Patent Publication Number: US-2022211338-A1

Title: Photon counting ct apparatus and method of correcting material decomposition map

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an X-ray computed tomography (CT) apparatus having a photon counting mode (hereinafter referred to as a “PCCT apparatus”), and, more particularly relates to a technique to reduce statistical noise occurring at a low energy side in the PCCT apparatus. 
     Description of the Related Art 
     The X-ray CT apparatus is an apparatus which, while rotating, around a subject, a pair including an X-ray tube and an X-ray detector arranged opposite each other across the subject with a positional relationship between the pair retained, obtains X-ray transmissive data about the subject and reconstructs a tomographic image thereof (hereinafter referred to as a “CT image”) by calculation, and is used as, for example, an inspection apparatus for industrial use or for security use or a diagnostic imaging apparatus for medical use. 
     With regard to an X-ray CT apparatus for medical use, a PCCT apparatus equipped with a photon counting mode is in the process of research and development. The PCCT apparatus uses a detector of the photon counting type to count quanta of X-rays (X-ray photons) transmitted through a subject with every detection element. This enables, for example, obtaining a spectrum available for estimating elements configuring an internal tissue of the subject through which X-rays have been transmitted, and thus obtaining an X-ray CT image subjected to material decomposition of the tissue of the subject (see Patent literature 1). 
     Moreover, the PCCT apparatus is able to discriminate the counted individual X-ray photons by energy values, thus obtaining X-ray intensities in respective energy bands (energy bins). With the use of this, the PCCT apparatus may be used to extract only X-rays in a specific energy range, convert the extracted X-rays into an image, and utilize the image for diagnosis. 
     On the other hand, Patent Literature 2 discloses a configuration which, to improve the accuracy of material decomposition in a PCCT apparatus, computationally calculates detection data caused by a non-linear response of a detector that detects photons. Then, such a disclosed configuration subtracts the calculated non-linear response data from data actually detected by the detector and performs material decomposition with use of detection data obtained by such subtraction. 
     LIST OF RELATED ART 
     Patent Literature 
     
         
         Patent Literature 1: JP-A-2019-176988 
         Patent Literature 2: JP-A-2016-193174 
       
    
     The PCCT apparatus performs correction processing on data obtained in each energy band counted by an X-ray detection element. For example, the PCCT apparatus performs linearity correction of a reference correction circuit, correction of logarithmic conversion processing, correction of offset processing, sensitivity correction, beam hardening correction, water phantom calibration, and CT value correction. Correction data for use in such correction is calculated based on data (Air data) which is obtained by radiating X-rays to the air without a subject being arranged and performing counting in every energy band with detection elements. Usually, Air data is obtained to determine correction data before the shipment of a PCCT apparatus or at the time of maintenance thereof. 
     However, a deviation may occur in the above-mentioned Air data between at the time of correction data calculation and at the time of actual image capturing of a subject, and there is an issue in which, if correction data calculated based on Air data with a deviation occurring therein is used to correct count data obtained at the time of image capturing of a subject, the accuracy of material decomposition decreases. The inventors of the present invention became aware that the cause of a deviation occurring in Air data depending on image capturing dates was the spectrum of X-rays generated by an X-ray tube varying by, for example, heat generated by the X-ray tube or by secular change thereof. 
     While the configuration disclosed in Patent Literature 2 obtains non-linear response data by computation and subtracts the obtained data from detection data, since a variation of the spectrum of X-rays caused by, for example, heat is also influenced by, for example, the ambient temperature of a place of installation of the PCCT apparatus or a time elapsed from powering-on thereof, it is not easy to accurately calculate detection data by computation. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention are directed to accurately performing material decomposition even if a variation in X-ray spectrum occurs in a PCCT apparatus. 
     According to an aspect of the present invention, a photon counting CT apparatus configured as described below is provided. The photon counting CT apparatus includes an X-ray tube that radiates X-rays to an imaging range, an X-ray detector that counts a plurality of X-ray photons passing through the imaging range in each of a plurality of energy bands according to energy levels of the respective X-ray photons, a map storage unit that stores a material decomposition map indicating measurement values in a plurality of energy bands previously obtained about combinations of two or more types of materials made to have a plurality of respective different thicknesses, a material decomposition unit that obtains, by referring to the material decomposition map, a combination of thicknesses of two or more types of materials corresponding to measurement values obtained by the X-ray detector performing counting about a plurality of energy bands in a state in which a subject is arranged in the imaging range, and a map correction unit that corrects the material decomposition map. The map correction unit actually measures corrective measurement values by radiating X-rays from the X-ray tube and counting X-ray photons in each of a plurality of energy bands in a state in which no subject is arranged in the imaging range and/or in a state in which one or more types of corrective materials are arranged at a position through which X-rays radiated from the X-ray tube pass, and corrects the measurement values in the material decomposition map based on the corrective measurement values. 
