A collaboration between Colorado State University and University of Illinois at Urbana-Champaign resulted in a new, 3D imaging technique to visualize tissues and other biological samples on a microscopic scale, with potential to assist with cancer or other disease diagnoses.

Their technique, which allows specimens to generate light at double the frequency, or half the wavelength, of the incident light, is referred to as harmonic optical tomography and looks at 3D signals that are generated from the sample. The team's work is described in a paper, "Harmonic optical tomography of nonlinear structures," published online June 1 in Nature Photonics.

Harmonic optical tomography, or HOT, is based on using holographic information, which measures both the intensity and the phase delay of the light, to generate 3D images of a sample by exploiting a new physical mechanism used to obtain three-dimensional images.
"Our lab specializes in using holographic data to investigate live cells and tissues," said Gabriel Popescu, a professor of electrical and computer engineering at University of Illinois and director of the Quantitative Light Imaging Laboratory at the Beckman Institute for Advanced Science and Technology. "We wanted to extend this technique to nonlinear samples by combining the holographic data and new physics models."

Range of applications

Usually images, such as those captured by a cellphone camera, flatten three-dimensional information onto a two-dimensional image. Three-dimensional imaging that can peer into the interior of an object provides critical information for a diverse range of applications, such as medical diagnostics, finding cracks in oil wells and airplane wings, using tomographic X-ray, and ultrasound methods.

In this collaboration, the team developed theoretical models to describe how to image the tissue and discovered a unique capability for 3D imaging that arises, counterintuitively, by illuminating the sample with blurry, out-of-focus laser light. The team designed and built a new system at Colorado State University to collect data. The data was then reconstructed with computational imaging algorithms. The experiments verified an entirely new form of optical tomography, producing outstanding validation of the experimental predictions.

"A key to the experimental demonstration of this new nonlinear tomographic imaging was a custom, high-power laser, designed and built by CSU graduate student Keith Wernsing," said Randy Bartels, professor in CSU's Department of Electrical and Computer Engineering and paper co-author. "This source was integrated into a custom off-axis holographic microscope that used a high numerical aperture condenser lens defocused for widefield illumination. It is this special illumination condition that allows the nonlinear optics to create the second-harmonic generation signal and obtain information to form a 3D image. This work is an exciting example of how close dialogue enables refinement of both theory and experimental design to produce innovative new concepts."

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