X-ray Phase-contrast Technique Research Articles

Comparison of Shadowgraphy and X-Ray Phase Contrast Methods for Characterizing a DT Ice Layer in an Inertial Confinement Fusion Target

Shadowgraphy and X-ray phase contrast (XPC) imaging are two techniques that are used for characterizing the deuterium-tritium ice layer in inertial confinement fusion targets. Each technique has limitations that affect how accurately they can characterize small crystalline defects and measure the ice thickness nonuniformities that may be only a few micrometers in height. The concern is that shadowgraphy may be overly sensitive to the shape and depth of defects in the ice surface and insufficiently sensitive to the shape of longer wavelength roughness, while XPC may be too insensitive to defects in the ice surface. Multiple ice layers with different thicknesses (40 to 63 μm), thickness uniformities (peak-to-valley variations that range from < 2 to 12 μm), and crystal defects were analyzed using shadowgraphy and XPC techniques. The results from each method agree when the ice layer is uniformly thick and the crystal lacks defects. That agreement worsens as the number of defects in the surface of the ice layer increases, and the roughness (that is determined from a shadowgram image of the target’s limb) becomes greater than can be justified by the number of defects that are seen in the target’s front and rear surfaces. The XPC technique is considerably less sensitive to surface defects, in part because of the poorer dynamic range and image resolution compared to shadowgraphy. Localized regions of the ice layer that are thicker or thinner than the average thickness of the layer are reported by shadowgraphy to be smaller in height and footprint (by up to 30%) than by XPC. As a result, the two techniques report different ice layer thicknesses that can vary by up to 10%. Shadowgraphy, which results from two caustics that trace different paths through the target, and in theory, image the same ice/vapor surface (but reflect from either the vapor or ice side of the interface), did not consistently characterize the size or shape of ice features to be the same magnitude. The XPC technique provides the best assessment of low-mode (l < 7) roughness in the ice layer. Shadowgraphy results using the strongest caustic is best for detecting the presence of grooves in the ice, although not for quantifying the size of them. If multiple grooves are present, it is best to discard and reform the ice layer.

Phase-Contrast Imaging in a Polychromatic X-ray Beam at a Laboratory Source

An experimental setup for carrying out measurements using the X-ray phase-contrast technique on a polychromatic laboratory source is proposed. A standard Mo-anode X-ray tube at an accelerating voltage of 45 kV is used in the experiments. A slit aperture is used to form a conical beam. The phase-contrast effect is demonstrated on a series of objects. The proposed technique of phase-contrast measurements can be used in tomographic experiments.

X-ray phase imaging with the unified modulated pattern analysis of near-field speckles at a laboratory source.

X-ray phase-contrast techniques are powerful methods for discerning features with similar densities, which are normally indistinguishable with conventional absorption contrast. While these techniques are well-established tools at large-scale synchrotron facilities, efforts have increasingly focused on implementations at laboratory sources for widespread use. X-ray speckle-based imaging is one of the phase-contrast techniques with high potential for translation to conventional x-ray systems. It yields phase-contrast, transmission, and dark-field images with high sensitivity using a relatively simple and cost-effective setup tolerant to divergent and polychromatic beams. Recently, we have introduced the unified modulated pattern analysis (UMPA) [Phys. Rev. Lett.118, 203903 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.203903], which further simplifies the translation of x-ray speckle-based imaging to low-brilliance sources. Here, we present the proof-of-principle implementation of UMPA speckle-based imaging at a microfocus liquid-metal-jet x-ray laboratory source.

Nozzle tip wetting in gasoline direct injection injector and its link with nozzle internal flow

Fuel film in the gasoline direct injection injector tip, or so-called nozzle tip wetting, has been found to be an important contributor of particle emissions. Attempts have been made to reduce the nozzle tip wetting by optimizing nozzle geometry designs. However, the inherent mechanism of the nozzle tip wetting formation and its link with nozzle internal flow is still unclear yet due to the lack of direct observations. To overcome this insufficiency, the nozzle internal flow and the formation process of the nozzle tip wetting were visualized in the real-scale aluminum nozzles using the X-ray phase-contrast technique. Results showed that the needle bouncing, injection pressure, and hole configuration affect the formation of the nozzle tip wetting, while the influence of needle bouncing is the most critical. A further study was conducted to examine the effect of nozzle counterbore diameter on the nozzle tip wetting. It was found that with an increase in counterbore diameter, the nozzle tip wetting slightly increased first and then decreased sharply after the counterbore diameter exceeded 0.40 mm. The mechanisms of the aforementioned phenomena were discussed in detail, which can contribute to the better understandings and control strategies of nozzle tip wetting.

