A system and method that determines a contour of a model vessel representing a branching structure, wherein the structure contains a branch point at which an upper segment of the structure splits into a first lower segment and a second lower segment, and determines 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, determines 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, determines the position of a contour branch point; and ensures 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.