磁共振成像原理左真濤中國科學院生物物理研究所腦與認知科學國家重點實驗室北京磁共振腦成像中心OutlineMRI

historySpindynamicsRelaxationSpatialEncodingBasicMRIsequeceFastImagingPrepulsesMRIhardwareWhat’sMRI?MRIisbasedontheprinciplesofnuclearmagneticresonance(NMR),whichisaspectroscopictechniqueusedtoobtainmicroscopicchemicalandphysicaldataaboutmolecules.Magneticresonanceimaging(MRI)isaspectroscopicimagingtechniqueusedinmedicalsettingstoproduceimagesoftheinsideofthehumanbody.In1977thefirstMRIexamwasperformedonahumanbeing.Ittook5hourstoproduceoneimage.TheMRIisaccomplishedthroughtheabsorptionandemissionofenergyoftheradiofrequency(RF)rangeoftheelectromagneticspectrum.3TMRI7TMRI9.4TMRI11.7TMRITimelineofMRImaging1920193019401950196019701980199020001924-Paulisuggeststhatnuclearparticlesmayhaveangularmomentum(spin).1937–Rabimeasuresmagneticmomentofnucleus.Coins“magneticresonance”.1946–Purcellshowsthatmatterabsorbsenergyataresonantfrequency.1946–Blochdemonstratesthatnuclearprecessioncanbemeasuredindetectorcoils.1972–DamadianpatentsideaforlargeNMRscannertodetectmalignanttissue.1959–SingermeasuresbloodflowusingNMR(inmice).1973–Lauterburpublishes

methodforgeneratingimagesusingNMRgradients.1973–MansfieldindependentlypublishesgradientapproachtoMR.1975–Ernstdevelops2D-FouriertransformforMR.NMRrenamedMRIMRIscannersbecomeclinicallyprevalent.1990–Ogawaandcolleaguescreatefunctionalimagesusingendogenous,blood-oxygenationcontrast.1985–InsurancereimbursementsforMRIexamsbegin.201020201998–Ohio8THumanMRIScanner.1999–CMRR7THumanMRIScanner.1998–Chicago9.4THumanMRIScanner.2017–CMRR10.5THumanMRIScanner.2019–NeuronSpin11.7THumanMRIScanner.NobelPrizesforMagneticResonancePaulC.Lauterbur2003PhysiologyorMedicineSirPeterMansfield(MRItechnology)2002Chemistry(3DmolecularstructureinsolutionbyNMR)KurtWüthrich1991ChemistryRichardErnst(High-resolutionpulsedFT-NMR)1952PhysicsFelixBlochEdwardMillsPurcell(BasicscienceofNMRphenomenon)1944PhysicsRabi(Measuredmagneticmomentofnucleus)中科院生物物理所腦成像平臺3T人體MRI2002年安裝2003年運行7T人體MRI2013年安裝2014年運行2009年安裝2010年運行MEG9.4T人體MRI2010年啟動研制2017年安裝正在調(diào)試11.7T動物MRI2019年底安裝2020年3月運行15.2T動物MRI2021年啟動自主研制，已得到北京市部分資金支持Triantafyllou,NI,2005生物醫(yī)學成像設施3TMRI7TMRIPET/MRB0=constantmagneticfieldAlongz-axisForfMRI1.5Tor3TZThe

Magnet

in

MRI

ScannerThespinsTheatomProtons(positivecharge,p+)Neutrons(nocharge,n)Electrons(negativecharge,e-)Theatomicnumber,Z

#ofprotonsTheatomicmassnumber,Athesumof#protonsand#neutronsMotionwithintheatomElectronsspinningabouttheirownaxesElectronsorbitingthenucleusThenucleusitselfspinningaboutitsownaxisnuclearspinp+p+nne-e-NuclearspinThenetspinofthenucleusderivesfromtheindividualspinsofprotonsandneutronswithinthenucleusThespinPisdifferentfordifferentnucleiP=0 Nospin,cannotbeusedforMRZandAbothevennumbersP=1/2,3/2,5/2,… halfintegerspinoddnumberedAP=1,2,3,… integerspinevenAandoddZ1HnucleiandMRNucleus:1protonSpin?(spinquantumnumber)Thehumanbodyiscomposedoftissuesthatcontainprimarilywaterandfat,bothofwhichcontainhydrogen.Hydrogenhasahighresponseonexternalmagneticfields,alargegyromagneticratioPeriodicTableoftheElements1H23Na31P17O13C19F128XeNclear

