Patent Publication Number: US-10786219-B2

Title: Radiographic image processing apparatus, scattered radiation correction method, and computer readable storage medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2018-009644, filed on Jan. 24, 2018, the entirety of which is hereby incorporated by reference herein and forms a part of the specification. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a radiographic image processing apparatus, a scattered radiation correction method, and a computer readable storage medium. 
     2. Description of the Related Art 
     Capturing a radiographic image of a subject with radiation transmitted through the subject raises a problem that the radiation scatters in the subject according to the thickness of the subject and generated scattered radiation deteriorates the contrast of the acquired radiographic image. Therefore, at the time of capturing a radiographic image, a scattered radiation removal grid (hereinafter, simply referred to as “grid”) can be provided between the subject and a radiation detector so that the radiation detector is not irradiated with scattered radiation when detecting radiation and acquiring the radiographic image. Using the grid to perform imaging can improve the contrast of the radiographic image because the radiation detector is less likely to be irradiated with radiations scattered by the subject. However, using the grid raises a problem that the workload of grid arrangement at the time of portable imaging to be performed in a hospital room or the like is large and the burden on a patient is heavy and further the grid may cut the radiations traveling straight when the applied direction of the radiations is inappropriate. Therefore, in order to improve the contrast lowered by the scattered radiation components of the radiations, it is conventionally known to perform image processing on the radiographic image for correction of the scattered radiation. 
     For example, Patent Literature 1 (JA2015-100543A) discloses a technique capable of generating a band image that represents a frequency component for each of a plurality of frequency bands by frequency decomposing an original image, converting the band image based on irradiation field information, subject information, imaging conditions and the like and generating a converted band image by multiplying a weight determined for each frequency band, generating a scattered radiation image by combining a plurality of converted band images having been generated, and subtracting the scattered radiation image from the original image to generate a radiographic image from which the scattered radiation has been removed. In general, it is known that the amount of contained scattered radiation is relatively large in a low-frequency band and is smaller in a high-frequency band. Therefore, in Patent Literature 1, the weight is determined so that the weight increases as the frequency band becomes lower. 
     Further, Patent Literature 2 (JP09-270004A) discloses a technique capable of generating a low-frequency component image by processing an original image with a lowpass filter, adjusting the generated low-frequency component image according to scattered radiation influence degree by the edge, and subtracting the adjusted low-frequency component image from the original image to generate a radiographic image from which the scattered radiation has been removed. 
     The techniques discussed in Patent Literatures 1 and 2 are techniques based on the recognition that the amount of contained scattered radiation is relatively large in the low-frequency band. However, experiments conducted by inventors of the present invention have revealed that the scattered radiation contains a certain amount of high-frequency components, the amount of frequency components in the high-frequency band increases in the vicinity of strong edges and the amount of frequency components in the low-frequency band increases in the vicinity of weak edges. Therefore, according to the technique discussed in Patent Literature 1 or Patent Literature 2, estimation errors of high-frequency components of the scattered radiation increase, especially, in the vicinity of strong edges, and high-frequency components are more likely to be emphasized after removal of the scattered radiation. Therefore, removal of scattered radiation in the vicinity of edges cannot be precisely performed. 
     SUMMARY 
     The present invention intends to obtain a radiographic image from which scattered radiation has been precisely removed by improving the accuracy in estimating scattered radiation in the vicinity of edges in the radiographic image, thereby. 
     To achieve at least one of the above-mentioned objects, a radiographic image processing apparatus according to one aspect of the present invention includes a hardware processor that determines the intensity of an edge in a radiographic image obtained by radiographically imaging a subject, sets a weighting factor to be used in extracting a frequency component from the radiographic image according to a determination result of the edge intensity, extracts the frequency component from the radiographic image using the weighting factor having been set, multiplies the extracted frequency component by a scattered radiation content rate to estimate a scattered radiation component in the radiographic image, multiplies the estimated scattered radiation component by a scattered radiation removal rate to generate a scattered radiation image representing the scattered radiation component to be removed from the radiographic image, and performs scattered radiation correction on the radiographic image by subtracting the scattered radiation image from the radiographic image. 
     Further, a scattered radiation correction method according to one aspect of the present invention is a scattered radiation correction method for a radiographic image processing apparatus that performs scattered radiation correction on a radiographic image obtained by radiographically imaging a subject, the method including: 
     determining the intensity of an edge in the radiographic image, 
     setting a weighting factor to be used in extracting a frequency component from the radiographic image according to a determination result of the edge intensity, 
     extracting the frequency component from the radiographic image using the weighting factor having been set, 
     multiplying the extracted frequency component by a scattered radiation content rate to estimate a scattered radiation component in the radiographic image, 
     multiplying the estimated scattered radiation component by a scattered radiation removal rate to generate a scattered radiation image representing the scattered radiation component to be removed from the radiographic image, and 
     performing scattered radiation correction on the radiographic image by subtracting the scattered radiation image from the radiographic image. 
