JP2016067947A - X-ray ct apparatus, image processing apparatus and image processing program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To exactly and easily execute calibration.SOLUTION: An X-ray CT apparatus includes a photon counting type detector, a correction part and a reconstitution part. The photon counting type detector has a plurality of X-ray detection elements which detect X-ray photons applied from an X-ray tube. The correction part corrects detection signals detected by the photon counting type detector for each of the X-ray detection elements on the basis of the gravity center of an X-ray spectrum detected by the photon counting type detector. The reconstitution part reconstitutes a CT image on the basis of the corrected detection signals.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to an X-ray CT apparatus, an image processing apparatus, and an image processing program.

  In recent years, a photon counting type X-ray detector is known as an X-ray detector used in an X-ray CT apparatus. Each X-ray detection element included in the photon counting X-ray detector outputs a detection signal that can count the incident X-ray photons and can measure the energy value (keV) of the X-ray photons. Further, when using a photon counting type detector, calibration for correcting variation in X-ray energy sensitivity of each X-ray detection element is essential.

Japanese Patent Laid-Open No. 11-109040 JP 2006-101926 A JP 2011-85479 A

Hideki Kato and two others, “X-ray photon energy absorption response characteristics of CdZnTe semiconductor radiation detectors”, T.K. IEEE Japan, Vol. 120-C, no. 12, 2000

  The problem to be solved by the present invention is to provide an X-ray CT apparatus, an image processing apparatus, and an image processing program that can perform calibration accurately and simply.

  The X-ray CT apparatus of the embodiment includes a photon counting type detector, a correction unit, and a reconstruction unit. The photon counting detector has a plurality of X-ray detection elements that detect X-ray photons emitted from the X-ray tube. The correction unit corrects the detection signal detected by the photon counting detector for each X-ray detection element based on the center of gravity of the X-ray spectrum detected by the photon counting detector. The reconstruction unit reconstructs a CT image based on the corrected detection signal.

FIG. 1 is a diagram illustrating a configuration example of an X-ray CT apparatus according to the first embodiment. FIG. 2 is a front view of the gantry device according to the first embodiment. FIG. 3 is a diagram for explaining an example of the detector according to the first embodiment. FIG. 4 is a diagram for explaining the calibration processing according to the related art. FIG. 5 is a diagram for explaining a calibration process according to the related art. FIG. 6 is a diagram illustrating a configuration example of the collection unit according to the first embodiment. FIG. 7 is a diagram for explaining the processing operation of the calculation unit according to the first embodiment. FIG. 8 is a diagram for explaining the processing operation of the calculation unit according to the first embodiment. FIG. 9 is a diagram illustrating a configuration example of the collection unit according to the first embodiment. FIG. 10 is a flowchart illustrating a procedure of processing for calculating a correction value by the X-ray CT apparatus according to the first embodiment. FIG. 11 is a flowchart illustrating a procedure of processing for reconstructing a CT image by the X-ray CT apparatus according to the first embodiment. FIG. 12 is a diagram for explaining the processing operation of the X-ray CT apparatus according to the modification of the first embodiment. FIG. 13 is a diagram for explaining an example of the detector according to the second embodiment. FIG. 14 is a diagram illustrating a configuration example of a collection unit according to the second embodiment. FIG. 15 is a diagram illustrating a configuration example of a scan control unit according to the second embodiment. FIG. 16 is a diagram for explaining the processing operation of the movement control unit according to the second embodiment. FIG. 17 is a diagram for explaining the processing operation of the calculation unit according to the second embodiment. FIG. 18 is a flowchart showing a procedure of processing for calculating a correction value by the X-ray CT apparatus according to the second embodiment. FIG. 19 is a diagram illustrating a configuration example of a scan control unit according to the third embodiment. FIG. 20 is a diagram for explaining the processing operation of the spectrum processing unit according to the third embodiment. FIG. 21 is a diagram for explaining the processing operation of the spectrum processing unit according to the third embodiment. FIG. 22 is a diagram (1) for explaining another example of the detector according to the second embodiment. FIG. 23 is a diagram (2) for explaining another example of the detector according to the second embodiment. FIG. 24 is a diagram for explaining the processing operation of the movement control unit according to the modification of the second embodiment.

  Hereinafter, an X-ray CT apparatus, an image processing apparatus, and an image processing program according to embodiments will be described with reference to the drawings.

