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InteractionMechanismsPhotodisruption

MD6305Laser‐TissueInteractionsClass11

JaeGwan Kim

jaekim@gist.ac.kr ,X2220

DepartmentofBioMedical Scienceand Engineering

Gwangju InstituteofSciencesandTechnology

Copyright.Mostfigures/tables/textsinthislecturearefromthetextbook“Laser‐TissueInteractionsbyMarkolf H.Niemz 2007”andthismaterialisonlyforthosewhotakethisclassandcannotbedistributedtoanyonewithoutthepermissionfromthelecturer.

AMapofLaser‐TissueInteractions

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OpticalBreakdown

soft tissues or fluids

OpticalBreakdown

• Athigherpulseenergies,shockwavesandothermechanicalsideeffectsbecomemoresignificant

• It’sbecausemechanicaleffectslinearlyincreasewiththeabsorbedenergy

• Ruptus(Latin)=ruptured• e.g.)90μmglass,ND:YLFlaserwith30ps

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Cavitation

• Thishappenswhenthelaserbeamwasfocusednotonthesurfacebutintothetissue

• Cavitationfromhumancorneaisseenbelow• ND:YLFlaserwith30psfocusingunderneaththeepithelium• Cavitationbubblesconsistofgaseousvapors,mainlywatervaporandcarbondioxide,whichwilldiffuseintothesurroundingtissue

ClinicalApplications

• Atoolofminimallyinvasivesurgery• Posteriorcapsulotomy ofthelens• Laser‐inducedlithotripsyofurinarycalculi

Intraocular lens (IOL) for Cataract surgery

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ClinicalApplications

• Posteriorcapsulotomy ofthelens• Laser‐inducedlithotripsyofurinarycalculi

Photodisruption

• Plasma‐inducedablation:spatiallyconfinedtothebreakdownregion

• However,shockwaveandcavitation:affectsadjacenttissue,notlocalized,canbe~mmorder

• Nanosecondpulsedoesnotinduceaplasma–inducedablation becausethethresholdofenergydensityofopticalbreakdownishighercomparedtopicosecondpulse

• Therefore,fornanosecondpulses,opticalbreakdownisalwaysassociatedwithshockwaveformationevenattheverythreshold

• Picoorfemtosecondpulses highpeakintensitybutlowpulseenergy reducedisruptiveeffects

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EnergyDensity

• Bothplasma‐inducedablationandphotodisruption relyonplasmageneration noteasytodistinguish

TimeScaleof4Effects

~nsec

30~50nsec

Expansion of plasma

IonizationOrdinary sound wave

50~150nsec

Vaporization

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3 EffectsfromHighEnergyDensity

• Theamountofenergyabsorbedduringphotodisruption is2ormoreordersofmagnitudehigherthanduringPIAthefreeelectrondensity&plasmatemperaturearealsohigherthanPIA

• Thiscauses3effectsinphotodisruption– Plasmashielding– Brillouin scattering– Multipleplasmageneration

PlasmaShielding

• Plasma:onceitisformed,itabsorbsandscattersthefollowingincidentlight

• Therefore,itprotects(“shields”)underlyingstructuresinthebeampath

• e.g.)retinaduringlasersurgeryofthelensorthevitreousisprotectedbythisplasmashield

• PIAalsoproducesplasmashieldingeffects,butstillpermitsthelighttobetransmittedthroughtheplasma

• However,photodisruptive interactionhasmoredenserplasmaandthus,hasmorestrongerplasmashieldingeffect

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Brillouin Scattering

• Heatingprocessoftheplasma generatesacousticwaves(phonons) scatterlightwithshiftedfrequency calledasBrillouinscattering

• Withevenhigherlaserenergydensity,thelaserlightitselfcancreatesalterationsinopticaldensity causesscattering calledasstimulatedBrillouinscattering

MultiplePlasmaGeneration

• DuringPIA,onlyonesparkisinducedattheveryfocuswhentheenergydensityisclosetotheablationthreshold

