X-ray Device

Abstract

An X-ray device includes an X-ray tube with at least two different focal point paths which can be selectively switched on and which are differently inclined to the central X-ray. A switch is provided for the selection of one of the focal point paths. An adjustable primary X-ray screen is located in the bundle of X-rays. The device is particularly characterized by an automatic switching device which permits operation only with the focal point path which is more strongly inclined to the central X-ray when a predetermined opening is exceeded during setting of the pair of screen plates of the primary X-ray screen, while removing the earlier selection of a more weakly inclined focal point path.

Description

This invention relates to an X-ray device having at least two different focal point paths which can be selectively switched on and which are differently inclined to the central X-ray, a switch for the selection of one of the focal point paths and an adjustable primary X-ray screen located in the X-ray bundle.

It is generally known that the geometrical lack of clearness during photographing with X-rays due to central projection is proportional to the size of projection of the focal point path ( = focus) of the X-ray tube in the direction of the central ray of the X-ray cone. Since the X-ray radiation from the anode surface of an X-ray tube is uniform in all directions it has been the practice for a long time to use the part of the emitted X-ray radiation which is radiated stripwise from the surface of the anode. This makes it possible to diminish the size of the projection of the focal point path serving as the image although the size of the focal point path remains uncharged. Thus a smaller geometrical lack of clearness is provided. It is also known that due to the final load capacity of the anode material when the focal point path gets smaller the maximum admissible X-ray tube output must be reduced. For very many years the anodes have been constructed as so-called rotary plates to provide a better and more uniform heat radiation from the anode. A corresponding construction of the cathode provides that the electrons strike only a comparatively narrow rectangular surface of the inclined edge of the rotary anode plate, the focal point path. The edge lengths of the focal point path are then so selected that the focal point path projects as a square in the direction of the central X-ray.

Depending on the nature of examination a doctor will either place a greater value upon a smaller geometrical lack of clearness or upon a small lack of clarity of movement. In the first case he will wish to operate with the smallest possible focal point path and in the second case with a larger focal point path to provide shorter illumination time periods with larger tube output. For a better adaptation to these different requirements the rotary anode plate of X-ray tube is often provided with an inner only slightly inclined edge and an outer edge which is more strongly inclined. A separate cathode is located opposite the two anode plate edge surfaces, which can send out an electron ray bundle corresponding in length and width. In case of such X-ray tubes it is left to the examining doctor to select the focal point path which he believes to be most suitable. However, there is the drawback that in X-ray tubes which due to a small inclination of the focal point path to the central X-ray permit exposures with a comparatively small geometrical lack of clarity, there is the danger that the photographs will be spoiled since during the screening of a film of a large size a part of the primary X-ray bundle is covered by an edge of the rotary anode.

An object of the present invention is to avoid these bad exposures without giving up at the same time the advantages of a particularly small lack of precision.

Other objects of the present invention will become apparent in the course of the following specification.

In the accomplishment of the objectives of the present invention an X-ray device of the described type is provided with an automatic switching device which when a predetermined opening during setting of the pair of screen plates of the primary X-ray screen is exceeded, permits operation only with the central X-ray of the focal point path which is greater inclined and removes the eventual preliminary selection of the lesser inclined focal point path.

This makes it certain that the examining doctor when selecting a large film size and thus a large opening of the primary ray screen located at will between the focal point path and the object being examined in the primary X-bundle, will be able to operate only with that focal point path wherein the X-ray bundle emerging from the focal point path fully illuminates the film area without shading from the edge of the anode plate. If the switch is set for a selection of the focal point path to the smallest focal point path, that the exposure for all film sizes will take place automatically with the smallest possible focal point path for the specific film size and the smallest possible geometrical lack of sharpness.

According to a particularly advantageous embodiment of the present invention the different angles of inclination of the individual focal point paths can be adapted relatively to the central X-ray to the location of the pairs of screen plates of the primary ray screen for generally used film sizes. Due to this construction in the case of these usual film sizes it is possible to make exposures with the theoretically smallest possible inclination of the focal point path relatively to the central X-ray, whereby the end rays of the X-ray fundle leave the anode surface with barely a strip. In this manner in case of these usual film sizes at a given focal point size, i.e. at a specific X-ray tube output, there is the best possible result as far as the smallest possible geometrical lack of clarity is concerned.

The invention will appear more clearly from the following detailed description when taken in connection with the accompanying drawing showing by way of example only, a preferred embodiment of the inventive idea.

In the drawing:

FIG. 1 is a diagrammatic sectional view of an X-ray device during exposure with the smallest possible geometrical lack of sharpness with a film of small size.

FIG. 2 is a partial top view of the rotary anode plate of FIG. 1 with an indicated focal point path.

FIG. 3 is similar to FIG. 1 but shows exposure with the smallest possible geometrical lack of sharpness when using a film of large size.

FIG. 4 is a partial top view of the rotary anode plate of FIG. 3 with an indicated focal point path.

