Near-infrared window in biological tissue

Blood consists of two different types of hemoglobin: oxyhemoglobin (${\displaystyle HbO_{2}}$) is bound to oxygen, while deoxyhemoglobin (${\displaystyle Hb}$) 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, ${\displaystyle HbO2}$ shows its highest absorption peak at 410 nm, and two secondary peaks at 550 nm and 600 nm. As light wavelengths passes 600 nm, ${\displaystyle HbO_{2}}$ absorption decays much faster than Hb absorption. The points where the molar extinction coefficient spectra of ${\displaystyle Hb}$ and ${\displaystyle HbO_{2}}$ intersect are called isosbestic points.

By using two different wavelengths, it is possible to calculate the concentrations of oxyhemoglobin (${\displaystyle C_{HbO2}}$) and deoxyhemoglobin (${\displaystyle C_{Hb}}$) as shown in the following equations:

${\displaystyle \mu _{a}(\lambda _{1})=\ln(10)\varepsilon _{HbO2}(\lambda _{1})C_{HbO2}+\ln(10)\varepsilon _{Hb}(\lambda _{1})C_{Hb}\,}$

${\displaystyle \mu _{a}(\lambda _{2})=\ln(10)\varepsilon _{HbO2}(\lambda _{2})C_{HbO2}+\ln(10)\varepsilon _{Hb}(\lambda _{2})C_{Hb}\,}$

Figure 2: The absorption spectrum of water.[4]

Here, ${\displaystyle \lambda _{1}}$ and ${\displaystyle \lambda _{2}}$ are the two wavelengths; ${\displaystyle \varepsilon _{HbO2}}$ and ${\displaystyle \varepsilon _{Hb}}$ are the molar extinction coefficients of ${\displaystyle HbO_{2}}$ and ${\displaystyle Hb}$, respectively; ${\displaystyle C_{HbO2}}$ and ${\displaystyle C_{Hb}}$ are the molar concentrations of ${\displaystyle HbO_{2}}$ and ${\displaystyle Hb}$ in tissue, respectively. Oxygen saturation (${\displaystyle SO_{2}}$) can then be computed as

${\displaystyle SO_{2}={\frac {C_{HbO2}}{C_{Hb}+C_{HbO2}}}}$

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.

Figure 3: The molar extinction coefficients of eumelanin and pheomelanin.[5]

Other tissue components with less significant contributions to the total absorption spectrum of tissue are melanin and fat.

Figure 4: The absorption coefficient spectrum of fat.[6]

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.[7] 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.[8] The molar extinction coefficient spectra corresponding to both types are shown in Figure 3.

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 pig fat.[9]

Figure 5: The scattering coefficient spectrum of biological tissue.[10]

In this case ${\displaystyle SaO_{2}\,}$ ≈ 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). 'cite: Anisotropic diffusion filter for dorsal hand vein features extraction - Sarah Hachemi Benziane, Abdelkader Benyettou'

In this case ${\displaystyle SvO_{2}\,}$ ≈ 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).

Figure 7: : Effective penetration depth in breast tissue (StO2 ≈ 70%). Effective attenuation coefficient: λ_min = 730 nm; NIR window = (626 - 1316) nm.

To define ${\displaystyle StO_{2}\,}$ (tissue oxygen saturation) (or ${\displaystyle TSI\,}$ (tissue saturation index)), it is necessary to define a distribution of arteries and veins in tissue. an arterial-venous blood volume ratio of 20%/80% can be adopted.[16] Thus tissue oxygen saturation can be defined as ${\displaystyle StO_{2}\,}$ = 0.2 x ${\displaystyle SaO_{2}\,}$ + 0.8 x ${\displaystyle SvO_{2}\,}$ ≈ 70%.

The total absorption (black) and the effective attenuation (magenta) coefficient spectra for breast tissue is shown in Figure 6 (c). In addition, the effective penetration depth is plotted in Figure 7.