Anisotropy of light propagation in human skin

Anisotropic mechanical characterization of human skin by in vivo multi-axial ring suction test

Human skin is a soft tissue behaving as an anisotropic material. The anisotropy emerges from the alignment of collagen fibers in the dermis, which causes the skin to exhibit greater stiffness in a certain direction, known as Langer's line. The importance of determining this anisotropy axis lies in assisting surgeons in making incisions that do not produce undesirable scars. In this paper, we introduce an open-source numerical framework, MARSAC (Multi-Axial Ring Suction for Anisotropy Characterization: https://github.com/aflahelouneg/MARSAC), adapted to a commercial device CutiScan CS 100® that applies a suction load on an annular section, causing a multi-axial stretch in the central zone, where in-plane displacements are captured by a camera. The presented framework receives inputs from a video file and converts them into displacement fields through Digital Image Correlation (DIC) technique. From the latter and based on an analytical model, the method assesses the anisotropic material parameters of human skin: Langer's line ϕ, and the elastic moduli E₁ and E₂ along the principal axes, providing that the Poisson's ratio is fixed. The pipeline was applied to a public data repository, https://search-data.ubfc.fr/femto/FR-18008901306731-2021-08-25_In-vivo-skin-anisotropy-dataset-for-a-young-man.html, containing 30 test series performed on a forearm of a Caucasian subject. As a result, the identified parameter averages, ϕˆ=40.9±8.2^∘ and the anisotropy ratio E₁ˆ/E₂ˆ=3.14±1.60, were in accordance with the literature. The intra-subject analysis showed a reliable assessment of ϕ and E₂. As skin anisotropy varies from site to site and from subject to subject, the novelty of the method consists in (i) an optimal utilization of CutiScan CS 100® probe to measure the Langer's line accurately and rapidly on small areas with a minimum diameter of 14mm, (ii) validation of an analytical model based on deformation ellipticity.

Mechanical anisotropy of hair affected by genetic diseases highlights structural information related to differential crosslinking in keratins

Previous work with Atomic Force Microscope (AFM) nanoindentation, on longitudinal and cross-sections of the human hair fibre, allowed for the derivation of a model for the mechanical behaviour of human hair, called the Anisotropic Index. Expanding that research further, and by applying this model, the nanomechanical behaviour of hairs from patients with the disease Trichothiodystrophy (TTD) has been examined and structural insights, gained from combining the AFM results with Differential Scanning Calorimetry (DSC) experiments and tensile measurements, suggests that TTD-affected hairs have a relatively increased amount of Keratin Intermediate Filaments, contained in compartments of differing crosslinking extent. The associated calculations of axial and transverse Young's Moduli deliver values in good agreement with the measured fibre mechanics. Furthermore, comparing these findings with the results previously obtained from the study of hairs from patients with the disease Monilethrix, it is shown that the Anisotropic Index correlates well with the known deficiencies in both hair types obtained from such patients and allows for discerning between the Control hair and from those affected by the two diseases. AFM nanoindentation along and across the fibre axis and the Anisotropic Index thus appear to reveal structural details of hair not otherwise acquirable, whilst DSC may offer a quick and simple method for distinguishing between different severities of TTD.

The effects of polarized photobiomodulation on cellular viability, proliferation, mitochondrial membrane potential and apoptosis in human fibroblasts: Potential applications to wound healing

Photobiomodulation (PBM) is a widely used therapeutic intervention used to treat several chronic conditions. Despite this, fundamental research underpinning its effectiveness is lacking, highlighted by the lack of a definitive mechanism of action. Additionally, there are many treatment variables which remain underexplored, one of those being the effect of polarization the property of light that specifies the direction of the oscillating electric field. When applied to PBM, using linearly polarized light, when compared to otherwise identical non-polarized light, may enhance its biological efficacy. As such, we investigated the potential biological effects of polarized PBM when compared to non-polarized and non-irradiated controls in the domains of cellular viability, proliferation, apoptosis and mitochondrial membrane potential (ΔΨ) within cells exposed to oxidative stress. It was noted that polarized PBM, when compared to non-polarized PBM and non-irradiated controls, demonstrated mostly increased levels of cellular proliferation and ΔΨ, whilst decreasing the amount of cellular apoptosis. These results indicate that polarization may have utility in the clinical application of PBM. Future research is needed to further elucidate the underpinning mechanisms of PBM and polarization.

Transmission of 940 nm diode laser to the radicular area during its application as root canal disinfectant

To measure the transmitted laser power in the coronal, middle and apical root thirds during vertical and horizontal irradiation of laser. 14 mm length whole roots and longitudinal root sections were irradiated using a 940 nm diode laser with 0.5 W in continuous mode for 5 s. A power detector was placed in front of the root apex for the vertical transmitted power measurement and placed laterally to root surface for the horizontal transmission experiment. The transmitted power from the root apex vertically was 53% of the irradiated power, while for the horizontal transmission experiment for the coronal, middle and apical root thirds were 25.6%, 40.4% and 41.3%, respectively. Irradiation of root canals with 940 nm laser power can be transmitted during vertical irradiation in more than 50% of the samples, whereas less transmission happens with horizontal irradiation at apical and middle root third than coronal.

Probing layered structures by multi-color backscattering polarimetry and machine learning

Polarization imaging can quantitatively probe the characteristic microstructural features of biological tissues non-invasively. In biomedical tissues, layered structures are common. Superposition of two simple layers can result in a complex Mueller matrix, and multi-color backscattering polarimetry can help to probe layered structures. In this work, multi-color backscattering Mueller matrix images are measured for living nude mice skins. Preliminary analysis of anisotropy parameter A and linear polarizance parameter b show signs of a layered structure in the skin. For more detailed examinations on polarization features of layered samples, we generate Mueller matrices by experimenting with two-layered thick tissues and concentrically aligned silk submerged in milk. Then we use supervised machine learning to identify polarization parameters that are sensitive to layered structure and guide the synthesis of more parameters. Monte Carlo simulation is also adopted to explore the relationship between parameters and microstructures of media. We conclude that multi-color backscattering polarimetry combined with supervised machine learning can be applied to probe the characteristic microstructure in layered living tissue samples.

