Design and Shimming Method of Low Length-to-Interdiameter Ratio Halbach Magnet

Abstract

The conventional Halbach permanent magnet usually has a high length to inter diameter ratio to gain a higher magnetic field homogeneity but can cause an increase in volume and weight, limiting the application in miniaturization. This paper proposes a novel design and shimming method for the Halbach magnet to have not only a lower length to inter diameter ratio but also a better homogeneity. In this paper, a multi-layer magnetic ring structure was first designed through theoretical calculation and simulation optimization, which compensated for the magnetic field inhomogeneity. Then, a passive shimming method was applied which simulated the first-order and second-order terms of magnetic field inhomogeneity using small magnetic blocks whose magnetization direction was consistent with the main magnetic field. This method has high shimming efficiency and can avoid the demagnetization of the magnetic blocks caused by their magnetization directions against the direction of the main magnetic field, especially when the intensity of the magnetic field is close to or beyond the remanence of the magnetic blocks. The magnetic field intensity of the proposed magnet is 1.08 T and the homogeneity reaches close to the sub-ppm. The performance of the magnet was verified in the portable MRI system combined with a set of homemade low-power gradient coils. MRI images of phantoms and okra reached a resolution of 156 μm (pixel size 156 μm, matrix 128 × 128, FOV 20 × 20 mm2), which shows the potential for wide application of the proposed design and shimming method.