Abstract:
The invention relates to a method for the creation of a panoramic tomographic image of an object ( 10 ) by means of X-rays, in which a digital X-ray-sensitive image detector ( 14 ) is moved relatively to the object to be X-rayed ( 10 ) and image data of the object ( 10 ), for a first layer ( 15 ) of the object ( 10 ) are summated to a first storage area ( 21.1 ), wherein the summation is carried out after a predefined first time interval (Δt 1 ) with a predefined first line offset (Δs 1 ). Image data for a second layer ( 16 ) of the object ( 10 ) are summated to a second storage area ( 21.2 ), which summation is performed after a predefined second time interval (Δt 2 ) with a predefined second line offset (Δs 2 ).  
     The invention further relates to a digital X-ray image acquisition device ( 1 ) for the creation of panoramic tomographic images of an object ( 10 ), comprising an X-ray-sensitive image detector ( 14 ), a first and second storage area ( 21.1, 21.2 ) for storing data, and a first and second linker ( 20.1, 20.2 ) for linking image data.

Description:
[0001]     The invention relates to a method for the production of a panoramic tomographic image of an object by means of X-rays, in which a digital X-ray-sensitive image detector is moved relatively to the object being tomographed, and image data for a first layer of the object is summated and appended to a first memory, said summation being undertaken after a predetermined first time interval with a predetermined first line offset.  
         [0002]     The invention also relates to a digital X-ray image acquisition device for panoramic tomography of an object, which contains an X-ray-sensitive image detector, a first memory for storing data and a linker for assigning image data.  
       DESCRIPTION OF THE RELATED ART  
       [0003]     EP 0 279 293 discloses a dental X-ray diagnostic device for panoramic tomography of a patient&#39;s jaw, which device contains an A/D converter connected to a detector, an image memory and a data processing device, which computes an overall image from the signals supplied by the detector system during a tomographic operation.  
         [0004]     It is also known from the prior art to use CCDs as sensors for tomography, said CCDs being operated in the TDI mode. The rate of displacement of the signal charges on the CCD is adapted to the relative speed of the CCD relative to the object being tomographed. In this way it is possible to create a sharp image of a certain layer of the object being tomographed.  
         [0005]     With the procedures known in the prior art it is only possible to record a single sharp layer per revolution of the X-ray device. However, this is a disadvantage for several reasons. On the one hand, in the case of dental panoramic tomography, the sharp layer is not always exactly in the arch of the jaw since every jaw is individually shaped. On the other hand, it is possible that a pathological site that must be identified for planning treatment or for making a diagnosis is not sharply imaged because it may lie next to the sharp layer, for example. Moreover, it is sometimes desirable to prepare several tomographic images of adjacent planes of a region in order to obtain a tomographic overview of the region, for example.  
         [0006]     All of these requirements necessitate one or more tomographic images, which, on the one hand, are time-consuming, and on the other hand, unnecessarily increase the radiation burden on the patient.  
         [0007]     It is thus an object of the invention to provide a method and an X-ray device with which it is possible to simultaneously tomograph several sharp layers lying side by side.  
       SUMMARY OF THE INVENTION  
       [0008]     This object is achieved by the invention with a method for panoramic tomography as defined in claim  1  and a digital X-ray imaging device as defined in subclaim  10 .  
         [0009]     The method for producing panoramic tomographic images of an object by means of X-rays, in which a digital X-ray-sensitive image detector is provided whose pixels are arranged in a two-dimensional line/column pattern envisions that the image detector be moved relatively to the object being tomographed at a predefined velocity to record image data of the object, the image data for a first layer being read from the image detector at a predetermined read frequency, and, after each readout of the image detector, summated in a first storage area and appended to an associated memory content present in the first storage area, said summation being performed after a set first time interval with a set first line offset, said first time interval also being a whole-number multiple of the reciprocal of the first read frequency.  
         [0010]     Image data for a second layer are read with a second read frequency from the image detector, and after each reading of the image detector summated in a second storage area to an associated memory content present in the second storage area. The summation is performed after a predetermined second time interval with a predetermined second line offset, the second time interval being a whole-number multiple of the reciprocal of the second read frequency.  
         [0011]     The memory content present in the storage areas can be an image obtained from image information previously read out from the image detector, a summated image of several such image data sets, or the memory may be empty.  
         [0012]     This method makes it possible to simultaneously record two layers of the object being imaged, in which case the layers may be in any position relative to each other.  
