Lasers In Dermatology

LASERS IN DERMATOLOGY

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The word LASER is an acronym for “LIGHT AMPLIFICATION BY STIMULATED EMMISION OF RADIATION”. The RUBY LASER was developed first (1959) Nd:YAG in 1961 Argon in 1962 Carbon dioxide in 1964. Laser light as a form of light is a part of the EMS. LASER light difers from standard light eg. sunlight or incandescent light in the following ways:

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The LASER light is formed of monochromatic photons ( ie. Light from a given source is all of one wavelength).

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LASER light is coherent ( i.e. The waves of energy are in phase with each other both in time and space) LASER light is also COLLIMATED ( i.e. The laser beam component waves are highly parallel, producing a narrow beam that can be propagated for long distances with minimal divergence or convergence)

The components of the LASER are :

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Lasing medium : THIS DITERMINES THE WAVELENGTH OF THE RADIATION . it can be SOLID ( e.g. ruby), GAS ( argon or carbon dioxide) or LIQUID ( e.g. rhodamine dye laser)

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OPTICAL CAVITY : Encloses the lasing medium and the amplification process takes place here.

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DELIVERY SYSTEM : To bring the laser beam from the machine to the patient, the laser systems that create visible and near infrared lasers of shorter wavelengths are transmitted via fibreoptic systems. The carbon dioxide laser which produces far infrared light of longer wavelengths uses a series of articulating joints and mirrors for this purpose.

ENERGY SOURCE : This is required for the amplification process. This may be a direct electric current, optical flash or energy derived from chemical reactions.

Basic physics about LASER :  ENERGY is contained in the light and is expressed in JOULES. 

POWER is defined as the time rate at which energy is emitted and is measured in watts ( joules/sec).

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IRRADIANCE OR POWER DENSITY is the concentration of the beam of light and is expressed as the power applied per unit area( watts/ sq.cm). this determines the ability of the LASER to coagulate, vaporize or incise tissue.

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FLUENCE or energy density is the actual amount of energy delivered to the unit area of target tissue and depends on the EXPOSURE TIME. FLUENCE = POWER × EXPOSURE TIME .

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TRANSVERSE ELECTROMAGNETIC MODE (TEM) is the distribution of energy across the laser beam diameter. The types of TEM are TEM00 ( the most fundamental mode used, the energy distribution follows a bell shape in which the peak power density is at the centre of the beam) and TEM01( the energy distribution has a doughnut shape with a “cold” spot in the center; this is less suitable for incisional work.

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TEMPORAL MODES OF OUTPUT: the LASER light is delivered in 2 broad categories: CONTINOUS WAVE MODE AND PULSE WAVE MODE. In the continuous wave mode, there is an uninterrupted beam of radiation of relatively low power. This requires a great deal of operative skills to deliver energy uniformly at the required dose over the area to be treated. However the advantages are that larger areas can be treated rapidly. Disadvantage is the lesser safety profile. In the pulsed mode, the continuous beam can be interrupted to form pulses when the light is emitted in bursts of low energy but with peak powers. This is done by cutting the beam either with simple mechanical shutters or electronically operated switches or through mode locking or Quality switching or through controlled pumping and discharge. Simple pulsed lasers produce the same peak power as continous wave, whereas superpulsed or ultrapulsed lasers produce higher peak power with lesser pulse durations. The PULSED SYSTEMS ALLOW IMPROVED ENERGY DELIVERY SYSTEMS WITH UNIFORM DOSE OF RADIATION ACROSS THE TREATMENT AREA AS COMPARED TO CONTINOUS WAVE LASER. THERE IS A HIGHER SAFETY PROFILE BECAUSE THESE SYSTEMS MINIMISE THERMAL INJURY BY ALLOWING THE TISSUE FOR A “COOLING PHASE” DURING THE “OFF” PORTION OF THE CYCLE. IT IS MORE TIME CONSUMING THAN CONTINOUS WAVE SYSTEM.

