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1、ReceiverSystemsArneReykowskiInvivoCorporation1.IntroductionThispresentationisdesignedtogiveanoverviewofthebuildingblocksofanMRIreceiveRFchain,startingwiththelocalMRIcoilgoingallthewaytotheimageprocessor.Furtherinformationonthistopiccanbefoundin(1).Thisbookcontainsadetailed
2、introductiontoMRIaswellasotherdiagnosticimagingsystems.Figure1showsatypicalreceivechainconsistingofthefollowingbuildingblocks:?RFcoils?Detuningcircuits?Matchingnetworks?Lownoisepreamplifiers(LNAs)?ModeMatrixandEigencoilcombinercircuitry?CoilcablesandBaluns?Antennaselector?
3、Automaticleveladjustments?ReceiversLocalRXCoilswithintegratedLNACoilMatchLNACombinerNetworkAutomaticCoilMatchLNALevelAdjustmentReAntennaImageCoilMatchLNAReceiverSelectorProcessorIm(NoutofM)MNCoilMatchLNAFigure1:BuildingblocksofatypicalRFreceivechain.ThepurposeoftheRFchainc
4、anbesummarizedasfollows:“SelectandextractasetofMRIsignalsfromasubsetoflocalMRIcoilsinapatientsaveanduserfriendlymanner,withhighSNRandhighlinearity,andconvertthemfromanalogRFsignalstodigitalcomplexk-spacedata.”SomeimportantparameterstocharacterizeanRFchainare:?Frequency?Lin
5、earity?Bandwidth?Channelcrosstalk?DynamicRange?SizeofAntennaselector?Noisefigure?LocalSAR2.BuildingblocksoftheRFchain2.1.MRIcoilincludingdetuning,matchingandLNAFigure2showsanMRIcoilinthevicinityofaconductivesampleandtheassociatedequivalentcircuit(2-5).Ascanbeseen,thecoilca
6、nbemodeledasinductorwithlossesoriginatingfromcoilconductor,sampleandradiation.AwelldesignedMRIcoilexhibitsaresistancewhichismainlyduetothesampleandresultsinahighunloadedtoloadedQratio.Figure2:EquivalentcircuitforasampleloadedlocalcoilInordertoextracttheMRIsignalfromthecoil
7、withhighestpossibleSNR,thecoilimpedancehastobetransformedtoaimpedancethatinsuresaminimumnoisefigureoftheconnectedlownoisepreamplifier(LNA).Typically,thisnoisematchimpedanceis50Ohms.Figure3showsthemostimportantcomponentsofanMRIcoil,whicharedetuningcircuit,tuningandmatchingc
8、ircuit,LNAandcoilcablewithoneormoreBaluns.Tuning&Matching3ActiveDetuningCS4Coax5CPPreampB
