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RF Coils invivo NMR course 2014
- 1. Bart van de Bank B.vandeBank@rad.umcn.nl Ingmar Voogt I.J.Voogt@umcutrecht.nl Michel Italiaander M.Italiaander@umcutrecht.nl MR Hardware RF Coils
- 2. Program: 2 13:30 – 14:15 MR Hardware 14:15 – 15:00 RF Coils 15:00 – 17:00 RF Practice
- 3. Learning objectives 3 After this lecture you should know: The properties of a RF coil The different coil types that are being used in MRI Be able to define the coil setup suitable for your own experiments and be able to develop a (simple) coil
- 4. RF Coils 4 Physics (only a few) Coil types How to build one? Resonance Larmor Principles B1 field Limitations
- 5. Resonance (phenomenon) 5 The ability of a system to store energy Kinetic, electric, etc. “Galoping Gertie” Opened July 1940 Suspension bridge 1.6km length (3rd longest in the world)
- 7. The bridge to MR 7 0B
- 12. B1 field 12 Determine the strength of the B1 field. Example: We want to have a 90° flip for 1H MRI at 3T and the pulse will take 100 μs. What amplitude should the pulse have? What flip angle we have if we lengthen the pulse to 400 μs?
- 13. B1 field 13 Determine the strength of the B1 field. Example: We want to have a 90° flip for 1H MRI at 3T and the pulse will take 100 μs. What amplitude should the pulse have? What flip angle we have if we lengthen the pulse to 400 μs?
- 14. Some equations 14 Maxwell’s equations Gauss’s law Gaus’s law for magnetism Faraday’s law Ampere’s law (corrected) Biot-Savart’s law* Relation between currents and their magnetic fields. Righthand rule Determination of B1 Direction * NB. approximation only at low field
- 15. RF Coils 15 Some physics Coil types How to build one? Solenoid Surface Helmholtz Alderman-Grant Bollinger Birdcage Arrays Antenna’s Microstrips
- 16. Soleniod coil 16 Benefits Homogeneous B1-field High B1-field strength Drawbacks Axial access only B1 always perpendicular to B0 High inductance L (LF) Source: www.hyperphysics.phy-astr.gsu.edu
- 17. Surface (flat) coil 17 Benefits Superb SNR Inherent localization Drawbacks Inhomogeneous B1-field Limited penetration depth Source: AJR june 2007 vol 188 no 6 1568-1572
- 18. Helmholtz coil 18 Benefits Open access Fairly homogeneous B1-field Reasonable SNR Drawbacks Long lead conductors
- 19. Alderman-Grant (saddle) coil 19 Benefits Volume setup Open design Easy to construct Drawbacks Inhomogeneous B1-field Only applicable for small objects Source: www.cis.rit.edu
- 20. Bollinger (cosine) coil 20 Benefits Volume setup Fairly Homogenous B1 field Open design Drawbacks Complexer design
- 21. Birdcage coil 21 Benefits Volume setup Superb homogeneity Applicable at high-field Drawbacks Very complex design Double resonant tough
- 22. Array coils 22 Benefits Superb SNR Large FOV Applicable at high-field Applicable for parallel imaging Drawbacks Complex design RF-coupling
- 23. Antenna’s 23 Benefits (relative) Simple design Propagating EM Wave (poynting vector) Combined with surface coils Drawbacks Only applicable at high field
- 24. Microstrips (radiative antenna) 24 Benefits B1 directed into tissue E-field in substrate Very high Q Drawbacks Coupling in arrays
- 25. RF Coils 25 Some physics Coil types How to build one? Components Resonance circuitry Quality Build your own loop coil Geometry Loopsize Quality factor Tuning Loading Matching Balancing Trapping Quadrature Multi nuclei Detune
- 26. Components & Impedance 26 Inductor [H] Impedance: Rule of thumb: 1 nH/mm Capacitor [F] Impedance: Resistor [Ω] Impedance: Total Impedance (=frequency profile) circuit-dependent! C pF L_coil nH 1 2 R ohm
