Hyperspectral Imaging Seminar

Dmitry Yudovsky and Laurent Pilon Retrieving Skin Properties From In Vivo Spectral Reflectance Measurements

Main Challenge: Demonstrate the capability of reflectance spectroscopy to: Determine the structure and thickness of the skin. Blood volume and oxygen saturation. Scattering coefficient from in vivo spectral reflectance measurements. Rapidly and non-invasive monitoring. In Vivo Spectral Reflectance Measurements

About Skin The largest organ of the human body. Total surface area of approximately 1.8 m^2. Total weight of approximately 11 kg for adult. Skin Structure: Two main layers: Outer – epidermal, Inner -dermis.

Skin Layers Epidermal layer is composed of dead cells embedded in a lipid matrix. Epidermal layer is pigmented by melanin. Melanin absorbs strongly in the UV. Melanin concentration is between mg/mL of the epidermal. Epidermal thickness may vary with anatomical, gender, age, health. Ranges between micro.

Skin Layers Cont. Dermis is responsible for the skin pliability, temperature control. Dermis is composed of collagen fibers perfused by nerves, capillaries and blood vessels. Dermis thickness ranges between micro.

Hemoglobin Properties Blood consists of hemoglobin. Hemoglobin responsible for oxygen transfer (Oxyhemoglobin). Hemoglobin is known as deoxyhemoglobin once it release its oxygen transfer. The ratio of oxyhemoglobin to total hemoglobin in blood – oxygen saturation.

Hemoglobin Properties Cont. Depends on body location and tissue health, the volume of blood in the dermis ranges 0.2-7% Hemoglobin absorb in the visible and near-infrared part of the spectrum. The spectral absorption coefficient of oxyhemoglobin and deoxyhemoglobin are differ. The color of the dermis depends of the local oxygen saturation of the blood.

Uses Detect a wide variety of medical conditions – Acute and long inflammation, Acne laser. Melanin concentration correlates with the risk of melanoma skin cancer. To estimate blood volume during alcohol consumption. Determine distribution of oxygen saturation in human skin.

Naïve Approach Optical tomography or ultrasound. Sample of the skin is removed and analyzed. Study light transfer in skin using Monte Carlo simulator. Diffusion approximation to model light transfer through tissue. Alternative An inverse method based on a semi empirical model for the diffuse reflectance from layer media subject

Motivation To Retrieve: Skin melanin volume, epidermal thickness. Dermal blood volume and oxygen saturation. Scattering coefficient from diffuse reflectance spectra of human skin. Racial differences, UV exposure and anatomical location. From spectral reflectance measures.

Measurements 50 male and 21 female of white Caucasian descent. 21 male and 21 female of black African descent. Illuminating a circular spot of skin, 1 inch in diameter and collecting the hemispherical reflectance ( nm). 3 locations: (i) Right inner forearm. (ii) Forehead over the left eye. (iii) Left outer forearm.

Main Challenge: Develop method accounting optical properties to retrieved physiologically meaningful parameters from the simulated diffuse reflectance spectra of skin. Diffuse reflectance spectra between nm was generated using Monte Carlo simulations. Inverse Model

In Vivo normal- hemispherical spectral reflectance concept

Optical Properties Of Human Skin The light scattering properties of the skin have been study in the visible and near infrared region of the spectrum (between nm). Study light transfer in skin by using Monte Carlo simulation, a stochastic method.

Monte Carlo Simulation Red, the impact of melanin is strong in range nm, the deep red wavelength specify the epidermal melanin. Green, Yellow, Orange, the impact of blood is maximal in range nm, the yellow wavelength can specify the blood content.

Optical Properties Of Human Skin The linear spectral absorption The transport scattering coefficient All units

Optical Properties Of Epidermis Absorption in the epidermis is mainly due to melanin and flesh. The volume fraction of melanin. The background absorption of humen flesh : Absorption coefficient of melanin:

Optical Properties Of Epidermis Melanin absorption of UV light is much stronger than that of near infrared light.

Optical Properties Of Dermis Absorption in the dermis is determined by blood and flesh. The volume fraction of the dermis occupied by blood. Blood absorption: The absorption coefficient of oxyhemoglobin: is the molar extinction coefficient of oxyhemoglobin in units

Optical Properties Of Dermis, cont. The average hemoglobin concentration in blood is 150 g/l, The oxygen saturation The absorption coefficient of deoxyhemoglobin:, the molar extinction coefficient of deoxyhemoglobin.

Optical Properties Of Dermis, cont. Oxyhemoglobin is nearly transparent for wavelength above 600 nm, giving oxygen rich blood its red color.

