1. A method comprising: defining an arbitrarily-shaped volume of interest (VOI) inside a 3D computer image containing also non-VOI material; using a numerical optimization algorithm to find an optimum convex polyhedron, such that the volume of the polyhedron contains a maximum volume of the VOI and a minimum (or zero) volume of non-VOI; and evaluating and optimizing the convex polyhedron with pre-defined parameters, including a maximum allowable amount of non-VOI allowed inside the convex polyhedron, a maximum allowable amount of VOI allowed outside the convex polyhedron and a maximum number of sides of the polyhedron.