Spherical and Improved Helmholtz Coil with High B₁ Homogeneity for Magnetic Resonance Imaging

Several types of volume RF coils such as Helmholtz pair have been used for Magnetic Resonance Imaging. However, the field uniformity of a coil has always been suffering from a critical issue in imaging accuracy. The presented work is a study and design of spherical and improved radiofrequency Helmholtz coil which produces a high B₁ magnetic field homogeneity. This coil is composed of four coaxial rings of wire, which are symmetrically located on a spherical surface. Each two rings on the same side have the same tuning capacitor. In contrast with the standard Helmholtz pair, which is of 2nd-order magnetic field homogeneity, the proposed coil provides fourth-order magnetic field homogeneity. The developed spherical structure is tested experimentally in free space and compared with results obtained with equivalent standard Helmholtz coil. The results lead to conclude that the proposed structure provides better B1 Homogeneity than the Standard Helmholtz type coil. Thus, the relative width at 1% of the profile is 1.37 times greater for the spherical Helmholtz coil. Similarly, the spherical Helmholtz coil has shown to have an increased efficiency and quality factor is of great interest.