Experimental characterization and numerical simulation of a dielectric tuning technique dedicated to miniature monolithic NMR coils

The use of monolithic transmission line design allows developing miniature radio frequency (RF) resonators with high sensitivity for nuclear magnetic resonance (NMR) applications. This design has allowed great improvement in NMR micro imaging achieving large signal to noise ratio (SNR) but requires a dedicated tuning technique. We have investigated on a tuning technique based on a dielectric coupling effect: a dielectric pad is placed at close proximity of the coil winding so as to intercept the electrical fringing field lines and thus shifts the resonance frequency (F₀). In order to evaluate the feasibility of this tuning technique, a parametric study, based on both experimental measurements and numerical simulations, was performed. The influence of the dielectric pad properties such as thickness, permittivity or distance to the coil on the achievable F₀ shift was evaluated. The overall objective of the present work is to develop a tuning system using a micro-device to achieve the displacement of the dielectric pad, thus allowing a fine control of the amount of intercepted electrical field and thus the adjustment of F₀.