Dark-field Contrast Research Articles

Simultaneous dark-field and attenuation contrast retrieval for single-shot laboratory-based tomography

Simultaneous dark-field and attenuation contrast retrieval for single-shot laboratory-based tomography

A comparison of X-ray attenuation, differential phase, and dark-field contrast imaging for the detection of porosity in carbon fiber reinforced cyanate ester

In this work we explore the capabilities of Talbot-Lau grating interferometry (TLGI) radiography for the inspection of porosity in structural specimens of cyanate ester carbon fiber reinforced polymer. The influence of system resolution and varying specimen thicknesses on mean values and standard deviations (STDV) in all three image modalities acquired by TLGI are addressed. Results show that mean absorption contrast (AC) values are highly affected by specimen thickness and strong negative correlation (r ≤ −0.8) is found only after correction via preliminary thickness measurements. Although dark-field contrast (DFC) is affected by changes in specimen thickness as well, the signal can be corrected by normalization with the inherently available AC. Consequently, strong positive correlation with porosity was found both in high- and low-resolution imaging (r = 0.83 and 0.71 respectively). Without the need for high image resolution or thickness measurements, the normalized DFC is a promising option for large field of view inspections. Investigations of STDV revealed strong positive correlations between porosity and AC STDV as well as differential phase contrast (DPC) STDV (r = 0.95 and 0.92 respectively) but high image resolution is required. Furthermore, results suggest increased robustness against variations in specimen thickness of AC and DPC STDV analyses.

A four-grating interferometer for x-ray multi-contrast imaging.

X-ray multi-contrast imaging with gratings provides a practical method to detect differential phase and dark-field contrast images in addition to the x-ray absorption image traditionally obtained in laboratory or hospital environments. Systems have been developed for preclinical applications in areas including breast imaging, lung imaging, rheumatoid arthritis hand imaging and kidney stone imaging. Prevailing x-ray interferometers for multi-contrast imaging include Talbot-Lau interferometers and universal moiré effect-based phase-grating interferometers. Talbot-Lau interferometers suffer from conflict between high interferometer sensitivity and large field of view (FOV) of the object being imaged. A small period analyzer grating is necessary to simultaneously achieve high sensitivity and large FOV within a compact imaging system but is technically challenging to produce for high x-ray energies. Phase-grating interferometers suffer from an intrinsic fringe period ranging from a few micrometers to several hundred micrometers that can hardly be resolved by large area flat panel x-ray detectors. The purpose of this work is to introduce a four-grating x-ray interferometer that simultaneously allows high sensitivity and large FOV, without the need for a small period analyzer grating. The four-grating interferometer consists of a source grating placed downstream of and close to the x-ray source, a pair of phase gratings separated by a fixed distance placed downstream of the source grating, and an analyzer grating placed upstream of and close to the x-ray detector. The object to be imaged is placed upstream of and close to the phase-grating pair. The distance between the source grating and the phase-grating pair is designed to be far larger than that between the phase-grating pair and the analyzer grating to promote simultaneously high sensitivity and large FOV. The method was evaluated by constructing a four-grating interferometer with an 8µm period source grating, a pair of phase gratings of 2.4µm period, and an 8µm period analyzer grating. The fringe visibility of the four-grating interferometer was measured to be ≈24% at 40kV and ≈18% at 50kV x-ray tube operating voltage. A quartz bead of 6mm diameter was imaged to compare the theoretical and experimental phase contrast signal with good agreement. Kidney stone specimens were imaged to demonstrate the potential of such a system for classification of kidney stones. The proposed four-grating interferometer geometry enables a compact x-ray multi-contrast imaging system with simultaneously high sensitivity and large FOV. Relaxation of the requirement for a small period analyzer grating makes it particularly suitable for high x-ray energy applications such as abdomen and chest imaging.

Edge illumination x-ray phase contrast simulations using the CAD-ASTRA toolbox.