         [0013]     It is especially advantageous if image data for other layers are read out from the image detector with other predetermined read frequencies and, after each readout of the image detector, are summated in other storage areas and appended to a respective associated memory content present in the other storage area. The summation is performed after predetermined other time intervals with predetermined other line offsets, in which case the other time intervals are each a whole number multiple of the reciprocal of the other read frequencies respectively. In this way it is possible to tomograph several layers simultaneously and obtain a tomographic image of, say, a jaw.  
         [0014]     The memory contents of each storage area can advantageously be read out and summated with the newly recorded image data with the given line offset in each case, and the summated data saved in the respective storage area. The memory contents may be summated data from previous summations. The image data may also be stored and summated in digital form.  
         [0015]     Instead of a summating unit other linkers may also be provided, e.g. subtractors. This only changes the specific design, but not the underlying principle of the invention.  
         [0016]     The read frequencies are advantageously equal to a common read frequency. Such a method can be carried out at lower cost.  
         [0017]     The respective time intervals are advantageously different from one another. This permits imaging of different layers.  
         [0018]     The whole-number multiples are advantageously time-dependent. This permits variation of the relative position of the layers during the tomographic operation. For instance it is conceivable that in a region of, say, a front tooth, it becomes necessary to bring the layers closer to one another than in another region, say, a molar region.  
         [0019]     It is especially advantageous if the data present in the storage areas are written into another memory. This other memory serves to store the finished panoramic tomograms of the layers in question and can be located externally of the device used for processing and can, for example, be in the form of a computer hard disk. The other memory forms the basis for retrieval of the image information for diagnostic purposes.  
         [0020]     It is especially advantageous if the predefined rate and/or the read frequencies are time-dependent. This increases flexibility in adapting the relative positions of the sharp layers.  
         [0021]     Instead of time dependence, location dependence of the location of the X-ray emitter and the image detector may be of advantage if a predetermined trajectory is being followed. The two dependencies are in a distinct relationship to each other due to a known equation of motion of the X-ray apparatus and are thus interchangeable.  
         [0022]     The image detector is advantageously reset at a given frequency. The given frequency may be time-dependent.  
         [0023]     The digital X-ray imaging device of the invention for panoramic tomography of an object includes an X-ray-sensitive image detector, whose pixels are arranged in a two-dimensional line pattern, a first memory for storing data and a linker for linking image data which cooperates with a first storage area and the image detector. In addition, another storage area and a clock unit are present, the clock unit offering several clock frequencies that control reading and writing of image information.  
         [0024]     Such an X-ray tomography device permits the recording of several sharp layers in a single pass.  
         [0025]     The image detector is advantageously designed as a CMOS image detector. CMOS image detectors permit a higher degree of integration of subassemblies on the image detector than CCD sensors and can also be manufactured at lower cost.  
         [0026]     Advantageously, one or more additional storage areas are present for the storage of data that cooperate with the linker, while the clock unit offers one or more additional clock frequencies.  
         [0027]     This makes it possible to tomograph further sharp layers.  
         [0028]     It is especially advantageous if the storage areas are logical regions of a common memory. This lowers the production costs.  
         [0029]     Advantageously, the time interval between two line offsets is a whole-number multiple of the reciprocal of the respective read frequency.  
         [0030]     The linker advantageously carries out two linkages, each with a pre-assigned line offset. In this way it becomes possible to dispense with other means of producing a line offset.  
         [0031]     It is especially advantageous if the respective line offset between two time intervals is a whole-number multiple of the reciprocal of the respective read frequency. This achieves the linkage of complete images with each other.  
         [0032]     The read frequencies are advantageously equal to a common read frequency. This reduces the equipment costs without imposing excessive limitation on the functionality of the X-ray imaging device.  
         [0033]     The time intervals at which a line offset is performed differ from one another for different memories and linkers. In the event of identical read frequencies, therefore, the tomography of different layers will be possible.  
         [0034]     The memory is advantageously designed as an analog memory and the linker as an analog linker. Such linkers and memories are very fast and can be installed on the board of the image detector in a space-saving manner. This reduces signal deterioration due to long conduction paths and intermediate digitization of the individual tomograms which, in sum total, leads to greater image deviations than does digitization of the finished summated image. In addition, the number of digitizations can be reduced, which means that only slower and/or fewer AD converters will be necessary.  
         [0035]     As an alternative, it is possible to design the memory as a digital memory and configure the linker such that it can read the digital memory contents from the storage areas. In this way corrections can be made at an early stage on the partial images.  
