Finite element analysis of thermal and acoustic processes during laser tattoo removal.

BACKGROUND AND OBJECTIVE: Q-switched laser therapy is commonly used for the removal of tattoos. However, despite ever increasing demand for this intervention, a better understanding of the mechanisms that result in pigment reduction is required in order to optimise outcomes and reduce the number of treatment episodes. STUDY DESIGN: A finite element analysis computer simulation was developed to model the fragmentation response of ink granules during irradiation of a professional black tattoo using a Q-switched Nd:YAG laser. Thermal and acoustic mechanisms were considered, allowing the optimal laser settings to be predicted throughout the course of treatment. Changes in the thermal properties of the ink during heating were taken into account to improve the reliability of the results obtained. RESULTS: The simulated results are in close agreement with clinical observations. Thermal fragmentation was shown to be the dominant mechanism in pigment reduction when using a 6 nanoseconds pulse at 1,064 nm. In order to provide maximum clearance whilst maintaining acceptable levels of tissue thermal damage, later treatments were shown to benefit from higher fluence levels than initial treatments. Larger spot diameters were also preferable throughout the course of treatment. CONCLUSIONS: The results from the simulation build upon previous work carried out in the field, applying ink thermal coefficients which vary with temperature for the first time. These results compliment clinical knowledge, suggesting that a proactive increase in fluence during a course of treatments is likely to improve the response to laser therapy.

Determination of the thermal and physical properties of black tattoo ink using compound analysis.

Despite the widespread use of laser therapy in the removal of tattoos, comparatively little is known about its mechanism of action. There is a need for an improved understanding of the composition and thermal properties of the tattoo ink in order that simulations of laser therapy may be better informed and treatment parameters optimised. Scanning electron microscopy and time-of-flight secondary ion mass spectrometry identified that the relative proportions of the constituent compounds of the ink likely to exist in vivo are the following: carbon black pigment (89 %), carvacrol (5 %), eugenol (2 %), hexenol (3 %) and propylene glycol (1 %). Chemical compound property tables identify that changes in phase of these compounds lead to a considerable reduction in the density and thermal conductivity of the ink and an increase in its specific heat as temperature increases. These temperature-dependent values of density, thermal conductivity and specific heat are substantially different to the constant values, derived from water or graphite at a fixed temperature, which have been applied in the simulations of laser therapy as previously described in the literature. Accordingly, the thermal properties of black tattoo ink described in this study provide valuable information that may be used to improve simulations of tattoo laser therapy.

Adverse effects following Q-switched ruby laser treatment of pigmented lesions.

A retrospective study was conducted to investigate the incidence of adverse effects following Q-switched ruby laser treatment of pigmented lesions at the Wessex Specialist Laser Centre. Sixty-one patients received a total of 151 treatments between January 2006 and January 2008. This is the largest series to date of patients on whom adverse effects have been reported following Q-switched ruby laser treatment of an assortment of pigmented lesions. Patients with traumatic or decorative tattoos were excluded from this study. Two of the treatments (1.3%) resulted in adverse effects. One patient developed hyperpigmentation and the other experienced scabbing and subsequent textural change following abrasion of the scab. No predisposing medical or other factors were observed in either patient. This low incidence of adverse effects is consistent with the highly selective absorption of ruby laser light by melanin. The presence of these adverse effects highlights the importance of test patch treatments, the necessity for patients to follow good post-treatment advice and the case for vigilance in monitoring the quality of the laser output.