PIEZOELECTRIC ATTENUATION TONGUE SYSTEM FOR SLIT RADIOGRAPHY EQUIPMENT

Abstract: A piezo-electric attenuation tongue system for slit radiography equipment comprises a number of tongues placed next to each other. The tongue system is manufactured by slitting a plate of piezo-electric material from one of the edges but leaving unslit a strip of the plate along the opposite edge.

Description: The invention relates to a piezoelectric attenuation tongue system for slit radiography equipment comprising a number of tongues placed next to each other which are each firmly mounted by one end on a carrier. From Dutch Patent Application 8,400,845 slit radi­ ography equipment is known which is provided with controll­ able attenuation elements placed next to each other which each act in combination with a section of a diaphragm slit in order to influence locally the x-ray beam transmitted or to be transmitted during operation through the diaphragm slit. The Dutch Patent Application 8,400,845 describes various embodiments of assemblies of attenuation elements, and one of the embodiments described in the Dutch Patent Application 8,400,845 consists of a number of tongues of piezoelectric material placed next to each other. The piezoelectric tongues are firmly mounted by one end on a carrier, while the other end can be moved under the influ­ ence of electrical control signals, which are fed to each piezoelectric tongue during operation, to a greater or lesser degree into the x-ray beam transmitted or to be transmitted through the diaphragm slit in order to attenuate said beam locally to a greater or lesser degree. A drawback of the known assembly of piezoelectric attenuation tongues is that during the assembly each tongue has to be separately adjusted both in relation to the distance from the adjacent tongues and in relation to the direction with respect to the x-ray beam to be in­ fluenced. In order to make such an adjustment possible, special facilities are necessary for each tongue on the carrier and/or the tongue and/or the fixing means for the mounting of the tongue on the carrier. Moreover, the adjustment is itself very time-consuming and therefore expensive. Despite accurate adjustment it is also very difficult, if not impossible, to make the gap between adjacent tongues very small. Said gap must, however, preferably be as small as possible in order to prevent, as much as possible, unattenuated x-ray radiation being transmitted through the slit between two adjacent tongues, which radiation could give rise to strips in the final radiograph. The need therefore exists for an improved atten­ uation tongue system to which the abovenamed drawbacks do not apply or apply to a lesser extent. The object of the invention is to meet this need. For this purpose, a piezoelectric attenuation tongue system of the type described above is characterized according to the invention in that the separate tongues each form a single whole with a base of piezoelectric material and are obtained by slitting a plate of piezo­ electric material from a first edge of the edges, leaving free a strip along a second edge situated opposite the first edge, which strip forms the base. The invention is described below in more detail with reference to the accompanying drawing of some exempl­ ary embodiments. Figure 1 shows a first embodiment of an assembly of attenuation tongues according to the invention; Figure 2 shows a second embodiment of an assembly of attenuation tongues according to the invention; Figure 3 shows the assembly of attenuation tongues of Figure 1 in the assembled state in plan view; Figure 4 shows a section along the line IV-IV in Figure 3; Figure 5 shows a detail of an exemplary embodiment of an assembly according to the invention; and Figure 6 shows a variant of Figure 5. Figure 1 shows a rectangular plate 1 of piezo­ electric material. The plate is slit from one of the sides 2 by a number of parallel saw cuts which extend in the direction of the opposite side 3, but which terminate at some distance from the side 3. The saw cuts each lie at an equal distance from each other or from the long sides 4, 5 of the plate 1. There is thus formed from the rectangular plate 1 a number (in the embodiment shown 19) of tongues 6 which extend in parallel from a common base which is formed by the unslit part 7 of the rectangular plate. The free ends of the tongues 6 can move in a direction transverse to the surface of the plate 1 with respect to each other. Figure 2 shows a similar assembly of attenuation tongues formed from a single plate 1′ of piezoelectric material. In this embodiment the side 3′ iS, hOwever, longer than the side 2′ and the saw cuts converge when viewed from the longest side 3′ in the direction of the shortest side 2′, while the long sides 4′, 5′, corres­ ponding to the long sides 4, 5 of Figure 1, also converge so that wedge-shaped tongues 6′ are obtained which again terminate at some distance from the longest side 3′ while leaving free a base 7′. It is pointed out that in Figure 2 the tongues are not equally long, as a result of which they have a different dynamic behaviour. This possible drawback can be eliminated by, for example, constructing the side 2′ of the plate 1′ as a circular arc as indicated by broken lines at 2" and making the slits equally long so that the ends of the slits also lie on a circular arc 8. The assemblies of attenuation