Mechanisms of ratio fluorescence imaging of diseased tissue

Abstract

The in vivo laser induced fluorescence experiments have shown that the intrinsic LIF spectral shape of bronchial dysplasia and carcinoma in situ does not differ significantly from that of normal bronchial tissue. However, the intensity of LIF from diseased tissue is significantly reduced. Comparing the in vivo measurement results and the Monte Carlo modeling based on measured optical properties, we have hypothesized that the factors which cause the fluorescence reduction in diseased tissue may be (i) thicker abnormal epithelium, (ii) slightly higher blood content, and (iii) reduction in the density of the fluorescence source density of submucosa. Based on the above hypothesis, the Monte Carlo simulation code was modified to allow one to model spectrally distorted (when compared to the intrinsic fluorescence spectra) fluorescence collected by an imaging system. The fluorescence images excited by He-Cd laser (442 nm) at three typical wavelengths (500 nm, 577 nm and 630 nm) were analyzed by using Monte Carlo simulations. The results indicated that even if there is not significant difference in spectral shape of intrinsic fluorescence between normal and abnormal bronchial tissue, the ratio of red (630 nm) to green (500 nm) fluorescence image from the modeled normal site is significantly higher than that from modeled abnormal site. And abnormal tissue can be identified by the ratio imaging technique. The major factor causing the increased red/green ratio at abnormal tissue sites is the different tissue optics between normal and abnormal tissue. In the in vivo study, five bronchial carcinoma in situ sites and 17 moderate/sever dysplasia lesions were examined by a ratio fluorescence imaging system through a bronchoscope. The red/green ratios of carcinoma in situ sites and dysplasia sites were found to be 1.85 +/- 0.2 and 1.7 +/- 0.2 higher than that of normal sites. This was consistent with the simulation results.