Hard X-ray Phase-contrast Imaging Research Articles

X-ray phase-contrast ghost imaging using a single-pixel camera

Advances in x-ray imaging have enabled breakthroughs in medicine, biology, materials science, and cultural heritage studies. Three-dimensional visualizations of micrometer-scale structures inside soft tissues and dense specimens have been made successful by state-of-the-art methods based on hard x-ray phase-contrast imaging. Nevertheless, it remains challenging to achieve high spatial resolutions without increasing radiation dose and risking damage to specimens. A promising approach to reduce dose and still achieve high spatial resolution is x-ray ghost imaging, which uses single-pixel, yet efficient direct x-ray detectors made of high-density materials. However, all currently existing x-ray ghost imaging protocols are unable to achieve phase contrast and suffer from low-quality image reconstructions. Here, we present an efficient approach which exploits structured-detection single-pixel imaging in order to produce an x-ray ghost image with phase contrast, accuracy, and high fidelity. The resulting x-ray phase-contrast ghost images provide accurate information regarding density variations in the sample and visibly render edges that are otherwise invisible with x-ray attenuation contrast. This demonstration of phase-contrast ghost imaging with x-rays has the potential to promote x-ray ghost imaging from a niche technique to a routinely applied method.

Hard X-Ray Phase-Contrast Laboratory Microscope Based on Three-Block Fresnel Zone Plate Interferometer

A device based on a three-block Fresnel zone plate interferometer is proposed for hard X-ray phase-contrast imaging. The device combines a low requirement for the coherence of the initial radiation (the interferometer operates in the amplitude division mode) with an optical magnification of the image. A numerical simulation of the image formation is carried out, taking into account the limited source-interferometer distance, the size and spectral width of the X-ray source. The calculations show that the proposed set-up can be used as a phase-contrast microscope using laboratory hard X-ray sources.

Thicket high Z wavelength-scale structured target: advantages and limitations for laser-driven hard x-ray source and phase contrast imaging

The hard x-ray copper K α source formed by the interaction of a high intensity laser pulse with flat and thicket sub-micron structured copper targets is thoroughly characterized. It is shown that the flux of quanta can be increased up to several times using the structured target instead of the flat one. The maximal conversion efficiency into K α reaches 1.4 × 10−4 corresponding to >109 photons with a high contrast ratio over the bremsstrahlung radiation at the peak intensity ∼1018 W cm−2. The efficient use of the thicket target at higher intensity (2 × 1018 W cm−2) is limited by temporal contrast of the laser pulse and the few-fold lowered damage threshold of the modified target. Measurements of the x-ray source size revealed that the lateral coherence is similar both for the flat and thicket target. The estimated source size of ∼15 μm makes it suitable for the x-ray phase contrast imaging with reasonable geometry and short time object exposure. A simple concept of the image acquisition correction is proposed in order to account for the x-ray source lateral shifts, which is based on measurements in each shot instead of long exposure. Image post-processing allowed us to enhance source lateral coherence and retrieve phase contrast effects when the x-ray source shifts exceed its size.

Unravelling the sound-induced motion of fish auditory structures: A non-invasive synchrotron radiation-based approach

Modern bony fishes display a high morphological diversity in their auditory structures. Yet, unraveling the sound-induced in situ interaction of the auditory structures has been challenging. Synchrotron radiation-based imaging techniques with high spatial and temporal resolutions now provide a powerful tool to study the functional morphology of such structures in a non-invasive way. We investigated two species representing two types of otophysic connections, namely, goldfish (Weberian apparatus) and Etroplus canarensis (swimbladder extensions contacting the inner ears) asking how different ancillary auditory structures affect otolith motion. Fishes were subjected to a 200 Hz pure tone in a 2L-standing wave tube-like setup while performing 2D radiography based on hard x-ray phase-contrast imaging. The shakers at each end of the tube were either driven 0° in-phase or 180° out-of-phase resulting in maximized sound pressure or sound-induced particle motion in the tube center, respectively. In both species, saccular otolith motion was more pronounced when the swimbladder walls oscillated under 0° in-phase condition and the motion patterns mainly matched the respective orientation patterns of ciliary bundles on the sensory epithelia. In future, we will quantify the motion patterns of fish auditory structures by applying “sound tomography” to cover the three-dimensional aspect of the moving structures.

