Abstract:
An image processing device for executing a concentration correction process on an obtained radiograph by using a recording plate capable of accumulating radiographic information, wherein the radiograph contains a valid image region corresponding to a region in which the recording plate is present, and an invalid image region corresponding to the region in which the recording plate is not present. Furthermore, the image processing device is provided with: a region-to-be-corrected identification unit for identifying a pre-set range of regions within the valid image region that are adjacent to the invalid image region as to-be-corrected regions; and concentration correction units for correcting the concentration level of pixels in the to-be-corrected regions by removing the concentration level trend in the to-be-corrected regions that become present when moving from the valid image region side toward the invalid image region side.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIM 
       [0001]    This application is a Continuation of International Application No. PCT/JP2012/073167 filed on Sep. 11, 2012, which was published under PCT Article 21(2) in Japanese, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2011-200061 filed on Sep. 14, 2011, the contents all of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to an image processing apparatus (image processing device) for performing a blackening process on a radiographic image. 
       BACKGROUND ART 
       [0003]    Imaging plates (hereinafter referred to as “IP”) have shapes with round corners. Therefore, a radiographic image, which has been captured using an IP, has white spots on corners thereof that are free of the IP. Even the side edges of the radiographic image may have white spots due to wobbling movement caused at times that the IP reading device is carried. It is further known that image portions, which do not have white spots but are positioned adjacent to white spots, have regions that are significantly affected by scattering rays. The white spots are highly luminous, and the regions that are significantly affected by scattering rays, referred to as scattered-ray regions, are relatively highly luminous. Therefore, the white spots and the scattered-ray regions tend to bring about a reduction in visibility of the radiographic image, and tend to cause strain on the eyes of the observer. 
         [0004]    Japanese Laid-Open Patent Publication No. 2004-283281 discloses a technique for automatically blackening white spots and scattered-ray regions on side edges of a photographic image that is captured using an IP. 
       SUMMARY OF INVENTION 
       [0005]    However, if a scattered-ray region of a radiographic image is simply blackened out, then subject information represented by the scattered-ray region is lost. Conversely, if a scattered-ray region is not blackened out, the visibility of the radiographic image is lowered as a result of the scattered-ray region being relatively highly luminous. 
         [0006]    The present invention has been made in view of the aforementioned problems. It is an object of the present invention to provide an image processing apparatus for blackening out a scattered-ray region without causing a loss in subject information represented by the scattered-ray region. 
         [0007]    To achieve the above object, there is provided in accordance with the present invention an image processing apparatus for performing a density correcting process on a radiographic image acquired using a recording plate that is capable of storing radiation information, wherein the radiographic image includes a valid image region corresponding to a region in which the recording plate is present, and an invalid image region corresponding to a region in which the recording plate is not present, the image processing apparatus comprising a correction target region identifying unit for identifying a region in a predetermined range of the valid image region, which is adjacent to the invalid image region, as a correction target region, and a density correction unit for removing a trend of density values of the correction target region, which is present from the valid image region toward the invalid image region, thereby correcting the density values of pixels in the correction target region. 
         [0008]    To achieve the aforementioned object, there also is provided in accordance with the present invention an image processing apparatus for performing a density correcting process on a radiographic image acquired using a solid-state detector, wherein the radiographic image includes a valid image region corresponding to a region that is irradiated with radiation, and an invalid image region corresponding to a region that is not irradiated with radiation, the image processing apparatus comprising a correction target region identifying unit for identifying a region in a predetermined range of the valid image region, which is adjacent to the invalid image region, as a correction target region, and a density correction unit for removing a trend of density values of the correction target region, which is present from the valid image region toward the invalid image region, thereby correcting the density values of pixels in the correction target region. 
         [0009]    The density correction unit may correct the density values of pixels in the correction target region by increasing density values to be added to pixels in the correction target region as the pixels are closer to the invalid image region. 
