Comparison of Responses of Tattoos to Picosecond and Nanosecond Q-Switched Neodymium:YAG Lasers FREE

Q-SWITCHED LASERS have revolutionized the treatment of tattoos. By restricting pulse duration, ink particles reach very high temperatures¹ with relative sparing of adjacent normal skin. This significantly decreases the scarring that often results after nonselective tattoo removal methods, such as dermabrasion or treatment with a carbon dioxide laser. The Q-switched neodymium:YAG laser has been shown to be particularly effective in the treatment of black tattoos.^2- 3 Neodymium:YAG lasers in clinical use are capable of delivering 10-nanosecond pulses. The most common pigment particle, carbon black in india ink, has been shown to be about 40 nm in diameter.⁴ These particles have thermal relaxation times of less than 10 nanoseconds; therefore, an argument can be made for using subnanosecond pulses.⁵ Assuming a thermal or mechanical mechanism for tattoo removal, subnanosecond pulses theoretically should be more effective in tattoo treatment. To test the hypothesis that picosecond pulses are more efficient than nanosecond pulses in the treatment of tattoos, we compared tattoo clearing after treatment with 2 neodymium:YAG laser systems in which all laser parameters were held constant except pulse duration. Using both pulse widths, we also examined immediate particle responses to pulsed laser irradiation in vitro.

Sixteen patients with cosmetic tattoos participated after being informed of the nature of the protocol and giving consent. The protocol was approved by the Subcommittee for Human Subjects of the Massachusetts General Hospital, Boston. Patients were enrolled on a consecutive basis as long as their tattoos were partly black and previously untreated. The restriction to black tattoos allowed for a reasonable chance for pigment lightening, given the response of black tattoos to 1064-nm radiation and the low fluences used in the study (vide supra), and easier correlation between clinical responses and in vitro black pigment alterations induced by laser. Of the 16 tattoos, 15 were created by a professional and 1 was done by an amateur. Eleven of the tattoos were multicolored, most of these containing black, red, and green pigments. The remaining 5 clinically showed only black pigmentation. Tattoos were divided into 3 parts, the first 2 parts being the treatment sites for comparison between pulse durations. These sites were designated either as the left or right symmetrical parts of a representative portion of the whole tattoo. Part 1 was treated with a mode-locked Q-switched neodymium:YAG laser (Model YG501, Quantel Technologies, Santa Clara, Calif) delivering 35-picosecond pulses. Spot size, fluence, and repetition rate were 1.4 mm, 0.65 J/cm², and 10 Hz, respectively. The beam profile was gaussian. Part 2 was treated with a Q-switched neodymium:YAG laser (Model NY82-10, Continuum, Santa Clara, Calif). Laser parameters were identical to part 1 except for pulse duration, which was 10 nanoseconds, and the beam profile, which was multimodal. The remainder of the tattoo was treated with the same laser values as those for part 2 except with parameters conventionally used in clinical practice (fluence of 8.0 J/cm² and 2.5-mm spot size). Both laser units used an articulated arm and focusing handpiece lens assembly for beam delivery. Care was taken to avoid gross overlapping of adjacent exposure sites. For all treatment sites, the patients underwent 4 treatments in the described manner at 3- to 4-week intervals. All pulse energies were verified with an energy meter (Model 365, Scientech, Boulder, Colo) prior to each treatment. The beam profiles and spot sizes were determined by imaging the beam with a charge-coupled device camera (Model TM-34KC, Pulnix, Sunnydale, Calif) as follows. A black surface was placed at the object distance in front of the articulated arm. The surface was brought into the focus of the camera so that the laser spot was in the field of view. The video signal was input into a frame grabber (CX100, Image Nation Corp, Beaverton, Ore) interfaced with a personal computer. After capturing the image, the beam size and profile were determined with the aid of imaging software (NIH Image, National Institutes of Health, Bethesda, Md).

Photographs were taken before each treatment. All photographs were taken with the same 35-mm camera (AE1, Canon USA, Lake Success, NY) under similar lighting conditions. The same film type (Ektachrome 100, Kodak, Rochester, NY) and processing method were used for all photographs. Local anesthesia (2% lidocaine with epinephrine) was used in patients requesting it. In 10 consenting patients, 2-mm punch biopsy specimens were obtained before and immediately after the first treatment, and at 30 days after the final treatment. Samples were taken from tattoo areas that appeared clinically black. Specimens were fixed in 10% buffered formalin, processed in paraffin, and stained with hematoxylin and eosin. Also, 1-µm sections were cut and stained with buffered 0.5% toluidine blue O. These sections and their unstained counterparts were examined for determination of the depth of altered pigment following treatment. Biopsy specimens were examined by a dermatopathologist blinded to the pulse duration. Transmission electron microscopy was performed in specimens from 4 patients. Specimens were fixed in 4% glutaraldehyde in 0.1-mol/L cacodylate buffer, postfixed in 2% osmic acid in buffer, dehydrated, and embedded in epoxy resin (Epon). Thin sections were stained with saturated uranyl acetate and Sato lead stain and examined with a transmission electron microscope (Philips CM10, Philips Elect ron Optics, Amsterdam, the Netherlands).

