LASERS : BASIC PRINCIPLE AND USES IN DERMATOLOGY

Dr Rohit Kr. SinghRes. (Dermatology)

Base HospitalLKO

Contents

• Introduction

• History of lasers

• Basics components of laser

• Principle and working of laser

• Effect of laser on skin tissue

• Applications of laser principles

• Laser hazards and protections

Introduction

DEFINITION

• Light Amplification by the Stimulated Emission of Radiation (acronym coined by Gordon Gould)

Conceptualization

A fantasy well-established in the past

• In 1898, H.G. Wells describes in his book “The War of the Worlds” a “fiery ray” impressively similar to modern day lasers:

• « It was as if they were hit by an invisible ray that exploded into white fire on impact. Suddenly, it seemed they were all turned to flame, and I stood there stupefied, unable to comprehend that it was death that leaped from one man to another (...) I only had the impression that it was something very strange, this silent jet of light that knocked down everything it touched, and when this invisible line passed over them, even the pine trees and the bushes burst into flame with a deafening sound... »

The “fiery ray” described by H.G. Wells is the incarnation of the absolute weapon: fired out of a vibrating funnel, a narrow ray straight as a rod sweeps over the surrounding countryside destroying everything in its path. It is hardly surprising that this fantasy of ultimate power (a “death ray” that quickly and “cleanly” kills the potential target from afar) was extremely popular during the first half of the 20th century and when the research scientist T.

History : Some important dates

• 1917: Albert Einstein developed the theoretical concept of photons & stimulated emission

• 1954: Charles Townes & Arthur Schawlow built the first MASER using ammonia and microwave energy

• 1960: Thomas Maiman produced the first laser using a synthetic ruby rod

• 1960: Dr. Ali Javan-first continuous laser (He-Ne 632.6 nm red gas ion laser

Pioneers

History of laser

• 1960s: Dr. Leon Goldman: Father of Laser Medicine & Surgery-usage of laser in medical practice

• 1962: Bennett et al: blue-green argon laser (retinal surgery)

• 1964: Kumar Patel: CO2 laser

• 1964: Nd:YAG laser

• 1969: Dye laser

• 1975: Excimer laser (noble gas-halide)

PIONEERS

BASICS COMPONENTS OF LASER

• A laser consists of two fundamental elements - LASER AMPLIFIER

• These two components are sufficient to amplify an existing light source

1. AMPLIFYING OR GAIN 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 - a system to excite the amplifying medium

– This creates the conditions for light amplification by supplying the necessary energy.

– There are different kinds of pumping system:

1. Optical (the sun, flash lamps, continuous arc lamps or tungsten-filament lamps, diode or other lasers)

2. Electrical (gas discharge tubes, electric current in semi-conductors)

3. Chemical

3. OPTICAL RESONATOR (OR CAVITY) in order to produce a very special radiation

– The laser oscillator uses reflecting mirrors to amplify the light source considerably by bouncing it back and forth within the cavity

– It also has an output beam mirror that enables part of the light wave in the cavity to be removed and its radiation used

• Technically, the whole device is known as a LASER OSCILLATOR, but this term is often shortened to simply “laser”.

Principle of laser

• Atom will absorb and emit light photons at particular wavelength corresponding to the energy differences between orbits. The wavelength l of emitted or absorbed photon can be obtained by the formula

• where E is the change in energy between the initial and final orbits.

hfhc

E

Principle of laser (cont...)

• “Stimulated emission” of radiation

• Atoms or molecules are characterized by a set of discrete allowed energy states

• An atom can move from one energy state to another when it receives or releases an amount of energy equal to the energy difference between the two states

Three distinct process may occur in the medium1. Stimulated absorption2. Spontaneous emission3. Stimulated emission

Principle of stimulated emission of radiation

• Two energy levels E1 and E2 (E1 < E2) whose atoms can interact with light of frequency

• The group E1-E2 is called radiative transition if atoms can only pass from E1 to E2 (or from E2 to E1 ) by interacting with light