Spin

and

MagneticmomentNuclearspin,PIntrinsicproperty,angularmomentumTheprotonshaveaspinandtherebyamagneticmoment

.Gyromagneticratio:WecancomparethemagneticmomentwithabarmagnetoracompassneedleThemagneticmomenthasasizeandadirectionandisnormallydescribedbyavector

SNmBThenucleusmagneticmomentProtonshaveaspinandthereforeamagneticdipolemomentMDMExternalmagneticfieldsintwoways:paralleloranti-parallelOuter

ScannerInside

ScannerSpins

in

MagnetTheprotonsoftheH2OmoleculesinourbodyalignalongB0NetmagnetizationWhenasample/patientisplacedinanexternalmagneticfield(asintheMRscanner)Theprotonsinthesample/patientwillactassmallmagnets/compassneedlesTheywillpartiallyalignwiththeexternalmagneticfieldThenetmagnetizationvectorisavectorsumofallmagneticdipolemomentsThenetmagnetizationisparalleltotheexternalmagneticfieldThisisanequilibriumsituationRoom

temperature

in

KelvinTotal

number

of

SpinsStrength

of

external

magnetic

fieldSmall

NMR

signalThenetmagnetizationvectorisgivenasInequilibriumBoltzmann’s

constantPlanck’s

constantPrecessionTheindividualmagneticmoments,m,hasanangle,q,tothedirectionoftheexternalmagneticfield,B0.producesatorque,C,onthemagneticmoment.yxzorPrecessionandLarmorfrequency

ω=γ·B0ωLarmorfrequencyγgyromagneticconstantB0staticmagneticfielde.g.1.5T

ω=64MHzzaxisx-yplaneTheindividualmagneticmoments,m,hasanangle,q,tothedirectionoftheexternalmagneticfield,B0.

PrecessionandphasePrecessionhappenswithacertainangularvelocity,wjcanbemeasuredinradiansordegreesTheaccumulatedphaseyxzxyzThephase,j,oftheprecessiontellsuswhereonthecircle,thetipofthemagneticmomentisPrecessionandphaseRotatingcoordinatesystemThemagneticmomentsareprecessingwiththeangularfrequencygivenbytheLarmorequationy’x’z

Rotating

coordinate

is

acoordinatesystem,thatrotateswiththeLarmorfrequency,wx,y,zturnstox’,y’,zThemagneticmomentstationaryseenintherotatingcoordinatesystemNetmagnetizationvectorandRF-pulseAtequilibriumthenetmagnetizationvectorisstationaryIncludingtheRF-pulsewiththeB1-fieldRotatingcoordinatesystemx,y-planerotateswiththeLarmorfrequency,w0M(t)rotatesatw0aroundB0yxzM0

ThemotionofM(t)isreducedtoarotationaroundB1(t),whichnowisstationaryinthex’y’-plane

RF

pulse

in

rotating

coordinate

systemThemotionofM(t)isreducedtoarotationaroundB1(t),whichnowisstationaryinthex’y’-planeFlip

down

the

net

magnetization

from

z

to

x’-y’

planex’zy’

x’zy’B0x’zy’B0zaxisx-yplanezaxisx-yplanebeforeafterRF

pulse

Flip

down

the

SpinsRF-pulseandflipangleTheRF-pulse(andtherebytheB1-field)flip

down

net

magnetization

(M).Thepositionisdescribedbytheflipangle,aTherotationvelocityaroundB1Wecantherebydeterminethetimeduration,t,theRF-pulseneedstobeturnedon,inordertogetthedesiredflipangle,a

