A modified phasor approach for analyzing time-gated fluorescence lifetime images.

Fluorescence lifetime imaging is a versatile tool that permits mapping the biochemical environment in the cell. Among various fluorescence lifetime imaging techniques, time-correlated single photon counting and time-gating methods have been demonstrated to be very efficient and robust for the imaging of biological specimens. Recently, the phasor representation of lifetime images became popular because it provides an intuitive graphical view of the fluorescence lifetime content of the images and, when used for global analysis, significantly improves the overall S/N of lifetime analysis. Compared to time-correlated single photon counting, time gating methods can provide higher count rates (∼10 MHz) but at the cost of truncating and under sampling the decay curve due to the limited number of gates commonly used. These limitations also complicate the implementation of the phasor analysis for time-gated data. In this work, we propose and validate a theoretical framework that overcomes these problems. This modified approach is tested on both simulated lifetime images and on cells. We demonstrate that this method is able to retrieve two lifetimes from time gating data that cannot be resolved using standard (non-global) fitting techniques. The new approach increases the information that can be obtained from typical measurements and simplifies the analysis of fluorescence lifetime imaging data.