1/7/2013 Meredith Staub
Written by Meredith Staub
MechSE assistant professor Kimani Toussaint specializes in optics, particularly relating to biophotonics. A major part of his research deals with the quantitative imaging of collagen fibers using nonlinear optics, trying to find the most effective imaging method in order to learn about their organization within various tissues.
"Ideally you want to be able to take pictures without destroying what you're looking at," Toussaint said. "And then, you want to be able to quantify certain features, so that you have pictures with approaches or methods that allow you to quantify information that you've extracted. We have found second harmonic generation to be a very good technique for this."
Second harmonic generation (SHG), also called frequency doubling, is a process in nonlinear optics in which pairs of photons are “combined” in a nonlinear material to form new photons with twice the energy, and therefore twice the frequency and half the wavelength of the original photons. SHG microscopy uses this effect to create an image of the object by measuring the frequency-doubled photons as they scatter off of the nonlinear object. For example, if a red laser light is sent in, photons at blue wavelengths would be scattered back. Filtering the imaging process for these blue scattered photons can create an image of the object. It requires a material with a noncentrosymmetric molecular structure in order to be effective; this makes biological materials such as collagen, microtubules, and muscle myosin, which have such a structure, ideal targets for this imaging method.
SHG has several advantages for biological tissue imaging. The process avoids exciting the molecules in the tissue, which can sometimes have ill effects. Labeling molecules with external probes, which may adversely affect intrinsic tissue properties, is not necessary because of how strong and distinct the SHG signals are. SHG can also construct three dimensional images, and can probe relatively deep into thick tissues compared to conventional imaging approaches.
"Second generation microscopy is in that spirit of trying to get high-resolution, high-contrast images," Toussaint said. "In contrast to multi-photon fluorescence imaging, one tends to use this type of tool for systems that typically would lend themselves to automatically giving you this contrasting color. So if I were to look at cells, this may not be the best tool to use. But if I were to look at collagen fibers, this would be a better tool to use."
In anisotropic structures, or structures that have preferences to specific polarization states of light, spatial harmonic analysis can be used to quantify the spatial organization of the structure. Toussaint and his group have combined this tool with SHG imaging in order to create detailed, three-dimensional analyses of the way collagen fibers are organized in a tissue. Quantifying this organization is extremely valuable for diagnostics of the tissue. Specifically, Toussaint and his group have found that this method can help stage breast cancer by imaging the collagen structure within the stroma.
"The initial results in our paper seemed to indicate that the technique is promising, and discriminated between some of those stages [of cancer]," Toussaint said. "But one always has to be careful about how the word 'diagnostic' gets used. When you take a technique from the lab to the hospital, to the clinic, you have to go through a very stringent vetting process. So that's why when you hear the word 'diagnostic,' it's usually followed by 'potential.' So it has good diagnostic potential. There's still more work to be done to really prove it."