Patent Number: 050383701
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A bundle of X-rays 3 emerges from the focus 2 situated in the housing 1 of an X-ray emitter and passes through the ray window 4 of the X-ray emitter. A diaphragm arrangement 5, which cuts out a ray fan 31 of a few millimeters in thickness from the bundle of X-rays 3 in a plane perpendicular to the plane of the drawing of FIG. 1, is connected to the housing 1. The diaphragm arrangement 5 has at its end facing away from the X-ray emitter 1 a cylindrical aperture 6, in which a first hollow-cylindrical diaphragm body 7 is arranged, which encloses a second diaphragm body 8, arranged concentrically to it. The common axis of symmetry and axis of rotation of the diaphragm bodies 7 and 8 is located in the plane of the ray fan 31, to be precise in such a way that the line joining the focus 2 to the center of the diaphragm body intersects the axis of symmetry at right angles. The rotatably mounted diaphragm bodies 7 and 8 are driven by a drive arrangement in such a way that the first diaphragm body 7 rotates faster by a factor of 6 than the diaphragm body 8. For this purpose, the drive arrangement could include a single motor, which would be coupled via suitably designed transmissions to the diaphragm bodies 7 and 8. Instead of this, in FIG. 1--for the sake of simplicity--a drive device with two stepping motors 9 and 10 is shown, of which the stepping motor 9, coupled to the outer diaphragm body 7, is coupled directly to a clock pulse generator 11, while the stepping motor 10, acting on the second diaphragm body 8, is thus connected via a frequency divider 12, which reduces the stepping frequency at a ratio of 1:6. As a consequence, the diaphragm body 7 rotates at six times the speed of the inner diaphragm body. As also explained in connection with FIGS. 2 and 3, a single X-ray beam 32 is cut out from the ray fan 31 by the diaphragm bodies 7 and 8, the dimensions of which beam in the vertical direction (perpendicular to the plane of the ray fan 31) are limited by a slit 13 which is only 0.5 mm wide and runs perpendicular to the plane of the drawing and the dimensions of which beam in the axial direction are determined by the design of the diaphragm body 7. If the diaphragm bodies rotate at constant speed, the X-ray beam 32 changes its point of impingement on a plane perpendicular to the plane of the drawing in accordance with a sawtooth-shaped time function. FIG. 2 shows a lateral plan view of the first diaphragm body 7. The diaphragm body consists of a material of a thickness such that the X-radiation emerging from the focus 2 is absorbed virtually completely as a result, for example of a 1 mm thick tungsten alloy. The diaphragm body may have a length of, for example, 50 mm and a diameter of 12 mm. At least one of the hollow shafts 71 on its end faces is coupled to the drive device explained in further detailed with reference to FIG. 1. Two mutually offset helical slits, which run around in the same encircling direction and have in each case a constant pitch are provided on the diaphragm body. Both slits have three turns or spirals each. The slit 73 has, however, a greater pitch (that is the ratio between the axial length of a turn and the circumference of the body 7) than the slit 72. The slit 73 has a width of 0.4 mm, while the slit 72 is considerably wider, for example 2 mm. The axial length of the slit 73 is slightly shorter than the length of the diaphragm body 7; if the slit were just as long, it would cut the diaphragm body into two divorced parts. Instead of three turns, the two slits may also have n turns (n=1 or 2 or else 4, 5, 6 etc.). In this case, the first diaphragm body would have to be rotated faster by a factor of 2n than the second diaphragm body 8. If the spirals in the diaphragm body 7 have the same encircling direction as the diaphragm body 8, the diaphragm bodies must be rotated in the same direction of rotation; if they have a difference encircling direction, a rotation in the opposite direction of rotation is necessary. The two slits are arranged mutually offset in such a way that they are offset on the circumference by precisely 180.degree. in the center of the diaphragm body, indicated by the arrow 70. In the position of the diaphragm body represented in FIG. 2, an X-ray beam can therefore pass through the slits 72 and 73 in the center of the diaphragm body perpendicular to the plane of the drawing--if the focus of the radiation source is located precisely in the center behind the diaphragm body. In this position of the diaphragm body there are two further points at which, on the side facing the focus, the slit 72 intersects the plane which is formed by the focus and the axis of symmetry or rotation 75. The axial position of these points is indicated by the arrows 721 and 723. Similarly, there are two points, which are indicated by the arrows 731 and 733, at which the slit 73 intersects the plane on the side facing away from the focus. If the distance of the focus from the generating line facing it of the diaphragm body relates to the distance of the focus from the generating line facing away from it in the same way as the axial lengths of a turn of the slits 72 and 73 relate to each other, a further X-ray beam additionally passes through the slit 72 at 721 and through the