Researchers have developed an endoscope that is as thin as a human hair and can image the activity of neurons in the brain of living mice. Because it's so thin, the endoscope can reach deep into the brain and give researchers access to areas that are invisible to microscopes or other types of endoscopes.
"In addition to animal studies that help us to understand how this new endoscope could someday be useful for certain applications in humans," said Shay Ohayon, who developed the device as a postdoctoral fellow in James DiCarlo's lab at the Massachusetts Institute of Technology. "It could, for example, provide a smaller and more comfortable nasal cavity imaging tool."
The new endoscope is based on only 1
In the journal The Optical Society (OSA) Biomedical Optics Express the researchers report that the endoscope can acquire images of nerve cells on a micrometer scale. This is the first time imaging has been demonstrated with such a thin endoscope in a living animal.
"With further development, the new microendoscope could be used to map neuron activity in previously inaccessible parts of the brain, such as the visual cortex, to primate animal models," said Ohayon. "It could also be used to study how neurons from different regions of the brain communicate with each other."
Extracting images from a fiber
The new micro-endoscope is based on a multimode optical fiber that can simultaneously carry several light beams. When light penetrates the fiber, it can be manipulated to create a tiny spot on the other end and can be moved to various positions on the tissue without moving the fiber. Scanning the tiny spot across the sample can excite fluorescent molecules used to label neuronal activity. As the fluorescence travels backward through the fiber from each point, an image of neuronal activity is formed.
To get a scan fast enough to fire neurons, we quickly used an optical component known as the Digital Mirror Device (DMD) to move the light spot, "Ohayon said It allows us to scan light at speeds of up to 20 kilohertz with the DMD. That's fast enough to detect the fluorescence of active neurons. "
Because the multimode fibers confused light for the endoscope, it used a method called Wavefront Shaping to convert the scrambled light into images, sending different patterns of light for wavefront shaping through the fiber to a camera at the other end, recording exactly how that specific fiber changed the transmitted light, then removing the camera and putting the imaging fiber into the brain – the information previously obtained about how the fiber changes the light is then used to create and scan a small point across the field of view.
Depicting living neurons
After successfully imaging cultured cells, researchers have tested their microendoscope on anesthetized mice stuck the fiber through a tiny hole in the skull of a mouse and lowered it Slowly get into the brain. To model the firing neurons, researchers used a technique called calcium imaging, which produces fluorescence in response to the calcium influx that occurs when a neuron fires.
"One of the benefits of such a thin endoscope is that you lose weight. In the brain, you can see all the blood vessels and navigate through the fiber so as not to hit them," says Ohayon.
In addition to demonstrating that their endoscope can capture detailed neural activity, researchers have also used multiple light colors for imaging. This capability could be used, for example, to observe interactions between two groups of neurons, each marked with a different color.
For standard imaging, the endoscope images the neurons at the tip of the fiber. However, the researchers also showed that the microendoscope can be imaged up to about 100 microns from the tip. "This is very useful because when the fiber is introduced into the brain, it can affect the function of neurons very close to the fiber," Ohayon explained. "Depicting an area slightly away from the fiber makes it easier to capture healthy neurons."
Handling Bends in the Fiber
A limitation of the micro-endoscope is that any bends in the fiber cause this to cause the ability to lose images. Although this did not affect the experiments described in the paper because the fiber was held straight when it was pushed into the brain, solving the bending problem could greatly expand the applications for the device. Several research groups are working on new types of fibers that are less prone to bending and calculation methods that can compensate for bending in real time.
"If this bending problem can be solved, it will likely change the nature of endoscopy in humans by using much thinner probes," Ohayon said. "This would allow more comfortable imaging than today's large endoscopes and enable imaging in parts of the body that are currently not possible."