Edge Illumination X-ray Phase Contrast Research Articles

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.

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.

Quantification of porosity in composite plates using planar X-ray phase contrast imaging

The application of planar Edge-Illumination X-ray Phase-Contrast imaging (EI-XPCi) for the non-destructive quantification of porosity in carbon fiber reinforced polymer (CFRP) specimens, a significant concern in aerospace applications, was investigated. The method enables fast, planar (2D) scans providing access to large samples. A set of woven CFRP plates with porosity content ranging from 0.7% to 10.7% was examined. In addition to standard X-ray attenuation, EI-XPCi provides differential phase and dark-field signals, sensitive to inhomogeneities and interfaces at scales above and below the system spatial resolution, respectively. The correlation with the porosity content from matrix digestion obtained from the dark-field signal was comparable to that from ultrasonic attenuation. The novel analysis of the standard deviation of differential phase (STDP), sensitive to inhomogeneities above the system resolution (approximately 12 μm), resulted in a very high correlation (R2 = 0.995) with the matrix digestion porosity content, outperforming ultrasonic attenuation measurements.

Modulation transfer function (MTF) evaluation for x-ray phase imaging system employing attenuation masks

Objective. Attenuation masks can be used in x-ray imaging systems to increase their inherent spatial resolution and/or make them sensitive to phase effects, a typical example being Edge Illumination x-ray phase contrast imaging (EI-XPCI). This work investigates the performance of a mask-based system such as EI-XPCI in terms of Modulation Transfer Function (MTF), in the absence of phase effects. Approach. Pre-sampled MTF measurements, using an edge, were performed on the same system implemented without masks, with non-skipped masks and finally with skipped masks (i.e. masks in which apertures illuminate every other pixel row/column). Results are compared to simulations and finally images of a resolution bar pattern acquired with all the above setups are presented. Main results. Compared to the detector’s inherent MTF, the non-skipped mask setup provides improved MTF results. In comparison to an ideal case where signal spill-out into neighbouring pixels is negligible, this improvement takes place only at specific frequencies of the MTF, dictated by the spatial repetition of the spill-out signal. This is limited with skipped masks, which indeed provide further MTF improvements over a larger frequency range. Experimental MTF measurements are supported through simulation and resolution bar pattern images. Significance. This work has quantified the improvement in MTF due to the use of attenuation masks and lays the foundation for how acceptance and routine quality control tests will have to be modified when systems using masks are introduced in clinical practice and how MTF results will compare to those of conventional imaging systems.

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.

Replacing the detector mask with a structured scintillator in edge-illumination x-ray phase contrast imaging

We present a proof-of-concept edge illumination x-ray phase contrast system where the detector mask has been replaced by an indirect conversion detector in which sensitive and insensitive regions have been obtained by “patterning” the scintillator. This was achieved by creating a free-standing grid with period and aperture size matching that of a typical detector mask and filling the apertures with gadolinium oxysulfide. Images of various samples were collected with both the modified and the original edge illumination systems based on the use of two masks to characterize the performances of this detector design. We found that, despite the proof-of-concept nature of this attempt resulting in a structured detector with suboptimal performance, it allows effective separation of the attenuation and refraction channels through phase retrieval and the visualization of hard-to-detect features such as cartilage through the latter channel, thus demonstrating that the proposed approach holds the potential to lead to improved stability since it will use a single optical element facilitating the design of rotating phase contrast systems or the retrofitting of conventional x-ray systems.

Post-acquisition mask misalignment correction for edge illumination x-ray phase contrast imaging.

Edge illumination x-ray phase contrast imaging uses a set of apertured masks to translate phase effects into variation of detected intensity. While the system is relatively robust against misalignment, mask movement during acquisition can lead to gradient artifacts. A method has been developed to correct the images by quantifying the misalignment post-acquisition and implementing correction maps to remove the gradient artifact. Images of a woven carbon fiber composite plate containing porosity were used as examples to demonstrate the image correction process. The gradient formed during image acquisition was removed without affecting the image quality, and results were subsequently used for quantification of porosity, indicating that the gradient correction did not affect the quantitative content of the images.

Volumetric High-Resolution X-Ray Phase-Contrast Virtual Histology of Breast Specimens With a Compact Laboratory System.

