Abstract:
An apparatus adapted to correct an image data acquired by an image detector including a plurality of detecting elements, includes: a first correction unit adapted for obtaining an offset-corrected image by removing an offset component due to the image detector from the image data, a second correction unit adapted for correcting a pixel value of the offset-corrected image on the basis of a gain of corresponding one of the plurality of detecting elements; and a third correction unit adapted for correcting a value of a selected pixel of the offset-corrected image, the selected pixel being selected based on the pixel value of the offset-corrected image, by generating a pixel value in place of a value of the selected pixel obtained by said second correction unit.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to correction of image data acquired by an image detector.  
         BACKGROUND OF THE INVENTION  
         [0002]    The inside of a matter, particularly a human body, can be visualized by measuring a distribution of radioactive rays, typically X rays, transmitted through the matter or human body. A radioactive ray distribution is detected nowadays generally with a large-size image sensor using a solid state image pickup device and called a flat panel X ray sensor. The merits of a solid state image pickup device reside in that signals indicative of an energy distribution on a plane can be directly and spatially sampled from a plurality of pixel elements disposed on the plane.  
           [0003]    The demerits reside in that since each of the plurality of pixel elements to be spatially sampled is fundamentally an independent element and has different characteristics, an acquired image is required to be corrected in accordance with a variation in the characteristics of each pixel element.  
           [0004]    The main variation in the characteristics of pixel elements as energy conversion elements is a variation in conversion efficiencies (gain) and offset values. The gain and offset are required to be corrected first, if a flat panel X ray sensor is used as a solid state image pickup device.  
           [0005]    Description will be given on prior art technologies. FIG. 4 is a schematic block diagram of the apparatus for correcting the gain and offset. In FIG. 4, reference numeral  1  represents an X ray generating apparatus which is controlled by a controller having a high voltage generator to generate X rays along a direction indicated by an arrow. An object, typically a human body  2 , is on a bed  3 . An X ray image sensor  5  is used for converting an intensity distribution of X rays transmitted through the object into electric signals. The X ray image sensor  5  is made of a large-size solid image pickup device. Signals indicative of an X ray intensity distribution on a two-dimensional plane can be spatially sampled from a plurality of pixel elements disposed in a two-dimensional matrix form.  
           [0006]    In the following, this X ray image sensor is called a flat panel sensor. A sampling pitch is set generally to about 100 μm to 200 μm to image the internal structure of a human body. The flat panel sensor is controlled by the controller (not shown). Pixels are sequentially scanned to convert charges in each pixel into voltage or current. This analog electric signal is supplied to an A/D converter  6  which converts the analog signal into a digital signal.  
           [0007]    The A/D converted digital signal is temporarily stored in a memory  7 . Data in the memory  7  is stored via a switch  8  into one of two memories  9  and  10 . The memory  9  stores an image signal output from the flat panel sensor under no exposure of X rays, as an offset fixed pattern image. The memory  10  stores an image signal obtained under exposure of X rays.  
           [0008]    Generally an X ray amount measuring apparatus (also called a phototimer) is used to monitor the X ray amount transmitted through a subject and control X ray exposure. When the cumulative X ray amount reaches a predetermined value, X ray exposure is stopped. When the X ray exposure stops, the controller scans the flat panel sensor, and image information of the subject is stored in the memory  10  via the memory  7  and switch  8 . Immediately thereafter, the flat panel sensor is driven under no X ray exposure during the period same as the subject imaging time to accumulate electric charges which are converted into a digital signal and stored in the memory  7  as an offset fixed pattern signal.  
           [0009]    This offset fixed pattern signal is stored in the memory  9  via the switch  8  which is turned to its contact B. A subtractor  11  subtracts a value in the memory  9  at each address from a value in the memory  10  at a corresponding address and the subtracted values are stored in a memory  12 .  
           [0010]    A look-up table (LUT)  13  is a table of logarithmic value conversion used for division operation. A radiation-sensed image of a subject is stored in a memory  15  via LUT  13  and a switch  14  which is turned to a contact C.  
