- Email: [email protected]
Laser physics
ELSEVIFR
Laser Physics JOHN
L. RATZ,
MD
T
he use of lasers in dermatology began slowly in the 1960s and continued through the 1970s. The 1980s began with a new surge in laser use as new functions were found for available lasers. Technologic advances brought new laser systems and upgrades of older lasers and ancillary equipment that fostered an almost exponential growth of laser use in dermatology in the mid- to late 1980s on into the 1990s. To novices, an area that was already intimidating became even more so because of the now increased number of lasers being used, with the varying theories, treatment parameters, and guidelines for laser use. Even laser pioneers might have had trouble keeping up without proper access to the newer systems; however, as difficult, confusing, and muddled as the world of lasers might have been and may have become, the key to understanding and keeping up has always been a thorough understanding of the basic science of lasers. What was necessary in the 1960s and 1970s is necessary now and has even greater importance for tomorrow -a complete understanding of laser terminology, basic science, treatment parameters, and laser-tissue interactions.
Important
Wavelength (4 If light is considered as a repeating sine wave J?UVV+L, wavelength is the distance between duplicate points on each successive wave ~--___ .1 -*-’ j , Thjs, t&n, is the “St&’ I I-*--’ _li or 1 inside each package of light. Wavelength is responsible for the color of visible light. Wavelengths of visible light range from approximately 400 nm (violet) to approximately 750 run (red). Wavelengths of ultraviolet light, however, are much shorter (10 -400 run) and those of infrared light much longer (750 - l,OOO,OOO + run) than either extreme for visible light (Fig 1).
Nanometer By usage, the nanometer has become the standard by which wavelength of medical lasers is usually measured. Occasionally, the values are given in micrometers (jun) or angstroms (A). The relationships among these units are as follows: 1 pm = 0.001 mm (10m3 mm) = 1000 nm = 10,000 A 1 nm = 0.000001 mm (lo+ mm) = 0.001 pm =lOA 1 A = 0.0000001 mm (lo-‘mm) = 0.0001 pm = 0.1 nm
Basic Concepts
The word Easeris an acronym for light amplification by the stimulated emission of radiation. That radiation is in the form of photons of light, which are the end product of light amplification that is produced, in turn, by stimulated emission. The concepts of stimulated emission and light amplification are described shortly, but first, other basic concepts must be understood.
Photons Light is one portion of the electromagnetic spectrum (Fig 1) that exhibits properties of both discrete particles, or photons, and waves. It might be easiest to consider a photon as a package of light that can be the same or different from other photons by the “stuff,” or wavelength, inside the package (Fig 2).
From the Department of Dermatology, Ochsner Clinic and Alton Ochsner Medical Foundation, New Orleans, Louisiana. Address cowespondence to John L Ratz, MD, Director, Dermatologic Surgery, Department of Dermatology, Ochsner Clinic and Alton Ochsner Medical Foundation, 1.516JeffersonHighway, New Orleans, LA 70121. 0 2995 by Efseoier Science Inc. 655 Avenue of the Americas, New York, NY 10010
Frequency The speed of light is constant at 186,000 miles/s or 3 X lo* m/s, regardless of color or wavelength. Because this speed is constant per unit of time, the number of wavelengths within that unit of time will be high for short wavelengths and low for long wavelengths: Short ?. _~--. ------~-_
_.-._ Long .--
h
Thus, the relationship between wavelength and frequency is an inverse one: the higher the frequency, the shorter the wavelength and, vice versa, the lower the frequency, the longer the wavelength. Thus, if a given laser (neodymium : yttrium-aluminum-garnet [Nd : YAG], n = 1060 nm) has been frequency-doubled 0738-081x/95/$9.50 SSDI 0718-08lx@#WO23-T
12
Clinicsin Dermatologyl 2995;13:12-20
RATZ
Excimer 193 248 308 351
102'
1o18
Frequency 0-w
TTTT 1o15
log
IO6
Argon Dye Ruby Nd -Yag CO, Figure 1. A representation of the electromagnetic spectrumdemonstratingrelative differencesin wavelengthandfrequencyasthey apply to the spectrum.Relative wavelengthoutputsfor various lasersusedin cutaneousmedicineand surgey are alsodepicted.
(as in the Q-switch Nd : YAG), its wavelength will be cut in half (A = 530 nm), and the properties of the end-product light will be exactly those of the resulting wavelength (530 run = green light) and not those of the original wavelength (1060 nm = near-infrared light).
Atomic and Molecular Interactions The properties of a given laser’s light, and thus what it can do and how it will function, depend on the wavelength of the light. The wavelength, in turn, depends on the atoms or molecules responsible for the production of that light. The interactions that take place within any laser are basically the same for each laser; they differ only in that the wavelength of light produced will be specific for the atoms or molecules responsible for the production of that 2. Photonsof light are like smallpackages, eachhaving a specificwavelengththat servesasthe “stuff” insidethe package.
light. Thus, a ruby laser always produces light with a wavelength of 694 nm, and an Nd: YAG laser always produces light with a wavelength of 1060 nm.
Resting State With that securely and clearly in mind, to know how lasers are different (ie, different 2’s), one need only to understand the interactions that take place within a laser to know how the light is produced and how lasers are the same.
Any atom or molecule has a natural tendency to exist in its natural or resting state. In this resting state, the orbiting electrons of the atom or molecule occupy a given natural position (Fig 3). A photon of energy must be absorbed by the atom or molecule for an electron to move into a higher orbital level (Fig 4).
Figure
577NM
488NM w
634NM
Red Light
m
Spontaneous Omission Once an electron is moved into a higher orbital level, its natural tendency will be to drop back to its normal or resting state. By doing so, a given packet of energy will be released as a photon that will be characteristic for that atom or molecule. Such an emission of energy is called spontaneous emission (Fig 5). When many atoms or mol-
Clinics in Dermatology
2995;23:11-20
l
EASER
PHYsrcs
13
Excited State
/4!??!-
Stimulation
Of
Stimulated Emission
0
8
Figure 3. Each atom or molecule normally exists in its resting state with its outer orbiting electron in a stable position. Figure 4. By absorbing a photon
of
energy, the outer orbiting electron is moved into the next orbital level, which is an unstable condition.
Figure 5. If no further power is absorbed by the atom or molecule in its excited state, the unstable electron falls backinto its normal orbital positionand releases a characteristicphotonof energy. Figure 6. Whenan atomor moleculeis in its excitedstate,absorptionof further energyresultsin stimulationof the excited-stateatom or molecule. Figure 7. Stimulationof fhe excited-stateatom or moleculeresultsin the stimulatedelectron‘sfalling backinfo ifs restingstateorbit and in the emission of two identicalphotonstraveling in the samedirection.
14
Clinicsin Dermafologyl 1995;23:11-20
RAT2
CD Figure 8. A normalpopulationof atomsor moleculescontains a muchgreaternumberof particlesin their restingstate. Figure 9. Throughthe absorptionof energy,moreatomsor moleculesare transformedinto their excitedstates.This populationinversionis necessary for light amplification.
ecules undergo spontaneous decay, the emissions are out of phase with one another. Stimulated Omission If, however, an electron is in an excited state and is stimulated by a photon of enough energy (Fig 6), that photon will cause orbital decay of the electron, sending the electron back to its resting state and resulting in the emission of a second photon identical to the incident photon. The net result is that two photons of equal wavelength (color) are released from the atom, and those two photons are traveling in the same direction and are in phase both spatially and temporally (Fig 7). This process is termed stimulated emission and is the first necessary element for laser light production.
