An image acquisition method includes determining the starting position, the ending position of a region of interest, and the value of overlap of the region of interest in the two adjacent sub-images, calculating the number of the sub-images required to be captured, the component of field of view at the direction of tube movement and the positions of the tube and the detector corresponding to each sub-image based on the starting position and the ending position of a region of interest and the value of the overlap. The method also includes moving the tube and the detector to each position and capturing the region of interest to obtain sub-images at the positions, and pasting the several sub-images together to form an image of the said region of interest.