1. A method for spatial compound imaging, comprising steps of: a) receiving a component image C(i, j), where i denotes a steering scan cycle and j a steering angle of the component image; b) obtaining a differential image by subtracting a component image C(i- 1 ,j) from the component image C(i, j) with the same steering angle from a previous steering scan cycle; c) pre-processing said differential image; d) compounding the pre-processed differential image and a previous compounded image to form a new compounded image.