RF Coils for Preclinical Multinuclear Imaging Based on Coupled-wire Structures Working in Resonant and Non-resonant Regime

Passive inductive metamaterials have been explored as alternative radio frequency (RF) coils for magnetic resonance imaging (MRI) with the aim to control and optimize the imaged volume and the sensitivity independently. Nevertheless, such structures result in a low signal-to-noise ratio (SNR) since the contribution of the loop in the global performance of the coil is reduced. Therefore, the purpose of this work is to explore a new strategy by combining off-resonance metamaterials with a resonant surface coil and observe the advantages that can be obtained. An elementary structure consisting of two parallel off-resonance wires coupled with a resonant surface coil was numerically analyzed. For experimental characterization, a prototype was built and tested in a 7 T MRI scanner for proton (
¹
H) and fluorine (
¹⁹
F) using a phantom. In addition, other coil setups were tested for reference and comparison in terms of B1
⁺
magnetic field homogeneity, signal and noise. The results show that with this new strategy a conventional surface coil can be optimized in terms of sensitive volume while maintaining its high SNR. Metamaterials permit a customized adjustment of volume and sensitivity in addition to the simple adaptation to other nuclei, making them beneficial elements in the design of RF coils.