Population Inversion In a normal population of atoms or molecules, the majority of those particles are in their resting state (Fig 8). Pumping enough energy into the system will raise the majority of atoms or molecules to their excited state. Such a change, called a population inversion, is necessary for the production of laser light (Fig 9).
design) is basically the same. Each can be thought of as an elongated box with a “hole” in one end (Fig 11A). At one end of the box is a fully reflective mirror and at the opposite end is a partially reflective mirror, which allows for transmission of only a very small amount of light (sort of an “optical” hole). In an atomic or molecular population inversion that is undergoing stimulated emission within a laser “box,” the in-phase photons will reflect off the mirrors at either end and result in more interactions with stimulated atoms or molecules and, thus, the release of even more in-phase photons, thereby amplifying the reaction and its production of in-phase light. Hence, light amplification (from) by the stimulated emission of (photons of) radiation (Figs llB-D). The “box” of the basic laser is more correctly called the optical resonator, an apt and descriptive term. It is also called the optical cavity, laser cavity, or resonator. Within the optical resonator are contained the characteristic atoms or molecules for that particular laser. When unexcited or inactivated, the contents of the box are called the laser or lasing medium. This medium can be solid (ruby and Nd : YAG-lasers), liquid (dye lasers), or gas or a mixture of gases (Ar and CO2 lasers). When energized into its excited state by a power source, it is known as the active medium. The active medium can be achieved through a variety of different sources including electricity, radio-frequency enhancement, light, chemical reaction, and mechanical power. Electricity is used to pump most gas lasers, and because lasers are variably but extremely inefficient, their electrical requirements will vary but may be quite high, enough so to require special electrical service. Radio-frequency enhancement is used mostly for sealed-tube CO, lasers and, for all practical purposes, translates into more efficient lasers that need only normal household current and do not require water cooling. Light is used to pump dye lasers and originates either from another laser (eg, argon-pumped tunable dye laser) or a high-output flashlamp source (flashlamp-pumped dye laser). Mechanically or chemically pumped lasers are not generally used for medical purposes.
Light Amplification Once a population inversion has occurred, as long as energy is still being pumped into the system, the stimulated emission of in-phase photons will result in their impact on other excited-state electrons, resulting in, again, additional in-phase photons that, in turn, will do the same thing, that is, produce more in-phase photons. When this chain-reaction “mass production” of in-phase photons occurs within the unique structure of the laser optical resonator, the process is further magnified, resulting in light amplification (Fig 10).
Figure 10. Populationinversionresultsin light amplification andultimately in the productionof laser light.
Laser Design To better understand the concept of light amplification, a knowledge of laser design is necessary. Every laser (by
Laser Pump On (Light Amplification)
Clinics in Dermatology
2995;23:22 -20
l
LASER PHYSICS
r SC
15
Power Source
Laser Pump On (Light Amplification)
Totally Reflective Mirror
Mirror
Mirror
Laser
Puma
[ Stimulated
Emission
ET
Laser Light
=z
Laser Pump On (Incomplete Amplification)
L Stimulated Emission
Reflected Emission
0C Figure 11. A. Every laseris simply a boxwith a “hole“ at oneend.The contentsof the boxarespecificfor eachtype of laserand producelight characteristicfor the atomsor moleculeswithin the box.B. Most medicallasersare pumpedby electricalor opticalenergy or by radio-frequencyenhancement. C. As poweris introducedinto the laser,atomsor moleculesareelevatedto their excitedstateand a populationinversionoccurs.The resultingphotonsreleasedareall identicaland are reflectedbackandforth betweenthz mirrorsat eachendof the “box.” Continuedabsorptionof energyresultsin firther stimulatedemissionand ultimately in light amp#&@im and the productionof laserlight. D. Theoptical resonatoris the cavity in which laserlight is producedthroughlight amplification,which, in turn, is the direct result of stimulatedemission.The light, onceproduced,entersthe laserdelivery system,which for mostlasersis quartz fiberoptics.
Because of the extreme inefficiency of many laser systems, as already mentioned, water cooling in addition to special elect&al requirements may be necessary. These factors should be discussed for each laser in the respective articles.
Delive y Systems Once the characteristic laser light has been produced, it exits the optical resonator via the optical “hole,” which is
the partially reflecting and partially transmitting mirror, and enters the laser’s delivery system. Most lasers employ either visible light or light capable of fIberoptic delivery, usually through high-quality quartz fibers. The COz laser (10,600 nm for infrared) is currently not capable of fiberoptic delivery and requkes an intrieate articulating arm system containing either fixed or adjustable mirrors that direct the beam to the handpiece (Fig 12). There the laser light is focused through a lens to its
16
Clinicsin Dermatologyl 2995;23:11-20
RATZ
Laser
Coherent
Light
1 Figure 12. Becauseof its positionin thefar-infrared regionof the electromagnetic spectrum,the CO2laseris not capableof delivey throughquartz fiberoptics.In mostCO2lasersystems, laserlight is reflectedoff a seriesof mirrorsin the complex articulating systemwhereit can then bedeliveredto a handpiece containinga lenswhich will allow the collimatedbeamto becomefocusedat the focal length of the lens.
focal point. The nature of the light (intensity, spot size, etc) can be altered either by changing handpieces (and thus lenses) or possibly by adjusting the handpiece (focus/defocus or adjustable handpiece). A semirigid waveguide fiber is available, but it is basically a mirrored tunnel or tube that has some limitations in its use. Nd : YAG lasers (1060 nm, near infrared) are capable of fiberoptic delivery but may be equipped with either a quartz or a sapphire tip for contact cutting procedures. This is discussed in greater detail in the article by Glassberg and Lask on the Nd : YAG laser.
Properties of Laser Light Each type of laser produces light that is characteristic for that particular laser. The light has a characteristic wavelength that is monochromatic and spatially and temporFigure 13. Coherentlight is spatially and temporallyin phase asopposedto incoherentlight, which containsmany different wavelengthsthat areneither temporallynor spatially in phase. I
Coherent Light
Time Incoherent Light
Time
I
Incoherent
J
Figure 24. Coherentlight is muchlike a well-disciplined marchingbandin which all members are identicaland are traveling in the samedirectionin perfecttime. In contrast, incoherentlight is similarto the relative “incoherence“of a milling crowd in which the crowd members are totally “out of phase.”