tongues shown in Figures 1 and 2 therefore have the form of a comb. Be­ cause the tongues have been formed from a single plate of material and, in addition, continue to form a single whole with the base of the comb (the unslit section 7 or 7′) the tongues do not have to be separately mounted and adjusted during assembly in slit radiography equipment. It is possible to make do with mounting and aligning the comb as a whole, which is relatively simple. The replacement of such a tongue assembly can also be performed very simply and rapidly. Because the separation between two tongues is in each case formed by a single saw cut or a slit made in another manner, the gap between two adjacent tongues is automatically equally large over the whole length of the slit. The slits required to form the tongues can be made with a fine small saw. However, it is also possible to make the slits by means of a laser or by means of ultra­ sonic techniques known per se. In practice, it has proved possible to make very narrow slits with a width in the order of 50 µm by means of ultrasonic techniques. Such a small gap between adjacent tongues cannot, or virtually cannot, be achieved if "loose" tongues are used. Figure 3 shows in plan view the assembly of Figure 1 in the assembled state, and Figure 4 shows a section along the line IV-IV in Figure 3. The tongue plate 1 provided with slits is mounted in a box-shaped casing 30 with an upper wall 31, a lower wall 32, side walls 33 and 34 and a front wall 35. A back wall is not drawn since it is not necessary. If desired, however, a back wall could indeed be present, which would then have to be pro­ vided with a slit. In the front wall 35 a slit 36 has been made. The slit 36, which acts as a slit diaphragm, of the casing faces an x-ray source, the focus of which is indicated by 37. The maximum width and thickness of the fanshaped x-ray beam 38 transmitted through the cas­ ing is therefore determined by the dimensions of the slit. Preferably, however, separate adjustment means are further provided in the casing in order to limit the x-ray beam accurately as will be described in further detail below. It is pointed out that hereinbefore and herein­ after the descriptions "upper" and "lower" are used in relation to the orientation of the figure. In reality, the position of the casing depends on the manner in which the slit radiography equipment is installed. In practical equipment, for example, the wall 34 may be the lower wall and the wall 33 the upper wall, or the wall 31 may be the lower wall and the wall 32 the upper wall. The x-ray source might also be situated on the other side of the casing (i.e. on the right-hand side in Figures 3 and 4). In the embodiment shown the non-slit section, the base 7, of the tongue plate 1 is mounted on the lower wall 32 of the casing and its tongues are directed obliquely upwards towards the slit in the front wall. Above the base 7 there is disposed a clamping bracket 39 which is attached by means of bolts 40 or the like to the lower wall of the casing on either side of the tongue plate and thus clamps the tongue plate against a spacer 41 of insulating material. In order to be able to control the piezoelectric tongues separately, the tongues have to be electrically separated from each other at least on one side of the tongue plate. For this purpose, in the exemplary embodi­ ment shown the slits in the plate 1 are continued on the lower side across the base 7 to a depth of about half the thickness of the plate, as is indicated by broken lines 42 in Figure 3. Since such slits to a depth of half the thickness weaken the tongue plate, the base 7 is prefer­ ably provided with a glued reinforcing strip 43 of a suit­ able material on the non-slit upper surface. Moreover, the ridge 7 is provided with a common electrical connecting point 44 on the upper side. In the spacer 41 there is disposed a cutout in which connector element 45 is placed which brings about separate electrical connections with each tongue 6. For this purpose, the connecting points of the connector ele­ ment in the exemplary embodiment shown are connected to electrically conducting tracks (not shown) disposed on an insulating plate 46 placed on the lower wall of the casing. One conducting track is provided for each tongue and pre­ ferably an additional track is also present which is con­ nected to the common connecting point 44. The conducting tracks are connected to diagrammatically indicated connec­ tor pins 47, two of which are visible, which reach through a cutout in the lower wall. The ends of the tongues 6 situated near the slit 36 reach, during operation, to a greater or lesser extent into the x-ray beam 38 and in the quiescent state are situated just outside the x-ray beam or in the x-ray beam, depending on the chosen manner of controlling the position of the tongues. Although the piezoelectric material of the tongues themselves in most cases already attenuates x-ray radiation to an extent which is adequate to influence the x-ray beam in the required manner, if desired, elements 48 of an x-ray radiation-absorbing material, for example lead or tungsten, can be provided on the ends of the tongues. Said elements may advantageously be formed from a strip of material which is provided, for example, by gluing along the edge 2, or 2′ of the plate 1, 1′ which has not yet been slit. As a result of the slitting the separate