# Patent

## Title
Image processing apparatus, control method thereof and computer-readable storage medium

## Abstract

A subtraction image is generated by performing subtraction between a mask image serving as a radiation image obtained by capturing an object, at least of a specific region of which does not include contrast medium, and a live image serving as a radiation image obtained by capturing the object which includes the contrast medium. The emphasis degree serving as the degree of emphasis processing for the subtraction image is determined based on at least either the mask image or live image. The emphasis processing is performed for the subtraction image based on the emphasis degree.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to emphasis processing for an image obtained by digital subtraction angiography.
2. Description of the Related Art
With recent advances in the digital technology, it has become popular to perform digital processing for images even in the medical field. Instead of conventional X-ray imaging using an X-ray diagnosis film, two-dimensional X-ray sensors configured to output an X-ray image as a digital image have also prevailed. Digital image processing such as edge emphasis processing for a digital image output from the two-dimensional X-ray sensor is important.
An application example of the digital image processing is DSA processing of acquiring a digital subtraction angiogram (to be referred to as a DSA image hereinafter). The DSA image is an image obtained by acquiring images before and after the inflow of a contrast medium into an object, and subtracting an image (to be referred to as a mask image hereinafter) before the inflow of the contrast medium from an image (to be referred to as a live image hereinafter) after the inflow of the contrast medium. In subtraction processing of the mask image from the live image, a blood vessel region serving as a region of interest in diagnosis is held as a change region between images that is generated by the inflow of the contrast medium. The remaining unnecessary region is removed as a background region, and the change region is expressed as a homogeneous region. The generated DSA image is an image helpful for diagnosis because the blood vessel region can be observed without influencing the remaining object structure.
The purpose of using a DSA image for diagnosis is clear visualization of a contrasted blood vessel image. This purpose is considered to have already been achieved in a subtraction image obtained by subtracting a mask image from a live image. However, an image more suitable for diagnosis is obtained by applying emphasis processing generally used as X-ray image processing to a subtraction image and further emphasizing the edge of a contrasted blood vessel region.
As an example of the emphasis processing, frequency emphasis processing will be explained. More specifically, an image is decomposed into a plurality of band-limited images representing different frequencies. The respective band-limited images undergo different adjustments and then are merged, generating one emphasized image.
As a method of decomposing an image into a plurality of band-limited images, there are various methods such as Laplacian pyramid decomposition, wavelet transform, and unsharp masking. For example, when unsharp masking is adopted, letting Sorg be an original image and Sus be a blurred image, a band-limited image H is given by:

H(x,y)=Sorg(x,y)−Sus(x,y)  (1)

where (x, y) is the pixel of an image, and H(x, y), Sorg(x, y), and Sus(x, y) are pixel values.

The above equation is used to generate one band-limited image from an original image. A method of generating a plurality of band-limited images representing different frequencies is as follows. A different frequency is represented by a level lv, and a plurality of band-limited images having different frequencies from a high frequency (lv=1) to a low frequency (lv=lvMax) are represented by {Hlv|lv=1, 2, . . . , lvMax}. At this time, the band-limited image Hlv at an arbitrary level lv is given by:

Hlv(x,y)=Suslv-1(x,y)−Suslv(x,y)  (2)

where {Suslv|lv=0, 1, 2, . . . , lvMax} are a plurality of blurred images having different frequencies. A blurred image Sus0 having lv=0 is the original image Sorg.

From equation (2), the relationship between the original image Sorg and the band-limited image Hlv is given by:

Sorg(x,y)=Σlv=1lvMaxHlv(x,y)+SuslvMax(x,y)  (3)

This means that the original image Sorg can be reconstructed by adding all the decomposed band-limited images Hlv (to be referred to as high-frequency images hereinafter) and a blurred image SuslvMax having a lowest frequency (to be referred to as a low-frequency image hereinafter).
Based on this relationship, frequency emphasis processing is given by equation (4) below using a coefficient {αlv|lv=1, 2, . . . , lvMax} which gives an emphasis degree on a high-frequency image:

Senh(x,y)=Σlv=1lvMaxαlvHlv(x,y)+SuslvMax(x,y)  (4)

where Senh is an image having undergone frequency emphasis processing. When the emphasis coefficient αlv is set to be 1 at all levels, Senh becomes equal to the original image Sorg in accordance with equation (3). A high-frequency image is emphasized by setting the emphasis coefficient αlv to be larger than 1, and suppressed by setting it to be smaller than 1. That is, by setting a different value of the emphasis coefficient αlv for each frequency level lv, the user can create images of his preferences having undergone various frequency emphasis or suppression processes.

