AN APPARATUS FOR SLIT RADIOGRAPHY

Abstract: An apparatus for slit radiography comprising an X-ray source by means of which a body being irradiated can be scanned through a slit diaphragm by a substantially planar, fan-shaped X-ray beam, the scanning X-ray beam passed through the body impinging upon an X-ray detector. The ap­ paratus comprises controllable attenuation elements coacting with the slit diaphragm, each element being adapted to in­ fluence a sector of the scanning X-ray beam under the control of signals generated by a radiation detector device. The ra­ diation detector device is disposed between the body being irradiated and the X-ray detector. The radiation detector device comprises at least one radiation detector extending at any moment into the scanning X-ray beam passed through the body and moving in synchronism with the scanning movement of the X-ray beam. The radiation detector device is divided into sections corresponding with the sectors of the scanning X-ray beam influenceable by the attenuation elements, while, in operation, each section of the radiation detector device gen­ erates a signal usable for controlling the attenuation elements.

Description: The present invention relates to an apparatus for slit radiography comprising an X-ray source by means of which a body being irradiated can be scanned through a slit diaphragm by a substantially planar, fan-shaped X-ray beam, while the scanning X-ray beam passed through the body impinges upon an X-ray detector. Such an apparatus is disclosed in NL-A-8400845 published on October 1, 1985. The apparatus for slit radiography as disclosed in NL-A-8400845 comprises slit control means, which during the scanning movement of the X-ray beam instantaneously and simultaneously adjust the quantity of X-rays passed per section of the slit diaphragm. The slit control means in turn are controlled by means of electric signals, which are generated by detection means positioned adjacent the X-ray detector. The slit control means are designed to adjust per section of the slit diaphragm the width of the slit and/or the transparency of the slit for X-rays. This may be done by means of moving attenuation elements into the slit of the slit diaphrahm, which attenuation elements are made from or provided with an X-ray absorbing and/or even blocking material like lead. In a first embodiment disclosed in NL-A-8400845 the attenuation elements are slidable elements, which will move in a direction transverse to the longitudinal direction of the slit under the control of the electric signals generated by the detection means. In another embodiment disclosed in NL-A-8400845 the attenuation elements are tongue-shaped elements one end of which may be Pivoted into the X-ray beam passing through the slit. The tongue-shaped elements nay be piëzo-electric tongues which may be clad with an X-rays absorbing material. NL-A-8400845 further describes several methods of generating control signals required for the attenuation elements. In most cases described in Dutch patent application NL-A-8400845, use is made for that purpose of a series of light detectors placed at the exit side of the X-ray detector device employed; each light detector corresponds with a section of the slit diaphragm and controls, through control means, the attenuation element cooperating with said section, or a group of attenuation elements cooperating with said section. However, this technique cannot be applied in a simple manner when a lighttight X-ray film cassette is employed as X-ray detector. Consequently, it is an object of the present invention to design an apparatus for slit radiography comprising controllable attenuation elements positioned along a slit diaphragm in such a manner that, even when a lighttight X-ray film cassette is employed, the control signals required for the attenuation elements are obtained in a relatively simple and reliable manner. To this effect according to the present invention, an apparatus of the above described type is characterized in that it comprises controllable attenuation elements coacting with the slit diaphragm, each element being adapted to influence a sector of the scanning X-ray beam under the control of signals generated by a radiation detector device; that the radiation detector device is disposed between the body being irradiated and the X-ray detector; that the radiation detector device comprises at least one radiation detector extending at any moment into the scanning X-ray beam passed through the body and moving in synchronism with the scanning movement of the X-ray beam. The radiation detector device is divided into sections corresponding with the sectors of the scanning X-ray beam influenceable by the attenuation elements, while in operation, each section of the radiation detector device generates a signal usable for controlling the attenuation elements. It is observed that the present invention can be used both in case a lighttight X-ray film cassette and in case any other type of X-ray detector is employed. Reference is made to FR-A-2116168 disclosing an apparatus for obtaining an equalized X-ray shadow picture. The apparatus is of the conventional type and not of the slit radiography type and in order to obtain an equalized X-ray shadow picture a two-dimentional radiation absorbing compensating filter device is used. This filter device compensates for the spatial variations of the X-ray absorption of a patient. However the filter device cannot be changed during the actual making of an X-ray picture to adapt it to a momentaneous local absorption