The invention relates to an imaging system (17) like a computed tomography system for generating an image of an object. Spectral measured projection data and non-spectral measured projection data are generated by a detector (6) having spectral detection elements and non-spectral detection elements, and spectral estimated projection data are estimated by using a model material distribution which could have caused the non-spectral measured projection data and by simulating a measurement of the spectral estimated projection data based on the model material distribution. An image is reconstructed based on the measured and estimated spectral projection data. Using the spectral estimated projection data in addition to the spectral measured projection data can lead to high quality spectral imaging, especially high quality spectral computed tomography imaging, which uses a simplified detector not only having generally more complex spectral detection elements, but also having simpler non-spectral detection elements.
