Principles of laser and intense pulsed light for cutaneous lesions

INTRODUCTION — When absorbed in sufficient amounts, light energy can induce changes in the skin. Lasers and intense pulsed light (IPL) devices allow for the delivery of light to the skin in a controlled manner. These devices are useful for achieving desired clinical effects in a variety of dermatologic conditions.

The term laser is an acronym for light amplification by stimulated emission of radiation. Lasers deliver monochromatic, coherent, collimated, high intensity beams of light. In contrast, IPL devices are filtered flashlamps that emit polychromatic, noncoherent light in a broad range of wavelengths. Thus, IPL devices are dimmer and less powerful than lasers.

For many years following the initial studies of the effects of lasers on the skin [1-4], the use of lasers was limited to nonselective coagulation and vaporization of tissue [5]. A revolution in the clinical utility of lasers occurred in the early 1980s with the development of the theory of selective photothermolysis [6]. This theory describes the parameters by which light can be used to selectively destroy targets in the skin through the selective absorption of light and spatial confinement of the effect. The majority of subsequent developments in laser technology for cutaneous disorders have been based upon this theory.

Progress in laser and IPL technology has also involved the development of safer and more efficient methods of achieving the desired effects on skin. Cooling technology limits inadvertent damage to tissues adjacent to targeted sites, allowing higher levels of light energy to be directed toward the target. In addition, the implementation of fractionated laser technology for cutaneous resurfacing has allowed for the achievement of the desired cosmetic outcome with reduced healing time.

The principles that govern the interactions between skin and laser light or IPL, and the types of these devices used in the treatment of skin will be discussed here. Background information on the production and basic characteristics of laser light, the treatment of cutaneous vascular and hyperpigmented lesions with laser light, and ablative laser resurfacing for skin rejuvenation are reviewed separately. (See "Basic principles of medical lasers" and "Laser and light therapy for cutaneous vascular lesions" and "Laser and light therapy for cutaneous hyperpigmentation" and "Ablative laser resurfacing for skin rejuvenation".)

SKIN OPTICS — A thorough understanding of interactions between light and biologic tissue is critical for the selection of a laser or IPL device as well as the effective and safe use of the equipment. Light interacts with tissue in four major ways (figure 1) [6]:

●Absorption – According to the Grotthuss-Draper law, absorption of light is required for light to exert effects (both beneficial and adverse) on tissue. Molecules that absorb light are called chromophores. Melanin, oxyhemoglobin, and water are examples of biologic chromophores that are frequently targeted with laser and IPL devices. Tattoo ink also functions as a chromophore, and is another common target in laser therapy. In most clinical settings, light absorbed into a target chromophore in the skin is converted to thermal energy, leading to heating and destruction of the chromophore.

●Scattering – Scattering of light describes the redirection of photons away from the primary direction of light travel. Dermal collagen is responsible for most light scattering in skin. When emitted light is scattered in the same direction as the primary beam or sideways, the power density of the beam attenuates with increasing depth. In contrast, light that is reflected back toward the source can increase the power density of the beam within the tissue. Scattering decreases with the use of long wavelengths of light and the delivery of light through large spot sizes. (See 'Wavelength' below and 'Spot size' below.)

●Reflection – A portion of light directed toward the skin does not penetrate the skin; light can be reflected away from the surface of the skin without exerting a clinical effect. The stratum corneum is responsible for most reflection from the skin.

●Transmission – Light can pass through tissue without being absorbed, scattered, or reflected. No clinical effect occurs from light that simply passes through tissue.

The amount of light that penetrates deeply into the skin is dependent upon absorption by chromophores and light scattering. Due to reduced scattering and the relative paucity of superficial chromophores that absorb long wavelengths of light, such wavelengths penetrate more deeply into the skin and can be used to target structures in the mid or deep dermis. As an example, light in the near infrared (800 to 1200 nm) range is weakly absorbed by the biologic chromophores in skin and is minimally scattered due to the relatively long wavelength. Thus, a laser that emits light with a wavelength near 1100 nm will penetrate more than 2 mm into the skin, and can be used for targeting tattoo pigment in the deep dermis [6].

SELECTIVE PHOTOTHERMOLYSIS — The clinical effect of a laser or IPL device is dependent upon both the properties of the light that irradiates the skin and the interaction of the light with chromophores, concepts that are well explained by the theory of selective photothermolysis. This theory describes the methods by which light can be manipulated to induce the desired clinical effect. The following principles compose the theory of selective photothermolysis [6]:

●The wavelength of light utilized should be absorbed preferentially by the target chromophore and must penetrate the skin to a sufficient depth.

●The light must be delivered in a period of time that is short enough to prevent the transfer of excessive heat to adjacent structures.

●The energy delivered per unit area (fluence) must be sufficient to exert the desired therapeutic effect but should also be at a level that minimizes collateral tissue damage.

Application of the theory of selective photothermolysis led to the identification of laser parameters that are most effective for specific clinical indications. As an example, knowledge of the light parameters that effectively target hemoglobin led to the development of the 585 nm pulsed dye laser, which has been used for the management of port wine birthmarks [7]. (See "Laser and light therapy for cutaneous vascular lesions", section on 'Capillary malformations (port wine birthmarks)'.)

THERAPEUTIC PARAMETERS — Wavelength and pulse duration are the most important laser parameters that govern the effects of laser light on skin. Fluence, irradiance, and spot size are additional laser settings that influence clinical outcomes.

Wavelength — Chromophores absorb light most effectively at different wavelengths (figure 2 and table 1). To exert the greatest effect, the laser wavelength should be near the maximum absorption of the target chromophore and should be of sufficient length to penetrate to the depth of the target.

Because light scattering decreases with increasing wavelength, greater light penetration generally is achieved with longer wavelengths of light. The depth of penetration of ultraviolet, visible, and near infrared light into the skin follows the electromagnetic spectrum (near infrared>red>yellow>green>blue>ultraviolet) (figure 3A). In contrast to this pattern, high absorption of light by water molecules in the epidermis markedly limits the depth of penetration of light emitted from mid-infrared (2940 nm erbium:yttrium aluminum garnet) to far infrared (10,600 nm carbon dioxide [CO₂]) lasers (figure 3B).

The selected wavelength should also be a wavelength for which minimum competition from other chromophores is present. If more than one chromophore is present in the target tissue, the absorption of light will be divided in relation to their relative absorption coefficients (measurements of the probability of light absorption by chromophores). In addition, a superficial chromophore can shield a deeper chromophore from irradiation. As an example, absorption of light by melanin in the epidermis can reduce the amount of light that reaches melanin in hair follicles, hemoglobin in dermal blood vessels, or tattoo pigment in the dermis.

Pulse duration — The pulse duration of the laser beam heavily affects the likelihood of achieving the desired clinical effect. The most appropriate pulse duration setting is determined by the thermal relaxation time of the target chromophore. (See 'Thermal relaxation time' below.)

Thermal relaxation time — Thermal relaxation time is defined as the amount of time it takes the temperature of a target to return to the ambient temperature following heating (figure 4) [6].

Large objects lose heat much more slowly than small structures. Thus, the thermal relaxation time is heavily influenced by the size of the target (table 2). As an example, the thermal relaxation time of a 50 to 100 micrometer blood vessel in a port wine birthmark is on the order of a few milliseconds [8]. In contrast, the thermal relaxation time of a melanosome measuring approximately 1 micrometer is approximately 1 microsecond [9].

If an object is heated for a period equal to or shorter than its thermal relaxation time, the accumulated heat and resultant damage is confined to the target object alone. When a 50 micrometer blood vessel in a port wine birthmark is irradiated with light from a pulsed dye laser with a pulse duration of approximately 1.5 milliseconds, the light is absorbed by oxyhemoglobin, and the destruction is confined to the target vessel [10]. This selective photothermolysis dramatically reduces the risk of scarring.

In contrast, if an object is heated for longer than its thermal relaxation time, thermal diffusion leads to heating of surrounding structures. Prior to the advent of pulsed dye lasers, nonpulsed (continuous wave) argon lasers were used for the treatment of vascular lesions [11,12]. Although light from argon lasers is well absorbed by oxyhemoglobin, the long duration with which the light was administered led to excessive heating of surrounding structures and a relatively high risk of scarring. (See 'Continuous and quasi-continuous wave lasers' below.)