value encountered when a patient is scanned by an X-ray beam. In one of the methods for making a suitable compensating filter device indeed a kind of slit diaphragm and a scanning method are used. The X-ray picture itself however is made after removal of the slit diaphragm in a conventional manner. The known method therefore is a two-step method. Further reference is made to DE-A-2351473, which discloses an apparatus for slit radiography in which an X-ray dose detector is placed between the body to be examined and an X-ray film. The dose detector provides a signal which is used to control the X-ray dose used in scan the body. This is done by controlling the speed of scanning, control of the energization of the X-ray source or by means of a controllable diapfragm. The dose detector however is not divided in sections nor are means disclosed for controlling a slit diaphragm per sector thereof. Some embodiments of the apparatus according to the present invention will now be described, by way of example, with reference to the accompanying drawing, in which: Fig. 1 diagrammatically shows a first embodiment of an apparatus according to the present invention; Fig. 2 diagrammatically shows a variant of Fig. 1; Fig. 3 shows another variant of Fig. 2; Fig. 4 shows a modification of Fig. 3; and Fig. 5 shows a modification of Fig. 4. Fig. 1 diagrammatically shows a first embodiment of the present invention, showing an X-ray source 1 having an X-ray focal point 2. The X-ray source, in operation, generates an X-ray beam B, a substantially planar fan-shaped portion B' of which in principle can pass through the slit S of a slit diaphragm 3. Furthermore, the figure shows a body 4 being irradiated and an X-ray detector 5 placed behind said body, said detector 5 being positioned in a casing, not shown. The X-ray detector may be of any conventional type, but is formed in the present embodiment by a lighttight X-ray film cassette containing an X-ray fluorescent screen 6 and an X-ray film 7 placed therebehind. In certain X-ray film cassettes, there is arranged behind the X-ray film a second X-ray fluorescent screen. Instead of such a film cassette, e.g. also a stationary X-ray image intensifier tube with large entrance screen or an X-ray image intensifier tube with a strip-like entrance screen can be employed. In the latter case, the X-ray image intensifier tube, in operation, performs a scanning movement. In the situation shown, the X-ray beam B' passed through the slit diaphragm only irradiates a strip-like part of the body 4, which is imaged on the entrance screen of the X-ray detector, as shown at 10. In order to obtain an image of a larger part of the body, the X-ray beam is adapted to effect a scanning movement in the plane of drawing. This can be effected in different manners not forming part of the present invention, e.g. by having the slit diaphragm perform a movement indicated by an arrow 8. The slit diaphragm coacts with a plurality of controllable attenuation elements 9. The attenuation elements are juxtaposed and may be disposed e.g. on one of the edges in such a manner that the attenuation elements can extend, under the influence of suitable control signals, to a greater or lesser extent, into the beam B' passed or to be passed through the slit S. Thus, the X-ray beam can be attenuated instantaneously and locally to a greater or lesser extent, as described in European patent application EP-A-85200377.1 It is observed that the slit of the slit diaphragm may occupy a horizontal, vertical or intermediate position, while the scanning is effected in vertical, horizontal or intermediate direction. The attenuation elements should be controlled in dependence on the intensity of the X-rays passed through the body. According to the present invention, use is made therefor of a radiation detector device provided between the body 4 being irradiated and the X-ray detector, said radiation detector device being arranged in such a manner that the radiation passed through the body at any moment can be detected, i.e. for each sector of the X-ray beam passed through the body separately, corresponding with an attenuation element 9 or with a group of such attenuation elements. According to the present invention, use is made therefore of a radiation detector device performing between the body and the X-ray detector a scanning movement in synchronism with the scanning movement effected, in operation, by the X-ray beam B'. To this effect, the radiation detector device is coupled mechanically or electromechanically to the means effecting that the X-ray beam performs a scanning movement. The radiation detector device may e.g. be placed before the X-ray detector 5 and, in operation, perform a vertical scanning movement. In the embodiment shown in Fig. 1, the radiation detector device is attached to an arm 11 extending from the slit diaphragm in the direction of the X-ray detector. The arm, as indicated by an arrow 12, is adapted to swivel relatively to a real or a virtual swivelling axis extending perpendicular to the plane of drawing through the X-ray focal point 2. The arm is at any moment beyond the X-ray beam passed through the slit diaphragm. The arm 11 may be suitably affixed to the slit diaphragm, as shown in Fig. 1, so that synchronism of the arm movement with the movement of the slit diaphragm (and hence also the scanning movement of the X-ray beam) is ensured. Such a fixed connection of the arm to the slit diaphragm, however, is not essential. Of importance is only that the arm moves in synchronism with the X-ray beam B'. The