         [0036]     Advantageously, another memory is provided that cooperates with the memory and serves for permanent storage of data. This allows for the storage of X-ray images for diagnostic purposes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]     The method of the invention and the X-ray tomographic device are explained in more detail with reference to the figures, in which:  
         [0038]      FIG. 1  shows an X-ray tomographic device,  
         [0039]      FIG. 2  is a basic diagram illustrating the principle of tomographing different layers,  
         [0040]      FIG. 3  shows the basic imaging-side structure of the X-ray image acquisition device in a first embodiment,  
         [0041]      FIG. 4  shows the basic imaging side structure of the X-ray image acquisition device in a second embodiment, and  
         [0042]      FIG. 5  shows a basic diagram illustrating the method of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]      FIG. 1  shows an X-ray device  1  according to the invention. On a supporting column  2  there is provided a boom  3  on which a carrier  4  is mounted for rotation. On the carrier  4  an X-ray emitter  5  and an image detector  6  are disposed diametrically opposed to a holding device  7  for a patient (not shown).  
         [0044]     The patient is positioned by the holding device  7  and a mouthpiece  8  such that the patient&#39;s head is kept stationary with respect to the supporting column  7 . The carrier  4  then revolves along a predetermined path around the patient&#39;s head.  
         [0045]     In the known TDI method based on the use of CCD sensors, the image information detected thus far is moved contrary to the direction of motion of the image detector relatively to the object being tomographed such that the image data of a point being imaged always expose the cell to which the image data of the same point being imaged was moved from the previous tomograph. The image data are therefore moved contrary to the imaging direction in such a way that they are located in a pixel of the image detector that lies on a straight line passing from the X-ray source to the image pixel through the point being imaged. The rate of displacement of the image data on the sensor must therefore be adapted to the position of the sharp layer.  
         [0046]      FIG. 2  shows a basic diagram for illustration of the principle for the production of a layer based on a jaw section  10  of the patient. For the sake of clarity, the diagram is not drawn to scale.  
         [0047]     The X-ray source  11  emits a fan beam  13  through a primary aperture  12 , to penetrate the jaw segment  10  and impinges on the image detector  13 , where it is recorded. Also shown are two layers of the jaw segment  10  being tomographed, a first layer  15  and a second layer  16 .  
         [0048]     In the small region of the jaw segment  10  being imaged shown in  FIG. 2 , the movement of the recording unit consisting of X-ray source  11 , primary aperture  12  and image detector  14  can be approximated as linear relative to the jaw segment  10  so that the X-ray unit moves relative to the jaw segment  10  at a velocity v(t) parallel to the layers  15  and  16  being imaged.  
         [0049]     The closer a layer to be imaged is to the image detector  14 , the more slowly its image moves, according to the principles of intercept theorems, on the image detector  14  when in relative motion across the image detector  14 . A point P 1  on the first layer  15 , here relating to the boundary between a tooth and a jaw, travels at a greater speed across the image detector than another point P 2  on the second layer  16 , which is on the same level.  
         [0050]     For geometric reasons, therefore, the necessary displacement speed of the image data on the image detector  14  for the sharp layer  16  is lower than the corresponding displacement speed for the sharp layer  15 .  
         [0051]     The desired spacing between the sharp layers to be imaged varies during image acquisition. In a front tooth region, for example, it could be necessary for the sharp layers to be closer together than in a molar region.  
         [0052]      FIG. 3  shows the image-acquisition side part of the X-ray imaging device  1  in a first embodiment. The jaw  10 ′ irradiated by the X-ray source  11  is imaged on the image detector  14 .  
         [0053]     The image detector  14  is configured as a CMOS sensor  14 . The CMOS sensor  14  can be read out without deleting the charges in the pixels. This makes it possible to read the CMOS sensor  14  as often as desired. Deletion of pixels is independent of the readout.  
         [0054]     The pixels of the CMOS sensor  14  are arranged in a two-dimensional line pattern on columns R x1 , R x2  and lines R y1 , R y2 , etc. . . . The CMOS sensor  14  registers image data on the jaw  10 ′ at a frequency f B (t), which means that the charge content of the pixels of the CMOS sensor  14  is reset after each clock pulse 1/f B ( 4   t )  
         [0055]     A summating unit  20  reads image data from the CMOS sensor  14  at a read frequency f L (t) and sums it up in a memory  21 . For this purpose the image data read out of the CMOS sensor  14  are summated in pixel form on the memory content present in the memory  21 . The memory content present in the storage areas can be an image of image information previously read out from the image detector, a summated image produced from several such image data, or the memory can be empty, if the memory has been emptied in the preceding clock cycle as described below.  
         [0056]     As an example, the memory  21  is divided up into four logical storage areas  21 . 1 ,  21 . 2 ,  21 . 3 ,  21 . 4 , into which the summating unit  20  summates data by the method described in more detail with reference to  FIG. 5 . The summated image data are then passed on to a second storage area  22  in which they are saved/filed and held ready for evaluation.  