     According to an embodiment of the present invention, even if a variation in X-ray spectrum occurs in a PCCT apparatus, the map correction unit acquires corrective measurement values and corrects measurement values in the material decomposition map based on the corrective measurement values, so that it is possible to accurately perform material decomposition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of a photon counting CT apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a diagram used to explain that measurement values are measured with respect to a plurality of energy bands for each detection element (channel) in the photon counting CT apparatus according to the first embodiment. 
         FIG. 3  is a diagram illustrating an example of a material decomposition map (bed) in the photon counting CT apparatus according to the first embodiment. 
         FIG. 4A  is an explanatory diagram used to explain that, for the purpose of correction of a material decomposition map, with no subject arranged in an image capturing range, an X-ray detector actually measures X-ray photons,  FIG. 4B  is an explanatory diagram used to explain that, for the purpose of correction of a material decomposition map, with a corrective material arranged in the image capturing range, the X-ray detector actually measures X-ray photons, and  FIG. 4C  is an explanatory diagram used to explain that image capturing of a subject is performed and material decomposition is performed with use of the corrected material decomposition map, each in the photon counting CT apparatus according to the first embodiment. 
         FIGS. 5A, 5B, and 5C  are explanatory diagrams illustrating a current corrective material map, a present-moment corrective material map, and a present-moment all-points corrective material map, respectively, in the photon counting CT apparatus according to the first embodiment. 
         FIGS. 6A, 6B, 6C, and 6D  are explanatory diagrams illustrating a current corrective material map, a present-moment corrective material map, a material decomposition map, and a corrected material decomposition map, respectively, in the photon counting CT apparatus according to the first embodiment. 
         FIG. 7  is a functional block diagram of a computing unit of the photon counting CT apparatus according to the first embodiment. 
         FIG. 8  is a flowchart illustrating the flow of image capturing processing according to the first embodiment. 
         FIG. 9  is a flowchart illustrating the flow of image capturing processing according to the first embodiment. 
         FIGS. 10A and 10B  are explanatory diagrams illustrating a configuration of a part of a photon counting CT apparatus according to a third embodiment of the present invention. 
         FIGS. 11A and 11B  are explanatory diagrams illustrating another configuration of a part of the photon counting CT apparatus according to the third embodiment. 
         FIG. 12  is a block diagram illustrating a configuration of a part of a photon counting CT apparatus according to a fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Examples of a photon counting CT apparatus according to embodiments of the present invention are described below. In the following description, in all of the figures used to describe embodiments of the present invention, elements or components having the respective same functions are assigned the respective same reference numerals and any repetitive descriptions thereof are omitted. 
     First Embodiment 
     An outline of a PCCT apparatus according to a first embodiment is described below with reference to  FIG. 1 . 
     The present embodiment is configured to actually measure measurement values obtained with X-rays at the time of image capturing of a subject  101  under a predetermined condition and correct a map (here, a bed)  410  for use in material decomposition with use of the actually measured measurement values. This enables accurately performing material decomposition even if a variation occurs in the spectrum of X-rays due to an X-ray tube  311  or a component arranged therearound being influenced by, for example, an increase in temperature of the X-ray tube  311 . 
     A PCCT apparatus according to the present embodiment includes, as illustrated in  FIG. 1  as an overall configuration thereof, the X-ray tube  311 , which radiates X-rays to an imaging range  103 , an X-ray detector  321 , a rotary unit (rotary plate)  332 , a map storage unit  409 , a material decomposition unit  408 , and a map correction unit  404 . 
     The X-ray tube  311  has a configuration in which a cathode and an anode are arranged within a vacuum tube. Thermal electrons generated from the cathode collide with the anode rotating, so that X-rays are generated. 
     The X-ray detector  321  includes a plurality of detection elements (channels Ch  1  to Ch n) arrayed and a counting circuit and counts a plurality of X-ray photons passing through the imaging range  103  in each of a plurality of energy bands (bin  1  to bin k) according to energy levels of the respective X-ray photons as illustrated in  FIG. 2 . 
     The rotary unit  332  rotates the X-ray tube  311  and the X-ray detector  321  around the imaging range  103 . 
