A dual-tuned concentric multimodal RF coil for 7 T ¹H/³¹P MRSI: concurrently enhancing B₁ efficiency over single-tuned references

This study presents the design, simulation, and experimental validation of a dual-tuned concentric multimodal surface coil for 7 T ¹H/³¹P magnetic resonance spectroscopic imaging (MRSI), developed to significantly enhance ³¹P B₁ efficiency while improving ¹H performance. The coil architecture utilizes two interleaved sets of three concentric loop resonators. Intra-nucleus electromagnetic coupling within each three-loop set generates a spectrum of eigenmodes; the operational modes for ¹H and ³¹P were specifically selected because their co-directed current distributions reinforce the magnetic field at the center, yielding B₁ patterns that resemble those of conventional single-loop surface coils but with superior efficiency. Full-wave electromagnetic simulations and bench measurements on a fabricated prototype were conducted to characterize the multimodal resonance behavior, scattering parameters, B₁ distribution, and 10-g local SAR, using size-matched conventional single-tuned loops as references. The results confirmed that the design reproducibly generated the predicted eigenmode ordering with sufficient spectral separation to prevent interference from parasitic or undesired modes. Notably, the multimodal design achieved an 83% boost in ³¹P B₁ efficiency and a 21% boost in ¹H B₁ efficiency at the coil center compared to same-sized single-tuned references. Sufficient inter-nuclear decoupling was achieved to prevent signal leakage between channels, and simulations with a human head model confirmed that the peak 10-g local SAR remained comparable to conventional designs. These findings demonstrate that this multimodal concentric design offers a robust and highly efficient solution for multinuclear MRSI at ultrahigh fields, effectively mitigating the sensitivity limitations of X-nuclei without compromising proton-based imaging capabilities.