Abstract:
A device and a method for reading out information stored in a storage layer ( 15 ), and an X-ray cassette are proposed which make use for the purpose of erasing information stored in the storage layer ( 15 ) of an erasing means ( 11, 12, 14 ) which generates an erasure radiation ( 17 ). This erasure radiation ( 17 ) can have both a first and a second intensity, the first intensity being larger than the second intensity.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a device and a method for reading out information stored in a storage layer, and to an X-ray cassette.  
         BACKGROUND OF THE INVENTION  
         [0002]    Particularly for medical purposes, an image which is stored in a storage layer as a latent image is produced from an object, for example a patient, by means of X-ray radiation. It is mostly a phosphor layer which is used as the storage layer in this case. The storage layer is excited by means of a radiation source in order to read out the X-ray image stored in the storage layer. Therefore, on the basis of this excitation the storage layer emits light which has an intensity corresponding to the stored X-ray image. The light emitted by the storage layer is received by a receiving means and subsequently converted into electric signals such that the X-ray image stored in the storage layer can subsequently be visualized. The X-ray image can be displayed, for example, directly on a monitor, or else be written onto a photographic X-ray film which can be used specifically for X-ray images. After the X-ray image has been read out from the storage layer, the latter is erased by an erasing device, in order to be able to store a subsequent X-ray image.  
           [0003]    Such storage layers can be arranged in an X-ray cassette. European patent EP 0 288 014 B1 discloses erasing a storage layer which is located in an X-ray cassette. The X-ray cassette contains an opening mechanism such that the X-ray cassette can be opened in order to erase information stored in the storage layer. Present outside the X-ray cassette is an erasing means which outputs an erasure radiation. This erasure radiation is irradiated into the open X-ray cassette and projected onto the storage layer. The irradiation of the storage layer by means of the erasure radiation erases the information stored in the storage layer. The known X-ray cassette contains a reflector or a diffusing means for the purpose of reflecting the erasure radiation irradiated into the cassette, and of scattering it over the width of the storage layer.  
           [0004]    A device for reading out information stored in a storage layer is disclosed in patent U.S. Pat. No. 5,038,037. An X-ray table for X-raying patients is described therein. The X-ray table contains two phosphor layers which are fitted on the top side and underside of a tape and serve for storing X-ray information. Both for the phosphor layer fitted on the top side and for that fitted on the underside of the tape, the known X-ray table has a dedicated device for reading out information from this phosphor layer. Furthermore, the X-ray table contains two erasing means which are used to erase the phosphor layers. A dedicated erasing means is provided for each of the phosphor layers. Serving as erasing means is an erasing lamp which—viewed in the transport direction of the tape—is arranged upstream of that point at which the X-ray image of the patient is taken. As a result, an “old” X-ray image stored in the phosphor layer is erased before a “new” X-ray image is taken. A plurality of erasing lamps arranged next to one another are likewise used as erasing means, being arranged in the X-ray table below that position at which the X-ray image is projected onto the stationary tape with the phosphor layers. Before the taping of a “new” X-ray image, the “old” X-ray image is therefore firstly erased with the aid of this erasing means, the tape being stationary. In this type of erasing means, the phosphor layer cannot be used directly for a “new” picture. The erasure of the phosphor layers does not take place until immediately before the subsequent taking of a “new” X-ray image. This means that time is lost and tuning takes place between the operations of erasure and subsequent storage. The erasure must firstly be terminated before the next X-ray image can begin to be taken.  
           [0005]    Patent application WO 99/28765 discloses an X-ray cassette in which both a device for reading out information stored in the storage layer and an erasing means for erasing information stored in the storage layer are present. The erasing lamp extends over the entire width of the storage layer in which information can be stored. By means of a drive, the erasing means is guided over the storage layer along a transport direction running perpendicular to the line direction. The storage layer can be erased in this way line by line.  
         SUMMARY OF THE INVENTION  
         [0006]    It is the object of the present invention to permit in a simple and effective way erasure of information stored in a storage layer, and a high quality of reproduction of stored information.  
