BASICS IN LASER

Dr P shilpa

Objectives

• What is Laser ?• LASER history..• LASER Properties.• How LASER is produced ?• Effects of laser.• Application of LASERs in

Ophthalmology.• LASER Safety.

What is Laser?

LASER is an acronym for:

L : Light

A : Amplification (by)

S : Stimulated

E : Emission (of)

R : Radiation

Term coined by Gordon Gould.

Lase means to absorb energy in one form and to emit a new form of light energy which is more useful.

LASER history

• 1917 -Sir Albert Einstein created the foundations for the laser.

• 1958 - C.H. Townes, A.L. Schawlow: Theoretical basis for lasers.

1960 - Theodore Maiman : Built first laser by using a ruby crystal medium .

• 1963 - C. Zweng: First medical laser trial (retinal coagulation).

• 1965 - W.Z. Yarn: First clinical laser surgery.

• 1970- The excimer laser was invented in  by Nikolai Basov

• 1971 -Neodymium yttrium

aluminum garnet

(Nd.YAG) and

Krypton laser developed.

Lasers have many important applications.

• Common consumer devices such as DVD players, laser printers, and barcode scanners.

•  laser surgery and various skin treatments, • Industry - cutting and welding materials.

• Military and law enforcement devices marking targets and measuring range and speed.• Laser lighting displays 

Laser physics

• Laser light differs from “ordinary” light in several important ways.

• These differences are a direct result of the manner in which laser light is generated

  Fig. 1. “White” light is composed of all wavelengths, traveling in all directions.

Fig. 2. Ordinary monochromatic light consists of light of approximately the same wavelength. However, the light is not traveling in one direction.

PROPERTIES OF LASER LIGHT

• Monochromatic (emit only one wave length)

• Coherence (all in step with one another-

improve

• focusing )

• Polarized (in one plane-easy to pass through

media)

• Collimated (in one direction & non spreading )

• High energy (Intensity measured by Watt J/s)

LASER Vs. LIGHT

LASER LIGHT

Simulated emission Monochromatic. Highly energized Parallelism Coherence Can be sharply

focussed.

Spontaneous emission.

Polychromatic. Poorly energized. Highly divergence Not coherent Can not be sharply

focussed.

How LASER is produced ?

Light is a form of energy at which the human eye is sensitive

LASER PHYSICS

• Light as electromagnetic waves, emitting radiant

energy in tiny package called ‘quanta’/photon.

• Each photon has a characteristic frequency and its energy is proportional to its frequency.

• When light is passed through certain kinds of materials, the photons may excite electrons around certain atoms into the next higher energy level

3 Mechanisms of Light Emission

1. Absorption2. Spontaneous Emission3. Stimulated Emission

Therefore 3 process of light emission:

Absorption

. A photon of the “right” energy can be absorbed and “bump” an electron into a higher energy level.

E2

Spontaneous Emission

An excited electron falls back to its lower energy level, releasing a photon in a random direction.

Stimulated EmissionA photon strikes an excited electron. The electron falls to its lower energy level, releasing a photon that is going in the same direction as, and is in exact phase with, the original photon. Note that whereas only one photon strikes the atom, two photons leave it—the original photon plus the emitted photon.

Background Physics

• Stimulated emission is the basis of the laser action. • The two photons that have been produced can then

generate more photons, and the 4 generated can generate 16 etc… etc… which could result in a cascade of intense monochromatic radiation.

• The natural state of matter -most electrons - lowest energy levels.

• One requirement - most “elevatable” electrons

must be at their higher energy level before the light enters the medium.

• Such a situation is called a population inversion.

• To create a population inversion-energy must be supplied to the medium

• External flashes of light (surrounding the solid crystal with a helical flash tube)

• Electric current passing through the gas

THREE BASIC COMPONENTS

• A Laser Medium e.g. Solid, Liquid or Gas

• Exciting Methods

for exciting atoms or molecules in the medium e.g. Light, Electricity

• Optical Cavity (Laser Tube)

around the medium which act as a resonator

The Resonator-

• The energy of the emitted laser beam - increased still further by causing the light beam to traverse the material multiple times.

• This is accomplished by placing a mirror over each end of the crystal or gas tube so that the distance between them is an even multiple of the laser light’s wavelength.

• The coherent light beam is reflected back and forth becoming more and more intense.

If the “exciting” energy is supplied in brief pulses, laser output of higher energy levels (in pulses) can be

obtained.

LASER INSTRUMENTATION

Three Main Components –

• Console: laser medium and tube, power supply and laser control system.

• Control Panel: dials or push buttons for controlling various parameters & standby switch as a safety measure.

• Delivery System

Delivery systems

• Transpupillary: - Slit lamp - Laser Indirect Ophthalmoscopy

• Trans scleral : - Contact - Non contact

• Endophotocoagulation.

Slit lamp biomicroscopic laser delivery

• Most commonly employed mode for anterior and posterior segment.

• ADVANTAGES:• Binocular and stereoscopic view.• Fixed distance. • Standardization of spot size is more accurate.• Aiming accuracy is good.