In-situ visualization of sound-induced otolith motion using hard X-ray phase contrast imaging

Regarding the basics of ear structure-function relationships in fish, the actual motion of the solid otolith relative to the underlying sensory epithelium has rarely been investigated. Otolith motion has been characterized based on a few experimental studies and on approaches using mathematical modeling, which have yielded partially conflicting results. Those studies either predicted a simple back-and-forth motion of the otolith or a shape-dependent, more complex motion. Our study was designed to develop and test a new set-up to generate experimental data on fish otolith motion in-situ. Investigating the basic parameters of otolith motion requires an approach with high spatial and temporal resolution. We therefore used hard X-ray phase contrast imaging (XPCI). We compared two anatomically well-studied cichlid species, Steatocranus tinanti and Etroplus maculatus, which, among other features, differ in the 3D shape of their otoliths. In a water-filled tank, we presented a pure tone of 200 Hz to 1) isolated otoliths embedded in agarose serving as a simple model or 2) to a fish (otoliths in-situ). Our new set-up successfully visualized the motion of otoliths in-situ and therefore paves the way for future studies evaluating the principles of otolith motion.

MHz frame rate hard X-ray phase-contrast imaging using synchrotron radiation.

Third generation synchrotron light sources offer high photon flux, partial spatial coherence, and ~10-10 s pulse widths. These enable hard X-ray phase-contrast imaging (XPCI) with single-bunch temporal resolutions. In this work, we exploited the MHz repetition rates of synchrotron X-ray pulses combined with indirect X-ray detection to demonstrate the potential of XPCI with millions of frames per second multiple-frame recording. This allows for the visualization of aperiodic or stochastic transient processes which are impossible to be realized using single-shot or stroboscopic XPCI. We present observations of various phenomena, such as crack tip propagation in glass, shock wave propagation in water and explosion during electric arc ignition, which evolve in the order of km/s (µm/ns).

PHASE-CONTRAST IMAGING BASED ON THREE-BLOCK FRESNEL ZONE PLATE INTERFEROMETERS USING HARD X-RAY LABORATORY SOURCES

The earlier represented three-block Fresnel Zone Plate interferometer based hard X-ray phase-contrast imaging setup designed for the initial plane wave radiation has been adapted to the laboratory sources of radiation. The limited distance between the source and the interferometer as well as the finite size of the source and the limited degree of monochromaticity of the illumination have been taken into account. It was shown that the decrease of the spatial coherency of the initial radiation does not worsen, and even improves the quality of the phase-contrast imaging, by suppressing the distortions caused by the X-ray diffraction at the edges of the Fresnel zone plates and the knifes of the interferometer.

Single and double grating-based X-ray microtomography using synchrotron radiation

Hard X-ray phase contrast imaging techniques have become most suitable for the non-destructive three-dimensional visualization of soft tissues at the microscopic level. Among the hard X-ray grating interferometry methods, a single-grating approach (XSGI) has been implemented by simplifying the established double-grating interferometer (XDGI). We quantitatively compare the XSGI and XDGI tomograms of a human nerve and demonstrate that both techniques provide sufficient contrast to allow for the distinction of tissue types. The two-fold binned data show spatial resolution of (5.2 ± 0.6) μm and (10.7 ± 0.6) μm, respectively, underlying the performance of XSGI in soft tissue imaging.

Dual phase grating interferometer for tunable dark-field sensitivity

Hard X-ray dark-field and phase contrast imaging using grating interferometry have shown great potential for medical and industrial applications. However, the wide spread applicability of the method is challenged by a number of technical related issues such as relatively low dose and flux efficiency due to the absorption grating, fabrication of high quality absorption gratings, slow data acquisition protocol and high mechanical stability requirements. In this paper, the authors propose an interferometric method for dark-field and differential phase contrast imaging based on phase shifting elements only with the purpose to improve the dose and flux efficiency and simplify the setup. The proposed interferometer consists of two identical phase gratings of small pitch (1.3 μm), which generate an interference fringe at the detector plane with a large enough pitch that can be resolved directly. In particular, the system exhibits flexible and tunable dark-field sensitivity which is advantageous to probe unresolvable micro-structure in the sample. Experiments on a micro focal tube validated the method and demonstrated the versatility and tunability of the system compared to conventional Talbot grating interferometer.

