Korea University: Optical microscope capable of high-resolution 3D imaging at ultra-high speeds developed

The research team led by prof. Choi Won-shik (vice-director of the Center for Molecular Spectroscopy and Dynamics at the Institute for Basic Science) developed an optical microscope technology that significantly reduces the number of measurements needed for ultra-high speed imaging, and succeeded in reconstructing high-resolution 3D images of the thin neural network structure in a mouse brain.
In general, high-resolution imaging of deep layers in biological tissues has proven challenging due to the noise induced by the strong scattering and extremely complex optical aberrations. Although recently many studies have attempted to address such issues, extensive measurements have been needed to obtain high-resolution images, and the process has also been highly time-consuming. Consequently, such approaches were considered impractical for 3D imaging, which involves imaging at various depths.

To determine the optical characteristics of a linear medium, such as biological tissue, electric field measurements are obtained by exposing the medium to light at various points or angles. The matrix that stores the relative positions or angles of incident and scattered light is known as the reflection or transmission matrix depending on the system structure, and measuring this matrix gives the most information on interactions between light and the medium. The Center for Molecular Spectroscopy and Dynamics applied a novel algorithm to the reflection matrix, and obtained high-resolution images of biological tissues by reducing scattering noise and correcting for optical aberrations. The process of measuring the reflection matrix is conventionally relatively time-consuming as measurements have to be obtained for multiple images using each incident wave corresponding to the diffraction limit,. This has made the approach less practical for biodynamics and 3D imaging.

The microscopic imaging technology developed by the research team significantly improves the speed of reflection matrix measurement. A reflection matrix was obtained through sparse sampling with random patterns instead of point or parallel beam illumination, and the complex optical aberrations were corrected through a time-reversal matrix. As a result, the team succeeded in obtaining high-resolution images using only 2% of the number of measurement images used in conventional imaging.

Based on the above, the team achieved ultra-high-speed visualization of myelinated axons in a mouse brain. A diffuser was used to produce random speckle illumination patterns, and scattering images were retrieved by shifting the brain tissues for each depth section. While the conventional reflection matrix imaging method takes several hours to measure images for the entire volume (128×128×125 μm3), the team’s technique significantly reduced the number of measurement images, taking only 3.58 seconds for the same volume. Moreover, the resulting high-resolution images had a lateral resolution of 0.45μm and an axial resolution of 2μm.

Prof. Yoon Seok-chan and IBS researcher Lee Ho-jun said, “Using random pattern illuminations and a time-reversal matrix, we managed to obtain high-resolution images while significantly reducing the number of measurements. We expect to see more related developments in ultra-high speed 3D imaging and neuroscience research.” Prof. Choi Won-shik, the corresponding author of this study, said, “We plan to further develop reflection matrix imaging methods to broaden the scope of the applications in healthcare and medicine.”

The team is preparing to miniaturize microscopes for real-world medical settings, and to apply their technique to the real-time early diagnosis of illnesses.

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