A novel passive shimming scheme using explicit control of magnetic field qualities with minimal use of ferromagnetic materials

In an MRI system, the static magnetic field homogeneity is strictly required especially in ultrahigh field situations. However, owing to the engineering tolerances and system errors, the magnetic field homogeneity of a magnet usually cannot meet the imaging requirement; thus, a shimming operation is always needed.Existing passive shimming methods commonly minimize the peak-peak variations of the magnetic fields over the diameter of spherical volume (DSV), targeting the field quality of 10-20 parts per million (ppm). However, these conventional passive shimming methods can sometimes lead to sub-optimal field quality and iron consumption solutions. Notably, the RMS error (RMSE) value of the field uniformity is inherently unoptimized. This work proposed a novel passive shimming method that can deliver a significantly improved shimming solution by actively controlling the central magnetic field and specific magnetic field deviations in the region of interest. A detailed comparison between the conventional and proposed methods was conducted on a 9.4T human MRI superconducting magnet.The results showed that the new solution had a significant advantage in searching for superior magnetic field homogeneity with less iron piece consumption. Significantly, the RMSE value of the magnetic field over the DSV can be substantially reducedgt;10 times. The proposed algorithms are also very efficient, taking only several seconds to find the shimming solution.The potential of the magnetic field homogeneity improvement methods will promote the development of high-end MRI systems.