         [0010]    Within the valid image region, a region other than the correction target region may be regarded as a region of interest, and the density correction unit may include a map calculator for calculating an additive quantity map representing additive quantities to be added to the density values of a plurality of pixels in the correction target region by calculating differences between the density values of a plurality of reference pixels serving as a reference for calculating the additive quantity map in the region of interest and the density values of the pixels in the correction target region, a low-pass filtering processor for performing a low-pass filtering process on the additive quantity map, and a density value adder for adding the additive quantity map, which has been processed by the low-pass filtering process, to the density values of the pixels in the correction target region, so that the density correction unit removes the trend of density values of the correction target region. 
         [0011]    Within the valid image region, a region other than the correction target region may be regarded as a region of interest, and the density correction unit may include a low-pass filtering processor for performing a low-pass filtering process on the density values of a plurality of pixels in the correction target region, a map calculator for calculating an additive quantity map representing additive quantities to be added to the density values of the pixels in the correction target region by calculating differences between the density values of a plurality of reference pixels serving as a reference for calculating the additive quantity map in the region of interest and the density values of the pixels in the correction target region, which have been processed by the low-pass filtering process, and a density value adder for adding the additive quantity map to the density values of the pixels in the correction target region, so that the density correction unit removes the trend of density values of the correction target region. 
         [0012]    The map calculator may calculate the additive quantity map by calculating differences between the density values of the pixels in the correction target region and the density values of the reference pixels that are shortest in distance from the pixels in the correction target region. 
         [0013]    Within the valid image region, a region other than the correction target region may be regarded as a region of interest, and the density correction unit may include a low-pass filtering processor for performing a low-pass filtering process on the density values of a plurality of pixels in the correction target region, a differential density value calculator for calculating differential density values of the pixels in the correction target region by calculating differences between the density values of the pixels in the correction target region and the density values of the pixels in the correction target region, which have been processed by the low-pass filtering process, and an adder for adding the density values of a plurality of reference pixels serving as a reference in the region of interest to the differential density values of the pixels in the correction target region, so that the density correction unit corrects the density values of the pixels in the correction target region. 
         [0014]    The adder may add the pixels in the correction target region and the reference pixels that are shortest in distance from the pixels in the correction target region. 
         [0015]    A boundary line between the correction target region and the region of interest and a boundary line between the valid image region and the invalid image region may lie parallel to each other. 
         [0016]    The reference pixels may comprise pixels in the region of interest that are adjacent to the pixels in the correction target region. 
         [0017]    The correction target region identifying unit may identify, as the correction target region, a region in the valid image region that lies within a constant distance from a boundary line between the valid image region and the invalid image region. 
         [0018]    The correction target region identifying unit may identify, as the correction target region, the invalid image region together with the region in the predetermined range of the valid image region that is adjacent to the invalid image region. 
         [0019]    The density correction unit may blacken out the invalid image region by replacing the density values of pixels in the invalid image region with a predetermined density value. 
         [0020]    According to the present invention, the trend of density values of the correction target region, which is present from the valid image region toward the invalid image region of the radiographic image, can be removed, thereby correcting and blackening out the density values of pixels that lie within the correction target region. Consequently, a scattered-ray region can be blackened out while retaining subject information. 
         [0021]    The aforementioned objects and other objects, characteristics, and advantages of the present invention will become more apparent from the following descriptions of a preferred embodiment, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1  is a perspective view of a mammographic apparatus, which serves as an example of a radiographic image capturing apparatus; 
           [0023]      FIG. 2  is an enlarged fragmentary side elevational view of the mammographic apparatus shown in  FIG. 1 ; 
           [0024]      FIG. 3  is an electric block diagram of an image processing system for reading and processing a radiographic image stored in an IP; 
           [0025]      FIG. 4  is a diagram showing a corner of a radiographic image read by an image reader; 
           [0026]      FIG. 5  is a block diagram of an image processing apparatus shown in  FIG. 3 ; 
           [0027]      FIG. 6  is a diagram illustrating a specified correction target region; 
           [0028]      FIG. 7  is a flowchart of an operation sequence of the image processing apparatus shown in  FIG. 3 ; 
           [0029]      FIG. 8  is a diagram illustrating a process of calculating an additive quantity map; 
           [0030]      FIG. 9  is a diagram showing a relationship between the density value of each pixel on a normal line A shown in  FIG. 8  and the additive quantity to be added to each pixel in a correction target region on the normal line A; 
           [0031]      FIG. 10  is a diagram showing an image of an additive quantity map calculated from the radiographic image shown in  FIG. 4 , which is read by the image reader and processed by a low-pass filtering process; 
           [0032]      FIG. 11  is a diagram showing the density value of each pixel of the correction target region on the normal line A shown in  FIG. 9 , at a time that the additive quantity map is added to density values of a plurality of pixels of the correction target region; 
           [0033]      FIG. 12  is a diagram showing a radiographic image, which is generated after the additive quantity map processed by the low-pass filtering process is added to density values of the pixels of the correction target region, for a case in which the radiographic image shown in  FIG. 4  is read by the image reader; 
           [0034]      FIG. 13  is a block diagram of a density correction unit according to a fourth modification; and 
           [0035]      FIG. 14  is a diagram showing a radiographic image generated by blackening out an invalid image region by correcting density values of a region using an additive quantity map, and thereafter replacing a plurality of pixel values in the invalid image region with a predetermined density value. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    An image processing apparatus according to a preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings. 
