1. A method of processing x-ray images, the method comprising: training an artificial neural network to process multi-spectral x-ray projections to determine composition information in terms of equivalent thickness of at least one basis material; providing a multi-spectral x-ray projection of an object, wherein the multi-spectral x-ray projection of the object contains energy content information describing energy content of the multi-spectral x-ray projection; processing the multi-spectral x-ray projection with the artificial neural network to determine composition information about the object, wherein the composition information includes one or more basis coefficients mapping the object in terms of equivalent thicknesses of the at least one basis material; and providing the composition information about the object.