Novel methods and analysis of B0 and B1 gradients in magnetic resonance imaging

Abstract

In this thesis, analysis of B0 gradients and B1 fields are performed and novel methods using B1 gradients instead of B0 gradients are proposed. The first contribution of this dissertation is expressing the nature of the interaction between the B0 gradient fields and the active implantable medical devices (AIMD). By utilizing the fact that gradient coils produce linear magnetic field in a volume of interest, the simplified closed form electric field expressions are defined inside a homogeneous cylindrical volume. Using these simplified expressions, the induced potential on an implant electrode has been computed approximately for various lead positions on a cylindrical phantom and verified by comparing with the measured potentials for these sample conditions. In addition, the validity of the method has been tested with isolated frog leg stimulation experiments. The results of both phantom and ex vivo experiments show that if the path of the implant lead is known, the induced voltage on the lead can be estimated analytically. The second topic in this dissertation is the Bloch-Siegert (BS) shift based B1 mapping method. The method is analyzed in terms of the effects of the off-resonance frequency, the RF pulse shape, and the duration of the RF pulse. Based on these analyses, a new theoretical model that relates the Fourier transform of the off-resonant BS RF pulse envelope to the phase shift is proposed. Utilizing Bloch simulations and phantom experiments the proposed frequency domain expression is verified. The results indicates that the proposed expression works well even for short pulse durations (< 2ms) and low offset frequencies (fRF < 500Hz) when the ratio of the RF field and the frequency offset of the RF pulse is smaller than 0.5. The last topic of this dissertation is on flow and shear wave imaging with B1 gradients instead of B0 gradients. In flow imaging, a novel sequence using a Bloch-Siegert pulse generated by a spatially dependent B1 field is proposed. The proposed method is experimentally verified by comparing the resultant velocity measurements with those obtained by using bipolar flow encoding B0 gradients. This comparison demonstrates the feasibility of using BS shift with B1 gradients in detecting the flow. The usage of B1 gradients is also proposed to detect shear waves at frequencies in kilohertz range and this method is experimentaly verified for 2kHz, 3kHz and 4kHz shear frequencies. The studies in this thesis indicate that extensive analysis of B0 gradients in Magnetic Resonance Imaging (MRI) is important for safety issues, and for scenarios where B0 gradients prove insufficient in encoding due to hardware limitations, utilizing B1 gradients can be considered as an alternative.