         [0037]      FIG. 1  is a perspective view of a mammographic apparatus  10 , which serves as an example of a radiographic image capturing apparatus.  FIG. 2  is an enlarged fragmentary side elevational view of the mammographic apparatus  10 . The mammographic apparatus  10  includes an upstanding base  12 , a main apparatus unit  16  fixed to a swing shaft  14  disposed substantially centrally on the base  12 , a radiation source housing unit  24  housing therein a radiation source  20  such as a molybdenum tube, a tungsten tube, a rhodium tube, or the like, for applying radiation (e.g., X-rays) to an object to be examined of a subject  18 , and which is fixed to an end of an arm  22  of the main apparatus unit  16 , an image capturing base  28  which is fixed to a bracket  26  of the main apparatus unit  16  and housing therein a solid-state detector, not shown, for detecting radiation that has passed through a breast  18   a  and acquiring radiographic image information, and a compression plate  30  mounted on the bracket  26  for compressing and holding the breast  18   a  against the image capturing base  28 . 
         [0038]    The solid-state detector, which comprises an IP (stimulable light emission plate), acquires a radiographic image of the subject by storing radiation that has passed through the breast  18   a . The IP stores radiation that has passed through the breast  18   a , and upon being irradiated with stimulating light, is stimulated to emit light having an intensity that depends on the dose of stored radiation. The IP includes a stimulable phosphor layer disposed on a substrate. A display operation unit  32  is mounted on the base  12  for displaying image capturing information, such as an image capturing direction, etc., and ID information of the subject  18 . The display operation unit  32  also enables such items of information to be set as necessary. 
         [0039]    An image processing system  50 , as shown in  FIG. 3 , will be described below.  FIG. 3  is an electric block diagram of an image processing system  50  for reading and processing a radiographic image that is stored in an IP  52 . The image processing system  50  includes the IP  52 , an image reader  54 , an image processing apparatus  56 , a display controller  58 , a display unit  60 , a record controller  62 , and a recording medium  64 . 
         [0040]    The image reader  54  reads image information of the subject that has been detected by the IP  52  (recording plate), converts the read radiographic image of the subject into digital signals, and supplies the digital signals to the image processing apparatus  56 . More specifically, the image reader  54  includes a light source for emitting stimulating light, a scanner for sweeping the stimulating light, a photomultiplier, an amplifier, and an A/D converter (not shown). The scanner scans the surface of the IP  52  with stimulating light. The scanner scans the surface of the IP  52 , whereupon stimulating light is applied to the IP  52 , and the region of the IP  52  that is irradiated with stimulating light emits an amount of light that depends on the stored radiographic image of the subject. The photomultiplier converts the emitted light into electric signals, and the amplifier amplifies the electric signals. The A/D converter converts the amplified electric signals into digital signals, thereby producing a radiographic image represented by the digital signals. 
         [0041]    The image processing apparatus  56  carries out predetermined image processing, to be described later, on the radiographic image read by the image reader  54 , and supplies the processed radiographic image to at least one of the display controller  58  and the record controller  62 . 
         [0042]    The display controller  58  displays the radiographic image on the display unit  60 , which may comprise a liquid crystal display panel or the like. The record controller  62  records the radiographic image in the recording medium  64 , which may comprise a flash memory, a hard disk, or the like. 
