Evaluation of Moisture-Related Attenuation Coefficient and Water Diffusion Velocity in Human Skin Using Optical Coherence Tomography

In this study, time-resolved optical coherence tomography (OCT) scanning images of the process of water diffusion in the skin that illustrate the enhancement in the backscattered intensities due to the increased water concentration are presented. In our experiments, the water concentration in the skin was increased by soaking the hand in water, and the same region of the skin was scanned and measured with the OCT system and a commercial moisture monitor every three minutes. To quantitatively analyze the moisture-related optical properties and the velocity of water diffusion in human skin, the attenuation coefficients of the skin, including the epidermis and dermis layers, were evaluated. Furthermore, the evaluated attenuation coefficients were compared with the measurements made using the commercial moisture monitor. The results demonstrate that the attenuation coefficient increases as the water concentration increases. Furthermore, by evaluating the positions of center-of mass of the backscattered intensities from OCT images, the diffusion velocity can be estimated. In contrast to the commercial moisture monitor, OCT can provide three-dimensional structural images of the skin and characterize its optical property, which together can be used to observe morphological changes and quantitatively evaluate the moisture-related attenuation coefficients in different skin layers.

from human fingertips [36]. The same group was also able to visualize the dynamics of the small arteries and veins of human fingers using OCT [37].
In this study, an SS-OCT system is implemented for the investigation of moisture-related optical property of human skin. In our experiments, OCT scans taken every 3 min after soaking the palm in water were used to observe water diffusion and evaluate the moisture-related attenuation coefficient of human skin. The time-resolved OCT scans revealed the process of water diffusion in the skin, which we then analyzed quantitatively along with the skin's moisture by evaluating the skin's attenuation coefficients. Then, the OCT scanning results were compared with the measurements made by a commercial moisture monitor. Furthermore, to investigate the diffusion velocity in skin, the positions of center-of-mass of backscattered intensities in the longitudinal direction from OCT images are evaluated.

Experimental Section
Figure 1(a) shows a schematic diagram of the portable SS-OCT system used for studying water diffusion in the skin [38]. The central wavelength and the scanning range of the swept source are 1,310 nm and 110 nm, respectively. This source can provide an output power of 6 mW and a sweeping rate of 30 kHz. It is connected to a Mach-Zehnder interferometer, consisting of two circulators and two couplers. Ten percent of the output power from the swept source is connected to a narrowband fiber Bragg grating (FBG) to generate an A-scan trigger for each A-scan. The narrowband FBG has a Bragg wavelength of 1,275 nm, and the reflected signal from the FBG is combined with the interfered signal by a 10/90 fiber coupler. To eliminate the DC component of the interfered signal, another 10/90 fiber coupler is used before the balanced detector (PDB150C, Thorlabs). Finally, the data from the balanced detector is sampled with a high-speed digitizer at a sampling rate of 100 MB/s (PXIe-5122, National Instruments). Based on this mechanism, the time-induced phase errors can be greatly reduced, and only half the on-board memory of the digitizer is required for data acquisition. In the sample arm, a palm-held probe is implemented for skin scanning. Figure 1(b) shows the layout of the probe for scanning human skin. A single-mode fiber with an FC/APC connector is connected to a collimator, and the output light beam was incident onto a two-axis galvanometer, which provides lateral and transverse scanning. The light beam is focused by an achromatic lens having a focal length of 10 mm, resulting in the focusing of the light beam at a depth of 300 μm beneath the sample surface. In this OCT system, the frame rate can achieve 50 frames per second, each consisting of 600 A-scans.
Water concentration in the skin is an important factor in preventing skin damage from external infections and aging. To increase the water concentration in skin, the left palm of a 23-year-old volunteer was soaked in water. Because lipids on the SC influence water diffusion and hydration, the volunteer washed his palm with soap to speed up water diffusion before the measurement. The index fingertip was scanned using the OCT system at 0, 3,6,9,12,15,18, and 30 min after soaking. After each OCT scan, a commercial moisture monitor (ZRH-009, Chung Yun Industrial) that assesses moisture levels based on the electrical conductance measurement was also used to measure the water concentration. To facilitate scanning of the same region of the index fingertip in each measurement, the scanned region was marked. However, the regions scanned in each measurement were not exactly identical, even with the marking, although each scan did cover most of the marked region. After each    To evalua o obtain the decay fits of DM layers a 30 min, the respectively are greater t diffusion int

Evaluat
To statist attenuation Figure 4, wh ine. In Figu seven measu and the mea ime. Howe monitor. Co EP and DM becomes sat mirrored by    layers before and after soaking the palm in water for 30 min revealed that the attenuation coefficients of the epidermis and dermis layers increase after soaking the palm in water. Additionally, we analyzed the moisture-related attenuation coefficients as a function of increasing immersion time and compared the OCT results with the measured moisture from the commercial moisture monitor. The moisture-related attenuation coefficients evaluated using OCT show the same trend as the results from the commercial product. Furthermore, the diffusion velocity of water in human skin can be estimated by evaluating the positions of center-of-mass of intensity variance OCT images, obtained from the successive OCT images. From the results, the water diffusion velocities in the EP and DM layers of one volunteer' skin are 12.32 μm/min and 19.61 μm/min, respectively. However, such information still cannot be obtained from any other commercial products.