Differentflipangles,αDependsonthetimedurationandstrengthoftheRF-pulse90°flipangleNolongitudinalcomponentofthenetmagnetizationvector,M45°flipangleBothtransverseandlongitudinalcomponentofM180°flipangleNotransversecomponentofMNMR

SignalSignalFrequency=LarmorfrequencySizeofthesignaldependsonthesizeofMinthetransverseplaneRelaxationThenetmagnetizationvectorhasacomponentinthetransverseplaneTheprocessto”getbackinequilibrium”

--

RelaxationTwoindependentprocessesareinplayT1relaxation–recoveryofthelongitudinalmagnetizationT2relaxation–decayofthetransversemagnetizationxzyxzyRelaxationRelaxationoccursbecauseofinteractionsbetweentheprotonsandsurrounding(tumblingandcollisions)Tumbling:translation,rotationandvibrationT1relaxation/decayT1relaxation–recoveryoflongitudinalmagnetizationExponentialrecoveryDifferenttissuehasdifferentrelaxationrateThesizeofthemagnetizationvectorinthelongitudinaldirectionrecoversovertimeEnergyexchangewiththesurroundingsattheLarmorfrequencyxzyB0zM1T%630MtimeM

T1relaxationThisextraenergywillbegiventothesurroundings(spinlatticerelaxation)bycollisionsTumbling:translation,rotationandvibrationFluctuatingmagneticfieldsMosteffectiveattumblingfrequenciesaroundtheLarmorfrequencyAnexponentialprocessTimeconstantcalledT1Itisdefinedasthetimeittakes63%oftheoriginallongitudinalmagnetizationtoberecoveredinthetissueT1-relaxationdependsontumblingfrequenciesaroundtheLarmorfrequencyMassofmolecules(tumblingfrequencydependsonthesizeofthemolecules)ViscosityTemperature(tumblingfrequenciesdependsonthetemperature)B0(T1becomeslonger,whenB0becomeslarger)T1relaxationTissueswithdifferentT1waterfatGreymattersignaltime[msec]T2relaxation/decayT2relaxation–decayoftransversemagnetizationDecayoftransversemagnetizationleadstolossofsignalFreeinductiondecay(FID)Atthesametime–butindependently–thetransversemagnetizationwillgraduallydecrease–decayThemagneticmomentslosescoherence(duetolocalmagneticfieldinhomogeneities)–aprocesscalleddephasingxzyB0xyMt2T%37Mxy37%T2timeT2relaxationLossofcoherenttransversemagnetization(dephasing)Theprotonswillcollideandmovearoundclosetoeachother(tumbling).TheywillfeelmagneticfieldsfromeachotherandthesurroundingsMagneticfieldinhomogeneitiesProtonsgetdifferentphasesThetotalnetmagnetizationbecomessmallerandsmaller…T2relaxationT2relaxationTissueswithdifferentT2time[msec]signalwaterfatGreymattertissueT1[msec]T2[msec]Whitematter60080Greymatter950100CSF45002200Muscle90050Fat25060Blood1200150T2*relaxationThefieldinhomogeneitiescausingthedephasingcanbecategorizedin2scales:MacroscopicFieldinhomogeneitieslargerthanthevoxelsizeFieldinhomogeneitieslargerthanthevoxelsizeCreatedfrom:InhomogeneitiesinthemainmagneticfieldB0Susceptibilityeffectsarounde.g.thesinuscavitiesThiscontributiontotherelaxationistermedT2’MicroscopicFieldinhomogeneitiessmallerthanthevoxel,andatthesamesizeasatomsandmoleculesTypicallygeneratedfromTumblingofmoleculesandatomsThiscontributiontotherelaxationistermedT2ThetotaltransverserelaxationistermedT2*MicroscopicMacroscopicTotalBloch

equationNetMagnetization(M)placedinamagneticField(B)willprecess:andrelax(R)andrelaxrelaxindividualcomponents

