Zhi Hua Ren^{1} and Shao Ying Huang^{1,2}

A permanent magnet helmet based on an Aubert ring aggregate is proposed to have a linear gradient along the axial direction for 3D head imaging in a low-field portable MRI system. It is a magnet array that consists of a series of asymmetric Aubert ring pairs, forming a Helmet shape. The inner radii of each ring are successfully optimized for a linear gradient along the axial direction, a comparably strong field strength (67.06mT), and a **B _{0}** in a miniaturized low-field portable MRI system.

The proposed design as shown in Fig.1(a) is based on an Aubert ring pair (shown in Fig. 1 (b)) [4]. It consists of $$$N$$$ ring pairs where the $$$i_{th}$$$ ring pair has an inner and outer radii of $$$r_\textrm{in}^\textrm{i-T}$$$ and $$$r_\textrm{out}^\textrm{i-T}$$$ for the ring on the top, and those of $$$r_\textrm{in}^\textrm{i-B}$$$ and $$$r_\textrm{out}^\textrm{i-B}$$$ for the ring at the bottom. Each ring has a thickness of $$$t$$$. The magnet rings on the top are radially polarized inwards, and those at the bottom are radially polarized outwards.

In the optimization, the two rings in a pair are allowed to have different dimensions. The distance between the two innermost edges of the rings was fixed to be 120mm. The outer radii of the rings are fixed at 250mm. The field of view (FOV) is a cylindrical (200mm in diameter, and 50mm in length) for head imaging. The volume center of the FOV is 50mm below the central plane of the proposed magnet array. The variables, $$$r_\textrm{in}^\textrm{i-T}$$$ and $$$r_\textrm{in}^\textrm{i-B}$$$ ($$$i = 1, ...., N$$$), were optimized to get a field strength over 65 mT, a field inhomogeneity less than 10000 ppm, and at the same time, a linear gradient field along the axial direction. The optimization tool is the Genetic algorithm (GA). To further accelerate the optimization, a current model for a single magnet ring [3] was used for a fast forward calculation.DISCUSSION & CONCLUSION

In this abstract, we propose to further optimize an Auber ring aggregate permanent magnet array to obtain a linear gradient field along the axial direction. The optimization successfully leads to a linear gradient field along the axial direction. It results in a Helmet shape for the array. The average field strength in a FOV (a cylinder with a diameter of 200mm and a length of 50mm) is 67.06mT, which is comparable to the field a Halbach-array can generate. It is for 3D head imaging in a low-field portable MRI system. The homogeneity of the current version is still relatively large, further shimming will be done. Alternatively, to work with the inhomogeneity, a wideband radiofrequency(RF) excitation and reception can be applied to a system using the optimized magnet.[1] C. Z. Cooley, J. P. Stockmann, B. D. Armstrong, M. Sarracanie, M. H. Lev, M. S. Rosen, and L. L. Wald, “Two-dimensional imaging in a lightweight portable MRI scanner without gradient coils,” Magnetic resonance in medicine, vol. 73, no. 2, pp. 872–883, 2015.

[2] Clarissa Zimmerman Cooley, Jason P Stockmann, Mathieu with, Matthew S Rosen, and Lawrence L Wald, 3D Imaging in a Portable MRI Scanner using Rotating Spatial Encoding Magnetic Fields and Transmit Array Spatial Encoding (TRASE), ISMRM 2015

[3] Z. H. Ren, W. C. Mu, and S.Y.Huang, “Design and Optimization of a Ring-Pair Permanent Magnet Array for Head Imaging in a Low-field Portable MRI System”, IEEE Transactions on Magnetics, 2018, in press

[4] G. Aubert, “Permanent magnet for nuclear magnetic resonance imaging equipment,” July 26 1994. US Patent 5,332,971

Fig. 1 Cutaway view of (a) the proposed magnet
array. The magnet rings on the top are radially polarized inwards, and those at
the bottom are radially polarized outwards. (b) an Auber ring pair [4].

Fig. 2 (a) The
optimized profile of the Aubert ring aggregate, (b) the simulated field pattern
on the center plane of the FOV, (c) the field distribution on the $$$rz$$$-plane,
(d) the field plots along the $$$z$$$-direction at different values of $$$r$$$.