ally in phase. This property of being spatially and temporally in phase is called coherence and differs greatly from ordinary light, which displays incoherence. The concepts of spatial and temporal coherence versus incoherence are demonstrated in Fig 13, but can probably be better visualized by comparing the spatial and temporal “coherence” of a finely tuned marching band with the “incoherence” of a milling crowd (Fig 14). In addition to coherence and monochromicity, laser light is highly collimated, which is its nearly parallel and nondivergent characteristic. The best illustration of this property is the comparison of two different light-pointing sources: the laser pointer, in which the spot size stays virtually the same despite the distance it traverses, and the common flashlight, the intensity of which diminishes markedly with distance (Fig 15). Thus, collimation, coherence, and monochromicity make up the key features of laser light. Different types of
25. Collimationof laserlight resultsin a nondivergent and virtually parallel beamwhoseintensity doesnot diminish greatly with distance;however,the divergentbeamof a noncollimatedlight sourcelosesits intensity rapidly over greater distances. Figure
Clinics in Dermatology
TEM oo
016
l
1995;13:11-20
TEM OI
017
LASER PHYSICS
Ideal
018
Figure 16. The mostcommontransverseelectromagnetic mode displaysa gaussiandistributionwith a central hot spotand approximately86% of the total energycontainedwithin the impactspot. Figure 17. The complextransverseelectromagnetic modecan result in a “donut” distributionpattern in which the center represents a coldspotand a “dull-knife” impactspot. Figure 78. In a theoretically “ideal” transverseelectomagnetic mode,all energywould beevenly distributedthroughoutthe impactspot.
lasers, as mentioned earlier, differ by their characteristic wavelengths (Fig 1). Although most lasers produce only one wavelength, some lasers may produce several monochromatic bands, such as the argon laser which produces 488~nm (blue) and 514~run (green) light outputs. Although a beam of laser light is spatially and temporally in phase, power distribution of the impact spot may vary, particularly if the beam is transmitted fiberoptically or through a lens. Such transmission can alter the fundamental power distribution of the impact spot. Power distribution, referred to as transverse electromagnetic mode (TEM), in its fundamental form, TEMc,a, displays a gaussian distribution (Fig 16), with approximately 86% of the power contained within the impact spot. Distortion through delivery systems can result in complex modes such as TEMa, (Fig 17), which displays a “donut” type of power distribution and a cold spot centrally. Such a distribution would have a dull-knife effect on the laser impact area as opposed to the TEM,,, gaussian distribution, which can be focused to a fine spot. In the theoretically “ideal” power distribution curve, on the other hand, all power would be evenly distributed over the entire surface area of the impact spot (Fig 18).
Continuous Versus Puked Delivery Lasers may differ in the manner in which their light is emitted or delivered. Some lasers, notably those with gaseous active media, are capable of the continuous discharge of light, also called continuous wave (CW). The power output of such lasers does not vary with time and is usually relatively low when compared with other forms of delivery (Fig 19). Such continuous waves can be broken up by an optical or mechanical shutter that simply breaks up the delivery
17
profile of the CW beam but does not alter the power output of the beam per se. Such delivery, although confusing, is called pulsed delivery (of the CW beam). Lasers not capable of CW delivery (notably, solid lasers) seem to build up a “head” of power before releasing their “shock wave” or pulse of relatively high power. The pulses delivered by such lasers are usually quite short and are followed by a lag phase before another pulse can be generated. Duration and power of the pulse, as well as duration of the lag phase, differ for the various pulsed lasers and are beyond the scope of this article (Fig 20). Different mechanisms for pulsing are abo available in some lasers. Specifically, “superpulsed” and “ultrapulsed’ CO, lasers have been developed. The net effect of such delivery can decrease thermal damage, which in some cases may be a plus but in other applications may have negative ramifications (eg, reduced thermal damage may reduce the hemostatic capabilities of the CO, laser) (Fig 21). This topic is discussed in detail in the article by Levins and Anderson, along with the concept of Qswitching, which allows for megawatt delivery over very short (nanosecond) impact durations (Figs 22 and 23).
Parameters Every laser is capable of a particular peak power, measured in watts. Characteristically, power outputs can be adjusted up to the peak output; however, operating at maximum output over the long term can diminish the half-life of the laser tube, which could prove quite costly. Because delivery of power to the target is achieved by an impact spot of measurable size, the amount of power delivered to tissue can be calculated as power per unit area. This measure of power density, also called irradiance, is calculable according to the following formula: Power density (ii-radiance) (W/cm2) = power output X 100 (mm2/cm2) impact spot size (mm*) Spot size is known and can be used to calculate the area by multiplying A by the impact radius (in mm) taken to the second power, or K r 2. As most lasers have a beam
Figure 19. Continuous-wavelasershave relatively low peak powersand are capableof shuttereddelivery, which is confusinglytermed“pulsed”delivery.
Time (set)
18
Clinicsin Dermatologyl 2995;23:11-20
RATZ
r Pulsed Laser
0 - Switched Laser
sco 5 1000 zi 2 a 0
A-!l
Time (psec)
Time (msec)
Time (nsec)
020
021
022
Figure 20. Pulsedlasersare not capableof continuousdelivery, have relatively greaterpowersper pulsethan continuous-wavelasers, and have variablelag timesbetweenpulses. Figure 21. Superpulsed COzlasersare capableof muchgreaterpowerper pulsethan conventionalcontinuous-waveCO* lasers; however,althoughthermaldamageto tissueis minimized,severaldesirableproperties,suchashemostasis, can belost. Figure 22. Q-switchingresultsin pulsesof extremelyshortduration and extremelyhigh poweroutput.
protile of TEM,,, , in which 86% of the effective beam is in the impact area, the calculation is a close approximation. If a laser delivers 10 W with a 1 -mm spot diameter, the equation becomes 10 W X 100 (mmz/cm2) 3.14 X (0.5 mm)2
= 1274 W/cm2
One must remember to calculate for radius and not diameter and to make the mm2 to cm2 adjustment. Most laser manufacturers supply power density charts that negate the need for calculation. An example of one such chart is Table 1. If the equation is known, the relationships of adjusting power and spot sizes and the relative impact on the tissue target are better understood. Doubling the power doubles the power density. Doubling the spot size, however, decreases the power density by the inverse of the square of the radius change, and halving the spot size has the op-
Figure 23. Respectivecomparisonof continuous-wave,pulsed, and Q-switchedlasers,demonstratingan inverserelationship betweentime andpowerfor the varioustypesof lasers. I Contlnuoua
wave
I
II I
1
Pulsed
-\ 0-1OOWan
>lOOOWanTYP.
Time (set)
1 0 Swltched
1 II
1I ’
> 1 Megawan
TYP.
posite, magnifying
effect. Thus, Power Spot 0 hm)
Beginningpowerandspotsize Powerdoubled Spotsizedoubled Spotsizehalved
10 20 10 10
1 1 2 0.5
PowerDensity W/cm9 1274watts/cm2 2548watts/cm2 318watts/cm2 5094watts/cm2
Measurement of power density, or irradiance, however, does not account for the effect of time or duration of exposure. The concept of time can be incorporated with the power output to determine the amount of power delivered and the duration of delivery in seconds. Such a calculation results in the measurement of power-time or energy delivery measured in joules; thus, watts
X seconds = watt seconds or joules
This measurement, unfortunately, neglects the effect of impact spot size. The concept of fluency, however, combines all these by measuring the energy delivered per unit area, or joules/cm2
=
watts X seconds (duration of delivery) area (of impact spot)
From this calculation can be appreciated that the net effect (absorbed energy per unit area or joules/c&) is affected in the same fashion as described earlier for power density, that is, direct linear relationship of power and inverse-square relationship of impact area. Additionally, time (measured in seconds) also has a direct linear effect on the measurement of fluence. From the calculation alone, equivalent changes in time or power should apparently have equivalent effects on tissue reaction as well. This, however, is not the case. Because of other
Clinics in Dermatology
l
1995;13:11-20
LASER
PHYSICS
19
Table 2. Power Density Determination Power Density (W/cm2) Spot Size (mm)
Area (mm2)
5.0 2.0 1.0 0.5 0.2 0.1
19.625 3.14 0.785 0.19625 0.0314 0.00785
2W 10.2 63.7 254.8 1019.1 6369.4 25,477.7
factors, such as limited transmission and variable absorption, an increase in time or duration of impact generally has a greater thermal impact on tissue than an equivalent increase in power output. Through this relationship several theoretic concepts have been developed. These are discussed later in the article.
