PHOSPHOR PLATE AND METHOD FOR MANUFACTURING THE PHOSPHOR PLATE

Abstract: An accelerated phosphor plate and a reader having high accuracy at the time of reading when the plate is irradiated with an excited beam and high spatial resolution. The plate is provided with micropores (26) each having a wall (2) impervious to excited light and filled with an accelerated phosphor (6), wherein the micropores are provided at each of the intersections of crossing directions.

Description: The present invention relates to a method of manufacturing a photostimulable phosphor plate and a photostimulable phosphor plate so manufactured. Radiation images such as X-ray images are often used for medical diagnosis. In order to obtain such an X-ray image, a so-called radiation photograph has been used. In this case, a phosphor layer (fluorescent screen) is irradiated with X-rays which have been transmitted through an object, a visible light beam is generated therefrom, and a film using silver salt is then irradiated with such visible light beam for the purpose of development. Thus, an "indirect" radiation image is obtained, using visible light generated from X-rays. For replacing X-ray imaging apparatus which obtains a bidimensional radiation image, either directly or indirectly, on a prior art film on which a silver salt photosensitive material is coated like a sheet, a high sensitivity and high resolution X-ray imaging system has been proposed. A highly sensitive and high resolution X-ray imaging apparatus as mentioned above is designed as a system utilising photostimulable phosphor. The basis system of this apparatus has been described in detail in U.S. Patent No.US-A-3,859,527. The phosphor used in this system stores a part of energy which it receives in the form of radiation, such as X-rays. This condition is comparatively stable and can therefore be maintained for a while or for a long period of time. However, when the phosphor under this condition is irradiated with a first light beam, working as an excitation light beam, the stored energy is emitted as a second light beam. For the first light beam, light having a wavelength in a wide range from infrared to ultraviolet can be used as well as visible light, depending on the phosphor material used. The light of the second light beam may also occur over a wide range of wavelengths from infrared to ultraviolet, again depending on the phosphor material used. The terms 'light' and 'light beam' as used herein thus comprehend not only visible light but also light outside the visible range. The second light beam can be received and converted into an electrical signal by a photoelectric converter, and then converted into a digital signal and thereby digital image information can be obtained. A photostimulable phosphor layer which has been used in the prior art is not transparent for the first light beam, namely the excitation light beam, and the second light beam, namely the photostimulated emitted light beam, and a distinctive scattering phenomenon has been manifest. Thus, even when such photostimulable phosphor layer is irradiated with an excitation light beam flux of a size corresponding to one pixel or to less than one pixel, the excitation light beam flux is scattered very widely. It has been observed that when a phosphor layer for example of a thickness of 0.3 mm is irradiated with an excitation light beam flux of a diameter of 0.1 mm, the flux is scattered, at the surface of the layer opposite to that upon which the excitation light beam is originally incident, up to a size larger than 1 mm in diameter, and in some cases up to a size larger than 3 mm in diameter. Fig. 1 shows such scattering conditions. As a result of such scattering of light beam flux, if one pixel has a size of 0.1 mm square in this case, a part of the information from 100 to 900 adjacent pixels is undesirably detected when the one pixel is read, and thereby the space resolution of the image obtained suffers significant deterioration and the image is naturally defocused. In attempts to alleviate the problem of scattering of the excitation light beam, several methods have been proposed. For instance, a method for decomposing white fine particles in the phosphor layer is described in Japanese Laid-open Patents Nos. 55-146447 and 58-58500, a method for adding a colouring agent which absorbs the excitation light beam is described in Japanese Laid-open Patent No. 61-170740, and a method for forming a colouring agent or white fine particles on a supporting substrate for the photostimulable phosphor is described in Japanese Laid-open Patent No. 62-211600. These methods have been used in attempts to improve image sharpness for an intensifying screen used with an X-ray film in the prior art. However, it is clear that these methods cannot remove the scattering of the excitation light beam. Moreover, a method for forming cracks in the vertical direction in the photostimulable phosphor layer, or for forming a honeycomb structure, is disclosed in Japanese Laid-open Patent No. 60-171500 - see also EP-A-0 126 564. It has also been attempted to prevent scattering by forming a pattern of projected and recessed areas, or a mosaic pattern, on a substrate surface. However, neither can these methods prevent the scattering of an excitation light beam and they still afford the possibility of Moire pattern formation on an image obtained. The present invention has been proposed, under the technical background as explained above, to provide a photostimulable phosphor plate which exhibits no substantial scattering effects with regard to emitted, fluorescent/photostimulated light generated in response to an excitation light beam. According