Edge illumination X-ray multi-contrast imaging without mask displacement

In the late '90s, the concept of "edge illumination" was developed at ELETTRA in Italy as an alternative method to increase the phase sensitivity of an imaging system. The main idea was to be able to reproduce the fine angular selection of "analyzer" crystals without actually using a crystal, as this would allow employing the method with divergent and polychromatic (i.e. conventional) x-ray sources. It was observed that this could be achieved by illuminating only the edges of the detector pixels, and that the method's sensitivity could be progressively increased by illuminating smaller pixel fractions closer to its physical edge. A few years later the idea was adapted for use with a conventional source by means of two sets of x-ray masks ("coded aperture" masks), which enabled obtaining the same effect for each row (or column) of pixels of an area detector illuminated by a cone beam. This article reviews the method and presents recent examples of application.

Edge illumination is an X-ray phase contrast imaging technique that introduces two absorbing masks with slit-shaped apertures in the imaging setup. During an edge illumination acquisition, a pixel-wise intensity profile is measured by acquiring projections at several different mask aperture alignments. The difference in profile peak position between acquisitions with and without a sample yields the refraction contrast. However, the peak position itself depends on the source focal spot position. In many lab X-ray sources, the position of the focal spot can drift over time, causing a corresponding drift of the measured intensity profile peak. As edge illumination computed tomography scans typically have long acquisition times, they are particularly sensitive to errors caused by focal spot drift. In this work, the effect of focal spot drift on the edge illumination refraction contrast is measured. Post-processing correction methods to compensate for the intensity profile shift based on repeating projection angles and polynomial fitting are proposed and their effectiveness is demonstrated.

This paper presents a new flexible compact multi-modal imaging setup referred to as PEPI (Photon-counting Edge-illumination Phase-contrast imaging) Lab, which is based on the edge-illumination (EI) technique and a chromatic detector. The system enables both X-ray phase-contrast (XPCI) and spectral (XSI) imaging of samples on the centimeter scale. This work conceptually follows all the stages in its realization, from the design to the first imaging results. The setup can be operated in four different modes, i.e. photon-counting/conventional, spectral, double-mask EI, and single-mask EI, whereby the switch to any modality is fast, software controlled, and does not require any hardware modification or lengthy re-alignment procedures. The system specifications, ranging from the X-ray tube features to the mask material and aspect ratio, have been quantitatively studied and optimized through a dedicated Geant4 simulation platform, guiding the choice of the instrumentation. The realization of the imaging setup, both in terms of hardware and control software, is detailed and discussed with a focus on practical/experimental aspects. Flexibility and compactness (66 cm source-to-detector distance in EI) are ensured by dedicated motion stages, whereas spectral capabilities are enabled by the Pixirad-1/Pixie-III detector in combination with a tungsten anode X-ray source operating in the range 40–100 kVp. The stability of the system, when operated in EI, has been verified, and drifts leading to mask misalignment of less than 1 upmum have been measured over a period of 54 h. The first imaging results, one for each modality, demonstrate that the system fulfills its design requirements. Specifically, XSI tomographic images of an iodine-based phantom demonstrate the system’s quantitativeness and sensibility to concentrations in the order of a few mg/ml. Planar XPCI images of a carpenter bee specimen, both in single and double-mask modes, demonstrate that refraction sensitivity (below 0.6 upmurad in double-mask mode) is comparable with other XPCI systems based on microfocus sources. Phase CT capabilities have also been tested on a dedicated plastic phantom, where the phase channel yielded a 15-fold higher signal-to-noise ratio with respect to attenuation.

Simulations of single-shot X-ray phase-contrast tomography based on edge illumination

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.

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.

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.

.Edge illumination (EI) is an x-ray phase-contrast imaging technique, exploiting sensitivity to x-ray refraction to visualize features, which are often not detected by conventional absorption-based radiography. The method does not require a high degree of spatial coherence and is achromatic and, therefore, can be implemented with both synchrotron radiation and commercial x-ray tubes. Using different retrieval algorithms, information about an object’s attenuation, refraction, and scattering properties can be obtained. In recent years, a theoretical framework has been developed that enables EI computed tomography (CT) and, hence, three-dimensional imaging. This review provides a summary of these advances, covering the development of different image acquisition schemes, retrieval approaches, and applications. These developments constitute an integral part in the transformation of EI CT into a widely spread imaging tool for use in a range of fields.

X-ray Phase Contrast Imaging (XPCI) is an imaging method that can provide quantitative information about the change in phase of X-ray wavefronts as they pass through an object. XPCI can image objects that cannot be easily seen in conventional absorption imaging, such as thin, weakly-absorbing objects. Most exploration into XPCI has involved synchrotron sources, which are large, fixed facilities and not widely available. Several tabletop methods exist, but these generally rely on interferometric methods or complicated gratings. We began investigating Edge-Illumination (EI), a non-interferometric, inexpensive XPCI method that can use a standard x-ray tube. However, EI requires at least two different spatial shifts, with small aperture openings and precise beam alignment, thereby increasing the complexity of the method. Due to the limitations of EI and the rise in availability of spectrally sensitive detectors, we propose a variant of EI, called Spectrally Responsive Edge Illumination (SREI), which relies on a diversity of X-ray energies instead of spatial shifts. Our goal is to develop an XPCI method that is simple, robust, and easily implementable with commercially available equipment. I will report on our progress.

