1. A method for obtaining multi-material decomposition images, said method comprising the steps of: acquiring an image pair from a dual energy computed tomography scan of an imaged object; selecting a material basis for multi-material decomposition of said image pair; at each image location in said image pair, applying a physicochemical model for said material basis to produce a triple-material decomposition; determining a plurality of decomposition coefficients associated with the triple-material decomposition; and selecting a material triplet corresponding to the triple-material decomposition when the decomposition coefficients comprise a non-negative value.