Improved Helmholtz-type magnetic resonance imaging coils with high-B1 homogeneity -: Spherical and ellipsoidal four-coil systems

The operation of two novel magnetic resonance imaging coils with high-B-1 homogeneity and easy tuning is described. These coils involve four coaxial loops of wire carrying the same current and symmetrically located on a spherical or an ellipsoidal surface. In comparison with Helmholtz coils, an improved field homogeneity is expected while keeping the simplicity and ease of use. Field calculation and electrical modeling are performed. Using an appropriate field decomposition, the design of the four-coil probes is optimized in terms of position and diameter (or azimuthal angle) of the loops to produce, in principle, a fourth or sixth-order homogeneous field with the spherical or ellipsoidal structure, respectively. At the electrical level, in addition to the inductive coupling between the coils, the neighboring coils located on a same side are connected in series with a tuning capacitor. An equivalent lumped circuit model of the four-coil system, including all couplings is provided. In practice, it permits, at a given Larmor frequency, to determine the tuning capacitors. Comparative tests in a spectro-imager were performed between a prototype Helmholtz coil and two spherical and ellipsoidal prototype coils. Concerning the radio-frequency field homogeneity, experimental results show good agreement with the theoretical predictions. Moreover, compared to the homologous Helmholtz coil, our four-coil prototypes possess a better quality factor and therefore lead to an improved nuclear magnetic resonance sensitivity. (C) 2008 Wiley Periodicals, Inc.