Abstract:
An image processing apparatus acquires a first region image by eliminating an non-X-ray irradiated region and a direct irradiated region from an X-ray image, decides an initial region in the first region image by analyzing the first region image, enlarges the initial region by using a region growing processing to obtain a second region in the first region image, and obtains a second region image which corresponds to the second region of the X-ray image.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to an image processing technique for acquiring a subject region from an X-ray image more precisely and accurately.  
       BACKGROUND OF THE INVENTION  
       [0002]     Owing to progress in the digital processing of image information in recent years, it is now common to apply digital processing even to medical X-ray images. In particular, sensors that are capable of outputting X-ray image data in digital form have been developed to take the place of X-ray photography using conventional film. The digital processing of medical X-ray images is essential in an X-ray imaging apparatus that uses such a sensor. There are various types of digital processing that can be applied to images captured by a sensor or camera. One important example of such image processing is gray level transformation processing, which converts acquired data to an image having easier-to-observe density values by applying a gray level transformation to the captured data.  
         [0003]      FIGS. 11A and 11B  are X-ray images of a knee joint, in which reference numeral  1201  denotes the entire image and reference numeral  1202  indicates the state in which the knee joint, which is the subject region, and the area peripheral thereto have been extracted from the entire image. In a case where data that has been captured by some image sensing apparatus such as a sensor or camera as mentioned above is displayed on a monitor screen or X-ray diagnostic film, the general practice is to apply a gray level transformation to the captured data to thereby transform the captured data to density values that are easy to observe. For example, in a case where X-ray image data such as that of a knee joint is displayed on X-ray diagnostic film, a feature (e.g., an average density value) for the gray level transformation is extracted from the X-ray image data and a transformation is made in such a manner that the extracted feature attains a predetermined value (e.g., a predetermined density value).  
         [0004]     One general method for performing the above-described gray level transformation will now be described with reference to  FIGS. 11A and 11B . The image region (subject region)  1202  in which the object of interest is present is acquired by eliminating a direct irradiated region from the X-ray irradiated region in image  1201 . A feature is calculated by performing a statistical investigation regarding the pixel values within the acquired subject region  1202 . For example, a histogram of pixel values is created and the created histogram is analyzed to calculate the feature, or a statistical quantity such as an average pixel value within the acquired subject region  1202  is calculated. The gray level transformation is performed based upon the feature or statistical quantity.  
         [0005]     However, in a case where the pixel-value distribution within the subject region is not ordinary, e.g., in a case where a radiopaque object typified by a metal such as screw or artificial joint is present in the subject region, as illustrated in  FIGS. 11C and 11D , the image of this radiopaque object has an influence upon the feature or statistical quantity with the method described above. As a result, a problem which arises is that the image after the gray level transformation is unstable. In order to perform the gray level transformation, therefore, a pure subject region from which the radiopaque object has been eliminated is acquired.  
         [0006]     Known methods of acquiring a pure subject region include a method of executing binarization processing using a threshold value that has been set based upon experience, and a method of creating a histogram  1301  of a subject region  1204 , as shown in  FIG. 12A , detecting a pixel value of a valley portion between peaks by histogram analysis, and separating the image of the radiopaque object and the image of the subject using this pixel value as a boundary ( FIG. 12B ) (see “Image Processing Engineering—Introduction”, edited by Yoshiharu Taniguchi, Kyoritsu Publishing K.K. (1996), Chapter 5 (pp. 79-97), Segmenting of Images).  
         [0007]     However, with binarization processing that employs a threshold value based upon experience, there is the possibility that the threshold value will be inappropriate in the event that the photographic conditions of the X-ray examination change, and it may no longer be possible to separate the radiopaque object and the subject (object of interest) correctly. Further, with the method of performing histogram analysis shown in  FIGS. 12A and 12B , a threshold value can be set appropriately to some degree. However, a problem which arises is that the radiopaque object and the subject cannot be separated in the case of a disease where the pixel values of the radiopaque object and subject approximate each other or change places.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, an object of the present invention is to make it possible to acquire a pure subject region highly precisely and stably.  