to the present invention there is provided a method of manufacturing a photostimulable phosphor plate having therein an array of fine holes, all of substantially the same size and with photostimulable phosphor therein, which phosphor is stimulable by excitation light to emit stimulated light, the wall surfaces of the holes being non-transmissive for or not penetrable by such excitation light, the method comprising:- etching fine holes in a plurality of metal sheets, stacking the plurality of metal sheets to provide the body of the plate, with the etched holes in the sheets being aligned so as to provide said array of fine holes, burying photostimulable phosphor material in the etched holes, providing that one surface of the body of the plate is reflective, providing a transparent protective layer at the opposed surface of the body of the plate. The present invention also provides a photostimulable phosphor plate manufactured by a method as set out above, wherein the array of fine holes is in the form of a lattice, with lines of holes aligned in a first lattice direction, corresponding to a sub-scanning direction for scanning of the plate by a beam of excitation light when the plate is in use, and lines of holes aligned in a second lattice direction, corresponding to the main scanning direction for such scanning, the second lattice direction being at an angle to a direction orthogonal to the first lattice direction such that from one end to the other of a line of holes extending in the second lattice direction, those ends are displaced one from the other in the first lattice direction by ▵ = b η, where b is the reading or scanning pitch in the sub-scanning direction, which corresponds to pixel size in that direction for an image to be produced on the basis of stimulated light, and η is the scanning efficiency of such scanning of the plate by the beam of excitation light. In an embodiment of the present invention, a photostimulable phosphor plate is provided, which has fine holes with photostimulable phosphor buried in the holes, between remaining portions of the plate which surround and, hence, form the holes. These hole forming portions 2 of the plate are such as not to be penetrated by an excitation light beam. The holes are regularly positioned, at respective crossing positions of arrays of imaginary lines extending in crossing directions, and are all of substantially the same size. Light transmissive sealing material may be provided at the light penetrating side of the hole forming portions (i.e. at the side towards which the excitation light beam will be directed), and sealing material provided at the opposed side. Phosphor embedded in the hole forming portions is sealed by the sealing materials. In an embodiment of the present invention the positions of adjacent holes may be deviated only by the value required for approximation. In an embodiment of the present invention the fine holes may be arranged as in a matrix, with the arrangement of fine holes in one direction of the matrix matching with a subscanning direction of excitation light beam scanning and also matching, in the main scanning direction, with a straight line in accordance with the relationship, Δ = b - b(1 - η), wherein the line reading pitch, which is equal to pixel size in the subscanning direction is assumed to be b, scanning efficiency of excitation light beam scanning is η, and the deviation in the subscanning direction between the starting and ending points of fine holes on one read line is ▵. A digital X-ray apparatus is disclosed, wich forms a latent image of an object on a photostimulable phosphor plate as an energy distribution pattern using X-ray energy, and which reads such latent image using an excitation light beam, such that each pixel is individually formed, when forming the latent image of the object, and corresponds to one or a larger integer number of fine holes of a photostimulable phosphor plate, which plate has, at crossing positions in crossing directions, such fine holes, of almost the same size, burying photostimulable phosphor in hole forming portions processed at least on a substrate which is not penetrated by an excitation light beam, and respective pixels, accumulated in one or more such fine holes, are read out for recovery of the image data. A digital X-ray apparatus which forms a latent image of a specimen on a photostimulable phosphor plate as an energy distribution pattern using the X-ray energy, and which reads such latent image using an excitation light beam may comprise a means for scanning the photostimulable phosphor plate, which is provided with fine holes burying photostimulable phosphor in a hole forming portion which is not penetrated by the excitation light beam and exhibits reflectivity for the excitation light beam at the plate surface, other than where the fine holes are formed, in the direction of hole arrangement with the excitation light beam; an emitted fluorescent light gathering means for gathering the fluorescent light emitted in response to the excitation light beam from the photostimulable phosphor buried in the fine holes; a reflected excitation light beam gathering means for gathering the excitation light beam reflected from the photostimulable phosphor plate; an excitation light beam irradiation period detecting means, for detecting periods when the excitation light beam is being irradiated on to the fine holes with the photostimulable phosphor, from the signal obtained from the reflected excitation light beam gathering means; and a means for sampling the emitted fluorescent light obtained by the photostimulable fluorescent light gathering means as pixel information during such periods detected by the excitation light beam irradiation period detecting means. