A fully open permanent magnet configuration for magnetic resonance imaging (MRI) system, which can produce lice imaging region with field homogeneity outside the magnet structure, is presented. It is a totally new problem about magnet design to build a slice imaging region with homogeneous magnetic field outside the magnet source. In order to satisfy the special design requirement for the fully open magnet structure of unilateral MRI devices, a new approach based on the combination of finite element and new-style equivalent source methods (FEM-NESM) is proposed in detail. To verify the effectiveness of the combined method, the calculated results are compared with those obtained by finite element method. Good agreement is obtained. The possibility of the existence of such a remote slice region and the location and size of the slice region are discussed.