Abstract
The majority of the clinical magnetic resonance imaging (MRI) scanners are based on superconducting magnets. They are bulky, heavy, and costly to purchase, site, and maintain. The disadvantages of a conventional MRI system have led to increasing interests in the development of a portable MRI scanner to expand MRI scanners’ application to a dynamic environment (e.g., the ambulance and the field hospital), to rural areas, or to underdeveloped areas/countries. In this thesis, a complete permanent-magnet-array-based (PMA-based) portable MRI system has been built based on a Halbach magnet array. Unlike the conventional MRI scanner, theB0 field is relatively inhomogeneous with a quadrupolar pattern, and it was rotated to serve as the spatial encoding field (SEM) to spatially encode the MR signal to get MR images. The MR imaging experiments were performed with a 2D phantom, and it validates the feasibility of the proposed portable MRI system. During the investigation of the built portable MRI system, the problems of a Halbach array have been identified. Firstly, the sensitivity of the loop coil will be degraded during the rotation of B0 field. Secondly, the B0 field in a Halbach array is in the transverse plane, thus the advancement of the RF coil for conventional superconducting magnet cannot be applied directly. To overcome the disadvantages of a Halbach array, an Inward-Outward (IO) ring-pair magnet array was proposed with a longitudinal magnetic field. On top of it, an irregular-shaped IO ring-pair magnet array is proposed and optimized with a monotonic SEM in favor of nonlinear image reconstruction, and an IO aggregate magnet helmet is designed to provide longitudinal gradients for 3D MR imaging. Overall, the construction and validation of the Halbach-array-based portable MRI system shows the feasibility of constructing a portable MRI scanner using inhomogeneous B0 field without gradient coils. With stronger attained sensitivities of receive array and the compatibility to the development of RF coils for conventional MRI scanners, new proposed IO ring-pair-based arrays are possible to release the potential of PMAs further for the development of a truly portable MRI system with capability of producing MR images for medical diagnosis.