| dc.description.abstract | Part I: Magnetic Particle Imaging for Human Functional Neuroimaging While Magnetic Resonance Imaging (MRI) has revolutionized diagnostic imaging since its clinical introduction in the 1980s — primarily focusing on hydrogen nuclei — it remains fundamentally limited by the weak nature of nuclear spin magnetism. For example, functional MRI (fMRI) provides valuable insights into brain activity through BOLD signaling, but its limited sensitivity and reliance on indirect physiological measures often necessitate large subject pools for meaningful analysis. In contrast, Magnetic Particle Imaging (MPI) utilizes the much stronger magnetism associated with superparamagnetic iron oxide nanoparticles (SPIONs), and by minimizing background signal levels which are not modulated by functional activity, it offers a promising alternative. However, there are no approved SPION tracers for human use that are well-suited to MPI, and we have little experience scaling this technology up to human-sized imagers. This thesis therefore demonstrates a human-scale MPI scanner using functional MPI (fMPI) in non-human primates and assesses its potential for future human studies. Additionally, we investigate safety aspects of MPI, specifically focusing on peripheral nerve stimulation (PNS) induced by the 25 kHz magnetic excitation fields used in MPI. Because this is a higher frequency than those used by MRI gradients, threshold data at this frequency are lacking. This thesis measures the PNS stimulation threshold in human subjects to better understand high-frequency magnetic PNS and ensure the safe implementation of human-scale MPI for future neuroimaging applications. Part II: Short Mid-Field MRI Magnet Designs Anxiety induced by the long, narrow tube of conventional 1.5T and 3T scanners is a common cause of incomplete patient examinations, leading to delays in diagnosis and reduced facility throughput. In contrast, the short aspect ratio of CT scanner bores is known to alleviate this anxiety, eliminating this problem. This thesis also addresses the need for a more patient-friendly MRI scanning option by introducing a new “hybrid” superconducting and permanent magnet concept applicable to mid-field (0.5T) superconducting solenoid magnets. While mid-field scanners offer lower sensitivity than high-field alternatives, recent advances in image reconstruction and denoising have significantly enhanced their utility, allowing them to deliver diagnostic information comparable to that of the previous generation of 1.5T scanners. Additionally, they increase the range of compatible metallic implants and offer hospitals a lower-cost, easier-to-site alternative to 1.5T and 3T scanners. They can also enhance patient comfort through shorter bore lengths and larger diameters, but their optimized winding designs still reach a limit in how short they can be made for a given homogeneity and diameter specification. This thesis introduces the use of rare-earth permanent magnets to enable further reductions in scanner length, aiming to match the aspect ratio of CT scanners. | |
