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

Traveling Wave MRI at 21.1 T: Propagation below Cut-off for Ultrahigh Field Vertical Bore System