Determination of tissue optical properties by steady-state spatial frequency-domain reflectometry

A new non-invasive method to measure the optical properties of biological tissue is described. This method consists of illuminating the investigated sample with light which is spatially periodically modulated in intensity. The spatial modulation of the backscattered light and the diffuse reflectivity of the sample, both detected with an imaging technique, are used to deduce the absorption and reduced scattering coefficient from a table generated by Monte Carlo simulations. This principle has three major advantages: Firstly, it permits the immediate acquisition of the average values of the optical coefficients over a relatively large area (typ. 20 mm in diameter), thus avoiding the perturbations generated by small tissue heterogeneities; It also provides good flexibility for measuring the optical coefficients at various wavelengths and it does not require the use of a detector with a large dynamic range. The method was first validated on phantoms with known optical properties. Finally, we measured the optical properties of human skin at 400 nm, 500 nm, 633 nm and 700 nm in vivo.