Optimization of the Transceiver Phased Array for Human Brain Imaging at 9.4 T: Loop Overlapping Rediscovered.
Nikolai I Avdievich1, Ioannis Giapitzakis1, Andreas Pfrommer1, and Anke Henning1,2

1High-field Magnetic Resonance, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, 2Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Synopsis

Ultra-high field (UHF) (>7T) transmit (Tx) and transceiver surface loop phased arrays improve Tx-efficiency and homogeneity for human brain imaging. Overlapping the loops enhances Tx-efficiency and SNR by increasing the penetration depth. However, overlapping can compromise decoupling and SNR by generating a substantial mutual resistance. Therefore, UHF Tx-arrays are commonly constructed using gapped loops. Based on analytical optimization we constructed a 9.4T 8-loop head transceiver array. Both the magnetic and electric coupling were compensated at the same time by overlapping and excellent decoupling was obtained. Tx- and Rx-performance of the array was compared favorably to that of a gaped array.

Introduction

Ultra-high field (UHF) (>7 T) transmit (Tx) (1) and transceiver (2,3) surface loop phased arrays combined with RF shimming or parallel transmission have been shown to improve Tx-efficiency (B1+/√P) and homogeneity for human brain imaging up to 9.4 T. Overlapping the loops (4) helps to improve Tx-efficiency and SNR by increasing the penetration depth and eliminating voids. On the other side, overlapping can complicate the decoupling. At fields of < 3T overlapped loops often generate substantial mutual resistance when loaded (4,5) and cannot be well decoupled using common decoupling methods, which compensate only for the mutual reactance (6). Following this idea UHF Tx-arrays are commonly constructed using gapped surface loops (1-3). In this work, based on analytical modeling of the impedance matrix, we optimized the loop geometry and relative positioning to minimize resistive and inductive coupling and constructed an 8-loop overlapped array. Our results show that at 9.4 T overlapping can be successfully used for decoupling of human head transceiver arrays. It also improves Tx and receive (Rx) performance in comparison to gapped arrays.

Methods

To describe the magnetic, km, and electric, ke, coupling between two rectangular loops placed on a cylindrical surface, we developed a full-wave analytical model based on dyadic Green’s functions (5). Analytical data analysis revealed a strong frequency dependence of ke and showed that at 400 MHz both km and ke can be cancelled at the same time by overlapping when the loop’s width increased from 8 cm (e.g. as in (2)) to 10.5 cm (Fig.1). In Fig.1 α is the angle between the loop centers. Thus, an optimal choice of loop size and overlap can provide a perfect decoupling of adjacent loops at 9.4 T. Based on this analytical data, we constructed an 8-loop single row (1 x 8) transceiver array (Fig.2) measured 20 cm in width, 23 cm in height, and 10 cm in length. Loop width of 11 cm was required to fit 8 loops on the array holder and also almost perfectly matched our analytical results. Excellent decoupling, i.e. better than -30 dB between adjacent loops and better than -22 dB between all others (Fig.3), was obtained without the need for any additional decoupling strategy. The overlapped array performance was compared to that of a gapped array with the same length and holder size (loop size: 10 cm - length, 8 cm - width). Experimental B1+, SNR (Sum-of-Square), and G-factors maps were obtained using a head and shoulder (HS) phantom (Fig.2) constructed to match tissue properties (ε = 58.6, σ = 0.64 S/m) (1). G-factor maps were obtained using non-accelerated and GRAPPA accelerated gradient echo imaging with acceleration factors (AF) from 1 to 5 and acceleration in the left-to-right (LR) or anterior-to-posterior (AP) directions as described in (1). All data were acquired on a Siemens Magnetom 9.4 T human imaging system. During transmission both arrays were driven in the circular polarized mode with 45º phase shift between the channels. QU/QL measured from 6.5 to 11 for anterior and posterior loops, respectively.

Results and Discussion

Fig.4 shows experimentally measured B1+, SNR and G-factors maps obtained using gapped and overlapped 8-channel arrays. As seen from Fig.4 overlapping improves both the Tx-performance and SNR. Both B1+ and SNR maps improved by eliminating the voids between the loops (Figs.4A, B). Since at lower fields (< 3 T) overlapping may compromise the parallel Rx-performance we also compared g-factor maps obtained by the two arrays. As an example, Fig.4C shows g-factor maps obtained using both arrays with AF = 3 and acceleration in the LR direction. As seen from Fig.4C and Table1 both arrays produced very similar average g-factor values. The number of overlapped loops can be further extended to 16 in a two-row (2 x 8) array design to improve the longitudinal (along the array axis) coverage.

Conclusions

We constructed a 9.4 T (400 MHz) 1x8 overlapped transceiver head array based on the results of the analytical analysis of the coupling between a pair of surface loops. We demonstrated that both the magnetic and electric coupling between the loops can be compensated at the same time simply by overlapping and nearly perfect decoupling (below -30 dB) can be obtained between all adjacent loops without additional decoupling strategies. Tx-efficiency and SNR of the overlapped array was compared favorably to that of a common UHF gapped array of the same dimensions. Parallel Rx-performance was also not compromised due to overlapping.

Acknowledgements

No acknowledgement found.

References

1) Shajan G et al, MRM 71:870, 2014. 2) Avdievich NI et al, Proc. ISMRM 22, 2014, 622. 3) Gilbert KM et al, MRM 67:1487, 2012. 4) Roemer PB et al, MRM 16:192, 1990. 5) Wright SM, Conc Magn Res, 15:2, 2002. 6) Avdievich NI et al, NMR in Biomed 26:1547, 2013.

Figures

Figure.1: km and ke plots.

Figure.2: Layout of the 8-ch. transceiver array.

Figure.3: S12 matrix of the array.

Figure.4: Transversal B1+ (A), SNR (SoS) (B), and G-factor (C) maps obtained for the same slice using gapped and overlapped arrays.

Table 1: Average G-factors.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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