Developments in static and pulsed magnetic field systems for detection of magnetic resonance in non-uniform magnetic fields

Abstract

<p>The evolution of wearable diagnostic devices and more importantly, increasing consumer awareness, have demanded advancements in sensing mechanisms, sensor data analysis and data processing. Magnetics technologies such as current sensing, actuation, switching, navigation and data recording have all evolved technologically with the demand of lower operational power and long-term system stability. However, none of these advancements, have incorporated operations in low magnetic fields since these fields are non-uniform, vary spatially and provide low data resolution. In this work, the possibility of sensor operations in low non-uniform magnetic fields is explored. Magnetic fields produced by neodymium iron boron permanent magnets are studied, simulated and tested with portable pulsed field generation systems to demonstrate the capability of detecting magnetic resonance signals in non-uniform DC magnetic fields. Advances in detection capabilities in non-uniform fields will allow multiple new application areas to develop, potentially revolutionizing medical diagnostic procedures.</p> <p>In this dissertation, we analyze different aspects of a portable magnetic resonance sensor system in detail. We first study magnetic fields produced by different permanent magnet geometries. The spatial magnetic field variations in the magnet's exterior are simulated using finite element methods. In particular, regions of localized field uniformity in the magnet's exterior are identified for ring magnet geometries. Various modifications to ring magnets such as magnet dimensions, inclusion of magnetic inserts, placement of multiple magnets and their orientations are simulated to identify the optimal geometry with maximum magnetic flux density in locally uniform regions.</p> <p>We next consider the generation of pulsed magnetic fields using portable electronic circuits. Pulsed magnetic fields are needed to initiate the magnetic resonance process. Thus, pulsed fields are used alongside the static fields in magnetic resonance measurements. We discuss design considerations for creating portable pulsed magnetic field circuits, delivering upto 10 A of current at operational frequencies ranging from 2 - 5 MHz, via design of two prototype circuits. Both these prototype devices rely on application of pulsed sinusoidals to switching devices connected to inductors.</p> <p>A combination of the static and pulsed magnetic fields constitutes the NMR sensing and detection system that is used to study ferromagnetic and paramagnetic materials. We present measurements from ferromagnetic materials placed in non-uniform magnetic fields with applications in oil-well industry. We also present measurements of paramagnetic materials within organic media. These measurements validate applicability of such portable sensor systems, thereby ushering in varied possibilities for future portable magnetic resonance measurements in low and non-uniform magnetic fields.</p>