Patent Publication Number: US-11045153-B2

Title: Device for acquiring pulse height spectrum, method for acquiring pulse height spectrum, program for acquiring pulse height spectrum, and radiation imaging apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a pulse height spectrum acquisition device, a pulse height spectrum acquisition method, a pulse height spectrum acquisition program, and a radiation imaging apparatus. More particularly, the present invention relates to the pulse height spectrum acquisition device, the pulse height spectrum acquisition method, the pulse height spectrum acquisition program, and the radiation imaging apparatus, for generating a pulse height spectrum on the basis of a detection result from a photon-counting type X-ray detector. 
     BACKGROUND ART 
     In medical-use radiation imaging apparatus utilizing X-rays, such as computed tomography (CT) scanners, conventionally, a detector detects X-rays emitted from an X-ray source and passing through an object, and acquires information of X-ray attenuation caused by the object, thereby visualizing the internal state of the object to provide it for diagnosis. 
     Currently used typical whole-body CT scanners generate X-rays from an X-ray tube to which high voltage has been applied, and scintillators detect the X-rays passing through the object. The X-rays detected by the scintillators are converted into fluorescence, and optical devices such as a photodiodes read out this fluorescence to output the fluorescence in the form of electrical signals. This detector system is employed in, what is called, a current mode (or an integration mode). In other words, a measured value indicates a total amount of the electrical signals generated during a certain time span, e.g., within a millisecond, but each of X-ray photons is not measured individually. Therefore, for example, detection of one X-ray photon of energy 100 keV and detection of two X-ray photons of energy 50 keV each, may bring about identical measurement results. 
     In recent years, in this type of radiation imaging apparatus, it has become very active to employ the detector system in a pulse mode, instead of the current mode, that is, detecting the X-ray photons passing through the object in the pulse mode, separately one by one, so as to implement more accurate diagnostic equipment. Detection of each X-ray photon separately allows acquisition of energy information of the X-ray photons, which has been unavailable in conventional computed tomography. Therefore, in the field of computed tomography, this is referred to as photon-counting CT (PCCT) or other similar names, as next-generation equipment, and it is expected that such equipment should bring about material decomposition or low exposure that has not been feasible in the conventional computed tomography. 
     When it is attempted to employ the detector in the pulse mode, there may be a major problem that X-ray photons are prone to have extremely high incident rate. In normal whole-body CT scanners, sometimes X-ray photons on the order of 10 9  per second per square millimeter of the detector (10 9  cps/mm 2 ) at the maximum are detected. 
     The typical size of pixel in the detector of a computed tomography scanner is on the order of 1 mm square. For example, assuming that 50 nanoseconds are required for the detector system to process a signal of one X-ray photon, there is a possibility that during signal processing of a particular X-ray photon, tens of signals of other X-ray photons may arrive, or the signal processing may be performed on erroneously recognized signals of two or more X-ray photons, instead of the signal of a single X-ray photon (what is called, pile up). This state indicates that detector is experiencing saturation, and once the detector is saturated, it is not possible to count the X-ray photons properly, or to obtain accurate energy information. 
     To address this problem, X-ray photons may be counted only by determining larger or smaller than thresholds provided in advance, without precisely measuring the energy of the X-ray photons one by one, so as to reduce the signal processing time on the X-ray photons. Specifically, in the PCCT, an output from the X-ray detector that has detected radiation (X-rays) is compared with predetermined thresholds by using comparators, and it is counted how many photons having energy equal to or larger than the energy corresponding to these thresholds are detected per a unit time. 
     As one method to calibrate what energy is associated with the provided threshold, there is a method to allow entrance of X-rays or gamma-rays having characteristic energy, and on the basis of a result of counting in the detector, a pulse height (energy) spectrum of the count rate of X-ray signals per unit time is generated, thereby obtaining relations between the threshold and the energy. By way of example, Patent Literature 1 discloses that following Equation is established, where the number of counts (count rate) at a predetermined threshold pulse height V n  is C(V n ), and the number of counts (count rate) at a predetermined pulse height V n , that is, the pulse height spectrum is c(V n ): 
     [1]
 
 c ( V   n )= C ( V   n−1 )− C ( V   n )  (1)
 