Gradients1.50TPositionB1.48T1.52TGradients:smallmagneticfieldsontopofmainmagneticfieldThespatialslopeisgradientstrengthOnegradientforeachspatialdirection:x,y,zThegradientsareturnedonandoffrapidlyduringscanningIsocenterEncoding

the

position

of

MRIω=γ·（B0+Gz）/frequency-encoding.htmlhttp://sfb649.wiwi.hu-berlin.de/fedc_homepage/xplore/ebooks/html/csa/node255.htmlGradientsBGz-gradienty-gradientx-gradientSuperconductingmagnetBGBGSliceSelectionTheresonancefrequencyasafunctionofzSlicethickness

zw(z)zs

ΔzGz

SliceselectionDzByRFexcitationofthenetmagnetizationvectorAsliceofthepatientisselectedTH=DzTheresonancefrequencyasafunctionofzSlice

profileTimedomainwsinc

FTTimedomainw

Sliceprofileafrequencyband(BW:bandwidth)IdealfrequencyprofileofRF-pulseImperfect

slice

profileSlice

profile

RealisticsliceprofileCrosstalkSpacebetweenslicesInterleavedslicesFWHMTH/2-TH/2THDephasingPrecessioninanon-uniformmagneticfieldLeadstoincoherentprecessionofspins->dephasingBecauseofdifferentprecessionfrequenciesExtradephasingontopofT2*-decayDephasinggradientsarealsocalledspoilers(theyspoilthesignal)Frequency

Encoding

PhaseencodingPhaseencodingprinciplePhasegradients,strengthG,durationWatermoleculesatdifferenty-positionsgetdifferentphase-offsetsControlleddephasing90°yK-Space

FourierandinverseFourierρ(x,y)ρ(x,y)s(kx,ky)s(kx,ky)k-spaceForaslicethedataistherebyrepresentedass(kx,ky)inthe(kx,ky)-spacek-spaceTheMRsignalfordifferent(kx,ky)aftertheexcitationpulse:Gx=Gy=0(kx,ky)=(0,0)kykxK-spaceandthereconstructedimageProtondensity,r(x,y)Signal,s(kx,ky)

k-spaceandresolutionHighresolutionintheimageNeedtosampleabigenoughk-space(highk-values)Sizeofk-space,Wi,isgivenasHavetosamplethek-spacedenseenoughbecausetheFOVandtheresolutionink-spacearerelated:K-spaceSamplingofk-spacek-spaceandtheMRimagekykxttPulseSequencediagrams90oAcquirethesignal90oDatapointsFreeInductionDecay(FID)

RFSignal90oFID90oFIDTRsignaltimeT2*decayMRSpectrascopyThenuclearprecessionratedependson:ThetypeofnucleusGyromagneticratio,e.g.γH=42.6MHz/TTheappliedmagneticfieldstrength,e.g.3TThemolecules,andtheirorientation:Chemicalshift:Field-shieldingbyelectroncloud,e.g.2ppm

Scalarcoupling:Nucleiinteractmagneticallyviaelectroniccloud.

Dipolarcoupling:Directmagneticinteractionofnuclei.Affectsrelaxationratesofensembles.Fourier

TransformationFouriertransformationTimeFrequencyInversFouriertransformationFouriertransformationInversFouriertransformation7TeslaSpectrumfromHumanBrainTkac&Gruetter2005WatersignalhasbeensuppressedGradientEchoGradientechoNo180

pulsesasinSpinEchoEchoesgeneratedthroughgradientreversalRFGTEEchoGradientEcho-rephaseHowgradientsrephaseByreversingthegradient,rephasingwilloccurSignalformedlikeanecho(calledagradientecho)AllthishappensontopoftheT2*decayEchoconditionConditionforgradientechoGradientA/2A-GTimet0t0+tGt0+3tGGradientechoandk-spacekykxARFGTEABCCBPluse

