Relevance and utility of the in-vivo and ex-vivo optical properties of the skin reported in the literature: a review [Invited]

Diffuse transmittance spectroscopy for ultra short-term measurement of pulse rate variability in healthy subjects

Multi-wavelength plethysmography (MWPPG) is a growing technique for noninvasive hemodynamic measurements, particularly spectral based methods. Different wavelengths have been investigated for both transmissive and reflectance PPG configurations. The objective of this work is to investigate the feasibility of Diffuse Transmittance Spectroscopy (DTS) for Pulse Rate Variability (PRV) measurements and to evaluate the performance of diffuse transmittance spectroscopy in quantifying ultrashort pulse rate variability. DTS was used for reconstructing PPG signal followed by PRV analysis. DTS and reference PPG recordings were acquired from 18 healthy subjects in total. PRV features include time-domain, and frequency-domain features are extracted from 50 s duration. The extracted PRV parameters were compared to PRV parameters derived from conventional pulse oximetry-based PPG. Pulse rate variability analysis was applied on DTS and reference PPG tracings. The comparison demonstrated a strong correlation between the diffuse transmittance spectral method and the gold standard PPG sensor. Significant correlation (r > 0.90, p < 0.05) was found between PRV from DTS and reference PRV for mean intervals, standard deviation of intervals (SDNN) and the root-mean square of the difference of successive intervals (RMSSD). A good agreement was found between PRV Parameters in time domain of PPG analysis using Bland-Altman plots with 95 % limits of agreements. However, Bland-Altman analysis showed a considerable divergence in frequency parameters. The study also revealed that PPG based diffuse transmittance measurements were insensitive to ambient noise in comparison with conventional pulse oximeter. The results suggest a potential application of the diffuse transmittance spectroscopy for pulse rate variability analysis.

Real Index of Refraction of Normal and Cancerous Axillary Lymph Nodes in Breast Cancer Patients: Results from an Experimental Study

Background: Breast malignancy is the most common cancer type and the second leading cause of cancer-related death for women all ages. Axillary surgery provides prognostic and predictive information, but carries significant morbidity. Imaging techniques are a promising field, providing the characterization of biological tissues using the interaction between the light and a medium, and may offer an accurate cancerous diagnosis without the need for formal histopathological examination. Methods: In this study, using a prism couple refractometer, we sought to determine tissues' reflection profiles in freshly excised human lymph nodes from female patients with breast cancer, in whom axillary lymph node dissection was performed. Results: Thirty-four patients were included, contributing a total number of 90 lymph nodes and, according to our results, the median refractive indices were significantly higher in cancerous lymph nodes compared to normal lymph nodes in 450 nm, 964 nm, and 1551 nm wavelengths (p < 0.05). Conclusions: Results from this small experimental study imply that the use of a prism couple refractometer may aid in the discrimination between benign and malignant axillary lymph nodes in female patients with breast cancer.

Non-Invasive Laser Surgery With Deep Operating Depth Using Multibeam Interference

Purpose

Laser surgery can use photo-chemical, photo-thermal, photo-ablative, and photo-mechanical effects to treat various tissues in the human body, and has unique advantages of extremely high precision, non-invasive penetration, and fast operation speed. However, at present, the effective penetration depth of directly illuminating light in the body is only several millimeters. Therefore, increasing the safe operating depth for non-invasive laser surgery will have important, widespread, and irreplaceable applications in the future.

Methods

The method is based on improving a recently emerged technique. Its principle involves using a negative dispersion device to broaden the width of the short light pulse first. Then, after the pulse enters the body, as its peak intensity is reduced, the skin and healthy tissues in the laser propagation path cannot be injured. Meanwhile, since body tissues have positive dispersion, the broadened width of the laser pulse will be shortened back. When the broadened pulse is completely shortened, a thin inner light layer with high intensity will be formed in the body and used as a scalpel to treat target tissue.

Results

The theoretical calculation results have shown that the designed apparatus has excellent performance. Its safe non-invasive operating depth can be more than 70 millimeters with the possibility of up to 130 millimeters. Surgery precisions are around 1 micron transversely and about 1 millimeter longitudinally in theory.