Edge illumination x-ray phase contrast imaging (XPCI) provides increased contrast for low absorbing materials compared to attenuation images and sheds light on the material microstructure through dark field contrast. To apply XPCI in areas such as non-destructive testing and inline inspection, where scanned samples are increasingly compared to simulated reference images, accurate and efficient simulation software is required. However, currently available simulators rely on expensive Monte Carlo techniques or wave-optics frameworks, resulting in long simulation times. Furthermore, these simulators are often not optimized to work with computer-aided design (CAD) models, a common and memory-efficient method to represent manufactured objects, hindering their integration in an inspection pipeline. In this work, we address these shortcomings by introducing an edge illumination XPCI simulation framework built upon the recently developed CAD-ASTRA toolbox. CAD-ASTRA allows for the efficient simulation of x-ray projections from CAD models through GPU-accelerated ray tracing and supports ray refraction in a geometric optics framework. The edge illumination implementation is validated and its performance is benchmarked against GATE, a state-of-the-art Monte Carlo simulator, revealing a simulation speed increase of up to three orders of magnitude, while maintaining high accuracy in the resulting images.

Implementing dark-field contrast imaging in computer tomography with simple setup Talbot-Lau-Interferometer

Implementing dark-field contrast imaging in computer tomography with simple setup Talbot-Lau-Interferometer

Implementation of a dual-phase grating interferometer for multi-scale characterization of building materials by tunable dark-field imaging

The multi-scale characterization of building materials is necessary to understand complex mechanical processes, with the goal of developing new more sustainable materials. To that end, imaging methods are often used in materials science to characterize the microscale. However, these methods compromise the volume of interest to achieve a higher resolution. Dark-field (DF) contrast imaging is being investigated to characterize building materials in length scales smaller than the resolution of the imaging system, allowing a direct comparison of features in the nano-scale range and overcoming the scale limitations of the established characterization methods. This work extends the implementation of a dual-phase X-ray grating interferometer (DP-XGI) for DF imaging in a lab-based setup. The interferometer was developed to operate at two different design energies of 22.0 keV and 40.8 keV and was designed to characterize nanoscale-size features in millimeter-sized material samples. The good performance of the interferometer in the low energy range (LER) is demonstrated by the DF retrieval of natural wood samples. In addition, a high energy range (HER) configuration is proposed, resulting in higher mean visibility and good sensitivity over a wider range of correlation lengths in the nanoscale range. Its potential for the characterization of mineral building materials is illustrated by the DF imaging of a Ketton limestone. Additionally, the capability of the DP-XGI to differentiate features in the nanoscale range is proven with the dark-field of Silica nanoparticles at different correlation lengths of calibrated sizes of 106 nm, 261 nm, and 507 nm.

Laboratory-based x-ray dark-field microscopy.

We demonstrate the capability of laboratory-based x-ray microscopes, using intensity-modulation masks, to access the sub-micron length scale in the dark field contrast channel while maintaining micron resolution in the resolved (refraction and attenuation) channels. The dark field contrast channel reveals the presence of ensembles of samples' features below the system resolution. Resolved refraction and attenuation channels provide multi-modal high-resolution imaging down to the micron scale. We investigate the regimes of modulated and un-modulated dark field as well as refraction, quantifying their dependence on the relationship between feature size in the imaged object and aperture size in the intensity-modulation mask. We propose an analytical model to link the measured signal with the sample's microscopic properties. Finally, we demonstrate the relevance of the microscopic dark field contrast channel in applications from both the life and physical sciences, providing proof of concept results for imaging collagen bundles in cartilage and dendritic growth in lithium batteries.

Grating designs for cone beam edge illumination X-ray phase contrast imaging: a simulation study.

Edge illumination is an emerging X-ray phase contrast imaging technique providing attenuation, phase and dark field contrast. Despite the successful transition from synchrotron to lab sources, the cone beam geometry of lab systems limits the effectiveness of using conventional planar gratings. The non-parallel incidence of X-rays introduces shadowing effects, worsening with increasing cone angle. To overcome this limitation, several alternative grating designs can be considered. In this paper, the effectiveness of three alternative designs is compared to conventional gratings using numerical simulations. Improvements in flux and contrast are discussed, taking into account practical considerations concerning the implementation of the designs.

