# Patent

## Title
Image processing device and image processing method

## Abstract

To provide image processing device and method capable of achieving a high-speed reconstruction process to a cone beam CT image, an area necessary for reconstruction in a projection image is determined from a slice position to be reconstructed, a convolution process is executed by using the determined area. Moreover, the slice position to be reconstructed is divided into the plural blocks, the areas of the projection image corresponding thereto are cut out, and the processing unit is set for each block as local data of each block, and the set unit is allocated with respect to each operation, whereby parallel processes are executed.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image processing device and an image processing method which are suitable for a process of a cone beam CT (computerized tomography) image.
2. Related Background Art
FIG. 2 shows the outline of a cone beam X-ray CT device (or scanner).
In FIG. 2, an X-ray irradiated from an X-ray tube 103 is absorbed and attenuated inside the body of a subject 102, and the X-ray passed the subject 102 is detected on a surface sensor 101. Then, the X-ray tube 103 and the surface sensor 101 are rotated around the subject 102 without changing the relative physical relationship between the X-ray tube 103 and the surface sensor 101, whereby the projection image data of the subject 102 as much as one rotation is acquired. Thus, the projection image data acquired like this is subjected to a reconstruction process, whereby the tomographic image of the subject 102 is acquired. Incidentally, to acquire the same tomographic image, the subject 102 may be rotated as much as one rotation instead of the rotation of the X-ray tube 103 and the surface sensor 101.
In the reconstruction process, a convolution process is first executed to the projection image data as shown in FIG. 3, and the convolution-processed projection image data is then subjected to back projection to each pixel of the reconstruction image as shown in FIG. 4.
To execute the reconstruction process at high speed, for example, a multiprocessor is used. Here, it should be noted that the method of achieving the high-speed reconstruction process by using the multiprocessor is described in “Reconstruction of 3-D X-ray Computerized Tomography Images Using a Distributed Memory Multiprocessor System”, TOHRU SASAKI and YASUSHI FUKUDA, Information Processing Society of Japan Transaction, Vol. 38, No. 9 (hereinafter called the document 1). In this method, the parallel processes are achieved by using the plural processors, and also the data is transferred at high speed. That is, the high-speed reconstruction process is achieved mainly by hardware.
However, the following important problems occur in the cone beam CT. That is, in the conventional technique, the projection image data acquired all over the projection angles is the line data corresponding to the one-dimensional fan beam.
However, in the cone beam CT, the projection image data is the two-dimensional image data, whereby an amount of data necessary in the process is huge. For this reason, if the convolution process is executed wholly to the huge amount of data as in the method described in the document 1, it is inefficient. Thus, it is impossible to achieve the high-speed reconstruction process by the hardware. Moreover, even if the parallel processes are executed, a huge amount of projection image data must be stored in the local memory of each of the processing units, whereby the capacity of each local memory must be made with high cost.
SUMMARY OF THE INVENTION
In consideration of the above conventional problems, an object of the present invention is to provide an image processing device and an image processing method which can achieve a high-speed reconstruction process of a cone beam CT image.
Here, an image processing device according to the present invention is characterized by comprising:
an extraction unit adapted to determine and extract an area necessary for reconstruction based on a geometric condition of a tomographic image slice surface to be reconstructed, in plural projection images; and
a reconstruction processing unit adapted to execute a reconstruction process by using the area extracted by the extraction unit.
Further, an image processing device according to the present invention is characterized by comprising:
a division unit adapted to divide a tomographic image slice position to be reconstructed into plural groups;
a distribution unit adapted to cut out an area of a projection image corresponding to the tomographic image slice position with respect to each group, and distribute the cut-out area as local data of each group; and
a reconstruction processing unit adapted to execute a reconstruction process independently with respect to each group.
Furthermore, an image processing device according to the present invention is characterized by comprising:
a designation unit adapted to designate a slice position of a tomographic image to be reconstructed;
an operation unit adapted to execute an operation of an existence range of projection data necessary for reconstructing the tomographic image of the designated slice position;
an extraction unit adapted to extract the projection data in the operated existence range; and
a reconstruction operation unit adapted to execute a reconstruction operation by using the extracted projection data.
Moreover, an image processing device according to the present invention is characterized by comprising:
a designation unit adapted to designate plural slice positions of a tomographic image to be reconstructed;
an allocation unit adapted to allocate the designated plural slice position respectively to plural processing units;
an operation unit adapted to execute an operation of an existence range of projection data necessary for reconstructing the tomographic image of the designated slice position in each of the plural processing units;
