| Magnetic resonance imaging (MRI) uses magnetic fields to acquire cross-sectional medical images with a rich variety of contrast methods for identifying pathology in the body, all without ionizing radiation or other harmful side effects. Prepolarized magnetic resonance imaging (PMRI) uses a fundamentally different magnet architecture than conventional MRI to take advantage of the benefits of acquiring image data at a low magnetic field while achieving the better signal-to-noise ratio of a high magnetic field. Whereas a conventional MRI scanner uses a static background magnetic field generated with superconducting coils or a permanent magnet, a PMRI scanner uses two independently pulsed copper electromagnets.; The primary advantages of PMRI include the ability to image adjacent to metal prostheses in the body, lower scanner cost, and the possibility for novel contrast mechanisms that require a variable magnetic field. These benefits come at the cost of limited flexibility, however PMRI can perform standard clinical musculoskeletal imaging with almost the same efficiency as conventional MRI.; This work addressed three engineering challenges in PMRI: design of the pulsing electronics for the polarizing magnet, design of the control system for the readout magnet, and design of efficient imaging methods. The high-field polarizing magnet was rapidly pulsed with a capacitor storage system but required a leakage current bypass network to minimize current in the off state. The low-field readout magnet was powered with a precise control system that offered both maximum regulation and optimal system damping to minimize transient switching disturbances. Even with optimal magnet control, the residual field errors can cause significant imaging artifacts unless the system RF transmit and receive are properly phase shifted with either software or hardware solutions.; Through improved design of the magnet electronics and the successful implementation of fast 3D imaging sequences, the Stanford PMRI system can now achieve standard clinical contrast and resolution for imaging in vivo human wrists. Additional upgrades to the PMRI system will bring the image SNR closer to that of clinical scanners, but PMRI already outperforms MRI in the presence of metal. |
