Abstract:
Image processing method for a digital medical examination image, the pixels of which are assigned a gray-scale value in each instance, with a minimum and a maximum gray-scale value being defined as limit values for the purpose of displaying the examination image, with the pixels being subjected to an evaluation by means of a neural network, in order to determine such pixels and to disregard them when defining the gray-scale values which are located in a direct radiation region or in a projected collimator region. 200421145

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to the German Application No. 10 2005 010 076.7, filed Mar. 4, 2005 which is incorporated by reference herein in its entirety.  
       FIELD OF INVENTION  
       [0002]     The invention relates to an image processing method for a digital medical examination image, the pixels of which are assigned a gray-scale value in each instance, with a minimum and maximum gray-scale value being defined as limit values for the purpose of displaying the examination image.  
       BACKGROUND OF INVENTION  
       [0003]     Digital medical examination images which require computed image processing are generated by medical examination methods such as computer tomography, magnetic resonance, ultrasound, methods of nuclear medicine for example, and also increasingly by digital x-ray methods. For this purpose, these complex method steps serve to optimally display the information content of the examination images which is significant for the diagnosis. There is thus a demand for automatic methods which ensure that the examination image or an image series is optimally displayed without further user intervention. A central problem with the image display is the definition of the minimum and the maximum gray-scale value of the pixels. The average level (level) and the contrast (window) can also be defined instead of the extreme values. However, the definition of the gray-scale values and/or the definition of the level and window are critical. Image information is lost if the window is selected too narrow, or the level is not optimal, and if the window is selected too wide the contrast of the examination image is too low and the details are thus more difficult to recognize.  
       SUMMARY OF INVENTION  
       [0004]     With conventional image processing methods, the values for the window or the minimum and maximum gray-scale values are either permanently predetermined, for example in an organ-dependent manner for a specific dose level in the case of an x-ray examination, or the sought parameters are automatically defined on the basis of the gray-scale values of the image pixels. In the simplest case, the minimum and the maximum gray-scale value of the examination image are defined and used for the image display. This known method nevertheless comprises a series of disadvantages. If the examination image contains direct radiation regions, the highest gray-scale values of the dynamic regions are thereby normally occupied. On the other hand if the projected collimators of an x-ray system are mapped for instance in the examination image, the lowest gray-scale values of the dynamic region are thus occupied. If, in the simplest case, only the minimum and maximum of the gray-scale values of the histogram are used to define the window, an excessively large window and an incorrect average level result, so that the image information actually required for diagnosis, which shows the examined organ in detail, is only suboptimally displayed.  
         [0005]     An object underlying the invention is to specify an image processing method for a digital medical examination image, by means of which the minimum and the maximum gray-scale value are automatically selected, thereby rendering the image display optimal.  
         [0006]     To solve this problem, provision is made according to the invention in the case of an image processing method of the type mentioned at the start for the pixels to be subjected to an evaluation by means of a neural network, in order to determine such pixels and to allow them to be disregarded when the gray-scale values, which are located in a direct radiation region or in a projected collimator region, are defined.  
         [0007]     The idea underlying the invention is that image regions can be determined by means of a neural network, said image regions belonging to a specific category, since their pixels exhibit similar gray-scale values. It is thereby possible to determine direct radiation regions or collimator regions. Direct radiation regions feature very high gray-scale values; collimator regions, on the other hand, feature very low gray-scale values. These regions with extreme gray-scale values are discounted when the gray-scale values are defined, so that limit values of the gray-scale values relevant for the image display only relate to such pixels which display the examined organ. The image processing method according to the invention can be automatically carried out so that the displayed examination images are not influenced by the subjectively selected settings by a user.  
         [0008]     Within the scope of the image processing method according to the invention, provision can be made for the signal value of a pixel to be used as a value for input nodes of the neural network. Alternatively or in addition, the signal-to-noise ratio of a pixel determined taking into account adjacent pixels can be used as a value for the input nodes of the neural network. Statistical values such as the variance or the standard deviation can be determined for the surroundings of a pixel. The data acquired in this way are input variables for the neural network.  
         [0009]     As a further alternative, provision can be made with the image processing method according to the invention in that an organ displayed on the examination image and/or the dose region selected with an x-ray examination and/or the x-ray voltage selected with an x-ray examination is/are used as a value for the input nodes of the neural network. It is also possible for the gradient of the signal value of a pixel determined taking into account adjacent pixels is used as a value for the input nodes of the neural network. The gradient of the signal values in the surroundings of a specific pixel supplies information about the “flatness” of the signals, with only very slight changes to the signal level from pixel to pixel being present in the direct radiation region and in the projected collimator region. However, larger gradients are usually present between adjacent pixels in the region of the displayed object.  
         [0010]     In a further embodiment of the method according to the invention, provision can be made in that the neural network determines the probability which specifies whether a specific pixel lies in the direct radiation region, in the region of the object or in the region of the collimator. For this purpose, the neural network is trained with sample images, so that the output nodes subsequently produce the corresponding probability values.  
         [0011]     With the image processing method according to the invention, provision can be made whereby examination image regions with similar or identical image information are determined on the basis of the determined probability values. From experience, the image regions with direct radiation and the image regions with projected collimators are simply connected regions. These regions can thus be determined on the basis of the probability values. In a further embodiment of the invention provision can thus be made for a region determined in this manner to be defined provided a specific minimum number of pixels with similar or identical image information is available. In the next method step, these regions with direct radiation or with projected collimator radiation can be disregarded so that for the purpose of defining the minimum and maximum gray-scale values, those image regions which show the object can be used.  