an extraction unit adapted to extract the projection data in the operated existence range in each of the plural processing units; and
a reconstruction operation unit adapted to execute a reconstruction operation by using the extracted projection data in each of the plural processing units.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing a structural example of a system;
FIG. 2 is a conceptual view for explaining a cone beam CT (Computed Tomography);
FIG. 3 is a conceptual view for explaining a convolution process;
FIG. 4 is a conceptual view for explaining a back projection process;
FIG. 5 is a view for explaining a line area of a projection image necessary for a back projection;
FIG. 6 is a conceptual view for explaining geometrical arrangement of the line area necessary for the back projection;
FIG. 7 is a conceptual view for explaining an area necessary for the back projection;
FIG. 8 is a conceptual view for explaining designation of a reconstruction slice position;
FIG. 9 is a view for explaining a location of the designated reconstruction slice position on a reconstruction image;
FIG. 10 is a conceptual view for explaining a reconstruction function;
FIG. 11 is a conceptual view for explaining an interpolation;
FIG. 12 is a conceptual view for explaining multi-computer MIMD (Multiple-Instruction stream, Multiple-Data stream) architecture;
FIG. 13 is a conceptual view for explaining allocation of processes to respective process units;
FIG. 14 is a conceptual view for explaining a flow of parallel processes;
FIG. 15 is a conceptual view for explaining an overlapped line area; and
FIG. 16 is a flowchart for explaining an operation to be executed in a cone beam X-ray CT device according to the present embodiment.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, the embodiment of the present invention will be concretely explained with reference to the attached drawings.
FIG. 1 is a schematic view for schematically showing entire structure of a cone beam X-ray CT device having an image processing device according to the embodiment of the present invention.
An X-ray radiography system control unit 106 executes a radiography control of an entire device, image collection, an image process and an image output. When the X-ray radiography system control unit 106 instructs an X-ray generator control unit 104 to generate the X-ray, an X-ray source 103 controlled by the X-ray generator control unit 104 generates the X-ray. The X-ray generated by the X-ray source 103 transmits through a patient 102 being a radiographic object, and the transmitted X-ray is detected by an X-ray detector (area sensor) 101. The detected X-ray is input to an image input unit 105 as projection image data. Then, projection images are collected every determined rotation angle while rotating the X-ray source 103 and the X-ray detector 101 around the patient by treating the patient 102 being the radiographic object as a center of rotation. It should be noted that the patient 102 may be rotated while maintaining the location relationship between the X-ray source 103 and the X-ray detector 101 in a manner that the patient is kept in a state capable of rotating by providing a rotation table (not shown) or the like at a portion where the patient 102 is located.
For the projection images which were input every rotation angle, a preprocess including the correction of the X-ray detector 101 and a log conversion process or an image process such as a reconstruction process or the like is executed by an image processing unit 107, then a tomographic images group is formed. This image processing unit 107 corresponds to the image processing device according to the embodiment of the present invention. The formed tomographic images group is displayed on a diagnostic monitor 109, stored in an image storage unit 108 or output to a printer 112, a diagnostic work station 113 and/or an image database 114 through a network 111. Various operations such as a window operation of display, a changeover display operation of a tomographic image in the direction of a body axis, an axial plane (slice plane) converting operation, a three-dimensional surface display operation and the like are executed by an operation unit 110.
Next, an operation of the cone beam X-ray CT device structured as above described will be explained.
FIG. 16 is a flowchart for explaining an operation to be executed in the cone beam X-ray CT device.
Initially, a simple radiography for designating a slice position and the CT radiography for obtaining the projection image data are executed (step S10). In the cone beam X-ray CT device of the present embodiment, since the area sensor is used as the X-ray detector 101, if the simple radiography is executed by stilling the device and an examinee, an image for designating the slice position can be obtained. Generally, although a high speed process is required in the CT radiography, the high speed process is not required in a still image. Therefore, if the radiography is executed by setting a read-out mode to a high precision read-out, a more preferable high precision still image can be obtained. It is allowed that a projection image radiographed from a specific projection angle is used as a simple radiography image in case of executing the CT radiography for obtaining the projection data without executing the simple radiography.
Thereafter, a slice surface of a tomographic image to be reconstructed is designated as shown in FIG. 8 by using the obtained simple radiography image (step S20). Projection data necessary for reconstructing a tomographic image on a certain arbitrary slice position is limited within a certain narrow area in a projection image as shown in FIG. 5. The above location relationship is indicated by the location relationship shown in FIG. 6.
In FIG. 6, a line area of the Z′-coordinate on the projection image of the slice position, where the Z-coordinate of a reconstruction image is Zi, is represented by the following area.




