Fit-free analysis of fluorescence lifetime imaging data using the phasor approach

Fluorescence lifetime imaging microscopy (FLIM) is used in diverse disciplines, including biology, chemistry and biophysics, but its use has been limited by the complexity of the data analysis. The phasor approach to FLIM has the potential to markedly reduce this complexity and at the same time provide a powerful visualization of the data content. Phasor plots for fluorescence lifetime analysis were originally developed as a graphical representation of excited-state fluorescence lifetimes for in vitro systems. The method's simple mathematics and specific rules avoid errors and confusion common in the study of complex and heterogeneous fluorescence. In the case of FLIM, the phasor approach has become a powerful method for simple and fit-free analyses of the information contained in the many thousands of pixels constituting an image. At present, the phasor plot is used not only for FLIM, but also for hyperspectral imaging, wherein phasors provide an unprecedented understanding of heterogeneous fluorescence. Undoubtedly, phasor plots will be increasingly important in the future analysis and understanding of FLIM and hyperspectral confocal imaging. This protocol presents the principle of the method and guides users through one of the popular interfaces for FLIM phasor analysis, namely, the SimFCS software. Implementation of the analysis takes only minutes to complete for a dataset containing hundreds of files.