On the evolution and relative merits of hard X-ray phase-contrast imaging methods

This review provides a brief overview, albeit from a somewhat personal perspective, of the evolution and key features of various hard X-ray phase-contrast imaging (PCI) methods of current interest in connection with translation to a wide range of imaging applications. Although such methods have already found wide-ranging applications using synchrotron sources, application to dynamic studies in a laboratory/clinical context, for example for in vivo imaging, has been slow due to the current limitations in the brilliance of compact laboratory sources and the availability of suitable high-performance X-ray detectors. On the theoretical side, promising new PCI methods are evolving which can record both components of the phase gradient in a single exposure and which can accept a relatively large spectral bandpass. In order to help to identify the most promising paths forward, we make some suggestions as to how the various PCI methods might be compared for performance with a particular view to identifying those which are the most efficient, given the fact that source performance is currently a key limiting factor on the improved performance and applicability of PCI systems, especially in the context of dynamic sample studies. The rapid ongoing development of both suitable improved sources and detectors gives strong encouragement to the view that hard X-ray PCI methods are poised for improved performance and an even wider range of applications in the near future.

Quantitative hard x-ray phase contrast imaging of micropipes in SiC

Peculiarities of quantitative hard x-ray phase contrast imaging of micropipes in SiC are discussed. The micropipe is assumed as a hollow cylinder with an elliptical cross section. The major and minor diameters can be restored using the least square fitting procedure by comparing the experimental data, i.e. the profile across the micropipe axis, with those calculated based on phase contrast theory. It is shown that one projection image gives an information which does not allow a complete determination of the elliptical cross section, if an orientation of micropipe is not known. Another problem is a weak accuracy in estimating the diameters, partly because of using pink synchrotron radiation, which is necessary because a monochromatic beam intensity is not sufficient to reveal the weak contrast from a very small object. The general problems of accuracy in estimating the two diameters using the least square procedure are discussed. Two experimental examples are considered to demonstrate small as well as modest accuracies in estimating the diameters.

Recent advances in synchrotron-based hard x-ray phase contrast imaging

Ever since the first demonstration of phase contrast imaging (PCI) in the 1930s by Frits Zernike, people have realized the significant advantage of phase contrast over conventional absorption-based imaging in terms of sensitivity to ‘transparent’ features within specimens. Thus, x-ray phase contrast imaging (XPCI) holds great potential in studies of soft biological tissues, typically containing low Z elements such as C, H, O and N. Particularly when synchrotron hard x-rays are employed, the favourable brightness, energy tunability, monochromatic characteristics and penetration depth have dramatically enhanced the quality and variety of XPCI methods, which permit detection of the phase shift associated with 3D geometry of relatively large samples in a non-destructive manner. In this paper, we review recent advances in several synchrotron-based hard x-ray XPCI methods. Challenges and key factors in methodological development are discussed, and biological and medical applications are presented.

Subnanoradian X-ray phase-contrast imaging using a far-field interferometer of nanometric phase gratings

Hard X-ray phase-contrast imaging characterizes the electron density distribution in an object without the need for radiation absorption. The power of phase contrast to resolve subtle changes, such as those in soft tissue structures, lies in its ability to detect minute refractive bending of X-rays. Here we report a far-field, two-arm interferometer based on the new nanometric phase gratings, which can detect X-ray refraction with subnanoradian sensitivity, and at the same time overcomes the fundamental limitation of ultra-narrow bandwidths (Δλ/λ~10−4) of the current, most sensitive methods based on crystal interferometers. On a 1.5% bandwidth synchrotron source, we demonstrate clear visualization of blood vessels in unstained mouse organs in simple projection views, with over an order-of-magnitude higher phase contrast than current near-field grating interferometers.

Absolute zinc quantification at the sub-cellular level by combined use of hard X-ray fluorescence and phase contrast imaging techniques

Hard X-ray fluorescence microscopy and magnified phase contrast imaging are combined to obtain quantitative maps of the projected zinc mass fraction in whole cell of PC12 cell lines. The experiments were performed on freeze dried cells at the nano-imaging station ID22NI of the European Synchrotron Radiation Facility (ESRF). X-ray fluorescence analysis gives the areal mass of most major, minor and trace elements while quantitative phase contrast imaging provides maps of the projected mass. The combined method was validated on calibration samples by comparison with other alternative techniques such as Atomic Force Microscopy (AFM) and Scanning Transmission Ion Microscopy (STIM). Up to now, absolute quantification at the sub-cellular level was impossible using X-ray fluorescence microscopy but can be reached for the first time with the use of the proposed approach.

X-ray phase imaging with a laboratory source using selective reflection from a mirror

A novel approach for hard x-ray phase contrast imaging with a laboratory source is reported. The technique is based on total external reflection from the edge of a mirror, aligned to intercept only half of the incident beam. The mirror edge thus produces two beams. The refraction x-rays undergo when interacting with a sample placed before the mirror, causes relative intensity variations between direct and reflected beams. Quantitative phase contrast and pure absorption imaging are demonstrated using this method.

