We have launched a project to promote grating-based X-ray phase imaging/tomography extensively. Here, two main activities are presented for enabling dynamic, or four-dimensional, X-ray phase tomography and nanoscopic X-ray phase tomography by grating interferometry. For the former, while some demonstrations in this direction were performed with white synchrotron radiation, improvement in image quality by spectrum tuning is described. A preliminary result by a total reflection mirror is presented, and as a next step, preparation of a 10% bandpass filter by a multilayer mirror is reported. For the latter, X-ray microscopes available both at synchrotron radiation facilities and laboratories equipped with a Fresnel zone plate are combined with grating interferometry. Here, a preliminary result with a combination of a Lau interferometer and a laboratory-based X-ray microscope is presented.
Four-dimensional X-ray phase tomography has been implemented by a combination of X-ray Talbot interferometry and white synchrotron radiation. While the Fourier-transform method has been used for the measurement of a differential phase image at every projection direction, an improved scan mode based on the fringe-scanning method is demonstrated to improve spatial resolution. The disadvantage of the fringe-scanning method, which requires multiple moiré images, is overcome by proposing a scan mode synchronously combining one-way continuous movements of sample rotation and grating displacement. In addition, the operation of an X-ray Talbot-Lau interferometer with white synchrotron radiation is reported. While an X-ray Talbot interferometer requires a horizontal sample rotation axis because of the condition of spatial coherency, such a horizontal rotation axis is not preferable for tomographic scans especially for soft objects. An X-ray Talbot-Lau interferometer overcomes this problem, allowing a vertical sample rotation axis. Although we encountered a vibration problem with the X-ray Talbot-Lau interferometer probably because of incomplete stage stability, our attempts have basically been successful, and we expect that various samples can be scanned by four-dimensional X-ray phase tomography, revealing dynamical properties in weakly absorbing objects that cannot be accessed by conventional X-ray phase tomography mainly performed for static samples.
Laminography is a technique for 3D volume reconstruction, extending the classical tomography to the estimation
of local areas in lamellar objects. We demonstrate X-ray phase laminography by using an X-ray Talbot
interferometer consisting of two transmission gratings, which has been used only for X-ray phase tomography.
In this presentation, experiments using 17.7 keV synchrotron radiation through a double-crystal monochromator
are reported. The X-rays passed through the sample placed in front of the first phase grating. The rotation axis
of the sample was set almost parallel to the sample plane normal, and inclined from the X-ray beam. Behind the
second amplitude grating, moiré fringe patterns were measured by displacing one of the gratings in the direction
parallel to its diffraction vector. Differential phase information were extracted through the fringe-scanning
method. For the reconstruction of the three-dimensional volume from the differential phase information, the filtered
back projection method was used with a specific filtering function. Promising results of phase laminography
reconstruction are obtained for simulation data as well as weakly absorbing lamellar objects such as a polymer
meshes and other samples. This advancement extends experiments with X-ray Talbot volume reconstruction to
a larger variety of samples.
Taking advantage of the fact that an X-ray Talbot interferometer functions with X-rays of a broad energy bandwidth, high-speed X-ray phase tomography has been demonstrated by using white synchrotron light. Time resolution in addition to three-dimensional spatial resolution has been attained, and we report this achievement as the first four-dimensional (4D) X-ray phase tomography. Moire image movies of samples rotating at a speed of 1 or 2 rps generated by a Talbot interferometer were recorded at a frame rate of up to 1 kf/s, and differential phase image movies of the same frame rate were created by the Fourier-transform method. Consequently, a sub-second time resolution was achieved in the 4D phase tomography, while the spatial resolution was below 0.1 mm and 0.05 mm in axial and in-plane directions, respectively. An X-ray Talbot interferometer generates visibility images in addition to differential phase images, showing the distribution of microstructures, which cause ultra-small angle scattering but cannot be resolved individually with system spatial resolution. Tomographic image reconstruction from the visibility images was also demonstrated.
The sensitivity of X-ray phase tomography based on Talbot(-Lau) interferometry is discussed. A criterion is described to evaluate the superiority of the technique in comparison to the conventional absorption-contrast method. An experimental result of X-ray phase tomography with a Talbot interferometer is compared with the criterion. The advantage of X-ray phase tomography based on Talbot(-Lau) interferometry is more prominent when smaller structures are observed with smaller pixels.
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.