Experimental Realisation of High-sensitivity Laboratory X-ray Grating-based Phase-contrast Computed Tomography.

The possibility to perform high-sensitivity X-ray phase-contrast imaging with laboratory grating-based phase-contrast computed tomography (gbPC-CT) setups is of great interest for a broad range of high-resolution biomedical applications. However, achieving high sensitivity with laboratory gbPC-CT setups still poses a challenge because several factors such as the reduced flux, the polychromaticity of the spectrum, and the limited coherence of the X-ray source reduce the performance of laboratory gbPC-CT in comparison to gbPC-CT at synchrotron facilities. In this work, we present our laboratory X-ray Talbot-Lau interferometry setup operating at 40 kVp and describe how we achieve the high sensitivity yet unrivalled by any other laboratory X-ray phase-contrast technique. We provide the angular sensitivity expressed via the minimum resolvable refraction angle both in theory and experiment, and compare our data with other differential phase-contrast setups. Furthermore, we show that the good stability of our high-sensitivity setup allows for tomographic scans, by which even the electron density can be retrieved quantitatively as has been demonstrated in several preclinical studies.

An x-ray backlit Talbot-Lau deflectometer for high-energy-density electron density diagnostics.

X-ray phase-contrast techniques can measure electron density gradients in high-energy-density plasmas through refraction induced phase shifts. An 8 keV Talbot-Lau interferometer consisting of free standing ultrathin gratings was deployed at an ultra-short, high-intensity laser system using K-shell emission from a 1-30 J, 8 ps laser pulse focused on thin Cu foil targets. Grating survival was demonstrated for 30 J, 8 ps laser pulses. The first x-ray deflectometry images obtained under laser backlighting showed up to 25% image contrast and thus enabled detection of electron areal density gradients with a maximum value of 8.1 ± 0.5 × 10(23) cm(-3) in a low-Z millimeter sized sample. An electron density profile was obtained from refraction measurements with an error of <8%. The 50 ± 15 μm spatial resolution achieved across the full field of view was found to be limited by the x-ray source-size, similar to conventional radiography.

Visualization of delamination in composite materials utilizing advanced X-ray imaging techniques

This work is focused on the development of instrumental radiographic methods for detection of delaminations in layered carbon fibre reinforced plastic composites used in the aerospace industry. The main limitation of current visualisation techniques is a very limited possibility to image so-called closed delaminations in which delaminated layers are in contact practically with no physical gap. In this contribution we report the development of innovative methods for closed delamination detection using an X-ray phase contrast technique for which the distance between delamination surfaces is not relevant. The approach is based on the energetic sensitivity of phase-enhanced radiography. Based on the applied methodology, we can distinguish both closed and open delamination. Further we have demonstrated the possibility to visualise open delaminations characterised by a physical gap between delaminated layers. This delamination type was successfully identified and visualized utilizing a high resolution and computed tomography table-top technique based on proper beam-hardening effect correction.

Spatial resolution of edge illumination X-ray phase-contrast imaging.

We analyze the spatial resolution of edge illumination X-ray phase-contrast imaging and its dependence upon various experimental parameters such as source size, source-to-sample and sample-to-detector distances, X-ray energy and size of the beam-shaping aperture. Different propagation regimes, as well as the beam divergence and polychromaticity encountered with laboratory sources, are also considered. We show that spatial resolution in edge illumination phase-contrast imaging presents peculiar features compared to other X-ray phase-contrast techniques. In particular, in the direction orthogonal to the s or mask lines used to shape the beam, this can be better than both the pixel dimension and the projected source size. Numerical simulations based on Fresnel diffraction integrals are presented, which confirm the analytical predictions. The obtained results allow a simple estimation of the spatial resolution for edge-illumination phase imaging in both synchrotron and laboratory setups.

Sensitivity of edge illumination X-ray phase-contrast imaging

Recently, we developed a theoretical model that can predict the signal-to-noise ratio for edge-like features in phase-contrast images. This model was then applied for the estimation of the sensitivity of three different X-ray phase-contrast techniques: propagation-based imaging, analyser-based imaging and grating interferometry. We show here how the same formalism can be used also in the case of the edge illumination (EI) technique, providing results that are consistent with those of a recently developed method for the estimation of noise in the retrieved refraction image. The new model is then applied to calculate, in the case of a given synchrotron radiation set-up, the optimum positions of the pre-sample aperture and detector edge to maximize the sensitivity. Finally, an example of the extremely high angular resolution achievable with the EI technique is presented.

