Purpose

To develop a robust design for a transmit dipole array coil for human whole brain imaging at 7T.

Introduction

In spite of great benefits offered by ultra-high field (UHF,>7T) MRI, its further development toward clinical applications is associated with significant difficulties in the design of RF coils. Recently, dipole antennas (1) have been proposed and used to construct UHF head arrays (2-4). The dipole provides a unique simplicity while offering comparable Tx-efficiency (<B₁⁺>/√P) and SNR to conventional loop designs. Recently, we developed a novel type of dipole elements, a folded-end dipole (4). In this work we developed and evaluated an 8-element transceiver (TxRx) bent folded-end dipole array for human head imaging at 7T (5). In addition, we evaluated passive dipole antennas (2,6,7) for decoupling the proposed array.

Methods

The 8-element folded-end dipole array measured 200mm x 230mm (width x height) and 250mm in length. The array had no RF shield. During numerical optimization, we considered several alterations to the previously reported design (Fig.1A). First, we extended the dipoles above the head, which enhances the RF field in the superior head area (8). We tested three positions, i.e. the array aligned with head and moved by 20mm and 40mm out. To further improve the B₁⁺ field at the superior head area (4,7,9), we added a local RF shield (Fig.1A). We also evaluated the Tx-performance of three other types of dipole arrays, i.e. the bent (unfolded) array, straight folded-end array, and straight (unfolded) array. All arrays were extended by 40mm above the head and included the local shield. In the next step, we added passive decoupling dipoles to the best array design. Figs.2A-C show placement of passive dipoles relative to active TxRx-dipoles. We simulated two types of passive dipoles, i.e. dipoles aligned with the active dipoles (decoupling 1), and passive dipoles moved away from the phantom to the level of the folded portion (decoupling 2). Decoupling 2 was suggested previously (10) to minimize destructive interference. For comparison, we tested both decoupling methods for the bent (unfolded) dipole array (Fig.2C). Figs.3A,B show the final geometry of the array design. Since the passive dipole near the nose caused very strong local SAR, it was removed, and all decoupled array designs (Fig.2) included only 7 passive dipoles. Electromagnetic (EM) simulations were performed using CST Studio Suite 2020 (Dassault Systèmes, Vélizy-Villacoublay, France) and the time-domain solver based on the finite-integration technique. All arrays were driven in the quadrature circular polarized (CP) mode (45° phase increment). All data were acquired on a Siemens Magnetom 7T human imaging system. We compared the new array to the commercial array coil (Nova Medical, Wilmington, MA, USA).

Results and Discussion

Fig.1B shows results of numerical optimization of the folded-end dipole array. Extension the dipoles above the head and addition of the local shield increases the Tx-efficiency and SAR-efficiency (<B₁⁺>/√pSAR, where pSAR is the peak local 10g SAR) by 55% and 46%, respectively. Bent arrays (both folded and unfolded) had similar Tx-efficiency ~10% higher than that of straight arrays. All arrays had similar SAR-efficiency. An additional benefit of the folded-end design is a reduction in the frequency change due to variation in head size (11). The folded-end dipole array demonstrated significantly less change in element resonance frequencies when both arrays were first tuned on the Duke voxel model and then loaded by the smaller Ella voxel model without readjustment (Fig.4). This effect is explained by the lower voltage along the conductor positioned near the head in the case of the folded-end dipoles. Figs.2D,E show ratios of transversal B₁⁺ maps obtained using decoupled bent folded-end and unfolded arrays to those obtained using the same arrays without decoupling. Destructive interference produces peripheral voids and significantly reduces the central B₁⁺ value (Fig.2E). In the case of the unfolded dipole array, <B₁⁺>/√P is reduced by 9% and 27.5% for decoupling 1 and 2, respectively. For folded-end array and decoupling 2, however, this effect is substantially reduced and barely noticeable (Fig.2D). Fig.3D shows S-matrices measured using the bent folded-end dipole array with and without decoupling. The average coupling between neighboring channels measured -18.3dB and -12.1dB for the decoupled and coupled array versions, respectively. Finally, Fig.5 shows in-vivo data obtained using the new folded-end dipole array and commercial array coil. Averaged over a 130-mm transverse slab, B₁⁺ measured 9.70 and 7.96 μT/√kW for the constructed dipole array and the commercial array, respectively. The dipole array provides significantly better coverage especially in the brain stem area.

Conclusion

We developed, constructed, and evaluated the 8-element TxRx bent folded-end dipole array for human head imaging at 7T. The array demonstrated more than 20% higher Tx-efficiency and significantly better whole-brain coverage than that provided by a widely-used commercial array. We demonstrated that in contrast to the common unfolded dipole array, the passive dipoles moved away from the sample produce practically no destructive interference with the array RF field.

Acknowledgements

Funding by the European Union (ERC Starting Grant, SYNAPLAST MR, Grant Number: 679927; ERC Advanced Grant SpreadMRI, Number: 834940) is gratefully acknowledged. SG acknowledges a support by Russian Science Foundation (Project 21-19-00707).

References

1) Raaijmakers AJE, Ipek O, Klomp DWJ et al. Magn Reson Med 2011;66:1488-1497. 2) Clément J, Gruetter R, and Ipek Ö. Magn Reson Med 2019;81:1447–1458. 3) Connell IRO, Mennon RS IEEE Trans Med Imag 2019;38: 2177-2187. 4) Avdievich NI, Solomakha G, Ruhm L, Bause J, Scheffler K, Henning A. Magn Reson Med 2020; 84:3453–3467. 5) Avdievich NI, Solomakha G, Ruhm L, Magill AW, Scheffler K. NMR in BioMed 2021;34(8):1-14. 6) Yan X, Zhang X, Wei L, Xue R. Appl Magn Res 2015;46:59-66. 7) Avdievich NI, Solomakha G, Ruhm L, Henning A., Scheffler K. Magn Reson Med 2021;86:581–597. 8) Avdievich NI, Hoffmann J, Shajan G, Pfrommer A, Giapitzakis IA, Scheffler K, Henning A, NMR in Biomed 2017, 30(2);1-12. 9) Alecci M, Collins CM, James Wilson J, Liu W, Smith MB, Jezzard P. Magn Reson Med 2003;49:363-370. 10) Neumann GF, Ladd ME, and Magill AW. In Proceedings of the 20th Annual Virtual Meeting ISMRM, 2020, p.4023. 11) Avdievich NI, Solomakha G, Ruhm L, Scheffler K, Henning A, Magn Reson Med 2019;82:811–824.