Page 54 - Mike's published articles

particle.Theflashoflightobservedclinicalyisalmostcertainlyindicativeofemisionofhighlyexcited statesinthisplasmandnotofamechanismwheresuchstatesarexcitedthermaly.Forthereasons
givenabove,thewavelengthdependenceofthismechanismismuchlessensitivethanmightbexpected fromtheusualcharacteristicsofabsorptionspectraofdyes.
Inalexperimentsacomparativelyfinedividinglinexistsbetwentheonsetofthismechanismfor thepigmentandtheoperationofthismechanismintheoverlyingtisuesabovethetato.Ifthefluxis tohigh,theskinwilbedisruptedalso.Ifthefluxistolow,litleornothingwilhapentothepigment ortheskin.Thealexandritelaserislesefectivethantherubyquotedhere,almostcertainlybecauseits pulsewidthisaround75-10nscomparedwiththe25nsoftheruby.Forequaliradiatedareasandpulse energies,thismeansthatthealexandritefluxis4timeslesthanthatoftheruby.
Sincethismultiphotonmechanismishighlynon-linearlydependentonthelaserflux,thecontrolof thelaserbeamintensityprofileisofgreatimportance.Failuretoensuresuchastableprofilewilleadto randomproductionofunusualyhighfluxzonesinthelasercros-section.Theseareverylikelytoinitiate multiphotonprocesesintheskinandcauseunwantedisruption,posiblyleadingtoscaring.Tomini- misescaring,thefluxonthebeamaxismustneverexcedthevaluewhichalowsmultiphotonabsorp- tiontocurintheskintisue.ThiscanonlybeguarantedifthelaseroperatesintheThM0mode.
ThemechanismexplainsalsothevastsuperiorityoftheQ-switchedlaseroverthecwlaser. Multiphotonprocesesandtheiratendantshockwavecanotocurwithcwradiation.Onlytrulythermal proceseswilocur,leadinginefecttoabuminthetraditionalsense.Thefectoftheabsorbance spectrumofthepigmentwilbegreaterinsuchacase,butthelosfactorsasmeasuredbyourreflectance spectrarestilveryrelevantinasesingthediferentialefectsofvariouslasersandpigments.The pigmentwilstilbesentialytotalyabsorbingatitssurface.
Inaplicationsrequiringthethermalyinducedcoagulationofblodvesels,laserswithpuLse lengthsinthemilisecondsregimearepredictedtobebest.Thisisbecauseshockwaveinducedparticle breakdownisnotrequiredinthesecases.Theheatingofablodveseldependsonthelaserfluxagain, becausenowthetemperaturereachedbytheblodintheveseldependsonthefinenergybalance26 betwentheinputlaserfluxandthethermaldifusivityofthevesel.Thislaterdepends,amongstother parameters,upontheinverseofthesquareoftheradiusofthevesel.Thus,therewilbearangeofvesel diametersbelowhichagivencwlaserisinefective,duetotorapidheatremoval,andabovewhichthe temperaturerisewilbexcesive,leadingtounfortunatefects.Inthiscontextitshouldbenotedthat tohigharepetitionrateofthelonglaserpulses,apliedtothesamespot,wilagainleadtoexcesive temperaturerise,becausethethermaldifusivityoftisue(esentialythatofwater)issuchthatseconds mustelapsebeforeambienttemperatureisreached(within1%)intreatedtisue.Beingcontroledby thesekineticfactors,thereisagreattendencyfortemperaturetointegratewithtimeifsuchlongperiods arenotalowed.Inthetatoremovalexperimentsaltheapliedenergyisultimatelydegradedtoheat and,forthesamereasonsasjustdiscused,theQ-switchedpulserepetitionrateshouldnotbemuchabove 1Hzatthesamespot27,28Indedthereissomevidencethatpatientstreatedwiththehigherrepetition ratelasersdoshowmorescaringthanwhenlowrepetitionratesystemsareused.
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