     In other words, the radiation counting equipment disclosed by Patent Literature 1 obtains from a difference between C(V n−1 ) and C(V n ), the number of counts (count rate) c(V n ) at the predetermined pulse height V n , i.e., the pulse height spectrum, as a result of pulse height analysis. That is, a difference of count rates between the thresholds is detected, and the count rates associated with the respective thresholds are calculated, thereby generating the pulse height spectrum. There is disclosed that by detecting a difference between the count rates of a plurality of thresholds, count rates respectively associated with the thresholds, are calculated, and the pulse height spectrum is generated. 
     PRIOR ART DOCUMENT 
     Patent Literature 
     Patent Literature 1 
     Japanese Unexamined Patent Application Publication No. 2015-184119 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the method for generating the pulse height spectrum of the radiation counting equipment as disclosed in Patent Literature 1, the count rates C(V n−1 ) and C(V n ) are obtained by separate measurements, causing that both values may have statistical errors independently. If it is attempted to obtain c (V n ) with high accuracy in generating the pulse height spectrum, in particular when the C(V n−1 ) is nearly equal to C(V n ), it is necessary to continue measurement to obtain the difference c(V n ) until achieving a value of statistical significance, resulting in that long measuring time is needed. On the other hand, as for the radiation detector, calibration is performed on the basis of the pulse height spectrum in some cases, and the time needed for the calibration may lead to cost increase of a system that employs this detector. 
     The present invention has been made in view of the situation above, and an objective of the present invention is to generate a highly accurate pulse height spectrum within a short amount of time, and in addition, the present invention aims at reducing cost of the radiation imaging apparatus employing the detector that performs the calibration by using the pulse height spectrum. 
     Means for Solving the Problems 
     In order to solve the problems described above, the present invention provides the following means. One aspect of the present invention provides a pulse height spectrum acquisition device of a radiation detector including a plurality of counting units for counting a detected signal obtained by detecting incident X-rays, when a value of the detected signal is equal to or larger than a threshold, and for outputting a count rate of each counting unit, the pulse height spectrum acquisition device comprising, a threshold setter configured to set to a first counting unit, a first threshold V1 as a threshold for a first measurement, along with setting to a second counting unit, a second threshold V2 larger than the first threshold V1, and to set a reconfigured first threshold V1′ as the threshold for a second measurement, the reconfigured first threshold being different from the first threshold V1, a measurement controller configured to perform the first measurement and the second measurement, and a pulse height spectrum generator configured to generate a pulse height spectrum for the first threshold V1 of the first counting unit, on the basis of a difference between a count rate from the first counting unit and the second counting unit obtained by the first measurement performed by the measurement controller, and a count rate from the first counting unit and the second counting unit obtained by the second measurement. 
     Advantage of the Invention 
     According to the present invention, the highly accurate pulse height spectrum is generated within a short amount of time, and further cost of the radiation imaging apparatus can be reduced which employs the detector that performs calibration by using the pulse height spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically illustrating a PCCT scanner relating to the first embodiment of the present invention; 
         FIG. 2  is a block diagram showing a schematic configuration of a computer in the PCCT scanner relating to the first embodiment of the present invention; 
         FIG. 3  is a perspective view schematically illustrating an X-ray detector relating to the first embodiment of the present invention; 
         FIG. 4  illustrates an example of the relationship between an array of sensing elements and pixels of the X-ray detector relating to the first embodiment of the present invention; 
         FIG. 5  illustrates an example of the relationship between the array of sensing elements and the pixel of the X-ray detector relating to the first embodiment of the present invention; 
         FIG. 6  is a block diagram showing a schematic configuration of a signal processor of the X-ray detector relating to the first embodiment of the present invention; 
         FIG. 7  is a reference diagram showing an example of a pulse height spectrum that is obtained when gamma rays are incident; 
         FIG. 8  is a flowchart to acquire the pulse height spectrum of the PCCT scanner relating to the first embodiment of the present invention; 
         FIG. 9  is a reference diagram explicitly illustrating energy ranges on the pulse height spectrum of  FIG. 7 ; 
         FIG. 10  is another flowchart to acquire the pulse height spectrum of the PCCT scanner relating to the first embodiment of the present invention; and 
         FIG. 11  is a block diagram showing a schematic configuration of the signal processor of the X-ray detector relating to a second embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     There will now be described one embodiment of the present invention with reference to the accompanying drawings. The pulse height spectrum acquisition device relating to the present invention is to acquire pulse height spectra of a radiation detector having a plurality of counting units, each counting a detected signal of incoming X-rays, when the detected signal being equal to or larger than thresholds, and to output count values. The pulse height spectrum acquisition device relating to the present invention, includes a threshold setter configured to set to a first counting unit, a first threshold V1 as a threshold for a first measurement, along with setting to a second counting unit, a second threshold V2 larger than the first threshold V1, and to set to the first counting unit a reconfigured first threshold V1′ as the threshold for a second measurement, the reconfigured first threshold being different from the first threshold V1, a measurement controller configured to perform the first measurement and the second measurement, and a pulse height spectrum generator configured to generate the pulse height spectrum for the first threshold V1 of the first counting unit, on the basis of a difference between the count value from the first counting unit and the second counting unit obtained by the first measurement performed by the measurement controller, and the count value from the first counting unit and the second counting unit obtained by the second measurement. According to the pulse height spectrum acquisition device with the configuration above, a highly accurate pulse height spectrum can be generated within a short period of time, and further, cost of a radiation imaging apparatus can be reduced, which employs the detector that performs calibration by using the pulse height spectrum. 
     There will be described embodiments of the present invention more specifically in the following. 
     First Embodiment 
     As one example of the radiation imaging apparatus relating to an embodiment of the present invention, an X-ray CT scanner, in particular, a PCCT scanner will be described with reference to the accompanying drawings. 
     As shown in  FIG. 1 , the PCCT scanner is provided with an imaging system including an X-ray source  120 , an X-ray detector  150 , a gantry rotor  110  placing the X-ray source  120  and the X-ray detector  150  in a manner opposed to each other and rotating about a predetermined rotation axis, a table  140 , a controller  170  for processing signals acquired by the X-ray detector  150  along with controlling the operations of the imaging system, and a computer  180  for creating reconstructed images on the basis of the data obtained by the X-ray detector  150 . 
     By way of example, an X-ray tube can be employed as the X-ray source  120 . The X-ray source  120  allows electron beams accelerated by tube voltage to hit a target material such as tungsten and molybdenum, to generate X-rays from the hit position (focal point), thereby emitting X-ray photons. A filter  125  is provided in proximity to the X-ray source  120 . The filter  125  adjusts a flux of the X-ray photons  130  and an energy distribution thereof, which are emitted from the X-ray source  120 . Therefore, after the flux and the energy distribution of the X-ray photons emitted from the X-ray source  120  are adjusted, a part thereof is absorbed by an object  200 , in response to a material distribution within the object, and another part the X-ray photons passes through the object  200  and detected by the X-ray detector  150  that will be described below. 
     On the gantry rotor  110 , the X-ray source  120  and the X-ray detector  150  are placed in a manner opposed to each other, and the gantry rotor rotates about a predetermined rotation axis. There is provided an aperture at the center of the gantry rotor  110  to place the object  200  therein, and a table  140  is arranged in the aperture to lay the object  200  thereon. The table  140  and the gantry rotor  110  are movable in predetermined directions relatively. Generally, CT scanners acquire data from all the directions, and the gantry rotor  110  rotates at a predetermined speed, allowing the X-ray source  120  and the X-ray detector  150  to rotate around the object  200 , so as to acquire data. Typically, the rotation speed is approximately one to four revolutions per second. The time required to accumulate data for acquiring projection data (one view) from one direction is typically on the order of 0.1 to 1 milliseconds. 
