PRINCIPLES OF LASERS copy.pptx

Principles of lasers for dermatology

1. LASER-BASICS Dr Prathyusha

2. Contents Introduction History of Laser Terminology Principle of Laser Components of Laser Types of Laser Laser-Tissue Interactions Laser Hazards Safety measures Conclusion

3. Introduction - What is LASER? L ight A mplification by the S timulated E mission of R adiation - Term coined by Gordon Gould - Father of LASER

4. HISTORY Max Planck (1900) energy emitted/absorbed in discrete units - “quanta” Einstein (1905) light delivers energy in packets: photons Einstein (1916) - concept of Stimulated Emission Maiman and Gould & Schawlaw (1960) built first ruby LASERs Soon employed in various fields- telecommunication, industry, aeronautics, photography, creation of 3D images, computer sphere and MEDICINE

5. In Dermatology Leon Goldman (Father of LASERs in medicine & surgery) in 1963: destruction of cutaneous pigmented structures and treatment of tattoos, port wine stain and tumors (nevi & melanomas) Described use of Argon laser in vascular malformations, CO2 laser - photo-excision of skin lesions, Nd: YAG laser - vascular lesions.   Milestone in 1983: Anderson & Parrish: Selective Photothermolysis : use of specific wavelength  destruction of specific molecules ( chromophores )-> better localization of energy; minimization of damage to surrounding tissue First ever FDA guidelines published: 1984

6. Basic terminology Energy : Fundamental unit of work – Joules (J) Power : The rate at which energy is emitted from a laser. Watt (W) Fluence : Determines the amount of laser energy per unit area and is expressed in joules/cm2. Irradiance : determines the ability of laser to incise, vaporize or coagulate a tissue W/cm2

7. Spot Size: The mathematical measurement of the radius of the laser beam. Pulse: A discontinuous burst of laser as opposed to a continuous beam. Pulse Frequency : The rate at which pulses are generated. Expressed in pulses per second (Hz). Pulse Duration : Pulse duration/width: time for which laser energy is applied ( pico,nano,micro -sec)

8. Wavelength Measured in nanometers . Specific for that laser. Matches absorption spectrum of chromophore targeted. Longer the wavelength , deeper the penetration (upto 1320 nm)

9. Light- form of electromagnetic energy Energy increased by many times by process of amplification –> Highly intense and powerful energy light beam Light energy -> converted to heat energy (tissue level) and this thermal energy is responsible for therapeutic results Principle of Laser

10. Stimulated emission Spontaneous emission: when electrons are close to nucleus atom is in state of lowest energy. Unstable excited electron reverts back to its original stable position by releasing a photon of energy. This emission is random and in all directions. Sunlight and any light observed in nature are spontaneous emissions. Stimulated emission: Excited electron absorbs one more photon of same energy, reverts back to resting orbit (ground state) releasing 2 photons both having same energy, wavelength, phase and direction as of the absorbed photon. Energy required for this process is provided to the laser by external power source.

11. Components of Laser ACTIVE GAIN MEDIUM/ LASING MEDIUM (solid, liquid or gas) This medium is composed of atoms, molecules, ions or electrons whose energy levels are used to increase the power of a light wave during its propagation. The physical principle involved is called stimulated emission. 2. PUMPING SYSTEM/ ENERGIZING SOURCE : The energy required for the process of light amplification is supplied by an external source such as an electric current (if the active medium is a gas) or by a light source (flash lamp or other laser, if the active medium is solid) This supply of energy is called optical pumping and amplifies the light until it is emitted through the output coupler 3. OPTICAL RESONATOR (OR CAVITY): It contains the gain medium. Reflecting mirrors amplify the light source considerably by bouncing it back and forth within the cavity

12. 4.Delivery systems Fibre optic cables: Easy to use, lighter, may break or twisting occurs during movement Not robust to transmit emissions from Co2 lasers, Er:Yag, Q-S lasers. Articulated arms: light is reflected by multiple mirrors. Ends with a hand piece in which light can be focussed by a lens or transmitted as a beam. Each delivery system ends in an operator hand‐piece through which light can be focused by a lens or transmitted as a collimated beam

13. Energizing pump provides energy for absorption by active medium. Atoms in gain medium reach an excited state and there is spontaneous emission. Some of these photons are reflected back-forth between two mirrors. As photons collide with excited atoms in gain medium, energy inside the resonator builds and gets amplified by reflections between parallel mirrors. At the output mirror a portion of light is allowed to escape, while rest remains inside resonator to maintain the lasing process. This continues as long as the pumping system is on. Thus the energy that comes out is in the form of intense beam of monochromatic, collimated coherent light.