• Athigherpulseenergies,severalplasmascanbeignited• Thefirstplasmawillbeignitedattheveryfocusandsucceedingradiationgenerateopticalbreakdownbeforereachingthesmallestbeamwaist

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PlasmaLength

• Plasmalength∝ pulseenergy• Plasmalength∝ 1/pulseduration• Shorterpulserequireslessenergytoproduceplasma• Withthesameenergy,psec pulseproduceslongerplasmalength largerplasmavolume requiresmoreenergyforionizationandvaporizationoftissue lessenergycontributestoproducemechanicaleffectssuchasshockwavesorcavitationlessmechanicaldamagetotissuethannsec pulse

OverallPlasmaFormationSequence

• Ifthefocussizeissame,plasmainducedbynsec pulsescontainsignificantlymoreenergythanpsec producedplasma thisadditionalamountofenergyneedstobedissipatedintothesurroundingtissuebythegenerationofshockwaves,cavitation,andjetformation

Plasma‐induced ablation Photodisruption

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ShockWaveGeneration

• Laserinducedopticalbreakdowncausesasuddenadiabaticriseintemperatureuptoafew10,000K Thehighkineticenergyoffreeelectrons

• Highkineticenergy electronsdiffuseintothesurroundingmedium inertionsfollowelectronswithacertaintimedelaymassmoves originofshockwavegeneration

• Thisshockwavesoonseparatesfromtheplasmaboundary• Initially,withahypersonicspeedandslowsdowntothespeedofsound

Adiabatic process: a process that occurs without the transfer of heat or matter between a system and its surroundings

ShockWavesinWater

• Speedofsoundinwater:~1483m/sat37oC• Laserinducedshockwaves:~5000m/satthefocus• Toderivearelationdescribingthepressuregradientattheshockfront,let’sconsideraslaboftissuewithacross‐sectionA0 whichispassedthroughbyashockfrontatspeedus

• Duringthetimeintervalofdt,theshockfrontmovesadistanceofdxs,andthusthespeedofshockwaveus is

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ShockWavesinWater

•

• p0,ρ0:pressureanddensityinsidethemedium,respectively• Shockwaveincreasesthepressurefromp0 top1 andofthedensityfromρ0 toρ1 atitsfront

• Toconserveamass,theleftsideparticleswillintrudetowardrightsidewithaspeedofup (< us)

• Derivationofrelationbetweenus and up,pressureandspeedarededucedinthetextbook

ShockWavesinWater

• Atp1 =0kbar,theshockspeedus approaches1483km/s,whereastheparticlespeedup remainsat0km/s.

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ShockWavesinWater

• However,theserelationshipscanalsobeusedtocalculatetheshockwavepressurep1 asafunctionofshockspeedusShockwavepressures(usuallyverydifficulttodetermine)canbederivedfrommeasuredshockspeeds

• Theinitialpressureattheboundaryofthelaserplasmawas17kbar for50μJ pulseswithadurationof30ps,whereasitwas21kbar for1mJpulseswithadurationof6ns

• Decayismuchsteeperin30ps

Plasma boundary

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ShockWavesinWater

• Vogeletal.estimatedthatthewidthofshockwavesissmallerinthecaseofthe30ps pulse(3μm)than6nspulse(10μm)

• Thisenergyisroughlygivenby≃ Δr (3.70)

– shockwavepressurep1– shockwavesurfaceareaAs– shockwavewidthΔr

• DuetodifferentplasmalengthsasshowninFig.3.60,weobtaininitialvaluesofAs ≃ 100μm2 for30ps pulsesandAs ≃2500μm2 for6nspulses(assumingafocalspotdiameterof4μm)

• From(3.70),wethenfindthatEs≃ 0.5μJ for30ps pulsesandEs≃ 50μJ for6nspulses

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ShockWavesinWater

• Only1–5%oftheincidentpulseenergyisconvertedtoshockwaveenergy

• Shockwavesfrompsec pulsesaresignificantlyweakerthanthoseinducedbynsec pulseswithcomparablepeakpressures

