X-ray installation comprising an image intensifier/image pick-up tube system and an automatic X-ray exposure device

#The invention relates to an X-ray installation comprising an image intensifier/image pick-up tube system and an automatic exposure device comprising means for recording a given mean dose determined from a number of subfields by means of video signals recorded during fluoroscopy prior to the exposure.

#The measuring member of an automatic X-ray exposure device of the commonly used type consists of an ionization chamber which usually comprises several measuring fields. The measuring field (the dominant) which offers optimum exposure must each time be selected by the radiologist. It is important that this measuring field is always situated in the part of the X-ray image of the patient which is important for diagnosis.

#The selection of a measuring field which is suitable for automatic exposure impedes the actual medical function of the radiologist. Moreover, incorrect exposures can occur due to incorrect selection of the dominant or due to the covering of part of the measuring field, for example, by a contrast medium filling.

#The invention will be described hereinafter with reference to a preferred embodiment of the installation which is shown in the drawing.

#FIG. 1 diagrammatically shows an X-ray installation in accordance with the invention, and

#FIG. 2 shows a block diagram of the validation circuit in accordance with the invention.

#FIG. 1 shows an X-ray tube 1 which is connected to a high-voltage generator 2. The radiation emitted by the tube 1 irradiates a patient 3 and reaches the central measuring field 4 of a measuring member which is adapted to receive radiation, for example, an ionization chamber. A film in a cassette 5 properly shows an X-ray shadow image formed by the object behind the object in the case of optimum exposure. The X-ray generator, only the tube 1 and the high-voltage generator 2 thereof being shown in the drawing, is adapted for the programmed exposure technique. Therefore, the generator comprises an exposure data presetting unit which is preset by operation of pushbuttons associated with each relevant organ (not shown).

#During fluoroscopy, the cassette is situated outside the beam path. The X-ray shadow image then appears on the luminescent input screen of an image intensifier 6, a television camera 8 being coupled to the luminescent output screen thereof via an objective 7. The video signal supplied by the television camera 8 is applied to a validation circuit 9. This circuit determines, on the basis of the video signal during fluoroscopy, the dose at which the high-voltage generator is to be switched off to ensure correct exposure of a subsequent X-ray exposure. A signal W.sub.a, supplied by the validation circuit 9 for this purpose, is applied to a comparison circuit 10 which compares this signal, generated during fluoroscopy, with a signal U.sub.i which is generated during the exposure and which is proportional to the dose. If the signal supplied by the measuring member 4 is proportional to the dose power, an integrator should be inserted between the measuring member 4 and the comparison circuit 10. When the signal supplied by the measuring member, which increases substantially with time under constant exposure conditions, reaches the value of the signal supplied by the validation circuit 9, the comparison circuit 10 causes the switching off of the high voltage from the high-voltage generator 2, thus terminating the exposure.

#The center of the measuring member 4 is arranged in the center of the stopped X-ray beam and has a measuring field (perpendicularly to the direction of the X-rays) which is smaller than the smallest exposure format used. However, it is alternatively possible for the effective measuring field of the measuring member 4 to be switchable in dependence upon the exposure format, so that in the case of a small exposure format the measuring field is reduced to include only the measuring field of the radiation. However, because a reduction of the effective measuring field of the measuring member 4 is accompanied by a corresponding reduction of the signal U.sub.i, an amplifier must then be provided which increases the gain accordingly again, so that ultimately the voltage U.sub.i on the output of the comparison circuit 10 is independent of the exposure format or the magnitude of the stopped X-ray beam.