Laser indirect ophthalmoscope.• Advantages :• Wider field(ability to reach periphery).• Better visualization and laser application in hazy

medium.• Ability to treat in supine position.• Disadvantage : difficulty in focusing.• Difficulty to standardize spot size.• Expensive.• Un co-operative patient.• Learning curve.

MODES OF LASER OPERATION

• Continuous Wave (CW) Laser: deliver - energy in a continuous stream of photons.

• Pulsed Lasers: Produce energy pulses of a few tens of micro to few mili second.

• Q Switches Lasers: Deliver energy pulses of extremely short duration (nano second).

• A Mode-locked Lasers: Emits a train of short duration pulses (picoseconds).

• Fundamental System: Optical condition in which only one type of wave is oscillating in the laser cavity.

• Multimode system: Large number of waves, each in a slight different direction ,oscillate in laser cavity.

CLASSIFICATION OF LASER

• Solid StateRubyNd.YagErbium.YAG

• GasIonArgonKryptonHe-NeonCO2

• Metal VapourCuGold

DyeRhodamine

Excimer Argon FluorideKrypton FluorideKrypton Chloride

DiodeGallium-Aluminum Arsenide (GaAlAs)

LASER TISSUE INTERACTION

:TISSUE VARIABLE: Transparency

Pigmentation

Water Content

LASER VARIABLE

TYPES OF OPHTHALMIC LASERS

THREE TYPE OF OCULAR PIGMENT

Effective retinal photocoagulation depends – • how well light penetrates the ocular media • how well the light is absorbed by pigment in the target tissue

• Haemoglobin: Argon Green are absorbed - useful to coagulate the blood

vessels.

• Xanthophyll: Present in inner and outer plexiform layers of macula. Maximum absorption is blue. Argon blue is not recommended

to treat macular lesions.

• Melanin: RPE, Choroid Argon Blue, Krypton Pan Retinal Photocoagulation, and Destruction of RPE

ELECTROMAGNETIC SPECTRUM

Light - tissue interactions

light

Thermal effectsPhotochemical effects Ionizing effects

Photoradiation eg. Dye laser

Photoablationeg. Excimer laser

PhotocoagulationArgon, krypton,dye,Nd:YAG

PhotovapourizationCO2 laser

Photodisruptioneg. Nd:YAG laser

Thermal Effects

(1) Photocoagulation:

Laser Light

Target Tissue

Generate Heat -Tissue temp.:increases from 37 deg c to 50 deg. C or higher

Denatures Proteins

(Coagulation) Risein temperature of about 10 to 20 0C will cause

coagulation of tissue.

Panretinal photocoagulation (PRP) ctd

1. Proliferative diabetic retinopathy with high risk characteristics

2. Neovascularisation of iris 3. Severe non proliferative diabetic retinopathy

associated with-poor compliance for follow up-before cataract surgery-renal failure-one eyed patient and-pregnancy

4. central retinal vein occlusion, branch retinal vein occlusion,

5. sickle retinopathy, 6. Eales disease and IRVAN (idiopathic retinal vasculitis,

aneurysms, and neuroretinitis )

Indications

Thermal Effects(2)PhotovaporizationLaserLight

TargetTissue increasetemp of cellular and extacellular

water to 100 degree

steam and (vaporization)

Heat Cell disintegration Cauterization Incision eg..Femtosecond laser

• The carbon dioxide laser used photovaporization and photocautery.

• Advantage - bloodless incision by sealing blood vessels and lymphatic tissue in its path.

• Intravitreal photocautery -used

• Mainly used in Laser iridotomy.• Complication- Bruch’s membrane rupture → CNV.

FEMTOSECOND LASER

Mode-locking -pulses of light of extremely short duration, on the order of picoseconds (10−12s) or femtoseconds (10−15s).

Indications

1. Clear Corneal Incisions in LASIK it replaces a mechanical device (

microkeratome) to create a precise corneal flap, also in cataract surgery to create the incision

2.  Capsulotomy3.  Phacofragmentation

Photochemical effcts

Photoablation:• Breaks the chemical bonds that hold tissue

together essentially vaporizing the tissue, e.g. Photorefractive Keratectomy, Argon Fluoride (ArF) Excimer Laser.

Contd. …

Is a form of ultraviolet laser

• Used in delicate surgeries  such as eye surgery  eg ;LASIK

PHOTOCHEMICAL EFFECT

Photoradiation OR Photodynamic therapy (PDT)

• 42°C and 52°C temp • Photoradiation by red (630 nm)- cytotoxic free radicals in a tumor previously

sensitized by a hematoporphyrin-derived uptake.• The most common - PDT. • This new treatment modality combines low-power diode lasers (689 nm) with

an infusion of verteporfin to ablate subretinal neovascular membranes and treatment of various tumors.