  
    
      
        
          Zi
          ×
          
            
              D
              +
              S
            
            
              D
              +
              R
            
          
        
        ∼
        
          Zi
          ×
          
            
              D
              +
              S
            
            
              D
              -
              R
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




In the expression 1, reference symbol R denotes a radius of a reconstruction area (half of vertical length and lateral length of a reconstruction image), reference symbol D denotes distance between the X-ray source and a center of rotation (center of the reconstruction image) and reference numeral S denotes distance between the center of rotation and the sensor.
Therefore, for example, in a case that a space between slice positions to be reconstructed is not required to be a dense state, data of an entire projection image is not always necessary for the reconstruction, and it is allowed that only the intermittent areas exist as shown in FIG. 7. And, it is allowed that a convolution process is also executed for only those intermittent areas.
If a location of the slice surface designated in the step S20 is assumed as a location on a central axis of the reconstruction image as shown in FIG. 9, the location Zi on the central axis of the reconstruction image can be calculated from the location Z′ of the slice position by the following expression 2 (step S30).




  
    
      
        Zi
        =
        
          
            Z
            ′
          
          ×
          
            D
            
              D
              +
              S
            
          
        
      
    
    
      
        (
        2
        )
      
    
  




In the expression 2, reference symbol R denotes the radius of the reconstruction area (half of vertical length and lateral length of a reconstruction image), reference symbol D denotes distance between the X-ray source and the center of rotation (center of the reconstruction image) and reference numeral S denotes distance between the center of rotation and the sensor.
If using the location Zi on the central axis of the reconstruction image obtained in the step S30, a line area of the Z′-coordinate on the projection image is represented by the following area (expression 3). This line area is calculated by the image process unit 107 on the basis of a location of the designated slice position (step S40).




  
    
      
        
          Zi
          ×
          
            
              D
              +
              S
            
            
              D
              +
              R
            
          
        
        ∼
        
          Zi
          ×
          
            
              D
              +
              S
            
            
              D
              -
              R
            
          
        
      
    
    
      
        (
        3
        )
      
    
  




