Instrumentation for real-time fluorescence lifetime imaging in endoscopy

The fluorescence lifetime of living tissues is, in certain cases, related t o their pathologic state and is therefore of interest for cancer detection. Measuring fluorescence lifetime in vivo during an endoscopic examination h as 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 su ch fast phenomenon (nanosecond time scale) on an image has been made possib le by using the homodyne detection approach, in which the excitation light and the detection gain are modulated in a phase-coherent way. Based on imag es acquired at different phase between the excitation and detection modulat ion, the fluorescence lifetime is calculated for each pixel of the image. D ifferent configurations of excitation modulation characteristics (pulse tra in versus sine-wave amplitude modulation) have been investigated and compar ed using Fourier transforms. Interestingly, a pulsed excitation combined wi th a sine-wave modulation detection gives valuable results. The expected au to-fluorescence signal emitted by human tissues under subthermal light exci tation irradiance has been estimated at the wavelengths of interest. The li mited 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 instru mentation. The results of this preliminary study indicate that the healthy bronchial mucosa, excited in the blue or in the green, fluoresces with a li fetime of 2.5 ns. (C) 1999 American Institute of Physics. [S0034-6748(99)02 512-5].