Estimation of losses in strip and circular wire conductors of radiofrequency planar surface coil by using the finite element method

An accurate coil design is a fundamental task to maximize signal-to-noise ratio in magnetic resonance applications. Coil design techniques take advantage of computer simulations especially when coil size is comparable to the radiofrequency (RF) wavelength. In particular, the estimation of the losses within the conductors as well as the radiative losses, both as a function of frequency, is instrumental to a complete coil performance characterization. However, the cross-sectional shape of the conductors strongly affects the radiofrequency coil's performance, especially at those frequencies where conductor losses represent the dominant power dissipation mechanism. Indeed, at radiofrequencies, the current flowing in the conductor is distributed in the proximity of its surface instead of being uniformly distributed over the cross section; it follows that an accurate conductor losses estimation can be performed only in the case of wire conductors by using analytical formulations. For strip conductors, although different theoretical approaches have been proposed in literature by taking into account the losses, no closed-form expression for conductors resistance is available which takes into account both classical and lateral skin effects. In this work, finite element method (FEM) simulations have been performed for estimating conductor and radiative losses in planar surface loops made of strips and circular wires; the results have been compared against analytical formulations and literature data. Workbench tests performed on two circular coil prototypes, the first one constituted by a strip and the second one by circular wire conductors, tuned at 63.9 MHz and 127.8 MHz, showed a good agreement with FEM simulations.