In the expression 3, reference symbol R denotes the radius of the reconstruction area (half of vertical length and lateral length of a reconstruction image), reference symbol D denotes distance between the X-ray source and the center of rotation (center of the reconstruction image) and reference numeral S denotes distance between the center of rotation and the sensor.
Next, the image process unit 107 executes the convolution process to the line area in the projection image data (step S50). The convolution process is executed by taking convolution of lateral line data of the projection image data and one-dimensional data called a reconstruction function.
The function of Ramachandoran and the function of Shepp & Logan being typical examples of the reconstruction function are shown in FIG. 10.
Thereafter, a back projection process is executed by using the projection image data to which the convolution process was executed (step S60). As shown in FIG. 4, the back projection process is executed in a manner, where as to each of pixels of the reconstruction image, the coordinate of a dot formed on the projection image by transmitting the X-ray through that each pixel of the reconstruction image is obtained, and pixel values of four dots in the vicinity of a location of the obtained coordinate are calculated by the interpolation then the obtained pixel values are summed up.
Here, the geometrical relationship in the interpolation is shown in FIG. 11. In FIG. 11, dots P1 to P4 are central locations of the four pixels in the vicinity of the projected dot. If it is assumed that distances between the projected dot and these four dots are respectively d1 to d4 and the pixel values of the four pixels in the vicinity of the projected dot are respectively Q1 to Q4, the interpolation may be executed by using back projection data V calculated by the following expressions 4.




  
    
      
        
          
            
              A1
              =
              
                
                  
                    Q1
                    ×
                    
                      d
                      2
                    
                  
                  +
                  
                    Q2
                    ×
                    
                      d
                      1
                    
                  
                
                
                  
                    d
                    1
                  
                  +
                  
                    d
                    2
                  
                
              
            
          
        
        
          
            
              A2
              =
              
                
                  
                    Q3
                    ×
                    
                      d
                      2
                    
                  
                  +
                  
                    Q4
                    ×
                    
                      d
                      1
                    
                  
                
                
                  
                    d
                    1
                  
                  +
                  
                    d
                    2
                  
                
              
            
          
        
        
          
            
              V
              =
              
                
                  
                    A1
                    ×
                    
                      d
                      4
                    
                  
                  +
                  
                    Q2
                    ×
                    
                      d
                      3
                    
                  
                
                
                  
                    d
                    3
                  
                  +
                  
                    d
                    4
                  
                
              
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




Then, if the back projection data V interpolated in this manner are summed up for the entire projection image data and the entire pixels, the reconstruction process is terminated.
As described above, the tomographic image of the designated slice position can be formed. The formed tomographic image is stored in the image storage unit 108 or displayed on the diagnostic monitor 109 (step S70).
In the present embodiment, an area necessary for reconstruction in the projection image is determined from the slice position to be reconstructed, and the convolution process and the back projection process are executed by using the determined area. Accordingly, the useless convolution and back projection processes can be omitted, thereby shortening a processing time of the reconstruction process.
In the present embodiment, it is structured that in a case of selecting plural portions of the slice positions of the tomographic image, the processes in the steps S30 to S60 indicated in a flowchart shown in FIG. 16 are executed in parallel in the respective process units. With respect the parallelizing, various architectures exist, however in the present embodiment, for example, the multi-computer MIMD architecture is used. If this multi-computer MIMD architecture is illustrated, FIG. 12 is given. As shown in FIG. 12, in the multi-computer MIMD architecture, memories are provided in the respective process units.
In the present embodiment, the reconstruction images of the respective slice positions are allocated to the respective process units. Then, only data for the line area of the projection image necessary for the allocated reconstruction image is stored in the memory of each of the process units. The above conception is shown in FIG. 13.
Next, a flow of the parallelized reconstruction process as above described will be explained with reference to FIG. 14.
In this process, it is assumed that the pixel number of the reconstruction image row is equal to the number of the process units, and initially, the each projection image data is equally divided into data, of which the number is the number of the respective units, and the divided data is transferred to the memories of the respective process units.
Next, the convolution processes are executed to the equally divided projection image data in the respective process units. Here, the projection image data is equally divided in order to improve an effect of the parallelizing by uniformly dispersing the processing load on the respective process unit.
Then, the back projection process is executed by using the projection image data resulted in executing the convolution process. However, the line area of the projection image data distributed to each of the process units at this time is not sufficiently satisfied by the equally divided line area. For example, as shown in FIG. 15, because there is a case that line areas necessary for the reconstruction image to be reconstructed by the respective process units are overlapped. In this case, it is required that data is to be transferred from the memory of another process unit. Therefore, in the present embodiment, the above transfer is executed by using an interconnection network. Then, after preparing necessary data, the back projection process is independently executed every the process unit.
In this manner, the reconstruction process in the cone beam CT can be effectively executed in parallel in the multi-computer MIMD architecture.
In the present embodiment, the slice position to be reconstructed is divided into plural blocks, and areas of the projection image corresponding to the divided blocks are cut out, and a unit of process is set every the block as local data of the each block, then a parallel process is executed by allocating the units of process to the units of operation. According to the above parallelizing, capacity of a local memory of each of the process units can be minimized and the costs can be reduced.
The embodiment of the present invention can be realized by executing the programs by, for example, a computer. Further, a means for supplying the programs to the computer, for example, a recording medium which can be read by the computer such as a CD-ROM or the like storing the programs or a transmission/reception medium such as an internet or the like for transmitting and receiving the programs are also can be applied as the embodiment of the present invention. Further, the above programs can be applied as the embodiment of the present invention. The above programs, the recording medium, the transmission/reception medium and the program products are included in the scope of the present invention.
This application claims priority from Japanese Patent Application No. 2004-239690 filed on Aug. 19, 2004, which is hereby incorporated by reference herein.

