磁共振成像和光谱学课件.ppt

24磁共振成像和光谱学 Explainthephenomenonoffreeinductiondecay Statetherationale 基本原理 foruseofthespin echo 自旋 techniqueinimaging DefineT2 Explainthepulsingandsignalacquisitionscheme 组合 usedinthefollowingpulsesequences Spin echo 自旋回波 Carr Purcell Meiboom Gill Inversionrecovery反转回复 Gradient echo 梯度回波 CharttheorderofoccurrenceofRFpulses gradient 梯度的 magneticfields andsignalacquisitioninspatially 空间地 encodedspin echoimaging Explainhowthetwo dimensionalFouriertransform 2DFT methodisusedtoconstructtheMRimage Calculate 计算 thetimerequiredtoobtainanimageusingthespin echotechnique Listsomemotionsuppression 抑制 techniques Givethephysicalprinciplethatunderliestheuseofcontrastagents 造影剂 inMR Explainthephysiological 生理的 basisoffunctionalMRI fMRI Listreasonsfortissuecontrast differencesinMRsignalamongvoxels andhowthesedifferencesarehighlightedthroughthechoiceofpulsesequenceparameters ExplainthechemicalbasisofMRspectroscopyandgivesomeexamplesofnucleithatarestudied OVERVIEW ThemagneticpropertiesofnucleidescribedinChapter2havesignificantapplicationsinmedicalimagingandbiochemicalanalysis Theseapplicationsarepossiblebecausetherelaxationpropertiesandtheresonancefrequencyforanucleusdependuponitsenvironment Factorssuchasthepresenceofchemicalbonds paramagnetic 顺磁性因子 ions andtherateofflowoffluidsinfluencethemagneticresonance MR signal Therefore differentregionsofabiologicalsampleproducedifferentMRsignals Whilethereasonsforthesedifferencesareoftensocomplexthatinterpretationisdifficult theirexistenceyieldsanintrinsic 固有的 subject 机理 contrastbetweentissues IfthevaluesoftheMRsignalscanbemappedaccordingtotheirspatiallocationsandcanbeencodedasbrightness grayscale onamonitor thenanimagemaybeproduced Thisisthemethodofmagnetic 强度 resonanceimaging MRI Thislatterapproachistermedmagneticresonancespectroscopy MRS 磁共振波谱分析 MRS 是测定活体内某一特定组织区域化学成分的唯一的无损伤技术 是磁共振成像和磁共振波谱技术完美结合的产物 是在磁共振成像的基础上又一新型的功能分析诊断方法 主要应用 脑部 心脏 骨骼肌和肝脏等方面的研究 以脑部最为广 脑部磁共振波谱研究较多的有脑梗死 脑肿瘤 脑白质和脑灰质疾病 癫痫和代谢性疾病等 尤其是颅脑肿瘤研究较多 对脑肿瘤与非肿瘤性病变鉴别 脑肿瘤良恶性鉴别 恶性肿瘤分级 肿瘤术后复发与坏死的鉴别 原发与转移瘤的鉴别等均有很大的临床应用价值 此外 还能鉴别颅咽管瘤与垂体瘤 脑内肿瘤与脑外肿瘤 确定脑室内的中枢神经细胞瘤等 在心脏方面的应用主要是在心肌缺血 心肌病等心肌代谢方面的研究 肝脏31P MRS主要研究包括肝代谢性疾病 肝炎肝硬化及肝肿瘤等 MRS能提供前列腺组织的代谢信息有助于鉴别前列腺癌和前列腺增生 MRS还能无创性地检测骨骼肌磷脂代谢和能量代谢的代谢产物及细胞内ph值 研究骨及软组织肿瘤的磷脂代谢和能量代谢的异常变化 freeinductiondecay 自由感应衰减 freeinductiondecay FID 是核磁共振 NMR 与磁共振成像 MRI 中最简单的信号形式 受激发的核种对磁共振频谱仪或磁共振成像扫瞄仪的射频线圈造成感应电流而产生信号 并且因发生弛豫而使信号强度逐渐衰减至零 这种逐渐衰减的信号即称为 自由感应衰减 信号衰减的快慢由横向弛豫时间常数T2决定 TherearetwoproblemswithusingtheFIDsequenceforimaging First itisdifficulttorecordtheMRsignalimmediatelyafterthe90 degreepulseistransmitted ThesecondproblemwiththeFIDsequenceisthattheT2relaxationofthetissueofinterestisusuallymaskedbyanotherphenomenonthathasthesameeffectuponthesignal T2 pronounced Ttwostar isdephasingthatisproducedbyinhomogeneitiesofthestaticmagneticfieldwithinasample Thiseffectresultsinalossofphasecoherencethatmimicsthe natural or true spin spinrelaxationofasample DefineT2 Toremovetheeffectofmagneticfieldinhomogeneities a180 degree rephasing 