Patent Number: 
Section: description

FIG. 1 shows the X-ray device which includes a radiation source 1 for generating X-rays. The radiation emitted by the radiation source is incident on the shutters 2 which are journaled so as to be slidable in a diaphragm device 8. The shutters 2 are provided with non-anatomical, unambiguously recognizable perforations and/or edge patterns which cannot be shown in FIG. 1. The shutters 2 define the beam path 5 so that the beam is incident on the part of the patient 6 which is to be examined by means of X-rays. The X-rays traverse the patient 6 and are absorbed to a different extent by tissue and bones of different density. The X-rays of different intensity thus produced are incident on the detector unit 3. In the detector unit the radiation image is recorded and applied to the image processing unit 4. The radiation image is searched for hole and/or edge patterns in the image processing unit 4, said patterns being provided in the shutters 2 and being transparent to X-rays so that they are reproduced in the radiation image. After such patterns have been found, the position 16 of the edges 15 of the shutters 2 is identified on the basis of the known arrangement of the hole and/or edge patterns in the shutters 2. The edges 15 are reproduced with a large difference in gray value tone in the radiation image; such a difference can also occur at the transition from bones to soft tissue. The position 16 of the edges 15 can be unambiguously determined by means of the hole and/or edge patterns. Only the irradiated part 10 of the patient with, for example the bones to be examined is then reproduced on a display unit 7. The area 9 covered by the shutters 2 is not displayed. The FIGS. 2 and 3 show two shutters 2 and 20 which are used to limit the beam path 5. The shutter 2 is provided with perforations which are transparent to X-rays. In each of the rows of perforations 11 to 14 there is a different, characteristic spacing of the holes. Information concerning the magnitude of the area to be covered is encoded in such different spacing of the holes. In the row of perforations 11 only one circular hole is provided in a series of elongate holes. This row of perforations 11 is situated at a single distance from the edge 15. The row of perforations 12 includes two holes and an elongate hole and is situated at twice the distance from the shutter edge 15. The row of perforations 13 includes three holes and is situated at three times the distance and the row of perforations 14 has a succession of four holes and is situated at four times the distance from the shutter edge 15. The shutter 20 in FIG. 3 has an edge 15 which can be particularly simply recognized in the radiation image. The shutter 20 is provided with indentations which are situated at a regular distance from one another. Such indentations are detected at least as an interruption in a long, straight edge segment by means of a standard edge detection algorithm. This ensures that they are reliably distinguished from anatomical edges in the radiation image. An additional verification of the recognized shutter edges can be performed on the basis of the known imaging geometry of the X-ray device by calculating in advance the spacing and/or position of the indentations in the radiation image, followed by comparison with the result detected in the radiation image. The edge 15 can be described by a mathematical function. FIG. 4 shows a radiation image in which the edge areas 9 are covered by shutters 2. Because the rows of perforations 11 to 14 are now visible, only a minimum area was irradiated by the X-rays. The shutters 2 in the diaphragm device 8 were moved very far into the beam path 5. The rows of perforations 11 to 14 are situated at known distances from the relevant edge 15 of the shutters 2 detected in the radiation image. In the image processing unit 4 the radiation image is searched for the reproductions of the patterns formed in the shutters 2, said search being performed on the basis of a pattern recognition algorithm. The radiation image is recorded typically by means of a digital detector. The values representative of the individual pixels are applied to the image processing unit 4. The radiation image, presented in digital form, is searched for a series of gray tones correlated with the rows of perforations 11 to 14 of the shutters 2. When such series of gray tones have been identified, the position 16 of the edge 15 of the shutter 2 in the radiation image is detected on the basis of the known distance between the relevant row of perforations 11 to 14 and the edge 15 of the shutter 2. The areas 9 of the radiation image which are covered by the shutters 2 are not shown on the display unit 7. Consequently, during the evaluation of the X-ray image of the patient 6 the physician will not be distracted by gray tones outside the irradiated area which are due to scattering. For radiography of artificial joints or prostheses the hole or edge patterns should be selected so that the artificial shapes are clearly distinct from the hole or edge patterns. All references cited herein, as well as the priority document German Patent Application 19919849.7 filed Apr. 30, 1999, are incorporated herein by reference in their entirety and for all purposes to the same extent as if each individual publication or patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes.