Frequency-domain fluorescence lifetime imaging for endoscopic clinical cancer photodetection: Apparatus design and preliminary results
We describe a new fluorescence imaging device for clin. cancer photodetection in hollow organs in which the tumor/normal tissue contrast is derived from the fluorescence lifetime of endogenous or exogenous fluorochromes. This fluorescence lifetime contrast gives information about the physicochem. properties of the environment which are different between normal and certain diseased tissues. The excitation light from a CW laser is modulated in amplitude at a radiofrequency by an electrooptical modulator and delivered by an optical fiber through an endoscope to the hollow organ. The image of the tissue collected by the endoscope is sepd. in two spectral windows, one being the backscattered excitation light and the other the fluorescence of the fluorochrome. Each image is then focused on the photocathode of image intensifiers (II) whose optical gain is modulated at the same frequency as the excitation intensity, resulting in homodyne phase-sensitive images. By acquiring stationary phase-sensitive frames at different phases between the excitation and the detection, it is possible to calc. in quasi-real time the apparent fluorescence lifetime of the corresponding tissue region for each pixel. A result obtained by investigating the endogenous fluorochromes present in the mucous membrane of an excised human bladder is presented to illustrate this method and most of the optical parameters which are of major importance for this photodetection modality have been evaluated.