Fourier Transform Infrared Microimaging

Professor and CBR member Yang Xia, of the Department of Physics, uses a variety of experimental techniques to study cartilage, whose degradation plays a major role in the development of osteoarthritis. Xia and his post doc Jianhua Yin recently published a paper in the journal Biomedical Optics Express (Volume 2, Pages 937-945) describing the use of Fourier Transform Infrared Imaging to study the concentration of various biomolecules that are important in cartilage. The introduction of their paper, titled Chemical Visualization of Individual Chondrocytes in Articular Cartilage by Attenuated-Total-Reflection Fourier Transform Infrared Microimaging, is reproduced below (with citations removed).

"Articular cartilage, a compliant load-bearing surface covering the ends of bones, comprises an extracellular matrix (ECM) embedded with living cells, the chondrocytes. The ECM of articular cartilage principally consists of water, type II collagen and aggregated proteoglycan molecules. The physical properties of articular cartilage depend on the structure and organization of the collagen network and the concentration of proteoglycans in the tissue, which are maintained by the functional activity of chondrocytes. As a result of trauma or degenerative joint diseases, cartilage is frequently damaged and difficult to be repaired due its avascular nature and no recruitment of healthy cells to the site of damage. Although the structure, geometry and function of chondrocytes have been studied extensively by many techniques such as microscopies, biological and biomechanical methods, the molecular/chemical components and their distribution inside/around the chondrocytes are difficult to study.

Fourier transform infrared Imaging (FTIRI), which uses the imaging approach to study spectroscopically chemical concentrations and distributions, has become a powerful tool in biomedical research. For example, it is possible to spatially resolve various chemical signatures with a fine spatial pixel size (e.g., 6.25 µm) and a spectral resolution (e.g. 1-16 cm⁻¹) in cartilage. FTIRI can also be very effective in the study of the orientation of the chemical bonds (e.g., amide I bond, which is the C = O in a molecular dipole) in cartilage, the changes in the collagen orientation due to external loading, and the molecular concentrations in cartilage. The main limitation of FTIRI is its spatial resolution, on the order of 5-10 microns due to its optical properties.

The use of an attenuated total reflection (ATR) accessory in FTIRI could break this optical limitation. With a germanium crystal, which has a high refractive index of 4.0 in the ATR accessory, the ATR FTIR microimaging (ATR-FTIRM) can improve the pixel resolution by a factor of four (from 6.25 µm to 1.56 µm in a commercial PerkinElmer FTIRI system). The purpose of this study is to determine if individual chondrocytes can be optically visualized and chemically measured by ATR-FTIRM to provide insights into the fine structure and molecular distribution of chondrocytes in articular cartilage."

Xia’s laboratory is supported by two grants from the National Institutes of Health.