     Further, a computer readable storage medium according to one aspect of the present invention is a non-transitory computer readable storage medium storing a program that causes a computer, to be used for a radiographic image processing apparatus that performs scattered radiation correction on a radiographic image obtained by radiographically imaging a subject, to determine the intensity of an edge in the radiographic image obtained by radiographically imaging the subject, set a weighting factor to be used in extracting a frequency component from the radiographic image according to a determination result of the edge intensity, extract the frequency component from the radiographic image using the weighting factor having been set, multiply the extracted frequency component by a scattered radiation content rate to estimate a scattered radiation component in the radiographic image, multiply the estimated scattered radiation component by a scattered radiation removal rate to generate a scattered radiation image representing the scattered radiation component to be removed from the radiographic image, and perform scattered radiation correction on the radiographic image by subtracting the scattered radiation image from the radiographic image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects, advantageous effects and features of the present invention will be more fully understood from the following detailed description and the accompanying drawing. However, these are not intended to limit the present invention. 
         FIG. 1  is a view illustrating the entire configuration of a radiographic imaging system according to an exemplary embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a functional configuration of a console illustrated in  FIG. 1 . 
         FIG. 3  is a view schematically illustrating an exemplary flow of scattered radiation correction processing A, which can be executed by a controller illustrated in  FIG. 2  according to a first exemplary embodiment. 
         FIG. 4  is a view illustrating filter processing. 
         FIG. 5  is a view illustrating a relationship between a weighting factor and a pixel value difference according to the first exemplary embodiment. 
         FIG. 6A  is a view illustrating a result of scattered radiation removal based on a frequency component image obtained by performing filter processing using a constant weighting factor. 
         FIG. 6B  is a view illustrating a result of scattered radiation removal based on a frequency component image obtained by performing filter processing using a weighting factor reflecting the edge. 
         FIG. 7  is a view schematically illustrating an exemplary flow of scattered radiation correction processing B that can be executed by the controller illustrated in  FIG. 2  according to a second exemplary embodiment. 
         FIG. 8  is a view illustrating exemplary non-local means filter processing. 
         FIG. 9  is a view schematically illustrating an exemplary flow of scattered radiation correction processing C that can be executed by the controller illustrated in  FIG. 2  according to a third exemplary embodiment. 
         FIG. 10  is a view illustrating frequency decomposition using only one kernel. 
         FIG. 11  is a view illustrating frequency decomposition using a plurality of kernels that are mutually different in size and/or shape. 
         FIG. 12A  is a view illustrating an exemplary relationship between the distance from an attentional pixel and the weighting factor in a kernel to be used in the frequency decomposition and differentiated in shape. 
         FIG. 12B  is a view illustrating an exemplary relationship between the distance from an attentional pixel and the weighting factor in another kernel to be used in the frequency decomposition and differentiated in shape. 
         FIG. 12C  is a view illustrating an exemplary relationship between the distance from an attentional pixel and the weighting factor in another kernel to be used in the frequency decomposition and differentiated in shape. 
         FIG. 12D  is a view illustrating an exemplary relationship between the distance from an attentional pixel and the weighting factor in another kernel to be used in the frequency decomposition and differentiated in shape. 
         FIG. 13  is a view schematically illustrating an exemplary flow of scattered radiation correction processing D that can be executed by the controller illustrated in  FIG. 2  according to a fourth exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     First Exemplary Embodiment 
     [Configuration of Radiographic Imaging System  100 ] 
     First, a configuration according to the first exemplary embodiment will be described.  FIG. 1  is a view illustrating an example of the entire configuration of the radiographic imaging system  100  according to the present exemplary embodiment. As illustrated in  FIG. 1 , the radiographic imaging system  100  includes an image capturing apparatus  1  and a console  2  that are connected in such a manner that data can be transmitted and received between them. 
     The image capturing apparatus  1  includes a radiation detector P, an imaging platform  11  on which the radiation detector P is attachable, and a radiation generator  12 . The imaging platform  11  includes a holder  11   a  that holds the radiation detector P. 
     The radiation detector P is constituted by a semiconductor image sensor, such as flat panel detector (FPD), and is provided so as to face the radiation generator  12  across a subject H. For example, the radiation detector P includes a glass substrate and a plurality of detection elements (pixels) arranged in a matrix pattern, at a predetermined position on the substrate, to detect radiations (X rays) emitted from the radiation generator  12  and penetrating at least the subject H according to their intensities and then convert the detected radiations into electric signals and accumulate them. Each pixel includes a switcher such as a thin film transistor (TFT), for example. The radiation detector P controls the switcher of each pixel based on image reading conditions input from the console  2  to switch the reading of the electric signal accumulated in each pixel and acquires image data by reading the electric signal accumulated in each pixel. Then, the radiation detector P outputs the acquired image data to the console  2 . 
     The radiation generator  12  is disposed at a position where it faces the radiation detector P across the subject H, and performs imaging by irradiating the plurality of radiation detectors P attached to the holder  11   a  via a patient serving as the subject H with radiations based on radiation exposure conditions input from the console  2 . The radiation exposure conditions input from the console  2  include, for example, tube current value, tube voltage value, radiation exposure time, mAs value, and SID (indicating the shortest distance between the radiation detector P and a bulb of the radiation generator  12 ). 
     The console  2  outputs imaging conditions, such as radiation exposure conditions and image reading conditions, to the image capturing apparatus  1  to control a radiation imaging operation and a radiographic image reading operation to be performed by the image capturing apparatus  1 , and also functions as a radiographic image processing apparatus that performs image processing on a radiographic image acquired by the image capturing apparatus  1 . 
