A METHODOLOGY FOR CURRENT-DENSITY CALCULATIONS IN HIGH-FREQUENCY RF RESONATORS

As nuclear magnetic resonance imaging and spectroscopy move inexorably toward higher field-strength magnets in search of improved signal-to- noise ratio, spectral resolution, and spatial resolution, the way in w hich radiofrequency (RF) probes are designed changes. At higher freque ncies, resonant cavities become the favored RF ''coil'' type and may b e built using streamline elements to reduce the inductance of the syst em. In modeling such systems, the quasi-static approach of assuming th at current flows evenly in all conductor cross sections and that adjac ent conductors do not affect each other becomes less reasonable. The p roximity of RF conductors in resonators typically causes RF eddy curre nts to flow, whereby the current density in each rung is altered by th e RF fields generated by nearby conductors. The proper understanding a nd prediction of how resonators will perform require a model of the cu rrent densities flowing in conducting sections, including all RF eddy current effects. Very few models of this type have been presented in t he literature. This article presents an overview of one such model and of how it may be applied to a variety of resonators, both shielded an d unshielded, circular, and elliptical, in cross section. Results are presented from a shielded head coil operating at 2 tesla. (C) 1997 Joh n Wiley & Sons, Inc.