We demonstrate low-coherence interferometry for diffusion measurements. We have measured the diffusion coefficient of a phthalocyanine dye in 1.5% agar gel with a two-wavelength interferometer; one wavelength was matched to the absorption peak of the dye at 675 nm, while the other, 805 nm, was not affected by the dye. The diffusion coefficient of the dye was found by fitting a mathematical model for the interferometer signal to the measured low-coherence interferometry amplitude. A 95% confidence interval for the diffusion coefficient was found to be D = (2.5 +/- 0.2) x 10(-10) m2/s. The influence of speckle averaging and experiment time on the determination of the diffusion coefficient has been studied. The presented technique allows in situ characterization of diffusion in semitransparent media.

We demonstrate that spectroscopic optical coherence tomography can be used for measurement of diffusion. We measured the diffusion coefficient of a Phthalocyanine dye in an Agar gel as a first model of dye diffusing into tissue. We used a two-wavelength interferometer, with one of the wavelengths matched to the absorption peak of the dye at 675nm while the other wavelength at 805nm is not affected by the dye. The diffusion constant of the dye in Agar gel is found by fitting the measured OCT amplitude (depth and time dependent) to a mathematical model for the OCT signal. This method may be used as a tool for dosimetry in Photo Dynamic Therapy (PDT). In PDT the therapeutic light exposure should be applied at a time when the concentration of sensitizer is optimal in the diseased tissue relative to normal tissue. By studying how the OCT signal changes with time and depth at two wavelengths differently affected by the diffusing dye, it should be possible to extract parameters determining diffusion of the sensitizer in live tissue. In comparison with fluorescence-based methods, this OCT approach has the advantage of better depth penetration and being able to account for attenuation effects due to scattering.