Patent Number: 063058421
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a Bucky X-ray examination apparatus which includes a patient table 1 and an X-ray source 2. Underneath the top of the patient table 1 there is arranged a moving grid 3 which is displaceable in the longitudinal direction of the table top and is provided with a film to be exposed during an X-ray exposure. The X-ray source 2 is supported by a stand (not shown) which is displaceable in the longitudinal direction of the table, the source being displaceable from a position in which its radiation cone is incident at the center of the table top, perpendicularly to the plane of drawing, to a position in which it is situated perpendicularly above the moving grid 3 pulled underneath the table top. The desired exposure format can be adjusted in this position. Beam limiting is performed by means of a diaphragm unit 4 which is mounted on the X-ray source 2. The X-rays, emanating from the focal spot or the focus 21 of an X-ray tube which is included in the X-ray source 2 but not shown, are limited by a first pair of shutter plates 41 having shutter edges which extend parallel to one another and perpendicularly to the plane of drawing of FIG. 1. There is also provided a second pair of shutter plates (not shown) which has horizontal shutter edges which extend parallel to the plane of drawing and limit the X-rays in the direction perpendicular to the plane of drawing. The radiation cone limited by the shutter plates is denoted by the reference 210; the radiation cone is interrupted by the wavy lines because in practice the distance of the X-ray source 2 is larger than shown in FIG. 1. The ray at the center of the radiation cone, the so-called central ray, is denoted by the reference 211. It is incident at right angles at the center of the moving grid 3. The diaphragm unit 4 includes a light source 42 which illuminates the table top (or a patient accommodated thereon) via a mirror 43, being transparent to X-rays, and through the diaphragm plates 41. The light source 42 is situated at the same distance from the mirror 43 as the focal spot 21. Therefore, if it (or the light-emitting structure included therein) were to have the same dimensions as the focal spot 21, the light cone limited by the shutter plates 41 should correspond to the radiation cone 210 and hence the illuminated field (i.e. the cross-section of the light cone in the plane of the film contained in the moving grid 3) should correspond to the irradiated field (the cross-section of the radiation cone 210 in the plane of the film present in the moving grid 3). For the reasons stated above the light-emitting structure of the light source (for example, the filament of an incandescent lamp) is significantly larger than the focal spot. In practice the dimensions of the filament may be, for example 6.times.3.2 mm whereas the (optically effective) dimensions of the focal spot are 1.times.1 mm or less. In the case of such a large filament, the umbra (that is the region which is not exposed to any light) is substantially larger than the region exposed to light from all light-emitting points of the light source. The edge of the illuminated field is then situated (comparatively readily reproducible) at approximately 8% of the brightness at the center. Consequently, the illuminated field that can be observed by the user is also significantly larger than the fully illuminated field which corresponds essentially to the irradiated field. The image defect defined by the difference between these fields amounts to approximately 15 mm in the direction of the greater filament width for the given dimensions of the filament and a distance of 1 m between the film and the focus 21, and to approximately 35 mm in the case of a distance of 2 m. In order to reduce or eliminate such image defects, the shutter plates 41 are provided with correction shutters 44 whose edges extend parallel to the edges of the shutter plates 41 and constrict the light cone. Because the correction shutters, however, are made of a material which is transparent to X-rays, the radiation cone 210 is not influenced thereby. The effect of the correction shutters will be described in detail hereinafter with reference to FIG. 2, wherein the central ray 211 does not extend vertically (as in FIG. 1) but horizontally. The filament 42 is taken to be particularly large in order to achieve a better illustration of the effects. Moreover, for the sake of simplicity of the drawing it is assumed that the light source 42 is situated at the location of the focal spot 21; because the optical distance between the filament 42 and the film, however, is exactly equal to the distance between the focal spot and the film, this assumption does not disturb the actual circumstances. The radiation cone 210 emanating from the focal spot 21 is denoted by solid lines. Dashed lines represent the light cone 420 which would occur if the correction shutters 44 were absent. It is clearly shown that the illuminated field in the plane of the film 30, present in the moving grid, is substantially larger than the irradiated field. The correction shutters 44 constrict the light cone in such a manner that the light cone 421 (denoted by dash-dot lines) is obtained. When the width of the correction shutters is suitably chosen (the term "width" is to be understood to mean herein the distance between the light-limiting edge of the correction shutter 44 and the