BACKGROUND 
     The invention relates to an X-ray examination apparatus which includes an X-ray source and an X-ray detector. 
     A known X-ray examination apparatus is described in international application WO 00/75872. The known X-ray examination apparatus reconstructs a three-dimensional data set from the series of projection images. The projection images are acquired notably by irradiating the patient to be examined from the respective orientations while using an X-ray beam emitted by the X-ray source. The orientations at which the projection images are formed are accurately calibrated, thus enabling accurate three-dimensional reconstruction without excessive artifacts. A contrast medium is administered to the patient to be examined and the projection images are acquired when the blood vessels in the part of the anatomy to be examined are filled with contrast medium. The three-dimensional reconstruction produces a three-dimensional data set which associates data values with three-dimensional spatial positions. This data set represents the structure of the blood vessels at an instant at which the vessels are filled with the contrast medium. It has been found in practice that the diagnostic information provided by display of the three-dimensional data set is still inadequate. 
     SUMMARY OF THE INVENTION 
     The invention is arranged to form a series of projection images at respective orientations of the X-ray source and the X-ray detector relative to a predetermined frame of reference. The orientations being calibrated relative to the frame of reference, and reconstructing a basic three-dimensional data set from the series of projection images. 
     It is an object of the invention to provide an X-ray examination apparatus which enables the spatial image information of the three-dimensional reconstruction to be combined with image information other than that contained in the projection images used for the three-dimensional reconstruction. 
     This object is achieved by means of an X-ray examination apparatus in accordance with the invention in which the positions of the X-ray source and the X-ray detector relative to the frame of reference are calibrated for one or more additional directions of observation. As a result, image information available for the one or more additional observation directions can be reproduced in registration with the three-dimensional data set. The combined rendition of the three-dimensional data set and the image information associated with the one or more additional directions of observation faithfully represents the spatial aspects of the object to be examined which is represented by the projection images wherefrom the basic three-dimensional data set is reconstructed and by the image information associated with the additional direction (directions) of observation. These and other aspects of the invention will be elaborated hereinafter on the basis of the following embodiments of the invention which are defined in the dependent claims. 
     For example, the image information associated with the additional direction (directions) of observation is available in the form of one or more additional X-ray images. Preferably, such additional X-ray images are formed as shadow images, each of which has its own projection direction. The projection directions wherefrom the additional X-ray images are formed then constitute the additional direction (directions) of observation. The additional X-ray images and the three-dimensional basic data set are reproduced with the correct mutual relationship. For example, it is advantageous to form the series of projection images at a comparatively low energy and intensity of the X-rays and to acquire the additional X-ray images at a higher intensity and/or energy of the X-rays. The patient to be examined is thus exposed to a low X-ray dose for the acquisition of the projection images for the basic three-dimensional data set. The additional X-ray images have a low noise level and hence a high diagnostic quality. The X-ray examination apparatus in accordance with the invention enables the high diagnostic quality of the additional X-ray images to be combined with the spatial image information in the basic three-dimensional data set. 
     Moreover, it is also possible to reproduce, together with the basic three-dimensional data set, image information acquired before or after the projection images wherefrom the basic three-dimensional data set is reconstructed. Variations in time in the object to be examined can thus be faithfully and accurately reproduced with the spatial structure of the object. For example, favorable results are obtained for the reproduction of the process of the filling of a part of the vascular system of the patient to be examined with blood with contrast medium. Preferably, there is formed a succession of additional X-ray images which show successive phases of the filling of the vascular system with blood with contrast medium. The spatial structure of the vascular system is known from the basic three-dimensional data set and the additional X-ray images provide the dynamics of the flow of the blood through the vascular system. Because the projection directions of the additional X-ray images, that is, the additional directions of observation and the projection images for the three-dimensional data set have been calibrated relative to the same frame of reference, both types of information can be made to correspond accurately. Consequently, the dynamics of the flow of blood through the vascular system can be accurately visualized in space while a comparatively low X-ray dose suffices nevertheless. 
     Preferably, the basic three-dimensional data set is continuously updated on the basis of the additional X-ray images; for example, it is handy to perform an update whenever a new additional X-ray image becomes available. A dynamic series of three-dimensional data sets is thus formed, successive data sets in such a dynamic series then relate to successive phases of the filling of the blood vessels with blood with contrast medium. 
