METHOD FOR THE READ-OUT OF AN EXPOSED PHOSPHOR PLATE IN DENTAL X-RAY PHOTOGRAPHY

Abstract: The invention relates to a method for the read-out of an image stored on a phosphor plate (1) in dental X-ray photography, by means of a laser beam (4) directed at the plate surface. The method comprises a preliminary read-out step and, thereafter, a final read-out step which provides the desired information and in which setting values determined on the basis of the preliminary read-out are used. According to the invention, the preliminary read-out is carried out on the marginal region (5) of the image, including mainly jawbone, and the final read-out is carried out on the central region (8) of the image, including the denture and the interdental spaces. The amplification used in the final read-out can in this case be adjusted on the basis of the preliminary read-out so that the darkest regions of the image, i.e. the interdental spaces, will be black. The preliminary read-out and the final read-out can be linked to a reciprocal movement of a phosphor plate (1) drawn out of a protective casing (2) and returned into it, and the deletion of the information from the plate, subsequent to the read-out, can be linked to a second reciprocal movement of the plate.

Description: Method for the read-out of an exposed phosphor plate in dental X-ray photography The present invention relates to a method in dental X-ray pho¬ tography for the read-out of an image, stored on a phosphor plate, by means of a beam directed at the surface of the plate, the method comprising a preliminary read-out step and, there¬ after, the final read-out step which will yield the desired information and in which setting values determined on the basis of the preliminary read-out will be used. The purpose of the preliminary read-out is to determine the radiation dose received by the phosphor plate in the exposure, and on the basis of this dose the optimal amplification by the read-out amplifier is adjusted for the final read-out step. The purpose is to find the gain level which makes maximal use of the conversion range of the analog/digital converter used in the read-out. The gain can be adjusted to the correct level if the signal intensity of those regions of the image which have received the largest amount of X-radiation is known. In a nor¬ mal dental radiograph these regions are the interdental spaces, which are shown black in the radiograph. The denture and the interdental spaces will usually be in the central region of the radiograph, but the exact locations of the interdental spaces cannot be anticipated. Regions which have received the largest amount of X-radiation can be sought by adjusting the amplification during the image read-out. However, in this case the gain in the middle of the image may change, whereupon the read-out result will not be optimal. Since part of the latent image on the phosphor plate will disappear during the image read-out, there is also no sense in reading out the image a second time using the amplifi¬ cation which could be set on the basis of the first read-out. US patent publication 4,527,060 discloses a method of prelim- inary read-out using a mechanism by which the intensity of the laser beam used for the read-out is lowered and/or the laser dot is enlarged, whereupon the intensity of the radiation im¬ pinging on the phosphor plate will lower to a level at which the latent image on the phosphor plate will substantially be retained. The preliminary read-out is carried out using this lowered-intensity laser dot, and on the basis of this read-out the necessary read-out amplifier parameters are set for the final full-intensity read-out of the image. According to the said patent, the preliminary read-out is targeted on the entire image surface, or at least on those regions of the image sur¬ face which contain essential information. A laser dot with its intensity lowered in the said manner is used in each case. The object of the present invention is to provide a new read¬ out method suitable for dental X-ray photography, wherein the preliminary image read-out can be carried out without lowering the intensity of the laser beam used and, thus, without the auxiliary devices required for that procedure. The invention is characterized in that the preliminary read-out is carried out on the marginal regions of the image, which in the main include the jawbone or the like, and the final read-out is carried out on the central region of the image, which includes the informa¬ tion desired from the image, such as the denture and the inter¬ dental spaces. The invention is based on the fact that, regardless of how well the photographed denture with its interdental spaces has coin¬ cided with the central region of the image, in the marginal regions of a normal dental radiograph there is always jawbone or some other non-dark region which can be taken as the basis for the adjustment of the amplification in the final read-out step. A preliminary read-out without lowering the intensity of the laser beam will destroy some of the latent image on the phosphor plate, but this is not of significance, since in terms of the information desired from the image the marginal regions of the image are in general not of interest. The final read-o is limited to the central region of the image, in which the denture and the interdental spaces can be made visible and which has been retained intact in the preliminary read-out. According to the invention, the amplification can adjusted by taking as the basis for the adjustment the degree of darkness, corresponding to the jawbone, prevalent in the marginal regio of the image. In the final read-out, the highest degree of image darkness, corresponding to the interdental spaces, is adjusted to a level substantially darker than it, for example, three times its darkness, in which case the interdental spaces will be shown black in the radiograph. On the other hand, the degree of darkness of the image will