Imaging and quantitative analysis of water evaporation process using spectral-domain optical coherence tomography under illumination with two near-infrared wavelengths

Dynamics of water content in a biological tissue is an important information of a biological activity. However, although the conventional measurement method measures the total amount of water contained in the tissue, there is no established method for quantitatively measuring the amount of water localized in biological tissue. A spectral-domain optical coherence tomography system (SD-OCT) using two near-infrared wavelengths was developed for applications in cross-sectional imaging and quantitative analysis of evaporation at room temperature. The wavelengths of the light sources were selected as 1060 nm, which is hardly absorbed by water, and 1470 nm, which is absorbed by a factor of 220. In this study, we examined the effect of water absorption and scattering on the attenuation coefficient in a dense medium using a two-wavelengths OCT and showed the possibility of measuring the dynamics of water in the evaporation process through light scattering. Experimental results showed that the attenuation coefficient in a dense medium was more affected by scattering than water absorption. They are highly influenced by scattering caused by the temporal variation of the refractive-index matching effect between the hydrogen-bonded cellulose and free water around it in the process of evaporation. It was concluded that the SD-OCT was a quite practical and useful tool to visualize and quantitatively analyze natural phenomena that have never previously been observed.