The invention relates to an apparatus for determining a relation between a surface motion of a body (BD) and an object motion of an object (OB) within the body (BD). The apparatus comprises a first sensing unit configured to acquire a first position signal indicative of a position of a first element placed at the location on the surface of the body with the surface motion a second sensing unit configured to acquire a second position signal indicative of a position of a second element attached to an interventional device and placed on or in the object, wherein the first position signal and the second position signal are acquired during a given duration synchronously and a third unit for calculating the relation between the surface motion and the object motion based on the first position signal and the second position signal. The invention also relates to a corresponding method for determining the motion relation.
