Objectives l Participants should: Basic Science of Lasers » Be familiar with the basic principles for the generation of laser light » Learn the properties of laser light Stephen R. Tan, MD FRCPC » Understand the biologic response of tissue to laser impact Director of the Division of Dermatologic Surgery HealthPartners Medical Groups & Clinics November 2007 Absorption & Spontaneous History Emission l Laser concept postulated by Einstein in l Atoms & molecules are normally in the 1917: “resting state” » “The Quantum Theory of Radiation” » This is the most stable position for a nucleus and surrounding electrons l First laser developed by Maiman in 1960: l Light can be absorbed by atoms and move » Stimulated visible light emission with ruby electrons from a resting state to an excited crystals state l Called the device a LASER: l Frequency & wavelength of emission and absorption are proportional to the change » Light Amplification by the Stimulated in energy Emission of Radiation Excitation of Electrons Generation of Photons l The “excited state” is unstable » Electrons spontaneously return to the “resting state” » During this process, they re-emit the energy used to push them to the “excited state” l Released energy (“radiation”) is the “spontaneous emission” of LASER l May be released as EM radiation (light) which travels in packets called photons Amplification of Photon Amplification of Photon Generation Generation l When already excited atoms are irradiated with the same energy again, it generates two waves of light energy with: » Same frequency and wavelength » Traveling in the same direction » Perfect spatial and temporal phase How does this happen in a laser? Excitation in the Laser l Atoms are placed into a cavity called the “optical cavity” or “resonator” » Lasers are named for the medium in the cavity (ex. CO2, pulsed “rhodamine” dye, ruby, Nd:YAG etc) » The cavity is electrically charged when you turn the laser on l Atoms become excited or “pumped” to an excited state by the charge Photons Generating more Population Inversion Photons l When the majority of the atoms in the resonator are excited, there is a “population inversion”: » Increasing likelihood of spontaneous emitted photons traveling along the long axis of the cavity will collide with other atoms l Remember that these other atoms are already in the “excited state” » Stimulated emission produces even more photons of the same frequency traveling along the same axis Amplification of Photon Generation All Smoke & Mirrors… l Reflecting mirrors are placed at either end of the cavity: » One is totally reflecting, the other partially so l Light travels back and forth within the cavity: » Promotes further stimulated emission » Amplifies the whole process l When you press the trigger: » The partially reflective mirror allows a small portion (~5-10%) of the light out of the cavity » Emitted through the handpiece as laser light Properties of Laser Light Electromagnetic Spectrum l Monochromaticity: Lasers produce pure band(s) of light: » Emissions are spectrally very narrow » Wavelength depends on the medium used to generate the light » Also called “temporal coherence” l Monochromaticity allows selective targeting of specific chromophores in the skin » Different chromophores absorb light of different wavelengths » Allows us to choose the right laser to target the right chromophore Lasers in the EM Spectrum Properties of Laser Light l Spatial Coherence: Lasers are highly directional and orderly » Light waves are in phase » All crests & troughs of the light are synchronous l This is why laser light has a low degree of divergence » Allows laser light to travel great distances while maintaining intensity » Typical laser diverges ~1mm for every meter traveled » The smaller the spot size, the greater the divergence Coherence Properties of Laser Light l Brightness: Lasers generate tremendously high powers » Results from the amplification process l High powers with low beam divergence = VERY BRIGHT LIGHT! Laser Power in Clinical Laser Power in Clinical Terms Terms l Power density: Power at the tissue level l Energy: Energy is measured in joules » Measured as W/cm2 [or (joule/sec)/cm2] » Directly proportional to the number of » The spot size selected determines the area of photons in the laser beam → more the laser striking the skin photons = more energy » Power density determines the rate of thermal l Rate of energy delivery: AKA power damage → increasing power density increases rate of tissue damage output is measured in watts l Fluence: When the pulse duration is » 1 Watt = 1 Joule/second known, the laser can multiply this by the power density » W/cm2 * sec = J/cm2 Pulse Duration Continuous Wave Lasers l CW lasers l Pulse duration may be set or variable produce a » May be set by the laser design continuous beam » May be varied to achieve different of laser light effects l No variation over l Produced by various optical, time mechanical, and electrical designs l Ex. Original CO2 lasers Pulsed Lasers Superpulsed Lasers l Produce individual l Specific term for pulses of laser CO2 lasers light » Used in the l Energy builds, Ultrapulse laser peaks, and tapers l Laser produces off very short pulses l Higher power than with a very high CW lasers peak power l Ex. PDL » Reduces amount of collateral » Millisecond domain pulses thermal damage Quality (Q)-Switched Lasers Laser Delivery l Further shortens pulse duration » Uses extremely l Either through an articulated arm or fast EM or chemical switch an optical fiber l Allows buildup of excessive energy in laser cavity before discharge » Q-switched Ruby produces power of 1,000,000 W/cm2! l Delivers very short single pulses of extremely high power » Nanosecond