1. An apparatus, comprising: processing circuitry configured to obtain first projection data representing an intensity of first radiation incident at a plurality of detector elements, the first radiation having a first photon energy; obtain second projection data representing an intensity of second radiation incident at the plurality of detector elements, the second radiation having a second photon energy; register a reconstructed image of the second projection data to a reconstructed image of the first projection data; update the second projection data using the registered reconstructed image of the second projection data; and perform material decomposition using the updated second projection data and an updated first projection data or the first projection data.