mri神经科学讲座

IntroductiontothePrinciplesofMRIJensJensenjense@Prismafit1GoodReference/2MRISignalTheMRIsignalisgeneratedbyprotonsinhydrogen.Mostofthesearepartofwatermolecules.InMRIparlance,weoftenrefertotheprotonsasspinssincetheybehaveinsomewayslikelittlespinningtops.Thesignalconsistsofatimedependentmagneticfieldthatthespinsgenerate.Thefrequencyofthissignalisthesameasthatofradiowaves.Itisdetectedbyacoilcalledtheradiofrequency(RF)coil

orreceivecoil.3MRIMainFieldTocreateadetectableMRIsignal,thespinshavetobeinaverystrong,veryhomogeneousmagneticfield.ThisfieldisgeneratedbythemainmagnetandiscalledthemainfieldorB0field.ForhumanMRI,theB0fieldstrengthisusually1.5to7T.ForanimalMRI,itisusually3to15T.Astrongrefrigeratormagnethasastrengthofabout0.01T.BringingmetalobjectsnearanMRIscannerisdangeroussincetheycanbeacceleratedwithgreatforcebythemainmagnet.4MainMagnet5Magnetfor10.5THumanMRI6

Whyweworryaboutmetalnearascanner7ProtonMagneticMomentProtonscangenerateamagneticfieldbecausetheyhaveanintrinsicmagneticmoment.Inotherwords,theyactliketinymagnets.Butifthereisnoexternalfield,thedirectionsofthemagnetsinatissuearerandomsothatthetissuegenerateszeronetfield.Whenanexternalfieldisapplied(e.g.,theB0field),thenthetissueprotonstendtoalignwiththefield,causingthetissuetogenerateitsownfield.8B1FieldInorderfortheprotonstoproduceasignal,thefieldtheygeneratemustchangewithtime.Toinitiatethis,asecondexternalfield,calledtheB1field,mustbeapplied.Thisiscalledspinexcitation.TheB1fieldisaboutamilliontimesweakerthanthestaticB0field,butoscillatesatradiofrequencies(10-1000MHz).TheB1fieldisusuallyappliedforjustinfewmillisecondsinwhatiscalledanRFpulse.This“kicks”thespinssothattheirdirectionsrotate.Thisiscalledspinprecessionandisanalogoustoagyroscopeprecessinginagravitationalfield.TheB1fieldisgeneratedbyasecondRFcoilcalledthetransmitcoil.

9TransmitCoil10SpinExcitation1112GyroscopeanalogySpinPrecessionLarmorFrequency

13SignalDecayAfterspinexcitation,theMRIsignalpersistsforroughly100msdependingonthetissue.ThedecaytimeforthesignaliscalledT2orT2*dependingonsometechnicaldetails.Signaldecayprimarilyreflectslossofspincoherence.Thisisalsocalledtransverserelaxation.14LongitudinalRelaxationAfterthesignaldecays,thespinscontinuetoprecessforaboutasecondortwo,buttheyeventuallyrelaxbacktotheirequilibriumstate.Thisiscalledlongitudinalrelaxation.ThetimescaleforlongitudinalrelaxationiscalledT1.WealwayshaveT1≥T2≥T2*.T1hasanimportanteffectonthetissuecontrastobservedwithMRI.15GradientField

16GradientfieldGradientcoils17ImageFormationWiththegradientsswitchedon,theLarmorfrequencybecomes:Thus,thefrequencyofthesignalencodesspatialinformation.Sincethefrequencycanbemeasuredwiththereceivecoil,thelocationofthesourceofthesignalcanbedetermined.Thisishowanimageisformed.ThegradientfieldisstaticexceptthatitistypicallyswitchedonandoffmultipletimesduringanMRIpulsesequence.Eachtimethegradientsareswitchedonandoffiscalledagradientpulse.AnMRIpulsesequenceconsistsofaseriesofRFandgradientpulses.Tocreateafullimage,manymeasurementshavetobemadewithdifferentRFandgradientpulses.Thistypicallytakesafewminutes,butcanbeasshortas1second.

18PulseSequenceDiagramGradientechosequence19PulseSequenceDiagramSpin

echosequencexxx20TypesofMRIImagesTherearemanytypesofMRIimagesthatcanbegeneratedwithdifferentcombinationsofRFandgradientpulses.Theydifferintermsoftissuecontrast,spatialresolution,imagedistortion,imageartifacts,imageacquisitiontime,andsignal-to-noiseratio(SNR).Forneuroscienceresearch,thethreeimagetypesthataremostcommonlyusedareanatomicalimages,functionalimages,anddiffusionimages.21AnatomicalImagesAnatomicalimagesshowbrainstructureathighresolutionandfidelity.Theyareusefulforidentifyingregionsofinterestandquantifyingthevolumesofspecificbrainstructures.TheseareoftenT1-weighted,whichmeansthatthecontrastisdominatedbydifferencesisT1.AcommonlyusedT1-weightedpulsesequenceiscalledMPRAGE.NeuroradiologistsoftenuseaT2-weightedpulsesequencecalledaT2-FLAIRsinceitisgoodatdetectingpathology.22T1-WeightedImageMPRAGE23T2-WeightedImageT2-FLAIR24FunctionalImagesFunctionalimageshaveacontrastthatreflectstissuefunction.Thisincludesbloodflow,metabolism,andrespiration.Inneuroscience,aspecifictypeoffunctionalMRI(fMRI),calledBOLDfMRI,isusedtostudybrainactivity.BOLDstandsforBloodOxygenLevelDependent.BOLDfMRIdetectssmallchangesinbloodflowcausedbyneurovascularcoupling.Thechangeinflowalterstheoxygenlevelofthebloodinsmallvessels,whichinturnincreasesT2*andT2.BOLDfMRImostoftenusesagradientechosequencewitharapidsignalreadoutcalledechoplanarimaging(EPI).2526NeurovascularCoupling27EffectonT2*GradientEchoEPIsequence28RestingStatefMRIData29DiffusionMRI(dMRI)MRIpulsesequencescanbedesignedsothatthesignalissensitivetothediffusionofwatermolecules.Duringthetimeofsignalacquisition(about100ms),watermoleculesinbiologicaltissuesmoveabout5-20μm.Forthisreason,dMRIissensitivetomicrostructuralfeatures(e.g.,cellsizes)onthislengthscale.Inthebrain,thisisparticularlyusefulforcharacterizingwhitematter,sincemyelinatedaxonsstronglyrestrictwaterdiffusion.dMRIcangenerateparametricmapsofavarietyofdiffusionproperties.Thesecandetectchangesintissuemicrostructurethatarenotclearlyvisibleonanatomicalimages.OneoftheoldestandmostwidelyuseddMRImethodsiscalleddiffusiontensorimaging(DTI).ManyotherdMRImethodsarealsoused.30MeanDiffusivityOneofthemostbasicdiffusionparametersisthemeandiffusivity(MD).Thisisameasureoftheamplitudeofdiffusion.TheMDincreaseswithedemaandneurodegeneration,butdecreasesforacuteischemia.MDmapshowingafocalischemiclesion31FractionalAnisotropyAnothercommondiffusionparameteristhefractionalanisotropy(FA).Thisisameasureoftheextentthatdiffusionhasadependenceondirection.TheFAishighinwhitematterandlowingraymatter.