The assessment of margin involvement is a fundamental task in breast conserving surgery to prevent recurrences and reoperations. It is usually performed through histology, which makes the process time consuming and can prevent the complete volumetric analysis of large specimens. X-ray phase contrast tomography combines high resolution, sufficient penetration depth and high soft tissue contrast, and can therefore provide a potential solution to this problem. In this work, we used a high-resolution implementation of the edge illumination X-ray phase contrast tomography based on “pixel-skipping” X-ray masks and sample dithering, to provide high definition virtual slices of breast specimens. The scanner was originally designed for intra-operative applications in which short scanning times were prioritised over spatial resolution; however, thanks to the versatility of edge illumination, high-resolution capabilities can be obtained with the same system simply by swapping x-ray masks without this imposing a reduction in the available field of view. This makes possible an improved visibility of fine tissue strands, enabling a direct comparison of selected CT slices with histology, and providing a tool to identify suspect features in large specimens before slicing. Combined with our previous results on fast specimen scanning, this works paves the way for the design of a multi-resolution EI scanner providing intra-operative capabilities as well as serving as a digital pathology system.

The effect of a variable focal spot size on the contrast channels retrieved in edge-illumination X-ray phase contrast imaging

Multi-modal X-ray imaging allows the extraction of phase and dark-field (or “Ultra-small Angle Scatter”) images alongside conventional attenuation ones. Recently, scan-based systems using conventional sources that can simultaneously output the above three images on relatively large-size objects have been developed by various groups. One limitation is the need for some degree of spatial coherence, achieved either through the use of microfocal sources, or by placing an absorption grating in front of an extended source. Both these solutions limit the amount of flux available for imaging, with the latter also leading to a more complex setup with additional alignment requirements. Edge-illumination partly overcomes this as it was proven to work with focal spots of up to 100 micron. While high-flux, 100 micron focal spot sources do exist, their comparatively large footprint and high cost can be obstacles to widespread translation. A simple solution consists in placing a single slit in front of a large focal spot source. We used a tunable slit to study the system performance at various effective focal spot sizes, by extracting transmission, phase and dark-field images of the same specimens for a range of slit widths. We show that consistent, repeatable results are obtained for varying X-ray statistics and effective focal spot sizes. As the slit width is increased, the expected reduction in the raw differential phase peaks is observed, compensated for in the retrieval process by a broadened sensitivity function. This leads to the same values being correctly retrieved, but with a slightly larger error bar i.e. a reduction in phase sensitivity. Concurrently, a slight increase in the dark-field signal is also observed.

Edge illumination X-ray phase contrast imaging for impact damage detection in CFRP

Carbon fibre reinforced polymer (CFRP) structures are widely used in the aerospace industry for their low weight and strong mechanical properties. However, they are prone to impact damage due to their brittle nature. Edge Illumination X-ray Phase Contrast imaging (EI XPCi), a novel, phase-based X-ray imaging technique, was compared against two established non-destructive evaluation methods for impact damage detection in CFRP plates. Planar EI XPCi was shown to allow the detection of a larger extent and more details of damage than both conventional X-ray imaging using contrast agents and ultrasonic immersion C-scans. However, both X-ray methods were unable to detect a delamination present in the plate, which was only observed from the ultrasonic C-scan. Planar EI XPCI has potential for rapid in-process scanning, where only 2D projection imaging is possible.

Effective modeling of high-energy laboratory-based x-ray phase contrast imaging utilizing absorption masks or gratings

Model refinements for the edge illumination x-ray phase contrast imaging method have been developed to improve simulation accuracy for high energy, polychromatic beams. High-energy x rays are desirable in imaging due to their penetrative power and, for biological samples, their lower dose deposition rate. Accurate models of such scenarios are required for designing appropriate imaging systems and to predict signal strength in complex settings such as clinical imaging or industrial quality assurance. When using optical components appropriate for high-energy x rays in a non-synchrotron setting, system performance was observed to deviate from that predicted by existing models. In this work, experimental data utilizing increasing thicknesses of a known filter material are used to illustrate the limitations of existing models and as validation for the new modeling features. Angular filtration of the cone beam was observed to be the most significant effect; however, specific features of the source and detector are also shown to affect system performance. We conclude by showing that a significantly improved agreement between experimental and simulated data is obtained with the refined model compared to previously existing ones.

Simplified retrieval method for Edge Illumination X-ray phase contrast imaging allowing multi-modal imaging with fewer input frames.

We present data from an implementation of Edge Illumination (EI) that uses a detector aperture designed for increasing dynamic range, suitable for clinically relevant X-ray energies and demonstrated here using synchrotron radiation. By utilising a sufficiently large crosstalk between pixels, this implementation enables single-scan imaging for phase and absorption, and double-scan for phase, absorption and dark field imaging. The presence of the detector mask enables a direct comparison between conventional EI and beam tracking (BT), which we conduct through Monte Carlo and analytical modelling in the case of a single-scan being used for the retrieval of all three contrasts. In the present case, where the X-ray beam width is comparable to the pixel size, we provide an analysis on best-positioning of the beam on the detector for accurate signal retrieval. Further, we demonstrate an application of this method by distinguishing different concentrations of microbubbles via their dark field signals at high energy using an EI system.