           [0007]    It is possible on the basis of the configuration according to the invention to avoid, or at least reduce, the occurrence of ghosts in the storage layer. Such ghosts can occur when the device according to the invention or the X-ray cassette is exposed to the scattered radiation of other X-ray machines which can be located, for example, in the vicinity. Such scattered radiations can certainly be very slight individually, but the addition of scattered radiations from a plurality of X-ray images of other X-ray machines can nevertheless accumulate to form visible ghosts in the storage layer. Moreover, conventional storage layers can contain small quantities of radioactive isotopes such as  226 Ra, which likewise emit slight amounts of radiation. These are further supplemented by natural, cosmic radiations. Particularly in the case of lengthy non-use of the storage layer, these radiations can contribute to the occurrence of the ghosts. Ghosts have a deleterious effect on the quality of the reproduction of the information read out. Ghosts can become noticeable as noise when stored information is read out. It is advantageously possible, moreover, to ensure on the basis of the invention that storage of “new” information is possible without the need to undertake prior erasure immediately before storage. There is no need to tune the erasing operating to the storage operation.  
           [0008]    The erasing means advantageously outputs an erasure radiation of a larger, first intensity immediately after information of an image has been read out from the storage layer. This ensures that the storage layer is ready very quickly to take a new image after the storage of an image.  
           [0009]    In a further advantageous refinement of the invention, the erasure radiation of a second, weaker intensity is output after the erasure radiation of the first intensity has been output. This prevents an undesired storage of scattered radiation striking the storage layer, since this scattered radiation is erased continuously by the erasure radiation.  
           [0010]    In a particularly advantageous refinement, the erasure radiation is output onto the storage layer between the reading out of the information of a first image and the subsequent storage of information of a second image. Storage of scattered radiation and the occurrence of ghosts during the entire time between the reading out of the first image and the storage of the information of the second image are therefore avoided. The quality of the reproduction of the information is further improved thereby. Furthermore, the erasure radiation of the second intensity can be output independently of the storage of information of a new image. Tuning between the storage and the erasure with the second intensity is not required for the sake of simplicity.  
           [0011]    It is preferable for there to be present a detection means for detecting that radiation with the aid of which information is stored in the storage layer. Consequently the erasing means can be switched off automatically when storage of information of a new image begins. For the sake of simplicity, the detection means can contain a photodiode upstream of which there is fitted a conversion layer for converting the storage radiation into a radiation that has a wavelength which can be detected by the photodiode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    An exemplary embodiment will be described in more detail below with reference to the appended drawings, in which:  
         [0013]    [0013]FIG. 1 shows a first exemplary embodiment of an X-ray cassette according to the invention, with an erasing source of planar configuration,  
         [0014]    [0014]FIG. 2 shows a sectional illustration of the first exemplary embodiment of the X-ray cassette in accordance with FIG. 1,  
         [0015]    [0015]FIG. 3 shows a second exemplary embodiment of an X-ray cassette according to the invention with an erasing source fitted laterally at the edge of the X-ray cassette,  
         [0016]    [0016]FIG. 4 shows a sectional illustration of the second exemplary embodiment of the X-ray cassette in accordance with FIG. 3,  
         [0017]    [0017]FIG. 5 shows an example of an X-ray table in which a third exemplary embodiment of an X-ray cassette according to the invention is arranged, and  
         [0018]    [0018]FIG. 6 shows a detailed schematic illustration of the third exemplary embodiment of the X-ray cassette with an exemplary embodiment of a reading head according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 shows the first exemplary embodiment of an X-ray cassette  1  according to the invention. This X-ray cassette contains a storage layer  15 . The storage layer  15  is a phosphor plate. X-ray images can be stored in the phosphor plate  15 . The X-ray cassette  1  has a reading head  10  for reading out the X-ray images stored in the phosphor plate  15 . Fitted along the longitudinal sides of the phosphor plate  15  are two guide bars  16  and  17  which serve for transporting and guiding the reading head  10 . The reading head  10  can be transported by means of a drive (not illustrated) along the guide bars  16  and  17  over the surface of the phosphor plate  15  in a feed direction A. An erasing lamp  11  is located behind the reading head  10 . The erasing lamp  11  is connected to the reading head  10  and can, just like the reading head  10 , be transported by means of the drive (not illustrated) along the guide bars  16  and  17  over the surface of the phosphor plate  15  in the feed direction A. The erasing lamp  11  serves to erase information stored in the phosphor plate which is still stored in the phosphor plate  15  after an X-ray image has been read out by means of the reading head  10 . During operation, the erasing lamp  11  emits an erasure radiation of a first intensity. This first intensity is advantageously so great that a single to and fro movement of the erasing lamp  11  over the phosphor plate  15  along the direction A erases residual information, remaining in the phosphor plate  15 , of the X-ray image. After the phosphor plate  15  has been erased by means of the erasing lamp  11 , the X-ray cassette is ready for taking a subsequent, second X-ray image. The erasing lamp  11  and the reading lamp  10  extend in a line direction B over the entire width of the phosphor plate  15 . The line direction B is perpendicular to the feed direction A.  
