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(b)(a)R1M1Ri1L1●●●iNRNL2MNiLFIGURE10.13Lumpedcircuitmodelforeddycurrents.Thegradientcoil(a)andeacheddy-currentconductor(b)aremodeledasLRcircuits.EddycurrentsresultfrommutualinductanceMkbetweenthegradientcoilandeacheddy-currentconductor.Themutualinductanceallowsacurrentiinthegradientcoiltoinducecurrentikintheeddy-currentconductors.Eachconductorproducesamagneticfieldproportiontoik+(AdaptedfromVanVaalsandBergman1990;Jehensonetal.1990.)Rdadc-rt+rR-R1+bPSgradientcoil>>PE(δ＝1)computerPSZ0-coil>>PE(δ＝0)FIG.3.(a)Preemphasiscircuit(foroneexponentialterm).Thehigh-passRCfiltergeneratesanexponentiallydecayingsignalwithadjustabletimeconstant(p1=RC)andrelativeamplitude(V1=(r/rt)(R1/Rd)).Summationofthesignaltotheinputstepfunctionproducesthedesiredpulseshape.OtherRCfilterswithdifferenttimeconstantsandgainscanbeaddedinparalleltogeneratemultiexponentials.Thedirectpathdshouldonlyexistinthepreemphasisforthegradientcoil,notforothercoilsliketheZ0coil.(b)Generalcircuitstructure.Thecomputerprovidestheinputfunctiontothepreemphasis(PE)units;thepowersupplies(PS)convertvoltagestocurrentswhichdrivethegradient,Z0,oranyothercoil.C△B0(t)40Hz20Hz0-20Hz-40Hz-60Hz1ms10ms100ms1s4s900180018001800HFGSGPGAADCFIR=1800预脉冲+FSE快速翻转恢复序列