Nw2014 laser fundamenal01

Fundamental laser ophthalmology5 November 2012Mallawee Charatcharungkiat, MDInstructor Nawat Watanachai, MD

Outline• Principle

– Laser physics– Properties of laser light

• Monochromaticity• Directionality• Coherence

– Laser interferometer– OCT

• Polarization• Intensity

– Laser system– Laser interactions and its

clinical use

– Photocoagulation• Basic• Choice of wavelength• Photocoagulation mechanism• Practical aspect• Clinical use

– TSCPC– ECP– LTP– LPI– Laser suture lysis

– Photodisruption• Nd:YAG capsulotomy• Femtosecond laser

– Photochemical• Photodynamic therapy

– Photoablation• Excimer laser







clinical use






Laser physics• “Light amplification by stimulated emission of

radiation”

Laser physics• “Light amplification by stimulated emission of

radiation”

• 1917

• Atomic transposition process– Absorption– Spontaneous emission– Stimulated emission

Albert Einstein

Laser Physics• Absorption

Unexcited atom(E0 : Ground state)

Photon

Excited atom (E1 : Excited state)

Laser Physics• Spontaneous emission


Photon


Laser Physics• Stimulated emission


2 Photon


Laser Physics

Laser Physics

• After absorption– Spontaneous emission

• Majority• Incoherent

– Stimulated emission• Few• Coherent Laser

Laser

Laser Physics• Element of laser

1. Active medium2. Energy input3. Optical feedback


1. Active medium• Allowed stimulated emission• Particular atomic energy transition

Wavelength of the emission

E = hv = hc /λ


1. Active medium• Gas : Argon, Krypton, Carbon dioxide,

Argon-fluoride excimer, Helium with neon

• Liquid : Dye• Solid : Supported by crystal

Nd:YAG, Er-YLF, Ruby Infrared holmium-YLF (IntraLase)

holmium-YAG(Laser thermal keratoplasty)

Semiconductor (diode)


2. Energy input• Make majority of atom are in higher energy state than

ground state• “Population inversion”• “Pumping”• “Light amplification”


2. Energy input• “Pumping”

– Gas laser : Electrical discharge between electrode in gas

– Dye laser : Other laser– Solid crystal : Incoherent light

(Xenon arc flash light)


3. Optical feedback• Promote stimulated emission, suppress spontaneous

emission• Spontaneous emission not amplify• Stimulated emission Coherent laser







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Properties of laser1. Monochromaticity

– Laser emit light only 1 wavelength / combination several wavelength

– Gas laser 0.01 nm– Color of light enhance target tissue absorption

or transmission– Not effected by chromatic aberration

in lens system– Focus in smaller spot > white light

Properties of laserChromatic aberration

– Distortion, a failure of lens to focus all colors to the same convergence point

– Lens have different refractive index for different wavelength (↓ refractive index - ↑ wavelength)

Properties of laser1. Monochromaticity

– Laser emit light only 1 wavelength / combination several wavelength

– Gas laser 0.01 nm– Color of light enhance target tissue absorption

or transmission– Not effected by chromatic aberration

in lens system– Focus in smaller spot > white light

Properties of laser2. Directionality

– Laser emit a narrow beam– Laser amplify only photon that travel along

narrow path between 2 mirrors – Cause collimating light– ↑ 1 mm in diameter of beam for every meter

travel– Focus light to small spot

Properties of laser3. Coherence

– Ability of 2 light beams, or different parts of the same beam, to produce interference

– Interference


– “Laser speckle” -- rough surface– Laser interferometer

Laser beam split to 2 beams

Diffuse by cataract

Overlap in retina “Interference fringes”• Non contact biometry (IOL master)

– Partial coherence interferometry


• Optical coherence tomography (OCT)– Michelson interferometer

– Interference property of temporally coherent light

– Light source (superluminescent diode), light detector, beam splitter, movable mirror

– Highest reflection : RPE, ONL, INL, ILM


• OCT

½ light

½ light


• OCT

Properties of laser4. Polarization

– Certain direction of light wave– Linearly polarized

• Electric fields of light wave in the same plane• Allow maximum transmission through laser medium

without loss caused by reflection

Properties of laser5. Intensity

– Power in a beam of given angular size– Most important property– “Brightness”

• Intensity / unit area


– Radiometric terminology

Term Unit

Energy joule (1 J = 1 watt x 1 sec.)

