FREQUENCY-DOMAIN METHOD FOR MEASURING SPECTRAL PROPERTIES IN MULTIPLE-SCATTERING MEDIA - METHEMOGLOBIN ABSORPTION-SPECTRUM IN A TISSUELIKE PHANTOM

We have measured the optical absorption and scattering coefficient spe ctra of a multiple-scattering medium (i.e., a biological tissue-simula ting phantom comprising a lipid colloid) containing methemoglobin by u sing frequency-domain techniques. The methemoglobin absorption spectru m determined in the multiple-scattering medium is in excellent agreeme nt with a corrected methemoglobin absorption spectrum obtained from a steady-state spectrophotometer measurement of the optical density of a minimally scattering medium. The determination of the corrected methe moglobin absorption spectrum takes into account the scattering from im purities in the methemoglobin solution containing no lipid colloid. Fr equency-domain techniques allow for the separation of the absorbing fr om the scattering properties of multiple-scattering media, and these t echniques thus provide an absolute measurement of the optical absorpti on spectra of the methemoglobin/lipid colloid suspension. One accurate ly determines the absolute methemoglobin absorption spectrum in the fr equency domain by extracting the scattering and absorption coefficient s from the phase shift Phi and average light intensity DC (or Phi and the amplitude of the light-intensity oscillations AC) data with relati onships provided by diffusion theory, but one determines it less accur ately by using the Phi and modulation M (M = AC/DC) data and the diffu sion theory relationships. In addition to the greater uncertainty in t he absorption and scattering coefficients extracted from the Phi and M data, the optical parameters extracted from the Phi and M data exhibi t systematically inaccurate behavior that cannot be explained by rando m noise in the system. Possible reasons for the systematically lower a ccuracy of the methemoglobin absorption spectrum obtained from Phi and M data are discussed.