B1 estimation using RF coil locators and current sensors measurements [electronic resource]

Parallel transmit (PTx) systems have been proposed to compensate for B1 (RF) field non-uniformity, to improve excitation pulse performance, and to minimize specific absorption rate (SAR), mostly for high field MRI. These systems require prior knowl- edge of the radiofrequency (RF) field of each channel to perform calibration and B1 shimming to cancel out spatial B1 variations. A second class of applications emerging for PTx at 1.5T and 3T is in interventional MRI and implant RF safety. The goal here is to minimize RF coupling to insulated conductive structures such as guide- wires, pacemakers, and deep brain stimulator leads. Lastly, dynamically polarized 13C imaging requires prior knowledge of spatial flip angles to optimize SNR, with flip angle errors resulting in costly re-polarization times. In all these cases, the imaging work-flow would be improved if B1 mapping could be avoided or minimized. Existing B1 mapping methods su↵er from a variety of issues including long scan times, limited B1 dynamic range, and high specific absorption rate (SAR). Moreover, experimental B1 mapping techniques assume nothing about the coil geometry or location, yet the coil structure is known a priori. The goal of this work is to create a "good enough" estimator of B1 fields without mapping, using techniques that enable registration between the physical and simula- tion domains. It is demonstrated that co-registration by fluorine or proton fiducials for coil localization, combined with on-coil RF current sensing, provide the necessary inputs to scale and transform a pre-computed library of B1 simulations into the phys- ical domain to estimate the B1 maps of a known coil geometry with a simple rapid pre-scan calibration. In this dissertation, it is then shown that the estimated B1 maps can be used to calculate complex RF shim weights in an RF shimming application without perform- ing MRI B1 mapping. All the experiments in this dissertation were performed on a GE 1.5T scanner using a Medusa console and a cylindrical four-channel transmit/receive array prototype. Our group is particularly interested in local transmit array applications at 1.5T and 3T for RF implant safety, and MRI interventions, where the intent is to minimize the extent of RF coupling and exposure. In both applications, local transmit arrays could minimize coupling, but the flexible or variable array layouts and load impedance variations call for B1+ calibration aids to simplify their use in setting local RF shim weights. To explore these issues, the feasibility of extending forward and reverse po- larization method is investigated. It is shown that pre-spoiler gradients combined with reverse polarization can significantly suppress the background signal and suc- cessfully visualize the conductive structure inside a body. Finally, it is shown that a 3D model of the conductive structure can be extracted by applying edge detection filter to reversed polarized images.

Keywords: Electrical Engineering.
Advisor: Pauly John (John M.) primary advisor. Thesis advisor
Nishimura Dwight George advisor. Thesis advisor
Scott Greig Cameron