Single and Multiple Coaxial Inputs to a Cylindrical Waveguide for Far Field MRI at 21.1 Tesla

Most clinical Magnetic Resonance Imaging (MRI) techniques use body coils for near field excitation and detection of an RF signal at 64-128 MHz, corresponding to magnetic field scanners of 1.5-3 T. Use of these conventional RF coils for high field MRI (>7 T) becomes progressively more difficult because the shorter wavelengths are on the order of the size of the resonating cavity and sample size, causing wave interference effects not normally seen in near field operation. However, such conditions lend themselves to far field operation. In traveling wave MRI, propagating RF fields are excited through an antenna located at a distance to a given sample such that excitation and detection in MRI can be induced in the far field using techniques developed in radar and communications applications. Numerous publications have utilized simple patch antennas integrated into 7 T human MRI scanners to image in the far field regime. At even higher fields, other excitation methods are available. For example, coaxial cable inputs are very simply implemented and can aid in both electromagnetic mode generation and the excitation of multiple simultaneously propagating RF waves. In this project, traveling wave MRI at ultra-high field is implemented using a concentric waveguide composed of a dielectric filled inner cylinder and an outer copper cylinder with dimensions similar to the magnet bore of 21.1 T ultra-wide bore magnet at National High Magnetic Field Laboratory, Tallahassee, Fl. Data was acquired from a simulated model of the system as well as from actual experiments for propagation instituted by coaxial inputs to the cylindrical waveguide.