Instrumentation for real-time fluorescence lifetime imaging in endoscopy

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Abstract
The fluorescence lifetime of living tissues is, in certain cases, related to their pathologic state and is therefore of interest for cancer detection. Measuring fluorescence lifetime in vivo during an endoscopic examination has thus been a challenging objective for several years. The present article deals with the development and first clinical trails of an instrumentation producing fluorescence lifetime images in real time. The acquisition of such fast phenomenon (nanosecond time scale) on an image has been made possible by using the homodyne detection approach, in which the excitation light and the detection gain are modulated in a phase-coherent way. Based on images acquired at different phase between the excitation and detection modulation, the fluorescence lifetime is calculated for each pixel of the image. Different configurations of excitation modulation characteristics (pulse train versus sine-wave amplitude modulation) have been investigated and compared using Fourier transforms. Interestingly, a pulsed excitation combined with a sine-wave modulation detection gives valuable results. The expected auto-fluorescence signal emitted by human tissues under subthermal light excitation irradiance has been estimated at the wavelengths of interest. The limited number of auto-fluorescence photons results in relatively high noise on the lifetime calculated. The typical standard deviation is about 125 ps for lifetimes of 2.5 ns with a 32 2 -pixel image (spatial integration). An in vivo image in the bronchi illustrates the potentiality of the new instrumentation. The results of this preliminary study indicate that the healthy bronchial mucosa, excited in the blue or in the green, fluoresces with a lifetime of 2.5 ns.