         [0043]      FIG. 4  is a diagram showing a corner of a radiographic image read by the image reader  54 . As shown in  FIG. 4 , the radiographic image includes a valid image region  72  that corresponds to a region in which the IP  52  is present, and an invalid image region  74  that corresponds to a region in which the IP  52  is not present. As described above, since the IP  52  has a shape with rounded corners, the IP  52  is incapable of storing radiation around the corners even if the IP  52  is irradiated with radiation. Therefore, the invalid image region  74  is highly luminous and is observed as a white spot. In addition, the valid image region  72  includes a scattered-ray region  76  that corresponds to an edge of the IP  52 . Since the edge of the IP  52  scatters radiation, the scattered-ray region  76  is observed as a blurred image. The density value of each pixel of the radiographic image varies depending on the dose of radiation applied to the IP  52 . As the applied dose of radiation becomes smaller, the density value also becomes smaller. The density value is low in the invalid image region  74  and varies within the scattered-ray region  76 . The density value becomes smaller toward the edge of the IP  52 . In corners of the radiographic image, a trend of density values is present from the valid image region  72  toward the invalid image region  74 . 
         [0044]      FIG. 5  is a block diagram of the image processing apparatus  56 . The image processing apparatus  56  includes an invalid image region detector  80 , a correction target region identifying unit  82 , and a density correction unit  84 . 
         [0045]    The invalid image region detector  80  detects the invalid image region  74  of the radiographic image that is sent from the image reader  54 . The invalid image region detector  80  may detect the invalid image region  74  by separating a background region and a profile region based on clustering according to a K-means algorithm and regarding the background region as the invalid image region  74 , as disclosed in Japanese Patent No. 2831892, for example. Alternatively, as disclosed in Japanese Laid-Open Patent Publication No. 2004-283281, the invalid image region detector  80  may detect the invalid image region  74  by detecting an edge. Further, alternatively, the invalid image region detector  80  may detect the invalid image region  74  by comparing the density value of each pixel of the radiographic image with a threshold value, and regarding a group of pixels the density values of which are lower than the threshold value as the invalid image region  74 . According to the present embodiment, the invalid image region detector  80  detects the invalid image region  74  by comparing the density value of each pixel of the radiographic image with a threshold value. The invalid image region detector  80  supplies the radiographic image that was sent thereto to the correction target region identifying unit  82 , and also supplies information representing the detected invalid image region  74 . 
         [0046]    Based on the detected invalid image region  74 , the correction target region identifying unit  82  identifies a region to be corrected, i.e., a correction target region, of the radiographic image where density values of the pixels are to be corrected. The correction target region identifying unit  82  identifies the invalid image region  74  together with a region in the valid image region  72 , which lies within a predetermined range adjacent to the invalid image region  74 , as a correction target region. The correction target region identifying unit  82  supplies the radiographic image that was sent thereto to the density correction unit  84 , and also supplies information representing the specified correction target region and the invalid image region  74 . 
         [0047]      FIG. 6  is a diagram illustrating a specified correction target region  90 . The region in the valid image region  72 , which lies within the predetermined range adjacent to the invalid image region  74 , may refer to a region (shown in hatching)  94  that lies within the valid image region  72  and which falls at a constant distance from a boundary line  92  between the valid image region  72  and the invalid image region  74 . The length of the constant distance is determined such that the region  94  includes the scattered-ray region  76  in its entirety. 
         [0048]    In the valid image region  72 , a region other than the correction target region  90  (region  94 ) is regarded as a region of interest  96 . A boundary line  98  between the region of interest  96  and the correction target region  90 , i.e., a boundary line between the region of interest  96  and the region  94 , lies parallel to the boundary line  92 . The boundary line  92  is made up of a plurality of pixels in the valid image region  72 , which are adjacent to the pixels of the invalid image region  74 . The boundary line  98  is made up of a plurality of pixels in the region of interest  96 , which are adjacent to the pixels of the region  94 . The pixels in the region of interest  96  that make up the boundary line  98 , i.e., the pixels on the boundary line  98 , are referred to as reference pixels. 
