Synopsis

Keywords: RF Arrays & Systems, Non-Proton

7T MRI has great potential to enhance the sensitivity to ²³Na, allowing access to functional and anatomical information when combined with proton imaging. We introduce an RF coil design of a 16-channel Tx/Rx head array composed of eight proton dipoles and eight overlapping sodium loops for 7T. The initial results were acquired on phantoms with comparable performance for both nuclei, and a 40% B₁⁺ gain was reported in sodium channels against the commercial coil. All elements were well-decoupled (-7.2dB to -36dB) without implementation of multiple layers or RF shield, paving the way for future simultaneous ¹H/²³Na MRI acquisition at 7T.

Introduction

Sodium MRI allows access to functional neurological activities¹, but suffers from low SNR due to its intrinsic low sensitivity. Several designs have attempted to combine sodium (²³Na) and proton (¹H) imaging with multi-layered arrangements^2-4. However, complex multi-layered coil array designs lead to increased coupling and therefore SNR losses in both nuclei. At high field there are also challenges in achieving uniform imaging performance due to RF inhomogeneities, particularly for the proton frequency. The aim of this study is to design a single layer 8-channel sodium and 8-channel proton transceiver head array that will address both of these issues at once for improved performance in both nuclei for 7T.

Methods

Coil Construction
A coil array system for both ²³Na (78.6MHz) and ¹H (297.2MHz) imaging was developed for 7T. The coil consists of eight transmit/receive proton dipoles and eight transmit/receive sodium loops distributed symmetrically around an acrylic cylinder (diameter=30cm) that was supported by custom-designed 3D-printed holders (PLA,Ultimaker). Each proton dipole (170mm×15mm)⁵ was placed in the centre of each sodium loop (250mm×150mm). Loops were made with copper wire, and tuning/matching was achieved with a combination of fixed capacitors (ATC, NY) and variable capacitors (NMAJ15HV,knowles, IL).(Fig.1a)
In-house built baluns tuned to their respective frequency were integrated along the coaxial cable (K_02252_D,Huber+Suhner,Switzerland) to decouple each element. The proton baluns were fixed on the coil by 3D-printed holders to ensure orthogonal orientation of the cable to the dipole.(Fig.1a) The sodium loop coils were further decoupled by optimising the overlapping distance (~15% of loop width) between neighbouring channels. To minimise coupling between sodium and proton components, four LC traps and an additional balun at proton frequency were implemented in each sodium loop. (Fig.1a,b)
Bench measurements
Bench measurements were carried out with dedicated phantoms for proton and sodium.(Fig.1c,d) The former was a cylindrical phantom (diameter=21cm,length=21cm,0.45w/v NaCl), while the latter was a sphere (diameter=15cm,125mmol NaCl). Central alignment of both phantoms was ensured by a custom-designed 3D-printed holder. Q-factor and S-matrix values were recorded from VNA (E5063A ENA, Keysight) to verify optimised tuning/matching and decoupling prior to MRI acquisitions (Fig.2).
MRI acquisition
MR data were acquired on a single-transmit for sodium and parallel-transmit for proton 7T MR scanner (MAGNETOM Terra, Siemens Healthineers, Erlangen, Germany) with a power splitter and respective TR switches (MR coiltech, UK) for each acquisition. B₁⁺ map for the proton array in the circularly polarised (CP) mode (B_1,CP) was assessed with a 3D actual-flip-angle AFI sequence (TR=200ms, 4.4×4.4×4mm³, FA=60deg)⁶. Signal levels in CP mode (S_CP) and individual channels (S_n) were acquired with 3D GRE sequence (TR/TE=10ms/2.48ms, 4×4×4mm³, FA=5deg, bandwidth=250Hz/pixel) using pTx system and setting the amplitude of the channel in question to 0.35 and others to zero in a circular fashion. B₁⁺ maps for individual proton transmit channels were calculated⁷ with: $$$ B_{1,n} =\frac{S_{n}\cdot B_{1,CP}}{S_{CP}} $$$.
For sodium elements, B₁⁺ maps were calculated with the double-angle method⁸ with middle axial slices acquired by 2D GRE sequences (TR/TE=150ms/1.92ms, FA=45/90deg, 6.6×6.6x25.0mm³, 32 averages, bandwidth=600Hz/pixel). The 45/90deg combination was verified by comparing signals from repeated measurements of different flip angles. Individual channel results were acquired by manually switching channels on the TR switch and bypassing the power splitter at each measurement. The same calculation was applied to a commercial 32-channel ¹H/²³Na coil array (Siemens Healthineers/Rapid Biomedical) on its sodium elements as comparison. (Fig.4)

Results

The S-matrix showed satisfying tune-and-match results (S_ii:¹H:-14 to -20dB;²³Na:-15.8 to -27dB).(Fig.2) The decoupling among nuclei was well-achieved (S_ij:¹H/¹H:-8dB to -16dB;²³Na/²³Na:-7.2dB to -27dB;¹H/²³Na:-25dB to -36dB). Proton B₁⁺ maps indicated that the in-house built coil generated a circular polarised field with relatively uniform contribution from all channels (Fig.3). VNA measurements indicated a high Q-factor ratio of 330/35 (unloaded/loaded) for a sodium loop located on the array. Individual B1+ transmit field contributions from sodium loops were less uniform, however, an overall 40% B₁⁺ gain was reported compared to the commercial coil in CP mode.(Fig.4)

Discussion

The in-house built coil exhibited a robust performance in simultaneous proton and sodium imaging in absence of an RF shield. The presence of the sodium coil does not alter the field efficiency of the proton/sodium coil array as compared to the similar dipole proton array⁹. Testing with the RF sweeper (Morris Instruments Inc., Canada) indicated ~0.5MHz shift in frequencies of channels 1, 3 and 7 when placed in the isocentre of the scanner. This is likely due to their relative proximity to the bore. Given the high Q-factor of the sodium loops, the homogeneity can be further improved by tuning/matching based on their relative positions. Moreover, SNR could be increased with implementation of multichannel receivers for proton and sodium.

Conclusion

In this work, we have shown the initial imaging results on phantoms to characterise a custom-built 8-channel ¹H/8-channel ²³Na head transceiver array for 7T. The coil showed great potential for high signal in sodium imaging with similar performance in the proton elements to proton-only arrays so that one unified examination will be sufficient, reducing time and labour costs in clinical practice. In-house developed pTx For future work, SAR_max of the array will be investigated with electromagnetic simulations to enable simultaneous ¹H/²³Na imaging for the benefit of patient diagnosis.

Acknowledgements

This work was supported by King’s China Scholarship Council, by core funding from the Wellcome/EPSRC Centre for Medical Engineering [WT203148/Z/16/Z], Wellcome Trust Collaboration in Science grant [WT201526/Z/16/Z], and by the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy’s and St Thomas’ NHS Foundation Trust and King’s College London and/or the NIHR Clinical Research Facility. The views expressed are those of the author(s) and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care.

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