edge of the shutter plate 41 which limits the X-rays), the illuminated field can be made to correspond to the irradiated field. The following equation suitably approximates the width b of the correction shutter: ##EQU1## Therein, B.sub.s is the width of the light source, B.sub.x is the width of the focal spot, d is the distance between the plane of the shutter edges and the focal spot, and D is the distance between the focal spot and the film 30 (film-focus distance). Using the practical values d=0.25 m, B.sub.s =5 mm and B.sub.x =1 mm, a value of 1.5 mm is then obtained for the width of the correction shutter in the case of a film-focus distance D of 1 m, and a value of 1.75 mm for a distance D amounting to 2 m. When the correction shutter 44 is rigidly connected to the shutter plate 41, the shutter width B is constant. In the case of a value B=1.5 mm, the image defects are completely corrected in the case of D=1 m (it would amount to 12 mm in the absence of correction shutters). In the case of a film-focus distance D of 2 m, the image defect is reduced from 28 mm to 3.5 mm. In this example the smallest exposure format that can still be adjusted amounts to 12 mm in the case of a film-focus distance D of 1 m. This value fully suffices in practice and amply meets the legally allowed maximum deviation of 20 mm between illuminated field and radiation field in the case of a film-focus distance D of 1 m. The width of the correction shutter can also be proportioned for a value D of between 1 m and 2 m. The maximum image defect would then be smaller than in the assumed case. Generally speaking, however, the aim is for exact beam limiting at the standard distance (1 m, and recently also 1.15 m). FIG. 3 shows diagrammatically various possibilities for arranging the correction shutters. Therein, the reference 212 denotes the anode disc of a rotary anode X-ray tube in which the focal spot 21 is situated on a focal spot track which is concentric with the horizontal axis of rotation of the anode disc 212. The focal spot track is inclined from 10.degree. to 20.degree. relative to the vertical, and the dimensions of the focal spot 21 are larger, in conformity with the principle of the strip focus, in the direction perpendicular to the focal spot track than in the direction of the focal spot track, so that the focal spot has a square appearance when viewed from the central ray 211. Because the plane of the focal spot track is inclined from the bottom left to the top right, the focal spot appears to be larger when viewed from the right-hand shutter edge than when viewed from the left-hand shutter edge. This means that the width B.sub.x of the focal spot appears to be larger at the right-hand side of the irradiated field than at the left-hand side thereof. The various possibilities for arranging the correction shutters 44 are shown on three different rows in FIG. 3. According to the lower row, the correction shutters are always situated underneath the shutter plates 41 and have the same width. This arrangement requires the least manufacturing and assembly work, but has the drawback that the diaphragm cannot be closed so far that it is no longer traversed by any X-rays; this is because the shutters cannot be closed further as soon as the facing shutter edges of the correction shutters 44 contact one another. It is to be noted, however, that complete closure of the shutter plates will not be required in practice. However, in given circumstances the correction shutters 44 could be resiliently connected to the shutter plates 41 so that the correction shutters would be displaced against the spring force upon closure. The upper row of FIG. 3 shows a different arrangement of the correction shutters 44. One correction shutter 44 is now connected to the lower side of the left-hand shutter plate 41 and the other correction shutter is rigidly connected to the upper side of the right-hand shutter plate 41. In this case the complete closure of the shutter plates for the X-rays is not impeded by the correction shutters. The second row shows an arrangement which is similar to that shown on the upper row, one of the two correction shutters (the lower left shutter in the present example) being situated at a given distance from the associated shutter plate. The edge ray of the radiation cone 210 (solid line) and of the light cone 421 (dashed line) is shown for the left-hand side as well as for the right-hand side. It appears that for the right-hand shutter plate these edge rays are situated nearer to one another in comparison with those for the left-hand shutter plate. Consequently, the correction of the light cone by the correction shutter 44 at the right-hand side is less than that at the left-hand side, as required in conformity with the equation 1, because the focal spot appears to be larger when viewed from the side of the right-hand shutter than from the side of the left-hand shutter. The asymmetry caused by the asymmetrical position of the focal spot track relative to the two shutter edges is thus compensated for. However, when the shutter plates are closed, i.e. the shutter plates and the correction shutters occupy the positions denoted by the references 41' and 44', respectively, the asymmetry is reduced so that the edge rays of the light cone and of the radiation cone are symmetrically situated relative to the central ray 211. All references cited herein 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.