Phase-space evolution of x-ray coherence in phase-sensitive imaging

X-ray coherence evolution in the imaging process plays a key role for x-ray phase-sensitive imaging. In this work we present a phase-space formulation for the phase-sensitive imaging. The theory is reformulated in terms of the cross-spectral density and associated Wigner distribution. The phase-space formulation enables an explicit and quantitative account of partial coherence effects on phase-sensitive imaging. The presented formulas for x-ray spectral density at the detector can be used for performing accurate phase retrieval and optimizing the phase-contrast visibility. The concept of phase-space shearing length derived from this phase-space formulation clarifies the spatial coherence requirement for phase-sensitive imaging with incoherent sources. The theory has been applied to x-ray Talbot interferometric imaging as well. The peak coherence condition derived reveals new insights into three-grating-based Talbot-interferometric imaging and gratings-based x-ray dark-field imaging.     

X射线同轴相衬成像实验

X射线相衬成像技术对软组织成像时比基于衰减的传统X射线成像技术优势明显,现在亟待发展一套广泛适用的相衬成像理论来指导其发展和临床应用．首先介绍了同轴相衬成像及相位成像的原理,随后根据菲涅耳．基尔霍夫衍射理论,利用数值模拟的方法研究微焦点源的尺寸对图像可见度的影响,最后在数值模拟结果指导下通过实验室直径为50μm的微焦点源X射线成像系统获得了厚度为150μm左右塑料气泡膜的相衬图像．     

Theory of quantitative phase-contrast computed tomography

Phase-contrast x-ray computed tomography (CT) is an emerging imaging technique that can be implemented at third-generation synchrotron radiation sources or by using a microfocus x-ray source. Promising results have recently been obtained in materials science and medicine. At the same time, the lack of a mathematical theory comparable with that of conventional CT limits the progress in this field. Such a theory is now suggested, establishing a fundamental relation between the three-dimensional Radon transform of the object function and the two-dimensional Radon transform of the phase-contrast projection. A reconstruction algorithm is derived in the form of a filtered backprojection. The filter function is given in the space and spatial-frequency domains. The theory suggested enables one to quantitatively determine the refractive index of a weakly absorbing medium from x-ray intensity data measured in the near-field region. The results of computer simulations are discussed.     

基于X射线像增强器成像的相衬滤波模型

对铝、黄铜、不锈钢3种金属材料制作的光栅在不同射线剂量和厚度差异的条件下进行图像采集,结合系统级的联合调制评价模型研究不同材料对X射线的调制作用,结果表明:在相同的射线剂量下,无论哪种材料的光栅,厚度差异越大成像反差越大;在厚度差异相同的情况下,随着射线剂量变化,不同材料对射线的调制能力变化趋势相差甚远。对光学调制传递函数、射线剂量和光栅厚度差异值进行三维曲线拟合,建立物质对X射线的调制模型和采集模型。在像增强器成像模型的基础上提出X射线相衬增强滤波模型,通过对薄膜基准目标源进行图像采集,在噪声环境下对其进行积分滤波处理和相衬增强处理,结果压低了图像噪声的同时也强化了目标轮廓信息,并大幅地增强X射线的成像反差。     

Imaging of polyethylene films by diffraction contrast

The technique of crystalline diffraction contrast imaging of lamellae in spherulitic and oriented thin films of polyethylene is illustrated for both conventional transmission and scanning transmission electron microscopY. Bright-field “ghost” imaging permits real space crystallography of the specimen and reveals the occurrence of variable chain inclination in a given lamellar preparation. N beam annular dark-field scanning transmission microscopy is useful for distinguishing between curved lamellae and mosaic blocks as well as for the direct imaging of the amorphous regions between lamellae.     

High-resolution tomographic imaging of a human cerebellum: comparison of absorption and grating-based phase contrast

Human brain tissue belongs to the most impressive and delicate three-dimensional structures in nature. Its outstanding functional importance in the organism implies a strong need for brain imaging modalities. Although magnetic resonance imaging provides deep insights, its spatial resolution is insufficient to study the structure on the level of individual cells. Therefore, our knowledge of brain microstructure currently relies on two-dimensional techniques, optical and electron microscopy, which generally require severe preparation procedures including sectioning and staining. X-ray absorption microtomography yields the necessary spatial resolution, but since the composition of the different types of brain tissue is similar, the images show only marginal contrast. An alternative to absorption could be X-ray phase contrast, which is known for much better discrimination of soft tissues but requires more intricate machinery. In the present communication, we report an evaluation of the recently developed X-ray grating interferometry technique, applied to obtain phase-contrast as well as absorption-contrast synchrotron radiation-based microtomography of human cerebellum. The results are quantitatively compared with synchrotron radiation-based microtomography in optimized absorption-contrast mode. It is demonstrated that grating interferometry allows identifying besides the blood vessels, the stratum moleculare, the stratum granulosum and the white matter. Along the periphery of the stratum granulosum, we have detected microstructures about 40 µm in diameter, which we associate with the Purkinje cells because of their location, size, shape and density. The detection of individual Purkinje cells without the application of any stain or contrast agent is unique in the field of computed tomography and sets new standards in non-destructive three-dimensional imaging.     

