FEM based analysis of highpass birdcage resonators for B1 field mapping

Abstract

3D full wave finite element method (FEM) based electromagnetic (EM) analysis is a technique to map EM fields generated by electrical devices. To better understand and apply this technique to magnetic resonance imaging (MRI) radio frequency (RF) birdcage resonators, a vast number of 3D full wave EM simulations are required for validation and optimization of the B1 field generated by them since they have to be tuned to a particular Larmor frequency. In the past RF birdcage resonators were constructed without doing any 3D full wave EM analysis and more emphasis was laid on tuning and matching the electrical circuits used to make these resonators. However modeling birdcage resonators in a 3D computer aided engineering (CAE) simulation environment is important to observe the resonance behavior and the B1 field distribution inside the birdcage resonator volume before its construction thus saving valuable resources. In this work we have attempted to map B1 field distribution inside the full and half birdcage resonators tuned to Larmor frequency for proton nuclei at 3 Tesla with the help of FEM. FEM essentially converts the problem of solving Maxwell’s partial differential equations into solving a large system of linear equations. In this work we make use of the ANSYS high frequency structure simulator (HFSS) which is an FEM based frequency domain solver. The results of the full birdcage resonator are further compared with experimental outcomes. The phantoms used for experiments and simulation are both symmetric and non-symmetric ones. It can be concluded that HFSS or similar FEM based EM simulator may be used to predict the B1 field inside loaded RF resonators to obtain information of the B1 field behavior. It is observed that B1 field distribution inside the birdcage resonator varies with different types of phantoms used to mimic small animals for MRI. B1 field maps and resonance results from simulation and experiment are presented. Finally this thesis concludes with areas of improvement and a road map for future work.