Phase Contrast Imaging with Coded Apertures Using Laboratory-Based X-ray Sources

X‐ray phase contrast imaging is a powerful technique that allows detection of changes in the phase of x‐ray wavefronts as they pass through a sample. As a result, details not visible in conventional x‐ray absorption imaging can be detected. Until recently the majority of applications of phase contrast imaging were at synchrotron facilities due to the availability of their high flux and coherence; however, a number of techniques have appeared recently that allow phase contrast imaging to be performed using laboratory sources. Here we describe a phase contrast imaging technique, developed at University College London, that uses two coded apertures. The x‐ray beam is shaped by the pre‐sample aperture, and small deviations in the x‐ray propagation direction are detected with the help of the detector aperture. In contrast with other methods, it has a much more relaxed requirement for the source size (it works with source sizes up to 100 μm). A working prototype coded‐aperture system has been built. An x‐ray detector with directly deposited columnar CsI has been used to minimize signal spill‐over into neighboring pixels. Phase contrast images obtained with the system have demonstrated its effectiveness for imaging low‐absorption materials

Effects of signal diffusion on x-ray phase contrast images

We discuss the problem of signal diffusion among neighbouring pixels in x-ray phase contrast imaging (XPCi) specifically for coded-aperture (CA) XPCi, but many of the discussed observations are directly transferable to other XPCi modalities. CA XPCi exploits the principle of pixel edge illumination by means of two CA masks. The first mask, placed in contact with the detector, creates insensitive regions between adjacent pixels; the second one, placed immediately before the sample, creates individual beams impinging on the boundaries between sensitive and insensitive regions on the detector, as created by the detector mask. In this way, edge illumination is achieved for all pixels of an area detector illuminated by a divergent and polychromatic beam generated by a conventional source. As the detector mask redefines the resolution properties of the detector, sample dithering can be used to effectively increase the system spatial resolution, without having to apply any post-processing procedure (e. g., deconvolution). This however creates artifacts in the form of secondary fringes (which have nothing to do with phase-related secondary fringes) if there is signal diffusion between adjacent pixels. In non-dithered images, signal diffusion between adjacent pixels causes a reduction in image contrast. This effect is investigated both theoretically and experimentally, and its direct implications on image quality are discussed. The interplay with the sample positioning with respect to the detector pixel matrix, which also has an effect on the obtained image contrast, is also discussed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3606442

Applications of a non-interferometric x-ray phase contrast imaging method with both synchrotron and conventional sources

We have developed a totally incoherent, non-interferometric x-ray phase contrast imaging (XPCI) method. This is based on the edge illumination (EI) concept developed at the ELETTRA synchrotron in Italy in the late ‘90s. The method was subsequently adapted to the divergent beam generated by a conventional source, by replicating it for every detector line through suitable masks. The method was modelled both with the simplified ray-tracing and with the more rigorous wave-optics approach, and in both cases excellent agreement with the experimental results was found. The wave-optics model enabled assessing the methods’ coherence requirements, showing that they are at least an order of magnitude more relaxed than in other methods, without this having negative consequences on the phase sensitivity. Our masks have large pitches (up to 50 times larger than in grating interferometry, for example), which allows for manufacturing through standard lithography, scalability, cost-effectiveness and easiness to align. When applied to a polychromatic and divergent beam generated by a conventional source, the method enables the detection of strong phase effects also with uncollimated, unapertured sources with focal spots of up to 100 mm, compatible with the state-of-the-art in mammography. When used at synchrotrons, it enables a contrast increase of orders of magnitude over other methods. Robust phase retrieval was proven for both coherent and incoherent sources, and additional advantages are compatibility with high x-ray energies and easy implementation of phase sensitivity in two directions simultaneously. This paper briefly summarizes these achievements and reviews some of the key results

Edge illumination and coded-aperture X-ray phase-contrast imaging: Increased sensitivity at synchrotrons and lab-based translations into medicine, biology and materials science