     The X-ray detector  150  is provided with a detection unit  151  configured to detect the X-ray photons entering the X-ray detector  150 , and to output detected signals in response to energy of thus detected X-ray photons, and a signal processor  152  configured to collect and process the detected signals output from the detection unit  151 . The detected signals of the X-ray photons output from the detection unit  151  are subjected to pulse-mode processing by more than one signal processors  152 , and then counted. In this example, the term “counting” described here may include a meaning to acquire energy information, in addition to counting the detected X-ray photons. Detection of X-rays having been scattered by the object  200  may generate undesirable signals, and therefore, preferably, a collimator  145  is placed in front of the detection unit  151  when viewed from the X-ray source  120  side, so as to block the scattered X-rays. Details of the X-ray detector  150  will be described below. 
     According to instructions from the computer  180 , the controller  170  controls the gantry rotor  110 , the X-ray source  120 , the table  140 , the X-ray detector  150 , and other components, performs predetermined processing on the signals that are detected and collected by the X-ray detector  150 , and transfers the processed signals to the computer  180 . 
     As shown in  FIG. 2 , the computer  180  is provided with a CPU  181  and a storage unit  182 . Also as shown in  FIG. 2 , the CPU  181  implements functions of the threshold setter  184 , the measurement controller  185 , and the pulse height spectrum generator  186 . The computer  180  stores in the storage unit  182 , the signals acquired via the controller  170  from the signal processor  152  of the X-ray detector  150 , and on the basis of those signals and other data, the computer reconstructs tomographic images of the subject and reconstructed images are created. 
     The threshold setter  184  provides thresholds respectively in the plurality of counting units  351  to  354  (see  FIG. 6 ) held by a channel  165  of a sub-pixel  21  of the detector, which will be described below. Details of the threshold setting according to the threshold setter  184  will be described later. The measurement controller  185  generates control signals to perform measurement (imaging) of the object, further generates control signals to perform necessary measurements to generate a pulse height spectrum, and then outputs those control signals to the controller  170 . The pulse height spectrum generator  186  receives via the controller  170  the outputs from the detector, obtained by the measurement performed based on the control signals for the measurement generated by the measurement controller  185 , and generates the pulse height spectrum on the basis of the outputs thus received. 
     The computer  180  is connected to a display unit  191  and to an input unit  192 , and the reconstructed images created by the CPU  181  functioning as the image generator (not illustrated) are displayed on the display unit  191 , according to the instruction from the CPU  181 . The input unit  192  accepts entries of imaging conditions and other information for the X-ray CT scanner, i.e., parameters necessary for data collection, such as a value of voltage applied to the X-ray source  120  from a high-voltage source (not illustrated) and tube current, and velocity and other values of the rotating operation of the X-ray source  120 . The display unit  191  is capable of displaying the parameters and the values thereof, and soon, input from the input unit  192 . 
     A part or all of the controller  170  and the computer  180  may be constructed as a system containing a memory and a main storage, and functions of the components constituting the controller  170  and the computer  180  can be implemented by the CPU that loads programs into the memory, the programs being stored in advance in the storage unit, and then executing the programs. A part or all of the functions may also be configured by hardware such as ASIC (Application Specific Integrated Circuit) and FPGA (Field Programmable Gate Array). 
     Next, the X-ray detector  150  will be described. The X-ray detector  150  is provided with a detection unit  151  comprising a two-dimensional array of multiple pixels  20  and detecting X-ray photons per pixel to output signals, and a signal processor  152  configured to collect and process the signals output from each of the pixels  20  of the detection unit  151 . 
     As shown in  FIG. 3 , the detection unit  151  is one of the units of the X-ray detector  150 , and comprises an array of multiple pixels  20  for detecting incoming X-ray photons. The X-ray photons  130  passing through the object  200  enter each of the pixels  20 , and the X-ray photons are counted. The number of the pixels included in the detection unit  151  may be 892 in the longitudinal direction and  64  in the transverse direction. 
     As shown in  FIG. 3 , the detection unit  151  is provided in the form of arc, setting the X-ray source  120  as the arc-center approximately, and along with the rotation of the gantry rotor  110 , the detection unit  151  rotates with keeping the positional relation with the X-ray source  120 . It is to be noted that the arrangement of the pixels  20  in the example as shown in  FIG. 3  are depicted in a manner that forms an approximate curved surface. However, in many cases, the pixels may form a flat surface without curvature, and sometimes, the arrangement of the pixels  20  in the detection unit  151  may form a polygonal shape. The X-ray photons passing through the object  200  enter each of the pixels  20 , and they are counted. 
     In order to eliminate the X-ray photons scattered by the object  200 , the collimator  145  (see  FIG. 1 ) is placed on the pixels  20 , on the X-ray source  120  side. The collimator  145  may be a two-dimensional square hole collimator, having the same pitch and shape as the pixels  20 , or it may be one-dimensional slit collimator. 
     In the detection unit  151 , each of the pixels  20  comprises a group of sensing elements referred to as sub-pixels  21 , where a plurality of sensing elements are provided in an array. This group of sensing elements constitutes the pixel  20  serving as one unit of the X-ray detector  150 . In other words, one pixel  20  has a configuration divided into multiple sub-pixels  21 . This configuration allows reduction of the count rate per processing circuit. The sub-pixels  21  included in the pixel  20  are sensing elements in a mode of, what is called, photon counting, and they detect incoming X-ray photons, and count the X-ray photons, with separating them into four energy ranges, for instance, in the signal processor  152  being connected. 
     Therefore, in the X-ray detector  150 , the sub-pixels  21  detect the X-ray photons independently, and signals output from the multiple sensing elements  21  constituting each of the pixels  20 , are added up to generate an output signal per pixel  20 .  FIG. 4  illustrates the pixels  20  where multiple sensing elements  21  each having the same size, are provided in the array with two elements in the channel direction and two elements in the slice direction, that is, four elements in total. In the example of  FIG. 4 , the size of the pixel  20  is one millimeter square and the size of each sub-pixel is 0.5 millimeter square, for instance. Following description of the present embodiment will be given, assuming that the pixel  20  comprises four sub-pixels  21 . 
     In the present embodiment, the size of the pixel  20  is one millimeter square, and each pixel is divided into four sub-pixels with 0.5 millimeter square each. Other division patterns may be available variously, such as nine sub-pixels of 3×3, or 16 sub-pixels of 4×4. Alternatively, the vertical and horizontal directions may have different division numbers, like n×m (n and m are natural numbers). 
     The sub-pixels thus divided are not necessarily the same in size, and the present invention is applicable to the case where the pixel is divided into sub-pixels various in size. It is to be noted that the present invention is applicable to the case where the pixel is not divided into the sub-pixels. In this case, the “sub-pixel” in the following description will be read as “pixel”. 
     For example, as shown in  FIG. 5 , each of the sensing elements  21  constituting each pixel  20  has a structure provided with positive and negative electrodes  41  and  42  in a manner placing a detection layer  40  therebetween, and the signal processor  152  is connected to each of the electrodes  41  and  42 . In the present embodiment, the negative electrode  41  (hereinafter, referred to as “common electrode  41 ”) provided on the entrance surface of the X-ray photons  130  (the upper surface of the detection layer  40  in  FIG. 5 ) serves as the common electrode covering the pixels  20  entirely. In addition, the positive electrode  42  (hereinafter, referred to as “individual electrode  42 ”) is provided for each sensing element  21  serving as the sub-pixel, and individual channels  165  of the signal processor  152  are connected to the individual electrodes  42  respectively. In other words, signals are read per sub-pixel, thereby counting the X-ray photons including acquisition of energy information. 
     As described above, the pixels  20  are provided with one common electrode  41  and the individual electrodes  42  the number of which corresponds to the number of the sub-pixels  21  (sensing elements). In other words, the pixel  20  includes multiple individual electrodes  42  on the surface of the detection layer  40 , and as indicated by broken lines in  FIG. 5 , a region associated with one individual electrode  42  forms one sub-pixel  21 . As illustrated in  FIG. 5 , if a radiation detector material of a direct conversion type is used as the detection layer  40 , boundaries between sub-pixels  21  (see  FIG. 4 ) may be invisible physically when viewed from the top of the pixel  20 , but the detection layer is divided into the sub-pixels, to function as the radiation detector. 
