RF coils based on low-profile electromagnetic band gap (EBG)-structures for the usage in magnetic resonance imaging (MRI)

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During the first period of this DFG project novel multi-channel radio frequency coils for 7Tesla MRI have been proposed and were positively evaluated using a near-field measurement system. Each element comprises of a meandered dipole with an electrical length of approximately three half wave lengths and a shielding plate behind it in order to primarily generate the magnetic field in the direction of the patient (transmit case) or primarily receive from there. The shielding is either a length optimized metallic plate or a flat electromagnetic bandgap structure forming a high impedance surface (HIS). In the former case, the eigen-resonant base plate results in improved electro-magnetic fields in the body of the patient. The HIS shield on the other hand enforces in-phase mirror-currents that lead to an improved B1 efficiency (magnetic field amplitude per accepted power). Furthermore adjacent coil elements are more isolated from each other due to the band-gap characteristic of the structured meta-surface. In the second phase of the DFG project, the available coils are to be tested extensively on a 7-Tesla scanner. Initially, this evaluation includes single elements, which are tested by utilizing so-called phantoms that mimic the human body in its electromagnetic properties. Subsequently, more complex multi-element coil-systems will be established (4-channel setups for knee or head imaging and 8-channel arrays for the abdominal and back imaging). Finally, in-vivo measurements of the above mentioned body parts shall be carried out with the innovative multi-channel coils of this project. Parallel to the tests with the 7-Tesla scanner further research shall be carried out following the previous innovative RF coil approaches, also based on the findings related to the practical evaluation. We would like to explore a location-dependent surface impedance of the shield. Initially the extreme cases PEC (homogeneous metal) and PMC (structured HIS meta-surface) will be considered. This concept represents a further unexplored degree of freedom in the generation of MRI-optimized electromagnetic fields in the human body. In addition to the above-mentioned dipole elements ring antennas (magnetic dipoles) will be considered above HIS shields, and applied especially in parts of the body, where such a ring topology offers advantages, such as for breast imaging. Finally, combinations of electric and magnetic dipoles will be examined, i.e., the established meander dipoles are combined with ring antennas, so that both, tangential and normal magnetic field components are generated. A correct phasing of these two components results in a single element that produces a circularly polarized magnetic near field.