Detection of individual sub-pixel features in edge-illumination x-ray phase contrast imaging by means of the dark-field channel

We report on a direct comparison in the detectability of individual sub-pixel-size features between the three complementary contrast channels provided by edge-illumination x-ray phase contrast imaging at constant exposure time and spatial sampling pitch. The dark-field (or ultra-small-angle x-ray scattering) image is known to provide information on sample micro-structure at length scales that are smaller than the system’s spatial resolution, averaged over its length. By using a custom-built groove sample, we show how this can also be exploited to detect individual, isolated features. While these are highlighted in the dark-field image, they remain invisible to the phase and attenuation contrast channels. Finally, we show images of a memory SD card as an indication towards potential applications.

A compact system for intraoperative specimen imaging based on edge illumination x-ray phase contrast

A significant number of patients receiving breast-conserving surgery (BCS) for invasive carcinoma and ductal carcinoma in situ (DCIS) may need reoperation following tumor-positive margins from final histopathology tests. All current intraoperative margin assessment modalities have specific limitations. As a first step towards the development of a compact system for intraoperative specimen imaging based on edge illumination x-ray phase contrast, we prove that the system’s dimensions can be reduced without affecting imaging performance.We analysed the variation in noise and contrast to noise ratio (CNR) with decreasing system length using the edge illumination x-ray phase contrast imaging setup. Two-(planar) and three-(computed tomography (CT)) dimensional imaging acquisitions of custom phantoms and a breast tissue specimen were made. Dedicated phase retrieval algorithms were used to separate refraction and absorption signals. A ‘single-shot’ retrieval method was also used, to retrieve thickness map images, due to its simple acquisition procedure and reduced acquisition times. Experimental results were compared to numerical simulations where appropriate.The relative contribution of dark noise signal in integrating detectors is significant for low photon count statistics acquisitions. Under constant exposure factors and magnification, a more compact system provides an increase in CNR. Superior CNR results were obtained for refraction and thickness map images when compared to absorption images. Results indicate that the ‘single-shot’ acquisition method is preferable for a compact CT intraoperative specimen scanner; it allows for shorter acquisition times and its combination of the absorption and refraction signals ultimately leads to a higher contrast. The first CT images of a breast specimen acquired with the compact system provided promising results when compared to those of the longer length system.

A Preliminary Investigation into the Use of Edge Illumination X-ray Phase Contrast Micro-CT for Preclinical Imaging

PurposeTo enable a preliminary assessment of the suitability of edge illumination (EI) x-ray phase contrast (XPC) micro x-ray computed tomography (micro-CT) to preclinical imaging. Specifically, to understand how different acquisition schemes and their combination with dedicated data processing affect contrast-to-noise ratio (CNR) and spatial resolution, while providing control over scan time and radiation dose delivery.ProceduresDeceased mice (n = 3) were scanned with an EI XPC micro-CT setup operated under different settings, leading to scan times between 18 h and 13 min. For the shortest scan, the entrance dose was measured with a calibrated PTW 23344 ion chamber. Different data processing methods were applied, retrieving either separate attenuation and phase images, or hybrid (combined attenuation and phase) images. A quantitative comparison was performed based on CNR and spatial resolution measurements for a soft tissue interface.ResultsAll phase-based images have led to a higher CNR for the considered soft tissue interface than the attenuation image, independent of scan time. The best relative CNR (a sixfold increase) was observed in one of the hybrid images. Spatial resolution was found to be connected to scan time, with a resolution of approximately 20 μm and 60 μm achieved for the longest and shortest scans, respectively. An entrance dose of approximately 300 mGy was estimated for the scan performed within 13 min.ConclusionsDespite their preliminary nature, our results suggest that EI XPC bears potential for enhancing the utility of preclinical micro-CT, and, pending further research and development, could ultimately become a valuable technique in this field.

Simple and robust synchrotron and laboratory solutions for high-resolution multimodal X-ray phase-based imaging

Edge illumination X-ray phase contrast imaging techniques are capable of quantitative retrieval of differential phase, absorption and X-ray scattering. We have recently developed a series of approaches enabling high-resolution implementations, both using synchrotron radiation and laboratory-based set-ups. Three-dimensional reconstruction of absorption, phase and dark-field can be achieved with a simple rotation of the sample. All these approaches share a common trait which consists in the use of an absorber that shapes the radiation field, in order to make the phase modulations introduced by the sample detectable. This enables a well-defined and high-contrast structuring of the radiation field as well as an accurate modelling of the effects that are related to the simultaneous use of a wide range of energies. Moreover, it can also be adapted for use with detectors featuring large pixel sizes, which could be desirable when a high detection efficiency is important.

