Handheld Microscopic Brain Imaging Device

Scientists have devised a promising, minimally invasive optical method that can capture micron-scale images from deep in the brains of living organisms. The technique, called two-photon microendoscopy, combines two powerful optical and mechanical methods into one device that fits in the palm of the hand.

A study of the technique, performed by researchers at Stanford University (CA, USA), was published in the September 1, 2005, issue of the journal Optics Letters. Researchers are developing ways to image individual cells inside living organisms to provide insights into how cellular behavior affects the characteristics of organisms as a whole. For instance, the nerve cells of the hippocampus area of the brain affect critical mental processes such as learning and memory.

Instead of the traditional one higher-energy photon method currently used, scientists hit the molecule with two photons of lower energy. Their combined energies total the energy needed to excite the fluorescent-dye molecules used to mark the tissue. The method eliminates the background haze and decreases scattering, because molecules outside the area of interest are much less likely to absorb a pair of photons simultaneously and fluoresce in response.

Whereas two-photon microscopy provides an alternative to conventional one-photon fluorescence microscopy, it still only penetrates brain tissue down to approximately 500-600 microns--scarcely grazing the surface. To get a look at the deep structures, the Stanford scientists utilized two microendoscopy tools, very small, minimally invasive optical probes that could be inserted deep into living brain tissue. To produce one group of images, they positioned the microendoscope into the hippocampus, approximately one millimeter below the brain surface of the mouse, to visualize this area of the brain. The two-photon imaging provided an additional 80 microns of depth, underneath the hippocampal surface.

When integrated with two-photon fluorescence, the result is a system that brings the power of an advanced imaging technique to the deep tissues of the brain. The researchers have used their two-photon microendoscopy method to collect the detailed images of the blood vessels in the hippocampus sections of the brains of live mice. The mice were injected with a fluorescein dye, a U.S Food and Drug Administration- (FDA)-approved contrast agent that is most frequently used for retinal exams in humans. The fluorescein labeled the blood plasma so the vessels in the brain could be clearly seen.




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