Construction and characterization of a frequency-domain fluorescence lifetime imaging microscopy system

The construction of a homodyne frequency domain fluorescence lifetime imaging microscope is described. The system consists of (i) an intensity-modulated laser excitation source, (ii) an epifluorescence microscope, (iii) a gain-modulated microchannel plate (MCP) image intensifier, and (iv) a slow-scan CCD camera. The phase and modulation homogeneity of the MCP image intensifier were determined at frequencies of 40, 100, 160, and 240 MHz. The detected modulation depths were 65, 52, 32, and 23%, respectively, and were highly homogeneously distributed. The phasedistribution image revealed iris effects at frequencies of 160 and 240 MHz but was homogeneous at lower frequencies. Lifetime imaging of a solution of the fluorescent flavoprotein lipoamide dehydrogenase demonstrated (i) the accuracy of the determined lifetimes (< 60 ps), (ii) the time resolution of the instrument (< 50 ps), and (iii) the average precision for single pixel fluorescence lifetimes (50 ps is feasible). The imaging of tiny fluorescent microspheres revealed that even in a volume of 0.3 x 10^-15 L, the standard error in the lifetimes can be as low as 79 ps. The spatial resolution of the instrument is estimated to be < 400 nm in the object plane at a 100 x magnification.