Time-resolved fluorescent imaging in tissue

Abstract

Fluorescence lifetime imaging is a useful tool for quantifying site-dependent environmental conditions in tissue. Fluorophores exist with known lifetime dependencies on factors such as concentrations of O2 and other specific molecules, as well as on temperature and pH. Extracting fluorophore lifetime for deeply embedded sites in turbid media such as tissue is made difficult by the multiple scattering of photons traveling through tissue. This scattering introduces photon arrival delays that have similar characteristics to the delays resulting from the excitation and subsequent emission of photons by fluorophores. Random walk theory (RWT) provides a framework in which the two sources of diffusion-like delays can be separated so that the part due to fluorescent lifetime can be quantified. We derive a closed-form solution that predicts time-resolved photon arrivals from a deeply embedded fluorophore site. The solution requires that an average absorption coefficient be used. However, it is shown that this assumption introduces only a small error. This RWT-derived solution is also shown to be valid for a range of geometries in which the fluorophore site is embedded at least 10 mean scattering lengths and in which the fluorophore lifetime is less than 1 ns.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.