Design and performance of a toroidal radiofrequency volume coil with intrinsic electromagnetic interference rejection for low-field portable Halbach-based MRI systems

Purpose: One of the intrinsic limitations of low-field MRI is low signal-to-noise ratio, which can be further reduced by electromagnetic interference (EMI) due to the lack of a Faraday shielded room. To address this issue, we propose a novel radiofrequency (RF) coil design that is inherently less sensitive to EMI while maintaining high receive sensitivity. Methods: The proposed coil structure is based on an anapole (toroidal) design and consists of six rings, each containing four continuous wires wound around an elliptical three-dimensional-printed former. The wire in the entire structure is uninterrupted. This coil is designed for use in systems with an axial B₀ field direction. The performance of the proposed RF coil was evaluated on a Halbach array system designed for neuroimaging and operating at 47 mT and compared with a widely used spiral head coil. Results: The noise level achieved by the proposed toroidal coil in combination with a belt wrapped around the subjects and grounded to the scanner was comparable to that of the widely used spiral head coil when combined with a grounding belt and passive aluminum shielding. The coil transmit and receive efficiency was comparable to the efficiency of the spiral head coil. Conclusion: The proposed RF coil inherently reduces the effect of EMI, potentially removing or reducing the need for passive shielding or external/internal sensors for EMI reduction in postprocessing.