Design, Fabrication and Test of a Lightweight 3.0 T Cryogen-Free MRI System With GM Refrigerator for Imaging Small Animals and Materials

Objective: To construct and evaluate a lightweight, high-performance and cost-effective 3.0 T MRI system with enhanced spatiotemporal magnetic field characteristics for advanced imaging applications. Methods: A lightweight 3.0 T cryogen-free magnet weighing $\sim$1100 kg was developed. Key optimizations included conduction-cooled pathway, vibration isolation, mechanical damping, and structural stability to ensure long-term magnetic field stability. Customized imaging sequences incorporated navigator echo corrections were developed to address residual vibrations. A gradient coil was designed with 200 mT/m peak amplitude and advanced shielding to minimize magnet-coil interactions. Passive shimming and active shim coils were integrated to improve spatial magnetic field homogeneity. Results: The 5 Gauss line of the superconducting magnet was constrained to 1.80 m × 1.20 m. Temporal magnetic field fluctuations were reduced by 99.81%, decreasing from 2.168 $\mu$T to 0.004 $\mu$T. Passive shimming achieved spatial peak-to-peak and root mean square error (RMSE) homogeneity of 22.41 parts per million (ppm) and 3.69 ppm over a 180 mm diameter of spherical volume (DSV), with further improvements to 4.18 ppm and 1.02 ppm through active shim coils. Gradient shield coils confined stray fields to 1.2 Gauss and reduced residual eddy fields. A complete MRI system was constructed with a home-built console and a radio frequency (RF) coil. High-resolution imaging of the mouse brain and detailed analysis of plastic parts and porous media were achieved, with accelerated algorithms reducing scan times significantly. Conclusion: The newly developed 3.0 T MRI system demonstrates superior spatiotemporal magnetic field stability and imaging capabilities. It offers significant improvements in image quality and resolution for various applications, including small animal studies and material characterization. Significance: The enhanced performance of this cryogen-free MR system represents a significant advancement in brain science, plastics, and porous media imaging technology.