An image processing device and method are disclosed for determining a proportion of necrotic tissue in a defined tissue area of an object under examination based on a high-energy image dataset and a low-energy image dataset, each recorded by way of x-ray measurements with different x-ray energies after a contrast medium has been applied to the object under examination. In at least one embodiment of the method, a virtual contrast medium image is determined from the high-energy image dataset and the low-energy image dataset and a segmentation image dataset is created, by the area of tissue being segmented. The segmentation result is transferred into the virtual contrast medium image for segmenting the tissue area in the virtual contrast medium image. Finally an analysis of values of the pixels lying in the segmented area is undertaken for identifying pixels which are to be assigned to necrotic tissue.