  We present a design and the optimization of an irregular-shaped ring-pair
magnet array that generates a 1D monotonic field pattern for 2D head imaging in
a low-field portable MRI system. The magnet rings are discretized into
fan-shaped ring segments with varying outer diameters for the design and
optimization. Besides, the inner radii of ring-pairs are tapered from outside
in to provide the controlled field inhomogeneity. Genetic algorithm (GA) was
used, and a current model for a fan-shaped ring segment was derived to have a
fast forward calculation in the optimization. A monotonic field pattern is
successfully obtained along the x-direction in a cylindrical field of view
(FoV), with a relatively strong magnetic field (132.98 mT) and the homogeneity
of 151840 ppm. The proposed array was further evaluated by applying its field
as a spatial encoding magnetic field (SEM) for imaging numerically. Due to the
field monotonicity, the reconstructed image by applying the fields of the
proposed array shows clearer features (a higher structural similarity index)
with a reduced error rate compared to that using a sparse dipolar Halbach
array. The proposed magnet array is a promising alternative to supply SEM for
imaging in a permanent-magnet-based low-field portable MRI system.
