High-field MRI issues: Finite wavelength effects, transverse coil design and acoustic noise reduction

This dissertation is composed of three important topics in high-field MRI (magnetic resonance imaging): finite wavelength effects, the design of gradients and active shims, and noise reduction of the MRI systems.; In the first part, a new method of dealing with MRI non-uniformities caused by EM wave interference patterns at 300-400 MHz in lossy dielectrics is presented. These non-uniformities are also known as "dielectric resonance" effects causing bright and dark spots in the image. The proposed method makes use of rotating cosnϕ current distributions (for n = 1,2,...) on the birdcage legs in order to compensate any non-uniform RF (radiofrequency) excitation occurring in different areas of the sample. The signals obtained during each experiment are then coalesced into a uniform image using appropriate weighting factors. The level of uniformity is improved by the application of additional cos(nϕ) birdcage-like channels.; In the second part, I will describe new applications for a method (recently developed by Shvartsman) in the design of transverse gradients and tesseral shims for whole-body MRI machines. The design and execution of these systems with previous methods heretofore had been quite difficult, especially for high-order tesseral shims. In this new "z-intercept" method, the coil is defined mathematically as a discrete system of loops laid out over a given topology and dictated by the symmetry and magnitude of the magnetic field they have to generate. After that the global optimization routine is used to adjust the positions of the loops for obtaining the maximal (minimal) target parameters such as linearity, uniformity, coil self-inductance, etc.; In the third part, I will describe the mechanism of noise generation from cylindrical parts of the whole-body NM machine: the gradient cylinder and the "warm" bore. These two parts are found to be most susceptible to mechanical vibration, therefore the reduction of their motion and corresponding noise is vitally important for patient comfort and image quality. The vibration is caused by Lorentz forces on time-variable currents in the tesla-range magnetic field. In the new approach, the mechanical power of vibration is calculated for cylindrical bodies and found to be in good agreement with experimental data.