- 27. Resonance circuit 27 Resonance condition: CL 1 2 C L 1 2 Serial resonance Parallel resonance
- 28. Quality factor 28 Q-factor reveals the quality of the resonant circuit High Q Low loss or small bandwidth Low Q High loss or high bandwidth Frequency 30MHz 50MHz 70MHz 90MHz 100MHz VDB(L1:2) -10 0 10 20 30 40 50 Frequency 30MHz 50MHz 70MHz 90MHz 100MHz VDB(L1:2) -10 0 10 20 30 40 50
- 29. Build your own coil I 29 1. Determine loopsize Region of interest Target depth 2. Create loop Determine inductance Estimation: Rule of thumb Determination: Use capacitor Determine Q Connect to system? Not ready, yet! Electrical Model V_RF R_Sy stem 50 L_Coil nH 1 2 R_Coil Ohm T1 L_Coil nH 1 2 R_Coil OhmL_Coil nH 1 2 50 R Target (visual) depth Optimal coil radius 2R0 Rcoil (<<1Ω) Inductance (L) ZL=jωL (L~1nH/mm) (Z~300Ω @5cm, 300MHz) Current I High Q Low R Q = ωL/Rcoil P = U * I ? [kW] 50Ω U
- 30. Build your own coil II 30 3. Tune the loop Larmor frequency of interest 4. Determine Q Differentiate between Unloaded Loaded 5. Connect to system? Yes Rcoil Current I ZL=jωL Electrical Model R_Coil Ohm C_Tune pF L_Coil nH 1 2 R_Coil OhmL_Coil nH 1 2 TUNE ZCt=-j/ωCt C ω0 ω Qunloaded = ωL/Rcoil Tissue (conductivity permeability) Rtissue Qloaded = ω L/(Rcoil+Rtissue) C_Tune pF R_Tissue Ohm R_Coil OhmL_Coil nH 1 2 Zt=? R_Tissue Ohm C_Tune pF L_Coil nH 1 2 R_Coil Ohm
- 31. Build your own coil III The magic 50 ohms31 R_s ystem 50 0 R_load n 0 V_RF 0 1 2 3 4 5 6 0 20 40 60 80 100 R [Ohm]P[mW]Power R [Ω]50 Ω RLoad < RSystem RLoad = RSystem RLoad > RSystem
- 32. Build your own coil IV 32 6. Match the coil Determine total impedance)}//({ tissuecoilLCt RRZZZ Electrical Model Rcoil Current I ZL=jωL TUNE ZCt=-j/ωCt Tissue (conductivity permeability) Rtissue Zt=? R_Tissue Ohm C_Tune pF L_Coil nH 1 2 R_Coil Ohm Zt=1/(1/(jωL+R)+jωC) = a + jB Re(Zt)=50 = a Im(Zt)=0 ≠ B Zt=50+jX jB -jB Z_Replace 50 + jB ω no match matched 50 + jB C_Match -jBZ_Replace tissuecoilLC t RRZZ Z 11 tissuecoilLC tissuecoilLC t RRZZ RRZZ Z jbaZt
- 33. Build your own coil V 33 Tissue (conductivity permeability) 2Ct 2Ct Cm 2Cm 2Cm Ct 50
- 34. Build your own coil - summary 34 Matching C Tuning C Loop L+R GND Matching Tuning
- 35. Build your own coil Quadrature35 I I -1 0 1 0 -1 0 1 0 90o delay + Transmit ? Hybride box Receive
- 36. Build your own coil Multi nuclei36 Measure different nuclei with MR 31P, 23Na, 19F, 13C etc. Design decisions single / multi coil arrangement single / multi probe input relative coil efficiency Always need 1H coil shimming, decoupling, localization, magnetization transfer, multi-nuclei (time interleaved),
- 37. Build your own coil Multi nuclei37 = C or L 1 2 C_HFC_LF L_coil 1 2 L_coil 1 2 L_parallel 1 2 L_coil 1 2 C_LF C_HF + @ High Frequency @ Low Frequency
- 38. Build your own coil Detuning38 Homogeneous excitation & localized acquisition (high SNR): Separate Tx and Rx coil But make sure that: During Tx: no B1 field coupling During Rx: no noise coupling 2C 4C 4C L/2 L/2 Ldec PIN match tune Forward Bias Reversed Bias
- 39. Literature 39 Haase, A., F. Odoj, et al. (2000). "NMR probeheads for in vivo applications." Concepts in Magnetic Resonance 12(6): 361-388. Mispelter, J., Lupu, M. & Briguet, A. NMR Probeheads for biophysical and biomedical experiments; Imperial College Press (2006)
- 40. Build your own coil Workshop40 Split in 6 groups Every group builds a coil Practice will be in room: P59, Gebouw de Valk (Building 304), 1st floor Please, be very careful with the equipment
- 41. 41