Optical property measurements in turbid media using frequency-domain photon migration

摘要

Abstract: In frequency-domain photon migration (FDPM), amplitude-modulated light is launched into a turbid medium, e.g., tissue, which results in the propagation of density waves of diffuse photons. Variations in the optical properties of the medium perturb the phase velocity and amplitude of the diffusing waves. These parameters can be determined by measuring the phase delay and demodulation amplitude of the waves with respect to the source. More specifically, the damped spherical-wave solutions to the homogeneous form of the diffusion equation yield expressions for phase ($phi@) and demodulation (m) as a function of source distance, modulation frequency, absorption coefficient ($Beta@), and effective scattering coefficient ($sigma$-eff$/). In this work, analytical expressions for the variable dependence of $phi and m on modulation frequency are presented. A simple method for extracting absorption coefficients from $phi and m vs. frequency plots is applied to the measurement of tissue phantoms. Using modulation frequencies between 5 MHz and 250 MHz, absorption coefficients as low as 0.024 cm$+$MIN@1$/ are measured in the presence of effective scattering coefficients as high as 144 cm$+$MIN@1$/. The results underscore the importance of employing multiple modulation frequencies for the quantitative determination of optical properties.!9