## Claims

1. An image processing device comprising:
an extraction unit configured to determine and extract an area necessary for reconstruction based on a geometric condition of a tomographic image slice position to be reconstructed, in plural projection images; and
a reconstruction processing unit configured to execute a reconstruction process by using the area extracted by said extraction unit, wherein, in a case where it is assumed that a difference in height between the center of an area being the target of the reconstruction and an X-ray source is Zi, a distance in a horizontal direction between the center of the area being the target of the reconstruction and the X-ray source is D, a distance in a horizontal direction between the center of the area being the target of the reconstruction and a sensor is S, and a radius of the area being the target of the reconstruction is R, said extraction unit extracts, as the area necessary for reconstruction, the area from Zi×(D+S)/(D+R) to Zi+(D+S)/(D−R) in height, based on the location of the X-ray source and the sensor.

2. An image processing method comprising:
an extraction step of determining and extracting an area necessary for reconstruction based on a geometric condition of a tomographic image slice position to be reconstructed, in plural projection images; and
a reconstruction processing step of executing a reconstruction process by using the area extracted in said extraction step, wherein, in a case where it is assumed that a difference in height between the center of an area being the target of the reconstruction and an X-ray source is Zi a distance in a horizontal direction between the center of the area being the target of the reconstruction and the X-ray source is D, a distance in a horizontal direction between the center of the area being the target of the reconstruction and a sensor is S, and a radius of the area being the target of the reconstruction is R, said extraction step extracts, as the area necessary for reconstruction, the area from Zi×(D+S)/(D+R) to Zi×(D+S)/(D−R) in height, based on the location of the X-ray source and the sensor.

3. A computer-readable medium encoded with a computer program for causing a computer to achieve an image processing method comprising:
an extraction step of determining and extracting an area necessary for reconstruction based on a geometric condition of a tomographic image slice position to be reconstructed, in plural projection images; and
a reconstruction processing step of executing a reconstruction process by using the area extracted in said extraction step, wherein, in a case where it is assumed that a difference in height between the center of an area being the target of the reconstruction and an X-ray source is Zi, a distance in a horizontal direction between the center of the area being the target of the reconstruction and the X-ray source is D, a distance in a horizontal direction between the center of the area being the target of the reconstruction and a sensor is S, and a radius of the area being the target of the reconstruction is R, said extraction step extracts, as the area necessary for reconstruction, the area from Zi×(D+S)/(D+R) to Zi×(D+S)/(D−R) in height based on the location of the X-ray source and the sensor.