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 (φ) and demodulation (m) as a function of source distance, modulation frequency, absorption coefficient (β), and effective scattering coefficient (Бeff).
In this work,we present analytical expressions for the variable dependence of φ and m on modulation frequency. A simple method for extracting absorption coefficients from φ 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 0.024cm -l are measured in the presence of effective scattering coefficients as high as 144cm -1. Our results underscore the importance of employing multiple modulation frequencies for the quantitative determination of optical properties.
© 1991 International Society of Optical Engineering (SPIE)
Tromberg, Bruce J., Lars O. Svaasand, Tsong-Tseh Tsay, Richard C. Haskell, and Michael W. Berns. "Optical property measurements in turbid media using frequency domain photon migration." Proceedings of SPIE: Future Trends in Biomedical Applications of Lasers, Vol. 1525 (1991): 52-58. DOI: 10.1117/12.48187