Double-Tuned Surface ¹H–²³Na Radio Frequency Coils at 7 T: Comparison of Three Decoupling Methods

Abstract

Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) with nuclei other than protons (X-nuclei) often require the acquisition of proton signal for shimming and co-registration procedures. Double-Tuned Radio Frequency (DT-RF) coils improve these procedures, avoiding the need for movement and repositioning of the subject during the examination. The drawback of DT-RF coils is basically the coupling between the two resonant structures, which increases signal losses leading to a degradation of the final MR image. To improve MR signal quality acquired via DT-RF coils, a suitable decoupling strategy should be implemented. For this purpose, three DT-RF coil prototypes, which differed only in the decoupling method, were built and their performances were compared through workbench measurements. Each prototype consisted of two concentric loops. The inner and outer loops were tuned at sodium (≈ 79 MHz) and proton (≈ 300 MHz) Larmor frequency at 7 Tesla, respectively. Active and passive decoupling designs were compared measuring the Q factor and the S₂₁ parameter for each prototype. Active decoupling was tested as an alternative to the standard passive decoupling with a trap circuit, in which a non-negligible amount of current flows at resonance, perturbing the magnetic field responsible for producing the MR image. Workbench measurements showed satisfactory Q factors and S₂₁ for both active and passive decoupling cases. Thus, active decoupling could be a promising alternative to achieve better MR signal quality. Furthermore, for active decoupling, two circuit elements were examined: PIN diodes and micro-electromechanical system (MEMS) switches.