1. A interactive medical apparatus for displaying interactive images of image data of internal structures of a patient at a dynamically changing subject position, to be viewed at an dynamically changing observer viewpoint coinciding with an external view of said patient comprising: a) imaging means for acquiring image data of the internal structures of the patient b) a semi-transparent screen for displaying an image of image data provided to it with an adjustable degree of transparency to appear superimposed on structures seen through the screen; c) a tracking device for repeatedly measuring location and orientation of the semi-transparent screen relative to the dynamically changing subject position, and the dynamically changing observer viewpoint; d) a mechanical arm for adjustably holding the semi-transparent screen in a selected position between said observer viewpoint and said patient position such that a selected region is visible through the semi-transparent screen; and e) a workstation means coupled to the tracking device for receiving the location and orientation of the screen, operator viewpoint and patient position, and for creating an image from said imaging data, on the semi-transparent screen of internal structures consistent with the relative location and orientation of the semi-transparent screen the operator viewpoint, and the patient position.