Effects of the lower energy and pulse stacking in carbon dioxide laser skin treatment: an objective analysis using second harmonic generation

PURPOSE

To evaluate the effect of fractional carbon dioxide (CO2) laser treatment using lower power associated with pulse stacking within collagen fibers, using second harmonic generation microscopy and computerized image analysis.

METHODS

Twenty male Wistar rats aging eight weeks were used. Each treatment area received a single-pass CO2 fractional laser with different parameters. The 20 animals were divided into two groups and euthanized after 30 and 60 days. Second harmonic generation images were obtained and program ImageJ was utilized to evaluate the collagen organization within all areas. Collagen anisotropy, entropy and optical density were quantified.

RESULTS

Increased anisotropy over time was observed in all four areas, but only reached statistical significance (p = 0.0305) when the mildest parameters were used (area four). Entropy decreased over time in all areas, but without significance(p = 0.1779) in area four. Density showed an overtime increase only in area four, but no statistical significance was reached (p = 0.6534).

CONCLUSIONS

When combined, the results obtained in this study regarding anisotropy, entropy and density tend to demonstrate that it is possible to achieve collagen remodeling with the use of lower power levels associated with stacked pulses.

Robust and practical measurement of volume transport parameters in solid photo-polymer materials for 3D printing

Volumetric light transport is a pervasive physical phenomenon, and therefore its accurate simulation is important for a broad array of disciplines. While suitable mathematical models for computing the transport are now available, obtaining the necessary material parameters needed to drive such simulations is a challenging task: direct measurements of these parameters from material samples are seldom possible. Building on the inverse scattering paradigm, we present a novel measurement approach which indirectly infers the transport parameters from extrinsic observations of multiple-scattered radiance. The novelty of the proposed approach lies in replacing structured illumination with a structured reflector bonded to the sample, and a robust fitting procedure that largely compensates for potential systematic errors in the calibration of the setup. We show the feasibility of our approach by validating simulations of complex 3D compositions of the measured materials against physical prints, using photo-polymer resins. As presented in this paper, our technique yields colorspace data suitable for accurate appearance reproduction in the area of 3D printing. Beyond that, and without fundamental changes to the basic measurement methodology, it could equally well be used to obtain spectral measurements that are useful for other application areas.

A serum containing vitamins C & E and a matrix-repair tripeptide reduces facial signs of aging as evidenced by Primos® analysis and frequently repeated auto-perception

Background

Allegations on the benefits of incorporating vitamin C, vitamin E, and combinations thereof in topical skincare formulations are mostly based on in vitro and ex vivo experiments and/or limited protocols of specific stress conditions (pollution, UV exposure, laser irradiation,…).

Objective

To evaluate the instrumentally measurable effects and quantitative consumer perceptions of a protective and reparative serum on a panel of volunteers under normal nonstressed conditions of use, employing FOITS technology and innovative self‐assessment methods.

Method

In an open‐label study women of ≥40 years with visible signs of photoaging applied a serum comprising l‐ascorbic acid USP (15% w/v), tocopheryl acetate USP, and 5 ppm palmitoyl tripeptide‐38 to the face once daily for 56 days. Skin roughness and isotropy changes were evaluated on days 0, 28, and 56, visual instrumental evaluation of skin‐tone parameters was assessed on days 0 and 56. Subjects completed self‐assessment questionnaires every third day of the trial period for radiance, homogeneity, and wrinkle appearance.

Results

Skin‐roughness parameters decreased significantly by 8%‐9% (P < .05) and subjects experienced a significant increase in skin isotropy (P < .05). Photographic analysis revealed significant improvements in skin tone, with a 9% decrease in redness and 8% increase in homogeneity (P < .0001 for both), in excellent agreement with subjects' perception of significant improvements of radiance, complexion, and wrinkles.

Conclusion

The study confirms statistically significant correlation between objectively measured and quantitative subjectively perceived benefits of the bespoke serum containing antioxidants and a matrix‐restoring peptide.

Spectral intensity correlations of backscattered diffuse light: the effect of scattering anisotropy

In this work, we report experiments and a theoretical scheme of photon transport in the frequency domain of rigid turbid media. We have employed spectral multi-speckle intensity correlations to estimate optical properties as the transport mean free path and the absorption length of turbid systems. We propose a scheme based on the photon diffusion model using an effective path-length distribution in the backscattering configuration and take explicitly into account the particles scattering anisotropy parameter g. By studying rigid Teflon slabs and polymer matrices doped with polystyrene particles of different degrees of scattering anisotropy, we find that the proposed model adequately describes our experimental results. Our hypothesis for the diffuse transport of backscattered photons in the weak multiple scattering regime is further validated using a numerical simulation scheme of speckle dynamics, based on the Copula method.

Hyperspectral imaging and characterization of allergic contact dermatitis in the short-wave infrared

Short-wave infrared hyperspectral imaging is applied to diagnose and monitor a case of allergic contact dermatitis (ACD) due to poison ivy exposure in one subject. This approach directly demonstrates increased tissue fluid content in ACD lesional skin with a spectral signature that matches the spectral signature of intradermally injected normal saline. The best contrast between the affected and unaffected skin is achieved through a selection of specific wavelengths at 1070, 1340 and 1605 nm and combining them in a pseudo-red-green-blue color space. An image derived from these wavelengths normalized to unaffected skin defines a "tissue fluid index" that may aid in the quantitative diagnosis and monitoring of ACD. Further clinical testing of this promising approach towards disease detection and monitoring with tissue fluid content quantification is warranted.

