Wataru Yashiro

Phase tomography by X-ray talbot interferometry for biological imaging

The X-ray phase tomography of biological samples is reported, which is based on X-ray Talbot interferometry. Its imaging principle is described in detail, and imaging results obtained for a cancerous rabbit liver and a mouse tail with synchrotron radiation are presented. Because an amplitude grating is needed to construct an X-ray Talbot interferometer, a high-aspect-ratio grating pattern was fabricated by X-ray lithography and gold electroplating. X-ray Talbot interferometry has an advantage that it functions with polychromatic cone-beam X-rays. Finally, the compatibility with a compact X-ray source is discussed.

On the origin of visibility contrast in x-ray Talbot interferometry

The reduction in visibility in x-ray grating interferometry based on the Talbot effect is formulated by the autocorrelation function of spatial fluctuations of a wavefront due to unresolved micron-size structures in samples. The experimental results for microspheres and melamine sponge were successfully explained by this formula with three parameters characterizing the wavefront fluctuations: variance, correlation length, and the Hurst exponent. The ultra-small-angle x-ray scattering of these samples was measured, and the scattering profiles were consistent with the formulation. Furthermore, we discuss the relation between the three parameters and the features of the micron-sized structures. The visibility-reduction contrast observed by x-ray grating interferometry can thus be understood in relation to the structural parameters of the microstructures.

High-speed X-ray phase imaging and X-ray phase tomography with Talbot interferometer and white synchrotron radiation

X-ray Talbot interferometry, which uses two transmission gratings, has the advantage that broad energy bandwidth x-rays can be used. We demonstrate the use of white synchrotron radiation for high-speed X-ray phase imaging and tomography in combination with an X-ray Talbot interferometer. The moiré fringe visibility over 20% was attained, enabling quantitative phase measurement. X-ray phase images with a frame rate of 500 f/s and an X-ray phase tomogram with a scan time of 0.5 s were obtained successfully. This result suggests a breakthrough for time-resolved three-dimensional observation of objects that weakly absorb X-rays, such as soft material and biological objects.

Four-dimensional X-ray phase tomography with Talbot interferometry and white synchrotron radiation: dynamic observation of a living worm

X-ray Talbot interferometry is attractive as a method for X-ray phase imaging and phase tomography for objects that weakly absorb X-rays. Because X-ray Talbot interferometry has the advantage that X-rays of a broad energy bandwidth can be used, high-speed X-ray phase imaging is possible with white synchrotron radiation. In this paper, we demonstrate time-resolved three-dimensional observation with X-ray Talbot interferometry (namely, four-dimensional X-ray phase tomography). Differential phase images, from which a phase tomogram was reconstructed, were obtained through the Fourier-transform method, unlike the phase-stepping method that requires several (at least three) moiré images to be measured sequentially in order to generate one differential phase image. We demonstrate dynamic observation of a living worm in three dimensions with a time resolution of 0.5 s, visualizing a drastic change in the respiratory tract.

Efficiency of capturing a phase image using cone-beam x-ray Talbot interferometry

We assesses the efficiency of x-ray Talbot interferometry (XTI), a technique based on the Talbot effect for measuring a wavefront gradient, in terms of how quickly it can capture a high-quality phase image with a large signal-to-noise ratio for a given incident photon number. Photon statistics cause errors in the phase of the moiré fringes and impose a detection limit on the wavefront gradient. The relation between the incident photon number and the detection limit is determined, and a figure of merit of XTI for a monochromatic cone beam is then defined. The dependence of the figure of merit on optical system parameters, such as grating pitch and position, is then discussed. The effects of varying the pattern height and linewidth of the second grating are shown for rectangular and trapezoidal teeth. Finally, we show how to design a practical cone-beam Talbot interferometer for certain boundary conditions.

X-Ray Phase Imaging with Single Phase Grating

X-ray phase imaging with a single phase grating based on the fractional Talbot effect is described. A phase grating with an 8 µm pitch was placed behind a weakly absorbing object and illuminated with partially coherent 17.7 keV X-rays. Intensity patterns downstream of the grating were recorded with a high-resolution image detector. By the fringe scanning method, an X-ray wavefront inclination by the object was obtained. Phase tomography was performed and the three-dimensional structure of a piece of a polymer blend was revealed with an 8 µm spatial resolution and a 9 mg/cm3 detection limit of density deviation.

Structural study of Si(1 1 1)√21 × √21-(Ag + Au) surface by X-ray diffraction

Abstract The in-plane structure of Si(1xa01xa01) 21 × 21 -(Ag+Au) surface was studied by using grazing incidence X-ray diffraction method. A total of 72 inequivalent fractional order reflections were obtained. The structural models are investigated including the previously proposed models. It is found that the Ag triangles and Si trimers are induced to displace considerably from the honeycomb chained triangle arrangement by the adsorption of Au atoms. From the analysis, the models with five Au atoms in the unit cell, which have the Au–Au interatomic distance peculiar to the 21 × 21 periodicity, are favored.

Iterative reconstruction in x-ray computed laminography from differential phase measurements

Phase-contrast X-ray computed laminography is demonstrated for the volume reconstruction of extended flat objects, not suitable to the usual tomographic scan. Using a Talbot interferometer, differential phase measurements are obtained and used to reconstruct the real part of the complex refractive index. The specific geometry of laminography leads to unsampled frequencies in a double cone in the reciprocal space, which degrades the spatial resolution in the direction normal to the object plane. First, the filtered backprojection formula from differential measurements is derived. Then, reconstruction is improved by the use of prior information of compact support and limited range, included in an iterative filtered backprojection algorithm. An implementation on GPU hardware was required to handle the reconstruction of volumes within a reasonable time. A synchrotron radiation experiment on polymer meshes is reported and results of the iterative reconstruction are compared with the simpler filtered backprojection.

Biomedical imaging by Talbot-type x-ray phase tomography

An X-ray Talbot interferometer for X-ray phase imaging and tomography was constructed using an amplitude grating of a gold pattern 8 μm in pitch and 30 μm in height developed by X-ray lithography and gold electroplating. The effective area of the grating was 20 mm x 20 mm, and was fully illuminated by synchrotron radiation at beamline 20XU, SPring-8, Japan. Almost whole body of a fish 3 cm in length was observed by phase tomography. Resulting images obtained with 0.07 nm and 0.045 nm X-rays revealed organs with bones in the same view successfully. A preliminary result of the combination with an X-ray imaging microscope is also presented, which was attempted to attain a higher spatial resolution. Finally, prospects of the compatibility with a conventional X-ray generator are discussed for practical applications such as clinical diagnoses.

Interface reconstructed structure of Ag/Si(1 1 1) revealed by X-ray diffraction

We studied a buried interface reconstructed structure of the Ag/Si(111)√3 x √3-Ag samples using grazing incidence X-ray diffraction with synchrotron radiation. We found that the √3 interface superstructure can be explained by an inequivalent-triangle (IET) model, which has been observed on the Si(111)√3 x √3-Ag surface at low substrate temperatures by STM. The calculated structure factors of the IET model were found to be very close to our observed ones. The reliability factor (R-factor) using the IET model was about 25%. The R-factor was improved to be much less value, 12% by considering defects of Ag atoms forming the √3 structure. The Patterson map expected from the IET model having the defects was very similar to that calculated from the observed structure factors.