正的基础上,可以完成任意X射线源下对具有台阶模糊边分布特征物体的同轴相衬成像优化工作.     

X-ray orientation microscopy using topo-tomography and multi-mode diffraction contrast tomography

Polycrystal orientation mapping techniques based on full-field acquisition schemes like X-ray Diffraction Contrast Tomography and certain other variants of 3D X-ray Diffraction or near-field High Energy Diffraction Microscopy enable time efficient mapping of 3D grain microstructures. The spatial resolution obtained with this class of monochromatic beam X-ray diffraction imaging approaches remains typically below the ultimate spatial resolution achievable with X-ray imaging detectors. Introducing a generalised reconstruction framework enabling the combination of acquisitions with different detector pixel size and sample tilt settings provide a pathway towards 3D orientation mapping with a spatial resolution approaching the one of state of the art X-ray imaging detector systems.     

Simulation study of magnetic resonance imaging-guided cortically constrained diffuse optical tomography of human brain function

Diffuse optical imaging can measure brain activity noninvasively in humans through the scalp and skull by measuring the light intensity modulation arising from localized-activity-induced absorption changes within the cortex. Spatial resolution and localization accuracy are currently limited by measurement geometry to approximately 3 cm in the plane parallel to the scalp. Depth resolution is a more significant challenge owing to the limited angle tomography permitted by reflectance-only measurements. We combine previously established concepts for improving image quality and demonstrate, through simulation studies, their application for improving the image quality of adult human brain function. We show in a three-dimensional human head model that localization accuracy is significantly improved by the addition of measurements that provide overlapping samples of brain tissue. However, the reconstructed absorption contrast is significantly underestimated because its depth is underestimated. We show that the absorption contrast amplitude accuracy can be significantly improved by providing a cortical spatial constraint in the image reconstruction to obtain a better depth localization. The cortical constraint makes physiological sense since the brain-activity-induced absorption changes are occurring in the cortex and not in the scalp, skull, and cerebral spinal fluid. This spatial constraint is provided by segmentation of coregistered structural magnetic resonance imaging (MRI). However, the absorption contrast deep within the cortex is reconstructed superficially, resulting in an underestimation of the absorption contrast. The synthesis of techniques described here indicates that multimodality imaging of brain function with diffuse optical imaging and MRI has the potential to provide more quantitative estimates of the total and deoxyhemoglobin response to brain activation, which is currently not provided by either method independently. However, issues of depth resolution within the cortex remain to be resolved.     

Novel X-Ray and Optical Diagnostics for Studying Energetic Materials: A Review

Thermomechanical, physical, and chemical processes in energetic materials (EMs) during manufacturing and processing or under external stimuli such as shock compression, involve multiple temporal and spatial scales. Discovering novel phenomena, acquiring new data, and understanding underlying mechanisms all require temporally and spatially resolved diagnostics. Here, we present a brief review of novel diagnostics that are either emerging or have existed but rarely been applied to EMs, including two-dimensional (2D) and three-dimensional (3D) X-ray imaging, X-ray diffraction, coherent X-ray diffraction imaging, small angle X-ray scattering, terahertz and optical absorption/emission spectroscopy, and one-dimensional (1D) and 2D laser-based velocity/displacement interferometry. Typical spatial scales involved are lattice (nanometer and micrometer) and typical temporal scales (femtosecond, picosecond, nanosecond, microsecond, and millisecond). The targeted scientific questions and engineering problems include defects, strengths, deformations, hot spots, phase changes, reactions, and shock sensitivities. Basic principles of measurement and data analysis, and illustrative examples of these are presented. Advanced measurements and experimental complexities also necessitate further development in corresponding data analysis and interpretation methodologies, and multiscale modeling.     

3D in situ observations of glass fibre/matrix interfacial debonding

X-ray microtomography was used for 3D in situ observations of the evolution of fibre/matrix interfacial debonding. A specimen with a single fibre oriented perpendicular to the tensile direction was tested at a synchrotron facility using a special loading rig which allowed for applying a load transverse to the fibre. Three distinguishable damage stages were observed: (i) interfacial debond initiation at the free surface, (ii) debond propagation from the surface into the specimen and (iii) unstable debonding along the full length of the scanned volume. The high resolution microtomography provides both qualitative and quantitative 3D data of the debonding initiation and propagation. Thus, microtomography is demonstrated as a promising technique which can assist micromechanical model development.     

