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When ultraviolet radiation is absorbed within the cutaneous tissues, it can trigger a number of phenomena that can have detrimental or beneficial consequences to an individual's health. Tanning is among the most visually noticeable of these phenomena. It may result in significant changes in skin pigmentation and thickness. These spectrally-dependent physiological responses, in turn, can elicit variations in the ultraviolet absorption profiles of the cutaneous tissues and, consequently, alter the occurrence of other ultraviolet-induced photobiological processes such as the breaking of DNA strands and the synthesis of previtamin D3. These tanning-elicited variations in the cutaneous tissues' absorption profiles is often tied to the increased presence of melanin throughout these tissues. However, during the tanning, shifts in the relative content of this pigment within certain skin layers can also be observed. In particular, the stratum basale, the innermost epidermal layer where melanogenesis takes place, can have its relative melanin content significantly reduced in comparison with other epidermal layers. Since the aforementioned photobiological phenomena are preferentially brought about within this layer, such pigmentation shifts may have a more pivotal role in skin photobiology than has been assumed to date. Accordingly, in this work, we investigate the impact of spectrally-dependent tanning-elicited physiological responses, with a particular focus on the inter-layer melanin distribution patterns, on the absorption profiles of the main cutaneous tissues. We also examine how variations in these absorption profiles may alter the outcomes of photo-triggered phenomena associated with the onset of different medical conditions. Our findings are expected to contribute to the advance of the current understanding about skin photobiology, which is indispensable for the success of photomedicine initiatives involving this highly complex organ.

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The presence of abnormal amounts of bilirubin in the blood stream and skin, usually referred to as hyperbilirubinemia, is associated with a wide range of pathologies that can pose considerable risks for human health. The early and effective screening of the severity degrees of this medical condition can play an important role on the selection of the appropriate treatment for the associated pathologies. This, in turn, can minimize the need for more aggressive and costly therapeutic interventions which can themselves pose considerable risks for morbidity and mortality. The current noninvasive protocols used to differentiate these severity degrees, however, are hindered by the relatively limited knowledge about the impact of different amounts of extravascular bilirubin on skin spectral responses and on the onset of jaundice, the resulting yellow-tinted skin appearance. In this paper, we address this open problem through controlled in silico experiments supported by measured data provided in the related literature. Our experimental findings bring biophysically-based insights to bear on the clarification of this biomedical entanglement, and unveil optical features that can potentially lead to more effective screening protocols for the noninvasive differentiation and monitoring of variable degrees of hyperbilirubinemia severity.

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The exposure of human skin to ultraviolet radiation (UVR) can trigger a wide array of biological responses, including photocarcinogenesis. Melanin, either in colloidal form or encapsulated into melanosomes, is known to be the main UVR attenuation substance acting within the cutaneous tissues. Although many studies have addressed the protective role of this pigment against the harmful effects of UVR exposure, the impact of different melanosome arrangements on the mitigation of these effects remains to be quantitatively verified. The difficulties to resolve this open question can be mainly attributed to the intrinsic practical limitations of in vivo and in vitro experiments involving skin specimens. In this paper, we describe controlled in silico experiments that allowed us to overcome such limitations and provide quantitative evidence for the clarification of this question. Besides contributing to a more robust understanding of the physiological parameters associated with cutaneous UVR attenuation, our findings can be incorporated into the development of more effective strategies for the evaluation of individuals' susceptibility to UVR exposure. Such strategies are essential for the prevention of UVR-induced pathologies, particularly skin cancer.

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The purple or blue coloration of hands and feet, known as peripheral cyanosis, can represent one of the initial signs of potentially life threatening medical conditions. Consequently, procedures aimed at its early detection and interpretation can help health-care professionals to select the appropriate treatment for these conditions. The effectiveness of such procedures, in turn, depends on the correct assessment of the biophysical processes responsible for eliciting this abnormal skin appearance. However, despite the diverse body of existing clinical research involving cyanosis, the interplay between physiological changes and the optical phenomena leading to cyanotic responses remains not fully understood. In this paper, we methodically examine this interplay through controlled in silico experiments. Among other relevant aspects, the results of our experiments demonstrate that Rayleigh scattering, a light attenuation phenomenon overlooked by previous studies on peripheral cyanosis, plays a pivotal role in the manifestation of cyanotic chromatic attributes. We believe that the insights derived from our experiments can contribute to the development of more effective protocols for the screening of medical conditions associated with peripheral cyanosis etiology.

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Peripheral cyanosis, the purple or blue coloration of hands and feet, can represent the initial signs of life-threatening medical conditions such as heart failure due to coronary occlusion. This makes its effective detection relevant for the timely screening of such conditions. In order to reduce the probability of false negatives during the assessment of peripheral cyanosis, one needs to consider that the manifestation of its characteristic chromatic attributes can be affected by a number of physiological factors, notably cutaneous pigmentation. The extent to which cutaneous pigmentation can impair this assessment has not been experimentally investigated to date, however. Although the detection of peripheral cyanosis in darkly-pigmented individuals has been deemed to be impractical, data to support or refute this assertion are lacking in the literature. In this paper, we address these issues through controlled in silico experiments that allow us to predictively reproduce appearance changes triggered by peripheral cyanosis (at different severity stages) on individuals with distinct levels of cutaneous pigmentation. Our findings indicate that the degree of detection difficulty posed by cutaneous pigmentation can be considerably mitigated by selecting the appropriate skin site to perform the observations.

