Info

Photon Beam Time Travel

Fig. 1.4. With the stimulated emission of energy, two photons are released in phase with one another as the electron drops back to its normal, stable configuration laser output (watts) x exposure time (secs)

pi x radius2 (of the laser beam)

In the case of irradiance and energy fluence, the higher the number the greater the effect. For example, high irradiances are needed to incise tissue, while only low irradiances are needed to coagulate tissue.

Fig. 1.4. With the stimulated emission of energy, two photons are released in phase with one another as the electron drops back to its normal, stable configuration monochromatic or composed of a single wavelength or color. The second unique characteristic is a property known as coherence, where all the waves of light move together temporally and spatially as they travel together in phase with one another. The third characteristic is collima-tion, where the transmission of light occurs in parallel fashion without significant divergence, even over long distances.

■ What Is Irradiance and Energy Fluence?

In order to use a laser to treat any skin condition, it is necessary to understand how the laser can be adjusted to obtain the most desired biologic effects in tissue (Fuller 1980). Two of the factors that are important in this process are irradiance and energy fluence. Irradiance, also called power density, determines the ability of a laser to incise, vaporize, or coagulate tissue and is expressed in watts/cm2. It can be calculated based on the formula:

The energy fluence determines the amount of laser energy delivered in a single pulse and is expressed in joules/cm2. It can be calculated based on the formula:

Currently Available Technology

How Does Laser Light Interact with Tissue?

In order to understand how to select the ideal laser from the myriad of currently available devices for the treatment of any cutaneous condition it is important to first understand how light produces a biologic effect in skin. The interaction of laser light with living tissue is generally a function of the wavelength of the laser system. In order for laser energy to produce any effect in skin it must first be absorbed. Absorption is the transformation of radiant energy (light) to a different form of energy (usually heat) by the specific interaction with tissue. If the light is reflected from the surface of the skin or transmitted completely through it without any absorption, then there will be no biologic effect. If the light is imprecisely absorbed by any target or chromophore in skin then the effect will also be imprecise. It is only when the light is highly absorbed by a specific component of skin that there will be a precise biologic effect. While this reaction may seem difficult to accurately anticipate, in fact, there are really only three main components of skin that absorb laser light: melanin, hemoglobin, and intracellular or extracellular water, and their absorption spectra have been well established. Manufacturers of lasers have taken this information and designed currently available technological devices that produce light which is the right color or wavelength to be precisely absorbed by one of these components of skin. This minimizes collateral injury to the surrounding normal skin.

In 1983, Drs. R. Rox Anderson and John A. Parrish (Anderson and Parrish 1983) of the Har laser output (watts) x ioo pi x radius2 (of the laser beam)

vard Wellman Laboratories of Photomedicine published in the journal Science their newly developed concept of selective photothermolysis (SPTL). This original concept explained how to safely and effectively treat the microvessels in children with port wine stains using laser light. It also led to the first "ground up" development of a specific laser, the pulsed dye laser, to treat a specific condition-port wine stains in children. This concept has now also been used to develop more effective treatments of many other cutaneous problems including the treatment of tattoos, benign pigmented lesions, and the removal of unwanted or excessive hair. The concept of SPTL defines the way to localize thermal injury to the tissue being treated and minimize collateral thermal damage to the surrounding nontargeted tissue. This is done by choosing the proper wavelength of light that will be absorbed by the specific targeted chromophore and delivering the right amount of energy with the proper pulse duration, known as the thermal relaxation time (TRT), which is based on the physical size of the target.

What Is a Q-Switched Laser?

