最新clinical magnetic resonance imaging an introduction (part ii)临床磁共振成像导论（第二部分教学讲义ppt.ppt

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1、ClinicalMagneticResonanceImagingAnIntroduction(PartII)临床磁共振成像导论（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

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