Fluorescent heterogeneities in turbid media: limits for detection, characterization, and comparison with absorption.

The fundamental limits for detection and characterization of fluorescent (phosphorescent) inhomogeneities embedded in tissuelike highly scattering turbid media are investigated. The absorption and fluorescence contrast introduced by exogenous fluorophores are also compared. Both analyses are based on practical signal-to-noise ratio considerations. For an object with fivefold fluorophore concentration and lifetime contrast with respect to the background tissue, we find the smallest detectable fluorescent object at 3-cm depth in tissuelike turbid media to be ~0.25 cm in radius, whereas the smallest characterizable object size is ~0.75 cm in radius, given a model with 1% amplitude and 0.5 degrees phase noise. We also find that, for fluorescence extinction coefficients epsilon </= 0.5 x 10(5) cm(-1) M(-1), the fluorescence measurement mode is superior to the absorption mode for detecting an inhomogeneity. The optimal choice of modulation frequency for the frequency-domain fluorescence measurements is also discussed.