Dept of Applied Biochemistry, School of Engineering, Tokai University
Our research in collaboration with SPring-8/JASRI and Advanced Photon Source (APS), Argonne National Laboratory involves a number of projects. We have performed X-ray microtomographic (micro-CT) and nanotomographic (nano-CT) studies of biological soft tissues, including human, mouse, and fruit fly brains. Cellular and subcellular structures, such as neuronal networks, organelle, and dendritic spines have been visualized at micrometer to nanometer resolution using synchrotron radiation X-ray sources. More details are at https://mizutanilab.github.io/
Neuronal circuits, which are essential for brain functions, are built up by neurons as a three-dimensional network, so tracing the three-dimensional neuronal network of human cerebral cortex is the first step to understanding the mechanism of human brain functions. In order to analyze the network of human cerebral cortex, the cortical tissue microstructure was visualized by X-ray microtomographic (micro-CT) imaging. Skeletonized wire models were built by tracing the three-dimensional distribution of X-ray attenuation coefficients. In this process, the 3D coefficient distribution was converted into 3D Cartesian coordinates by model building. The 3D coordinates are easier to handle than the 3D image, making it possible to analyze human brain circuits. The analysis has revealed that a local feedback circuit is one of the typical units composing our brain network.
The transmissive and less refractile nature of X-rays with respect to biological tissue enables three-dimensional analysis without any clearing procedure such as those required for light microscopy. Therefore, X-ray microtomography is a potential method of visualizing the neuronal circuits of brain, like X-ray crystallography in molecular biology. DOI PubMed Pdf YouTube
We have also reported a three-dimensional analysis of the brain network of the fruit fly Drosophila melanogaster by synchrotron-radiation tomographic microscopy. A skeletonized wire model of the left half of the brain network was built by tracing the X-ray attenuation coefficients. The obtained models of neuronal processes were classified into groups on the basis of their three-dimensional structures. The model structure indicated that the Drosophila brain is composed of networks with different complexity and extensity depending on the brain regions. Simple networks should be appropriate for relaying information straightforwardly while intricate and widespread networks can integrate information from a number of brain regions. These structures of the reconstructed networks provide a basis for understanding how the Drosophila brain works. The challenge is to establish a methodology for reconstructing the brain network in a higher-resolution image, leading to a comprehensive understanding of the brain structure. An article reviewing this study appeared in MIT Technology Review. DOI PubMed arXiv YouTube
Spatial resolution is a fundamental parameter in structural sciences. We developed a method for estimating the spatial resolution of real images from a logarithmic intensity plot in the Fourier domain. The logarithmic intensity plots of test images indicated that the full width at half maximum of a Gaussian point spread function can be estimated from the images themselves. The spatial resolution of imaging X-ray nanotomography using Fresnel zone-plate optics was estimated with this method. A cross section of a test object visualized with the nano-CT indicated that square-wave patterns up to 120-nm pitch were resolved. The logarithmic intensity plot was calculated from a nano-CT cross section of brain tissue. The full width at half maximum of the point spread function estimated from the plot coincided with the resolution determined from the test object. These results indicated that the logarithmic intensity plot in the Fourier domain provides an alternative measure of the spatial resolution without explicitly defining a noise criterion. DOI PubMed arXiv
Recently, our interests have been focused on (1) three-dimensional structural analysis of biological soft tissues, including human, mouse, and fruit fly brains, and (2) evaluation of the three-dimensional spatial resolution of real images.