Combined Imaging Approach Provides Insights into Multiple Sclerosis

The technique also promises to provide new information about how the disease degrades the myelin sheath, which insulates nerve fibers and enables them to properly conduct impulses in the spinal cord, brain, and in the peripheral nervous system throughout the body, according to Dr. Ji-Xin Cheng, an assistant professor in Purdue University's (West Lafayette, IN, USA)Weldon School of Biomedical Engineering and department of chemistry.

The three imaging techniques, called sum frequency generation, two-photon-excitation fluorescence, and coherent anti-Stokes Raman scattering, typically are used alone. Purdue researchers have developed a way to combine all three methods in the same platform, promising to reveal new details about the spinal cord and myelin sheath, according to Dr. Cheng.

The investigators results will be published in the May 2007 issue of the Biophysical Journal and is currently online. Raman microscopy, an imaging technique invented more than 30 years ago, cannot be used effectively to study living tissue because the very weak Raman scattering signals require hours to yield an image, whereas coherent anti-Stokes Raman scattering (CARS), overcomes this limitation, according to Dr. Cheng.

CARS imaging takes advantage of the fact that molecules vibrate at specific frequencies. In a CARS microscope, two laser beams are overlapped to generate a single beam having a new frequency representing the difference between the original two beams. This new frequency then drives specific molecules to vibrate together in phase, amplifying the Raman signals from those molecules.

Sum frequency generation imaging does the contrary, adding the frequencies of the two original beams, generating a new signal with a frequency that is the sum of the original beams.

The third imaging technique, two-photon excitation fluorescence, provides higher contrast and brighter images than conventional fluorescent imaging methods. Photons are the individual particles that make up light. In two-photon excitation fluorescence, two photons are used to illuminate a target.
The researchers have used the imaging methods to observe living spinal tissue extracted from guinea pigs.


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