• E1 is called the lower energy level

• E2the upper energy level

The emission-absorption principle

• Absorption: – An atom in a lower level absorbs a photon of frequency hν and moves to an upper level

• Spontaneous emission– An atom in an upper level can decay spontaneously to the lower level and emit a photon of

frequency hν if the transition between E2 and E1 is radiative– This photon has a random direction and phase

• Stimulated emission– An incident photon causes an upper level atom to decay, emitting a “stimulated” photon

whose properties are identical to those of the incident photon– The term “stimulated” underlines the fact that this kind of radiation only occurs if an incident

photon is present– The amplification arises due to the similarities between the incident and emitted photons

Population inversion and pumping

• To favour stimulated emission over absorption, there need to be more excited-state atoms than ground-state atoms

• Spontaneous emission naturally tends to empty the upper level so this level has to be emptied faster by stimulated emission

• It has been proved that stimulated emission is much more likely to happen if the medium used is flooded with light (i.e. with a large number of photons)

+• A good way to do this is to confine

the photons in an optical cavity

• The confinement of the light increases the probability of stimulated emission rather than spontaneous emission occurring

•If there are more atoms in the upper level (N2) than in the lower level (N1), the system is not at equilibrium

•A situation not at equilibrium must be created by adding energy via a process known as “pumping” light in order to raise enough atoms to the upper level

•This is known as population inversion and is given by N2 - N1 = Δ

•Light is amplified when the population inversion is positive

•Pumping may be electrical, optical or chemical

Spectroscopic systems used to create a laser

• Every pumping system (particularly optical or electrical) corresponds to a certain energy, which must be transferable to the atoms of the medium

• The difference in energy between the excited state and the ground state must match the pumping energy

• The objective is to store atoms in level E2 by absorbing “pumping” radiation whose wavelength is shorter than that of the laser transition

• At least half of this population must be excited to level 2 to obtain population inversion

• Moreover, level 2 must be able to store these atoms so spontaneous emission must be very unlikely

Gaussian beam

An optical cavity selects a specific beam (a Gaussian beam) from the many photons spontaneously emitted by the “lamp-amplifying medium” and the number of photons carried by this beam is increased considerably, as it travels back and forth, by the process of stimulated emission. This beam can have a very low divergence and can be very precisely focused if the right optical tools are used.

Monochromaticity

• The spectral bandwidth of a laser is given by the width of the spontaneous emission: if the transition between the upper and lower levels is narrow, then the spontaneous emission will be fractions of a nanometre (this is the case for the red line in neon, which has a width equal to 1/1000th of a nanometre and a frequency of 1 GHz)

• The spectrum of a helium-neon laser is therefore “monochromatic” in the sense that only one colour is visible to the naked eye as the line is very narrow

• The spectral properties of lasers become even more interesting when just one frequency can be selected (using a series of filters placed in the optical cavity)

• The optical cavity is capable of filtering the spontaneous emission in the form of discrete frequencies (the longitudinal modes)

COLLIMATION TEMPORAL COHERENCE Directionality

Monochromicity

Temporal coherence

LASER characteristics

LASER characteristics Symbol Unit of measurement

Wavelenght lambda nm

Spot size d ( diameter)s ( square)

cm

Pluse duration p (power)energy delivered per unit

time

Watt ( w) = joules × sec (j/sec)

Fluence ɸEnergy delivered per unit area

joule/cm2

Irradiance Power delivered per unit area

w/cm2

LASER beam typesContinuous – wave (CW) LASERS Quasi – continuous – mode (

QSM)Pulsed LASERS

Mechanism of action

Continuous beam of light Continuous – wavelength lasers that are mechanically shuttered to deliver pulses of light as short as 20 ms

Select pulse duration based on target size

Long pulse (millisecond) : 0.5 to 400 ms allows targeting of most hair and blood vessels ( ie visible to near IR)

Short pulse ( microsecond) Modified to produce very short pulses with high peak power in a repititivefashion; developed to reduce the amount of thermal damage allows safer skin resufacing ( CO2)

Q- switched (QS) nanosecond: allow build up of extremely high energy in laser cavity before discharging in very short single pulses