Going beyond histology. Synchrotron micro-computed tomography as a methodology for biological tissue characterization: from tissue morphology to individual cells

Current light microscopic methods such as serial sectioning, confocal microscopy or multiphoton microscopy are severely limited in their ability to analyse rather opaque biological structures in three dimensions, while electron optical methods offer either a good three-dimensional topographic visualization (scanning electron microscopy) or high-resolution imaging of very thin samples (transmission electron microscopy). However, sample preparation commonly results in a significant alteration and the destruction of the three-dimensional integrity of the specimen. Depending on the selected photon energy, the interaction between X-rays and biological matter provides semi-transparency of the specimen, allowing penetration of even large specimens. Based on the projection-slice theorem, angular projections can be used for tomographic imaging. This method is well developed in medical and materials science for structure sizes down to several micrometres and is considered as being non-destructive. Achieving a spatial and structural resolution that is sufficient for the imaging of cells inside biological tissues is difficult due to several experimental conditions. A major problem that cannot be resolved with conventional X-ray sources are the low differences in density and absorption contrast of cells and the surrounding tissue. Therefore, X-ray monochromatization coupled with a sufficiently high photon flux and coherent beam properties are key requirements and currently only possible with synchrotron-produced X-rays. In this study, we report on the three-dimensional morphological characterization of articular cartilage using synchrotron-generated X-rays demonstrating the spatial distribution of single cells inside the tissue and their quantification, while comparing our findings to conventional histological techniques.     

On coherence in neutron imaging

The variety of imaging signals in neutron radiography and tomography became quite large compared to the pure absorption and scattering contrast in neutron radiographies and topographies in the early sixties or seventies of the last century. The diversity of absorption based techniques for neutron radiography and tomography is comparable to coherence based imaging techniques such as phase contrast, differential phase contrast, dark field imaging, diffraction enhanced contrast, refraction contrast, ultra small angle scattering contrast, grating interferometry and crystal interferometry, also the spin of the neutron was successfully used for imaging [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]. We show which effects (total reflection, diffraction, refraction) contribute to e.g. a step boundary or a phase boundary. Taking this simple object, one can learn to understand the imaging procedure and what is displayed in a radiograph.     

同步辐射原位X射线散射技术在纳米与能源材料中的应用

同步辐射原位X射线散射技术可以实现对材料结构进行多尺度的、无损的、高时间空间分辨率的表征,动态地揭示材料微观结构在不同外界环境下的演变过程。X射线散射基础理论已经相对成熟。第三代同步辐射光源大幅提高了X射线散射技术的时空分辨率,进一步拓宽X射线散射技术的应用场景。当前同步辐射原位X射线散射技术的难点主要集中于实验装置设计和大数据处理。概述了X射线散射技术的主要分类和基本的实验方法,主要介绍了不同分类的同步辐射原位X射线散射技术在纳米材料(纳米颗粒生长和纳米颗粒自组装)与能源材料(以钙钛矿薄膜材料为代表)研究中的应用。最后结合当前国内外先进同步辐射光源的发展现状,展望了同步辐射原位X射线散射技术未来发展的方向和应用前景。     

Photoacoustic Imaging: Contrast Agents and Their Biomedical Applications

Photoacoustic (PA) imaging as a fast‐developing imaging technique has great potential in biomedical and clinical applications. It is a noninvasive imaging modality that depends on the light‐absorption coefficient of the imaged tissue and the injected PA‐imaging contrast agents. Furthermore, PA imaging provides superb contrast, super spatial resolution, and high penetrability and sensitivity to tissue functional characteristics by detecting the acoustic wave to construct PA images. In recent years, a series of PA‐imaging contrast agents are developed to improve the PA‐imaging performance in biomedical applications. Here, recent progress of PA contrast agents and their biomedical applications are outlined. PA contrast agents are classified according to their components and function, and gold nanocrystals, gold‐nanocrystal assembly, transition‐metal chalcogenides/MXene‐based nanomaterials, carbon‐based nanomaterials, other inorganic imaging agents, small organic molecules, semiconducting polymer nanoparticles, and nonlinear PA‐imaging contrast agents are discussed. The applications of PA contrast agents as biosensors (in the sensing of metal ions, pH, enzymes, temperature, hypoxia, reactive oxygen species, and reactive nitrogen species) and in bioimaging (lymph nodes, vasculature, tumors, and brain tissue) are discussed in detail. Finally, an outlook on the future research and investigation of PA‐imaging contrast agents and their significance in biomedical research is presented.     

