Design and Testing of Radiofrequency Spherical four Coils

In this paper, we present the design and testing of a radiofrequency prototype coil with good performances in terms of B1 magnetic field homogeneity and can be utilized for Magnetic Resonance Imaging. It is constituted of four coaxial separately tuned rings of wire and symmetrically located on a spherical surface. Compared to standard Helmholtz pair, which has 2nd-order magnetic field homogeneity, it yields to improvement in field homogeneity, while preserving the simplicity of design. The four coils of proposed structure are tuned to the same frequency. The proposed structure gets at 4th-order magnetic field homogeneity by optimizing the distance between rings and the diameters of outer loops. An electrical modeling of the four-coil system taking into account the coupling effects between all rings permits to determine the resonance frequency in the homogenous mode. Measurements of B1 field homogeneity were introduced in free space. Compared to the Helmholtz coil, the proposed structure presents good performances in terms of B1 homogeneity, quality factor and sensitivity. The design of proposed coil has been optimized for best SNR performances. Globally, this work claims to be a contribution to the study of the four-coil RF systems derived from the Helmholtz pairs.