Hard x-ray phase contrast imaging using a tabletop Talbot–Lau interferometer with multiline embedded x-ray targets

We demonstrate hard x-ray phase contrast imaging (XPCI) using a tabletop Talbot-Lau interferometer in which the x-ray source and source grating are replaced with an x-ray source with multiline metal targets embedded in a diamond substrate. This source realizes an array of linear x-ray sources of a few micrometers width without fabrication difficulty because of the shallow penetration depth of electrons irradiated to the metal targets. This enhances the coherence of x rays from each linear source and allows XPCI within 45 cm source-detector distance under 1.2 W input power for 8 keV x rays.

Interferometric hard x-ray phase contrast imaging at 204 nm grating period

We report on hard x-ray phase contrast imaging experiments using a grating interferometer of approximately 1/10th the grating period achieved in previous studies. We designed the gratings as a staircase array of multilayer stacks which are fabricated in a single thin film deposition process. We performed the experiments at 19 keV x-ray energy and 0.8 μm pixel resolution. The small grating period resulted in clear separation of different diffraction orders and multiple images on the detector. A slitted beam was used to remove overlap of the images from the different diffraction orders. The phase contrast images showed detailed features as small as 10 μm, and demonstrated the feasibility of high resolution x-ray phase contrast imaging with nanometer scale gratings.

Fabrication of 200 nm period centimeter area hard x-ray absorption gratings by multilayer deposition

We describe the design and fabrication trials of x-ray absorption gratings of 200 nm period and up to 100:1 depth-to-period ratios for full-field hard x-ray imaging applications. Hard x-ray phase-contrast imaging relies on gratings of ultra-small periods and sufficient depth to achieve high sensitivity. Current grating designs utilize lithographic processes to produce periodic vertical structures, where grating periods below 2.0 µm are difficult due to the extreme aspect ratios of the structures. In our design, multiple bilayers of x-ray transparent and opaque materials are deposited on a staircase substrate, and mostly on the floor surfaces of the steps only. When illuminated by an x-ray beam horizontally, the multilayer stack on each step functions as a micro-grating whose grating period is the thickness of a bilayer. The array of micro-gratings over the length of the staircase works as a single grating over a large area when continuity conditions are met. Since the layers can be nanometers thick and many microns wide, this design allows sub-micron grating periods and sufficient grating depth to modulate hard x-rays. We present the details of the fabrication process and diffraction profiles and contact radiography images showing successful intensity modulation of a 25 keV x-ray beam.

Combined use of hard X-ray phase contrast imaging and X-ray fluorescence microscopy for sub-cellular metal quantification

Hard X-ray fluorescence microscopy and magnified phase contrast imaging are combined to obtain quantitative maps of the projected metal concentration in whole cells. The experiments were performed on freeze dried cells at the nano-imaging station ID22NI of the European Synchrotron Radiation Facility (ESRF). X-ray fluorescence analysis gives the areal mass of most major, minor and trace elements; it is validated using a biological standard of known composition. Quantitative phase contrast imaging provides maps of the projected mass and is validated using calibration samples and through comparison with Atomic Force Microscopy and Scanning Transmission Ion Microscopy. Up to now, absolute quantification at the sub-cellular level was impossible using X-ray fluorescence microscopy but can be reached with the use of the proposed approach.

X-Ray Phase Contrast Imaging of Freestanding Lipid Model Membranes

Membranes are considered as the most important interfaces in biology, and can be visualized under physiological conditions by optical techniques such as phase contrast and fluorescence light microscopy. While the contour lines and large lateral domains of biological membranes can be imaged, the density profile of the membrane and associated changes cannot be resolved by visible light. We report on hard x-ray phase contrast imaging of black lipid membranes (BLMs), which are freely suspended over a micro machined aperture in an aqueous solution. This new way of membrane structure analysis allows investigating bio molecular and organic sub-stances in aqueous environments by parallel and divergent beam propagation imaging, using partially coherent multi-keV x-ray radiation. The width of the thinning film is significantly smaller than the detector pixel size, but can be resolved from quantitative analysis of the intensity fringes in the Fresnel diffraction regime down to its native thickness of about 5nm. We have put forward a simplified but extendable model, which enables the theoretical description of image formation and charac-terization of membrane thickness and its decrease during the thinning process from a bulk to a bimolecular film. The structural changes can be obtained from both the loss of contrast and the asymmetry of the detected Fresnel fringes. On the basis of the recent experiments, future investigations will be performed to study the inte-ractions of membranes, as they are for example known from synaptic fusion, with high spatial resolution.