         [0049]    The density correction unit  84  removes the trend of pixel values of the pixels in the correction target region  90 , which is present from the valid image region  72  toward the invalid image region  74 , thereby correcting (blackening) the density values of the pixels in the correction target region  90  so as to blacken out the correction target region  90 . 
         [0050]    The density correction unit  84  includes a map calculator  100 , a low-pass filtering processor  102 , and a density value adder  104 . The map calculator  100  calculates an additive quantity map by calculating differences between the density values of the reference pixels and the density values of the pixels in the correction target region  90 . The additive quantity map refers to a map representing additive quantities (density values) to be added to the density values of the pixels in the correction target region  90 . The map calculator  100  may also calculate an additive quantity map by calculating differences between the density values of the pixels in the correction target region  90  and the density values of the reference pixels that are shortest in distance from the pixels in the correction target region  90 . 
         [0051]    The low-pass filtering processor  102  performs a low-pass filtering process on the calculated additive quantity map to thereby remove high-frequency components from the additive quantity map. The density correction unit  84  adds the additive quantity map processed by the low-pass filtering process to the density values of the pixels in the correction target region  90 , thereby correcting the density values of the pixels in the correction target region  90 . 
         [0052]    Operations of the image processing apparatus  56  will be described below with reference to the flowchart shown in  FIG. 7 . First, if a radiographic image is sent from the image reader  54  to the image processing apparatus  56 , the invalid image region detector  80  detects an invalid image region  74  in the radiographic image that was sent to the image processing apparatus  56  (step S 1 ). 
         [0053]    Next, based on the detected invalid image region  74 , the correction target region identifying unit  82  identifies a correction target region  90  where the density values of pixels are to be corrected in the radiographic image (step S 2 ). The correction target region identifying unit  82  then identifies, as a correction target region  90 , a region  94  that lies within the invalid image region  74 , and a predetermined range in the valid image region  72  that lies adjacent to the invalid image region  74  (see  FIG. 6 ). Due to the fact that the scattered-ray region  76 , which is difficult to detect accurately, is to be included within the correction target region  90 , the correction target region identifying unit  82  identifies not only the scattered-ray region  76  and the invalid image region  74  as the correction target region  90 , but also identifies, as the correction target region  90 , the region  94  including the scattered-ray region  76  and the invalid image region  74 . 
         [0054]    Next, the map calculator  100  of the density correction unit  84  identifies a plurality of reference pixels from the pixels of the region of interest  96  of the radiographic image (step S 3 ). More specifically, the map calculator  100  identifies, as reference pixels, a plurality of pixels in the region of interest  96 , which are present on the boundary line  98  between the region of interest  96  and the correction target region  90 . 
         [0055]    Thereafter, the map calculator  100  of the density correction unit  84  calculates an additive quantity map representing additive quantities to be added to the density values of the pixels in the correction target region  90 , by calculating differences between the density values of the pixels in the correction target region  90  and the density values of the reference pixels that are shortest in distance from the pixels in the correction target region  90  (step S 4 ). The map calculator  100  calculates the differences between the density values of the pixels in the correction target region  90  and the density values of the reference pixels that are shortest in distance from the pixels in the correction target region  90 , by subtracting the density values of the pixels in the correction target region  90  from the density values of the reference pixels. 
         [0056]      FIG. 8  is a diagram illustrating a process of calculating an additive quantity map. Since the map calculator  100  calculates differences between the density values of the pixels in the correction target region  90  and the density values of the reference pixels that are shortest in distance from the pixels in the correction target region  90 , the pixels in the correction target region  90  and the reference pixels are both present on the same normal lines to the boundary line  98 . For example, a pixel  110  in the correction target region  90  and a reference pixel  112  shortest in distance from the pixel  110  both are present on a normal line A, which is normal to the boundary line  98  on the reference pixel  112 . The map calculator  100  calculates an additive quantity map by calculating, on each line normal to the boundary line  98 , the difference between the density value of the reference pixel and the density value of each pixel in the correction target region  90 . In  FIG. 8 , the breast  18   a  is illustrated as an object whose image is to be captured. 