absorption elements are then produced simultaneously with the tongues. The use of absorption elements on the free ends of the tongues has the additional advantage that the in­ fluencing of the x-ray beam can be made proportional to the angular position of the tongues in a simple manner. In addition, the risk is reduced that unattenuated x-ray radiation is transmitted between two adjacent tongues which have a different angular position, i.e. whose ends reach to a different extent into the x-ray beam. Admittedly, said risk is nevertheless already very small in a tongue assembly according to the invention as a consequence of the very narrow slits between the tongues. In the tongue assembly shown, the risk of trans­ mission of x-ray radiation which cannot be influenced via the slits between the tongues virtually only exists in the case of the slits on either side of the centre tongue­ (s) because said slits are most in line with the x-ray radiation at that point. This is dependent on the dist­ ance between the tongues and the x-ray focus, the dimen­ sions of the x-ray focus and the width of the ends of the tongues. All this can, if desired, be prevented by prov­ iding at least the centre tongue(s) with two small elements or horns of absorbing material which reach forwards, i.e. in the direction of the x-ray source and which precisely shield slits on either side of the tongue. Such small elements are indicated at 63, 64 for the centre tongue in Figure 5 which shows the centre tongue 60 and two ad­ jacent tongues 61, 62. If elements 48 are also used, the horns 63, 64 may form a single whole with the element 48 concerned. As an alternative it is possible to provide all the tongues apart from the centre one with absorption elements 48 and to provide the centre tongue with an ele­ ment 48′ (Figure 6) which reaches forward and which is equally as wide as the tongue itself plus the two slits on either side of the tongue. Depending on the distance between the x-ray focus and the tongues, the width of the ends of the tongues and the dimensions of the x-ray focus, it may be desirable to provide the tongues, at least in the central region, alternately with elements 48 and 48′. The difference in dynamic behaviour of the tongues produced by this can be compensated for by constructing the ends of the tongues provided with elements 48′ in a wedge-shaped manner as is indicated by broken lines at 65 in Figure 6 for the centre tongue. A similar method could in principle be used also in the case of tongue 60 of Figure 5. It is pointed out that use of additional absorp­ tion elements reduces the mechanical resonant frequency of the tongues, as a result of which the response of the tongues becomes slower. This effect can be compensated for by constructing all the tongues with a wedge-shaped end. By choosing the length and/or tapering of the wedge-­ shaped sections differently for tongues which are provided with an absorption element projecting forwards such as 48′ from those chosen for tongues which are provided with an element which does not project forward such as 48, the same resonant frequency can be obtained for every tongue. In general it is of importance in slit radiography equipment provided with attenuation tongues to be able to set the dimensions of the scanning x-ray beam, i.e. the thickness and the width, or rather the angles  (Figure 3) and α (Figure 4) as accurately as possible in order to be certain that the whole of the scanning x-ray beam, or at least a fixed section thereof can also in fact be influenced by the tongues. If a tongue plate according to the invention is used, this is still more important because the maximum deflection of the tongues has prefer­ ably to be as small as possible in order to prevent the base 7 from cracking at the position of the junction with the tongues. A relatively small maximum deflection of the tongues requires a relatively large setting accuracy for the dimensions of the x-ray beam. For this purpose, according to the invention, there are disposed on either side of the x-ray beam strongly absorbing, for example lead, elements 49 and 50 respec­ tively which are attached to the side walls 33, 34 of the casing in the exemplary embodiment shown and whose distance from the side walls is adjustable, as is indicated diagram­ matically by arrows 51. For the setting, an adjustment screw may, for example, be provided such as is indicated by 52 for the element 50. The thickness of the x-ray beam can be set in a similar manner by an adjustable strip of absorb­ ing material disposed on the upper side of the x-ray beam. The lower side of the x-ray beam is defined in this ex­ emplary embodiment by the lower edge of the slit 36 in the casing 30 which is also manufactured from a material which absorbs x-ray radiation. Figure 4 shows such a strip of absorbing material 53 which is joined to the upper wall via a sprung plate or strip 54 and whose distance from the upper wall can be set by means of one or more adjustment screws 55. It is pointed out that only a few exemplary em­ bodiments of the invention have been described above. In addition to the above various modifications are obvious to those skilled in the art. Thus, for example, the risk of spark flashover between adjacent tongues can be reduced by providing the tongues with a layer of lacquer. Such modifications are considered to fall within the scope of the invention.