However, the emphasis coefficient αlv allows adjustment at each level, but emphasis or suppression is uniformly performed at the same frequency component. That is, this method has a problem that an edge component to be emphasized and a noise component to be suppressed cannot be separated.
To solve this problem, Japanese Patent Laid-Open No. 09-248291 discloses a method of detecting only an edge component from a high-frequency image and emphasizing it, thereby obtaining the emphasis effect of only the edge. Japanese Patent Publication No. 04-030786 discloses a method of performing threshold processing using a predetermined value for each portion of a subtraction image in order to clearly discriminate a blood vessel region in a DSA image from the remaining region, separating only the blood vessel region serving as a region of interest based on the result, and highlighting it.
The following problem arises when the above-described emphasis processing is applied to a DSA image.
The DSA image is a subtraction image obtained by subtracting a mask image from a live image and removing an object structure in order to enhance the contrast of a contrasted region.
In general, a small-pixel-value region in an X-ray image corresponds to a region having a thick object structure. Since the amount of X-rays reaching the sensor is small in the region having the thick object structure, the small-pixel-value region is a region where noise components with a low S/N ratio are dominant. Inter-image subtraction removes a clinically unnecessary object structure, but a noise component in such a small-pixel-value region remains on the subtraction image.
For this reason, if emphasis processing is applied to a DSA image, noise in a subtraction image corresponding to the small-pixel-value region before inter-image subtraction is further emphasized by emphasis processing, greatly impairing visibility.
The method disclosed in Japanese Patent Laid-Open No. 09-248291 in which only an edge component is detected and undergoes emphasis processing does not especially target a DSA image, and cannot perform emphasis processing considering noise in a subtraction image. The method disclosed in Japanese Patent Publication No. 04-030786 targets a DSA image, separates only a blood vessel region serving as a region of interest, and highlights it. However, this method does not consider noise dependent on a pixel value before the above-mentioned inter-image subtraction.
SUMMARY OF THE INVENTION
The present invention provides an image processing technique of adaptively performing emphasis processing for a subtraction image in consideration of noise dependent on a pixel value before inter-image subtraction, and generating a high-quality subtraction image.
To achieve the above object, an image processing apparatus according to the present invention comprises the following arrangement. That is, there is provided an image processing apparatus for processing a radiation image, comprising: a subtraction unit configured to generate a subtraction image by performing subtraction between a mask image serving as a radiation image obtained by capturing an object, at least of a specific region of which does not include contrast medium, and a live image serving as a radiation image obtained by capturing the object which includes the contrast medium; a determination unit configured to determine, based on at least one of the mask image and the live image, an emphasis degree serving as a degree of emphasis processing for the subtraction image; and a processing unit configured to perform the emphasis processing for the subtraction image based on the emphasis degree.
According to the present invention, emphasis and suppression can be adaptively performed in consideration of pixel value-dependent noise by using information of a live image before inter-image subtraction in emphasis processing for an image obtained by digital subtraction angiography.
According to the present invention, emphasis and suppression can be adaptively performed relatively quickly in consideration of pixel value-dependent noise by storing and using information of a mask image before inter-image subtraction in emphasis processing for an image obtained by digital subtraction angiography.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram for explaining the arrangement of an X-ray image processing apparatus according to the first embodiment;
FIG. 2 is a flowchart showing processing according to the first embodiment;
FIG. 3 is a block diagram for explaining the arrangement of an X-ray image processing apparatus according to the second embodiment;
FIG. 4 is a flowchart showing processing according to the second embodiment;
FIG. 5 is a flowchart showing in detail edge detection processing according to the second embodiment;
FIG. 6 is a block diagram for explaining the arrangement of an X-ray image processing apparatus according to the third embodiment;
FIG. 7 is a flowchart showing processing according to the third embodiment; and
FIG. 8 is a block diagram exemplifying a computer system capable of implementing the present invention.



DESCRIPTION OF THE EMBODIMENTS
Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
First Embodiment
An X-ray image processing apparatus serving as a radiation image processing apparatus according to the first embodiment will be explained with reference to FIG. 1.
An X-ray image processing apparatus 100 includes an X-ray generation unit 101 capable of generating X-ray pulses as radiation of 3 to 30 pulses per sec, and a two-dimensional X-ray sensor 104 which receives an X-ray 103 having passed through an object 102 and captures, as an X-ray image (radiation image), a moving image synchronized with X-ray pulses. The two-dimensional X-ray sensor 104 functions as an image capturing unit which captures a moving image of the object 102 irradiated with X-rays.
The X-ray image processing apparatus 100 includes a pre processing unit 105 which performs pre-processing for respective frames of a moving image captured by the two-dimensional X-ray sensor 104 at different times.
The X-ray image processing apparatus 100 includes an image storage unit 106 which stores at least one frame of a pre-processed moving image as a mask image before the inflow of contrast medium. A frame to be stored as a mask image is, for example, a frame immediately after the start of moving image capturing, a frame immediately before the inflow of the contrast medium that is automatically acquired upon detecting the inflow of the contrast medium from a moving image, or a frame selected by designating the storage timing by the operator at the start of the inflow of the contrast medium. It is also possible to store a plurality of frames, and appropriately select a frame to be used as a mask image, or merge a plurality of frames.
Further, an image obtained by capturing an object, at least of a specific region which does not include the contrast medium can be used as a mask image. The specific region can be an entire of the image, or a region of interest in a blood vessel region in the image. The region of interest in the blood vessel region can be a partial region with a quarter of size of the image and the same center position of the image.
The X-ray image processing apparatus 100 includes a subtraction unit 107 which subtracts a mask image stored in the image storage unit 106 from a frame (to be referred to as a live image hereinafter) after the inflow of the contrast medium that is output from the pre-processing unit 105, and which outputs the resultant image as a subtraction image. The X-ray image processing apparatus 100 also includes an emphasis degree setting unit 108 which sets an emphasis degree for a subtraction image.
The X-ray image processing apparatus 100 includes an emphasis correction value calculation unit 109 which generates, from a live image output from the pre-processing unit 105, an emphasis correction image in which a pixel value represents a correction value for an emphasis degree set by the emphasis degree setting unit 108. The X-ray image processing apparatus 100 includes an emphasis processing unit 110 which performs emphasis processing for a subtraction image based on the emphasis degree and emphasis correction image, generating an emphasis subtraction image. Further, the X-ray image processing apparatus 100 includes an image display unit 111 which displays an emphasis subtraction image output from the emphasis processing unit 110 as a DSA image.
The operations of the emphasis degree setting unit 108, emphasis correction value calculation unit 109, and emphasis processing unit 110 in the first embodiment will be further explained with reference to a flowchart shown in FIG. 2.
In step S201, the emphasis degree setting unit 108 sets an emphasis degree a for a subtraction image based on a user operation. The emphasis degree is generally set while the user confirms a DSA image displayed on the image display unit 111 so as to obtain an emphasis subtraction image of his choice. Note that the emphasis degree may be set to be a different value for each image capturing (imaging). Alternatively, it is also possible to set in advance, in a database (not shown), emphasis degrees for a plurality of types of image capturing conditions such as an X-ray image capturing condition, image capturing portion, and image capturing manipulation, and read out and set an emphasis degree corresponding to an image capturing condition designated at the time of image capturing.
In step S202, the X-ray image processing apparatus 100 inputs a live image output from the pre-processing unit 105 to the emphasis correction value calculation unit 109. The emphasis correction value calculation unit 109 outputs an emphasis correction image C from the live image.
For example, the first embodiment will explain a method of generating an emphasis correction image having, as a pixel value, a result of calculating, from a live image for each pixel, an emphasis correction value based on estimation of a noise amount.
Random noise in an X-ray image is represented by σ2. The random noise σ2 in an X-ray image is theoretically formed from quantum noise σQ2 proportional to an X-ray amount incident on the two-dimensional X-ray sensor 104, and system noise σS2 independent of the incident X-ray amount. Here, V is the pixel value of an image that is proportional to the incident X-ray amount, and G is a coefficient for converting the pixel value V into a value equivalent to the incident X-ray amount. Assuming that the quantum noise σQ2 and system noise σS2 do not have a correlation, the random noise variance σ2 in an X-ray image is given by:

σ2(V)=σQ2(V/G)+σS2  (5)

The parameters of the quantum noise σQ2, system noise σS2, and gain coefficient G in equation (5) are values uniquely determined in the image capturing process, and can be calculated in advance.
For the pixel (x, y) of a live image, a theoretical noise amount σ2(Vx,y) corresponding to the average pixel value Vx,y of each pixel can be estimated by calculating the average pixel value V from n×n pixels centered on (x, y).
For the pixel (x, y) of a live image, the variance value σx,y2 of arbitrary n×n pixels centered on (x, y) is calculated. If n×n pixels used for calculation do not contain an edge, the variance value σx,y2 takes a value equal to or smaller than the theoretical noise amount σ2(Vx,y). If n×n pixels used for calculation contain an edge, the variance value σx,y2 becomes larger than the theoretical noise amount σ2(Vx,y). From this fact, an emphasis correction value C(x, y) of each pixel is given by:




  
    
      
        
          C
          ⁡
          
            (
            
              x
              ,
              y
            
            )
          
        
        =
        
          {
          
            
              
                1
              
              
                
                  
                    σ
                    
                      x
                      ,
                      y
                    
                    2
                  
                  >
                  
                    
                      σ
                      2
                    
                    ⁡
                    
                      (
                      
                        V
                        
                          x
                          ,
                          y
                        
                      
                      )
                    
                  
                
              
            
            
              
                0
              
              
                
                  (
                  otherwise
                  )
                
              
            
          
        
      
    
    
      
        (
        6
        )
      
    
  




The emphasis correction value C(x, y) is “1” when arbitrary n×n pixels centered on the pixel (x, y) contain an edge, and “0” whey they contain only noise. The emphasis correction image C is generated as a binary image equal in image size to the live image, by calculating emphasis correction values for all the pixels (x, y) of the live image and setting them as the pixel values of the pixels (x, y).
In step S203, the X-ray image processing apparatus 100 inputs the subtraction image, emphasis degree α, and emphasis correction image C to the emphasis processing unit 110 to generate an emphasis subtraction image Senh, and then ends the process. Although various methods are conceivable for the emphasis processing, a case in which unsharp masking already described above is employed will be explained below.
Let Sorg be a subtraction image, {Hlv|lv=1, 2, . . . , lvMax} be a high-frequency image generated by frequency decomposition processing, and SuslvMax be a low-frequency image. Then, the emphasis subtraction image Senh is given by equation (7) below in which the emphasis correction image C is added to equation (4):

Senh(x,y)=Σlv=1lvMaxαlv×C(x,y)×Hlv(x,y)+SuslvMax(x,y)  (7)

In the above description, the user sets the emphasis degree α for a subtraction image by using the emphasis degree setting unit 108 in step S201. This corresponds to the case of level lv=1 at which only one high-frequency image is generated according to equation (7). When frequency emphasis processing is used as emphasis processing, the level lv is set to be a value lvMax larger than 1, as needed. In this case, the value of the emphasis degree α can be set to a different value for each level lv. Hence, the emphasis degree setting unit 108 is configured to set the emphasis degree to be αlv and allow the user to set the level lv from lv=1 to lv=lvMax. The user may determine a lvMax value, and the frequency decomposition level may be determined in accordance with the lvMax value.
Although equation (7) uses the common emphasis correction image C for all high-frequency images {Hlv|lv=1, 2, . . . , lvMax}, the emphasis correction image C may also be calculated for each frequency. In this case, a plurality of blurred images {Suslv|lv=1, 1, 2, . . . , lvMax} having different frequencies are calculated from a live image. An emphasis correction value is calculated for each level in accordance with equation (6), generating an emphasis correction image {Clv|lv=1, 1, 2, . . . , lvMax}. At this time, the emphasis subtraction image Senh is given by:

Senh(x,y)=Σlv=1lvMaxαlv×Clv(x,y)×Hlv(x,y)+SuslvMax(x,y)  (8)

The emphasis correction value C(x, y) is a binary image in the above description, but may be a multilevel image given by:




  
    
      
        
          C
          ⁡
          
            (
            
              x
              ,
              y
            
            )
          
        
        =
        
          
            σ
            
              x
              ,
              y
            
            2
          
          
            
              σ
              2
            
            ⁡
            
              (
              
                V
                
                  x
                  ,
                  y
                
              
              )
            
          
        
      
    
    
      
        (
        9
        )
      
    
  




