Date post: | 23-Jan-2018 |
Category: |
Engineering |
Upload: | sairam-geethanath |
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Radio Frequency coil
PresentedbyDarshanKS
Radio Frequency coil • Radiofrequency coils (RF coils) are the "antenna" of the MRI system,
broadcasting the RF signal to the patient and / or receiving the return signal
• Types of RF coil • Volume coil • Surface coil
• Frequency formula
f = 1/2𝜋√𝐿𝐶
d
RF coils
=
d =
d =
d =
d =
d =
d= d=.
d =
d =
d =
d =
d =
d =
d= d=
.Fig1:BiotSavartholdgood,forlowfrequenciesandregionclosetocoil
• GeneratedmagneCcfield(B-field)canbecalculatedbythelawof‘Biot-Savart’equaCon
• dB= 𝜇0 /4𝜋 𝐼 𝑑𝑙∗𝑟/|𝑟|^3
Design Parameters
Design• Volume coil 1.5Tesla(1.5MHz)
Ø Coil radius – 1.6cm Ø Leg length - 2.75cm Ø Leg width – 0.5cm Ø Seg width – 10.05cm Ø Seg length - 1.26cm
• 8 Rung • fo = 63.8MHz (1.5 Tesla)
Fig:Samplebirdcagecoil
Design • Surface coil
Ø N=1 Ø Loop Diameter =2.6cm Ø Inductance
RF Coils
Fig1:Birdcagecoil
Fig2:Surfaceloop
Fig3:Phasedarraycoil
Fig:Birdcagecoilat63.8MHz Fig:Surfacecoilat405kHz Fig:Arraycoilresonanceat405kHz
In-housebuiltcoils
Solenoidcoilworkingatdifferentfieldfrequency
Fig:Earthfield,1.6kHz Fig:4.5mT,215kHz Fig:9.5mT,405kHz
HFSS Design steps
• Modelling• Assigningboundaries• ExcitaCon• MeshoperaCon• Analysis• OpCmetrics• RadiaCon
Wrist Phantom:
SingleloopSpiralcoil
H-Field at XY Plane
Wrist model: Spiral lumped
Coil characterizaBon using NA E5063A
Fig: surface coil of loop innerdiameter=13cmisdividedinto fourbreak point at each point 48pF isp laced to get resonance at64.54MHz
Fig:Na-23surfacecoilofloopinnerdiameter=12cmisdivided intofourbreak point at each point 470pF isplaced and the pointed red andyellow arrow indicaCng the tuningand matching circuit to getresonanceat105.34MHz
Frequency response of the coil
Fig: Assembled four channel wrist coil
Fig:Interfacesolenoidcoiltothenetworkanalyzertovalidatetheworkingfrequency.CoilresonaCngat215kHzwithagain24dB
Experimental setup for coil characterizaBon
Figure1:ExperimentalsetupforAFcoilcharacterizaCon Figure2:AFcoil
Data 1
0 1000 2000 30000
5
10
15
Frequency in Hz
Mag
netic
fiel
d in
ute
sla
Demo: coil characterizaBon using NA
• AmagneCcfieldpassingthroughtheprobeloopgeneratesavoltage
• AccordingtoFaradayslaw,whichstatesthattheinducedvoltageisproporConaltotherateofchangeofmagneCcfluxthroughacircuitloop• Atverylowfrequencies,avoltagewouldbeinduceddirectlyintheinternalloopconductor• Athighfrequency,avoltageistheninducedintheoutersheathloop,andthisappearsacrossthesheathgap
Radio frequency amplifier
• RFamplifier:lowlevelinputsignalupfrommilli-wacrangetoalevelhighleveloutputsignal
ElementTrapcircuit
withdiode
RFamp
Surfaceloopcoil
• RFAcircuitlevelschemaCc,amplifierstageconsistsof
• Input/outputMatchingnetwork:responsibleformatchingthesystem50ohmimpedancewiththelowimpedancesoftheRFtransistorsformaximumpowertransfer.
• Couplinganddecouplingcomponent:TransfertheRFandDCenergyintheappropriatedirecCons
• Transistor:AmplifiestheRFsignal
hcp://www.cpcamps.com/introducCon-to-nmr-mri-amplifiers.html