Wavelength weightings in machine learning for ovine joint tissue differentiation using diffuse reflectance spectroscopy (DRS)

Objective: To investigate the DRS of ovine joint tissue to determine the optimal optical wavelengths for tissue differentiation and relate these wavelengths to the biomolecular composition of tissues. In this study, we combine machine learning with DRS for tissue classification and then look further at the weighting matrix of the classifier to further understand the key differentiating features. Methods: Supervised machine learning was used to analyse DRS data. After normalising the data, dimension reduction was achieved through multiclass Fisher's linear discriminant analysis (Multiclass FLDA) and classified with linear discriminant analysis (LDA). The classifier was first run with all the tissue types and the wavelength range 190 nm - 1081 nm. We analysed the weighting matrix of the classifier and then ran the classifier again, the first time using the ten highest weighted wavelengths and the second using only the single highest. Our method was applied to a dataset containing ovine joint tissue including cartilage, cortical and subchondral bone, fat, ligament, meniscus, and muscle. Results: It achieved a classification accuracy of 100% using the wavelength 190 nm - 1081 nm (2048 attributes) with an accuracy of 90% being present for 10 attributes with the exception of those with comparable compositions such as ligament and meniscus. An accuracy greater than 70% was achieved using a single wavelength, with the same exceptions. Conclusion: Multiclass FLDA combined with LDA is a viable technique for tissue identification from DRS data. The majority of differentiating features existed within the wavelength ranges 370-470 and 800-1010 nm. Focusing on key spectral regions means that a spectrometer with a narrower range can potentially be used, with less computational power needed for subsequent analysis.

Good, better, best? The effects of polarization on photobiomodulation therapy

Photobiomodulation therapy (PBMT) is a widely adopted form of phototherapy used to treat many chronic conditions that effect the population at large. The exact physiological mechanisms of PBMT remain unsolved; however, the prevailing theory centres on changes in mitochondrial function. There are many irradiation parameters to consider when investigating PBMT, one of which is the state of polarization. There is some evidence to show that polarization of red and near-infrared light may promote different and/or increased biological activity when compared to otherwise identical non-polarized light. These enhanced cellular effects may also be present when the polarized light is applied linear to the tissue direction. Herein, we synthesize the current experimental and clinical evidence pertaining to polarized photobiomodulation therapy; ultimately, to better inform future research into this area of phototherapy.

Anisotropic light scattering from myelinated axons in the spinal cord

Optogenetics has become an integral tool for studying and dissecting the neural circuitries of the brain using optical control. Recently, it has also begun to be used in the investigation of the spinal cord and peripheral nervous system. However, information on these regions' optical properties is sparse. Moreover, there is a lack of data on the dependence of light propagation with respect to neural tissue organization and orientation. This information is important for effective simulations and optogenetic planning, particularly in the spinal cord where the myelinated axons are highly organized. To this end, we report experimental measurements for the scattering coefficient, validated with three different methods in both the longitudinal and radial directions of multiple mammalian spinal cords. In our analysis, we find that there is indeed a directional dependence of photon propagation when interacting with organized myelinated axons. Specifically, light propagating perpendicular to myelinated axons in the white matter of the spinal cord produced a measured reduced scattering coefficient ( μ s ' ) of 3.52 ± 0.1    mm - 1 , and light that was propagated along the myelinated axons in the white matter produced a measured μ s ' of 1.57 ± 0.03    mm - 1 , across the various species considered. This 50% decrease in scattering power along the myelinated axons is observed with three different measurement strategies (integrating spheres, observed transmittance, and punch-through method). Furthermore, this directional dependence in scattering power and overall light attenuation did not occur in the gray matter regions where the myelin organization is nearly random. The acquired information will be integral in preparing future light-transport simulations and in overall optogenetic planning in both the spinal cord and the brain.

Hands-Free User Interface for AR/VR Devices Exploiting Wearer's Facial Gestures Using Unsupervised Deep Learning

Developing a user interface (UI) suitable for headset environments is one of the challenges in the field of augmented reality (AR) technologies. This study proposes a hands-free UI for an AR headset that exploits facial gestures of the wearer to recognize user intentions. The facial gestures of the headset wearer are detected by a custom-designed sensor that detects skin deformation based on infrared diffusion characteristics of human skin. We designed a deep neural network classifier to determine the user's intended gestures from skin-deformation data, which are exploited as user input commands for the proposed UI system. The proposed classifier is composed of a spatiotemporal autoencoder and deep embedded clustering algorithm, trained in an unsupervised manner. The UI device was embedded in a commercial AR headset, and several experiments were performed on the online sensor data to verify operation of the device. We achieved implementation of a hands-free UI for an AR headset with average accuracy of 95.4% user-command recognition, as determined through tests by participants.

Processing of collagen based biomaterials and the resulting materials properties

Collagen, the most abundant extracellular matrix protein in animal kingdom belongs to a family of fibrous proteins, which transfer load in tissues and which provide a highly biocompatible environment for cells. This high biocompatibility makes collagen a perfect biomaterial for implantable medical products and scaffolds for in vitro testing systems. To manufacture collagen based solutions, porous sponges, membranes and threads for surgical and dental purposes or cell culture matrices, collagen rich tissues as skin and tendon of mammals are intensively processed by physical and chemical means. Other tissues such as pericardium and intestine are more gently decellularized while maintaining their complex collagenous architectures. Tissue processing technologies are organized as a series of steps, which are combined in different ways to manufacture structurally versatile materials with varying properties in strength, stability against temperature and enzymatic degradation and cellular response. Complex structures are achieved by combined technologies. Different drying techniques are performed with sterilisation steps and the preparation of porous structures simultaneously. Chemical crosslinking is combined with casting steps as spinning, moulding or additive manufacturing techniques. Important progress is expected by using collagen based bio-inks, which can be formed into 3D structures and combined with live cells. This review will give an overview of the technological principles of processing collagen rich tissues down to collagen hydrolysates and the methods to rebuild differently shaped products. The effects of the processing steps on the final materials properties are discussed especially with regard to the thermal and the physical properties and the susceptibility to enzymatic degradation. These properties are key features for biological and clinical application, handling and metabolization.