Similarly, the importance of pulse duration in successful selective photothermolysis is evident in traditional (nonfractionated) laser skin resurfacing with CO₂ or erbium:yttrium aluminum garnet (Er:YAG) lasers [13,14]. The CO₂ laser penetrates the skin to a depth of approximately 0.05 mm, and the thermal relaxation time of this thickness of tissue is approximately 1 millisecond. The delivery of light with pulse durations of less than 1 millisecond greatly minimizes collateral thermal damage during CO₂ laser resurfacing.

Large versus small structures — When heat is delivered slowly, small structures sometimes can be heated for longer durations than their thermal relaxation times without sustaining damage. This phenomenon allows for the selective destruction of large structures while smaller structures that contain the same chromophore or other chromophores with similar absorptive spectra are spared.

For example, if small targets containing melanin, such as melanosomes in the epidermis, are heated slowly over 10 to 20 milliseconds (longer than their thermal relaxation time of 1 microsecond), accumulated heat will be released to the surrounding tissue without heating the melanosome or the surrounding structures sufficiently to cause tissue coagulation. In contrast, a hair follicle from a patient with dark-colored hair is a relatively large structure that contains a significant amount of melanin. The size of the follicle causes the structure to lose heat much more slowly than the melanosome; the thermal relaxation time of a hair follicle is approximately 20 milliseconds. When light is delivered with a pulse duration of 10 to 20 milliseconds, there is not sufficient time for the heat to dissipate to tissue surrounding the hair follicle, and the follicle is destroyed while the adjacent tissues and epidermal melanosomes remain intact. This property is used effectively for laser hair removal and is of particular importance in individuals with dark skin [15].

Fluence — As stated in the Grotthuss-Draper law, a sufficient amount of light energy has to be absorbed by a structure in order to produce a detectable effect [16] (see 'Skin optics' above). Fluence is a measurement of the amount of energy delivered per unit area (figure 5).

For most pulsed lasers used for selective photothermolysis of vascular lesions, the fluences used during treatment range from 3 to 15 J/cm². For IPL treatment of vascular lesions, higher fluences are often required, ranging from 15 to 30 J/cm² or more. The higher level of energy necessary to achieve the desired clinical effect is attributed to the less selective targeting of chromophores by IPL devices. (See "Laser and light therapy for cutaneous vascular lesions".)

The destruction of very large structures, such as hair follicles, with laser or IPL usually requires high fluences (20 to 50 J/cm²) due to the amount of tissue that has to be heated to achieve thermal coagulation of these structures.

Another indication for the use of high fluences (sometimes in excess of 100 J/cm²) is the use of long wavelength lasers such as the 1064 neodymium:yttrium aluminum garnet (Nd:YAG) laser to treat structures such as telangiectasias. The target chromophore in telangiectasias (oxyhemoglobin) has low affinity for this wavelength of light, and the delivery of high amounts of energy is necessary to heat the target structure to a level sufficient for coagulation.

Irradiance — Power describes the rate of energy delivery and is measured in Joules per second (watts). The irradiance relates this measurement to the size of the treated area, and describes the rate of energy delivery per unit area to an object (watts/cm²) [16]. Irradiance is easily calculated from the laser power output and spot size (figure 6).

Irradiance essentially is a measurement of the intensity of energy delivery. Very high irradiance will achieve much faster heating of an object than low irradiance. Decreasing the pulse duration of a laser or IPL device without changing the energy setting will result in a higher level of irradiance.

The relevance of irradiance in the clinical setting is supported by the observation that slow heating (low irradiance) coagulates tissue while fast heating (high irradiance) can vaporize tissue. At extremely high levels of irradiance (eg, megawatt or gigawatt per cm²), such as is achieved with nanosecond or picosecond pulse duration Q-switched lasers, heating is so rapid that targets shatter from photomechanical forces rather than vaporization. (See 'Pulsed lasers' below.)

Although fluence is commonly used when discussing settings of pulsed lasers and IPL devices, the lack of a fixed pulsed duration with continuous wave lasers makes irradiance the preferred unit of measurement when referring to the continuous wave devices. (See 'Continuous and quasi-continuous wave lasers' below.)

Spot size — Spot size is the diameter of the beam of light emitted from the laser or IPL device that hits the skin surface. Because fluence is a measurement of Joules delivered per square centimeter (J/cm²), one might assume that increasing the energy output of a laser in proportion to an increase in spot size would result in a similar effect on a larger area of tissue. However, scattering of light occurs to a greater degree with small spot sizes. Thus, the light energy that enters the target tissue is attenuated far more rapidly with the use of small spot sizes than with the use of large spot sizes.

