Characteristic X-rays from a pure metal target bombarded with a few MeV proton micro-beams can be used as a point source of quasi-monochromatic X-rays. On the basis of this idea, we have previously developed a microscopic computed tomography (CT) which we called particle induced X-ray emission (PIXE)-micron-CT. We applied our CT to observe organs in a body of a living Drosophila, which is popularly used in various fundamental studies, such as gene research. We optimized the beam spot size of proton micro-beam, redesigned the geometry of the PIXE-micron-CT and improved the sample preparation method. As a result of these efforts, the CT scan time could be reduced to less than a sixth of its previous value without degrading the spatial resolution. Three-dimensional (3D) images of a living Drosophila in different growth stages were obtained. The forms of organs changed significantly depending on the growth stage. The CT images of living Drosophila were much different from those of dead or formalin fixed ones. The in vivo 3D imaging by PIXE-micron-CT is very useful and is one of the powerful tool in biological and medical studies.
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