A novel holographic microscope could image mouse brain through its skull

Researchers can now view the mouse brain through the skull thanks to a new holographic microscope.

Led by 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 developed a new type of holographic microscope.

The results were published in Science Advances on July 27.

According to the results, it is possible to "see through" the intact skull with the novel microscope. It also means that it can provide high-resolution 3D imaging of the neural network within a living mouse brain without removing the skull.

It is essential to precisely assess the signal reflected from the target tissue and apply enough light energy to the sample to explore the interior properties of living organisms using light. However, in living tissues, multiple scattering effects and severe aberration tend to occur when light hits the cells, which makes it difficult to obtain sharp images.

Now it is much easier

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 due to this process.

By correcting the wavefront distortion of the light reflected from the object to be examined, it is feasible to see the relatively deep characteristics inside the tissues, even if there is only a minimal amount of reflected light. However, this correcting process is hampered by the various scattering effects, said the statement. As a result, it's crucial to reduce the ratio of the multiple-scattered waves and raise it to acquire a high-resolution deep-tissue image.

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

More than 80 times the light energy

Specifically, the researchers devised a method to preferentially select single-scattered waves by taking advantage of the fact that they have similar reflection waveforms even when light is input from various angles.

By using complex algorithm and a numerical operation that analyzes the eigenmode of a medium (a unique wave that delivers light energy into a medium), allows the finding of a resonance mode that maximizes constructive interference (interference that occurs when waves of the same phase overlap) between wavefronts of light, this novel microscope is focusing on more than 80 times the light energy on the neural fibers than before, while selectively removing unnecessary signals.

“When we first observed the optical resonance of complex media, our work received great attention from academia," said Professor KIM Moonseok and Dr. JO Yonghyeon, who have developed the foundation of the holographic microscope.

"From basic principles to practical application of observing the neural network beneath the mouse skull, we have opened a new way for brain neuroimaging convergent technology by combining the efforts of talented people in physics, life, and brain science.”

Abstract

Compensation of sample-induced optical aberrations is crucial for visualizing microscopic structures deep within biological tissues. However, strong multiple scattering poses a fundamental limitation for identifying and correcting the tissue-induced aberrations. Here, we introduce a label-free deep-tissue imaging technique termed dimensionality reduction adaptive-optical microscopy (DReAM) to selectively attenuate multiple scattering. We established a theoretical framework in which dimensionality reduction of a time-gated reflection matrix can attenuate uncorrelated multiple scattering while retaining a single-scattering signal with a strong wave correlation, irrespective of sample-induced aberrations. We performed mouse brain imaging in vivo through the intact skull with the probe beam at visible wavelengths. Despite the strong scattering and aberrations, DReAM offered a 17-fold enhancement of single scattering–to–multiple scattering ratio and provided high-contrast images of neural fibers in the brain cortex with the diffraction-limited spatial resolution of 412 nanometers and a 33-fold enhanced Strehl ratio.