     As illustrated in  FIG. 2 , the console  2  includes a controller  21 , a storage device  22 , an operating device  23 , a display device  24 , and a communication device  25 , which are mutually connected via a bus  26 . 
     The controller  21  includes a central processing unit (CPU), a random access memory (RAM) and the like. The CPU of the controller  21  reads out system programs and various processing programs stored in the storage device  22  according to an operation of the operating device  23  and develops them in the RAM, and then centrally controls operations to be performed by respective devices constituting the console  2  as well as the radiation exposure operation and the reading operation to be performed by the image capturing apparatus  1  according to the developed programs. Further, the CPU executes image processing, such as scattered radiation correction processing A described below, on a radiographic image transmitted from the radiation detector P of the image capturing apparatus  1 . 
     The storage device  22  is constituted by a nonvolatile semiconductor memory, a hard disk, or the like. The storage device  22  stores various programs to be executed by the controller  21 , parameters required in executing each processing by program, and data representing processing results. The various programs are stored in the form of readable program codes, and the controller  21  sequentially executes operations according to the program codes. 
     In addition, the storage device  22  stores imaging conditions (e.g., radiation exposure conditions and image reading conditions) corresponding to an imaging region. Further, the storage device  22  stores imaging order information transmitted from a radiology information system (RIS) or the like, which is not illustrated. The imaging order information includes patient information, examination information (e.g., examination ID, imaging region (including imaging direction, such as front, side, A→P, P→A, and the like), and examination date). 
     In addition, the storage device  22  stores a formula (Expression 1) expressing a relationship between pixel value and subject thickness for each imaging condition. Further, the storage device  22  stores a formula (Expression 2) expressing a relationship between subject thickness and scattered radiation content rate for each imaging region. 
     The operating device  23  is constituted by a keyboard, including cursor keys, numerical input keys, and various functional keys, and a pointing device such as a mouse, and outputs a key operation on the keyboard or an instruction signal input by a mouse operation to the controller  21 . Further, the operating device  23  may include a touch panel on a display screen of the display device  24 . In this case, the operating device  23  outputs an instruction signal input via the touch panel to the controller  21 . Further, the operating device  23  is equipped with an exposure switch for instructing dynamic imaging to the radiation generator  12 . 
     The display device  24  is constituted by a monitor, such as a liquid crystal display (LCD) or a cathode ray tube (CRT), and displays an input instruction from the operating device  23 , data, or the like, according to an instruction of a display signal input from the controller  21 . 
     The communication device  25  has an interface that performs transmission and reception of data with each of the radiation generator  12  and the radiation detector P. The communications between the console  2  and the radiation generator  12  or the radiation detector P can be wired communications or wireless communications. 
     Further, the communication device  25  includes a LAN adapter, a modem, a terminal adapter (TA) and the like, and controls transmission and reception of data with the RIS (not illustrated) or the like connected to a communication network. 
     [Operations of Radiographic Imaging System  100 ] 
     In a state where the radiation detector P is set in the holder  11   a  of the image capturing apparatus  1 , if imaging order information of an imaging target is selected by the operating device  23  in the console  2 , imaging conditions (radiation exposure conditions and radiographic image reading conditions) according to the selected imaging order information are read out from the storage device  22  and transmitted to the image capturing apparatus  1 . After the positioning of the subject H is completed in the image capturing apparatus  1 , if the exposure switch is pressed, the radiation generator  12  emits radiations and the radiation detector P reads image data of a radiographic image. The read image data (i.e., radiographic image) is transmitted to the console  2 . 
     In the console  2 , when the communication device  25  receives the radiographic image from the radiation detector P, the controller  21  executes scattered radiation correction processing A.  FIG. 3  is a flowchart illustrating an exemplary flow of the scattered radiation correction processing A. The scattered radiation correction processing A can be executed by cooperation between the controller  21  and the programs stored in the storage device  22 . 
     First, the controller  21  acquires the radiographic image transmitted from the radiation detector P via the communication device  25  (step S 1 ). 
     Next, the controller  21  performs edge determination processing on the radiographic image (step S 2 ). 
     In the edge determination processing, first, the controller  21  designates each pixel of the radiographic image as an attentional pixel and sets a region of m pixels×n pixels (m and n are positive integers) including the attentional pixel positioned at the center thereof as an edge determination region. The edge determination region is the same as a convolution region (R in  FIG. 4 ) in filter processing to be performed in a subsequent stage. Next, in each edge determination region, the controller  21  calculates a difference between a pixel value of the attentional pixel and a pixel value of each pixel (that is referred to as “referential pixel”) in the edge determination region. In this case, the controller  21  can determine that the edge is stronger as the difference between the pixel value of the attentional pixel and the pixel value of the referential pixel is larger. Therefore, in the present exemplary embodiment, the difference between the pixel value of the attentional pixel and the pixel value of the referential pixel is used as a determination result of the edge intensity at the referential pixel. Instead of the difference between the pixel value of the attentional pixel and the pixel value of the referential pixel, a ratio (for example, a ratio having a numerator being the larger one, in this embodiment) may be used. 
     Next, the controller  21  sets a weighting factor to be used in the convolution calculation during the filter processing based on the edge determination result (step S 3 ). 
