IMAGE SCREEN AND A METHOD FOR PRODUCING AN IMAGE SCREEN

Abstract: An image screen contains a material layer (1), which has an entrance surface (1a) for receiving X-rays (R) in X-ray imaging, and on the opposite side of the material layer an exit surface (1b), on which an image that can be picked-up optically is generated. To form the image, there are areas (2) of a material which emits light by the effect of X-ray radiation, which areas (2) are adjacent to each other in the plane of the screen and located between the entrance surface (1a) and the exit surface (1b) in the thickness direction of the material layer. The areas (2) are separated from each other in the plane of the screen to improve spatial resolution. Longitudinal axes (Z) of the areas (2) extending in the thickness direction of the material layer are arranged in varying angles ( alpha ) of inclination with the plane of the screen so that the angles of inclination increase from the middle of the screen towards the edges so that extensions to the longitudinal axes (Z) converge towards the central axis (A). The invention also relates to a method for producing an image screen used for X-ray imaging.

Description: Image screen and a method for producing an image screen The invention relates to an image screen which is of the type presented in the characterizing part of the appended claim 1. The invention also relates to a method according to the preamble of the appended claim 6 for producing an image screen. For example in medical X-ray imaging, a screen containing luminescent material is used to form an image from X-rays. The screen comprises a material layer in which one surface, the entrance surface, faces the source of the X-ray radiation, and the opposite surface, the exit surface, faces an image receptor, which can be an X-ray film or an optical imaging device, i.e. a camera. The luminescent material is capable of converting high-energy radiation to radiation in the wavelength of vis- ible light. There are numerous known luminescent materials, and they differ from each other for example in the wavelength of visible light they produce. In X-ray imaging technique, such a screen is called an intensifying screen, since it in a way "intensifies" the radiation to visible light suitable for imaging methods. Because of its function, the in- tensifying screen can also be called an image screen. In image screens containing an even layer of luminescent material in the image plane of the screen, spatial resolution problems occur due to the fact that light tends to diffuse in the material also in the direction of the plane of the screen. This transverse scattering can be reduced by minimizing the material thickness of the luminescent material in the image screen, but then the intensity of the received light suffers. To eliminate this drawback, various solutions have been suggested, in which the luminescent material is arranged in areas separate from each other on the plane of the screen, between which no light is conveyed in the screen. The result is a kind of a mesh screen. The areas are formed by making point-like holes in the substrate material of the screen with a working technique which requires great precision, after which the holes are filled with the luminescent material. Different structures and their production methods are presented for instance in international publication WO 93/15509, German patent publication 4222946, European patent 126564, and German patent publication 2347923. German application publication 3325035 discloses a mesh-like image screen, similar in its structure to the aforementioned principle, in which all the inlet apertures of the holes filled with luminescent material are shifted in the plane of the screen in the same direction aside from the outlet apertures, to increase the active light surface on the plane of the screen. The screen functions as an input screen for the X-ray image intensifier tube, and the light obtained with it is still electronically intensified for the actual imaging. European patent publication 272581 also discloses an input screen for an X-ray image intensifier tube, which input screen is formed of several superimposed thin subplates, the holes of which can match or be sporadically located with respect to each other. The desired high spatial resolution is achieved with different image screen structures similar to the ones described above, and with the luminescent materials contained in them. The numerous prior art publications discussing image screen structures also reflect the fact that a great deal of effort has been devoted to improving spatial resolution. Until now, however, attention has not been paid to other quality factors of the image generated on the image screen, which factors are important especially in immediate imaging from X-rays. One such factor is the uniform brightness of an image received by the image receptor in all sections of the screen. The purpose of the invention is to eliminate the aforementioned drawbacks and to present a screen, which, in addition to good spatial reso- lution also has a uniform image quality all over the screen. To attain this purpose, the image screen according to the invention is characterized in what will be presented in the characterizing part of the appended claim 1. The purpose of the invention is also to present a method for producing such an image screen, and this method is characterized in the features presented in the appended claim 6. Previously, as in the aforementioned German application publication 3325035, the aim has been to make the spots of luminescent material as regular as possible, wherein they are similar to each other also in the cross-section perpendicular to the plane of the luminescent screen. Thus, the luminescent material is located in the thickness direction of the image screen, i.e. in the area between the entrance surface of X- rays and the exit surface of light in concentrations of same shape, their longitudinal axes being perpendicular to the plane, or at least parallel to each other. Consequently, the radiation patterns emitted from the corresponding two-dimensional luminuous spots