1. An apparatus for determining a contour of a model vessel representing a branching structure, wherein the structure comprising a branch point at which an upper segment of the structure splits into a first lower segment and a second lower segment, and the apparatus comprises: an image data processing unit for determining a path of a first model vessel representative of at least part of the upper segment and at least part of the first lower segment, and for determining a path of a second model vessel, representative of at least part of the upper segment and at least part of the second lower segment; and a contour calculation unit for determining the position of a contour branch point and ensuring that for at least one position above the contour branch point a contour of the first model vessel is substantially the same as a contour of the second model vessel.