         [0009]     According to one aspect of the present invention, there is provided an image processing apparatus comprising: first acquisition means for acquiring a first region image by eliminating an non-X-ray irradiated region and a direct irradiated region from an X-ray image; decision means for deciding an initial region in the first region image by analyzing the first region image; region growing means for enlarging the initial region by using a region growing processing to obtain a second region in the first region image; and second acquisition means for obtaining a second region image which corresponds to the second region of the X-ray image.  
         [0010]     According to another aspect of the present invention, there is provided an image processing method comprising: a first acquisition step of acquiring a first region image by eliminating a non-X-ray irradiated region and a direct irradiated region from an X-ray image; a decision step of deciding an initial region in the first region image by analyzing the first region image; a region growing step of enlarging the initial region by using a region growing processing to obtain a second region in the first region image; and a second acquisition step of obtaining a second region image which corresponds to the second region of the X-ray image.  
         [0011]     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0013]      FIG. 1  is a block diagram illustrating the structure of an X-ray imaging apparatus according to a first embodiment of the present invention;  
         [0014]      FIG. 2  is a flowchart for describing gray level transformation processing according to the first embodiment;  
         [0015]      FIG. 3  is a flowchart for describing subject region acquisition processing according to the first embodiment;  
         [0016]      FIGS. 4A  to  4 E are diagrams illustrating hip-joint images;  
         [0017]      FIG. 5  is a diagram illustrating a histogram generated from the approximate subject region of a hip-joint image;  
         [0018]      FIG. 6  is a block diagram illustrating the structure of an X-ray imaging apparatus according to a second embodiment of the present invention;  
         [0019]      FIG. 7  is a flowchart for describing gray level transformation processing according to the second embodiment;  
         [0020]      FIG. 8  is a flowchart for describing subject region acquisition processing according to the second embodiment;  
         [0021]      FIG. 9  is a diagram illustrating an initial-area forming rule, which has been set for every X-ray imaging region, used in region growing according to the second embodiment;  
         [0022]      FIGS. 10A  to  10 C are diagrams for describing threshold values in region growing processing;  
         [0023]      FIGS. 11A  to  11 D are diagrams illustrating knee joints; and  
         [0024]      FIGS. 12A and 12B  are diagrams for describing a method of histogram analysis of an approximate subject region of a knee-joint image. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.  
         [0026]     &lt;First Embodiment&gt; 
         [0027]      FIG. 1  is a block diagram illustrating the structure of an X-ray imaging apparatus  100  according to a first embodiment of the present invention. As shown in  FIG. 1 , the X-ray imaging apparatus  100  is one having an image processing function and includes a data acquisition circuit  105 , a preprocessing circuit  106 , a CPU  108 , a main memory  109 , a control panel  110  and an image processing circuit  111 . The X-ray imaging apparatus  100  has a two-dimensional X-ray sensor  104  and an X-ray generating circuit  101  connected to the data acquisition circuit  105 .  
         [0028]     In the X-ray imaging apparatus  100  constructed as set forth above, the main memory  109  stores various data necessary for processing by the CPU  108  and includes a work memory for the CPU  108 . The CPU  108  uses the main memory  109  to control various operations of the overall apparatus in accordance with operations performed at the control panel  110 . The X-ray imaging apparatus  100  operates in the manner described below.  
         [0029]     First, the X-ray generating circuit  101  emits an X-ray beam  102  toward an object  103 . The X-ray beam  102  emitted from the X-ray generating circuit  101  passes through the object  103  while being attenuated, reaches the two-dimensional X-ray sensor  104  and is output as an X-ray image by the two-dimensional X-ray sensor  104 . It is assumed here that the X-ray image that is output from the two-dimensional X-ray sensor  104  is an image of the portion of a human body, e.g., of a knee.  
