A single-image retrieval method for edge illumination X-ray phase-contrast imaging: Application and noise analysis

Hybrid framework for feasible modeling of an edge illumination X-ray phase-contrast imaging system at a human scale

X-ray phase contrast imaging (XPCI) shows promising potential for analyzing blood vessels affected by a cardiovascular disease and may become a valuable diagnostic tool in angiography. Identifying the most suitable XPCI method for providing optimal visibility at clinically acceptable doses is critical. This study presents a two-dimensional computational simulation in Geant4 to quantitatively compare two XPCI techniques—Inline XPCI and Edge Illumination Single Mask (EISM)—in visualizing atherosclerotic plaques within the projected thickness image of the lumen. The angiographic specimen analyzed was modeled as three concentric cylinders, with half of the inner cylinder simulating the lumen of the blood vessel and the other half simulating an atherosclerotic plaque, the intermediate cylinder simulating the vessel tissue, and the outer cylinder simulating the tissue around the blood vessel. The computational setup consisted of a W-anode polychromatic x-ray source and a photon counting detector, with six spectra and three sample-to-detector distances (magnifications) considered. To enable a direct comparison with Inline XPCI, a multi-material phase retrieval procedure was adapted to work within the EISM framework. Estimates of absorbed dose were also performed for each XPCI method. The results demonstrate that Inline XPCI achieves a superior contrast-to-noise ratio (CNR) as compared to EISM in both analyzed regions of the recovered image, across all spectra and magnifications, while also exhibiting lower absorbed doses. The difference in CNR between the two XPCI methods decreases as the voltage and magnification increases. While Inline XPCI shows superior contrast in the plaque region for all voltages and magnifications studied, EISM excels in recovering the projected thickness in the free-plaque region, achieving better contrast at low voltages for any magnification.

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.

We present a theoretical and experimental analysis of the angular sensitivity of edge illumination X-ray phase-contrast imaging in its implementation with conventional X-ray sources (sometimes referred to as the “coded-aperture” method). We study how the polychromaticity and finite source dimensions encountered in laboratory-based setups affect the detected signal. We also show that the sensitivity is independent of the period of the masks. Experimental images are presented and analyzed, proving that, despite the simple setup, high angular resolutions of a few hundred nanoradians can be obtained.

X-ray phase contrast imaging (XPCI) can separate the attenuation, refraction, and scattering signals of the object. The application of image fusion enables the concentration of distinctive information into a single image. Some methods have been applied in XPCI field, but wavelet-based decomposition approaches often result in loss of original data. To explore the application value of a novel image fusion method for XPCI system and computed tomography (CT) system. The means of fast adaptive bidimensional empirical mode decomposition (FABEMD) is considered for image decomposition to avoid unnecessary information loss. A parameter δ is proposed to guide the fusion of bidimensional intrinsic mode functions which contain high-frequency information, using a pulse coupled neural network with morphological gradients (MGPCNN). The residual images are fused by the energy attribute fusion strategy. Image preprocessing and enhancement are performed on the result to ensure its quality. The effectiveness of other image fusion methods has been compared, such as discrete wavelet transforms and anisotropic diffusion fusion. The δ-guided FABEMD-MGPCNN method achieved either the first or second position in objective evaluation metrics with biological samples, as compared to other image fusion methods. Moreover, comparisons are made with other fusion methods used for XPCI. Finally, the proposed method applied in CT show expected results to retain the feature information. The proposed δ-guided FABEMD-MGPCNN method shows potential feasibility and superiority over traditional and recent image fusion methods for X-ray differential phase contrast imaging and computed tomography systems.

There is currently much interest in developing X-ray Phase Contrast Imaging (XPCI) systems which employ laboratory sources in order to deploy the technique in real world applications. The challenge faced by nearly all XPCI techniques is that of efficiently utilising the x-ray flux emitted by an x-ray tube which is polychromatic and possesses only partial spatial coherence. Techniques have, however, been developed which overcome these limitations. Such a technique, known as coded aperture XPCI, has been under development in our laboratories in recent years for application principally in medical imaging and security screening. In this paper we derive limitations imposed upon source polychromaticity and spatial extent by the coded aperture system. We also show that although other grating XPCI techniques employ a different physical principle, they satisfy design constraints similar to those of the coded aperture XPCI.

In this note we present the first proof-of-concept results on the potential effectiveness of the edge-illumination x-ray phase contrast method (in its ‘coded-aperture’ based lab implementation) combined with tomosynthesis. We believe that, albeit admittedly preliminary (e.g. we only present phantom work), these results deserve early publication in a note primarily for four reasons. First, we fully modelled the imaging acquisition method, and validated the simulation directly with experimental results. This shows that the implementation of the method in the new geometry is understood, and thus that it will be possible to use the model to simulate more complex scenarios in the future. Secondly, we show that a strong phase contrast signal is preserved in the reconstructed tomosynthesis slices: this was a concern, as the high spatial frequency nature of the signal makes it sensitive to any filtration-related procedure. Third, we show that, despite the non-optimized nature of the imaging prototype used, we can perform a full angular scan at acceptable dose levels and with exposure times not excessively distant from what is required by clinical practice. Finally, we discuss how the proposed phase contrast method, unlike other approaches apart from free-space propagation (which however requires a smaller focal spot, thus reducing the flux and increasing exposure times), can be easily implemented in a tomosynthesis geometry suitable for clinical use. In summary, we find that these technical results indicate a high potential for the combination of the two methods. Combining slice separation with detail enhancement provided by phase effects would substantially increase the detectability of small lesions and/or calcifications, which we aim to demonstrate in the next steps of this study.

