Spatially resolved diffuse reflectance for the determination of tissue optical properties and metabolic state

Abstract

A multiple wavelength diffuse reflectance instrument was evaluated for the in-situ measurement of tissue optical properties. A diffusion dipole model with a non-linear least- squares fitting procedure was used to generate optical properties from the radial profiles of diffuse reflectance measurements. The diffusion model was compared with Monte Carlo simulations showing acceptable agrement for a range of distances far enough away from the source point. Instrumentation was evaluated by experimental measurement of tissue simulating phantoms where measured optical properties were compared to those obtained using a collimated transmission setup. The results showed agreement to within 5 - 10%. The optical properties of rabbit ligament and tendon are reported where anisotropic scattering due to collagen alignment was observed. This study represents the first known measurement of rabbit ligament and tendon optical properties and the use of diffuse reflectance to measure the 'lightguide' effects of collagen alignment. To evaluate instrument response to changes in tissue oxygenation levels the optical properties of human skin were measured before and during arterial occlusion. During arterial occlusion absorption at 633 nm increased while absorption at 810 nm remained relatively constant. These results are consistent with the deoxygenation of hemoglobin that occurs during occlusion.