     The map storage unit  409  stores a material decomposition map  410 , which is used for material decomposition processing. The material decomposition map  410  is a map indicating measurement values previously obtained by the X-ray detector  321  performing actual measurement or calculation in a plurality of energy bands (bin  1  to bin k) in a case where a plurality of types of respective thicknesses (lengths through which X-rays pass) of two or more types of materials targeted for discrimination (here, a bone and a fat) is prepared and each of combinations thereof is arranged between the X-ray tube  311  and the X-ray detector  321 , as illustrated in  FIG. 3  as an example. While a bed is used as the material decomposition map  410  as illustrated in  FIG. 3 , the material decomposition map  410  can be expressed by, for example, a graph or mathematical expression. 
     The material decomposition unit  408  obtains, by referring to the material decomposition map  410 , a combination of respective thicknesses (transmissive distances) of two or more types of materials which correspond to measurement values obtained by the X-ray detector  321  performing counting with regard to a plurality of energy bands in a state in which the subject  101  is arranged in the imaging range  103 . 
     The map correction unit  404  radiates X-rays from the X-ray tube  311  in a vacant state in which the subject  101  is not arranged in the imaging range  103  ( FIG. 4A ) and/or in a state in which one or more types of corrective materials  104  are arranged between the X-ray tube  311  and the X-ray detector  321  ( FIG. 4B ) at desired timing such as timing before or after image capturing of the subject  101  or timing of powering-on of the PCCT apparatus, and performs counting in each of a plurality of energy bands, thus acquiring corrective measurement values  412  ( FIG. 5B ). The map correction unit  404  corrects measurement values in the material decomposition map  410  based on the acquired corrective measurement values  412 . 
     The one or more types of corrective materials  104  as mentioned herein refer to one or more materials different in element or composition or one or more objects different in shape. 
     The elements or compositions of the corrective materials  104  can be the same materials as, or materials different from, materials in the material decomposition map  410  (for example, a bone and a fat). Furthermore, in a case where the elements or compositions of the corrective materials  104  are different from the materials in the material decomposition map  410 , a relationship  61  between corrective measurement values  411  ( FIG. 5A  and  FIG. 6A ) obtained by performing counting in each of a plurality of energy bands with regard to the corrective materials  104  and measurement values (measurement values in the material decomposition map  410 ) ( FIG. 3  and  FIG. 6C ) obtained by performing counting in each of a plurality of energy bands with regard to the materials in the material decomposition map  410  is previously measured with X-ray spectra. The map correction unit  404  uses the measured relationship  61  to correct measurement values in the material decomposition map  410  ( FIG. 3  and  FIG. 6C ) based on corrective measurement values  412  or  412 - 1  ( FIG. 5B  or  FIG. 6B ), thus being able to create a corrected material decomposition map  410 - 1  ( FIG. 6D ). 
     For example, as illustrated in  FIGS. 5A and 5B  and  FIGS. 6A and 6B , at least one of aluminum (Al) and acrylic plates can be used as the corrective material  104 . 
     Moreover, in a case where the elements or compositions of the corrective materials  104  are different from materials in the material decomposition map  410 , it is desirable that a current corrective material map  411  ( FIG. 5A  and  FIG. 6A ) created at the same time point as the time point of creation of the material decomposition map  410  be currently stored in the map storage unit  409 . The current corrective material map  411  is a map indicating measurement values previously obtained by the X-ray detector  321  performing actual measurement or calculation with regard to a plurality of energy bands (bin  1  to bin k: in  FIG. 5A  and  FIG. 6A , k=3) in a case where a plurality of types of respective different thicknesses of two or more types of corrective materials (Al and acrylic)  104  is prepared and each of combinations thereof is arranged between the X-ray tube  311  and the X-ray detector  321 , as illustrated in  FIG. 5A  and  FIG. 6A . The current corrective material map  411  is generated by a service engineer before the shipment of the PCCT apparatus or at the time of replacement of the X-ray tube  311  or the X-ray detector  321 , and is then stored in the map storage unit  409 . 
     It is desirable that the map correction unit  404  performs actual measurement of the corrective measurement values  412  ( FIG. 5B ) at the present moment, thus generating a map (hereinafter referred to as a “present-moment corrective material map  412 ”). As with the current corrective material map  411 , the present-moment corrective material map  412  is a map indicating measurement values in each of a plurality of energy bands obtained with regard to each of combinations of a plurality of respective different thicknesses of two or more types of corrective materials (Al and acrylic)  104 , and is generated by performing actual measurement of at least some measurement values at the present moment, so that X-ray spectra at the present moment can be reflected in the measurement values. 