         [0057]     Depending on the physical configuration of the sensor, it may be necessary to effect summation by reading out the respective memory contents of the storage areas  21 . 1 ,  21 . 2 ,  21 . 3 ,  21 . 4 , and to write them back to the respective storage area  21 . 1 ,  21 . 2 ,  21 . 3 ,  21 . 4 , following summation.  
         [0058]     The summating unit  20  is basically controlled during summation by other parameters n 1 (t), n 2 (t), n 3 (t), n 4 (t), Δs 1 , Δs 2 , Δs 3 , Δs 4 , whose function is explained in more detail with reference to  FIG. 5 .  
         [0059]     It is still possible to provide an analog amplifier between the CMOS sensor  14  and the summating unit  20 .  
         [0060]      FIG. 4  shows the image acquisition-side structure of the radiographic device  1  in a second embodiment. Unlike the embodiment shown in  FIG. 3 , here a plurality of summating units  20 . 1 ,  20 . 2 ,  20 . 3 , and  20 . 4  is provided. Each of these summating units  20 . 1 ,  20 . 2 ,  20 . 3  und  20 . 4  operates with its own read frequency f L1 (t), f L2 (t), f L3 (t) and f L4 (t). To each summating unit  20 . 1 ,  20 . 2 ,  20 . 3  und  20 . 4  there is assigned a line offset Δs 1 ′, Δs 2 ′, Δs 3 ′ und Δs 4 ′ as well as, for each, an whole number n 1 ′(t), n 2 ′(t), n 3 ′(t), and n 4 ′(t) is preset, to control the memory logic. Each of these summating units  20 . 1 ,  20 . 2 ,  20 . 3  und  20 . 4  is associated with a storage area  21 . 1 ′,  21 . 2 ′,  21 . 3 ′ and  21 . 4 ′ of a memory  21 ′.  
         [0061]     The storage and summating process is explained in more detail with reference to  FIG. 5 .  
         [0062]     The velocity v(t), the imaging frequency f B (t), the whole numbers n 1 (t), n 2 (t), n 3 (t), n 4 (t), n 1 ′(t), n 2 ′(t), n 3 ′(t), n 4 ′(t), and the read frequencies f L1 (t), f L2 (t), f L3 (t), and f L4 (t) are time dependent, said time dependence being a function of the region of the jaw  10 ′ to be imaged. It is therefore also possible to represent the aforementioned magnitudes as a function of the location of the X-ray apparatus. The speed of revolution of the X-ray emitter  5  and of the image detector  6  around the jaw  10 ′ of the patient is dependent on the position of the X-ray emitter and the image detector  6  relative to the jaw.  
         [0063]     The summating units  20 . 1 ,  20 . 2 ,  20 . 3 , and  20 . 4  as well as the memory  21 ′ are designed as analog structures. In the storage areas  21 . 1 ′,  21 . 2 ′,  21 . 3 ′, and  21 . 4 ′ the signals of the image detector  14  are summated in the analog mode. This has the advantage that the summating units  20 . 1 ,  20 . 2 ,  20 . 3 , and  20 . 4  and the memory  21 ′ can be put on the CMOS chip without there being any necessity for extremely fast digitization. The analog structures are space-saving, the short signal paths and direct processing of the image signals without prior digitization improve the precision and the signal-to-noise ratio, and the analog structures are, in addition, sufficiently fast.  
         [0064]     Instead of summating, other linkages may be undertaken, e.g., subtraction of two consecutive recorded images and subsequent addition of the differential images resulting from the subtraction. This is advantageous whenever the CMOS sensor  14  is reset with a frequency f B (t), which is lower than the accordingly read frequency f L1 (t), f L2 (t), f L3 (t), and f L4 (t). By forming the difference between two successively readout memory contents it is possible to ascertain the newly acquired information content.  
         [0065]     It is still possible to provide an analog amplifier between the CMOS sensor  14  and the linker  20 .  
         [0066]      FIG. 5  illustrates the method of acquiring TDI images by means of the CMOS sensor  14 . The principle employed for the simultaneous production of two layers by means of the storage areas  21 . 1  and  21 . 2  is illustrated. Unlike  FIG. 3 , the sensor is shown in a rotated position so that v(t) points upwardly. 
 Three times are shown,  T   0   , T   1  und  T   2 , where:  T   1   =T   0   +n   1 ( t )/ f   L1 ( t );    T   2   =T   0 +2 ×n   1 ( t )/ f   L1 ( t ), and    T   2   =T   0   +n   2 ( t )/ f   L2 ( t ) where  n   2 ( t )=2 ×n   1 ( t ).  