Characteristics Little or no variation in power output over time (stable avg. beam power)

Produces individual pulses of light Energy within the pulse is not constant but rather builds, peaks and tapers off within a very short timePeak power outputs are upto100 time the max. output of CW lasers

Complications Non – selective tissue injuries ( scar) as heat spreads from chromophore

Lower risk of thermal injury to surrounding non – targeted structures

Examples CO2 LASERARGON LASER

KTP laser

Q- switched (QS) LASER•In order to store many atoms in an upper level, the flow to a lower level must first be limited•Thus, stimulated emission must be prevented by placing an attenuator in the cavity to stop light from travelling back and forth

•When the pumping system supplies more atoms per second than lose energy by spontaneous emission, the population in the upper level can become very large

•Stimulated emission becomes very probable and the laser is suddenly triggered

•The flow due to stimulated emission is much greater than the other flows (filling by pumping and emptying by spontaneous emission): all the atoms stored in the upper level fall sharply, emitting stimulated photons (starting with the spontaneous emission trapped in the cavity)

The different types of laser can be classified according to the nature of the amplifying medium: gas, liquid(dye) or solid state

Gas lasers

•Same pump source: electricity

•The gaseous species enter the excited state either directly, by collision with electrons, or indirectly, by collision with other gases, themselves electrically excited

•Gas lasers cover the whole optical spectrum, from the ultraviolet to the far infrared

Dye lasers

•Dye lasers use organic materials that generally emit in the visible spectrum and are thus coloured

•The pump source of dye lasers is optical: either an arc lamp or, in the majority of cases, another laser (gas or solid state)

•Dye lasers are the only ones to cover the visible spectrum entirely

Solid state lasers

•Solid state lasers are either semiconductor (or diode) lasers pumped electrically or those with a crystalline or glass matrix pumped optically

•Also have range in visible and ultraviolet spectrum

Excimerlasers

• Name is derived from the terms excited and dimers

• Use reactive gases, chlorine and fluorine mied with inert gases such as argon, krypton,, or xenon

• When electrically stimulated, a psedo – molecule (dimer) is produced

• Hen lased, the dimerproduces light in the UV range

Non – Laser light sources

Intense pulse light

Non – coherent light within 500 to 1200 nm

Chromophores (absorbing molecules)

Mechanism

• Chromophores exhibit characteristic bands of absorption at certain wavelengths • When absorption occurs, the photon surrenders its energy to a chromophore.• Once absorbed, the photon ceases to exist and the chromophore becomes excited• It is this fact that allows for the delineation of specific targets for laser activity

• Three primary skin chromophores are – Water– Hemoglobin– Melanin

• Melanin absorbs broadly across the visible and ultraviolet (UV) spectrum• Oxyhemoglobin and reduced hemoglobin in blood exhibit strong bands in the UV, blue,

green and yellow regions• Water has strong absorption in the infrared (IR) region

“Effect Of Laser On Skin Tissue”Skin optics

Fresnel reflectance : 4–7% of light is typically reflected because of the difference in the refractive index between air (n = 0) and stratumcorneum (n = 1.45)

Penetration of lasers

• In pigmented epidermis, melanin absorption - optical spectrum (200–1000 nm)

• In the dermis, there is strong, wavelength-dependent scattering by collagen fibres

• In general, between 280 and 1300 nm, the depth of penetration increases with wavelength

• Above 1300 nm, penetration decreases due to the absorption of light by water

• The most deeply penetrating wavelengths are 650–1200 nm, while the least penetrating wavelengths are within the far-UV and far-IR regions

Penetration of laser depends upon 1. Absorption and scattering2. Dept of penetration increases

with wavelength3. Amount of scattering is

inversely proportional to wavelength

The depth of optical penetration for CO2 lasers is only ~20 microns, butFRACTIONAL CO2 LASERS can vaporize nearly full-thickness microchannels through the dermis

Penetration of lasers

• Ablative laser treatments work mainly on the epidermis(surface skin cells)

• Non-ablative treatments work solely on dermal collagen (mid-layer of skin) only