           [0011]    A memory  16  stores an image signal subjected to a calibration operation of the apparatus. Also in this case, an image is sensed in the above-described manner and stored via the switch  14  turned to a contact D. In the calibration operation, an X ray amount distribution itself is sensed without the subject  2  to obtain a variation in gains of pixel elements. The calibration operation is usually performed approximately once per day, for example, at the start of a day work. With this calibration operation, data (also called a gain image) corresponding to a variation in gains of pixel elements of the flat panel sensor is stored in the memory  16 .  
           [0012]    A subtractor  17  subtracts the gain image from the subject image to generate an image signal whose gain variation is corrected. This image signal is stored in a memory  18 . This corrected image signal is subjected to succeeding image diagnosing, filing, transmission, display or the like.  
           [0013]    A general radiographic apparatus converts the corrected image signal into a diagnosis image signal by using a gradation process, a dynamic range change process, a spatial frequency process and the like. This diagnosis image signal is supplied to an external apparatus, typically a filing apparatus and a hard copy apparatus.  
           [0014]    An X ray image is characterized in its very wide dynamic range. For example, a medical radiographic image has a low level signal area where X rays were almost shielded, for example, due to metal embedded in a human body.  
           [0015]    Such a low level signal area may be neglected because it is quite insignificant as image information. However, if the gain correction is performed for this low signal level area, the gain variation pattern (gain image) of pixel elements is superposed upon a subject image, although this area corresponds to an area having almost no incidence energy, i.r., an area without a signal level variation. Noises are therefore generated by this gain correction.  
           [0016]    This phenomenon will be described with reference FIGS. 5A, 5B,  5 C,  5 D,  5 E and  5 F. FIGS.  5 A,  5 B and  5 C show an image of general X ray distributions in one-dimensional representation. For the purposes of simplicity, random noises generated during image sensing are omitted. A line  101  represents a gain variation acquired in the calibration operation. A line  102  represents an offset acquired under no X ray exposure. A line  103  represents an image of an actual subject. It is assumed herein that the subject has a uniform radiation transmission factor distribution. FIG. 5B shows an image  104  after the offset  102  is removed from the image signal  103 . FIG. 5C shows an image  105  after gain correction in which the offset-corrected image  104  is divided by the gain variation (gain image)  101 . This image  105  has uniform pixel values.  
           [0017]    [0017]FIGS. 5D, 5E and  5 F show an example of image data of a radiation-sensed image having very low levels without almost no sensor sensitivity. A line  106  represents a radiation-sensed image. After the offset  102  is removed from the radiation-sensed image  106 , a stable image  107  having almost constant signal values is acquired as shown in FIG. 5E. After the gain correction is performed, an image is rather obtained with noise having a pattern  108  indicative of gain variation or X ray shading as shown in FIG. 5F.  
           [0018]    This noise level is very low, while emphasized in FIG. 5F. Although this noise level is very low, the output image has a large variation since the logarithmic LUT shown in FIG. 4 is used. A general correction method including an offset correction and a gain correction is disclosed, for example, in Japanese Patent Application Laid-open No. 7-72256. A problem of noise generation caused by the gain correction is not reported.  
         SUMMARY OF THE INVENTION  
         [0019]    It is an object of the invention to solve the above-described problem.  
           [0020]    According to the present invention, the foregoing object is attained by providing an apparatus adapted to correct an image data acquired by an image detector including a plurality of detecting elements, comprising: a first correction unit adapted for obtaining an offset-corrected image by removing an offset component due to the image detector from the image data; a second correction unit adapted for correcting a pixel value of the offset-corrected image on the basis of a gain of corresponding one of the plurality of detecting elements; and a third correction unit adapted for correcting a value of a selected pixel of the offset-corrected image, the selected pixel being selected based on the pixel value of the offset-corrected image, by generating a pixel value in place of a value of the selected pixel obtained by said second correction unit.  