The laser cavity "Q" is a measure of the optical loss per pass of a photon within the optical cavity (Goldman et al. 1965). Thus, the "Q" of a system is a way to characterize the quality of the photons being released so that a high "Q" implies low loss and low "Q" implies high loss. A Q-switch is a physical method to create extremely short (5-20 ns) pulses of high intensity (5-10 MW) laser light with peak power of 4 joules. In addition to the normal components (Fig. 1.5) of a laser that were previously described, this system utilizes a shutter which is constructed of a polarizer and a pockels cell within the optical cavity. A pockels cell is an optically transparent crystal that rotates the plane of polarization of light when voltage is applied to it. Together, the polarizer and pockels cell act as the "Q"-switch. Light energy is allowed to build (Fig. 1.6) within the optical cavity when voltage is applied to the pockels cell. Once the voltage is turned off, the light energy is released (Fig. 1.7) in one extremely powerful

Flashlamp

Flashlamp

Rear mirror

Uncoated front surface

Fig. 1.5. Classic appearance of a solid state laser with central rod that could be a ruby, Nd:YAG or alexandrite crystal surrounded by a flashlamp with emission of light from only one end of the optical cavity

Rear mirror

Uncoated front surface

Fig. 1.5. Classic appearance of a solid state laser with central rod that could be a ruby, Nd:YAG or alexandrite crystal surrounded by a flashlamp with emission of light from only one end of the optical cavity

Alexandrite Crystals Used Lasers
Fig. 1.6. The Q-switched lasers contain a Pockels cell that can be made opaque by the application of voltage and thus allow energy to build within the optical cavity

short pulse. Currently available Q-switched lasers include the ruby, Nd:YAG and alexandrite lasers.

The Q-switched lasers and the photons of light released from them have unique characteristics that allow them to be effectively used to treat tattoos (Goldman et al. 1967) and benign pigmented lesions. This is due to the mechanism of action whereby photoacoustic waves are generated within the skin by the released photons of light which heats the small tattoo

Fig. 1.7. Once the voltage is turned off, the Pockels cell becomes optically transparent and the accumulated energy is allowed to be released in a single, short, powerful pulse

pigment particles or the melanosomes. This heating causes cavitation within the cells containing the ink particles or pigment, followed by rupture and eventual phagocytosis by macrophages and removal of the debris from the site. Clinically, this process produces gradual fading of the tattoos with a series of 4-8 treatments at 6-8-week intervals and removal of the benign pigmented lesion with only 1-2 treatments, again at 6-8-week intervals. The precise targeting of subcellular organelles and pigment particles by the Q-switched lasers reduces collateral damage and minimizes the risk of scarring or textural changes. The treatment of tattoos and benign pigmented lesions represent additional examples of how selective photothermolysis can be effectively applied to more accurately treat conditions other than the microvessels of port wine stains that this concept was originally developed to treat.

Indications

Vascular Lesions

The most common laser used today for the treatment of many different vascular conditions is the pulsed dye laser (Garden and Geronemus 1990). While initially designed for the treatment of microvessels in port wine stains of infants and children, the initial parameters have been modified to provide longer pulses and wavelengths of light to treat deeper and larger blood vessels and also to do this with epidermal cooling. Cryogen spray or contact cryogen cooling prior to the laser pulse reduces pain while also decreasing the potential for epidermal injury as the light passes through it to reach the deeper blood vessels. Thermal quenching from postpulse cooling further reduces the risk of collateral thermal injury following delivery of the pulse of light. Cooling devices are now routinely used for port wine stains in children and adults, leg veins, solar telangiectasia, and other small blood vessels diseases. Long pulses of light from the Nd:YAG laser and the nonlaser Intense Pulsed Light (IPL) devices have also been used to treat larger and deeper blood vessels. The small beam diameter of the krypton laser makes it a useful tool in the treatment of limited areas of involvement with small caliber, linear blood vessels on the nose or cheeks.

Pigmented Lesions and Tattoos

To treat cosmetically important, but benign, pigmented lesions and tattoos it is imperative that the risk of scarring and other complications be minimized as much as possible. This is made possible today with the use of short pulses of light from the Q-switched lasers, ruby, Nd:YAG, and alexandrite, that deliver pulses of light which approximate the thermal relaxation time of melanosomes and tattoo ink particles, and through their photoacoustic effects can produce destruction of melanin pigment or tattoo pigment particles for subsequent removal by macrophages. The most common benign, pigmented lesions treated with these conditions are solar lentigines, nevus of Ota/Ito, cafe-au-lait macules, Becker's nevus, postinflammatory hyperpigmentation, mucosal lentigines of Peutz-Jeghers syndrome, and melasma. The variability of the response in congenital or acquired nevocellular nevi makes treatment with Q-switched lasers less desirable. Decorative, traumatic, and cosmetic tattoos can all be effectively treated with the Q-switched lasers.