Spot size is most important when targeting structures in the mid to deep dermis with long pulsed lasers, such as the ruby, alexandrite, diode, and Nd:YAG lasers. A spot size of at least 7 to 10 mm is essential in this setting. The therapeutic benefit of increasing spot size is minimal or nonexistent with spot sizes greater than 10 mm. For targets in the epidermis or upper dermis, spot size is less important. Of course, larger spot sizes allow for faster coverage of a given treatment area.

FRACTIONAL PHOTOTHERMOLYSIS — Fractional photothermolysis is another method of maintaining tight control over the extent of laser light injury. The technique involves the use of infrared light, which is well absorbed by water, to coagulate or ablate narrow columns of tissue called microthermal zones (MTZs) [17]. A pattern of thousands of MTZs (approximately 0.1 mm in diameter) is laid down during treatment. In general, the proportion of injured tissue represents 10 to 50 percent of the skin surface (figure 7). Skin between the columns is spared from injury. (See 'Fractionated lasers' below.)

SKIN COOLING — Although laser or IPL treatments are often used to target structures in the dermis, such as hair follicles, telangiectasias, or tattoos, the epidermis, which contains melanin, can absorb a significant amount of visible laser light (table 1). Excessive heating of the epidermis secondary to the absorption of light can lead to epidermal cell injury and death, resulting in hypopigmentation or hyperpigmentation.

Almost all pulsed laser and IPL systems use some form of cooling. Cooling agents may be cold gases, liquids, or solids that are applied to the skin surface preceding, during, or after treatment.

Cooling of the epidermis protects it from damage, thereby allowing for the delivery of higher amounts of light energy to the dermis. Cooling of the dermis has a similar effect, as it reduces the transfer of heat to adjacent, nontarget tissues. Pain reduction is another benefit of cooling [18].

The simplest method of cooling the skin involves the application of an ice cube or an ice-cold roller to the skin surface before treatment. However, most laser and IPL devices now use integrated cooling systems, which are easier to use and provide more reliable and consistent methods of cooling. For example, the epidermis and upper dermis may be cooled by the application of a metal plate or a chilled transparent window that is attached to the laser handpiece, or through a mechanism that blows cold air onto the treated site. A cryogen liquid spray is also used to rapidly cool the epidermis [19].

CLASSIFICATION OF DEVICES — The three major categories of lasers used in the treatment of skin include continuous wave lasers, pulsed lasers, and fractionated lasers (table 3).

Continuous and quasi-continuous wave lasers — Continuous wave lasers emit a continuous laser beam as long as the foot pedal or finger switch is depressed. Examples of continuous wave lasers include the argon, argon pumped tunable dye, krypton, and some carbon dioxide (CO₂) lasers.

Quasi-continuous wave lasers are very rapidly pulsing lasers, and include copper vapor, copper bromide, and potassium titanyl phosphate (KTP) lasers. Intervals between pulses are extremely short, and the emitted light functions similarly to a continuous beam.

With these lasers, long periods of continuous exposure to the laser beam or insufficient cooling time between pulses can lead to excessive heating of nontarget structures in the skin. In an attempt to reduce collateral damage with continuous wave lasers, automated scanning devices were designed to move the laser beams across the skin in predetermined patterns so that any individual point of skin would receive the laser beam for only a short duration [20]. However, scanning devices were not always effective in preventing cutaneous damage. As a result, continuous wave lasers have been mostly supplanted by pulsed lasers due to the much better selectivity of the latter.

Pulsed lasers — Pulsed lasers produce a laser beam that is emitted in short pulses with a long period (0.1 to 1 second) between pulses.

Relatively long-pulsed lasers such as the 595 nm pulsed dye laser or the 755 nm alexandrite laser have pulses in the 1 to 50 millisecond range and peak powers in the kilowatt range. They are designed to selectively coagulate relatively large structures such as telangiectasias and hair follicles [21]. The long pulse duration of these lasers minimizes collateral tissue damage during the delivery of the high amounts of energy required for the coagulation of these structures. (See 'Large versus small structures' above.)

