Patent Number: 
Section: description

FIG. 1 shows the construction of the X-ray examination apparatus according to the invention. The X-ray source 1 emits X-rays 9. The X-rays pass the absorption means 3 which will also be referred to hereinafter as collimator plates 3. The collimator plates 3 are displaced by the control unit 4. The X-rays 7 transmitted by the collimator plates 3 irradiate the object 8 to be examined and are incident on the X-ray detector 2. The X-ray image is then picked up and converted into an electric image signal. The image signal 20 is applied to the image processing unit 5. The acquired image is displayed on the display unit in the form of a monitor 6. The image processing unit 5 is connected to the control unit 4 in order to adjust the degree of absorption of the collimator plates 3. FIG. 2 shows the construction of the image processing unit 5. The image processing unit 5 receives the image signal 20 from the X-ray detector 2 which is not shown herein. The signal is applied to the image memory 21 for storage and at the same time to the monitor 6 for display. The actual attenuation is calculated in the attenuation calculation unit 22 by means of the adjustment parameter vector pakt stored in the memory 29. The actual attenuation D(pakt) is the value of the attenuation applied to the acquired X-ray image, because absorption means may have been partly slid into the X-ray beam path. The unit 35 forms the reciprocal value of the actual attenuation value D in order to multiply it by the X-ray image stored in the image memory 21 so as to obtain an image wherefrom the attenuation D has been removed. The non-attenuated image is subdivided into image regions N in the unit 23. The unit 23 is succeeded by a parameter extraction unit 24 for calculating the image parameters such as contrast or structure parameters. In the optional attenuation correction unit the attenuation could also be calculated on the basis of the adjustment parameter vector Pakt. The parameter matrix C thus formed is applied to the quality function calculation unit 26. The unit 26 receives from the unit 32 the system parameters in the form of the system parameter vector s and the vector r representing the knowledge base. The knowledge or control base in the form of the vector r is stored in the unit 31 and is applied to the unit 26 for the calculation of the quality function Z as well as to the adaptation unit 30 for the adaptation of the knowledge base. The quality function is optimized in the unit 27. The adjusting parameter vector popt is calculated which contains the optimized adjustment of the absorption means. This adjustment parameter vector popt is applied to a further attenuation correction unit 33 which is similar to the attenuation unit 22 and calculates the adjustment of the attenuation D(p) on the basis of the adjustment parameter vector popt in order to apply it to the calculation of the quality function in the unit 26 again so as to be used for further optimization of the adjustment parameter vector p during the next run, until the optimized adjustment parameter vector popt has been determined. The latter is then applied to the unit 28 in which it is checked or confirmed that the instantaneous adjustment of the plates, also being applied to the monitor 6, is correct. When the instantaneous adjustment in the form of the adjustment parameter vector popt is confirmed by the physician, the adjustment parameter vector pakt is stored in the memory 29 and conducted therefrom to the control unit (not shown). At the same time the adjustment of the absorption means, embodied in the adjustment parameter vector popt, is superposed on the actual X-ray image in the superposition unit 34. The adjustment parameter vector pakt actually used and stored in the memory 29 is applied to the adaptation unit 30 in which the knowledge base is adapted by means of a learning process. The use of the quality function calculation will be illustrated on the basis of a simple example. The image I(x,y) stored in the image memory is subdivided into N non-overlapping, rectangular regions Ii(x,y) of a width of Nx. pixels and a length of Ny pixels. For these regions the structure contained therein is described by the variance and                               σ          i          2                =                              1                          (                                                                    N                    x                                    ⁢                                      N                    y                                                  -                1                            )                                ⁢                                    ∑                              x                =                1                                            N                x                                      ⁢                                          ∑                                  y                  =                  1                                                  N                  y                                            ⁢                                                (                                                                                    I                        i                                            ⁡                                              (                                                  x                          ,                          y                                                )                                                              -                                                                  1                                                                              N                            x                                                    ⁢                                                      N                            y                                                                                              ⁢                                                                        ∑                                                      x                            =                            1                                                                                N                            x                                                                          ⁢                                                                              ∑                                                          y                              =                              1                                                                                      N                              y                                                                                ⁢                                                                                    I                              i                                                        ⁡                                                          (                                                              x                                ,                                y                                                            )                                                                                                                                                            )                                2                                                                        (        4        )             is calculated as the parameter. Because only one parameter is calculated for all image regions, the parameter matrix C has one row only: C=("sgr"l . . . "sgr"N) For a single absorption means, hereinafter being a semi-transparent plate whose position is characterized by the adjustment parameter vector p=(1, "PHgr"xcfx86), with the angle "PHgr" and the insertion length l, the effect of the radiation attenuation on an image I can be represented by a function D(p,x,y) in such a manner that the attenuated image ID is given by ID(p, x, y)=D(p, x, y)I(x, y)xe2x80x83xe2x80x83(5) D(p,x,y) can be approximated, for example, via simple simulations. For the optimization a quality function Z(r, s, C, D(p)) can be indicated which is dependent on the additional parameter vectors r and s. A simple example of such a quality function can be given by the weighted addition of a plurality of single quality functions; for example, should essentially regions of little structure be removed, a component can penalize the attenuation of diagnostically relevant regions containing structures, that is such structures lead to an increase of the quality function which is then to be minimized. A further component penalizes regions with little structure which are not masked by the inserted diaphragm plate. Such a quality function is, for example, as follows:                                                                         Z                ⁡                                  (                                      r                    ,                    s                    ,                                          C                                              _                        _                                                              ,                                          D                      ⁡                                              (                                                  p                          _                                                )                                                                              )                                            =                              xe2x80x83                            ⁢                                                ∑                                      n                    =                    1                                    N                                ⁢                                  (                                                            rU                      ⁡                                              (                                                                                                            D                              n                                                        ⁡                                                          (                                                              p                                _                                                            )                                                                                ⁢                                                      (                                                          1                              -                                                                                                c                                                                      1                                    ,                                    n                                                                                                  /                                                                  s                                  2                                                                                                                      )                                                                          )                                                              +                                                                                                                                          xe2x80x83                            ⁢                              U                ⁡                                  (                                                            (                                              1                        -                                                                              D                            n                                                    ⁡                                                      (                                                          p                              _                                                        )                                                                                              )                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                                                                                    c                                                          1                              ,                              n                                                                                /                                                      s                            2                                                                          -                        1                                            )                                                        )                                            )                                                          (        6        )             where                               D          n                =                              1                                          N                x                            ⁢                              N                y                                              ⁢                                    ∑                              x                =                1                                            N                x                                      ⁢                                          ∑                                  y                  =                  1                                                  N                  y                                            ⁢                              D                ⁡                                  (                                                            p                      _                                        ,                    x                    ,                    y                                    )                                                                                        (        7        )             and the elements of C are in conformity with cl,n={C}l,n. In the present example the control base has only a single component r; the parameter s represents the sole component of the system parameter vector s and provides dose-dependent and radiation quality dependent normalization of the diagnostically relevant contrast. This parameter can be read from a stored table in dependence on the beam quality used. The function U increasingly penalizes positive contributions. It may be given, for example by       U    ⁡          (      x      )        =      {                                        x            2                                                              for              ⁢                              xe2x80x83                            ⁢              x                         greater than             0                                                0                                                    for              ⁢                              xe2x80x83                            ⁢              x                        ≤            0                               The first term of the quality function (6) thus penalizes unstructured regions which are not attenuated. The second term of the quality function (6) evaluates the effect of the introduced plate on contrasts in the diagnostically relevant region. Both components are contradictory and are weighted relative to one another via r. The larger r is chosen, the more the suppression of unstructured regions is given priority over the preservation of diagnostically relevant information. The optimum diaphragm position is then determined by minimizing the quality function:                                           p            _                    opt                =                  arg          ⁢                      xe2x80x83                    ⁢                                    min                                                                    p                    ⁢                                          xe2x80x83                                                        _                                ⁢                                  xe2x80x83                                                      ⁢                                          Z                ⁡                                  (                                      r                    ,                    s                    ,                                          C                                              _                        _                                                              ,                                          D                      ⁢                                              xe2x80x83                                            ⁢                                              (                                                  p                          _                                                )                                                                              )                                            .                                                          (        8        )             This can be performed, for example numerically by way of the Nelder-Mead Simplex algorithm. The diaphragm position thus determined is then graphically superposed on the monitor or adjusted directly by motor. As has already been described, the correction can be used to perform the user-specific or application-specific adaptation of r.