Nonlinear absorption‐based analysis of energy deposition in melanosomes for 532‐nm short‐pulsed laser skin treatment

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Abstract
Background and Objectives: The clinical use of 532‐nm short‐pulsed lasers has provided effective treatment of epidermal pigmented lesions. However, the detection of significant differences in treatment effects between picosecond and nanosecond lasers has still varied among clinical studies. For robust evaluation of the differences based on the treatment mechanism, this study presents a nonlinear absorption‐based analysis of energy deposition in melanosomes for 532‐nm short‐pulsed laser treatment.Study Design/Materials and Methods: Nonlinear absorption by melanin is modeled based on sequential two‐photon absorption. Absorption cross‐sections and nonradiative lifetimes of melanin, which are necessary for the nonlinear absorption‐based analysis, are determined from transmittance measurement. Using the model and parameters, energy deposition in melanosomes was calculated with varying fluence and pulse width settings, including actual clinical parameters.Results: The energy deposition in melanosomes increased with shorter laser pulses, and subnanosecond laser pulses were found to be most efficient. The comparison of energy deposition calculated using clinical parameters demonstrated the differences in treatment effects between picosecond and nanosecond lasers reported in clinical studies.Conclusion: The nonlinear absorption‐based analysis provides quantitative evidence for the safety and efficacy evaluation of short‐pulsed laser treatments, which may lead to the establishment of numerical indices for determining treatment conditions. Future studies considering the effects of the surrounding tissue on energy deposition in melanosomes will be needed.

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