Design of radio frequency coils for magnetic resonance imaging applications: A computational electromagnetic approach

The advancement of MRI as a radiological instrument has been associated with a constant drive towards higher magnetic field strengths resulting in higher operational frequencies. More powerful magnets bring the promise of enhanced signal to noise ratio, exquisite resolution, and reduced scan times. At the same time however, MRI at higher frequencies adds significant engineering complexities to the MRI experiment, most notably in designing safe, versatile, and high-performance radio frequency (RF) Coils.; In this work, computational and theoretical electromagnetic analysis of several RF coils used in MRI are presented at Larmor frequencies that range between 64 and 470 MHz representing clinical imaging at 1.5:11 Tesla. The electromagnetic interactions with phantoms and anatomically detailed head models, including a developed high-resolution human head mesh, are studied at different field strengths. The computational tool of choice here was the finite difference time domain (FDTD) method. Combined with measurements using an 8 Tesla MRI system, currently the most powerful clinical magnet in the world and a 1.5 Tesla system, the FDTD method is utilized to study, analyze, and eventually design RF coils. Innovative Engineering approaches using phased array techniques are presented to improve the performance of RF head coils in terms of transverse magnetic field uniformity and reduction of specific absorption rate for operation at 4.7 and 8 Tesla. Novel analytical derivations are presented to explain the source of the MR signal. The combination of the analytical derivations, FDTD modeling, experiments, and infrared imaging gives a new prospective onto the electromagnetics associated with low and high field clinical imaging.