Dielectric resonance phenomena in ultra high field MRI

Purpose: Dielectric resonances have previously been advanced as a significant cause of image degradation at higher fields. In this work, a study of dielectric resonances in ultra high field MRI is presented to explore the real importance of dielectric resonances in the human brain in this setting. Method: Gradient-recalled echo images were acquired using a transverse electromagnetic resonator at 1.5, 4.7, and 8 T. Images were obtained from the human head and from phantoms filled with pure water, saline, and mineral oil. In addition, an exact theoretical analysis of dielectric resonances is presented for a spherical phantom and for a model of the human head. Results: Theoretical results demonstrate that distilled water can sustain dielectric resonances in head-sized spheres near 200 and 360 MHz, but the presence of significant conductivity suppresses these resonances. These findings are confirmed experimentally with proton images of water and saline (0.05 sand 0.125 M NaCl). For lossy phantoms, coupling between the source and phantom overwhelms the dielectric resonance. Because of their low relative permittivity, mineral oil phantoms with 20 cm diameter do not exhibit dielectric resonances below similar to 900 MHz. Significant dielectric resonances were not observed in human head images obtained at 1.5, 4.7, and 8 T. Conclusion: It appears that dielectric resonances do not have any real importance in determining image quality for MR studies of the human head. It is advanced that RF coils are much more likely to be the source of any inhomogeneity.