Date of Award
Fall 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Biomedical Engineering (ENAS)
First Advisor
Constable, Todd
Abstract
Over the last 50 years Magnetic Resonance Imaging (MRI) has become a mainstay of modern medicine as a noninvasive diagnostic imaging tool. Most clinical and research MR scanners have trended towards higher field strengths as a means of improving spatial resolution while minimizing scan time. However, widespread availability of these systems is limited, with 90% of people worldwide still lacking access to MRI. Much of this discrepancy comes from the high cost and complexity associated with installing, operating, and maintaining conventional MR scanners. By relaxing the hardware requirements, moving to lower field-strengths, and creating more open-bore systems, MRI can become cheaper and more widely accessible. Such systems could be used in unconventional locations such as physician offices, intensive care units (ICU), emergency rooms, surgical suites, or even rural healthcare sites. In this study an open, low-field proof-of-concept MR system was constructed and validated. The scanner is a single-sided electromagnet that uses field-cycling technology to polarize spins at a higher field and readout at a much lower field. Field-cycling allows for MR imaging to take place in the inhomogeneous B_0 inherent to the system’s open-bore magnet design. To further reduce the hardware complexity and acoustic noise that is characteristic of conventional MR scanners, gradients coils were replaced with an RF planar array, and spatial encoding was performed exploiting the Bloch-Siegert shift effect. Encoding field patterns analogous to nonlinear gradient encoding fields were created by varying the number of coils used to transmit the off-resonance Bloch-Siegert pulse, and the phase of those pulses. An optimization technique was designed to create nonlinear encoding schemes that provide the most spatial information with minimal repeated information. Several pulse sequences were developed and tested for imaging with this scanner. These sequences include a multi-echo encoding CPMG sequence, a reverse polarization inversion recovery sequence for fat suppression imaging, and a short prepolarization saturation recovery sequence for water suppression imaging. Multiple transmit and receive channels were used for signal localization. All images were reconstructed using the conjugate gradient method for nonlinear encoding fields. The system’s imaging capabilities were validated through simulation and experimental studies involving various 2D phantoms. Similar to other nonlinear encoding techniques, the resolution was variable over the FOV. Still, experimental results matched expected results and there was clear delineation of phantom geometries and features.This novel MRI scanner demonstrates the feasibility of gradient-free imaging in a nonuniform B_0. Future work will focus on reducing scan time so the system can provide similar benefits to ultrasound in terms of ease-of-use and availability. This new architecture of relaxing the homogeneity criteria of the main magnet field and reducing the hardware demand could allow MR scanners to be built in any shape or size. Overall, the techniques presented in this thesis re-imagines conventional MRI, making it low-cost, silent, and highly accessible.
Recommended Citation
Selvaganesan, Kartiga, "Gradient-free Magnetic Resonance Imaging in a Nonuniform B0, With a Novel Low-field Magnet System" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1184.
https://elischolar.library.yale.edu/gsas_dissertations/1184
