Toward Automated Manufacturing of RF Coils: Microstrip Resonators for 4.7 T Using 3D-Printed Dielectrics and Conductors

Abstract

Microstrip transmission line (MTL) resonators are widely used as radio-frequency (RF) transceiver coils in high-field magnetic resonance imaging (MRI). Typically, discrete capacitors are used to tune the MTL resonators to the Larmor frequency and to match to the 50 Ω characteristic impedance of the RF chain. The cost, availability, and labor-intensive work of soldering capacitors on each coil contribute significantly to the expense of RF coil arrays for MRI; therefore, a manufacturing method with lower cost and fewer processing steps is desirable. The additive manufacturing method of rapid prototyping offers a new method to build custom-designed MTL resonators with reduced fabrication steps and, potentially, cost. This feasibility study explores fused deposition modelling to 3D print the MTL resonator structure simultaneously with matching/tuning capacitors and conductors. Typical low-cost 3D printers are capable of printing only polymers, not metal and polymer printing in one machine. In this work, a low-cost 3D printer was modified by adding the capability to print conductive ink and used to print MTL resonators with monolithic parallel-plate capacitors. These integrated capacitors eliminate the repetitive work of soldering, and tuning is achieved by trimming the capacitor plates. In addition, 3D printing allows unconventional designs that minimize the amount of dielectric below the microstrip and, therefore, losses in the substrate. Resulting signal-to-noise ratio values using ink conductors are within 30% of those achieved with copper despite a resistivity that is two orders of magnitude higher. This performance gap can be addressed using newer inks that have much lower resistivity.