Researchers led by Associate Director CHOI Wonshik of the Center for Molecular Spectroscopy and Dynamics within the Institute of Basic Sciences, Professor Kim Munseok of the Catholic University of Korea, and Professor CHOI Myunghwan of Seoul National University have developed a new type of holographic microscope. It is said that the new microscope can achieve “see through” the intact skull, and is capable of high-resolution 3D imaging of the neural network inside the brain of a living mouse without removing the skull.
For reference, the mouse skull has a thickness and transparency similar to that of a human nail.
In order to examine the internal features of an organism using light, it is necessary a) to deliver sufficient light energy to the sample and b) to accurately measure the signal reflected from the target tissue. However, in living tissues, multiple scattering effects and severe deflection1)They tend to occur when light hits cells, making it difficult to get sharp images.
In complex structures such as living tissues, light undergoes multiple scattering, causing photons to randomly change their direction many times as they travel through tissues. Because of this process, much of the image information carried by the light is destroyed. However, even if it is a very small amount of reflected light, it is possible to observe features that are relatively deep in the tissue by correcting the wavefront.2)Distortion of light reflected from the target to be observed. However, the polydispersion effects mentioned above interfere with this correction process. Therefore, in order to obtain a high-resolution image of the deep tissue, it is important to remove multiple scattered waves and increase the proportion of single scattered waves.
All the way in 2019, for the first time IBS researchers develop the time-resolved high-speed holographic microscope3)It can eliminate polyscattering and simultaneously measure the amplitude and phase of light. They used this microscope to observe the neural network of live fish without invasive surgery. However, in the case of a mouse with a skull thicker than that of a fish, it was not possible to obtain an image of the brain’s neural network without removing or thinning the skull, due to the severe distortion of light and the multiple scattering that occurs when light travels through the bone structure.
The research team was able to quantitatively analyze the interaction between light and matter, allowing them to improve upon their previous microscopy. In this recent study, they report the successful development of an ultra-deep, three-dimensional, time-resolved microscope that allows the observation of tissues at ever greater depth.
Specifically, the researchers have devised a way to preferentially select scattered waves by taking advantage of the fact that they have similar reflective waveforms even when light is introduced from different angles. This is done through a complex algorithm and numerical process that analyzes the intrinsic mode of the medium (a unique wave that delivers light energy to the medium), allowing to find a resonance mode that maximizes constructive interference (interference that occurs when waves of the same phase interfere) between the wavefronts of light. This new microscope has made it possible to focus more than 80 times more optical energy on nerve fibers than before, while selectively removing unnecessary signals. This allowed to increase the ratio of single scattered waves versus multiple scattered waves by several orders of magnitude.
The research team continued to demonstrate this new technology by monitoring the mouse brain. The microscope was able to correct wavefront distortion even at a depth that was previously impossible with current technology. The new microscope succeeded in obtaining a high-resolution image of the neural network of the mouse brain under the skull. All of this was achieved at the visible wavelength without removing the mouse skull and without the need for a fluorescent label.
Professor Kim Munseok and Dr. Joo Younghyun, who developed the basis for the holographic microscope, said, “When we first observed optical resonance in complex media, our work received a lot of attention from academia. From basic principles to practical application of observing the neural network beneath the mouse skull, we opened a new method for converging technology. for neuroimaging of the brain by combining the efforts of talented people in physics, life, and brain science.”
Associate Director CHOI Wonshik said: “Our center has long developed a super-deep bioimaging technology that applies physical principles. Our current findings are expected to contribute significantly to the advancement of multidisciplinary biomedical research including neuroscience and the microscale industry.”
This research was published in the online edition of Science Advances (IF 14.136) on July 28.
Glossary of terms:
1) Aberration is a phenomenon that occurs due to the difference in the speed of light depending on the refractive index of the medium. This means that when an image is formed, not all of the light rays gather in a single point, causing the image to become garbled and distorted.
2) Wavefront refers to the plane that is formed by connecting all points in the same phase of the wave. For example, the wavefront that is created when a stone is thrown into a lake is circular.
3) Time-Resolved Holographic Microscopy: A holographic microscope is a technology that detects the amplitude and phases of light using the light interference effect that occurs when two laser beams meet. In particular, a time-resolved holographic microscope can selectively acquire an optical signal at a given depth using a light source with a very short interference length of about 10 μm.