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Anemia is a prevalent medical condition that seriously affects millions of people all over the world. In many regions, not only its initial detection but also its monitoring are hindered by limited access to laboratory facilities. This situation has motivated the development of a wide range of optical devices and procedures to assist physicians in these tasks. Although noticeable progress has been achieved in this area, the search for reliable, low-cost, and risk-free solutions still continues, and the strengthening of the knowledge base about this disorder and its effects is essential for the success of these initiatives. We contribute to these efforts by closely examining the sensitivity of human skin hyperspectral responses (within and outside the visible region of the light spectrum) to reduced hemoglobin concentrations associated with increasing anemia severity levels. This investigation, which involves skin specimens with distinct biophysical and morphological characteristics, is supported by controlled in silico experiments performed using a predictive light transport model and measured data reported in the biomedical literature. We also propose a noninvasive procedure to be employed in the monitoring of this condition at the point-of-care.

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The identification and interpretation of skin spectral responses play a central role in a wide range of biomedical engineering applications, from the noninvasive assessment of human health parameters to the location of individuals in distress during search and rescue operations. In this paper, we investigate the sensitivity of these responses to physiological changes triggered by adverse environmental conditions. Our findings, which are supported by predictive computer simulations and experimental observations reported in the scientific literature, indicate that the resulting variations of skin reflectance can be substantial. Accordingly, if not properly taken into account, they may considerably impair the efficacy of systems designed for the detection and analysis of skin signatures within and outside the visible spectral region.

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Several techniques employed in the in vivo estimation of epidermal melanin content rely on the assumption that the effects of different distribution patterns of aggregated melanin (clustered within the melanosomes) on skin spectral responses, particularly across the 600-1350 nm range, can be ignored. Accordingly, for all practical purposes, only the non-aggregated (colloidal) form of melanin is taken into account by these techniques. In this paper, however, we demonstrate through predictive computer simulations that these responses are directly influenced by the occurrence of both forms of melanin. Our in silico findings, in turn, indicate that such an assumption may lead to inaccurate estimations of epidermal melanin content.

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In this paper, we investigate techniques for reducing the dimensionality of skin hyperspectral reflectance databases and maintaining a high degree of fidelity during data reconstruction. We compare results obtained using principal components analysis (PCA) with results provided by a piecewise PCA approach that explores the different roles performed by the main light attenuation agents acting within the cutaneous tissues in the ultraviolet (UV), visible and near-infrared (NIR) domains. Our investigation encapsulates not only skin spectral responses obtained by varying the contents of these agents, but also responses resulting from the absence of melanin pigmentation associated with the vitiligo condition.

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There are several pathologies whose study and diagnosis is impaired by a relatively small number of documented cases. A practical approach to overcome this obstacle and advance the research in this area consists in employing computer simulations to perform controlled in silico experiments. The results of these experiments, in turn, may be incorporated in the design of differential protocols for these pathologies. Accordingly, in this paper, we investigate the spectral responses of human skin affected by the presence of abnormal amounts of two dysfunctional hemoglobins, methemoglobin and sulfhemoglobin, which are associated with two life-threatening medical conditions, methemoglobinemia and sulfhemoglobinemia, respectively. We analyze the results of our in silico experiments and discuss their potential applications to the development of more effective noninvasive monitoring and differentiation procedures for these medical conditions.

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Light transport models are employed in applications in such varied areas as realistic image synthesis, noninvasive treatment of diseases, and remote sensing of natural resources. Openly accessible research resources can lead to significant advances involving these applications by fostering the cross-fertilization of different scientific disciplines. However, few light transport models have their source code openly available for download. Moreover, simply making the code available might not be enough; these models' complexity usually prevents their use beyond the research groups that developed them. The NPSGD (Natural Phenomena Simulation Group Distributed) framework makes light transport models easily accessible for online use. NPSGD acts a front end, connecting model implementations to the Web. It lets researchers perform predictive and time-intensive light transport simulations in a user-friendly, fault-tolerant way. More important, as a proof of concept, NPSGD demonstrates that the reproducibility of research results through model transparency is feasible. Such reproducibility can result in fruitful collaborations between model developers and users, regardless of their field of expertise.

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An experimental set-up was devised to record the transmission of red and green HeNe lasers through different types of paper. The measured data was compared with data obtained using the Henyey-Greenstein function (often employed in paper optics models to represent the bulk scattering of material samples) and data obtained using an alternative exponentiated cosine function. The comparisons are used to qualitatively assess the degree of fidelity of the bulk scattering approximations provided by both functions.