# Self-Isolated Dual-Mode High-Pass Birdcage RF Coil for Proton and Sodium MR Imaging at 7 T MRI
###### Abstract  
This study presents the feasibility of a dual-mode high-pass birdcage RF coil to acquire MR images at both \({}^{1}\)H and \({}^{23}\)Na frequencies at ultra-high-field MR scanner, 7 T. A dual-mode circuit (DMC) in the dual-mode birdcage (DMDC) 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 DMDC RF coil resonated at proton (\({}^{1}\)H) and sodium (\({}^{23}\)Na) frequencies, and also single-tuned high-pass birdcage RF coils were constructed for both \({}^{1}\)H and \({}^{23}\)Na 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 DMDC coil at \({}^{1}\)H port was 10.2% lower than that of \({}^{1}\)H single-tuned birdcage (STBC) coil, with a modest SNR reduction of 6.5%. Similarly, the Q-factor for the DMDC coil at \({}^{23}\)Na port was 12.3% less than that of \({}^{23}\)Na STBC coil, and the SNR showed a minimal reduction of 5.4%. Utilizing the DMDC coil, promising \({}^{1}\)H and \({}^{23}\)Na MR images were acquired compared to those by using STBC coils. In conclusion, deploying a DMDC \({}^{1}\)H/\({}^{23}\)Na 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. magnetic resonance imaging; sodium MRI; dual-mode RF coil; dual-tuned RF coil; ultra-high-field MRI; EM simulation +
Footnote †: journal: _Sci._  
dedicated RF coils to achieve adequate image quality [14]. Additionally, the intracellular sodium ions have short relaxation times of about 0.5-5 ms, which demands the implementation of rapid acquisition techniques to capture sufficient signals, which is technically challenging. In addition, high-resolution \({}^{1}\)H anatomy imaging is required for the appropriate interpretation of measured sodium concentration from \({}^{23}\)Na MRI. Despite these challenges, ongoing research has focused on enhancing the feasibility and reliability of \({}^{1}\)H/\({}^{23}\)Na MRI by developing more sensitive coils, optimizing the pulse sequences, and employing quantitative analysis techniques [15; 16; 17; 18; 19; 20; 21]. Enhancing the quality of \({}^{23}\)Na MRI necessitates the optimization of RF coils specifically designed for \({}^{23}\)Na imaging. Several designs and schemes exist for \({}^{23}\)Na RF coil construction such as single-tuned RF coils, single-tuned RF coils with \({}^{1}\)H frequency blocking circuit, and dual-tuned \({}^{1}\)H/\({}^{23}\)Na RF coils [13; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. When using the single-tuned RF coil for \({}^{1}\)H and \({}^{23}\)Na separately, co-registration of two images is essential; however, this approach often results in errors in \({}^{23}\)Na images due to poor B\({}_{0}\) shimming. So, \({}^{23}\)Na coil would be better designed to operate within \({}^{1}\)H coil to facilitate the automatic co-registration of both nuclei images. At the same time, positioning the coil loops close to the imaging object is advantageous in the increase in coil sensitivity. However, for the brain imaging with a size of 150-200 mm diameter, the spatial constraints limit the feasibility of incorporating \({}^{23}\)Na coil loops into outer volume \({}^{1}\)H coil, if two separate nuclei coils are designed. As an alternative design approach, dual-tuned RF coil becomes advantageous in addressing spatial limitation and also facilitating the integration of multi-channel reception capabilities for advanced imaging techniques. Various techniques have been developed to enable the dual-tuning of RF coils for acquiring proton and x-nuclei MRI at the same experimental setting.