Power watt

Energy density J/cm²

Irradiance watt / cm²

Intensity watt / sr(sr = Steradian, unit of solid angle)

brightness Watt / sr cm²


– Radiometric terminology

Term Unit

Energy joule (1 J = 1 watt x 1 sec.)

Power watt

Energy density J/cm²

Irradiance watt / cm²

Intensity watt / sr(sr = Steradian, unit of solid angle)

brightness Watt / sr cm²

Laser output


– Tissue effect• Determined by focal point spot size• Energy density , irradiance

– Spot size• 50 µm spot size = ¶ (25 x 10 -4) ² cm²

= 2 x 10 -5 cm²


– Continuous laser• Argon, Krypton watts

– Pulsed laser• Nd:YAG joules• Average, peak power

Properties of laser

Sun has power 10 26 watts, but emits in all direction

Helium neon laser 1 mW has 100 x radiance of sun







clinical use






Laser system• Types of medical laser


– CO2 (λ = 9.2-10.8 µm)• Gas laser• Nonophthalmic surgery : Gynecology, ENT• Absorbed by water

– Er:YAG (λ = 2.94 µm)• Solid laser• Strongest absorption by water shallow penetration

depth low vaporization, small collateral damage


– Nd:YAG (λ = 1064 µm)• Important ophthalmic laser• Low absorption and scatting deep penetration

– Argon/Krypton (λ = 488,515,568,647 nm)• Gas laser• Retinal photocoagulation in the past

– Excimer laser (λ = 157,193,248,308,351 nm)• ArF (193 nm) Strong absorption in protein

submicrometer penetration in tissue • Refractive surgery (Corneal ablation)

Laser system• Contact lens

– Aberration• Focal spot size of laser beam is limited by diffraction

and aberration• ↑ central aberration ↑ focal spot size• Photocoagulation Flat contact lens to control

aberration– Magnify spot size on retina– Increase view of field







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clinical use






Laser-tissue interaction

1960, Theodore Maiman built the first successful laser with ruby crystal medium

1927-2007

Laser-tissue interaction1. Photocoagulation

Target tissue absorb light energy

Convert to thermal energy

Tissue Temperature > 65 °C

Tissue protein denaturation and coagulative necrosis


– Laser type• Green, red, yellow, infrared

– Approach• Transpupillary with slit lamp• Indirect ophthalmoscope• Endophotocoagulation with vitrectomy surgery• Transscleral application with contact probe


– Pigment in Ocular tissue absorption

• Melanin : green, yellow, red, infrared

• Macular xanthophyll : Blue > yellow, red

• Hemoglobin : blue, green, yellow > red







clinical use







– Choice of laser wavelength• Green laser

– Well absorb by melanin, hemoglobin, less absorb by xanthophyll

– Retinal vascular abnormality, CNV– Argon green laser (514 nm)

» Popular wavelength for retina photocoagulation– Frequency-doubled Nd:YAG laser (532 nm)

» Continuous , pulsed output» Its absorption and clinical use similar to Dye yellow but

more reliable due to solid medium


– Choice of laser wavelength• Red laser

– Good penetrate through cataract, VH than other laser– Less absorb by xanthophyll– CNV near fovea– Deep burn discomfort– Krypton red laser (647 nm)

» Well absorb only melanin deeper outer retinal and choroidal burn

» Less absorb by hemoglobin good for VH, CNV overlying thin subretinal Hge

bad for retinal vascular abnormality


– Choice of laser wavelength• Infrared laser

– Similar to red laser– Deeper penetrate through tissue– Semiconductor diode laser (805-810 nm)

