醫(yī)學(xué)影像物理學(xué)14磁共振成像II

MagneticResonanceImagingIINuclearMagneticResonancePhenomena&MagneticImagingParameter磁化強(qiáng)度矢量單位體積內(nèi)的磁矩矢量之和M0,Macromagneticmomentμ,nuclearmagneticmomentM0:Numberofparallelnuclei>Numberofantiparallelnuclei2M/L在外部磁場中的運(yùn)動(dòng)因?yàn)榇啪嘏c角動(dòng)量方向的一致性,用磁矩來描述:在僅有靜磁場的情況下,夾角為0,所以無進(jìn)動(dòng).但上式的意義適用于對M產(chǎn)生影響的所有磁場,包括變化磁場.3ResonancemechanicresonanceelectronspinresonanceferromagneticresonanceProtonresonanceTheincreaseinamplitudeofoscillationofanelectricormechanicalsystemexposedtoaperiodicforcewhosefrequencyisequalorveryclosetothenaturalundampedfrequencyofthesystem.Fromthepointofviewofenergy,wecanregardresonanceisaformofenergyconversion.4NuclearMagneticResonanceWhenwegiverfpulseintheperpendiculardirectionofstaticmagneticfield,Nuclearmagneticresonancewilltakeplace.Nucleiinlowerenergystatewillabsorbrfpulseandjumptohigherenergystate,whichisanunstablestate.Whenrfpulseisceased,somenucleiinhigherenergystatewilljumpbacktolowerenergystate.Atthesametime,energywillbereleasedinformofEMwave.5NuclearMagneticResonanceRadioFrequencypulseResonanceabsorptionResonanceemissionFreeInductionDecayNuclearmagneticresonanceistheinductionoftransitionsbetweentwodifferentenergystate.6RadioFrequencyPulseRfpulseisgeneratedbyancurrentpulseinthecoilnearthesample.Thefunctionofrfpulsecorrespondtotheperiodicforce7ConditionofrfpulseTheenergyabsorptionofnucleiisinformofquantum,justlikephoto-electroneffect.Thefrequencyofrfpulsemustequatetothefrequencyofprocessionofnuclei.8ResonanceAbsorption/激勵(lì)

Theelectricmagneticwavewillinducesthenuclearmagneticmomentfromtheirlowerenergytotheirhigherenergystatewhengetenergyfromoutsideinformofradiofrequency.Thisprocessiscalledresonanceabsorption.

吸收RF能量達(dá)到不穩(wěn)定狀態(tài)的過程.9飽和現(xiàn)象低能態(tài)核吸收射頻電磁波的輻射能躍遷到高能級，如高能態(tài)核無法返回到低能態(tài)，那末隨著躍遷的不斷進(jìn)行，處于低能態(tài)的1H核數(shù)目與處于高能態(tài)1H核數(shù)目相等。上述這種現(xiàn)象稱為飽和。相干飽和與非相干飽和10ClassicalDescription

激勵(lì)過程經(jīng)典解釋射頻脈沖與旋轉(zhuǎn)磁場/施加方向在靜止磁場與旋轉(zhuǎn)磁場共同作用下的章動(dòng)圍繞旋轉(zhuǎn)磁場旋進(jìn)造成的能量變化11射頻脈沖與旋轉(zhuǎn)磁場射頻脈沖與旋轉(zhuǎn)磁場的等效旋轉(zhuǎn)坐標(biāo)系與其中的B1場12在靜止磁場與旋轉(zhuǎn)磁場共同作用下的章動(dòng)RfpulsebringarevolvingmagneticfieldB1,MwillrotatefollowingB1also.SowecanregardMwillrotatearoundB0andB1.ThiskindofRotationiscallednutation(章動(dòng)).圍繞旋轉(zhuǎn)磁場旋進(jìn)造成的能量變化,由M偏轉(zhuǎn)的角度體現(xiàn).13偏轉(zhuǎn)角/翻轉(zhuǎn)角RF脈沖功率的大小RF脈沖的作用時(shí)間.影響偏轉(zhuǎn)角度的因素:1490oPulseand180oPulseIftherfpulsemakemacromagneticmomentdeflect90o,wecallthisrfpulse90opulse.Iftherfpulsemakemacromagneticmomentdeflect180o,wecallthisrfpulse180opulse.關(guān)于翻轉(zhuǎn)角的說明15resonanceemissionWhenresonanceabsorptionisceased,Nuclearmagneticmomentswillcometotheirlowerenergystatefromtheirhigherenergy.Theenergygottenfromoutsidewillbereleasedandreturnedtooutsideinformofelectric–magneticwave.Thisprocessiscalledresonanceemission.16RelaxationProcessAfterexcitationbytherfpulse,theprocessthatNMRrecovertothermalequilibriumiscalledrelaxation.RelaxationprocessischaracterizedbytwotimeconstantsdesignatedT1andT2.MacromagneticmomentM0canbedividedintoverticalcomponentandhorizontalcomponent,inthesetwodirection,theyareindependentprocess.17Longitudinalrelaxationand