be correct also if, for some reason, interdental spaces are not shown in the image. In a case in which the marginal regions subjected to the pre¬ liminary read-out have substantial variations in darkness due to teeth and interdental spaces coinciding at some points wit the marginal region of the image, the amplification can be based directly on the image darkness preliminarily read out a corresponding to the interdental spaces. The image read-out according to the invention is most prefer¬ ably carried out so that the preliminary read-out takes place during a mutual linear movement of the phosphor plate and the read-out device, and the final read-out during a mutual linear movement of the phosphor plate and the read-out device in the direction reverse to that of the preliminary read-out. The sa read-out device can thus be used for both the preliminary rea out and the final read-out. If the phosphor plate is, after exposure, enclosed in a prote tive casing, the read-out can advantageously be performed so that the phosphor plate is drawn out of the protective casing and is read out preliminarily in connection with the drawing out, in a read-out station outside the protective casing, whereafter the final read-out is carried out in the same read¬ out station during the return of the phosphor plate into the protective casing. It is possible to use for the read-out the apparatus described in FI application 903796, which according to the said application reads out the image stored on a sliding exposed plate while the sheet is being drawn out of an RIM cas¬ sette serving as a protective casing and, after the read-out, deletes the information from the plate while the plate is being returned into the cassette. According to the present invention, however, the apparatus is used so that a preliminary read-out is carried out in connection with the drawing out of the plate and the final read-out in connection with the return movement of the plate. Thereafter the plate may make a new movement out of and into the cassette for deletion by means of an intense ray of light aimed at the surface of the plate. The image read-out may, according to the invention, take place advantageously so that in the preliminary read-out step those regions of the image are read out which frame one side and the two opposite ends of the image, and the rest of the image is read out in the final read-out step. The read-out device may be made up of, for example, a scanner equipped with a pivotable mirror, the scanner guiding the laser beam which carries out the read-out back and forth across the phosphor plate, which is in steady linear motion, and the said read-out steps will in this case require only the control of the direction and extent of the transverse movement of the scanner in the different steps of the read-out. The invention is described below in greater detail with the help of examples and with reference to the accompanying draw¬ ings, in which Figures la-le depict, step by step, the prelimi¬ nary image read-out while the phosphor plate is being drawn out of the protective casing, and the final image read-out while the phosphor plate is being returned into the protective cas- ing, and Figures 2a-2c depict, step by step, the drawing out the phosphor plate and the preliminary read-out in one appa¬ ratus suitable for the purpose. Figure la shows a phosphor plate 1, which has been exposed in dental X-ray photography and has been placed in a protective casing 2 after the exposure. The protective casing may be an RIM cassette according to FI application 903796, and the phos¬ phor plate may be a sliding plate which can be drawn out of th cassette in accordance with the said patent application, al¬ though the present invention is by no means limited to merely these embodiments. The preliminary read-out of the image produced by exposure on the phosphor plate 1 takes place while the plate is being draw out of the protective casing 2 in accordance with Figure lb. The phosphor plate 1 is brought into steady linear motion, and a laser beam 4 scanning the plate surface reciprocally in the lateral direction is directed at it in a read-out station 3. The laser beam 4 is directed so that it initially reads the phosphor plate preliminarily over its entire width, thereafter over a rather narrow region limited to one side of the plate, and finally again over the entire width of the plate. Figure l depicts a step in which the preliminary read-out has been com¬ pleted. The preliminary read-out thus covers the marginal regions 5 of the plate 1, which surround as a rather narrow zone one side 6 and the two opposite ends 7 of the plate. On the other hand, the central region 8 of the plate has been lef intact in the preliminary read-out. The final image read-out using setting values determined on th basis of the preliminary read-out is carried out by using the same read-out station 3 and laser beam 4 during the return of the phosphor plate 1 into the protective casing 2. The read-ou is now directed only at the central region 8 of the phosphor plate 1, left intact in the preliminary read-out, and the lase beam 4 reads it out with reciprocal scans in accordance with Figure Id. Figure le shows a step in which the final read-out has been completed, whereafter the plate can be returned into the protective casing 2 (Figure la). After the read-out steps the phosphor plate 1 may make a new movement out of and into the protective casing 2, during which the information is deleted from the plate by means of a delet¬ ing light directed at the surface. After the deletion the phos phor plate is ready for a second round of use. The apparatus, depicted in Figures 2a-2c, by means of which th plate 1 read-out and deletion steps described above can be im¬ plemented, comprises a frame 9, a space 10 receiving a phospho plate in a protective casing 2, a cover 11 closing the said space, and a drawing device 13 which moves reciprocally on a shaft 12 turning about its