A panel of 8 dermatologists and nursing staff familiar with laser treatment of tattoos but not familiar with the study simultaneously and independently evaluated each tattoo from pretreatment and posttreatment photographs. The posttreatment scores were based on the photographs taken 1 month after the fourth and final laser treatment session. Before evaluating the experimental set of data, a short series of slides from patients not in this study were shown as a training set. Using this set, consensus was reached among the evaluators regarding the grading system for tattoo ink darkening, as follows:

Raters were instructed to give separate scores based on improvement percentage in black portions and nonblack portions of the tattoos. For every tattoo, the modal score was recorded for each pulse duration. A Wilcoxon matched-pair signed-rank test was used to analyze the data. Hypopigmentation, hyperpigmentation, textural changes, and scarring were graded as absent, trace present, or present.

We performed 2 in vitro experiments. In the first, we irradiated india ink suspensions composed of stock tattoo ink for medical tattooing (Sanford-Farber, Lewisburg, Tenn). The stock ink suspension was diluted with distilled water to obtain a usable optical density for testing. These suspensions were put into plastic Petri dishes (60×15 mm, Fisher Scientific, Pittsburgh, Pa) under which a black piece of paper was placed (Zap-it laser alignment paper, Kentek, Pittsfield, NH) to prevent backscatter. The suspensions were irradiated with picosecond and nanosecond pulses with the same parameters as in the human tattoos. The handpiece tip was placed at the surface of the suspension and the laser beam was moved so that the entire surface was irradiated uniformly. Portions of the suspensions were submitted for transmission electron microscopy after in vitro irradiation. Drops of the specimens were placed on a carbon and formvar-coated grid, allowed to dry, and examined unstained with an electron microscope.

The remainder of the irradiated suspensions were placed in plastic cuvettes (1×1×3 cm, Fisher Scientific), and the optical densities were measured before and following irradiation by recording the absorption of a helium neon laser beam by the suspensions. This was quantified by measuring the signal drop across the suspension with a photodiode attached to an oscilloscope (Model 9420, LeCroy, Spring Valley, NY).

Of the 16 tattoos, 12 were judged to clear better with picosecond pulses based on the response of black ink regions (Figure 1 shows a representative result). In the remainder of tattoos, there was only slight or no clearing with either pulse duration (Table 1). Within individual tattoos, black tattoo regions responded best in all cases. Overall, clearance of black ink was better after picosecond treatment (P<.002). Although green tattoo regions lightened in some patients, red, purple, blue, orange, and yellow pigments did not clear regardless of number of treatments or pulse duration, and overall, nonblack tattoo regions responded similarly for both pulse durations (P>.20). Although not formally assessed by the panel of raters, we found that conventional high-energy nanosecond pulses produced clearing comparable with that of the lower-energy picosecond pulses. During treatment, picosecond pulses produced more intense immediate whitening than low-energy nanosecond pulses. Also, in most cases, picosecond pulse sites showed plasma formation and slight postoperative pinpoint bleeding not observed after nanosecond pulses of the same fluence (Figure 1). Edema was noted after both nanosecond and picosecond pulses. Hypopigmentation was noted in 1 tattoo in the picosecond treated portion. No scarring was noted except in parts of 2 tattoos treated with conventional high-fluence nanosecond pulses. These areas showed persistent erythema and induration. No other adverse effects were observed.

Routine light microscopic examination showed pretreatment pigment in all cases. The concentration of granules roughly correlated to the clinical darkness of the tattoo. Also, the clinical color corresponded to the microscopic particle color. The predominant granules appeared as black, irregularly shaped "clumps" that ranged from 1 to 5 µm in diameter. In some specimens, occasional distinct red and green granules (1-2 µm in diameter) were identified. Intertattoo ink depth varied considerably and ranged from 250 to 1700 µm deep to the stratum corneum. Intratattoo ink depths, however, were more consistent, with biopsy specimens showing no more than ±200 µm depth variability from different sections of the same specimens. Immediately after treatment, nonblack particles appeared unaltered in the biopsy specimens; however, in all cases there was a smudging of black particles, noted as a transition from black granules to amorphous light brown bodies with a lacy appearance. In general, a line could be drawn demarcating the depth of transition of the particles. This depth ranged from a mean (±SEM) of 670±96 µm (n=10) for picosecond pulses to 590±107 µm for nanosecond pulses. Other immediate changes included the formation of suprabasilar clefts and vacuoles in the dermal interstitium in proximity to pigmentation. Altered pigment and cellular debris lined some of these dermal vacuoles. These immediate histological changes ranged from most pronounced after conventional high-fluence nanosecond pulses to least pronounced after low-fluence nanosecond pulses. No significant fibrosis was noted immediately following treatment or 1 month after the last treatment.