14. TYPES OF LASER MEDIA GAS LIQUID SOLID Argon Carbon dioxide Helium – Neon Krypton Xenon chloride Rhodamine dye dissolved in organic solvent-PDL Crystal Alexandrite Er -doped YAG Holmium –doped YAG Neodymium doped YAG Potassium titanyl phosphate (KTP ) Ruby Semiconductor Diode ( eg.alumnium gallium,arsenide )

15. Properties of LASER Laser light differs from light emitted by a conventional light source in that it has the following characteristics: Monochromatic : all photons - single wavelength Coherent : travel in same direction and in same phase- spatially and temporally Collimated : all individual waves are parallel to each other ; can be propagated long distances with minimal convergence/divergence- no loss of intensity

16. Types : Lasers can be classified into ablative and non-ablative based on their action on epidermis ABLATIVE: lasers which removes the epidermis and heats dermis -more effective; but invasive and more downtime; higher risk to patient Eg : CO2 laser, Erb YAG laser Non-ABLATIVE: do not remove epidermis and acts on deeper tissue -modest results, multiple sittings; but very less downtime and complications

17. Modes of delivery: Continuous wave method Pulse method Fractional Q-switched

18. Continuous wave mode : Uninterrupted Low power. Requires operator skill to deliver adequate amount of energy, uniformly by free hand. Large areas ; with lesser time consumption. Reduced safety profile.

19. Pulse mode: beam interrupted to form pulses (bursts of peak power) either by mechanical shutters, electronic switches, mode locking, quality switching. can produce single, multiple or pulse trains with width of nano, micro sec improved energy delivery with uniform dose of radiation across treatment area compared to CW. safety profile is higher as they minimize thermal injury by allowing ‘cooling phase’ more time consuming Ultrapulse : High energy for very short time; minimising collateral damage

20. Fractional mode: both CW and Pulse modes; further fractionated by micro lens arrays, computer patterns into smaller fragments. forms grid pattern with zones of treatment and intervening untreated areas fewer side effects, faster recovery time

21. Q or Quality switching : Short pulses (5-20ns) of high intensity (5-10MW) created. system has polariser and pockel cells in optical cavity Pockel cells are optically transparent crystal which rotates the plane of polarisation of light On application of voltage pockel cells become opaque-> allowing buildup of energy in optical cavity energy is released in short powerful pulse when voltage is switched off

22. LASER- TISSUE INTERACTIONS Variety of interactions take place when skin is exposed to laser Depend on type of laser, angle of incidence and property of tissue Reflection, transmission, scattering and absorption of light occurs Absorption of laser-conversion to thermal energy-> therapeutic directly or indirect (conduction)

23. Tissue optics REFLECTION . 4–6% of light is reflected. Increases with increase in angle of incidence Lowest when the beam is perpendicular.

24. ABSORPTION This is governed by Beer’s law, which relates the absorption of light to the properties of the substance through which the light is travelling. When a photon is absorbed by a target molecule or chromophore, all of its energy is transferred to that molecule. The basis for selective skin laser surgery is that light can be manipulated in terms of its wavelength, energy content and pulse duration so that a particular target chromophore absorbs light and is selectively damaged or destroyed CHROMOPHORES: An exogenous or endogenous material present in tissue which absorbs light of particular wavelength, depending on its absorption coefficient

25. ENDOGENOUS CHROMOPHORES: Melanin: UV -1200nm, visible light Hb: UVA, blue (400 nm), green (541 nm), Yellow (577nm) Collagen: Visible and near infra-red spectra Water: in the mid and far infrared regions EXOGENOUS CHROMOPHORES: eg. Tattoo ink.

26. Absorption spectra of principle tissue chromophores

27. SCATTERING - Is the deviation of light by non-uniformities in the medium through which it passes. Eg . Collagen It reduces the energy available for the target chromophores, thereby decreasing the clinical effect. TRANSMISSION - Light that is not Reflected, absorbed or scattered passes to deeper tissue. Longer the wavelength( 600-1200nm) penetrate more deeply because they are scattered less. Shorter wavelength (300-400 nm) being scattered and penetration is less than 0-1mm.

28. Penetration of laser depends upon: Absorption and scattering Dept of penetration increases with wavelength Spot size In general, between 280 and 1300nm , the depth of penetration increases with wavelength Above 1300 nm , penetration decreases due to the absorption (trapping) of light in upper layers by water The most deeply penetrating wavelengths are 650–1200 nm the least penetrating wavelengths are within the far-UV and far-IR regions Short wavelength will limit the penetration depth of the beam due to limitation by the strong scattering effect. Longer wavelength, the scattering effect of the light is minimal deeper penetration of the light and it allows treatment of a deep dermal lesion.