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.

A Monte Carlo model of a polychromatic laboratory based (coded aperture) edge illumination x-ray phase contrast imaging system has been developed and validated against experimental data. The ability for the simulation framework to be used to model two-dimensional images is also shown. The Monte Carlo model has been developed using the McXtrace engine and is polychromatic, i.e., results are obtained through the use of the full x-ray spectrum rather than an effective energy. This type of simulation can in future be used to model imaging of objects with complex geometry, for system prototyping, as well as providing a first step towards the development of a simulation for modelling dose delivery as a part of translating the imaging technique for use in clinical environments.

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 μm, 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.

A method of tomographic phase retrieval is developed for multi-material objects whose components each has a distinct complex refractive index. The phase-retrieval algorithm, based on the Transport-of-Intensity equation, utilizes propagation-based X-ray phase contrast images acquired at a single defocus distance for each tomographic projection. The method requires a priori knowledge of the complex refractive index for each material present in the sample, together with the total projected thickness of the object at each orientation. The requirement of only a single defocus distance per projection simplifies the experimental setup and imposes no additional dose compared to conventional tomography. The algorithm was implemented using phase contrast data acquired at the SPring-8 Synchrotron facility in Japan. The three-dimensional (3D) complex refractive index distribution of a multi-material test object was quantitatively reconstructed using a single X-ray phase-contrast image per projection. The technique is robust in the presence of noise, compared to conventional absorption based tomography.

X-ray phase contrast imaging (XPCI) methods give access to contrast mechanisms that are based on the refractive properties of matter on top of the absorption coefficient in conventional x-ray imaging. Ultra small angle x-ray scattering (USAXS) is a phase contrast mechanism that arises due to multiple refraction events caused by physical features of a scale below the physical resolution of the used imaging system. USAXS contrast can therefore give insight into subresolution structural information, which is an ongoing research topic in the vast field of different XPCI techniques. In this study, we quantitatively compare the USAXS signal retrieved by the beam tracking XPCI technique with the gold standard of the analyzer based imaging XPCI technique using a synchrotron x-ray source. We find that, provided certain conditions are met, the two methods measure the same quantity.

In this work, we performed a proof-of-concept experiment for phase-contrast x-ray imaging (PCXI) based on a single antiscatter grid and a polychromatic x-ray source. We established a table-top setup which consists of a focused-linear grid having a strip density of 200 lines/inch, a microfocus x-ray tube having a focal-spot size of about 5 μm, and a CMOS-type flat-panel detector having a pixel size of 48 μm. By using our prototype PCXI system and the Fourier demodulation technique, we successfully obtained attenuation, scattering, and differential phase-contrast images of improved visibility from the raw images of several selected samples at x-ray tube conditions of 90 kVp and 0.1 mAs. Further, fusion image (e.g., the attenuation＋the scattering) may have an advantage in displaying details of the sample's structures that are not clearly visible in the conventional attenuation image. Our experimental results indicate that single-grid-based approach seems a useful method for PCXI with great simplicity and minimal requirements on the setup alignment.

A single-exposure quantitative method of x-ray phase contrast imaging, suitable for animal in vivo observations, is described and shown experimentally both for a known static sample and an ex vivo biological airway. The ability to acquire the desired information within a single exposure is important for dynamic samples, as is sufficient sensitivity to reveal small variations in the composition or thickness of such a sample. This approach satisfies both these needs by analyzing how a reference grid pattern is deformed by the presence of the sample, similar to a Shack-Hartmann sensor. By resolving the shift of the pattern into horizontal and vertical components, a quantitative phase depth map is recovered, sensitive to both sharp edges as well as low phase gradients.

X-ray computed tomography (CT) is a powerful and routinely used clinical diagnostic technique, which is well tolerated by patients, and which provides high-resolution images and volumetric information about the body. However, two important limitations still affect this examination procedure: (1) its low sensitivity with respect to soft tissues, and (2) the hazards associated with x-ray exposure. Conventional radiology is based on the detection of the different photon absorption properties that characterize biological tissues, and thus the obtainable image contrast from soft and/or similar tissues is intrinsically limited. In this scenario, x-ray phase contrast imaging (XPCI) has been extensively tested and proven to overcome some of the main issues surrounding standard x-ray imaging. In addition to the absorption signal, XPCI relies on detecting the phase shifts induced by an object. Interestingly, as the order of magnitude of the phase contrast is higher than that of absorption, XPCI can, in principle, offer higher sensitivity at lower radiation doses. However, other technical aspects may counterbalance this gain, and an optimized setup and image processing solutions need to be implemented. The work presented here describes the strategies and developments we have realized, with the aim of controlling the radiation dose for the highly sensitive and quantitative XPCI-CT. Different algorithms for the phase retrieval and CT reconstruction of the XPCI data are presented. The CT algorithms we have implemented, namely the equally sloped tomography and the dictionary learning method, allow the image quality to be preserved while reducing the number of angular projections required by a factor of five. The results applied to breast imaging report accurate reconstructions at clinically compatible doses of the 3D distribution of the refractive properties of full human organs obtained by using three different phase retrieval methods. The described methodologies and the presented results have been validated by a team of clinical radiologists and represent an important step in the exploitation of XPCI-CT for in vivo and possible clinical applications.