Near-infrared window in biological tissue: Difference between revisions
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The '''near-infrared (NIR) window''' (also known as optical window or therapeutic window) defines the range of wavelengths where light has its maximum depth of penetration in tissue. Within the NIR window, scattering is the most dominant light-tissue interaction, and therefore the propagating light becomes diffuse rapidly. Since scattering increases the distance travelled by photons within tissue, the probability of photon absorption also increases. Because scattering has weak dependence on wavelength, the NIR window is primarily limited by the light absorption of blood at short wavelengths and water at long wavelengths. |
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==Absorption properties of tissue components== |
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The absorption coefficient (<math>\mu_{a}</math>) is defined as the probability of photon absorption in tissue per unit path length<ref>LV. Wang and HI. Wu, Biomedical Optics. Wiley. ISBN 9780471743040, 2007.</ref>. Different tissue components have different <math>\mu_a</math> values. Moreover, <math>\mu_a</math> is a function of wavelength. Below are discussed the absorption properties of the most important [[chromophores]] in tissue. |
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[[Image:Fig_1.png|thumb|right|alt='''Figure 1:''' The molar extinction coefficients of HbO2 and Hb .|'''Figure 1:''' The molar extinction coefficients of HbO2 and Hb <ref>Molar extinction coefficients of oxy and deoxyhemoglobin compiled by Scott Prahl. URL: http://omlc.ogi.edu/spectra/hemoglobin.</ref>.]] |
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'''Blood:''' Blood consists of two different types of [[hemoglobin]]: oxyhemoglobin (<math>HbO_2</math>) is bound to oxygen, while deoxyhemoglobin (<math>Hb</math>) is unbound to oxygen. These two different types of hemoglobin exhibit different absorption spectra that are normally represented in terms of molar extinction coefficients, as shown in Figure 1. The molar extinction coefficient of Hb has its highest absorption peak at 420 nm and a second peak at 580 nm. Its spectrum then gradually decreases as light wavelength increases. On the other hand, <math>HbO2</math> shows its highest absorption peak at 410 nm, and two secondary peaks at 550 nm and 600 nm. As light wavelengths passes 600 nm, <math>HbO_2</math> absorption decays much faster than Hb absorption. The points where the molar extinction coefficient spectra of <math>Hb</math> and <math>HbO_2</math> intersect are called [[isosbestic points]]. |
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By using two different wavelengths, it is possible to calculate the concentrations of oxyhemoglobin (<math>C_{HbO2}</math>) and deoxyhemoglobin (<math>C_{Hb}</math>) as shown in the following equations: |
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:<math>\mu_a(\lambda_1) = \ln(10)\varepsilon_{HbO2}(\lambda_1)C_{HbO2}+\ln(10)\varepsilon_{Hb}(\lambda_1)C_{Hb} \,</math> |
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:<math>\mu_a(\lambda_2) = \ln(10)\varepsilon_{HbO2}(\lambda_2)C_{HbO2}+\ln(10)\varepsilon_{Hb}(\lambda_2)C_{Hb} \,</math> |
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[[Image:Fig_2.png|thumb|200 px| right|alt='''Figure 2:''' The absorption spectrum of water .|'''Figure 2:''' The absorption spectrum of water <ref>G. M. Hale, and M. R. Querry, Optical constants of water in the 200 nm to 200 µm wavelength region, Appl. Opt., 12, 555-563, 1973.</ref>.]] |
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Here, <math>\lambda_1</math> and <math>\lambda_2</math> are the two wavelengths; <math>\varepsilon_{HbO2}</math> and <math>\varepsilon_{Hb}</math> are the molar extinction coefficients of <math>HbO_2</math> and <math>Hb</math>, respectively; <math>C_{HbO2}</math> and <math>C_{Hb}</math> are the molar concentrations of <math>HbO_2</math> and <math>Hb</math> in tissue, respectively. |
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Oxygen saturation (<math>SO_2</math>) can then be computed as |
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:<math>SO_2=\frac {C_{HbO2}} {C_{HbO2}+C_{Hb}}</math> |