         [0030]     The data acquisition circuit  105  converts the X-ray image, which has been output from the two-dimensional X-ray sensor  104 , to an electric signal and supplies the electric signal to the preprocessing circuit  106 . The latter subjects the signal (X-ray image signal) from the data acquisition circuit  105  to preprocessing such as offset correction processing and gain correction processing. Under the control of the CPU  108 , the X-ray image signal that has undergone this preprocessing by the preprocessing circuit  106  is transferred as an original image to the main memory  109  and image processing circuit  111  via a CPU bus  107 .  
         [0031]     The image processing circuit  111  includes an X-ray irradiated region acquisition circuit  113  for acquiring an X-ray irradiated region; a direct irradiated region acquisition circuit  114  for acquiring a direct irradiated region from the X-ray irradiated region; an approximate subject region acquisition circuit  115  for acquiring an approximate subject region from the X-ray irradiated region and direct irradiated region; a subject region acquisition circuit  116  for acquiring a subject region from the approximate subject region; a feature extraction circuit  117  for calculating a feature for a gray level transformation from the subject region; and a gray level transformation circuit  118  for performing a gray level transformation of an image based upon the feature calculated by the feature extraction circuit  117 . The image processing circuit  111  may be implemented by hardware or a part or the entirety thereof may be implemented by software.  
         [0032]      FIG. 2  is a flowchart for describing gray level transformation processing according to the first embodiment, and  FIG. 3  is a flowchart for describing processing executed by the subject region acquisition circuit  116 . Further,  FIGS. 4A  to  4 E are examples of images that undergo subject region acquisition processing according to the first embodiment. Illustrated are an original image  401 , an image (referred to as an “x-ray irradiated region image”)  402  after acquisition of an X-ray irradiated region, an image (referred to as an “approximate subject region image”)  403  after elimination of a direct irradiated region from the X-ray irradiated region, an image  404  indicating an initial region used in region growing, and an image (referred to an “a subject region image”)  405  indicating a subject region that is the result of processing. Further,  FIG. 5  is a histogram created from the image  403  indicating the approximate subject region image. Pixel values are plotted along the horizontal axis of the graph, and frequency along the vertical axis. Reference will be had to these diagrams to describe in detail the gray level transformation processing according to this embodiment.  
         [0033]     First, under the control of the CPU  108 , the original image (e.g., a hip-joint image)  401  that has been processed by the preprocessing circuit  106  is supplied to the image processing circuit  111  via the CPU bus  107 . Upon receiving the original image  401 , the image processing circuit  111  acquires the X-ray irradiated region of the original image  401  using the X-ray irradiated region acquisition circuit  113  and generates the X-ray irradiated region image  402  (step S 201 ). Next, the image processing circuit  111  acquires the direct irradiated region of the X-ray irradiated region image  402  using the direct irradiated region acquisition circuit  114  (step S 202 ) and deletes the acquired direct irradiated region from the X-ray irradiated region image  402  using the approximate subject region acquisition circuit  115 , thereby generating the approximate subject region image  403  (step S 203 ).  
         [0034]     Next, the image processing circuit  111  generates the subject region image  405  from the approximate subject region image  403  (step S 204 ). The processing of step S 204  will now be described in detail with reference to FIGS.  3  to  5 .  