In recent years, the complementary nature of multi-contrast imaging has increased the popularity of x-ray phase contrast imaging, including edge illumination. However, edge illumination system optimization most often relies on phase and transmission contrast only, without considering dark field contrast. Computer simulations are a widespread approach to design and optimize imaging systems, including the benchmarking of simulation results, i.e., the comparison to a reference value. Providing such a reference is, however, particularly challenging for the dark field signal. In this work, we present a practical method to directly estimate transmission, refraction, and dark field contrast reference values from simulated x-ray trajectories in Monte Carlo simulations. This allows an immediate comparison of the retrieved simulated contrasts to their respective references. We show how the generated reference values can be used effectively for benchmarking simulation results and discuss other potential applications of the presented approach.

An unmet demand for high resolution tomographic imaging modalities providing enhanced soft tissue contrast exists in a number of biomedical disciplines. X-ray phase contrast imaging (XPCi) methods can provide a solution: contrast is driven by phase (refraction) effects rather than attenuation effects, the formers being much larger than the latters for weakly attenuating materials and energies typically used for biomedical imaging. However, the majority of the existing XPCi methods suffer from drawbacks affecting their implementation outside specialized facilities such as synchrotrons and therefore their applicability to biomedical research. The Edge Illumination (EI) XPCi method has the potential to overcome or at least mitigate most of these drawbacks. Its major strengths are its simple setup, compatibility with commercially available x-ray tubes and potential for low-dose imaging. EI XPCi has recently been implemented as a tomographic modality, and it was demonstrated that the method can provide quantitatively accurate volumetric images acquired with low entrance doses. This paper explains the experimental requirements for tomographic EI XPCi, outlines the image reconstruction process and discusses potential applications in biomedicine. As an example, first experimental images of an atherosclerotic plaque specimen are presented.

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.

Purpose: The anatomical noise power spectra (NPS) for differential phase contrast (DPC) and dark field (DF) imaging have recently been characterized using a power-law model with two parameters, alpha and beta, an innovative extension to the methodology used in x-ray attenuation based breast imaging such as mammography, DBT, or cone-beam CT. Beta values of 3.6, 2.6, and 1.3 have been measured for absorption, DPC, and DF respectively for cadaver breasts imaged in the coronal plane; these dramatic differences should be reflected in their detection performance. The purpose of this study was to determine the impact of anatomical noise on breast calcification detection and compare the detection performance of the three contrast mechanisms of a multi-contrast x-ray imaging system. Methods: In our studies, a calcification image object was segmented out of the multi-contrast images of a cadaver breast specimen. 50 measured total NPS were measured from breast cadavers directly. The ideal model observer detectability was calculated for a range of doses (5–100%) and a range of calcification sizes (diameter = 0.25–2.5 mm). Results: Overall we found the highest average detectability corresponded to DPC imaging (7.4 for 1 mm calc.), with DF the next highest (3.8 for 1 mm calc.), and absorption the lowest (3.2 for 1 mm calc.). However, absorption imaging also showed the slowest dependence on dose of the three modalities due to the significant anatomical noise. DPC showed a peak detectability for calcifications ∼1.25 mm in diameter, DF showed a peak for calcifications around 0.75 mm in diameter, and absorption imaging had no such peak in the range explored. Conclusion: Understanding imaging performance for DPC and DF is critical to transition these modalities to the clinic. The results presented here offer new insight into how these modalities complement absorption imaging to maximize the likelihood of detecting early breast cancers. J. Garrett, Y. Ge, K. Li: Nothing to disclose. G.-H. Chen: Research funded, GE Healthcare; Research funded, Siemens AX.

One of the most commonly used correction methods in X-ray imaging is flat field correction, which corrects for systematic inconsistencies, such as differences in detector pixel response. In conventional X-ray imaging, flat fields are acquired by exposing the detector without any object in the X-ray beam. However, in edge illumination X-ray CT, which is an emerging phase contrast imaging technique, two masks are used to measure the refraction of the X-rays. These masks remain in place while the flat fields are acquired and thus influence the intensity of the flat fields. This influence is studied theoretically and validated experimentally using Monte Carlo simulations of an edge illumination experiment in GATE.

Edge illumination (EI) is an X-ray imaging technique that, in addition to conventional absorption contrast, provides refraction and scatter contrast. It relies on an absorption mask in front of the sample that splits the X-ray beam into beamlets, which hits a second absorption mask positioned in front of the detector. The sample mask is then shifted in multiple steps with respect to the detector mask, thereby measuring an illumination curve per detector element. The width, position, and area of this curve estimated with and without the sample in the beam is then compared, which ultimately provides absorption, refraction, and scatter contrast for each detector pixel. From the obtained contrast sinograms, three contrast tomograms can be computed. In summary, conventional EI relies on a two-stage process comprised of a computational and time intensive contrast retrieval process, followed by tomographic reconstruction. In this work, a novel joint reconstruction method is proposed, which utilizes a combined forward model to reconstruct the three contrasts simultaneously, without the need for an intermediate contrast retrieval step. Compared to the state-of-the-art, this approach reduces reconstruction times, as the retrieval step is skipped and allows a much more flexible acquisition scheme, as there is no need to sample a full illumination curve at each projection angle. The proposed method is shown to improve reconstruction quality on subsampled datasets, enabling the reconstruction of three contrasts from single-shot datasets.