X-ray computer tomography (CT) is fast becoming an accepted tool within the materials science community for the acquisition of 3D images. Here the authors review the current state of the art as CT transforms from a qualitative diagnostic tool to a quantitative one. Our review considers first the image acquisition process, including the use of iterative reconstruction strategies suited to specific segmentation tasks and emerging methods that provide more insight (e.g. fast and high resolution imaging, crystallite (grain) imaging) than conventional attenuation based tomography. Methods and shortcomings of CT are examined for the quantification of 3D volumetric data to extract key topological parameters such as phase fractions, phase contiguity, and damage levels as well as density variations. As a non-destructive technique, CT is an ideal means of following structural development over time via time lapse sequences of 3D images (sometimes called 3D movies or 4D imaging). This includes information nee...

https://www.tandfonline.com/doi/pdf/10.1179/1743280413Y.0000000023

Quantitative X-ray tomography

Regolith particles on the asteroid Itokawa were recovered by the Hayabusa mission. Their three-dimensional (3D) structure and other properties, revealed by x-ray microtomography, provide information on regolith formation. Modal abundances of minerals, bulk density (3.4 grams per cubic centimeter), and the 3D textures indicate that the particles represent a mixture of equilibrated and less-equilibrated LL chondrite materials. Evidence for melting was not seen on any of the particles. Some particles have rounded edges. Overall, the particles’ size and shape are different from those seen in particles from the lunar regolith. These features suggest that meteoroid impacts on the asteroid surface primarily form much of the regolith particle, and that seismic-induced grain motion in the smooth terrain abrades them over time.

https://science.sciencemag.org/content/sci/333/6046/1125.full.pdf

Three-Dimensional Structure of Hayabusa Samples: Origin and Evolution of Itokawa Regolith

The melting temperature of Earth’s mantle provides key constraints on the thermal structures of both the mantle and the core Through high-pressure experiments and three-dimensional x-ray microtomographic imaging, we showed that the solidus temperature of a primitive (pyrolitic) mantle is as low as 3570 ± 200 kelvin at pressures expected near the boundary between the mantle and the outer core Because the lowermost mantle is not globally molten, this provides an upper bound of the temperature at the core-mantle boundary (TCMB) Such remarkably low TCMB implies that the post-perovskite phase is present in wide areas of the lowermost mantle The low TCMB also requires that the melting temperature of the outer core is depressed largely by impurities such as hydrogen

http://science.sciencemag.org/content/sci/343/6170/522.full.pdf

Low core-mantle boundary temperature inferred from the solidus of pyrolite.

Conventional x-ray transmission tomography provides the spatial distribution of the absorption coefficient inside a sample. Other tomographic techniques, based on the detection of photons coming from fluorescent emission, Compton and Rayleigh scattering, are used for obtaining information on the internal elemental composition of the sample. However, the reconstruction problem for these techniques is generally much more difficult than that of transmission tomography, mainly due to self-absorption effects in the sample. In this article an approach to the reconstruction problem is presented, which integrates the information from the three types of signals. This method provides the quantitative spatial distribution of all elements that emit detectable fluorescent lines (Z15 in usual experimental conditions), even when the absorption effects are strong, and the spatial distribution of the global density of the lighter elements. The use of this technique is demonstrated on the reconstruction of a grain of the martian meteorite NWA817, mainly composed of low Z elements not measured in fluorescence and for which this method provides a unique insight. The measurement was done at the ID22 beamline of the European Synchrotron Radiation Facility.

Internal elemental microanalysis combining x-ray fluorescence, Compton and transmission tomography

Solid-state radiation detectors, using crystalline semiconductors to convert radiation photons to electrical charges, outperform other technologies with high detectivity and sensitivity. Here, we demonstrate a thin-film x-ray detector comprised with highly crystalline two-dimensional Ruddlesden-Popper phase layered perovskites fabricated in a fully depleted p-i-n architecture. It shows high diode resistivity of 1012 ohm·cm in reverse-bias regime leading to a high x-ray detecting sensitivity up to 0.276 C Gyair−1 cm−3. Such high signal is collected by the built-in potential underpinning operation of primary photocurrent device with robust operation. The detectors generate substantial x-ray photon–induced open-circuit voltages that offer an alternative detecting mechanism. Our findings suggest a new generation of x-ray detectors based on low-cost layered perovskite thin films for future x-ray imaging technologies.

