1. A method for building a subject-specific statistical model of shape variation for an anatomical structure, the method comprising: providing a set of geometric representations of shape for said anatomical structure, wherein each representation is associated with one of multiple subjects, each subject having a respective subset of associated representations, and each subset comprising multiple representations including a reference representation, said multiple representations corresponding to different shapes of the anatomical structure arising from physical motion and/or deformation thereof, the method comprising: specifying a set of shape parameters whose values, for any given representation, characterise the shape of the representation; for each subject, representing a probability distribution of the values of the shape parameters across the subset of representations associated with that subject by a set of subject-specific distribution parameters; determining a regression between the subject-specific distribution parameters and the respective reference representation for each subject; applying said determined regression to a reference representation for a new subject to determine subject-specific distribution parameters for the new subject; and using the subject-specific distribution parameters to build a subject-specific statistical motion model for the new subject.