Patent Publication Number: US-10314557-B2

Title: Radiography apparatus and method for controlling the radiography apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/838,636, filed on Aug. 28, 2015, which claims priority from Korean Patent Application No. 10-2015-0009376, filed on Jan. 20, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     Apparatuses and methods consistent with exemplary embodiments relate to a radiography apparatus and a method for controlling the radiography apparatus. 
     2. Description of the Related Art 
     A radiography apparatus irradiates radiation such as X-rays to a subject, and receives radiation transmitted through the subject to acquire images about an inside of the subject. The radiography apparatus can acquire information about the inside of the subject using such a property of radiation that is absorbed or transmitted according to properties of a material it passes through. 
     The radiography apparatus is used in various fields. For example, the radiography apparatus is used to detect lesions in a human body, and to understand inside structures of objects and elements. Also, the radiography apparatus is used to scan baggage in an airport or harbor. Examples of the radiography apparatus include Digital Radiography (DR), Computed Tomography (CT), Full Field Digital Mammography (FFDM), an angiography apparatus, and Positron Emission Tomography (PET). 
     SUMMARY 
     Exemplary embodiments address at least the above problems and/or disadvantages and other disadvantages not described above. Also, the exemplary embodiments are not required to overcome the disadvantages described above, and may not overcome any of the problems described above. 
     Exemplary embodiments provide a radiography apparatus that is capable of reducing or removing an artifact on an image, which may be generated by radiography, and a method of controlling the radiography apparatus. 
     According to an aspect of an exemplary embodiment, there is provided a method of controlling a radiography apparatus, the method including acquiring a first radiation image of a subject before a contrast reagent is injected into the subject, and acquiring a second radiation image of the subject after the contrast reagent is injected into the subject. The method further includes calculating a difference between data of a pixel of the first radiation image and data of a pixel of the second radiation image, for each of pixels of the first radiation image, and acquiring an image of the subject based on the difference for each of pixels of the first radiation image. 
     The calculating may include calculating a difference between data of a pixel in an area of the first radiation image and data of a pixel in an area of the second radiation image, for each of pixels in the area of the first radiation image. 
     The data of the pixel of the first radiation image and the data of the pixel of the second radiation image may include at least one among pixel intensities, values acquired by edge detection, and wavelet coefficients. 
     The method may further include dividing the first radiation image into first areas, and dividing the second radiation image into second areas. 
     At least two of the first areas of the first radiation image may have an overlapping part, or at least two of the second areas of the second radiation image may have an overlapping part. 
     At least one of the first areas of the first radiation image may have a size that is different from a size of one or more remaining ones of the first areas, or at least one of the second areas of the second radiation image may have a size that is different from a size of one or more remaining ones of the second areas. 
     The method may further include subtracting the first radiation image from the second radiation image to acquire a difference image. The acquiring may include correcting the difference image based on the difference for each of the pixels of the first radiation image, to acquire the image. 
     The method may further include determining a degree of difference based on the difference for each of the pixels of the first radiation image, for each of pixels of the second radiation image. The acquiring may include acquiring the image based on the degree of difference for each of the pixels of the second radiation image. 
     The first radiation image may be of a high resolution, and the second radiation image may be of the high resolution. The calculating may include down-sampling the first radiation image of the high resolution and the second radiation image of the high resolution to acquire the first radiation image of a low resolution and the second radiation image of the low resolution, and calculating a difference between data of a pixel of the first radiation image of the low resolution and data of a pixel of the second radiation image of the low resolution, for each of pixels of the first radiation image of the low resolution. 
     The method may further include determining a degree of difference based on the difference for each of the pixels of the first radiation image of the low resolution, for each of pixels of the second radiation image of the low resolution, and acquiring an image of the low resolution based on the degree of difference for each of the pixels of the second radiation image of the low resolution. The acquiring the image of the subject may include up-sampling the image of the low resolution to acquire the image of the subject. 
     According to an aspect of an exemplary embodiment, there is provided a radiography apparatus including a radiographer configured to acquire a first radiation image of a subject before a contrast reagent is injected into the subject, and acquire a second radiation image of the subject after the contrast reagent is injected into the subject. The radiography apparatus further includes an image processor configured to calculate a difference between data of a pixel of the first radiation image and data of a pixel of the second radiation image, for each of pixels of the first radiation image, and acquire an image of the subject based the difference for each of the pixels of the first radiation image. 
     The image processor may be configured to calculate a difference between data of a pixel in an area of the first radiation image and data of a pixel in an area of the second radiation image, for each of pixels in the area of the first radiation image. 
     The image processor may be further configured to divide the first radiation image into first divided areas, and divide the second radiation image into second divided areas. 
     The image processor may be further configured to subtract the first radiation image from the second radiation image to acquire a difference image, and correct the difference image based on the difference for each of the pixels of the first radiation image, to acquire the image. 
     The image processor may be further configured to determine a degree of difference based on the difference for each of the pixels of the first radiation image, for each of pixels of the second radiation image, and acquire the image based on the degree of difference for each of the pixels of the second radiation image. 
     The first radiation image may be of a high resolution, and the second radiation image may be of a high resolution. The image processor may be further configured to down-sample the first radiation image of the high resolution and the second radiation image of the high resolution to acquire the first radiation image of the low resolution and the second radiation image of the low resolution, and calculate a difference between data of a pixel of the first radiation image of the low resolution and data of a pixel of the second radiation image of the low resolution, for each of pixels of the first radiation image of the low resolution. 
     The image processor may be further configured to determine a degree of difference based on the difference for each of the pixels of the first radiation image of the low resolution, for each of pixels of the second radiation image of the low resolution, acquire an image of the low resolution based on the degree of difference for each of the pixels of the second radiation image of the low resolution, and up-sample the image of the low resolution to acquire the image of the subject. 
     According to an aspect of an exemplary embodiment, there is provided a radiography apparatus including an image processor configured to calculate a difference between data of a pixel of a first radiation image of a subject without a contrast reagent and data of a pixel of a second radiation image of the subject with the contrast reagent, for each of pixels of the first radiation image. The image processor is further configured to determine a degree of difference based on the difference for each of the pixels of the first radiation image, for each of pixels of the second radiation image, and acquire an image of the subject based on the degree of difference for each of the pixels of the second radiation image. 
     The image processor may be further configured to subtract the first radiation image from the second radiation image to acquire a difference image, and apply, to the difference image, the degree of difference for each of the pixels of the second radiation image, to acquire the image. 
     The degree of difference may be a smallest or greatest value among the difference for each of the pixels of the first radiation image, or an average value of the difference for each of the pixels of the first radiation image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects will be more apparent by describing certain exemplary embodiments with reference to the accompanying drawings in which: 
         FIG. 1  is a block diagram of a radiography apparatus according to an exemplary embodiment; 
         FIG. 2  is a view of the radiography apparatus of  FIG. 1 ; 
         FIG. 3  is a view of a radiation irradiator of  FIGS. 1 and 2 ; 
         FIG. 4  is a view of a radiation detector of  FIGS. 1 and 2 ; 
         FIG. 5  is a view of an image intensifier according to an exemplary embodiment; 
         FIG. 6  is a view of scanning before a contrast agent is injected according to an exemplary embodiment; 
         FIG. 7  is a view of scanning after a contrast agent is injected according to an exemplary embodiment; 
         FIG. 8A  is a diagram of a process of acquiring a difference image according to an exemplary embodiment; 
         FIG. 8B  is a diagram of a process of calculating differences between images using an image acquired before a contrast agent is injected and an image acquired after a contrast agent is injected according to an exemplary embodiment; 
         FIG. 9  is a diagram of a method of calculating a difference between a pixel of a first radiation image and an arbitrary pixel of a second radiation image according to an exemplary embodiment; 
         FIG. 10  is a table used in the method of  FIG. 10 ; 
         FIG. 11  is a diagram of a method of calculating a difference between a pixel of a first radiation image and an arbitrary pixel of a second radiation image according to another exemplary embodiment; 
         FIG. 12  is a table used in the method of  FIG. 11 ; 
         FIG. 13A  is a diagram of a process of creating a resultant image using degrees of difference according to an exemplary embodiment; 
         FIG. 13B  is an example of the created resultant image of  FIG. 13A ; 
         FIG. 14  is a diagram of a process of creating a resultant image using degrees of difference according to another exemplary embodiment; 
         FIG. 15  is a diagram of area division that is performed on a first radiation image or a second radiation image according to a first exemplary embodiment; 
         FIG. 16  is a diagram of a process of calculating differences for each area, and combining a plurality of areas for which differences are calculated according to the first exemplary embodiment; 
         FIG. 17  is an example of a resultant image created by combining the areas according to the first exemplary embodiment; 
         FIG. 18  is a diagram of area division that is performed on a first radiation image or a second radiation image according to a second exemplary embodiment; 
         FIG. 19  is a diagram of area division that is performed on a first radiation image or a second radiation image according to a third exemplary embodiment; 
         FIG. 20  is a diagram of a multi-resolution method according to an exemplary embodiment; 
         FIG. 21  is a flowchart illustrating a method of controlling a radiography apparatus according to a first exemplary embodiment; 
         FIG. 22  is a flowchart illustrating a method of controlling a radiography apparatus according to a second exemplary embodiment; 
         FIG. 23  is a flowchart illustrating a method of controlling a radiography apparatus according to a third exemplary embodiment; and 
         FIG. 24  is a flowchart illustrating a method of controlling a radiography apparatus according to a fourth exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Exemplary embodiments are described in greater detail herein with reference to the accompanying drawings. 