However, multiple treatments are required, and certain colors may not respond at all. In addition, there is a risk of darkening of some tattoo colors that occurs as a result of a chemical reaction following laser treatment, making removal exceedingly difficult.

Unwanted Hair

A number of devices, including the long-pulse ruby, long-pulse Nd:YAG, long-pulse diode, and IPL, have been used to permanently reduce the numbers of darkly pigmented hair (Wheeland 1997). This is done by targeting the melanin within the hair shaft and bulb with light energy which thermally damages the cells and either slows or destroys their ability to regrow. At present, treatment of blonde or gray hair with laser light is poor, even with the application of an exogenously applied synthetic melanin solution.

Ablative and Nonablative Facial Resurfacing

Over the past decade the short-pulsed carbon dioxide and erbium:YAG lasers have been used to perform ablative laser skin resurfacing. These devices thermally destroy the epidermis and superficial dermis with minimal collateral damage. Long healing times and even longer periods of persistent erythema and possibly permanent hypopigmentation have reduced the use of these devices. Since many patients are unwilling to accept any downtime from a cosmetic procedure, a number of noninvasive devices have been developed, including the Nd:YAG at 1,320 nm, the diode at 1,450 nm, the pulsed dye laser, and the IPL, to help restore the youthful condition of the skin noninvasively without producing a wound or other visible injury that would keep patients from following their normal activities. The most recent nonin-vasive technique for rejuvenation is using light from the light emitting diodes (LED) to stimulate the skin. This device delivers intense, nonlaser, red- or blue-colored light that can stimulate fibroblasts to produce collagen, elastin, and glycosaminoglycans to help rejuvenate the skin with a series of treatments. Sometimes the topi cal application of a photosensitizer like a 5%-20% cream of aminolevulinic acid (ALA) prior to exposure to red light from the LED will increase the response with only minimal crusting and erythema (Walker et al. 1986).

Safety

Safety is the most important aspect of properly operating a laser or other optical device since there is always some associated risk to the patient, the laser surgeon, and the operating room personnel whenever a laser is being utilized for treatment. In the outpatient arena, the safe operation of lasers is not generally determined by the manufacturer, medical licensing board, or other regulatory body. Thus, it is important for the laser operator to understand the risks involved in using lasers and then develop an appropriate group of standards to ensure that the equipment is being used in the safest fashion possible.

Training

The safe use of any laser begins with appropriate training and familiarization in the indications and uses of each device. This allows the development of the necessary proficiency that will also result in the concomitant maximal safe use of each device.

Signage

The greatest risk when operating a laser is that of eye injury. To help prevent eye injury, appropriate signage on the laser operating room door should describe the nature of the laser being used, its wavelength, and energy. Plus, a pair of protective glasses or goggles appropriate for the device being used should always be placed on the door outside of the laser operating room in case emergency entrance is required. The door to the laser operating room should be locked, if possible, and all exterior windows closed and covered.

Eye Protection

Inside the operating room, care must also be taken to protect the eyes. If not appropriately protected, the cornea may be injured by either direct or reflected light from the carbon dioxide and erbium:YAG lasers. A more serious injury to the retina can be caused by any of the visible or near-infrared lasers. For the laser surgeon and operating room personnel there are special optically coated glasses and goggles that match the emission spectrum of the laser being used. To check whether the correct eye protection is being used the manufacturer has stamped on the arm of the glasses or the face of the goggles the wavelengths of light for which protection is provided and the amount of the protection provided in terms of optical density (O.D.). For most laser devices, the current recommendations are to use eye protection with at least an O.D. of 4.0. For the patient, there are several ways to provide appropriate eye protection. If the procedure is being performed in the immediate vicinity of the orbit, it is probably best to use metal scleral eye shields (Figs. 1.8, 1.9), which are placed directly on the corneal surface after first using anesthetic eye drops (Nelson et al. 1990). However, if the procedure is being done on the lower part of the face, trunk, or extremities, burnished stainless steel eye cups (Figs. 1.10, 1.11) that fit over the eyelids and protect the entire periorbital area are probably best.

