Diffuse Photon Density Wave Fields in Turbid Media and Tissue

When a picosecond light pulse is incident on biological tissue, the temporal characteristics of the light backscattered from, or transmitted through, the sample carry information about the optical absorption and scattering coefficients of the tissue. We develop a simple model, based on the diffusion approximation to radiative transfer theory, which yields analytic expressions for the pulse shape in terms of the interaction coefficients of a homogeneous slab. The model predictions are in good agreement with the results of preliminary in vivo experiments and Monte Carlo simulations.

A detailed description of the construction and performance of a variable frequency cross-correlation phase fluorometer is reported. The phase fluorometer operates over the frequency range 1-160 MHz with a maximum resolution of a few picoseconds. The effects of distortions introduced by the light modulator and the nonlinear dynode characteristic are discussed in terms of the harmonic content of the detected signal. A source of systematic errors due to nonhomogeneous modulation is also discussed with particular attention to the color effect of the photomultipliers. The application of the phase fluorometer to the measurement of very long and very short lifetimes is reported. Some application to the measurement of multiexponential decays is also illustrated.