A deep learning reconstruction framework for low dose phase contrast computed tomography via inter-contrast enhancement

Phase contrast computed tomography (PCCT) offers excellent imaging contrast on soft tissue while it generate absorption, phase and dark-field contrast tomographic images. It has shown a great potential in clinical diagnosis. However, existing PCCT methods require high radiation doses. Reducing tube current is a universal low dose approach while it will introduce quantum noise in projections. In this paper, we report a deep learning (DL) framework for low dose PCCT based on inter-contrast enhancement. It utilizes the multi-contrast feature of PCCT and the varying effects of noise on each contrast. The missing structure in the contrasts that are more affected by noise can be recovered by those that are less affected. Considering the grating-based PCCT as example, the proposed framework is validated with experiments and a dramatic quality improvement of multi-contrast tomographic images has been obtained. This study shows potential of DL techniques in the field of low dose PCCT.

Initial Characterization of Dark-Field CT on a Clinical Gantry.

X-ray computed tomography (CT) is an important non-destructive imaging technique, particularly in clinical diagnostics. Even with the latest innovations like dual-energy and photon-counting CT, the image contrast is solely generated from attenuation in the tissue. An extension - fully compatible with these novelties - is dark-field CT, which retrieves an additional, so-called dark-field contrast. Unlike the attenuation channel, the dark-field channel is sensitive to tissue microstructure and porosity below the resolution of the imaging system, which allows additional insights into the health of the lung tissue or the structure of calcifications. The potential clinical value has been demonstrated in several preclinical studies and recently also in radiography patient studies. Just recently the first dark-field CT for the human body was established at the Technical University of Munich and in this paper, we discuss the performance of this prototype. We evaluate the interferometer components and the imposed challenges that the integration into the CT gantry brings by comparing the results to simulations and measurements at a laboratory setup. The influence of the clinical X-ray source on the Talbot-Lau interferometer and the impact of vibrations, which are immanent on the clinical CT gantry, are analyzed in detail to reveal their characteristic frequencies and origin. A beam hardening correction is introduced as an important step to adapt to the poly-chromatic spectrum and make quantitative dark-field imaging possible. We close with an analysis of the image resolution and the applied patient dose, and conclude that the performance is sufficient to suggest initial patient studies using the presented dark-field CT system.

Artifacts in metallography: etching pits

The article considers etch pits that appear in metals and alloys during metallographic etching as an artifact. The optical contrasting tools capabilities, such as dark field, polarized light, and differential interference contrast in detecting etch pits, are demonstrated.

Adapting an XCT-scanner to enable edge illumination X-ray phase contrast imaging

Conventional X-ray computed tomography (XCT) is a non-destructive imaging technique to visualize and inspect the internal structure of materials in 3D, where materials are distinguished solely on the basis of their attenuation coefficient. However, with more specialized X-ray phase contrast imaging methods, sensitivity to complementary contrasts can be achieved, namely phase contrast and dark field contrast. Edge illumination X-ray phase contrast imaging (EI-XPCI) is a technique that is especially suited for laboratory application, because it has no strict coherence requirements for the source. While there is a wide variety of XCTscanners available on the market, a commercial EI-XPCI scanner does not yet exist. In this work, we present the procedure of outfitting an existing XCT-scanner with components to enable EI-XPCI scans and discuss the alignment procedure of these added components. Finally, phase contrast images are shown demonstrating the successful upgrade of a conventional XCT system with edge illumination based phase contrast possibilities.

STEM analysis of deformation and B distribution In nanosecond laser ultra-doped Si1−x B x

We report on the structural properties of highly B-doped silicon (up to 10 at.% of active doping) realised by nanosecond laser doping. The crystalline quality, lattice deformation and B distribution profile of the doped layer are investigated by scanning transmission electron microscopy followed by high-angle annular dark field contrast studies and geometrical phase analysis, and compared to the results of secondary ions mass spectrometry and Hall measurements. When increasing the active B concentration above 4 at.%, the fully strained, perfectly crystalline, Si:B layer starts showing dislocations and stacking faults. These only disappear around 8 at.% when the Si:B layer is well accommodated to the substrate. With increasing B incorporation, an increasing number of small precipitates is observed, together with filaments with a higher active B concentration and stacking faults. At the highest concentrations studied, large precipitates form, related to the decrease of active B concentration. The structural information, defect type and concentration, and active B distribution are connected to the initial increase and subsequent gradual loss of superconductivity.