     In the following description, like drawing reference numerals are used for like elements, even in different drawings. The matters defined in the description, such as detailed construction and elements, are provided to assist in a comprehensive understanding of the exemplary embodiments. However, it is apparent that the exemplary embodiments can be practiced without those specifically defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the description with unnecessary detail. 
     It will be understood that the terms “comprises” and/or “comprising” used herein specify the presence of stated features or components, but do not preclude the presence or addition of one or more other features or components. In addition, the terms such as “unit”, “-er (-or)”, and “module” described in the specification refer to an element for performing at least one function or operation, and may be implemented in hardware, software, or the combination of hardware and software. 
       FIG. 1  is a block diagram of a radiography apparatus  1  according to an exemplary embodiment, and  FIG. 2  is a view of the radiography apparatus  1  of  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , the radiography apparatus  1  includes a radiographer  100  and a workstation  200  communicatively connected to the radiographer  100 . 
     The radiographer  100  may be communicatively connected to the workstation  200  through a cable or a wireless communication network. Various data acquired by the radiographer  100  may be transferred in the form of electrical signals to the workstation  200 , and various control signals generated by the workstation  200  may be also transferred in the form of electrical signals to the radiographer  100 . 
     The radiographer  100  uses radiation  98  to acquire data about an inside of a subject  99 . In order to use the radiation  98  to acquire data about the inside of the subject  99 , the radiographer  100  includes a radiation irradiator  110  configured to irradiate the radiation  98  to the subject  99 , and a radiation detector  130  configured to receive radiation transmitted through the subject  99 , and output electrical signals according to the received radiation. 
     The radiation irradiator  110  generates the radiation  98  of a predetermined energy spectrum, and irradiates the generated radiation  98  toward the subject  99 . The radiation irradiator  110  includes a radiation tube  111  configured to generate radiation  98 , and irradiate the generated radiation  98  toward the subject  99 , and a collimator  112  configured to filter the radiation irradiated from the radiation tube  111 . The radiation tube  111  is electrically connected to a power supply  101 , and receives a supply voltage needed for generating the radiation  98  from the power supply  101 . 
     The radiation  98  emitted by the radiation irradiator  110  is irradiated to the subject  99 , and then transmitted through the subject  99 . Herein, the subject  99  may be a living thing, such as a human body or animal, or a non-living thing, such as a baggage, a machine tool, or a building. 
     The radiation  98  irradiated to the subject  99  may be absorbed in a material inside the subject  99 , or may attenuate at a rate to be transmitted through the subject  99 . In this case, the radiation  98  may attenuate according to an attenuation coefficient of the material inside the subject  99 . Different kinds of materials may have different attenuation coefficients. For example, bones of a human body may have a relatively greater attenuation coefficient than other tissues so that they absorb a major part of radiation, and blood vessels of a human body may have a relatively low attenuation coefficient so that they can transmit a major part of radiation. The attenuation coefficient may be decided depending on a property (for example, density) of the material inside the subject  99  at which the radiation  98  arrives. 
     In detail, an intensity I of radiation transmitted through an internal tissue in the subject  99  may be calculated according to Equation (1) below.
 
 I=I   0   e   −μ     t   ,  (1)
 
     where I 0  represents an intensity of radiation irradiated by the radiographer  100 , μ represents an attenuation coefficient according to the internal tissue, etc. of the subject  99 , and t represents a thickness of the internal tissue of the subject  99  through which radiation is transmitted. The attenuation coefficient may depend on a kind or structure of the material existing in the subject  99 . 
     The radiation detector  130  receives the radiation  98  transmitted through the subject  99 , and outputs electrical signals (hereinafter, referred to as radiation signals) corresponding to the received radiation  98 . 
     The radiation detector  130  includes a radiation detecting panel  132  configured to receive the radiation  98  transmitted through the subject  99 , and output the electrical signals corresponding to the received radiation  98 , and an anti-scatter grid  131  configured to absorb radiation scattered when it passes through the subject  99  to cause only radiation traveling in a proper direction to arrive at the radiation detecting panel  132 . According to another exemplary embodiment, the radiation detector  130  may include, instead of the radiation detecting panel  132 , an image intensifier ( 150  of  FIG. 5 ). 
     Details about the radiation irradiator  110  and the radiation detector  130  will be described later. 
     As shown in  FIG. 2 , the radiographer  100  further includes an installation frame  101  in which the radiation irradiator  110  and the radiation detector  130  are installed, and a driver  102  configured to operate the installation frame  101  according to an irradiation direction or location of radiation, to support operations of the radiation irradiator  110  and the radiation detector  130 . 
     The installation frame  101  has a “C”-shaped structure. The radiation irradiator  110  and the radiation detector  130  are installed at both ends of the “C”-shaped structure such that the radiation irradiator  110  faces the radiation detector  130 . 
     The driver  102  includes a connector  103 , a rotation driver  104 , an up-down driver  105 , a main body  106 , and a mover  107 . The frame  101  rotates at various angles or move to various positions by the above-mentioned components. Accordingly, the radiation irradiator  110  irradiates the radiation  98  to the subject  99  placed on a table  97 , at various positions and in various directions. 
     The connector  103  connects the installation frame  101  to the rotation driver  104 , and may include various components to control the radiation irradiator  110  and the radiation detector  130 , or to support control operations of the radiation irradiator  110  and the radiation detector  130 . 
     The rotation driver  104  may include a rotation motor configured to rotate the connector  103  with respect to a predetermined axis  103   a , and the installation frame  101  connected to the connector  103 , according to a control signal. Accordingly, the radiation irradiator  110  irradiates the radiation  98  to the subject  99  in various directions, and the radiation detector  130  receives radiation in various directions. 
     The up-down driver  105  may include a bar and a driver configured to move the bar in a direction toward the ground or in a direction counter to the ground. The driver may be installed in the main body  106 . The driver may include a motor, and a wheel module configured to rotate according to driving of the motor. The wheel module may be connected to the bar through a toothed wheel, etc. to rotate to move the bar in the direction toward the ground or in the direction counter to the ground. 
     The main body  106  may include various components for controlling and driving operations of the radiographer  100 , such as the driver to move the bar in the direction toward the ground or in the direction counter to the ground. For example, the main body  106  may include the power supply  101  configured to supply a supply voltage that is applied to the radiation irradiator  110 . Also, the main body  106  may include a semiconductor chip and a printed circuit board (PCB) configured to perform operations of the radiographer  100 . The main body  106  is physically separated by a separate housing, as shown in  FIG. 2 . However, the main body  106  may be implemented on a ceiling, a floor, and/or a wall of a radiation room. 
     The main body  106  includes the mover  107  configured to move the main body  106 , and the mover  107  may be implemented using a wheel, a rail, or the like. 
     The workstation  200  controls operations of the radiographer  100 , and creates radiation images being visual information that a user can see, based on electrical signals received from the radiographer  100 . 
     The workstation  200  may be a desktop computer, a laptop computer, or a computing apparatus configured to control operations of the radiographer  100 , and perform various image processing. The workstation  200  may be an apparatus physically separated from the radiographer  100 , as shown in  FIG. 2 , or may be an apparatus physically coupled with the radiographer  100 . The workstation  200  may be implemented with various structures that can be considered by a system designer for controlling radiography or image processing. 