Fig. 1.8. The appearance of the concave surfaces of various sizes of corneal eye shields used to protect the eye during laser surgery in the periorbital area

How Apply Ocular Laser Shields
Fig. 1.9. The appearance of the convex surfaces of various sizes of corneal eye shields used to protect the eye during laser surgery in the periorbital area
Photos Eyelid Surgery Being Preformed
Fig. 1.10. The appearance of the Wheeland-Stefa-novsky eye goggles worn over the eyelids for laser surgery not being performed in the immediate periorbital area
Periorbital Area
Fig. 1.11. The appearance of the externally applied Durette Oculo-Plastik eye cups worn over the eyelids during laser surgery performed closed to periorbital area
Eye Protection During Laser

Fig. 1.8. The appearance of the concave surfaces of various sizes of corneal eye shields used to protect the eye during laser surgery in the periorbital area

The same eye glasses or goggles used by the laser surgeon and operating room personnel are not recommended for patients since these may leave gaps on the lower edge near the cheek that permit the passage of light under them and cause injury to the patient.

Laser Plume

Any of the lasers that ablate tissue and create a plume of smoke can potentially harm the laser surgeon, patient, and operating room personnel. Various bacterial spores and human papilloma viral (HPV) particles (Garden et al. i988) have been recovered from carbon dioxide laser plumes. The two best methods to prevent this inhalation injury are to use laser-specific surgical masks and a laser-specific plume/ smoke evacuator held close to the operative site. There is no evidence that HIV or hepatitis C viral particles are transmitted in the laser plume.

Laser Splatter

When treating tattoos or benign pigmented lesions with a Q-switched laser the impact of the pulses of light can disrupt the surface of the skin, sending an explosion of blood and skin fragments flying away from the operative site at a very high speed. The speed of these particles is so fast that it cannot be removed by a smoke or plume evacuator. As a result, most the manufacturers will supply the device with a nozzle or tip that can contain these particles at the skin surface and thus prevent dissemination of these materials into the air. Another technique that has also been used successfully when treating tattoos to prevent tissue splattering from the operative site is to apply a sheet of hydrogel surgical dressing on the surface of the treatment site and discharge the laser through this material to the target. Any extrusion of tissue that occurs with the Q-switched laser pulses will be trapped within the hydrogel and not be allowed to splatter from the operative field.

Fire

Most of the medical lasers used in the treatment of skin diseases do not share the risk of older devices, like the continuous emitting carbon dioxide laser, of igniting a fire. Despite this, it is still recommended that any flammable material, including acetone cleansers, alcohol-based prep solutions, or gas anesthetics be restricted from the laser operating room. By following these simple guidelines and using common sense and skill, the risk of using a laser should be no greater than that associated with using older, traditional, nonlaser devices to perform the same procedure.

Future

As new concepts emerge to help explain how light can be used to more precisely interact with tissue, it is certain that the development of additional devices based on those concepts will follow soon after. Nonthermal photoablative decomposition using the femtosecond titan-tium:sapphire laser is but one area of recent investigation that could significantly change the way laser light can be used to ablate tissue with minimal collateral injury. Exciting new research ideas initiating photochemical reactions with laser light with either topically or parenterally administered drugs or other photosensitizers could further expand our knowledge of how lasers can be used to effectively treat a number of conditions, like inflammatory, premalignant, and malignant conditions, that currently are either poorly treated or untreatable today.

References

Anderson RR, Parrish JA (1983) Selective photo-thermolysis: Precise microsurgery by selective absorption of pulsed radiation. Science 220:

How To Reduce Acne Scarring

How To Reduce Acne Scarring

Acne is a name that is famous in its own right, but for all of the wrong reasons. Most teenagers know, and dread, the very word, as it so prevalently wrecks havoc on their faces throughout their adolescent years.

Get My Free Ebook


Post a comment