重新定相 pulsecanbeappliedatsometimeTaftertheinitialRFpulse IftheinitialRFpulseproducednutationaboutthex axis therephasingpulseisappliedtoproducenutationaboutthey axis MarginFigure24 3 Theeffectoftherephasingpulseistotakespinsthatwereprecessingmorerapidlyandflipthembehindthespinsrotatingattheaveragerateofprecession Similarly spinsthatarerotatingmoreslowlyareautomaticallyflipped ahead ofthemeanprecessionrate Thenucleiarestillprecessingatdifferentratesbecauseofinhomogeneitiesinthemagneticfieldofthesystem Aftertherephasingpulse thefasterspinscatchupwiththeaveragespins andtheslowerspinsarecaughtbytheaveragespins Whenthishappens thespinsaresaidtohave rephased SpinEcho IfdephasingprogressesforatimeTaftertheinitialRFpulse thenatimeTisrequiredafterapplicationoftherephasingpulseforthespinstorephase Atthemomentofexactrephasing thesignalwillbestrongest sincetheeffectofT2 iscompletelyremoved Inthespin echopulsesequencethe90 degreepulse A causesnutationofthebulkmagnetizationaboutthex axis Dephasingthenoccurs B asslowercomponentsofthemagnetizationlagandfastercomponentsadvance A180 degreerephasingpulse C causesnutationaboutthey axisandreversestheorderoffastandslowcomponents Thecomponentsthenmerge D toproducethemaximumsignal DiagramsAtoDareshownintherotatingframeofreference CarrandPurcellintroducedtheideaofusingmultiplerephasingpulses withcollectionofthesignalaftereachpulse MeiboomandGillproposedapplyingnutationaboutanaxisthatisperpendiculartotheaxisofnutationfortheoriginalRFpulse asdescribedaboveforspinecho ThesetechniquesareusedintheCarr Purcell Meiboom Gill CPMG pulsesequence BecausethespinsarerephasedafterTE itisasiftheexperimenthadstartedoverwiththeinitial90 degreepulse ThereforeasecondrephasingpulsedeliveredatimeTEafterthefirstproducesasecondrephasing andanothersignal echo isobtained Thediminution 减少 ofsignalbetweensubsequentechoesiscausedbyT1andT2relaxation Carr Purcell Meiboom Gill 测量 T2 弛豫时间的脉肿序列 测量 T2 弛豫时间的脉肿序列 测量 T2 弛豫时间的脉肿序列 测量 T2 弛豫时间的脉肿序列 测量 T2 弛豫时间的脉肿序列 InversionRecovery SPATIALENCODINGOFMAGNETICRESONANCEIMAGINGSIGNAL 对信号进行空间编码是为了对信号进行空间定位形成MR成像 Sensitive PointMethod Thesignalfromeachvolumeelement voxel inthepatientcouldbereceivedseparatelyinsequence i e onevoxelatatime Onecouldthenrecordthevalueofeachsignalinaseparatestoragelocationinthecomputerandreconstructtheimagebydisplayingthevaluesasamatrixofshadesofgray Inpractice thismethodofdataacquisition called sensitive point scanning istootime consuming andpatientmotionwouldbeunacceptable Itisdescribedheretoillustrateseveralfundamentalpointsaboutdataacquisitionandtoemphasizetheneedformorecomplexbutefficientmethodsofacquiringdata Ifthemagneticfieldismadetovarygraduallyacrossthepatient thenaccordingtotheLarmorequationthefrequencyofprecessionandthereforetheresonancefrequencyoftheprotonswillalsovary Supposethatthevariationor gradient ofthemagneticfieldisinthecraniocaudal 头尾位 direction MarginFigure24 6 thatis supposethemagneticfieldisstrongeratthepatient sheadthanatthefeet