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a diagram for explaining scattering of an excitation light beam at the photostimulable phosphor of the prior art; Figs. 2A and 2B are sectional views of a photostimulable phosphor plate; Fig. 3 is a diagram for explaining scattering of an excitation light beam at the photostimulable phosphor shown in figure 2A; Figs. 4A to 4F are diagrams illustrating the manufacture of a photostimulable phosphor plate in accordance with an embodiment of the present invention; Figs. 5A and 5B are diagrams illustrating a photostimulable phosphor plate provided with fine circular holes in accordance with an embodiment of the present invention; Fig. 6 is a diagram indicating various flat or plan shapes of fine holes; Fig. 7 is a diagram indicating various sectional shapes of fine holes; Fig. 8 shows a photostimulable phosphor plate, in accordance with an embodiment of the present invention, with circular fine holes arranged to have the highest density; Fig. 9 shows a photostimulable phosphor plate in accordance with an embodiment of the present invention; Fig. 10 shows a structure of a digital X-ray reader; Fig. 11 shows a pixel forming profile and indicates numbers of fine holes provided per pixel; Fig. 12 shows a structure of a digital X-ray reader which employs synchronisation with a reflected excitation light beam; Fig. 13 is a circuit diagram relating to synchronisation with the reflected excitation light beam; and Fig. 14 is a time chart relating to synchronisation with the reflected excitation light beam. Fig. 2A shows a photostimulable phosphor plate having therein fine holes 26 of substantially the same size. The fine holes are regularly positioned, at the crossing points of two imaginary arrays of lines which extend in mutually crossing directions. Extinctive phosphor 6, in the holes, is surrounded by portions of the plate (hole forming portions 2) which are not penetrable by excitation light beam used to stimulate the phosphor. Fig. 2B shows hole forming portions 2, of substantially the same size, which are processed so as not to be penetrable by the excitation light beam, with a light transmissive sealing material 4 provided at the light penetrating side of the hole forming portions 2 (the side from which excitation light is directed at the plate), a sealing material 5 provided on the surface opposite to the light penetrating side of the hole forming portions, and photostimulable phosphor 6 in the fine holes 26, surrounded by the hole forming portions 2 and sealed by the light transmissive sealing material 4 and sealing material 5. X-ray energy in accordance with an object pattern obtained when a specimen is irradiated with X-rays is distributed and stored in the fine holes 26 regularly arranged on the photostimulable phosphor plate. The photostimulable phosphor plate with such a stored energy distribution pattern is scanned with an excitation light beam and thereby the object pattern, stored as the energy distribution pattern in the plate, may be extracted as an electrical signal pattern. To obtain an output electrical signal pattern, the fine holes 26 regularly arranged on the photostimulable phosphor plate are scanned, in one direction of the two crossing directions mentioned above, with an excitation beam. Since the photostimulable phosphor irradiated with the excitation beam is provided within the fine holes separated by portions 2 which are not penetrated by the excitation light beam, the excitation light beam is not subject to scattering. Therefore, reduction of space resolution can be prevented. Fig. 3 illustrates how scattering is prevented or restricted, to avoid reduction of space resolution. A method of manufacturing a photostimulable phosphor plate in accordance with an embodiment of the present invention will be explained hereunder with reference to Figs. 4A to 4F and Figs. 5A and 5B. This manufacturing method utilises etching. First, a resist pattern suitable for forming fine holes (see Fig. 5A) in a thin stainless steel plate is produced by well-known CAD technique. Here, the diameter of a part of the pattern corresponding to a hole is set smaller than the desired diameter of a fine hole to be formed. Using this resist pattern, masks 22, 23 are formed on both surfaces of a stainless steel plate 20, as shown in Fig. 4A. Here, 24 designates mask holes in the masks 22, 23. Fine holes 26 are formed as shown in Fig. 4B by applying etching agent to the stainless steel plate 20 through the mask holes 24. Here, 28 designates hole walls. The photostimulable phosphor plate in this case is built up of three steel plates 20, as shown at 20₁, 20₂ and 20₃ in Fig. 4D. For joining the three plates together, hole wall areas 29 of a plate surface are coated with an adhesive agent 30 by a screen printing method, as shown in Fig. 4C. In the case of stainless steel plate 20₂, both surfaces are coated with the bonding agent 30, whilst in the cases of stainless steel plates 20₁, 20₃, only the surfaces which come into contact with the stainless steel plate 20₂ are coated with the agent. In Fig. 4D, 32 is a glass plate. After the bonding process, the deep fine holes 26, formed through the three stacked stainless steel plates 20₁ to 20₃, are filled with photostimulable phosphor powder 6 (BaFBr:Bu²⁺), as shown in Fig. 4E, and a polyester protection layer 34, as shown in Fig. 4F, is formed thereon, thereby completing the manufacture of the photostimulable phosphor plate, which is schematically illustrated in Figs. 5A and 5B. The wall surfaces of fine holes 26 are optical surfaces and efficiently reflect the