Purpose

To develop an unshielded transceiver (TxRx) folded-end dipole array for human head imaging at 9.4T and improve decoupling of adjacent dipole elements, a novel array design with modified passive dipole antennas was developed, evaluated, and tested.

Introduction

Dipole antennas have recently been introduced (1) and used for human body imaging (1-3) at ultra-high field (UHF, >7T). For head imaging, dipoles must be substantially shortened (4-7). Such short dipoles are often decoupled only to the level of ~-10dB (4,7), which is insufficient for optimal transmit (Tx) performance. Common decoupling methods, which require electrical connections, are difficult to use for decoupling of distantly located adjacent Tx-dipoles. Alternatively, adjacent dipoles can be decoupled using passive dipole antennas placed between them (5,8). This decoupling practice should be used with care since passive dipoles may interact destructively with the RF-field, B₁⁺, produced by the Tx-array (9). In this work, we developed a novel decoupling method of adjacent Tx-dipoles by using modified passive dipole antennas.

Methods

The general idea behind the folded-end dipole design is shown in Fig.1A. Commonly, the resonance frequency of a short dipole antenna is decreased by connecting inductors in series. In such design, we “cut out” the central part of the dipole current and voltage distributions, which is “hidden” within the inductors. As a result, the “loaded” portion of the distribution has large voltage values at both ends and a very non-uniform current distribution. Alternatively, by folding the dipole (Fig.1B), we load only its central portion, which has lower voltage values and a more uniform current. Coupling between two Tx-dipole antennas produces two modes (Fig.1C). The anti-parallel mode creates no RF field in the center between the dipoles. Thus, a parallel passive decoupling dipole (Fig.1D, top) interacts only with the parallel mode. Therefore, such passive dipole resonating higher than both modes can shift down only the frequency of the parallel mode, which is adjusted by varying the passive dipole resonance frequency. Degeneracy of the two modes corresponds to decoupling of the Tx-dipoles. To minimize destructive interaction, we turned the passive dipole by 90º (Fig.1D, bottom) and bent to increase coupling with the Tx-dipoles. Such passive dipole interacts only with the anti-parallel mode. By varying the passive dipole resonance frequency, which is now lower than that of both modes, the resonance frequency of the anti-parallel mode can be shifted up. Degeneracy of the two modes corresponds to decoupling of the Tx-dipoles.
Electromagnetic simulations were performed using CST Studio Suite 2019 (CST, Darmstadt, Germany) and the time-domain solver based on the finite-integration technique. We simulated several 8-element dipole arrays (folded-end and unfolded) decoupled using parallel and perpendicular passive decoupling dipoles (Fig.2). After modeling, we constructed an 8-element transceiver unshielded dipole array decoupled using novel perpendicular passive dipoles (Fig.3B). The array measured 200mm in width, 230mm in height, and 170mm in length. To increase B₁⁺ field at the superior head locationad (10), we added a flat elliptical RF shield (Fig.3A). All data were acquired on a Siemens Magnetom 9.4T human imaging system.

Results and Discussion

The array was well decoupled (Fig.3D). All adjacent elements of the array were decoupled better than -15dB with an average S₁₂ value of -18.1dB. Figs.2C,D show simulated transversal B₁⁺ maps and their ratios obtained using decoupled folded-end dipole arrays. Parallel passive dipoles produce a strong destructive interference with the B₁⁺ field both at the periphery and the center of the phantom where the field is reduced by 16%. Turning the passive dipoles by 90º substantially minimizes the alteration of the RF-field both at the periphery and center (3% reduction). Similar results were obtained for decoupling of the unfolded dipole array (Fig.2B). Fig.4 displays sagittal B₁⁺ and transversal SAR_10g maps obtained using different 1x8 dipole arrays loaded by a head voxel model. Increasing the fold size, which makes the current distribution along the dipole more uniform (Fig.1B), extends the RF-field in both directions and ~1.5 times decreases the peak SAR_10g (slice 2, ears). SAR_10g maps through the array center (slice 1) are very similar. Optimal 30-mm bent folded-end dipole array also produced higher B₁⁺/√P (7%) and B₁⁺/√pSAR_10g (~30%) than the unfolded array. Fig.5 shows in-vivo experimental data. The unshielded folded-end dipole array provided good coverage over the entire brain. Averaged over a 130-mm transversal slab, B₁⁺ measures 9.3 μT/√kW.

Conclusion

We developed a novel passive dipole design for decoupling transmit human head dipole arrays. The new perpendicular decoupling antennas produced substantially less destructive interference with the RF-field of the array than the common parallel design. The constructed eight-element dipole array demonstrated good decoupling and whole-brain coverage.

Acknowledgements

Funding by the ERC Starting grant / SYNAPLAST / 679927 and the Cancer Prevention and Research Institute of Texas (CPRIT) grant / RR180056 is kindly acknowledged.

References

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