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Design of Self-Resonance Modes (SRM) of monolithic ultra-high dielectric constant (uHDC) materials and RF Coils for B1 field enhancement1Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, PA, United States, 2Department of Engineering Sciences and Mechanics, The Pennsylvania State University, State College, PA, United States, 3Radiology Department, Center for Magnetic Resonance Research, MN, United States
Synopsis
Ultra-high dielectric constant (uHDC) materials were established as an effective B1 shimming and enhancement tool in MRI. A dielectric material operates at certain frequency range and enhances fields at an anticipated resonance frequency. We trimmed a rectangular dielectric block such that the fundamental frequency mode of the block resonated at 3 T, and compared it with a similarly sized non-resonant block. Both cases were coupled with a transmit receive surface coil resonant at 123.2 MHz. The effect of the surface coil area on the B1 field enhancement was explored to optimize the dielectric and coil configuration.
Purpose
Monolithic ultra-high dielectric constant (uHDC) materials are shown to produce a large enhancement in B1 field of a pre-fabricated RF coil [1,2]. Conversely, an uHDC block itself has many specific self-resonant modes (SRM), for a given geometry and permittivity. To effectively utilize uHDC material for RF engineering, it is necessary to investigate how RF coil design is influenced by the SRM of uHDC monolithic material for optimal RF field enhancement. In this work, we hypothesized that the fundamental SRM of uHDC block at MR system operating frequency should be most efficient in B1 field enhancement.Methods
Since the frequency of the fundamental SRM is determined by the largest dimension of the material, an on-resonance block was obtained by trimming the longest side of a rectangular uHDC block to 90 mm such that the frequency of fundamental SRM of the block resonated at 123.2 MHz, the operating frequency of the 3T system. An off-resonance block with largest dimension of 109 mm (fTE11δ=119.0 MHz) was used for comparison. The frequency of the SRM was determined experimentally by measuring S11 with a pickup coil on a network analyzer. Subsequently, two rectangular surface coils were constructed and tuned with one coil (86 mm x 86 mm) having twice of the area of the other (61 mm x 61 mm). The transmit efficiency was numerically calculated in xFDTD and experimentally measured (3 mm3 voxels) using Bloch-Siegert technique [3,4] on a 3T Siemens PrismaFit under six different experimental setups: two baselines with the small and large surface coil alone, on-resonant block with small and large coil as well as the off-resonant block with small and large coil. Each setup was placed on an Agar rectangular bottle phantom (120 mm x 120 mm x 200 mm, T1=1.9 s) as shown in Figure 1. Additionally, we numerically explored the optimal RF coil size for on-resonance and off-resonance block. The side length of the rectangular RF coil was varied between 40 mm and 90 mm. The transmit efficiency was normalized for each setup using a given ROI in the phantom as indicated.Results and Discussion
With the measured permittivity of 1177 and its initial geometry the uHDC block had its fundamental SRM at 119.0 MHz. Figure 2 shows dependence of the frequency of the SRM of the uHDC block on its dimension. Both simulated (Figure 3) and experimental (Figure 4) results are in good agreement. For both coil configurations with on-resonant uHDC block, there is a about a twofold improvement of the transmit efficiency over the surface coils alone. The larger coil has a greater enhancement over smaller coil with both blocks. Both on-resonant and off-resonant uHDC block dramatically improve the baseline transmit efficiencies in the phantom. The B1 field enhancement is slight stronger with on-resonant block than off-resonant block. The off-resonant block, however, offers a smoother B1 distribution and a wider enhancement pattern. This is due to the very concentrated B flux lines in the center of the block of the SRM. We also explored a large range of possible RF coil size for each of the blocks. As the simulated B1 maps suggest, the coil size that matches the block size would yield best results.
Conclusion
Our experimental and simulated results demonstrate the B1 field can be enhanced greatly by both on-resonant and off-resonant uHDC blocks with similar magnitude. The geometry of RF coils that couple to the uHDC blocks should provide the largest B1 flux to the sample for optimal enhancement.Acknowledgements
NIH grants of R24 MH106049, RO1 NS070839, S10 RR029672, P41 EB015894 and P30 NS076408.References
[1] Rupprecht et al., ISMRM 2013 P5458
[2] O'Reilley et al., ISMRM 2016 P0393
[3] Sacolick et al., MRM 63:1315–1322 (2010)
[4] Jankiewicz et al, ISMRM 2012 P4265
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