# Self-Isolated Dual-Mode High-Pass Birdcage RF Coil for Proton and Sodium MR Imaging at 7 T MRI
## 2 Materials and Methods
### MR Imaging Experiment  
The imaging performances of STBC RF coils and DMBC RF coil were tested in a human 7 T scanner (Achieva 7.0 T, Philips, Amsterdam, The Netherlands) at the Korea Basic Science Institute in Cheongju, Republic of Korea. To ensure the stable positioning of the RF coils on the patient table, we utilized an additional cylindrical frame assembly (with an outer diameter of 370 mm and an inner diameter of 300 mm). Additionally, the RF coil frame supporters were employed to secure the RF coil frame, preventing any movement during the experiments. After the RF coil frame was securely positioned, a head support was placed between the participant's head and the RF coil frame, and additional foam pads were employed to stabilize the head, preventing movement during the MR imaging. For both the \({}^{1}\)H and \({}^{23}\)Na MR experiment, we employed the scanner vendor's \({}^{1}\)H-\({}^{23}\)Na Transmit/Receive (T/R) interface box. This approach ensured that all experiments were conducted under consistent conditions, particularly regarding the RF power limitations specific to each nuclei's STBC and DMBC RF coils. The STBC RF coils were tested for both \({}^{1}\)H and \({}^{23}\)Na MRI. For the \({}^{1}\)H MR experiments, B\({}_{1}^{+}\)-field mapping was performed on a cylindrical phantom (inner diameter of 160 mm; length of 200 mm) using an actual flip-angle imaging (AFI) method [43], and T1-weighted magnetization-prepared rapid gradient-echo (MPRAGE) imaging was performed on the human brain [44], to investigate B\({}_{1}\) inhomogeneity at large flip-angle (FA). For the \({}^{23}\)Na experiments, the B\({}_{1}^{+}\)-field mapping was performed on the cylindrical phantom using the double-flip angle method (DAM) [45], and \({}^{23}\)Na images were obtained using an ultra-short echo-time (UTE) sequence on the human brain [17; 20]. Because of the low signal intensity of \({}^{23}\)Na in the brain, the B\({}_{1}^{+}\)-field mapping was conducted on a high sodium concentration phantom, consisting of a 1 g/L of copper sulfate and 154 mM of sodium chloride solution. A two-dimensional (2D) stacked radial projection sequence was employed for all \({}^{23}\)Na imaging to reduce the echo-time (TE), and the number of projections was 3000 for phantom imaging and 6000 for in vivo head imaging. All experiments were performed in strict compliance with established safety guidelines for experimental RF hardware in MRI [46; 47], ensuring that the SAR remained within the prescribed limits of the MRI system. The human study was approved by the institutional review board of Korea University, and written informed consent was obtained from the volunteers before the experiment commenced. The detailed sequence parameters are shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{\({}^{1}\)H MRI} & \multicolumn{2}{c}{\({}^{23}\)Na MRI} \\ \hline Sequence & Dual-TR (AFI) & T\({}_{1}\)-weighted MPRAGE & UTE (DAM) & UTE \\ \hline Nominal FA (\({}^{\circ}\)) & 60 & 7 & 45 and 90 & 32 \\ \hline TE/TR (ms) & 3.9/20 and 120 & 2.6/5.5 & 0.19/120 & 0.19/100 \\ \hline \hline \end{tabular}
\end{table}
Table 2: MR sequence parameters for \({}^{1}\)H and \({}^{23}\)Na MRI.  
SNR maps were calculated in in vivo MR images, as follows:  
\[\text{SNR}=\frac{\text{Signal Intensity}}{\text{Average standard deviation of ROI}_{\text{s}}} \tag{5}\]  
where the average standard deviation of the region-of-interest (ROI) represents the average standard deviation of the noise in four ROIs of noise-only background region outside the brain. The uniformity of FA was calculated in 2D ROI within the MR images as follows [48]:  
\[\text{Uniformity}(\%)=\left[1-\frac{\text{I}_{\text{max}}-\text{I}_{\text{ min}}}{\text{I}_{\text{max}}+\text{I}_{\text{min}}}\right]\times 100 \tag{6}\]  
where I\({}_{\text{max}}\) and I\({}_{\text{min}}\) represent the maximum and minimum values in FA maps, respectively. FA and SNR maps were calculated and reconstructed using MATLAB (R2022b, MathWorks Inc., Natick, MA, USA).