class11 interaction mechanisms photodisruption - gistbiophotonics.gist.ac.kr/course materials/laser...
TRANSCRIPT
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InteractionMechanismsPhotodisruption
MD6305Laser‐TissueInteractionsClass11
JaeGwan Kim
[email protected] ,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
1
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)