29. Laser tissue interaction Photo thermal Photomechanical Photochemical

30. Photothermal effects Laser absorbed-> converted to heat Cellular damage such as reversible thermal damge , local vasodilation and inflammatory cascade activation are produced at temperatures <50 C Once temp crosses 50C, irreversible changes like denaturation of proteins, collagen coagulation, membrane permeability changes -> cell necrosis Can result in partial thickness burns with scars.

31. If tissue temp-100C, tissue drying and vacuole formation occurs Tissue water-> steam (vaporisation) ; water expand with pressure and micro explosions occur within tissue surface(ablation) Beyond 100C without further vaporisation, carbonization – black charring occurs with smoke formation (~300C)

32. THERMAL RELAXATION TIME (TRT) It is the time taken for the target to dissipate 63% of incident thermal energy. If a chromophore is exposed to a laser for a time longer than its TRT it will result in collateral thermal damage and increased risk of complications.

33. Selective Photo- thermolysis Due to prolonged exposure to laser energy, there is collateral thermal damage to the surrounding tissue Selectively destroying chromophore/target tissue without destroying surrounding structures such as blood vessels, collagen etc by controlled destruction. Wavelength should be specific to absorption spectrum of the chromophore and penetrate deep enough to reach the target Optimal pulse duration should be equal to or less that the TRT Fluence should be high enough to destroy the target within the pulse duration Factors governing Selective photothermolysis: Wavelength Pulse width/duration Fluence/ energy density

34. Extended theory of Selective Photo- thermolysis When target tissue doesn’t absorb laser, but adjacent tissue absorbs strongly (chromophore) Target tissue damaged selectively by diffusion of heat from the surrounding highly absorbing structure Thermal damage time is time taken by target to reach damage temperature by diffusion of heat from adjacent chromophore Eg : in LHR, target is hair bulb but chromophore is melanin

35. Thermokinetic Selectivity theory For same chromophore, a longer pulse duration allows more rapid intra-pulse cooling of target (melanin) in smaller target (epidermis) than melanin in larger target (hair follicle) Thus for selective destruction of hair without epidermal effects, pulse duration must be longer than TRT (3-10ms) of epidermis but not exceed that of hair follicle (10-100ms)

36. Photomechanical Effect Also Photoacoustic effect High peak laser in ultra short pulses ->builds up intense heat High pressure at constant volume generates shock waves or photoacoustic waves when there is no time for thermal relaxation Tissue structures and chromophores ( eg . Melanin and tattoo ink) are torn apart by pressure waves->eliminated by scavengers or transepidermally -> cavitations A popping sound (acoustic) is generated when high pulsed laser used

37. Photochemical effects This forms the basis of photodynamic therapy (PDT) and usually involves the topical or systemic administration of a photosensitizer or precursor thereof. Subsequent irradiation with an appropriate light source elicits two types of photo‐oxidative reaction and an immediate cytotoxic effect. PDT can also target endogenous chromophores (such as produced by Pityrosporum acnes and utilized in the blue light treatment of acne vulgaris)

38. Heat diffusion During laser-tissue interactions heat generated and transferred (diffuses) to surrounding tissue mainly by conduction (also convection, radiation) Characterises the collateral damage Epidermis is first barrier while treating dermal tissues; inadvertently gets heated Protected by surface cooling

39. Role of cooling Surface cooling of epidermis achieved by conductive cooling by self-cooling sapphire tip, cryogens, ice compresses, cool gel etc Applied before, during and after pulse Also minimises pain and allows higher energy usage

40. Cooling may be achieved by three means: 1 Cold air convection : Air, chilled to temperatures as low as –30°C, is directed onto the area to be treated. 2 Contact cooling. This can involve simple application of ice‐ packs or more sophisticated systems in which cold water flows between colourless and transparent plates (usually sapphire as it is a more efficient conductor than glass). Although a good method of cooling, the condensate that develops on the plates can obscure the skin and may require frequent removal. 3 Cryogen spray (dynamic) cooling. A frozen gas (e.g. tetrafluoroethane ) is sprayed onto the skin immediately before the laser pulse. Evaporative cooling has a high heat transfer coefficient and this is therefore the most efficient way of pre‐cooling. With timed, automated control this method is also relatively predictable and reproducible

41. Some commonly used Lasers

42. CO2 Laser Wavelength – 10,600nm Chromophore- water When CO2 laser interacts with tisse -> 3 zones -1 st zone of ablation, intracellular water vaporised -2 nd zone irreversible thermal damage -3 rd zone reversible thermal damage – collagen shrinkage and visible tightening of skin

