Using a high-efficiency grating interferometer for hard X rays (10-30 keV) and a phase-stepping technique, separate radiographs of the phase and absorption profiles of bulk samples can be obtained from a single set of measurements. Tomographic reconstruction yields quantitative three-dimensional maps of the X-ray refractive index, with a spatial resolution down to a few microns. The method is mechanically robust, requires little spatial coherence and monochromaticity, and can be scaled up to large fields of view, with a detector of correspondingly moderate spatial resolution. These are important prerequisites for use with laboratory X-ray sources.

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X-ray phase imaging with a grating interferometer.

The development of wave optics for light brought many new insights into our understanding of physics, driven by fundamental experiments like the ones by Young, Fizeau, Michelson-Morley and others. Quantum mechanics, and especially the de Broglie’s postulate relating the momentum p of a particle to the wave vector k of an matter wave: k = 2 λ = p/ℏ, suggested that wave optical experiments should be also possible with massive particles (see table 1), and over the last 40 years electron and neutron interferometers have demonstrated many fundamental aspects of quantum mechanics [1].

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Optics and interferometry with atoms and molecules

Phase-contrast x-ray imaging (PCI) is an innovative method that is sensitive to the refraction of the x-rays in matter. PCI is particularly adapted to visualize weakly absorbing details like those often encountered in biology and medicine. In past years, PCI has become one of the most used imaging methods in laboratory and preclinical studies: its unique characteristics allow high contrast 3D visualization of thick and complex samples even at high spatial resolution. Applications have covered a wide range of pathologies and organs, and are more and more often performed in vivo. Several techniques are now available to exploit and visualize the phase-contrast: propagation- and analyzer-based, crystal and grating interferometry and non-interferometric methods like the coded aperture. In this review, covering the last five years, we will give an overview of the main theoretical and experimental developments and of the important steps performed towards the clinical implementation of PCI.

http://iopscience.iop.org/article/10.1088/0031-9155/58/1/R1/pdf

X-ray phase-contrast imaging: from pre-clinical applications towards clinics

This self-contained, comprehensive book describes the fundamental properties of soft x-rays and extreme ultraviolet (EUV) radiation and discusses their applications in a wide variety of fields, including EUV lithography for semiconductor chip manufacture and soft x-ray biomicroscopy. The author begins by presenting the relevant basic principles such as radiation and scattering, wave propagation, diffraction, and coherence. He then goes on to examine a broad range of phenomena and applications. The topics covered include EUV lithography, biomicroscopy, spectromicroscopy, EUV astronomy, synchrotron radiation, and soft x-ray lasers. He also provides a great deal of useful reference material such as electron binding energies, characteristic emission lines and photo-absorption cross-sections. The book will be of great interest to graduate students and researchers in engineering, physics, chemistry, and the life sciences. It will also appeal to practicing engineers involved in semiconductor fabrication and materials science.

X-Rays and Extreme Ultraviolet Radiation: Principles and Applications

Since the middle of the 1990s, X-ray phase imaging including phase tomography has been attracting increasing attention. The advantage of X-ray phase imaging is that an extremely high sensitivity is achieved for weak-absorbing materials, such as biological soft tissues, which generate a poor contrast by conventional methods. Medical and biological imaging is the main target of X-ray phase imaging, and several trials using synchrotron radiation sources and laboratory sources have been made. Measuring and controlling the X-ray phase are also significant for X-ray microscopy with a high spatial resolution, and innovative techniques are attracting intense interest. The progress of X-ray phase imaging is supported by the developments in X-ray sources such as third-generation synchrotron radiation sources, optical elements, and image detectors. This article describes the advantages of using X-ray phase information and reviews various techniques studied for X-ray phase imaging.

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Recent Advances in X-ray Phase Imaging

First Talbot interferometry in the hard X-ray region was demonstrated using a pair of transmission gratings made by forming gold stripes on glass plates. By aligning the gratings on the optical axis of X-rays with a separation that caused the Talbot effect by the first grating, moire fringes were produced inclining one grating slightly against the other around the optical axis. A phase object placed in front of the first grating was detected by moire-fringe bending. Using the technique of phase-shifting interferometry, the differential phase corresponding to the phase object could also be measured. This result suggests that X-ray Talbot interferometry is a novel and simple method for phase-sensitive X-ray radiography.

https://iopscience.iop.org/article/10.1143/JJAP.42.L866/pdf

Demonstration of X-Ray Talbot Interferometry

Hard X-ray phase-contrast microscopy has been performed with phase plates of tantalum using an X-ray beam from an undulator in SPring-8. The photon energy was set at 10 keV near the L3 absorption edge of tantalum (9.9 keV) in order to increase the phase contrast. To demonstrate its capability, a transparent specimen was imaged clearly in the reverse contrast with phase plates to shift the phase by one-quarter and three-quarters of a period, while conventional absorption imaging showed little contrast. Further, an image contrast as high as approximately 40% can be obtained for the cell walls of another specimen.

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10 keV X-Ray Phase-Contrast Microscopy for Observing Transparent Specimens

The Hyogo beamline has been constructed as the first contract beamline at SPring-8. It has three experimental hutches and their corresponding three monochromators. Two upstream transparent monochromators enable us to perform three experiments for different purposes simultaneously. Employing a figure-8 undulator, horizontally or vertically polarized X-rays can be obtained in a wide energy range. Rocking curves of the monochromators were almost identical to theoretical ones.

Hyogo beamline at SPring-8: multiple station beamline with the TROIKA concept

Results of phase-contrast X-ray imaging are presented. The optical system employed consisted of a successive arrangement of horizontal and vertical (+, -) double crystals taking asymmetric Bragg reflection with an asymmetry factor of ~0.2. The original beam size was thus expanded in both directions and the field of view actually obtained was ~5×5 mm2. Boundary structures in samples were clearly observed with much higher contrast than those obtained in conventional absorption-contrast imaging. Since this method works in real time, it will provide a new X-ray imaging diagnosis technique for in situ observation over a large area of the samples.

Phase-contrast X-ray imaging using both vertically and horizontally expanded synchrotron radiation X-rays with asymmetric Bragg reflection

A hard X-ray transmission imaging microscope has been in use at the beamline BL24XU of SPring-8. It makes use of a phase zone plate made of tantalum as its X-ray lens, and is capable of imaging the structure as fine as 125-nm line-and-space pattern. The Zernike's phase-contrast method has been implemented to the microscope with phase plates made of gold. The photon energy was tuned to 12 keV just above the L3 absorption edge of gold (11.9 keV) in order to increase the image contrast. Polystyrene micro particles as transparent specimens were imaged clearly in the opposite image contrast with phase plates to shift the phase of the central order spectra in the back focal plane of the objective by one-quarter and three-quarters of a period while the absorption contrast image showed little image contrast. Performance of the newly developed phase zone plate has been tested and it was confirmed that the structure as fine as 60-nm line-and-space pattern was able to be imaged.

Kengo Takai

Papers

Demonstration of X-Ray Talbot Interferometry

10 keV X-Ray Phase-Contrast Microscopy for Observing Transparent Specimens

Hyogo beamline at SPring-8: multiple station beamline with the TROIKA concept

Phase-contrast X-ray imaging using both vertically and horizontally expanded synchrotron radiation X-rays with asymmetric Bragg reflection

Hard X-ray phase-contrast microscope for observing transparent specimens