S.M. Ould Ahmed Ghaly, L. Guendouz, A. Hedjiedj, J.M. Escanyé, and D. Canet (France)
RF Magnetic field, MRI probe, B1 homogeneity, spherical structure, ellipsoidal structure.
In this work, we present the design and test of two proto type probes with high B1 field homogeneity for MRI, com prising four coaxial rings of wire in a symmetric arrange ment on a spherical and on an ellipsoidal surface. Com pared to Helmholtz pair which presents a 2nd-order homo geneity, the aim is first an improvement of the field ho mogeneity, while preserving the simplicity of implemen tation. So, the two neighbouring rings, on the same side, are mounted in series and share the same tuning capaci tor. Thus, tuning of the coils is achieved as simply as for Helmholtz pair. The position and diameter of each ring are optimized so that the assembly produces a fourth- or sixth order homogeneous field. A complete electrical model of the four-coil system taking into account all coupling ef fects permits to determine the resonance frequency in the co-current mode, and consequently to predict the value of the tuning capacitors. Results of comparison tests, in free space and in a spectro-imager, between a Helmholtz pair and our two prototype coils are presented, and appear to be in good agreement with theoretical predictions. More over, noticeable improvements of the quality factor (+37% on average) and the sensitivity (+40%) are observed.
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