         [0035]     First, a histogram of the approximate subject region image  403  is created (step S 301 ;  FIG. 5 ). Next, a maximum-frequency pixel value Vf is extracted from the histogram created (step S 302 ). Next, a maximum pixel value Vm of the approximate subject region image  403  is calculated (step S 303 ) and then an initial region of the subject region is set (step S 304 ). In this example, a set of pixels having-pixel values greater than an intermediate value (Vf+Vm)/ 2  between Vf and Vm is adopted as an initial region  404  (step S 304 ). Next, the initial region  404  is subjected to region growing in accordance with prescribed conditions (step S 305 ). In this example, region growing is defined in a case where pixel values Vi and Vo of neighboring pixels inside and outside an area satisfy the following equation:
 
 Vi−Vo&lt;Th 
 
         [0036]     Here Th is a threshold value [Th=f(Vm,Vo)] decided by Vm and Vo. The threshold value is given as shown in  FIG. 10C , by way of example. The difference between the pixel value inside the area and the neighboring pixel value outside the area is calculated. In the case of a 4-pixel neighborhood region growing algorithm, however, there are a maximum of four neighboring pixels outside the area (pixels above, below and to the left and right). Accordingly, the difference with respect to each of these is calculated and it is determined whether the pixel is inside or outside the area. Further, in the case of an 8-pixel neighborhood region growing algorithm, four diagonal directions are applied and a maximum of eight neighboring pixels will exist.  
         [0037]     The above-described region growing processing is executed repeatedly as long as the region changes, and processing is terminated when the region no longer changes (step S 306 ). The image portion of the original image  401  corresponding to the region thus obtained by region growing processing at the moment the processing of  FIG. 3  is terminated is output as the subject region image  405 . As illustrated in  FIGS. 4A  to  4 E, the region of a radiopaque object  406  has been excluded from the subject region image  405 , and it will be understood that the image of a pure subject region has been obtained.  
         [0038]     With reference again to  FIG. 2 , feature extraction processing is executed using the subject region image  405  acquired at step S 204  above, and a gray level transformation is performed based upon the feature extracted. First, the feature extraction circuit  117  calculates the feature from the subject region image  405  acquired by the subject region acquisition circuit  116  (step S 205 ). An intermediate value or average value, etc., can be used as the feature. The gray level transformation circuit  118  then subjects the original image  401  to a gray level transformation based upon the feature calculated by the feature extraction circuit  117  (step S 206 ). By way of example, the original image  401  is subjected to a gray level transformation in such a manner that the feature (intermediate value or average value) becomes a prescribed density.  
         [0039]     Thus, in accordance with the first embodiment, a pure subject region is acquired by applying a region growing method to a region that has been extracted based upon an X-ray irradiated region and a direct irradiated region. More specifically, an effect of the first embodiment is that a pure subject region from which a radiopaque object typified by a screw, artificial joint or protector in the captured image has been removed can be obtained by an algorithm that is based upon the region growing method. Accordingly, even in cases where a radiopaque object exists in a captured image, a stable, highly precise gray level transformation can be performed without being affected by the radiopaque object.  
         [0040]     &lt;Second Embodiment&gt; 
         [0041]     In a second embodiment, an arrangement in which the setting of an initial region or the condition of region growing is changed over for every body part photographed.  FIG. 6  is a block diagram illustrating the structure of an X-ray imaging apparatus  700  according to the second embodiment. Components in  FIG. 6  similar to those of the first embodiment ( FIG. 1 ) are designated by like reference characters. The X-ray imaging apparatus  700  differs from the X-ray imaging apparatus  100  in that an image processing circuit  711  is further provided with a body part information acquisition circuit  712 , and in that processing for creating an initial region and the initial value used in region growing are changed in the subject region acquisition circuit  716  depending upon the body part information that has been acquired by the body part information acquisition circuit  712 . Accordingly, the content of processing in the subject region acquisition circuit  716  changed by the addition of the body part information acquisition circuit  712  will be described in detail.  
         [0042]      FIG. 7  is a flowchart illustrating a gray level transformation processing according to the second embodiment, and  FIG. 8  is a flowchart for describing the processing of the subject region acquisition circuit  716  according to the second embodiment. The processing steps will be described using the examples of  FIGS. 4A  to  4 E in a manner similar to that of the first embodiment.  FIG. 9  illustrates a method of setting an initial region used in region growing, described below, and  FIGS. 10A  to  10 C are graphs illustrating threshold values used in region growing. These graphs have been set in correspondence with respective ones of body part of interest.  