Short-pulse lasers have pulse durations in the 5 to 100 nanosecond (10^-9 second) range, and include Q-switched ruby, Q-switched alexandrite, and Q-switched neodymium:yttrium aluminum garnet (Nd:YAG) lasers. These devices are used to target small structures such as melanosomes and tattoo ink particles [22]. Q-switching refers to an electro-optical switch within the laser cavity that allows the release of all of the laser energy stored in the laser cavity in one brief powerful pulse. Power outputs in the megawatt to gigawatt range are common. The target is heated at such a rapid rate that it shatters and supersonic shockwaves are created in the tissue.

Lasers with pulse durations in the 300 to 900 picosecond (10^-12 second) range also have been developed. Compared with nanosecond-range lasers, these lasers can shatter even smaller particles with even higher peak powers for more efficient tattoo fading [23].

Pulsed ablative CO₂ and erbium:yttrium aluminum garnet (Er:YAG) lasers have been used for laser skin resurfacing [13]. With the advent of fractionated laser systems for resurfacing, CO₂ and Er:YAG lasers have fallen out of favor with most laser surgeons. They are still useful for precisely ablating superficial skin lesions while leaving behind a very narrow zone of thermal damage. Examples of indications for these lasers include rhinophyma [24], actinic cheilitis [25], adenoma sebaceum, epidermal nevi, and deep rhytides.

Fractionated lasers — Fractional photothermolysis involves the use of light energy to create numerous narrow columns of coagulated or ablated skin (microthermal zones, MTZs) (figure 7). As healing occurs, remodeling of the affected epidermis and dermis contributes to changes in the appearance or texture of the skin. Fractionated lasers most commonly are used for skin resurfacing and the treatment of scars [26,27]. They are divided into nonablative and ablative devices.

Nonablative — Nonablative fractionated lasers thermally coagulate narrow vertical columns of epidermis and variable portions of the dermis. This is achieved with the use of near infrared lasers (1320, 1440, 1540, 1550, and 1927 nm). Near infrared lasers emit light that is absorbed by water in the skin to a lesser extent than light from mid and far infrared lasers, which are used for fractional ablation.

Following treatment, the thermally coagulated epidermis is rapidly sloughed off and reepithelialization is completed within two to three days. The thermally coagulated dermis is gradually replaced with new collagen deposition and collagen remodeling. A series of treatments is usually necessary to gradually remodel the entire skin surface.

Ablative — Ablative fractionated laser delivery creates ablative MTZs in the skin that extend into the dermis. This is achieved with mid infrared (2940 or 2790 nm) and far infrared (10,600 nm) lasers, which emit light that is strongly absorbed by water [28,29]. A zone of thermally coagulated tissue also occurs around the ablated zone. (See "Ablative laser resurfacing for skin rejuvenation", section on 'Ablative fractional lasers'.)

With the Er:YAG (2940 nm) laser, the peripheral coagulated zone is very narrow, which accounts for the frequent appearance of pinpoint bleeding during treatment. The CO₂ (10,600 nm) laser yields a thicker coagulated zone, which reduces bleeding. However, the time required for reepithelialization is slightly longer with the CO₂ laser. Reepithelialization usually occurs around four days after treatment with a fractionated Er:YAG laser and after about seven days following fractionated CO₂ laser therapy. This relatively short time course to reepithelialization is an advantage over traditional laser resurfacing with pulsed CO₂ or Er:YAG lasers, which requires a healing period of around 7 to 10 days.

Intense pulsed light — IPL sources are filtered xenon flashlamps that release pulses of noncoherent polychromatic light. They are less powerful than lasers.

Light emitted from IPL devices usually falls in the visible to near infrared range (400 to 1200 nm). The broad range of emitted light makes IPL sources versatile devices, allowing for treatment of both vascular and pigmented lesions. The primary use of IPL devices is for photorejuvenation [30].

Because the delivered light energy is spread over a broad range of wavelengths, a greater number of treatment sessions is usually required for treatment with an IPL device than for treatment with lasers. Pulse durations are similar to the long-pulsed lasers.

OTHER LASER/LIGHT TISSUE INTERACTIONS — Nonthermal effects of lasers are also used for dermatologic indications. Excimer lasers optimize the localized delivery of narrowband ultraviolet B (UVB) light in phototherapy for psoriasis. In addition, lasers and IPL devices can be used to stimulate photochemical reactions in photodynamic therapy.

