1. An image data processing apparatus, comprising: processing circuitry configured to receive medical image data representative of a region of a subject, wherein the medical image data comprises, for each of a plurality of points in the region of the subject, a first intensity value and a second intensity value, each point having a respective position on an at least two-dimensional distribution of first intensity against second intensity, and wherein the first intensity values comprise intensity values from a non-contrast scan and the second intensity values comprise intensity values from a contrast scan; and render or segment the medical image data in dependence on the positions of points in the at least two-dimensional distribution, wherein the rendering or segmenting of the medical image data comprises determining a plurality of regions, including a misalignment region, wherein the misalignment region includes points for which the second intensity value from the contrast scan is smaller than the first intensity value from the non-contrast scan.