A High-Performance Conformal Metamaterial for 1.5 T Wrist Magnetic Resonance Imaging

Abstract

Improving the signal-to-noise ratio (SNR) is crucial for enhancing image quality and diagnostic reliability in magnetic resonance imaging (MRI). However, traditional methods that rely on increasing the number of coil channels are approaching their physical and engineering optimization limits. In recent years, wireless, passive metamaterials have been explored for local magnetic field enhancement via radiofrequency field modulation, offering a new pathway to improve SNR. However, the regular configurations of most existing metamaterial structures hinder their conformance to anatomical contours, thereby limiting the spatial filling factor and preventing further SNR enhancement, while anatomically conformal designs often result in nondegenerate orthogonal modes. This study presents a conformal elliptical cylindrical metamaterial designed to achieve uniform magnetic-field enhancement and restore orthogonal-mode degeneracy in a non-axisymmetric geometric configuration. This is achieved through cross-sectional gradient impedance design and end-ring impedance tuning. Additionally, nonlinear response control is introduced to eliminate interference with the radiofrequency transmit field. Experimental results show that, when used in various clinical imaging sequences, the metamaterial yields wrist MR images with an SNR more than 4 times that obtained with a commercial flexible coil. This work provides a general design methodology for the synergistic regulation of field distributions and modal characteristics in asymmetric electromagnetic structures, establishing a robust technical foundation for the development of high-performance, anatomically conformal clinical MRI hardware.