Quantifying the influence of the epidermal optical properties on laser treatment parameters

Abstract

Laser based treatment modalities offer patients a less invasive, often localised treatment method. This has advantages of faster healing times and fewer side effects from treatment. For most laser treatments though the target site is some distance into the skin so light needs to pass through some of the skin layers. The penetration depth of light in skin is determined by the total attenuation, constituted by the scattering and absorption coefficients of the tissue at the treatment wavelength. The absorption coefficient is a measure of the absorption (attenuation) of light through the medium (skin). Similarly the scattering coefficient is a measure of the scattering of light. Furthermore, the thickness of the epidermal layer in skin varies with location across the body. It stands to reason that a thicker epidermis will absorb more light than thinner epidermal layers. Hence in determining the fluence rate reaching a specific position (depth) in the skin, both the epidermal thickness and the absorption coefficient need to be taken into account. In vivo measurements to determine the fluence rate are not easy to implement. Computer modeling of the light interaction with the tissue is an alternative method that can be used to predict the fluence rate at any given depth in the tissue.