Determination of optical properties of turbid media spanning visible and near-infrared regimes via spatially modulated quantitative spectroscopy

Abstract

We present a novel, noncontact method for the determination of quantitative optical properties of turbid media from 430 to 1050 nm. Through measuring the broadband reflectance from an unknown sample as a function of the spatial frequency of the projected illumination patterns, the absolute absorption and reduced scattering coefficients can be calculated without a priori assumptions of the chromophores present. This technique, which is called spatially modulated quantitative spectroscopy (SMoQS), was validated through the quantification of optical properties of homogenous liquid phantoms with known concentrations of absorbers and scatterers. The properties of the phantoms were recovered across the range of values prepared with R(2) values of 0.985 and 0.996 for absorption and reduced scattering, respectively. A measurement was also performed on skin tissue as a demonstration of the method's performance in vivo. The resultant absorption spectrum was well described by a multichromophore fit, and the quantitative values for oxy- and deoxyhemoglobin, water, and melanin were within published ranges for skin.

Figure 1

System diagram for SMoQS.

Figure 2

(a) An example of raw demodulated spectra, M_AC(λ,f_x), collected from a tissue-simulating liquid phantom containing nigrosin, Intralipid, and water; (b) reflectance calibrated by the reference phantom measurement; (c) the diffuse MTF, shown at 680 nm in this example; and (d) the resultant absorption and reduced scattering spectra.

Figure 3

Optical properties of high and low albedo phantoms. (a) Measured and expected absorption spectra, (b) accuracy between expected and measured absorption values, (c) measured and expected reduced scattering spectra, and (d) accuracy between expected and measured reduced scattering.

Figure 4

Measured absorption spectrum from the volar forearm. The dotted lines indicate spectral regions of low SNR in the measured spectra. The dashed line represents the combined chromophore fit to the measured spectra, and the scaled chromophore components are plotted underneath. The subplot displays the corresponding reduced scattering spectrum for this particular measurement.