The edge illumination principle was first proposed at Elettra (Italy) in the late nineties, as an alternative method for achieving high phase sensitivity with a very simple and flexible set-up, and has since been under continuous development in the radiation physics group at UCL. Edge illumination allows overcoming most of the limitations of other phase-contrast techniques, enabling their translation into a laboratory environment. It is relatively insensitive to mechanical and thermal instabilities and it can be adapted to the divergent and polychromatic beams provided by X-ray tubes. This method has been demonstrated to work efficiently with source sizes up to 100m, compatible with state-of-the-art mammography sources. Two full prototypes have been built and are operational at UCL. Recent activity focused on applications such as breast and cartilage imaging, homeland security and detection of defects in composite materials. New methods such as phase retrieval, tomosynthesis and computed tomography algorithms are currently being theoretically and experimentally investigated. These results strongly indicate the technique as an extremely powerful and versatile tool for X-ray imaging in a wide range of applications

The Golgi Localization of GOLPH2 (GP73/GOLM1) Is Determined by the Transmembrane and Cytoplamic Sequences

Golgi phosphoprotein 2 (GOLPH2) is a resident Golgi type-II membrane protein upregulated in liver disease. Given that GOLPH2 traffics through endosomes and can be secreted into the circulation, it is a promising serum marker for liver diseases. The structure of GOLPH2 and the functions of its different protein domains are not known. In the current study, we investigated the structural determinants for Golgi localization using a panel of GOLPH2 truncation mutants. The Golgi localization of GOLPH2 was not affected by the deletion of the C-terminal part of the protein. A truncated mutant containing the N-terminal portion (the cytoplasmic tail and transmembrane domain (TMD)) localized to the Golgi. Sequential deletion analysis of the N-terminal indicated that the TMD with a positively charged residue in the cytoplasmic N-terminal tail were sufficient to support Golgi localization. We also showed that both endogenous and secreted GOLPH2 exist as a disulfide-bonded dimer, and the coiled-coil domain was sufficient for dimerization. This structural knowledge is important for the understanding the pathogenic role of GOLPH2 in liver diseases, and the development of GOLPH2-based hepatocellular cancer diagnostic methods

Method for automatization of the alignment of a laboratory based x-ray phase contrast edge illumination system

Here we present a general alignment algorithm for an edge illumination x-ray phase contrast imaging system, which is used with the laboratory systems developed at UCL. It has the flexibility to be used with all current mask designs, and could also be applied to future synchrotron based systems. The algorithm has proved to be robust experimentally, and can be used for the automatization of future commercial systems through automatic alignment and alignment correction

Medicine, material science and security: the versatility of the coded-aperture approach

The principal limitation to the widespread deployment of X-ray phase imaging in a variety of applications is probably versatility. A versatile X-ray phase imaging system must be able to work with polychromatic and non-microfocus sources (for example, those currently used in medical and industrial applications), have physical dimensions sufficiently large to accommodate samples of interest, be insensitive to environmental disturbances (such as vibrations and temperature variations), require only simple system set-up and maintenance, and be able to perform quantitative imaging. The coded-aperture technique, based upon the edge illumination principle, satisfies each of these criteria. To date, we have applied the technique to mammography, materials science, small-animal imaging, non-destructive testing and security. In this paper, we outline the theory of coded-aperture phase imaging and show an example of how the technique may be applied to imaging samples with a practically important scale

A systems approach to policy evaluation

There is growing interest in evaluating policy implementation in ways that grapple with the complexity of the process. This article offers an example of using systems methodology to explore how the child protection policy in child contact centres has functioned in practice. Rather than just asking the traditional evaluation question “is it working?” this study sought to understand how the policy was working and how it was interpreted as it interacted with other systems, producing conflicts, local variation and emergent effects. It illustrates how the systems concepts of ‘emergence’, ‘local rationality’, ‘socio-technical systems’ and ‘feedback for learning’ can contribute new knowledge and understanding to a complex policy evaluation problem