Accuracy of interstitial measurements of absolute light fluence rate in the determination of tissue optical properties

Abstract

The optical absorption and transport (or reduced) scattering coefficient of tissue in vivo can be deduced from in situ measurements of the light fluence rate at 2 or more points. This requires: (1) absolute measurement of the light fluence rate with small, interstitial light detectors; (2) knowledge of the irradiation and detection geometries and (3) a mathematical model relating the fluence-rate distribution to the absorption and scattering coefficients. The purpose of this study was to assess the accuracy of this technique using tissue-simulating phantoms with a wide range of known optical properties. Light fluence-rate measurements were made using either novel fluorescent-tip detectors with isotropic response of cut-end fibers of high or low numerical aperture. The light source was either a cut-end optical fiber or a fiber with a scattering tip which produced a nearly isotropic radiance distribution. Optical interaction coefficients of the phantom were derived from the fluence measurements using different solutions of the diffusion theory for infinite or semi-infinite media. Errors in the derived optical interaction coefficients, and their dependence on the optical interaction coefficients and on the source detector types are presented and discussed.