     In step S 3 , the controller  21  sets the weighting factor for each referential pixel of each edge determination region. As illustrated in  FIG. 5 , the setting of the weighting factor is performed in such a manner that the weighting factor becomes larger as the difference (or ratio) in pixel value between the attentional pixel and the referential pixel is smaller (when the edge is weak or there is no edge) and the weighting factor becomes smaller as the difference (or ratio) in pixel value between the attentional pixel and the referential pixel is larger (when the edge is strong). The relationship between the difference (or ratio) in pixel value between the attentional pixel and the referential pixel and the weighting factor can be optimally obtained through experiments and stored beforehand in the storage device  22 . 
     Alternatively, in step S 2 , the controller  21  can obtain a variance value of the pixel value in the edge determination region and may determine that the edge around the attentional pixel is strong as the variance value is large and also determine that the edge around the attentional pixel is weak as the variance value is small. Further, weighting factors (for example, refer to  FIG. 12A  to  FIG. 12D ) differentiated according to the distance between the attentional pixel and the referential pixel can be stored beforehand in the storage device  22 . In step S 3 , the controller  21  can set the weighting factor by adjusting the weighting factor stored beforehand in such a manner that the weight becomes smaller as the edge becomes larger (as the variance value becomes larger) and the weight becomes larger as the edge becomes smaller (as the variance value becomes smaller). 
     Next, the controller  21  performs filter processing on the radiographic image using the weighting factor having been set to generate a frequency component image (step S 4 ). 
     In the filter processing, as illustrated in  FIG. 4 , the controller  21  designates each pixel of the radiographic image as an attentional pixel and sets a region of m pixels×n pixels (m and n are positive integers) including the attentional pixel positioned at the center thereof as a filter region (i.e., a convolution region, indicated by R in  FIG. 4 ). Then, the controller  21  multiplies the pixel value of each pixel in the filter region by an element of the corresponding position in a template of filter factors stored beforehand in the storage device  22  and in a template of the weighting factors set in step S 3  and calculates a sum thereof, and then generates the frequency component image by designating the calculated sum as the pixel value of the attentional pixel. The filter factor can be, for example, constant for all elements in the filter region. 
     Further, the filter factor can be a coefficient (for example, refer to  FIG. 12A  to  FIG. 12D ) differentiated according to the distance between the attentional pixel and the referential pixel, and the controller  21  can multiply the pixel value of each pixel in the filter region by an element of the corresponding position in the template of the weighting factors having been set based on an edge determination result determined using the template of the filter factors and the difference or ratio between the pixel value of the attentional pixel and the pixel value of the referential pixel and calculate a sum thereof. Then, the controller  21  can generate the frequency component image by performing bilateral filter processing using the calculated sum as the pixel value of the attentional pixel. 
     In the filter processing, since the frequency band of a filter processed image is determined by the kernel (i.e., the coefficient to be multiplied with the pixel value in the filter region), performing the filter processing using the kernel obtained by multiplying the filter factor by the weighting factor based on the edge determination result can change the frequency band to be extracted by the location in the radiographic image (the intensity of the edge at the location). As the weighting factor becomes larger, the frequency component of low frequency can be extracted. As the weighting factor becomes smaller, the frequency component of high frequency can be extracted. 
     Next, the controller  21  acquires imaging conditions of the radiographic image and imaging region information as external parameters (step S 5 ), and estimates the subject thickness from the radiographic image based on the acquired imaging conditions (step S 6 ), and further estimates a scattered radiation content rate from the radiographic image based on the acquired imaging region and the estimated subject thickness (step S 7 ). 
     The controller  21  acquires the imaging region from the imaging order information used for the imaging and acquires imaging conditions corresponding to the imaging region from the storage device  22 . In the present exemplary embodiment, acquired as the imaging conditions are tube voltage, mAs value, and SID. In the present exemplary embodiment, the imaging region and the imaging conditions are acquired based on the imaging order information. However, a user may manually input them from the operating device  23 . The imaging conditions may be acquired from the image capturing apparatus  1 . Further, the processing in steps S 5  to S 7  may be executed concurrently with the processing in steps S 2  to S 4  or may be executed earlier. 
     The subject thickness of each pixel of the radiographic image can be estimated using the following formula (Expression 1).
 
Subject thickness=coefficient  A ×log(pixel value)+coefficient  B   (Expression 1)
 
Here, the coefficients A and B are coefficients determined by the imaging conditions (i.e., tube voltage, mAs value, and SID). The above-mentioned formula (Expression 1) is stored in the storage device  22  for each imaging condition.
 
     The method for estimating the subject thickness is not limited to the one described above and a conventionally known method (for example, refer to JP 2016-202219A) may be used. 
     Further, the scattered radiation content rate of each pixel of the radiographic image can be estimated using the following formula (Expression 2).
 
Scattered radiation content rate=coefficient  C ×log(subject thickness+coefficient  D )+coefficient  E   (Expression 2)
 
Here, coefficients C, D, and E are coefficients determined by the imaging region. The above-mentioned formula (Expression 2) is stored in the storage device  22  for each imaging region.
 
     Further, the following formula (Expression 3) may be stored in association with the imaging conditions and the imaging region beforehand, and the scattered radiation content rate may be directly obtained from the pixel value using the formula (Expression 3) corresponding to the imaging conditions and the imaging region.