         [0020]    A second erasing lamp  12  is arranged inside the X-ray cassette  1  below the phosphor plate  15 . This second erasing lamp  12  is of planar configuration. The surface of the second erasing lamp  12  runs parallel to that of the phosphor plate  15 . The extent of the surface of the second erasing lamp  12  corresponds essentially to that of the phosphor plate  15  in which X-ray images are stored. The erasure radiation emitted by the second erasing lamp  12  has a wavelength that lies within that wavelength region in which the phosphor plate  15  can be stimulated. The flat radiator “PLANON” marketed by the Osram company can, for example, be used as such a second erasing lamp  12  of planar configuration.  
         [0021]    The X-ray cassette  1  further contains a control means  13  with the aid of which the components contained in the X-ray cassette  1  are driven. The control means  13  serves, in particular, to control the first erasing lamp  11  and the second erasing lamp  12 . The control means  13  is used to switch the two erasing lamps  11  and  12  into an on state, in which they output erasure radiation, and into an off state, in which they do not output erasure radiation. The second erasing lamp  12  is advantageously switched into the on state by the control means  13  immediately after the phosphor plate  15  has been erased by the first erasing lamp  11 . Thereafter, it continuously emits the erasure radiation of the weak, second intensity. The second intensity is advantageously so weak that its contribution to the total noise is negligible. The second erasing lamp  12  is switched into the off state by the control means  13  for the purpose of storing a new X-ray image in the phosphor plate  15 . The new X-ray image can therefore be stored completely in the phosphor plate  15  without the second erasing lamp  12  already erasing even the tiniest portions of information. As already described above, the X-ray image is subsequently read out with the aid of the reading head  10 , and the residual information, which continues to be stored in the phosphor plate  15  after the reading out, of the X-ray image is erased by means of the first erasing lamp  11 .  
         [0022]    It is likewise possible to leave the second erasing lamp  12  in the on state even during storage of the new X-ray image. In this case, advantageously there need not be any tuning between the second erasing lamp and the radiation source which outputs the imaging radiation with the information to be stored in the direction of the phosphor plate  15 . The erasure radiation which can be output by the second erasing lamp  12  can be fashioned such that there is no longer a need to switch off the second erasing lamp  12 . It is therefore unnecessary for the sake of simplicity to incorporate and tune the functioning of the second erasing lamp  12  into the operation of imaging, reading out and erasing.  
         [0023]    [0023]FIG. 2 shows a schematic sectional illustration of the phosphor plate  15  and of the second erasing lamp  12  in accordance with FIG. 1. The second erasing lamp  12  is arranged parallel to the phosphor plate  15  inside the X-ray cassette  1 . The second erasing lamp  12  emits, in the direction of the phosphor plate  15 , an erasure radiation  17  which has the weak, second intensity.  
         [0024]    [0024]FIG. 3 shows a second exemplary embodiment of the X-ray cassette  1  according to the invention. Instead of the second erasing lamp  12  fitted below the phosphor plate  15  (FIG. 1), a third erasing lamp  14  is arranged here at the rear lateral longitudinal wall of the X-ray cassette  1 . This third erasing lamp  14  is located laterally next to the phosphor plate  15  and extends along the longitudinal side thereof. A planar reflector  16  is arranged parallel to the phosphor plate  15  on the inside of the top cover of the X-ray cassette  1 . The reflector  16  can be a mirror, for example.  
         [0025]    The third erasing lamp  14  has the same function as the second erasing lamp  12  of the first exemplary embodiment in accordance with FIG. 1. It emits erasure radiation of the weak, second intensity. This weak erasure radiation can be radiated partially by the third erasure lamp  14  directly onto the phosphor plate  15 , or else can be directed by the third erasing lamp  15  onto the reflector  16  and be reflected thereby, in turn, in the direction of the phosphor plate  15 . The erasure radiation output by the third erasing lamp  14  is distributed in this way over the entire surface of the phosphor plate  15 . The entire surface of the phosphor plate  15  is reached by the second erasure radiation  17  because of the arrangements of the erasing lamp  14  and the reflector  16 .  
