A HIGH B₁ FIELD HOMOGENEITY GENERATION USING FREE ELEMENT ELLIPTICAL FOUR-COIL SYSTEM

Design and testing of a prototype probe with high B1 magnetic field homogeneity that can be used for MRI is presented. It consists of four coaxial separately tuned rings of wire in a symmetric arrangement on an ellipsoidal surface. Compared to Helmholtz pair, which presents 2nd-order homogeneity, an improvement in field homogeneity is achieved, while preserving the simplicity of implementation. The proposed probe has an identical self-resonance frequency for all coils to allow easy tuning. The position and diameter of each ring are optimized so that the assembly produces a 6th-order homogeneous field. A complete electrical model of the four-coil system taking into account all coupling effects permits to determine the resonance frequency in the co-current mode and consequently to predict the value of the tuning capacitors. Resulted field homogeneity of the proposed four-coils are compared to that of Helmholtz coil which shows an improvement of 58% in field homogeneity profile when considering 10% profile uniformity width. Moreover, compared to the homologous Helmholtz coil, the proposed four-coil prototype possesses a better quality factor and therefore leads to an improved Nuclear Magnetic Resonance (NMR) sensitivity.