Patent Number: 061480622
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the figures, the same elements are identified by the same reference numbers. Reference is more particularly made to FIG. 3, in which a first embodiment of an X-ray beam-shaping filter 10 of variable area has been depicted. This filter, like the filter of the prior art in FIG. 2, comprises a main frame 11 provided with sliding rails 13, 14 and two compensating plates 17, 18 made of X-ray-absorbent material. Since the general structure is similar to that of the conventional filter in FIG. 2, in particular with regard to the arrangement of the plates 17, 18 on the rails 13, 14 by means of carriages (not depicted), reference should be made to the previous description of this figure for greater details. The shaping filter 10 of one embodiment of the invention differs from that of the prior art by the arrangement of the compensating plates 17, 18. Given that the compensating plates 17, 18 are identical, the following description relating to one of the plates applies in extends to the other compensating plate. The compensating plate 17 comprises a first plate element 17a having the general shape of a biconvex meniscus comprising a first end joined conventionally to a carriage which can move translationally on the rail 13 and a second end, opposite the first, provided with a pivot pin 20. A second plate element 17b, having the general shape of a scythe blade, has a first end mounted so as to pivot on the pivot pin 20 and an enlarged second end, opposite the first, having a curved elongate slot 22. A stud 23 projects from the first plate element 17a in order to fit into the elongate curved slot 22 at the enlarged second end of the second plate element 17b. A stress spring 21, placed around the pivot pin 20, is joined by one of its ends, respectively, to the first and second plate elements 17a, 17b. Finally, a stop 24 is provided on the main frame 11 in order to keep the second plate element 17b in its initial position, as will be seen later. In FIG. 3, the plate 17 has been depicted in its retracted position in which the first and second plate elements 17a, 17b are in their initial position in which these plate elements overlap almost entirely. In contrast, the plate 18 has been depicted in its final active position and the plate elements have been depicted in their maximum deployed position in which the first and second plate elements 17a and 18b now overlap only over a minimum area 25 along the outer edge of the first plate element 18a and along the inner edge of the second plate element 18b. Those parts of the plate elements 18a and 18b which correspond to this overlap area 25 are bevelled so that the overlap area 25 has a thickness identical to the rest of the plate 18, as may be seen in FIG. 6. The plate elements 18a, 18b have an identical thickness in their non-bevelled parts. Although the second plate elements 17b, 18b have been depicted as pivoting above the first plate elements 17a, 18a, it is also possible to place them in the same manner below the first plate elements 17a, 18a. The operation of the filter 10 is described with reference to FIGS. 3 to 6. Initially, the plates are in the retracted position, depicted in FIG. 3 in respect of the plate 17, in which the first plate element 17a is pushed back by sliding at one end of the rail 13 as far as a position in which, by rotation under the effect of the bearing force of the stop 24 against the spring 21, the stud 23 bears on one end of the slot 22 and the plate elements are in their initial position and overlap almost entirely. When the user wishes to shape the field of view of a region 1 of a patient under examination which includes a wide area 2 of low absorption density, so as to compensate for the lowest absorption of the X-ray beam 3 by the low-density area 2 as depicted in FIG. 5, he moves the first plate element, for example 18a, by sliding it along the rail 14 in order to end up over the low-density area. Under the action of the spring 21, the second plate element 18b pivots about the pin 20 and is deployed, the sliding of the stud 23 in the slot 22 during pivoting of the second plate element causing this element to pivot uniformly. At this stage, the plate is in an active position. Referring more particularly to FIG. 4, the plate 17 is brought by the user into an intermediate active first position in which the overlap area 25 of the plate elements 17a and 17b is large, but as may be seen in FIG. 4 this relatively large overlap area 25, because of the shape and size of the plate elements, lies entirely over the frame 11 and, consequently, only part of the plate element 17a lies in the field of view 19 of the X-ray beam and is active in order to absorb part of this X-ray beam. Since the thickness of this plate element 17a is constant, the compensation produced is uniform. The other plate 18, in the case of FIG. 4, is in an intermediate active position close to the final active position and, as may be seen in FIG. 4, the plate element 18b was deployed under the effect of the spring 21 and the overlap area of the plate elements 18a, 18b is almost the minimum, but it lies slightly within the field of view 19. However, because of the fact that this small overlap area 25 corresponds to appropriate bevelled parts of the plate elements 18a, 18b, the thickness in this overlap area 25 is almost equal to the thickness of the rest of the plate element 18a lying in the field of view 19. Uniform compensation over the entire desired part of the field of view 19 is therefore obtained. Referring again to FIG. 3, the plate 18 has been depicted in its final active position in which the spring 21 has pivoted the plate element 18b until the end of the slot 22 butts against the stud 23. In this position, the area of the field of view 19 covered by the plate is the maximum area. The overlap area 25 of the plate elements 18a, 18b is the minimum and, as previously, because of the bevelling of the corresponding parts of the plate elements, has a thickness equal to the remaining parts of the plate elements. Thus, uniform compensation over the entire area of the field of view covered by the plate 18 is obtained. Depicted in FIGS. 7 and 8 is a filter according to another embodiment of the invention which differs from the filter described above by the fact that the plate elements 17a, 17b and 18a, 18b can be moved in relative translation, one with respect to the other, and by the means allowing this relative translation of the plate elements. The second plate element 18a, 18b is a curved plate of almost constant width provided at both its ends with parallel straight slots 31, 32. Since the slots 30, 31 allow the second plate element 18a, 18b to move translationally along two studs 32, 33 fixed to the first plate element 17a, 17b. Return springs 34, 35 are fixed both to the support plate and to the second plate element 17b, 18a. FIG. 7 depicts the plate 17 in its initial retracted position in which the overlap of the plate elements is the maximum. In this position, the plate rests on the stops 36. When the user moves the first plate element 18a, by sliding it along the rail, in order to bring it into a first intermediate active position, as depicted in FIG. 7, the second plate element 18b remains stationary. The overlap area 25 remains large but it lies entirely over the main frame 11 and only part of the first plate element lies in the field of view 19. Because of the constant thickness of the plate element, the X-ray beam is therefore uniformly attenuated. When the user brings the first plate element into the position depicted in respect of the plate 18 in FIG. 8, the second plate element 18b has still not been moved, but the studs 32, 33 butt against the front end of the slots 30, 31. The overlap area 25 of the plate elements is the minimum area and, because of the fact that the plate elements have suitable bevelled parts in this minimum overlap area 25, the thickness remains constant. When the user slides the first plate element into the final active position of the plate 17 in FIG. 8, the second plate element 17b is driven by the first plate element 17a under the action of the studs 32, 33 on the front ends of the slots 30, 31 and against the return force of the springs 34, 35. Of course, conventional locking means are provided in order to keep the plates in the positions chosen by the user. In the position of the plate 17 depicted in FIG. 8, the area of the field of view covered by the plate is the maximum. Although the minimum overlap area 25 lies within the field of view, the thickness of material through which the X-ray beam passes remains constant, for the reasons given above, and uniform attenuation is obtained. Although the embodiments of the invention have been described with filters having two rails and two plates, it is possible to produce filters having a single rail and a single plate, or more than two rails and two plates, for example four rails and four plates diametrically opposed in pairs. Various modifications in structure and/or function and/or steps may be made by one skilled in the art to the disclosed embodiments without departing from the scope and extent of the invention.