Synopsis

Self-decoupled coils that do not require additional decoupling treatments provide increased freedom for RF array design. However, only a single-row array was demonstrated in the initial work and it is not clear whether this design can be applied to multi-row arrays. Furthermore, the initial self-decoupled coil was fed in a horizontal conductor and thus its B₁ field was same as that of a conventional loop. If fed at a corner or in a vertical conductor, the self-decoupled coil may exhibit asymmetric currents and act as a “loopole” which can be used to improve transmit or receive performance. In this work, we proposed several solutions for multi-row arrays and also investigate loopole-type self-decoupled coils. We found that the self-decoupled coil design can be applied to multi-row arrays with a simple modification (fed at corner) or adding another decoupling method (such as overlapping). The self-decoupled method could also be used to improve transmit efficiency and SNR by optimizing the feed port’s positions.

Purpose

Methods

Multi-row Self-decoupled Coils

Fig. 1 illustrates different kinds of coil arrangements for multi-row self-decoupled coils, which are referred to as corner-fed (Fig. 1a), horizontal-fed (Fig. 1b) and vertical-fed (Fig. 1c). In the corner-fed arrangement (Fig. 1a) dipole modes are generated in both the horizontal and vertical directions, which is expected to decouple adjacent elements in the same row and decouple adjacent elements in different rows simultaneously. So this design may not require additional decoupling treatments. In the horizontal- and vertical-fed arrangements (Fig. 1b), the self-decoupled coil only generates the desired dipole-mode along the one direction. Therefore another decoupling treatment is needed to decouple elements in the other direction (overlapping was used here). Note that corner- and vertical-fed arrangements exhibit asymmetric currents in the vertical conductors that lead to loopole-type B1 patterns⁵ (discussed later). To validate these designs, different 2x2 self-decoupled arrays were simulated for 7T (298 MHz) MRI. The corner fed array was built and tested. We also investigated the coupling performance of 24-ch and 32-ch multi-row arrays for different applications through simulation.

Analysis of Loopole-type B₁ field

The B₁=(B_x±iB_y)/2 is mainly determined by currents along the vertical conductors. For horizontal-fed self-decoupled coils, the current along the vertical conductors are symmetric and exhibits the same B₁ field (both pattern and efficiency) as the conventional loop coil. However, the current along the vertical conductors becomes asymmetric when the coil is corner- or vertical-fed, stronger near the feed port and weaker near the C_mode. This asymmetric current should lead to a loopole-type B₁ pattern. To validate this, B₁ fields of self-decoupled coils fed at different positions (corresponding to the corner-fed, horizontal-fed and vertical-fed multi-row arrangement) were comparatively investigated through simulation. The EM simulations were performed with HFSS software (Ansys, Canonsburg, PA, USA).

Results

Fig. 2 shows the simulated S-parameter plots of three 2x2 self-decoupled arrays using different arrangements. These arrays (each coil 10x10 cm²) were mounted on 25-cm-diameter cylindrical former. A 16 cm-diameter cylinder phantom was placed 4.5 cm below as a load (б=0.6 S/m and ξr=78). For all these arrays, the worst-case isolation was approximately -14 dB. The corner-fed 2x2 array was also validated in bench tests (Fig. 3). Fig. 4 shows simulation models of a 32-ch head array, a 24-ch knee array and a 32-ch body array using the multi-row self-decoupled design. For all these arrays, the worst-case inter-element isolation was still better than -15 dB (marked at the bottom of Fig. 4).

Fig. 5 shows simulation results of a conventional coil (uniform current distribution) and different self-decoupled coils (fed at different positions). These coils have a dimension of 10x10 cm² and placed 1 cm below a 20-cm-diameter phantom (б=0.6 S/m and ξr=78). By feeding in the vertical conductor, the self-decoupled coil did exhibit "loopole-type" B₁ patterns, which can increase either B₁⁺ or B₁^- (red box in Fig. 5) at the expense of decreasing the other. This lead to notable improvement in transmit efficiency and SNR (around 20%), but little improvement in the safety efficiency (B₁⁺/√MaxSAR_10g). This is mainly because the self-decoupled coil increased MaxSAR_10g by 25% in this specific case.

Discussions and Conclusion

Acknowledgements

References

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