Computerised tomography (CT) scanning an object 40 with a dual-energy CT system to obtain a first complete set of projection data in a first scan mode and a second incomplete set of projection data in a second scan mode； extracting prior structure information indicating edge intensity of the object from an image reconstructed from the first set of data； and reconstructing a second attenuation coefficient image of the object from the incomplete projection data using the prior structure information as a constraint. Preferably the first and second scan modes are low and high energy scans respectively, and the dual-energy CT scanner has multiple rows of low energy detectors 20, behind which are fewer high energy detectors 30. Preferably, edge extraction, for example using a Laplace operator, Candy operator, discrete gradient transform or adaptive thresholding, is performed on the first attenuation coefficient image to obtain the prior structure information. A broader independent claim is included for CT imaging comprising extracting prior structure information indicating edge intensity of an object from a first image of the internal structure of the object, CT scanning the object to obtain projection data, from which an image is reconstructed using the extracted prior structure information as a constraint.