 
Scattered radiation content rate=coefficient  C ×log(coefficient  A ×log(pixel value)+coefficient  B +coefficient  D )+coefficient  E   (Expression 3)
 
     Next, the controller  21  acquires a scattered radiation removal rate as an external parameter (step S 8 ). For example, a user may input the scattered radiation removal rate from the operating device  23 . Further, a value in the range from 0 to 100% entered from the operating device  23  may be acquired as the scattered radiation removal rate. A table indicating a relationship between grid ratio and scattered radiation removal rate may be stored in the storage device  22 , and a scattered radiation removal rate corresponding to a grid ratio entered from the operating device  23  may be acquired. 
     Next, the controller  21  estimates a scattered radiation component by multiplying the scattered radiation content rate for each pixel of the frequency component image generated in step S 4  and generates a scattered radiation image representing the scattered radiation amount to be removed from the radiographic image by multiplying the scattered radiation component by the scattered radiation removal rate (step S 9 ). 
     Then, the controller  21  subtracts the scattered radiation image from the radiographic image (subtracts the pixel value of the corresponding pixel of the scattered radiation image from the pixel value of each pixel of the radiographic image) to generate a radiographic image from which the scattered radiation has been removed (step S 10 ), and then terminates the scattered radiation correction processing A. 
       FIG. 6A  is a view illustrating a result of the scattered radiation removal based on a frequency component image obtained by performing filter processing on a radiographic image using a constant weighting factor.  FIG. 6B  is a view illustrating a result of the scattered radiation removal based on a frequency component image obtained by performing filter processing on a radiographic image using a weighting factor reflecting the edge intensity (in which the weighting factor becomes smaller as the edge becomes stronger). As illustrated in  FIG. 6A , when the weighting factor is constant irrespective of the edge intensity, the accuracy in estimating the scattered radiation becomes worse as the edge is stronger. Therefore, reproduction errors of high-frequency components in the vicinity of edges occur in the radiographic image from which the scattered radiation has been removed. On the other hand, as illustrated in  FIG. 6B , performing the filter processing while setting the weighting factor of the kernel to be used in the filter processing to become smaller as the edge becomes stronger can improve the accuracy in estimating high-frequency components of the scattered radiation for the portion where the edge is strong and can accurately remove the scattered radiation. 
     Second Exemplary Embodiment 
     Next, a second exemplary embodiment of the present invention will be described. 
     Since the second exemplary embodiment is similar to the first exemplary embodiment in its configuration and imaging operation, their descriptions will be cited. Hereinafter, operations that can be performed by the console  2  according to the second exemplary embodiment will be described. 
     In the console  2 , when receiving image data from the radiation detector P, the controller  21  executes scattered radiation correction processing B.  FIG. 7  is a flowchart illustrating an exemplary flow of the scattered radiation correction processing B. The scattered radiation correction processing B can be executed by cooperation between the controller  21  and the programs stored in the storage device  22 . 
     First, the controller  21  acquires a radiographic image transmitted from the radiation detector P via the communication device  25  (step S 21 ). 
     Next, the controller  21  performs non-local means (NLM) filter processing on the radiographic image to generate a frequency component image (step S 22 ). 
     The non-local means filter is a filter that performs smoothing while preserving edges by using a calculation result of the degree of similarity of a local image as a weighting factor at the time of the convolution calculation. 
     The non-local means filter processing will be described below with reference to  FIG. 8 . The following (1) to (6) is processing to be executed while designating each pixel of the radiographic image as an attentional pixel G 1 , in the non-local means filter processing. 
     (1) The controller  21  sets a region of m pixels×n pixels (m and n are positive integers) including the attentional pixel G 1  positioned at the center thereof as an attentional region. 
     (2) The controller  21  sets a filter region around the attentional region (including the attentional region). 
     (3) The controller  21  designates each pixel in the filter region as a referential pixel G 2 , and sets a region of m pixels×n pixels (m and n are positive integers) including the referential pixel G 2  positioned at the center thereof as a referential region. 
     (4) The controller  21  calculates the degree of similarity between the attentional region and the referential region. For example, the square sum of the difference between the pixel value of each pixel in the attentional region and the pixel value of the corresponding position in the referential region can be calculated as the degree of similarity between the attentional region and the referential region, although it is not limited specifically. In this case, the controller  21  can determine that the edge is stronger as the degree of similarity between the attentional region and the referential region is lower. Therefore, in the present exemplary embodiment, the calculated degree of similarity can be used as a determination result of the edge intensity.
 
(5) The controller  21  calculates a weighted average of respective pixels in the filter region while designating the calculated degree of similarity as a weighting factor of the referential pixel G 2 .
 
(6) The controller  21  replaces the value of the attentional pixel G 1  with the calculated weighted average. By executing the processing of (1) to (6) for all pixels, the controller  21  generates the frequency component image.
 
     In the filter processing, since the frequency band of a filter processed image is determined by the kernel, performing the filter processing using a weighting factor based on the edge determination result as the kernel can change the frequency band to be extracted by the location in the radiographic image (the intensity of the edge at the location). 
     Next, the controller  21  acquires imaging conditions of the radiographic image and imaging region information as external parameters (step S 23 ), and estimates the subject thickness from the radiographic image based on the acquired imaging conditions (step S 24 ), and further estimates a scattered radiation content rate from the radiographic image based on the acquired imaging region and the estimated subject thickness (step S 25 ). The processing in steps S 23  to S 25  may be executed concurrently with the processing in step S 22  or may be executed earlier. 
