Evaluation of moisture-related attenuation coefficient and water diffusion velocity in human skin using optical coherence tomography

Determination of confocal parameters of OCT imaging for eliminating confocal effect on attenuation coefficient estimation

Optical coherence tomography (OCT) provides three-dimensional images of biological tissues based on low-coherence optical interference. Attenuation coefficient estimation is one of the important functional extensions of OCT and has received many research efforts as attenuation coefficient has been found to be associated with histopathological transformation of human tissues. However, attenuation coefficient estimation accuracy is deteriorated by the confocal effect on OCT A-scan. Thus, it is desired to eliminate the confocal effect, which requires accurate determination of the confocal parameters. In this study, we propose what we believe to be a novel method for confocal parameter extraction, called dual NA ratio fitting (DNRF). DNRF requires a repetition of B-scan with varied numerical apertures (NA), altering the Rayleigh length of B-scan but keeping the focal depth fixed. Then, the focal depth and Rayleigh length can be determined by fitting an A-scan ratio function from the two repeated B-scans. The NA tuning was achieved by adding a beam expansion module into the sample arm. The method was evaluated on intralipid samples and multi-layer phantoms. The results demonstrate that our method is capable of determine the confocal parameters with good accuracy. With the extracted confocal parameters, the confocal effect was removed effectively, upon which attenuation coefficient estimation using traditional depth-resolved method appeared to be more accurate than the confocal parameter extraction based on the A-scan ratio function of two repeated B-scans with varied focal depths. Last, our method was tested on human skin in vivo, yielding attenuation coefficients consistent with literature. This indicates good potential of our method to be used for clinical applications.

Assessment of Optical Attenuation and Skin Thickness in Type 2 Diabetes Mellitus Patients Using Optical Coherence Tomography

Diabetes management often involves invasive blood glucose monitoring, which can be uncomfortable for patients. Non-invasive techniques like multiple μ-spatially offset Raman spectroscopy (mμSORS) offer a promising alternative. To provide clinical data supporting mμSORS, we conducted a clinical trial with 198 participants to evaluate mμSORS for non-invasive blood glucose measurement. Using Optical Coherence Tomography, we studied skin thickness and optical attenuation in 172 diabetic and 26 healthy subjects. Results showed thicker stratum corneum and stratum spinosum (SS) in diabetics. Epidermal thickness increased with age and body mass index (BMI), decreased with skin brightness, and varied minimally with gender. Optical attenuation in SS was lower in diabetics, decreased with increasing a*, and was minimally affected by gender and BMI but increased with age in the upper dermis. These findings support mμSORS for accurate non-invasive glucose monitoring.

Influence of optical clearing agents on the scattering properties of human nail bed and blood microrheological properties: In vivo and in vitro study

Optical clearing agents (OCAs) are substances that temporarily modify tissue's optical properties, enabling better imaging and light penetration. This study aimed to assess the impact of OCAs on the nail bed and blood using in vivo and in vitro optical methods. In the in vivo part, OCAs were applied to the nail bed, and optical coherence tomography and optical digital capillaroscopy were used to evaluate their effects on optical clearing and capillary blood flow, respectively. In the in vitro part, the collected blood samples were incubated with the OCA and blood aggregation properties were estimated using diffuse light scattering techniques. The results indicate that OCAs significantly influence the optical properties of the nail bed and blood microrheology. These findings suggest that OCAs hold promise for improving optical imaging and diagnostics, particularly for nail bed applications, and can modify blood microrheology.

Two-dimensional correlation (2D) method for improving the accuracy of OCT-based noninvasive blood glucose concentration (BGC) monitoring