own axis and being driven by a step motor (not shown) . Furthermore, the apparatus is equipped with a read-out station 3 for the preliminary read-out and the fina read-out of the image on the phosphor plate 1 drawn out of the protective casing 2, as well as with a deleting station 14 for the deletion step subsequent to the read-out step. In Figure 2a the protective casing 2 with the phosphor plate i it has been inserted into the opening provided for it in the frame 9 of the apparatus, in such a manner that the casing ex¬ tends to a shoulder 15, equipped with a magnet, in the recipro cally moving drawing device 13. In Figure 2b, the cover 11 mov able in the lateral direction has been shifted to a position i alignment with the protective casing 2 in such a manner that the protective casing with the phosphor plate is left in its entirety inside the cover. A spring-loaded pin 16 belonging to the cover 11 is at this time in alignment with an opening 17 i the frame and the end of a rod 18 belonging to the drawing de¬ vice 13, and the cover will prevent the access of outside ligh into the space below the cover and to the read-out and deletio stations 3, 14 behind it. When the drawing device 13 begins to move from the initial po¬ sition according to Figure 2b, the rod 18 of the drawing devic will withdraw from the opening 17 in the frame, and the pin 16 belonging to the cover 11 will, pressed by the spring, push into the said opening, thereby locking the cover in place. The magnet of the drawing device 13 will draw the phosphor plate 1 out of the protective casing 2, and the preliminary read-out o the plate will be carried out by a laser beam 4 in the read-ou station, as described above. In Figure 2c the drawing device 1 and the phosphor plate 1 have arrived at the end of their move ment track, where the plate 1 is entirely outside the protec¬ tive casing 2, and thereafter there will be a return to the position in accordance with Figure 2b, during which the final read-out of the phosphor plate 1 will take place in the read¬ out station 3, in the manner described above. After the read-out steps, the phosphor plate 1, drawn by the drawing device 13, will make a new reciprocal movement, during which the information is deleted from the plate by means of light directed at its surface in the deletion station 14. Afte the deletion, the phosphor plate 1 will return into the protec tive casing 2, the cover 11 will be opened (Figure 2a), and th protective casing with its phosphor plate, ready for a new ex¬ posure, will be removed from the apparatus. For an expert in the art it is clear that the various embodi¬ ments of the invention are not limited to the examples present ed above but may vary within the accompanying patent claims.

Claims: Claims 1. A method for the read-out of an image stored on a phos¬ phor plate (1) in dental X-ray photography, by means of a beam (4) directed at the plate surface, the method comprising a pre¬ liminary read-out step and thereafter a final read-out step which will provide the desired information and in which setting values determined on the basis of the preliminary read-out will be used, characterized in that the preliminary read-out is car¬ ried out on the marginal region (5) of the image, including in the main jawbone or the like, and the final read-out is carried out on the central region (8) of the image, including the in¬ formation desired from the image, such as the denture and the interdental spaces. 2. A method according to Claim 1, characterized in that the preliminary read-out takes place during a mutual linear move¬ ment of the phosphor plate (1) and the read-out device (3) , and the final read-out during a mutual linear movement of the phos¬ phor plate and the read-out device in the reverse direction. 3. A method according to Claim 2, characterized in that the phosphor plate (1) is drawn out of a protective casing (2) and the preliminary read-out is carried out in connection with the drawing out, in a read-out station (3) outside the protective casing, and that the final read-out is carried out in the same read-out station (3) while the phosphor plate is returning into the protective cover. 4. A method according to Claim 2 or 3, characterized in tha those image (1) regions (5) which frame one side (6) and the two opposite ends (7) of the image are read out in the prelim¬ inary read-out step, and that the rest (8) of the image is read out in the final read-out step. 5. A method according to any of Claims 2-4, characterized i that after the read-out steps the information is deleted from the phosphor plate (1) by means of a deleting light directed the surface of the plate, which has been brought into linear motion. 6. A method according to Claims 3 and 5, characterized in that after the read-out steps, the phosphor plate (1) is draw again out of the protective casing (2) and the information is deleted from it in a deleting station (14) which is outside t protective casing and suitably located next to the read-out station. 7. A method according to any of the above claims, charac¬ terized in that in the final read-out step the highest-degree darkness of the image, corresponding to the interdental spaces is adjusted to a substantially higher degree of darkness than the prevailing darkness determined in the preliminary read-ou corresponding to the jawbone, preferably at about three times the darkness. 8. A method according to any of Claims 1-6, characterized that in cases in which there are substantial variations of darkness in the marginal regions (5) of the image, which have been read out preliminarily, the highest degree of darkness corresponding to the interdental spaces is selected and, for the final read-out step, the highest degree of darkness of the image is adjusted on the basis of it.

Inventors: KOIVULEHTO LAURI, KOIVULEHTO, LAURI
Applicants: ORION YHTYMAE OY, ORION-YHTYMAE OY
Application year: 1993
Main classification: G03B42
IPC codes: A61B6, G01T1, G03B42