slit 73 at 731. Similarly, an X-ray beam passes through the slits 72 and 73 at 723 and 733. These three X-ray beams define a plane which naturally coincides with the plane of the ray fan 31. In this case, when the diaphragm body rotates, the three X-ray beams move to the left or to the right, depending on the direction of rotation, until the first beam reaches one end of the slit, after which a further beam appears at the other end. It is clear from the above that the differences in the pitch of the slits or in the axial length of their turns are determined by the distance of the focus from the diaphragm body 7 and by the diameter of the diaphragm body. The smaller the ratio of these two values, the greater the difference in the lengths or pitches. If, on the other hand, the emitter is very far removed from the diaphragm body in comparison with the diameter, the lengths and the pitches of the two slits are virtually the same. It also is evident from the above that the cross-section of an X-ray beam 32 emerging from the diaphragm arrangement 5 (cf. FIG. 1) is determined in the axial direction by the dimensions of the thinner slit and in the plane perpendicular to the ray fan 31 by the aperture of the slit diaphragm 13. It would also be possible to make the slit 72 just as narrow as the slit 73, so that the slit diaphragm 13 could even be dispensed with. However, with finite dimensions of the focus 2, this would result in an increase in the geometrical unsharpness of the X-ray beam and the arrangement would become more sensitive to production discrepancies in the position of the focus 2 with respect to the diaphragm body. Therefore, the arrangement with a wider slit 72 with smaller pitch and an additional slit diaphragm 13 is to be preferred. As already mentioned, the diaphragm body 7 cuts out (at least) as many X-ray beams as the slits have turns. As a rule, however, only one X-ray beam is desired. Although this could be achieved if slits with only a single turn were provided, in this case the slits or their projection would intersect the plane of the ray fan at a considerably more acute angle, so that, with the same slit width, the axial dimensions would be considerably increased in an undesired way. In the case of the exemplary embodiment according to FIGS. 1-3, a different approach is therefore adopted: of the X-ray beams which could pass through the diaphragm body, only a single one is allowed through. The second diaphragm body 8 (FIG. 3) serves this purpose. The second diaphragm body 8 is again a hollow cylinder, which may consist of the same material as the first diaphragm body and has at least one end face a shaft coupled to the drive device 9 . . . 12 (FIG. 1). Otherwise this diaphragm body corresponds to that according to European laid-open patent application 74,021, i.e. it is provided with two slits 82 and 83 mutually offset by 180.degree. on the circumference, each of which extends over the same axial length and has the form of a helix. However, the two slits 82 and 83 have only half a turn, i.e. they extend over an arc of only 180.degree. each on the circumference of the diaphragm body 8. The slits 82 and 83 are considerably wider than the narrow slit 73 on the first diaphragm body. In a suitable position of the two diaphragm bodies with respect to each other, of the three X-ray beams which could pass through the first diaphragm body, two are absorbed, for example the two outer ones, and only the middle one is allowed through. If the second diaphragm body is rotated at a sixth of the speed of the first diaphragm body, this X-ray beam moves in both diaphragm bodies at the same speed, so that only this one X-ray beam is ever allowed through. The number a of the turns of the slits 72, 73 in the first diaphragm body 7 which the X-ray beam passes through in the course of its axial movement does not necessarily have to be an integral number, and by the same token, the corresponding number b for the second diaphragm body 8 does not have to be precisely 0.5. However, for the ratio, the condition a/b=2n must be satisfied, n being an integer (greater than 0). Only then is a periodic movement of the X-ray beam obtained at constant speed. If a is not an integral number and/or b is less than 0.5, during the course of the periodic movement there are intervals of greater or lesser length in which the X-ray beam is suppressed. Instead of the diaphragm body represented in FIG. 3, other hollow-cylindrical diaphragm bodies co-rotating with the diaphragm body 7 may also be provided, as described in detail in German patent application P 38 29 688 which corresponds to the aforementioned copending application. For example, the diaphragm body may have a semicircular cross-section and be provided with only a single slit, which extends over the length of the diaphragm body and describes an arc of at least approximately 180.degree.. Similarly, a hollow-cylindrical body of semicircular cross-section which is provided on its circumference with a plurality of apertures mutually offset in axial and circumferential directions may be used. However, in the case of the embodiment last mentioned, the X-ray beam jumps from one aperture to the other. The advantage of the embodiment represented in FIG. 3 over the one last-mentioned is also that this diaphragm body does not have any imbalance.