Andreas Rennings1, Le Chen1, Friedrich Wetterling2, Mark E. Ladd3, Daniel Erni1
1General and Theoretical Electrical Engineering (ATE), Faculty of Engineering, Univ. of Duisburg-Essen, Duisburg, Germany; 2Computer Assisted Clinical Medicine, Faculty of Medicine, Univ. of Heidelberg, Mannheim, Germany; 3Erwin L. Hahn Institute for Magnetic Resonance Imaging, Univ. of Duisburg-Essen, Essen, Germany
The RF field excitation homogeneity problem of MRI at high resonance frequencies is investigated on the basis of the simplest two-dimensional setup including a cylinder phantom with spatially constant but frequency-dependent material parameters and an impressed surface current distribution between the phantoms 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 and therefore maximize the physical insight, which allows quite general and helpful statements about the B1+ pattern quality for the widely used first-order circularly polarized (CP) excitation mode. The coefficient of variation (CoV) of the B1+ distribution is most suited as a figure of merit for the homogeneity, since it normalizes the standard deviation to the mean value of the field pattern. Only two parameters of the arrangement have been varied the excitation frequency ranging from 30MHz up to 450MHz and the diameter of the phantom ranging from a few centimeters up to 40 cm, which covers more or less every MR imaging scenario. A characteristic B1+ pattern will be introduced, which indicates the transition between a regime with the well-known central brightening and another one without.
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