Laser- Tissue Interactions Any incident laser impact is subject to all of the following possibilities on a tissue target: direct reflection, indirect reflection, scatter, transmission, and absorption (Fig 24). In fact, the net effect of a laser impact is the total of all of these and varies from laser to laser. Laser-tissue interactions can be further characterized by lasers’ relative absorptions (coefficients of absorption) and relative distances traversed in tissue before absorption is complete (coefficients of extinction). The CO2 laser, for example, has a high coefficient of absorption (98%) in tissue and a low coefficient of extinction, which translates into maximal energy absorption in a given impact target area with very little extension of energy effect beyond this point. The argon laser, on the other hand, has a relatively low (-45%) coefficient of absorption in tissue
Figure 24. Any laser-tissue interaction is the net result of the total of reflection,scattertransmission,andabsorption,which are different in characteristicfor eachlaser.
I
Direct Reflection From Surface
Indirect Reflection From Deep Reflective Chromophores
V
5W
10 w
20 w
50 w
25.5 159.2 636.9 2547.7 15,923.6 63,694.3
50.9 318.5 1273.9 5,095.5 31B47.3 127,388.5
101.9 636.9 2547.8 10,191.l 63,694.3 254.777.1
254.7 1592.4 6369.4 2L477.7 159,235.7 636,942.7
-
and a high coefficient of extinction, which means that approximately half of argon laser energy is reflected into the surrounding tissue and is capable of transmission for a relatively greater distance before complete absorption. This laser-tissue interaction in argon lasers can result in significant nonspecific absorption and subsequent nonspecific thermal tissue damage. As useful as the coefficients of absorption and extinction for each laser can be in understanding the relative laser-tissue interactions, their discussion and use have been largely abandoned. For TF&&, lasers, a basic impact on tissue will result in characteristic thermal reactions that wiIl be dependent on energy absorption, duration of impact, and laser-tissue properties. In general, a laser impact of a TEM, (gausSian) laser will result in a central zone of vaporization, in which tissue reaches 100°C and, because of the significant (80%) water content of tissue, results in vaporization. Continued contact can then result in carbonization or char formation and subsequent superheating of tissues and significant thermal damage (the “charcoal grill” effect). beyond this point of vaporization will he a zone of thermal necrosis, followed by a zone of irreversible thermal protein denaturation and then successively lesser amounts of thermal damage (Fig 25). The relative effect of any laser-tissue impact will depend on a multitude of variable properties, perhaps the most important of which is selective versus nonselective
Figure 25. A commontissueimpactresultsin a centralareaof vaporizationfollowed by a surroundingareaof thermalnecrosis, irreversibleprotein denaturation,and,f%utlly, reversiblethermal changes.
Scatter Over Wide And Deep Surface Area ermal Necrosis
Transmission Through Tissue With No Effect
Absorption by Target Chromophores
20 RATZ
Clinicsin Dermatologyl 2995;13:12 -20
Theoretic Considerations
\
450
500
550
600
Wavelength
650
700
(nm)
Figure 26. Respectiveabsorptioncurvesfor melanin, oxygenatedhemoglobin,and deoxygenated hemoglobin.Although maximumabsorptionof oxyhemoglobinis at 418 nm (the Soret band),because this is in the ultraviolet portion of the spectrum, attention hasbeenfocusedon the lesserabsorptionbandsat 542 and 577 nm. Deoxygenatedhemoglobinabsorptionat approximately560nm hasbeenrelatively ignored.
absorption. Tissue targets (chromophores) maximally absorb certain characteristic wavelengths of light, mostly dependent on color. In the case of the CO? laser (far infrared), maximum absorption (-98%) (nonselective) occurs in water (chromophore); penetration is minimal; and reflection, scatter, and transmission are even less.On the other hand, the argon laser can be maximally absorbed by hemoglobin or melanin (selective absorption), but reflection, scatter, and possibly transmission can be significant. Because of these properties, absorption curves for known tissue target chromophores are known, and lasershave been adapted to maximize the usefulness of their impacts (Fig 26). In recent years, most of the focus has been on the development of nonionizing lasersthat can be maximally absorbed by oxygenated hemoglobin (577,585 nm) for the treatment of vascular lesions and by melanin (510, 514,694 nm, etc) for the treatment of pigmented lesions. Unfortunately, perhaps, the deoxyhemoglobin absorption curve has been, for the most part, ignored. As many vascular lesions probably contain significant amounts of venous blood, treatment at the deoxygenated hemoglobin absorption peak (-560 nm) might very well improve the percentage successof vascular lesion treatment with lasers.1-5This area needs investigation.
Many theoretic considerations have arisen around maximizing the abosorption of laser energy by target chromophores and minimizing thermal injury. Such considerations include the concept of selective photothermolysis, reported first for hemoglobin (and 577-nm laser light) and melanin (351-nm excimer laser light). This concept suggests,appropriately, that maximal absorption of laser energy by a chromophore will result in damage or destruction of that chromophore, particularly if the impact is of a short enough duration with a subsequent lag phase to allow for tissue cooling to minimize damage to surrounding tissue. The concept of tissue cooling will be recognized as the theory of thermal relaxation time and, though possibly conservative, almost absolutely allows for minimizing the risk of thermal damage to surrounding tissue. These concepts are the foundation on which further laser development will be built. They are important concepts to study and to understand.
Conclusion Laser basic physics and tissue interactions are boring and not particularly easy to understand or remember; however, to understand the new and upcoming developments of laser application in medicine, understanding lasers totally and developing a working knowledge of them are mandatory. Newer lasers and concepts are on their way, and this basic material is imperative for anyone who wishes to lead the way or to follow with an understanding of what is being done and why it may or may not be working.
References 1. Apfelberg DB, editor. Atlas of cutaneouslasersurgery. New York: Raven Press,1992. 2. Apfelberg DB, editor. Evaluation and installationof surgical lasersystems.New York: Springer-Verlag, 1987. 3. Dixon JA. Surgical Application of Lasers.2nd ed. Chicago: Year Book Medical, 1987. 4. Dover JS, Amdt KA, GeronemusRG, et al. Illustrated cutaneouslaser surgery: A Practitioner’s guide. Norwalk (CT): Appleton & Lange, 1990. 5. Ratz JL, editor. Lasersin cutaneousmedicine and surgery. Chicago:Year Book Medical, 1986.
