Laser Treatment of Tattoos and Pigmented Lesions

Introduction

Most lasers used in dermatology today are based on the principle of selective photothermolysis. According to this theory, using the proper wavelength can cause light energy to be preferentially absorbed by a target chromophore in the skin and transferred into heat energy. Different chromophores in the skin, such as melanin in pigmented lesions, oxyhemoglobin in vascular lesions, and water in all cells, preferentially absorb certain wavelengths of light. By designing a laser to produce a wavelength of light that is better absorbed by the target chromophore than the surrounding tissue, it is possible to selectively heat that chromophore to the point of irreversible damage. By limiting the time that the laser is fired into the chromophore (the pulse duration), it is possible to contain the damage to the selected chromophore. If the laser is fired in a time shorter than the target's thermal relaxation time (the time required for the target to lose 50% of heat), the generated heat will cause selective damage to the target chromophore. If the laser pulse duration is too long (greater than the chromophore's thermal relaxation time), the heat produced in the chromophore will have time to spread to surrounding structures, causing non-selective damage that may lead to scarring. Choosing an appropriate wavelength and pulse duration makes it possible to selectively heat and destroy certain chromophores in the skin without damaging surrounding structures. In this manner, vascular lesions, pigmented lesions, and hair may all be selectively targeted and destroyed.

The chromophore targets in lentigines, and most pigmented lesions, are extremely small melanosomes. These melanosomes, approximately 0.5 µm in diameter, cool very quickly when heated; therefore, they have a short thermal relaxation time. The estimated thermal relaxation time of a melanosome is approximately 250–1000 nanoseconds (ns). Therefore, lasers with very short pulse durations, in the NS or shorter domain, are ideally suited to target this chromophore. Quality switched, “Q-switched” (QS), lasers emit pulses in the NS domain. These lasers store large amounts of energy in the laser cavity through the use of an optical shutter. When the laser fires, the shutter is then able to release a high-powered pulse with an extremely short pulse duration. Even shorter pulse duration lasers than the QS lasers are now in use, in the picosecond (ps) range. The chromophore targets in tattoo removal are the tattoo particles, ranging in size from 40–300 nm. These particles, smaller than melanosomes, therefore require shorter thermal relaxation times. Energy delivered in a ps pulse duration (1 trillionth of a second, 10 ⁻¹² seconds) is approximately 100 times shorter than a ns pulse duration (10 ⁻⁹ seconds), allowing for a closer fit for a tattoo particle's thermal relaxation time.

The delivery of an exceptionally high-energy laser pulse within the NS time span results in rapid heating of the target melanosome (estimated at 10 million degrees/s), causing it to explode. While electron microscopy has confirmed highly-selective destruction of melanosomes within melanocytes and melanized keratinocytes, it is not known precisely how the pigment-containing cells are destroyed. It is believed that melanocytes and melanized keratinocytes are destroyed due to both heat formation due to the absorption of the laser light as well as mechanical damage from acoustic waves that emanate from the absorbing melanosome. Similarly, the mechanisms of laser removal of tattoo pigment are not fully known, but precise photoacoustic and photothermal disruption of the particle likely leads to removal via the lymphatic system. After treatment with either QS or PS lasers, electron microscopy demonstrated pigmented disrupted particles within lysosomes.

By inducing damage to the specific target through the use of laser radiation, a number of commonly encountered pigmented lesions can be treated with excellent cosmetic outcomes. There are many lasers that can be employed to treat pigmented lesions. As a target chromophore, melanin has a broad absorption spectrum within the ultraviolet, visible, and near-infrared (NI) light range ( Fig. 32.1 ). Ideal wavelengths to treat pigmented lesions would be those with greater absorption by melanin than by oxyhemoglobin. However, melanin light absorption decreases with increasing wavelength. Therefore, longer wavelengths penetrate deeper, but are absorbed less by melanin. Given that the pigment in lentigines is very superficial, pigmented lasers with shorter wavelengths, such as pulsed-dye (510 nm), QS potassium titanyl phosphate (KTP) (532 nm), and QS ruby (694 nm) lasers are typically used. Longer wavelength lasers, such as the QS neodymium : yttrium-aluminum-garnet (Nd : YAG) (1064 nm), are used for lesions where the pigment is located in the dermis, such as nevus of Ota and tattoos. QS alexandrite (755 nm) lasers, with an intermediate wavelength, may be used for both superficial and deep pigment.

