음향패턴간섭복사력기반초음파신경자극

Abstract

경두개집속초음파를이용한심부뇌자극에대한수요가증가하고있다.신경질환에효과적인심부뇌자극은전극을뇌에직접이식하는방식이다.경두개집속초음파는경두개전기자극,경두개자기자극과같은다른비침습적방법에비해높은공간분해능과침투깊이로주목받고있다.열에너지,캐비테이션,음향방사력이경두개집속초음파의잠재적인메커니즘으로언급되지만,본논문에서는ARF를사용하는경우에대해서만다룬다. 집속초음파에의해조직에서발생하는것으로알려진수많은음향방사력방정식을수치시뮬레이션을통해비교하였다.단일집속변환기를사용할때계산된음향방사력에따라조직응답을얻었다.비교된세가지음향방사력방정식은레이놀즈응력,운동량자속밀도텐서및감쇠에의한힘이다.조직에서발생하는변위와응력텐서의비교를기반으로레이놀즈응력방정식은조직의응답을가장잘설명하는음향방사력방정식으로결정되었다. 기존의초음파신경조절방식보다더강하게자극하기위해서는레이놀즈응력에의한힘의크기를키워야한다.레이놀즈응력에의한힘은입자속도장의제곱구배에비례하므로크기를증가시키기위해정상파를사용하였다.기존의방식과달리두개의집중된탐촉자를서로마주보게배치하여정상파를생성하였다.정재파에서발생하는레이놀즈응력에의한힘을패턴간섭방사력이라고하며,수치해석시뮬레이션결과기존방식보다약34배큰것으로나타났다.패턴간섭방사력을직접측정하는대신시간평균2차압력을측정하여검증하였다.시간평균2차압력을측정하기위해정재파압력장중앙에유리관을놓고관내부의수면변화를측정하였다. 패턴간섭방사력의분해능을향상시키기위해단일주파수연속파파형대신주파수변조파형을사용하였다.실험을통해선형주파수변조처프파형을이용하여생성된주파수변조패턴간섭복사력의분해능이기존패턴간섭방사력대비약8.5배향상됨을확인하였다.폴리디메틸실록산캔틸레버를이용한새로운기술은기존패턴간섭방사력및주파수변조패턴간섭복사력을측정하기위해정상파에서복사력을측정하기위해개발되었다.시뮬레이션과실험을통해주파수변조패턴간섭복사력의목표점은변환기중하나에지연시간이주어졌을때3mm와6mm만큼이동할수있었다. 주파수변조패턴간섭방사력를얻기위해최적화방법을수행하고조직내부에설계된스트레스텐서를생성하기위해입력및출력압력을최적화하였다.목표응력패턴의분해능,파장및위치를변경하여최적화된입력및출력압력을분석하였다.최적화결과는분해능이최적화된출력압력의주파수대역폭과관련이있으며주주파수는목표응력패턴의파장과관련이있음을보여주었다.또한최적화방법을사용하면시간지연을사용하여목표위치를이동하는것보다더정확한결과를보여준다 |Thedemandfordeepbrainstimulation(DBS)usingtranscranialfocusedultrasound(tFUS)isincreasing.DBS,whichiseffectiveforneurologicaldisorders,usethemethodimplantingelectrodesdirectlyintothebrain.tFUSisattractingattentionduetoitshighspatialresolutionandpenetrationdepthcomparedtotheothernon-invasivemethodssuchastranscranialelectricalstimulation(tES)andtranscranialmagneticstimulation(TMS).Thermalenergy,cavitation,andacousticradiationforce(ARF)arecommonlymentionedaspotentialmechanismsoftFUS,andsomerecentstudiesalsosuggestastresstensor.ThisthesiscoversonlywiththecaseofusingARFandtheresultingstresstensor. SeveraltypesofARFsareusedinvariousfields.First,itisnecessarytospecifywhichARFisoccurringinthetissue.EquationsthatcancalculateARFincludeReynoldsstressforce(RSF),momentumfluxdensitytensorforce,andattenuationforce.ThetissuedisplacementandstresstensorbyeachARFwerecomparedbynumericalsimulation.TissuedisplacementshowedthesameresponsetothethreeARFs,butstresstensorwasgeneratedonlywhenRSFwasused.Consideringtheimportanceofstresstensor,theRSFequationwasdeterminedastheARFoccurringintissue. Tostimulatemorestronglythantheexistingultrasoundneuromodulationmethod,themagnitudeofRSFmustbeincreased.SinceRSFisproportionaltothegradientofthesquareoftheparticlevelocityfield,astandingwavewasusedtoincreasethemagnitude.Unliketheconventionalmethod,twofocusedtransducerswereplacedfacingeachothertocreateastandingwave.TheRSFgeneratedfromastandingwaveiscalledpatterninterferenceradiationforce(PIRF),andnumericalsimulationsshowedthatitisapproximately34timeslargerthantheconventionalmethod.InsteadofmeasuringthePIRFdirectly,thetime-averagedsecond-orderpressurewasmeasuredforverification.Tomeasurethetime-averagedsecond-orderpressure,aglasstubewasplacedinthemiddleofastandingwavepressurefieldtomeasurethechangeinwatersurfaceinsidethetube. ToimprovetheresolutionofPIRF,afrequency-modulatedwaveformwasusedinsteadofasinglefrequencycontinuouswave(CW)waveform.Throughtheexperiment,itwasconfirmedthattheresolutionofthefrequency-modulatedpatterninterferenceradiationforce(FM-PIRF)generatedusingthelinearlyfrequency-modulatedchirpwaveformwasimprovedbyabout8.5timescomparedtothePIRF.Anewtechniqueusingpolydimethylsiloxane(PDMS)cantileverwasdevelopedtomeasuretheradiationforceinastandingwavetomeasurePIRFandFM-PIRF.Throughsimulationandexperiment,targetpointofFM-PIRFwasabletomoveby3mmand6mmwhenthedelaytimewasgiventooneofthetransducers. OptimizationmethodwasperformedtoobtainFM-PIRFandoptimizedinputandoutputpressuretogeneratethedesignedstresstensorinsidethetissue.Theoptimizedinputandoutputpressurewasanalyzedbychangingtheresolution,wavelength,andpositionofthetargetstresspattern.Theoptimizationresultsshowedthattheresolutionisrelatedtothefrequencybandwidthoftheoptimizedoutputpressure,andthemainfrequencyisrelatedtothewavelengthofthetargetstresspattern.Inaddition,usingtheoptimizationmethodshowsmoreaccurateresultsthanmovingthetargetpositionusingtimedelay.