1. A CT imaging method, comprising: CT scanning an object with a dual-energy CT system to obtain a first complete set of projection data at a first energy level, and to obtain a second incomplete set of projection data at a second energy level different from the first energy level; reconstructing a first image of the object from the first set of projection data, and extracting, from the first image, prior structure information of the object indicating edge intensity, wherein the extracted prior structure information describes boundaries and details having obvious difference in the first image; and reconstructing a second image of the object from the second incomplete set of projection data using the extracted prior structure information as a constraint.