excitation light beam and emitted fluorescent light. Therefore, the excitation light beam does not penetrate through the wall surfaces of the fine holes and emitted fluorescent light, arising from the photostimulable phosphor due to irradiation with the excitation light beam can be collected efficiently (refer to Fig. 3). Accordingly, reduction of the space resolution of an image can be prevented. The quantity of photostimulable fluorescent light emitted from the photostimulable phosphor due to irradiation with the excitation light beam is increased by stacking stainless steel plates as described above, and space resolution is not thereby reduced. The thin stainless steel plates used in this embodiment may be replaced by other thin metal plates. Many fine holes may be formed in thin metal plates by various methods including etching methods and mechanical processing methods. In relation to the present invention, there is no limitation on the methods by which fine holes may be formed. The fine holes 26 formed may exhibit various shapes, depending on the materials and forming methods used. Possible sectional shapes for holes are illustrated in Figs. 7. For example, when it is assumed that fine holes with a diameter of 0.08 mm are formed in a stainless steel plate with a thickness of 0.1 mm with vertical and horizontal pitches of 0.1 mm by the etching method, the shape of the holes will never be straight (c.f. Fig. 7A). When etching is effected from only one side of the plate, holes will be larger to that side (Fig. 7B, Fig. 7E). When etching is effected from both sides of the plates, the centres of the holes are narrowed (Fig. 7C, Fig. 7D, Fig. 7F). Other shapes appear depending on the method used for forming the holes of mask. When the holes are formed by an electrospark machining method, their shape is comparatively straight. Namely, the holes can take various sectional shapes but it is possible to execute the present invention regardless of the shape of hole. Further, the flat or plan shapes of the fine holes may vary, for example as shown in Fig. 6. The shapes may be circular, elliptical, square, rectangular or polygonal. These are not restrictive examples, but can be used for convenience of manufacture. In cases in which the wall surfaces of the fine holes formed may be penetrated by the excitation light beam, due to the properties of the material used, it is better that wall surfaces be coated or evaporated with material which does not allow penetration by the excitation light beam, in order to prevent penetration by the excitation light beam. Moreover, in cases in which the wall surfaces of the holes do not have sufficient surface accuracy from an optical viewpoint, it is effective to smooth the surface by coating it with resin and form thereon a layer such as metal having a high reflectivity. Although the size of fine hole used in embodiments of the present invention is not particularly restricted, the present size lower limit lies at about 0.01 mm diameter because of technical difficulties in burying photostimulable phosphor in fine holes of a certain thickness. The upper visual limit lies at about 0.4 mm diameter from the viewpoint of space resolution required for X-ray image diagnosis. The fine holes may be perpendicular to the surface of the photostimulable phosphor plate but they may also be formed at an inclination. The shape of a hole may be straight or the sizes of upper and lower portions of hole may be different. Moreover, any kind of material may be used, provided it has sufficient mechanical strength after hole formation. In the embodiment described above, a glass plate is provided on one surface of the stainless steel plate (20₁, 20₂, 20₃) after formation of the fine holes. It is, however, also possible to use a metal sheet. In this case, it is preferable that the sheet used reflects the excitation light beam and emitted fluorescent light. Alternatively, a sheet which reflects the excitation light beam but is penetrated by the emitted fluorescent light may be used, as may a sheet having the inverse properties, depending on the manner of use; namely, depending on selection of the direction from which the excitation light beam is irradiated or the direction in which the emitted fluorescent light is collected. A cover which functions as a selection mirror may also be effectively formed to both sides using adhesive agent. Although not particularly limited, it is effective to use a material, as the cover, such as lead glass which is penetrated by excitation light beam and the photostimulable emitted light and which absorbs X-rays in order to prevent the back scattering of an X-ray beam. Alternatively, a lead plate may be attached. As photostimulable phosphors which may be buried in the fine holes, those which are not penetrated by the excitation light beam to show the scattering may be used, or those which are penetrated by the excitation light beam or the emitted fluorescent light in any combination may be used, without restriction. The method of burying the phosphor may be freely chosen. For example, a method in which photostimulable phosphor powder of a grain size of 5 µm or less is dispersed into a solution of binder and the solution is then supplied into the holes may be used. Alternatively a method may be used in which the photostimulable phosphor powder is put directly into the fine holes and thereafter binder soaked into the powder, as may a method in which a layer which may be dissolved later is formed at parts other than the holes by a lift-off method, the holes are filled with the photostimulable phosphor by evaporation and thereafter the photostimulable phosphor is removed from the parts other