1. A medical imaging data processing apparatus for identifying an interface between anatomical features comprising: processing circuitry: configured to receive first medical imaging data that represents a region of a subject at a first time, and second medical imaging data that represents the region of the subject at a second, later time; configured to perform a registration procedure to obtain registration data representative of a registration between the first medical imaging data and the second medical imaging data; and configured to determine a variation of the registration data with location and to identify an interface between an anatomical feature and at least one further anatomical feature based on said variation of the registration data with location.