Time-Domain Near-Infrared Spectroscopy and Imaging: A Review

This article reviews the past and current statuses of time-domain near-infrared spectroscopy (TD-NIRS) and imaging. Although time-domain technology is not yet widely employed due to its drawbacks of being cumbersome, bulky, and very expensive compared to commercial continuous wave (CW) and frequency-domain (FD) fNIRS systems, TD-NIRS has great advantages over CW and FD systems because time-resolved data measured by TD systems contain the richest information about optical properties inside measured objects. This article focuses on reviewing the theoretical background, advanced theories and methods, instruments, and studies on clinical applications for TD-NIRS including some clinical studies which used TD-NIRS systems. Major events in the development of TD-NIRS and imaging are identified and summarized in chronological tables and figures. Finally, prospects for TD-NIRS in the near future are briefly described.

Diffuse reflectance spectroscopy-enhanced drill for bone boundary detection

Intramedullary nailing is a routine orthopedic procedure used for treating fractures of femoral or tibial shafts. A critical part of this procedure involves the drilling of pilot holes in both ends of the bone for the placement of the screws that will secure the IM rod to sections of the fractured bone. This step introduces a risk of soft tissue damage because the drill bit, if not stopped in time, can transverse the bone-tissue boundary into the overlying muscle, causing unnecessary injury and prolonging healing time due to periosteum damage. In this respect, detecting the bone-tissue boundary before break-through can reduce the risks and complications associated with intramedullary nailing. Hence, in the present study, a two-wavelength diffuse reflectance spectroscopy technique was integrated into a surgical drill to optically detect bone-tissue boundary and automatically trigger the drill to stop. Furthermore, Monte-Carlo simulations were used to estimate the maximum distance from within the bone at which the bone-tissue boundary could be detected using DRS. The simulation results estimated that the detection distance, termed the "look-ahead-distance" was ∼1.5 mm for 1.3 mm source-detector fiber separation. Experimental measurements with 1.3 mm source-detector fiber separation showed that the look-ahead-distance was in the order of 250 µm in experiments with set drill rate and in the range of 1 mm in experiments where the holes were drilled by hand. Despite this difference, the automated DRS enhanced drill successfully detected the approaching bone tissue boundary when tested on samples of bovine femur and muscle tissue.

Perspective: Prospects of non-invasive sensing of the human brain with diffuse optical imaging

Since the initial demonstration of near-infrared spectroscopy (NIRS) for noninvasive measurements of brain perfusion and metabolism in the 1970s, and its application to functional brain studies (fNIRS) in the 1990s, the field of noninvasive optical studies of the brain has been continuously growing. Technological developments, data analysis advances, and novel areas of application keep advancing the field. In this article, we provide a view of the state of the field of cerebral NIRS, starting with a brief historical introduction and a description of the information content of the NIRS signal. We argue that NIRS and fNIRS studies should always report data of both oxy- and deoxyhemoglobin concentrations in brain tissue, as they complement each other to provide more complete functional and physiological information, and may help identify different types of confounds. One significant challenge is the assessment of absolute tissue properties, be them optical or physiological, so that relative measurements account for the vast majority of NIRS and fNIRS applications. However, even relative measurements of hemodynamics or metabolic changes face the major problem of a potential contamination from extracerebral tissue layers. Accounting for extracerebral contributions to fNIRS signals is one of the most critical barriers in the field. We present some of the approaches that were proposed to tackle this challenge in the study of cerebral hemodynamics and functional connectivity. Finally, we critically compare fNIRS and functional magnetic resonance imaging (fMRI) by relating their measurements in terms of signal and noise, and by commenting on their complementarity.

Double anisotropic coherent backscattering of light

A double anisotropic coherent backscattering cone was found. In contrast to the (single) anisotropic coherent backscattering, which was observed in liquid crystals, here, the long axis of the elongated structures changes its orientation with angular distance. We compared our results with the two-dimensional Fourier transform of spatially resolved reflectance measurements and found good agreement, which is predicted by the reciprocity thesis. Furthermore, a Monte Carlo model was applied to reproduce successfully the results of the experiment, whereas the double anisotropy is not predicted by diffusion models.

Anisotropic light propagation in bovine muscle tissue depends on the initial fiber orientation, muscle type and wavelength

The effects of fiber orientation on vis/NIR light propagation were studied in three bovine muscles: biceps brachii, brachialis and soleus. Broadband light was focused onto the sample and the diffuse reflectance spot was captured using a hyperspectral camera (470-1620 nm), after which rhombuses were fitted to equi-intensity points. In samples with fibers running parallel to the measurement surface, the rhombus' major axis was oriented perpendicular to the fiber direction close to the point of illumination. However, at larger distances from the illumination spot, the major axis orientation aligned with the fiber direction. This phenomenon was found to be muscle dependent. Furthermore, the rhombus orientation was highly dependent on the sample positioning underneath the camera, especially when the muscle fibers ran parallel to the measurement surface. The bias parameter, indicating the deviation from a circular shape, was higher for samples with the fibers running parallel to the measurement surface. Moreover, clear effects of wavelength and distance from the illumination point on this parameter were observed. These results show the importance of fiber orientation when considering optical techniques for measurements on anisotropic, fibrous tissues. Moreover, the prediction of muscle fiber orientation seemed feasible, which can be of interest to the meat industry.