A sensitive and robust thin-film x-ray detector using 2D layered perovskite diodes.

Zernike phase contrast x-ray microscope has been developed at the undulator beamline 20XU and 47XU of SPring-8. The system consists of a pseudo-Kohler-illuminating system, a Fresnel zone plate objective with outermost zone width of 100 nm, a Zernike phase plate (0.96-μm-thick tantalum, λ/4 or 3λ/4 phase-shifter at 8 keV) installed at the back-focal plane of the objective, and a visible-light conversion type cooled CCD camera as an image detector. A sectored (polygon) condenser plate is employed as the condenser in order to secure a large and flat field of view. Details and experimental results of the system will be shown.

https://iopscience.iop.org/article/10.1088/1742-6596/186/1/012020/pdf

Zernike phase-contrast x-ray microscope with pseudo-Kohler illumination generated by sectored (polygon) condenser plate

An x‐ray fast micro‐tomography system was developed at medical and imaging beamline BL20B2 in SPring‐8. We attempted a “continuous rotation method” whereby the sample keeps rotating in the beam during the measurement. The image acquisitions are synchronized with the trigger pulses branched from the stepper motor controller. An electron multiplier (EM)‐CCD camera (C9100‐02, 1000×1000 pixels, 30 fps, 8 μm/pixel, Hamamatsu Photonics) and beam‐monitor AA40P (Hamamatsu Photonics) were used as an image detector. The effective pixel size of the detector was 4.9 μm/pixel and the spatial resolution was about 15 μm. The total measurement time for 900 projections has been reduced to 36 s. The reconstruction time was also reduced using GPGPU and SSD system. Using these techniques, preliminary experiment for 4‐D (time resolved 3‐D) micro‐imaging has also been performed.

Development of fast (sub‐minute) micro‐tomography

In SPring-8, the number of micro-tomography experiments is increasing. We adopted a continuous acquisition method and high speed CCD camera for increasing a throughput. Those techniques reduced the measurement time and increased the throughput. The total measurement time for 1800 projections with 2000×1311 pixels projection images was reduced to 20 minutes or less while that of previous system using "step-and-shoot" method was a few hours. These enable us to image soft materials (tissue) and to do time resolved measurements in three-dimension. The spatial resolution of those systems is around 1 μm and the field of view 1 mm in horizontal direction. Using those systems, more than 30 measurements can be done during one day. It also means that the hundreds gigabytes of data per day is obtained.

http://iopscience.iop.org/article/10.1088/1742-6596/186/1/012050/pdf

High-definition high-throughput micro-tomography at SPring-8

A high efficiency X-ray image detector was introduced in an x-ray imaging micro-tomography system. The detector is a visible light conversion type which consists of phosphor screen, straight-fiber (1:1) optics and scientific CMOS device. The obtained CT images show high reproducibility of X-ray linear absorption coefficient of test material (Cu/Al concentric pattern). The x-ray intensity could be reduced about 10 % of previous system with lens-coupled image detector system.

https://iopscience.iop.org/article/10.1088/1742-6596/849/1/012051/pdf

Introducing high efficiency image detector to X-ray imaging tomography

We have developed an analyser-based phase contrast X-ray imaging technique to measure the mean length scale of pores or particles that cannot be resolved directly by the system. By combining attenuation, phase and ultra-small angle X-ray scattering information, the technique was capable of measuring differences in airway dimension between lungs of healthy mice and those with mild and severe emphysema. Our measurements of airway dimensions from 2D images showed a 1:1 relationship to the actual airway dimensions measured using micro-CT. Using 80 images, the sensitivity and specificity were measured to be 0.80 and 0.89, respectively, with the area under the ROC curve close to ideal at 0.96. Reducing the number of images to 11 slightly decreased the sensitivity to 0.75 and the ROC curve area to 0.90, whilst the specificity remained high at 0.89.

Kentaro Uesugi

Papers

Zernike phase-contrast x-ray microscope with pseudo-Kohler illumination generated by sectored (polygon) condenser plate

Development of fast (sub‐minute) micro‐tomography

High-definition high-throughput micro-tomography at SPring-8

Introducing high efficiency image detector to X-ray imaging tomography

Emphysema quantified: mapping regional airway dimensions using 2D phase contrast X-ray imaging.