according to the principles of physics, are also parallel to each other, which in optical imaging has the result that the luminosity directed from the edge zones of the screen to an imaging device, such as a CCD camera, located at the optical axis, is reduced. Thus, the differences have to be compensated for example electronically in image processing. In the image screen according to the invention, these differences are already compensated in such a way that the areas of luminescent material extending in the depth direction of the screen and located in the edges of the image screen, are inclined more towards the middle to the central axis of the imaging than the sections in the middle of the screen, i.e. the extensions of the longitudinal axes of the areas, and correspondingly, the primary directions of the radiation patterns are situated in varying angles of inclination with respect to the plane of the screen. The acute angle between the extension to the longitudinal axis of the area and the normal in the plane of the screen increases gradually towards the edge of the screen. Thus, the extensions of the longitudinal axes, in a way, form a fan convergent towards the central axis of the imaging. Furthermore, the appended dependent claims 2 to 5 present advantageous alternatives for the structure of the screen. The extensions to the longitudinal axes of the areas can be convergently directed towards the middle also on the side of the entrance surface of X-rays, which provides advantages that will be described hereinbelow. According to the method of the invention, the areas of luminescent material extending in the thickness direction of the screen are formed at different angles at the production stage of the screen. The appended dependent claims 7 to 11 relating to the method, describe several advantageous alternatives to implement this. For example, it is possible to direct the working beam that generates the areas from the same source towards the substrate material of the screen, and by turning the direction of the beam it is possible to set the longitudinal axes of the areas in angles of inclination differing from each other and corresponding to the turning angle. In the following, the invention will be described in more detail with reference to the appended drawings, in which Fig. 1 shows a detail of an image screen of prior art in a cross- section, Fig. 2 shows a detail of the image screen according to the in- vention in a cross-sectional view and in a front view, Fig. 3 shows a second embodiment of the image screen, Fig. 4 shows a third embodiment of the image screen, Fig. 5 illustrates the use of an image screen in an X-ray apparatus, and Fig. 6 illustrates a production method according to the invention. Fig. 1 presents in a cross-section a known image screen for improving spatial resolution. The image screen contains a material layer 1 , in which, in the area between an entrance surface 1a and an exit surface 1b there are holes filled with a luminescent material, which run in the thickness direction of the screen and form area 2 of luminescent material, which are separated from each other and form a two- dimensional mesh of pixels in the plane of the exit surface 1b. Dashed lines illustrate the radiation patterns of visible light which is emitted from the pixels, and the primary directions of these patterns, i.e. the di- rections of highest intensity are parallel to the longitudinal axes Z of the areas. The longitudinal axes, in turn, are parallel to each other and typically perpendicular to the plane of the screen. Fig. 2 presents an image screen according to the invention, in whose material layer 1 there is an entrance surface 1a for receiving X-rays R in X-rayi imaging, and on the opposite side of the material layer there is an exit surface 1b, in which an image that can be picked-up optically is formed. In this context, an image screen refers to a screen in which the exit surface 1b of the image is free, i.e. the image formed on it can be received with an image receptor based on visible light, such as a photographic film or a camera, and the surface is not covered with photo- cathode as in electronic X-ray image intensifier tubes. The material layer 1 is a straight, planar piece which forms an essential part of the image screen. In the plane of the screen there are areas of a material which emits light by the effect of X-ray radiation, i.e. areas 2 a of luminescent material, which are placed next to each other and extend in the thickness direction of the screen between the exit surface 1b and the entrance surface 1a. For the above-described purpose, the areas are separated from each other in the plane of the screen. The longitudinal axes Z of the luminescent material running in the thickness direction of the material layer 1 are arranged in varying angles of inclination with the plane of the screen and material layer, so that the angles of inclination increase from the middle of the screen towards the edge of the screen. The extensions of the longitudinal axes Z, which run via pixels P formed by the areas 2 on the exit surface 1b, converge on the side of this surface towards the central imaging axis A, i.e. they cut this central axis in different angles. The central imaging axis A can be the optical axis of an imaging device equipped with lens optics and an image pick-up surface on the imaging plane of the lens optics. The imaging device is especially a camera which immediately forms the image electronically, such as a CCD camera. The differences in intensity seen by the imaging device which are due to the structure of the image screen now ready-compensated so that the areas 2 of luminescent material located on the edges of the image screen 1 in the thickness direction of the screen incline more towards the middle and the central imaging axis A than the areas 2 in the middle of the screen. The extensions to the longitudinal axes Z of the areas, and correspondingly the main directions of radiation patterns L are situated in varying angles of inclination with respect to the plane of the screen. The acute angle between the extension to the longitudinal axis Z of the