1. A computer implemented method for enhancing a virtual non-contrast image, comprising the steps of: performing, by one or more computing devices; receiving a pair of computed tomography (CT) images acquired using a dual-scan CT apparatus; calculating a virtual non-contrast image from the pair of CT images using known tissue attenuation coefficients; estimating a conditional probability distribution for tissue at a first point in each of said pair of CT images and said virtual non-contrast image and for tissue at a second point in each of said pair of CT images and said virtual non-contrast image as being the same type; estimating a conditional probability distribution for tissue at a first point in each of said pair of CT images and said virtual non-contrast image and for tissue at a second point in each of said pair of CT images and said virtual non-contrast image as being of different types; calculating from said conditional probability distributions an a posteriori probability of the tissue at said first point and said second point as being the same type; and calculating an enhanced virtual non-contrast image using the a posteriori probability of the tissue at said first point and said second point as being the same type.