Topology Optimization and Experimental Validation of a Magnet Structure for Portable Ultralow-Field MRI Devices

Portable ultralow-field (ULF) magnetic resonance imaging (MRI) offers advantages over conventional high-field MRI, including mobility, reduced weight, and lower cost, which make it suitable for specialized settings such as ambulances and intensive care units. To maximize its benefits, further reducing overall weight while maintaining imaging quality is a key development trend. This study applies topology optimization (TO) to the iron yoke in order to reduce its weight while achieving a stronger static magnetic field in the ULF MRI device. Unlike parametric optimization (PO), the TO method does not rely on predefined structures, enabling the discovery of novel topology designs. The optimization results demonstrate that while maintaining an average magnetic field above 100 mT in the target region, the TO method achieves a 29.4% reduction in iron weight compared to the optimal structure obtained by the conventional PO method. The optimal structure derived from the TO method was simplified to facilitate manufacturing, and a prototype was fabricated accordingly. In this newly developed ULF MRI system, a shimming ring and passive shimming magnet blocks were incorporated, and second-order active shimming was applied, reducing the overall field inhomogeneity to 300 parts per million (ppm). Water phantom imaging was successfully performed using this fully redesigned system.