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Laser-based mid-infrared reflectance imaging of biological tissues

Bujin Guo, Y. Wang, C. Peng, H. L. Zhang, G. P. Luo, H. Q. Le, C. Gmachl, D. L. Sivco, M. L. Peabody, and A.Y. Cho

Open Access Open Access

Abstract

Mid-infrared (MIR) (3–12 um) spectral imaging is a power analytical tool, but difficult in the back-reflectance mode for in-vivo diagnostics. Feasibility of MIR back-reflectance imaging is demonstrated using MIR semiconductor lasers. Transmittance through 500-µm thick films of water and blood showed a capability to resolve more than 6-OD signal dynamic range. Reflectance scanning imaging through a 150-µm thick film of blood showed negligible scattering effect, indicating the feasibility of optical coherent imaging. The result of coherent imaging of a plant leaf shows a MIR sub-surface image that would not be visible in white light. With two wavelengths, a similar result for a chicken skin subcutaneous tissue at different focal depths was obtained, showing blood vessels beneath a lipid layer. These results suggest that advanced multi-laser wavelength systems in the fingerprint spectral region can be a useful tool for in-vivo spectral imaging in biomedical research and diagnostic applications.

©2004 Optical Society of America

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Figures (6)
Fig. 1.
Fig. 1. Conceptual sketch of the reflectance imaging. (a) illustrates a tissue immersed beneath a thin film layer of fluid (i.e., blood); (b) shows an ideal optically flat sample; and (c) shows a sample with substantial surface morphological features.

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Fig. 2.
Fig. 2. Infrared micro-reflectance imaging setup. (a), scanning imaging system; and (b) staring imaging system. Letter abbreviations in the figure are as follows: L for lens, M for mirror, FPA for focal plan array, S for sample, and W for CaF2 window.

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Fig. 3.
Fig. 3. Experimental results for attenuation coefficients of water and blood. (a) and (b) Measurements (symbols) and least-square fit (line) of relative transmitted power as a function of film thickness for water and animal blood, respectively (wavelength 5.4 µm); (c), Comparison of the measured attenuation coefficients at different wavelength to those in Ref. 18, demonstrating excellent agreement. (d), the difference between blood and water attenuation coefficient at various wavelengths, showing the effect of blood absorption in the fingerprint region.

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Fig. 4.
Fig. 4. Infrared micro-reflectance imaging of a gold-coated patterned sample through a thin layer of water or blood at 5.4 µm. (a), a white light image of the sample, notice the alignment marker in the bottom of the image; (b), the IR laser image without liquid above the sample; (c), a visible-spectrum picture of the sample with a thin blood film on top, only the blood can be seen; (d)-(f), the IR laser images with a thin layer of water above the sample, the water thickness is 50-, 100-, and 150-µm from left to right. (g)-(i), the IR laser images with a thin layer of blood above the sample, the blood thickness are the same as those of water. Little difference between water and blood images indicates the lack of significant scattering by particles in blood at this long wavelength.

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Fig. 5.
Fig. 5. Infrared micro-reflectance imaging of a house plant leaf (Epipremnum Pinnatum). (a), a white-light picture of a leaf; (b), a white-light image of a small section of the leaf, notice the upper epidermis and the vein; (c), MIR laser image of a small section of the leaf taken with the staring imaging system; (d), MIR laser image of the same section when the depth of focus is changed, showing an image from a deeper layer from the upper epidermis, which would not be visible with white light.

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Fig. 6.
Fig. 6. Infrared micro-reflectance imaging of the micro-blood vessels from chicken skin. (a), the white light image of a small section of the chicken skin, notice the micro-blood vessel shown in the image; (b), the image of the surface using 3.4 µm laser illumination, the blood vessel was not seen; (c), the MIR laser image when the focus depth is changed to a subsurface plane, showing the blood vessels; (d) the same as (c) but using 4.6 µm laser; (e), the false color composite image both images (c) and (d).

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Tables (1)

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Table I Laser characteristics

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