An image processing system (IPS) and related method. The system comprises an input interface (IN) for receiving an earlier input image (Va) and a later input image (Vb) acquired of an object (OB) whilst a fluid is present within the object (OB). A differentiator (Δ) is configured to form a difference image (Vd) from the at least two input images (Va,Vb). An image structure identifier (ID) is operable to identify one or more locations in the difference image. Based on a respective feature descriptor that describes a respective neighborhood around said one or more locations. An output interface (OUT) outputs a feature map (Vm) that includes the one or more locations.