In the above equation, when the variance value σx,y2 of arbitrary n×n pixels centered on the pixel (x, y) is larger than the theoretical noise amount σ2(Vx,y), the emphasis correction value takes a value of 1 or larger. Hence, the value of the pixel (x, y) is emphasized according to equation (7) or (8). To the contrary, when the variance value σx,y2 is smaller than the theoretical noise amount σ2(Vx,y), the emphasis correction value takes a value smaller than 1. Thus, the value of the pixel (x, y) is suppressed according to equation (7) or (8). That is, the emphasis correction value can be expressed as an edge likelihood index or noise likelihood index without clearly separating an edge and noise.
In the first embodiment, the high-frequency image H is generated from a subtraction image. Therefore, the high-frequency image H contains blood vessel edge information serving as a clinical region of interest, but a clinically unnecessary object structure is removed by inter-image subtraction. However, radiation dose-dependent noise in a region where a thick object structure is captured remains. That is, the high-frequency image H is an image representing, by large pixel values, a blood vessel edge serving as a clinical region of interest and noise in a low-radiation-dose region.
In contrast, the emphasis correction image C is generated from a live image. In the emphasis correction image C, edge information and noise are separated by estimating the amount of radiation dose-dependent noise. However, the emphasis correction image C contains the edge of an unnecessary object structure which should be removed by inter-image subtraction. That is, in the emphasis correction image C, the edge region is represented by 1 (or a value of 1 or larger), and the remaining region is represented by 0 (or a value smaller than 1) regardless of whether the target region is a clinical region of interest.
According to equation (7) or (8), a blood vessel edge serving as a clinical region of interest contained in the high-frequency image H is also an edge in the emphasis correction image C, and the pixel value is saved or emphasized because it is multiplied by a value of 1 or larger. Noise contained in the high-frequency image H is noise other than an edge in the emphasis correction image C, and the pixel value is suppressed or set to be 0 because it is multiplied by a value smaller than 1. Further, the edge of a clinically unnecessary region contained in the emphasis correction image C has a small value in the high-frequency image H and is suppressed by multiplication. Adjustment (frequency emphasis or frequency suppression) is performed for each band-limited image by multiplying the emphasis degree αlv of each frequency.
As described above, according to the first embodiment, when performing emphasis processing for a subtraction image at an emphasis degree set by the user, an emphasis correction value considering radiation dose-dependent noise is calculated from a live image and used. Hence, emphasis processing can be adaptively performed for a subtraction image.
Second Embodiment
An X-ray image processing apparatus according to the second embodiment will be explained with reference to FIG. 3.
Similarly to the first embodiment, an X-ray image processing apparatus 300 includes an X-ray generation unit 101, two-dimensional X-ray sensor 104, pre-processing unit 105, image storage unit 106, subtraction unit 107, and image display unit 111. Also, the X-ray image processing apparatus 300 includes an emphasis degree setting unit 108 which sets an emphasis degree for a subtraction image, and an emphasis correction value calculation unit 109 which generates an emphasis correction image representing a correction value (correction coefficient) for the emphasis degree. These units are the same as those in the first embodiment, and description thereof will not be repeated.
The X-ray image processing apparatus 300 includes an edge detection unit 301 which detects an edge from a subtraction image output from the pre-processing unit 105, generating an edge image in which a pixel value expresses edge information. The X-ray image processing apparatus 300 further includes an emphasis processing unit 302 which performs emphasis processing for a subtraction image based on the edge image in addition to the emphasis degree and emphasis correction image, generating an emphasis subtraction image.
The operations of the emphasis degree setting unit 108, edge detection unit 301, emphasis correction value calculation unit 109, and emphasis processing unit 302 in the second embodiment will be further explained with reference to a flowchart shown in FIG. 4. Note that some processes overlap the contents described with reference to the flowchart of FIG. 2 in the first embodiment, and the operation will be described in detail based on only a different flowchart.
In step S401, the emphasis degree setting unit 108 sets the emphasis degree a for a subtraction image based on a user operation. As described in the first embodiment, when frequency emphasis processing is used as emphasis processing, σlv corresponding to a plurality of frequencies from lv=1 to lv=lvMax may be set.
In step S402, the X-ray image processing apparatus 300 inputs a live image output from the pre-processing unit 105 to the emphasis correction value calculation unit 109. The emphasis correction value calculation unit 109 outputs an emphasis correction image C from the live image.
In step S403, the X-ray image processing apparatus 300 inputs a subtraction image to the edge detection unit 301, generating an edge image E.
In step S404, the X-ray image processing apparatus 300 inputs the subtraction image, edge image E, emphasis degree α, and emphasis correction image C to the emphasis processing unit 302 to generate an emphasis subtraction image Senh, and then ends the process. The emphasis subtraction image Senh is given by equation (10) below in which the edge image E is added to equation (7) or (8):




  
    
      
        
          Senh
          ⁡
          
            (
            
              x
              ,
              y
            
            )
          
        
        =
        
          
            
              ∑
              
                lv
                =
                1
              
              lvMax
            
            ⁢
            
              
                α
                lv
              
              ×
              
                
                  C
                  lv
                
                ⁡
                
                  (
                  
                    x
                    ,
                    y
                  
                  )
                
              
              ×
              
                
                  E
                  lv
                
                ⁡
                
                  (
                  
                    x
                    ,
                    y
                  
                  )
                
              
              ×
              
                
                  H
                  lv
                
                ⁡
                
                  (
                  
                    x
                    ,
                    y
                  
                  )
                
              
            
          
          +
          
            
              Sus
              lvMax
            
            ⁡
            
              (
              
                x
                ,
                y
              
              )
            
          
        
      
    
    
      
        (
        10
        )
      
    
  




where the suffix lv of α, C, E, and H is the frequency, similar to the description of the first embodiment. Especially, as for C and E, lv may be set to be 1 to obtain one image. Alternatively, a plurality of blurred images having different frequencies may be obtained from a live image and subtraction image, and the emphasis correction value or edge may be detected from these images.