     For example, corrective measurement values in each energy band are actually measured with regard to at least two combinations in the present-moment corrective material map  412  (two fields in the bed). Specifically, for example, corrective measurement values in each energy band actually measured in a vacant state in which the subject  101  is not arranged in the imaging range  103  ( FIG. 4A ) are used as corrective measurement values in fields in which both the thicknesses of two types of corrective materials (Al and acrylic)  104  in the map  412  illustrated in  FIG. 5B  are 0 centimeters (cm). Moreover, corrective measurement values in each energy band actually measured in a state in which only an acrylic plate with a thickness of 10 cm serving as a corrective material  104  is arranged in the imaging range  103  ( FIG. 4B ) are used as corrective measurement values in fields for a combination of an acrylic plate with a thickness of 10 cm and an Al plate with a thickness of 0 cm in the map  412  illustrated in  FIG. 5B . 
     In this way, after obtaining corrective measurement values by actual measurement with regard to at least two portions (in  FIG. 5B , three portions) of the present-moment corrective material map  412  illustrated in  FIG. 5B , the map correction unit  404  calculates, by interpolation or extrapolation, measurement values for the remaining vacant fields in the present-moment corrective material map  412  illustrated in  FIG. 5B  while referring to the current corrective material map  411  illustrated in  FIG. 5A . 
     An example of a calculation performed by the map correction unit  404  is described. A value for a field in the map illustrated in  FIG. 5A  is expressed by Ak_ac#x_al#y. “k” denotes a bin number (in  FIG. 5A, 1 to 3 ), “x” of ac#x denotes the length (millimeter (mm)) of an acrylic plate in an X-ray transmission direction, and “y” of al#y denotes the length (mm) of an aluminum plate in an X-ray transmission direction. Moreover, a value for a field in the map illustrated in  FIG. 5B  is similarly expressed by Bk_ac#x_al#y. Additionally, a value for a field not yet subjected to measurement in the map illustrated in  FIG. 5B  is expressed by Ck_ac#x_al#y. For example, a value C for a position of “acrylic 50 mm” and “aluminum 0 mm” in  FIG. 5B  is obtainable by the following equation (1): 
         Ck _ ac# 50_ al# 0= Ak _ ac# 50_ al# 0×( Bk _ ac# 100_ al# 0/ Ak _ ac# 100_ al# 0+ Bk _ ac# 0_ al# 0/ Ak _ ac# 0_ al# 0)/2  (1)
 
     In this way, the map correction unit  404  obtains, by interpolation, a position computable by interpolation from information about positions measured at the present moment or, if not so, calculates a ratio between “A” and “B” by extrapolation, and multiplies the original value of “A” by a result of such calculation, thus performing correction. 
     This enables generating a present-moment all-points corrective material map  412 - 1  ( FIG. 5C ). X-ray spectra at the present moment are reflected in the generated present-moment all-points corrective material map  412 - 1  illustrated in  FIG. 5C . 
     Therefore, comparing the present-moment all-points corrective material map  412 - 1  ( FIG. 6B ) and the current corrective material map  411  ( FIG. 6A ) with each other as illustrated in  FIGS. 6A and 6B  enables recognizing a change in X-ray spectra occurring during that time as a change in measurement values. Correcting the measurement values in the material decomposition map  410  illustrated in  FIG. 6C  according to such a change in X-ray spectra enables obtaining a corrected material decomposition map  410 - 1  ( FIG. 6D ) associated with the change in X-ray spectra. 
     A method of correcting measurement values in the material decomposition map  410  to generate a corrected material decomposition map  410 - 1  is described with reference to  FIGS. 6A to 6D . 
     First, the method obtains, by calculation, to which position (a ratio between acrylic and aluminum) in the current corrective material map  411  illustrated in  FIG. 6A  each position (for example, a fat 50 mm and a bone 2 mm) in the material decomposition map  410  illustrated in  FIG. 6C  corresponds. Since attenuation results of bins of respective signals obtained by transmission through a fat 50 mm and a bone 2 mm are previously obtained from the material decomposition map  410  illustrated in  FIG. 6C , the calculation method obtains a ratio between the attenuation results and looks for a point in which an attenuation result coincident with the obtained ratio is obtainable in the current corrective material map  411  illustrated in  FIG. 6A . However, while, since there is never a perfect coincidence, the obtained point includes an error, the calculation method obtains a point in which the error becomes minimum. The minimum error is defined as, for example, a place in which the sum of the squares obtained by squaring and summing differences of measurement values in the respective bins becomes minimum. The method obtains a correction value with regard to a position obtained as such on the current corrective material map  411  illustrated in  FIG. 6A , by performing the following calculation: 
     Correction value=(Measurement value at an associated position in the present-moment all-points corrective material map  412 - 1  illustrated in  FIG. 6B )/“Measurement value at the associated position in the current corrective material map  411  illustrated in  FIG. 6A ). The method multiplies the obtained correction value by a value at the associated position in the material decomposition map  410  illustrated in  FIG. 6C . The method repeats this calculation, thus creating a corrected material decomposition map  410 - 1  ( FIG. 6D ) for a case where different materials are used. 