than the holes. Referring to Fig. 8, fine holes 36, which are almost the same in size, and are provided with internal wall surfaces 2 (not indicated) not penetrated by the excitation light beam, are provided at respective hole forming positions affording a regular arrangement of positions wherein adjacent hole forming positions are displaced by amounts required for approximating or bringing together the hole forming positions. Photostimulable phosphor is buried in the fine holes 36. Moreover, explained hereunder is a photostimulable phosphor plate comprising fine holes 26 of almost the same size, providing wall surfaces 2 which are not penetrated by the excitation light beam, formed at respective hole forming positions of the substrate giving a regular hole arrangement where at least adjacent hole forming positions are previously deviated by a predetermined value. Light transmissive sealing material 4 is arranged at the surface on the light penetrating side of the fine hole forming substrate, and sealing material 5 is arranged at the surface opposed to the light penetrating side of the fine hole forming substrate. Photostimulable phosphor 6 fills the fine holes 26 sealed by the light beam transmissivity sealing material 4 and sealing material 5. The fine holes of the photostimulable phosphor plate of the second embodiment of the present invention are thus arranged with the highest density as shown in Fig. 8. Since the fine holes 36 are formed close to each other, the quantity of photostimulable phosphor buried in the photostimulable phosphor plate is increased and the phosphor effectively prevents reduction of energy to be stored therein. A cross-sectional view of the second embodiment resembles Fig. 2. Like the first embodiment described above, since the photostimulable phosphor irradiated with the excitation light beam is buried between internal wall surfaces 2 which are not penetrated by the excitation light beam, the light beam is not subject to scattering. Accordingly, reduction of space resolution can be prevented. The method of manufacturing the photostimulable phosphor plate is similar to that of the first embodiment and a resist pattern corresponding to Fig. 8 is used. Next, a third embodiment of the present invention will be explained with reference to Fig. 9. In Fig. 9, a photostimulable phosphor plate 10 is formed by a plurality of stainless sheets 12. Each stainless sheet 12 has a thickness of 0.1 mm and a size of 380 mm square. The centre area 14 thereof (356 mm square) is provided with a plurality of fine holes 16 arranged in the form of a lattice. When forming such fine holes 16, the size of one pixel is considered. For example, the size of one pixel of a photostimulable phosphor plate or sheet for use in diagnosing breast cancer is set to approximately 50 µm square. In the case of X-ray imaging of the chest, the pixel size is set from 87.5 µm square to 175 µm square in order to process the digital information. Although such sizes are not essential, diagnosis may be realised with such space resolution. Therefore, the pixel size must be changed depending on the object tissue and in the case of embodiments of the present invention, the photostimulable phosphor plates or sheets may be used to provide various pixel sizes. The possible minimum pixel size is determined by the possible excitation light beam diameter. The current minimum pixel size is about 20 µm square. The maximum pixel size is not limited from the viewpoint of the possibility of realisation, but the diagnostic purpose is not attained when the pixel size is 0.4 mm square or more. Therefore, in embodiments of the present invention, pixel size ranges from 20 µm square to 0.4 mm square, the pixel being capable of taking a square shape and rectangular shape. When the pixel size is determined, circular or square fine holes 16 which are smaller in diameter than the pixel size are formed on, for example, four stainless sheets 12. In this case, the lateral and vertical (as seen in Fig. 9) pitches of the lattice of fine holes are set to 175 µm and positional deviation of fine holes 16 between the starting and ending points is set to 52.5 µm. The holes 16 are, of course, to be scanned by an excitation light beam when the plate is in use. The fine holes 16 are formed so that the arrangement of fine hole matches with a subscanning direction (vertical direction in Fig. 9) of the scanning by the excitation light beam and also matches with a straight line according to the relationship Δ = b - b(1 - η), wherein the line reading pitch which is equal to the pixel size in the subscanning direction is taken as b and the scanning efficiency of scanning by the excitation light beam as η, in the main scanning direction (lateral direction in Fig. 9). The scanning efficiency η is expressed by η = s/c when a read line length is c and an actual scanning length of the excitation light beam is s. As the processing method used to form the fine holes 16 on a stainless sheet 12, the etching process is employed for example. A thermosetting type epoxy resin is dissolved into an organic agent, both sides of the sheet are coated with the solution, obtained by dispersing the graphite powder, by a screen printing method (except for the fine hole portions) and thereafter the coated area is dried up. Such a stainless sheet 12 and a stainless sheet 12 coated with the resin only at one single surface are stacked with each other. Moreover, a stainless sheet without fine holes 16 and having a thickness of 0.2 mm is placed on one surface. The three stainless sheets are placed under pressure, with a weight, and bonded through thermosetting at 180°C. In addition, a reflective film is