Application of High Impedance Surfaces to Improve Radiofrequency Coil Performance for 7-Tesla Magnetic Resonance Imaging
In modern medicine Magnetic Resonance Imaging (MRI) has been widely used for the detection of diseases, like brain tumor, breast cancer or heart disease, to name a few. In order to improve the image quality, the concept of high-field and ultra-high-field imaging has been intensively investigated over the last three decades. However, associated with the increased magnetic field strength various problems and challenges occur. The inhomogeneous B1-field distribution and decreased penetration depth into the subject to be imaged are two critical issues. The concept of High Impedance Surface (HIS) has been successfully applied in a number of antenna applications. The most relevant achievements are an improved radiation efficiency especially for low profile antennas and a decreased mutual coupling between adjacent antennas or antenna ports. In this thesis, an approach to improve the B1 distribution of radiofrequency (RF) coils in terms of homogeneity and penetration depth by utilizing a HIS shield is presented. First, fundamental investigations are carried out to verify the concept of using a shield with large surface impedance, which is modeled here by a Surface Impedance Boundary Condition (SIBC), to enhance the B1 distribution of RF coils. Different SIBCs are considered and their effects on the electromagnetic (EM) field of RF coils are studied. Next, the shield with a large surface impedance is realized by a periodic HIS structure, where two approaches (multi-layer uni-planar and mushroom-like topologies) are introduced in the lattice design of the HIS structure. The proposed HIS structures are evaluated via their reflection phase and dispersion diagram, which are based on unit cell simulation; as well as the bandgap property based on transmission line (TL) models. Then, the realized HIS shield based on the uni-planar topology is applied to the RF coils, and the concept of using HIS shield to improve the B1 distribution is confirmed through full-wave simulations and experimental results. Additionally, we consider another important parameter for a multi-channel dipole coil array consisting of several array elements—the coupling characteristic of the coil array, especially between the neighboring elements (the worst case scenario). The last chapter provides a brief summary and discussion, as well as an outlook of the future work.
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