     Next, the controller  21  acquires a scattered radiation removal rate as an external parameter (step S 26 ). 
     Next, the controller  21  multiplies each pixel in the frequency component image generated in step S 22  by the scattered radiation content rate to estimate a scattered radiation component and multiplies the scattered radiation component by the scattered radiation removal rate to generate a scattered radiation image representing the scattered radiation amount to be removed from the radiographic image (step S 27 ). 
     Then, the controller  21  subtracts the scattered radiation image from the radiographic image to generate a radiographic image from which the scattered radiation has been removed (step S 28 ), and terminates the scattered radiation correction processing B. 
     In the second exemplary embodiment, similar to the first exemplary embodiment, by performing the filter processing on the radiographic image with the weighting factor becoming smaller as the edge becomes stronger (as the degree of similarity becomes smaller), the accuracy in estimating high-frequency components of the scattered radiation can be improved for strong edge portions, and the scattered radiation can be accurately removed. 
     Third Exemplary Embodiment 
     Next, a third exemplary embodiment of the present invention will be described. 
     Since the third exemplary embodiment is similar to the first exemplary embodiment in its configuration and imaging operation, their descriptions will be cited. Hereinafter, operations that can be performed by the console  2  according to the third exemplary embodiment will be described. 
     In the console  2 , when receiving image data from the radiation detector P, the controller  21  executes scattered radiation correction processing C.  FIG. 9  is a flowchart illustrating an exemplary flow of the scattered radiation correction processing C. The scattered radiation correction processing C can be executed by cooperation between the controller  21  and the programs stored in the storage device  22 . 
     First, the controller  21  acquires a radiographic image transmitted from the radiation detector P via the communication device  25  (step S 31 ). 
     Next, the controller  21  performs edge determination processing on the radiographic image (step S 32 ). 
     The edge determination processing in step S 32  is, for example, the edge determination processing (including the one using the variance value) described in step S 2  in  FIG. 3  or the edge determination processing using the degree of similarity calculated through the processing of (1) to (4) described in step S 22  of  FIG. 7 , or the like. 
     Next, the controller  21  sets a weighting factor to be used in the convolution calculation during the filter processing based on the edge determination result (step S 33 ). 
     In step S 33 , the controller  21  sets the weighting factor for the referential pixel in each edge determination region. In the present embodiment, the setting of the weighting factor is performed in such a manner that the weighting factor becomes smaller as the edge is weaker or when there is no edge (as the difference or ratio in pixel value between the attentional pixel and the referential pixel calculated in the edge determination processing is smaller, the variance value is smaller, or the degree of similarity is larger) and becomes larger as the edge is stronger (as the difference or ratio in pixel value between the attentional pixel and the referential pixel calculated in edge determination processing is larger, the variance value is larger, or the degree of similarity is smaller). In the storage device  22 , the difference in pixel value between the attentional pixel and the referential pixel (or the ratio, the variance value or the degree of similarity) is stored beforehand in relation to the weighting factor. 
     Next, the controller  21  performs frequency decomposition on the radiographic image using the weighting factor having been set and generates frequency component images of a plurality of frequency bands (a plurality of frequency band images) (step S 34 ). 
     For example, one of the following (A) and (B) is employable as the method of frequency decomposition. 
     (A) Using a kernel being constant in size and shape to generate frequency component image of a plurality of frequency bands. 
     (B) Using kernels differentiated in size and/or shape to generate frequency component image of a plurality of frequency bands. 
     According to above-mentioned method (A), for example, as illustrated in  FIG. 10 , the controller  21  first generates an unsharp image  1  by performing filter processing (e.g., a simple averaging filter, a binominal filter, a Gaussian filter, or the like) on an input radiographic image (an original image) using a kernel K 1  being constant in size and shape, which is prepared beforehand (stored in the storage device  22 ). Then, the controller  21  performs filter processing on the unsharp image  1  using the kernel K 1  to generate an unsharp image  2 . Next, the controller  21  performs filter processing on the unsharp image  2  using the kernel K 1  to generate an unsharp image  3 . Then, the controller  21  generates a high-frequency component image by obtaining a difference between the original image and the unsharp image  1 , a medium-frequency component image by obtaining a difference between the unsharp image  1  and the unsharp image  2 , and a low-frequency component image by obtaining a difference between the unsharp image  2  and the unsharp image  3 . 
     Namely, in step S 34 , the controller  21  generates a plurality of unsharp images by performing filter processing using a kernel obtained by multiplying the kernel K 1  having been set beforehand by the weighting factor set in step S 33 , and generates a plurality of frequency band images (e.g., high-frequency component image, medium-frequency component image, and low-frequency component image) using the plurality of generated unsharp images. 
     According to above-mentioned method (B), for example, as illustrated in  FIG. 11 , the controller  21  performs filter processing (for example, the simple averaging filter, the binominal filter, the Gaussian filter, or the like) on an original image using kernels K 2  to K 4  differentiated in size and/or shape to generate a plurality of unsharp images  1  to  3  different in frequency band. Then, the controller  21  generates a high-frequency component image by obtaining a difference between the original image and the unsharp image  1 , a medium-frequency component image by obtaining a difference between the unsharp image  1  and the unsharp image  2 , and a low-frequency component image by obtaining a difference between the unsharp image  2  and the unsharp image  3 . 
