<title>Combination of diffuse reflectance and fluorescence imaging of turbid media</title>

Abstract

ABSTRACT We have used the cross-polarization method to eliminate the specular reflection and enhance the diffusive back-scattering ofpolarized fluorescence excitation light from the turbid media. The image of the ratio between fluorescence and cross-polarized reflection provides a map of the fluorescence yield, defined as the ratio of the autofluorescence emission to theillumination incidence, over the surface of the turbid media when excitation and collection of fluorescence are highlyinhomogeneous. The simple ratio imaging technique shows the feasibility to detect early cancer, which usually starts fromthe superficial layer oftissue, based on the contrast in the fluorescence yield between lesion and normal tissue.Keywords: Fluorescence, diffuse reflection, tissue, imaging 1. INTRODUCTION Optical spectroscopy has been widely used in chemical analysis of biological samples. Since the mid-80s, the use of thistechnology for tissue classification has become a new approach in non-invasive medical diagnostics. In particular, themethods involved with laser-induced fluorescence (LIF) to characterize pathological states in human tissue have beenextensively investigated1171. The tissues are usually classified based on the spectral characteristics of endogenous tissuefluorescence (autofluorescence). Careful in vivo studies on several organ sites with an optical fiber catheter revealed thatthe fluorescence yield of early lesions were almost always lower than that of the surrounding normal tissues, although thespectral lineshapes of early lesions and normal tissues vary both from individual by individual and within an individual471.The algorithms based on the contrast in fluorescence yield between lesion and normal tissue can be used to discriminateearly lesions. However, the point by point diagnosis by the use of an optical fiber catheter of fixed fluorescence excitationand collection geometry is not practical in clinical application. An imaging technique is desirable.For examination of large area by an imaging device, recorded fluorescence power are strongly affected by the geometricaleffects such as separation of the source-sample-detector, incident/emission angles and irregularity of sample surface.