A Free Element Spherical and Ellipsoidal Radio Frequency Coils with High B1 Homogeneity for MRI

An ellipsoidal four-loop MRI coil, comprising four coaxial separately tuned rings on an ellipsoidal surface, is developed and tested. In comparison with spherical four-loop coil which provides a fourth-order homogeneity of the B1 field. This coil aims to provide an effective sixth-order homogeneity of the Bi field while preserving simplicity of design. A complete electrical model of the coil, including all couplings, is investigated to assure these requirements. To facilitate the easy tuning of the developed coil and predict the current-ratio between the centre and outer loops in the co-current mode, an identical self-resonance frequency for all loops is adopted. A prototype shielded elliptical coil at 100,241MHz, consisting in two centre (respectively outer) rings with diameter 4.8 cm (3.4 cm) separated by 1.4cm (4cm), is built. A shielded spherical Coil with the same outer diameter (4.8 cm) was built to permit practical comparisons. Relative measurements of the B1-axial field were carried out in free space. The Ellipsoidal coil (E) exhibits better performances than the Spherical Coil (S). Loaded tests, using water samples of cylindrical shapes, were carried out on a Bruker-Biospec-Avance on MRI images. Axial single-slice (1mm) gradient echo images permitted to provide the axial-field-profile. Quite similar lobe widening is observed for the ellipsoidal coil with regards to the free space results.