Optimization design of a permanent magnet used for a low field (0.2 T) movable MRI system

Objective To design a lightweight permanent magnet for a lowfield movable head imaging MRI system. Materials and methods To reduce the weight of the magnet, the pole pieces, anti-eddy current plates, and shimming rings were removed, and the distance between the two vertical yokes was shortened as much as possible. To compensate for the magnetic field deformation caused by the shortened distance between two vertical iron yokes, two side magnetic poles were added to the vertical yokes. The magnetic field distributions in magnetic poles, the iron yoke, and the spherical imaging region were simulated. Phantom and in vivo head imaging were conducted with a lowfield movable MRI prototype scanner equipped with the proposed permanent magnet. Results A permanent magnet with a center field of 0.19815 T, a homogeneity of 46 ppm over the 20 cm spherical imaging region, and a weight of 654 kg have been achieved. Acceptable images of a phantom and a human brain have been acquired with the prototype MRI scanner. Discussion The proposed permanent magnet design significantly reduces the magnet’s weight compared with the conventional magnet structure and shows promise in promoting the development of lowfield compact MRI systems.