This document evaluates coupling strategies for an ultra-high field MRI probe made of a cylindrical dielectric resonator. Five coupling methods are experimentally tested: a single loop aside, around, and around the edge of the resonator as well as double and triple loops. The triple loop coupling achieves the highest transmission at -9.2dB while maintaining a Q-factor of 346. Electromagnetic simulations match the experimental results and field distributions. The triple loop design is optimized for use in a 14 Tesla MRI machine.

1. Title: Evaluation on coupling
strategies for ultra-high field
MRI probe made of cylindrical
dielectric resonator
RUI LIU1, MICHAEL T. LANAGAN2, STEVEN E. PERINI2, THOMAS
NEUBERGER3
1ENGINEERING SCIENCE AND MECHANICS, PENN STATE UNIVERSITY;2MATERIALS RESEARCH
INSTITUTE, PENN STATE UNIVERSITY; 3HUCK INSTITUTE, PENN STATE UNIVERSITY
14 Tesla system MRI machine

2. Magnetic Resonance Imagine (MRI)
Magnetic resonance imaging (MRI) utilizes strong magnetic
fields and radiowaves to form images of the subject.
The technology is widely used in medical imaging to
investigate anatomy and function of the body for medical
diagnosis.
An MRI machine contains a large superconducting magnet,
a radio frequency (RF) transceiver, and gradient magnets
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

3. Magnetic Resonance Imagine (MRI) (continued)
MRI System Block Diagram
B1
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

4. Signal strength “S” & Signal to Noise Ratio (SNR)
S = 𝜔0 𝑀0 𝐵1 𝑉𝑠
SNR α 𝑄
RF magnetic field strength,
related to coupling of resonator
Bandwidth relative to
center frequency
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

5. Ceramic Dielectric Resonators
Ceramic dielectric resonators are high permittivity, high Q objects
that can effectively transmit an electric filed and store energy with a
lower rate energy loss.
High permittivity
High Q-factor (Q factor of >1000 vs. 100 for RF coils)
Strong uniform magnetic fields (B1 field)
Compact structure/simple geometry
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

6. Mode Designation and Mode Chart
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

7. Methods
Anritus 37369D Lightning Network Analyzer
Hakki-Coleman method
Port 2Port 2
Incident
Transmitted
Microwave Power (S21
)Reflected
Microwave
Power (S11
)
Network analyzer
Source
Receiver AReceiver AReceiver RReceiver R Receiver BReceiver B
Port 1Port 1
D.U.T
Transmitted
Brass Plates
Coupler
Dielectric Sample
Power Out
Hakki-Coleman method
Power in
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

8. RF Coupling Methods
Goals:
Maximized power transmission
Maintaining High Q-factor
Center frequency can be tuned to 600 MHz for 14T MRI machine
Resonator:
CaTiO3 (relative permittivity of 156)
Outer/inner diameter: 46.1 mm/5.32mm; Height: 33.7 mm
Five coupling schemes (see next slide)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

9. RF Coupling Methods (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

10. Experimental Results
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

11. Experimental Results (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

12. Experimental Results (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

13. Experimental Results (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

14. Experimental Results (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

15. Experimental Results (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

16. Experimental Results (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
Final design:
Wire diameter 0.69 mm
Triple loop
Shielded in the holder with tuning
Center Freq: 600 MHz
Loss at REF: -10.6 dB
Q: 346
Resonant frequency range: 600 ±5 MHz

17. CST Microwave Studio Simulation
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
 CST MWS is a specialist tool for the 3D EM simulation of high
frequency (HF) components. It enables the fast and accurate
analysis of HF devices: antennas, filters, couplers, planar and
multi-layer structures.
 Using CST Microwave Studio to simulate the electromagnetic
field distributions of TE01δ mode around the excited dielectric
resonator for 14T MRI machine.
 Compare the results to experiment data to try to find the
optimized coupling configuration.

