Realizing the Applications of Nonlinear Encoding in MRI

Abstract

Magnetic Resonance Imaging (MRI) possesses the unique ability to capture a wide range of physiological attributes with high spatial resolution. This flexibility has allowed researchers and medical professionals to not only study, but diagnose, an array of disease states and illnesses that previously required invasive means. However, despite its’ tremendous advantages most of the world’s population lacks ready access to this diagnostic tool, often having to travel great distances just to reach a scanner. MRI currently faces a severe inequity in both accessibility and utilization due to the high costs associated with obtaining, transporting, and maintaining a traditional scanner. Hence MRI is generally limited to the middle and upper classes in wealthier countries; creating a tremendous need for this diagnostic tool to be disseminated to regions with lower per capita income.The primary goal of this work is to explore several approaches to the challenges of under-utilization and inaccessibility that the field of MRI faces. All of the techniques presented here have shown promise in improving the accessibility of MRI by either eliminating or replacing costly components without sacrificing on the diagnostic quality of the images. The first approach explored the feasibility of employing a low power, nonlinear gradient to perform slice selection and phase refocusing in a traditional spin-echo type sequence. Showing that it is not only possible to image in a permanent hyperboloidal gradient, but also observing the benefits to phase compensation and resolution, thus opening the door to exploring nonlinear encoding fields. The next approach sought to eliminate pulsed B0 gradients in favor of radio-frequency (RF) based gradients. This approached utilized a nonlinear, spatially-varying RF field to encode information in a fast-spin echo sequence. As a proof of concept, only one dimension utilized this approach but the work can be easily extended to 2D. The final approach looked at the logistics of creating and implementing a portable MRI system. This required the design and installation of nonstandard hardware in all aspects of the imaging system; including a novel high temperature superconducting magnet, a multi-channel digital spectrometer, and a multi-channel gradient array capable of creating arbitrary encoding field.