     The material decomposition unit  408  performs material decomposition on measurement values of a subject with use of the corrected material decomposition map  410 - 1 . 
     Accordingly, in the PCCT apparatus according to the present embodiment, even if a change in X-ray spectra to be radiated occurs due to, for example, an increase in temperature of the X-ray tube  311 , the map correction unit  404  acquires corrective measurement values and corrects the measurement values in the material decomposition map  410 , so that the material decomposition unit  408  can accurately perform material decomposition. 
     Furthermore, to generate the present-moment corrective material map  412 , the map correction unit  404  can radiate X-rays from the X-ray tube  311  in each of three states, i.e., a vacant state (Air) in which the subject  101  is not arranged in the imaging range  103 , a state in which a first corrective material (for example, Al) with a predetermined thickness is arranged between the X-ray tube  311  and the X-ray detector  321 , and a state in which a second corrective material (for example, acrylic) with a predetermined thickness is arranged between the X-ray tube  311  and the X-ray detector  321  and perform counting in each of a plurality of energy bands, thus actually measuring corrective measurement values. The map correction unit  404  corrects measurement values in the material decomposition map  410  with use of corrective measurement values actually measured in each of three states. 
     &lt;Details of Configuration&gt; 
     The PCCT apparatus  100  according to the present embodiment is described below in more detail. As illustrated in  FIG. 1 , the PCCT apparatus  100  according to the present embodiment includes a measuring unit  300 , a computing unit  400 , and a user interface (UI) unit  200 . 
     The measuring unit  300  radiates X-rays to the subject  101  under the control of the computing unit  400  and measures X-ray photons transmitted through the subject  101 . The measuring unit  300  includes, in addition to the X-ray tube  311  and an X-ray detection unit  320 , a gantry  330 , a control unit  340 , and a bed  102 , on which to place the subject  101 . The control unit  340  includes a radiation controller  341 , a gantry controller  342 , a bed controller  343 , and a detection controller  344 . 
     An bore  331  is provided at the central portion of the gantry  330 , and the subject  101  and the bed  102  are arranged within the bore  331 . A rotary plate  332 , on which the X-ray tube  311  and the X-ray detector  321  are mounted, and a drive mechanism (not illustrated), which is configured to rotate the rotary plate  332 , are arranged inside the gantry  330 . When the rotary plate  332  rotates a predetermined angle, the gantry controller  342  outputs a signal to the detection controller  344 , the detection controller  344  outputs a signal to the X-ray detection unit  320 , and the counting circuit of the X-ray detector  321  outputs counting data as data for one angle. Furthermore, for example, the diameter of the bore  331  of the gantry  330  is 700 mm. The distance between the X-ray generation point of the X-ray tube  311  and the X-ray entrance surface of the X-ray detector  321  is, for example, 1,000 mm. 
     The time required for one rotation of the rotary plate  332  is set by parameters input by the user via the UI unit  200 . For example, if the required time is set as 1.0 seconds (s) per rotation, it is possible to set the number of times of image capturing per rotation to 900 times. Furthermore, in the present specification, the circumferential direction of the bore  331  is assumed to be an x-direction and the radial direction thereof is assumed to be a y-direction. A z-direction (the body axis direction of the subject  101 ) is a direction perpendicular to the x-direction and the y-direction. 
     An X-ray filter  312 , which adjusts an X-ray spectrum, and a bowtie filter  313 , which suppresses a dosage to a surrounding portion thereof, are arranged between the X-ray tube  311  and the imaging range  103 , and thus configure an X-ray radiation unit  310 . The X-ray tube  311  receives a high voltage supplied under the control of the radiation controller  341 . 
     The X-ray detector  321  has a configuration in which a plurality of detection elements is arrayed. A plurality of X-ray detectors  321  is arranged in an arc form, so that the X-ray detection unit  320  is configured. As illustrated in  FIGS. 4A to 4C , a collimator  323 , which restricts incident directions of X-rays, is erected on the entrance surface side of the X-ray detector  321 . The detection elements, which configure the X-ray detector  321 , to be used includes semiconductor elements. Furthermore, the size in the x-direction of each detection element is, for example, 1 mm. 