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The tissue effects of LASER are as follows : TISSUE EFFECTS DEPEND ON  TYPE OF LASER  ANGLE OF INCIDENCE  BASIC PROPERTIES OF SKIN THE LASER ENERGY AFTER ABSORPTION IS CONVERTED TO THERMAL ENERGY AND IS MAINLY RESPONSIBLE FOR ALL THE THERAPEUTIC ( BIOLOGICAL) EFFECTS PRODUCED BY THE LASER DIRECTLY OR INDIRECTLY( following conduction). The therapeutic effects are as follows (A) PHOTOTHERMOLYTIC EFFECT :

The laser energy is converted to heat energy. Denaturation of proteins occurs at 40-90 degree C. they are reversible if early and the temp. is < 50 degrees. Once the temp. is > 50 degrees, the tissue concentration of denatured proteins increases--- the process becomes irreversible( COAGULATIVE TISSUE DAMAGE ---- LEADS TO CELL NECROSIS, HEMOSTASIS). If the temp. reaches > 100 degrees, there occurs vaporization with tissue ablation. To achieve localized ablation with minimal residual thermal damage, sufficient energy ( which achieves rapid cooling ) is rapidly delivered to the superficial layers of the tissue( depth of penetration) for a short period of time( the time of exposure should be equal to or less than thermal relaxation time i.e. Tr time). { Tr is the time taken to dissipate half its thermal energy and cool to 50% of its initial temperature achieved immediately after exposure. The magnitude of relaxation time depends on the depth of penetration . for human skin, the Tr is roughly 0.1 sec}. During vaporization, the tissue is rapidly brought to 100 degree centigrade and the CELLULAR AND EXTRACELLULAR WATER IS CONVERTED TO STEAM WHICH BRINGS ABOUT EXPLOSION OF THE CELL WALLS. THIS TYPE OF DESTRUCTION MINIMISES THERMAL DAMAGE TO ADJACENT TISSUES. To produce coagulation in soft tissue , irradiance of greater than 10 W/sq.cm are required; irradiance between 10-150W/sq.cm produces thermal necrosis and coagulation . Rapid vaporization of tissue is achieved with irradiance more than 150W/sq.cm. irradiance of greater than 50,000 W/sq.cm will cut through the tissue. Next to vaporized ablated zone is the region of residual thermal damage which is 2-4 times the depth of penetration. The unwanted effects of LASER like desiccation and charring can be avoided by RAPID VAPORIZATION. (B) THERMOMECHANICAL EFFECT : Apart from direct effects of heat, ultra rapid heating in pulsed irradiation ---- causes tearing of the tissue structures by pressure waves, cavitation or rapid differential expansion. (C) PHOTOACOUSTIC EFFECT : The high energy rapidly pulsed lasers mechanically fragment and shatter pigment chromophore( melanin) or pigmented particles ( tattoo ink). These smaller fragments are then cleared away from the tissue by the body lymphatics, vasculature etc. (D) PHOTOCHEMICAL EFFECT : LASER can cause a series of chemical reaction in the tissue either directly or through internal conversion of energy to chemical energy. (E) SELECTIVE PHOTOTHERMOLYSIS : The theory of selective thermolysis postulates that light of a wavelength which is absorbed by a target chromophore will selectively damage or destroy that chromophore if the fluence is sufficiently high and the pulse duration less than or equal to the thermal relaxation time of that chromophore. This has been successfully used to treat superficial vascular malformations, certain benign pigmented lesions and for removal of hair or exogenous tattoos. Target chromophore

Selective absorption nanometer

wavelength

Melanin Oxyhemoglobin Water Tattoos

700-1000 585, 577, 532 above 1100-2940, 10,600 Variation as per individual color.

in

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AIM OF LASER THERAPY : uniform deposition of energy over a site in a manner that does not cause undue charring or excessive coagulation injury.