           [0021]    Further, the foregoing object is also attained by providing a method adapted to correct an image data acquired by an image detector including a plurality of detecting elements, comprising steps of: a first correction adapted for obtaining an offset-corrected image by removing an offset component due to the image detector from the image data; a second correction adapted for correcting a pixel value of the offset-corrected image on the basis of a gain of corresponding one of the plurality of detecting elements; and a third correction adapted for correcting a value of a selected pixel of the offset-corrected image, the selected pixel being selected based on the pixel value of the offset-corrected image, by generating a pixel value in place of a value of the selected pixel obtained by the second correction step.  
           [0022]    Furthermore, the foregoing object is also attained by providing a computer readable storage medium storing a program for making a computer perform a method adapted to correct an image data acquired by an image detector including a plurality of detecting elements, the method comprising steps of: a first correction adapted for obtaining an offset-corrected image by removing an offset component due to the image detector from the image data; a second correction adapted for correcting a pixel value of the offset-corrected image on the basis of a gain of corresponding one of the plurality of detecting elements; and a third correction adapted for correcting a value of a selected pixel of the offset-corrected image, the selected pixel being selected based on the pixel value of the offset-corrected image, by generating a pixel value in place of a value of the selected pixel obtained by the second correction step.  
           [0023]    Other objects, features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the descriptions, server to explain the principle of the invention.  
         [0025]    [0025]FIG. 1 is a block diagram showing a first embodiment of the invention.  
         [0026]    [0026]FIG. 2 is a block diagram showing a second embodiment of the invention.  
         [0027]    [0027]FIG. 3 is a block diagram showing a third embodiment of the invention.  
         [0028]    [0028]FIG. 4 is a diagram used for explaining prior art technologies.  
         [0029]    [0029]FIGS. 5A, 5B,  5 C,  5 D,  5 E and  5 F are diagrams used for explaining the problem to be solved by the invention.  
         [0030]    [0030]FIG. 6 is a graph used for explaining a preset value of the first embodiment.  
         [0031]    [0031]FIG. 7 is a graph showing an example of a LUT of the first embodiment.  
         [0032]    [0032]FIGS. 8A, 8B and  8 C are diagrams used for explaining the third embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Preferred embodiments of the present invention will be described in detail in accordance with the accompanying drawings.  
         [0034]    First Embodiment  
         [0035]    [0035]FIG. 1 is a block diagram of a radiographic apparatus embodying the present invention. In FIG. 1, like constituent elements to those shown in FIG. 4 are represented by identical reference numerals and the description thereof is omitted. Different points from the block diagram shown in FIG. 4 are the addition of constituent elements  21  to  23 . A decoder  21  evaluates the pixel value offset-corrected and logarithmically converted with a logarithmic LUT to output a signal indicative of whether a gain correction is necessary or not. This signal selects either an E contact or an F contact of a switch  23 . The switch  23  selects either a subtraction result F of a subtractor  17  for the gain correction or a value E in place of the subtraction result F. A converter  22  outputs a preset value (pixel value to be replaced with the subtraction result F) to convert the image data supplied from the memory  15 , for example.  
         [0036]    The converter  22  may be arranged to dump the input data to output a new fixed value or output a value varying in some degree in accordance with the input data.  
         [0037]    A method of determining a preset value will be described with reference to FIG. 6. In FIG. 6, a curve LUT shows the characteristics of the logarithmic conversion LUT. An input data width ΔW0 represents a variation range (a pixel value range of a gain image) of data whose gain variation is to be corrected. An input data D0 indicates that gain correction is not required for the data having a value equal to or smaller than the input data D0, because the sensor linearity cannot be guaranteed for such the value. An input of D0 data yields an output of D1 data. If the value of input data has the value D0 or smaller, the decoder  21  shown in FIG. 1 outputs a signal to select the contact E.  
         [0038]    In this case, it is considered reasonable that the preset value is set to a value corresponding to B1 which is the minimum absolute value (subtraction result takes a negative value) of the subtraction result. The reason for this is that since an output range corresponding to ΔW0 is ΔW1, there is a high possibility that an output range of the subtraction of gain correction is from B1 to A1. The converter  22  therefore outputs the value corresponding to B1 independently from the input image data.  
