Abstract: A method includes generating spectral projection data of a subject, including at least first spectral projection data corresponding to a first energy range and second spectral projection data corresponding to a second energy range, wherein the first and the second energy range are different. The method further includes constructing an image from a combination of the spectral projection data, and constructing a set of basis images for the 5 energy ranges and from the spectral projection data. The method further includes constructing a multi-dimensional histogram from the set of basis images, wherein the multi-dimensional histogram includes at least two axes, a first corresponding to a first basis component and a second corresponding to a second basis component, and the multi-dimensional histogram includes a set of clusters, including one cluster for each material 10 represented in the spectral projection data.