Examining the Impact of Sample Thickness Variations on the Hyperspectral Radiometric Responses of Flowing Blood.

The hyperspectral reflectance and transmittance of flowing blood samples are employed in a wide range of biomedical applied research initiatives such as the detection and monitoring of hematological abnormalities. The success of these initiatives is tied to the correct interpretation of these radiometric quantities. This, in turn, requires a comprehensive understanding about their sensitivity to variations in the experimental conditions in which they have been obtained. In this paper, we aim to contribute to these efforts by systematically examining the effects of sample thickness variations on these quantities. More specifically, we employed controlled in silico experiments to assess these effects on samples with different biophysical characteristics, notably their hematocrit, hemolysis level and orientation of their constituent cells with respect to the flow direction. To ensure a high degree of fidelity in our experiments, we used a first-principles simulation framework supported by measured data. Our findings unveil distinct spectrally-dependent trends associated with reflectance and transmittance changes elicited by sample thickness variations.

On the Sensitivity of Skin Spectral Responses to Variations in the Thickness of the Cutaneous Tissues.

A wide range of devices are being routinely used in the noninvasive screening and monitoring of medical conditions through the analysis of skin spectral responses. The correct interpretation of these responses often depends on the availability of high-fidelity characterization datasets for the selected specimens. More specifically, the higher their fidelity, the more effective the quantification of changes observed in a given biophysical variable of interest. Skin thickness is among the most relevant of these parameters since it plays a pivotal role in the attenuation (scattering and absorption) of light traversing the cutaneous tissues. Transient and permanent physiological processes, such as tanning and ageing, can result in significant time-dependent thickness variations. These, in turn, can introduce biases in the comparison of skin spectral responses obtained at different time instances. In this paper, we investigate the impact of thickness variations on skin reflectance with respect to different regions of light spectrum. Our findings are expected to contribute to the mitigation of interpretation errors and, thus, to the enhancement of noninvasive screening and monitoring procedures based on skin spectral responses.

Tanning-Elicited Variations in the Ultraviolet Absorption Spectra of the Cutaneous Tissues: Skin Photobiology and Photomedicine Implications.

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.

Unveiling the Impact of Distinct Melanosome Arrangements on the Attenuation of Cancer-Inducing Ultraviolet Radiation.

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.

Elucidating the contribution of Rayleigh scattering to the bluish appearance of veins.

The bluish appearance of veins located immediately beneath the skin has long been a topic of interest for biomedical optics researchers. Despite this interest, a thorough identification of the specific optical processes responsible for this phenomenon remains to be achieved. We employ controlled in silico experiments to address this enduring open problem. Our experiments, which are supported by measured data available in the scientific literature, are performed using first-principles models of light interaction with human skin and blood. Using this investigation approach, we quantitatively demonstrate that Rayleigh scattering caused by collagen fibrils present in the papillary dermis, a sublayer of the skin, can play a pivotal role in the bluish appearance of veins as suggested by previous works in this area. Moreover, also taking color perception aspects into account, we systematically assess the effects of variations in fibril radius and papillary dermis thickness on the coloration of veins under different illuminants. Notably, this assessment indicates that Rayleigh scattering elicited by reticulin fibrils, another type of fibril found in the papillary dermis, is unlikely to significantly contribute to the bluish appearance of veins. By strengthening the current understanding of light attenuation mechanisms affecting the appearance of skin and blood, our investigation contributes to the development of more effective technologies aimed at the noninvasive measurement of the physiological properties of these tissues.

Elucidating the biophysical processes responsible for the chromatic attributes of peripheral cyanosis.

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.

In silico investigation of the effects of hemolysis on the hyperspectral absorptance of blood in motion.

Measurement of the optical absorptance of blood can provide insight into its composition and behaviour. Accordingly, optical devices and sensors are commonly used in a clinical setting to measure the absorptance of blood, either directly or indirectly through measurement of skin spectral responses. These measurements enable the evaluation or constant monitoring of a patient's blood. In this paper, we perform predictive simulations to investigate the absorptance of blood and how it is affected by hemolysis. These simulations are performed using a cell-based light interaction model, known as CLBlood, which accounts for the orientation and distribution of red blood cells. This allows us to evaluate the effect of hemolysis under different flow conditions. Furthermore, we produce results in the ultraviolet, visible and infrared domains using CLBlood's hyperspectral capabilities. We then evaluate the sensitivity of the absorptance signature of blood to hemolysis in each of these domains under several experimental conditions. The observations in this paper enhance our understanding of the impact of hemolysis on the optical absorptance of blood, potentially leading to simplified and more accurate methods for its detection and monitoring.

On the detection of peripheral cyanosis in individuals with distinct levels of cutaneous pigmentation.

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.

Three-wavelength method for the optical differentiation of methemoglobin and sulfhemoglobin in oxygenated blood.

Methemoglobinemia and sulfhemoglobinemia are rare, but potentially life threatening, diseases that refer to an abnormal amount of methemoglobin or sulfhemoglobin in the blood, respectively. Unfortunately, blood samples containing abnormal quantities of methemoglobin or sulfhemoglobin have similar spectral characteristics. This makes it difficult to optically differentiate them and, hence, difficult to diagnose a patient with either disease. However, performing treatments for one of the diseases without a correct diagnosis can introduce increased risk to the patient. In this paper, we propose a method for differentiating the presence of methemoglobin and sulfhemoglobin in blood, under several conditions, using reflectance values measured at three wavelengths. In order to validate our method, we perform in silico experiments considering various levels of methemoglobin and sulfhemoglobin. These experiments employ a cell-based light interaction model, known as CLBlood, which accounts for the orientation and distribution of red blood cells. We then discuss the reflectance curves produced by the experiments and evaluate the efficacy of our method. In particular, we consider various experimental conditions by modifying the flow rate, hemolysis level and incident light direction.