Laser Physics JOHN
L. RATZ,
MD
T
he use of lasers in dermatology began slowly in the 1960s and continued through the 1970s. The 1980s began with a new surge in laser use as new functions were found for available lasers. Technologic advances brought new laser systems and upgrades of older lasers and ancillary equipment that fostered an almost exponential growth of laser use in dermatology in the mid- to late 1980s on into the 1990s. To novices, an area that was already intimidating became even more so because of the now increased number of lasers being used, with the varying theories, treatment parameters, and guidelines for laser use. Even laser pioneers might have had trouble keeping up without proper access to the newer systems; however, as difficult, confusing, and muddled as the world of lasers might have been and may have become, the key to understanding and keeping up has always been a thorough understanding of the basic science of lasers. What was necessary in the 1960s and 1970s is necessary now and has even greater importance for tomorrow -a complete understanding of laser terminology, basic science, treatment parameters, and laser-tissue interactions.
Important
Wavelength (4 If light is considered as a repeating sine wave J?UVV+L, wavelength is the distance between duplicate points on each successive wave ~--___ .1 -*-’ j , Thjs, t&n, is the “St&’ I I-*--’ _li or 1 inside each package of light. Wavelength is responsible for the color of visible light. Wavelengths of visible light range from approximately 400 nm (violet) to approximately 750 run (red). Wavelengths of ultraviolet light, however, are much shorter (10 -400 run) and those of infrared light much longer (750 - l,OOO,OOO + run) than either extreme for visible light (Fig 1).
Nanometer By usage, the nanometer has become the standard by which wavelength of medical lasers is usually measured. Occasionally, the values are given in micrometers (jun) or angstroms (A). The relationships among these units are as follows: 1 pm = 0.001 mm (10m3 mm) = 1000 nm = 10,000 A 1 nm = 0.000001 mm (lo+ mm) = 0.001 pm =lOA 1 A = 0.0000001 mm (lo-‘mm) = 0.0001 pm = 0.1 nm
Basic Concepts
The word Easeris an acronym for light amplification by the stimulated emission of radiation. That radiation is in the form of photons of light, which are the end product of light amplification that is produced, in turn, by stimulated emission. The concepts of stimulated emission and light amplification are described shortly, but first, other basic concepts must be understood.
Photons Light is one portion of the electromagnetic spectrum (Fig 1) that exhibits properties of both discrete particles, or photons, and waves. It might be easiest to consider a photon as a package of light that can be the same or different from other photons by the “stuff,” or wavelength, inside the package (Fig 2).
From the Department of Dermatology, Ochsner Clinic and Alton Ochsner Medical Foundation, New Orleans, Louisiana. Address cowespondence to John L Ratz, MD, Director, Dermatologic Surgery, Department of Dermatology, Ochsner Clinic and Alton Ochsner Medical Foundation, 1.516JeffersonHighway, New Orleans, LA 70121. 0 2995 by Efseoier Science Inc. 655 Avenue of the Americas, New York, NY 10010
Frequency The speed of light is constant at 186,000 miles/s or 3 X lo* m/s, regardless of color or wavelength. Because this speed is constant per unit of time, the number of wavelengths within that unit of time will be high for short wavelengths and low for long wavelengths: Short ?. _~--. ------~-_
_.-._ Long .--
h
Thus, the relationship between wavelength and frequency is an inverse one: the higher the frequency, the shorter the wavelength and, vice versa, the lower the frequency, the longer the wavelength. Thus, if a given laser (neodymium : yttrium-aluminum-garnet [Nd : YAG], n = 1060 nm) has been frequency-doubled 0738-081x/95/$9.50 SSDI 0718-08lx@#WO23-T
12
Clinicsin Dermatologyl 2995;13:12-20
RATZ
Excimer 193 248 308 351
102'
1o18
Frequency 0-w
TTTT 1o15
log
IO6
Argon Dye Ruby Nd -Yag CO, Figure 1. A representation of the electromagnetic spectrumdemonstratingrelative differencesin wavelengthandfrequencyasthey apply to the spectrum.Relative wavelengthoutputsfor various lasersusedin cutaneousmedicineand surgey are alsodepicted.
(as in the Q-switch Nd : YAG), its wavelength will be cut in half (A = 530 nm), and the properties of the end-product light will be exactly those of the resulting wavelength (530 run = green light) and not those of the original wavelength (1060 nm = near-infrared light).
Atomic and Molecular Interactions The properties of a given laser’s light, and thus what it can do and how it will function, depend on the wavelength of the light. The wavelength, in turn, depends on the atoms or molecules responsible for the production of that light. The interactions that take place within any laser are basically the same for each laser; they differ only in that the wavelength of light produced will be specific for the atoms or molecules responsible for the production of that 2. Photonsof light are like smallpackages, eachhaving a specificwavelengththat servesasthe “stuff” insidethe package.
light. Thus, a ruby laser always produces light with a wavelength of 694 nm, and an Nd: YAG laser always produces light with a wavelength of 1060 nm.
Resting State With that securely and clearly in mind, to know how lasers are different (ie, different 2’s), one need only to understand the interactions that take place within a laser to know how the light is produced and how lasers are the same.
Any atom or molecule has a natural tendency to exist in its natural or resting state. In this resting state, the orbiting electrons of the atom or molecule occupy a given natural position (Fig 3). A photon of energy must be absorbed by the atom or molecule for an electron to move into a higher orbital level (Fig 4).
Figure
577NM
488NM w
634NM
Red Light
m
Spontaneous Omission Once an electron is moved into a higher orbital level, its natural tendency will be to drop back to its normal or resting state. By doing so, a given packet of energy will be released as a photon that will be characteristic for that atom or molecule. Such an emission of energy is called spontaneous emission (Fig 5). When many atoms or mol-
Clinics in Dermatology
2995;23:11-20
l
EASER
PHYsrcs
13
Excited State
/4!??!-
Stimulation
Of
Stimulated Emission
0
8
Figure 3. Each atom or molecule normally exists in its resting state with its outer orbiting electron in a stable position. Figure 4. By absorbing a photon
of
energy, the outer orbiting electron is moved into the next orbital level, which is an unstable condition.
Figure 5. If no further power is absorbed by the atom or molecule in its excited state, the unstable electron falls backinto its normal orbital positionand releases a characteristicphotonof energy. Figure 6. Whenan atomor moleculeis in its excitedstate,absorptionof further energyresultsin stimulationof the excited-stateatom or molecule. Figure 7. Stimulationof fhe excited-stateatom or moleculeresultsin the stimulatedelectron‘sfalling backinfo ifs restingstateorbit and in the emission of two identicalphotonstraveling in the samedirection.
14
Clinicsin Dermafologyl 1995;23:11-20
RAT2
CD Figure 8. A normalpopulationof atomsor moleculescontains a muchgreaternumberof particlesin their restingstate. Figure 9. Throughthe absorptionof energy,moreatomsor moleculesare transformedinto their excitedstates.This populationinversionis necessary for light amplification.
ecules undergo spontaneous decay, the emissions are out of phase with one another. Stimulated Omission If, however, an electron is in an excited state and is stimulated by a photon of enough energy (Fig 6), that photon will cause orbital decay of the electron, sending the electron back to its resting state and resulting in the emission of a second photon identical to the incident photon. The net result is that two photons of equal wavelength (color) are released from the atom, and those two photons are traveling in the same direction and are in phase both spatially and temporally (Fig 7). This process is termed stimulated emission and is the first necessary element for laser light production.