FIG. 1 shows a rotary anode plate 1 of an X-ray tube 2, the outer and inner edge portions 3 and 4 of the plate having different inclinations to the plate axis 5. Separate cathodes 6 and 7 are located above these edge portions, so that a sharply limited electron ray bundle can be sent to the corresponding edge portion of the rotary anode plate. Two adjustable screen plate pairs 12, 13 of a primary ray screen are located in the direction of the central X-ray 8 extending in a direction perpendicular to the plate axis 5, the plates being used for limiting the X-ray bundle 3 relatively to the selected film 10 and the object 11 which is being examined and which is placed directly in front of the film. The doctor can select by the use of a switch 14 the desired cathode 6, 7 and thus the edge range of the rotary anode plate 1 from which the X-rays are to be emitted. Two change-over switches 15, 16 connected in series with the screen plate pairs of the primary ray screen are located in the circuit of the cathode 7 used for the edge section 4 of the rotary anode plate which is less inclined to the central ray 8. These change-over switches are used to switch on the cathode 6 related to an edge section 3 of the rotary anode plate which is stronger inclined to the central ray, for a specific opening of the screen plate pairs independently from the location of the switch 14.

FIG. 2 shows in top view the rotary anode plate 1 of FIG. 1 and indicates the location of the focal point path 17, i.e. the range which is struck by the electron ray bundle emitted from the cathode 7. The relationships of the edges of this focal point path are so adapted to the relatively small inclination 23 of this edge range to the central ray, that the projection 18 of this focal point path appears in the direction of the central ray 8 as a small square. The inclination 23 of this edge range 4 is the illustration of FIGS. 1 and 2 relatively to the central ray 8 is equal to one half of the angle at which appears the selected film size 10 looking from the focal point path 17. This is set at the same time the inclination at which the selected film size at the provided focal point path distance from the film can be illuminated with the smallest possible lack of geometrical sharpness.

FIG. 3 shows the same arrangement as in FIG. 1 with the difference that the screen plate pairs 12, 13 are arranged for the use of a larger film size 19 and a wider opened X-ray stray bundle 20. Due to the wider opened screen plate pairs 12, 13 the switched 15, 16 (FIG. 3) are moved into a position in which the cathode 6 is always switched on irrespective of the position of the switch 14, the cathode 6 being related to the rotary anode plate edge 3 which is stronger inclined to the central ray 8.

FIG. 4 shows in its top view of the rotary anode plate of FIG. 3 that the surface of the focal point path 21 upon which strike electrons emitted by the cathode 6 toward the outer edge section 3 of the rotary anode plate is wider than the focal point path 17 of FIG. 2 due to the different bundle arrangement of the electrons by this cathode. The projection 22 of this focal point path 21 is the direction of the central ray 8 is again a square one due to the greater inclination 24 of the edge of the rotary anode plate.

In modern X-ray examining devices the adjustment of the primary ray screen takes place automatically depending upon the size of the inserted cassette or the selected film size. This fixes the opening angle of the X-ray bundle 9, 20 due to the provided distance between the focal point path and the film. The examining doctor has the selection by correspondingly actuating the switch 14 of carrying out the exposure with a greater focal point path or with a smaller focal point path and smaller geometrical lack of clarity. Since the output of the larger focal point path is generally greater in the first case it is possible to operate usually with a shorter illumination time period and thus with lesser lack of movement clarity.

Even if the doctor decides to use the smaller focal point path, the changeover switches 15, 16 operated by the screen plate pairs 12, 13 of the primary ray screen will switch the cathode 6 without his action to the focal point path 17 of the following greater size as soon as the screen plate pairs have reached a position due to the selected film size wherein one half of the opening angle of the X-ray bundle is greater than the angle of the selected inner edge section of the rotary anode plate. This eliminates illumination errors caused by a selection of a focal point path which is too small in comparison with the selected film size. In addition the adaptation of the different anode angles to the distance between the focal point path and the film and the usual film sizes, provides that the operation always takes place with the theoretically smallest possible focal point path.

The operation of the change-over switch 15 by the pair of screen plates 13 can be eliminated in all cases wherein the pair of screen plates 13 can be moved only perpendicularly to the plate axis. For primary ray screens which are rotatable about the central X-ray the setting of the change-over switches is to be considered only for those vectors of the path which lie in the direction of the plate axis. In cases wherein the individual screen plates are adjustable independently from each other, a change-over switch is provided only for those screen plates which are shiftable under the horizon provided by the plane of the rotary anode plate. When X-ray tubes with more than two focal point paths are used it is advantageous to use multi-stage rotary switches instead of the ordinary change-over switches.

Claims

What is claimed is:

1. An X-ray device, comprising in combination, an X-ray tube of the rotating anode type having a rotatable stem and an anode plate mounted upon said rotatable stem, at least two electron-emitting cathodes spaced from said anode plate, and at least two concentric annular target tracks of different obtuse face angles formed on said anode plate, each of said target tracks being associated to one of said cathodes, a manually operated switch for selecting one of said cathodes to be activated, an X-ray screen for fluoroscopy mounted at a radial distance from the anode in a plane parallel to the axis of revolution of said rotating anode and being adjusted to X-rays emitted from the target tracks in a radial direction rectangular to said axis of revolution, two pairs of adjustable collimator plates mounted between the target tracks and said X-ray screen parallel to said X-ray screen and an automatic switching device comprising at least one changeover switch for said cathodes, actuated by said pair of collimator plates adjustable in a direction parallel to the axis of revolution, said automatic switching device switching on said cathode being associated to the target track having the bigger face angle when the selected adjustment of said pair of collimator plates exceeds a predetermined opening of the radiation beam larger than twice the angle of the surface of said target track having the smaller face angle with a plane normal to the axis of revolution, said switching device switching out said cathode being associated to the target track having the smaller face angle even when it has been manually selected, and wherein said predetermined opening of the radiation beam is effected when the planes of the X-ray emitting areas of the respective target tracks touch the edges of the collimator plates and the opposed edges of the usual film sizes being fixed in the plane of said X-ray screen.