Simulation of oblique-incidence probe geometries for depth-resolved fluorescence spectroscopy

Abstract

Optimization of device-tissue interface parameters may lead to an improvement in the efficacy of fluorescence spectroscopy for minimally-invasive disease detection. Although illumination-collection geometry has been shown to have a strong influence on the spatial origin of detected fluorescence, the performance of devices which deliver and/or collect light at oblique incidence are not well characterized or understood. Simulations were performed using a Monte Carlo model of light propagation in homogeneous tissue in order to identify and describe general trends in the intensity and spatial origin of fluorescence detected by angled geometries. Specifically, the influence of illumination angle, collection angle and illumination-collection spot separation distance were investigated for low and high attenuation tissue cases. Results indicated that oblique-incidence geometries have the potential to enhance the selective interrogation of superficial or subsurface fluorophores at user-selectable depths up to about 0.5 mm. Detected fluorescence intensity was shown to increase significantly with illumination and collection angle. Improved selectivity and signal intensity over normal-incidence geometries resulted from the overlap of illumination and collection cones within the tissue. Cases involving highly attenuating tissue produced a moderate reduction in the depth of signal origin. While Monte Carlo modeling indicates that oblique-incidence designs can facilitate depth-selective fluorescence spectroscopy, optimization of device performance will require application-specific consideration of optical and biological parameters.