Claims: 1. Piezoelectric attenuation tongue system for slit radiography equipment comprising a number of tongues placed next to each other which are each firmly mounted by one end on a carrier, characterized in that the separ­ ate tongues each form a single whole with a base of piezo­ electric material and are obtained by slitting a plate of piezoelectric material from a first edge of the edges, leaving free a strip along a second edge situated opposite the first edge, which strip forms the base. 2. Attenuation tongue system according to Claim 1, characterized in that the plate of piezoelectric material is rectangular and in that the slits are disposed in parallel. 3. Attenuation tongue system according to Claim 1, characterized in that the plate of piezoelectric material is trapezoidal and is slit from the shortest parallel side, and in that the tongues are wedge-shaped. 4. Attenuation tongue system according to Claim 1, characterized in that the slits are continued across the base to a depth of approximately half the thickness of the plate. 5. Attenuation tongue system according to Claim 1, characterized in that a reinforcing strip is glued to the base on the side which is not slit to a depth of half the thickness. 6. Attenuation tongue system according to Claim 1, characterized in that each tongue is provided at the free end with an additional element of material which absorbs x-ray radiation. 7. Attenuation tongue system according to Claim 6, characterized in that the additional elements of material which absorbs x-ray radiation are formed from at least one strip of absorbing material provided along the correspond­ ing edge of the plate of piezoelectric material prior to the slitting of the plate of piezoelectric material. 8. Attenuation tongue system according to Claim 6 or 7, characterized in that at least the centre tongue is provided with an element of absorbing material which reaches forward with respect to the other tongues and which is equally as wide as the end of the centre tongue concerned augmented by the width of the slits on either side of the centre tongue. 9. Attenuation tongue system according to Claim 1 or Claim 6, characterized in that at least the centre tongue is provided with two horns of material which absorbs x-ray radiation, which horns reach forward and screen the slits on either side of the tongue. 10. Attenuation tongue system according to Claim 1, characterized in that the base is clamped to the carrier by means of a clamping bracket, the slit surface of the base facing the carrier, and in that the carrier is prov­ ided with connector means which bring about a separate electrical connection to each region of the base situated between two slits. 11. Attenuation tongue system according to Claim 1 in which the carrier is mounted in a casing which is provided with openings for transmitting the x-ray beam to be influenced by the attenuation tongues during oper­ ation and originating from the x-ray source of the slit radiography equipment, characterized in that the casing is provided with limiting means for limiting the dimensions of the x-ray beam to be transmitted. 12. Attenuation tongue system according to Claim 11, characterized in that the limiting means comprise two limiting elements of material which absorbs x-ray radiation which are placed on either side of the plate of piezoelec­ tric material, which extend approximately transversely with respect to the plate of piezoelectric material in order to limit the width of the x-ray beam at that point and which are provided with adjustment means for setting the distance between each limiting element and the adjac­ ent edge of the plate of piezoelectric material. 13. Attenuation tongue system according to Claim 11, characterized in that the limiting means comprise at least one strip-like element which absorbs x-ray radiation, which extends transversely with respect to the tongues, and which is provided with adjustment means for setting the distance from the plate of piezoelectric material. 14. Attenuation tongue system according Claim 3, characterized in that the snort side has a circular arc form and in that the slits are all equally long and ex­ tend radially from the side having a circular arc shape. 15. Attenuation tongue system according to Claim 6, characterized in that at least the ends of the tongues themselves are constructed in a wedge-shaped manner. 16. Attenuation tongue system according to Claim 8 or 9, characterized in that at least the ends of all the tongues which carry absorption elements which reach for­ ward are constructed in a wedge-shaped manner. 17. Attenuation tongue system according to Claims 15 and 16, characterized in that the wedge shape of the ends of the tongues provided with absorption elements which reach forward differs from that of the tongues which are not provided with absorption elements which project forwards.

Inventors: DUINKER, SIMON
Applicants: B.V. OPTISCHE INDUSTRIE "DE OUDE DELFT"
Application year: 1986
Main classification: G21K1
IPC codes: G21K1, G02B26, A61B6