Edge-illumination x-ray phase contrast imaging with Pt-based metallic glass masks.

Edge-illumination x-ray phase contrast imaging (EI XPCI) is a non-interferometric phase-sensitive method where two absorption masks are employed. These masks are fabricated through a photolithography process followed by electroplating which is challenging in terms of yield as well as time- and cost-effectiveness. We report on the first implementation of EI XPCI with Pt-based metallic glass masks fabricated by an imprinting method. The new tested alloy exhibits good characteristics including high workability beside high x-ray attenuation. The fabrication process is easy and cheap, and can produce large-size masks for high x-ray energies within minutes. Imaging experiments show a good quality phase image, which confirms the potential of these masks to make the EI XPCI technique widely available and affordable.

On the relative performance of edge illumination x-ray phase-contrast CT and conventional, attenuation-based CT.

This article is aimed at comparing edge illumination (EI) x-ray phase contrast computed tomography (PCT) and conventional (attenuation-based) computed tomography (CT), based on their respective contrast and noise transfer. The noise in raw projections obtained with EI PCT is propagated through every step of the data processing, including phase retrieval and tomographic reconstruction, leading to a description of the noise in the reconstructed phase tomograms. This is compared to the noise in corresponding attenuation tomograms obtained with CT. Specifically, a formula is derived that allows evaluating the relative performance of both modalities on the basis of their contrast-to-noise ratio (CNR), for a variety of experimental parameters. The noise power spectra of phase tomograms are shifted towards lower spatial frequencies, leading to a fundamentally different noise texture. The relative performance of EI PCT and CT, in terms of their CNR, is linked to spatial resolution: the CNR in phase tomograms is generally superior to that in attenuation tomograms for higher spatial resolutions (tens to hundreds of μm), but inferior for lower spatial resolutions (hundreds of μm to mm). These results imply that EI PCT could outperform CT in applications for which high spatial resolutions are key, e.g., small animal or specimen imaging.

Edge-illumination x-ray phase contrast imaging restoration using discrete curvelet regularization transform.

This article considers the problem of recovering edge-illumination x-ray phase contrast (EIXPC) images from a set of potentially Poisson noisy projection measurements. The authors cast a recovery as a sparse regularization problem based on Anscombe multiscale variance stabilizing transform (MS-VST) with fast discrete curvelet transform which was applied to simulated edge-illumination x-ray phase contrast images. For accurate modelling, the noise characteristics of the EIXPCi data are used to determine the relative importance of each projection. Two implementations of curvelet sparse regularization transforms were applied, including the unequally-spaced fast Fourier transform and the wrapping-based transform. The algorithms were evaluated in terms of contrast improvement, quality of image restoration, object perceptibility, and peak signal-to-noise ratio. The methods provide nearly optimal solution without excessive memory and recovery time requirement. The performance of the proposed algorithms is demonstrated through a series of complex numerical geometric and anthropomorphic phantom studies. The results of numerical simulations demonstrate that the discrete curvelet transform with MS-VST is fast and robust, and it can effectively improve image quality, preserve and enhance edges and restore lost information while significantly reducing the noise. Additionally, both sparse sampling and decreasing x-ray tube current (i.e. noisy data) lead to the reduction of radiation dose in the x-ray imaging.

Compact and cost effective lab-based edge-illumination x-ray phase contrast imaging with a structured focal spot

We present a different implementation of the Edge Illumination (EI) X-ray Phase Contrast imaging method based on the use of multiple focal spots created through an additional x-ray mask. While this resembles directly inspired by the Talbot-Lau implementation of grating interferometry, the aim of the source mask and its effect on the acquired images are different. The individual “sourcelets” are much larger than in grating methods, and then still spatially incoherent; however, their use allows (a) exploiting cheap and large focal spot sources and (b) reducing the source spot size from the usual 70–100 μm typically used in EI to few tens of μm, which enables the realisation of more compact setups. However, in EI, multiple sources create images shifted by one detector pixel with respect to the other, imposing the use of an image restoration algorithm. Here, we show that the approach is feasible by deconvolving differential phase-contrast image profiles acquired with three separate sources, and comparing results with simulation predictions for equivalent profiles generated by a single source. We also show that this enables reducing the system length from the 2 m used so far to 1 m.