The invention relates to a system for magnetic resonance imaging (MRI). Such systems face an additional contribution to the inhomogeneity of the radio frequency (RF) field when high magnetic fields are applied. The invention tries to improve the homogeneity of the RF field for high field strengths, particularly for field strengths at or above 3 tesla. To improve the homogeneity an electrically conducting material (4) is positioned within the cavity (2) of the system. The material has a conductivity and a thickness which ensure that the total radial conductance in an xy-plane perpendicular to the symmetry axis of the cavity becomes isotropic.