Electron microscopic analysis revealed pretreatment particles that were electron dense and ranged from 10 to 100 nm in diameter (mean, 40 nm). (We define "particle" as the smallest identifiable structure on electron microscopy. This is distinguished from "granule," which we define as the smallest structure observed on routine light microscopy [usually 0.5-4.0 µm in diameter].)^6- 7 Particles resided predominantly in fibroblasts. Immediately after irradiation, many of these particles appeared unchanged; however, a fraction (approximately 30%) showed a lamellated electron-lucent appearance (Figure 2). Marked debris was noted in the cytoplasm of pigment-laden cells. Specimens obtained 1 month after the final treatment showed a persistence of these lamellated particles. These electron microscopic findings were similar for both picosecond and nanosecond exposures.

The in vitro optical density decreased after irradiation of ink in the cuvettes. The suspensions were visibly lighter after irradiation, and the optical density of the suspensions decreased from 0.1 to 0.06 after both nanosecond and picosecond pulses.

Electron micrographs of the in vitro irradiated india ink suspension showed a similar mixture of electron-dense and electron-lucent particles as noted in vivo, with the exception that after in vitro irradiation, many particles were much larger than before treatment (Figure 3). Particle diameters ranged from baseline size (40 nm) up to 300 nm. It was unclear from review of the electron micrographs whether single particles had enlarged or if many particles had coalesced to form larger, more electron-lucent lamellated particles. The qualitative nature of the changes was independent of pulse duration; however, a greater proportion of particles were altered in the picosecond-treated sample.

Our results show that picosecond laser pulses are more efficient in clearing cosmetic tattoos than nanosecond domain pulses. By holding other laser parameters constant, pulse duration was shown to be a significant factor in tattoo removal. With picosecond pulses, we were able to clear some tattoos with fluences of less magnitude than typical nanosecond domain pulses used in clinical practice.

The mechanisms for tattoo removal by pulsed laser radiation are poorly understood. It has been shown that ultrashort laser pulses can selectively disrupt cells containing tattoo pigments,^6- 7 releasing ink into the dermis, some of which is removed by the vascular and lymphatic systems. Fragmentation of ink particles is an intuitively attractive mechanism. The resulting smaller particles should be more easily phagocytosed and packaged; moreover, the smaller particle diameters, as they approach the wavelength of visible light, should result in more intrinsic dermal light scattering, making the particles less visible from the skin surface. Some tattoos are clinically resistant to all laser therapies despite the predicted high particle temperatures achieved through selective photothermolysis. Reasons cited for failure of some tattoos to clear include the absorption spectrum of the pigment, the depth of pigment, and the structural properties of the ink. Also, some inks may remain in the dermis after being rephagocytosed by resident cells.^5,8

Regardless of the mechanism for tattoo clearing by laser, the initial event is absorption of the incident beam by the ink. To achieve the maximum temperature increase in the ink particles while sparing the adjacent dermal collagen, pulse durations should be equal to or less than the thermal relaxation time, defined as the time that it takes for the central temperature in a structure to decrease by 50%.⁹ If the pulse duration is less than the thermal relaxation time of the absorbing particle, then heat is confined without significant thermal diffusion during the laser pulse. For the 40-nm particles in our study, assuming they behave thermally as independent entities, thermal relaxation time is roughly 1 nanosecond, so that only the picosecond pulses were thermally confined.

Thus, higher peak temperatures should result from picosecond pulses, and this may explain the differences in tattoo clearing. We calculated the predicted peak temperatures at the center of a particle for the 2 power densities (2.1×10¹⁰ W/cm² for picosecond vs 7.5×10⁷ W/cm² for nanosecond pulses) by solving the general heat equation for an arbitrary sphere of radius by numerical methods.¹⁰ It was found that for a 40-nm-diameter sphere (the size of a typical tattoo particle), thermal confinement is achieved with a 35-picosecond pulse duration (the pulse duration of our picosecond system), so that the energy deposited in the 40-nm sphere stays largely within this diameter at the end of the laser pulse. On the other hand, if the same amount of energy is delivered in a 10-nanosecond pulse, then significant heat transfer takes place in the surrounding medium during the laser pulse, and the peak temperature attained by the particle is less than 3% of the peak temperature achieved with the picosecond pulse. The result of the theoretical calculation thus supports the argument that picosecond laser pulses should be more effective at altering the tattoo particles than nanosecond laser pulses.

