# Self-Isolated Dual-Mode High-Pass Birdcage RF Coil for Proton and Sodium MR Imaging at 7 T MRI
###### Abstract
These include active switching mechanisms employing pin-diodes to modulate circuit resonance [26; 27], the implementation of trap circuits along the leg of the birdcage RF coil to shift or block specific frequencies [28; 29; 30], and the incorporation of a four end-ring architecture designed to resonate simultaneously at two distinct frequencies [31; 32]. The pin-diode switching technique is widely utilized due to the availability of pin-diode DC bias drivers in MR scanner, while the pin-diode activation results in a sensitivity reduction. Specifically, when the pin-diode is forward-biased, there is a significant decrease in the SNR of approximately 35% compared to a single-tuned RF coil configuration [26; 33]. Meanwhile, frequency-specific blocking traps can be integrated into each leg of a single birdcage RF coil structure to achieve dual-tuning. This approach halves the number of active legs at each frequency, necessitating a doubling of the leg count to preserve the homogeneity of the B\({}_{1}\) field. The trap can be utilized as a frequency-shifting (FS) circuit with an additional capacitor to resonate the single birdcage RF coil at two frequencies. Employing the FS trap circuits to tune at a lower frequency results in a performance degradation of the RF coil at high frequencies. In other words, the resonance of the RF coil at a lower frequency comes at the expense of its efficiency at a higher frequency [34]. Additionally, the FS trap is not designed for any two resonant frequencies, and adjusting the trap and capacitor values for dual-tuning is complicated. Moreover, the operation of the FS traps presents a challenge when the target frequencies are in close proximity. Alternatively, a four end-ring design, usually comprising a low-pass and high-pass birdcage RF coil without lossy components, offers superior SNR compared with the alternating legs of a birdcage coil with traps [35]. However, it requires an optimized inner to outer rung length ratio, leading to an increase in the overall coil length. This increase in the RF coil length consequently restricts the accessible space for patients, presenting a challenge for applications in brain imaging. The advantages and challenges of existing dual-tuned RF coil designs are summarized in Table 1.  
In response to the limitations presented by existing dual-tuning methods in RF coil design, the study introduces a self-isolated dual-tuned high-pass birdcage RF coil, termed the dual-mode birdcage (DMBC) RF coil. The DMBC RF coil is physically a single birdcage RF coil of precisely the combination of two single-tuned high-pass birdcage RF coil operating at each \({}^{1}\)H and \({}^{23}\)Na frequency without additional end-rings and active switching components. The DMBC RF coil is simultaneously tuned to two distinctive homogeneous modes at two different frequencies. The design features a dual-mode circuit (DMC) that utilizes a trap circuit specifically for tuning at the \({}^{23}\)Na resonant frequency. This configuration enables tuning at the \({}^{1}\)H frequency exclusively through the end-ring capacitors, as the same approach used in single-tuned birdcage (STBC) RF coils. Thus, the DMC allows the separate control of the DMBC RF coil to be tuned effectively at two frequencies. The utility of the DMBC RF coil with DMC was verified through \({}^{1}\)H and \({}^{23}\)Na MRI experiments at 7 T human scanner to maximize \({}^{23}\)Na signal sensitivity, and the images were compared with those \({}^{1}\)H and \({}^{23}\)Na image acquired by STBC RF coils.