Various methods are applicable to generation of the edge image E in step S403. For example, the operation of the edge detection unit 301 when the Canny edge detection method is used will be explained with reference to a flowchart shown in FIG. 5.
In step S501, the edge detection unit 301 performs noise reduction processing using a Gaussian filter for an input subtraction image, generating a noise reduction image N.
In step S502, the edge detection unit 301 performs first derivative processing for the noise reduction image N in the horizontal and vertical directions, generating a horizontally differentiated image DX and vertically differentiated image DY. In the first derivative processing, for example, an edge detection operator such as a Roberts, Prewitte, or Sobel operator is used. The horizontally differentiated image DX and vertically differentiated image DY are images in which the value of each pixel has information about the magnitude and direction of a gradient in the horizontal or vertical direction.
In step S503, the edge detection unit 301 calculates a gradient magnitude image MAG and gradient direction image THETA from the horizontally differentiated image DX and vertically differentiated image DY in accordance with:




  
    
      
        MAG
        =
        
          
            
              DX
              2
            
            +
            
              DY
              2
            
          
        
      
    
    
      
        (
        11
        )
      
    
  
  
    
      
        THETA
        =
        
          arctan
          ⁡
          
            (
            
              DY
              DX
            
            )
          
        
      
    
    
      
        (
        12
        )
      
    
  




The gradient magnitude image MAG is an image in which the value of each pixel represents the magnitude of a gradient. The gradient direction image THETA is an image in which a pixel whose pixel value increases in the horizontal direction is represented by 0, a pixel whose pixel value increases in the vertical direction is represented by π/2, and the value of each pixel represents the direction of a gradient by using a value of −Π/2 (inclusive) to π/2 (exclusive).
In step S504, the edge detection unit 301 performs non-local maximum point suppression processing based on the gradient magnitude image MAG and gradient direction image THETA, and outputs an edge candidate image E as edge information. The edge candidate image E is a binary image obtained by representing local maximum edge pixels by 1 and the remaining pixels by 0 in a noise reduction image. In the non-local maximum point suppression processing, two pixels adjacent to a pixel (x, y) of interest are selected based on the gradient direction image THETA(x, y). When the gradient magnitude image MAG(x, y) of the pixel (x, y) of interest is larger than the values of the two adjacent pixels, the pixel (x, y) of interest is regarded as a local maximum edge pixel, and E(x, y)=1 is set. An example of this processing is as follows.
When the gradient direction image THETA(x, y) is −π/8 (inclusive) to π/8 (exclusive), two pixels in the horizontal direction are set as adjacent pixels, and E(x, y) is calculated according to:




  
    
      
        
          E
          ⁡
          
            (
            
              x
              ,
              y
            
            )
          
        
        =
        
          {
          
            
              
                1
              
              
                
                  
                    
                      
                        (
                        
                          
                            MAG
                            ⁡
                            
                              (
                              
                                
                                  x
                                  -
                                  1
                                
                                ,
                                y
                              
                              )
                            
                          
                          <
                          
                            MAG
                            ⁡
                            
                              (
                              
                                x
                                ,
                                y
                              
                              )
                            
                          
                        
                        )
                      
                      ⁢
                      
                        
                      
                      ⁢
                      and
                      ⁢
                      
                        
                      
                      ⁢
                      
                        MAG
                        ⁡
                        
                          (
                          
                            x
                            ,
                            y
                          
                          )
                        
                      
                    
                    >
                    
                      MAG
                      ⁡
                      
                        (
                        
                          
                            x
                            +
                            1
                          
                          ,
                          y
                        
                        )
                      
                    
                  
                  )
                
              
            
            
              
                0
              
              
                
                  (
                  otherwise
                  )
                
              
            
          
        
      
    
    
      
        (
        13
        )
      
    
  




When the gradient direction image THETA(x, y) is π/8 (inclusive) to 3π/8 (exclusive), two pixels in the oblique direction are set as adjacent pixels, and E(x, y) is calculated according to:




  
    
      
        
          E
          ⁡
          
            (
            
              x
              ,
              y
            
            )
          
        
        =
        
          {
          
            
              
                1
              
              
                
                  
                    
                      
                        (
                        
                          
                            MAG
                            ⁡
                            
                              (
                              
                                x
                                ,
                                y
                              
                              )
                            
                          
                          <
                          
                            MAG
                            ⁡
                            
                              (
                              
                                
                                  x
                                  -
                                  1
                                
                                ,
                                
                                  y
                                  -
                                  1
                                
                              
                              )
                            
                          
                        
                        )
                      
                      ⁢
                      
                        
                      
                      ⁢
                      and
                      ⁢
                      
                        
                      
                      ⁢
                      
                        MAG
                        ⁡
                        
                          (
                          
                            x
                            ,
                            y
                          
                          )
                        
                      
                    
                    >
                    
                      MAG
                      ⁡
                      
                        (
                        
                          
                            x
                            +
                            1
                          
                          ,
                          
                            y
                            +
                            1
                          
                        
                        )
                      
                    
                  
                  )
                
              
            
            
              
                0
              
              
                
                  (
                  otherwise
                  )
                
              
            
          
        
      
    
    
      
        (
        14
        )
      
    
  




When the gradient direction image THETA(x, y) is 3π/8 (inclusive) to π/2 (exclusive) or −π/2 (inclusive) to −3π/8 (exclusive), two pixels in the vertical direction are set as adjacent pixels, and E(x, y) is calculated according to:




  
    
      
        
          E
          ⁡
          
            (
            
              x
              ,
              y
            
            )
          
        
        =
        
          {
          
            
              
                1
              
              
                
                  
                    
                      
                        (
                        
                          
                            MAG
                            ⁡
                            
                              (
                              
                                x
                                ,
                                y
                              
                              )
                            
                          
                          >
                          
                            MAG
                            ⁡
                            
                              (
                              
                                x
                                ,
                                
                                  y
                                  -
                                  1
                                
                              
                              )
                            