Spectral X-ray dark-field signal characterization from dual-energy projection phase-stepping data with a Talbot-Lau interferometer

Material-selective analysis of spectral X-ray imaging data requires prior knowledge of the energy dependence of the observed signal. Contrary to conventional X-ray imaging, where the material-specific attenuation coefficient is usually precisely known, the linear diffusion coefficient of the X-ray dark-field contrast does not only depend on the material and its microstructure, but also on the setup geometry and is difficult to access. Here, we present an optimization approach to retrieve the energy dependence of the X-ray dark-field signal quantitatively on the example of closed-cell foams from projection data without the need for additional hardware to a standard grating-based X-ray dark-field imaging setup. A model for the visibility is used to determine the linear diffusion coefficient with a least-squares optimization. The comparison of the results to spectrometer measurements of the linear diffusion coefficient suggests the proposed method to provide a good estimate for the energydependent dark-field signal.

Periphyton (microbiome) of filamentous algae studied by polarization microscopy, aided also by some contrasting optical methods.

Polarization microscopy, possibly together with some contrast techniques (dark field and color phase contrast), was used to study the periphyton (microbiome) growing on filamentous green algae. The material containing filamentous algae with periphyton on the surface was collected in the villages of Sýkořice and Zbečno (Křivoklátsko Protected Landscape Area). The objects were studied in a LOMO MIN-8 St. Petersburg polarizing microscope and a Carl Zeiss Jena NfpK laboratory microscope equipped with an In Ph 160 basic body with variable dark field or color phase contrast and a Nikon D70 DSLR digital camera. Cells of filamentous algae of the genera Cladophora, Vaucheria, and Oedogonium were studied and the periphyton attached to them formed by cyanobacteria of the genera Chamaesiphon and Pleurocapsa and algae of the genera Characium, including diatoms of the genera Eunotia and Synedra. In all cases, the cell walls of the host algae showed a very strong birefringence. In contrast, the walls of cyanobacteria of the genera Chamaesiphon and Pleurocapsa were characterized by a much weaker birefringence (Pleurocapsa somewhat thicker), and the diatom frustules of the genera Eunotia and Synedra were almost without a birefringence. Strongly birefringent granules were found in the cytoplasm of the green alga of the genus Characium, which forms periphyton on the filamentous green algae of the genus Vaucheria. The periphyton on the filamentous alga of the genus Oedogonium, formed by cyanobacteria of the genus Pleurocapsa and diatoms of the genera Eunotia and Synedra, deposited in a massive layer of mucus containing birefringent crystals, showed a particularly strong birefringence. At the end of the vegetation of filamentous algae, their parts and remnants of periphyton (diatom frustules and crystals) became part of the detritus at the bottom of the culture vessel. The use of polarization microscopy in the study of filamentous algae with periphyton on the surface allows us not only to determine the birefringence of the observed structures, but also to partially deduce their chemical composition, or regular arrangement ofparticles, so-called shape birefringence.

Iterative signal retrieval for X-ray grating interferometry with dual-shot.

X-ray grating interferometry normally requires multiple steps and exposures, causing a prolonged imaging time. There is motivation to use fewer steps to reduce scanning time and complexity, while keeping fidelity of the retrieved signals. We propose an iterative signal retrieval method, extracting attenuation, dark field contrast (DFC), and differential phase contrast (DPC) signals from two X-ray exposures. Two shots were captured at G2 grating positions with difference of 1/4 grating period. The algorithm consists of two stages. At the first stage, amplitude of sample phase stepping curve retrieved by virtual phase stepping (VPS) method, visibility and local phase of background phase stepping curve are used to limit the results to the proximity of the ground truth. After the second stage, three high-quality parameters, amplitude, visibility, and local phase, are retrieved through finetuning, and three signals are calculated. Simulated and real-sample experiments were conducted to validate this method. We used standard phase stepping result as benchmark and calculated structural similarity (SSIM) and peak signal-to-noise ratio (PSNR) between benchmark and parameters retrieved by our dual-shot method and virtual phase stepping (VPS) method. For both simulated and real-sample experiments, the SSIM and PSNR value of dual-shot method are higher than those of VPS method. For real-sample method, we also conducted a three-step PS, and the SSIM and PSNR value of dual-shot method are slightly lower than those of three-step PS. Using our dual-shot method demonstrates higher performance than other single-shot method in retrieving high-quality signals, and it also reduces radiation dose and time.