arm extends beyond the body being irradiated and is provided at the end extending beyond the body with a depending arm 13, which carries an X-ray fluorescent screen 14 at the end distal from the arm 11. The X-ray fluorescent screen 14 extends at least partly at any moment into the X-ray beam passed through the body and forms the radiation detector proper. The depending arm 13 also carries a series of juxtaposed lenses 15. The number of lenses 15 corresponds with the number (groups of) attenuation elements. The lenses are mounted on the depending arm 13 in such a manner that the lenses are present at any moment beyond the X-ray beam passed through the body. Each lens is adapted to collect the light generated in a given sector of the X-ray fluorescent screen under the influence of X-radiation impinging upon the X-ray fluorescent screen. Said sectors correspond again with the sectors of the X-ray beam B' passed or to be passed through the slit diaphragm and influenceable by an attenuation element 9 or a group of attenuation elements. Each lens is adapted to bundle the light collected on a corresponding signal transmitter 16 likewise attached to the assembly of arms 11, 13, e.g. a photodetector transmitting an electric signal corresponding with the luminance of the light. Said electric signal is applied via a line 17 to a control device 18 shown diagrammatically. Control device 18 forms from the signals received control signals for the attenuation element corresponding with the light detector concerned. The fluorescent screen 14, in the embodiment shown, is mounted at an angle to the scanning X-ray beam, so that a relatively high light output is obtained with a relatively thin screen. However, this is not essential. Naturally, the fluorescent screen should be designed so as to minimally attenuate the scanning X-ray beam. The X-ray fluorescent screen 14 may be a screen extending in a direction transverse to the plane of drawing, but if a better separation between the various sectors of the X-ray beam is desired, the screen may also consist of a plurality of juxtaposed but mutually separated screen portions. Fig. 2 diagrammatically shows a variant of the radiation detector device shown in Fig. 1, wherein the arms 11, 13 or similar, corresponding members have been omitted. The scanning X-ray beam passed through the slit diaphragm is indicated in Fig. 2 at 20. Before impinging upon the X-ray film cassette 7 or another imaging X-ray detector, the X-ray beam 20 passes a series of juxtaposed scintillation crystal elements 21, one being shown in Fig. 2, for which may be used e.g. germanium iodide crystals, which produce light under the influence of X-radiation. Each scintillation crystal element is coupled optically to a photodetector 22 producing, in response to the associated scintillation crystal element, an electric signal which can be used again for forming a control signal for a corresponding attenuation element or a corresponding group of attenuation elements. To increase the effective light output of the scintillation crystal elements, each scintillation crystal element, preferably on all sides, except the photodetector-coupled side, is provided with a layer reflecting on the inside. Such a layer at the same time protects the scintillation crystal element against external influences, such as the action of moisture, which is of relevance, since scintillation crystals mostly have hygroscopic properties. Fig. 3 shows a second variant of the radiation detector device according to the present invention. The scanning X-ray beam is again indicated at 20. Instead of a series of scintillation crystal elements, there is now employed a series of juxtaposed ionization chambers 30, one being shown diagrammatically. Each ionization chamber comprises two electrodes 31, 32. There prevails a voltage differential between the electrodes. When the ionization chamber is irradiated by energy-rich radiation, such as X-radiation, this will result in a current flowing in the electric circuit connected to the electrodes 31, 32, from which, again, control signals for one or more corresponding attenuation elements can be derived. Naturally, at least the front and rear wall, as viewed in the direction of the scanning beam 20, should be made of material not or hardly attenuating X-radiation. Instead of a plurality of separate ionization chambers, also a single elongate ionization chamber can be used which is provided with one common electrode extending along the entire length of the ionization chamber, i.e. transversely to the plane of drawing,and a plurality of separate electrodes from which electrodes the control signals for the corresponding attenuation elements are derived. Such an elongate ionization chamber is shown in Fig. 4 and indicated at 40. The common electrode is indicated at 41 and the electrodes opposite the common electrode are indicated at 42a....e. The common electrode can be considered to be an interconnection of a plurality of separate electrodes. Such an interconnection can be positioned both in the elongate ionization chamber and outside said chamber. A mixed form between the embodiments of a radiation detector shown in Figs. 3 and 4 is shown in top view in Fig. 5. An elongate ionization chamber 50 contains a plurality of partitions 51 of synthetic plastics material, e.g. Mylar®. The compartments between the partitions again form the ionization chamber. The scanning X-ray beam 20 is shown in top view and has the direction indicated by an arrow 52. The partitions are directed preferably at the X-ray focal point 2.

