A multimodal axial array resonator and its application in radiofrequency (RF) volume coil designs for low-field open...

Low-field open magnetic resonance imaging (MRI) systems, typically operating at magnetic field strengths below 1 Tesla, has greatly expanded the accessibility of MRI technology to meet a wide range of patient needs. However, the inherent challenges of low-field MRI, such as limited signal-to-noise ratios and limited availability of dedicated radiofrequency (RF) coils, have prompted the need for innovative coil designs that can improve imaging quality and diagnostic capabilities. In this work, we introduce a multimodal axial array resonator and its implementation in a volume coil, or referred to as a coupled stack-up volume coil, to address these challenges in low-field open MRI.

A prototype coupled stack-up volume coil was designed with optimized coil spacing to improve B1 field homogeneity. Finite difference time-domain (FDTD) simulations were conducted to evaluate the coil's performance, including B1 field efficiency and specific absorption rate (SAR). Bench tests were performed to validate the simulated results using oil phantoms. Comparisons were made with a solenoid coil, a birdcage coil, and an equal gap coupled coil throughout the study.

Numeral electromagnetic studies demonstrate the superior performance of the proposed coupled stack-up volume coil, achieving 47.7% higher transmit/receive efficiency and 68% more uniform magnetic field distribution compared to conventional birdcage coils. The results of the bench tests show that the achieved B1 field efficiency of the coupled stack-up volume coil is 11.48 $\text{μT}/\sqrt{W}$ , representing a 57.3% improvement in comparison to that of a conventional birdcage coil.

A multimodal axial array resonator technique or coupled stack-up technique is successfully developed for the design of low field MR RF volume coils. The proposed coupled stack-up volume coil outperforms the conventional volume coils in terms of B1 efficiency, imaging coverage, and low-frequency operation capability. This design provides a robust and simple solution to the low-field MR RF coil design.