         [0026]    [0026]FIG. 4 shows a schematic sectional illustration of the X-ray cassette  1  in accordance with the second exemplary embodiment of FIG. 3. The X-ray cassette  1  is illustrated here sectioned in the line direction B. The third erasing lamp  14  is arranged on the left-hand side of FIG. 4. It is located in a fashion offset to the side of the reflector  16  and the phosphor plate  15 , at approximately the same distance from the reflector  16  and the phosphor plate  15 , inside the X-ray cassette  1 .  
         [0027]    [0027]FIG. 5 shows an example of the-application of a third exemplary embodiment of the X-ray cassette  1  according to the invention. In this example of application, the X-ray cassette  1  is inserted in an X-ray table  20 . This X-ray table  20  includes an X-ray base  23  in which the X-ray cassette  1  is located, and a supporting surface  24  arranged on this X-ray base  23 . Patients are laid on this supporting surface  24  for X-rays to be taken. An X-ray source  21  is fitted on the X-ray table  20  over the supporting surface  24  and the X-ray base  23  with the X-ray cassette  1  located therein. The X-ray source  21  emits an X-ray radiation  25  in the direction of the supporting surface  24  in order to take the X-ray image. The X-ray cassette  1  present in the X-ray base  23  here contains a detection means  22  which serves as a sensor for the X-ray radiation  25 . A sensor  22  can determine whether the X-ray source  21  is emitting X-ray radiation  25 . The operability of the erasing means contained inside the X-ray cassette  1  can be controlled with the aid of the sensor  22 .  
         [0028]    [0028]FIG. 6 serves to illustrate further the functioning of the sensor  22  and to describe the reading head  10 . FIG. 6 shows the reading head  10  and further components, present in the reading head  10 , for reading out the X-ray information stored in the phosphor plate  15 . The reading head  10  contains a laser diode row  34  extending along the line direction B. The laser diode row  34  serves for exciting a phosphor material  32  which is applied to a transparent substrate material  33  and forms the phosphor plate  15  together with this substrate material  33 . An exciting radiation  38  output by the laser diode row  34  traverses the substrate material  33  and penetrates into the phosphor material  32 . The phosphor material  32  emits an emission radiation because of this excitation of the phosphor material  32 . This emission radiation is projected onto a CCD row  36  by an optical imaging means  35  which contains a multiplicity of optical conductors, for example. The CCD row  36  includes a multiplicity of light-sensitive surfaces  37  arranged in a row next to one another.  
         [0029]    [0029]FIG. 6 further shows the sensor  22  for detecting X-ray radiation  25 . The sensor  22  contains a photodiode  30  over which a scintillation layer  31  is arranged. The X-ray radiation  25  emitted by the X-ray source  21  strikes this scintillation layer  31 . The scintillation layer  31  performs a wavelength conversion. The X-ray radiation  25  is converted into a radiation of a wavelength which can be detected by the photodiode  30 . The sensor  22  is connected to the control means  13 . The information emitted by the sensor  22  via the output of X-ray radiation  25  by the X-ray source  21  can be used by the control means  13  to drive the second erasing lamp  12 . In the exemplary embodiment in accordance with FIG. 6, this second erasing lamp  12  is arranged below the phosphor plate  15 , as already described in conjunction with FIG. 1.  
         [0030]    If the second erasing lamp  12  is in its on state, such that it outputs weak erasure radiation  17  in the direction of the phosphor plate  15 , the second erasing lamp  12  can then be switched into the off state by the control means  13  when the sensor  22  establishes that the X-ray source  21  is emitting X-ray radiation  25  for the purpose of renewed storage of an X-ray image in the phosphor plate  15 . The second erasing lamp  12  can be switched off automatically in this way such that there is no erasure with the second erasure radiation  17  during the taking of a new X-ray image.  
         [0031]    Instead of the photodiode  30  and the scintillation layer  31 , the sensor  22  can also be configured otherwise. The configuration by means of the photodiode  30  and the scintillation layer  31  constitutes a variant of the sensor  22  which is particularly easy to implement.  
         [0032]    Two erasing lamps  11  and  12  or  14  are used in the previously described exemplary embodiments for the purpose of erasing the phosphor plate  15  by means of the strong and the weak erasure radiation. Instead of two erasing lamps, it is likewise possible to make use of a single erasing lamp, or else also of more than two erasing lamps. In the case of a single erasing lamp, the latter is configured in such a way that it outputs both the weak and the strong erasure radiation.