Self-Isolated Dual-Mode High-Pass Birdcage RF Coil for Proton and Sodium MR Imaging at 7 T MRI

This study presents the feasibility of a dual-mode high-pass birdcage RF coil to acquire MR images at both H-1 and Na-23 frequencies at ultra-high-field MR scanner, 7 T. A dual-mode circuit (DMC) in the dual-mode birdcage (DMBC) RF coil operates at two frequencies, addressing the limitations of sensitivity reduction and isolation between two frequencies as in traditional dual-tuned RF coil. Finite-difference time-domain (FDTD) based electromagnetic (EM) simulations were performed to verify the RF coil at each frequency on the three-dimensional human head model. The DMBC RF coil resonated at proton (H-1) and sodium (Na-23) frequencies, and also single-tuned high-pass birdcage RF coils were constructed for both H-1 and Na-23 frequencies. The bench test performance of the RF coils was evaluated using network analysis parameters, including the measurement of scattering parameters (S-parameters) and quality factors (Q-factors). Q-factor of the DMBC coil at H-1 port was 10.2% lower than that of H-1 single-tuned birdcage (STBC) coil, with a modest SNR reduction of 6.5%. Similarly, the Q-factor for the DMBC coil at Na-23 port was 12.3% less than that of Na-23 STBC coil, and the SNR showed a minimal reduction of 5.4%. Utilizing the DMBC coil, promising H-1 and Na-23 MR images were acquired compared to those by using STBC coils. In conclusion, deploying a DMBC H-1/Na-23 coil has been demonstrated to overcome traditional constraints associated with dual-tuned RF coils, achieving this with only nominal signal attenuation across both nuclei operational frequencies.