February 19, 2013

Three-Photon Technology Improves Brain Imaging

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The brain has been an object of human curiosity for hundreds of years. Microscopy and imaging technologies used to study the brain, however, have not been able to provide scientists the depth and clarity they desire. Prof. Chris Xu ’96, applied and engineering physics, has developed a new method of microscopy that improves on past technology.

Traditional methods of brain imaging can be divided into two categories. The first, microscopic-level imaging, involves X-Ray or MRI technology. This allows researchers to see the entire brain, but it is slow and has poor spatial resolution. The other category is optical imaging, which provides high resolution and fast speeds, but cannot penetrate the surface of the brain. The drawbacks of these technologies drove Cornell researchers Winfried Denk and Watt Webb to develop two-photon microscopy, in 1990, a valuable contribution to the field at the time.

“We pushed for three-photon technology in 1995, but at the time, the advantage wasn’t very clear,” Xu said. “Only in the last few years have we been pushing deeper and deeper into the brain.”

The limitation of two-photon microscopy is the depth it can achieve. Scientists discovered two years ago that they had reached this fundamental limit in a mouse brain.

“This prompted us to think of what to do next,” Xu said. “By going up to three-photon technology, we reached a new fundamental barrier, but one much deeper than what we previously faced with two-photon technology.”

The improvement allowed researchers to go deeper into the brain by a factor of about two or three times, Xu said.

Another key component of three-photon microscopy is the use of fiber optic technology to generate a laser of ideal wavelength. Using well-known techniques in the telecom industry, researchers were able to operate at a wavelength of 1.7 microns, but did not have sufficient power for optimal performance. Xu proposed using an ordinary glass rod to generate a soliton, or a high-energy light pulse. The use of the glass rod magnified the strength of the laser, thus providing an essential component to the three-photon system.

In order to fully use this newly-developed technology, a new microscope had to be built. Nicholas Horton grad, one of the co-authors on the paper on three-photon microscopy, contributed to the construction of the microscope.

“The home-made microscope may not appear sophisticated, but it is flexible and open, which is advantageous because it lets us swap out components easily,” Horton said.

In addition to the three-photon technology, the microscope contains a system to secure and stabilize the mouse’s brain so it can be studied, as well be used as an imaging platform. This allows researchers to change the microscope orientation and image the parts of the brain in different sizes.

“The beauty of multiphoton microscopy is its versatility. We image the brain because it is probably the most mysterious organ. Understanding how brain works will have huge impact in science and medicine,” Horton said. “But we can also use multiphoton imaging to detect cancer cells in living animals, which is another research direction of our group. In the future, multiphoton microscopy could be used to detect cancer in human organs without having to remove tissue for conventional biopsy.”

The significance of deep tissue imaging is apparent in clinical practice. Many neurological diseases are not well-understood, Xu said.

“If we don’t know how the brain normally functions, there is no way to understand why the brain malfunctions,” Xu said.

Xu envisions the technology to be used for brain mapping and the study of “emergent phenomenon”, or new capabilities granted through the collective action of neural networks. Three-photon microscopy allows for dynamic functional imaging, which will provide information at physiologically relevant speeds that scientists can use to study how neurons interact in the brain.

Original Author: Nicolas Ramos