Introduction

Resonators based on metamaterial inspired designs can be utilized to provide local signal enhancement of conventional coils and practically enable wireless MR imaging [1]. These resonators can be manufactured as thin, flexible and cost-effective surfaces (metasurfaces). The reduction of cabling, due to omission of local receive coils, can increase patient comfort and ease patient positioning for MRI examinations. Lower B₀ field strengths generally lead to lower signal-to-noise ratios (SNR) in MRI [2]. Therefore, this work investigates the ability of metasurfaces to enhance the sensitivity profile of the scanner-integrated spine-coil at 0.55T, 1.5T and 3T.

Methods

Three metasurfaces [3] were manufactured with identical geometries (191mm x 191mm) and adjusted to be resonant at the Larmor frequencies of interest (23.6MHz, 63.7MHz, 123.3MHz) corresponding to the three MRI systems respectively (MAGNETOM Free.Max, MAGNETOM Aera, MAGNETOM Skyra, Siemens Healthineers, Germany). The metasurfaces consist of an inner 8x8 grid of unit cells featuring lumped capacitors for resonance tuning (S42E Series, Johnson Technology, USA) and a coupled outer loop enabling transmit-decoupling via anti-parallel diodes (UM9989, Microchip Technology, USA). Workbench S-parameter measurements were conducted using a vector network analyzer (ZNB4, Rohde & Schwarz, Germany) with an untuned sniffer coil, to evaluate differences in resonance dampening when loading the metasurfaces with a cylindrical MRI phantom (Model 10606530 K2305, 42cm length, 14.5cm diameter, Siemens Healthineers, Germany). To measure the receive enhancements in the different MRI systems, the phantom was positioned centrally on the patient table including the integrated spine-coils (Figure 1). MR images were acquired with the metasurface directly on top of the phantom and also by solely using the spine-coil. The setup and gradient-echo sequence parameters were identical and the achieved enhancement was compared across field strengths. The compared enhancements were calculated based on the ratios of the resulting SNR maps in the transversal center slices. A 4cm wide region of interest (Figure 3, left) was used to compute an averaged SNR ratio with respect to the depth in the phantom.

Results

The metasurfaces were successfully adjusted for their respective field strengths (Figure 2). With increasing resonance frequency, the bandwidth generally increased and the S₁₁ magnitude decreased. Loading the metasurfaces with the phantom broadened the resonance peak compared to the unloaded state, which was least pronounced for the 0.55T variant. In the MRI measurements, the metasurfaces enhanced the sensitivity profile of the spine-coils in the anterior region, where the enhancement decreased at depth from the metasurfaces. The acquired image profiles are exemplarily shown for the 1.5T acquisitions, with and without the metasurface present (Figure 3a-b). The achieved SNR ratio was highest at 0.55T, reaching a 27-fold increase beneath the metasurface (Figure 3c). At 1.5T the SNR was increased up to 11-fold and at 3T up to 2.5-fold. This low‑field superiority remained throughout the entire phantom depth. A slight decrease in SNR below the spine-coil reference value was observed close to the spine-coil at depths of 11.1cm, 8.4cm and 6.4cm corresponding to the measurements at 0.55T, 1.5T and 3T respectively. This was stronger and more extended at higher field strengths.

Discussion & Conclusion

This contribution showcases the potential of metasurfaces to increase the sensitivity of scanner-integrated coils, especially at low-field systems. The evaluated metasurface design was most efficient in terms of SNR enhancement at 0.55T. The comparison applied identical metasurface geometries and setups at all field strengths. Part of the increased performance at lower frequencies can be attributed to lower losses in the conductor and lumped capacitors. Also, the dielectric properties of the phantom are frequency dependent and might alter the performance to some degree. At lower frequencies, the resonator was less influenced by the loading, as is comparable to conventional MR coils. Since the comparison was performed across different scanner systems, also the position of the integrated spine-coil can not be assumed to be identical. The significant SNR gain achieved at low-field paves the way for further development and implementation of wireless metasurface technologies in MRI.

Acknowledgements

This work was funded by the Federal Ministry of Education and Research within the Research Campus STIMULATE under the number ‘13GW0473A’ and ‘13GW0473B’.

References

[1] E. Stoja et al., Improving magnetic resonance imaging with smart and thin metasurfaces. Sci Rep 11,16179 (2021).
[2] J. Marques, F. Simonis and A. Webb, Low-Field MRI: An MR-Physics perspective. J Magn Reson Imaging 49,6 (2019).
[3] R. Kowal et al., Metamaterial Inspired Surface Resonators as Wireless Coil. In: Proceedings of the 13th Interventional MRI Symposium, p. 106 (2022), Leipzig.