pulses Articulated Arm Optical Fiber Beam Intensity Profiles Beam Intensity Profiles l Energy distribution over a cross- section of the beam » “Top hat”: Uniform intensity across the diameter → theoretically ideal, but not necessarily the most practical » Gaussian: Peak energy in the center of the spot, energy decreases towards the edge Overlapped vs. Non- Why does this matter? Overlapped Pulses l You can get away with overlapping the pulses with a Gaussian distributed beam l You can burn the patient if you overlap the pulses with a “top hat” beam Lasers and the Skin Laser Tissue Penetration l Generally, the depth of penetration increases with longer wavelengths l The exceptions are lasers that target water Theory of Selective Choose a Specific Photothermolysis Chromophore l Postulated in 1983 by Anderson & Parish l Light is absorbed in tissue by l Selective tissue damage is possible if: chromophores » A specific wavelength is chosen which is absorbed by the target chromophore l Major chromophores in the skin are water, melanin, and hemoglobin » Pulse duration is shorter than the thermal relaxation time (i.e. cooling time) of the » Each has wavelengths where the target chromophore more efficiently absorbs laser light l The goal is selective thermal damage to » Modern lasers use this fact to generate the target WITHOUT non-specific thermal light that will be absorbed by a specific damage to surrounding structures chromophore Hemoglobin & Melanin Choose a Laser with the Right Absorption Curves Pulse Duration l Thermal relaxation time of cutaneous vessels is between 1 – 10ms » Pulsed dye and KTP lasers have adjustable pulse durations within and beyond this range l Thermal relaxation time of melanin is 1µs » Q-switched Nd:YAG, Ruby and Alexandrite all have pulse durations on the order of a few nanoseconds Pulse Stacking Vascular Lasers l Newer pulsed dye lasers have pulse widths well beyond the thermal relaxation time of vessels l The idea is to deliver higher cumulative fluence through pulse stacking of lower energy pulses » Effective fluence = fourth root of # of stacked pulses fluence used l Ex. If a given area is treated with 16 pulses, the effective fluence is two times the actual fluence used » Stacked pulses at a lower fluence can cause equivalent thermal damage to a single pulse at high fluence Selective Targeting of Chromophores Laser-Tissue Interaction l When laser light is absorbed by a target chromophore, there are three possible biologic effects: » Photothermal: Effects resulting directly from heat and thermal damage » Photomechanical: Rapid temperature change of the chromophore results in sudden thermal expansion, tissue vaporization, shock wave, or pressure wave formation » Photochemical: Light reacts with an endogenous or exogenous photosensitizer (ex. PDT) Photothermal Damage Heating the Skin l Absorption of laser light results in l The goal of laser therapy is to heat up the transformation of light to heat specific target that you want to destroy » < 50oC: Reversible thermal damage l The site and rate of absorption are » 50o – 100oC: Most tissues undergo denaturation how laser damage is targeted: or irreversible coagulation of proteins » > 100oC: Tissue vaporization occurs » The site of heating is determined by the l Heating targets beyond the reversible laser wavelength and absorbing damage threshold results in coagulative chromophore necrosis of epithelial or connective tissue » The rate of heating is determined by the l Tissue repairs itself with granulation tissue fluence and pulse duration at the site of the original damage Heating < 100oC: Dermal Heating > 100oC: Remodeling Vaporization l Used as “non-ablative resurfacing” l Tissue water is heated above the boiling l Uses 1320nm Nd:YAG or 1450nm diode lasers point → vaporizes into a plume of water » Infrared lasers targeting tissue water vapor and tissue l Initiates and enhances dermal collagen remodeling » Uses an epidermal cooling device to minimize epidermal water heating (kept below 50oC) » Thermal injury localized to papillary and upper reticular dermis » Induces dermal collagen remodeling, with clinically mild or moderate atrophic scar or wrinkle improvement Fractionated Resurfacing Photomechanical Effects l Newer lasers selectively ablate multiple small l Produces immediate disruption of viable targets within a larger field tissue at the cellular level l Speeds healing time versus conventional l May be thermal or non-thermal: resurfacing: » Multiple small areas to heal as opposed to complete » Thermal: Sudden heating causes steam epidermal + dermal loss formation → PDL causes a 100oC » Surrounding skin is intact to provide cells for wound temperature increase in 450µs, rupturing healing vessel walls and causing purpura l May have decreased incidence of side effects » Non-thermal: Laser pulses of short duration l Multiple treatments potentially needed and high peak power (Q-switched) causes shock waves, cavitation, and vaporization Photomechanical Effects What else has chromophores? l How fast does the Q-switched l What’s the difference between these laser heat the fancy sunglasses? melanin? l 10 billion oC per second!!! (but the laser only fires for 20-40 nanoseconds) l At that rate, the tattoo or melanin granules literally explode The covered wavelengths! Conclusion l Dermatologic laser therapies all have a scientific rationale behind them l The proper laser medium generates light with the proper: » Wavelength » Intensity » Pulse duration » Depth l Allows us to SELECTIVELY damage targets in the skin l Know what you’re settings mean and what you’re shooting at before pulling the trigger!
Pages to are hidden for
"Basic Science of Lasers"Please download to view full document