Researchers led by Associate Director CHOI Wonshik from the Center for Molecular Spectroscopy and Dynamics within the Institute for Basic Science, Professor KIM Moonseok from the Catholic University of Korea, and Professor CHOI Myunghwan from Seoul National University developed a new type of holographic microscope. The new microscope is said to be able to “see through” the intact skull and is able to create high-resolution 3D images of the neural network within a living mouse brain without removing the skull.
For reference, the mouse skull is similar in thickness and opacity to a human fingernail.
In order to examine the internal features of a living organism with light, it is necessary to A) apply sufficient light energy to the sample and B) accurately measure the signal reflected from the target tissue. However, several scattering effects and strong aberrations occur in living tissues1)often occur when light hits the cells, making it difficult to obtain sharp images.
In complex structures such as living tissue, light is scattered multiple times, causing the photons to randomly change direction several times as they travel through the tissue. As a result of this process, much of the image information carried by the light is destroyed. But even if there is a very small amount of reflected light, it is possible to observe the features that are relatively deep within the tissue by correcting the wavefront2)Distortion of the light reflected from the target being observed. However, the multiple scattering effects mentioned above interfere with this correction process. Therefore, in order to obtain a high-resolution deep-tissue image, it is important to remove the multiply scattered waves and increase the ratio of the singly scattered waves.
As early as 2019, the IBS researchers developed the time-resolved holographic high-speed microscope for the first time3)Eliminate multiple scattering while measuring the amplitude and phase of the light. They used this microscope to observe the neural network of live fish without a sectioning operation. However, in the case of a mouse, which has a skull thicker than that of a fish, it was not possible to obtain a neural network image of the brain without removing or thinning the skull because of severe light distortion and multiple scattering as the light travels through the bone structure.
The research team managed to quantitatively analyze the interaction between light and matter, which allowed them to further improve their previous microscope. In this recent study, they reported the successful development of a super-deep, three-dimensional, time-resolved holographic microscope that enables observation of tissues at a greater depth than ever before.
In particular, the researchers developed a method for preferentially selecting simply scattered waves by taking advantage of the fact that they have similar reflection waveforms even when light is incident from different angles. This is done through a complex algorithm and numerical operation that analyzes a medium’s eigenmode (a unique wave that delivers light energy into a medium), which allows finding a resonance mode that maximizes constructive interference (interference that occurs when waves of equal phase overlap) between light wavefronts. This allowed the new microscope to focus more than 80 times the light energy onto nerve fibers than before, while selectively removing unnecessary signals. As a result, the ratio of simply scattered waves to multiply scattered waves could be increased by several orders of magnitude.
The research team began demonstrating this new technology by observing the mouse brain. The microscope was able to correct wavefront distortion even to a depth not previously possible with existing technology. The new microscope managed to obtain a high-resolution image of the neural network of the mouse brain beneath the skull. This was all accomplished in the visible wavelength range without removing the mouse skull and without the need for fluorescent labeling.
Professor Kim Moonseok and Dr. JO Yonghyeon, who developed the fundamentals of the holographic microscope, said: “When we first observed the optical resonance of complex media, our work attracted great attention in academia. From the basic principles to the practical application of observing the neural network under the mouse skull, we have opened a new path for the convergent technology of brain neuroimaging by combining the efforts of talented people in physics, life sciences and brain science.”
Deputy director CHOI Wonshik said, “For a long time, our center has been developing super-depth bioimaging technology that applies physical principles. Our current finding is expected to make a great contribution to the development of biomedical interdisciplinary research including neuroscience and the precision metrology industry.”
This research was published in the online edition of the journal Science Advances (IF 14.136) on July 28.
Glossary:
1) Aberration is a phenomenon that occurs due to the variation in the speed of light depending on the refractive index of the medium. That is, when the image is formed, not all light rays gather at one point, causing the image to be blurred and distorted.
2) Wavefront refers to a plane formed by connecting all points of the same phase of the wave. For example, the wavefront created when you throw a stone into a lake is circular.
3) Time-Resolved Holographic Microscopy: Holographic microscopy is a technology that captures the amplitude and phase of light using the light interference effect that occurs when two laser beams meet. In particular, a time-resolved holographic microscope can selectively detect an optical signal at a certain depth by using a light source with a very short interference length of about 10 µm.
