Reduction of RF penetration effects in high field imaging

A method of correcting for the RF inhomogeneity in the body by dielectric loading of the coil-to-shield space in an RF resonator (coil and shield assembly) is described. The presence of the RF coil and RF shield have significant effects on RF homogeneity. Based on theoretical calculations, a method for adjusting the RF homogeneity by manipulating the axial propagation constant, k(z), is proposed. This is accomplished by loading the coil-to- shield space with dielectric material of suitable relative permittivity so as to increase k(z) and decrease the radial propagation constant, k(ρ). In this manner, the radial wavelength (λ(ρ) = 2π/k(ρ)) can be increased relative to the body dimensions, and the field amplitude variations in the axial plane minimized. Theoretical calculations indicate that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil-to- shield space would reduce the RF field inhomogeneity from ±15% to about ±3% over a central 30-cm-diameter region of a homogeneous 40-cm-diameter body at both 64 and 170 MHz. The theoretical model was verified in laboratory measurements of the vertical bar |B₁| vertical bar field generated in a test coil at 170 MHz which was scaled to correspond to a body at 64 MHz. However, the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity.