Spatially resolved absolute diffuse reflectance measurements for noninvasive determination of the optical scattering and absorption coefficients of biological tissue

TLDR

Absorption and scattering coefficients of tissues govern the radial diffuse reflectance from a point source. The study presents a system for remote, noncontact measurement of spatially resolved absolute diffuse reflectance to estimate tissue optical properties. The system uses a laser source and CCD camera, with beam deflection for source calibration, and employs a neural network trained on Monte Carlo simulations to infer absorption and scattering coefficients from reflectance data, applied to phantoms and ex vivo tissues at 633 and 751 nm. The method shows that the diffusion equation is unsuitable, achieves 2.6 % rms error for scattering and 14 % for absorption on phantoms, but fails to match experimental reflectance within 2 mm of the beam for muscle tissue.

Abstract

The absorption and transport scattering coefficients of biological tissues determine the radial dependence of the diffuse reflectance that is due to a point source. A system is described for making remote measurements of spatially resolved absolute diffuse reflectance and hence noninvasive, noncontact estimates of the tissue optical properties. The system incorporated a laser source and a CCD camera. Deflection of the incident beam into the camera allowed characterization of the source for absolute reflectance measurements. It is shown that an often used solution of the diffusion equation cannot be applied for these measurements. Instead, a neural network, trained on the results of Monte Carlo simulations, was used to estimate the absorption and scattering coefficients from the reflectance data. Tests on tissue-simulating phantoms with transport scattering coefficients between 0.5 and 2.0 mm−1 and absorption coefficients between 0.002 and 0.1 mm−1 showed the rms errors of this technique to be 2.6% for the transport scattering coefficient and 14% for the absorption coefficients. The optical properties of bovine muscle, adipose, and liver tissue, as well as chicken muscle (breast), were also measured exυiυo at 633 and 751 nm. For muscle tissue it was found that the Monte Carlo simulation did not agree with experimental measurements of reflectance at distances less than 2 mm from the incident beam.