  We demonstrate an experimental method for direct 2D and 3D imaging of
magnetic radio-frequency (rf) field distribution in metal-dielectric structures
based on traveling wave (TW) magnetic resonance imaging (MRI) at ultra-high
field (> 7T). The typical apparatus would include an ultra-high field whole
body or small bore MRI scanner, waveguide elements filled with MRI active
dielectrics with predefined electric and magnetic properties, and TW rf
transmit-receive probes. We validated the technique by obtaining TW
magnetic-resonance (MR) images of the magnetic field distribution of the rf
modes of circular waveguide filled with deionized water in a 16.4 T small-bore
MRI scanner and compared the MR images with numerical simulations. Our MRI
technique opens up a practical non-perturbed way of imaging of previously
inaccessible rf field distribution of modes inside of various shapes metal
waveguides with inserted dielectric objects, including waveguide mode
converters and transformers.
