B0 Shimming of the Human Brain at Ultrahigh Field MRI with a Multi-Coil Shim Setup

Abstract

Magnetic resonance imaging (MRI) is widely used for contemporary diagnostics and research. Higher static magnetic field enables imaging with a higher resolution because of the increased signal-to-noise-ratio in comparison to low field MRI. However, this benefit is not cost-free. Less B1+ field uniformity, higher B0 inhomogeneity, higher specific absorption rate (SAR), and shortened T2 and T2* are some of the challenges of measurement at ultrahigh-field (UHF). The aim of this thesis is to address higher B0 inhomogeneity at a magnet with a strength of 9.4 tesla. To this end, the shimming hardware and software required for homogenization of the B0 field were designed, and the performance of the constructed setups has been validated by simulation and in vivo measurements. The first part of the thesis (Chapter 1) describes the source of B0 inhomogeneity, how it changes the FID signal, its consequences, and why UHF intensifies the B0 inhomogeneity. Then, the process of field homogenization, known as shimming, is introduced, and shimming with spherical harmonics is explained. Next, the standard method for B0 field measurement and inhomogeneity quantification is presented, and least squares minimization is described in order to optimal currents calculation for a constrained shimming. Then, dynamic slice-wise shimming is introduced as an approach to achieve a better B0 uniformity by breaking VOI to sub-volumes. Finally, the multi-coil shim setup is presented which benefits from small local coils for a more efficient shimming. The second part of the thesis (Chapter 2) focuses on construction and application of multi-coil shim setup as achievements of this thesis. First, the construction process of the setup and comparison with spherical harmonic basis sets are provided. Later, the impact of the improved B0 uniformity with the dynamic multi-coil shimming on fMRI contrast is studied. Finally, a novel multi-coil design approach is presented in which a multi-coil shim setup is optimized for shimming of the human brain. Sections 3.4 and 3.5 present a summary of the collaborations in other related projects. First, a novel method to design the shim coils’ wiring pattern based on stream function is introduced which allows higher order shimming with limited number of the coils to be achieved. Next, an application of the small local coils for parallel imaging is demonstrated. Small local coils are employed for a local modulation of the magnetic field and superimpose a unique phase variation to the spin distribution that can be used to disentangle different part of the object. The last part of the thesis starts with conclusions and outlook. Later, the resultant publications are listed, and the relevant publications are appended at the end.