1. A method performed by a computer system for determining a position in an image of an object, comprising: displaying a volume rendering of image data acquired from the object, pointing at a structure of interest displayed in the volume rendering of the image data, generating a viewing ray profile comprising information characterizing a ray miming through said structure of interest, selecting a contextual profile from various contextual profiles, each of said contextual profiles comprising a representative ray profile representing a viewing ray profile of a structure and comprising profile information, and determining a position within said structure of interest based on said profile information of said selected contextual profile in the case that the representative ray profile of said selected contextual profile matches with at least a part of said viewing ray profile.
8. The method according to claim 1 wherein said information characterizing said ray running through said structure of interest comprises intensity values, gradient magnitude values along said ray, or both.