     It is preferable to employ as a material of the detection layer  40 , a compound semiconductor being a radiation detector material of a direct conversion type, which is easy to micro machining and capable of direct reading of electrical signals, such as Cadmium Telluride, Cadmium Zinc Telluride, Thallium Bromide, Mercury Iodide, and Bismuth Iodide. Alternatively, a scintillator (a radiation detector material of a indirect conversion type) optically coupled with an optical device may be used as the detection layer. Preferably, the thickness of the detection layer is around 0.5 mm to 3 mm. 
     The X-ray photons  130  enter the detection layer  40  from the common electrode  41  side, then the X-ray photons being detected, and an electrical charge whose amount is responsive to the energy of thus detected X-ray photons is generated. The high-voltage power source, not illustrated, applies voltage of −600 V to the common electrode  41 , for instance. It is desirable that there should be no attenuation of X-ray photons by the common electrode  41  or by the individual electrodes  42 . The common electrode  41  and the individual electrodes  42  should be sufficiently thinner than the detection layer  40 , and they may be processed to 1 μm or less in thickness. 
     As shown in  FIG. 6 , the signal processor  152  is provided with channels  165  respectively for sub-pixels  21 , and the channels  165  detect output signals from the sub-pixels  21  belonging to the pixel  20 . Then, a signal adder adds up the signals according to predetermined conditions, collects the signals as an output signal from each pixel  20 , and processes thus obtained output signal. 
       FIG. 6  illustrates an example of the channels  165  of the signal processor  152 , being connected to the sub-pixels  21 , respectively. Each of the channels  165  is provided with a charge sensitive pre-amplifier  310  configured to convert the X-ray photons detected as charge signals into voltage signals, a shaping amplifier  320  configured to perform shaping of the voltage signals converted by the charge sensitive pre-amplifier  310 , and four counting units  351  to  354  to obtain energy information by comparing the voltage related to the voltage signal and thresholds (reference voltages). The counting units  351  to  354  are provided with converters  331  to  334 , and counters  341  to  344 , respectively. 
     Signal processing in the channel  165  as thus configured is performed as the following. A signal read out from any of the sub-pixels in the pixel  20 , is initially converted into a voltage signal from a charge signal, by the charge sensitive pre-amplifier  310 , and output to the shaping amplifier  320 . The voltage signal converted by the charge sensitive pre-amplifier  310  is subjected to shaping in the shaping amplifier  320  (hereinafter, the voltage signal after the shaping will be referred to as “detected signal”), and in order to obtain the energy information, signals having a pulse height equal to or larger than thresholds (reference voltages) are counted in the counting units  351  to  354 . 
     In  FIG. 6 , four comparators  331  to  334  are provided to obtain the energy information, and thresholds V1 to V4, different from one another are given to the comparators, respectively. Then, in the comparators  331  to  334 , the detected signal is compared with the thresholds V1 to V4. When the detected signal is larger than the threshold, any of the counters  341  to  344  corresponding to the comparator is incremented. 
     With this configuration, the X-ray photons can be categorized into four types and counted separately, in response to the energy amount. After the elapse of predetermined processing time from the detection of an event by a trigger circuit, not illustrated, the individual channels  165  in the signal processor  152  are reset to become ready for the counting of next event. Upon completion of a predetermined measuring time (one view), incrementing is stopped, and the count rate of each counter is output to the controller  170 . In designing the present embodiment, any number of the comparators, at least two, may be employed. 
     (Calibration in Counting Unit) 
     The thresholds V1 to V4 given to the comparators  331  to  334  respectively are able to supply the same voltage to a plurality of sub-pixels. However, due to the characteristics of the detection layer, or of the individual channels  165  (including the characteristics of the comparators  341  to  344 ) in the signal processing circuit, the same threshold may be associated with a different energy amount, as to each counting unit. In other words, though the same thresholds are provided, the same count results may not be obtained in some cases, as to each of the counting unit (even though impact of statistical errors is excluded). In such a case, it is preferable to perform calibration for each counting unit. 
     In performing calibration, X-rays or gamma-rays having characteristic energy is made to enter, and a pulse height spectrum is obtained in response to each threshold setting. Preferable examples of the X-rays or gamma-rays having characteristic energy may include line gamma-ray or characteristic X-ray, specifically, gamma-rays of 60 keV emitted from  241 Am, gamma-rays of 122 keV emitted from  57 Co, characteristic X-rays of 31 and 35 keV emitted from  133 Ba, and characteristic X-rays of 75 keV and 85 keV emitted from lead irradiated with radiation. In addition to the line gamma-ray or characteristic X-ray, there are examples of the characteristic energy, such as Compton edge or a back scattering peak of such line gamma-ray or characteristic X-ray, and maximum energy of the X-ray photons determined by the tube voltage applied to the X-ray tube. 
     If non-linearity between the detected energy and output pulse height is not ignorable, it is preferable to render the characteristic energy held by the incident X-rays or gamma-rays to fall into the range, as close as possible to the energy range that is targeted for calibration. 
       FIG. 7  shows an example of the pulse height spectrum that is obtained with the entry of gamma-rays from  57 Co. According to the pulse height spectrum as shown in  FIG. 7 , it can be read that the photo peak of 122 keV is associated with the threshold setting to  47 . If it is known that 60 keV is associated with the threshold setting to  23  in another measurement, it is found according to interpolation therebetween, that the threshold can be set to 31 for the counting an event of 80 keV or larger, whereby calibration of the counting units  351  to  354  can be achieved. 
     It is not necessary to acquire the pulse height spectrum of the whole energy range (or the whole threshold range), but it may be acquired as to a range only in proximity to the characteristic energy of incident X-rays or gamma-rays. For example, in the example as described above, even in the case where the pulse height spectrum is acquired only within the range where the threshold setting is from  40  to  55 , it is possible to obtain what value of the threshold setting is associated with the photo peak of 122 keV. 
     (Acquisition of Pulse Height Spectrum) 
     The pulse height spectrum generator  186  generates the pulse height spectrum for each of the counting units  351  to  354 . There will now be described how the pulse height spectrum is generated for the counting unit  351  (hereinafter, referred to as “first counting unit  351 ”). In the present embodiment, when the pulse height spectrum of the first counting unit  351  is acquired, the count rate (count value) of the counting unit  352  (hereinafter, referred to as “second counting unit  352 ”) adjacent to the first counting unit  351  is also utilized. In the present embodiment, there has been described an example where the number of counting units is four, but this number can be changed appropriately. Specifically, the pulse height spectrum is generated and acquired according to the flowchart of  FIG. 8 . 
     In step S 11 , the threshold setter  184  sets the threshold V1 to the first counting unit  351 , and in step S 12 , sets the threshold V2 to the second counting unit  352 . In other words, as the threshold for the measurement (COUNT) A, the threshold setter  184  sets the first threshold V1 to the first counting unit, and sets to the second counting unit, the second threshold V2 that is larger than the first threshold V1. 
     Subsequently in step S 13 , the measurement controller  185  generates a control signal used to perform COUNT A, and outputs the control signal to the controller  170 . The controller  170  controls the gantry rotor  110 , the X-ray source  120 , the table  140 , the X-ray detector  150 , and other components, according to the control signal from the measurement controller  185 , and performs COUNT A. 
     In next step S 14 , the threshold setter  184  reconfigures the first threshold of the first counting unit. That is, the threshold setter  184  performs reconfiguration to change the first threshold V1 of the first counting unit to the first threshold V1′, to be used as the threshold for the measurement (COUNT) B. The threshold setter  184  performs this reconfiguration so that the first threshold V1′ becomes a value smaller than the second threshold V2 and different from the first threshold V1, and leaves the second threshold V2 as it is. In step S 15 , the measurement controller  185  generates a control signal to perform COUNT B, and outputs the control signal to the controller  170 . The controller  170  performs COUNT B according to the control signal from the measurement controller  185 . 
     In step S 16 , the pulse height spectrum generator  186  generates a pulse height spectrum for the first threshold in the first counting unit  351 , on the basis of a difference between the count rate from the first counting unit and the second counting unit obtained by COUNT A, and the count rate from the first counting unit and the second counting unit obtained by COUNT B. 
     Specifically, in the first counting unit  351 , the pulse height spectrum c1 (V1 n ) at the threshold V1 n  is obtained according to the following process. As described above, in steps S 13  and S 15 , the measurement controller  185  employs the threshold V2 m , where V2 m &gt;V1 n  for the second counting unit  352  that is connected to the same sub-pixel to which the first counting unit  351  is connected, and performs COUNT A and B with the following settings:
     COUNT A: (V1, V2)=(V1 n−1 , V2 m )   COUNT B: (V1, V2)=(V1 n , V2 m )   

