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

Purpose: 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.

Methods: 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.

Results: 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 reduced >10 times. The proposed algorithms are also very efficient, taking only several seconds to find the shimming solution.

Conclusion: The potential of the magnetic field homogeneity improvement methods will promote the development of high-end MRI systems.