         [0039]    Another method is conceivable which estimates a proper output value in accordance with the input image data. For example, the converter  22  is constituted of a LUT such as shown in FIG. 7. This LUT provides an output which changes in some degree with an input. In this embodiment, the decoder  21  evaluates data passed through LUT  13 . Since this LUT has a one-to-one correspondence between input and output, the decoder  21  may be arranged so as to evaluate data before it passes through LUT  13 .  
         [0040]    For example, the decoder  21  outputs a signal instructing to select the contact E if the input (before logarithmic conversion) is 10 or smaller. In this case, a proper low level value is set to the preset value.  
         [0041]    The process, judgement and the like of this embodiment can obviously be realized by using a computer and software. It is apparent that the image processing constitution shown in FIG. 1 can be realized by computer programs.  
         [0042]    In this embodiment, although a two-dimensional image sensor (X ray image sensor  5 ) is used, the embodiment is also applicable to a one-dimensional line sensor.  
         [0043]    Second Embodiment  
         [0044]    [0044]FIG. 2 is a block diagram showing the second embodiment of the invention. Different points from those shown in FIG. 4 are the addition of constituent elements  31  to  34 . A value discrimination decoder  31  performs a similar operation to that of the decoder  21  shown in FIG. 1. In accordance with a discrimination result, the decoder  31  outputs a signal to select either a contact G of a switch  34  or a contact H thereof. A filter  32  for spatial filtering has a function of a smoothing filter (simple moving average) with nuclei of 100×100 or more. This filter  32  gradates gain correction data (gain image) and stores it in a memory  33 .  
         [0045]    If the pixel value is very low and if a pixel belongs to a pixel value area having no linearity, the decoder  31  and switch  34  operate to perform gain correction by using the correction data stored in the memory  33  which data was obtained by gradating the gain correction data stored in the memory  16  by spatial filtering. In this case, precise gain correction is not performed.  
         [0046]    However, better gain correction can be rather attained by using the gradated (smoothed) correction data, because gain variation does not substantially exist in the pixel value area having no linearity and the gain correction is not necessarily required. The technological advantage of this embodiment is that correction noises (to be caused by gain correction) can be removed more naturally because the preset value is not necessary to be set in advance.  
         [0047]    In this embodiment, although spatial filtering (moving average) is used for the correction of correction data (gain image), an averaging process of a portion or the whole of correction data may also be used. In this case, the memory capacity can be reduced.  
         [0048]    In this embodiment, the decoder  21  evaluates data passed through LUT  13 . Since this LUT has a one-to-one correspondence between input and output, the decoder  21  may evaluate data before it passes through LUT  13 .  
         [0049]    Third Embodiment  
         [0050]    In the first embodiment, instead of the gain correction value, the preset value is used for a pixel value in a low pixel value (low radiation ray amount) area. In the third embodiment, if there is a pixel having unnecessary correction noises generated by gain correction because of non-linearity of the input/output characteristics of an image sensor, the correction noises are removed by filtering the image after gain correction instead of replacing the pixel value with a preset value. A judgement of whether there is linearity or not is performed by evaluating pixel values before the gain correction.  
         [0051]    [0051]FIG. 3 is a block diagram of the third embodiment. Substantial different points from those shown in FIG. 1 are the addition of a first controller  41 , a second controller  42  and an image memory  43 . The operation of each controller is illustrated in FIGS. 8A, 8B and  8 C. FIG. 8A shows the bit map of a pixel value according to the third embodiment. Each pixel value is represented by data of a 14-bit width (0-th to 13-th bits).  
         [0052]    In this embodiment, a memory element is constituted of 16 bits (2 bytes) and a flag FLG is set to the unused 14-th bit. The flag “0” indicates a normal pixel value, and the flag “1” indicates a pixel value in an area having no linearity. Namely, the flag “1” suggests the generation of correction noises by gain correction. It is difficult to judge from only a pixel value after gain correction whether this pixel corresponds to the radiation ray amount in the area having linearity, because the pixel value was changed due to the gain correction (this pixel value cannot be used as an index of received radiation ray amount).  
