Endoscopic tissue characterization by frequency-domain fluorescence lifetime imaging (FD-FLIM)

Tissue characterization by endoscopic fluorescence imaging of endogenous or-exogenous fluorochromes is a promising method for early cancer detection. However, the steady-state fluorescence contrast between healthy tissue and lesions such as early-stage carcinomas is generally poor. The authors propose to improve this contrast by using the additional information contained in the fluorescence lifetime (FLT). The FLT of several fluorochromes is sensitive to their physico-chemical environment. The FLT can be measured by frequency-domain methods. The excitation light from a continuous wave (CW) laser is modulated in amplitude at radio-frequencies by an electro-optic modulator, and delivered to the tissue via an optical fibre. The endoscopic site is imaged by an endoscope on to an optical device. The gain of the fluorescence image detector is also modulated at the same frequency for homodyning. The tissue fluorescence image is recorded at several phases between the excitation and the detection modulations during an acquisition cycle. With these images, an image processor calculates the apparent FLT for each pixel and constructs a lifetime image of the endoscopic site. This process is performed at quasi-video frequencies. The influence of various physical parameters (modulation frequency, number of images by; cycle, shot noise, tissue optical properties etc.) has been investigated by analytical analysis, simulation methods and experimentation. Preliminary results obtained on human tissues are also presented to illustrate the potentiality of the method.