Feng Liu1, Jianfeng Zhu2, Ran
Zhang3, Ling Xia2, Stuart Crozier1
1School of Information
Technology & Electrical Engineering, University of Queensland, Brisbane,
Queensland, Australia; 2Department of Biomedical Engineering,
Zhejiang University, Hangzhou, Zhejiang, China, People's Republic of; 3School
of Electrical Engineering, Shandong University, Jinan, Shandong, China,
People's Republic of
This paper presents a new passive shimming (PS) design scheme for the correction of static magnetic field inhomogeneities in MRI systems. The PS procedure usually employs field-based (a) or harmonics-based (b) methods to find an optimum iron piece configuration to improve the field uniformity in the imaging region. For the PS technique (a), the peak-to-peak field inhomogeneity is minimized and the harmonic components are inherently unconstrained; in the technique (b), selected unwanted harmonics are minimized and the overall field uniformity is consequently reduced. The approach (a) usually provides good field homogeneity but lacks flexibility in managing all terms of spherical harmonic field expansion; the approach (b) is capable of controlling targeted low-order harmonic terms but can have difficulty in producing optimal overall field homogeneity and in controlling high-order harmonics. The proposed algorithm attempts to combine the strengths of these two methods for a better PS solution. During the PS implementation, an explicit expression of the system matrix with both field and harmonics sensitivities is generated, and then an optimization procedure is performed for the determination of shim piece thicknesses and locations. An experimental study showed that the hybrid method provided good quality, flexible solutions for controlling individual harmonics impurities and also overall field uniformity.
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