In this paper, we propose an efficient method for implementing bi-directional pixel-based focusing (BiPBF) based on a sparse array imaging technique. The proposed method can improve spatial resolution and frame rate of ultrasound imaging with reduced hardware complexity by synthesizing a large transmit aperture with sparsely distributed small subapertures. As the distance between adjacent subapertures increases, however, the image resolution tends to decrease due to the elevation of grating lobes. Such grating lobes can be eliminated in conventional synthetic aperture imaging techniques. On the contrary, the grating lobes of the sparse BiPBF scheme can not be eliminated, which is to be proven analytically in this paper. We also propose the condition and method for suppressing the grating lobes below -40dB, which can be achieved by placing the transmit focal depth at a near depth and properly selecting the subaperture distance in proportion to receive aperture size. The results of both the phantom and in vivo experiments show that the proposed method implements two-way dynamic focusing using a smaller number of subapertures, resulting in reduced system complexity and increased frame rate.