Wecouldrepeattheprocedureintheothertwoorthogonal 正交的 planesbyusingadorsal ventral 背腹图式 magneticfieldgradienttodefineacoronal 管状的 planeandaleft rightgradienttodefineasagittal 矢状的 plane Theintersectionofthreeplanesisapointandyieldsa sensitivepoint fordataacquisition Thesensitive pointtechniquebeginswiththeapplicationofalternatingmagneticfields Forpartofacyclethemagneticfieldincreasesfromrighttoleftasshownhere Thedirectionofincreasethenreverses Theregiondefinedasthe slice maintainsthesamemagneticfieldvalue Thesensitive pointtechniquebeginswiththeapplicationofalternatingmagneticfields Forpartofacyclethemagneticfieldincreasesfromrighttoleftasshownhere Thedirectionofincreasethenreverses Theregiondefinedasthe slice maintainsthesamemagneticfieldvalue Two DimensionalFourierTransform Thesensitive pointtechniquemakesuseof frequencyencoding toestablishauniqueresonancefrequencyforeachvoxelinthepatient Themostobviouslimitationofthesensitive pointtechniqueisthatexcessivetimeisrequiredtoobtainenoughinformationtoconstructanimage Fortunately therearefastermethodsofspatialencoding Signalsmaybeencodedaccordingtotheirphase TheymayalsobeencodedbyfrequencyasecondtimebyusingtheFouriertransform Anumberofalternateimageacquisitionschemeshavebeenemployed Theseincludesensitive line sequential plane andthree dimensionalzeugmatography 核磁共振成像 Atthepresenttime themostcommonlyusedmethodofspatialencodingincommercialMRIsystemsisthetwo dimensionalFouriertransform 2DFT Thefirststepinthe2DFTmethodofsignalacquisitionisactivationofaslice selectgradientwhileanarrow bandwidthRFpulseissent ThelocationoftheplanealongthedirectionofthegradientisdeterminedbythecenterfrequencyoftheRFpulse andthethicknessofthesectionor slice isdeterminedbythebandwidthoftheRFpulse Thesecondstepof2DFTistoturnonaphase encodinggradientinadirectionperpendiculartotheslice encodinggradient ThisgradientisturnedoffbeforeanyRFpulsesareapplied InMarginFigure24 8thephase encodingdirectionisalongthex axis butthisalignmentisarbitrary Theeffectofthephase encodinggradientistovarythephaseofprecessionoftheprotonsalongthexdirection Thephaseofprecessionofprotonsalongtheydirectionisthesameatanylocationalongthex axis Thethirdstepin2DFTistoapplyafrequency encodinggradientatthetimethattheRFsignalisreceivedfromthepatient Thefrequency encodinggradientactsaccordingtothesameprincipleastheslice selectgradientorthephase encodinggradient Whenitison protonsatoneendofthesliceprecessfasterthandoprotonsattheotherend Thefourthstepof2DFTistheapplicationoftheFouriertransform seeAppendixI Thisapproachpermitsacomplexdatasettobeacquiredinasinglesetofmeasurementsoverarelativelyshortperiodoftime ThedataareanalyzedlaterwithamathematicaltransformationormappingbetweenFourier transformpairs InMRI therearetwotransformations two dimensionalFouriertransformation Becauseofthisgradient theMRsignalisacomplexsignalmadeupof 由 组成 thecontributionsfromprotonsinvoxelshavingdifferentfrequenciesofprecession MarginFigure24 10 Thefifthstepistorepeatsteps1to4anumberoftimesandaverageth