Direct Imaging of Radio-Frequency Modes via Traveling Wave Magnetic Resonance Imaging

TL;DR

This paper introduces a novel MRI-based method for direct 2D and 3D imaging of radio-frequency magnetic field distributions inside metal-dielectric structures, enabling visualization of RF modes in complex waveguides at ultra-high fields.

Contribution

The authors develop and validate a traveling wave MRI technique for directly imaging RF magnetic fields in metal-dielectric waveguides, a previously inaccessible measurement approach.

Findings

Successfully imaged RF modes in a circular waveguide filled with water at 16.4 T.

Compared MRI images with numerical simulations, showing good agreement.

Demonstrated potential for non-perturbative imaging of RF fields in complex structures.

Abstract

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.