Introduction

Local multi-channel local transmit/receive (TXRX) radio frequency (RF) coil arrays are widely used in ultrahigh field MRI because they enable RF shimming and signal-to-noise ratio (SNR) gains¹. Such arrays can be constructed using dielectric resonator antennas (DRA) which have some advantages over standard loop elements: DRA can be manufactured as ceramic blocks with desired dielectric constant ε_r and electrical conductivity σ, thereby providing an opportunity to tailor their transmit field (B₁⁺) patterns^2,3. O’Reilly et al.⁴ showed that a loop-coupled DRA with a very high ε_r (above 1000) can outperform a loop coil of similar size in terms of B₁⁺ and SAR efficiency. The main drawback of using DRA with such high ε_r is the B₁⁺ penetration depth what makes this approach limited to the peripheral regions⁵. To utilize this B₁⁺ peripheral gain and still be able to reach deeper located regions without changing the electrical properties of the DRA, a different solution would be required. Dipole antennas were first proposed for MRI at 7T by Raaijmakers et al.⁶ who showed that high transmit efficiency in deeper located anatomical regions can be obtained. In previous work, we showed that dipole antenna can be decoupled with a loop-coupled dielectric resonator antenna⁷. Therefore, the goal of this proof-of-principle study was to demonstrate for the first time the feasibility of mixing loop-coupled dielectric resonator antennas with dipole antennas. For this purpose, we developed an 8-channel version of what we call dipolectric antenna (dipole + dielectric) array. The array’s performance was evaluated by means of electromagnetic field simulations, bench and MR phantom measurements. Finally, the 8-channel dipolectric antenna array was used in in vivo MRI/MRS of the human brain at 7 T.

Methods

Electromagnetic field and specific absorption ratio (SAR) simulations in a spherical phantom (ε_r=77, σ=1.09S/m) and human voxel model “Duke” from the Virtual Family were performed using FDTD method of Sim4Life (Zurich Medtech, Switzerland). B₁⁺ efficiency was defined as transmit field per unit power B₁⁺/√P_in, while SAR efficiency was defined as B₁⁺/√SAR_10g where SAR_10g denotes maximum SAR averaged over 10g volume. In this proof-of-principle study, a similar rectangular DRA to the one in O’Reilly et al. was used, but with a 7.5x lower σ value (ε_r=1070,σ=0.2S/m) enabling greater transmit field penetration depth (HyQRS,USA). To build a single-channel dipolectric antenna, DRA (90x44x5)mm³ and dipole antenna (length=250mm,width=6mm) were combined as described in Fig.1. It was driven in three different transmit (TX) modes: a) DRA-dipole TX, b) dipole-only TX (all DRA matched to 50Ω) and DRA-only TX (all dipole antennas matched to 50Ω). In each TX mode all 8 DR/dipole elements were actively receiving the signal (8-TXRX switches were used). 8-channel dipolectric antenna array (Fig.2) was developed and driven in two modes: DRA-only and dipole-only. Each TX mode (phase setting) was obtained for each array using a phase optimization algorithm⁸ for (Fig.2). 8-channel dipolectric antenna array was constructed and evaluated in in vivo MRI experiments using a 7-T head-only MR scanner (Siemens,Germany). B₁⁺ mapping was performed using SA2RAGE⁹ (Fig.1,4). In vivo experiments were conducted: MRI using 3D-GRE sequence and MRS using short-TE STEAM (Fig.5).

Results

To compare a single-channel dipolectric antenna with its loop-dipole counterpart, three different TX modes were used. Simulations revealed differences in B₁⁺ field patterns which were confirmed by B₁⁺ mapping experiments (Fig.1). To study the performance of 8-channel dipolectric antenna array, simulations involving Duke were conducted, and the array was benchmarked against an 8-channel loop-dipole array. It was found that coupling between DR and adjacent dipole antennas was significantly lower than loop-dipole coupling (Fig.2). Simulations showed that, depending on the TX mode, dipolectric antenna array produced: more efficient B₁⁺ in the periphery as well at greater depths (Fig.3). To validate electromagnetic simulations, B₁⁺ mapping in the spherical phantom was performed and a very good agreement between the simulations and measurements was observed (Fig.4). To investigate in vivo performance, the 8-channel dipolectric array was employed in MRI/MRS experiments involving one male subject (Fig.5).

Discussion and Conclusion

This study demonstrates for the first time the feasibility of mixing dipole with loop-coupled rectangular dielectric resonator antennas for in vivo MRI/MRS at 7T. We showed that this approach, which we called dipolectric antenna, can benefit from all of the advantages provided by: dielectric resonator (high inter-channel isolation as well as high B₁⁺ and SAR efficiency in the peripheral regions) and dipole antennas (high B₁⁺ and SAR efficiency at greater depths). The 8-channel dipolectric antenna array provided substantial gains vs. the 8-channel loop-dipole array: higher B₁⁺ and SAR efficiency in the periphery (DRA-only mode), and, interestingly, higher B₁⁺ efficiency (dipole-only mode) not only in the periphery (since dipole antennas themselves excite TE mode), but also in deeper located regions what can be associated with lower coupling between dipole and DRA when compared with the loop-dipole case (Fig.2). Dipolectric antenna is a new approach which can be tailored for a wide range of applications: the geometry of DRA as well as its electrical properties can be optimized for either periphery or depth as well as the geometry of the dipole antenna. We conclude that dipolectric antenna array is a promising solution not only for 7T-MRI, but also for higher magnetic field strengths.

Acknowledgements

We acknowledge access to the facilities and expertise of the CIBM Center for Biomedical Imaging, a Swiss research center of excellence founded and supported by Lausanne University Hospital (CHUV), University of Lausanne (UNIL), Ecole polytechnique fédérale de Lausanne (EPFL), University of Geneva (UNIGE) and Geneva University Hospitals (HUG).

References

1.Ladd, et al., Progress in NMR Spectroscopy, 2018 2.Aussenhofer and Webb, MRM 2012 . 3.Aussenhofer and Webb, JMR 2014 4.O’Reilly et al., MRM 2018. 5.Wenz and Gruetter, Frontiers in Physics 2021. 6. Raaijmakers et al. MRM 2011 7. Wenz and Gruetter, ISMRM 2020. 8.Clement et al., MRM, 2019. 9.Eggenschwiler et al., MRM 2012.