医学影像物理学课件：14yj-MRI 02.ppt

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1、MagneticResonanceImagingIINuclearMagneticResonancePhenomena&MagneticImagingParameter磁化强度矢量单位体积内的磁矩矢量之和M0,Macromagneticmomentµ,nuclearmagneticmomentM0:Numberofparallelnuclei>NumberofantiparallelnucleiM/L在外部磁场中的运动因为磁矩与角动量方向的一致性,用磁矩来描述:在仅有静磁场的情况下,夹角为0,所以无进动.但上式的意义适用于对M产生影响的所有磁场,包括变化磁场.Resonanceme

2、chanicresonanceelectronspinresonanceferromagneticresonanceProtonresonanceTheincreaseinamplitudeofoscillationofanelectricormechanicalsystemexposedtoaperiodicforcewhosefrequencyisequalorveryclosetothenaturalundampedfrequencyofthesystem.Fromthepointofviewofenergy,wecanregardresonanceisaformofener

3、gyconversion.NuclearMagneticResonanceWhenwegiverfpulseintheperpendiculardirectionofstaticmagneticfield,Nuclearmagneticresonancewilltakeplace.Nucleiinlowerenergystatewillabsorbrfpulseandjumptohigherenergystate,whichisanunstablestate.Whenrfpulseisceased,somenucleiinhigherenergystatewilljumpbackt

4、olowerenergystate.Atthesametime,energywillbereleasedinformofEMwave.NuclearMagneticResonanceRadioFrequencypulseResonanceabsorptionResonanceemissionFreeInductionDecayNuclearmagneticresonanceistheinductionoftransitionsbetweentwodifferentenergystate.RadioFrequencyPulseRfpulseisgeneratedbyancurrent

5、pulseinthecoilnearthesample.ThefunctionofrfpulsecorrespondtotheperiodicforceConditionofrfpulseTheenergyabsorptionofnucleiisinformofquantum,justlikephoto-electroneffect.Thefrequencyofrfpulsemustequatetothefrequencyofprocessionofnuclei.ResonanceAbsorption/激励Theelectricmagneticwavewillinducesthen

6、uclearmagneticmomentfromtheirlowerenergytotheirhigherenergystatewhengetenergyfromoutsideinformofradiofrequency.Thisprocessiscalledresonanceabsorption.吸收RF能量达到不稳定状态的过程.饱和现象低能态核吸收射频电磁波的辐射能跃迁到高能级，如高能态核无法返回到低能态，那末随着跃迁的不断进行，处于低能态的1H核数目与处于高能态1H核数目相等。上述这种现象称为饱和。相干饱和与非相干饱和ClassicalDescription激励过程经典解释射

7、频脉冲与旋转磁场/施加方向在静止磁场与旋转磁场共同作用下的章动围绕旋转磁场旋进造成的能量变化射频脉冲与旋转磁场射频脉冲与旋转磁场的等效旋转坐标系与其中的B1场在静止磁场与旋转磁场共同作用下的章动RfpulsebringarevolvingmagneticfieldB1,MwillrotatefollowingB1also.SowecanregardMwillrotatearoundB0andB1.ThiskindofRotationiscallednutation(章