Meta-metallic coils and resonators: Methods for high Q-value resonant geometries

Abstract

A novel method of decreasing ohmic losses and increasing Q-value in metallic resonators at high frequencies is presented. The method overcomes the skin-depth limitation of rf current flow cross section. The method uses layers of conductive foil of thickness less than a skin depth and capacitive gaps between layers. The capacitive gaps can substantially equalize the rf current flowing in each layer, resulting in a total cross-sectional dimension for rf current flow many times larger than a skin depth. Analytic theory and finite-element simulations indicate that, for a variety of structures, the Q-value enhancement over a single thick conductor approaches the ratio of total conductor thickness to skin depth if the total number of layers is greater than one-third the square of the ratio of total conductor thickness to skin depth. The layer number requirement is due to counter-currents in each foil layer caused by the surrounding rf magnetic fields. We call structures that exhibit this type of Q-enhancement "meta-metallic." In addition, end effects due to rf magnetic fields wrapping around the ends of the foils can substantially reduce the Q-value for some classes of structures. Foil structures with Q-values that are substantially influenced by such end effects are discussed as are five classes of structures that are not. We focus particularly on 400 MHz, which is the resonant frequency of protons at 9.4 T. Simulations at 400 MHz are shown with comparison to measurements on fabricated structures. The methods and geometries described here are general for magnetic resonance and can be used at frequencies much higher than 400 MHz. Published by AIP Publishing. [http://dx

MRI surface-coil pair with strong inductive coupling

“…However, there is also a consideration not reflected in the circuit model that as the current in the coupling coil decreases, the perfect magnetic boundary condition is no longer presented to the SRS and so Q s will decrease for the reasons described in Ref. 1. In the present experiment, sample loading was dominant.…”

“…For the SRS by itself, a Q-value of 480 was measured using a coupling loop and network analyzer described in Ref. 1. From the transmission line connected to the coupling coil, a Q-value of 114 was measured for the SRS coil assembly with a sample.…”

“…However, conventional spiral surface coils tend to have low Qvalue for reasons discussed in Ref. 1. The third factor (current ratio) seems new and accessible for a coupled coil system but not for a simple coil.…”

“…1,2 In that work, a high Q-value geometry appropriate for a surface coil in MRI was found to consist of a self-resonant spiral (SRS) made of metallic foil of thickness on the order of a skin depth surrounded by an outer resonant "equalization" coil. Although the original purpose of the outer coil was to improve Q-value by reducing rf currents on the foil edges, we find that the vector reception field (VRF) of the dual coil assembly is altered significantly compared to a simple loop surface coil.…”

“…5 We were led to this coil assembly through our earlier work on meta-metallic surface coils. 1,2 Although the method of two step coupling-from transmission line to outer loop by a capacitive network followed by inductive coupling to the meta-metallic loop-solves a particular problem, the coil assembly described here stands on its own. It is not limited to meta-metallic surface coils.…”