Longer pulse width ruby, alexandrite, and Nd : YAG lasers, predominantly used for hair removal, have the same wavelengths as the QS versions used in the treatment of pigmented lesions. However, they deliver the laser energy over a longer time interval – milliseconds rather than nanoseconds. The longer pulse width creates less confined thermal damage to the melanosomes, but used with higher energies and less epidermal cooling than used during hair removal, they are capable of removing some pigmented lesions within the epidermis. The longer pulse widths also allow for higher fluences so that larger structures such as hair follicles and nests of pigmented cells may be targeted. Other longer pulse duration lasers, such as the KTP (532 nm) and the pulsed-dye laser (585–595 nm), can be used to treat lentigines because of their melanin absorption.

Another option for treating superficial pigmented lesions is intense pulsed light sources. Unlike laser devices with a collimated beam at a specific wavelength, these devices emit polychromatic light ranging from 515 nm to 1200 nm. There are a diversity of different intense pulsed light sources that differ in that they use varying cut-off filters to utilize predetermined wavelengths or filter certain wavelengths out to make the delivered light more specific for selected targets, such as vascular or pigmented lesions or hair. By providing light energy over broader wavelengths, the intense pulsed light is advantageous since it can improve both the vascular and pigmentary component of photodamage.

There are also lasers that do not rely on the theory of selective photothermolysis to remove pigment but instead non-selectively remove the epidermis and epidermal melanocytes. Such lasers ablate the skin and any associated pigment is destroyed as a secondary event. These types of lasers include the carbon dioxide (CO ₂ ) (10 600 nm), erbium : yttrium-aluminum-garnet (Er : YAG) (2940 nm), and yttrium-scandium-gallium-garnet (YSGG) (2740 nm). The newer fractional lasers (available in both ablative and non-ablative wavelengths) damage columns of the epidermis and dermis while leaving interspersed areas unaffected. It is hypothesized that pigment is improved by the elimination of damaged melanocytes and melanin through the microscopic treatment zones created. In this manner, there is faster healing over traditional ablative lasers as the surrounding normal tissue migrates into the areas of damage to heal each irradiated column. Because only fractions of the pigmented epidermis and dermis are affected at a time, a series of treatments is necessary to achieve the desired result.

History

The use of lasers for pigmented lesions began in 1963 when Leon Goldman and colleagues found that 0.5 ms pulses of ruby laser radiation were selectively absorbed by pigmented skin. Just 2 years later, these investigators also discovered the specific utility of the QS ruby laser. They found that the whitening tissue reaction seen when using a QS ruby laser was substantially different from a necrotic tissue reaction seen when treating with a longer pulsed ruby laser. Despite these early findings, the ability of the QS ruby laser to selectively target pigment was not initially appreciated. Instead, for many years, pigmented lesions were treated with non-selective continuous wave sources such as the argon and CO ₂ lasers. In the late 1980s, Anderson and co-workers demonstrated that pigment could be selectively diminished by using the QS Nd : YAG laser pulses at 1064, 532, and 355 nm. This group also showed that the longer wavelengths (which are less well absorbed by melanin) required a higher energy fluence to be effective. Over the last 20 years, after further investigation, pulsed lasers and intense pulsed light sources have become the treatment of choice for epidermal, and pulsed lasers the choice for dermal, pigmented lesions and tattoos.

Preoperative Preparation