高分辨三维成像原位试验机研制进展及应用

先进材料及结构的损伤表征和在役性能评价是重大装备研发与服役中的关键科学问题。目前,依托同步辐射大科学装置的X射线三维成像技术在金属材料细观损伤力学行为研究方面具有独特优势,而兼容于同步辐射光源相应光束线站的各类原位加载装置对材料内部微结构损伤演化的动态高分辨表征具有重要意义。简要介绍了国内外依托世界各大高性能光源的原位加载试验机研制进展与应用成果,重点阐述了可实现单向拉压、循环加载及具备极寒、高温、真空等样品环境的基于同步辐射X射线成像的原位加载装置的设计原理及结构特点。最后,结合第三代高能X射线三维成像技术特点、先进光源线站建设、高通量试验要求等,对材料原位加载条件下的高时空分辨率动态成像进行了展望,指出开发集拉伸、压缩、低周疲劳、高周疲劳和超高周疲劳加载机构于一体的多功能原位试验机是一项重要的工作。     

A Review of X-Ray Imaging at the BAMline (BESSY II)

The hard X-ray beamline BAMline at BESSY II (Berlin, Germany) has now been in service for 20 years. Several improvements have been implemented in this time, and this review provides an overview of the imaging methods available at the BAMline. Besides classic full-field synchrotron X-ray computed tomography (SXCT), also absorption edge CT, synchrotron X-ray refraction radiography (SXRR), and synchrotron X-ray refraction tomography (SXRCT) are used for imaging. Moreover, virtually any of those techniques are currently coupled in situ or operando with ancillary equipment such as load rigs, furnaces, or potentiostats. Each of the available techniques is explained and both the current and the potential usage are described with corresponding examples. The potential use is manifold, the examples cover organic materials, composite materials, energy-related materials, biological samples, and materials related to additive manufacturing. The article includes published examples as well as some unpublished applications.     

A Bayesian approach to triaxial strain tomography from high-energy X-ray diffraction

Diffraction of high-energy X-rays produced at synchrotron sources can provide rapid strain measurements, with high spatial resolution, and good penetrating power. With an uncollimated diffracted beam, through-thickness averages of strain can be measured using this technique, which poses an associated rich tomography problem. This paper proposes a Gaussian process (GP) model for three-dimensional strain fields satisfying static equilibrium and an accompanying algorithm for tomographic reconstruction of strain fields from high-energy X-ray diffraction. This method can achieve triaxial strain tomography in three dimensions using only a single axis of rotation. The method builds upon recent work where the GP approach was used to reconstruct two-dimensional strain fields from neutron-based measurements. A demonstration is provided from simulated data, showing the method is capable of rejecting realistic levels of Gaussian noise.     

High-resolution neutron microtomography with noiseless neutron counting detector

The improved collimation and intensity of thermal and cold neutron beamlines combined with recent advances in neutron imaging devices enable high-resolution neutron radiography and microtomography, which can provide information on the internal structure of objects not achievable with conventional X-ray imaging techniques. Neutron detection efficiency, spatial and temporal resolution (important for the studies of dynamic processes) and low background count rate are among the crucial parameters defining the quality of radiographic images and tomographic reconstructions. The unique capabilities of neutron counting detectors with neutron-sensitive microchannel plates (MCPs) and with Timepix CMOS readouts providing high neutron detection efficiency (∼70% for cold neutrons), spatial resolutions ranging from 15 to 55 μm and a temporal resolution of ∼1 μs—combined with the virtual absence of readout noise—make these devices very attractive for high-resolution microtomography. In this paper we demonstrate the capabilities of an MCP-Timepix detection system applied to microtomographic imaging, performed at the ICON cold neutron facility of the Paul Scherrer Institute. The high resolution and the absence of readout noise enable accurate reconstruction of texture in a relatively opaque wood sample, differentiation of internal tissues of a fly and imaging of individual ∼400 μm grains in an organic powder encapsulated in a ∼700 μm thick metal casing.     

Characterizing the nano and micro structure of concrete to improve its durability

New and advanced methodologies have been developed to characterize the nano and microstructure of cement paste and concrete exposed to aggressive environments. High resolution full-field soft X-ray imaging in the water window is providing new insight on the nano scale of the cement hydration process, which leads to a nano-optimization of cement-based systems. Hard X-ray microtomography images of ice inside cement paste and cracking caused by the alkali–silica reaction (ASR) enables three-dimensional structural identification. The potential of neutron diffraction to determine reactive aggregates by measuring their residual strains and preferred orientation is studied. Results of experiments using these tools are shown on this paper.