Application of the stretched exponential function to fluorescence lifetime imaging of biological tissue

The fluorescence decay in fluorescence lifetime imaging (FLIM) is typically fitted to a multi-exponential model with discrete lifetimes. The interaction between fluorophores in heterogeneous samples (e.g. biological tissue) can, however, produce complex decay characteristics that do not correspond to such models. Although they appear to provide a better fit to fluorescence decay data than the assumption of a mono-exponential decay, the assumption of multiple discrete components is essentially arbitrary and often erroneous. The stretched exponential function (StrEF) describes fluorescence decay profiles using a continuous lifetime distribution as has been reported for tryptophan, being one of the main fluorophores in tissue. We have demonstrated that this model represents our time-domain FLIM data better than multi-exponential discrete decay components, yielding excellent contrast in tissue discrimination without compromising the goodness of fit, and it significantly decreases the required processing time. In addition, the stretched exponential decay model can provide a direct measure of the sample heterogeneity and the resulting heterogeneity map can reveal subtle tissue differences that other models fail to show.