A MIM/coaxial stub-line CRLH zeroth-order series-mode resonator used as an RF coil element for 7-Tesla magnetic resonance imaging

A novel MIM/coaxial-stub composite right/left-handed (CRLH) metamaterial transmission-line (TL) structure is proposed. In addition to the common metal-insulator-metal (MIM) series capacitor, a short-circuited coaxial line, which is vertically aligned to the otherwise planar structure, forms the shunt inductor. The zeroth-order (ZO) series mode with its spatially uniform and longitudinally polarized series current of the short-circuited CRLH TL is utilized in the context of RF field excitation in a 7-Tesla magnetic resonance imaging (MRI) scanner. The usage of coaxial stubs instead of sidewise microstrip lines has several advantages, especially for the MRI application, such as a unit cell with smaller width, the unwanted longitudinal magnetic field components excited by the shunt resonantor are shielded by the outer coaxial connector and the upper MIM plate, yielding a high Q_sh-to-Q_se-ratio (here 250/100), which implies an extremely uniform series current distribution. Here a 40-cm long ZO resonant element for 300 MHz is proposed. FEM eigenmode and FDTD driven-mode full-wave simulation, together with corresponding near-field measurements on fabricated prototypes are presented to comfirm the approach.