Review of short-wave infrared spectroscopy and imaging methods for biological tissue characterization

Abstract

We present a review of short-wave infrared (SWIR, defined here as ∼1000 to 2000 nm) spectroscopy and imaging techniques for biological tissue optical property characterization. Studies indicate notable SWIR absorption features of tissue constituents including water (near 1150, 1450, and 1900 nm), lipids (near 1040, 1200, 1400, and 1700 nm), and collagen (near 1200 and 1500 nm) that are much more prominent than corresponding features observed in the visible and near-infrared (VIS-NIR, defined here as ∼400 to 1000 nm). Furthermore, the wavelength dependence of the scattering coefficient has been observed to follow a power-law decay from the VIS-NIR to the SWIR region. Thus, the magnitude of tissue scattering is lower at SWIR wavelengths than that observed at VIS or NIR wavelengths, potentially enabling increased penetration depth of incident light at SWIR wavelengths that are not highly absorbed by the aforementioned chromophores. These aspects of SWIR suggest that the tissue spectroscopy and imaging in this range of wavelengths have the potential to provide enhanced sensitivity (relative to VIS-NIR measurements) to chromophores such as water and lipids, thereby helping to characterize changes in the concentrations of these chromophores due to conditions such as atherosclerotic plaque, breast cancer, and burns.

Figures

Fig. 1

(a) Absorption coefficients of oxygenated and deoxygenated hemoglobin, water, and lipid in the visible and near-infrared (VIS-NIR) (defined here as ∼400 to 1000 nm) and short-wave infrared (SWIR) (defined here as ∼1000 to 2000 nm) regions, obtained from Refs. , , and , with each spectrum normalized to its maximum value for ease of comparison. Both water and lipid have prominent absorption peaks in the SWIR, despite not having many notable features in the VIS or NIR regions. These spectra suggest that SWIR measurements have the potential to provide tissue composition information that is not nearly as readily available in the VIS-NIR. (b) Absorption coefficients, from Ref. , of the same tissue constituents as in (a), from 500 to 1600 nm, with collagen and beta-carotene added, with each spectrum normalized to its maximum value (reproduced with permission). Collagen exhibits a major SWIR absorption peak near 1500 nm as well as secondary absorption peaks near 1050 and 1200 nm; most of this information content is not accessible with VIS-NIR measurements.

Fig. 2

(a) Absorption spectrum μa(λ) from 400 to 2000 nm for human skin tissue (N=21 samples) measured ex vivo in Ref. , with prominent absorption peaks labeled. (b) Reduced scattering spectrum μs′(λ) from 400 to 2000 nm for the same human skin tissue dataset as in (a), modeled as a sum of Rayleigh scattering (λ−4, most prominent at visible wavelengths) and Mie scattering (λ−0.22, most prominent at NIR and SWIR wavelengths). Error bars represent standard deviation over the number of samples. Notable water and lipid absorption peaks are present throughout a large portion of the SWIR region, and the reduced scattering spectrum can be approximated by a Mie scattering power law in the SWIR region. These spectra suggest that SWIR measurements can be employed to detect spectral signatures of water and lipid in skin and that the power-law behavior of the reduced scattering coefficient extends into the SWIR regime. (Figure © Institute of Physics and Engineering in Medicine. Reproduced with permission of IOP publishing. All rights reserved).

Fig. 3

SWIR absorbance spectra [unitless, defined as 1/log(reflectance)] of human carotid atherosclerotic plaques (fibrous, dotted line; calcified, dashed line; and soft, solid line), measured ex vivo using a fiber probe and spectrometer. The spectra exhibit features of absorption from water (peaks near 1450 and 1950 nm) and lipids (peak near 1200 nm). This work suggests that SWIR may have the potential to differentiate between vulnerable plaque and more stable forms of plaque. (Figure reproduced with permission.)

Fig. 4

Diffuse reflectance spectra (colored symbols) of (a)–(c) noncancerous and (d) and (e) cancerous human breast tissues over the range from 500 to 1600 nm, measured ex vivo with a fiber-probe-based setup and shown with fits of a diffusion theory model (black solid lines). Differences between tissue types can be clearly seen and quantitatively related to changes in water and lipid volume fractions, which can then be employed for tissue classification. (Figure reproduced with permission.)

Fig. 5

(a) Effective attenuation, μeff(λ) and (b) absorption, μa(λ) coefficients of a human forearm in vivo, from 1150 to 1520 nm, obtained from time-resolved reflectance with a fiber probe. The measured μa values in (b) are shown alongside models for μa(λ) using water fractions of 47%, 52%, and 57%, represented with the broken line, solid line, and dotted line, respectively, employing the water absorption spectrum from Ref. . The water fraction of 52% was in good agreement with that obtained for muscle tissue using magnetic resonance imaging. This result suggests that the time-resolved SWIR imaging has the potential to quantitatively and noninvasively sense water content beneath the tissue surface in vivo. (Figure © Institute of Physics and Engineering in Medicine. Reproduced with permission of IOP publishing. All rights reserved).

Fig. 6

Reduced scattering (top panel) and absorption (three bottom panels) coefficients from 850 to 1800 nm obtained for in vivo rat tissue preburn and ∼2-h postburn using a hybrid wide-field imaging method with spatially modulated and unmodulated light. (Figure reproduced with permission).

Fig. 7

Reflectance and transmittance of rat skin in the SWIR range, calculated using Monte Carlo simulation. The absorption and reduced scattering coefficients input into the simulations are those shown in Fig. 6 for normal rat tissue in vivo, as obtained from multispectral SWIR imaging. These reflectance and transmittance curves indicate that although scattering decreases monotonically over the SWIR regime, the mean penetration depth of the light does not increase monotonically as a result. Instead, the SWIR penetration depth curve for skin exhibits a number of local maxima and minima that coincide with the absorption features of water in this wavelength range.