area and the normal N to the plane of the screen, i.e. the angle of inclination α gradually increases towards the edge of the screen. Thus, the extensions to the longitudinal axes, in a way, form a fan convergent towards the central imaging axis A. The areas 2 of luminescent material are formed of microscopically small holes made in the thin, planar substrate material forming the material layer 1 to extend in the thickness direction of the material. The holes are filled with luminescent material. The remaining substrate material functions as walls between the areas 2 of luminescent material, separating them from each other in the direction of the plane. The diameter of the holes is smaller than 100 μm, generally in the range of 40 to 80 μm. The result is typically over 10,000 holes, i.e. pixels, per square centimetre. The substrate material can be opaque, or poorly permeable to light, or the inner surfaces of the holes formed in it can be coated to prevent the transmission of light. The production of the image screen will be described in more detail hereinbelow. It should be noticed that in Fig. 2, the differences in the angles of in- clination of the adjacent areas 2 are exaggerated for illustrative purposes. In practice, the differences grow gradually in the image screen, and the differences in the angles of inclination α of the adjacent areas are extremely small. The differences become distinct only when comparing the areas 2 of luminescent material, which are located at a greater distance from each other on the straight line running from the centre of the screen 1 to the edge. The areas 2 of luminescent material incline symmetrically with respect to the central imaging axis A and the angles of inclination increase in every direction when moving from the middle to the edges in the two-dimensional plane of the screen. Fig. 3 presents another embodiment of the image screen within the scope of the invention. In Fig. 2, the extensions of the longitudinal axes Z converged towards the front of the exit surface 1b, whereas in this embodiment they converge towards the front of the entrance surface 1a. Thus, the extensions to the longitudinal axes Z of the areas 2, directed to the side of the entrance surface 1a, converge towards the central axis A of X-ray radiation R, i.e. towards the central axis of the fan of X-rays emitted from the X-ray source. The angles of inclination α of the areas are thus arranged to correspond to the angle of incidence of the X-rays on the entrance surface 1a of the material layer 1. The effect achieved with this is that in practice, only the beams that have come directly from the X-ray source and passed through the object to be X-rayed, are allowed to excite the luminescent material, and the effect of radiation coming from a direction other than the X-ray source, i.e. scattered radiation, remains insignificantly small. Thus, it is even possible to exclude the grid before the image screen, which is typically used to remove scattered radiation. Fig. 4 presents a third embodiment, which, in a way, combines the features of the embodiments of Figs. 2 and 3. On the side of the entrance surface 1a of the material layer 1 , the areas 2 turn gradually to a larger angle with the central axis A of the X-ray radiation from the middle towards the edges, and on the side of the exit surface 1b, the areas 2 turn in a corresponding way with respect to the central imaging axis A. In practice, this structure is achieved with two sub-layers 3a and 3b, in both of which the areas 2 are directed as desired. By placing the sub-layers together in such a way that their reverse sides are against each other, it is possible to make the areas 2 form continuous areas containing luminescent material in the thickness direction of the screen, in which areas the parts on the side of the entrance surface 1a are formed of areas of the material layer 3a in the entrance surface 1a, and the parts on the side of the exit surface 1b are formed of the areas of the material layer 3b in the exit surface 1b. As presented in Fig. 4, the areas do not have to be fully aligned with respect to each other, because the spatial resolution is achieved also when the areas overlap each other only partially. Furthermore, it is possible to use two or more superimposed material layers, in which the change in the inclination of the areas takes place in the same direction, i.e. for example two material layers 1 of Fig. 2 superimposed on each other. Also in this case it is not necessary for the areas 2 to be aligned. Fig. 5 presents the location of an image screen F in an X-ray apparatus. The apparatus in question implements immediate X-ray imaging, and comprises an X-ray source 4 emitting X-ray radiation R which in medical X-raying passes through an object 5 to be X-rayed, for example a part of the body of a human being or an animal, and produces a visible image on the image screen F. A dashed line 6 illustrates a grid between the object and the image screen F, which grid prevents the radiation scattered from the object from entering the image screen and impairing the resolution of the picture. On the other side of the image screen, opposite to the exit surface 1a there is an optical imaging device 7, such as a CCD camera, whose imaging optics produces an electronically processable image onto its image surface from the image visible on the exit surface 1a of the image screen. The image surface is smaller than the wide image surface of the image screen. Thus, light is not electronically intensified in the imaging before the image surface of the imaging device. As can be seen in Fig. 5, the central axis A of X-ray radiation and the central imaging axis A, i.e. the optical axis of the camera, coincide, i.e. they are on the same straight line parallel to the normal of the image screen F. The areas 2 are preferably orientated in such a way that on the side of the exit surface 1b, the extensions of their longitudinal axes Z are directed towards the imaging device 7, most advantageously towards the aperture of the same, and on the side of the entrance surface 1a, they are directed towards the X-ray source 4. The