Population Inversion In a normal population of atoms or molecules, the majority of those particles are in their resting state (Fig 8). Pumping enough energy into the system will raise the majority of atoms or molecules to their excited state. Such a change, called a population inversion, is necessary for the production of laser light (Fig 9).
design) is basically the same. Each can be thought of as an elongated box with a “hole” in one end (Fig 11A). At one end of the box is a fully reflective mirror and at the opposite end is a partially reflective mirror, which allows for transmission of only a very small amount of light (sort of an “optical” hole). In an atomic or molecular population inversion that is undergoing stimulated emission within a laser “box,” the in-phase photons will reflect off the mirrors at either end and result in more interactions with stimulated atoms or molecules and, thus, the release of even more in-phase photons, thereby amplifying the reaction and its production of in-phase light. Hence, light amplification (from) by the stimulated emission of (photons of) radiation (Figs llB-D). The “box” of the basic laser is more correctly called the optical resonator, an apt and descriptive term. It is also called the optical cavity, laser cavity, or resonator. Within the optical resonator are contained the characteristic atoms or molecules for that particular laser. When unexcited or inactivated, the contents of the box are called the laser or lasing medium. This medium can be solid (ruby and Nd : YAG-lasers), liquid (dye lasers), or gas or a mixture of gases (Ar and CO2 lasers). When energized into its excited state by a power source, it is known as the active medium. The active medium can be achieved through a variety of different sources including electricity, radio-frequency enhancement, light, chemical reaction, and mechanical power. Electricity is used to pump most gas lasers, and because lasers are variably but extremely inefficient, their electrical requirements will vary but may be quite high, enough so to require special electrical service. Radio-frequency enhancement is used mostly for sealed-tube CO, lasers and, for all practical purposes, translates into more efficient lasers that need only normal household current and do not require water cooling. Light is used to pump dye lasers and originates either from another laser (eg, argon-pumped tunable dye laser) or a high-output flashlamp source (flashlamp-pumped dye laser). Mechanically or chemically pumped lasers are not generally used for medical purposes.
Light Amplification Once a population inversion has occurred, as long as energy is still being pumped into the system, the stimulated emission of in-phase photons will result in their impact on other excited-state electrons, resulting in, again, additional in-phase photons that, in turn, will do the same thing, that is, produce more in-phase photons. When this chain-reaction “mass production” of in-phase photons occurs within the unique structure of the laser optical resonator, the process is further magnified, resulting in light amplification (Fig 10).
Figure 10. Populationinversionresultsin light amplification andultimately in the productionof laser light.
Laser Design To better understand the concept of light amplification, a knowledge of laser design is necessary. Every laser (by
Laser Pump On (Light Amplification)
Clinics in Dermatology
2995;23:22 -20
l
LASER PHYSICS
r SC
15
Power Source
Laser Pump On (Light Amplification)
Totally Reflective Mirror
Mirror
Mirror
Laser
Puma
[
Emission
ET
Laser Light
=z
Laser Pump On (Incomplete Amplification)
L Stimulated Emission
Reflected Emission
0C Figure 11. A. Every laseris simply a boxwith a “hole“ at oneend.The contentsof the boxarespecificfor eachtype of laserand producelight characteristicfor the atomsor moleculeswithin the box.B. Most medicallasersare pumpedby electricalor opticalenergy or by radio-frequencyenhancement. C. As poweris introducedinto the laser,atomsor moleculesareelevatedto their excitedstateand a populationinversionoccurs.The resultingphotonsreleasedareall identicaland are reflectedbackandforth betweenthz mirrorsat eachendof the “box.” Continuedabsorptionof energyresultsin firther stimulatedemissionand ultimately in light amp#&@im and the productionof laserlight. D. Theoptical resonatoris the cavity in which laserlight is producedthroughlight amplification,which, in turn, is the direct result of stimulatedemission.The light, onceproduced,entersthe laserdelivery system,which for mostlasersis quartz fiberoptics.
Because of the extreme inefficiency of many laser systems, as already mentioned, water cooling in addition to special elect&al requirements may be necessary. These factors should be discussed for each laser in the respective articles.
Delive y Systems Once the characteristic laser light has been produced, it exits the optical resonator via the optical “hole,” which is
the partially reflecting and partially transmitting mirror, and enters the laser’s delivery system. Most lasers employ either visible light or light capable of fIberoptic delivery, usually through high-quality quartz fibers. The COz laser (10,600 nm for infrared) is currently not capable of fiberoptic delivery and requkes an intrieate articulating arm system containing either fixed or adjustable mirrors that direct the beam to the handpiece (Fig 12). There the laser light is focused through a lens to its
16
Clinicsin Dermatologyl 2995;23:11-20
RATZ
Laser
Coherent
Light
1 Figure 12. Becauseof its positionin thefar-infrared regionof the electromagnetic spectrum,the CO2laseris not capableof delivey throughquartz fiberoptics.In mostCO2lasersystems, laserlight is reflectedoff a seriesof mirrorsin the complex articulating systemwhereit can then bedeliveredto a handpiece containinga lenswhich will allow the collimatedbeamto becomefocusedat the focal length of the lens.
focal point. The nature of the light (intensity, spot size, etc) can be altered either by changing handpieces (and thus lenses) or possibly by adjusting the handpiece (focus/defocus or adjustable handpiece). A semirigid waveguide fiber is available, but it is basically a mirrored tunnel or tube that has some limitations in its use. Nd : YAG lasers (1060 nm, near infrared) are capable of fiberoptic delivery but may be equipped with either a quartz or a sapphire tip for contact cutting procedures. This is discussed in greater detail in the article by Glassberg and Lask on the Nd : YAG laser.
Properties of Laser Light Each type of laser produces light that is characteristic for that particular laser. The light has a characteristic wavelength that is monochromatic and spatially and temporFigure 13. Coherentlight is spatially and temporallyin phase asopposedto incoherentlight, which containsmany different wavelengthsthat areneither temporallynor spatially in phase. I
Coherent Light
Time Incoherent Light
Time
I
Incoherent
J
Figure 24. Coherentlight is muchlike a well-disciplined marchingbandin which all members are identicaland are traveling in the samedirectionin perfecttime. In contrast, incoherentlight is similarto the relative “incoherence“of a milling crowd in which the crowd members are totally “out of phase.”
