IR tomography goes full color

Combining Fourier transform infrared (FTIR) spectroscopy with CT creates a nondestructive, 3-D imaging technique providing full-color, molecular-level chemical information of unprecedented detail.

“The notion of having the colors in a 3-D reconstructed image being tied to real chemistry is powerful,” said Berkeley Lab’s IR imaging expert Michael C. Martin; the work is a collaboration between researchers at Lawrence Berkeley National Laboratory and the University of Wisconsin-Milwaukee (UWM). “We’ve all seen pretty 3-D renderings of medical scans with colors – for example, bone-colored bones – but that’s simply an artistic choice. Now we can spectrally identify the specific types of minerals within a piece of bone and assign a color to each type within the 3-D reconstructed image.”

Spectromicrotomographic images of a human hair show absorptions of protein (red) and phospholipid (blue-green). (Center) The medulla is observed to have little protein. (Bottom) The medulla has higher concentrations of phospholipids. Both scale bars represent 25 μm. Courtesy of Lawrence Berkeley National Laboratory.

IR spectroscopy can be used to identify the chemical constituents of a sample, and the application of the Fourier transform algorithm allows all IR fingerprints to be simultaneously recorded. Working with physicist Carol J. Hirschmugl, director of the Laboratory for Dynamics and Structure at Surfaces and a principal investigator with UWM’s Synchrotron Radiation Center (SRC), Martin and colleagues combined FTIR with CT, which reconstructs 3-D images from multiple cross-sectional slices, to achieve what is believed to be the first demonstration of FTIR spectromicrotomography.

The technique was successful thanks to the speed with which the 2-D FTIR images could be obtained at the SRC’s Infrared Environmental Imaging beamline. Hundreds of 2-D spectral images were recorded as a sample rotated in front of an IR microscope. For each wavelength, CT algorithms reconstructed a full 3-D representation of the sample into a complete spectrum for every voxel.

The researchers applied the technique to obtain 3-D images of the molecular architecture of the cell walls in a flowering plant (zinnia) and in a woody plant (poplar).

FTIR spectromicrotomography is expected to be used in biomedical imaging, biofuel development, agriculture and even art history, where, according to Martin, a painting’s different layers could be revealed.

The findings were reported in Nature Methods (doi: 10.1038/nmeth.2596).