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'''Water:''' Although water is nearly transparent in the range of visible light, it becomes absorbing over the near-infrared region. Water is a critical component since its concentration is high in human tissue. The absorption spectrum of water in the range from 250 to 1000 nm is shown in Figure 2. Although absorption is rather low in this spectral range, it still contributes to the overall attenuation of tissue. |
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[[Image:Fig_3.png|thumb|left|alt='''Figure 3:''' Figure 3: The molar extinction coefficients of eumelanin and pheomelanin.|'''Figure 3:''' The molar extinction coefficients of eumelanin and pheomelanin <ref>Extinction coefficient of melanin by Steven Jacques. URL: http://omlc.ogi.edu/spectra/melanin/extcoeff.html.</ref>.]] |
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Other tissue components with less significant contributions to the total absorption spectrum of tissue are melanin and fat. |
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[[Image:Fig_4.png|thumb|right|alt='''Figure 4:''' Figure 4: The absorption coefficient spectrum of fat .|'''Figure 4:''' The absorption coefficient spectrum of fat <ref>R.L.P. van Veen, H.J.C.M. Sterenborg, A. Pifferi, A. Torricelli, and R. Cubeddu, OSA Annual BIOMED Topical Meeting, 2004.</ref>.]] |
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'''Melanin:''' Melanin is a chromophore that exists in the human epidermal layer of skin responsible for protection from harmful UV radiation. When melanocytes are stimulated by solar radiation, melanin is produced<ref>T. Vo-Dinh, Biomedical Photonics Handbook. Taylor & Francis, Inc. ISBN 0849311160, 2002.</ref>. Melanin is one of the major absorbers of light in some biological tissue (although its contribution is smaller than other components). There are two types of melanin: eumelanin which is black-brown and pheomelanin which is red-yellow<ref>George Zonios and Aikaterini Dimou, Ioannis Bassukas, Dimitrios Galaris, and Argyrios Ysolakidis and Efthimios Kaxiras, J. Biomed. Opt., Vol.13, 014017, 2008.</ref>. The molar extinction coefficient spectra corresponding to both types are shown in Figure 3. |
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'''Fat:''' Fat is one of the major components in tissue that can comprise 10-40% of tissue. Although not many mammalian fat spectra are available, Figure 4 shows an example extracted from <ref>R.L.P. van Veen, H.J.C.M. Sterenborg, A. Pifferi, A. Torricelli, and R. Cubeddu, OSA Annual BIOMED Topical Meeting, 2004.</ref>. |
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[[Image:Fig_5.png|thumb|right|alt='''Figure 5:''' Figure 5: The absorption coefficient spectrum of fat .|'''Figure 5:''' The scattering coefficient spectrum of biological tissue <ref>S. Jacques, C. Newman, D. Levy, and A. von Eschenbach. Univ. of Texas M. D. Anderson Cancer Center, 1987.</ref>.]] |
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==Scattering properties of tissue components== |
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Optical scattering occurs due to mismatches in refractive index of the different tissue components, ranging from cell membranes to whole cells. Cell nuclei and mitochondria are the most important scatterers <ref>LV. Wang and HI. Wu, Biomedical Optics. Wiley. ISBN 9780471743040, 2007.</ref>. Their dimensions range from 100 nm to 6 μm, and thus fall within the NIR window. Most of these organelles fall in the [[Mie regime]], and exhibit highly anisotropic forward-directed scattering <ref>T. Vo-Dinh, Biomedical Photonics Handbook. Taylor & Francis, Inc. ISBN 0849311160, 2002.</ref>. |
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Light scattering in biological tissue is denoted by the scattering coefficient (<math>\mu_s</math>), which is defined as the probability of photon scattering in tissue per unit path length <ref>LV. Wang and HI. Wu, Biomedical Optics. Wiley. ISBN 9780471743040, 2007.</ref>. Figure 5 shows a plot of the scattering spectrum<ref>S. Jacques, C. Newman, D. Levy, and A. von Eschenbach. Univ. of Texas M. D. Anderson Cancer Center, 1987.</ref>. |
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==Effective attenuation coefficient== |
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Attenuation of light in deep biological tissue depends on the effective attenuation coefficient (<math>\mu_{eff}</math>), which is defined as |
