Introduction

Application of metamaterials to MRI is often limited by their dimensions, especially at lower B₀ fields such as 1.5T or 3T where the wavelength is on the order of several meters^1-4. Therefore, highly subwavelength solutions are needed. Another limitation of metamaterials for this application is the lack of tunability of the final design. Once it is fabricated, the working resonance frequency is fixed, and varying load conditions significantly shift the resonant frequency and consequently affect the performance. In this paper, we propose a spiral metamaterial resonator that is compact (less than 10cm) and has a tunability.

Methods

The proposed metamaterial resonator is shown in Fig.1. It consists of a rectangular dielectric substrate (PLA, ε_r = 3.5, σ = 5e-5 S/m, 84mm x 84mm x 6mm) with a cavity (80mm x 80mm x 4mm) and two conductive spirals (R_in = 3mm, R_out = 35mm, 8 turns, trace width = 2mm) placed on the opposite sides of the substrate. The size of the proposed metamaterial resonator is approximately λ/55 at the working frequency of 64MHz. The cavity in the substrate has two inlets for water deposition/removal. Deionized water was used to vary the effective permittivity of the substrate. A circular loop with a diameter of 8cm was used as a receive (Rx) coil. The separation between the loop and the metamaterial was fixed to 5mm. The Rx loop was tuned and matched using an L-shape matching circuit. The Rx loop coupled with the metamaterial resonator was tuned and matched by varying the water volume inside the cavity and by using a single series capacitor. A homogeneous cuboid (ε_r = 74, σ = 3.53S/m, 15cm x 15cm x 15cm) was used as a load to mimic human body in the simulation. A container with 0.9% Sodium Chloride solution (ε_r = 126, σ = 0.26S/m) was used as a load in the measurement. For a fair comparison between the two cases, the single loop was moved closer to the load to make sure that the distance between the coil and the load and that between the coil with metamaterial and the load is the same (5mm).

Results

The proposed model was simulated using CST Microwave Studio. B₁- fields in the xz- and xy-planes inside the load are shown in Fig.2 for the cases without and with the proposed resonator. It can be observed that the magnetic field is redistributed and refocused in the presence of the resonator. The B₁- field intensity measured through the central line (indicated as white dash line in Fig.2(a) and (b)) is improved by 273%, 80%, 40%, and 17% at the distances of 0mm, 10mm, 20mm, and 30mm inside the load, respectively. The proposed spiral resonator and a loop coil were fabricated. Fig. 3 (a) shows a photo of the fabricated metamaterial resonator. The load was placed 5 mm above the resonator. Both S11 and the magnetic fields were measured using Rohde & Schwarz vector network analyzer. Fig.3(b) shows the measured S11 of the Rx coil coupled with the spiral resonator when the water volume is varied from 0ml to 12ml. It shows that the resonant frequency can be tuned within a range of approximately 15MHz. Fig.4 (a) shows a comparison between the simulated and the measured S11 of the Rx loop coupled with the proposed resonator with and without the load. The measurements agree well with the simulation results. Fig.4(b) shows simulated and measured 1D profiles of the y-component of H-fields through the central axis of the load produced by the Rx coil with and without the proposed resonator. The magnitude of H_y measured through the central axis is increased by 77%, 58%, and 40% at a distance of 10mm, 20mm, and 30mm, respectively. An enhancement of more than 28% is maintained up to 7cm into the load, resulting in a longer penetration depth.

Discussion & Conclusion

We demonstrate experimentally that a water-tunable spiral metamaterial resonator provides a significant magnetic field enhancement and an increased penetration depth for an RF receive coil with tuning flexibility. The design is thin thus ensuring proximity between RF coils and the subject under a scan. The rigid PLA substrate can be replaced by a soft PDMS material, thus allowing more flexible placement of the metamaterial on curved surfaces of a body and/or coils.

Acknowledgements

No acknowledgement found.

References

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