We describe a method of determining deformation data for an imaged object, the method comprising: inputting first and second sets of image data corresponding to different deformations of the imaged object, each comprising imaging signal data for an imaging signal, said imaging signal data including at least signal phase data; and determining, for at least one point in said first set of image data, a corresponding displacement for said point in said second set of image data; wherein the displacement determining comprises: initialising a value of displacement; determining an adjusted value for said displacement to provide said corresponding displacement, said determining of an adjusted value comprising: determining an average of differences in signal phase between corresponding positions in said first and second sets of image data, said corresponding positions being determined by a current value of said displacement; and using said average to determine said adjusted displacement value.