General Principles
LASER stands for Light Amplification by the Stimulated Emission of Radiation. Einstein first proposed the concept of stimulated emission in 1927. The first laser was developed in 1959 (ruby), and was first used on human skin in 1963. The use of these early lasers was crude, and damage to cutaneous tissues and hence scarring was common.
It was the introduction of the concept of selective thermolysis by Anderson and Parrish in the 1980s that saw the explosion of use in cutaneous medical lasers. This concept has led to the development of lasers that can act on specific targets or “chromophores” in certain layers of the skin, allowing for targeted therapy with minimal collateral damage in the form of scarring.
Laser Physics
Laser light is produced by the release of a series of coherent, unidirectional photons of light. These photons of light are generated by an external light source or electromagnetic field, which is used to excite a specific medium. Different mediums, which can be in solid, gaseous or liquid state, are selected to produce different lasers.
The atoms in the medium absorb the energy from the external source, thereby exciting some of their electrons into higher energy levels. Upon relaxation back into resting state, the energy lost is released as a photon of light in a process known as spontaneous emission . These photons are incoherent and go on to excite other electrons into higher energy levels resulting in multiple photons being spontaneously emitted throughout the material. This process continues until the number of electrons in the higher energy levels outnumbers those in the lower energy levels, resulting in a situation called population inversion . When this occurs, the high energy electrons will cause the photons to drop down into resting orbit, emitting a further photon of the same frequency and direction which is in phase with the incident photon without absorbing the incident photon. This process is called stimulated emission of photons and each photon released contains a specific amount of energy that is consistent throughout the material.
By placing the medium between two mirrors this process can amplify itself by allowing light that is produced at right-angles to the mirrors to pass back and forth through the medium producing increasing numbers of unidirectional, coherent photons which are then released, by transiently removing one mirror, as a laser beam.
Laser light has specific therapeutic properties, which are exploited in medical use. It is:
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Monochromatic: lasers have a specific wavelength and so have the ability to selectively target chromophores.
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Coherent: alignment of light waves allows high intensity to be focused over small area.
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Compressible: the high temporal coherence of waves allows for ultra-short pulses that deliver localized energy.
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Collimated: transmission of parallel rays of light without divergence allows for high localized intensity.
Laser Therapy in Cutaneous Treatment
Selective photothermolysis predicts that selective thermal damage of a chromophore will result when sufficient fluence (energy delivered per unit area – J/m 2 ), at a wavelength preferentially absorbed by that target, is delivered during a time (pulse duration) equal to or less than the thermal relaxation time of target.
The use of laser in cutaneous treatment involves the selection of a specific wavelength, which is preferentially absorbed by a specific target or chromophore in the skin. This specific target can be oxyhemoglobin (vascular lesions), melanin (pigmented lesions) or water (skin resurfacing). Since the surrounding epidermis is spared, the treatment is specific, with little injury to normal skin.
The laser effect is influenced by a number of laser parameters including spot size, divergence/convergence of laser beam, wavelength, pulse duration, and fluence.
The first two factors are believed to be optimized in all modern lasers. The spot size of laser affects the optical penetration of light if its radius is equal or less than the distance for which light is free to diffuse within the tissues. Larger spot diameter will allow greater laser penetration due to greater internal reflection. Furthermore, a larger surface area can be treated faster. Most lasers therefore use a spot size that is greater than the penetrative depth of the laser (>4 mm).
The efficacy of laser treatment also depends on the convergence of incident light and the uniformity of irradiance. Convergence of laser light allows for less external reflection from the skin surface and therefore deeper penetration and more selective target injury.
Different chromophores in the skin are targeted by lasers with different wavelengths. Wavelength also affects the depth of penetration, with laser light with a longer wavelength penetrating deeper into the dermis.
The most selective target heating is achieved when energy is deposited at a rate faster than the rate of cooling of the target chromophore. Longer pulse duration will achieve more efficient heating but if the pulse duration is too long there will be insufficient time for heat to dissipate, causing an undesirable temperature rise that will lead to damage to surrounding epidermis and dermis. These injuries could result in scarring or other cutaneous side effects. The time taken for heat to dissipate from a particular chromophore (the thermal relaxation time) in seconds is approximately equal to the square of the target’s diameter in millimeters. In treating vascular malformation where a variety of abnormal vessel diameters are present, at different depths under the epidermis, the setting of an ideal pulse duration for treatment becomes difficult.
