We present a design and optimization of an irregular Aubert ring-pair-aggregate permanent magnet array that generates 2D B0-field with an improved gradient for head imaging for a low-field portable MRI system. The ring-aggregates are discretized into fan-shaped sections with identical angles and varying outer diameters for design and optimization. Genetic algorithm (GA) was used. Compared to a Halbach-array, the proposed array shows an increased in field strength (111.2 mT) with a controlled inhomogeneity, and an enhanced gradient (off-center-concentric-pattern). It leads to better reconstructed images using simulations where the central blurring area is eliminated.