A Novel Passive Shimming Method of for Unilateral NMR

Introduction:
Unilateral NMR devices are important tools in various applications such as non-destructive measurement and well logging, but are not applied commonly for imaging, primarily because of its poor B0 homogeneity which leads to a low signal noise ratio. This paper presents a novel passive shimming method using permanently magnetized shimming pieces and iron shimming pieces to improve the homogeneity for unilateral NMR magnet.
Method:
The B0 field was built by the 9 identical NdFeB magnet blocks. The shimming process is modeled as an integer programming problem which can be used to predict the locations and sizes of the shimming pieces. There are two kinds of shimming pieces were employed: the permanent shimming piece, and the iron shimming piece. Four identical permanent magnets were used to correct the in-homogeneities of the main field. To improve the homogeneity further, 25 possible iron shimming positions were predetermined which can be seen in Fig.1

. The Lingo software [1] was employed to determine the real positions of the iron shimming pieces. The iron pieces were placed on the surface above the main permanent magnet array. And the ROI (Region of interest) plane which has a size of 10mm*10mm are located at 15mm above the magnet array. In the processing of Lingo software, the numbers and locations of iron shims are regarded as objective variables, the B0 non-homogeneity and the number of iron pieces is minimized.
Experimental Results:
The efficiency of shimming is improved through actual measuring experiment. The optimally designed shimming pieces are proven to improve the field homogeneity from 2758 to 796 ppm after just one iteration of shimming courses is carried out as shown in Fig 2 .
Conclusion:
The experiment results show that the proposed passive shimming is reliable and accurate in improving the field non-homogeneity for unilateral NMR magnets caused by manufacturing tolerances.