A Novel Strategy and Test of Passive Shimming for Multi-Volumes in Cylindrical MRI Scanner

With few exceptions, a fresh magnetic resonance imaging (MRI) scanner may fail to achieve the magnetic field specifications, due to the existence of engineering errors or surrounding ferromagnetic materials. Excellent field homogeneity is one of the most important criteria for imaging quality; therefore, the shimming technique must be employed to bring the magnetic field homogeneity within the MRI requirements. The commonly used method is passive shimming (PS), which utilizes the shim pieces (iron pieces) with a different thickness in specific positions to generate a 'corrective' magnetic field. In conventional PS, the inhomogeneous field over the measured volume (MV) is shimmed to be the best as much as possible. However, in fact, since the MV and the real imaging volumes (IVs) are different, a problem may exist as the field over the MV is good enough but may not be sufficiently qualified over some IVs. As a result, a multi-volumes PS strategy is proposed in this article, which pays more attention to the magnetic field over some potential IVs other than the MV. Compared with the conventional single-constraint method, the new method increases the shimming targets and constraints tailored to imaging, without increasing any computing burden and additional iron consumption. In addition, the PS test on a 1.5 T MRI scanner is performed using the conventional and proposed algorithms, respectively. The magnetic field obtained by a new strategy is obviously superior to that of the conventional method, evaluated from some specific volumes for parts of human body imaging, demonstrating the effectiveness of the multi-volumes shimming strategy.