Modeling thermal damage in skin from 2000-nm laser irradiation

Abstract

An optical-thermal-damage model of the skin under laser irradiation is developed by using finite-element modeling software (FEMLAB 3.1, Comsol, Incorporated, Burlington, Massachusetts). The general model simulates light propagation, heat generation, transient temperature response, and thermal damage produced by a radically symmetric laser beam of normal incidence. Predictions from the model are made of transient surface temperatures and the thermal damage on a pigskin surface generated by 2000-nm laser irradiation, and these predictions are compared to experimental measurements. The comparisons validate the model predictions, boundary conditions, and optical, thermal, and rate process parameters. The model enables the authors to verify the suitability of the American National Standards Institute (ANSI) maximum permissible exposure (MPE) standard for a wavelength of 2000 nm with exposure duration from 0.1 to 1 s and 3.5-mm beam diameter. Compared with the ANSI MPE standard, however, the MPE values predicted by the model are higher for exposure durations less than 0.1 s. The model indicates that it may be necessary to modify the ANSI MPE standard for cases in which the laser-beam diameter is larger than 3.5 mm when a "safety factor" of ten is used. A histopathological analysis of the skin damage is performed to determine the mechanisms of laser-induced damage in the skin.

Infrared skin damage thresholds from 1319-nm continuous-wave laser exposures

“…In previous skin studies, [4][5][6][7] the results of experiments and tests on Yucatan miniature pigs were presented for a 2000-nm thulium fiber laser for various exposure durations and incident beam diameters. Some damage threshold studies have been completed at 1315 nm for pulses in the microsecond regime, as reported by Cain et al 8 A preliminary skin damage threshold study at 1214 and 1319 nm has also been documented.…”

“…[21][22][23][24] Models that represent the skin as a two-or three-layer construct, with a Beer's law absorption term for the laser energy deposition, have been shown to accurately predict the optical-thermal response of the tissue. 5,19,25 Thermal diffusion solutions are most often computed through finite element or finite difference methods. Increased accuracy has been demonstrated when temperature-dependent surface cooling associated with the evaporation of water is included.…”

Modeling of surface thermodynamics and damage thresholds in the IR and THz regime

“…Tissue damage is modeled by the Arrhenius damage integral: Q(z, r; r) = A exp ( -E, )dt (12) where R is the universal gas constant and T is the temperature measured in Kelvin. A is a normalization constant and Ea is the action potential specific to the tissue.…”

“…[7][8][9] This has led to a need for close examination of strongly absorbed frequencies both in the infrared (IR) and the terahertz (THz) region 10 of the electromagnetic spectrum. 7,11,12 In these regions, absorption depth of the incident radiation can be extremely shallow.' 3 We have speculated that boundary conditions must be accurately represented in order to correctly predict the temperature response, and subsequent damage thresholds.…”

High-energy pulsed Raman fiber laser for biological tissue coagulation

“…The use of pulsed lasers can be an effective approach to deliver thermal energy with temporal confinement and therefore facilitate efficient tissue coagulation [8]. For example, although the CW laser marking parameters of Suter et al [7] suggest that more than a Joule of energy was required to induce coagulation and a visible mark, numerical models [4,9] can be used to show that when laser energy is delivered on a time scale that is short relative to thermal diffusion, the threshold for marking may be reduced more than one hundred-fold. Motivated by such models, we sought to develop a 1440-nm laser capable of delivering pulses with ~10 mJ of energy and with temporal duration in the 100~500 μs.…”