SequenceK-space

GradientechoSignalRFSignal90oFIDEchoT2*decayGradientEchoPulseSequenceTETE90o90oRFSignalGradientA/2A/2AAEchoEchoFIDFIDTR-G-GGGTimet0t0+tGt0+3tGMacroscopicdephasingeffects(T2’)canberefocusedAdvantagesT2decayinsteadofT2*decayTimeindependentfieldinhomogeneitieswillberefocusedAllthreeweightingsarepossiblewithappropriatetimingparameters(TRandTE)T1w,T2w,T2*wLargersignalSpinEchoRFSignal90o180oFIDEchoDecaywithT2DecaywithT2*SpinEchoformationxyB0RFSignal90o180oFIDEchotimezxzyB0RFSignal90o180oFIDEchotimezSpinEchoformationxzyB0RFrefocusing-pulszRFSignal90o180oFIDEchotimezSpinEchoformationxzyB0RFrefocusing-pulszRFSignal90o180oFIDEchotimezB1SpinEchoformationxzyB0zzRFSignal90o180oFIDEchotimezSpinEchoformationxzyB0NMVSpinEchozzzRFSignal90o180oFIDEchotimezSpinEchoformationSpin

echo180°pulse90°pulseSpinechoRephasingDephasing

Spin

Echo

SignalRFSignal90o180oFIDEchoT2decayT2*decayRefocusingpulseflipsallspins180degreeThephaseisinverted:φafter=-φbeforeInk-space:correspondstoadiagonalflip:(kx,after,ky,after)=(-kx,before,-ky,before)SpinEchoandk-space(kx,ky)(-kx,-ky)180°Inversion

recoveryRFSignalTITR180o180o90o90oFIDFIDTIzyxM0zyxM0zyxM0zyxM0M0Mz-M0tTissueshavedifferentT1-valuesT1-recoveryFat180°90°TIShortT1tissueWaterLongT1tissue:Mz=0Mztime-MzNull

Tissue

signalTissueshavedifferentT1-valuesFat180°90°TIShortT1tissueWaterLongT1tissue:Mz=0Mztime-MzNulling

Tissues：

CSF：FLAIR，MPRAGE

Fat：STIR

Image

Contrast

T1weightingNMVtimeRepetitiontime,TRT1>T1timeNMVtimeMeasuringtime,TET1weightedImagefatCSF

T2weightingNMVNMVtimeRepetitiontime,TRNMVtimeMeasuringtime,TET2/T2*weightedimagefatCSFSpinechoinversionpulserefocusesonlystaticinhomogeneitiesT2andT2*decayT2*decayT2

decay1st

echoRelativesignalamplitude2nd

echoTimeFIDT2

and

T2*weighted

imagesGradientechoLongTRLongTET2*weightingSpinechoLongTRLongTET2weighting

Proton

density

weightingNMVNMVtimeRepetitiontime,TRNMVtimeMeasuringtime,TEProtondensityweightedFast

ImagingStandard

Spin

echo90o90oFast

Spin

Echo180

180

90

90

Fastspinecho90o180oADCGsliceRFGphaseGfrequencyTTRPhaseencodingisrewoundbeforethenextrefocusingpulse180oStandardGradientEchoTR90o90oRFGsliceGfrequencySignalGphaseOnephaseencodinglineink-spaceperexcitationSpeedUpGradientEchoSequenceAcquiremorelinesateachexitationEPI,FSESampleK-spacemoreefficientlyspiralscanningParallelimaging(multi-coilimaging)SENSE,SMASHDisadvantageReducedsignal-to-noiseratioMoreartifactsPatientheatingfromRF-pulses(higherSAR)LesscontrastMzDuetoshortTR,fulllongitudinalrelaxationwillnotoccurAftersomeexcitationsasteady-statewillprevailSteady-statedefinition:Sn=Sn-1MztTRSteady-state(n-1)’thexcitationn’thexcitationOptimalflipangle

SpoilingSpoilingremovestransversalmagnetizationbeforenextexcitationGradientspoilingGradientsdephasesignalbeforeexcitationRFspoilingRFpulsephaseincreaseby~117