Scanning microdiffraction of polymers

Scanning transmission electron microscopy (STEM) has been suggested to have advantages over conventional transmission electron microscopy (CTEM) for the observation of diffraction contrast features and diffraction patterns from radiation-sensitive crystalline polymers. For many applications it is desirable to obtain successive diffraction patterns from very small adjacent areas. Several microarea diffraction techniques are available using CTEM and STEM. The most useful technique is scanning microarea diffraction used in conjunction with STEM dark-field imaging. Using this technique we have obtained diffraction patterns from regions as small as 100 nm×100 nm for a 12 nm thick polyethylene single crystal. Adjacent microarea diffraction patterns can be obtained while only radiation-damaging the diffracting region. This allows mapping of the specimen crystallography on a very fine scale as well as allowing one to obtain a diffraction pattern for selecting various STEM dark-field conditions while only damaging a small portion of the specimen before the dark-field image is recorded.     

Interpretation of dark-field contrast and particle-size selectivity in grating interferometers

In grating-based x-ray phase sensitive imaging, dark-field contrast refers to the extinction of the interference fringes due to small-angle scattering. For configurations where the sample is placed before the beamsplitter grating, the dark-field contrast has been quantified with theoretical wave propagation models. Yet when the grating is placed before the sample, the dark-field contrast has only been modeled in the geometric optics regime. Here we attempt to quantify the dark-field effect in the grating-before-sample geometry with first-principle wave calculations and understand the associated particle-size selectivity. We obtain an expression for the dark-field effect in terms of the sample material's complex refractive index, which can be verified experimentally without fitting parameters. A dark-field computed tomography experiment shows that the particle-size selectivity can be used to differentiate materials of identical x-ray absorption.     

Variation on Zernike's phase-contrast microscope

We describe the design, construction, and testing of a variant of Zernike's phase-contrast microscope. The sample is illuminated with a white-light source through an annular aperture, which is projected onto the entrance pupil of the objective lens. In the return path the light diffracted by the sample and appearing in the interior of the objective's aperture (i.e., the test beam) is separated from the light returning in the annular region near the rim of the objective (i.e., the reference beam). The separated beams are relatively phase shifted and then combined to create an interferogram of the sample's surface on a CCD camera. It is fairly straightforward to use this system as a conventional bright-field or dark-field microscope, but its most interesting application is as a Zernike phase-contrast microscope with adjustable amplitude ratio and phase shift between test and reference beams. The ability to continuously adjust the phase of the reference beam also enables quantitative measurement of the phase imparted by the sample to the incident beam.     

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.     

Direct tomography with chemical-bond contrast

Three-dimensional (3D) X-ray imaging methods have advanced tremendously during recent years. Traditional tomography uses absorption as the contrast mechanism, but for many purposes its sensitivity is limited. The introduction of diffraction, small-angle scattering, refraction, and phase contrasts has increased the sensitivity, especially in materials composed of light elements (for example, carbon and oxygen). X-ray spectroscopy, in principle, offers information on element composition and chemical environment. However, its application in 3D imaging over macroscopic length scales has not been possible for light elements. Here we introduce a new hard-X-ray spectroscopic tomography with a unique sensitivity to light elements. In this method, dark-field section images are obtained directly without any reconstruction algorithms. We apply the method to acquire the 3D structure and map the chemical bonding in selected samples relevant to materials science. The novel aspects make this technique a powerful new imaging tool, with an inherent access to the molecular-level chemical environment.     