                          
                        
                        )
                      
                      ⁢
                      
                        
                      
                      ⁢
                      and
                      ⁢
                      
                        
                      
                      ⁢
                      
                        MAG
                        ⁡
                        
                          (
                          
                            x
                            ,
                            y
                          
                          )
                        
                      
                    
                    >
                    
                      MAG
                      ⁡
                      
                        (
                        
                          
                            x
                            +
                            1
                          
                          ,
                          
                            y
                            +
                            1
                          
                        
                        )
                      
                    
                  
                  )
                
              
            
            
              
                0
              
              
                
                  (
                  otherwise
                  )
                
              
            
          
        
      
    
    
      
        (
        15
        )
      
    
  




When the gradient direction image THETA(x, y) is −3π/8 (inclusive) to −π/8 (exclusive), two pixels in the oblique direction are set as adjacent pixels, and E(x, y) is calculated according to:




  
    
      
        
          E
          ⁡
          
            (
            
              x
              ,
              y
            
            )
          
        
        =
        
          {
          
            
              
                1
              
              
                
                  
                    
                      
                        (
                        
                          
                            MAG
                            ⁡
                            
                              (
                              
                                x
                                ,
                                y
                              
                              )
                            
                          
                          >
                          
                            MAG
                            ⁡
                            
                              (
                              
                                
                                  x
                                  -
                                  1
                                
                                ,
                                
                                  y
                                  +
                                  1
                                
                              
                              )
                            
                          
                        
                        )
                      
                      ⁢
                      
                        
                      
                      ⁢
                      and
                      ⁢
                      
                        
                      
                      ⁢
                      
                        MAG
                        ⁡
                        
                          (
                          
                            x
                            ,
                            y
                          
                          )
                        
                      
                    
                    >
                    
                      MAG
                      ⁡
                      
                        (
                        
                          
                            x
                            +
                            1
                          
                          ,
                          
                            y
                            -
                            1
                          
                        
                        )
                      
                    
                  
                  )
                
              
            
            
              
                0
              
              
                
                  (
                  otherwise
                  )
                
              
            
          
        
      
    
    
      
        (
        16
        )
      
    
  




In step S505, the edge detection unit 301 performs threshold processing for the edge candidate image E based on the gradient magnitude image MAG and two thresholds Tlow and Thigh (Tlow<Thigh), and outputs a weak edge image Elow and strong edge image Ehigh. The weak edge image Elow is a binary image obtained by comparing the gradient magnitude image MAG(x, y) and the Tlow value for all pixels (x, y) serving as the edge candidate image E(x, y)=1, and setting the values of pixels having MAG(x, y)>Tlow to be 1 and those of the remaining pixels to be 0. The strong edge image Ehigh is a binary image obtained by comparing the gradient magnitude image MAG(x, y) and Thigh for all pixels (x, y) serving as the edge candidate image E(x, y)=1, and setting the values of pixels having MAG(x, y)>Thigh to be 1 and those of the remaining pixels to be 0.
In step S506, the edge detection unit 301 performs edge tracking processing based on the weak edge image Elow and strong edge image Ehigh, and outputs an edge image HE. In the edge tracking processing, when the concatenation component of the pixels (x, y) serving as the weak edge image Elow(x, y)=1 contains a pixel (x, y) in the strong edge image Ehigh(x, y)=1, all the pixels (x, y) forming the concatenation component are regarded as edge pixels, and E(x, y)=1 is set. The remaining pixels (x, y) are non-edge pixels, and E(x, y)=0 is set. The edge image E acquired by the above processing is output as a result E of the Canny edge detection method for the subtraction image. After that, the Canny edge detection processing ends.
Note that this edge detection processing may be applied to the high-frequency image Hlv generated from a subtraction image. In this case, edge detection is performed for respective high-frequency images Hlv having lv=0 to lv=lvMax, and the result is output as Elv.
The edge detection target in the second embodiment is the edge of a contrasted blood vessel region serving as clinically helpful image information. The characteristic of the edge varies depending on the state of the inflow of the contrast medium.
In the above-described edge detection processing, the operator used in the noise reduction processing or first derivative processing may be adaptively switched in accordance with the time after the start of the inflow of the contrast medium. When the frame rate in image capturing is high, part of the noise reduction processing, threshold processing, and edge tracking processing may be omitted or replaced with relatively simple processing in order to increase the processing speed. Another example of the edge detection processing is a zero crossing method of detecting a zero crossing based on second derivative processing.
As described above, according to the second embodiment, the edge image E is generated from a subtraction image. The edge image E is an image obtained by extracting an edge using information such as the edge intensity, direction, or continuity which is not considered in only the subtraction image. By adding this image to the equation of frequency emphasis processing, the second embodiment can implement adaptive edge emphasis and noise suppression at higher precision.
Third Embodiment
An X-ray image processing apparatus according to the third embodiment will be explained with reference to FIG. 6.
Similarly to the first and second embodiments, an X-ray image processing apparatus 600 includes an X-ray generation unit 101, two-dimensional X-ray sensor 104, pre-processing unit 105, image storage unit 106, subtraction unit 107, and image display unit 111. Also, the X-ray image processing apparatus 600 includes an emphasis degree setting unit 108 which sets an emphasis degree for a subtraction image. These units are the same as those in the first and second embodiments, and description thereof will not be repeated.
The X-ray image processing apparatus 600 includes an emphasis correction value calculation unit 601 which generates, from a mask image stored in the image storage unit 106, an emphasis correction image in which a pixel value represents a correction value (correction coefficient) for an emphasis degree set by the emphasis degree setting unit 108.
The X-ray image processing apparatus 600 includes an emphasis correction value storage unit 602 which stores an emphasis correction image of at least one frame output from the emphasis correction value calculation unit 601. Also, the X-ray image processing apparatus 600 includes an emphasis processing unit 603 which performs emphasis processing for a subtraction image based on the emphasis degree and the emphasis correction image stored in the emphasis correction value storage unit 602, generating an emphasis subtraction image.
The operations of the emphasis degree setting unit 108, emphasis correction value calculation unit 601, emphasis correction value storage unit 602, and emphasis processing unit 603 in the third embodiment will be further explained with reference to a flowchart shown in FIG. 7. Note that some processes overlap the contents described with reference to the flowchart of FIG. 2 in the first embodiment and the flowchart of FIG. 4 in the second embodiment, and the operation will be described in detail based on only a different flowchart.
In step S701, the user sets the emphasis degree α for a subtraction image by using the emphasis degree setting unit 108. As described in the first embodiment, when frequency emphasis processing is used as emphasis processing, αlv corresponding to a plurality of frequencies from lv=1 to lv=lvMax may be set.
In step S702, the X-ray image processing apparatus 600 inputs a mask image output from the image storage unit 106 to the emphasis correction value calculation unit 601. The emphasis correction value calculation unit 601 outputs an emphasis correction image C from the mask image.
In step S703, the X-ray image processing apparatus 600 stores, in the emphasis correction value storage unit 602, the emphasis correction image C output from the emphasis correction value calculation unit 601.
In step S704, the X-ray image processing apparatus 600 inputs the subtraction image, emphasis degree α, and emphasis correction image C to the emphasis processing unit 603 to generate an emphasis subtraction image Senh, and then ends the process. Here, the emphasis correction image C stored in the emphasis correction value storage unit 602 is read out and used. The emphasis subtraction image Senh is given by the same equation as equation (7) or (8).
In step S702, unlike the first and second embodiments, the emphasis correction value calculation unit 601 receives a mask image and generates the emphasis correction image C in accordance with, for example:

C(x,y)=kVx,y  (17)

In the above equation, a value obtained by multiplying the average value of arbitrary n×n pixels centered on the pixel (x, y) by a predetermined coefficient k is used as the emphasis correction value C(x, y) in the pixel (x, y). By using the emphasis correction image C, the emphasis subtraction image Senh calculated according to equation (7) can be emphasized or suppressed in proportion to the pixel value of the mask image.
In general, a region where the pixel value is small (that is, the incident X-ray amount is small) in an X-ray image is a region where a large amount of noise exists and the S/N ratio is low. Considering this, the coefficient k is set so that C<1 in a small-pixel-value region having a low S/N ratio in the mask image. This setting produces the suppression effect on the subtraction image.
Unlike the first and second embodiments, the third embodiment uses a mask image to calculate an emphasis correction value. Since the mask image is an image before the inflow of contrast medium, unlike a live image, it is impossible to extract the edge of a blood vessel region and set it as an emphasis correction value. Generally in creation of a DSA image, a mask image is created once, saved in the image storage unit, and is not updated after that. As long as the emphasis correction image C generated from the mask image is saved in the emphasis correction value storage unit 602, the saved emphasis correction image C can be read out and used in subsequent processing.
As described above, according to the third embodiment, an emphasis subtraction image can be generated more quickly than by the arrangements described in the first and second embodiments. The third embodiment is effective particularly in an image processing apparatus requiring a high frame rate.
(Information Processing Apparatus)
The respective building components shown in FIGS. 1, 3, and 6 may be formed from dedicated hardware, but the functional arrangement of hardware may be implemented by software. In this case, the functions of the respective building components shown in FIGS. 1, 3, and 6 can be implemented by installing software in the information processing apparatus, and executing the software to implement the image processing method by using the calculation function of the information processing apparatus. By executing the software, for example, pre-processing is performed for each frame of a moving image output from the two-dimensional X-ray sensor 104 to acquire a mask image and live image before and after the inflow of contrast medium, and a subtraction image is acquired by inter-image subtraction. Then, frequency decomposition processing for the subtraction image, edge detection processing, emphasis correction value calculation processing using a mask image or live image, and an emphasis processing step using the result are executed.
FIG. 8 is a block diagram showing the hardware arrangement of the information processing apparatus and the arrangement of its peripheral devices. An information processing apparatus 1000 is connected to an image capturing apparatus 2000, and can communicate data with it.
A CPU 1010 can control the overall information processing apparatus 1000 by using programs and data stored in a RAM 1020 and ROM 1030, and execute calculation processing regarding predetermined image processing by executing programs.
The RAM 1020 has an area for temporarily storing programs and data loaded from a magneto-optical disk 1060 and hard disk 1050. Further, the RAM 1020 has an area for temporarily storing image data such as a mask image, live image, and subtraction image acquired from the image capturing apparatus 2000. The RAM 1020 also has a work area to be used when the CPU 1010 executes various processes. The ROM 1030 stores the setting data and boot program of the information processing apparatus 1000 and the like.
The hard disk 1050 holds an OS (Operating System), and programs and data which cause the CPU 1010 of the computer to execute processes to be performed by the respective building components shown in FIGS. 1, 3, and 6. These programs and data are loaded into the RAM 1020 under the control of the CPU 1010, as needed, and processed by the CPU 1010 (computer). Data of a mask image, live image, and subtraction image can also be saved in the hard disk 1050.
The magneto-optical disk 1060 is an example of an information storage medium. Some or all programs or data saved in the hard disk 1050 can be stored in the magneto-optical disk 1060.
A mouse 1070 and keyboard 1080 can be operated by the operator of the information processing apparatus 1000 to input, to the CPU 1010, various instructions such as the setting of the emphasis degree for each frequency by the emphasis degree setting unit 108.
A printer 1090 can print out, on a printing medium, an image displayed on the image display unit 111. An accelerator 1110 can improve a predetermined function or processing performance of the respective building components in the information processing apparatus 1000.
A display device 1100 is formed from a CRT, liquid crystal screen, or the like, and can display the result of processing by the CPU 1010 as an image, character, or the like. For example, the display device 1100 can display an image which is processed by the respective building components shown in FIGS. 1 and 2 and is finally output from the image display unit 111. In this case, the image display unit 111 functions as a display control unit for displaying an image on the display device 1100. A bus 1040 connects the respective building components in the information processing apparatus 1000, and can transmit/receive data between them.
(Image Capturing Apparatus 2000)
Next, the image capturing apparatus 2000 will be explained. The image capturing apparatus 2000 can capture a moving image during the inflow of contrast medium, like an X-ray fluoroscopic apparatus. The image capturing apparatus 2000 transmits the captured image data to the information processing apparatus 1000. Note that a plurality of image data may be transmitted at once to the information processing apparatus 1000, or image data may be transmitted sequentially every time an image is captured.
Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (for example, computer-readable medium).
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2012-245636, filed Nov. 7, 2012, which is hereby incorporated by reference herein in its entirety.