18. Simulation Results: Single Loop Aside
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

19. Simulation Results: Single Loop Aside (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
Center Resonant Frequency: Experimental: 498.5MHz; Simulation: 493.2MHz

20. Simulation Results: Single Loop Aside (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
E Field (top view)

21. H Field (top view)
Simulation Results: Single Loop Aside (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

22. Simulation Results: Single Loop Around
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

23. Simulation Results: Single Loop Around (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
Center Resonant Frequency: Experimental: 549MHz; Simulation: 538.8MHz

24. E Field (top view)
Simulation Results: Single Loop Around (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

25. H Field (top view)
Simulation Results: Single Loop Around (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

26. Simulation Results: Single Loop Around Edge
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

27. Simulation Results: Single Loop Around Edge (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
Center Resonant Frequency: Experimental: 516MHz; Simulation: 510.4MHz

28. E Field (top view)
Simulation Results: Single Loop Around Edge (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

29. H Field (top view)
Simulation Results: Single Loop Around Edge (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

30. Simulation Results: Double Loop
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

31. Center Resonant Frequency: Experimental: 555.5MHz; Simulation: 553.2MHz
Simulation Results: Double Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

32. Simulation Results: Double Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
E Field (top view)

33. Simulation Results: Double Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
H Field (top view)

34. Simulation Results: Triple Loop
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

35. Center Resonant Frequency: Experimental: 595.3MHz; Simulation: 572.4MHz
Simulation Results: Triple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

36. E Field (top view)
Simulation Results: Triple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

37. Simulation Results: Triple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
E Field (3D view)

38. H Field (top view)
Simulation Results: Triple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

39. Simulation Results: Triple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
H Field (3D view)

40. Simulation Results: Quadruple Loop
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

41. Center Resonant Frequency: Experimental: 627.9MHz; Simulation: 773.6MHz (deviant from experimental)
Simulation Results: Quadruple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

42. Suspect TE01δ Mode H Field (top view)
Simulation Results: Quadruple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

43. Suspect TE01δ Mode H Field (3D view)
Simulation Results: Quadruple Loop (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

44. Probe Prototype
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
Probe prototype and design model

45. Preliminary MRI Imaging
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
Preliminary water/oil mixture phantom imaging

46. Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
Preliminary MRI Imaging (continued)
Water phantom imaging SNR comparison

47. Conclusions
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

48. Conclusions (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

49. Conclusions (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

50. Conclusions (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
By replacing a single loop coupled on the side of the resonator with
coupling loops around the resonator, an increase in the S21 value could be
achieved with a sacrifice in Q value, meaning a gain in B1 field strength
with a trade-off in SNR. As the number of the turns of the coupling loop
increased, the S21 value increased from -16.6 dB to -9.2 dB and reached
diminishing return with four turns
The Q value also increased from 60.8 to 716.2 and started to decrease
beyond three turns. The final designed implemented the triple loop
configuration and showed a 157% increase in the effective power
transmission compared to previous design.

51. Conclusions (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
The measured Q values are related to other contributions in the
resonant system: 1/𝑄= 1/𝑄 𝑐+ 1/𝑄 𝑑+1/𝑄 𝑟𝑎𝑑+ 1/𝑄 𝑒𝑥
The experimental measurement showed a trade-off between S21
parameters and Qex. The equation also implies the overall quality factor is
dominated by the smallest Q-factor among the four. Therefore considering
lossy samples being imaged in MRI machine, the small Q-factor of the
sample rather than the improved Q on coupling and resonator will
dominate the overall Q-factor. Therefore, the improvement on S21
parameter or better power transmission is more significant than
improvements on Q-value.

52. Conclusions (continued)
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator
The simulation showed that as the looping scheme became more
complicated, the fields were no longer uniformly distributed rendering a
decrease in the B1 field component that is perpendicular to B0 field.
The more turns presented, the more distortion was rendered on the
field distribution. The distortion could render deviant resonance from
TE01δ mode resonance.
Although experimentally triple loop configuration was the optimum
solution for this CDR targeting the 14 T MRI machine, a simpler coupling
strategy would be more desirable to yield uniform B1 field distribution.

53. Experimental:
Redesign the probe for simple and reproducible fixtures
MRI imaging with more samples such as tobacco seeds
Simulation:
Add shielding to current structures
Add S21 and Q calculations
Replace discrete sources with coaxial cables and tuning and matching
circuitry
Exploring structures of simpler geometry
Future Work
Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

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Neuberger, T., Tyagi, V., Semouchkina, E., Lanagan, M., Baker, A., Haines, K., & Webb, A. (n.d.). Design of a ceramic dielectric resonator for NMR microimaging at
14.1 tesla. Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, 33B(2), 109-114.
Wen, H. (n.d.). The Evaluation of Dielectric Resonators Containing H2O or D2O as RF Coils for High-Field MR Imaging and Spectroscopy. Journal of Magnetic
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Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator