# Self-Isolated Dual-Mode High-Pass Birdcage RF Coil for Proton and Sodium MR Imaging at 7 T MRI
## 4 Discussion
The acquisition time for \({}^{23}\)Na imaging of the human brain was considerably extended. In the study, the SAR constraint and RF power limitation inherent to the scanner prevented the application of short RF pulse duration. Consequently, the acquisition was performed with three times to enhance SNR for the measurement. Optimizing system parameters to enable short RF pulse durations, for example, \(<\)1 ms could increase \({}^{23}\)Na SNR, reducing the total acquisition time for human application. Although \({}^{1}\)H and \({}^{23}\)Na images were compared, along with various coil parameter measurements for performance evaluation, between the proposed DMBC RF coil and the STBC RF coils, the study was limited to a few cases to demonstrate the feasibility of the human brain MRI. The results showed comparable imaging performance, but more population data will be needed to obtain statistical significance in future study. Particularly for \({}^{23}\)Na imaging, which inherently suffers from low signal sensitivity, achieving high resolution images presents a challenge. To address this issue, we reconstructed \({}^{23}\)Na images at a lower resolution compared to the \({}^{1}\)H images, thereby maintaining the accuracy of the results while enhancing the SNR. Moreover, further optimization of \({}^{23}\)Na imaging is required. This includes not only achieving a higher pixel resolution (below 3 mm isotropic, currently 3.5 \(\times\) 3.5 \(\times\) 8 mm\({}^{3}\)), but also aspects like reproducibility, and quantification of sodium concentration [20]. Additionally, the application of multi-channel reception RF coils for \({}^{23}\)Na imaging could potentially reduce acquisition time and simultaneously increase SNR, which would significantly improve both the efficiency and quality of \({}^{23}\)Na MRI. In the study, the new coil was designed primarily for the brain study considering the potential future application to brain tumors, ischemia, brain disorders including stroke and epilepsy, and neurodegenerative diseases such as multiple sclerosis and Alzheimer's disease, etc. For achieving high quality proton and sodium imaging, such as high resolution and high signal intensity, a compact and highly sensitive coil design tailored to the human head is essential. Consequently, we proposed a single coil system capable of operating in a dual-mode, accommodating both frequencies effectively. This DMBC RF coil's benefits are not limited to brain imaging; they extend to other body parts, such as extremity imaging. With its high sensitivity, the proposed DMBC RF coil enables true simultaneous proton and sodium MR imaging, provided the scanner's receiver supports simultaneous broadband acquisition. Current state-of-the-art commercial scanners facilitate interleaved acquisition of signals at two different frequencies, allowing for alternative acquisition of signals from two nuclei [50]. Particularly, (semi-) simultaneous MRI of proton and sodium nuclei is useful in auto-coregistered abdominal imaging (in space and physiology) where the target organs exhibit non-rigid body motion. Thus, using the proposed dual-mode RF coil design for high sensitivity and simultaneous two-nuclei acquisition, whole-body MRI, including body imaging at clinical field strength like 3 T, becomes a viable option [51]. Although direct comparison of DMBC RF coil performance to dual-tuned RF coil was not performed in the study, it is speculated that DMBC RF coil, notably devoid of switching components such as pin-diodes, could eliminate performance degradation commonly associated with DC biasing in these elements. This exclusion contributes to consistent performance as it eliminates the diode-related signal losses and potential reliability issues over time, critical considerations in high-use clinical settings. Furthermore, the coil's construction with only two end-rings simplifies the overall birdcage configuration, eliminating the common patient positioning challenges encountered with more complex four end-ring dual-tuned RF coils.