fitted to the wall surface of fine holes 16 in order to reflect the excitation light beam. A phosphor powder consisting of BaC Br:Eu with a grain size of 5 µm or less is dispersed into organic solvent including epoxy resin as binder, it is then poured on to the sheet under a reduced pressure condition and the phosphor buried in the holes is dried up. This process is repeated three times. After confirming that the holes are filled with the photostimulable phosphor up to the surface thereof, the mixture of photostimulable phosphor and epoxy resin at the surface is wiped away. The sheets are hardened at 180°C and moreover a transparent polyester sheet is bonded to the surface as the protection layer. A photostimulable phosphor plate 10 manufactured as explained above was fixed on a stage and irradiated with the laser beam of 100 µm extent in the subscanning direction and 40 µm extent in the main scanning direction using a laser scanning system having a scanning efficiency of 70% consisting of the semiconductor laser with a wavelength of 780 nm, a lens and a galvano mirror. Thereby, it was confirmed that the excitation light beam penetrates through the photostimulable phosphor in the fine holes 16 and that scattering from one hole to other fine holes 16 can be prevented. Namely, the emitted fluorescent light which is emitted, as a consequence of irradiation of the surface of photostimulable phosphor plate 10 with X-rays, when the phosphor is excited with a pulse laser was gathered by a condenser mirror and glass fibre array and received by a photomultiplier. The converted electrical signal was then converted into the digital signal through the analog to digital conversion. Here, it was confirmed from the quantity of light received that the excitation light beam reflected from the wall surface of fine holes 16 could be gathered by a fibre array different from that mentioned above, then received by a photodiode. The sheet was irradiated with a reference dose of X-rays, and a reference output of each pixel input to a memory. By this, a normal image can be obtained in which changes in the quantity of photostimulated light beam produced, resulting from aging, is compensated and fluctuation or changes due to aging of pixels is compensated. In embodiments of the present invention, the size of the excitation beam on the phosphor plate or sheet must be smaller than it could otherwise be, by a factor determined by the degree of wobble which arises. Here, it is preferable that the length of the beam in the main scanning direction is smaller than the length of one pixel in the subscanning direction. The excitation light beam used may be a continuous light or pulse beam but the length of excitation light beam in the main scanning direction is best as short as possible. Moreover, it is a matter of course that the scanning must be made in such a manner that the excitation light beam does not pass over pixels adjacent in the subscanning direction. An image of a predetermined space resolution can be read without influence of scattering of the excitation light beam by paying attention to these conditions. Even in a case in which the excitation light beam scanning system and the light beam gathering and receiving system are fixed and the photostimulable phosphor plate or sheet is moved, or vice versa, the angle between the photostimulable phosphor plate or sheet and a subscanning direction may be determined uniquely in relation to the efficiency of the scanning by the excitation light beam and the size of one pixel. Fig. 10 shows a photostimulable phosphor reader. Using X-ray energy, a latent image of an object is formed as an energy distribution pattern on the photostimulable phosphor plate 105 and such latent image is read using an excitation light beam. As shown in Fig. 10, a laser beam output from an excitation light beam source 101 is used for scanning, through the scanner 102 consisting of a galvano mirror or polygon mirror. The photostimulable phosphor plate 105 is scanned by the laser beam through an optical part 103 for compensating the shape of the beam (for example an f0 lens, etc.) and a reflection mirror 104. The fluorescent light emitted from the photostimulable phosphor plate 105 when the plate is scanned by the laser beam from the laser beam system is gathered by a gathering means such as a fibre array 107. The gathered light beam is converted into an electrical signal, corresponding to the quantity of the light received, in a photoelectric converter 108, such as a photoelectron multiplier, through a filter which does not transmit the excitation light beam as received from the fibre array 107 but transmits only the photostimulated light beam and is then amplified by an amplifier 109. Thereafter, the signal is converted to a digital signal by an A/D converter 110. The digital signal is first stored in a frame memory 111 or stored in an optical disk memory 112 without passing through the frame memory. Thereafter, processing such as a gradation process is carried out as required in an image processing part 113. The image is displayed as an X-ray image on the image display part 114 such as a CRT, or is written direct on an X-ray film through film writing apparatus and the film is then developed to obtain the X-ray image. A reader of this kind can be used for reading a photostimulable phosphor plate as described in relation to any of the first to third embodiments of the present invention. The laser beam diameter at the surface of photostimulable phosphor plate is set to 170 µm in the subscanning direction (in which the photostimulable phosphor plate moves) or to 40 µm in the main scanning direction. The laser beam diameter