Design, fabrication and measurement of a force-balanced strong gradient magnetic field assembly for a cryogen-free 7 T / 160 mm MRI superconducting magnet.

High-field magnetic resonance imaging (MRI) systems impose stringent performance requirements on gradient coils, including high efficiency, excellent field linearity, and sufficient mechanical robustness. In this work, an actively shielded and force-balanced gradient coil assembly was developed for a cryogen-free 7 T/160 mm superconducting MRI magnet. The gradient coils were designed using a conventional boundary element method (BEM)-based framework with active shielding, force-balance constraints, and engineering manufacturability considerations to improve gradient efficiency and suppress electromagnetic forces under strong background magnetic fields. Based on the optimized design, three-axis gradient coils were fabricated using CNC machining and axial winding and integrated into a compact coaxial multilayer assembly through a standardized engineering fabrication and assembly process. Experimental results show that the X-, Y-, and Z-axis coils achieve gradient efficiencies of 3.95 mT/m/A, 3.72 mT/m/A, and 4.12 mT/m/A, respectively, with field linearity errors within ±2.5% inside a 50 mm diameter spherical volume (DSV). The measured electrical parameters show good agreement with the design values, confirming the feasibility of the proposed fabrication and assembly method. These results demonstrate the engineering feasibility and practical implementation capability of the proposed gradient coil assembly for compact high-field micro-MRI applications.