X-ray imaging system

A method of generating composition information (e.g. thickness and/or material data) for a sample based on an X-ray image is disclosed. The method comprises obtaining an input image, 700, for processing based on a source X-ray image of the sample acquired using an X-ray detector and determining a model of the sample based on the input image, 706. The model is then evaluated by computing, based on the model, simulated X-ray image data; evaluating the simulated image data against the input image to determine whether a convergence criterion is fulfilled; and generating an updated model of the sample if the convergence criterion is not fulfilled, the updated model generated using the simulated image data 204. The model evaluating process is repeated based on one or more successive updated models until the convergence criterion is fulfilled in a final iteration and composition information is output for the sample based on the model, 710.