http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Three Studies in High-Field MR Engineering: SAR Reduction Through Sequential RF Pulses, A System for B1+ Steering, and A Low-Complexity Multi-Nuclei Array Coil
Recent years have seen great development in ultra-high-field (UHF) magnetic resonance imaging (MRI). In particular, the advantage of increased sensitivity available at UHF and the scan time acceleration enabled by parallel imaging can together offer unprecedented imaging detail. However, technical challenges, such as B1+ inhomogeneity and specific absorption rate (SAR) also come with the UHF scanner. A head coil array at 9.4T using a novel transmit approach of serial transit parallel receive (STxPRx) is modeled and analyzed in this doctoral work. By transmitting each coil element in an array sequentially rather than simultaneously, a 25.7% peak SAR reduction can be found. For MRI applications with significant SAR concerns, such as lower-gamma nuclei imaging, this work provides a potential alternative approach. To mitigate the image inhomogeneity issue at 7T, an add-on B1+ steering system is reported in this doctoral work. By using a house-designed phase shifter array and associated circuitry, 8 transmit channels with independent phase control can be created to drive an 8-channel transmit coil array, yet with only two transmitters installed on the scanner. By using the conjugate-phase steering method, an approximately 2.6x4.2cm2 very homogenous B1+ region and approximately 5.9x8.9cm2 fairly homogenous B1+ region can be generated at any targeting point in phantom. This work suggests a different hardware approach to gain the new degree of freedom of controllable transmit B1+ pattern in parallel imaging. Along with the development of higher B0 strength, multi-nuclei MRI have become increasingly popular since both the anatomical detail from proton and the physiological process from X-nuclei can be acquired simultaneously. As the essential tool in multi-nuclei MRI, the multi-tuned RF array coil suffers from the excessive complexity of the multi-tuned retrofit of the current decoupling mechanism. A novel strategy aimed at simplifying multi-nuclei array coil⁰́₉s decoupling procedure is investigated in this doctoral work. High impedance preamplifier and transformer are experimented to achieve coil decoupling in a broadband way. The preliminary test result validates the feasibility of achieving decoupling for a broader frequency range, yet the decoupling bandwidth and coil sensitivity in this approach are currently limited by the characteristic of the transformer. The electronic version of this dissertation is accessible from https://hdl.handle.net/1969.1/197342
Doctor of Philosophy Texas A&M University 2022 https://hdl.handle.net/1969.1/197342