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Description
The conventional iron matrix shimming method is sensitive to positional variations, which significantly impacts the magnetic field homogeneity of small-aperture 7T/54mm cryogen-free MRI superconducting magnets. This study aims to develop a robust, adjustable shimming method to improve magnetic field uniformity and precisely cancel low-order harmonics in these sensitive systems. A set of circumferential thin iron strips is attached to the surface of the gradient coil, with their positions and dimensions optimized using a genetic algorithm. The optimization adjusts the position of the strips in both the radial and axial directions to minimize interactions of iron strips and improve overall field homogeneity. The results show that the peak field homogeneity improved from 11.12 ppm to 6.66 ppm after shimming, a 40.11% improvement. The method successfully reduces first-order harmonics, although the cancellation of second-order harmonics requires further refinement. Additionally, the magnetic field drift after shimming is reduced to less than 0.1 ppm, demonstrating the method’s robustness. This approach is more flexible and easier to implement than traditional methods, offering a reliable solution for small-aperture MRI superconducting magnets. Future work will focus on further reducing field homogeneity to 2-3 ppm for even higher precision.
