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Q-switchedlaserremovaloftatos- Aclinicalandspectroscopicinvestigationofthemechanism
K. Siomos1, R.T. Bailey2, F.R. Cruickshank2, M.J. Murphy3
1lnstituteofMaterialsStructureandLaserPhysics,TechnicalUniversityofCrete, 73110Chania,Crete,Grece 2TheDepartmentofPure&ApliedChemistry,TheUniversityofStrathclyde, 295CathedralStret,GlasgowGi1XL,Scotland 3MysanMedicalLtd.,Glasgow,Scotland
ABSTRACT
Theliquidphasespectraoftatopigmentsareshowntobeunreliableasabasisformechanistic deductions.Thereflectancespectraofthesolidsfrom20nmto50nm(50to20cm1)areshown toacuratelyasestherelativelosoflaserlightfordiferentpigmentsandtobeusefulinexamining thesetocheckforsimilaritiesinthepigments.Theabsorbancediferencesbetwenthepigmentsare showntobelargelyirelevantinasesingtheaseoftatoremovalbylaserradiationofavarietyof wavelengths.Amultiphotonabsorptionmechanismwithitsconcomitantshockwaveisproposedtobe responsibleforthereductionofpigmentparticlestosmalsizeswhichthelymphsystemcanremove.The diferentbehaviourofblueandgrentatos,treatedbyQ-switchedrubyandNd:YAGlasers,isatiributed totheparticleagregationsizeofthepigmentsinthetato.
Keywords:Tato,Clinicaltreatmentoftauos,Tato-pigments'spectroscopy,Pigmentcharac- terization,Nd:YAGlaser,Rubylaser,Alexandritelaser,Multiphotonabsorption,
2. iNTRODUCTION
Thepropertiesoflaserlightinbiologicalaplicationscanbeficientlyexploitedonlyifthemecha- nismsoftherelevantprocesesarewelunderstod.Suchmechanismsarenotsimple,sincelaserlight ofhighintensityisfrequentlyused(intermsofJoulespersquarecentimetre-fluenceorWatspersquare centimetre-flux).Photophysicalandphotochemicalprocesesresultingfromthistypeofradiationdifer markedlyfromthoseoflowintensityradiationasobtainedfromeventhemostpowerfullamps.The sucesfulaproachtothetreatmentofskindisordersorartificialyinducedskindiscolourationasin tatoengravingdependsstronglyonthemechanismofthetransferofenergyfromthelaserbeamtothe relevanttisueorpigment79.Whenlightisincidentuponamediumsuchasskinwithanembededpig- ment,itisreflectedpartialyateachmismatchofrefractiveindex,i.e.,ateachmaterialboundary.Inthe caseoftatos,thiswouldbeattheair/skininterfaceandatthetisue/pigmentinterface.Aditionaly,the lightispartialyscateredbyinhomogeneitiesintherefractiveindexwhichinourexamplewouldprinci- palybethetransparentboundariesinthetisue.Finaly,thelightisabsorbedpartialybyboththetisue andthepigment.Onlythislastprocesleadstoenergytransferintothepigmentparticle,ultimately causingphysicalandchemicaltransformations.Howeverthefirsttwoftheseprocesesleadtolosof energyfromthelaserbeamarivingatthetargetsiteandleadindirectlytoefectsontheperceived
Shock waves
sucesoftheclinicalprocedure.Itisacordinglynecesarytoconsidertheircontributionaswelasthe absorptionproces.
40/SPIEVol.2623
Q.8194-1987-7/96/$6.QO