ally in phase. This property of being spatially and temporally in phase is called coherence and differs greatly from ordinary light, which displays incoherence. The concepts of spatial and temporal coherence versus incoherence are demonstrated in Fig 13, but can probably be better visualized by comparing the spatial and temporal “coherence” of a finely tuned marching band with the “incoherence” of a milling crowd (Fig 14). In addition to coherence and monochromicity, laser light is highly collimated, which is its nearly parallel and nondivergent characteristic. The best illustration of this property is the comparison of two different light-pointing sources: the laser pointer, in which the spot size stays virtually the same despite the distance it traverses, and the common flashlight, the intensity of which diminishes markedly with distance (Fig 15). Thus, collimation, coherence, and monochromicity make up the key features of laser light. Different types of
25. Collimationof laserlight resultsin a nondivergent and virtually parallel beamwhoseintensity doesnot diminish greatly with distance;however,the divergentbeamof a noncollimatedlight sourcelosesits intensity rapidly over greater distances. Figure
Clinics in Dermatology
TEM oo
016
l
1995;13:11-20
TEM OI
017
LASER PHYSICS
Ideal
018
Figure 16. The mostcommontransverseelectromagnetic mode displaysa gaussiandistributionwith a central hot spotand approximately86% of the total energycontainedwithin the impactspot. Figure 17. The complextransverseelectromagnetic modecan result in a “donut” distributionpattern in which the center represents a coldspotand a “dull-knife” impactspot. Figure 78. In a theoretically “ideal” transverseelectomagnetic mode,all energywould beevenly distributedthroughoutthe impactspot.
lasers, as mentioned earlier, differ by their characteristic wavelengths (Fig 1). Although most lasers produce only one wavelength, some lasers may produce several monochromatic bands, such as the argon laser which produces 488~nm (blue) and 514~run (green) light outputs. Although a beam of laser light is spatially and temporally in phase, power distribution of the impact spot may vary, particularly if the beam is transmitted fiberoptically or through a lens. Such transmission can alter the fundamental power distribution of the impact spot. Power distribution, referred to as transverse electromagnetic mode (TEM), in its fundamental form, TEMc,a, displays a gaussian distribution (Fig 16), with approximately 86% of the power contained within the impact spot. Distortion through delivery systems can result in complex modes such as TEMa, (Fig 17), which displays a “donut” type of power distribution and a cold spot centrally. Such a distribution would have a dull-knife effect on the laser impact area as opposed to the TEM,,, gaussian distribution, which can be focused to a fine spot. In the theoretically “ideal” power distribution curve, on the other hand, all power would be evenly distributed over the entire surface area of the impact spot (Fig 18).
Continuous Versus Puked Delivery Lasers may differ in the manner in which their light is emitted or delivered. Some lasers, notably those with gaseous active media, are capable of the continuous discharge of light, also called continuous wave (CW). The power output of such lasers does not vary with time and is usually relatively low when compared with other forms of delivery (Fig 19). Such continuous waves can be broken up by an optical or mechanical shutter that simply breaks up the delivery
17
profile of the CW beam but does not alter the power output of the beam per se. Such delivery, although confusing, is called pulsed delivery (of the CW beam). Lasers not capable of CW delivery (notably, solid lasers) seem to build up a “head” of power before releasing their “shock wave” or pulse of relatively high power. The pulses delivered by such lasers are usually quite short and are followed by a lag phase before another pulse can be generated. Duration and power of the pulse, as well as duration of the lag phase, differ for the various pulsed lasers and are beyond the scope of this article (Fig 20). Different mechanisms for pulsing are abo available in some lasers. Specifically, “superpulsed” and “ultrapulsed’ CO, lasers have been developed. The net effect of such delivery can decrease thermal damage, which in some cases may be a plus but in other applications may have negative ramifications (eg, reduced thermal damage may reduce the hemostatic capabilities of the CO, laser) (Fig 21). This topic is discussed in detail in the article by Levins and Anderson, along with the concept of Qswitching, which allows for megawatt delivery over very short (nanosecond) impact durations (Figs 22 and 23).
Parameters Every laser is capable of a particular peak power, measured in watts. Characteristically, power outputs can be adjusted up to the peak output; however, operating at maximum output over the long term can diminish the half-life of the laser tube, which could prove quite costly. Because delivery of power to the target is achieved by an impact spot of measurable size, the amount of power delivered to tissue can be calculated as power per unit area. This measure of power density, also called irradiance, is calculable according to the following formula: Power density (ii-radiance) (W/cm2) = power output X 100 (mm2/cm2) impact spot size (mm*) Spot size is known and can be used to calculate the area by multiplying A by the impact radius (in mm) taken to the second power, or K r 2. As most lasers have a beam
Figure 19. Continuous-wavelasershave relatively low peak powersand are capableof shuttereddelivery, which is confusinglytermed“pulsed”delivery.
Time (set)
18
Clinicsin Dermatologyl 2995;23:11-20
RATZ
r Pulsed Laser
0 - Switched Laser
sco 5 1000 zi 2 a 0
A-!l
Time (psec)
Time (msec)
Time (nsec)
020
021
022
Figure 20. Pulsedlasersare not capableof continuousdelivery, have relatively greaterpowersper pulsethan continuous-wavelasers, and have variablelag timesbetweenpulses. Figure 21. Superpulsed COzlasersare capableof muchgreaterpowerper pulsethan conventionalcontinuous-waveCO* lasers; however,althoughthermaldamageto tissueis minimized,severaldesirableproperties,suchashemostasis, can belost. Figure 22. Q-switchingresultsin pulsesof extremelyshortduration and extremelyhigh poweroutput.
protile of TEM,,, , in which 86% of the effective beam is in the impact area, the calculation is a close approximation. If a laser delivers 10 W with a 1 -mm spot diameter, the equation becomes 10 W X 100 (mmz/cm2) 3.14 X (0.5 mm)2
= 1274 W/cm2
One must remember to calculate for radius and not diameter and to make the mm2 to cm2 adjustment. Most laser manufacturers supply power density charts that negate the need for calculation. An example of one such chart is Table 1. If the equation is known, the relationships of adjusting power and spot sizes and the relative impact on the tissue target are better understood. Doubling the power doubles the power density. Doubling the spot size, however, decreases the power density by the inverse of the square of the radius change, and halving the spot size has the op-
Figure 23. Respectivecomparisonof continuous-wave,pulsed, and Q-switchedlasers,demonstratingan inverserelationship betweentime andpowerfor the varioustypesof lasers. I Contlnuoua
wave
I
II I
1
Pulsed
-\ 0-1OOWan
>lOOOWanTYP.
Time (set)
1 0 Swltched
1 II
1I ’
> 1 Megawan
TYP.
posite, magnifying
effect. Thus, Power Spot 0 hm)
Beginningpowerandspotsize Powerdoubled Spotsizedoubled Spotsizehalved
10 20 10 10
1 1 2 0.5
PowerDensity W/cm9 1274watts/cm2 2548watts/cm2 318watts/cm2 5094watts/cm2
Measurement of power density, or irradiance, however, does not account for the effect of time or duration of exposure. The concept of time can be incorporated with the power output to determine the amount of power delivered and the duration of delivery in seconds. Such a calculation results in the measurement of power-time or energy delivery measured in joules; thus, watts
X seconds = watt seconds or joules
This measurement, unfortunately, neglects the effect of impact spot size. The concept of fluency, however, combines all these by measuring the energy delivered per unit area, or joules/cm2
=
watts X seconds (duration of delivery) area (of impact spot)
From this calculation can be appreciated that the net effect (absorbed energy per unit area or joules/c&) is affected in the same fashion as described earlier for power density, that is, direct linear relationship of power and inverse-square relationship of impact area. Additionally, time (measured in seconds) also has a direct linear effect on the measurement of fluence. From the calculation alone, equivalent changes in time or power should apparently have equivalent effects on tissue reaction as well. This, however, is not the case. Because of other
Clinics in Dermatology
l
1995;13:11-20
LASER
PHYSICS
19
Table 2. Power Density Determination Power Density (W/cm2) Spot Size (mm)
Area (mm2)
5.0 2.0 1.0 0.5 0.2 0.1
19.625 3.14 0.785 0.19625 0.0314 0.00785
2W 10.2 63.7 254.8 1019.1 6369.4 25,477.7
factors, such as limited transmission and variable absorption, an increase in time or duration of impact generally has a greater thermal impact on tissue than an equivalent increase in power output. Through this relationship several theoretic concepts have been developed. These are discussed later in the article.
