Apparatus and method for multi-atlas based segmentation of medical image data

An image data processing apparatus including 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 the atlas selection processor is configured to select the subset of atlas data sets in dependence on the positions of one or more anatomical landmarks comprised in the plurality of atlas data sets.