TransverserelaxationLongitudinalrelaxationThenucleiinparallelstatebecomemoreandmorewiththereleaseofenergy.Sointhisdirectionthemacromagneticmomentumrecovergradually.TransverserelaxationWhenthereisaB1,μleantoit,butB1disappear,thistendencydisappeartoo.ThecomponentofμinXYplatearedispersedindifferentdirections.18RelaxationTimeLongitudinalrelaxationTransverserelaxationM0isinitialvalueofmacromagneticmoment.MzisthecomponentofmacromagneticmomentinZdirection(B0).MxyisthecomponentofmacromagneticmomentinXYplate.19T1T2ThegraylevelinNMRimagerelatetoT1andT2.20BlockEquation描述磁化強(qiáng)度矢量在靜磁場與RF射頻以及弛豫中的變化實(shí)驗(yàn)室坐標(biāo)系：旋轉(zhuǎn)坐標(biāo)系：21Block方程分離形式在x′上施加射頻場，在旋轉(zhuǎn)坐標(biāo)系中，有三個(gè)方向上的方程：22Block方程的解穩(wěn)態(tài)解，各方向磁化強(qiáng)度在旋轉(zhuǎn)坐標(biāo)系中變化率近似為0，可以觀察磁化矢量。暫態(tài)解，忽略掉弛豫，觀察在B1場下的磁化強(qiáng)度變化，用于射頻脈沖設(shè)計(jì)。暫態(tài)過程中，如果|rB1|>>1/T1,T2;脈沖寬度<<T1,T2,可忽略弛豫，用于計(jì)算B1進(jìn)動(dòng)中的偏轉(zhuǎn)角。在撤消掉B1后，可用于計(jì)算弛豫過程。23RelaxationRuleLongitudinalrelaxationTransverserelaxationM0isinitialvalueofmacromagneticmoment.MzisthecomponentofmacromagneticmomentinZdirection(B0).MxyisthecomponentofmacromagneticmomentinXYplate.24PhysicalMechanismofRelaxationSpin-latticeRelaxation(thermalrelaxation)

ThelongitudinalrelaxationisalsonamedSpin-latticerelaxationorthermalrelaxation.Theprocessofitisainteractionwithenvironment.Duringitthespindecaybydissipatingenergytothesurroundings.Spin-spinRelaxationThetransverserelaxationisalsonamedspin-spinrelaxation.Theprocessisattributabletotheinteractionofspinswithneighboringnuclearmagneticmoments.Independentprocess.25T1T2meaningT1與溫度，外部磁環(huán)境，粘度有關(guān)，103數(shù)量級T2與溫度，粘度無關(guān)，與主磁場相關(guān)性不大，與主磁場均勻性關(guān)系大DifferenttissueshavedifferentT1andT2.26組織含水量水的運(yùn)動(dòng)程度脂肪含量順磁性粒子的作用Forsametissues,NormalandabnormalhavedifferentT1andT2.組織弛豫的主要決定因素：27T2*IfBisabsolutelyuniform,transverserelaxationisdeterminedbyT2(固有馳豫時(shí)間，本征馳豫時(shí)間）.T2*（實(shí)際馳豫時(shí)間，準(zhǔn)馳豫時(shí)間）：Becauseoftheununiforminmagneticfield,therearelittledifferenceintheresonancefrequency,whichwillmakeMxydecreasemorequickly.SoinfactthetimeT2isshorterthanT2intheory.WeuseT2*representtheactualtimecoefficientintransverserelaxation.28FreeInductionDecay(FID)SignalThedecayofmagneticfieldinXYpla