Dachao Gao

Contrast and resolution in imaging with a microfocus x-ray source

A simple general treatment of x-ray image formation by Fresnel diffraction is presented; the image can alternatively be considered as an in-line hologram. Particular consideration is given to phase-contrast microscopy and imaging using hard x rays. The theory makes use of the optical transfer function in a similar way to that used in the theory of electron microscope imaging. Resolution and contrast are the criteria used to specify the visibility of an image. Resolution in turn depends primarily on the spatial coherence of the illumination, with chromatic coherence of lesser importance. Thus broadband microfocus sources can give useful phase-contrast images. Both plane- and spherical-wave conditions are explicitly considered as limiting cases appropriate to macroscopic imaging and microscopy, respectively, while intermediate cases may also be of practical interest. Some results are presented for x-ray images showing phase contrast.

Quantitative X‐ray projection microscopy: phase‐contrast and multi‐spectral imaging

We outline a new approach to X‐ray projection microscopy in a scanning electron microscope (SEM), which exploits phase contrast to boost the quality and information content of images. These developments have been made possible by the combination of a high‐brightness field‐emission gun (FEG)‐based SEM, direct detection CCD technology and new phase retrieval algorithms. Using this approach we have been able to obtain spatial resolution of < 0.2 µm and have demonstrated novel features such as: (i) phase‐contrast enhanced visibility of high spatial frequency image features (e.g. edges and boundaries) over a wide energy range; (ii) energy‐resolved imaging to simultaneously produce multiple quasi‐monochromatic images using broad‐band polychromatic illumination; (iii) easy implementation of microtomography; (iv) rapid and robust phase/amplitude‐retrieval algorithms to enable new real‐time and quantitative modes of microscopic imaging. These algorithms can also be applied successfully to recover object–plane information from intermediate‐field images, unlocking the potentially greater contrast and resolution of the intermediate‐field regime. Widespread applications are envisaged for fields such as materials science, biological and biomedical research and microelectronics device inspection. Some illustrative examples are presented. The quantitative methods described here are also very relevant to projection microscopy using other sources of radiation, such as visible light and electrons.

The role of radiographic phase-contrast imaging in the development of intracochlear electrode arrays

Objective This study describes the application of a new radiographic imaging modality, phase-contrast radiography, to in vitro human temporal bone imaging and investigates its use in the development of new electrode arrays for cochlear implants. Background The development of perimodiolar electrode arrays for cochlear implants requires detailed information from postoperative radiologic assessment on the position of the array in relation to the cochlear structures. Current standard radiographic techniques provide only limited details. Materials and Methods Nucleus standard electrode arrays and perimodiolar Contour electrode arrays were implanted into the scala tympani of 11 human temporal bones. Both conventional and phase-contrast radiographs were taken of each temporal bone for comparative purposes. Results Phase-contrast imaging provides better visualization of anatomic details of the inner ear and of the structure of the intracochlear electrode array, and better definition of electrode location in relation to cochlear walls. Conclusion Phase-contrast radiography offers significant improvement over conventional radiography in images of in vitro human temporal bones. It seems to be a valuable tool in the development of intracochlear electrode arrays and cochlear implant research. However, this new radiographic technique still requires certain computational and physics challenges to be addressed before its clinical use can be established.

Software image alignment for X-ray microtomography with submicrometre resolution using a SEM-based X-ray microscope.

Improved X‐ray sources and optics now enable X‐ray imaging resolution down to ∼50 nm for laboratory‐based X‐ray microscopy systems. This offers the potential for submicrometre resolution in tomography; however, achieving this resolution presents challenges due to system stability. We describe the use of software methods to enable submicrometre resolution of approximately 560 nm. This is a very high resolution for a modest laboratory‐based point‐projection X‐ray tomography system. The hardware is based on a scanning electron microscope, and benefits from inline X‐ray phase contrast to improve visibility of fine features. Improving the resolution achievable with the system enables it to be used to address a greater range of samples.

Quantitative methods in phase-contrast x-ray imaging.