Conclusion

An improved method of non-invasive laser surgery with deep operation depth has been investigated theoretically. The calculations show that the designed apparatus has excellent performance. The proposed method depends on two well-known physical phenomena: light pulse broadening and shortening caused by optical negative and positive dispersions, and thus has solid basis. The developed method will have important, widespread and irreplaceable applications in the medical surgery field.

Seeing through the skin: Optical methods for visualizing transdermal drug delivery with microneedles

Optical methods play a pivotal role in advancing transdermal drug delivery research, particularly with the emergence of microneedle technology. This review presents a comprehensive analysis of optical methods used in studying transdermal drug delivery facilitated by microneedle technology. Beginning with an introduction to microneedle technology and skin anatomy and optical properties, the review explores the integration of optical methods for enhanced visualization. Optical imaging offers key advantages including real-time drug distribution visualization, non-invasive skin response monitoring, and quantitative drug penetration analysis. A spectrum of optical imaging modalities ranging from conventional dermoscopy and stereomicroscopy to advance techniques as fluorescence microscopy, laser scanning microscopy, in vivo imaging system, two-photon microscopy, fluorescence lifetime imaging microscopy, optical coherence tomography, Raman microspectroscopy, laser speckle contrast imaging, and photoacoustic microscopy is discussed. Challenges such as resolution and depth penetration limitations are addressed alongside potential breakthroughs and future directions in optical techniques development. The review underscores the importance of bridging the gap between preclinical and clinical studies, explores opportunities for integrating optical imaging and chemical sensing methods with drug delivery systems, and highlight the importance of non-invasive "optical biopsy" as a valuable alternative to conventional histology. Overall, this review provides insight into the role of optical methods in understanding transdermal drug delivery mechanisms with microneedles.

Effects of cold storage on double integrating sphere optical property measurements of porcine dermis and subcutaneous fat from 400 to 1100 nm

Significance

Accurate values of skin optical properties are essential for developing reliable computational models and optimizing optical imaging systems. However, published values show a large variability due to a variety of factors, including differences in sample collection, preparation, experimental methodology, and analysis.

Aim

We aim to explore the influence of storage conditions on the optical properties of the excised skin from 400 to 1100 nm.

Approach

We utilize a double integrating sphere system and inverse adding-doubling approach to determine absorption,μ a , and reduced scattering,μ s ' , coefficients of the porcine dermis and subcutaneous fat before and after refrigeration, freezing, or flash freezing.

Results

Our findings indicate a small average change of - 0.005 , - 0.003 , and 0.002    mm - 1 inμ a for the dermis and 0.001, - 0.003 , and - 0.008    mm - 1 for the subcutaneous tissue after refrigeration, freezing, and flash freezing, respectively, with the most notable differences observed in the hemoglobin absorption region. The value ofμ s ' shows a negligible average change of - 0.05 , - 0.001 , and - 0.02   mm - 1 for the dermis, and 0.06, - 0.1 , and 0.03    mm - 1 change for the subcutaneous tissue for refrigerated, frozen, and flash-frozen samples, respectively.

Conclusions

The results provide additional context for the variability of published values of optical parameters and enable informed selection of sample storage conditions for future measurements. In addition, the results discussed here can be used to improve study planning, particularly with regard to maximizing the use of finite samples that have been collected.

Evaluating facial dermis aging in healthy Caucasian females with LC-OCT and deep learning

Recent advancements in high-resolution imaging have significantly improved our understanding of microstructural changes in the skin and their relationship to the aging process. Line Field Confocal Optical Coherence Tomography (LC-OCT) provides detailed 3D insights into various skin layers, including the papillary dermis and its fibrous network. In this study, a deep learning model utilizing a 3D ResNet-18 network was trained to predict chronological age from LC-OCT images of 100 healthy Caucasian female volunteers, aged 20 to 70 years. The AI-based protocol focused on regions of interest delineated between the segmented dermal-epidermal junction and the superficial dermis, exploiting complex patterns within the collagen network for age prediction. The model achieved a mean absolute error of 4.2 years and exhibited a Pearson correlation coefficient of 0.937 with actual ages. Furthermore, there was a notable correlation (r = 0.87) between quantified clinical scoring, encompassing parameters such as firmness, elasticity, density, and wrinkle appearance, and the ages predicted by deep learning model. This strong correlation underscores how integrating emerging imaging technologies with deep learning can accelerate aging research and deepen our understanding of how alterations in skin microstructure are related to visible signs of aging.