» Near infrared spectrum» Well absorb by melanin only deeper outer retinal and choroidal burn» Similar property to krypton red laser but less discomfort

due to near invisible laser (no sensation of flashing)» Treatment of choice for ROP» Transscleral contact retinal photocoagulation

Penetrate sclera and silicone scleral exoplant


– Choice of laser wavelength• Yellow laser

– Penetrate through cataract– Less absorb by xanthophyll– Destroy vascular structure with little damage to adjacent

tissue– Dye yellow laser (560-580 nm)

» Well absorb by Hemoglobin, melanin» Safe for macular photocoagulation» Less useful in VH, preretinal hemorrhage, CNV overlying

with subretinal hemorrhage







clinical use







– Photocoagulation mechanism• Macular edema

– Direct closure of leaking vascular» Microaneurysm laser induced endovascular

thrombosis heat induce vessel wall contraction

– Grid » Multifactorial and unclear» RPE damage retinal capillary and venule endothelial

proliferation restore inner blood-retinal barrier» Decrease total surface area of leaking retinal vessels


– Photocoagulation mechanism• Scatter photocoagulation for NV

– Destruction of oxygen consuming photoreceptor ↓VEGF– Wilson et al. Gene expression

- Angiotensin II type 2 receptor (Inhibit VEGF) - Calcitonin receptor-like receptor - Interleukin-1 - Fibroblast growth factor - Plasminogen activator inhibitor II


– Photocoagulation mechanism• Central serous chorioretinopathy

– Unclear– Laser direct at site fluorescence leak

Destroy sick RPE

Healthy neighboring RPE proliferate, seal defect, reestablish outer blood-retinal barrier

– Focal obliteration of hyperpermeable choriocapillaris– Coagulum mechanically plug RPE leakage site


– Photocoagulation mechanism• Choroidal neovascularization, retinal vascular anomaly

– Laser induced endovascular thrombosis– Heat induce vessel wall contraction


– Photocoagulation mechanism• Nonvascular intraocular tumor

– Retinoblstoma» Amelanotic poorly absorb laser» Heavy, confluent laser photocoagulation Close

surround retinal vascular blood supply tumor necorsis– Choroidal malignant melanoma

» Heavy, confluent laser photocoagulation Close surround choroidal vascular blood supply

» Focal laser destruction tumor necrosis


– Photocoagulation mechanism• Retinal break

– Laser and cryotherapy adhesion and scar– Adhesive force generate within 24 hours several weeks to

its maximum strength– Laser > Cryotherapy

» Less breakdown of blood retinal barrier» ↓Risk proliferative vitreoretinopathy







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– Practical aspect1. Lens

• Lesion location, patient anatomy, desired field of view, image magnification, working distance

1) Contact lens– All purpose fundus contact lens

» Goldmann contact lens» Macular, retinal periphery» Virtual, erect image» Look away from mirror more anterior


1) Contact lens– Contact lens for macular photocoagulation

» Mainster high magnification, Volk Area Centralis, Mainster Standard lens

» Real, inverted image– Contact lens for peripheral photocoagulation

» Rodenstock Panfundoscopic, Mainster wide field, Volk Superquad 160 lens

» Real, inverted image

2) Non contact lens– Lens 60,78,90 D Spot magnification 0.92,1.15,1.39


– Practical aspect2. Laser setting

• Slit lamp magnification• Wavelength selection

– Retinal vascular lesion» Argon green, Nd:YAG green, Dye yellow

– CNV» Argon green, Nd:YAG green, Dye yellow, Dye red, Krypton

red, Diode– Scatter photocoagulation

» Argon green **» Red, Diode Cataract, VH



• Spot size– Macular photocoagulation = 50-200 µm– Peripheral photocoagulation = 200-1000 µm

– Shorter wavelength higher intraocular scattering Larger, less focused retinal burn



