Origin of phosphorescence signals reemitted from tissues.

We investigate the origin of fluorescent or phosphorescent signals reemitted from highly scattering media (such as tissues), using diffusion theory and probability analysis. Results show that the lifetime of a uniformly distributed phosphorescent or fluorescent optical probe will profoundly affect the volume interrogated by noninvasive reflectance measurements. When the lifetime is greater than photon migration times, the origin of the reemitted signal is confined closest to the surface. Our computations suggest that noninvasive measurements of tissue oxygen concentration may not necessarily interrogate deep tissues when systemically administered phosphorescent dyes are used.