A common modality that uses the optical properties of tissue at multiple spectral bands is the determination of tissue hemoglobin oxygen saturation ( StO2 ). By measuring the absorption coefficient at two or more wavelengths, tissue chromophore concentrations, namely oxy- and deoxyhemoglobin, can be estimated by solving a series of equations.37 (link)–40 (link, link, link) This reliance on multiple wavelengths has made using inorganic phantoms to accurately emulate tissue chromophores difficult. As such, we wanted to determine if the multipigment approach could create phantoms that accurately emulate physiological chromophore concentrations given different levels of StO2 .
To accomplish this, a computational model of skin absorption was made using blood, melanin, water, and fat as the primary chromophores. The absorption coefficient spectrum of the theoretical skin was determined through a summation of the absorption coefficients of the pure chromophores, obtained from the literature, such that μa(λ)=StO2BHμaHbO2(λ)+(1−StO2)BHμaHb(λ)+Wμawater(λ)+Fμafat(λ)+Mμamel(λ), where H is the concentration of hemoglobin within blood, StO2 is the ratio of oxyhemoglobin to total hemoglobin, and B , W , F , and M are the volume fractions of blood, water, fat, and melanosomes, respectively, within the tissue.41 –45 (link, link, link, link)
Using fixed volume fractions, tissue absorption spectra were produced between 470 and 950 nm using three different values for StO2 : 30%, 50%, and 80%. The volume fractions for water and fat were set to 0.5 and 0.02, respectively, based on the values used in similar models.38 (link),39 (link) A low melanosome volume fraction of 0.0255% was selected so that the theoretical tissue would have an absorption spectrum representative of pale, minimally pigmented skin.44 (link)–47 (link, link, link) The volume fraction of total blood was set to be 0.047 which, given a blood hemoglobin concentration of 2.3  mmol/L , corresponded to a total hemoglobin concentration of 110  μM , which falls within reported physiological ranges.37 (link),39 (link) Although not as important for StO2 measurements, the reduced scattering coefficient of the tissue model was set to be similar to be similar to that of skin. This was accomplished through a Mie-Rayleigh power law equation with values derived from the literature.44 (link),47 (link)
Multipigment phantoms were made to match the theoretical absorption coefficients for each of the three StO2 values using 38 g of total epoxy. The required pigment concentrations (see Table S3 in the Supplementary Material) were determined using the previously described [Eq. (3)] weighted-band approach with bands at 540 to 560 nm, 625 to 645 nm, and 820 to 840 nm, which correspond to wavelength regions often used to measure StO2 in tissue.39 (link),40 (link) After measuring the optical properties of the oxygenation phantoms, the absorption coefficients at the three bands were used along with Eq. (5) to set up a system of equations to solve for the equivalent melanosome volume fraction, blood volume fraction, and StO2 , assuming a water volume fraction of 0.5 and fat volume fraction of 0.02.
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