extensions to the longitudinal axes of the areas can meet at the same point in the central axis A. Even though in the figure the imaging device is facing straight the image screen F, it is also possible to use an arrangement in which the optical axis A runs via a mirror placed between the imaging device 7 and the image screen F, as presented, for example, in the international publication WO 95/27922, which describes immediate imaging from X-rays. The image screen is also suitable for use in X-ray tomography, where the arrangement differs from the imaging arrangement of Fig. 5. It is also possible to place a photographic film against the exit surface 1 b of the image screen F. This alternative is feasible especially in the embodiment of Fig. 3, in which the areas 2 are parallel to X-ray radiation R. Fig. 6 illustrates method for producing an image screen. Areas 2 are formed by directing a working beam W from a radiation source S towards the surface of a substrate material 1. By turning the beam emitted from the radiation source with respect to the plane of the substrate material, it is possible to generate areas varying in their direction. The alterations obtained in the angles of inclination α of the extensions to the longitudinal axes Z with respect to the plane of the material are the same as the respective momentary angle α between the working beam, which has generated them, and the normal of the plane of the material. Thus, the radiation source S can be placed to turn with respect to the material in different angles in such a way that, viewed from the direction of the substrate material 1 , the beam W comes from the same fixed point which corresponds to the point on the front side of the finished screen at which the extensions to the longitudinal axes Z of the areas meet. To process the areas, a suitable high-energy beam is used, which is at the same time capable of producing microscopically small holes in the substrate material to be included in the image screen. One example that could be mentioned is the excimer laser. As for the substrate material, it is possible to use a suitable plastic material, for example polyester (PET), polyimide or polycarbonate. Before working, a photo- mask is placed on top of the substrate layer to ensure that only the desired locations in the substrate, i.e. the future locations of the holes are exposed to the radiation. The photomask can be used several times. The thickness of the substrate material to be processed can vary, and when working the holes all the way through, its thickness can be for example ca 100 — 500 μm. There are also other methods for processing the substrate, such as photo etching methods known as such, in which the sections defined by a mask in a special photosensitive photo etching material are exposed to UV radiation, and unirradiated or irradiated sections, depending on the type of the photosensitive material are etched off in a chemical treatment. The light source from which the working beams are emitted, is thus point-like, and the mask can be arranged directly on the surface of the substrate material by coating it with a thin copper layer and with a photosensitive membrane on top of that, in which it is possible to arrange optically a pattern required by the mask, to cause the copper to be selectively removed so that the mask remains on the surface of the substrate in a copper pattern. As the luminescent material for the areas 2, it is possible to use a suitable substance which, excited by X-ray radiation, emits light. One commonly used group is formed by the yttrium, lanthanum, or gadolinium oxy-sulphides activated with terbium, and yttrium or gadolinium oxy-sulphides activated with europium. Gd-oxy-sulphide (Gd 2 0 2 S:Eu 3+ ) activated with europium is well suited for CCD imaging, due to red emission. The materials can be applied in the holes in the form of a paste produced together with a binding agent and a solvent with known technical methods. In addition to worked substrate material, the image screen can contain other layers, such as on the entrance side of the X-ray radiation a thin metallization against the layer of substrate material filled with lu- minescent material, which metallization is produced for example by sputtering and is permeable to X-ray radiation but reflects visible light and prevents its escape to the wrong direction. Similarly, on the entrance side of the X-ray radiation, it is possible to use a support layer permeable to X-ray radiation to give mechanical strength to the screen. With the support layer, it is also possible to cover the holes, wherein they can be metallized on the inside, for example by sputtering, before filling with the luminescent material. The light exit side can be provided with a thin, transparent protective layer. In such multi-layer screens, the entrance surface 1a and the exit surface 1b refer to the interface between the substrate material layer and another layer, which the X-ray radiation hits, or from which light is emitted correspondingly. In the figures, the holes 1 extend through the substrate material, but they can also have a closed bottom. This can be achieved, for example, with the photo etching method. Also such holes can be metallized on the inside with a very thin metal layer. In this case, the layer of substrate material can also be thicker than usual. In all working methods, the holes are usually slightly conical in such a way that the entering end of the working has a larger diameter. Furthermore, the areas 2 can be formed by another principle, for example by using a beam to work grooves in the layer of substrate material containing luminescent material throughout, such as a scintillation plastic screen, the grooves enclosing areas containing luminescent material. Thus, the air gaps formed by the grooves prevent light from spreading in lateral direction, and they extend in the thickness direction of the screen and have varying angles of inclination in different areas of the plane of the screen, according to the above- presented principle. It is also possible to metallize said air gaps, for example by sputtering, to increase the blocking effect.