     As shown in  FIG. 1 , the workstation  200  includes an amplifier  206 , an analog-to-digital converter  207 , an image processor  210 , a controller  220 , and a memory device  221 , such as Read Only Memory (ROM) or Random Access Memory (RAM). The image processor  210 , the controller  220 , and the memory device  221  may be installed in a housing constituting a main body  205  of  FIG. 2  of the workstation  200 . 
     The amplifier  206  amplifies electrical signals output from the radiation detector  130 . According to another exemplary embodiment, the amplifier  206  may be installed in the radiographer  100 . Also, the amplifier  206  may be omitted. 
     The analog-to-digital converter  207  converts an electrical signal in an analog format into a digital signal in a digital format. In detail, the analog-to-digital converter  207  may convert an analog signal into a digital signal according to a predetermined sampling rate. However, the analog-to-digital converter  207  may be omitted. 
     The image processor  210  creates a radiation image using the electrical signals received from the radiation detector  130 , and may perform various image processing related to the radiation image. 
     The image processor  210  includes a first processor  211 , a second processor  212 , and a third processor  213 . The first processor  211 , the second processor  212 , and the third processor  213  may be physically or logically separated from each other. If the first processor  211 , the second processor  212 , and the third processor  213  are physically separated from each other, the first processor  211 , the second processor  212 , and the third processor  213  may be implemented as a plurality of semiconductor chips, and if the first processor  211 , the second processor  212 , and the third processor  213  are logically separated from each other, the first processor  211 , the second processor  212 , and the third processor  213  may be implemented as a single semiconductor chip. 
     The first processor  211  creates a radiation image based on the received electrical signals. Also, the first processor  211  may apply one or more lookup tables LUT to the created radiation image, or may apply various filters, such as a low-pass filter or a high-pass filter, to the created radiation image, to further perform image processing on the created radiation image. 
     The second processor  212  acquires a difference image between a plurality of radiation images (for example, a first radiation image and a second radiation image) created by the first processor  211 . The difference image means an image that displays differences between the first radiation image and the second radiation image. 
     In detail, the second processor  212  may select a pixel of the first radiation image, select a pixel of the second radiation image corresponding to the selected pixel of the first radiation image, and then mathematically subtract a pixel value of the selected pixel of the second radiation image from a pixel value of the selected pixel of the first radiation image, thereby acquiring the difference image. Herein, a pixel means a minimum unit to form an image, and a number of pixels included in an image may decide a resolution of the image. 
     A pixel value may include a pixel intensity. The pixel intensity represents a degree of brightness of light that is represented by the corresponding pixel, and degrees of brightness of light may be classified into several levels according to predetermined criteria. For example, a pixel intensity of an arbitrary pixel may be set to a value between 0 and 256. In this case, the pixel intensity of 0 may be set to the darkest brightness, and the pixel intensity of 256 may be set to the brightest brightness. 
     The second processor  212  may add a predetermined weight to at least one among the first radiation image and the second radiation image, and then acquire the difference image between the first radiation image and the second radiation image. 
     The third processor  213  compares all pixels of the first radiation image to an arbitrary pixel of the second radiation image using the acquired radiation images. The third processor  213  may correct the difference image acquired by the second processor  212  according to results of the comparison, or create a radiation image corresponding to the results of the comparison. 
     In detail, the third processor  213  may calculate differences between data related to the pixels of the first radiation image and data related to the pixels of the second radiation image, and correct the difference image acquired by the second processor  212  using results of the calculation, or create a radiation image corresponding to the results of the calculation. Herein, the data related to the pixels of the first radiation image or the data related to the pixels of the second radiation image may be at least one among pixel intensities, values acquired by edge detection, values acquired by wavelet transform, and wavelet coefficients. 
     The values acquired by edge detection mean values acquired by detecting an edge of an image, wherein the edge of the image means an area having a sharp change in brightness in the image. The third processor  213  may detect only an edge(s) from the second radiation image, and then compare pixel values of pixels corresponding to the detected edge(s) to pixel values of all pixels of the first radiation image to thereby acquire resultant data about differences between the pixel values of the first radiation image and the second radiation image. 
     The wavelet transform means transformation based on a wavelet function. The wavelet transform may be expressed using a basis function of a finite length. A wavelet may be given as vibrations that repeatedly increase or decrease with respect to zero, and the wavelet function means a mathematical expression of such vibrations. The wavelet coefficient means a coefficient that is added to at least one basis function of the wavelet transform. An arbitrary signal can be expressed as a combination of a wavelet coefficient and a basis function. The third processor  213  may calculate differences between data of all pixels of the first radiation image and data of an arbitrary pixel of the second radiation image, using values acquired by the wavelet transform or wavelet coefficients for individual pixels. 
     The resultant values of edge detection, the resultant values of the wavelet transform, or the wavelet coefficients may be acquired by various methods that can be considered by one of ordinary skill in the art, and because the methods have been well-known in the art, detailed descriptions thereof will be omitted. 
     Details about operations of the second processor  212  and the third processor  213  will be described later. 
     The controller  220  controls overall operations of the radiographer  100  and the workstation  200 . For example, the controller  220  applies a control signal to the power supply  101  according to electrical signals received from an inputter  201  to enable the power supply  101  to apply a tube voltage and a tube current to the radiation tube  111  of the radiation irradiator  110 . Accordingly, the radiation irradiator  110  irradiates the radiation  98  to the subject  99 . Also, the controller  220  transfers control signals for controlling operations of the image processor  210 . 
     The memory device  221  may temporarily or non-temporarily store signals output from the controller  220  or information that is to be input to the controller  220 , to thereby support operations of the controller  220 . Radiation images output from the image processor  210  may also be temporarily or non-temporarily stored in the memory device  221 . 
     The image processor  210  and the controller  220  may be implemented as a Central Processing Unit (CPU) or a Graphic Processing Unit (GPU), wherein the CPU or the GPU may be implemented with one or more semiconductor chips and related components. Also, the memory device  221  may also be implemented with one or more memory semiconductors, a PCB, and related components. 
     As shown in  FIGS. 1 and 2 , the workstation  200  includes the inputter  201  and a display  202 . The inputter  201  and the display  202  may be connected to the main body  205  through a cable or a wireless communication network. 
     The inputter  201  receives various information related to operations of the radiography apparatus  1  from a user. The inputter  201  may output an electrical signal according to a user&#39;s manipulation, and transfer the electrical signal to the controller  220  to control operations of the radiography apparatus  1 . The inputter  201  may be, for example, a keyboard, a mouse, a keypad, a trackball, a track pad, one or more physical buttons, a knob, an operation stick, a touch pad, or a touch screen. 
     The display  202  may display acquired radiation images or a GPU related information for control of radiography. For example, the display  202  may display, along with the radiation images, information related to operations or settings of the radiography apparatus  1 , three-dimensional ( 3 D) images, information about the subject  99 , or other various information. 
     As shown in  FIG. 2 , according to an exemplary embodiment, the display  202  includes a plurality of displays  203  and  204 , and the displays  203  and  204  may display the same image or different images. For example, the display  203  may display an image acquired by the first processor  211 , and the display  204  may display an image acquired by the second processor  212  or the third processor  213 . 
     The display  202  may be implemented with a Plasma Display Panel (PDP), Light Emitting Diodes (LEDs), a Liquid Crystal Display (LCD), or a Cathode Ray Tube (CRT). However, the display  202  may be implemented with any other device that can be used to display images. 
     Hereinafter, the radiation irradiator  110  and the radiation detector  130  of the radiographer  100  will be described in more detail. 
       FIG. 3  is a view of the radiation irradiator  110  of  FIGS. 1 and 2 . 
     Referring to  FIG. 3 , the radiation irradiator  110  includes the radiation tube  111  electrically connected to the power supply  101 , and the collimator  112  through which the radiation  98  passes. 
     The power supply  101  applies predetermined voltage and current to the radiation tube  111  under the control of the controller  220  installed in the workstation  200  or the radiographer  100 . The power supply  101  may be a commercial power source, or an emergency power source that can be implemented as a separate, independent generator. Also, the power supply  101  may be a storage battery provided in the radiographer  100 . 
     When the predetermined voltage and current is applied from the power supply  101  to the radiation tube  111 , the radiation tube  111  generates radiation of a predetermined magnitude according to the predetermined voltage or current. The radiation tube  111  includes a tube body  120 , a cathode  121 , and an anode  123 . 
