# Self-Isolated Dual-Mode High-Pass Birdcage RF Coil for Proton and Sodium MR Imaging at 7 T MRI
## 4 Discussion  
This study aimed to develop a novel dual-tuned birdcage RF coil capable of operation at both \({}^{1}\)H and \({}^{23}\)Na frequency at 7 T scanner. To achieve the goal, we proposed DMBC RF coil employing DMC, which functions as an open or capacitive circuit at two distinct frequencies. The conventional FS trap based dual-tuning method [28, 29] can resonate the RF coil at two frequencies, but it is complicated to adjust the values of the components to make the desired equivalent capacitance, resulting in the signal loss at \({}^{1}\)H resonant frequency due to the existing FS trap in practical scenarios. In contrast to the FS trap based dual-tuned birdcage RF coil, the proposed DMC is designed to have minimal impact at the \({}^{1}\)H frequency, allowing the use of only end-ring capacitors, while it operates as an additional capacitance for tuning at a \({}^{23}\)Na frequency. While the proposed method necessitates one additional capacitor compared to FS trap-based method, it represents an optimal and intuitive strategy for achieving independent and isolated operation of the RF coil at two distinct frequencies. This design minimizes the performance degradation of DMBC RF coils compared to STBC RF coils. Thus, DMC empowers completely self-isolated and separate control for tuning at two distinct frequencies of DMBC RF coil. Either dual-mode tuning at any two frequencies, even with relatively small differences, is possible by employing DMC. In this study, we innovated a single birdcage coil design using four ports to enable quadrature driving at two different frequencies, thus realizing the dual-mode RF coil with enhanced functionality. EM simulations were specifically conducted for the single-tuned RF coil configuration. This approach was adopted due to the operational characteristics of the DMC. At the frequency corresponding to \({}^{1}\)H resonance, the DMC effectively acts as an open circuit, rendering its influence negligible. Conversely, at \({}^{23}\)Na resonance frequency, the DMC functions as a capacitive circuit, essentially contributing additional capacitance to the end-ring capacitors. Consequently, the simulation did not encompass the practical effects of the DMC in real-world scenarios, as its impact is considered minimal. Therefore, to empirically validate the performance of the DMBC RF coil, STBC RF coils specific to \({}^{1}\)H and \({}^{23}\)Na frequencies were constructed and tested. Bench test measurements revealed that Q-factor of DMBC RF of \({}^{1}\)H port was only 10.2% lower than that of \({}^{1}\)H STBC RF coil, with a relatively modest SNR reduction of 6.5%. Similarly, the Q-factor for DMBC RF coil for \({}^{23}\)Na port was 12.3% less than that of \({}^{23}\)Na STBC RF coil, yet the SNR showed a minimal reduction of 5.4%. The modest SNR reduction associated with DMBC RF coil can be attributed to the intrinsic trade-off characteristic of the dual-mode operation implemented by the DMC. This design element may influence the sensitivity and uniformity of the magnetic field distribution. A slight drawback of the DMBC RF coil is that when all the ports for \({}^{1}\)H and \({}^{23}\)Na are connected to the MRI system's interface box during the experiment, there is a potential for interference due to the influence of the alternate frequency. To address this, an additional LC filter circuit, functioning as an open circuit at the alternate frequency, could be employed to potentially enhance overall coil performance and mitigate performance degradation. Despite these considerations, the DMBC RF coil demonstrates robust performance, particularly notable for its capacity to efficiently accommodate dual-nuclei imaging applications, emphasizing its versatility and potential utility in advancing MRI technology. Moreover, the specific absorption rate (SAR) from the RF coil adheres to the safety thresholds as defined by the International Electrotechnical Commission (IEC) standards [49], ensuring the coil's safe application in clinical settings.