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:<math>\mu_{eff}=\sqrt{3\mu_a(\mu_a+\mu'_s)}</math> |
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where <math>\mu^'_s</math> is the transport scattering coefficient defined as |
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:<math>\mu'_s=\mu_s (1-g) \,</math> |
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where <math>g</math> is the anisotropy of biological tissue, which has a representative value of 0.9. |
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The effective attenuation coefficient is the dominant factor for determining light attenuation at depth <math>d</math> >> 1/ <math>\mu_{eff}</math>. |
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==Estimation of the NIR window in tissue== |
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NIR window can be computed based on the absorption coefficient spectrum or the effective attenuation coefficient spectrum. A possible criterion for selecting the NIR window is given by the FWHM of the inverse of these spectra. |
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Oxygen saturation will define the concentration of oxy and deoxyhemoglobin in tissue and so the total absorption spectrum. Depending on the type of tissue, we can consider different situations. |
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{{multiple image |
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| width = 200 |
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| image1 = Fig_6.png |
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| alt1 = Figure_3_The_absorption_spectrum_for_arteries |
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| caption1 = '''Figure 6 (a):''' The absorption spectrum for arteries (SaO<sub>2</sub> ≈ 98%). λ<sub>min</sub> = 686 nm; '''NIR window''' = (634 - 756) nm. |
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| image2 = Fig_7.png |
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| alt2 = Figure_4_The_absorption_spectrum_for_veins |
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| caption2 = '''Figure 6 (b):''' The absorption spectrum for veins (SvO<sub>2</sub> ≈ 60%). |
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λ<sub>min</sub> = 730 nm; '''NIR window''' = (664 - 934) nm. |
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| image3 = Fig_8.png |
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| alt3 = Figure_5_The_absorption_spectrum_for_brain_tissue |
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| caption3 = '''Figure 6 (c):''' The absorption spectrum for brain tissue (StO<sub>2</sub> ≈ 70%). λ<sub>min</sub> = 730 nm; '''NIR window''' = (656 - 916) nm. |
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}} |
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'''Absorption spectrum for arteries:''' In this case <math>SaO_2</math> ≈ 98% (arterial oxygen saturation). Then oxyhemoglobin will be dominant in the total absorption (black) and the effective attenuation (magenta) coefficient spectra, as shown in Figure 6 (a). |
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'''Absorption spectrum for veins:''' In this case <math>SvO_2</math> ≈ 60% (venous oxygen saturation). Then oxyhemoglobin and deoxyhemoglobin will have similar contributions to the total absorption (black) and the effective attenuation (magenta) coefficient spectra, as shown in Figure 6 (b). |
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'''Absorption spectrum for brain tissue:''' To define <math>StO_2</math> (tissue oxygen saturation) it is necessary to define a distribution of arteries and veins in tissue. For brain tissue, the ratio is given by 1:3 for arteries and veins, respectively <ref>M. A. Mintun, M. E. Raichle, W. R. W. Martin, and P. Herscovitch, Brain oxygen utilization measured with O-15 radiotracers and positron emission tomography. J. Nucl. Med. 25, 177-187, 1984.</ref>. Thus tissue oxygen saturation can be defined as <math>StO_2</math> = 0.3 x <math>SaO_2</math> + 0.7 x <math>SvO_2</math> ≈ 70%. |
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The total absorption (black) and the effective attenuation (magenta) coefficient spectra for brain tissue is shown in Figure 6 (c). |
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== References == |
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{{reflist}} |
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