Phase-space formulation for phase-contrast x-ray imaging

Phase-space formulation based on the Wigner distribution has been presented for analyzing phase-contrast image formation. Based on the statistical nature and affine canonical covariance of Wigner distributions in the phase space, we show that the partial coherence effects of incident x-ray wave field on image intensity are simply accounted for by a multiplication factor, which is the reduced complex degree of coherence of the incident x-ray wave field. We show especially that with the undulator sources one cannot obtain the phase-contrast intensity by summing over the contributions from all electron positions, since the van Cittert-Zernike theorem fails in general for undulators. We derive a comprehensive formula that quantifies the effects of partial spatial coherence, polychromatic spectrum, body attenuation, imaging-detector resolution, and radiation dose on phase-contrast visibility in clinical imaging. The results of our computer modeling and simulations show how the formula can provide design guidelines and optimal parameters for clinical x-ray phase-contrast imaging systems.     

Microstructural study of as-extruded and heat-treated ribbons of poly(p-phenylene benzobisthiazole)

Ribbons of poly(p-phenylene benzobisthiazole) (PBT) prepared by extrusion from methane sulphonic acid solution into acid-water coagulation baths, were examined by electron microscopy, wide-angle X-ray scattering and small angle X-ray scattering. Electron diffraction patterns and corresponding dark-field images of as-extruded ribbons suggest that the chains are well oriented in the extrusion direction, with laterally-ordered regions averaging 2 nm in width. Upon heat treatment, the equatorial reflections sharpen and dark-field imaging indicates an increase in lateral order to about 15 nm. Small-angle X-ray scattering patterns show very little scattered intensity except for strong equatorial scattering attributed to voids elongated parallel to the extrusion direction. The absence of distinct crystalline images in 0 0l dark-field is supporting evidence for axial shift of the molecules along the chain axis, as suggested by previous diffraction studies.     

Advances in neutron imaging

Imaging techniques based on neutron beams are rapidly developing and have become versatile non-destructive analyzing tools in many research fields. Due to their intrinsic properties, neutrons differ strongly from electrons, protons or X-rays in terms of their interaction with matter: they penetrate deeply into most common metallic materials while they have a high sensitivity to light elements such as hydrogen, hydrogenous substances, or lithium. This makes neutrons perfectly suited probes for research on materials that are used for energy storage and conversion, e.g., batteries, hydrogen storage, fuel cells, etc. Moreover, their wave properties can be exploited to perform diffraction, phase-contrast and dark-field imaging experiments. Their magnetic moment allows for resolving magnetic properties in bulk samples. This review will focus on recent applications of neutron imaging techniques in both materials research and fundamental science illustrated by examples selected from different areas.     

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。     

Implementation of diffraction-enhanced imaging experiments: at the NSLS and APS

Diffraction-enhanced imaging is a recently developed X-ray imaging technique that has demonstrated enhanced contrast for dense, highly absorbing materials of interest in materials science and medicine. The implementation of this technique in experiments at the National Synchrotron Light Source and at the Advanced Photon Source is described in detail.     

Broadband light profile microscopy: a rapid and direct method for thin film depth imaging

Light profile microscopy (LPM) is a recently developed technique of optical inspection that is used to record micrometer scale images of thin film cross-sections on a direct basis. This technique uses a novel right-angle imaging geometry that shows outstanding contrast for subtle interface structures and morphologies that are invisible to conventional methods of inspection. When laser sources are used for sample illumination, image contrast is provided by luminescence and elastic and/or inelastic scatter. When a white-light excitation source is used for LPM, primary contrast is obtained from elastic scatter, while secondary contrast results from refraction, secondary transmission, and secondary reflection from material phases. We term this mode of inspection broadband light profile microscopy (BB-LPM). It is implemented with a compact, easily aligned apparatus and minimal sample preparation, and it shows outstanding interface contrast similar to laser LPM. In this work we demonstrate BB-LPM as a method for direct imaging of the layers structures of a variety of thin film samples of industrial and manufacturing interest.     

Surface-plasmon microscopy with a two-piece solid immersion lens: bright and dark fields

We report bright-field and dark-field surface-plasmon imaging using a modified solid immersion lens and a commercial objective of moderate NA in the epi configuration. The contrast and resolution are extremely good, giving well-resolved images of protein monolayers both in air and in water. We also describe a two-part solid immersion lens that allows the sample to be moved without degrading the image quality in any observable way. The merits of the two-part lens are discussed and compared to commercially available microscope objectives. Finally, we introduce a simple Green's function model that illustrates the key features of both bright-field and dark-field surface-plasmon imaging.     

Phase inhomogeneity of pyroboroncarbon structure

Peculiarities of the structure and phase composition of pyroboroncarbon were studied by a combination of electron-microscopic techniques including electron diffraction, high-energy electron loss spectroscopy, bright-and dark-field imaging with diffraction contrast, and high-resolution measurements up to the atomic resolution level.