Electromagnetic Near-Field Inhomogeneity Reduction for Image Acquisition Optimization in High-Resolution Multi-Channel Magnetic Resonance Imaging (MRI) Systems

This paper is a study of the inhomogeneity reduction for near-field acquisition in high-resolution magnetic resonance imaging (MRI) systems. The acquisition homogeneity in MRI imaging modality is an open issue concerning the optimal MRI image generation in terms of the RF signal acquisition. The acquisition inhomogeneity is related to the radiation patterns of the receiving antennas and its location in the MRI system, among other relevant aspects. The acquisition inhomogeneity is translated into two main effects: pattern ripples at the outer cylindrical rings and radial inhomogeneity when comparing the center value (maximum) with the rest of the pattern. To overcome these effects, two strategies are proposed. In the first one, it is proposed to progressively vary the antenna location in the azimuthal array distribution. In the second one, it is proposed to progressively vary the antenna amplitude and phase feeding in the array distribution. To compute a figure of merit of the pattern radial uniformity and the ripples, two metrics are defined in this paper. It is proved that both the progressive modification in the location at each array ring and the variation of the feeding phase of each array ring reduce the pattern ripples and radial inhomogeneity. Optimal values for either the angular rotation or the feeding phase values can be calculated, depending on the particular dimensions of the cylinder that conforms the region of interest.