1. A method of preparing a volume image of a patient, comprising the steps of; (a) acquiring a reference image of the patient suitable for treatment planning purposes; (b) acquiring a subsequent image of the patient; (c) selecting a plurality of voxels in one of the subsequent image and the reference image, and preparing a histogram of the voxel values in a region around each selected voxel, to form a first set of histograms; (d) identifying the corresponding regions in the other image and preparing a histogram of the voxel values in each of those corresponding regions, to form a second set of histograms; (e) for each histogram in the first set, identifying a transformation that transforms it to be substantially the same as the corresponding histogram of the second set, thereby to derive a set of transformations; and (f) applying a transformation to voxels in the subsequent image based on the set of transformations, thereby to render the subsequent image suitable for treatment planning purposes.