## Claims

1. An image processing apparatus for processing a radiation image, comprising:
a CPU; and
at least one memory operatively coupled to said CPU, said CPU and said at least one memory cooperating to function as:
a subtraction unit configured to generate a subtraction image by performing subtraction between a mask image serving as a radiation image obtained by capturing an object, at least a specific region of which does not include contrast medium, and a live image serving as a radiation image obtained by capturing the object which includes the contrast medium;
a determination unit configured to determine, based on 1) a value indicating a magnitude of local dispersion of pixel values of the live image and 2) noise statistics of the live image, an emphasis degree serving as a degree of emphasis processing for the subtraction image; and
a processing unit configured to perform the emphasis processing on the subtraction image based on the emphasis degree determined by said determination unit.

2. The apparatus according to claim 1, wherein said processing unit inhibits emphasis processing for the subtraction image in a region where a pixel value of the mask image is smaller than a predetermined value.

3. A method of controlling an image processing apparatus for processing a radiation image, comprising:
a subtraction step of generating a subtraction image by performing subtraction between a mask image serving as a radiation image obtained by capturing an object, at least a specific region of which does not include contrast medium, and a live image serving as a radiation image obtained by capturing the object which includes contrast medium;
a determination step of determining, based on 1) a value indicating a magnitude of local dispersion of pixel values of the live image and 2) noise statistics of the live image, an emphasis degree serving as a degree of emphasis processing for the subtraction image; and
a processing step of performing the emphasis processing on the subtraction image based on the emphasis degree determined in said determination step.

4. A non-transitory computer-readable storage medium storing, in executable form, a program for causing a computer to control an image processing apparatus for processing a radiation image, the program causing the computer to function as
a subtraction unit configured to generate a subtraction image by performing subtraction between a mask image serving as a radiation image obtained by capturing an object, at least a specific region of which does not include contrast medium, and a live image serving as a radiation image obtained by capturing the object which includes contrast medium,
a determination unit configured to determine, based on 1) a value indicating a magnitude of local dispersion of pixel values of the live image and 2) noise statistics of the live image, an emphasis degree serving as a degree of emphasis processing for the subtraction image, and
a processing unit configured to perform the emphasis processing on the subtraction image based on the emphasis degree determined by said determination unit.

5. The apparatus according to claim 1, wherein said CPU and said at least one memory further cooperate to function as a detection unit configured to detect an edge from the subtraction image,
wherein said processing unit performs the emphasis processing for the subtraction image based on the emphasis degree, an emphasis correction value serving as a correction value for the emphasis degree, and the edge detected by said detection unit.

6. The apparatus according to claim 1, wherein said processing unit decomposes the subtraction image into a plurality of band-limited images, performs adjustment of one of frequency emphasis and frequency suppression for each of the plurality of band-limited images based on the emphasis degree and an emphasis correction value serving as a correction value for the emphasis degree, and generates, from the plurality of band-limited images having undergone the adjustment, a subtraction image by performing emphasis processing for the subtraction image.

7. The apparatus according to claim 5, wherein said processing unit decomposes the subtraction image into a plurality of band-limited images, performs adjustment of one of frequency emphasis and frequency suppression for each of the plurality of band-limited images based on the emphasis degree, the emphasis correction value, and the edge, and generates, from the plurality of band-limited images having undergone the adjustment, a subtraction image by performing emphasis processing for the subtraction image.

8. The apparatus according to claim 1, wherein said processing unit decomposes the subtraction image into a plurality of band-limited images, detects an edge of each band-limited image from the plurality of band-limited images, performs adjustment of one of frequency emphasis and frequency suppression for each of the plurality of band-limited images based on the emphasis degree, an emphasis correction value serving as a correction value for the emphasis degree, and the edge of each band-limited image, and generates, from the plurality of band-limited images having undergone the adjustment, a subtraction image by performing emphasis processing for the subtraction image.

9. The apparatus according to claim 6, wherein said CPU and said at least one memory further cooperate to function as a setting unit configured to set an emphasis degree serving as a degree of emphasis processing for the subtraction image,
wherein said setting unit sets the emphasis degree for each of the plurality of band-limited images.

10. The apparatus according to claim 1, wherein said CPU and said at least one memory further cooperate to function as a setting unit configured to set an emphasis degree serving as a degree of emphasis processing for the subtraction image,
wherein said setting unit reads out, from a database which stores in advance emphasis degrees for respective types of image capturing conditions, a corresponding emphasis degree in accordance with designation of an image capturing condition, and sets the emphasis degree.

11. The apparatus according to claim 1, wherein said CPU and said at least one memory further cooperate to function as a calculation unit configured to calculate, from the mask image, an emphasis correction value serving as a correction value for the emphasis degree,
wherein said calculation unit calculates the emphasis correction value based on a noise amount estimated from a radiation dose by which the object is irradiated.