Introduction

Several studies have shown proof-of-principle implementations of metamaterials for local RF transmit efficiency and sensitivity improvement^1-6. The most common implementations are based on sets of split-rings^3-4 or wires⁵, designed to be resonant or close to resonant at the frequency of interest. In our recent studies, we implemented a hybrid metamaterial based on a high permittivity substrate and set of conducting strips⁶. We have also shown the feasibility of dual-band enhancement, with a set of long strips producing the resonant mode for the low frequency band, and a set of short strips (which behaves equivalently to split-rings) responsible for the resonant modes for the high frequency band. Whereas in the original study⁷ we focused on 7T ³¹P/¹H dual-band implementation, in the current study we were interested to examine the possibility to extend it to much lower frequencies, namely a dual-band metamaterial for 3T ²³Na/¹H local signal enhancement. To implement such a metamaterial in a compact setup, we used high permittivity material suspensions of BaTiO₃ in water. The implementation was examined via electromagnetic (EM) simulations and in phantom experiments at 3 T.

Methods

3D EM simulations of the B₁⁺ field were performed using FIT (finite integration technique) software (CST Microwave Studio, Darmstadt, Germany). All B₁⁺ maps were normalized to an accepted power of 1 Watt. The first steps in the design included an optimization study of the resonant structures at 33.8 MHz and 128 MHz separately. The structure was designed to have dimensions 16x31x1 cm³. The conducting strips were implemented using copper strips in a zigzag shape (which allowed the resonant mode to be achieved within the aforementioned structure dimension) and were attached to a transparent plastic sheet, placed inside the pad. Once the metasurfaces are optimized, a combined setup was produced with the long strips on one side of the dielectric and the short strips on the other side, in contact with the dielectric layer. The simulation showed that a coupling effect between the metasurfaces reduces the efficiency of the higher frequency. A tilt of the high frequency setup in-plane (by 10°) reduced this coupling, and resonant modes for ²³Na and ¹H could be achieved. Figure 1 shows the geometry of the metasurface and the magnetic field distributions for each band. The effect of the dielectric permittivity on the enhancement was examined. Although a relative permittivity (ε_r) of 250 shows maximal enhancement, the estimated actual permittivity used was slightly lower, ε_r = 230. To improve the attainable enhancement, we added an extra BaTiO₃ pad to the setup (as used in several works⁸ -18x18x1 cm³, ε_r = 280) to increase the enhancement at the ¹H frequency. The phantom included one container of water with NaCl (0.1%) and another container of white cheese (0.2% NaCl). The metasurface was placed on the opposite side of the phantom from a commercial ²³N and ¹H double-resonant surface coil . Phantom images were acquired on a 3T Siemens TIM TRIO scanner, including a low flip angle gradient-echo sequence for SNR estimation for protons and a chemical shift imaging sequence for sodium. Scan parameters are shown in the caption for Figure 3.

Results

Figure 1 shows the H-field resonant modes of the setup for low and high frequencies. Figure 2 shows EM simulation in a full setup, which included a surface-coil, a phantom and a metamaterial. Using metamaterial setup, the maximal enhancement is achieved at ε_r=250 (which matched the resonant mode). Simulation results of the implemented setup (using ε_r=230 and added BaTiO₃ pad) show an enhancement of ~2 for ²³Na and ~2.5 for ¹H in the vicinity of the metamaterial. Figure 3 shows experimental results, where the SNR enhancement ratio (using square root of the normalized intensity divided by the noise) show maximal enhancement of ~1.5 for ²³Na and ~2.3 for ¹H. The deviation between the simulated and measured enhancement ratio for the ²³Na can be due to the relatively high partial volume in the ²³Na scan.

Conclusions

In this study, we designed a dual-band ²³Na/¹H metametarial for local signal enhancement. This work has shown an improvement in SNR of a factor of ~1.5 for ²³Na and ~2.3 for ¹H. Further control and optimization is required to achieve a greater penetration of the signal enhancement, especially for the ²³Na frequency.

Acknowledgements

No acknowledgement found.

References

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