## 1 Introduction  
Hyperpolarized \({}^{13}\)C MRS/MRSI has been effectively utilized for real-time investigation of cellular metabolism because it increases magnetization of \({}^{13}\)C-labeled compounds [1, 2] without using invasive radiation treatments [3]. The MRS technique can be used to examine the cardiovascular system and improve perfusion measurements [4]. The low gyromagnetic ratio (four times lower than that of \({}^{1}\)H) and natural abundance (\({}^{13}\)C for 1.1% and \({}^{1}\)H for 99.9% at natural abundance) of the \({}^{13}\)C nucleus can weaken the acquired \({}^{13}\)C signals when verifying in vivo metabolism. Given the two aforementioned natural characteristics, the \({}^{13}\)C signalsshould be enhanced via dynamic nuclear polarization (DNP) with injectable agents through in vivo subjects [2; 5]. Regional or time-resolved in vivo \({}^{13}\)C metabolic information can be acquired via \({}^{13}\)C MRI and MRS. Prior to the acquisition of \({}^{13}\)C MR images, \({}^{1}\)H MR imaging is required for shimming and accurate localization. Localization can be performed via co-registration of the \({}^{1}\)H and \({}^{13}\)C MR images. For the co-registration of different nuclei MR images, it is desirable to use the same RF coil for both nuclei. Given the aforementioned aspects, a dual-tuned RF coil, which is tuned to both frequencies of nuclei, was utilized to obtain multi-nuclei MR images containing anatomical and metabolic information of the subject [6; 7; 8; 9]. The most significant advantage of the dual-tuned RF coil is that both nuclei can be MR imaged without changing the RF coil and/or repositioning the subjects. The co-registration of the two types of MR images is straightforward and accurate throughout the dual-tuned RF coil. There are many types of dual-tuned RF coils. Phased-array surface loop coils were first proposed for multi-nuclei MR studies using PIN-diode switching or trap circuits [7; 10]. However, these kinds of dual-tuned surface RF coils have several weaknesses. Specifically, the dual-tuned RF surface loop coils exhibit a field distribution that is less homogeneous than that of volume coils [11]. A set of RF coils can be bulky in multi-nuclei MR studies using different combinations of RF coils [12; 13]. Additionally, phased-array surface loops can be applied to small-volume subjects, such as rats, human heads, cartilages in the knee joints, or regions adjacent to the muscle of interest, owing to their limited depth of signal sensitivity [14; 15]. Second, in the case of dual-tuned volume RF coils, such as birdcage RF coils with \(N\) legs, which is called the alternate leg method [16; 17; 18], the homogeneity of MR images of \({}^{1}\)H and the x-nucleus can be compromised because the \(N/2\) legs are tuned to the frequency of \({}^{1}\)H and the rest are tuned to that of the x-nucleus. To overcome this limitation, a four-ring birdcage coil for multi-nuclei spectroscopy was proposed by [PERSON] et al. [19]. However, this type of birdcage RF coil can be bulky and complex to construct. To address these limitations, we propose a \({}^{13}\)C birdcage RF coil with built-in switching circuits that can be activated passively and pass the electromagnetic waves of \({}^{1}\)H resonant frequency to ensure the acquisition of the x-nuclei MR signals without the loss of birdcage legs. Furthermore, the proposed RF coil can be utilized with any high-performance \({}^{1}\)H RF coil for \({}^{1}\)H MR imaging without any movement and repositioning of the RF coils and subjects. In this study, we provide a distinct description of the development and performance verification of a 16-leg low-pass proton-frequency-transparent (PFT) \({}^{13}\)C birdcage RF coil to obtain \({}^{13}\)C MRS/MRSI data and \({}^{1}\)H MR images. A high-performance \({}^{1}\)H RF coil was utilized with the proposed birdcage RF coil for \({}^{1}\)H MR imaging and localization with the following \({}^{13}\)C MRS/MRSI acquisition.