Comparison of penetration depth between two-photon excitation and single-photon excitation in imaging through turbid tissue media

We show, both theoretically and experimentally, that for a turbid tissue me dium where Mie scattering is dominant, multiple scattering not only reduces the illumination power in the forward direction but also exhibits an aniso tropic distribution of scattered photons. Thus, a signal level under two-ph oton excitation drops much faster than that under single-photon excitation although image resolution is much higher in the former case. As a result, t he penetration depth under two-photon excitation is limited by the strength of two-photon fluorescence and is not necessarily larger than that under s ingle-photon excitation. (C) 2000 American Institute of Physics. [S0003-695 1(00)04536-8].