Claims: Claims: 1. Image screen containing a material layer (1), which has an entrance surface (1a) for receiving X-rays (R) in X-ray imaging, and on the opposite side of the material layer an exit surface (1b), on which an image that can be picked-up optically is generated, wherein to form the image there are areas (2) of a material which emits light by the effect of X-ray radiation, which areas (2) are adjacent to each other in the plane of the screen and located between the entrance surface (1a) and the exit surface (1b) in the thickness direction of the material layer and separated from each other in the plane of the screen to improve spatial resolution, characterized in that longitudinal axes (Z) of the areas (2) extending in the thickness direction of the material layer are arranged in varying angles of inclination (α) with the plane of the screen so that the angles of inclination increase from the middle of the screen towards the edges so that extensions to the longitudinal axes (Z) converge towards the central axis (A) of imaging. 2. Image screen according to claim 1 , characterized in that on the side of the exit surface (1b), the extensions to the longitudinal axes (Z) converge towards the central axis (A) of optical imaging. 3. Image screen according to claim 1 , characterized in that on the side of the entrance surface (1a) the extensions of the longitudinal axes (Z) converge towards the central axis (A) of X-ray radiation. 4. Image screen according to any of the claims 1 to 3, characterized in that the extensions to the longitudinal axes (Z) extend to a common point outside the screen. 5. Image screen according to any of the claims 1 to 4, characterized in that the areas (2) of light-emitting material are formed of holes made in the substrate material forming the material layer (1), which holes are filled with a light-emitting material, wherein the areas (2) are separated from each other with partition walls between the holes. 6. Method for producing an image screen used for X-ray imaging, which image screen contains a material layer (1), which has an entrance surface (1a) for receiving X-rays (R) in X-ray imaging, and on the opposite side of the material layer an exit surface (1b), on which an image that can be picked-up optically is generated, wherein the screen is divided into areas (2) separate from each other in the plane of the screen, which in the thickness direction of the material layer (1) between the entrance surface and the exit surface contain material which emits light by the effect of X-ray radiation, characterized in that longitudinal axes (Z) of the areas (2), extending in the thickness direction of the material layer, are formed in varying angles of inclination so that the extensions to the same converge towards the same central axis (A) of the imaging. 7. Method according to claim 6, characterized in that the areas (2) are formed by directing a working beam (W) from a radiation source (S) towards the surface of the substrate material forming the material layer (1), and by turning the beam (W) emitted from the radiation source with respect to the plane of the substrate material, areas (2) varying in their longitudinal axis (Z) are formed, wherein the angles of inclination (α) of the longitudinal axes with respect to the normal of the plane of the material correspond to the respective momentary turning angle (α) of the working beam which has generated them with respect to the normal of the same plane. 8. Method according to claim 7, characterized in that the working beam (W) is directed to the material from the side of the entrance surface (1a). 9. Method according to claim 7, characterized in that the working beam (W) is directed to the material from the side of the exit surface (1b). 10. Method according to claim 7, characterized in that in the first substrate material (3a) areas (2) are formed with the working beam (W), and in the second substrate material (3b) areas (2) are formed with the working beam (W), after which the materials (3a, 3b) are placed against each other so that the surfaces of the materials which were on the side of the working beam form an entrance surface (1a) and an exit surface (1b) on the material layer (1) formed of the materials joined together. 11. Method according to claim 7, characterized in that the surface of the substrate material is provided with a mask defining the location of the areas in the plane of the screen, which mask allows the working beam (W) to pass to particular locations in the material and protects the other locations from the effect of the working beam.

Inventors: SALMI MATTI, VARJONEN VESA, SALMI, MATTI, VARJONEN, VESA
Applicants: IMIX AB OY, SALMI MATTI, VARJONEN VESA, OY IMIX AB, SALMI, MATTI, VARJONEN, VESA
Application year: 1998
Main classification: G21K4
IPC codes: G21K1