1. A medical image processing apparatus, comprising: an image selection circuit that obtains a series of sets of image data, each set of image data representative of a region at a different time, and performs a segmentation process to segment a bone region as an anatomical feature in at least one of the sets of image data to determine a sub-region of interest comprising the anatomical feature; a registration circuit that, for each set of image data of a plurality of the sets of image data in the series, performs a global registration process to obtain a global registration between a reference data set of the series and the set of image data; and a registration refinement circuit that, for each set of image data of the plurality of sets of image data: performs a local registration process, to obtain a local registration between the reference data set of the series and the set of image data within the sub-region of interest determined by the segmentation process; obtains a combined registration based on the global registration and the local registration; applies the combined registration to the reference data set or to the set of image data to obtain a transformed reference data set or transformed set of image data; and performs at least one of (a), (b) and (c) to obtain a subtracted, transformed set of image data: (a) subtracts the set of image data and the transformed reference data set; (b) subtracts the transformed set of image data and the reference data set; and (c) subtracts the transformed set of image data and a further reference data set.