43. Erb d YAG Erbium doped Yttrium-Aluminium-Garnet laser Wavelength-1064nm Chromophore- Water Absorbed 10x by water than 10,600nm CO2 laser-> less collateral damage One pass ablates 10-30micron and zone of thermal damage is ~15-30micron

44. Pigment removing Laser Lasers which remove pigment with minimum damage to epidermis Pigment should strongly absorb corresponding wavelength Q-switching is commonly used as it generates nanosecond pulse which matches with thermal relaxation time of melanosome or tattoo pigment Photoacoustic effect damages the melanosomes-> phagocytosed Q switched Nd YAG, Ruby and Alexandrite lasers

45. Vascular Laser Selective destruction by targeting Hemoglobin Oxyhemoglobin has 3 peaks -418nm, 542 nm, 577 nm Hb absorbs energy and leads to destruction of endothelium Host factors and type of lesion determines laser wavelength to be used Deeper vessel – higher spot size; larger vessel- longer pulse duration Pulse dye laser 585nm, long pulse Nd YAG

46. Hair Removal Laser Chromophore- Melanin Hair bulb and shaft of pigmented hair contains melanin Melanin absorbs and transfers heat to hair bulb; damaging stem cells Issue arises as epidermis also contains melanin; absorbs light leading to pigment alteration in dark skin Longer wavelength penetrates deeper and is less absorbed by epidermal melanin, so safer and also longer pulse Additionally, epidermal cooling mechanism prevent thermal damage to epidermal melanin

47. Fractionated lasers Developed by Manstein & colleagues in 2004. Creation of MTZs (Microscopic Thermal Zones) with sparing of normal skin around each MTZ. Intact, non-treated skin leads to more rapid healing. Degenerated debris is incorporated into columns and eliminated transepidermally .

48. Hazards of laser

49. Fire hazards Drapes, dry guaze , towels, dry hair, face masks and plastic materials can be ignited. Greatest risk is with the CO2 and erbium: YAG lasers used for skin resurfacing and ablative fractional treatments Prevention of fire hazards: Remember to place the laser in STANDBY mode when not actually treating the patient Avoid inadvertently activating the laser foot-switch Moisten any hair near the treatment field; remove mascara and eye makeup when working around Eyelids Alcohol, acetone or other flammable skin-cleaning solutions must be allowed to completely dry before laser use Reduce intraoperative oxygen concentration to <40% A fire extinguisher and water should be readily available

50. Plume hazards Inhalation of laser generated plume materials: Particularly with resurfacing or vaporization of hair during its removal;latter can release irritating sulfur and other oxides Cytotoxic, mutagenic, disease transmission properties Chemical contents detected : CO, NH3, HCN, formaldehyde, benzene. They can produce upper respiratory tract irritation. Viral particles (HIV DNA, intact HPV DNA) have been detected in laser plumes of CO2 laser in treating warts/papillomatosis

51. Prevention of laser plume hazards Smoke evacuator, laser masks, good ventilation,are most effective measures Most surgical masks filter upto 0.5um size,77% laser plumes are 1.1um or less in size Smoke evacuator should have specific attributes: suction unit or pump, filter, hose and inlet nozzle. The capture velocity should be about 30-50 cu feet per minute at the inlet nozzle. The filter should be high efficiency particulate air filter(HEPA)

52. Side effects Immediate: Pain, burning sensation, edema Early: • Oozing, crusting • Secondary infection LATE: Dyspigmentation (hypo/hyper) Change in skin texture Demarcation lines(in facial rejuvenation) Keloids and hypertrophic scars Milia Persistent erythema

53. Cutaneous complications Hyperpigmentation/ hypopigmentation Blister formation, Milia, scarring Wound infections/ delayed wound healing Allergic Contact dermatitis Postoperative erythema Purpura CUTANEOUS BURNS Can occur with essentially all dermatological laser, IPL, RF or therapeutic ultrasound devices Primarily due to improper device, dosimetry and/or treatment technique. PREVENTION: Appropriate cooling, proper technique, dosing

54. To prevent health hazards Labelling operating room (OR) door when laser is in use Doors and windows covered, reflecting surfaces should be draped Protective devices for patient, physician and OR attendant; eye-shields or goggles of optimum optical density Smoke evacuator - laser plume can be hazardous O2 cylinders and explosive chemicals should be stored far away

55. References Bolognia-Dermatology (3rd Ed.) Fitzpatrick's Dermatology in General Medicine. Rook's Textbook of Dermatology - 4 Volume Set (9th Edition). IADVL 5 th edition ACSI textbook on Cutaneous & aesthetic surgery

56. THANK YOU