Effect of wavelength and beam width on penetration in light-tissue interaction using computational methods

Abstract

Penetration depth of ultraviolet, visible light and infrared radiation in biological tissue has not previously been adequately measured. Risk assessment of typical intense pulsed light and laser intensities, spectral characteristics and the subsequent chemical, physiological and psychological effects of such outputs on vital organs as consequence of inappropriate output use are examined. This technical note focuses on wavelength, illumination geometry and skin tone and their effect on the energy density (fluence) distribution within tissue. Monte Carlo modelling is one of the most widely used stochastic methods for the modelling of light transport in turbid biological media such as human skin. Using custom Monte Carlo simulation software of a multi-layered skin model, fluence distributions are produced for various non-ionising radiation combinations. Fluence distributions were analysed using Matlab mathematical software. Penetration depth increases with increasing wavelength with a maximum penetration depth of 5378 μm calculated. The calculations show that a 10-mm beam width produces a fluence level at target depths of 1-3 mm equal to 73-88% (depending on depth) of the fluence level at the same depths produced by an infinitely wide beam of equal incident fluence. Meaning little additional penetration is achieved with larger spot sizes. Fluence distribution within tissue and thus the treatment efficacy depends upon the illumination geometry and wavelength. To optimise therapeutic techniques, light-tissue interactions must be thoroughly understood and can be greatly supported by the use of mathematical modelling techniques.

Keywords: IPL; Laser; Monte Carlo; Penetration.

Fig. 1

Light penetration into skin illustrating the depth to which wavelengths penetrate human skin. Red light is extinguished some 4–5 mm beneath the surface of the skin whereas ultraviolet hardly penetrates at all and blue barely 1 mm into tissue [8]

Fig. 2

Diagram depicting the Monte Carlo model Cartesian geometry used

Fig. 3

Absorption coefficients of melanin, oxyhaemoglobin and water. The IPL emission spectrum used for this evaluation is overlaid for reference [3]

Fig. 4

Deflection of a photon at a scattering point

Fig. 5

Showing penetration of 300–750 nm photons into tissue matrix from the photon distribution

Fig. 6

Showing a detailed description of photon deposition for wavelengths ranging 300 to 750 nm

Fig. 7

Calculated penetration profiles for uniform 1, 5, 10, 20 and 40 mm width beam, of equal incident fluence obtained by Monte Carlo simulation using typical skin parameters for wavelengths of 525–1100 nm

Fig. 8

A schematic representation of spot-size-dependent depth of penetration