Analytical Models for Time-Resolved Fluorescence Spectroscopy in Tissues

Laser induced fluorescence is a rapidly growing technique for the characterization of scattering material, most notably in in-vivo biomedical testing. Most previous applications have relied on the measurements of the steady-state emission spectrum, with subsequent analysis of the spectrum for relative concentrations of potential fluorophores. Only recently a few investigators have explored the use of the fluorescence lifetimes as a diagnostic tool by taking advantage of the perturbation of the lifetime by multiple scattering of the excitation and emission light in the tissue. We have developed a model to study the fluorescence signal generated by a fluorophore distributed in a scattering medium. This model is based on two coupled transient transfer phenomena: the transport of the pulsed source laser, and the induced transient fluorescence excited by the laser. The two are coupled through the source term for the induced fluorescence field. Whereas previous research focused mainly on the fluorescence properties of various dyes, compounds, and materials, only recently have such transport questions been addressed by researchers. In addition, the studies that have been done in the literature have taken the optical properties of the tissue to be the same at the excitation and emission wavelengths. We have presented analytical and numerical solutions for finite, infinite, cylindrical and spherical geometries.