     Furthermore, it is advantageous to store a number of preferred additional directions of observation and/or the corresponding positions for the X-ray source and the X-ray detector in advance in a memory. This facilitates the positioning of the X-ray source and the X-ray detector for the acquisition of the additional X-ray images. When the user (users) himself (themselves) is (are) allowed to store a plurality of preferred additional directions of observation and/or the associated positions of the X-ray source and the X-ray detector, the personal preferences of the relevant user (users), for example, individual radiologists, can be taken into account. 
     Preferably, it is ensured that for the positioning of the X-ray source and the X-ray detector for the acquisition of the one or more additional X-ray images the X-ray source and the X-ray detector are moved from a predetermined starting position to the desired position. Consequently, the desired position for the acquisition of the additional X-ray images is always reached from the same direction. It has been found that the desired position is thus accurately reached, because inter alia mechanical hysteresis effects are avoided in particular. 
     The following description, claims and accompanying drawings set forth certain illustrative embodiments applying various principles of the present invention. It is to be appreciated that different embodiments applying principles of the invention may take form in various components, steps and arrangements of components and steps. These described embodiments being indicative of but a few of the various ways in which some or all of the principles of the invention may be employed in a method or apparatus. The drawings are only for the purpose of illustrating an embodiment of an apparatus and method applying principles of the present invention and are not to be construed as limiting the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon consideration of the following detailed description of apparatus applying aspects of the present invention with reference to the accompanying drawings, wherein: 
     FIG. 1 is a diagrammatic representation of the 3D rotation angiography procedure in which the invention is used, and 
     FIG. 2 is a diagrammatic representation of an X-ray examination apparatus in which the invention is used. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a diagrammatic representation of a set-up of the 3D rotation angiography procedure. FIG. 1 shows the X-ray source  1  and the X-ray detector  2  in a plurality of orientations relative to the patient  20  to be examined. As is indicated by the arrow, the X-ray source and the X-ray detector are rotated together around the axis of rotation  3 . To this end, for example, the X-ray source and the X-ray detector are both suspended from a stand such as a C-arm  20 . One of the two-dimensional projection images  10   1 -10 7  is formed at each of these orientations. Such projection images are often subtraction images obtained by subtracting a previously acquired mask image from an instantaneous projection image, so that the subtraction image represents practically the difference only between the instantaneous projection image and the mask image. Using a correction unit  21 , a number of geometrical corrections is applied to the projection images  10   1 - 10   7  in order to correct the projection images for known distortions, such as barrel and cushion distortion, which occur notably when an X-ray image intensifier with a television camera is used as the X-ray detector. A reconstruction unit  22  reconstructs the three-dimensional data set  23  from the projection images  10   1 - 10   7 . This three-dimensional data set is reproduced on, for example, a monitor  24  which is suitable for (quasi-) spatial display of the three-dimensional data set. 
     In conformity with the invention, not only the projection images  10   1 - 10   7  are formed, but also the additional X-ray images  11   1 ,  11   2 ,  11   3 . The example shown utilizes three additional X-ray images. Such additional X-ray images are formed in each time one of the additional directions of observation  12   1 ,  12   2  and  12   3 . Such additional directions of observation may be situated in the plane in which the X-ray source and the X-ray detector are rotated for the acquisition of the projection images  10   1 - 10   7 ; this is the case for the additional direction of observation  12   2 . Additional directions of observation may also be situated outside the plane in which the X-ray source and the X-ray detector are rotated for the acquisition of the projection images  10   1 - 10   7 ; this is the case for the additional directions of observation  12   1  and  12   3 . The additional directions of observation are defined together with the orientations of the projection images  10   1 - 10   7 , for example, relative to the calibration phantom  6  in the same position as during the calibration of the orientations of the projection images for the 3D data set. The frame of reference for the calibration of the orientations for the projection images and for the calibration of the additional directions of observation is defined by the calibration phantom in the position in which it is arranged for the calibration. 
     FIG. 2 is a diagrammatic representation of an X-ray examination apparatus in which the invention is used. The X-ray source  1  and the X-ray detector  2 , in this case being constructed as an X-ray image intensifier, are suspended from a stand which is in this case a C-arm  20 . The C-arm  20  is movable through a sleeve  30 , the X-ray source  1  and the X-ray detector  2  then rotating together in the plane of drawing. This motion is also referred to as a rolling rotation. The sleeve  30  is also rotatable about the axis of rotation  3 , the X-ray source and the X-ray detector  2  then rotating transversely of the plane of drawing; the latter rotation is also referred to as a “propeller motion”. This propeller motion enables, notably in the case of cardiology, a three-dimensional reconstruction of high diagnostic quality of the heart of the patient to be examined. 