Multi-modal microscopy imaging with the OpenFlexure Delta Stage.

Microscopes are vital pieces of equipment in much of biological research and medical diagnostics. However, access to a microscope can represent a bottleneck in research, especially in lower-income countries. 'Smart' computer controlled motorized microscopes, which can perform automated routines or acquire images in a range of modalities are even more expensive and inaccessible. Developing low-cost, open-source, smart microscopes enables more researchers to conceive and execute optimized or more complex experiments. Here we present the OpenFlexure Delta Stage, a 3D-printed microscope designed for researchers. Powered by the OpenFlexure software stack, it is capable of performing automated experiments. The design files and assembly instructions are freely available under an open licence. Its intuitive and modular design-along with detailed documentation-allows researchers to implement a variety of imaging modes with ease. The versatility of this microscope is demonstrated by imaging biological and non-biological samples (red blood cells with Plasmodium parasites and colloidal particles in brightfield, epi-fluorescence, darkfield, Rheinberg and differential phase contrast. We present the design strategy and choice of tools to develop devices accessible to researchers from lower-income countries, as well as the advantages of an open-source project in this context. This microscope, having been open-source since its conception, has already been built and tested by researchers around the world, promoting a community of expertise and an environment of reproducibility in science.

Envelope modulated x-ray grating interferometry

X-ray dark-field and phase contrast imaging using grating interferometry (GI) have demonstrated great potential for medical and industrial applications. GI relies on the fabrication of high-quality absorption gratings, which has revealed to be quite challenging. This paper proposes an interferometer for dark-field and differential phase contrast imaging using a single phase-shifting element. This proposal simplifies the setup without penalizing the image quality. The proposed interferometer consists of a single dual-period phase grating having two distinct periods, which generates intensity modulation with a period large enough to be resolved directly by the detector. This makes the proposed GI system easily adaptable as there are no stringent alignment requirements. We show the utility of this method for a possible diagnostic task by imaging smoked murine lung samples. The simplicity of the proposed setup compared to a conventional Talbot(-Lau) interferometer suggests that it could be easily and widely used for a variety of applications.

Cold and Thermal Neutron Single Grating Dark-Field Imaging Extended to an Inverse Pattern Regime

Neutron dark-field imaging is a powerful tool for the spatially resolved characterization of microstructural features of materials and components. Recently, a novel achromatic technique based on a single absorption grating for the concurrent measurement of attenuation, dark-field and differential phase contrast was introduced. However, the range of measurable length scales of the technique in quantitative dark-field measurements appeared limited to some 10–100 nanometers, due to the relatively high spatial resolution requirement to detect the projected beam modulation. Here, we show how using grating–detector distances beyond the resolution limit for a given collimation produces a sequence of inverse and regular projection patterns and, thus, leads to a significant extension of the range of accessible length scales probed by dark-field imaging. In addition, we show that this concept can also be applied to 2D grating structures, which will enable concurrent three-fold directional dark-field measurements at a wide range of length scales. The approach is demonstrated with measurements on an electrical steel sheet sample, which confirm the validity of combining the results from the regular and inverse grating patterns.

Ag2Al plates in Al–Ag alloys

Abstract The late stages of precipitation in aluminum-rich Al –Ag alloys with silver contents from 3 at.% to 22 at.% were studied with transmission electron microscopy. Shapes, sizes, and arrangements of plates of the stable hexagonal Ag2Al phase forming on {111} planes of the f.c.c. matrix were analysed. The high-angle annular dark-field contrast in scanning transmission electron microscopy is calibrated. This calibration allows for the quantitative measurement of plate thicknesses from high-angle annular dark-field scanning transmission electron micrographs of Ag2Al plates inclined to the electron beam. Results from these measurements are in good agreement with direct bright-field micrographs of plates viewed edge-on. For samples with silver contents above 12 at.% a parallel lamellar arrangement of fine γ-plates and Al-rich matrix is found after extended heat treatments.