An image processing system comprising: an input port (IN) for receiving two input images acquired of an object. Respective contrast in said images encodes information on different physical properties of the object. The images being converted from a signal detected at a detector (D) of an imaging apparatus (IM). A differentiator of the image processing system forms respective differences from pairs of image points from the respective input images. An edge evaluator (EV) computes, based on said differences, an edge score for at least one of said pairs of image points. The score is based on a measure that represents or is derivable from a conditional noise likelihood function. The likelihood function is based on a probability density that models noise for said signal. Said score is output through an output port (OUT).