Study of two contact-less tuning principles for small monolithic radiofrequency MRI coils and development of an automated system based on piezoelectric motor

• Two contact-less tuning principles for small monolithic NMR coils were studied.
• The coil can be tuned up or down over a large frequency range.
• Piezoelectric-based automated system was built to implement the tuning principles.
• The automated system allows real-time tuning during an MRI experiment.
• MRI compatibility of the automated system was validated by imaging experiment.

The Multi-turn Transmission Line Resonator (MTLR) design allows for developing very small and highly sensitive radiofrequency (RF) coils for magnetic resonance imaging. However the monolithic feature and the small size of the developed coils, imposes to employ dedicated contact-free tuning techniques. In this work, we investigated two contact-free tuning principles for small monolithic RF coils: a dielectric tuning principle based on the interception of the electric field lines by a dielectric layer, and an inductive tuning principle based on the interception of magnetic field lines by a closed conducting ring. For both tuning principles, we used experimental measurements co-supported by electromagnetic simulation and analytical modelling to determine the accessible tuning range as a function of both the tuning element's characteristics and its distance to the RF coil. A maximal tuning range about 9% and 24% was achieved using the dielectric tuning principle and the inductive tuning principle, respectively. Contrarily to the dielectric tuning principle, the inductive tuning principle was found to strongly limit the quality factor of the coil. To implement the two investigated tuning principles, we have developed an automated system based on piezoelectric actuators that achieves microscopic displacements of the tuning elements. The automated system was also used to perform inductive impedance matching of the coil. For purposes of both tuning and matching, when starting from a strongly un-tuned or un-matched configuration, the automated system reaches convergence within a few tens of seconds. First MRI experiments were performed to evaluate the MRI compatibility of the automated system.