B1-field inhomogeneity problem of MRI: Basic investigations on a head-tissue-simulating cylinder phantom excited by a birdcage-mode

The RF-field inhomogeneity problem of MRI at high resonance frequency is investigated from an electromagnetic viewpoint by numerical FEM simulations. The evaluation is based on a simple two-dimensional model including a cylinder phantom with spatially constant but frequency-dependent material parameters of a head-tissue-simulating liquid (HTSL) and an imposed field-exciting surface current distribution between the phantom´s surface and a surrounding perfect electric conductor (PEC) shield. This simple arrangement has been chosen intentionally in order to reduce the set of parameters to a minimum, allowing clear statements about the applicability of the widely used first-order circularly polarized (CP) excitation mode. The coefficient of variation (CoV) of the |B1+| distribution is a suitable figure of merit for the specification of the homogeneity. Only two parameters of the arrangement have been varied - the excitation frequency ranging from 30 MHz up to 450 MHz and the diameter of the phantom ranging from a few centimeters up to 25 cm, which covers more or less every MR head imaging scenario. Two characteristic |B1+| distributions will be introduced, which separate the frequency/diameter combinations into three different regions, a first one with a quasi homogeneous pattern, a second one where an interference pattern - the central brightening - occurs, and a last one, where only a quite weak center amplitude smaller then the mean value is excited.