Non-Fourier thermal transport induced structural hierarchy and damage to collagen ultrastructure subjected to laser irradiation

Comprehending the mechanism of thermal transport through biological tissues is an important factor for optimal ablation of cancerous tissues and minimising collateral tissue damage. The present study reports detailed mapping of the rise in internal temperature within the tissue mimics due to NIR (1064 nm) laser irradiation, both for bare mimics and with gold nanostructures infused. Gold nanostructures such as mesoflowers and nanospheres have been synthesised and used as photothermal converters to enhance the temperature rise, resulting in achieving the desired degradation of malignant tissue in targeted region. Thermal history was observed experimentally and simulated considering non-Fourier dual phase lag (DPL) model incorporated Pennes bio-heat transfer equation using COMSOL Multiphysics software. The gross deviation in temperature i.e. rise from the classical Fourier model for bio-heat conduction suggests additional effects of temperature rise on the secondary structures and morphological and physico-chemical changes to the collagen ultrastructures building the tissue mass. The observed thermal denaturation in the collagen fibril morphologies have been explained based on the physico-chemical structure of collagen and its response to thermal radiation. The large shift in frequency of amides A and B is pronounced at a depth of maximum temperature rise compared with other positions in tissue phantom. Observations for change in band of amide I, amide II, and amide III are found to be responsible for damage to collagen ultra-structure. Variation in the concentration of gold nanostructures shows the potentiality of localised hyperthermia treatment subjected to NIR radiation through a proposed free radical mechanism.

Magnification—a New Look at a Long-known Optical Property of Dentin

Light propagation in human dentin exhibits a strong directional dependence featuring the long-known optical magnification property. We hypothesized that this anisotropic effect is caused by multiple scattering at the dentin tubules, and not by fiberoptic effects, as had been previously assumed. We performed measurements of the transmitted intensity from dentin disks and compared them with Monte Carlo simulations of light propagation in dentin, considering the scattering by the tissue’s microstructure. We found that the optical anisotropy of dentin can be fully explained with this model. We concluded that the magnification property of dentin is due to multiple scattering by the dental microstructure.

Distortion correction and cross-talk compensation algorithm for use with an imaging spectrometer based spatially resolved diffuse reflectance system

Optical spectroscopy of human tissue has been widely applied within the field of biomedical optics to allow rapid, in vivo characterization and analysis of the tissue. When designing an instrument of this type, an imaging spectrometer is often employed to allow for simultaneous analysis of distinct signals. This is especially important when performing spatially resolved diffuse reflectance spectroscopy. In this article, an algorithm is presented that allows for the automated processing of 2-dimensional images acquired from an imaging spectrometer. The algorithm automatically defines distinct spectrometer tracks and adaptively compensates for distortion introduced by optical components in the imaging chain. Crosstalk resulting from the overlap of adjacent spectrometer tracks in the image is detected and subtracted from each signal. The algorithm's performance is demonstrated in the processing of spatially resolved diffuse reflectance spectra recovered from an Intralipid and ink liquid phantom and is shown to increase the range of wavelengths over which usable data can be recovered.

Polarized light interaction with tissues

This tutorial-review introduces the fundamentals of polarized light interaction with biological tissues and presents some of the recent key polarization optical methods that have made possible the quantitative studies essential for biomedical diagnostics. Tissue structures and the corresponding models showing linear and circular birefringence, dichroism, and chirality are analyzed. As the basis for a quantitative description of the interaction of polarized light with tissues, the theory of polarization transfer in a random medium is used. This theory employs the modified transfer equation for Stokes parameters to predict the polarization properties of single- and multiple-scattered optical fields. The near-order of scatterers in tissues is accounted for to provide an adequate description of tissue polarization properties. Biomedical diagnostic techniques based on polarized light detection, including polarization imaging and spectroscopy, amplitude and intensity light scattering matrix measurements, and polarization-sensitive optical coherence tomography are described. Examples of biomedical applications of these techniques for early diagnostics of cataracts, detection of precancer, and prediction of skin disease are presented. The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.

Fabrication of various optical tissue phantoms by the spin-coating method

Although numerous studies have been performed to fabricate various optical tissue phantom (OTP) models, the fabrication of OTPs that simulate skin layers is laborious and time-consuming owing to the intricate characteristics of skin tissue. This study presents various OTP models that optically and structurally simulate the epidermis–dermis skin layer. The spin-coating method was employed to reproduce a uniform thin layer that mimics the epidermis layer, and the fabrication parameters were optimized for epoxy and silicone reference materials. Various OTP models simulating blood vessels and hyperpigmentation lesions were fabricated using the two reference materials to determine their feasibility. The suitability of each of the two reference materials for OTP fabrication was qualitatively evaluated by comparing the quality of the OTP models.

Three-dimensional facial recognition using passive long-wavelength infrared polarimetric imaging

We use a polarimetric camera to record the Stokes parameters and the degree of linear polarization of long-wavelength infrared radiation emitted by human faces. These Stokes images are combined with Fresnel relations to extract the surface normal at each pixel. Integrating over these surface normals yields a three-dimensional facial image. One major difficulty of this technique is that the normal vectors determined from the polarizations are not unique. We overcome this problem by introducing an additional boundary condition on the subject. The major sources of error in producing inversions are noise in the images caused by scattering of the background signal and the ambiguity in determining the surface normals from the Fresnel coefficients.

Anisotropic diffusive transport: connecting microscopic scattering and macroscopic transport properties

This work concerns the modeling of radiative transfer in anisotropic turbid media using diffusion theory. A theory for the relationship between microscopic scattering properties (i.e., an arbitrary differential scattering cross-section) and the macroscopic diffusion tensor, in the limit of independent scatterers, is presented. The theory is accompanied by a numerical method capable of performing the calculations. In addition, a boundary condition appropriate for modeling systems with anisotropic radiance is derived. It is shown that anisotropic diffusion theory, when based on these developments, indeed can describe radiative transfer in anisotropic turbid media. More specifically, it is reported that solutions to the anisotropic diffusion equation are in excellent agreement with Monte Carlo simulations, both in steady-state and time-domain. This stands in contrast to previous work on the topic, where inadequate boundary conditions and/or incorrect relations between microscopic scattering properties and the diffusion tensor have caused disagreement between simulations and diffusion theory. The present work thus falsify previous claims that anisotropic diffusion theory cannot describe anisotropic radiative transfer, and instead open for accurate quantitative diffusion-based modeling of anisotropic turbid materials.