Laser- Tissue Interactions Any incident laser impact is subject to all of the following possibilities on a tissue target: direct reflection, indirect reflection, scatter, transmission, and absorption (Fig 24). In fact, the net effect of a laser impact is the total of all of these and varies from laser to laser. Laser-tissue interactions can be further characterized by lasers’ relative absorptions (coefficients of absorption) and relative distances traversed in tissue before absorption is complete (coefficients of extinction). The CO2 laser, for example, has a high coefficient of absorption (98%) in tissue and a low coefficient of extinction, which translates into maximal energy absorption in a given impact target area with very little extension of energy effect beyond this point. The argon laser, on the other hand, has a relatively low (-45%) coefficient of absorption in tissue
Figure 24. Any laser-tissue interaction is the net result of the total of reflection,scattertransmission,andabsorption,which are different in characteristicfor eachlaser.
I
Direct Reflection From Surface
Indirect Reflection From Deep Reflective Chromophores
V
5W
10 w
20 w
50 w
25.5 159.2 636.9 2547.7 15,923.6 63,694.3
50.9 318.5 1273.9 5,095.5 31B47.3 127,388.5
101.9 636.9 2547.8 10,191.l 63,694.3 254.777.1
254.7 1592.4 6369.4 2L477.7 159,235.7 636,942.7
-
and a high coefficient of extinction, which means that approximately half of argon laser energy is reflected into the surrounding tissue and is capable of transmission for a relatively greater distance before complete absorption. This laser-tissue interaction in argon lasers can result in significant nonspecific absorption and subsequent nonspecific thermal tissue damage. As useful as the coefficients of absorption and extinction for each laser can be in understanding the relative laser-tissue interactions, their discussion and use have been largely abandoned. For TF&&, lasers, a basic impact on tissue will result in characteristic thermal reactions that wiIl be dependent on energy absorption, duration of impact, and laser-tissue properties. In general, a laser impact of a TEM, (gausSian) laser will result in a central zone of vaporization, in which tissue reaches 100°C and, because of the significant (80%) water content of tissue, results in vaporization. Continued contact can then result in carbonization or char formation and subsequent superheating of tissues and significant thermal damage (the “charcoal grill” effect). beyond this point of vaporization will he a zone of thermal necrosis, followed by a zone of irreversible thermal protein denaturation and then successively lesser amounts of thermal damage (Fig 25). The relative effect of any laser-tissue impact will depend on a multitude of variable properties, perhaps the most important of which is selective versus nonselective
Figure 25. A commontissueimpactresultsin a centralareaof vaporizationfollowed by a surroundingareaof thermalnecrosis, irreversibleprotein denaturation,and,f%utlly, reversiblethermal changes.
Scatter Over Wide And Deep Surface Area ermal Necrosis
Transmission Through Tissue With No Effect
Absorption by Target Chromophores
20 RATZ
Clinicsin Dermatologyl 2995;13:12 -20
Theoretic Considerations
\
450
500
550
600
Wavelength
650
700
(nm)
Figure 26. Respectiveabsorptioncurvesfor melanin, oxygenatedhemoglobin,and deoxygenated hemoglobin.Although maximumabsorptionof oxyhemoglobinis at 418 nm (the Soret band),because this is in the ultraviolet portion of the spectrum, attention hasbeenfocusedon the lesserabsorptionbandsat 542 and 577 nm. Deoxygenatedhemoglobinabsorptionat approximately560nm hasbeenrelatively ignored.
absorption. Tissue targets (chromophores) maximally absorb certain characteristic wavelengths of light, mostly dependent on color. In the case of the CO? laser (far infrared), maximum absorption (-98%) (nonselective) occurs in water (chromophore); penetration is minimal; and reflection, scatter, and transmission are even less.On the other hand, the argon laser can be maximally absorbed by hemoglobin or melanin (selective absorption), but reflection, scatter, and possibly transmission can be significant. Because of these properties, absorption curves for known tissue target chromophores are known, and lasershave been adapted to maximize the usefulness of their impacts (Fig 26). In recent years, most of the focus has been on the development of nonionizing lasersthat can be maximally absorbed by oxygenated hemoglobin (577,585 nm) for the treatment of vascular lesions and by melanin (510, 514,694 nm, etc) for the treatment of pigmented lesions. Unfortunately, perhaps, the deoxyhemoglobin absorption curve has been, for the most part, ignored. As many vascular lesions probably contain significant amounts of venous blood, treatment at the deoxygenated hemoglobin absorption peak (-560 nm) might very well improve the percentage successof vascular lesion treatment with lasers.1-5This area needs investigation.
Many theoretic considerations have arisen around maximizing the abosorption of laser energy by target chromophores and minimizing thermal injury. Such considerations include the concept of selective photothermolysis, reported first for hemoglobin (and 577-nm laser light) and melanin (351-nm excimer laser light). This concept suggests,appropriately, that maximal absorption of laser energy by a chromophore will result in damage or destruction of that chromophore, particularly if the impact is of a short enough duration with a subsequent lag phase to allow for tissue cooling to minimize damage to surrounding tissue. The concept of tissue cooling will be recognized as the theory of thermal relaxation time and, though possibly conservative, almost absolutely allows for minimizing the risk of thermal damage to surrounding tissue. These concepts are the foundation on which further laser development will be built. They are important concepts to study and to understand.
Conclusion Laser basic physics and tissue interactions are boring and not particularly easy to understand or remember; however, to understand the new and upcoming developments of laser application in medicine, understanding lasers totally and developing a working knowledge of them are mandatory. Newer lasers and concepts are on their way, and this basic material is imperative for anyone who wishes to lead the way or to follow with an understanding of what is being done and why it may or may not be working.
References 1. Apfelberg DB, editor. Atlas of cutaneouslasersurgery. New York: Raven Press,1992. 2. Apfelberg DB, editor. Evaluation and installationof surgical lasersystems.New York: Springer-Verlag, 1987. 3. Dixon JA. Surgical Application of Lasers.2nd ed. Chicago: Year Book Medical, 1987. 4. Dover JS, Amdt KA, GeronemusRG, et al. Illustrated cutaneouslaser surgery: A Practitioner’s guide. Norwalk (CT): Appleton & Lange, 1990. 5. Ratz JL, editor. Lasersin cutaneousmedicine and surgery. Chicago:Year Book Medical, 1986.