1. An apparatus for determining a coordinate transformation for mixing an image of a first subject into an X-ray image of a second subject for use with a navigation system and an X-ray imaging system, comprising: markers which are acquirable by a navigation system; an arrangement adapted for physically associating said markers with said second subject; and X-ray positive marks acquirable by said X-ray imaging system connected to said arrangement with respective positions and orientations of said markers and said X-ray positive marks relative to each other being known, and said X-ray positive marks being remote from said arrangement.