Development of a Helium-Free 1.5 T Extremity MRI Magnet

We report the design, fabrication, and testing of a 1.5 tesla extremity magnetic resonance imaging (MRI) magnet that operates without liquid helium, and cooled entirely by conduction. The magnet employs an 8-coil NbTi superconducting winding on a high-conductivity aluminum former, actively shielded and supported in a cryostat by three stainless steel cantilever tubes for thermal isolation. Thermal links of high-purity aluminum foil ensure efficient heat transfer from the coils to a 1 W cryocooler. Finite element simulations guided the design, predicting a homogeneous field (≈5 ppm peak-to-peak) over a 15 cm field of view and acceptable mechanical stresses in the coils. The fabricated magnet was successfully cooled below 5 K and energized to 1.5 T. After minimal training quenches, it achieved stable operation with a measured field drift on the order of 0.1–0.2 ppm/h. Passive shimming brought the field homogeneity to within 8 ppm peak-to-peak over a 150 mm diameter volume, meeting clinical MRI requirements. These results demonstrate the viability of a helium-free, compact MRI magnet for extremity imaging, representing a significant step toward more accessible and low-maintenance MRI systems.