     The tube body  120  accommodates various components, such as the cathode  121  and the anode  123 , needed to generate radiation, and fixes the components stably to prevent electrons e from leaking out. The tube body  120  may be a glass tube made of silica (hard) glass. An inside of the tube body  120  may be evacuated to a high vacuum state of about 10 −7  mmHg 
     Electronic beams e are irradiated from the cathode  121  toward the anode  123 . A filament  122  on which electrons are concentrated is provided at one end of the cathode  121 . The filament  122  may be heated according to an applied tube voltage to emit the concentrated electrons e to the inside of the tube body  120 , and the emitted electrons e may be accelerated in the tube body  120  to move toward the anode  123 . The filament  122  of the cathode  121  may be fabricated with a metal such as tungsten W. According to another exemplary embodiment, a carbon nano tube instead of the filament  122  may be provided at the cathode  121 . 
     The anode  123  generates the radiation  98 . When the electrons e emitted and moved from the filament  122  collide with a target surface  124  of the anode  123 , the electrons e may decelerate sharply, and the radiation  98  of energy corresponding to the tube voltage may be generated from the target surface  124  according to the deceleration of the electrons e. The target surface  124  is cut in a direction, as shown in  FIG. 3 , so that the radiation  98  is emitted in a predetermined direction. The anode  123  may be fabricated with a metal such as copper Cu, and the target surface  124  may be formed with a metal, such as tungsten W, chrome Cr, iron Fe, or nickel Ni. 
     The anode  123  includes a rotary anode that is in a shape of disk. The edge of the rotary anode  123  is cut at a predetermined angle, and accordingly, the target surface  124  is formed on the cut edge. The rotary anode  123  may rotate at predetermined speed with respect to a predetermined rotation axis R. In order to rotate the rotary anode  123 , the radiation tube  111  includes a stator  128  configured to form a rotating field, a rotor  127  configured to rotate according to the rotating field formed by the stator  128  to rotate the rotating anode  123 , a plurality of bearings  126  configured to rotate according to the rotation of the rotor  127 , and a shaft member  125  functioning as the rotation axis R. The rotor  127  may be a permanent magnet. 
     According to another exemplary embodiment, the anode  123  may be a fixed anode that is in a shape of a cylinder having a cutting face cut at a predetermined angle. In this case, the target surface  124  may be formed on the cutting face. 
     The radiation  98  emitted from the target surface  124  of the anode  123  may pass through the collimator  112 . 
     The collimator  112  filters the radiation  98  emitted from the radiation tube  111  to adjust a direction and a range of the radiation  98  to some degrees. The collimator  112  may include an opening through which radiation irradiated in a direction or in a range passes, and a plurality of collimator blades that absorb radiation irradiated in different directions. A user may control an irradiation direction and a range of radiation by changing a location or size of the opening. The collimator blades may be fabricated with a material such as lead Pb that can absorb radiation. 
     The radiation  98  passed through the collimator  112  is irradiated on the subject  99 , and radiation transmitted through the subject  99  arrives at the radiation detector  130 . 
       FIG. 4  is a view of the radiation detector  130  of  FIGS. 1 and 2 . 
     Referring to  FIG. 4 , the radiation detector  130  includes the anti-scatter grid  131 , the radiation detecting panel  132 , and a PCB  133 . 
     The anti-scatter grid  131  absorbs radiation passed through and scattered by the subject  99  to cause only radiation traveling in a proper direction to arrive at the radiation detecting panel  132 . The anti-scatter grid  131  includes a plurality of partition walls  131   a  to block radiation, and a plurality of transmitting holes  131   b  through which radiation passes. The partition walls  131   a  may be made of a material such as lead Pb to absorb radiation scattered or refracted in the subject  99 , and the transmitting holes  131   b  may pass radiation neither scattered nor reflected. 
     The radiation detecting panel  132  receives radiation, converts the received radiation into electrical signals corresponding to the radiation, and then outputs the electrical signals. The radiation detecting panel  132  may directly convert the radiation into the electrical signals (direct method). Alternatively, the radiation detecting panel  132  may generate visible light corresponding to the radiation, and then convert the visible light into the electrical signals (indirect method). 
     According to the direct method as shown in  FIG. 4 , the radiation detecting panel  132  includes a first electrode  141  to whose one surface radiation is incident, a semiconductor material layer  142  formed on another surface of the first electrode  141  to which no radiation is incident, and a flat plate  143  contacting the semiconductor material layer  142  that is opposite to the other surface of the first electrode  141 , as shown in  FIG. 4 . On the flat plate  143 , a plurality of second electrodes  144  and a plurality of thin film transistors  145  are arranged in one or more columns. The first electrode  141  may have positive (+) or negative (−) polarity, and the polarity of the second electrodes  144  may be opposite to that of the first electrode  1213 . A predetermined bias voltage may be applied between the first electrode  141  and the second electrodes  144 . 
     Electron-hole pairs created in the semiconductor material layer  142  according to incidence and absorption of radiation may move toward the first electrode  141  or the second electrodes  144  according to the polarities of the first electrode  141  and the second electrodes  144 . The second electrodes  144  may receive holes or negative charges transferred from the semiconductor material layer  142 , and output an electrical signal corresponding to the received negative charges. The thin film transistors  145  may read out electrical signals transferred from the corresponding second electrodes  144  to acquire image data. Each of the second electrodes  144  and the thin film transistors  145  corresponding to the second electrode  144  may be packaged in a CMOS chip. 
     If the radiation detecting panel  132  converts radiation into electrical signals according to the indirect method, a phosphor screen for outputting visible light corresponding to received radiation may be disposed between the anti-scatter grid  131  and the radiation detecting panel  132 , and photo diodes, instead of the second electrodes  144 , may be arranged on the flat plate  143  to convert visible light into electrical signals. Also, the radiation detecting panel  132  may include a scintillator configured to emit visible-light photons according to radiation, and photo diodes configured to detect the emitted visible-light photons. 
     According to an exemplary embodiment, the radiation detecting panel  132  may be a Photon Counting Detector (PCD). 
     The PCB  133  is disposed on another surface of the radiation detecting panel  132 . The PCB  133  may be attached on the other surface of the radiation detecting panel  132  to control various operations of the radiation detecting panel  132  or to store image data output from the radiation detecting panel  132 . 
     Electrical signals output from the radiation detector  130  may be temporarily or non-temporarily stored in a memory device such as a semiconductor storage provided on the substrate  133 , and then transferred to the image processor  210 . Also, image data acquired by the radiation detector  130  may be transferred directly to the image processor  210 . 
       FIG. 5  is a view of the image intensifier  150  accordingly to an exemplary embodiment. 
     The radiation detector  130  of  FIGS. 1 and 2  may include the image intensifier  150 , instead of the radiation detecting panel  132  of  FIGS. 1 and 4 . The image intensifier  150  emits photons corresponding to incident radiation to thereby acquire image data. 
     Referring to  FIG. 5 , the image intensifier  150  includes a housing  151 , a tube body  152 , an anti-scatter grid  153 , a radiation-electron converter  155 , an anode  156 , an electron-light converter  157 , a light guide  160 , and a plurality of lenses  161   a  to  161   c.    
     The housing  151  accommodates the tube body  152 , the anti-scatter grid  153 , the radiation-electron converter  155 , the anode  156 , the electron-light converter  157 , the light guide  160 , and the lenses  161   a  to  161   c , and fixes the above-mentioned devices stably while protecting them against an external impact. The housing  151  may be shaped to correspond to the shape of the tube body  152  or the light guide  160 . 
     The tube body  152  causes electrons e traveling in the tube body  152  to be focused toward the anode  156 , while stably fixing various components, such as the radiation-electron converter  155 , the anode  156 , and the electron-light converter  157 . Also, the tube body  152  may prevent the electrons e from leaking out. 
     The tube body  152  may have a nearly cylindrical shape. The tube body  152  may be fabricated in a shape of a cylinder, wherein a diameter of a negative (−) polarity part of the tube body  152  in which the radiation-electron converter  155  is disposed is greater than a diameter of a positive (+) polarity part of the tube body  152  in which the electron-light converter  157  is disposed, as shown in  FIG. 5 . The negative (−) polarity part of the tube body  152  in which the radiation-electron converter  155  is disposed may be in the shape of a convex lens protruding in a direction in which radiation is incident. 
