Simulation and Optimization Study of an Ultra-Low-Field Bell-Shaped Head MRI Electromagnet

Ultra-low magnetic field systems have attracted increasing attention in recent years because they lack cryocoolers and exhibit lightweight, low cost, and mobilization convenience. In contrast to permanent magnet systems, electromagnetic systems can easily vary background magnetic fields, thus offering opportunities on multiple scales of time and distance to characterize the molecular dynamics and transport properties of complex liquids in bulk or embedded in confined environments. An unconventional ultra-low field bell-shaped head MRI magnet with a dedicated compact structure is needed for applications in clinics, intensive care units, and mobile vans and evaluating stroke in the hyperacute and acute stages. In this work, we use the discrete minimum energy (DME) method to design the ultra-low field bell-shaped head MRI electromagnet. This method involves obtaining a DME current map, initial coil extraction, and coil adjustment. The DME map is used to determine the initial current distribution of the MRI electromagnet over a predefined domain subject to constraints. Then, initial coil extraction aims to extract the initial regularized rectangle coil layout from the DME map. Considering that the extraction process decreases magnetic field homogeneity in the region of interest (ROI), coil adjustment is then implemented. This step adjusts the position and volume of the initial coil layout to meet the homogeneity requirement. A bell-shaped electromagnet ( $$< 20$$ mT, 2 MA/m $$^2$$ ) with a homogeneity of 7 ppm over 260 mm DSV is designed in this work.