Magnetic resonance array square coils: Dependence of the frequency and sample conductivity on decoupling

In magnetic resonance systems, radio frequency (RF) coils are key components in both study transmission and receiving phases. The design of home-made, organ-specific RF coils can be a constraint in many research projects. Among the different types, array coils are constituted by a combination of multiple surface coils into an array structure, which allows for a high signal-to-noise ratio over a wide field-of-view. Mutual coupling between coil elements has to be minimized in order to maximize performance. This Tutorial describes the dependence of such coupling on the coil tuning frequency and sample conductivity. The author proposes the application of a numerical full-wave method based on the finite-difference time-domain algorithm to evaluate the loop mutual coupling of an array constituted by two square loops in loaded configuration. By evaluating S12 scattering parameter for different values of the coil elements position, results show that the optimal distance, which minimizes the mutual coupling, is located in the range of 87%-93% of the loop side and varies slightly with the frequency (64, 128, and 298 MHz) and sample conductivity (0.6, 1.5, and 3 S/m) values, with a difference of almost 7%. The presented simulation results underline that the knowledge of a sample-coil interaction model can be useful for the design of an array coil strictly coupled to the sample. I believe that this Tutorial could be interesting for graduate students and researchers in the field of magnetic resonance coil design and development.