• The advantage -over conventional photocoagulation -lower energy levels result in much less damage to adjacent normal tissue

E.G. Treatment of Ocular tumours and CNV

Photodisruption: Extremely high irradiance delivered - very short exposure - small spot

electrons - energized from molecules - target tissue (microscopically localized temp. rise from 37 deg. c to 1500degc)

collection of free electrons and ions – plasma

Rapid expansion of the plasma creates acoustic and shock waves

combines with latent tissue stress, incise the target tissue

Tissue damage

Contd. …

Nd:YAG laser• (neodymium-doped yttrium aluminum garnet) is a crystal that

is used as a lasing medium for solid-state lasers.• Nd:YAG lasers typically emit light with a wavelength of 1064nm

, in the infrared.

Applications

• Correct posterior capsular opacification• Peripheral  iridotomy in patients with 

acute angle-closure glaucoma.• Frequency-doubled Nd:YAG lasers (wavelength 532 nm) are

used for pan-retinal photocoagulation in patients with diabetic retinopathy.

[-

The most common laser source used is the Nd:YAG 1064-nm.

Methods of pulsing and Nd:YAG laser:-1)Q- switching2)mode- locking

LASER IN ANTERIOR SEGMENT

CORNEA:

Laser in Keratorefractive Surgery:• Photo Refractive Keratectomy (PRK)• Laser in situ Keratomileusis (LASIK)• Laser Subepithelial Keratectomy (LASEK)• Epi Lasik

Laser Thermal Keratoplasty

Corneal Neovascularization

Retrocorneal Pigmented Plaques

Laser Asepsis

PRK LASIK

LASER IN GLAUCOMA

Laser Iridotomy, Laser Iredectomy

Laser Trabeculoplasty (LT)

Selective Laser Trabeculoplasty

ARGON LASER TRABECULOPLASTY

Mechanism of action:Mechanical. Biological.

SCLEROSTOMY

LASER IN LENS

• Posterior capsulotomy(YAG)

• Laser phacoemulcification

• Phacoablation

LASER IN VITEROUS• Viterous membranes

• Viterous traction bands

Posterior capsulotomy(YAG)

LASER TREATMENT OF FUNDUS DISORDERS

Diabetic Retinopathy

Retinal Vascular Diseases

Choroidal Neovascularization (CNV)

Clinical Significant Macular Edema (CSME)

Central Serous Retinopathy (CSR)

Retinal Break/Detachment

Tumour

Retinoblstoma before treatment

Retinoblastoma after thermotherapy

DIAGNOSTIC USE OF LASERS

• Scanning Laser Ophthalmoscopy • allows for high-resolution, real-time motion images of

the macula without patient discomfort.

• SLO angiography: to study retinal and choroidal blood flow.

• May be used to perform microperimetry, an extremely accurate mapping of the macula’s visual field.

LASER SAFETY

• Class-I : Causing no biological damage.

• Class-II : Safe on momentary viewing but chronic exposure may cause damage.

• Class-III : Not safe even in momentary view.

• Class-IV : Cause more hazardous than Class-III.

LASER SAFETY REGULATION:

• Patient safety is ensured by correct positioning.

• Danger to the surgeon is avoided by safety filter system.

• Safety of observers and assistants.

Complications • General complications:• Pain • Seizures.

• Anterior segment complications:

Elevated IOP. Corneal damage. Iris burns. Crystalline lens burns. IOL and PC damage. Internal opthalmoplegia.

Complications(contd…)

• Choroidal detachment and exudative RD.

• Choroidal ,subretinal and vitreous

hemorrhage.

• Thermal induced retinal vascular damage.

• Preretinal membranes.

Complications(contd..)

• Ischaemic papillitis.

• Paracentral visual field loss and scotoma.

• Photocoagulation scar enlargement.

• Subretinal fibrosis.

• Iatrogenic choroidal neovascularisation.

• Accidental foveal burns.

PREVENTION OF LASER HAZARDS

Engineering Control Measure: laser housing

filters and shutter for safe observer viewing

Personal protective devices, like protective eye wear or goggles with side shields, protective clothes may be included.

New Developments

Pattern Scan Laser:(pascal)

PASCAL

PATTERN SCAN LASER:(Pascal)

Offering multiple, patterned burns in a single-

session procedure.

Improved precision

Safety

Patient comfort

Significant reduction in treatment time.

CONCLUSIONLight Amplification by the Stimulated Emission of Radiation).PROPERTIES Simulated emission Monochromatic. Highly energized Parallelism Coherence Can be sharply focussed

MECHANISM:1 Absorption

2 Spontaneous Emission 3 Stimulated Emission• THREE BASIC COMPONENTS • A Laser Medium • Exciting Methods • Optical Cavity (Laser Tube)

Light - tissue interactions

light

Thermal effectsPhotochemical effects Ionizing effects

Photoradiation eg. Dye laser

Photoablationeg. Excimer laser

PhotocoagulationArgon, krypton,dye,Nd:YAG

PhotovapourizationCO2 laser

Photodisruptioneg. Nd:YAG laser

REFERENCES

• YANOFF AND DUKER OPHTHALMOLOGY- 3rd edition.

• LASERS IN OPTHALMOLOGY• A practical guide-AIIMS.

• LASER SURGERY OF THE POSTERIOR SEGMENT- Steven M. Bloom

Thank you