     The computing unit  400  controls the overall operation of the PCCT apparatus  100  and processes data acquired by the measuring unit  300 , thus performing image capturing. The computing unit  400  includes a central processing unit (CPU)  401 , a memory  402 , and a hard disk drive (HDD) device  403 . The computing unit  400  includes, as illustrated in the functional block diagram of  FIG. 7 , the functions of a map correction unit  404 , an image capturing unit  405 , a measurement value data correction unit  406 , a material decomposition unit  408 , and an image generation unit  407 . 
     The CPU  401  loads a program previously stored in the HDD device  403  onto the memory  402  and executes the loaded program, thus implementing the above-mentioned functions by software. Furthermore, the whole or a part of the functions of the computing unit  400  can be implemented by, for example, an integrated circuit such as an application specific integrated circuit (ASIC) or a field-programmable gate array (FPGA). 
     The image capturing unit  405  performs image capturing such as CT imaging or scanogram. The measurement value data correction unit  406  performs correction processing on measurement value data collected in each energy band. The correction processing as mentioned herein includes, for example, linearity correction of a reference correction circuit, logarithmic conversion processing, offset processing, sensitivity correction, beam hardening correction, water phantom calibration, and CT value correction. 
     The material decomposition unit  408  performs conversion from measurement value data corrected by the measurement value data correction unit  406  into transmissive distances (for example, a bone 1 cm and a fat 5 cm) of materials configuring the subject  102 . 
     The image generation unit  407  virtually reconstructs, with use of a conversion result obtained by the material decomposition unit  408 , an image obtained by extracting only a bone, an image obtained by extracting only a fat, or a CT image obtained by radiating X-rays in a specific energy band. 
     The HDD device  403  stores, for example, data which is used for processing and data which is a result of processing. 
     The UI unit  200  includes an input device  210 , such as a keyboard and a mouse, and an output device  220 , such as a display device, and receives, from the user, for example, timing at which to correct the material decomposition map  410 , the type of an image intended to be output, and an image capturing conditions and outputs such received information to the computing unit  400 . The image capturing condition includes, for example, a tube current and tube voltage for the X-ray tube  311  and an image capturing range for the subject  101 . 
     Next, image capturing processing which the computing unit  400  performs is described with reference to the flowchart of  FIG. 8 . 
     &lt;Step S 1101 &gt; 
     First, the map correction unit  404  determines whether the present moment is previously-determined timing at which to perform correction of the material decomposition map  410 , and, if it is determined that the present moment is timing of map correction (Yes in step S 1101 ), the map correction unit  404  advances the processing to step S 1102 . If it is determined that the present moment is not timing of map correction (No in step S 1101 ), the map correction unit  404  advances the processing to step S 1104 . 
     Here, the previously-determined timing of map correction is timing set by the user, and includes, as setbed timing, for example, before image capturing, halfway through image capturing, after image capturing, and timing of powering-on in the morning. 
     &lt;Step S 1102 &gt; 
     When the present moment is timing at which to perform correction of the material decomposition map  410 , the map correction unit  404  actually measures measurement values with regard to two types of corrective materials (Al and acrylic)  104  at the present moment and generates a corrective material map  412  based on the measured measurement values. 
     Step S 1102  is specifically described as follows with reference to the flowchart of  FIG. 9 . 
     (Step S 2001 ) 
     The map correction unit  404  radiates X-rays from the X-ray tube  311  as illustrated in  FIG. 4A  in a vacant state (Air) in which the subject  101  is not arranged in the imaging range  103 , and actually measures corrective measurement values in each of a plurality of energy bands with the X-ray detector  321 . 
     (Step S 2002 ) 
     The map correction unit  404  causes the output device (display device)  220  to display an indication for prompting the user to arrange only an acrylic plate with a thickness of 10 cm, which is a corrective material  104 , between the X-ray tube  311  and the imaging range  103 , and, if the user has arranged the acrylic plate, the map correction unit  404  radiates X-rays from the X-ray tube  311  in that state ( FIG. 4B ) and actually measures corrective measurement values  412  in each of a plurality of energy bands with the X-ray detector  321 . 
     (Step S 2003 ) 
     The map correction unit  404  uses the measurement values obtained in steps S 2001  and S 2002  as corrective measurement values in fields for thicknesses of 0 cm of the corrective materials (Al and acrylic)  104  in the present-moment corrective material map  412  and fields for thicknesses of 10 cm in acrylic and 0 cm in Al therein, and calculates measurement values in remaining vacant fields by interpolation and extrapolation. This results in generation of the present-moment corrective material map  412  ( FIG. 5B ). 
