George Zonios

Modeling diffuse reflectance from semi-infinite turbid media: application to the study of skin optical properties

Diffuse reflectance spectroscopy in the visible and NIR spectral ranges is an effective and extensively used technique for the non-invasive study and characterization of various biological tissues. In this article, a short review of currently available modeling techniques for diffuse reflectance from semi-infinite turbid media is presented. Starting from the basic physical picture of the diffuse reflectance problem, a simple and practical model is then proposed for use with fiber optic probes. The validity of the model is tested on tissue phantoms and it is then applied to the analysis of diffuse reflectance spectra collected from human skin in vivo.

Melanin absorption spectroscopy: new method for noninvasive skin investigation and melanoma detection

We present a new method for studying melanin in vivo based on diffuse reflectance spectroscopy of human skin. We find that the optical absorption spectrum of in vivo melanin exhibits an exponential dependence on wavelength, consistent with, but with a higher decay slope than, in vitro results. We offer theoretical justification for this exponential dependence on the basis of a recently proposed model for the structure of eumelanin protomolecules. Moreover, we report on a new method for analysis of diffuse reflectance spectra, which identifies intrinsic differences in absorption spectra between malignant melanoma and dysplastic nevi in vivo. These preliminary results are confirmed both by analysis of our own clinical data as well as by analysis of data from three independent, previously published studies. In particular, we find evidence that the histologic transition from dysplastic nevi to melanoma in situ and then to malignant melanoma is reflected in the melanin absorption spectra. Our results are very promising for the development of techniques for the noninvasive detection of melanoma and, more generally, for the study and characterization of pigmented skin lesions. It is also a promising approach for a better understanding of the biological role, structure, and function of melanin.

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.

Melanin optical properties provide evidence for chemical and structural disorder in vivo

Melanin is a ubiquitous chromophore of human skin but its in vivo optical properties are relatively unexplored. We present here a detailed study of the optical absorption of melanin present in melanocytic nevi of human subjects with Fitzpatrick skin type III. Using diffuse reflectance spectroscopy, we show that the melanin absorption spectrum exhibits an exponential dependence on wavelength in vivo with a decay constant that follows a normal distribution, characteristic of a random biological variable. This is the first time such direct in vivo quantitative evidence is obtained supporting the recently proposed hypothesis of chemical and structural disorder for melanin. In addition, the ability to measure the melanin optical properties in vivo opens new ways for the study of melanin in its native environment as well as for the non-invasive study and characterization of various skin disorders and diseases.

Simple two-layer reflectance model for biological tissue applications

A two-layer tissue diffuse reflectance model is described. The model is based on a simple one-layer model that we have recently developed and successfully applied to the analysis of in vivo skin reflectance. The model, which is specifically designed for use with a fiber optic probe, has as its main features simplicity and ease of application, and it is capable of estimating the thickness and the absorption coefficient of a superficial absorbing and scattering layer. Both of these parameters are of great interest for the noninvasive study of epithelial biological tissues. The validity range and accuracy of the model are tested on tissue phantoms in both the forward and inverse modes of application.

Comparative evaluation of two simple diffuse reflectance models for biological tissue applications

We present a comparative evaluation of two simple diffuse reflectance models for biological tissue applications. One model is based on a widely accepted and used in biomedical optics implementation of diffusion theory, and the other one is based on a semiempirical approach derived from basic physical principles. We test the models on tissue phantoms and on human skin, utilizing a standard six-around-one optical fiber probe for light delivery and collection. We show that both models are suitable for use with an optical fiber probe and illustrate the potential, applicability, and validity range of the models.

In Vivo Optical Properties of Melanocytic Skin Lesions: Common Nevi, Dysplastic Nevi and Malignant Melanoma

We present an in vivo study of the optical properties of common nevi, dysplastic nevi and malignant melanoma skin lesions in human subjects. Reflectance spectra were measured on 1379 skin lesions, in the visible and near‐infrared spectral regions, using a spectral imaging system, in a clinical setting. Analysis of the data using a reflectance model revealed differences between the optical properties of melanin present in nevi and melanoma lesions. These differences, which are in agreement with our previous observations on average reflectance spectra, may be potentially useful for the noninvasive characterization of pigmented skin lesions and the early diagnosis of melanoma.

Modeling diffuse reflectance from homogeneous semi-infinite turbid media for biological tissue applications: a Monte Carlo study

Diffuse reflectance spectroscopy is one of the simplest and widely used techniques for the non-invasive study of biological tissues but no exact analytical solution exists for the problem of diffuse reflectance from turbid media such as biological tissues. In this work, a general treatment of the problem of diffuse reflectance from a homogeneous semi-infinite turbid medium is presented using Monte Carlo simulations. Based on the results of the Monte Carlo method, simple semi-empirical analytical solutions are developed valid for a wide range of collection geometries corresponding to various optical detector diameters. This approach may be useful for the quick and accurate modeling of diffuse reflectance from tissues.

Noise and stray light characterization of a compact CCD spectrophotometer used in biomedical applications

In recent years, new generation spectrophotometers are increasingly used in biomedical applications. Handheld spectrophotometers, offering compactness, versatility, and low cost, have facilitated a broad array of applications in biomedical optics. However, despite the popularity and the diverse range of applications, a detailed characterization of many of these new spectrophotometers in terms of stray light and noise characteristics is missing from the literature. Such a popular instrument (USB2000) is characterized in detail with particular focus and emphasis on its noise and stray light characteristics. The results of the analysis may be useful to numerous users of this and other similar instruments in a diverse range of applications.

Probing skin interaction with hydrogen peroxide using diffuse reflectance spectroscopy

Hydrogen peroxide is an important oxidizing agent in biological systems. In dermatology, it is frequently used as topical antiseptic, it has a haemostatic function, it can cause skin blanching, and it can facilitate skin tanning. In this work, we investigated skin interaction with hydrogen peroxide, non-invasively, using diffuse reflectance spectroscopy. We observed transient changes in the oxyhaemoglobin and deoxyhaemoglobin concentrations as a result of topical application of dilute H(2)O(2) solutions to the skin, with changes in deoxyhaemoglobin concentration being more pronounced. Furthermore, we did not observe any appreciable changes in melanin absorption properties as well as in the skin scattering properties. We also found no evidence for production of oxidized haemoglobin forms. Our observations are consistent with an at least partial decomposition of hydrogen peroxide within the stratum corneum and epidermis, with the resulting oxygen and/or remaining hydrogen peroxide inducing vasoconstriction to dermal blood vessels and increasing haemoglobin oxygen saturation. An assessment of the effects of topical application of hydrogen peroxide to the skin may serve as the basis for the development of non-invasive techniques to measure skin antioxidant capacity and also may shed light onto skin related disorders such as vitiligo.