     Assuming that expected values of the count rate C2 (V2 m ) obtained in the second counting unit  352  should be the same in COUNT A and COUNT B described above, the pulse height spectrum c1 (V1 n ) is obtained according to the following Equation 2. This corresponds to obtaining of the pulse height spectrum that is associated with the energy range  400 , as shown in  FIG. 9 , using a difference between the count rate associated with the energy range  440  and the count rate associated with the energy range  450 . 
     In here, Ctot denotes a total count rate (total number of counts per unit time), and Ctot is the same value irrespective of the counting units  351  to  354 , in each of the channels  165  of one signal processor  152 . In addition, C1 denotes the count rate obtained in the counting unit  351 , and c1 denotes the pulse height spectrum of the counting unit  351 . The count rate is obtained by normalizing the count value by the measuring time. 
     
       
         
           
             
               
                 
                   [ 
                   2 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           c 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
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                             ( 
                             
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                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
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                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   1 
                                   
                                     COUNT 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     A 
                                   
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     V 
                                     ⁢ 
                                     
                                         
                                     
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                                       1 
                                       
                                         n 
                                         - 
                                         1 
                                       
                                     
                                   
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   2 
                                   
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     A 
                                   
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     V 
                                     ⁢ 
                                     
                                         
                                     
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                                   ) 
                                 
                               
                             
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                           - 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           ( 
                           
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 
                                   COUNT 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   B 
                                 
                               
                               ⁢ 
                               
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                                   B 
                                 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
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                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           ( 
                           
                             
                               [ 
                               
                                 
                                   
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                                     ⁢ 
                                     
                                         
                                     
                                   
                                   tot 
                                   
                                     COUNT 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     A 
                                   
                                 
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                                     ⁢ 
                                     
                                         
                                     
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                                         COUNT 
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                                         A 
                                       
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
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                           ( 
                             
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                                   tot 
                                   
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                                   ⁢ 
                                   
                                     ( 
                                     
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                                         1 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     B 
                                   
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
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                                     ⁢ 
                                     
                                         
                                     
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                                       2 
                                       i 
                                     
                                   
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                                   = 
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                               ⁢ 
                               
                                   
                               
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                                 c 
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                                 ⁢ 
                                 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     B 
                                   
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       1 
                                       i 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In step  17 , thus obtained pulse height spectrum is stored in the storage unit  182 . Then, in next step S 18 , it is determined whether the pulse height spectrum covering a targeted range is obtained. If it is determined it has not been obtained yet, the process proceeds with step S 19  to reconfigure the first threshold, and processing from steps S 12  to S 18  is repeated. When it is determined in step S 18  that the pulse height spectrum covering the targeted range is acquired, the process for acquiring the pulse height spectrum is completed. 
     On the other hand, in contrast to the pulse height spectrum thus obtained related to the present embodiment, the pulse height spectrum is obtained according to the following Equation 3 in the conventional method as disclosed in the aforementioned Patent Literature 1. In Equation 3, Ctot denotes the total count rate (the total number of counts per unit time), and the value of Ctot is the same, irrespective of the counting units  351  to  354 , in each of the channels  165  of in one signal processor  152 . This corresponds to obtaining of the pulse height spectrum that is associated with the energy range  400 , as shown in  FIG. 9 , by using a difference between the count rate associated with the energy range  410  and the count rate associated with the energy range  420 . In addition, C1 denotes the count rate obtained in the counting unit  351 , and c1 denotes the pulse height spectrum of the counting unit  351 . The count rate is obtained by normalizing the count value by the measuring time. 
     