         [0012]     The invention also relates to a medical examination device, in particular an x-ray system for recording digital medical examination images.  
         [0013]     According to the invention the medical examination device is designed to implement the described image processing method. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Further advantages and details of the invention are described on the basis of an exemplary embodiment with reference to the figures. The figures are schematic representations in which:  
         [0015]      FIG. 1  shows a histogram with a gray-scale value distribution of an x-ray recording;  
         [0016]      FIG. 2  shows the processing of the image information by a neural network within the scope of the method according to the invention, and  
         [0017]      FIG. 3  shows a radiography recording with determined direct radiation regions  
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0018]      FIG. 1  shows the gray-scale value distribution of an x-ray recording in the form of a histogram. The gray-scale values are plotted on the horizontal axis, and the number of pixels is plotted on the vertical axis;  FIG. 1  accordingly shows the distribution of the gray-scale values. The region  1  contains the gray-scale values in the projected collimator region, which are very low. A window  2  is connected thereto, which is restricted by a minimum 3 and a maximum 4. Direct radiation is present in the region  5 , the gray-scale values accordingly being very high there. To achieve an optimum display of the medical examination image, the region of the window  2  between the minimum 3 and maximum 4 must be determined. The region of the window  2  can also be defined by an average value 6 and by specifying window width. The position of the window is determined by the image processing method.  
         [0019]      FIG. 2  shows the processing of the image information by a neural network. With an x-ray examination in which a flat panel detector is used a gray-scale value is obtained for each pixel of the detector. Together, all pixel values form an image matrix  7 , which is subjected to preprocessing  8  in the next method step.  
         [0020]     Within the scope of the preprocessing  8 , the image matrix  7  is analyzed pixel by pixel. Using the method step of preprocessing  8 , values are determined for input nodes  9  of a neural network  10 . In the simplest case, the signal values of the pixels can be used as values for the input nodes  9 . Since the gray-scale values in the projected collimator region are very low and the gray-scale values in the region of the direct radiation are very high, an evaluation can take place already as a result of these simple specifications. Alternatively or in addition, the signal-to-noise ratio of the pixels can be taken into account. For this purpose, the pixels are evaluated in the surroundings of a pixel, in order to determine the variance and the standard deviation of the gray-scale values. In the simplest case, the direct 3×3 surroundings of a pixel can be evaluated. In the region of the direction radiation, the local x-ray spectrum is not damped or filtered so that the signal-to-noise ratio is at its greatest at this location and is defined by the “softest” possible x-ray spectrum. On the other hand, the signal-to-noise ratio in the region of the projected collimator region is at its lowest through the lead plates and the x-ray spectrum is maximally filtered, the “hardest” possible spectrum accordingly being present here. The selected definitions “soft” and “hard” relate to the output spectrum selected for a given organ.  
         [0021]     Alternatively or in addition, the gradient of the signals in the surroundings of a pixel can also be taken into consideration. A surrounding of 3×3, 5×5 pixels or larger surroundings can also be selected here. Since only very minor changes to the signal level from pixel to pixel are present in the direct radiation area and also in the projected collimator region, the gradient of the signals is a characteristic variable in order to determine in which area a specific pixel is located. In addition, further parameters can be taken into consideration, including the selected examined organ, the selected dose region or the selected x-ray voltage.  
         [0022]     Besides the input nodes  9 , the neural network  10  features a hidden layer  11 , with the input nodes  9  and the nodes of the hidden layer  11  being coupled via connections  12 . The hidden layer  11  is connected to an output node  13 , which produces the probability determined by the neural network  10  for a specific pixel. The probability distribution for the complete examination image is subsequently present in the form of a probability matrix  14 . The neural network  10  must be trained first in a known manner, with corresponding examination images being used, in which the different regions (direct radiation, collimator, object) have been defined by a user.  
         [0023]     In the exemplary embodiment displayed, the probabilities from 0 to 0.1 are assigned to the collimator region, the probabilities between 0.1 to 0.9 are assigned to the object region and the probabilities between 0.9 and 1 are assigned to the direction radiation region. The ideal value for the collimator region is 0, the ideal value for the direct radiation region is 1.  
         [0024]     Based on an individual pixel, in the next method step the surrounding region is expanded by a pixel by pixel connection of adjacent pixels with the same or similar image information. For this purpose, morphological filters can also be used, with use being made of the fact that direct radiation regions and image regions of projected collimators are simply connected areas in a mathematic sense. Based on a pixel which has a very high probability of belonging to the category direct radiation (p=1), further pixels belonging to this category and thus bearing similar probability values are now sought in the direct neighborhood. Since the areas are connected and do not feature any islands, individual enclosed pixels which have rather deviating probabilities can be thus accepted and extend the area. To ensure that an area actually belongs to the direction radiation category, a specific defined number of directly connected pixels are required to be present. If the pixels of the area lying in the peripheral area consistently feature probability values deviating from the expected value (p=1), this is evaluated as a stop criterion. The same applies analogously for image regions of the projected collimator, in which the expected value for the probability is p=0. Within the scope of the method, a number of areas are found in this manner, in which either direct radiation is present or projected collimator regions are present.  
         [0025]      FIG. 3  shows an example of a radiography recording of the thorax with a number of direct radiation regions.  
         [0026]     The regions  15   a  to  15   d  were determined by the image processing method implemented in an x-ray system, in which regions direct radiation strikes the detector during the examination. When the limit values are defined for the gray-scale values, the direct radiation regions  15   a  to  15   d  are not taken into consideration so that only the remaining region  16  is used to define the window and the minimum and maximum gray-scale value.