         [0057]      FIG. 9  is a diagram showing a relationship between the density value of each pixel on a normal line A shown in  FIG. 8  and the additive quantity to be added to each pixel in the correction target region  90  on the normal line A. As shown in  FIG. 9 , a region in which the pixel density values are lower than the threshold value referred to above is indicated as an invalid image region  74 , and a region in which the pixel density values are higher than the threshold value referred to above is indicated as a valid image region  72 . The invalid image region  74  and a region  94  including the scattered-ray region  76  are indicated as a correction target region  90 . It can be seen from the figure that the scattered-ray region  76  has pixel values therein that vary significantly. 
         [0058]    As shown in  FIG. 9 , the density value of each pixel in the correction target region  90  is subtracted from the density value of a reference pixel on a normal line A, whereby an additive quantity, which is to be added to each pixel in the correction target region  90  on the normal line A, is determined. 
         [0059]    Next, the low-pass filtering processor  102  of the density correction unit  84  performs a low-pass filtering process on the additive quantity map that was calculated in step S 4 , to thereby remove high-frequency components from the additive quantity map (step S 5 ). Upon the low-pass filtering process being performed on the additive quantity map, as shown in  FIG. 9 , a curve (indicated by the broken line in  FIG. 9 ), which is represented by the additive quantities to be added to the pixels in the correction target region  90 , is smoothed. In the correction target region  90 , pixels with lower density values are combined with greater additive quantities, which are added thereto. In other words, pixels that are closer to the invalid image region  74  are combined with greater additive quantities, which are added thereto, and the pixels of the invalid image region  74  are combined with the greatest additive quantity, which is added thereto. 
         [0060]      FIG. 10  is a diagram showing an image of an additive quantity map, which is calculated from the radiographic image shown in  FIG. 4 , and which is read by the image reader  54  and processed by a low-pass filtering process. As shown in  FIG. 10 , image regions corresponding to the invalid image region  74  and the scattered-ray region  76  are blackened out, and an image region corresponding to the valid image region  72  other than the scattered-ray region  76  is observed as a white spot. 
         [0061]    Next, the density value adder  104  adds the additive quantity map to the density values of the pixels in the correction target region  90  of the radiographic image that is read by the image reader  54  (step S 6 ). 
         [0062]    In step S 6 , rather than the additive quantity map calculated in step S 4 , the density value adder  104  adds the additive quantity map that was processed by the low-pass filtering process in step S 5  to the density values of the pixels in the correction target region  90  for the following reasons. Namely, the high-frequency components in the correction target region  90  represent subject information. If an additive quantity map that was not processed by the low-pass filtering process is added to the density values of the pixels in the correction target region  90 , then subject information in the correction target region  90  is lost. On the other hand, if the additive quantity map that was processed by the low-pass filtering process is added to the density values of the pixels in the correction target region  90 , the correction target region  90  is blackened out while retaining the subject information (high-frequency components) in the correction target region  90 . 
         [0063]      FIG. 11  is a diagram showing the density value of each pixel in the correction target region  90  on the normal line A shown in  FIG. 9 , at a time that the additive quantity map is added to the density values of the pixels of the correction target region. As shown in  FIG. 11 , the correction target region  90  is blackened out, i.e., density values of the pixels in the correction target region  90  are increased, without removing subject information (high-frequency components) in the scattered-ray region  76 . Similarly, the invalid image region  74  is blackened out. 
         [0064]      FIG. 12  is a diagram showing a radiographic image generated after the additive quantity map, which was processed by the low-pass filtering process, is added to the density values of the pixels in the correction target region  90 , for a case in which the radiographic image shown in  FIG. 4  is read by the image reader  54 . As shown in  FIG. 12 , the invalid image region  74  and the scattered-ray region  76  are blackened out substantially uniformly. 
