A new holographic microscope allows scientists to see through the skull and image the brain



A new kind of holographic microscope was created by researchers under the direction of Associate Director CHOI Wonshik of the Center for Molecular Spectroscopy and Dynamics within the Institute for Basic Science, Professor KIM Moonseok of The Catholic University of Korea, and Professor CHOI Myunghwan of Seoul National University. The new microscope reportedly has the ability to "see through" the intact skull and can image the neuronal network in a living mouse brain in high-resolution 3D without removing the skull. *

A substantial amount of light energy must be delivered to the sample in order to examine its interior structures using light, and the signal reflected from the target tissue must be precisely measured. It is challenging to produce clear images in live tissues due to the numerous scattering effects and severe aberration1 that frequently happen when light strikes the cells.

Light experiences repeated scattering in intricate structures like living tissue, which causes photons to erratically shift their direction several times as they pass through the tissue. A large portion of the image data carried by the light is damaged as a result of this process. However, by correcting the wavefront2 distortion of the light that was reflected from the object to be seen, it is feasible to see the features that are relatively deep inside the tissues, even if there is only a very little amount of reflected light. However, this correcting process is hampered by the various scattering effects stated above. As a result, it's crucial to reduce the ratio of the multiple-scattered waves and raise it in order to acquire a high-resolution deep-tissue image.

The high-speed time-resolved holographic microscope3, which can remove multiple scattering and concurrently monitor the amplitude and phase of light, was created by IBS researchers for the first time back in 2019. Without doing any incisional surgery, they used this microscope to view the live fish's brain network. However, because of extreme light distortion and numerous scattering that happen when light passes through the bone structure in a mouse, whose skull is thicker than that of a fish, it was not able to generate a neural network image of the brain without removing or thinning the skull.

The study team was able to quantitatively analyze how light and matter interact, which gave them the opportunity to further develop their earlier microscope. A super-depth, three-dimensional time-resolved holographic microscope that enables deeper-than-ever tissue viewing was successfully developed, according to a recent study.

By utilizing the fact that single-scattered waves exhibit comparable reflection waveforms even when light is supplied from different angles, the researchers specifically developed a strategy to preferentially choose single-scattered waves. In order to discover the resonance mode that optimizes constructive interference (interference that happens when waves of the same phase overlap), a complicated algorithm and numerical operation that examines the eigenmode of a medium (a distinct wave that distributes light energy into a medium) are used. This allowed the new microscope to selectively filter out unwanted information while focusing more than 80 times as much light energy on the brain fibers as it had previously. This made it possible to increase the ratio of single-scattered waves to multiple-scattered waves by several orders of magnitude.

The study team used mouse brain imaging to demonstrate this novel method. Even at a depth where employing current technology was previously impossible, the wavefront distortion could be corrected using the microscope. The neuronal network of the mouse brain beneath the skull was successfully imaged by the new microscope in high resolution. Without taking the mouse's skull out and without using a fluorescent marker, all of this was accomplished in the visible wavelength.

According to Professor KIM Moonseok and Dr. JO Yonghyeon, who created the holographic microscope's framework, "Our research attracted a lot of interest from academics when we initially discovered the optical resonance of complex media. By integrating the talents of bright individuals in physics, biology, and brain research, we have pioneered a new path for brain neuroimaging convergent technology, from fundamental concepts to practical application of watching the neural network beneath the mouse skull."

CHOI Wonshik, an associate director, said: "Our center has been working on creating super-depth bioimaging technology that utilizes physical concepts. Our current discovery is anticipated to have a significant impact on the growth of biological multidisciplinary research, including neurology and the precision metrology sector."

1) Aberration is a phenomena that happens as a result of the light's speed changing based on the medium's refractive index. This means that when the image is produced, all of the light rays do not converge at once, resulting in a blurry and distorted image.

2) A wavefront is a plane created by joining all of the wave's points that belong to the same phase. For instance, when a stone is thrown into a lake, a circular wavefront is produced.

3) Time-resolved holographic microscope: Holographic microscopy uses the light interference created when two laser beams collide to measure the amplitude and phase of light. In example, a light source with a very low interference length of roughly 10 m can be used to selectively acquire an optical signal at a given depth with a time-resolved holographic microscope.

Institute for Basic Science

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