Noninvasive measurement of fluorophore concentration in turbid media with a simple fluorescence /reflectance ratio technique.

Measurement of the concentration of fluorescent compounds in turbid media is difficult because the absorption and multiple scattering of excitation and emission of light has a large effect on the detected fluorescence. For surface measurements with optical fibers we demonstrate by experiments and numerical simulation that this effect can be minimized by measurement of the fluorescence at one source-detector distance, the diffusely reflected excitation light at a second distance, and with the ratio of these two signals as an indicator of fluorophore concentration. For optical properties typical of soft tissue in the red and the near infrared the optimum performance is obtained by measurement of fluorescence at 0.65 mm and reflectance at 1.35 mm. This choice reduces the rms error in fluorophore concentration to 14.6% over a wide range of absorption and scattering coefficients.