Age dependency of the reduced scattering coefficient and viscoelastic character, and their relationship in Japanese female skin

We measured the reduced scattering coefficient (µs') with the edge-loss method at skin sites on the faces and upper arms of Japanese females aged 20s to 50s, and observed significant negative correlations at all sites, including the mouth. At the same mouth site, we also measured the viscoelastic properties with a Cutometer MPA580 and evaluated their correlation with µs'. Significant correlations were observed for the parameters of pliability, the portion of the viscoelasticity during the suction phase, and total recovery from the Cutometer against µs' with the effect of age removed. The results experimentally suggest the relation between µs' and the collagen condition.

Enhancing tumor's skin photothermal therapy using Gold nanoparticles : a Monte Carlo simulation

The aim of this study is to investigate how the introduction of Gold nanoparticles GNPs into a skin tumor affects the ability to absorb laser light during multicolor laser exposure. The Monte Carlo Geant4 technique was used to construct a cubic geometry simulating human skin, and a 5 mm tumor spheroid was implanted at an adjustable depth x. Our findings show that injecting a very low concentration of 0.01% GNPs into a tumor located 1 cm below the skin's surface causes significant laser absorption of up to 25%, particularly in the 900 nm to 1200 nm range, resulting in a temperature increase of approximately 20%. It is an effective way to raise a tumor's temperature and cause cell death while preserving healthy cells. The addition of GNPs to a tumor during polychromatic laser exposure with a wavelength ranging from 900 nm to 1200 nm increases laser absorption and thus temperature while preserving areas without GNPs.

Correlation analysis between texture features and elasticity of skin hyperspectral images in the near-infrared band

BACKGROUND/PURPOSE

Skin elasticity was used to evaluate healthy and diseased skin. Correlation analysis between image texture characteristics and skin elasticity was performed to study the feasibility of assessing skin elasticity using a non-contact method.

MATERIALS AND METHODS

Skin images in the near-infrared band were acquired using a hyperspectral camera, and skin elasticity was obtained using a skin elastimeter. Texture features of the mean, standard deviation, entropy, contrast, correlation, homogeneity, and energy were extracted from the acquired skin images, and a correlation analysis with skin elasticity was performed.

RESULTS

The texture features, and skin elasticity of skin images in the near-infrared band had the highest correlation on the side of eye and under of arm, and the mean and correlation were features of texture suitable for distinguishing skin elasticity according to the body part.

CONCLUSION

In this study, we performed elasticity and correlation analyses for various body parts using the texture characteristics of skin hyperspectral images in the near-infrared band, confirming a significant correlation in some body parts. It is expected that this will be used as a cornerstone of skin elasticity evaluation research using non-contact methods.

Effect of skin color on optical properties and the implications for medical optical technologies: a review

Significance

Skin color affects light penetration leading to differences in its absorption and scattering properties. COVID-19 highlighted the importance of understanding of the interaction of light with different skin types, e.g., pulse oximetry (PO) unreliably determined oxygen saturation levels in people from Black and ethnic minority backgrounds. Furthermore, with increased use of other medical wearables using light to provide disease information and photodynamic therapies to treat skin cancers, a thorough understanding of the effect skin color has on light is important for reducing healthcare disparities.

Aim

The aim of this work is to perform a thorough review on the effect of skin color on optical properties and the implication of variation on optical medical technologies.

Approach

Published in vivo optical coefficients associated with different skin colors were collated and their effects on optical penetration depth and transport mean free path (TMFP) assessed.

Results

Variation among reported values is significant. We show that absorption coefficients for dark skin are ∼ 6 % to 74% greater than for light skin in the 400 to 1000 nm spectrum. Beyond 600 nm, the TMFP for light skin is greater than for dark skin. Maximum transmission for all skin types was beyond 940 nm in this spectrum. There are significant losses of light with increasing skin depth; in this spectrum, depending upon Fitzpatrick skin type (FST), on average 14% to 18% of light is lost by a depth of 0.1 mm compared with 90% to 97% of the remaining light being lost by a depth of 1.93 mm.

Conclusions

Current published data suggest that at wavelengths beyond 940 nm light transmission is greatest for all FSTs. Data beyond 1000 nm are minimal and further study is required. It is possible that the amount of light transmitted through skin for all skin colors will converge with increasing wavelength enabling optical medical technologies to become independent of skin color.