• Fractional laser treatment works at both the epidermal and dermal layers of the skin

THERMAL INTERACTIONS

Photocoagulation• A Laser heating of tissues above

50 oC but below 100oC induces disordering of proteins and other bio-molecules

Photo-vaporization • With very high power densities,

instead of cooking, lasers will quickly heat the tissues to above 100o C , water within the tissues boils and evaporates. Since 70% of the body tissue is water, the boiling change the tissue into a gas

Thermal relaxation time (TRT)

• Thermal relaxation time (TRT) is defined, for a given tissue structure, as the time required for the heated tissue to cool halfway towards its initial temperature

Most of the absorbed energy is converted to heat

Diffuse into the surrounding tissue

Heat diffusion via conduction is also called thermal relaxation

The key to clean ablation of tissue is to ablate it quickly, before much heat is conducted into surrounding tissue

Photochemical ablation

• When using high power lasers of ultraviolet wavelength, some chemical bonds can be broken without causing local heating; this process is called photo-chemical ablation

• The photo-chemical ablation results in clean-cut incision

• The thermal component is relatively small and the zone of thermal interaction is limited in the incision wall

Selective photothermolysis

• First arose to guide the design of a laser for treating pediatric port-wine stains

• Selective, localized heating (with focal destruction of “target” structures) is achieved by a combination of selective light absorption and a pulse duration shorter than or approximately equal to the TRT of the targets

• The optimal pulse duration for selective photothermolysisis approximately equal to the TRT

Skin cooling

• Epidermal melanin damage most undesirable side effect of LASER treatment

• All cooling methods extract heat at the skin surface via a cooling agent– Spray cooling, a cold liquid - liquid fluorocarbon is

sprayed on the skin

– Solid contact cooling - cold sapphire window held against the skin, through which laser or IPL energy is delivered

– Cold air or cold gels - passive skin cooling

Skin cooling

• Three basic types of skin cooling1. Pre-cooling

– Pulsed dye and all Q-switched lasers (pulse durations shorter than ~5 milliseconds )

– Cryogen spray

2. Parallel cooling– For pulses longer than ~5 milliseconds– Solid contact cooling with a cold sapphire window pressed

against the skin just before and during

3. Post-cooling– Bulk skin cooling before and after laser treatments (e.g. with ice

or cold air) is useful for minimizing pain, erythema and edema

Applications of laser principles

• Ablative (Vaporizing) Skin Resurfacing

• Treatment of Vascular Lesions

• Interactions During Treatment of Pigmented Lesions and Tattoos

• Interactions During Hair Removal

• Interactions During Non-ablative Skin Rejuvenation

• Fractional Photothermolysis

• Laser-Based Diagnostics

Ablative (Vaporizing) Skin Resurfacing

• The ablative lasers are far-IR – CO2 (10 600 nm) – Erbium:yttrium aluminum garnet (Er:YAG; 2940 nm)

• Chromophore - water• Pulsed and/or scanned focused beams are used to precisely vaporize

superficial tissue, causing a “plume” of material leaving the skin. • Most of the heat is removed during vaporization, but a thin layer of

residual thermal damage remains that is useful for hemostasis. • Laser resurfacing is very useful for treating

– Photoaging– Scars– Epidermal nevi – Seborrheic keratoses

• Resurfacing removes the old epidermis and stimulates contraction and remodeling of the dermis for many months after treatment

• Complications - Scarring, transient hyperpigmentation, delayed-onset permanent hypopigmentation, prolonged erythema, and bacterial, viral and fungal infections have been reported after laser resurfacing

Treatment of Vascular Lesions

• A peak of absorption by oxyhemoglobin occurs at 577 nm within the yellow spectrum

1. Flashlamp-pumped pulsed dye lasers (PDLs) (585–600 nm) with pulse durations ranging from 0.45 to 40 milliseconds

2. Pulsed neodymium:YAG (Nd:YAG) lasers (1064 nm)

3. Alexandrite lasers (755 nm, ~3 millisecond pulse duration)– Hypertrophic lesions– PDL-resistant vascular lesions– This difference may be due to preferential absorption by

oxyhemoglobin at 1064 nm, in contrast to preferential absorption by deoxyhemoglobin at 755 nm