     The anti-scatter grid  153  is disposed at one end of the tube body  152  to which radiation is irradiated, and absorbs radiation passed through and scattered by a subject to cause only radiation traveling in a proper direction to arrive at the radiation-electron converter  155 . The anti-scatter grid  153  may include a plurality of partition walls that are made of a material such as lead Pb and block radiation, and a plurality of transmitting holes through which non-scattered radiation passes. Radiation passed through the transmitting holes may arrive at the radiation-electron converter  155 . 
     In the tube body  152 , a plurality of focusing electrodes  154   a  and  154   b  are further disposed. In an inside of the tube body  152 , electrons e may move to the anode  156 , and be focused at a focusing point f that is a location near the anode  156 . The focusing electrodes  154   a  and  154   b  induce the electrons e moving to the anode  156  to be focused at the focusing point f. 
     The radiation-electron converter  155  is disposed in the negative (−) polarity part of the tube body  152 . The radiation-electron converter  155  includes a phosphor plate  155   a  configured to emit photons corresponding to incident radiation, and a photocathode  155   b  configured to emit electrons e corresponding to photons emitted from the phosphor plate  155   a.    
     The anode  156  decides a movement direction and speed of electrons e. Electrons e move to the anode  156  according to their polarities, and are focused around the anode  156 . The anode  156  may accelerate the moving electrons e. 
     The electron-light converter  157  is disposed around the anode  156 . The electron-light converter  157  emits visible-light photons corresponding to the incident electrons e to an inside of the light guide  160 . The electron-light converter  157  may include an output phosphor. Light emitted to the inside of the light guide  160  are refracted by the first lens  161   a  to travel in parallel. The output photons are received by first and second photographing units  159   a  and  159   b , and the first and second photographing units  159   a  and  159   b  output and store electrical signals corresponding to the received photons to acquire image data. 
     A reflector  158  for reflecting the emitted photons in all directions is provided in the light guide  160 . The reflector  158  transfers a part of the output photons to a first light guide  160   a  in which the first photographing unit  159  is disposed, and a remaining part of the output photons to a second light guide  160   b  in which the second photographing unit  159   b  is disposed. Accordingly, a plurality of image data may be acquired. 
     In the first light guide  160   a  and the second light guide  160   b , the second lens  161   b  and the third lens  161   c  are respectively disposed to focus light. Light focused by the second lens  161   b  and the third lens  161   c  is transferred to the first photographing unit  159   a  and the second photographing unit  159   b , respectively. In the first light guide  160   a  and the second light guide  160   b  a first diaphragm  161   d  and a second diaphragm  161   e  are respectively disposed to adjust amounts of light that are incident to the first photographing unit  159   a  and the second photographing unit  159   b.    
     The first and second photographing units  159   a  and  159   b  receive photons, and then create and store electrical signals corresponding to the received photons to acquire image data. Each of the first and second photographing units  159   a  and  159   b  may include an image sensor having a plurality of image pickup devices, wherein each image pickup device may be a Charge-Coupled Device (CCD) or a Complementary Metal-Oxide Semiconductor (CMOS). 
     So far, an example of the radiography apparatus  1  has been described. However, the radiography apparatus  1  is not limited to the radiography apparatus  1  as described above. For example, the radiography apparatus  1  can be applied to Computed Tomography (CT), Full Field Digital Mammography (FFDM), or Single-Photon Emission Computed Tomography (SPECT) in the same manner or through appropriate modifications. 
     Hereinafter, operations of the image processor  210  will be described in more detail with reference to  FIGS. 6 to 21 . 
       FIG. 6  is a view of scanning before a contrast agent is injected according to an exemplary embodiment, and  FIG. 7  is a view of scanning after a contrast agent is injected according to an exemplary embodiment. 
     As shown in  FIG. 6 , the radiographer  100  performs radiography on a human body  99   a , which is a subject. In this case, the radiography may be performed before a contrast agent ( 94  of  FIG. 7 ) is injected into the human body  99   a . In detail, the radiation irradiator  110  irradiates the radiation  98  toward the human body  99   a . The radiation detector  130  receives radiation transmitted through the human body  99   a , and outputs electrical signals corresponding to the received radiation. The first processor  211  of the image processor  210  (see  FIG. 1 ) may acquire a radiation image based on the electrical signals. Hereinafter, the radiation image acquired by the radiography performed before the contrast agent  94  is injected will be referred to as a first radiation image. 
     In this case, because attenuation coefficients of blood vessels  95  distributed in the human body  99   a  are not greatly different from those of muscles around the blood vessels  95 , the blood vessels  95  may not be distinguished in the acquired radiation image. 
     As shown in  FIG. 7 , the radiographer  100  performs radiography after the contrast agent  94  is injected into the human body  99   a . The contrast reagent  94  means a material that is injected into the inside of the human body  99   a  to increase a contrast of a material existing in the inside of the human body  99   a . The contrast reagent  94  is used to clearly distinguish a material (for example, tissues or blood vessels) from other tissues during radiography. The contrast reagent  94  may increase a contrast of a material in the human body  99   a  by artificially increasing or decreasing a degree at which an internal material of the human body  99   a  transmits radiation. Accordingly, when the contrast agent  94  is used, a living body structure, lesions. etc. can be clearly distinguished from other materials around them, which can lead to accurate diagnosis of the subject  99 . 
     The contrast reagent  94  may be iodine, iodine-gadolinium, or barium sulfate (BaSO 4 ). Also, the contrast reagent  94  may be a gas such as carbon oxide. One kind of contrast reagent  94  or a plurality of kinds of contrast reagents  94  may be injected into the human body  99   a.    
     The contrast reagent  94  may be injected into the blood vessels  95  in the human body  99   a  through an arterial vessel located in the thigh, etc. of the human body  99   a . The contrast reagent  94  may flow through the blood vessels  95  to increase the attenuation coefficient of the blood vessels  95 . Accordingly, if radiography is performed after the contrast reagent  94  is injected, the blood vessels  95  can more clearly appear on a radiation image than before the contrast reagent  94  is injected. Hereinafter, a radiation image acquired by radiography performed after the contrast reagent  94  is injected will be referred to as a second radiation image. The first radiation image and the second radiation image may be photographed images of the same part (for example, the heart or brain) in the human body  99   a.    
       FIG. 8A  is a diagram of a process of acquiring a difference image accordingly to an exemplary embodiment. 
     As shown in  FIGS. 6, 7, and 8A , if a first radiation image A acquired before the contrast reagent  94  is injected and a second radiation image B acquired after the contrast reagent  94  is injected are obtained, the second processor  212  of the image processor  210  (see  FIG. 1 ) acquires a difference image C between the first radiation image A and the second radiation image B. In this case, the second processor  212  acquires the difference image C using a method of subtracting the first radiation image A from the second radiation image B (S 1 ). 
     Because the second radiation image B is an image acquired after the contrast reagent  94  is injected, and the contrast reagent  94  flows through blood vessels  95   a  of the first radiation image A, only the blood vessels  95  may clearly appear on the difference image C between the first radiation image A and the second radiation image B. As such, by acquiring the difference image C, the radiography apparatus  1  may perform Digital Subtraction Angiography (DSA). 
       FIG. 8B  is a diagram of a process of calculating differences between images using an image acquired before the contrast agent  94  is injected and an image acquired after the contrast agent  94  is injected accordingly to an exemplary embodiment. 
     Although the difference image C is acquired, if the human body  99   a  moves during radiography, a difference is made even in an area other than the blood vessels  95  between the first radiation image A and the second radiation image B according to the movement of the human body  99   a , so that an artifact may be generated in the difference image C. 
     To compensate for the artifact, the third processor  213  (see  FIG. 1 ) calculates and acquires data representing differences between the first radiation image A and a pixel of the second radiation image B, independently from acquiring the difference image C (S 2 ). 
     Hereinafter, for convenience of description, various data including degrees (or values) representing differences between all pixel values of the first radiation image A and a pixel value of the second radiation image B will be referred to as degrees of difference  96 . 
     After calculating the degrees of difference  96 , the third processor  213  acquires a resultant image  93  using the degrees of difference  96 . The resultant image  93  means an image that is acquired by image processing of the image processor  210  and can be displayed for a user through the display  202  (see  FIG. 1 ). According to exemplary embodiments, the image processor  210  may further perform image processing on the resultant image  93  to correct the resultant image  93 . The corrected resultant image  93  may be displayed for a user through the display  202 . 
