AUTHORS: Tiston L., Wenz D.

ESMRMB 40th Annual Scientific Meeting, : , Barcelona, October 2024

ABSTRACT

This study explores improvement of a 38-channel dipolectric antenna array at 7T by analyzing the impact of conductivity on its performance.  This was done through electromagnetic field simulations on a human voxel model. Considerable deep brain SNR gains were found using low-loss dielectrics.

A continuous pursuit for higher SNR in MRI remains valid regardless of the B0 field strength. However, it was arguably ultrahigh field (UHF) that pushed researchers to explore novel RF coil concepts which enabled further receive performance gains. Some of those approaches were particularly promising. For instance, it was demonstrated that dielectric materials, not only in the form of a pad1 but a helmet2 as well, are suitable to increase SNR in the human brain at 7T. In general, increasing SNR in the center of the human brain can be considered more challenging since it is not expected to be achieved simply by increasing the number of loop coil elements in a receive array3. Recently, a novel alternative: dipolectric antenna – a combination of a loop-coupled dielectric resonator antenna (DRA) and a dipole antenna – was introduced4,5. While the dipolectric antenna array shows promise, the impact of the DRA’s electrical properties on the array’s performance has been unexplored. Therefore, the aim of this work is to determine how electrical conductivity of the DRA can affect transmit and receive performance of a 38-channel dipolectric antenna array for neuroimaging at 7T.

Electromagnetic field (EMF) simulations of a human voxel model Duke were conducted using Sim4Life (Zurich Medtech, Zurich, Switzerland). A 38-channel dipolectric array was modeled similarly as described earlier4 (Fig. 1), and it was composed of 30, receive-only cylindrical DRAs (thickness = 5 mm; 2 different diameters: 24 mm and 30mm) and 8 transceive (TxRx) dipole antennas. The dielectric constant εr of each DRA was set to be constant in each simulation (εr = 1070). Four different electrical conductivity σ values were investigated: 0.2, 0.1, 0.05, and 0.005 S/m. The data obtained from simulations were exported to PathWave Advanced Design System (ADS). Each element of the array was tuned to the resonance frequency (297.2 MHz) and matched to 50 Ohm. The circuit parameters were then introduced into the Match module of the Sim4Life to produce the final EMF maps which finally were exported to MatLab for analysis. Optimal SNR was calculated for all 38 elements of the array using an implementation of Roemer’s approach5. Transmit field (B1+) efficiency and peak specific absorption rate averaged over 10 g mass (pSAR10g) for the 8-channel TxRx dipole antenna array was normalized to 1W input power for a circularly polarized mode (45° phase difference per channel).

To determine how σ can influence transmit and receive performance of the 38-channel dipolectric antenna, Duke simulations were conducted. B1+ efficiency for the 8-channel TxRx dipole antenna array was higher for lower σ values (Fig. 1). That increase was mostly in the periphery. No significant B1+ efficiency increase in the center of Duke’s head was observed. The slight increase in B1+ efficiency was accompanied with an increase in pSAR10g for lower σ values. Specifically, 0.93, 0.89, 0.87, and 0.86 W/kg for 0.005, 0.05 0.1 0.2 S/m, corresponding to an 8% increase in pSAR10g from 0.2 to 0.005 S/m.

It was found that SNR was increased for lower σ values (Fig. 2 and Fig.3). There was an SNR gain of 31 % and 41 % from 0.2 to 0.005 S/m in the deep brain and periphery (thickness 10), respectively. For σ = 0.05 S/m, almost the same improvement in SNR as for σ = 0.005 S/m was observed.

Lower σ values led to higher coupling between the arrays’s elements (Fig. 2). For σ = 0.2 S/m, the worst-case coupling was -7.6 dB. For σ = 0.005 S/m is was found to be higher: -5.7 dB. Generally, for the array, the highest coupling was found to be between the dipole antennas and adjacent DRAs as well as between DRAs in row 1 (Fig. 4).

This work demonstrates that receive performance of a 38-channel dipolectric antenna array can be considerably improved using dielectric resonator antennas (DRAs) with low electrical conductivity σ (0.005 S/m). Most notably a significant increase in deep brain SNR was shown with a reduction in the σ value. The mechanism behind the increase in deep SNR is yet not fully understood. However, we expect that low σ led to a lower loss factor in each DRA, which might have allowed for an increased receive sensitivity of the array. Although there was an increase in coupling between the array’s elements and pSAR10g for lower σ, this could likely be a worthy trade-off, given the expected SNR gain. Our next step is to validate the data obtained from simulations by constructing a 38-channel dipolectric antenna array with the currently lowest achievable σ for this dielectric constant (εr = 1070) which is 0.08 S/m.

Teeuwisse et al., MRM, 2012

Lakshmanan et al., MRM 2022.

Wiggins et al., MRM 2010.

Wenz et al., MRM 2023.

Roemer et al., MRM 1990.

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