     Namely, in step S 34 , the controller  21  generates a plurality of unsharp images by performing filter processing using kernels obtained by multiplying the kernels K 2  to K 4  having been set beforehand by the weighting factor set in step S 33 , and generates frequency component images of a plurality of frequency bands (e.g., high-frequency component image, medium-frequency component image, and low-frequency component image) using the generated plurality of unsharp images. 
     In the description of (A) and (B) using  FIG. 10  and  FIG. 11 , although three unsharp images are generated to generate frequency component images of three frequency bands (high-frequency component image, medium-frequency component image, and low-frequency component image), this is only an example. Frequency component images of a plurality of necessary bands can be generated by generating a plurality of unsharp images and obtaining their differences. 
     When the filter processing is performed using kernels different in size, it is feasible to extract different frequency components according to the size of each kernel. The larger the kernel size is, the higher the effect of smoothing is. This is useful in extracting low-frequency components. The smaller the kernel size is, the lower the effect of smoothing is. This is useful in extracting high-frequency components. More specifically, when the kernels K 2  to K 4  are differentiated in size, the size of kernel K 2 &lt;the size of kernel K 3 &lt;the size of kernel K 4 . 
     Further, when the filter processing is performed using kernels different in shape, it is feasible to extract different frequency components according to the shape of each kernel. The kernels different in shape are, for example, kernels different in weighting factor to be multiplied with the filter factor having been set beforehand according to the distance between the attentional pixel and the referential pixel.  FIG. 12A  to  FIG. 12D  illustrate examples of the weighting factor. As illustrated in  FIG. 12A , when using a kernel whose weighting factor changes steeply as the distance from the attentional pixel is shorter, the effect of smoothing is lower and accordingly high-frequency components can be extracted. As illustrated in  FIG. 12D , when using a kernel whose weighting factor changes gradually according to the distance from the attentional pixel, the effect of smoothing is higher and accordingly low-frequency components can be extracted. 
     The frequency bands of the frequency components to be extracted in step S 34  and the kernels to be used in extracting the frequency component images are set (stored in the storage device  22 ) beforehand based on experiments. 
     Next, the controller  21  acquires imaging conditions of the radiographic image and imaging region information as external parameters (step S 35 ), and estimates the subject thickness from the radiographic image based on the acquired imaging conditions (step S 36 ), and further estimates a scattered radiation content rate from the radiographic image based on the acquired imaging region and the estimated subject thickness (step S 37 ). The processing in steps S 35  to S 37  may be executed concurrently with the processing in steps S 32  to S 34  or may be executed earlier. 
     Next, the controller  21  multiplies each pixel of the frequency component image of each frequency band generated in step S 34  by the scattered radiation content rate to generate a scattered radiation component image estimating the scattered radiation component of each frequency band, and then combines them (step S 38 ). 
     Next, the controller  21  acquires a scattered radiation removal rate as an external parameter (step S 39 ), and multiplies the combined scattered radiation component image by the scattered radiation removal rate to generate a scattered radiation image indicating the scattered radiation amount to be removed (step S 40 ). 
     Then, the controller  21  subtracts the scattered radiation image from the radiographic image to generate a radiographic image from which the scattered radiation has been removed (step S 41 ), and terminates the scattered radiation correction processing C. 
     In the third exemplary embodiment, the radiographic image is designated as the original image, the filter processing is repetitively performed using the kernel whose weighting factor becomes larger as the edge is stronger to generate unsharp images of a plurality of frequency bands, and the generated unsharp images are subtracted from the original image to generate frequency band images of the plurality of frequency bands. Accordingly, it is feasible to extract high-frequency components of the scattered radiation for strong edge portions. The accuracy in estimating high-frequency components of the scattered radiation can be improved for strong edge portions, and the scattered radiation can be accurately removed. Further, compared to the first and second exemplary embodiments, the third exemplary embodiment brings an effect that it is easy to extract the scattered radiation of an aimed frequency component. 
     Fourth Exemplary Embodiment 
     Next, a fourth exemplary embodiment of the present invention will be described. 
     Since the fourth exemplary embodiment is similar to the first exemplary embodiment in its configuration and imaging operation, their descriptions will be cited. Hereinafter, operations that can be performed by the console  2  according to the fourth exemplary embodiment will be described. 
     In the console  2 , when receiving image data from the radiation detector P, the controller  21  executes scattered radiation correction processing D.  FIG. 13  is a flowchart illustrating an exemplary flow of the scattered radiation correction processing D. The scattered radiation correction processing D can be executed by cooperation between the controller  21  and the programs stored in the storage device  22 . 
     First, the controller  21  acquires a radiographic image transmitted from the radiation detector P via the communication device  25  (step S 51 ). 
     Next, the controller  21  performs edge determination processing on the radiographic image (step S 52 ). 
     In step S 52 , first, the controller  21  designates each pixel of the radiographic image as an attentional pixel, and sets a region of m pixels×n pixels (m and n are positive integers) including the attentional pixel positioned at the center thereof as an edge determination region. Next, in each edge determination region, the controller  21  calculates a variance value of the pixel value in the region and determines the edge intensity based on the variance value. For example, the controller  21  determines that the edge is stronger as the variance value is larger and the edge is weaker as the variance value is smaller. Alternatively, the controller  21  may obtain a difference or ratio between an average of respective pixel values in the edge determination region and the pixel value of the attentional pixel, and may determine that the edge is stranger as the difference or ratio is larger and weaker as the difference or ratio is smaller. 
