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 in a first scan mode, and to obtain a second incomplete set of projection data in a second scan mode; reconstructing a first attenuation coefficient image of the object from the first set of projection data, and extracting, from the first attenuation coefficient image, prior structure information of the object indicating edge intensity; and reconstructing a second attenuation coefficient image of the object from the second incomplete set of projection data using the extracted prior structure information as a constraint.