1. A medical image data processing apparatus comprising: a data receiver receiving image data to be segmented; and an atlas selection processor accessing a plurality of atlas data sets and selecting a subset of the atlas data sets for use in segmenting the image data; wherein each atlas data set comprises image data comprising a set of pixels or voxels, a plurality of predetermined anatomical landmarks, and associated data that identifies positions of the anatomical landmarks; and the atlas selection processor configured to: for each atlas data set, obtain a registration between the set of pixels or voxels of the atlas data set and the image data to be segmented, using a first registration method in real time; for each atlas data set, transform the positions of the anatomical landmarks represented by the associated data, in accordance with the registration for that atlas data set of the image data to be segmented; for each atlas data set, compute a distance between the transformed positions of the anatomical landmarks of the atlas data set in a coordinate space of the image data to he segmented and the transformed positions of anatomical landmarks for other of the atlas data sets in the coordinate space of the image data to be segmented; perform an on-line clustering method, following the first registration method, comprising forming a plurality of clusters of the atlas data sets based on the values of the computed distances between the transformed positions of the anatomical landmarks, and select one of the clusters based on the values of the computed distances between the transformed positions of the anatomical andmarks for the clusters; and select the atlas data sets of the selected cluster to be said subset of atlas data sets for use in segmenting the image data.