Using a multimode optical fiber as thin as a human hair, scientists at the Leibniz-Institute of Photonic Technology have observed at high resolution the neuronal structures inside deep brain areas of living mice. The researchers drew on holographic methods for controlling light to design a fluorescence imaging system compact enough to fit on the tip of a fiber. The ultranarrow, minimally invasive probe offers a smaller footprint and higher resolution compared to endoscopes based on graded-index lenses or fiber bundles. According to the researchers, the new probe is capable of acquiring 7-kilopixel images with micron-level spatial resolution at imaging speeds of 3.5 frames/s. These results could provide adequate spatial and temporal resolution for fluorescent imaging of subcellular structures in living tissues. Fiber probe, surrounded by neurons. Courtesy of Tomáš Cižmár. The researchers used the probe to obtain images of brain cells and neuronal processes in the visual cortex and hippocampus of living mice. They achieved spatial resolution approaching one micron, with minimal damage to the tissue surrounding the fiber penetration area. The probe’s robust design allowed continuous imaging for periods of several hours. According to the team, detailed observations of these areas within the brain are crucial for research into sensory perception and severe neuronal diseases. Current investigation methods are highly invasive, said the team, and prevent scientists from being able to observe neuronal networks at work deep within the brain without destroying large amounts of the surrounding tissue. Traditional endoscopes are composed of hundreds of optical fibers, making them too large to penetrate such sensitive brain regions. Deep neuronal structures are too small to be imaged using noninvasive methods such as MRI. The projection behind the Leibniz IPHT scientists shows an image of neurons obtained deep inside the brain via a single multimode fiber. Courtesy of Sven Doering. “This minimally invasive approach will enable neuroscientists to investigate functions of neurons in deep structures of the brain of behaving animals,” said project partner Nathalie Rochefort from the University of Edinburgh. “Without perturbing the neuronal circuits in action, it will be possible to reveal the activity of these neuronal circuits while the animal is exploring an environment or learning a new task.” Use of the new probe could lead scientists to a better understanding of the functions of deeply hidden brain compartments, such as the formation of memories, as well as related dysfunctions, including Alzheimer’s disease. The new probe could also pave the way for in vivo implementation of numerous techniques of modern microscopy, including multiphoton, superresolution, and light-sheet approaches. Future advancements will rely on the development of new fiber types directly optimized for the purposes of holographic endoscopy, the researchers said. The research team intends to build on its work to investigate advanced microscopy techniques through single fiber endoscopes. “We will strive hard to prepare more significant advancements on this result, essentially funneling the most advanced methods of modern microscopy deep inside the tissues of living and functioning organisms,” said professor Tomáš Cižmár. The research was published in Light: Science and Applications (https://doi.org/10.1038/s41377-018-0094-x). Neuronal somata, neuronal processes and blood cells deep inside deep brain areas of a living mouse. The images were obtained via a single multimode fiber. Courtesy of Tomáš Cižmár.