Loop radiofrequency coils for clinical magnetic resonance imaging at 7 Tesla

To date, the 7 T magnetic resonance imaging (MRI) scanner remains a pure research system and there is still a long way ahead till full clinical integration. Key challenges are the absence of a body transmit radiofrequency (RF) coil as well as of dedicated RF coils in general, short RF wavelengths of the excitation field in the order of the dimensions of a human body leading to signal inhomogeneities, and severe limitations with respect to the specific absorption rate. They all result in a strong need for RF engineering and sequence optimization to explore the potential of MRI at 7 T, and to pave the way for its future clinical application. In this thesis, high-resolution MRI with a rather small field-of-view (FOV) in the head and neck region (parotid gland/duct and carotid arteries), and of the musculoskeletal system as well as with a very large FOV in the abdomen (spine) were presented. Therefore, a variety of RF coils were used: from a commercially available single-loop coil to novel, specially developed phased array coils each consisting of eight loop elements. Methods to thoroughly characterize and test the developed RF coils were presented, including numerical simulations, bench and MRI measurements. Characterization with respect to performance for parallel acquisition techniques and an extensive compliance testing for patient safety were described in detail. All aspects of the engineering part, from design to optimization, and finally, to the in vivo application in volunteers and patients were covered. Since clinical applicability has always been the purpose, optimized imaging protocols along with a discussion on the clinical relevance was included in each study. The presented RF loop coils widely expand the options for clinical research at 7 T and advance the integration of this technology in a clinical setting.