         [0067]     Time T 0  represents the time when the memory areas  21 . 1  and  21 . 2  are just being written. In this case the image information which is present in the CMOS sensor  14  in line R y1  is written into line  1  of the two storage areas  21 . 1  and  21 . 2 .  
         [0068]     At time T 1 , a line offset Δs 1  in memory area  21 . 1  of one line is effected. The first line R y1  is written into the last written line of the storage area  21 . 1 , which had been deleted at the previous clock pulse, and the second line R y2  is appended to the contents of the first line  1  of the first storage area  21 . 1 .  
         [0069]     The image information is summated and appended to the second storage area  21 . 2  in the same way as at time T 0 .  
         [0070]     After each summation cycle individual lines of storage areas  21 . 1  and  21 . 2  are read out and sent to memory  22 , which stores the data.  
         [0071]     At the time T 2 , another line offset is effected in the storage area  21 . 1 , so that the line R y1  is added to line n−1 of storage area  21 . 1 . The line R y2  is appended to the line n of the storage area  21 . 2 .  
         [0072]     In the storage area  21 . 2 , after a line offset Δs 2  of one line, summation to line  2  is effected, as was carried out at time T 1  in storage area  21 . 1 .  
         [0073]     Together with the readout of the lines in the storage areas  21 . 1  and  21 . 2  the lines are reset. At the time T 0  the line n is read out and reset. The readout in the next clock cycle is then performed in the previous line, here therefore n−1. What this achieves is that summation is performed just as often into each line before the readout. Then at time T 2  the line n−2 is read out.  
         [0074]     It is possible to specify other parameterizations leading to the same result. For example, the line offset Δs can be represented as a function of time Δs(t) so that the line offset varies throughout the cycles. In the following case it would then be true that: 
 
Δ s   1 ( T   i )=1 ; i= 1, 2, 3, . . . , 
 
Δ s   2 ( T   i′ )=1 ; i′= 2, 4, 6, . . . . 
 
         [0075]     However, this does not depart from the basic scope of the present invention.  
       LIST OF REFERENCE NUMERALS  
       [0000]    
       
           1  X-ray apparatus  
           2  Supporting column  
           3  Boom  
           4  Rotatable carrier  
           5  X-ray emitter  
           6  Image detector  
           7  Holding device  
           8  Mouthpiece  
           10  Jaw segment  
           11  X-ray source  
           12  Primary aperture  
           13  Fan beam  
           14  Image detector  
           15  First layer  
           16  Second layer  
           20  Summating unit  
           20 . 1  First summating unit  
           20 . 2  Second summating unit  
           20 . 3  Third summating unit  
           20 . 4  Fourth summating unit  
           21  Memory  
           21 ′ Memory  
           21 . 1  First storage area  
           21 . 2  Second storage area  
           21 . 3  Third storage area  
           21 . 4  Fourth storage area  
           21 . 1 ′ First storage area  
           21 . 2 ′ Second storage area  
           21 . 3 ′ Third storage area  
           21 . 4 ′ Fourth storage area  
           22  Memory  
          Δs 1  First line offset  
          Δs 2  Second line offset  
          Δs 3  Third line offset  
          Δs 4  Fourth line offset  
          Δs 1 ′ First line offset  
          Δs 2 ′ Second line offset  
          Δs 3 ′ Third line offset  
          Δs 4 ′ Fourth line offset  
          n 1 (t) First whole number  
          n 2 (t) Second whole number  
          n 3 (t) Third whole number  
          n 4 (t) Fourth whole number  
          n 1 (t) First whole number  
          n 2 ′(t) Second whole number  
          n 3 ′(t) Third whole number  
          n 4 ′(t) Fourth whole number  
          Δt 1  First time interval  
          Δt 2  Second time interval  
          Δt 3  Third time interval  
          Δt 4  Fourth time interval  
          Δt 1 ′First time interval  
          Δt 2 ′ Second time interval  
          Δt 3 ′ Third time interval  
          Δt 4 ′ Fourth time interval  
          T 0 , T 1 , T 2  Points of time  
          P 1  First point  
          P 2  Second point  
          R x1  First column  
          R x2  Second column  
          R y1  First line  
          R y2  Second line  
          f B (t) Imaging frequency  
          f L (t) Read frequency  
          f L1 (t) First read frequency  
          f L2 (t) Second read frequency  
          f L3 (t) Third read frequency  
          f L4 (t) Fourth read frequency