Anna Zamir

X–ray absorption, phase and dark–field tomography through a beam tracking approach

We present a development of the beam–tracking approach that allows its implementation in computed tomography. One absorbing mask placed before the sample and a high resolution detector are used to track variations in the beam intensity distribution caused by the sample. Absorption, refraction, and dark–field are retrieved through a multi–Gaussian interpolation of the beam. Standard filtered back projection is used to reconstruct three dimensional maps of the real and imaginary part of the refractive index, and of the dark–field signal. While the method is here demonstrated using synchrotron radiation, its low coherence requirements suggest a possible implementation with laboratory sources.

Robust phase retrieval for high resolution edge illumination x-ray phase-contrast computed tomography in non-ideal environments

Edge illumination x-ray phase contrast tomography is a recently developed imaging technique which enables three-dimensional visualisation of low-absorbing materials. Dedicated phase retrieval algorithms can provide separate computed tomography (CT) maps of sample absorption, refraction and scattering properties. In this paper we propose a novel “modified local retrieval” method which is capable of accurately retrieving sample properties in a range of realistic, non-ideal imaging environments. These include system misalignment, defects in the used optical elements and system geometry variations over time due to vibrations or temperature fluctuations. System instabilities were analysed, modelled and incorporated into a simulation study. As a result, an additional modification was introduced to the retrieval procedure to account for changes in the imaging system over time, as well as local variations over the field of view. The performance of the proposed method was evaluated in comparison to a previously used “global retrieval” method by applying both approaches to experimental CT data of a rat’s heart acquired in a non-ideal environment. The use of the proposed method resulted in the removal of major artefacts, leading to a significant improvement in image quality. This method will therefore enable acquiring high-resolution, reliable CT data of large samples in realistic settings.

Edge illumination X-ray phase tomography of multi-material samples using a single-image phase retrieval algorithm

In this paper we present a single-image phase retrieval algorithm for multi-material samples, developed for the edge illumination (EI) X-ray phase contrast imaging method. The theoretical derivation is provided, along with any assumptions made. The algorithm is evaluated quantitatively using both simulated and experimental results from a computed tomography (CT) scan using the EI laboratory implementation. Qualitative CT results are provided for a biological sample containing both bone and soft-tissue. Using a single EI image per projection and knowledge of the complex refractive index, the algorithm can accurately retrieve the interface between a given pair of materials. A composite CT slice can be created by splicing together multiple CT reconstructions, each retrieved for a different pair of materials.

Recent advances in edge illumination x-ray phase-contrast tomography

Abstract. 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.

Strategies for fast and low-dose laboratory-based phase contrast tomography for microstructural scaffold analysis in tissue engineering

The application of x-ray phase contrast computed tomography (PCT) to the field of tissue engineering is dis- cussed. Specific focus is on the edge illumination PCT method, which can be adapted to weakly coherent x-ray sources, permitting PCT imaging in standard (non-synchrotron) laboratory environments. The method was applied to a prominent research topic in tissue engineering, namely the development of effective and reliable decellularization protocols to derive scaffolds from native tissue. Results show that edge illumination PCT provides sufficient image quality to evaluate the microstructural integrity of scaffolds and, thus, to assess the performance of the used decellularization technique. In order to highlight that edge illumination PCT can ultimately comply with demands on a high specimen throughput and low doses of radiation, recently developed strategies for scan time and dose reduction are discussed.

X-ray phase contrast tomography; proof of principle for post-mortem imaging.

OBJECTIVE To demonstrate the feasibility of using X-ray phase-contrast tomography to assess internal organs in a post-mortem piglet model, as a possible non-invasive imaging autopsy technique. METHODS Tomographic images of a new-born piglet were obtained using a free-space propagation X-ray phase-contrast imaging setup at a synchrotron (European Synchrotron Radiation Facility, Grenoble, France). A monochromatic X-ray beam (52 keV) was used in combination with a detector pixel size of 46 × 46 µm(2). A phase-retrieval algorithm was applied to all projections, which were then reconstructed into tomograms using the filtered-back projection algorithm. Images were assessed for diagnostic quality. RESULTS Images obtained with the free-space propagation setup presented high soft-tissue contrast and sufficient resolution for resolving organ structure. All of the main body organs (heart, lungs, kidneys, liver and intestines) were easily identified and adequately visualized. In addition, grey/white matter differentiation in the cerebellum while still contained within the skull was shown. CONCLUSION The feasibility of using X-ray phase-contrast tomography as a post-mortem imaging technique in an animal model has been demonstrated. Future studies will focus on translating this experiment to a laboratory-based setup. ADVANCES IN KNOWLEDGE Appropriate image processing and analysis enable the simultaneous visualization of both soft- and hard-tissue structures in X-ray phase-contrast images of a complex, thick sample.

Low-dose x-ray phase contrast tomography: Experimental setup, image reconstruction and applications in biomedicine

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.

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.

Fast and low-dose phase contrast CT with non-microfocal laboratory x-ray sources (Conference Presentation)

The implementation of X-Ray Phase Contrast (XPC) imaging at synchrotrons has demonstrated transformative potential on a wide range of applications, from medicine and biology to materials science. However, translation to conventional laboratory sources has proven more problematic, because of XPC’s stringent requirements in terms of spatial coherence. This has imposed the use of either micro-focal sources, or collimators (e.g. source gratings) where sources with extended focal spots were used. This reduces the available x-ray flux leading to long exposure times, which is often exacerbated by the use of additional optical elements that need to be scanned during image acquisition. Where these elements are placed downstream of the object, they also lead to an increase in the delivered dose. XPC has also been successfully adapted to full 3D, computed tomography (CT) implementations, which has however exacerbated the above concerns in terms of acquisition times and delivered doses. We tackled this problem by developing an incoherent approach to XPC that works with non micro-focal laboratory sources without requiring any additional collimation. The method uses one or two low aspect ratio x-ray masks that are built on low-absorbing graphite substrates for maximum transmission through the mask apertures. The combination of this with a “single-shot” phase retrieval algorithm has enabled the development of a lab-based XPC-CT system that can perform a full scan in a few minutes while delivering low radiation doses. The talk will briefly describe how the method works, then show application examples including direct comparisons with the synchrotron gold standard.