• Fromthecorrespondingparticlespeeds,Vogeletal.(1994a)havecalculatedatissuedisplacementofapproximately1.2μm for30ps pulsesandadisplacementofroughly4μm for6nspulses

• Theserathersmalldisplacementscancausemechanicaldamageonasubcellularlevelonly,buttheymightinducefunctionalchangeswithincells

MeasurementsofShockWaves

• Twotypesofmeasurements– Opticalmeasurements– Mechanicalmeasurements

• Opticalmeasurements– Adecreaseinprobebeamintensityisdetectedwithafastphotodiodeaslongastheshockwavepassesthroughthefocusoftheprobebeam

– Bymovingthefocusoftheprobebeamwithrespecttothesiteofplasmageneration,thepropagationoftheshockwavecanbemonitoredonafastdigitaloscilloscope

– Fastphotodiodesevenenableatemporalanalysisoftherisetimeoftheshockfront

Weaker

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MeasurementsofShockWaves

• Mechanicalmeasurements– Itreliesonpiezoelectrictransducerstransformingtheshockwavepressuretoavoltagesignal

– Piezoelectrictransducer:athinpolyvinyldifluoride (PVDF) foilwhichisgoldcoatedonbothsidesformeasuringtheinducedvoltagebyattachingtwothinwires

– Thecorrespondingpressureismonitoredonafastdigitaloscilloscope

MeasurementsofShockWaves

• Source:30ps pulsefromaNd:YLF laser• Probebeam:ahelium–neonlaserfocusedatthedepthofinterest

• Detector:afastphotodiode• Duringitsoveralldurationofroughly40ns,theshockwavecanhardlycauseanygrosstissuedisplacement

• Thus,furtherevidenceisgiventhatshockwavedamageislimitedtoasubcellularlevel

Shock wavepass starts

Shock wavepassed

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MeasurementsofShockWaves

• ThevoltagesignalfromaPVDFtransducerisshown• Aplasmawasinducedatthesurfaceofatoothslicewithathicknessof0.5mm

130ns  3800m/s(sound speed in teeth)   = 0.5mm/130ns

Reflection fromopposite side

130nsx2=260ns 

MeasurementsofShockWaves

• Tracesoftheshockwaveatdifferentdistancesfromitsorigincanbemeasuredbymovingthefocusoftheprobebeamawayfromtheplasmasite

• FromFig.3.67,ashockspeed~4160m/sforthefirst10ns ~60kbar

• Theshockwavethenslowsdowntoabout1480m/s(soundspeedinwater)afteranother30ns

• Thespatialextentofshockwavesis~0.2mm(~4160m/sx40ns)

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Cavitation

• Laser‐inducedcavitations occurifplasmasaregeneratedinsidesofttissuesorfluids

• Highplasmatemperature(5,000~40,000K) vaporizingthefocalvolume kineticenergyisconvertedtopotentialenergystoredintheexpandedcavitationbubble duetoouterstaticpressure,bubbleimplodeslessthanamsec wherebythebubblecontent(watervapor,carbonoxides)iscompressed pressureandtemperatureriseclosetothoseduringopticalbreakdown reboundthebubble secondtransientisemittedandthewholesequencerepeatsafewtimesuntilallenergyisdissipatedandallgasesaresolvedbysurroundingfluids

CavitationStudyTechniques

• High‐speedphotographictechnique(~upto1millionframe/sec)

• Q‐switchedrubylaserwith100mJ~400mJpulseenergy

1st collapse2nd collapse

Brassblock

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CavitationStudyTechniques

• Collapseofsphericalcavitationbubbles(Rayleigh1917)

. ⁄(3.71)

(3.72)

– :maxradiusofcavitation– :durationofthecollapse– :thedensityofthefluid– :thestaticpressure– : thevaporpressureofthefluid

– :thebubbleenergy

CavitationStudyTechniques

• Vaporpressure:thepressureexertedbythegasinequilibriumwithasolidorliquidinaclosedcontaineratagiventemperature.

