Beam-Forming for Whole-Body Travelling-wave MRI at Ultra-High-Field

In clinical magnetic resonance imaging (MRI), uniform nuclei-spin excitation via the circularly polarized transverse magnetic field is crucial. Ultra-High-Field (UHF, ≥7T) MRI is of great significance in medical imaging due to higher signal-to-noise ratio (SNR). However, higher operating frequency (≥ 297 MHz) at UHF MRI makes it challenge to achieve uniform nuclei-spin excitation, since the human body behaves as an electrically large object. The wave-behavior dominates field distribution inside biological tissues instead of the quasi-static principle. Following the classical volume layout for current sources, it forms multi-beam transmission condition, which inevitably led to prominent standing-wave patterns. It is good for targeted focusing application but detrimental to large-scale nuclei-spin excitation at UHF MRI. Waveguide based travelling-wave MRI is a promising alternative to achieve single-beam transmission, but its major power is blocked by the head and extremities. Therefore, it is not suitable to whole-body MRI. Our study presents a novel single-beam transmission strategy for whole-body application through beam-forming in a center-fed waveguide system. The inner metallic bore of MRI system as well as the dielectric material were used to achieve effective beam-forming inside biological tissues. The role of phase-velocity matching and wave-impedance matching in alleviating scattering effect arising from highly non-unform dielectric media, were also explored in this study. Preliminary phantom validation and in vivo scans of the spine, thorax, abdomen, and pelvis show the potential of this method for whole-body MRI at UHF.