Division of Superconducting Magnet Science and Technology, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China.
School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing, China.
Magn Reson Med. 2022 Dec;88(6):2732-2744. doi: 10.1002/mrm.29419. Epub 2022 Sep 5.
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 reduced >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.
在 MRI 系统中，静态磁场的均匀性是严格要求的，特别是在超高场情况下。然而，由于工程公差和系统误差，磁铁的磁场均匀性通常无法满足成像要求；因此，通常需要进行匀场操作。
现有的被动匀场方法通常将磁场的峰值-峰值变化最小化，以达到球形体积（DSV）直径上的磁场质量为 10-20 百万分之一（ppm）。然而，这些传统的被动匀场方法有时会导致次优的磁场质量和铁芯消耗解决方案。值得注意的是，磁场均匀性的均方根误差（RMSE）值本质上没有得到优化。本工作提出了一种新的被动匀场方法，该方法可以通过主动控制感兴趣区域中的中心磁场和特定磁场偏差来提供显著改进的匀场解决方案。在 9.4T 人体超导磁共振成像磁体上对常规方法和提出的方法进行了详细比较。
结果表明，新的解决方案在寻找具有较少铁芯消耗的更好的磁场均匀性方面具有显著优势。显著地，DSV 上的磁场 RMSE 值可以显著降低>10 倍。所提出的算法也非常高效，只需几秒钟即可找到匀场解决方案。
磁场均匀性改进方法的潜力将促进高端 MRI 系统的发展。