         [0043]     First, under the control of the CPU  108 , the original image (e.g., the hip-joint image)  401  that has been processed by the preprocessing circuit  106  is received by the image processing circuit  711 . Upon receiving the original image  401 , the image processing circuit  711  acquires the body part information of the original image  401  using the body part information acquisition circuit  712  and stores the information in the main memory  109  (step S 801 ). Specific examples of processing executed by the body part information acquisition circuit  712  include a method of detecting a photographic body part from the original image  401  automatically by a technique such as pattern matching, and a method of allowing the operator to select or designate the region utilizing a control panel  710 . It does not matter which method is used.  
         [0044]     The approximate subject region image  403  is acquired by the X-ray irradiated region acquisition circuit  113 , direct irradiated region acquisition circuit  114  and approximate subject region acquisition circuit  115  (steps S 802  to S 804 ). These processing steps are the same as steps S 201  to S 203  of the first embodiment ( FIG. 2 ) and need not be described again.  
         [0045]     Next, the subject region image  405  is generated from the approximate subject region image  403  by the subject region acquisition circuit  716  (step S 805 ). The flow of this processing will be described in accordance with  FIG. 8 . It should be noted that steps S 901  to S 903  are similar to steps S 301  to S 303  in the first embodiment ( FIG. 3 ).  
         [0046]     The initial region of the subject region is set at step S 904 . A rule for forming an initial region set for every body part is selected using the body part information that the body part information acquisition circuit  712  stored in a main memory  709  at step S 801 , and the initial region  404  is set in accordance with the rule selected (step S 904 ).  FIG. 9  is a diagram showing an example of retention of rules for forming initial regions according to the second embodiment. As shown in  FIG. 9 , a range of pixel values to be selected as an initial region (a rule for forming an initial region) has been set for every body part (chest, head, extremities) to undergo X-ray imaging. Accordingly, if the body part to be photographed is “EXTREMITIES”, for example, as exemplified in  FIGS. 4A  to  4 E, then the rule selected is “ADOPT VALUES GREATER THAN INTERMEDIATE VALUE OF PIXEL VALUE HAVING HIGHEST FREQUENCY AND MAXIMUM PIXEL VALUE”, and the initial region  404  is formed in accordance with this rule. It should be noted that the conditions of the initial regions shown in  FIG. 9  illustrate examples of conditions that have been set in accordance with the features of the X-ray image of each body part and that these examples do not impose any limitation.  
         [0047]     Next, the initial region  404  is subjected to region growing in accordance with certain conditions (step S 905 ). In a manner similar to that of the first embodiment, region growing is defined in a case where pixel values Vi and Vo of neighboring pixels inside and outside the region satisfy the following equation:
 
 Vi−Vo&lt;Th  ( Vm, Vo , body part information)
 
         [0048]     Here Th is a threshold value [Th=f(Vm,Vo, region)] decided by Vm, Vo and body part information. The threshold value is given as shown in  FIGS. 10A  to  10 C, by way of example. Specifically, at step S 905 , the region-growth rule is changed or selected using the body part information that the body part information acquisition circuit  712  stored in the main memory  709  at step S 801 , and region growing is executed based upon the rule selected. It should be noted that the functions of the threshold values indicated in  FIGS. 10A  to  10   c  are such that the threshold value is low (region growth readily occurs) in a region of comparatively high pixel values where a radiopaque object tends not to be present and high (region growth does not readily occur) in a region of comparatively low pixel values where a radiopaque object tends to be present. Further, Th at minus infinity is the condition at which region growth will absolutely not occur. The reason while this condition differs depending upon the body part photographed is that it reflects the features of the body part statistically obtained. Further, by deciding the threshold value using the difference value with respect to the maximum pixel value, as mentioned above, the threshold value is set appropriately in regions where pixel values are comparatively high and regions where pixel values are comparatively low. It should be noted that the functions of the threshold values shown in  FIGS. 10A  to  10 C illustrate examples of functions that have been set in accordance with the features of the X-ray images of each body part and that these examples are not limitative.  