     In detail, the third processor  213  may calculate differences between a pixel value of an arbitrary pixel of the second radiation image B and pixel values of all pixels of the first radiation image A, and acquire the degrees of differences  96  between the first radiation image A and a selected pixel of the second radiation image B using the calculated differences. The process of acquiring the degrees of difference  96  may be performed on all pixels of the second radiation image B. Herein, data related to the pixels of the first radiation image A and the second radiation image B may be at least one among pixel intensities, values acquired by edge detection, values acquired by wavelet transform, and wavelet coefficients. 
     Hereinafter, the process of calculating the degrees of difference  96  will be described in more detail. 
       FIG. 9  is a diagram of a method of calculating a difference between a pixel of a first radiation image and an arbitrary pixel of a second radiation image according to an exemplary embodiment, and  FIG. 10  is a table used in the method of  FIG. 9 .  FIG. 11  is a diagram of a method of calculating a difference between a pixel of a first radiation image and an arbitrary pixel of a second radiation image according to another exemplary embodiment, and  FIG. 12  is a table used in the method of  FIG. 11   
     In the tables shown in  FIGS. 10 and 12 , data of a first radiation image A means pixel intensities of individual pixels of the first radiation image A, and data of a second radiation image B means a pixel intensity of an arbitrary pixel selected from the second radiation image B. However, the pixel intensities shown in the tables of  FIGS. 10 and 12  have been decided for convenience of description, and may be different from experimental values. 
     As shown in  FIG. 9 , the third processor  213  selects an arbitrary pixel B 1  of the second radiation image B, and calculates differences between a pixel value of the arbitrary pixel B 1  and pixel values of all pixels A 11  to A 55  of the first radiation image A. In this case, the pixel values may be pixel intensities given as any values between 0 and 255. However, values acquired by edge detection, values acquired by wavelet transform, or wavelet coefficients may be used instead of pixel intensities, as described above. 
     For example, as shown in  FIG. 10 , the pixel value of the arbitrary pixel B 1  of the second radiation image B is 100, and the third processor  213  calculates differences between the pixel value  100  of the arbitrary value B 1  and the pixel values of the individual pixels A 11  to A 55  of the first radiation image A. 
     After calculating the differences between the pixel value of the arbitrary pixel B 1  of the second radiation image B and the pixel values of all the pixels A 11  to A 55  of the first radiation image A, the third processor  213  may acquire one of the degrees of difference  96  between the first radiation image A and the pixel B 1  of the second radiation image B based on the calculated differences. 
     According to an exemplary embodiment, the third processor  213  may decide the smallest or greatest value of the differences between the pixel value of the arbitrary pixel B 1  of the second radiation image B and the pixel values of all the pixels A 11  to A 55  of the first radiation image A, as one of the degrees of difference  96  between the first radiation image A and the second radiation image B. For example, the third processor  213  may decide “2”, which is a difference between the pixel value of the pixel B 1  of the second radiation image B and the pixel value of the pixel A 55  of the first radiation image A, as one of the degrees of difference  96  between the first radiation image A and the second radiation image B, as shown in  FIG. 10 . 
     Also, according to another exemplary embodiment, the third processor  213  may square the acquired differences, sum the squared values, and then calculate a radical root of the summed result to thereby acquire one of the degrees of difference  96 . In this case, the third processor  213  may acquire one of the degrees of difference  96  according to Equation (2) below.
 
 R   k =√{square root over (Σ ij ( X   ij   −Y   k ) 2 )},  (2)
 
     where R k  represents a degree of difference between the first radiation image A and the pixel of the second radiation image B, i and j are indexes for identifying each pixel of the first radiation image A, k is an index for identifying an arbitrary pixel of the second radiation image B, X ij  represents a pixel value of a pixel corresponding to the indexes ij of the first radiation image A, and Y k  represents a pixel value of an arbitrary pixel B k  of the second radiation image B. 
     Also, accordingly to another exemplary embodiment, the third processor  213  may acquire one of the degrees of difference  96  by calculating an average value of the differences, or using one of various methods that can be considered by one of ordinary skill in the art. 
     As shown in  FIGS. 11 and 12 , the third processor  213  may acquire one of the degrees of difference  96  with respect to another pixel B 2  of the second radiation image B, in the same manner. The process of calculating one of the degrees of difference  96  is performed on all pixels of the second radiation image B. Accordingly, the degrees of difference  96  is calculated for all the pixels of the second radiation image B. 
     If the arbitrary pixel B 1  of the second radiation image B is a pixel in an area where none of the blood vessels  95  into which the contrast reagent  94  has been injected are displayed, the pixel value of the arbitrary pixel B 1  may be identical to the pixel values of the pixels A 11  to A 55  of the first radiation image A, or may have a small difference within a predetermined range from the pixel values of the pixels A 11  to A 55  of the first radiation image A. For example, the difference between the pixel value of the arbitrary pixel B 1  of the second radiation image B and the pixel value of each pixel of the first radiation image A may be zero or an arbitrary value (for example, 1 or 2) close to zero, as shown in  FIG. 10 . Accordingly, a degree of difference of a small value may be acquired. 
     If the arbitrary pixel B 2  of the second radiation image B is a pixel in an area where the blood vessels  95  into which the contrast reagent  94  has been injected are displayed, the pixel value of the arbitrary pixel B 2  may be greatly different from the pixel values of the pixels A 11  to A 55  of the first radiation image A. For example, as shown in  FIG. 12 , if a pixel intensity of a pixel at which the blood vessels  95  into which the contrast reagent  94  has been injected are displayed is 100, and a pixel intensity of a pixel at which a material around the blood vessels  95  is displayed is a value close to zero, a difference between a pixel value of each pixel of the first radiation image A and the pixel value of the arbitrary pixel B 2  of the second radiation image B may be 100 or a value close to 100. Accordingly, a degree of difference of a great value may be acquired. 
     According to another exemplary embodiment, the third processor  213  may decide an arbitrary threshold value, and compare the acquired degree of difference to the arbitrary threshold value to determine whether to discard the degree of difference for each pixel of the second radiation image B. If the third processor  213  determines that the acquired degree of difference is smaller than the arbitrary threshold value, the third processor  213  may discard the acquired degree of difference  96 . If the third processor  213  determines that the acquired degree of difference is greater than the arbitrary threshold value, the third processor  213  may maintain the acquired degree of difference. Herein, the arbitrary threshold value may have been decided in advance by a user or a system designer. Also, the arbitrary threshold value may be decided according to the formula of calculating a degree of difference. 
     According to another exemplary embodiment, the third processor  213  may maintain all of the acquired degrees of difference  96 , without discarding them. Because a pixel value of a pixel in an area into which no contrast reagent  94  has been injected shows no substantial difference from the pixel values of the pixels of the first radiation image A, the degree of difference may be zero or a value close to zero although all of the acquired degrees of difference  96  are maintained. 
       FIG. 13A  is a diagram of a process of creating a resultant image using degrees of difference according to an exemplary embodiment, and  FIG. 13B  is an example of the created resultant image of  FIG. 13A . 
     Referring to  FIGS. 13A and 13B , after the degrees of differences  96  for all the pixels of the second radiation image B are calculated, the third processor  213  creates a resultant image D using the degrees of difference  96  for all the pixels of the second radiation image B. In other words, the third processor  213  creates resultant image D using a group consisting the degrees of difference  96 . 
     In detail, after the third processor  213  calculates the degrees of differences  96  for all pixels of the second radiation image B, the third processor  213  reflects the degrees of difference  96  to the difference image C acquired by the second processor  212  to correct the difference image C. Accordingly, the third processor  2134  acquires the first resultant image D. 
     The third processor  213  may select a pixel of the difference image C, decide a pixel of the second radiation image B corresponding to the selected pixel, and then acquire a degree of difference corresponding to the pixel of the second radiation image B. Herein, the pixel of the second radiation image B corresponding to the selected pixel means a pixel of the second radiation image B at a location corresponding to that of the selected pixel of the difference image C. 
     Successively, the third processor  213  may reflect the degree of difference to the selected pixel of the difference image C to correct the selected pixel of the difference image C, as shown in  FIG. 13A . In this case, the third processor  213  may multiply a pixel value of the selected pixel by the degree of difference, or divide the product of the pixel value of the selected pixel and the degree of difference by a predetermined value, thereby correcting the selected pixel of the difference image C. According to another exemplary embodiment, the third processor  213  may further add a predetermined weight to apply the degree of difference to the pixel value of the selected pixel. The third processor  213  may apply the above-described process to all pixels of the difference image C to thereby correct the difference image C. 
     If the third processor  213  has discarded degrees of difference that are smaller than the threshold value, pixels of the difference image C corresponding to the discarded degrees of difference may not be corrected. 