• Power, Duration– Burn intensity

• Burn intensity

Light Barely visible retinal blanching CSC, GridMild Faint whiteModerate Dirty white Scatter PRP, RB

Heavy Dense white Choroidal melanoma

Burn intensity α (Burn duration)(Power)

Spot size


– Practical aspect

Panretinal Photocoagulation for PDR :Pattern Scan Laser VS Argon Laser

Am J Ophthalmol 2012

Background

• Traditionally laser burns have been placed one by one in a grid pattern

100-500 µm 100-200 ms

Total spots ≥ 1500 spots

Several sessions

Background

Blumenkranz MS

PASCAL (PAttern SCAn Laser)

• A new frequency doubled 532 nm Nd:YAG laser• Delivering arrays up to 56 spots over less than 0.6 sec following a single foot step• Less painful and safer alternative to argon laser for both PRP and macular photocoagulation

Background

Pattern scan laser• Short pulse duration result in a quicker PRP procedure

• As pulse durations < 50 ms X Thermal energy Mechanical rupture

• because of transient vapor formed adjacent to melanosomes

• Without thermal energy – laser-induced damage is limited to RPE and

photoreceptors, sparing choroid and inner retina

Background

Pattern scan laser• Decrease perception of pain

– Mechanical rupture effect do not diffuse to sensory neuron-rich choroid as seen in Argon laser

• Safe– Inner retina and choroid are spared– However, as pulse duration < 50 ms

– Smaller safety margin when titrating power of the PASCAL

Power for rupture Bruch membranePower for

sub-therapeutic burn

Background

• However, they are not aware of any study that has compared clinical outcomes for the 2 lasers when treating high-risk PDR

• This retrospective comparative study evaluates efficacy between PASCAL and traditional argon laser in treating newly diagnosed high-risk PDR

Methods : Procedure

Argon laser• 514-nm (green) pulses• 1 burn width apart• Indirect headset • Slit-lamp microscope with contact

lens• spot-size magnification factor

of 2x• Pulse duration 200 ms• Spot size 200-300 µm• Power 200 mW increased by

10-20 mW until a gray/white lesion

• 2 or 3 sessions (37% completed PRP in 1 session)

PASCAL

• 1 burn width apart• Small or larger array was

determined by operator• Slit-lamp microscope with contact

lens• spot-size magnification 2x• Pulse duration 20 ms• Spot size 200 µm• Power 200 mW increased

until gray/white lesion• 2 or 3 sessions

(44% completed PRP in 1 session)

Discussion

• PASCAL has greater speed and comfort

• When both lasers were applied with similar number and size of laser spots in similar patients

– PASCAL treatment was less effective in inducing regression

and preventing recurrence of NV in high-risk PDR

• Inherent difference in properties of PASCAL and argon lasers

– Limits efficacy of PASCAL when used in context of traditional argon laser treatment parameters

Discussion

-PRP scars following treatment with argon laser are larger size than with PASCAL -Total area of PRP scars in the argon-treated patient exceeds that of the PASCAL-treated patient by an order of magnitude

Discussion

• Increasing rate of recurrence NV experienced in the PASCAL-treated patients

– Given equivalent number of treatment spots, PASCAL created smaller total burn area results in a significant decrease in efficacy compared to Argon laser

– Either additional lesions or larger spot sizes may be required to achieve comparable efficacy with PASCAL

Conclusion

• PRP with PASCAL less effective than traditional argon green laser in high-risk PDR to achieving treatment goals when using traditional argon green laser parameters

• Increasing number, spot size, or duration of laser burns may improve efficacy of PASCAL as measured by rates of regression and recurrence NV







clinical use







– Clinical use

Transscleral cyclophotocoagulation (TSCPC)• Diode laser : less pain & inflammation• Power 1.5-2 W, Duration 2 sec, 180-270 °