         [0049]     The above-described region growing processing is executed repeatedly as long as the region changes, and processing is terminated when the region no longer changes (step S 906 ). The image portion of the original image  401  corresponding to the region thus obtained by region growing processing at the moment the processing of  FIG. 8  is terminated is output as the subject region image  405 . As illustrated in  FIGS. 4A  to  4 E, the area of the radiopaque object  406  has been excluded from the subject region image  405 , and it will be understood that the image of a pure subject region has been obtained.  
         [0050]     In accordance with the second embodiment, as described above, besides the effect of the first embodiment, an additional effect is that an initial region or initial condition conforming to a photographic body part is set and an algorithm that is based upon a region growing method can be applied. Further, a radiopaque object typified by a screw, artificial joint or protector within the image thus captured can be eliminated in highly precise fashion and a pure subject region can be acquired stably. Accordingly, even in cases where a radiopaque object exists in a captured image, a stable, highly precise gray level transformation can be performed without being affected by the radiopaque object.  
         [0051]     &lt;Other Embodiments&gt; 
         [0052]     It goes without saying that the object of the invention is attained also by supplying a storage medium storing the program codes of the software for performing the functions of the foregoing embodiments to a system or an apparatus, reading the program codes with a computer (e.g., a CPU or MPU) of the system or apparatus from the storage medium, and then executing the program codes.  
         [0053]     In this case, the program codes read from the storage medium implement the novel functions of the embodiments and the storage medium storing the program codes constitutes the invention.  
         [0054]     Examples of storage media that can be used for supplying the program code are a floppy disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, non-volatile type memory card or ROM, etc.  
         [0055]     Furthermore, besides the case where the aforesaid functions according to the embodiments are implemented by executing the program codes read by a computer, it goes without saying that the present invention covers a case where an operating system or the like running on the computer performs a part of or the entire process in accordance with the designation of program codes and implements the functions according to the embodiments.  
         [0056]     It goes without saying that the present invention further covers a case where, after the program codes read from the storage medium are written in a function expansion board inserted into the computer or in a memory provided in a function expansion unit connected to the computer, a CPU or the like contained in the function expansion board or function expansion unit performs a part of or the entire process in accordance with the designation of program codes and implements the function of the above embodiments.  
         [0057]     Thus, in accordance with the embodiments described above, a pure subject region can be acquired from an approximate subject region acquired based upon an X-ray irradiated region and a direct irradiated region. In particular, an initial region is set based upon a pixel-value distribution of an approximate subject region and the set initial region is subjected to region growth based upon pixel values inside and outside the region. As a result, it is possible to acquire a more stable, highly precise subject region. In particular, in the discrimination of the inside and outside of a region based upon pixel values inside and outside the region, an initial region that has been set based upon a pixel-value distribution of an approximate subject region is subjected to region growth based upon pixel values inside and outside the region and the distribution of pixel values, thereby making it possible to acquire a more stable, highly precise subject region.  
         [0058]     Further, in the setting of the initial region and in region growing, the content of this processing is changed in accordance with the photographic body part of the X-ray image, thereby making it possible to acquire a more stable, highly precise subject region.  
         [0059]     In accordance with the present invention, a pure subject region can be acquired highly precisely and stably. Further, if it is so arranged that a gray level transformation is performed using the subject region thus obtained, it becomes possible to perform a gray level transformation that excludes effects ascribable to metal, etc., and excellent results of gray level transformation are obtained.  
         [0060]     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.  
         [0000]     Claim of Priority  
         [0061]     This application claims priority from Japanese Patent Application No. 2004-077042 filed Mar. 17, 2004, which is hereby incorporated by reference herein.