医学影像物理学课件：15yj-MRI 03.ppt

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1、MagneticResonanceImagingIIIMagneticImagingMethodT1,T2-WeightedMR-ImageThesignalreceivedinanexperimentcontaininformationabouttheconcentrationofnucleibeingexaminedandalsoabouttheirrelaxationtimesT1andT2.SothegrayscalesignalinMRIimagecanrepresentconcentration,T1,orT2.IfwechooseT1,thengetT1-weighte

2、dimage.ProtonDensityWeightedMR-ImageConcentrationofhydrogennucleiBoneTissuesandAirChamberOther:FlowEffectMagneticSusceptibilityChemicalShift(BasicofSpectrum)DiffusionGenerationofMRIsignalFIDSpinEchoInversionRecoveryGradientEcho*FIDDependonHdensityandT2*.BecausetheFIDsignaldecayintimeT2*,itishar

3、dtogetT1andT2fromsinglesignal.90º脉冲的终止会引起振荡，对后继信号的采集有影响SpinEcho90º-FID-180º-EchoEchowave的出现TI,TRActionof180ºPulse90ºpulsemakeM0toXYplateDifferentprocessionspeedleadtodephase180ºpulsemakeµdirectiontogether(rephase)DephaseprocesstakeplaceandNMRsignalbeemittedagainTR>>T1、TE<>T1，T

4、E≥T2DifferentTWeightedImageAdjust流速函数：g(v)Inversion-RecoverySequence180ºpulsemakeM0reverseitsdirection.When90ºpulseisgivennucleisystem,FIDsignalbeemitted.TheinitialamplitudeofFIDisproportionalwithM(t).MeasureT1IRWeightedImageDifferenttissuedifferentT1processBraintissueCerebrospinalfluid90ºpulse

5、indifferenttimewillleadtodifferentweightedT1image.IRSEPulseSequenceAmplitudeofIR反转点90º后的恢复，在T1<PWI;长TI,长TE,长TR->PWI（T2WI)TI决定T1，T

6、E(τ)决定T2，为保证M0的充分恢复，TR要足够长。MainParameterTR：重复时间TI：复相脉冲时间TE：回波时间Alpha角：翻转角总结：控制上述不同参数，获取不同的加权成像。MRIImageMethod图像灰度与信号的强度空间定位投影法2D傅立叶变换法MRI的坐标系IfwereconstructureaimagewithT1orT2,wheretheNMRsignalcontainingT1T2informationcomefrommustbeknown.ThisalsomeansweneedtolocatethesignalsourceofFID.坐标系的建立右手螺

7、旋zyx梯度场GradientMagneticField梯度的概念梯度磁场的产生梯度磁场与主磁场的迭加Consistoftwocurrentcoilwhosecurrentdirectionsareinverse.Adjustthedistancetogetalineargradientfield梯度磁场与主磁场空间迭加Thestrengthofmagneticfieldchangewithspatialposition.LinearChange空间编码/