Liquid Solid

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CavitationStudyTechniques

• Thetemporaloscillationofcavitationbubble:capturedbyprobebeamexperiment

Slope ~1/3

3 times of collapse

CavitationStudyTechniques

• Energyconversiontobubbleenergy– Picosec laser:19%– Nanosec laser:24%

• Averageenergylossofbubblesduringtheir1st cycle~84%– Among84%loss,emissionofsoundisthemajorpart

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TissueDamage

• Forshockwavegeneration– ≃ Δr (Δr:shockwavewidth) (3.70)– ∝ ∝

• Forcavitationgeneration

– (r:cavitationradius) (3.72)

– ∝

• Therefore,withthesameamountofenergygiventotissues,cavitationwillcausemoredamagethanshockwave

• Tissuedamageismainlyfromcavitationandjetformationsincecavitationcausestissuedisplacementafewmm

• Shockwavecauses1~4μmdisplacementoftissuesubcellularlevel

JetFormation

• Whencavitationbubblescollapseinthevicinityofasolidboundary,ahigh‐speedliquidjetdirectedtowardthewallisproduced

• Ifthebubbleisindirectcontactwiththesolidboundaryduringitscollapse,thejetcancausehigh‐impactpressureagainstthewall

• Thus,bubblesattachedtosolidshavethelargestdamagepotential

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JetFormation

• High‐speedphotographyisusedtostudyjetformation• Jetvelocity~156m/swasreportedbyVogeletal.(1989) thiscorrespondsto2kbar(~1974atm)pressureofwaterhammer

Brass block

d1

d2

d1 >d2

Jet

Counter jet

OriginofJetFormation

• Whenthebubblecollapsesduetoexternalpressurethesurroundingfluidisacceleratedtowardthecenterofthebubble

• However,atthesidepointingtothesolidboundarythereislessfluidavailable

• Hence,thecollapsetakesplacemoreslowlyatthissideofthebubble

• Thiseffectultimatelyleadstoanasymmetriccollapse• Atthefastercollapsingside,fluidparticlesgainadditionalkineticenergy

• Thisexplainswhyjetformationoccurstowardthesolidboundary.

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OriginofCounterJetFormation

• Ifthejetisrelativelyslow,thevelocityofthecentralpartoftheslowercollapsingsidemightevenbehigherthanthejetitselfbecausethatsideofthebubbleisaccelerateduntiltheveryendofthecollapse

• Inthiscase,acounterjet isformedpointingintheoppositedirection

• Pathline portraitsoftheflowaroundcollapsingbubbleshavebeenexperimentallyandtheoreticallydetermined

• InFigs.3.75a–b,twoofthemareshownwhichofferagoodvisualizationofthefluidflowduringthecollapse.

DamagebyJetFormation

• Thedamagingeffectofjetformationisextremelyenhancedifagasbubbleremainingfromanearlierlaserpulseishitbyacoustictransientsgeneratedbysubsequentpulses

• AccordingtoVogeletal.(1990),thedamagerangeinducedbya4mJpulsecanreachdiametersofupto2–3.5mmifgasbubblesareattachedtothecornealtissue

• Verysmallgasbubbles,however,quicklydissolveduetotheirsmallvolumeandstrongsurfacetension

• Therefore,thesemicrobubbles shouldnotcauseanyprobleminachievingacertainpredictableeffectiftherepetitionrateofthelaserpulsesisadequatelychosen.

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Summary

• Mainidea:fragmentationandcuttingoftissuebymechanicalforces

• Observations:plasmasparking,generationofshockwaves,cavitation,jetformation

• Typicallasers:solid‐statelasers,e.g.Nd:YAG,Nd:YLF,Ti:Sapphire

• Typicalpulsedurations:100fs...100ns• Typicalpowerdensities:1011 ...1016 W/cm2

• Specialapplications:lensfragmentation,lithotripsy(surgicalproceduretoremovestonesfromurinarytract,i.e.,kidney,ureter,bladder,orurethra)