Introduction

Hyperpolarized ¹³C MR imaging is an emerging molecular imaging technology to study dynamic metabolic and physiological processes rapidly, non-invasively and specifically. The unique real-time in vivo metabolic information provided enables the study of a wide range of metabolism-related diseases, such as tumors, hepatic, and renal metabolic diseases (1-4). All heteronuclear molecular MRI studies require efficient heteronuclear RF coils to produce data with high sensitivity and 1H RF coils to acquire 1H anatomical images (5). The aim of this work is to design a double-tuned RF coil for ¹H/¹³C MR imaging at 7T to meet the requirement of achieving uniform excitation and highly sensitive acquisition. The numerical electromagnetic simulation was utilized to evaluate the feasibility and performance of the developed RF coil in the s-parameters and B₁⁺ field distributions. The bench tests were also performed by utilizing the constructed coil system to validate the performance and the feasibility for heteronuclear MRI.

Method

To achieve uniform excitation and highly sensitive acquisition, the developed double-tuned RF coil system for ¹H/¹³C MR imaging at 7T incorporates a 16-leg double-tuned high-pass birdcage coil for ¹H/¹³C transmission, in which 1H is a transceiver, and a 30-channel receive array for 13C reception. Numerical simulations and analysis of the double-tuned high-pass birdcage coil were conducted by Computer Simulation Technology, (Darmstadt, Germany). The simulation model of the double-tuned birdcage coil was built as shown in Fig.1. The coil with 300 mm diameter was set with leg and end ring copper width 20 mm, and its overall length was 260 mm. The length of RF shield was 262 mm and the distance from the coil was 35 mm. The high-pass structures at each end of the coil in the Fig. 1b were operated at ¹H/¹³C (297.2/74.74MHz) frequencies, which could be achieved by adding the double-tuned tuning circuit in each port and additional double-tuned matching circuits in the excited ports of the end-rings. The RF front ends circuit of ¹H/¹³C double-tuned birdcage coil was shown in Fig.2.
All lumped elements at the end-rings were represented by 50 Ω discrete ports in the field simulation, which were then replaced by the electronic components and external ports in the circuit co-simulation. The coil was loaded with a spherical phantom (164 mm diameter, conductivityσ= 1 S/m, permittivity ɛr = 80). The values of capacitors and inductors was adjusted until obtained the desired resonance pattern. The phase of four excitation ports was set to 0°, 90°, 180°, and 270° to achieve a homogeneous circularly polarized field at two frequencies. The performance the proposed double-tuned birdcage coil was evaluated by the s-parameters and B₁⁺ field in the EM simulations. The probes were added in the center of the coil for monitor the magnetic field intensity.
For comparison, the ¹H and ¹³C single-tuned birdcage coils with the same dimension as the ¹H/¹³C double-tuned coil were designed and simulated, respectively. Both coils were loaded a 164mm-diameter spherical phantom.
S-parameters were measured on the bench using the network analyzer to evaluate the performance of the constructed double-tuned birdcage coil.

RESULT

The simulated s-parameters of the double-tuned birdcage were obtained as shown in Fig.3 (a). The strength of magnetic field is the strongest at the center of the coil at the resonate frequencies as shown in the Fig.3(b). These results indicated that the double-tuned birdcage coil was able to operate at ¹H/¹³C frequencies. The simulated B₁⁺ fields of the double-tuned and single-tuned birdcage coils were calculated and shown in the Fig.4. The results indicated the double-tuned birdcage produced a homogeneous B₁⁺ field in the center of the transverse plane at ¹H /¹³C frequencies. In addition, the B₁⁺ distribution pattern of the ¹H/¹³C double-tuned birdcage coil at ¹H and ¹³C frequencies is essentially the same as that of the single-tuned birdcage coils. We confirmed the performance of the by measuring the scattering parameters and resonance characteristics on the constructed coil system as shown in Fig. 5.

Conclusion

In this study, a novel double-tuned RF coil system was designed and constructed for ¹H/¹³C MR imaging at 7T. The performance of the coil was evaluated by the numerical simulation in s-parameters and B₁⁺ field distributions. The bench tests were conducted to further validate the performance. Our performance evaluations showed that the proposed double-tuned RF coil system can be expected to achieve uniform excitation and highly sensitive acquisition for ¹H/¹³ C MR imaging at 7T and demonstrated the feasibility for heteronuclear MRI applications. In future work, coil performance will be further optimized, and phantom and in vivo studies on a 7T MR system will be performed.

Acknowledgements

This work was supported in part by the National Key Research and Development Program of China (2021YFE0204400), the NSFC grant (81627901), Project on Global Common Challenges of Chinese Academy of Sciences (No. 321GJHZ2022081GC), the Key Laboratory for Magnetic Resonance and Multimodality Imaging of Guangdong Province, (2023B12120600520), the Funding Program of Shenzhen, China (RCYX20200714114735123), the Chinese Academy of Sciences Youth Innovation Promotion Association funded project (Y2021098).