Patent Number: 
Section: description

FIG. 1 is a diagrammatic representation of the 3D rotation angiography procedure. FIG. 1 shows the X-ray source I and the X-ray detector 2 in a number 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 about the axis of rotation 3. To this end, for example, the X-ray source and the X-ray detector are both suspended from a support such as a C-arm 20. A two-dimensional projection image 101-107 is formed in each of said orientations. Such projection images are often subtraction images obtained by subtracting a current projection image from a previously picked up mask image, so that the subtraction image represents practically only the difference between the current projection image and the mask image. A number of geometrical corrections is applied to the projection images 101-107 by means of a correction unit 21 in order to correct the projection images for known image distortion such as barrel and cushion distortion which occurs notably when an X-ray image intensifier with a television camera is used as the X-ray detector. The three-dimensional data set 23 is reconstructed from the projection images 101-107 by means of a reconstruction unit 22. This three-dimensional data set is displayed, for example on a monitor 24 which is suitable for the (quasi) spatial display of the three-dimensional data set. FIG. 2 shows diagrammatically 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 support which is in this case a C-arm 20. The C-arm 20 is displaceable in a sleeve 30 while the X-ray source 1 and the X-ray detector 2 rotate 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 1 with the X-ray detector 2 then rotating in a plane transversely of the plane of drawing; the latter rotation is also referred to as a xe2x80x9cpropeller motionxe2x80x9d. Notably for cardiology such a propeller motion offers the possibility of performing a three-dimensional reconstruction of a high diagnostic quality of the heart of the patient to be examined. The calibration phantom 6 is arranged on a tower 19 in order to carry out the calibration in accordance with the invention. The tower 19 is placed on the patient table 31. The calibration phantom is thus positioned outside the isocenter 9 and near the X-ray image intensifier 2 in the situation shown. An X-ray image, being the first calibration image of the calibration phantom, is formed under the control of the calibration control unit 7. Subsequently, again under the control of the calibration control unit 7, the C-arm 20 is rotated 180xc2x0 about the axis of rotation 3; the positions of the X-ray source 1 and the X-ray detector 2 are thus reversed and the calibration phantom 6 is situated nearer to the X-ray source in comparison with the isocenter 9. Under the control of the calibration control unit 7 another X-ray image, being the second calibration image, is formed of the phantom. Inspection of the two calibration images so as to establish whether the image of the calibration phantom has relatively shifted reveals whether the central beam line 4 extends exactly perpendicularly to the axis of rotation 3. This can be done with the naked eye by displaying the two calibration images on the monitor 24. In a contemporary X-ray examination apparatus the calibration control unit 7 and the correction unit 21 with the reconstruction unit 22 are usually included in a programmable processor 25. For example, the result of the calibration, being the zero orientation, can be stored in a memory of, for example the calibration control unit. The zero orientation can thus be easily fetched again. FIG. 3 illustrates the building up of the tower for the calibration phantom. The tower 19 is preferably constructed while using a plurality of stacked, mating building elements which are known, for example from the toy industry. When the tower is not in use, it can be simply taken apart and stowed away without occupying a large storage volume. In that case the tower will not be in the way when it is not used after the calibration. Notably the upper building element is provided with a recess in which the calibration phantom, such as the lead ruler 6, can be accurately fitted. The simplest procedure is to provide all building elements with such a recess so that it will not be necessary to find exactly the upper building element upon assembly of the tower. It has been found in practice that suitable results are obtained by means of a tower which has a height of 36 cm and is composed of four large Perspex structural elements 35 and four small Perspex structural elements 36.