Monochromatic x-ray computed tomography (CT) at two different energies provides information about electron density of human tissue without ambiguity due to the beam hardening effect. This information makes the treatment planning for proton and heavy-ion radiotherapy more precise. We have started a feasibility study on dual energy x-ray CT by using synchrotron radiation. A translation-rotation scanning CT system was developed for quantitative measurement in order to clarify what precision in the measurement was achieved. Liquid samples of solutions of K2HPO4 and solid samples of tissue equivalent materials were used to simulate human tissue. The experiments were carried out using monochromatic x-rays with energies of 40, 70 and 80 keV produced by monochromatizing synchrotron radiation. The solid samples were also measured in a complementary method using high-energy carbon beams to evaluate the electron densities. The measured electron densities were compared with the theoretical values or the values measured in the complementary method. It was found that these values were in agreement in 0.9% on average. Effective atomic numbers were obtained as well from dual-energy x-ray CT. The tomographic image based on each of the electron densities and the effective atomic number presents a different feature of the material, and its contrast drastically differs from that in a conventional CT image.

In this paper, several basic issues are discussed regarding tendon redundancy and joint stiffness adjustability for a robotic mechanism driven with redundant tendons. After briefly discussing tendon- controllability, the authors define local maximum sets of tendons that are redundant simultaneously (local maximum redundant tendon sets) and obtain an irreducible description of a redundant tendon in terms of the others. The authors obtain conditions under which the joint stiffness is adjustable using the tendons' nonlinear elasticity.

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The use of natural gas hydrate (NGH) as a natural gas transportation and storage medium is expected to meet further energy demand in the near future. To exploit NGH for industrial porposes, it is necessary to establish NGH storage systems using their self-preservation properties. In this report, NGH pellets containing CH 4 , C 2 H 6 , and C 3 H 8 were produced by continuous production using a bench-scale unit and successfully stored for 3 months at 253 K under atmospheric pressure. Qualitative and quantitative analyses [phase-contrast X-ray computed tomography (CT) by means of diffraction-enhanced imaging, cryogenic scanning electron microscopy, powder X-ray diffraction, and gas chromatography] were conducted to investigate the progression of dissociation. Some of the NGH pellets were also examined for their thermal history dependence between approximately 85 and 253 K after storage for 1 day or less to assess their stability under a rapid temperature change. The internal texture of the original NGH pellets was dense with hydrate even after 3 months of storage. On the other hand, thermal cracks covered with ice had formed in the hydrate pellet after the temperature rise (from 85 to 253 K). These cracks were assumed to be dissociation sites, but the dissociation was stopped only near the cracks. The mass fractions of NGH were larger than 70% after 3 months of storage and undergoing thermal history. These results demonstrate the excellent characteristics of NGH pellets produced by continuous production, improving their suitability for use as natural gas storage media.