Introduction to MRI - MRI NMR spectroscopy proton

1BrianHargreavesStanfordUniversityIntroductiontoMagneticResonanceImaging2MagneticResonanceImaging•Non-invasivemedicalimagingmethod,likeultrasoundandX-ray.•Clinicallyusedinawidevarietyofspecialties.AbdomenSpineHeart/Coronary3MagneticResonanceImagingAdvantages:•Excellent/flexiblecontrast•Non-invasive•Noionizingradiation•ArbitraryscanplaneChallenges:•Newcontrastmechanisms•Fasterimaging4MRISystemsAt$2million,themostexpensiveequipmentinthehospital…5Topics•MagneticResonance•MRImageFormation•Contrast•ApplicationsofMRI6MagneticResonance•Certainatomicnucleiincluding1Hexhibitnuclearmagneticresonance.•Nuclear“spins”arelikemagneticdipoles.1H7Polarization•Spinsarenormallyorientedrandomly.•Inanappliedmagneticfield,thespinsalignwiththeappliedfieldintheirequilibriumstate.•ExcessalongB0resultsinnetmagnetization.NoAppliedFieldAppliedFieldB08StaticMagneticFieldLongitudinalTransverseB0zx,y9Precession•Spinsprecessaboutappliedmagneticfield,B0,thatisalongzaxis.•Thefrequencyofthisprecessionisproportionaltotheappliedfield:B10Relaxation•Magnetizationreturnsexponentiallytoequilibrium:•LongitudinalrecoverytimeconstantisT1•TransversedecaytimeconstantisT2•Relaxationandprecessionareindependent.PrecessionDecayRecovery11SignalReception•Precessingspinscauseachangeinflux(F)inatransversereceivecoil.•Fluxchangeinducesavoltageacrossthecoil.SignalyxB0zF12Excitation•“Excite”spinsoutoftheirequilibriumstate.•TransverseRFfield(B1)rotatesatB0aboutz-axis.B1MagnetizationB0RotatingFrame13MRImageFormation•GradientcoilsprovidealinearvariationinBzwithposition.•Resultisaresonantfrequencyvariationwithposition.BzPosition14SelectiveExcitationFrequencyMagnitudeTimeRFAmplitude(a)(b)Position(d)(c)Slope=1GFrequency15ImageAcquisition•Gradientcausesresonantfrequencytovarywithposition.•Receivesumofsignalsfromeachspin.FrequencyPosition16ImageReconstruction•Receivedsignalisasumof“tones.”•The“tones”ofthesignalaretheimage.•Thisalsoappliesto2Dand3Dimages.FourierTransformReceivedSignalImage17PulseSequences•Excitationandimagingareseparate.•Pulsesequencecontrols:•RFexcitation•Gradientwaveforms•Acquisition•Reconstructioninformationaswell.181D-PulseSequenceRFGzGxAcq.ExcitationImaging191D-PulseSequence–Detailed!RFGzGxAcq.Phase,ModulationFrequencyFiniteamplitude,slewrate•Demodulationfrequency,phase•Samplingrateandduration20MRSignalrdermtsiftV2rGBrBrf022rdeermtsdtrGtBV0rdermetsdtrGVtB021k-spacedtGtkt02rdermtsdtrGVrdermtksrkiV2,tkkrmtsF22k-SpaceTrajectorieskxkykxkykxky2DFourierTransformEcho-PlanarSpiral232DFT-PulseSequenceRFGzGxAcq.Gy24Spiral-PulseSequenceRFGzGxGykxky252DImageReconstructionkykxFrequency-space(k-space)Imagespace26Resolution•Imageresolutionincreasesashigherspatialfrequenciesareacquired.1mm2mm4mmkykxkykxkykx27ImageNoiseandSNRLowSignal-to-NoiseRatioHighSignal-to-NoiseRatio28Contrast•Contrastisthedifferenceinappearanceofdifferenttissuesinanimage.X-raycontrastisbasedontransmission.29ContrastinMRI•Hydrogen(water)densityresultsincontrastbetweentissues.•Manyothermechanisms,somebasedonrelaxation.30T2ContrastCSFWhite/GrayMatterSignalTimeLongEcho-TimeShortEcho-Time31T1ContrastSignalTimeSignalTimeShortRepetitionLongRepetitionCSFWhite/GrayMatter32SomeApplicationsofMRI•Brain/Spineimaging•KneeImaging•CardiacImaging33KneeImaging-Ligaments•MRIis97%accurateindiagnosinganACLtear.HealthyACLFull-ThicknessACLTear??34KneeImaging-Menisci•MRIisthebestnon-invasivemethodofdiagnosingmeniscaltearsFSEDEFT35KneeImaging-Cartilage•Highresolutionimagesbegintoshowcartilagestructure:•0.4x0.4x2mm3resolution•5minutescantimeCartilageDEFT5min.DEFT5min.Bone(fromErickson–1997)36Real-TimeInteractiveMRI•Shows“live”images.•Usefulwhenthereismotion,suchasinthechest.•Imagingisveryfast,butSNRislower.•Interactiveimagingallowsustomovethescanplaneinreal-time.37CoronaryArteryImaging3839====EXTRAS====40FieldofView•SamplingdensitydeterminesFOV.•Sparsesamplingresultsinaliasing.kxkykxky41Off-Resonance•Practically,themagneticfieldstrengthisnotperfectlyuniform.•Resonantfrequencyisproportionaltofieldstrength:zyxzyxB0Off-resonance:42SpinEchoes•180°RFtipcanreversethedephasingeffectsofoff-resonance.•Spinsrealignatsometimetoformaspinecho43ImagingSequences•Imageacquisitionusuallyrequiresmultiplerepetitions.90°x90°x90°x90°xTRTRTR44ImageNoiseandSNRLowSignal-to-NoiseRatioHighSignal-to-NoiseRatio45SNREfficiency•CanimproveSNRbysimplyaveraging.•UseSNRefficiency,hSNR,asafaircomparisonofSNRbetweendifferentimagingmethods.scanSNRTSNRh46Summary•B0polarizesatomicnuclei•SpinsprecessandrelaxtoalignwithB0.•B1allowsmanipulationofmagnetization.•Excitationsequencesprovideimagecontrast.47MotivationforCartilageImaging•Osteoarthritishasahighincidence(15%oftheCanadianpopulation)•Assessnewtreatmentsforcartilagedegeneration•Alternativetoarthroscopy(invasiveandexpensive)48SynovialFluidPatellaPatellarCartilageFemoralCartilageFemurCartilageImaging:KneeAnatomyFemurTibia(T2-weightedwithFatSuppression)(T1-weighted)AxialImageSagittalIm