       
         
           
             
               
                 
                   [ 
                   3 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           c 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                           ⁢ 
                           
                             ( 
                             
                               V 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 n 
                               
                             
                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               1 
                               
                                 COUNT 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 A 
                               
                             
                             ⁢ 
                             
                               ( 
                               
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                                   1 
                                   
                                     n 
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                               ) 
                             
                           
                           - 
                           
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               1 
                               
                                 COUNT 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 B 
                               
                             
                             ⁢ 
                             
                               ( 
                               
                                 V 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   1 
                                   n 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             [ 
                             
                               
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                 
                                 tot 
                                 
                                   COUNT 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   A 
                                 
                               
                               - 
                               
                                 
                                   ∑ 
                                   
                                     i 
                                     = 
                                     0 
                                   
                                   
                                     n 
                                     - 
                                     1 
                                   
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   c 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     1 
                                     
                                       COUNT 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       A 
                                     
                                   
                                   ⁢ 
                                   
                                     ( 
                                     
                                       V 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         1 
                                         i 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             ] 
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           [ 
                           
                             
                               
                                 C 
                                 ⁢ 
                                 
                                     
                                 
                               
                               tot 
                               
                                 COUNT 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 B 
                               
                             
                             - 
                             
                               
                                 ∑ 
                                 
                                   i 
                                   = 
                                   0 
                                 
                                 n 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 c 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   1 
                                   
                                     COUNT 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     B 
                                   
                                 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     V 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       1 
                                       i 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Next, there will be described that the measuring time can be reduced according to the method of the present embodiment for generating the pulse height spectrum. There will be described an example where the count rate C1(V1 n−1 ) (associated with the energy range  410  in  FIG. 9 ) is 100 cps when the threshold of the counting unit  351  is set to V1 n−1 , the count rate C1(V1 n ) (associated with the energy range  420  in  FIG. 9 ) is 90 cps when this threshold is set to V1 n , and the count rate C2(V2 m ) (associated with the energy range  430  in  FIG. 9 ) is 80 cps when the threshold of the counting unit  352  is set to V2 m . When the measurement is performed for one second, an expected value of the pulse height spectrum c1(V1 n ) for the energy range  400  becomes 10 counts according to both the conventional method and the present embodiment. On the other hand, a degree of accuracy of c1(V1 e ) is calculated as the following, according to the Poisson distribution. 
     [4]
 
√{square root over (100+90)}=13.8 COUNT  Conventional Method
 
√{square root over ((100−80)+(90−80))}=5.5 COUNT  Present Embodiment
 
     According to the calculation above, it was confirmed that the degree of accuracy became 2.5 times larger (13.8/5.5=2.5) in the example of the present embodiment. In general, the accuracy in measurement is proportional to the square of the measuring time, and therefore, it is found that only one-six or less measuring time is required to achieve the same level of accuracy. 
     When the pulse height spectrum is obtained according to the conventional method, one time measurement is performed for each threshold to detect a difference between the count rates of the thresholds. On the other hand, when the method for generating the pulse height spectrum according to the present embodiment is employed, it is necessary to perform two times measurement, COUNT A and COUNT B, for the threshold V1 n  in the counting unit  351 , with setting threshold V2 n  larger than V1 n  to the other counting unit  352 , resulting in that the number of measuring times is doubled at the maximum, in comparison to the conventional method. Therefore, it is desirable to set V2 m  to be a value close to V1 n , making C2(V2 m ) be a large value so that the whole measurement may require less time. Conditions to achieve higher accuracy in the measurement of the present embodiment, even with the doubled measurement, can be provided assuming a situation that doubled measuring time can be consumed for the conventional method, and it is calculated according to the following expression 4. 
     
       
         
           
             
               
                 
                   [ 
                   5 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           
                             
                               
                                 
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                                   × 
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                                   ⁢ 
                                   
                                       
                                   
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                               × 
                               2 
                               ⁢ 
                               
                                   
                               
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                               t 
                             
                           
                         
                       
                     
                     &gt; 
                     
                       
                         
                           
                             
                               ( 
                               
                                 
                                   
                                     
                                       
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                                           ( 
                                           
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                                               1 
                                               
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                                                 - 
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                                         ( 
                                         
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                             × 
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                                         ( 
                                         
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                                         ) 
                                       
                                     
                                   
                                 
                               
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                             × 
                             t 
                           
                         
                       
                       
                         
                           
                             ( 
                             
                               
                                 
                                   
                                     
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                                       ⁢ 
                                       
                                           
                                       
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                                         ( 
                                         
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                                             1 
                                             
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                                       ) 
                                     
                                   
                                 
                               
                             
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                           × 
                           t 
                         
                         - 
                         
                           
                             ( 
                             
                               
                                 
                                   
                                     
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                                       ⁢ 
                                       
                                           
                                       
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                                         ( 
                                         
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                                             1 
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                                     - 
                                   
                                 
                               
                               
                                 
                                   
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                     ⁢ 
                                     
                                       ( 
                                       
                                         V 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         
                                           2 
                                           m 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             ) 
                           
                           × 
                           t 
                         
                       
                     
                   
                   ∴ 
                   
                     
                       C 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                       ⁢ 
                       
                         ( 
                         
                           V 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             2 
                             m 
                           
                         
                         ) 
                       
                     
                     &gt; 
                     
                       
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                           ⁢ 
                           
                             ( 
                             
                               V 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 
                                   n 
                                   - 
                                   1 
                                 
                               
                             