The optical scattering coefficient (μ_s) in the dermis layer of human skin obtained with optical coherence tomography (OCT) has shown to have a strong correlation with the blood glucose concentration (BGC), which can be used for noninvasive BGC monitoring. Unfortunately, the nonhomogeneity in the skin may cause inaccuracies for the BGC analysis. In this paper, we propose a 2D correlation analysis method to identify 2D regions in the skin with μ_s sensitive to BGC variations and only use data in these regions to calculate μ_s for minimizing the inaccuracy induced by nonhomogeneity and therefore improving the accuracy of OCT-based BGC monitoring. We demonstrate the effectiveness of the 2D method with OCT data obtained with in vivo human forearm skins of nine different human subjects. In particular, we present a 3D OCT data set in a two-dimensional (2D) map of depth vs. a lateral dimension and calculate the correlation coefficient R between the μ_s and the BGC in each region of the 2D map with the BGC data measured with a glucose meter using finger blood. We filter out the μ_s data from regions with low R values and only keep the μ_s data with R values sufficiently high (R-filter). The filtered μ_s data in all the regions are then averaged to produce an average μ_s data. We define a term called overall relevancy (OR) to quantify the degree of correlation between the filtered/averaged μ_s data and the finger-blood BGC data to determine the optimal R value for such an R-filter with the highest obtained OR. We found that the optimal R for such an R-filter has an absolute value (|R|) of 0.6 or 0.65. We further show that the R-filter obtained with the 2D correlation method yields better OR between μ_s and the BGC than that obtained with the previously reported 1D correlation method. We believe that the method demonstrated in this paper is important for understanding the influence of BGC on μ_s in human skins and therefore for improving the accuracy of OCT-based noninvasive BGC monitoring, although further studies are required to validate its effectiveness.

General model for depth-resolved estimation of the optical attenuation coefficients in optical coherence tomography

We present the proof of concept of a general model that uses the tissue sample transmittance as input to estimate the depth-resolved attenuation coefficient of tissue samples using optical coherence tomography (OCT). This method allows us to obtain an image of tissue optical properties instead of intensity contrast, guiding diagnosis and tissues differentiation, extending its application from thick to thin samples. The performance of our method was simulated and tested with the assistance of a home built single-layered and multilayered phantoms (~100 μm each layer) with known attenuation coefficient on the range of 0.9 to 2.32 mm^-1 . It is shown that the estimated depth-resolved attenuation coefficient recovers the reference values, measured by using an integrating sphere followed by the inverse adding doubling processing technique. That was corroborated for all situations when the correct transmittance value is used with an average difference of 7%. Finally, we applied the proposed method to estimate the depth-resolved attenuation coefficient for a thin biological sample, demonstrating the ability of our method on real OCT images.

In vivo estimation of water diffusivity in occluded human skin using terahertz reflection spectroscopy

Water diffusion and the concentration profile within the skin significantly affect the surrounding chemical absorption and molecular synthesis. Occluding the skin causes water to accumulate in the top layer of the skin (the stratum corneum [SC]) and also affects the water diffusivity. Scar treatments such as silicone gel and silicone sheets make use of occlusion to increase skin hydration. However with existing techniques, it is not possible to quantitatively measure the diffusivity of the water during occlusion: current methods determine water diffusivity by measuring the water evaporated through the skin and thus require the skin to breathe. In this work, we use the high sensitivity of terahertz light to water to study how the water content in the SC changes upon occlusion. From our measurements, we can solve the diffusion equations in the SC to deduce the water concentration profile in occluded skin and subsequently to determine the diffusivity. To our knowledge, this is the first work showing how the diffusivity of human skin can be measured during occlusion and we envisage this paper as being used as a guide for non-invasively determining the diffusivity of occluded human skin in vivo.

Measurements of the thermal coefficient of optical attenuation at different depth regions of in vivo human skins using optical coherence tomography: a pilot study

We present detailed measurement results of optical attenuation's thermal coefficients (referenced to the temperature of the skin surface) in different depth regions of in vivo human forearm skins using optical coherence tomography (OCT). We first design a temperature control module with an integrated optical probe to precisely control the surface temperature of a section of human skin. We propose a method of using the correlation map to identify regions in the skin having strong correlations with the surface temperature of the skin and find that the attenuation coefficient in these regions closely follows the variation of the surface temperature without any hysteresis. We observe a negative thermal coefficient of attenuation in the epidermis. While in dermis, the slope signs of the thermal coefficient of attenuation are different at different depth regions for a particular subject, however, the depth regions with a positive (or negative) slope are different in different subjects. We further find that the magnitude of the thermal coefficient of attenuation coefficient is greater in epidermis than in dermis. We believe the knowledge of such thermal properties of skins is important for several noninvasive diagnostic applications, such as OCT glucose monitoring, and the method demonstrated in this paper is effective in studying the optical and biological properties in different regions of skin.