A sensitive x-ray phase contrast technique for rapid imaging using a single phase grid analyzer

Phase contrast x-ray imaging (PCXI) is a promising imaging modality, capable of sensitively differentiating soft tissue structures at high spatial resolution. However, high sensitivity often comes at the cost of a long exposure time or multiple exposures per image, limiting the imaging speed and possibly increasing the radiation dose. Here, we demonstrate a PCXI method that uses a single short exposure to sensitively capture sample phase information, permitting high speed x-ray movies and live animal imaging. The method illuminates a checkerboard phase grid to produce a fine grid-like intensity reference pattern at the detector, then spatially maps sample-induced distortions of this pattern to recover differential phase images of the sample. The use of a phase grid is an improvement on our previous absorption grid work in two ways. There is minimal loss in x-ray flux, permitting faster imaging, and, a very fine pattern is produced for homogenous high spatial resolution. We describe how this pattern permits retrieval of five images from a single exposure; the sample phase gradient images in the horizontal and vertical directions, a projected phase depth image, an edge-enhanced image, and a type of scattering image. Finally, we describe how the reconstruction technique can achieve subpixel distortion retrieval and study the behavior of the technique in regard to analysis technique, Talbot distance, and exposure time.

Analytical and experimental determination of signal-to-noise ratio and figure of merit in three phase-contrast imaging techniques

We present a theoretical and experimental comparison of three X-ray phase-contrast techniques: propagation-based imaging, analyzer-based imaging and grating interferometry. The signal-to-noise ratio and the figure of merit are quantitatively compared for the three techniques on the same phantoms and using the same X-ray source and detector. Principal dependencies of the signal upon the numerous acquisition parameters, the spatial resolution and X-ray energy are discussed in detail. The sensitivity of each technique, in terms of the smallest detectable phase shift, is also evaluated.

A variable-wavelength-based approach of phase retrieval for contrast transfer function based methods

X-ray phase-contrast imaging has emerged as an important method for improving contrast and sensitivity in the field of X-ray imaging. This increase in the sensitivity is attributed to the fact that, in the hard X-ray regime, the phase shift is more prominent as compared with the attenuation for materials having a low X-ray absorption coefficient. Among all the methods using the X-ray phase-contrast technique, in-line phase-contrast imaging scores over the other methods in terms of ease of implementation and efficient use of available X-ray flux. In order to retrieve the projected phase map of the object from the recorded intensity pattern, a large number of algorithms have been proposed. These algorithms generally use either the transport of intensity or contrast transfer function based approach for phase retrieval. In this paper it is proposed to use multiple wavelengths for phase retrieval using the contrast transfer function based formalism.

Application of X-ray phase-contrast imaging technique in the study of pyrocarbon-coated zirconia kernels

X-ray in-line phase-contrast imaging is an emerging technique made practical by high brilliance synchrotron X-ray sources and high-performance X-ray detectors. This technique is capable of providing improved contrast along with edge enhancement for weakly absorbing samples and hence it is of immense value in study of materials like carbon fiber, carbon composite, etc. This technique is also very useful for determination of coating thickness, uniformity of pyrocarbon coating on the fuel kernel particles. The applications of such fuel kernel particles demand a high-quality of uniformity and precise thickness. The thickness and uniformity of such coatings depends upon several parameters of the system during their preparation. In this paper, we have explored the use of X-ray phase-contrast techniques for the measurement of coating thickness and its uniformity. We present the results of such experiments on SiC and pyrocarbon (PyC)-coated zirconia micro-spheres. This information may be of importance in optimization of the various parameters during the deposition process. The results bring out the importance of the X-ray phase-contrast technique in materials research.

Application of x-ray phase-contrast imaging topolycrystalline CVD diamond

Surface morphology, individual grains and cracks in free-standing polycrystalline diamond samples have been imaged by a series of x-ray phase-contrast techniques, using well collimated synchrotron radiation of moderate coherence length at a `second-generation' source (Daresbury, UK). Whole samples (10 × 10 mm2 and larger) as well as selected regions have been studied. These contained crystallites of diameters in the range 50-200 µm. Contrast control has been achieved by varying the angle of an analysing crystal in the transmitted beam.