         [0065]    As described above, an additive quantity map is calculated by subtracting the density values of a plurality of pixels in the correction target region  90  from the density values of a plurality of reference pixels. Further, after the additive quantity map has been processed by the low-pass filtering process, the processed additive quantity map is added to the density values of the pixels in the correction target region  90 . Therefore, a trend of the density values of the pixels in the correction target region  90 , which is present from the valid image region  72  toward the invalid image region  74 , is removed, and the correction target region  90  is blackened out. In other words, the scattered-ray region  76  is blackened out while retaining subject information (high-frequency components) of the scattered-ray region  76 , and the invalid image region  74  also is blackened out. Although it is difficult to accurately detect the scattered-ray region  76  from the radiographic image, the scattered-ray region  76  is blackened out without removing the subject information of the scattered-ray region  76 , regardless of the difficulty in accurately detecting the scattered-ray region  76  according to the present embodiment. Since the additive quantities that are added to the density values of pixels in the valid image region  72 , which is not the scattered-ray region  76 , of the correction target region  90  are extremely small, even if the region  94  is greater than the scattered-ray region  76 , the density values of the valid image region  72 , which is not the scattered-ray region  76 , are not significantly affected by the density correction, but remain essentially uncorrected. In this manner, no subject information is lost. 
         [0066]    Conversely, if the density values of the pixels in the correction target region  90  are corrected by being replaced with a predetermined value, then the subject information in the valid image region  72  is lost. According to the present embodiment, such a problem does not occur. 
         [0067]    According to the above embodiment, since greater additive quantities are added to the pixels in the correction target region  90 , the density values of which are lower than the density value of the reference pixels, the correction target region  90  is blackened out substantially uniformly while retaining subject information therein. 
         [0068]    In the case that a radiographic image of the breast  18   a  is captured, a line tangential to an edge of the breast  18   a  in the scattered-ray region  76  is close to the normal line A, which is normal to the boundary line  98 , as shown in  FIG. 8 . Consequently, calculation of the additive quantity map, which is calculated by determining the difference between the density value of a reference pixel and the density value of a pixel in the correction target region  90  on respective normal lines that are normal to the boundary line  98  and then adding the calculated additive quantity map, makes it possible to blacken out the scattered-ray region  76  while sufficiently retaining subject information therein. 
         [0069]    The above embodiment can be modified in the following ways. 
       (Modification 1) 
       [0070]    In the above-described embodiment, the map calculator  100  calculates an additive quantity map by subtracting the density values of a plurality of pixels in the correction target region  90  from the density values of a plurality of reference pixels (step S 4 ), and thereafter, the low-pass filtering processor  102  performs a low-pass filtering process on the calculated additive quantity map (step S 5 ). However, the low-pass filtering processor  102  may perform a low-pass filtering process on the density values of a plurality of pixels in the correction target region  90 , and thereafter, the map calculator  100  may determine an additive quantity map by subtracting the density values of the pixels in the correction target region  90 , which have been processed by the low-pass filtering process, from the density values of a plurality of reference pixels. According to this modification, although the low-pass filtering process is not performed on the additive quantity map, the same additive quantity map as that generated in the above-described embodiment is obtained, the trend of the density values of the pixels in the correction target region  90 , which is present from the valid image region  72  toward the invalid image region  74 , is removed, and the correction target region  90  is blackened out. 
       (Modification 2) 
       [0071]    In the above-described embodiment, an additive quantity map is calculated by calculating the difference between the density values of reference pixels and the density values of pixels in the correction target region  90 , both of which are on the same normal lines. However, an additive quantity map may be calculated by calculating the difference between the density values of reference pixels and the density values of pixels in the correction target region  90 , which are not on the same normal lines, but are on the same straight lines, for example. For example, an additive quantity map may be calculated by calculating the difference between the density values of reference pixels and the density values of pixels in the correction target region  90 , which are on the same straight lines that lie parallel to a y-axis direction (vertical direction of the radiographic image) or an x-axis direction (horizontal direction of the radiographic image). 
       (Modification 3) 
       [0072]    In the above-described embodiment, pixels on the boundary line  98  between the region of interest  96  and the correction target region  90  are used as reference pixels. However, the reference pixels may be pixels that lie within the region of interest  96 . 