     In a preferred version of the calibration, the calibration phantom  6  is arranged on the tower  19 . The tower  19  is placed on the patient table  31 . The calibration phantom is thus situated outside the isocenter  9  and close to the X-ray image intensifier  2  in the situation shown. Under the control of the calibration control unit  7  there is formed an X-ray image, that is, the first calibration image of the calibration phantom. Subsequently, the C-arm  20  is rotated through 180° about the axis of rotation  3 , that is, again under the control of the calibration unit  7 ; the positions of the X-ray source  1  and the X-ray detector are thus interchanged and the calibration phantom  6  is situated nearer to the X-ray source than to the isocenter  9 . Under the control of the calibration control unit  7  a further X-ray image is formed of the phantom, that is, the second calibration image. By comparison of the two calibration images so as to detect a relative shift of the image of the calibration phantom, it can be checked whether the central beam line  4  extends accurately perpendicularly to the axis of rotation  3 . This can be done by the naked eye by displaying both calibration images on the monitor  24 . Subsequently, the orientations of the X-ray source and the X-ray detector are determined in the positions in which the projection images  10   1 - 10   7  are accurately perpendicular to the central beam line. In the present example only seven orientations are shown for the projection images, but in practice often tens of orientations or even one hundred or more orientations are used for the projection images. In the case of such a large number it is handy to determine the orientations of the individual projection images relative to the calibrated central axis  4  on the basis of the time during which the C-arm has rotated as from a preselected starting position. This is because it has been found that the mechanical movement of the C-arm is accurately reproducible. Moreover, the additional directions of observation for the additional X-ray images  12   1 - 12   3  are accurately determined relative to the central beam line. These additional directions of observation correspond to the projection directions  12   1 ,  12   2 ,  12   3  wherefrom the patient to be examined is irradiated by means of the X-ray source in order to form the additional X-ray images. The additional directions of observation  12   1  and  12   3  are not situated in the plane of drawing in FIG.  2  and are reached by rotation about the axis of rotation  3 . The additional direction of observation  12   2  is situated in the plane of drawing and is reached by rotating the C-arm through the sleeve  30 . Evidently, it is also possible to use additional directions of observation which are reached by combinations of movement through the sleeve  30 , rotation around the axis  3  and also rotation around the central beam line  4 . The rotation around the central beam line becomes possible by mounting the assembly formed by the sleeve and the C-arm  20  on a support (not shown) which is suspended from the ceiling so as to be rotatable or is mounted so as to be rotatable on a base (not shown) on the floor. 
     In a contemporary X-ray examination apparatus the calibration control unit  7  and the correction unit  21  with the reconstruction unit  21  are usually incorporated in a programmable processor  25 . For example, the result of the calibration, the zero-point orientation, can be stored in a memory of, for example, the calibration control unit. The zero-point orientation can thus be readily fetched again. 
     The invention is of course not limited to the described or shown embodiments, but generally extends to any embodiment, which falls within the scope of the appended claims as seen in light of the foregoing description and drawings. While a particular feature of the invention may have been described above with respect to only one of the illustrated embodiments, such features may be combined with one or more other features of other embodiments, as may be desired and advantageous for any given particular application. From the above description of the invention, those skilled in the art will perceive improvements, changes and modification. Such improvements, changes and modification within the skill of the art are intended to be covered by the appended claims.
An X-ray examination apparatus includes an X-ray source and an X-ray detector and is arranged to provide a series of projection images at respective orientations of the X-ray source ( 1) and the X-ray detector ( 2) relative to a predetermined frame of reference. The orientations of the projection images are calibrated relative to this frame of reference. A basic three-dimensional data set ( 23) is reconstructed from the projection images. A number of directions of observations ( 12 1 -12 3) are calibrated relative to the same frame of reference. One or more additional X-ray images ( 11 1 -11 3) are formed for the calibrated directions of observation, that is, preferably at successive instants in time. A dynamic series of three-dimensional data sets is formed by updating the basic three-dimensional data set by means of the additional X-ray images.