Spectral Scattering Coefficients The spectral scattering coefficients of the epidermis and dermis are equal. The power constant b = 1.5, for all locations. C range between 10^5- 10^6, defends on the average size of the microscopic feathers of tissue. Responsible for light scattering in the skin.

Using Inverse Method Optical properties of skin depend on the property vector: The measured diffuse reflectance spectrum is denoted by: A given input parameters Where is the jth wavelength and j is an integer between 1 and k, wavelength range nm.

Hemispherical Diffuse Reflectance From Skin Yudovsky and Pilon approximate expression for the diffuse reflectance of skin. The transport single scattering albedos of: Epidermis. Dermis., Index of refraction of both layers.

Hemispherical Diffuse Reflectance From Skin Cont. The transport single scattering albedo is defined as:

Reduced Reflectance From Human Skin The reduced reflectance is: The modified optical thickness of the epidermis: Semi empirical parameter:

Diffuse Reflectance Of Homogeneous Layer The diffuse reflectance of semi infinite homogeneous layer: The refraction index of tissue The normal normal reflectivity of tissue/air interface: The refraction index of air n0 = 1

Inverse Method cont. This can be achieved by finding vector that minimized the sum: Where : is the estimate spectral diffuse reflectance. is the weight associated with jth. (W =2 for wavelength 600nm) K is the number of wavelengths. Wavelengths between nm

Inverse Method cont. The values of the estimate vector: Were constrained between physiologically upper and lower bounds.

Inverse Method cont. Six values of oxygen saturation, epidermal thickness, melanosome volume fraction, blood volume fraction. Three values of scattering constant (C) and b. Total 11,664 simulated spectra using Monte Carlo. For each spectrum the estimate vector was found by minimizing the Eq. Explore the effect of each parameter separately, two scenarios were considered.

Inverse Method cont. First scenario – Assumed that C and b were known. Second scenario – All parameters assumed unknown. The minimization was stopped once successive iteration of the algorithm no longer reduced by more than 10^-9. Multiple random initial guesses for the estimate vector were attempted to prevent convergence to local minimum.

Reflectance Results (i) Right inner forearm

Reflectance Results (ii) Forehead over left eye

Reflectance Results (i) Left outer forearm

Results The reconstructed reflectance spectra matches the average measured reflectance within standard deviation, for all locations and for both populations. For wave length lower than 600 nm, the absolute difference was less than 0.05 for all spectra considered. Beyond 600 nm, the accuracy diminishes due to week absorption by melanin and hemoglobin.

Biological properties determined from the reflectance spectra and Inversed method

Conclusion From Results Melanin Concentration Differences in melanin pigmentation occurred due to racial and exposure to UV. The value of calculated for the left outer forearm and forehead increased by almost identical amounts compare to the right inner forearm for black and white objects. Changes in melanin concentration caused by race and tanning can be detected by the inverse method.

Melanin concentration

Conclusion From Results Epidermal Thickness Values of epidermal thickness for skin of lightly versus strongly pigmented subjects were similar. The estimates of epidermal thickness for all location was consists with those reported in related work. Spectral reflectance combined with an inverse method can retrieve the epidermal thickness.

Epidermal Thickness

Conclusion From Results Blood Volume Blood volume ranges between 0.2-7%, depends on bodily location. Blood volume is larger in the forehead than in the forearm. The retrieved values are consists for black and white objects. Estimate blood volume and oxygen saturation could be retrieved from the spectral reflectance of human skin.

Blood Volume

Conclusion From Results Scattering constant The scattering power constant C ranges between approximately, and power law constant b = 1.5. Varying b between 1.3 and 1.5 influenced only the value of constant C. The reduced scattering coefficient was unaffected by the choice of b for wavelength between 450 and 660nm. The shape and magnitude of scattering coefficient is similar to other studies.

Transport scattering coefficients For b =1.5 and C =

Summarize Inverse method was applied to in vivo spectral reflectance. Estimate changes in tissue pigmentation due to racial differences or exposure to UV. Epidermal thickness from one anatomical location to other of white and black objects. Blood volume and oxygen saturation in the dermis of different body location and populations. The scattering coefficient of skin.

Summarize Assess the capability of inversed method to retrieved properties of the skin from. Using in vivo experimental data from human skin and numerically generated reflectance spectra. Using large population. Difference skin complextion. Featured different locations and sun exposures.

Summarize The proposed technique can be used to monitor changes in skin: Melanin volume fraction, epidermal thickness and blood volume. Changes occur in the normal course of aging, UV exposure, results of diseases such as cancer, ulceration or photo damage. The changes affect the reflectance spectra of humen skin.

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