Cont…

4. Copper vapour or copper bromide lasers emit either green light at 511 nm or yellow light at 578 nm

5. Potassium titanyl phosphate (KTP) lasers, at a wavelength of 532 nm

• Complications – Purpura is the result of microvascular hemorrhage, – Subsequent thrombosis – Delayed appearance of vasculitis

When the pulse duration exceeds -20 milliseconds, there is little or no immediate purpura because cavitation and vessel rupture are avoided

Laser used for vascular lesions

Indication laser

Port – wine stain PDLIPL

Hemangiomas PDL

Telangiectasia Green light lasers ( 532nm) PDLDiode laserLong – plse Nd:YAG ( 1064nm)

Pyogenic granuloma PDLCO2 laser, combined continuous – wave/pulse

Angiofibromas PDLCO2

PROLIFERATING HEMANGIOMAS

Interactions During Treatment of Pigmented Lesions and Tattoos

• Melanin has a broad absorption spectrum which lends itself to possible targeting by a wide variety of lasers

• Choice of treatment wavelength is based on – Avoiding absorption by other chromophores– Matching depth of penetration to the depth of the lesion

• Red and near-IR lasers are the most selective

• Mechanism – Treatment of tattoos with Q-switched lasers fragments the ink particles and

selectively kills pigment-containing cells, with – Resultant ink particle release– Subsequent removal of tattoo ink particles can occur via an epidermal crust

and/or lymphatic transport, and some particles are re-phagocytosed by dermal cells

Laser treatment for tattoo pigment

Laser type Wavelength ( nm) Tattoo pigment colour

Pigmented PDL 510 Orange, yellow, purple

QS Nd:YAG, frequency doubled

532 Red, orange, yellow

QS ruby 694 Red, blue-blackOccasionally green and brown

QS alexandrite 755 Blue, black and green

QS Nd: YAG 1064 Blue-black

TATTOOS

• LASER commonly are used1. Q-switched ruby ( 694 nm)2. Alexandrite lasers (775 nm)3. Q-switched Nd:YAG laser emits at 1064 nmwith typical pulse durations

of 10 nanoseconds

Other conditions • Lentigines, • Nevus of Ota• Café-au-lait macules• Melanocytic nevi

• Side effects: pigment & textural changes, allergic reactions, ink darkening, tissue aerosolization with possible infectious particles.

Interactions During Hair Removal

• Red to near-IR region of the spectrum (i.e. deeply penetrating)

• High-energy,

• Millisecond-domain pulses of wavelengths

– Ruby Lasers ( 694nm)

– Alexandrite Lasers ( 755nm)

– Diode Lasers ( 800 – 810 nm)

– Nd:YAG Lasers ( 1064 nm)

Mechanism • Damage to follicular stem cells in the bulge region of the

outer root sheath and/or the dermal papilla at the base of the hair follicle

Temporary hair loss – Induction of catagen, which can occur at very low fluences; – An 810 nm diode laser source intended for home use

Permanent loss of terminal hair – Miniaturization to produce vellus-like hairs– Complete degeneration with local fibrosis

Unwanted stimulation of a vellus-to-terminal hair transition canoccur after laser or IPL treatments, particularly on the face

Interactions During Non-ablativeSkin Rejuvenation

• Non-ablative facial rejuvenation • Fine lines • Non-dynamic rhytides• Mid-IR lasers

– 1320 nm Nd:YAG– 1450 nm diode – 1540 nm erbium:glass lasers– IPL sources

Mechanism• These work by subtle thermal effects on the dermis, presumably

stimulating a wound healing response

• Responses to non-ablative rejuvenation are usually gradual and subtle

TREATMENT OF RHYTIDES

Fractional Photothermolysis

• Thousands of nearly invisible, microscopic zones of thermal injury are created

• Stimulating turnover

• Remodelling of both the epidermis and dermis

• Non-ablative FP uses focused mid-IR laser microbeams to create a pixilated pattern of small columns of thermal damage, called microthermal treatment zones (MTZs)

• Ablative FP uses carbon dioxide or erbium (Er:YAG) lasers to vaporize a similar pattern of small vertical channels that can extend deeply into the skin

Uses • Scars,• Fine rhytides, • Telangiectasias,• Dermatoheliosis• Poikiloderma• Ablative FP may also bean effective mechanism for topical drug delivery,

by providing many channels directly into the dermis.