                             ) 
                           
                         
                         + 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                           ⁢ 
                           
                             ( 
                             
                               V 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 1 
                                 n 
                               
                             
                             ) 
                           
                         
                       
                       4 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Assuming that C1(V1 n−1 ) is approximately equal to C1(V1 n ), it is preferable to set V2 m  so that C2 (V2 m ) is equal to or larger than a half of C1(V1 n ) (to set V2 m  to be a value close to V1 n ). 
     The measuring time may be provided to different thresholds independently. Under the condition that the pulse height spectrum is given accuracy being required, necessary measuring time can be obtained in response to the count rates C1(V1) and C2(V2), or in response to a difference therebetween. This is preferable since the total measuring time can be reduced, by setting the measuring time flexibly depending on the values of the thresholds. 
     In obtaining the count rates C(V) by COUNT A and COUNT B, it should be noted that the counting is required to be normalized based on live time (the time obtained by subtracting dead time in measurement system, from the actual measuring time), not based on the actual measuring time. For this purpose, in the case where the same actual measuring time should be provided for COUNT A and COUNT B, it is preferable to carry out the measurement under the same trigger condition, so as to achieve the same dead time. Alternatively, it is preferable to carry out the measurement at a low count rate to the extent that the dead time is ignorable, for example, 1% or less. 
     In reality, the same value of V2 m  is provided to multiple V n  as to which the pulse height spectrum is required to obtain, thereby lessening the number of measurements to reduce the measuring time. For example, with the setting of (V1, V2)=(V1 n−2 , V2 m ), (V1 n−1 , V2 m ), and (V1 n , V2 m ), the measurement can be performed three times. The case is preferable because two pulse height spectra c1(V1 n−1 ) and c1(V1 n ) respectively associated with V1 n−1  and V1 n  can be obtained according to the measurement of three times, and this may prevent increase of the number of measurements. 
     (Acquisition of Pulse Height Spectrum of Other Counting Units) 
     As described above, it is necessary to obtain the pulse height spectrum of each counting unit. In the aforementioned example, there has been described the method for acquiring the pulse height spectrum of the counting unit  351 . According to the same procedures as described in the aforementioned method, the pulse height spectrum can be obtained for other counting units within a short amount of time. In the present embodiment, in order to reduce the measuring time, four counting units are respectively provided with the thresholds V1 to V4 and the measurements are performed, so as to obtain the pulse height spectra. 
     This type of pulse height spectrum can be acquired according to the flowchart of  FIG. 10 , for instance. In step S 21 , the threshold setter  184  sets the thresholds V1 to V4, different from one another, respectively to the first to the fourth counting units  351  to  354 . In next step S 22 , the measurement controller  185  generates a control signal to perform COUNT A, and outputs the control signal to the controller  170 . According to the control signal from the measurement controller  185 , the controller  170  controls the gantry rotor  110 , the X-ray source  120 , the table  140 , the X-ray detector  150 , and other components, and performs COUNT A. 
     Then, in step S 23 , the threshold setter  184  increases the threshold V1 of the first counting unit  351  and the threshold V3 of the third counting unit  353  to reconfigure the thresholds as the next values. According to thus reconfigured thresholds, the measurement controller  185  performs COUNT B in step S 24 . In step S 25 , the pulse height spectra are acquired from the count rate obtained by COUNT A and COUNT B, and the storage unit  182  stores thus acquired pulse height spectra (step S 26 ). It is determined whether or not thus acquired pulse height spectra cover target ranges. On the other hand, when it is determined that such pulse height spectra have not been acquired yet, the process proceeds with step S 28 . When it is determined that the pulse height spectra covering the target ranges have been obtained in step S 27 , the process for acquiring the pulse height spectra is completed. 
     In step S 28 , the threshold V2 of the second counting unit  352  is increased and reconfigured as the next value. At this time, the threshold V4 of the fourth counting unit  354  may also be increased as needed. In next step S 29 , according to the threshold set in step S 28 , COUNT A′ is performed. Then, in step S 30 , the pulse height spectrum generator  186  generates the pulse height spectrum on the basis of the count rate obtained from the latest two measurements, and the storage unit  182  stores the pulse height spectrum generated in step S 31 . In next step S 32 , it is determined whether or not the pulse height spectrum covering the target range is obtained, and if it is determined that such pulse height spectrum has not been acquired, the process proceeds with step S 23 , and the processing from steps S 23  to S 32  is repeated. When it is determined that the pulse height spectra covering the target ranges have been obtained in step S 27  or S 32 , the process for acquiring the pulse height spectra is completed. 
     The relations between the aforementioned thresholds and measurements are summarized, and followings are established between the thresholds V1 to V4 set to the four counting units, and the measurements (COUNTs):
     COUNT A: (V1, V2, V3, V4)=(V1 n−1 , V2 m-1 , V3 p−1 , V4 q−1 )   COUNT B: (V1, V2, V3, V4)=(V1 n , V2 m-1 , V3 p , V4 q−1 )   COUNT A′: (V1, V2, V3, V4)=(V1 n , V2 m , V3 p , V4 q )   COUNT B′: (V1, V2, V3, V4)=(V1 n+1 , V2 m , V3 p+1 , V4 q )   . . .   where V1&lt;V2&lt;V3&lt;V4   

     The pulse height spectra calculation process in steps S 25  and S 30  is performed according to the following Equation 5, from the count rate C each obtained from the measurements (COUNTs) described above. In other words, according to Equation 5, the pulse height spectra of the first counting unit  351  to the third counting unit  353  can be obtained efficiently. As for the counting unit  354 , the pulse height spectrum may be obtained from the measurement using the aforementioned method separately. 
     [6]
 
 c 1( V 1 n )=( C 1 COUNTA ( V 1 n−1 )− C 2 COUNTA ( V 2 m-1 ))−( C 1 COUNTB ( V 1 n )− C 2 COUNTB ( V 2 m-1 ))
 
 c 1( v 1 n+1 )=( C 1 COUNTA′ ( V 1 n )− C 2 COUNTA′ ( V 2 m ))−( C 1 COUNTB′ ( V 1 n+1 )− C 2 COUNTB′ ( V 2 m ))
 
 c 2( V 2 m )=( C 2 COUNTB ( V 2 m-1 )− C 3 COUNTB ( V 3 p ))−( C 2 COUNTA′ ( V 2 m )− C 3 COUNTA′ ( V 3 p ))
 
 c 3( V 3 p )=( C 3 COUNTA ( V 3 p−1 )− C 4 COUNTA ( V 4 q−1 ))−( C 3 COUNTB ( V 3 p )− C 4 COUNTB ( V 4 q−1 ))
 
 c 3( V 3 p+1 )=( C 3 COUNTA′ ( V 3 p )− C 4 COUNTA′ ( V 4 q ))−( C 3 COUNTB′ ( V 3 p+1 )− C 4 COUNTB′ ( V 4 q ))  (5)
 
