Application of the stretched exponential function to fluorescence lifetimeimaging

Conventional analyses of fluorescence lifetime measurements resolve the flu orescence decay profile in terms of discrete exponential components with di stinct lifetimes. In complex, heterogeneous biological samples such as tiss ue, multi-exponential decay functions can appear to provide a better fit to fluorescence decay data than the assumption of a mono-exponential decay, b ut the assumption of multiple discrete components is essentially arbitrary and is often erroneous. Moreover, interactions, both between fluorophores a nd with their environment, can result in complex fluorescence decay profile s that represent a continuous distribution of lifetimes. Such continuous di stributions have been reported for tryptophan, which is one of the main flu orophores in tissue. This situation is better represented by the stretched- exponential function (StrEF). In this work, we have applied, for the first time to our knowledge, the StrEF to time-domain whole-field fluorescence li fetime imaging (FLIM), yielding both excellent tissue contrast and goodness of fit using data from rat tissue. We note that for many biological sample s for which there is no a priori knowledge of multiple discrete exponential fluorescence decay profiles, the StrEF is likely to provide a truer repres entation of the underlying fluorescence dynamics. Furthermore, fitting to a StrEF significantly decreases the required processing time, compared with a multi-exponential component fit and typically provides improved contrast and signal/noise in the resulting FLIM images. In addition, the stretched-e xponential decay model can provide a direct measure of the heterogeneity of the sample, and the resulting heterogeneity map can reveal subtle tissue d ifferences that other models fail to show.