     Because the greater degrees of difference  96  are acquired at the blood vessels  95  through which the contrast reagent  94  passes, and the difference image C displays only the blood vessels  95  through which the contrast reagent  94  passes, as described above, if the degrees of difference  96  are applied to the difference image C, the first resultant image D with the highlighted blood vessels  95  can be acquired. 
       FIG. 14  is a diagram of a process of creating a resultant image using degrees of difference according to another exemplary embodiment. 
     As shown in  FIG. 14 , the third processor  213  creates a second resultant image E using only the degrees of difference  96 . In detail, the third processor  213  combines the degrees of difference  96  to create the second resultant image E. The third processor  213  locates the degrees of difference  96  at locations of the corresponding pixels to create an image E 1  as shown in  FIG. 14 . In  FIG. 14 , the brighter areas E 1  may represent areas at which the smaller degrees of difference  96  are located, and darker areas E 2  may represent areas at which the greater degrees of difference  96  are located. 
     Because a great degree of difference is acquired from an area through which the contrast reagent  94  passes, and a small degree of difference is acquired from an area through which no contrast reagent  94  passes, as described above, the third processor  213  may represent the blood vessels  95  in the second resultant image E, like the difference image C. Accordingly, the second resultant image E acquired using the degrees of difference  96  may be used instead of the difference image C or the first resultant image D. 
     So far, the exemplary embodiments in which the third processor  213  acquires the first resultant image D and the second resultant image E by calculating the degrees of difference  96  without dividing the plurality of radiation images A and B have been described. In other words, the above-described processes are to acquire the first resultant image D and the second resultant image E using the first radiation image A and the second radiation image B. According to another exemplary embodiment, the third processor  213  may divide each of the first radiation image A and the second radiation image B into two areas or more, then acquire resultant images D and E for each area, and combine the resultant images D and E acquired for each area to thereby acquire a final resultant image. 
       FIG. 15  is a diagram of area division that is performed on a first radiation image or a second radiation image according to a first exemplary embodiment.  FIG. 16  is a diagram of a process of acquiring differences for each area, and combining a plurality of areas for which differences are acquired according to the first exemplary embodiment.  FIG. 17  is an example of a resultant image created by combining the areas according to the first exemplary embodiment. 
     As shown in  FIG. 15 , the third processor  213  divides the first radiation image A or the second radiation image B into a plurality of areas F 11 , F 12 , F 13 , F 21 , F 22 , F 23 , F 31 , F 32 , and F 33  having the same size. Hereinafter, the areas F 11 , F 12 , F 13 , F 21 , F 22 , F 23 , F 31 , F 32 , and F 33  will be referred to as divided areas F 11  to F 33 . 
     Each of the divided areas F 11  to F 33  may be in a shape of a square or rectangle. The sizes or locations of the divided areas of the first radiation image A may be the same as those of the divided areas of the second radiation image B so that the divided areas of the first radiation image A correspond to the divided areas of the second radiation image B. The same size of two divided areas means that a length of a bottom side and a height of one of the two divided areas are the same as those of another one of the two divided areas, and different sizes of two divided areas mean that the length of the bottom side or the height of one of the two divided areas is different from that of another one of the two divided areas. 
     When the divided areas F 11  to F 33  have the same size, each of the divided areas F 11  to F 33  includes the same number of pixels as shown in  FIG. 15 . 
     The third processor  213  may calculate differences between pixel values of all pixels existing in a divided area of the first radiation image A and a pixel value of an arbitrary pixel of a divided area of the second radiation image B, and calculate a degree of difference for the arbitrary pixel in the divided area of the second radiation image B based on the calculated differences. In this case, the divided area of the second radiation image B may correspond to the divided area of the first radiation image A. 
     After the third processor  213  acquires degrees of differences for all pixels in the divided area of the second radiation image B, the third processor  213  may acquire a resultant image D for the divided area of the second radiation image B. The third processor  213  may apply the above-described process to the remaining divided areas of the second radiation image B in the same manner, to create resultant images D 1  to D 4  for all the divided areas F 11  to F 21  of the second radiation image B, as shown in  FIG. 16 . Thereafter, the third processor  213  may combine the resultant images D 1  to D 4  for the divided areas F 11  to F 21  to acquire a final resultant image D as shown in  FIG. 17 . 
     Because each of the first radiation image A and the second radiation image B is divided into a plurality of areas, data (for example, pixel intensity distribution) of pixels in each divided area may not greatly change although the subject  99 , for example, the human body  99   a , moves during radiography. Accordingly, through the area division, the relatively more accurate resultant image D can be acquired. 
       FIG. 18  is a diagram of area division that is performed on a first radiation image or a second radiation image according to a second exemplary embodiment. 
     The third processor  213  divides the first radiation image A or the second radiation image B into a plurality of divided areas G 11  to G 33  that have different shapes or sizes, as shown in  FIG. 18 . Also, the third processor  213  may divide an area of the first radiation image A or the second radiation image B into a plurality of divided areas having different shapes or sizes, and a remaining area of the first radiation image A or the second radiation image B into a plurality of divided areas having the same shape or size. In other words, at least one of a plurality of divided areas of the first radiation image A or the second radiation image B may have a size that is different from that of the remaining divided areas. The third processor  213  may calculate a degree of difference for each divided area, and combine resultant images of the divided areas acquired according to results of the calculation to thereby acquire a final resultant image. 
       FIG. 19  is a diagram of area division that is performed on a first radiation image or a second radiation image according to a third exemplary embodiment. 
     The third processor  213  divides an image into a plurality of divided areas H 11  to H 22  such that at least two of the divided areas H 11  to H 22  overlap each other. According to exemplary embodiments, all of the divided areas H 11  to H 22  may overlap each other. Accordingly, at least two of the divided areas H 11  to H 22  include an overlapping part. The overlapping divided areas H 11  to H 22  share one or more pixels. The overlapping divided areas H 11  to H 22  may have the same size or shape, or different sizes or shapes. Also, some of the overlapping divided areas H 11  to H 22  may have the same size or shape. The third processor  213  may calculate a degree of difference for each divided area, and combine resultant images of the divided areas acquired according to results of the calculation to thereby acquire a final resultant image. 
       FIG. 20  is a diagram of a multi-resolution method according to an exemplary embodiment. 
     An image may be represented with a predetermined resolution. Herein, the resolution means an index indicating a number of pixels forming an image. The resolution can be represented by a number of pixels existing in a unit length. For example, the resolution may be represented in units of pixels per inch (ppi) representing a number of pixels per inch. As an image has a higher resolution, the image can be implemented with more data, and accordingly, the image may include more information. 
     An image of a predetermined resolution may be converted into an image of another resolution. For example, an image of a high resolution may be converted into an image of a low resolution, which is called down-sampling. In contrast, an image of a low resolution may be converted into an image of a high resolution, which is called up-sampling. The down-sampling and the up-sampling may be performed by various kinds of transform functions. For example, the down-sampling may acquire an image of a lower resolution by converting a plurality of pixels of an image into a pixel to reduce the number of pixels. In this case, a pixel value of a pixel of the image of the lower resolution may be decided as a mean value of the pixel values of the plurality of pixels or as any one of the pixel values of the plurality of pixels. The multi-resolution method is to perform image processing on an image of a low resolution and to then reflect a result of the image processing to an image of a high resolution corresponding to the image of the low resolution, to increase image processing speed. 
     As shown in  FIG. 20 , the third processor  213  calculates degrees of difference I of a second radiation image using a first radiation image of a low resolution and a second radiation image of a low resolution. Because the first radiation image of the low resolution and the second radiation image of the low resolution have a relatively smaller number of pixels, it is possible to reduce an amount of calculation. When the degrees of difference I are acquired using the first radiation image of the low resolution and the second radiation image of the low resolution, the third processor  213  applies the degrees of difference I to a difference image J between a first radiation image of a high resolution and a second radiation image of a high resolution to acquire a new resultant image. For example, the third processor  213  may apply a degree of difference corresponding to a pixel of the low resolution to a plurality of pixels of the difference image J acquired with a high resolution to thereby acquire the new resultant image. 
     Hereinafter, various exemplary embodiments of a method of controlling a radiography apparatus will be described with reference to  FIGS. 21 to 24 . 
       FIG. 21  is a flowchart illustrating a method of controlling a radiography apparatus according to a first exemplary embodiment. 