Opto-physiological modeling applied to photoplethysmographic cardiovascular assessment

This paper presents opto-physiological (OP) modeling and its application in cardiovascular assessment techniques based on photoplethysmography (PPG). Existing contact point measurement techniques, i.e., pulse oximetry probes, are compared with the next generation non-contact and imaging implementations, i.e., non-contact reflection and camera-based PPG. The further development of effective physiological monitoring techniques relies on novel approaches to OP modeling that can better inform the design and development of sensing hardware and applicable signal processing procedures. With the help of finite-element optical simulation, fundamental research into OP modeling of photoplethysmography is being exploited towards the development of engineering solutions for practical biomedical systems. This paper reviews a body of research comprising two OP models that have led to significant progress in the design of transmission mode pulse oximetry probes, and approaches to 3D blood perfusion mapping for the interpretation of cardiovascular performance.

Time-resolved measurements of the optical properties of fibrous media using the anisotropic diffusion equation

Transmittance and reflectance from spruce wood and bovine ligamentum nuchae as two different fibrous media are examined by time-of-flight spectroscopy for varying source detector separations and several orientations of the fibers in the sample. The anisotropic diffusion theory is used to obtain the absorption coefficient and the diffusion coefficients parallel and perpendicular to the fibers. The results are compared to those obtained with the isotropic diffusion theory. It is shown that for increasing source detector separations, the retrieved optical properties change as expected from Monte Carlo simulations performed in a previous study. This confirms that the anisotropic diffusion theory yields useful results for certain experimental conditions.

Low-level laser therapy for spinal cord injury in rats: effects of polarization

The effects of laser polarization on the efficacy of near-infrared low-level laser therapy for spinal cord injury (SCI) are presented. Rat spinal cords were injured with a weight-drop device, and the lesion sites were directly irradiated with a linearly polarized 808-nm diode laser positioned either perpendicular or parallel to the spine immediately after the injury and daily for five consecutive days. Functional recovery was assessed daily by an open-field test. Regardless of the polarization direction, functional scores of SCI rats that were treated with the 808-nm laser irradiation were significantly higher than those of SCI alone group (Group 1) from day 5 after injury. The locomotive function of SCI rats irradiated parallel to the spinal column (Group 3) was significantly improved from day 10 after injury, compared to SCI rats treated with the linear polarization perpendicular to the spinal column (Group 2). There were no significant differences in ATP contents in the injured tissue among the three groups. We speculate that the higher efficacy with parallel irradiation is attributable to the deeper light penetration into tissue with anisotropic scattering.

Imaging scattering orientation with spatial frequency domain imaging

Optical imaging techniques based on multiple light scattering generally have poor sensitivity to the orientation and direction of microscopic light scattering structures. In order to address this limitation, we introduce a spatial frequency domain method for imaging contrast from oriented scattering structures by measuring the angular-dependence of structured light reflectance. The measurement is made by projecting sinusoidal patterns of light intensity on a sample, and measuring the degree to which the patterns are blurred as a function of the projection angle. We derive a spatial Fourier domain solution to an anisotropic diffusion model. This solution predicts the effects of bulk scattering orientation on the amplitude and phase of the projected patterns. We introduce a new contrast function based on a scattering orientation index (SOI) which is sensitive to the degree to which light scattering is directionally dependent. We validate the technique using tissue simulating phantoms, and ex vivo samples of muscle and brain. Our results show that SOI is independent of the overall amount of bulk light scattering and absorption, and that isotropic versus oriented scattering structures can be clearly distinguished. We determine the orientation of subsurface microscopic scattering structures located up to 600 μm beneath highly scattering (μ(') (s) = 1.5 mm(-1)) material.

Skin benefits of a myconoside-rich extract from resurrection plant Haberlea rhodopensis

Resurrection plant Haberlea rhodopensis develops molecules to survive drought stress. These molecules allow the plant to resurge from a desiccation state. We have extracted a specific fraction from the plant (Haberlea extract) and found it rich, among other molecules, of a caffeoyl phenylethanoid glycoside called myconoside, a molecule extremely abundant in the plant with a potential role in survival. Peroxide-stressed normal human dermal fibroblasts treated with the Haberlea extract, showed increased collagen VI (+822%), collagen XVI (+928%) and elastin (+144%) mRNA synthesis, measured by RT-qPCR. This effect was superior to those obtained with benchmarks retinoic acid and retinol. When used at 3% in human skin biopsies, Haberlea extract protected against UV-induced dermis oxidation by 100% (P < 0.01), as evidenced by immunohistochemistry. Finally, when tested in human volunteers (n = 20) at 3% in a cream against a placebo, Haberlea extract increased skin elasticity (3× placebo, P < 0.0002) and skin radiance (4× placebo, P < 0.05) after only 15 days of treatment, with the effect sustained after 30 and 60 days of treatment. We demonstrated that by using Haberlea extract (particularly rich in glycoside myconoside), it is possible to strongly stimulate antioxidant skin defences and extracellular matrix protein synthesis. This effect, in turn, will further stimulate skin elasticity and skin radiance significantly in human volunteers. The extract can be suggested for anti-ageing treatments, intended for claims such as protection from oxidation, increased skin elasticity and enhanced skin radiance.

A review of in-vivo optical properties of human tissues and its impact on PDT

A thorough understanding of optical properties of biological tissues is critical to effective treatment planning for therapies such as photodynamic therapy (PDT). In the last two decades, new technologies, such as broadband diffuse spectroscopy, have been developed to obtain in vivo data in humans that was not possible before. We found that the in vivo optical properties generally vary in the ranges μ(a) = 0.03-1.6 cm⁻¹ and μ'(s) = 1.2-40 cm⁻¹, although the actual range is tissue-type dependent. We have also examined the overall trend of the absorption spectra (for μ(a) and μ'(s)) as a function of wavelength within a 95% confidence interval for various tissues in vivo. The impact of optical properties on light fluence rate is also discussed for various light application geometries including superficial, interstitial, and within a cavity.

