Will Sub-Nanosecond Lasers Replace Nanosecond Lasers for Tattoo Removal?

With their short nanosecond pulse durations, Qswitched lasers have revolutionized the treatment of tattoos [1-3]. Nevertheless, the mechanisms for clinically observed tattoo clearance are still not well understood. Two mechanisms have been proposed: a) a thermal effect where tattoo particles are heated to sufficiently high temperatures to cause chemical changes within the particles and the surrounding cells; and b) a mechanical fragmentation of the particles due to extremely fast changes in the particles’ temperature [1-4]. Theoretically, both mechanisms should be more effective towards shorter pulse durations. It is for this reason that sub-nanosecond, 750-900 picoseconds (e.g., 0.75 0.9 nanoseconds) tattoo-removal lasers [5, 9], have recently attracted considerable interest. However, as measurements of the dependence of the ablation threshold fluence on pulse duration in a wide range of pulse durations (as short as 0.1 ps) have demonstrated, the pulse duration is of importance only when the irradiated tissue is basically transparent to the laser light [6]. For highly absorbing tissues, such as tattooed skin, the ablation threshold fluence was found to be completely independent of the laser pulse duration, even when pulse durations were shortened from the nanosecond to sub-picosecond (0.1 ps) range [6]. The question thus arises whether the shortening of laser pulses will have any significant effect on tattoo removal efficacy, especially since the pulse duration of the current “picosecond” devices are only slightly shorter than one nanosecond.