Accurate magnetic resonance fingerprinting-based B1+ mapping in the human body for ultra-high-field MRI

Ultra-high field (UHF) MRI (B0 >= 7 Tesla) offers unique advantages compared to lower field strengths, such as an increased signal-to-noise ratio, but it also faces critical challenges due to inhomogeneity of the transmit magnetic field (B1+). When the human body is targeted at UHF, a large B1+ variation becomes apparent, and many applications rely on accurate quantification of B1+ across this wide range. However, established methods exhibit limited sensitivity at low flip angles (FAs), and hardware constraints of the RF amplifier and transmit chain often preclude straightforward compensation strategies. This thesis presents the development and investigation of a novel magnetic resonance fingerprinting (MRF)-based B1+ mapping method capable of accurately quantifying B1+ across a broad FA range (6° to 74°). Compared to existing techniques, this more than doubles the usable dynamic range, significantly improving performance in low FA regions, which are critical in body imaging at 7 T. The method was validated through phantom and in vivo studies, demonstrating strong agreement with reference methods and showing better consistency in subjects with higher body mass index. A detailed investigation of the MRF signal model highlighted key factors affecting accuracy, including RF-pulse properties and signal spoiling efficacy. Hardware investigations revealed nonlinearities and amplifier imperfections as significant sources of error, emphasizing the importance of monitoring the actual RF output, which allowed correction and significant improvement in the resulting B1+ maps. The method was further extended to a 3D hybrid channel-wise framework using a stack-of-stars trajectory, enabling full liver coverage within 10 min under free-breathing conditions. This hybrid approach, combining two absolute B1+ maps with low FA GRE acquisitions, allowed accurate channel-wise estimation and showed good agreement with the 3D method with one channel active, where the 3D approach was more consistent in low FA areas. Overall, this work represents an important step toward achieving accurate, reference-standard B1+ mapping at UHF. Its improved sensitivity at low flip angles offers significant benefits for applications that demand precise estimation, potentially advancing the capabilities of UHF body imaging at 7 T and beyond.