The lack of qualitative electron and light microscopic differences between the 2 pulse durations supports a similar mechanism (thermal) for the laser-induced particle changes. That picosecond pulses resulted in a greater microscopic depth of altered pigment is also consistent with our thermal argument, since a smaller subsurface energy density would critically increase the particle temperature. Accordingly, because more particles are altered with picosecond pulses, gross tattoo clearing is enhanced with the same surface fluence.

The differences in the beam profiles of the 2 lasers should also be considered in interpreting the results. The nanosecond profile, whose shape was more like that of a top hat, produced a more uniform surface fluence. Still, the peak energy density (at the center of the gaussian profile) of the picosecond laser was only about 20% greater than the peak of the multimode (nanosecond) profile. It is unlikely that this small difference in local energy density within the spot significantly influenced our findings.

High temperatures have been shown to induce our observed electron microscopic changes in other studies.^4,6 Chen and Diebold⁴ report enlarged electron-lucent particles after in vitro radiation of carbon black with a pulsed neodymium:YAG laser. They found that laser irradiation of the suspension caused it to become grossly transparent, and their description of "new particles with a shell-like structure" was consistent with our own findings. In explaining the mechanism for these changes, they suggest that the gradual reduction in absorbance (blackness) of the suspensions was caused by sufficient particle heating to initiate chemical reactions with the surrounding water. They found hydrogen and carbon monoxide gas above the irradiated sample and note that this reaction might be responsible for the gradual loss of carbon in the suspension. This chemical reaction has been described as an endothermic steam-carbon reaction.⁴

In addition to temperature increases, irradiation of an absorber by short laser pulses causes rapid thermal expansion, which propagates as a stress wave. Inertial confinement is achieved when the laser pulse is delivered within the time when pressure can be relieved from the absorbing particle. For india ink particles, assuming they behave inertially as independent structures, this is approximately 25 picoseconds, slightly less than the 35-picosecond pulses in our experiment, so that stress wave differences probably did not play a significant role in tattoo clearing.

Ideally we would have liked to treat resistant tattoos with high-energy picosecond pulses since the present study only shows greater energy efficiency with shorter pulses. In all cases, conventional high-fluence nanosecond domain pulses were as or more effective than the very low picosecond fluences used in the direct comparison part of the study. It is unknown whether resistant tattoos can be treated effectively by higher-fluence picosecond pulses (5.0-10.0 J/cm²). Unfortunately, high-energy picosecond pulses are difficult and expensive to generate, so that very small spot sizes (<0.5 mm) would be required to produce higher fluences with our system. These small spot sizes result in unacceptable scattering losses so that tissue penetration is compromised. Another challenge with picosecond pulses is optical breakdown (plasma production), which typically occurs with power densities greater than 10⁹to 10¹¹ W/cm²,¹¹ so that plasma threshold is reached even with our low-energy picosecond pulses. This might limit the usefulness of higher-energy picosecond pulses because the plasma would consume energy intended for deeper tattoo particles. On the other hand, with a neodymium:YAG laser with parameters similar to ours, it has been shown that plasma-generated shock waves in distilled water are able to propagate hundreds of micrometers below the surface. The associated shock peak pressures were attenuated by only 50% at 300 µm,¹² suggesting that plasma-generated shock waves might contribute to fibroblast cell death and/or intrinsic ink particle changes. Moreover, the plasma, in creating a pinpoint surface defect, might elicit greater dermal inflammation and facilitate transepidermal elimination.

Results of our study suggest that intrinsic optical property changes, rather than particle fragmentation, are responsible for india ink tattoo clearing, and that these temperature-induced changes occur with lower fluence thresholds with shorter laser pulses. Further studies with more powerful picosecond lasers are necessary to demonstrate whether picosecond pulses are capable of clearing resistant tattoos. Also, because many tattoos contain nonblack inks, studies should examine ultrastructural changes before and after laser treatment with chemically dissimilar pigments.

Accepted for publication July 29, 1997.

This work was supported by the Department of Defense Medical Free-Electron Laser (MFEL) program under contract N00014-94-1-0927.

The views expressed in this article are those of the authors and do not reflect the official policy or position of the US Department of the Navy, Department of Defense, or the US government.

Presented at the 16th annual meeting of the American Society for Laser Medicine and Surgery, Orlando, Fla, April 17, 1996.

We thank William Farinelli for his technical assistance.

Reprints: CDR E. Victor Ross, USN, Department of Clinical Research, Naval Medical Center, 34800 Bob Wilson Dr, San Diego, CA 92134-5000.