1 November 2004 Mitigating thermal mechanical damage potential during two-photon dermal imaging
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J. of Biomedical Optics, 9(6), (2004). doi:10.1117/1.1806135
Two-photon excitation fluorescence microscopy allows in vivo high-resolution imaging of human skin structure and biochemistry with a penetration depth over 100 μm. The major damage mechanism during two-photon skin imaging is associated with the formation of cavitation at the epidermal-dermal junction, which results in thermal mechanical damage of the tissue. In this report, we verify that this damage mechanism is of thermal origin and is associated with one-photon absorption of infrared excitation light by melanin granules present in the epidermal-dermal junction. The thermal mechanical damage threshold for selected Caucasian skin specimens from a skin bank as a function of laser pulse energy and repetition rate has been determined. The experimentally established thermal mechanical damage threshold is consistent with a simple heat diffusion model for skin under femtosecond pulse laser illumination. Minimizing thermal mechanical damage is vital for the potential use of two-photon imaging in noninvasive optical biopsy of human skin in vivo. We describe a technique to mitigate specimen thermal mechanical damage based on the use of a laser pulse picker that reduces the laser repetition rate by selecting a fraction of pulses from a laser pulse train. Since the laser pulse picker decreases laser average power while maintaining laser pulse peak power, thermal mechanical damage can be minimized while two-photon fluorescence excitation efficiency is maximized.
Barry R. Masters, Peter T. C. So, Christof Buehler, Nicholas P. Barry, Jason D. B. Sutin, William W. Mantulin, Enrico Gratton, "Mitigating thermal mechanical damage potential during two-photon dermal imaging," Journal of Biomedical Optics 9(6), (1 November 2004). https://doi.org/10.1117/1.1806135

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