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We proposed and developed a novel transillumination laser CT imaging system, using optical fibers, based on the optical heterodyne detection method for biomedical use. The use of optical fibers enables portability and robustness against environmental changes such as varying temperature, air-flow shifts, and unexpected vibrations. In addition, motion-artifact-free images can be obtained with the present system as measurements can be performed with the object fixed. We experimentally investigate in detail the fundamental imaging properties of the system, that has a spatial resolution of 500 μm, a dynamic range of approximately 110 dB, and a minimum-detectable-optical power of 10-14 W as a result of the excellent properties of the heterodyne detection. Based on experimental observations, the proposed system can reconstruct tomographic images of highly scattering objects in the transillumination mode, similar to X-ray CT, at sub-millimeter spatial resolution and can derive quantitative information from the images. Finally, we experimentally demonstrate the first in-situ tomographic images of plants using the fiber-optic-based laser CT system.