A new method for extracting quantitative information from phase-contrast x-ray images obtained with microfocus x-ray sources is presented. The proposed technique allows rapid noninvasive characterization of the internal structure of thick optically opaque organic samples. The method does not generally involve any sample preparation and does not need any x-ray optical elements (such as monochromators, zone plates, or interferometers). As a consequence, samples can be imaged in vivo or in vitro, and the images are free from optical aberrations. While alternative techniques of x-ray phase-contrast imaging usually require expensive synchrotron radiation sources, our method can be implemented with conventional, albeit microfocus, x-ray tubes, which greatly enhances its practicality. In the present work, we develop the theoretical framework, perform numerical simulations, and present the first experimental results, demonstrating the viability of the proposed approach. We believe that this method should find wide-ranging applications in clinical radiology and medical research.

Low temperature MOCVD of Hg1−xCdxTe on 311, 511, 711 and shaped GaAs

Abstract The influence of six new GaAs substrate orientations on the surface morphology and defect hillocks of epitaxial Hg 1− x Cd x Te (MCT) layers, grown by low temperature MOCVD, is investigated using Nomarski interference contrast microscopy and SEM. Smoother epitaxial layers are obtained on (311), (511) and (711) substrates of A or B polarity compared with growth on (100) or (100) 2° off toward (110) substrates. The quality and orientation of the layers was monitored by X-ray techniques, Rutherford backscattering/channeling and selected area electron channeling patterns. Absolute polarity determination by X-ray diffraction confirmed that the epilayers adopted the polarity of the substrate. Conformal growth on contoured substrates revealed preferential facet formation and growth rate variation which leads to local composition changes in the interdiffused multilayer process. The growth hillocks commonly observed in epitaxy of MCT are related to the oval defects in III–V compounds, and dissolve on annealing.

X-ray ultramicroscopy using integrated sample cells.

The X-ray ultramicroscope (XuM), based on using a scanning electron microscope as host, provides a new approach to X-ray projection microscopy. The right-angle-type integrated sample cells described here expand the capabilities of the XuM technique. The integrated sample cell combines a target, a spacer, a sample chamber, and an exit window in one physical unit, thereby simplifying the instrumentation and providing increased mechanical stability. The XuM imaging results presented here, obtained using such right-angle integrated sample cells, clearly demonstrate the ability to characterize very small features in objects, down to of order 100nm, including their use for dry, wet and even liquid samples.

New Methods of X-Ray Imaging based on Phase Contrast

Phase-contrast X-ray images can be produced in various ways. Some aspects of the relatively simple “in-line” method are presented which may be implemented using a quasi-spherical X-ray wave with high spatial but not chromatic coherence. Appropriate sources can be found in commercially-available microfocus tubes or at synchrotrons. Compared to previous phase-contrast methods, the present one offers relatively simple implementation, relatively high intensity and a large field of view, making it potentially suitable for clinical applications.

X-ray phase-contrast imaging study of soft tissue and bone samples

Conventional radiography is based on absorption contrast and geometrical (ray) optics. After an outline of the relevant theory, this article reports results displaying both phase- and absorption-contrast, collected with a technique which utilizes a micro-focus x-ray source to achieve a high degree of spatial coherence, and relatively large object-to-image distances to enable (wave) interference effects (Fresnel diffraction) to occur and manifest themselves as phase contrast in the image plane. Both soft tissue (chicken knee) and hard tissue (finger bone) samples are investigated for a range of source sizes and object-to-image distances, encompassing conditions somewhat analogous to conventional radiography. Variation in image contrast and resolution as a function of these variables is observed and discussed.

A new approach for mapping x-ray rocking curves

A new experimental geometry for simultaneously mapping the rocking curves of a sample over a one‐ or two‐dimensional array of points on the sample has been developed. The 422 asymmetric diffraction of Cu Kα1 radiation with a monolithic channel‐cut silicon crystal angularly collimates and spatially expands the x‐ray beam. A cooled Reticon linear photodiode array with 25‐μm spatial resolution was used for the one‐dimensional detector. Compared with the commonly used double‐crystal diffractometer method, more information can be obtained from the present method, for example, characterization of the minute local misorientation of subgrains and the precise determination of the curvature of a crystal. The uniformity of the crystal quality and the local variation of the curvature of the crystal planes over a sample can also be revealed from the contour map of the rocking curves. Maps of the rocking curves of several samples have been collected using this method.