       (Modification 4) 
       [0073]    According to Modification 4, the density correction unit  84  of the image processing apparatus  56  may be replaced with a density correction unit  120  shown in  FIG. 13 . Further, according to Modification 4, the density correction unit  120  includes a low-pass filtering processor  122 , a differential density value calculator  124 , and an adder  126 . The low-pass filtering processor  122  performs a low-pass filtering process on the density values of a plurality of pixels in the correction target region  90  in order to remove high-frequency components. The differential density value calculator  124  calculates differences between the density values of the pixels in the correction target region  90  and the density values of the pixels in the correction target region  90 , which have been processed by the low-pass filtering process, thereby calculating differential density values of the pixels in the correction target region  90 . The adder  126  adds the density values of a plurality of reference pixels to the calculated differential density values of the pixels in the correction target region  90 . The adder  126  adds the reference pixels and the pixels in the correction target region  90 , which are present on the same straight lines, e.g., normal lines. According to Modification 4, the trend of the density values of the pixels in the correction target region  90 , which is present from the valid image region  72  toward the invalid image region  74 , is removed, and the correction target region  90  is blackened out. 
       (Modification 5) 
       [0074]    In step S 6  shown in  FIG. 7 , after the density correction unit  84  has added the additive quantity map to the density values of the pixels in the correction target region  90 , thereby blackening out the pixels in the correction target region  90 , the density correction unit  84  may blacken out the image of the invalid image region  74 . For example, the density correction unit  84  may blacken out the invalid image region  74  by replacing the density values of the pixels in the invalid image region  74  with a predetermined density value. 
         [0075]    In the above-described embodiment, in step S 2  of  FIG. 7 , the correction target region identifying unit  82  identifies the region  94  and the invalid image region  74  as a correction target region  90 . However, according to the present modification, the correction target region identifying unit  82  may identify only the region  94  as a correction target region  90 . In this case, in step S 6 , the density values of the region  94  identified as a correction target region  90  are corrected by the additive quantity map, and the correction target region  90  is blackened out while retaining the subject information therein. Since the density values of the invalid image region  74  are not corrected, the invalid image region  74  is blackened out by replacing the density values of the pixels in the invalid image region  74  with a predetermined density value. 
         [0076]      FIG. 14  is a diagram showing a radiographic image, which is generated upon blackening out the invalid image region  74  by correcting the density values of the region  94  using an additive quantity map, and thereafter replacing a plurality of pixel values in the invalid image region  74  with a predetermined density value. 
       (Modification 6) 
       [0077]    In the above embodiment, the entire region of the IP is irradiated with radiation. However, the irradiation field may be reduced such that only a portion of the entire region of the IP is irradiated with radiation. In this case, while the region of the IP that is irradiated with radiation is able to produce image information concerning the subject, the region of the IP that is not irradiated with radiation is unable to produce image information of the subject, but instead produces a white spot. 
         [0078]    Even if the irradiation field is reduced while capturing the radiographic image, a scattered-ray region in which the radiation is scattered is produced between the region that is irradiated with radiation and the region that is not irradiated with radiation. The same processing sequence as with the above embodiment then is carried out in order to blacken out the scattered-ray region while retaining the subject information of the scattered-ray region, and also to blacken out the region that is not irradiated with radiation. 
         [0079]    More specifically, according to Modification 6, among the radiographic image read from the IP  52 , which has been captured in the reduced irradiation field, a region corresponding to the region irradiated with radiation is regarded as the valid image region  72  in the above-described embodiment, a region corresponding to the region that is not irradiated with radiation is regarded as the invalid image region  74  in the above-described embodiment, and a scattered-ray region, which is produced between the valid image region  72  and the invalid image region  74 , is regarded as the scattered-ray region  76  in the above-described embodiment. The operational sequence according to the flowchart shown in  FIG. 7  is performed in order to blacken out the correction target region  90 , which includes the scattered-ray region  76 . 
         [0080]    In Modification 6, the solid-state detector is not limited to the IP  52 , but may comprise a DR (digital radiography) panel such as an electronic cassette (FPD) or the like, which is capable of detecting radiation electrically. 
       (Modification 7) 
       [0081]    Modifications 1 through 6 described above may be combined in any way, insofar as no inconsistencies occur in the resultant combinations. 
         [0082]    The present invention has been described above in connection with a given preferred embodiment. However, the technical scope of the present invention is not limited to the description of the above embodiment. It will be obvious to those skilled in the art that various changes or improvements may be made to the above embodiment. It is apparent from the description of the scope of the claims for patent that such changes or modifications fall within the technical scope of the present invention.