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1、ClinicalMagneticResonanceImagingAnIntroduction(PartII)臨床磁共振成像導(dǎo)論（第二部分）OUTLINE(part1)History+LocaloriginsofMRIFundamentalPhysicsofMRITissuecontrast+VersatilityHistoryLessonCarlFredrichGauss.(1777-1855)GermanPhysicistFindingsledtoaknowlegdgeofmagnetismanditsquantifiationGauss-unitofmeasu
2、rementofmagnetismNikolaTesla(1856–1943)SerbianElectricalEngineerWorkinelectromagneticinductionTesla–unitofmeasurementforMagneticFieldstrengthNuclearMagneticResonanceMAGNETICMOMENTALIGNSINB0+NoB0:randommotionB0:alignmentNuclearMagneticResonanceLarmourEquation:?=?B0RESONANCE:REQUIRESCON
3、SIDEDERATIONOFSPECIFICPRECESSIONALFREQUENCYOFATOMICSPINSNMR–CLINICALMRICHHCHHCHHCHHFattyacidchainHHOWaterFat+Water=99%bodytissueH+:ALIGNMENT+PRESCESSIONNMR–CLINICALMRIApplytheLarmorequationH1@1T:?=42.58MHzT-1@1.5TLarmorfrequency=63.87MHz(fallswithintherangeofradiowaves)Resonance+Excit
4、ationEnergyabsorptionhappensifspinningnucleiare“hit”withradiationatofthesamefrequencyofthespinLeadstomisalignmentwithB0Alsoleadstophasecoherence.ThisisExcitationRelaxation: FreeInductionDecay(FID)RemovetheRFandthespinswillloosetheirenergy.RealignwithB0EnergydecaysslowlyLoosephasecoher
5、anceFIDExcitation+Relaxation=MRSignal+CurrentinducedinRFcoilduetoalternatingBfield=MRIsignalBoNMV90RFpulseImageFormationSpatialLocalisationofPixelsZYXImageFormationPhaseFrequencySliceResolutionFewpixelsShortscantimeManypixelsLongscantimePixelMappingEachlineofdataisstoredastheImageisbu
6、iltupgraduallyFouriertransformdecodesdata+formstheimagePhaseFrequencyTheMReffect!DifferingMRImagesT1FluiddarkT2FluidbrightRelaxation: FreeInductionDecay(FID)Thespinswillloosetheirenergyintwoways:EnergydecaysslowlyRelaxingbacktoB0T1RecoveryLoosephasecoheranceT2decayBrownianMotionFAT:La
7、rge,slowmoleculesLotsofbumpsFastenergylossShortT1+ShortT2WATER:Small,fastmoleculesFewerbumpsSlowenergylossLongT1+longT2MrBlobbyVsSpeedyGonzalez!TypicalT1+T2valuesfortissues(@1.5T)TissueT1valueT2valueDistilledwaterCerebroSpinalFluidGrayMatterWhiteMatterFatMuscleLiverKidney3000240090078
8、02607