Claims: An apparatus for slit radiography comprising an X-ray source (1) by means of which a body (4) being irradiated can be scanned through a slit diaphragm (B) by a substantially planar fan-shaped X-ray beam (B'; 20), the scanning X-ray beam (B'; 20) passed through the body (4) impinging upon an X-ray detector (5), characterized in that the apparatus comprises controllable attenuation elements (9) positioned along the slit diaphragm (3), each element (9) being adapted to influence a sector of the scanning X-ray beam (B'; 20) under the control of signals generated by a further radiation detector device (14, 15, 16; 21, 22; 30; 40; 50); that this radiation detector device is disposed between the body (4) being irradiated and the X-ray detector (5); that the radiation detector device (14, 15, 16; 21, 22; 30; 40; 50) comprises at least one radiation detector (14; 21; 30; 40; 50) extending at any moment into the scanning X-ray beam (B'; 20) passed through the body (4) and moving in synchronism with the scanning movement of the X-ray beam (B'; 20), the radiation detector device being divided into sections corresponding with the sectors of the scanning X-ray beam (B'; 20) influenceable by the attenuation elements (9), while, in operation, each section of the radiation detector device generates a signal usable for controlling the attenuation elements (9). An apparatus according to claim 1, characterized in that the radiation detector comprises an X-ray fluorescent screen (14) and that the radiation detector device comprises furthermore a plurality of juxtaposed lenses (15) disposed beyond the scanning X-ray beam (B'; 20), each lens (15) being adapted to collect the light generated in a given sector of the X-ray fluorescent screen (14) under the influence of the scanning X-ray beam (B'; 20), and to direct said light at an associated photodetector (16) converting the light signal into an electric signal, for controlling the attenuation elements (9). An apparatus according to claim 2,characterized in that the X-ray fluorescent screen (14) is divided into a plurality of juxtaposed, mutually separated sections. An apparatus according to claim 2, characterized in that the X-ray fluorescent screen (14) extends at an acute angle into the scanning X-ray beam (B'; 20). An apparatus according to claim 1, characterized in that the radiation detector comprises a plurality of juxtaposed scintillation crystal elements (21), each being coupled optically at a side located beyond the scanning X-ray beam (B'; 20) to a photodetector (22) producing an electric signal, for controlling the attenuation elements (9). An apparatus according to claim 5, characterized in that each scintillation crystal element (21) is provided with an internally reflecting layer on the entire outer surface thereof, except the portion to which the photodetector (22) is coupled. An apparatus according to claim 1, characterized in that the radiation detector comprises a plurality of juxtaposed ionization chambers (30), each having two opposed electrodes (31, 32) which, in operation, conduct a current from which a control signal is derived for the attenuation elements (9). An apparatus according to claim 7, characterized in that the ionization chambers (30) are combined to a single elongate ionization chamber (40; 50) having a plurality of spaced apart, first electrodes (42a...e) and a common second electrode (41) An apparatus according to claim 8, characterized in that a partition (51) is placed in the elongate ionization chamber (40; 50) between each pair of neighbouring first electrodes (42a...e). An apparatus according to claim 9, characterized in that the partitions (51) are each directed towards the focal point (2) of the X-ray source (1). All apparatus according to claim 1, characterized in that the radiation detector device (14, 15, 16; 21, 22; 30; 40; 50) is attached to an arm (11) extending from the slit diaphragm (3) in the direction of the X-ray detector (5) to beyond the body (4) being irradiated, said arm (11) being adapted to swivel in a vertical, and horizontal plane, respectively, relatively to a line extending horizontally, and vertically, respectively, through the focal point (2) of the X-ray source (1), and which is provided at the end proximal to the X-ray detector with a depending arm (13) carrying the radiation detector (14; 21; 30; 40; 50).

Inventors: DUINKER, SIMON, VLASBLOEM, HUGO
Applicants: B.V. OPTISCHE INDUSTRIE "DE OUDE DELFT"
Application year: 1985
Main classification: G21K1
IPC codes: H05G1, A61B6, G21K1, G03B42, G01N23, G21K3