Diffusion coefficient in photon diffusion theory.

The choice of the diffusion coefficient to be used in photon diffusion theory has been a subject of discussion in recent publications on tissue optics. We compared several diffusion coefficients with the apparent diffusion coefficient from the more fundamental transport theory, D(app). Application to point sources in turbid media, for which exact solutions are available, showed that D(app) has to be preferred. We give a simple equation to approximate D(app) for several phase functions that apply to tissue optics. Reasons for the remaining discrepancies in diffusion coefficients applied to time-resolved and time-averaged descriptions of photon propagation in homogeneous turbid media are discussed.