     The threshold V4 of the counting unit  354  is not necessarily incremented by one, such as V q−1 , V q , and V q+1 . For example, even though the threshold V4 can be a value sufficiently larger value than the value of V3, it is desirable to vary the value of the threshold V4 gradually following the variation of V3 as given by the aforementioned Equation, in light of an objective of the present embodiment, i.e., “to reduce the measuring time by mitigating errors with narrowing the energy range  450 ”. 
     As described so far, the simple configuration is provided as shown in  FIG. 6 , holding less number of parameters, such as four thresholds for four counting units, and generating a highly accurate pulse height spectra in a short amount of time with calibration of the detector, whereby the radiation detector or the radiation imaging apparatus can be implemented at low cost. 
     Second Embodiment 
     Next, there will be described the PCCT scanner according to a second embodiment of the present invention. In the following, components of the PCCT scanner identical to the first embodiment will be labeled the same, and will not be described redundantly. 
     (Operation of Signal Processor) 
     In the second embodiment, the operation of the processing circuit is different from the first embodiment.  FIG. 11  illustrates an example of an individual channel  1165  connected to each sub-pixel of the signal processor  152 . A signal output from the shaping amplifier  320  is counted in response to a result of comparison of the pulse height, larger or smaller than a threshold (reference voltage), in each of the counting units  1351  to  1354 . 
     Specifically, in the counting unit  1351 , there are counted a signal having the pulse height between the threshold V1 and V2, in the counting unit  1352 , a signal having the pulse height between the thresholds V2 and V3, in the counting unit  1353 , a signal having the pulse height between the threshold V3 and V4, and in the counting unit  1354 , a signal having the pulse height equal to or larger than the threshold V4. It is to be noted here that the thresholds are configured in a manner that satisfies V1&lt;V2&lt;V3&lt;V4. 
     For example, when the pulse height is between the threshold V2 and V3, the signal is output from the comparators  331  and  332 , but not from the comparators  333  and  334 . Since there is an entry of the signal in the counter  1341 , coming from both of the comparators  331  and  332 , it can be determined that the original signal does not have the pulse height between the thresholds V1 and V2, and this event is not counted in the counter  1341 . 
     Since the signal only from the comparator  332 , but not from the comparator  333 , enters the counter  1342 , it can be determined that the original signal has the pulse height between the threshold V2 and V3, and this event is counted in the counter  1342 . It is to be noted here in  FIG. 11 , there are provided four comparators  331  to  334  in order to obtain the energy information, but any number of the comparators, at least two, may be used in designing the present embodiment. 
     (Acquisition of Pulse Height Spectrum) 
     Let D1 denote the count rate obtained in the counting unit  1351 , and this value is related to the count rate obtained in the configuration of the first embodiment, as given in the following: 
     [7]
 
 D 1( V 1, V 2)= C 1( V 1)− C 2( V 2)  (6)
 
     Therefore, in order to acquire the pulse height spectrum c1(V1 n ) at the threshold V1 n , the threshold V2 m  satisfying V2 m &gt;V1 n  is employed, similar to the first embodiment, and the measurement (COUNT) is performed with the settings as the following:
     COUNT A: (V1, V2)=(V1 n−1 , V2 m )   COUNT B: (V1, V2)=(V1 n , V2 m ) By using the count rate as obtained by the measurements, the pulse height spectrum c1 (V1 n ) can be obtained by the following Equation, and a similar effect can be achieved:
 
[8]
 
 c 1( V 1 n )= D 1 COUNTA ( V 1 n−1   ,V 2 m )− D 1 COUNTB ( V 1 n   ,V 2 m )  (7)
   

     In addition to the aforementioned embodiments, a difference between the counting results obtained in the two counting units may be calculated and output in the controller  170 , unlike the second embodiment where the outputs from the two comparators are input in one counter, and this enables implementation of the same function as the second embodiment. 
     In the present embodiment, the value of n is made to vary, one by one, to obtain the pulse height spectrum, but modification is possible such as varying the value of n, two by two, depending on how much density is required in obtaining the pulse height spectrum. 
     Further in the present embodiments, the sub-pixels are separated, by providing the common electrode on the upper surface of the radiation detector material of a direct conversion type, with the individual electrodes on the lower side thereof. 
     However, the sub-pixels may be provided with the individual electrodes also on the upper surface, without using the common electrode. Similarly, in a plurality of radiation detectors, the pixels  20  in the radiation detectors adjacent to each other may share the common electrode on the upper surface, or may be provided with the electrodes individually. 
     As a material of the detector, it is also possible to employ a material comprising a scintillator (radiation detector material of an indirect conversion type) optically coupled with an optical device, instead of the radiation detector material of a direct conversion type. In this case, as a method for separating the sub-pixels, the scintillator covered with a light-shielding agent may be provided for each sub-pixel. Alternatively, for one scintillator, a method for producing micro-cracks between sub-pixels by laser may be used to separate the sub-pixels. As the optical device, a photomultiplier tube (PMT), photodiode (PD), avalanche photodiode (APD), silicone photomultiplier (SiPM), and similar elements may be employed. 
     DESCRIPTION OF SYMBOLS 
       20  . . . pixel,  21  . . . sub-pixel,  40  . . . detection layer,  41 ,  42  . . . electrode,  110  . . . gantry rotor,  120  . . . X-ray source,  125  . . . filter,  130  . . . X-ray photon,  140  . . . table,  145  . . . collimator,  150  . . . X-ray detector,  151  . . . detection unit,  152  . . . signal processor,  165  . . . channel,  170  . . . controller,  180  . . . computer,  181  . . . CPU,  182  . . . storage unit,  184  . . . threshold setter,  185  . . . measurement controller,  186  . . . pulse height spectrum generator,  191  . . . display unit,  192  . . . input unit