Endoscopic cyclophotocoagulation (ECP)• Argon laser


– Clinical use

Laser trabeculoplasty (LTP)• Argon

laser trabeculoplasty(ALT)• TM

shrink, cause stretching of adjacent area

• TM release IL-1ᵝ, TNF-α stimulate MMP ↑ Outflow

• Goniolens junction of ant. nonpigment-post. pigment TM

• P. 300-1000 mW, D 0.1 sec, Spot size 50 µm, 180°

• Selective laser trabeculoplasty (SLT)• Target

intracellular melanin less coagulative damage, TM structural change

• Frequency-doubled(532 nm) Q-switched Nd:YAG laser

• P. 0.4-1.0 mJ, D 0.3 ns, Spot size 400 µm


– Clinical use

Laser iridectomy• Angle closure• Argon laser

• Iris colour effect• P.800-1000 mW, D 0.02-0.1 sec,

Spot size 50 µm• Q-switch Nd:YAG laser

• Abraham, Wise lens• P. 2-8mJ


– Clinical use

Laser suture lysis• Hoskins lens, Ritch lens• Argon laser• P. 300-800 mW, D. 0.02-0.1 sec,

Spot size 50-100 µmPanretinal Photocoagulation


– Clinical use

Grid laser

Focal laser• RB, Lattice







clinical use






Laser-tissue interaction2. Photodisruption

High peak power pulsed laser (Nd:YAG, Er:YAG)

Ionize target tissue and increase tissue temperature to exceed vaporization threshold

Vapor bubbles

Rupture tissue within zone of bubbles


– Clinical use

Neodymium : Yttrium-Aluminum-Garnet Laser Capsulotomy (Nd:YAG)• Posterior capsule opacification• Infrared light 1064 nm • P.0.8-2.0 mJ, aim slightly

behind capsule


– Clinical use

Refractive surgery• Femtosecond laser (10 -15)• Infrared 1053 nm• Create corneal epithelial flap







clinical use






Laser-tissue interaction3. Photochemical

– Nonthermal light induce chemical reaction• Photosynthesis in plants• Phototransduction in photoreceptor• Photodynamic therapy (PDT)

Laser-tissue interaction3. Photochemical

– Photodynamic therapy (PDT)• CNV, ocular and nonocular tumor

Photosensitizer (Porphyrin derivative : Verteporfin) IV

Blood stream, retina, choroid

Accumulate in LDL receptor in NV

Photon in porphyrin absorpt Far-red peak laser(688-691nm)

-Lower sensitivity to retina-Superior choroid penetration

Toxic singlet oxygen and free radical

Endothelial cell damage, photothrombosis, closure NV







clinical use






Laser-tissue interaction4. Photoablation

– High-powered ultraviolet laser etch cornea like synthetic polymer

Excimer laser• Excited dimer (Rare

gas+Halide)• ArF (193 nm)

• Photon break covalent bond strength of cornea

• Remove submicron layer of cornea without damage adjacent tissue absence of thermal injury







clinical use






Reference• Daniel Palanker, Mark S. Blumenkranz, John J. Weiter. Chapter 21 Retinal

laser therapy : Biophysical basis and application. Retina. Forth edition. Elsevier Mosby

• Steven M. Bloom, MD, Alexander J. Brucker, MD. Laser surgery of the posterior segment. Second edition. Philadelphia: Lippincott-Raven; 1997.

• American academy of ophthalmology : section 12 Retina and vitreous, Basic and clinical science course, 2010-2011

• American academy of ophthalmology : section 10 Glaucoma, Basic and clinical science course, 2010-2011

• Antonia M. Joussen, Thomas W. Gardner, Bernd Kirchhof, Stephen J. Ryan. Retinal vascular disease. New York: Springer

• Aimee V. Chappelow, Kevin Tan, Nadia K. Waheed, Peter K. Kaiser. Panretinal photocoagulation for proliferative diabetic retinopathy : Pattern scan laser versus Argon laser. Am J Ophthalmol 2012;153:137-142

"The important thing is not to stop questioning.

Curiosity has its own reason for existing."

Albert Einstein

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Nw2014 laser fundamenal01