Excimer laser — The 308 nm excimer laser delivers UVB light that is absorbed by proteins, DNA, and RNA in the skin. The laser works similarly to traditional narrowband UVB phototherapy in psoriasis, but allows for the delivery of higher doses of ultraviolet light to localized resistant plaques [31]. The excimer laser has also been used for vitiligo and other dermatologic conditions.

Photodynamic therapy — In the dermatologic setting, photodynamic therapy typically involves the application of a topical photosensitizer (aminolevulinic acid or methyl-aminolevulinic acid) to a predetermined treatment site. After the photosensitizer has been absorbed into the tissue, the site is irradiated with a light source. Irradiation activates the photosensitizer, leading to a photochemical cascade that results in local cellular injury and death. (See "Photodynamic therapy".)

Blue or red light-emitting diode (LED) sources, a variety of visible light lasers, and IPL devices have all been utilized as light sources for photodynamic therapy. Photodynamic therapy has been used for the treatment of actinic keratoses, skin cancer, and acne, as well as for photorejuvenation [32,33]. (See "Treatment of actinic keratosis", section on 'Photodynamic therapy' and "Treatment and prognosis of low-risk cutaneous squamous cell carcinoma (cSCC)", section on 'Photodynamic therapy' and "Treatment and prognosis of basal cell carcinoma at low risk of recurrence", section on 'Photodynamic therapy' and "Light-based, adjunctive, and other therapies for acne vulgaris", section on 'Light/laser therapies'.)

SUMMARY AND RECOMMENDATIONS

●Overview – Lasers and intense pulsed light (IPL) devices are used to induce clinical changes in the skin through the manipulation of light energy. Lasers are powerful devices that deliver monochromatic, coherent, collimated beams of light. IPL devices are less powerful, filtered xenon flashlamps that deliver broadband, polychromatic, noncoherent light. An understanding of the interactions between light energy and skin is crucial for the safe and effective use of light-emitting devices. (See 'Introduction' above.)

●Skin optics – Light targeting the skin can be absorbed, scattered, reflected, or transmitted (figure 1). Only light that is absorbed will exert a detectable clinical effect (figure 2). (See 'Skin optics' above.)

●Selective photothermolysis – The theory of selective photothermolysis describes the use of light energy to selectively destroy specific structures in the skin while minimizing damage to other tissues. Most modern lasers used for dermatologic indications are designed to function through selective photothermolysis. (See 'Selective photothermolysis' above.)

●Therapeutic parameters:

•Wavelength – The wavelength of emitted light determines its depth of penetration into the skin (figure 3A-B). In general, long wavelengths of light penetrate more deeply into the skin than shorter wavelengths. (See 'Wavelength' above.)

•Pulse duration and thermal relaxation time – The thermal relaxation time of a target determines the ideal pulse duration for the delivery of light (figure 4). If the pulse duration is equal to or shorter than the thermal relaxation time, heating and damage are confined to the target. If pulse durations longer than the thermal relaxation time are used, heat dissipates to surrounding tissues, and damage to the adjacent tissues may occur. (See 'Pulse duration' above.)

•Spot size – Light delivered through a small spot size is more susceptible to scattering than light delivered through large spot sizes. Larger spot sizes are preferred when targeting structures in the mid or deep dermis. Spot size is less important for superficial targets. (See 'Spot size' above.)

●Skin cooling – Cooling reduces collateral damage from laser or IPL treatment and enables the delivery of higher levels of light energy. Cooling agents may be cold gases, liquids, or solids. (See 'Skin cooling' above.)

●Laser devices – Examples of laser devices include continuous, quasi-continuous, pulsed, and fractionated lasers. Continuous and quasi-continuous lasers are infrequently used due to the increased risks of adverse effects secondary to excessive heating of tissue. Long-pulsed lasers allow for slow heating of targets, and are particularly effective for minimizing epidermal damage during laser hair removal. Q-switch lasers can deliver powerful, short pulses of light energy that shatter targets such as tattoo pigment in the dermis. Nonablative and ablative fractionated lasers create microscopic columns of coagulated or ablated tissue that are followed by reepithelialization and dermal remodeling (figure 7). (See 'Classification of devices' above.)