in the main scanning direction is preferably smaller than the length of one pixel in the main scanning direction. When the phosphor plate is scanned with the laser beam mentioned above, a standard single pixel from which light is gathered has a size of 176 µm square and includes 16 holes in total (refer to I of Fig. 11). If a single pixel has a size of 132 µm square, nine holes (refer to II of Fig. 11) exist within the one pixel. In this case, the laser beam diameter in the subscanning direction is 125 µm. Moreover, if a single pixel has the size of 88 µm square, four holes (refer to III of Fig. 11) exist in the one pixel. In this case, the laser beam diameter in the subscanning direction is 83 µm. When the one pixel has the size of 44 µm square, only one hole (refer to IV of Fig. 11) exists in the one pixel and the laser beam diameter in the subscanning direction is 39 µm or 20 µm in the main scanning direction. In embodiments of the present invention the number of holes corresponding to one pixel may range from 1 to 400. As explained previously, several fine holes may be provided in one pixel, but if two pixels use in common a part of certain fine hole, space resolution is reduced. A situation may be considered in which several holes are used as one pixel but that scanning line used for reading the photostimulable phosphor is deviated at the time of the reading operation. Namely, a situation may be considered in which a hole located on a certain main scanning line corresponding to a certain straight line but that the excitation light beam deviates slightly from this line. This may mean that a certain hole is not always irradiated by the excitation light beam over its entire extent. In other words, only a part of a certain hole may be irradiated by the excitation light beam only. However, in a case in which one pixel is formed with several holes, it is apparent that the part not irradiated with the light beam relatively increases in comparison with the case where the one pixel is formed by one hole. Accordingly, when one pixel is formed with several holes, the read accuracy can be improved and reliability of reader can also be enhanced. In the reader described previously, as seen in Fig. 10, a light beam is gathered for the scanning of the photostimulable phosphor plate. In this reader, light of the excitation light beam reflected from the wall surface not forming the holes of the photostimulable phosphor plate is gathered by a fibre array different from that of Fig. 10, or by a plastic light receiver, and light from the photostimulable phosphor is received with synchronisation between such gathered light beam and the light from the photostimulable phosphor. The excitation light beam from the wall surface is reflected by utilising the photostimulable phosphor plate of any of the first to third embodiments described above which are formed to reflect the light from that surface, other than at hole forming portions of the extinctive phosphor plate. Fig. 12 shows a structure wherein a reflected excitation light beam gathering optical guide path 113, for gathering the excitation light beam, is provided. Elements similar to those in Fig. 10 are designated by like reference signs. 1071 designates an excitation light beam absorbing filter; 107 gathers photostimulated fluorescent light and therefore it must absorb reflections of the excitation light beam. The excitation light beam absorbing filter 1071 absorbs light of 600 to 900 nm wavelength (wavelength of the excitation light beam) and transmits light of 400 nm wavelength (wavelength of the photostimulated fluorescent beam). 115 is a gathering mirror which gathers light so that the excitation light beam and emitted fluorescent light are not scattered. 207 is a reflected excitation light gathering and guiding path; 2071 is an emitted fluorescent light absorbing filter which absorbs light in the vicinity of 400 nm (wavelength of emitted fluorescent light) and transmits light of 600 to 900 nm wavelength (wavelength of the excitation light beam). 208 is a photosensor. This photosensor is a semiconductor sensor such as a photoelectron multiplier or photodiode. The emitted fluorescent light absorbing filter 2071 realises power saving through selection of a kind of photoelectron multiplier. Fig. 13 illustrates a circuit for determining the timing of sampling. In Fig. 13, 108 designates a photoelectric converter and 208 designates a photosensor similar to that shown in Fig. 12. The emitted fluorescent light and light of the excitation light beam are input to the photoelectric converter 108 and photosensor 208 respectively through the fibre array 107 and reflected and excited light beam gathering and guiding path 207 as shown in Fig. 12. The emitted fluorescent light input to the photoelectric converter 108 is converted into an electrical signal and input to the A/D converter 110 through the amplifier 109. On the other hand, the light of the excitation light beam converted to the electrical signal in the photosensor 208 is compared with a reference voltage in a comparison circuit 209. Fig. 14 shows a timechart of signals output from the circuit shown in Fig. 13. As shown in Fig. 13, when the electrical signal from the photosensor 208 is lower than the reference voltage, the comparison circuit 209 outputs signals. The photosensor 208 receives the light of the excitation light beam. If the line L in Fig. 11 is scanned by the excitation light beam, the surface other than the hole portions (the photostimulable phosphor is buried therein) intensively reflects the excitation light. The excitation light beam is absorbed in the hole portions in which the photostimulable phosphor is buried