SpoilersRewinderGradientspoilingRFspoilingTwosequencetypesSpoiledsequenceSignalfromFIDT1-(orPD)-weightedNames:SPGR,FLASH,RF-FAST,T1-FFEUnspoiledsequenceSignalfromFID+echoesMoreT2-weightedNames:GRASS,FISP,FAST,FFEMoresignalB-SSFPBalancedsteady-state-free-precessionUnspoiledfastgradientechosequenceOthernames:TrueFISP,FIESTA,balancedFFEGradientssymmetricinTRVeryshortTE,TRHighsignal-to-noiseratioEcho

planar

ImagingFastmethodwhereentire2Dk-spacecanbesampledafteroneexcitationGsliceGphaseGfreqRFEPI,artifactsEPIissensitivetoanumbersoferrorsB0inhomogeneityGradientimperfectionShortT2ThespatialresolutionisusuallylowImageghosted(replicated)FOV/2inphasedirectionCausedbyalternativeacquisitiondirectioninK-spaceEPISpin

EchoPhaseretning1.

Tag

inflowingarterialbloodbymagneticinversion2.

Acquirethetagimage3.Repeatexperimentwithouttag4.

Acquirethe

controlimage

/~fmri/asl.htmlPrincipleofASLpulsedASL(PASL)continuousASL(CASL)pseudocontinuousASL(PCASL)velocity-selectiveASL(VS-ASL)

Brightness

rCBFDifferentASLTechniquesGaussianstaticmotionstaticmotionln(Sb/S0)bMRDiffusionWeightedImagingADCb=0s/mm2GzGyGxFA<D>DECNosymDECLinearLineFieldPlanarSpherical2Dand3Dimaging2D:excitationslicebyslice3D:excitationofvolume3D:phaseencodingalsoinslicedirection3DK-spaceReconstructionby3DFourierTransformRFGsACQGpGmParallelMRIThespatialsensitivityofcoilelementscaninprinciplebeuseddirectlyforverylowresolutionimagingBasedonuseofmulti-elementcoilsTheindividualcoilshavedifferentspatialsensitivityprofilesThespatialsensitivityoftheelementsarecombinedwithnormalphaseencodingSpole1Spole2Spole3Spole4Parallelimaging,SENSEOnlyeach4.ky-lineissampledFOVreducedto1/4Coil1Coil2Coil3Coil4Coil1Coil2Coil3Coil4K-space

4equationswith4unknownscanbesolvedbylinearalgebraParallelimaging,SENSEOnlyeach4.ky-lineissampledFOVreducedto?NumberofcoilsmustbeequaltoorlargerthanaccelerationfactorMorecoilsgiveshigherSNRNaming:Philips:SENSESiemens:mSENSEGE:ASSET4equationswith4unknownscanbesolvedbylinearalgebraCoil1Coil2Coil3Coil4Coil1Coil2Coil3Coil4K-space

Parallel

imaging,GRAPPAGRAPPA:GeneRalizedAutocalibratingPartiallyParallelAcquisitionsK-spacebasedparallelimagingmethodSignal-to-noiseratio

MRI

systemMagnetProducesthestrongmainmagneticfieldRFsystemTransmittersystem:GeneratestheB1fieldsReceiversystem:acquisition,amplificationanddigitalizationoftheMRsignal

RFCoilsGradientAddslinearlyvaryingB-fieldsinscannerForspatiallyencodingthesignalConsoleGeneratethePulsesequenceImagereconstructionMainmagnetMagneticfieldgeneratedbyelectromagnetwithmanywindings3Trequiresapprox250Aina15000windingssolenoidCoilmadeofNiobium-Titan-Copper-Alloycooleddownto4.2KissuperconductingCoilsubmergedinliquidhelium(300-500L)Heevaporates,recycledbycold-headInmodernmagnets,100%HerecyclingBruker

11.7T

animal

MRIDesirablemagnetpropertiesMagneticfieldstrength:Animal:0.5T,1.5Tforclinical;3.0T,4.7T,7.0T,9.4T,11.7T,15.2T,21TforresearchHuman:0.35T,0.5T,0.75T,1.0T,1.5T,2.0T,3.0T,4.0T,4.7T,5.0T,7.0T,8.0T,9.4T,10.5T