     Next, the controller  21  sets a weighting factor to be used for a frequency component image of each frequency band to be generated in step S 54 , for each pixel, based on the edge determination result (step S 53 ). For example, the setting of the weighting factor is performed in such a manner that the weighting factor applied to the frequency component image of the high-frequency band becomes larger as the peripheral edge of the pixel is stronger and the weighting factor applied to the frequency component image of the low-frequency band becomes smaller as the peripheral edge of the pixel is weaker. The relationship between the edge intensity (variance value) and the weighting factor applied to the frequency component image of each frequency band is determined based on experiments and stored beforehand in the storage device  22 . 
     Next, the controller  21  performs frequency decomposition on the radiographic image and generates frequency component images of a plurality of frequency bands (step S 54 ). 
     In step S 54 , for example, the controller  21  performs the frequency decomposition according to the method (A) or (B) described in step S 34  of the scattered radiation correction processing C according to the third exemplary embodiment. In this step S 54 , the controller  21  does not use the weighting factor set in step S 53 . 
     Next, the controller  21  multiplies each of the generated frequency component images of the plurality of frequency bands by the weighting factor set in step S 53  to generate a plurality of weighted frequency component images (step S 55 ). 
     Next, the controller  21  acquires imaging conditions of the radiographic image and imaging region information as external parameters (step S 56 ), and estimates the subject thickness from the radiographic image based on the acquired imaging conditions (step S 57 ), and further estimates a scattered radiation content rate from the radiographic image based on the acquired imaging region and the estimated subject thickness (step S 58 ). The processing in steps S 56  to S 58  may be executed concurrently with the processing in steps S 52  to S 55  or may be executed earlier. 
     Next, the controller  21  multiplies each pixel of the frequency component image of each frequency band, to which the weighting factor has been multiplied in step S 55 , by the scattered radiation content rate to generate a scattered radiation component image estimating the scattered radiation component of each frequency band, and then combines them (step S 59 ). 
     Next, the controller  21  acquires a scattered radiation removal rate as an external parameter (step S 60 ), multiplies the combined scattered radiation component image by the scattered radiation removal rate to generate a scattered radiation image indicating the scattered radiation amount to be removed (step S 61 ). 
     Then, the controller  21  subtracts the scattered radiation image from the radiographic image to generate a radiographic image from which the scattered radiation has been removed (step S 62 ), and terminates the scattered radiation correction processing D. 
     In the fourth exemplary embodiment, the weight of the high-frequency band image included in the scattered radiation image to be used in estimating the scattered radiation becomes stranger as the edge in the region is stronger. Therefore, the accuracy in estimating high-frequency components of the scattered radiation can be improved for strong edge portions, and the scattered radiation can be accurately removed. Further, compared to the first and second exemplary embodiments, the fourth exemplary embodiment brings an effect that it is easy to extract the scattered radiation of an aimed frequency component. 
     As mentioned above, the controller  21  of the console  2  determines the intensity of an edge in a radiographic image obtained by radiographically imaging a subject, sets a weighting factor to be used in extracting a frequency component of a predetermined frequency band from the radiographic image according to a determination result, extracts the frequency component from the radiographic image using the weighting factor having been set, and multiplies the extracted frequency component by the scattered radiation content rate to estimate the scattered radiation component in radiographic image. Then, the controller  21  multiplies the estimated scattered radiation component by the scattered radiation removal rate to generate the scattered radiation image representing the scattered radiation component to be removed from the radiographic image, and performs scattered radiation correction on the radiographic image by subtracting the scattered radiation image from the radiographic image. 
     Accordingly, it is feasible to improve the accuracy in estimating the scattered radiation in the vicinity of edges in the radiographic image, and therefore it is feasible to obtain a radiographic image from which the scattered radiation has been precisely removed. 
     The contents described in the above-mentioned exemplary embodiments are preferred examples, and the present invention is not limited to these examples. 
     For example, in the above-mentioned exemplary embodiments, the present invention is applied to a radiographic image of a chest. However, the present invention is also applicable to a radiographic image capturing another region. 
     Further, in the above-mentioned exemplary embodiments, the console  2  controlling the image capturing apparatus  1  is functionally operable as the radiographic image processing apparatus. However, the radiographic image processing apparatus may be separated from the console. 
     Further, for example, in the above-mentioned description, the hard disk and the semiconductor nonvolatile memory are practical examples of a computer readable medium storing the programs according to the present invention. However, the computer readable medium is not limited to these examples. For example, a portable storage medium such as a compact disk read only memory (CD-ROM) is employable as a computer readable medium. Further, carrier waves are employable as a medium capable of providing data of the programs according to the present invention via a communication line. 
     Further, detailed configurations and detailed operations of respective devices constituting the radiographic imaging system can be appropriately modified without departing from the gist of the present invention. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims. 
     The entire disclosure of Japanese Patent Application No. 2018-009644 filed on Jan. 24, 2018 including the specification, claims, drawings, and abstract is incorporated herein by reference in its entirety.