Cannula Microscopy Allows Deep-Brain Fluorescent Imaging

A tiny glass surgical needle can deliver fluorescent imaging and video from a mouse brain at depths as great as 2 mm, with a field of view of 200 μm in diameter.

Developed by researchers at the University of Utah, the system is based on a simple surgical glass cannula with a diameter of 220 μm that provides computational-cannula microscopy (CCM) for high-resolution, wide-field fluorescence microscopy. The researchers extended the CCM set-up by providing an epi-illumination configuration, using the cannula to guide the excitation beam to the tissue. Concomitant image-processing algorithms convert the spatially scrambled images into fluorescent images and video.

The small size of the cannula enables minimally invasive deep-brain imaging, without the need for a more complex optical system. In addition, since no scanning is involved, the system can achieve wide-field fluorescence video at the native frame rate of the camera, reducing tissue damage. After proving the process works in animal models, the researchers now believe CCM can be developed for human patients, creating a simpler, less expensive, and less invasive method than currently used endoscopes. The study was published on March 20, 2017, in Nature Scientific Reports.

“Typically, researchers must surgically take a sample of the animal’s brain to examine the cells under a microscope, or they use an endoscope that can be anywhere from 10 to 100 times thicker than a needle,” said senior author associate professor of electrical and computer engineering Rajesh Menon, PhD. “That’s very damaging. What we have done is to take a surgical needle that’s really tiny and easily put it into the brain as deep as we want and see very clear high-resolution images.”

Imaging deep inside biological tissue using multi-photon microscopy offers a maximum penetration depth of 600-800 μm, but at the expense of poorer spatial resolution as the diffraction limit is dictated by the longer wavelength and by scattering in tissue. With three-photon microscopy, imaging at up to 1.2 mm has been reported, but due to the extremely inefficient, low absorption cross-section, the large excitation intensities required can lead to photo-toxicity. Furthermore, many biological features lie at depths greater than 1.2 mm from the surface of the brain, such as the basal ganglia, hippocampus, and the hypothalamus.