In addition to pulse duration, sufficient fluence is also required to achieve a damaging temperature in a target chromophore (therapeutic threshold). The use of higher fluence will increase the risk of developing skin complications during treatment.
Laser Modality
A laser can be classified as either operating in continuous or pulsed mode. A continuous wave laser simply means that the laser emits a constant source of light without interruption, while a pulsed wave laser emits laser light at a predetermined duration and repetition rate. In a Q-switched laser, pulses are only emitted at peak intensity and for extremely short durations.
Most medical lasers are used in pulsed mode, as the targeting of chromophores becomes more specific, with less thermal damage to the surrounding tissues owing to the short thermal relaxation time of chromophores which allows for heat dissipation between pulses. A pulsed laser is used when the destruction of the target chromophore relies on heat denaturation or coagulation.
In a Q-switched laser, short pulses of high peak power laser light are emitted. Hence, it is used in conditions where a shattering effect is needed to destroy the target chromophores, i.e. pigmented lesions and tattoos (see below).
Laser Safety
Lasers can be broadly classified into four classes ( Box 5.1 ). Most medical lasers used are Class 4 and hence suitable precautions need to be undertaken while undertaking laser procedures.
Class 1 Lasers that are not capable of emitting hazardous levels of radiation under normal conditions.
Class 2 Lasers that have visible wavelength lasers with lower power output, which will not cause permanent damage to the eye in the 0.25 seconds it takes to blink.
Class 3 Lasers that, if viewed directly, can cause permanent damage to the eye.
Class 4 Lasers that produce a hazardous emission, either through direct or scattered radiation. Suitable eye protection must be worn. There is also a potential fire risk.
The MHRA (Medicines and Healthcare Products Regulatory Agency) has produced a guideline on the safe use of lasers. All personnel who are associated with the purchase, supply, installation, use, and maintenance of medical lasers and intense pulsed light (IPL) systems should be familiar with the document.
Each hospital that undertakes laser treatment should have local safety rules, with a trained laser safety officer. The local safety rules should be specific to each clinical application and for each laser, and all personnel involved in laser use should be familiar with them. All staff should be appropriately trained for the use and maintenance of lasers, safety precautions, and adverse events reporting.
Specific safety precautions are:
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Eye protection: suitable eye protection should be used for each specific laser.
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Smoke/plume extraction: smoke plume may contain hair particles, viable cells, bacteria, viruses, prions, and other hazardous materials. Numerous toxic and carcinogenic gases will also be given off. It is therefore essential that precautions are taken to reduce such risks. A well-fitting, high filtration-efficiency face mask (e.g. particulate respirators that filter particles of 0.1 μm in size) should be used. A standard surgical face mask is not sufficient to act as the primary method of particle filtration. This should be supplemented by a dedicated smoke evacuator. A standard medical vacuum system (e.g., an operating theater wall suction system) is not suitable for smoke plume removal.
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Fire hazard: flammable substances such as alcoholic skin prep should not be used in areas where lasers are used. Fire extinguishing equipment should be readily available. Care should also be taken when performing laser treatments near nasopharyngeal or endotracheal tubes.
Types of Laser
The main types of laser used for cutaneous treatments are divided according to the chromophores they target – oxyhemoglobin (vascular lesions), melanin (pigmented lesions and hair removal lasers) or water (resurfacing lasers). Ablative lasers are the resurfacing lasers, such as CO 2 and erbium:YAG lasers, which remove the epidermis. Nonablative lasers selectively target the chromophores within the epidermis, or the dermis only. Most lasers that are used to treat vascular, pigmented lesions or hair, and the IPL systems fall into this group.
Commonly used lasers along with their wavelengths and target chromophores are listed in Table 5.1 .
| Medium | Laser | Wavelength (nm) | Chromophore |
|---|---|---|---|
| Solid | Alexandrite | 755 | Melanin Tattoo pigment |
| Nd:YAG | 1064 (deep lesions) | Melanin Tattoo pigment | |
| KTP | 532 (epidermal lesions) | ||
| Copper vapor | 578 | Oxyhemoglobin | |
| Erbium:YAG | 2940 | Water | |
| Liquid | Pigmented lesion dye | 510 | Melanin |
| Gas | Carbon dioxide (CO 2 ) | 10,600 | Water |
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