Quantitative Raman spectroscopy in turbid media.

ABSTRACT

Intrinsic Raman spectra of biological tissue are distorted by the influences of tissue absorption and scattering, which significantly challenge signal quantification. A combined Raman and spatially resolved reflectance setup is introduced to measure the absorption coefficient micro(a) and the reduced scattering coefficient micro(s) (') of the tissue, together with the Raman signals. The influence of micro(a) and micro(s) (') on the resonance Raman signal of beta-carotene is measured at 1524 cm(-1) by tissue phantom measurements and Monte Carlo simulations for micro(a)=0.01 to 10 mm(-1) and micro(s) (')=0.1 to 10 mm(-1). Both methods show that the Raman signal drops roughly proportional to 1 micro(a) for micro(a)>0.2 mm(-1) in the measurement geometry and that the influence of micro(s) (') is weaker, but not negligible. Possible correction functions dependent on the elastic diffuse reflectance are investigated to correct the Raman signal for the influence of micro(a) and micro(s) ('), provided that micro(a) and micro(s) (') are measured as well. A correction function based on the Monte Carlo simulation of Raman signals is suggested as an alternative. Both approaches strongly reduce the turbidity-induced variation of the Raman signals and allow absolute Raman scattering coefficients to be determined.