KEYWORDS: Skin, In vivo imaging, Reflectivity, Optical spheres, In vitro testing, Diffuse reflectance spectroscopy, Laser therapeutics, Color difference, Visualization, Spectrometers
For quantitative prediction and evaluation of pulsed dye laser therapy of port wine stain (PWS) skin, the CIE L*a*b* color difference, ΔE*, has been utilized to characterize numerically the color differences between normal untreated and treated PWS skin. Three optical instruments (Minolta chromameter CR-200, a cross-polarized diffuse reflectance imaging system, and visual reflectance spectrometers) are compared to investigate their clinical feasibility for quantitative color assessment. Compared to the chromameter as a standard measurement instrument, other instruments also provide valuable measurements of skin color for the relative quantification of PWS treatment outcome. The fiber-optic visual reflectance spectrometer is preferable for continuous measurement of a small area of skin. The cross-polarized imaging system is useful as a simple non-contact measurement technique to provide spatially resolved color difference images.
Cryogen spray cooling (CSC) provides thermal protection to the epidermis during dermatologic laser surgery (DLS) for removal of port wine stain (PWS) birthmarks. The objectives of this study are: to improve the thermal modeling of skin undergoing CSC-assisted DLS for PWS treatment; and, to address the effect of temporal and lateral variations in surface heat transfer during CSC on epidermal protection. The finite element method is used to solve the light and heat diffusion equations in a skin-cross section composed by epidermis, dermis and two blood vessels. Thermal conductivities of each biological structure are modeled as temperature dependent functions. The model accounts for the latent heat of fusion and vaporization, and temporal and spatial thermal variations---due to the inherent non-homogeneous nature of sprays---in surface cooling. Thermal damage due to laser irradiation is evaluated by an Arrhenius integral model. For a 60 ms cryogen spurt, temperature maps of epidermis show that at the end of the spurt there are significant temperature differences, which resulted in epidermal damage after a 5 J/cm2 0.45 ms laser pulse at 585 nm on light color skin type. A 60 ms delay between end of spurt and laser onset produced a relative more homogeneous temperature distribution at the epidermis, and, subsequently, a more effective CSC-DLS for which only the blood vessels were thermally damaged. Temporal and lateral variations in surface cooling must be taken into account to guarantee that enough epidermal protection is provided.
We present an initial study on applying genetic algorithms (GA) to retrieve human skin optical properties using visual reflectance spectroscopy (VRS). A three-layered skin model consisting of 13 parameters is first used to simulate skin and, through an analytical model based on optical diffusion theory, we study their independent effects on the reflectance spectra. Based on a preliminary analysis, nine skin parameters are chosen to be fitted by GA. The fitting procedure is applied first on simulated reflectance spectra with added white noise, and then on measured spectra from normal and port wine stain (PWS) human skin. A normalized residue of less than 0.005 is achieved for simulated spectra. In the case of measured spectra from human skin, the normalized residue is less than 0.01. Comparisons between applying GA and manual iteration (MI) fitting show that GA performed much better than the MI fitting method and can easily distinguish melanin concentrations for different skin types. Furthermore, the GA approach can lead to a reasonable understanding of the blood volume fraction and other skin properties, provided that the applicability of the diffusion approximation is satisfied.
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