INTRODUCTION

Several specific configurations of double-tuned MRI RF coils have been described^1-4 for the acquisition of co-registered X-nuclei and proton images with good B1 homogeneity and large FOV. Among them an appealing configuration is based on the coaxial assembly of two birdcages, each one tuned at the frequency of the desired nucleus^5-6. Despite its conceptual simplicity, the resonant frequency shifts induced by the mutual coupling make its tuning cumbersome or requires extra lossy elements (i.e. traps on each rung). In this work, we focus on the optimization of a dual-tuned 1H/23Na based on the nested birdcage RF transceiver configuration, suitable for a pre-clinical 2.35T scanner. We have studied the RF coils’ isolation and efficiency for a set of useful geometries. To this purpose FEM numerical simulations have been first validated against the workbench measurements of a 2.35T prototype and then used to explore the performances for different geometrical configurations.

METHODS

We first build and performed FEM simulations (Ansys HFSS v.17.2, Eigenmode analysis) of coaxially arranged double-birdcage (Fig. 1) with an inner 23Na Low Pass (LP, 26.5MHz), an external 1H High Pass (HP, 100.3MHz), and an outer RF shield (S). The FEM simulations and workbench measurements were used to determine the % shift of the resonant frequencies (Df%) of the useful 1H mode of the unshielded (LP+HP) and shielded (LP+HP+S) configurations compared to the isolated HP birdcages (HP; HP+S) for a range of azimuthal angles. The simulations and measurements agreement is very good, within few % (Fig. 2). FEM simulations were extended for several values of the birdcages length ratios, shield diameters and azimuthal angles estimating the Df% of the 1H and 23Na useful modes (Fig. 3). The same simulations were used to study the effect of coils mutual coupling on the electric and magnetic field maps, by means of normalized field profiles (Fig. 4). The Ansys Eigenmode modality is able to calculate the correct field profiles, not their absolute field intensities. To assess the issue of RF coil efficiency at both resonant frequencies, a 50 Ohm port with appropriate matching capacitor was inserted in the FEM design. Using the Driven Modal approach and a matching condition better than -25dB, the B1 field for unit RF input power at 1H and 23Na frequencies (Fig. 5) could be accurately evaluated.

RESULTS

The effect of mutual coupling affects strongly the high frequency mode, and is practically not relevant for the low frequency one. The azimuthal angle has an effect on mutual coupling with a minimum condition for 22.5deg, corresponding to the rungs of the inner coil midway between the rungs of the outer coil. This simulation confirms previous experimental evidence^5-6. Surprisingly the presence of the shield strongly reduces the coupling, as it gets closer to the outer birdcage. The coupling is reduced if the LP is shortened (note the scale of the upper and lower panel in Fig. 3) thus suggesting another possible design strategy. The B1 profiles are relatively insensitive to the presence of the second coil (Fig. 4) while the B1 intensities have consistent reduction especially for the 1H mode and the longer LP (Fig. 5).

DISCUSSION

The level of mutual coupling in the nested double-tuned birdcage design can be greatly reduced if the optimal azimuthal angle is chosen, the RF shield is located close to the external birdcage and the LP is as short as possible. RF coil efficiency of the 1H mode is affected by the geometry of the LP coil, but this is often not an issue in preclinical applications.

CONCLUSION

Our extensive FEM simulation study, confirmed by workbench measurement with a 2.35T prototype, shows the optimization strategies to be followed for the design of nested double-tuned birdcages RF coils. Our FEM results can be easily extended to large size nested RF coils for UHF MRI clinical applications.

Acknowledgements

No acknowledgement found.

References

1. M. Alecci, et al. Practical design of a 4 Tesla double-tuned RF surface coil for interleaved 1H and 23Na MRI of rat brain. J. Magn. Reson. 181:203-211 (2006).
2. F. Wetterling, et al. A double-tuned 1H/23Na dual resonator system for tissue sodium concentration measurements in the rat brain via Na-MRI. Phys. Med. Biol. 55, 7681–7695 (2010).
3. G.C. Wiggins, et al. High-performance radiofrequency coils for 23Na MRI: brain and musculoskeletal applications. NMR Biomed. 29:96-106 (2016).
4. N.K. Bangerter, et al. Sodium MRI radiofrequency coils for body imaging. NMR Biomed. 29:107-118 (2016).
5. A. Asfour, A three-coil RF probe-head at 2.35 T: potential applications to the 23Na and to the hyperpolarized 129Xe MRI in small animals. International Conference IEEE-EMBS, pp. 5693–5699 (2010).
6. R.C Brand et al. Design and Performance of a Transformer-Coupled Double Resonant Quadrature Birdcage Coil for Localized Proton and Phosphorus Spectroscopy in the Human Calf Muscle at 7 T, CMRA 42:155-164 (2013).