     As shown in  FIG. 21 , in operation S 400 , before a contrast regent is injected into a subject, radiation is irradiated to the subject, radiation transmitted through the subject is received, and electrical signals corresponding to the received radiation are converted and combined to acquire a first radiation image of the subject. 
     In operation S 401 , after a contrast reagent is injected to the subject, and after a predetermined time elapses, radiation is again irradiated to the subject to acquire a second radiation image of the subject. In this case, because the contrast reagent injected into the subject may move through blood vessels inside the subject, the second radiation image may show the blood vessels more clearly. 
     In operation S 402 , after the first radiation image and the second radiation image are acquired, subtraction is performed on the first radiation image and the second radiation image to acquire a difference image between the first radiation image and the second radiation image. Operation S 402  of acquiring the difference image may be performed when operations S 403  to S 410 , which will be described later, are performed, or after operations S 403  to S 410  are performed. 
     In operations S 403  to S 405 , after the first radiation image and the second radiation image are acquired, a difference between a pixel value of a first or i-th pixel of the first radiation image and a pixel value of a first of k-th pixel of the second radiation image is calculated. 
     In operation S 406 , a value of i is iterated so that a difference between a pixel value of a second or i+1-th pixel of the first radiation image and a pixel value of a second or i+1-th pixel of the second radiation image is calculated. Operations S 403  to S 406  are performed until all of N pixels of the first radiation image are processed. 
     In operation S 407 , it is determined whether the value of i is equal to a number N of the pixels of the first radiation image. When the value of i is determined to be equal to the number N of the pixels of the first radiation image, the method continues in operation S 408 . Otherwise, the method returns to operation S 405 . According to exemplary embodiments, values acquired by edge detection or wavelet coefficients may be used instead of pixel values. 
     In operation S 408 , a degree of difference for the first or k-th pixel of the second radiation image is acquired. The degree of difference means a value representing a difference between all the pixels of the first radiation image and the corresponding pixel of the second radiation image. The degree of difference may be the smallest value of the calculated differences, or as a radical root of a sum of squares of a difference between the pixel value of each pixel of the first radiation image and the pixel value of the first or k-th pixel of the second radiation image. However, the degree of difference may be acquired by one of various methods that can be considered by one of ordinary skill in the art. 
     In operation S 409 , a value of k is iterated so that operations S 403  to S 408  are performed on a second or k+1-th pixel of the second radiation image to calculate a degree of difference for the second or k+1-th pixel of the second radiation image. Operations S 403  to S 408  are performed on all of M pixels of the second radiation image. 
     In operation S 410 , it is determined whether the value of k is equal to a number M of the pixels of the second radiation image. When the value of k is determined to be equal to the number M of the pixels of the second radiation image, the method continues in operation S 411 . Otherwise, the method returns to operation S 404 . As a result, degrees of difference for all the pixels of the second radiation image are acquired. 
     In operation S 411 , the acquired degrees of difference are reflected to the difference image to correct the difference image. Accordingly, in operation S 412 , a new resultant image is acquired. 
       FIG. 22  is a flowchart illustrating a method of controlling a radiography apparatus according to a second exemplary embodiment. 
     As shown in  FIG. 22 , in operations S 420  and S 421 , a first radiation image is acquired before a contrast regent is injected into a subject, and a second radiation image is acquired after the contrast reagent is injected into the subject, like the first exemplary embodiment shown in  FIG. 21 . 
     In operation S 422 , the same operations as operations S 402  to S 410  of the first exemplary embodiment may be performed to acquire degrees of difference for all pixels of the second radiation image. 
     In operation S 423 , a resultant image is acquired based on the degrees of difference. In detail, by locating the degrees of difference at locations of the corresponding pixels, an image as shown in  FIG. 14  may be acquired. Because a degree of difference for an area through which the contrast reagent passes is different from a degree of difference for an area through which no contrast reagent passes, the image shown in  FIG. 14  may also show blood vessels, like a difference image. 
       FIG. 23  is a flowchart illustrating a method of controlling a radiography apparatus according to a third exemplary embodiment. 
     As shown in  FIG. 23 , in operations S 430  and S 431 , a first radiation image is acquired before a contrast regent is injected into a subject, and a second radiation image is acquired after the contrast reagent is injected into the subject, like the first and second exemplary embodiments shown in  FIGS. 21 and 22 . 
     In operation S 432 , each of the first radiation image and the second radiation image are divided into a plurality of divided areas such that the plurality of divided areas of the first radiation image correspond to the plurality of divided areas of the second radiation image. The plurality of divided areas of the first radiation image may have the same shape or size. According to exemplary embodiments, at least one of the plurality of divided areas may have a shape or size that is different from that of the remaining divided areas. Also, the plurality of divided areas may overlap each other to share a part of pixels of the first radiation image. 
     Likewise, the plurality of divided areas of the second radiation image may have the same shape or size. Also, according to exemplary embodiments, at least one of the plurality of divided areas may have a shape or size that is different from that of the remaining divided areas. Also, two or more of the plurality of divided areas may overlap a part of the remaining divided areas. 
     A degree of difference for a pixel in a divided area of the second radiation image is acquired. As described above, the degree of difference for the pixel may be acquired by calculating a difference between a pixel value of the pixel and a pixel value of a pixel in an arbitrary divided area of the first radiation image. By performing the above-described operation on all pixels in the divided area of the second radiation image, degrees of difference for all the pixels of the divided area of the second radiation image are acquired. 
     In operation S 433 , by performing the above-described operation on all the divided areas of the second radiation image, degrees of difference for all pixels in each divided area of the second radiation image are acquired. 
     In operation S 434 , a resultant image is acquired by applying the degrees of difference to a difference image, or by combining the divided areas based on the degrees of difference. 
       FIG. 24  is a flowchart illustrating a method of controlling a radiography apparatus according to a fourth exemplary embodiment. 
     As shown in  FIG. 24 , in operations S 440  and S 441 , a first radiation image is acquired before a contrast regent is injected into a subject, and a second radiation image is acquired after the contrast reagent is injected into the subject, like the first, second, and third embodiments shown in  FIGS. 21, 22, and 23 . In this case, the first radiation image and the second radiation image may be radiation images of relatively high resolutions. 
     In operation S 442 , a difference image of a high resolution between the first radiation image and the second radiation image is acquired. 
     Then, a first radiation image of a relatively low resolution and a second radiation image of a relatively low resolution are acquired from the first radiation image of the relatively high resolution and the second radiation image of the relatively high resolution. The radiation images of the low resolution may be acquired by down-sampling the radiation images of the high resolution. 
     In operation S 443 , degrees of difference for all pixels of the second radiation image are acquired using the first radiation image of the relatively low resolution and the second radiation image of the relatively low resolution. Degrees of difference for individual pixels may be acquired by calculating differences between pixel values of all pixels of the first radiation image and a pixel value of an arbitrary pixel of the second radiation image, as described above. 
     In operation S 444 , the degrees of difference are reflected to a difference image of a high resolution to acquire a resultant image, or a resultant image of a relatively low resolution formed with the degrees of difference may be up-sampled to acquire a resultant image of a relatively high resolution. 
     According to the radiography apparatus and the method for controlling the radiography apparatus, it is possible to reduce or remove an artifact on an image, which may be generated upon radiography, thereby acquiring an accurate image about an inside of a subject. It is also possible to reduce or remove an artifact on an image, which may be generated due to movement of a subject during radiography. Further, it is possible to acquire a clear and accurate angiography image about blood vessels in a subject. 
     In addition, the exemplary embodiments may also be implemented through computer-readable code and/or instructions on a medium, e.g., a non-transitory computer-readable medium, to control at least one processing element to implement any above-described embodiments. The medium may correspond to any medium or media which may serve as a storage and/or perform transmission of the computer-readable code. 
     The computer-readable code may be recorded and/or transferred on a medium in a variety of ways, and examples of the medium include recording media, such as magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.) and optical recording media (e.g., compact disc read only memories (CD-ROMs) or digital versatile discs (DVDs)), and transmission media such as Internet transmission media. Thus, the medium may have a structure suitable for storing or carrying a signal or information, such as a device carrying a bitstream according to one or more exemplary embodiments. The medium may also be on a distributed network, so that the computer-readable code is stored and/or transferred on the medium and executed in a distributed fashion. Furthermore, the processing element may include a processor or a computer processor, and the processing element may be distributed and/or included in a single device. 
     The foregoing exemplary embodiments and advantages are merely exemplary and are not to be construed as limiting. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.