A system for travelling wave MR imaging includes an MR imaging apparatus having a magnet coil assembly having a magnet coil bore extending therethrough, a gradient coil assembly positioned within the magnet coil bore and having a gradient coil bore extending therethrough, and a waveguide positioned within the gradient coil bore. The waveguide has a waveguide bore extending therethrough. A computer is programmed to access a scan sequence comprising an RF pulse sequence and execute the scan sequence. During execution of the scan sequence, the computer is programmed to operate the waveguide in a hybrid mode to transmit an RF pulse of the RF pulse scan sequence as a travelling wave at a frequency lower than a cutoff frequency of a principal mode of the waveguide absent a dielectric core and to acquire MR signals from an imaging subject positioned within the waveguide bore.