         [0053]    The first controller  41  shown in FIG. 3 is operated to evaluate a pixel value before gain correction to set the flag. This operation is illustrated in the flow chart of FIG. 8B. A pixel value is checked (step s 821 ). If the pixel value is smaller than 10, it is judged that there is no linearity (the pixel value corresponds to the area without linearity), and the flag FLG is set to “1” (step s 822 ). If it is judged that there is linearity, the flag FLG is set to “0” (step s 823 ).  
         [0054]    After this operation by the first controller  41 , gain correction including subtraction is performed. The image after gain correction is transferred to the succeeding stage with the flag FLG at D14 being retained, and then is stored in the memory  18 . The second controller  42  evaluates the retained D14 (FLG) to determine whether a correction noise removal process is to be executed. Correction noises are removed by spatial filtering which outputs an average value of pixels surrounding a subject pixel. This operation is illustrated in the flow chart of FIG. 8C.  
         [0055]    First, D14 is evaluated (step s 831 ). If it is judged that D14=“1”, i.e., correction noises were generated by gain correction because of loss of linearity, an average value of pixels surrounding the subject pixel, e.g., 10×10 pixels, is written in the succeeding memory  43  (step s 832 ). If it is judged as a normal state (D14=“0”), the pixel value of the subject pixel is written in the memory  43  (step s 833 ). An image written in the memory  43  in the above manner is a final output image.  
         [0056]    In this embodiment, the evaluation result of data before gain correction is retained at the succeeding process stage. Therefore, the second controller  42  may output the preset value similar to that of the first embodiment, instead of the average value (moving average value). With this arrangement, the effects similar to those of the first embodiment can be attained.  
         [0057]    As described above, pixel data immediately after offset correction or data as an index of radiation ray amount is evaluated to identify a low level pixel value which does not show linearity of an image sensor. For such a pixel value, the gain correction is not performed, while modified gain correction data (gain image) is used or an image after gain correction is corrected. In this manner, it is possible to remove artifacts such as noises generated by gain correction. It is important to select a gain correction method in accordance with the data which may be an index of radiation ray amount, such as input data to the gain correction unit.  
         [0058]    It is effective to fix pixel data not subjected to gain correction to the preset value such as the minimum value of image data. When a low level pixel value is clipped, the image after gain correction is often clipped. However, if an image after gain correction is clipped, a variation in pixel values increases through the gain correction so that it is difficult to judge whether a pixel value correspond to a level (range) having no linearity of the image sensor. Such a problem can be solved by making a judgement by using data which may be an index of radiation ray amount, such as a pixel value before gain correction.  
         [0059]    Other Embodiments  
         [0060]    It is needless to say that the object of the invention can be achieved, by supplying a storage medium storing software program codes realizing the function of an apparatus or system of any one of the first to third embodiments to the apparatus or system, and by making a computer (CPU, MPU or the like) of the apparatus or system read and execute the program codes stored in the storage medium.  
         [0061]    In this case, the software program codes themselves read from the storage medium realize the function of any one of the first to third embodiments. Therefore, the storage medium storing the program codes and the program codes themselves constitute the present invention.  
         [0062]    The storage medium for storing such program codes may be a ROM, a floppy (registered trademark) disk, a hard disk, an optical disk, a magneto optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card or the like.  
         [0063]    It is obvious that the embodiments of the invention include not only the case that the function of any one of the first to third embodiments is realized by executing the program codes read by a computer but also the case that the function of any one of the first to third embodiments is realized by performing a portion or the whole of actual processes in accordance with the instructions of the program codes and by utilizing an OS or the like running on the computer.  
         [0064]    It is obvious that the embodiments of the invention include the case wherein the function of any one of the first to third embodiments is realized by writing the program codes read from the storage medium into a memory of a function expansion board inserted into a computer or of a function expansion unit connected to the computer, and thereafter by making a CPU or the like of the function expansion board or function expansion unit execute a portion or the whole of actual processes.  
         [0065]    If the invention is applied to such a program or a storage medium storing the program, the program is constituted of, for example, program codes corresponding to the flow charts shown in FIGS. 8B and 8C.  
         [0066]    Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.