     &lt;Step S 1103 &gt; 
     The map correction unit  404  compares the generated present-moment corrective material map  412  ( FIG. 5B ) and the current corrective material map  411  ( FIG. 5A ) with each other, and corrects measurement values in the material decomposition map  410  ( FIG. 3  and  FIG. 6C ) according to a result of comparison. 
     &lt;Steps S 1104  and S 1105 &gt; 
     If receiving an instruction for starting image capturing from the user via the UI unit  200  (Yes in step S 1104 ), the image capturing unit  405  advances the processing to step S 1105 , in which the image capturing unit  405  radiates X-rays from the X-ray tube  311  to the subject  101  arranged in the imaging range  103  as illustrated in  FIG. 4C  and measures measurement values in a plurality of energy bands with the X-ray detector  321 . 
     &lt;Step S 1106 &gt; 
     The material decomposition unit  408  obtains, by referring to the material decomposition map  410  corrected in step S 1103 , a combination of thicknesses (transmissive distances) of two or more types of materials corresponding to the measurement values about the subject  101  acquired in step S 1105 . 
     &lt;Steps S 1107  and S 1108 &gt; 
     The image generation unit  407  generates, by computation, an image obtained by extracting a desired material (for example, only a bone or only a fat) or an X-ray CT image obtained in the case of radiating X-rays with a specific energy (for example, 60 electron volts (eV)) based on the transmissive distances of materials obtained by the material decomposition unit  408  in step S 1106 . The image generation unit  407  causes the output device  220  to display the generated image. 
     Even in a case where X-ray spectra have changed due to, for example, an increase in temperature of the X-ray tube  311 , the present embodiment actually measures a change in measurement values at the present moment and corrects a material decomposition map, and is, therefore, able to improve the accuracy of material decomposition. Therefore, the doctor is enabled to make a diagnosis while viewing an image obtained by accurate material decomposition. 
     Second Embodiment 
     A PCCT apparatus according to a second embodiment is described. 
     The second embodiment uses, as a corrective material  104 , a cylindrical phantom made of resin the attenuation of which becomes almost the same as that of the subject  101 . For example, the second embodiment uses a cylindrical phantom made of polyethylene. 
     In step S 2001  in the flowchart of  FIG. 9  in the first embodiment, the map correction unit  404  actually measures measurement values in a vacant state (Air) in which the subject  101  is not arranged in the imaging range  103 , and then in step S 2002 , the map correction unit  404  actually measures measurement values with the cylindrical phantom arranged in the imaging range  103 . In step S 2003 , the map correction unit  404  generates a present-moment corrective material map  412  which includes, as two elements thereof, measurement value data actually measured in a vacant state in which the subject  101  is not arranged and measurement value data actually measured with the cylindrical phantom made of resin arranged. 
     As the current corrective material map  411 , a map which includes, as two elements thereof, measurement value data actually measured in a vacant state (Air) and measurement value data actually measured with the cylindrical phantom arranged is also current. 
     In step S 1102  illustrated in  FIG. 8 , the map correction unit  404  corrects the material decomposition map  410  based on a difference between the present-moment corrective material map  412  and the current corrective material map  411 , thus obtaining the corrected material decomposition map  410 - 1 . 
     The other configurations, processing operations, and advantageous effects are similar to those in the first embodiment. 
     Third Embodiment 
     A PCCT apparatus according to a third embodiment is described with reference to  FIGS. 10A and 10B . 
     While each of the PCCT apparatuses according to the first and second embodiments is configured to actually measure corrective measurement values with the X-ray detector  321 , the map correction unit  404  in the third embodiment further includes a reference detector  902  in addition to the X-ray detector  321  and actually measures corrective measurement values with the reference detector  902 . 
     The reference detector  902  is arranged at a position at which X-rays radiated from the X-ray tube  311  and passing outside the imaging range  103  arrive, as illustrated in  FIG. 10A . Accordingly, actually measuring corrective measurement values with the reference detector  902  enables actually measuring corrective measurement values even during image capturing of a subject and thus correcting the material decomposition map  410  ( FIG. 3  and  FIG. 6C ) in real time. 