Simple and effective method for measuring translucency using edge loss: optimization of measurement conditions and applications for skin

We have developed a simple and effective method for everyday measurement of translucency with a handy spectral reflectometer using edge loss. Edge loss can be used to quantify the translucency index in terms of changes in reflectance under two types of measurement conditions. Here, a measurement condition represents the pairing of an illumination area and a measurement area. As a measure of the degree of lateral spread of reflected light, the translucency index can influence the appearance of human skin because this index represents eventual translucency. First, we estimated how edge loss changes when measurement conditions are varied. We then selected the combination of two measurement conditions of large and small edge loss to minimize errors. Finally, we estimated actual skin translucency changes before and after treatments comprising acetone-ether immersion and ultraviolet irradiation. The results were qualitatively consistent with the expectations under variations in absorbance and scattering capacity, indicating the effectiveness of this method in evaluating translucency. This method allows simultaneous measurement of translucency and reflectance as a spectrum, and also appears applicable for daily use, although common optical parameters cannot be derived using this method alone.

Penetration depth of linear polarization imaging for two-layer anisotropic samples

Polarization techniques can suppress multiply scattering light and have been demonstrated as an effective tool to improve image quality of superficial tissues where many cancers start to develop. Learning the penetration depth behavior of different polarization imaging techniques is important for their clinical applications in diagnosis of skin abnormalities. In the present paper, we construct a two-layer sample consisting of isotropic and anisotropic media and examine quantitatively using both experiments and Monte Carlo simulations the penetration depths of three different polarization imaging methods, i.e., linear differential polarization imaging (LDPI), degree of linear polarization imaging (DOLPI), and rotating linear polarization imaging (RLPI). The results show that the contrast curves of the three techniques are distinctively different, but their characteristic depths are all of the order of the transport mean free path length of the top layer. Penetration depths of LDPI and DOLPI depend on the incident polarization angle. The characteristic depth of DOLPI, and approximately of LDPI at small g, scales with the transport mean free path length. The characteristic depth of RLPI is almost twice as big as that of DOLPI and LDPI, and increases significantly as g increases.

Near-IR fluorescence and reflectance confocal microscopy for imaging of quantum dots in mammalian skin

Understanding the skin penetration of nanoparticles (NPs) is an important concern due to the increasing presence of NPs in consumer products, including cosmetics. Technical challenges have slowed progress in evaluating skin barrier and NP factors that contribute to skin penetration risk. To limit sampling error and other problems associated with histological processing, many researchers are implementing whole tissue confocal or multiphoton microscopies. This work introduces a fluorescence and reflectance confocal microscopy system that utilizes near-IR excitation and emission to detect near-IR lead sulfide quantum dots (QDs) through ex vivo human epidermis. We provide a detailed prediction and experimental analysis of QD detection sensitivity and demonstrate detection of QD skin penetration in a barrier disrupted model. The unique properties of near-IR lead-based QDs will enable future studies that examine the impact of further barrier-disrupting agents on skin penetration of QDs and elucidate mechanistic insight into QD tissue interactions at the cellular level.

Path-length resolved reflectance in tendon and muscle

Optical diffuse reflectance in fibrous tissues depends on measurement angles in relation to fiber orientation. In this study, path-length resolved optical reflectance was measured in tendon and skeletal muscle samples using a low-coherence Mach-Zehnder interferometer. The results show that the angular dependency in reflectance was eliminated in tendon tissue when representing reflectance as a function of mean path-length. Our analysis indicated that this observation can be understood in the frame work of anisotropic diffuse theory. However the same phenomenon was not observed in muscles, suggesting involvement of additional scattering mechanisms.

Effects of anisotropy on the depth of penetration of photons into turbid media

Biomedical applications of near infrared radiation (NIR) techniques (i.e., based on light wavelengths roughly between 400 and 1100 nm) require that a preliminary estimate of the tissue volume being investigated be found. One possible estimate is the depth to which a photon penetrates a tissue before it eventually emerges at a separating plane at a given time. A simple model for this problem can be based on a lattice random walk and was initially analyzed when the associated optical coefficients are isotropic with respect to the geometry. Here we include the effects of anisotropy in the optical coefficients, finding that at long times the statistical properties of the depth of penetration can be accounted for by very simple scaling factors while at short times the anisotropy factors can be quite significant.

Anisotropic Raman scattering in collagen bundles

Collagen is the main connective tissue protein of vertebrates and shows exceptional mechanical and optical properties. The alignment of collagen fibrils correlates to the function of a specific tissue and leads to optical anisotropy. The effect of the molecular alignment on Raman scattering, however, has barely been investigated. We found that the peak intensities of the C-C, C=O, and N-H vibrational modes, which are typical for the Raman bands of the protein backbone, change with the orientation of the collagen fibrils. These observations demonstrate that Raman spectra contain specific information regarding molecular and fiber alignment.

In vivo characterization of human pigmented lesions by degree of linear polarization image maps using incident linearly polarized light

BACKGROUND AND OBJECTIVE

Melanoma is the most serious form of skin cancer and often appears as an evolving multicolored skin growth. It is well documented that pre-existing atypical or dysplastic nevi can evolve into a melanoma. The development of an in vivo imaging system to characterize benign and malignant nevi has been emphasized to aid in early detection of melanoma. The goal of this study is to utilize a novel Stokes polarimetry imaging (SPI) system for the characterization of pigmented lesions, and to evaluate the SPI system in comparison to dermoscopy and histology images.

STUDY DESIGN/MATERIALS AND METHODS

Linearly polarized light with varying incident polarization angles (IPA) illuminated various types of pigmented lesions. The melanocytic nesting patterns of pigmented lesions were characterized by constructing the degree-of-linear-polarization (DOLP) image map with comparison to dermoscopy and histology. The incident polarized light was filtered by visible filters for spectral imaging and incident deeply penetrating red light was used to correlate the SPI image with histopathological examination.

RESULTS

The DOLP images with varying IPA at different visible wavelengths were used to characterize various kinds of pigmented lesions by showing subsurface melanocytic nesting distribution as well as morphological information with better resolution and contrast. In correlation with dermoscopy and histology, various defining features such as compound, junctional, lentiginous, reticular, globular patterns of melanocytic nests were identified.