and the emitted fluorescent light is output therefrom through a certain degree of reflection. Therefore, an electrical signal indicated as 208 in Fig. 14 can be obtained from the light of the excitation light beam received by the photosensor 208. Light of the excitation light beam reflected from the reflecting part results in a high voltage from sensor 208, whilst light of the excitation light beam reflected from the hole portion where the photostimulable phosphor is buried results in a low voltage. Comparison is carried out with reference to the reference voltage in a comparison circuit 209 in order to discriminate hole portions and reflecting portions. An output of comparison circuit 209 is input to a flipflop 210 which outputs a signal synchronised with the input signal thereto. An output of the flipflop 210 is ANDed with an original clock in an AND gate 211 and is then input to an A/D converter 110 as an operation clock. Namely, when an electrical signal corresponding to light of the excitation light beam (output of photosensor 208) is lower than the reference voltage, the A/D converter 110 operates and an electrical signal corresponding to emitted fluorescent light (output of amplifier 109) is converted to a digital signal. The converted digital value is added by an adder 217 whilst an output of AND gate 211 is ON, and the added value is stored in a flipflop 218 (such flipflops are provided in a number corresponding to the number of bits of the digital signal, but this is not illustrated in the drawings). While the addition is carried out in the adder 218, an output of AND gate 211 is input to a counter 214 and the number of clocks is counted. When an output of the flipflop 210 becomes OFF, the counter is cleared and such value is stored in the flipflop 215. Finally, a divider 219 outputs a value, obtained by dividing a sum of outputs of the A/D converter from flipflop 218 with a value from the counter stored in the flipflop 215, to the memory 111. Namely, the emitted fluorescent light is sampled while the excited light beam passes the holes filled with the photostimulable phosphor by receiving the excited light beam. In this apparatus, outputs of the A/D converter are added and an average value of these outputs is obtained. However, it is also possible that only addition, or that integration also, may be carried out. Here, it is important to select the timing of such addition and the time for integration by receiving the excitation light beam.

Claims: A method of manufacturing a photostimulable phosphor plate having therein an array of fine holes (26; 16; 36), all of substantially the same size and with photostimulable phosphor (6) therein, which phosphor is stimulable by excitation light to emit stimulated light, the wall surfaces of the holes being non-transmissive for or not penetrable by such excitation light, the method comprising:- etching fine holes (26; 16; 36) in a plurality of metal sheets (20₁, 20₂, 20₃), stacking the plurality of metal sheets (20₁, 20₂, 20₃) to provide the body of the plate, with the etched holes in the sheets being aligned so as to provide said array of fine holes (26; 16; 36), burying photostimulable phosphor material in the etched holes, providing that one surface of the body of the plate is reflective, providing a transparent protective layer at the opposed surface of the body of the plate. A method as claimed in claim 1, comprising bonding together the metal sheets (20₁, 20₂, 20₃) by a screen printing method. A photostimulable phosphor plate manufactured by a method as claimed in claim 1 or 2, wherein the array of fine holes (26; 16; 36) is in the form of a lattice, with lines of holes aligned in a first lattice direction, corresponding to a sub-scanning direction for scanning of the plate by a beam of excitation light when the plate is in use, and lines of holes aligned in a second lattice direction, corresponding to the main scanning direction for such scanning, the second lattice direction being at an angle to a direction orthogonal to the first lattice direction such that from one end to the other of a line of holes (26; 16; 36) extending in the second lattice direction, those ends are displaced one from the other in the first lattice direction by ▵ = b η, where b is the reading or scanning pitch in the sub-scanning direction, which corresponds to pixel size in that direction for an image to be produced on the basis of stimulated light, and ηis the scanning efficiency of such scanning of the plate by the beam of excitation light. A plate as claimed in claim 3, wherein the wall surfaces of the holes are reflective for such excitation and stimulated light. A plate as claimed in claim 3 or 4, wherein the holes are positioned in the array so as to be closely adjacent one to another, being close packed or approximately so. A plate as claimed in claim 3, 4 or 5, with light transmissive sealing material (4) provided on the plate on one side thereof, towards which side the excitation light is to be directed when the plate is in use, and sealing material (5) on the other side of the plate, sealing the phosphor in the holes (26; 16; 36). A plate as claimed in claim 3, 4, 5 or 6, wherein the plate surface on one side of the plate, towards which side the excitation light is to be directed when the plate is in use, is reflective for the excitation light, where the holes (26; 16; 36) are not provided.

Inventors: IWASE NOBUHIRO, KOSHINO NAGAAKI, NAMIKI FUMIHIRO, SUGIYAMA YUUICHI, TADAKI SHINJI, TAKEDA SHIRO, IWASE, NOBUHIRO, KOSHINO, NAGAAKI, NAMIKI, FUMIHIRO, SUGIYAMA, YUUICHI, TADAKI, SHINJI, TAKEDA, SHIRO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G21K4
IPC codes: G21K4