     Moreover, since the reference detector  902  is present outside the imaging range  103 , if none is arranged between the reference detector  902  and the X-ray tube  311 , measurement values in the state of Air are obtained and, if a corrective material  104  with a predetermined thickness is arranged in front of the reference detector  902 , measurement values about the arranged corrective material  104  are obtained. Therefore, if, as illustrated in  FIG. 10B , a reference detector  902  including a plurality of detection elements  902 - 1  to  902 - 3  is used, none is arranged in front of the detection element  902 - 1 , and corrective materials  104 - 1  and  104 - 2  different in thickness or material are previously arranged in front of the detection elements  902 - 2  and  902 - 3 , respectively, it is possible to obtain corrective measurement values in the state of Air and about two types of corrective materials at one time. Therefore, it is possible to quickly generate the present-moment corrective material map  412 . 
     Moreover, since the reference detector  902  is arranged outside the imaging range  103  and it is, therefore, possible to perform image capturing of the subject  101  with the reference detector  902  remaining arranged, there is also such an advantage that the user is not required to attach or detach the corrective material  104  at a position through which X-rays radiated from the X-ray tube  311  pass and the material decomposition map  410  is able to be automatically corrected. 
     Additionally, the reference detector  902  is located outside the subject  101  and is, therefore, not influenced by the subject  101 . Taking the advantage of this feature, it is possible to use, as the reference detector  902 , an end portion of the X-ray detector  321 , which detects X-rays transmitted through the subject  101 , as illustrated in  FIGS. 11A and 11B . In this case, there is such an advantage that the X-ray detector  321  and the reference detector  902  can be integrated. 
     Furthermore, in the present embodiment, it is desirable that the current corrective material map  411  be also generated with use of measurement values detected by the reference detector  902 . 
     The other configurations are similar to those in the first embodiment and is, therefore, omitted from description. 
     Fourth Embodiment 
     A PCCT apparatus according to a fourth embodiment is described with reference to  FIG. 12 . 
     While the PCCT apparatus according to each of the first to third embodiments has a configuration in which, after actually measuring corrective measurement values, the map correction unit  404  generates the present-moment corrective material map  412  by computation and then corrects the material decomposition map  410 , the map correction unit  404  in the fourth embodiment uses a trained learning model  920 , inputs actually measured corrective measurement values to the learning model  920 , and then directly obtains the corrected material decomposition map  410 - 1  as an output of the learning model  920 . The learning model  920  to be used can be a known one and can be, for example, a neural network. 
     To train the learning model  920 , the corrective measurement values obtained by the respective methods in the first to third embodiments can be used as input data and the corrected material decomposition map  410 - 1  can be used as correct answer data, so that the learning model  920  can be trained in advance sufficiently by machine learning or deep learning. 
     This enables the map correction unit  404  to obtain the corrected material decomposition map  410 - 1  by only actually measuring corrective measurement values and inputting the measured corrective measurement values to the learning model  920 , so that it is possible to accurately perform correction of the material decomposition map  410  in a short amount of time. 
     A corrective measurement value which the map correction unit  404  inputs to the learning model  920  only needs to be 1 or more, and can be, for example, only measurement values in a plurality of energy bands actually measured in a vacant state (Air) in which no subject is arranged in the imaging range  103 , and, additionally, measurement values actually measured with one or more types of corrective materials  104  arranged can be input to the learning model  920 . 
     Furthermore, data to be output from the trained learning model can be set to be not the material decomposition map  410  but the present-moment corrective material map  412 . In this case, the map correction unit  404  can correct the material decomposition map  410  by performing processing similar to that in the first embodiment with use of the obtained corrective measurement values  412 , thus obtaining the corrected material decomposition map  410 - 1 . 
     In the fourth embodiment, the configurations, processing operations, and advantageous effects other than processing in which the map correction unit  404  obtains the corrected material decomposition map  410 - 1  are similar to those in the first to third embodiments and, therefore, the detailed description thereof is omitted. 
     DESCRIPTION OF REFERENCE NUMERALS 
       100 : PCCT apparatus,  101 : subject,  102 : bed,  200 : UI unit,  210 : input device,  220 : output device,  300 : measuring unit,  310 : X-ray radiation unit,  311 : X-ray tube,  312 : X-ray filter,  313 : bowtie filter,  320 : X-ray detection unit,  321 : X-ray detector,  322 : detection element,  330 : gantry,  331 : bore,  332 : rotary plate,  340 : control unit,  341 : radiation controller,  342 : gantry controller,  343 : bed controller,  344 : detection controller,  400 : computing unit,  401 : central processing unit,  402 : memory,  403 : HDD device,  404 : map correction unit,  405 : image capturing unit,  406 : measurement value data correction unit,  407 : image generation unit,  408 : material decomposition unit,  409 : map storage unit,  410 : material decomposition map.