1. An apparatus for determining a contour of a model vessel representing a branching structure, wherein the structure comprises a branch point at which an upper segment of the structure splits into a first lower segment and a second lower segment, the apparatus comprising: a processing circuit configured to determine 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, determine 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 determine a position of a contour branch point to ensure 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, wherein the processing circuit is further configured to determine a first contour parameter for the first model vessel, determine a second contour parameter for the second model vessel, compare the first contour parameter and the second contour parameter, determine the position of the contour branch point in dependence on the comparison, for a series of positions away from the branch point, repeat the determining of the first contour parameter and the second contour parameter, and repeat the comparing of the first contour parameter and the second contour parameter, and determine the position of the contour branch point to be substantially equal to a position at which the difference becomes less than or equal to a threshold.