Phase-contrast X-ray imaging | EPFL Graph Search

Phase-contrast X-ray imaging or phase-sensitive X-ray imaging is a general term for different technical methods that use information concerning changes in the phase of an X-ray beam that passes through an object in order to create its images. Standard X-ray imaging techniques like radiography or computed tomography (CT) rely on a decrease of the X-ray beam's intensity (attenuation) when traversing the sample, which can be measured directly with the assistance of an X-ray detector. However, in phase contrast X-ray imaging, the beam's phase shift caused by the sample is not measured directly, but is transformed into variations in intensity, which then can be recorded by the detector. In addition to producing projection images, phase contrast X-ray imaging, like conventional transmission, can be combined with tomographic techniques to obtain the 3D distribution of the real part of the refractive index of the sample. When applied to samples that consist of atoms with low atomic number Z,

High resolution differential laser interferometry for the VLTI (Very Large Telescope Interferometer)

Olivier Boris Scherler

One method for locating extrasolar planets is to observe the lateral movement of a star in the sky caused by a planet in orbit around it. In order to detect this displacement, the angular position of the star has to be measured with high accuracy. This technique is called astrometry. The Very Large Telescope Interferometer (VLTI) is operated by the European Southern Observatory and located at the Paranal Observatory in Chile. The purpose of the PRIMA instrument (Phase Referenced Imaging and Micro-arcsecond Astrometry) of the VLTI is to perform high-resolution astrometric measurements and high-resolution imaging of faint stars using white light interferometry, by combining the light collected by two telescopes. In order to allow the detection of extrasolar planets, the astrometric measurement has to be performed with micro-arcsecond accuracy. In astrometric mode the PRIMA instrument observes two targets at the same time: the object of scientific interest, and a bright reference star. The angular position of the science object relative to the reference star is obtained by monitoring the differential optical path travelled by the light of each object in two separate white-light interferometers. The aim of this work was to develop a high-resolution laser metrology based on superheterodyne interferometry, with an accuracy of 5 nm over a differential optical path of 100 mm. Moreover the laser source had to be stabilised on an absolute frequency reference, in order to ensure the long-term stability and calibration required to achieve the target performance. Superheterodyne interferometry allowed the direct measurement of the differential optical path using two heterodyne interferometers working with two different frequency shifts. The differential phase measurement between the two interferometers was obtained by electronic mixing of the two heterodyne signals, leading to the differential optical path needed for the astrometric measurement

Institut de Microtechnique, Université de Neuchâtel2006

Visualization of water drying in porous materials by X-ray phase contrast imaging

Anne Bonnin

We present in this study results from X-ray tomographic microscopy with synchrotron radiation performed both in attenuation and phase contrast modes on a limestone sample during two stages of water drying. No contrast agent was used in order to increase the X-ray attenuation by water. We show that only by using the phase contrast mode it is possible to achieve enough water content change resolution to investigate the drying process at the pore-scale. We performed 3D image analysis of the time-differential phase contrast tomogram. We show by the results of such analysis that it is possible to obtain a reliable characterization of the spatial redistribution of water in the resolved pore system in agreement with what expected from the theory of drying in porous media and from measurements performed with other approaches. We thus show the potential of X-ray phase contrast imaging for pore-scale investigations of reactive water transport processes which cannot be imaged by adding a contrast agent for exploiting the standard attenuation contrast imaging mode. X-ray tomographic imaging is an ideal method for investigating at the pore scale and in 3D liquid transport processes in porous materials because it allows nowadays achieving very high temporal and spatial resolutions, especially when synchrotron radiation is used. In the last years, an increasing number of pore-scale studies have exploited such method to improve our understanding of such diverse phenomena as displacement of oil by water in rocks, transport of contaminants through soils, transport of liquids in engineering porous materials affecting their durability and lifetime. Since water weakly attenuates X-rays and the standard approach in X-ray imaging exploits X-ray photon photoelectric absorption, the degree of attenuation by the liquid is typically enhanced by “doping” it with tracer particles or by dissolving a salt, all made of high density or high atomic number elements. Such approach, based upon “contrast agents” has been exploited in biomedical Xray imaging since very long time and has been very successful. Contrast agents imply a lot of drawbacks when the liquid transport process is reactive, i.e., when the liquid chemically interacts with the host porous material. In many cases, e.g., water transport in cement-based materials or liquefied CO2 transport in rocks, it is necessary not to perturb the couplings between the chemical reactions and the liquid transport because they are key investigation targets for the process to be studied, e.g., understanding how water affects the microstructure evolution during cement hardening and the stability/potentiality of CO2 storage in a given type of geologic site. Thus one may not be able to use contrast agents, in order not to change the chemical reactions. We show in this work that X-ray tomographic microscopy based upon phase, instead of attenuation, contrast retains the high temporal/spatial resolution while achieving sufficient contrast to resolve distilled water content changes at the pore scale, without the need of any contrast agent. Our work is focused in this case on visualizing and characterizing, by 3D image analysis, the spatial temporal distribution patterns of water in a natural limestone subject to evaporative drying. However, the imaging and image analysis approaches proposed in this article have a broader impact for 3D imaging of reactive water (or other liquid) transport processes in both natural and engineered porous materials.

Wiley-Blackwell2015

Low-dose, simple, and fast grating-based X-ray phase-contrast imaging

Marco Stampanoni, Kai Zhang

Phase sensitive X-ray imaging methods can provide substantially increased contrast over conventional absorption-based imaging and therefore new and otherwise inaccessible information. The use of gratings as optical elements in hard X-ray phase imaging overcomes some of the problems that have impaired the wider use of phase contrast in X-ray radiography and tomography. So far, to separate the phase information from other contributions detected with a grating interferometer, a phase-stepping approach has been considered, which implies the acquisition of multiple radiographic projections. Here we present an innovative, highly sensitive X-ray tomographic phase-contrast imaging approach based on grating interferometry, which extracts the phase-contrast signal without the need of phase stepping. Compared to the existing phase-stepping approach, the main advantages of this new method dubbed "reverse projection" are not only the significantly reduced delivered dose, without the degradation of the image quality, but also the much higher efficiency. The new technique sets the prerequisites for future fast and low-dose phase-contrast imaging methods, fundamental for imaging biological specimens and in vivo studies.

National Academy of Sciences2010