  At high magnetic field strengths (> 4 T), the propagation wave vector of the
excitation field B1 can no longer be ignored as the wavelength becomes
commensurate or smaller than the imaging field of view (FOV), particularly for
high dielectric media. The emergence of this propagation wave vector renders
the excitation field variant in both time and space. In this paper, the
propagation of RF excitation waves in a high field vertical NMR system is
demonstrated for the first time with an appropriate screened dielectric
waveguide excited by a simple loop coil to enable below cutoff propagation for
imaging in the far field. Uniquely, aqueous samples provide the high
permittivity required to modify the cut-off requirements of this transmission
system. Theory and simulations corroborate the observed propagating modes field
patterns. The ease of construction and implementation of this setup permits use
in a variety of high field systems. At a minimum, this design provides a larger
FOV that is limited primarily by the three-dimensional linearity of the
gradient coil. More critically, it also opens up the use of multimode
excitation for selective nulling or maximum excitation of the FOV along the
direction of wave propagation.
