Slotted‐waveguide array radio frequency coil for ultra‐high‐field magnetic resonance imaging

A novel radio frequency (RF) coil for ultra‐high‐field MRI in the form of a slotted waveguide array (SWGA) filled with a low‐loss high‐permittivity dielectric is proposed, evaluated, and demonstrated. A comprehensive computational electromagnetics study, along with basic RF measurements, to characterize the SWGA RF coil at 7T is presented. Slotted waveguides are robust structures capable of handling high powers. They are inherently narrow‐band and have well defined linear polarization. When arranged in an array, they effectively generate high‐quality B1 field with strong B1+ and extremely low B1− and Bz components. With added dielectric lenses, the observed transmit efficiencies exceed 2.3μTW in the human head model phantom, which is much higher than all results reported in literature. Moreover, we show that the proposed exciter, as an array with well‐decoupled elements (measured isolation between elements is 33 dB or higher), can effectively be used for RF shimming. The novel coil generates RF magnetic field with excellent circular polarization, good uniformity, and negligible axial z‐component, and it provides arbitrarily large or small field of view and excellent transmit efficiency, with and without dielectric lenses. It features well‐defined narrowband operation, excellent isolation between ports/channels, and inherent possibilities for field optimizations via RF shimming and parallel imaging.