Optimized Electromagnetic Coupling for Enhanced Performance of Ceramic Dielectric Resonators in Magnetic Resonance Imaging Applications

Title: Evaluation on coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonatorRui Liu1, Michael T. Lanagan2, Steven E. Perini2, Thomas Neuberger31Engineering Science and Mechanics, Penn State University;2Materials Research Institute, Penn State University; 3Huck Institute, Penn State University

14 Tesla system MRI machine

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

Magnetic Resonance Imagine (MRI) (continued)MRI System Block Diagram

B1Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

Signal strength S & Signal to Noise Ratio (SNR)RF magnetic field strength, related to coupling of resonatorBandwidth relative to center frequencyTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

Ceramic Dielectric ResonatorsCeramic 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

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

MethodsAnritus 37369D Lightning Network AnalyzerHakki-Coleman method

Brass Plates

Coupler

Dielectric Sample

Power Out

Hakki-Coleman methodPower inTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

RF Coupling MethodsGoals:Maximized power transmission Maintaining High Q-factorCenter frequency can be tuned to 600 MHz for 14T MRI machineResonator: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

RF Coupling Methods (continued)

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

Experimental Results

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

Experimental Results (continued)

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

Experimental Results (continued)

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

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

Experimental Results (continued)

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

Experimental Results (continued)

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

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

Final design:Wire diameter 0.69 mmTriple loopShielded in the holder with tuningCenter Freq: 600 MHzLoss at REF: -10.6 dBQ: 346Resonant frequency range: 600 5 MHz

CST Microwave Studio SimulationTitle: 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 TE01 mode around the excited dielectric resonator for 14T MRI machine.Compare the results to experiment data to try to find the optimized coupling configuration.

Simulation Results: Single Loop AsideTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

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

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

E Field (top view)

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

Simulation Results: Single Loop AroundTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

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

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

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

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

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

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

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

Simulation Results: Double LoopTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

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

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

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

Simulation Results: Triple LoopTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

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

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

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

E Field (3D view)

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

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

Simulation Results: Quadruple LoopTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

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

Suspect TE01 Mode H Field (top view)Simulation Results: Quadruple Loop (continued)Title: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

Suspect TE01 Mode H Field (3D view)

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

Probe PrototypeTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

Probe prototype and design model

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

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

Preliminary MRI Imaging (continued)Water phantom imaging SNR comparison

ConclusionsTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

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

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

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 turnsThe 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.

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/= 1/+ 1/+1/+ 1/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.

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 TE01 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.

Experimental:Redesign the probe for simple and reproducible fixturesMRI imaging with more samples such as tobacco seeds Simulation:Add shielding to current structuresAdd S21 and Q calculationsReplace discrete sources with coaxial cables and tuning and matching circuitryExploring structures of simpler geometry

Future WorkTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator

Aussenhofer, S., & Webb, A. (2013). High-permittivity solid ceramic resonators for high-field human MRI. NMR in Biomedicine, 26(11), 1555-1561.Aussenhofer, S., & Webb, A. (n.d.). Design and evaluation of a detunable water-based quadrature HEM11 mode dielectric resonator as a new type of volume coil for high field MRI. Magnetic Resonance in Medicine, 68(4), 1325-1331.Aussenhofer, S., & Webb, A. (n.d.). An eight-channel transmit/receive array of TE01 mode high permittivity ceramic resonators for human imaging at 7T. Journal of Magnetic Resonance, 243, 122-129.Haines, K., Neuberger, T., Lanagan, M., Semouchkina, E., & Webb, A. (n.d.). High Q calcium titanate cylindrical dielectric resonators for magnetic resonance microimaging. Journal of Magnetic Resonance, 200(2), 349-353.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 Resonance, Series B, 110(2), 117-123.Pozar, D. (1998). Microwave engineering (2.nd ed.). New York: John Wiley & Sons,.Pyrz, M., Lanagan, M., Perini, S., Neuberger, T., Chen, F., & Semouchkina, E. (2013) Optimization of Electromagnetic Coupling to Ceramic Resonators for Magnetic Resonance Imaging Applications. CICMT, 2013, 000069-000075.

ReferencesTitle: Coupling strategies for ultra-high field MRI probe made of cylindrical dielectric resonator