1. An apparatus, comprising: processing circuitry configured to obtain material-component data corresponding to a material decomposition of projection data having a plurality of energy components, the projection data representing an intensity of X-ray radiation detected at a plurality of energy-resolving detector elements; transform the obtained material-component data to an uncorrelated basis using a whitening transform to generate uncorrelated data; de-noise the generated uncorrelated data; and transform the denoised uncorrelated data using an inverse whitening transform to generate denoised material-component data.