Laser-Based Diagnostics

1.Optical coherence tomography (OCT)

– Near-IR,

– low-coherence light

– Used for high-resolution crosssectional imaging of body tissues

• Doppler version has been shown to be capable of imaging blood flow before and after laser treatment of vascular lesions

2. Laser confocal microscopy• Captures light scattered or emitted from a thin plane “section”

inside skin

• In vivo with histology-like resolution down to a depth of approximately 0.3 mm

• Because histochemical stains are not used, confocal microscopy of skin tumors reveals diagnostic features that are different from those of conventional histology

• Microvascular blood flow and trafficking of lymphocytes can be observed

• In a preliminary study, the sensitivity and specificity of in vivo confocal microscopy for differentiating melanocytic nevi and seborrheic keratoses from cutaneous melanoma and basal cell carcinoma were 94% and 98%, respectively

Laser hazards and protections

Hazards to the eye

The cornea and lens•Cornea is accessible to danger of UV and most of IR lasers,•UV-A, UV-B (between 295nm and 320 nm) and IR-A (between 1 to 2 mm) are dangerous for lens,•308-nm (UV-B) excimer XeCl laser is particular dangerous because of it can simultaneously damage the lens, the cornea and the retina.

Hazards to the eye

The retinaThe directionality of a laser beam permits the ray to be focused to an extremely small spot on the retina. A collimated laser will be concentrated by a factor of 100,000 when passing from cornea to retina.

Visible or near IR lasers (400 nm to 1400nm) are particularly dangerous to the retina and always requires eye-protection when working with these kind of lasers.

Protection to the eye

Eye protectionEyewear (goggles) is the most common laser protective measure, especially for open laser beams. It should be good design with all around shielding and adequate visible light transmission.

Identification of the eyewear : All laser protective eyewear shall be clearly labelled with information adequate to ensure the proper choice of eyewear with particular lasers.

Fire hazards

• FIRE HAZARDS

• Drapes, clothing, dry hair and plastic materials, including endotrachealtubes, can be ignited, especially when oxygen is in use

• Greatest risk is with the CO2 and erbium:YAG lasers used for skin resurfacing and ablative fractional treatments

• PREVENTION OF FIRES

• 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

Cutaneous burns

• POSSIBLE 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 OF CUTANEOUS BURNS

• Knowledge and careful observation of specific clinical endpoint responses for a particular laser–lesion combination

“Plume” materials

• INHALATION OF LASER-GENERATED “PLUME” MATERIALS

• Particularly with resurfacing or vaporization of hair during its removal; latter can release irritating sulfur and other oxides

• PREVENTION OF LASER “PLUME” BIOHAZARDS

• Smoke evacuator and good ventilation are most effective measures

• Sub-micrometer surgical filter masks provide some protection when worn properly

• Q-switched lasers capture particulate material in plastic cones

Conclsion

• Lasers and IPL should just be considered as “drugs”, i.e. something that can be manipulated to ease the burden of those suffering from skin diseases

• Total internal reflection of spontaneous emission of radiation (TRASER) devices, which are neither lasers nor IPL, are more efficient than lasers and are tunable

REFERENCES

1) FITZPATRICK’S DERMATOLOGY IN

GENERAL MEDICINE

2) ROOK’S TEXTBOOK OF DERMTOLOGY

3) IADVL TEXTBOOK & ATLAS OF

DERMATOLOGY BY R.G. & AMEET

VALIA

4) TEXT BOOK OF DERMATOLOGY BY BOLOGNIA 3ed

THANK YOU