CONCLUSION

When imaging pigmented melanocytic lesions, the SPI system with varying IPA at the red light wavelength can better define the melanocytic nesting patterns in both the dermal epidermal junction and the dermis. The SPI system has the potential to be an effective in vivo method of detecting pre-malignant nevi and melanoma.

Equi-intensity distribution of optical reflectance in a fibrous turbid medium

Light propagation in a fibrous anisotropic scattering medium is quite different from that in an isotropic medium. Both the anisotropic diffuse equation (ADE) and the continuous time random walk (CTRW) theory predict that the equi-intensity profiles of the surface reflectance have an elliptical shape in a fibrous turbid medium. In this study, we simulated the spatially resolved surface reflectance in a fibrous sample using a Monte Carlo model. A parametric equation was used to quantitatively characterize the geometric profiles of the reflectance patterns. The results indicated that the equi-intensity profiles of surface reflectance had elliptical shapes only when evaluated at distances far away from the incident point. The length ratio of the two orthogonal axes of the ellipse was not affected by the sample optical properties when the ratio of reduced scattering coefficients along the two axes is the same. But the relationship between the aforementioned two ratios was different from the predication of ADE theory. Only for fibers of small sizes did the fitted axes ratios approach the values predicted from the ADE theory.

Image reconstruction in diffuse optical tomography based on simplified spherical harmonics approximation

The use of higher order approximations to the Radiative transport equation, through simplified spherical harmonics expansion (SP(N)) in optical tomography are presented. It is shown that, although the anisotropy factor can be modeled in the forward problem, its sensitivity to the measured boundary data is limited to superficial regions and more importantly, due to uniqueness of the inverse problem it cannot be determined using frequency domain data. Image reconstruction through the use of higher ordered models is presented. It is demonstrated that at higher orders (for example SP7) the image reconstruction becomes highly under-determined due to the large increase in the number of unknowns which cannot be adequately recovered. However, reconstruction of diffuse parameters, namely optical absorption and reduced scatter have shown to be more accurate where only the sensitivity matrix used in the inverse problem is based on SP(N) method and image reconstruction is limited to these two diffuse parameters.

Optical anisotropic diffusion: new model systems and theoretical modeling

We have developed several new experimental model systems that demonstrate anisotropic diffusion of light. These systems, consisting of aligned fibers, stretched plastic foam, and stretched plastic frit, have relatively simple microstructures and are easily sliced, making them ideal for testing theoretical models of diffusion. We demonstrate that the solution to the diffusion equation for arbitrary orientation of the diffusion tensor is consistent with experimental measurements. We also show that simple models of microstructure, based on cylindrical and planar scatterers, are consistent with the experimental results. These models provide simple analytical expressions for predicting the degree of alignment of the scatterers from diffuse transmission measurements. The combination of experimental results and theoretical support demonstrates both the power and the limitations of the diffusion model for providing information about microstructure via simple experiments and modeling.

A 35-month profilometric and clinical evaluation of non-ablative remodeling using a 1540-nm Er:glass laser

BACKGROUND/OBJECTIVE

As remodeling is getting more popular with patients, long-term studies are becoming necessary. The aim of this 35-month clinical study was to evaluate the long-term benefits obtained using a 1540-nm Er:glass laser for non-ablative remodeling of perioral and periorbital rhytids. The role of maintenance treatments was also investigated.

STUDY DESIGN/METHODS

Eleven women with periorbital and perioral rhytids underwent a series of five treatments at 6-week intervals with an Er:glass laser. Five patients subsequently received two maintenance retreatments and six did not. The maintenance treatments were performed at 14 and 20 months. Silicone imprints were performed to measure anisotropy before treatment, at 6 months, at 14 months and at 35 months. Patient self-evaluation/questionnaire was also done to assess adverse effects and subjective clinical improvement.

RESULTS

For all 11 patients, the percentage of anisotropy reduction was 41.21% at 6 months, 51.76% at 14 months and 29.87% at 35 months. No adverse effects were noted. Patient satisfaction was high at the end of the evaluation. Retreated patients were more satisfied than non-retreated ones. However, there was no difference in the anisotropy factor between the two groups.

CONCLUSION

Treatment of facial rhytids with a non-ablative 1540-nm Er:glass laser system can produce benefits that persist over 2 years after the last treatment.

Turbidity-corrected Raman spectroscopy for blood analyte detection

A major challenge in quantitative biological Raman spectroscopy, particularly as applied to transcutaneous Raman spectroscopy measurements, is overcoming the deleterious effects of scattering and absorption (turbidity). The Raman spectral information is distorted by multiple scattering and absorption events in the surrounding medium, thereby diminishing the prediction capability of the calibration model. To account for these distortions, we present a novel analytical method, that we call turbidity-corrected Raman spectroscopy (TCRS), which is based on the photon migration approach and employs alternate acquisition of diffuse reflectance and Raman spectra. We demonstrate that, upon application of TCRS, the widely varying Raman spectra observed from a set of tissue phantoms having the same concentration of Raman scatterers but different turbidities has a tendency to collapse onto a single spectral profile. Furthermore, in a prospective study that employs physical tissue models with varying turbidities and randomized concentrations of Raman scatterers and interfering agents, a 20% reduction in prediction error is obtained by applying the turbidity correction procedure to the observed Raman spectra.

Light scattering spectroscopy of human skin in vivo

We present an in vivo study of the reduced scattering coefficient of normal skin and of common melanocytic nevi in Caucasian subjects. The spectral shape of the reduced scattering coefficient is described well by a power-law dependence on the wavelength, in accordance with previous studies of light scattering by biological tissues. We investigate statistical variations in the scattering spectrum slope and also identify an inherent correlation between scattering intensity and scattering spectral slope, observed mainly in normal skin. In addition, we do not find any significant differences between the scattering properties of normal skin and common melanocytic nevi. Finally, we also provide a short review of previously published studies reporting on the light scattering properties of human skin both in vivo and in vitro.