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Types of LASERS are:

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THOSE THAT EMIT VISIBLE LIGHT : These are primarily used to treat vascular and benign pigmented conditions, these include ---- ARGON, ARGON PUMPED TUNABLE DYE, FLASH LAMP PUMPED PULSED DYE COPPER VAPOR, COPPER BROMIDE AND KRYPTON LASERS. These devises emit visible light ranging from blue to red portions of the EMS. They are absorbed by the natural chromophores of the skin ----- hemoglobin and melanin.

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THOSE THAT EMIT INFRARED LIGHT : these include Carbon Dioxide and Neodymium: yttriumaluminium garnet laser.these are not absorbed by the skin chromophores ---- melanin and hemoglobin. Carbon dioxide is selectively absorbed by water. The efficacy of Nd: YAG laser depends on the volume of the tissue.

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THOSE THAT PRODUCE VISIBLE OR INFRARED LIGHT IN EXTREMELY SHORT AND INCREDIBLY POWERFUL PULSES. This group of devices are known as Q switched lasers. As the wave moves through the tissue, kinetic energy is produced which can fracture or disintegrate particles like tattoo and melanin. These fractures allow Q switched lasers to be used clinically for the treatment of benign pigmented lesions and the scarless removal of tattoos. COMPLICATIONS ARE:  Scarring  Pigmentary alterations  Altered texture  Local infection ADVANTAGES ARE :  Office procedure  Good and can be used in patients with pacemakers and anticoagulant therapy.  Multiple lesion can be treated in the same setting  Bloodless field.

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DISADVANTAGES ARE:  Expensive  All types of lesions cannot be treated by the same type of LASER SAFETY MEASURES DURING USE OF LASER : Goggles , mask, smoke evacuators, labeling the operating room. Keeping the door and windows of the operating room covered.

Type of light visibl e

laser

wavelength

mode

absorption

Lasing medium

argon

448-514

cw

visibl e visibl e

ruby

694( red)

dye

visibl e Near IR

alexandrit e DIODE

577-585( pulsed) 570-630( tunable) 755

Q switched Q switched

Near IR

Nd:YAG

1064

CW, Q switched

Hemoglobi n melanin Melanin tattoos Melanin Tattoos hemoglobi n Melanin tattoos Melanin hemoglobi n Melanin tattoos

Mid IR

Er: YAG

2940

CW, Q switched

water

Erbium+ yttrium aluminium garnet crystals

0.03 pass

mm

min

Far IR

Carbon dioxide

10600

CW Super pulse

water

Mixture of 1 part carbon dioxide, 1.5 parts nitrogen and 4 parts helium

0.1mm per pass

min

GIST :       

810

Q switched CW,pulse d

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Scatte r in tissue mod

Clinical use

Argon gas

Depth of penetratio n 1-2 mm

Ruby crystal

1mm

mod

Copper vapor Rhodamine G

1.2 mm

min

Tattoos and hair removal Vascular lesions, few pigmented lesions and tattoos

Alexandrite crystals Diode crystals

1.5mm

mod

4 mm

min

5 mm

high

Neodymium YAG crystals

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Vascular lesions, pigmented lesions

Tattoos and hair removal Hair removal and few vascular lesions Very powerful and effective tissue coagulator. In Q mode, removes black tattoos and recalcitrant keloids. Superficial skin surfacing of rhytides, atrophic scars, syringomas and xanthelesma Excisional mode : blepharoplasty, scalp reduction, keloids, rhinophyma, hair transplantation. Vaporization mode : vascular lesions, pigmented lesions, appendage lesions.

For vascular lesions : ARGON, DYE, Nd: YAG, CARBON DIOXIDE For pigmented lesons : ARGON, CARBON DIOXIDE. For tattoo removal : ARGON( green), RUBY, ALEXANDRITE ( esp. green), Nd:YAG ( black), CARBON DIOXIDE. For hair removal : RUBY, ALEXANDRITE, DIODE For superficial skin surfacing : Er: YAG, CARBON DIOXIDE. HIGH TISSUE SCATTER : Nd: YAG MINIMAL TISSUE SCATTER : DYE, DIODE, Er: YAG.