  An X-ray CT apparatus described in the following embodiment is an apparatus capable of performing photon counting CT. That is, the X-ray CT apparatus described in the following embodiment counts X-rays that have passed through the subject using a photon counting type detector instead of a conventional integral type (current mode measurement type) detector. Thus, the X-ray CT image data having a high SN ratio can be reconstructed.

(First embodiment)
FIG. 1 is a diagram illustrating a configuration example of an X-ray CT apparatus according to the first embodiment. As shown in FIG. 1, the X-ray CT apparatus according to the first embodiment includes a gantry device 10, a couch device 20, and a console device 30.

  The gantry device 10 is a device that irradiates the subject P with X-rays and collects data related to the X-rays transmitted through the subject P. The gantry device 10 includes a high-voltage generator 11, an X-ray tube 12, a detector 13, The collecting unit 14, the rotating frame 15, and the gantry driving unit 16 are included. FIG. 2 is a front view of the gantry device 10 according to the first embodiment.

  As shown in FIG. 2, the rotating frame 15 supports the X-ray tube 12 and the detector 13 so as to face each other with the subject P interposed therebetween, and a circular orbit centering on the subject P by a gantry driving unit 16 described later. An annular frame that rotates at a high speed.

  The X-ray tube 12 is a vacuum tube that irradiates the subject P with an X-ray beam by a high voltage supplied by a high voltage generator 11 described later, and the X-ray beam is applied to the subject P as the rotating frame 15 rotates. Irradiate against.

  The high voltage generator 11 is a device that supplies a high voltage to the X-ray tube 12, and the X-ray tube 12 generates X-rays using the high voltage supplied from the high voltage generator 11. That is, the high voltage generator 11 adjusts the X-ray dose irradiated to the subject P by adjusting the tube voltage and tube current supplied to the X-ray tube 12.

  The gantry driving unit 16 rotates the rotary frame 15 to rotate the X-ray tube 12 and the detector 13 on a circular orbit around the subject P.

  The detector 13 is a photon counting type detector, and includes a plurality of X-ray detection elements (also referred to as “sensors”) for counting light derived from X-rays transmitted through the subject P. For example, the X-ray detection element included in the detector 13 according to the first embodiment is an indirect conversion type surface detector that includes a scintillator and an optical sensor. Here, the optical sensor is, for example, a SiPM (Silicon photomultiplier). Each X-ray detection element of the detector 13 outputs an electrical signal (pulse) corresponding to the incident X-ray photon. The peak value of this electric signal (pulse) has a correlation with the energy value of the X-ray photon. FIG. 3 is a diagram for explaining an example of the detector 13 according to the first embodiment.

  In FIG. 3, the detector 13 shown in FIG. 2 is shown enlarged. FIG. 3 shows a case where the detector 13 is viewed from the Y-axis side. As shown in FIG. 3, the detector 13 has two-dimensionally arranged X-ray detection elements on the surface. For example, a plurality of X-ray detection element arrays arranged in the channel direction (X-axis direction in FIG. 3) are arranged in the body axis direction (Z-axis direction shown in FIG. 3) of the subject P.

  Returning to FIG. 2, the collection unit 14 collects a count result that is a result of a count process using the detection signal of the detector 13. The collection unit 14 counts photons (X-ray photons) derived from X-rays irradiated from the X-ray tube 12 and transmitted through the subject P, and collects the results of discriminating the energies of the counted photons as a count result. . Then, the collection unit 14 transmits the counting result to the console device 30.

  The couch device 20 is a device on which the subject P is placed, and includes a couchtop 22 and a couch driving device 21. The couchtop 22 is a plate on which the subject P is placed, and the couch driving device 21 moves the couchtop 22 in the rotary frame 15 by moving the couchtop 22 in the Z-axis direction.

  For example, the gantry device 10 performs a helical scan that rotates the rotating frame 15 while moving the top plate 22 to scan the subject P in a spiral shape. Alternatively, the gantry device 10 performs a conventional scan in which the subject P is scanned in a circular orbit by rotating the rotating frame 15 while the position of the subject P is fixed after the top plate 22 is moved.

  The console device 30 is a device that accepts an operation of the X-ray CT apparatus by an operator and reconstructs X-ray CT image data using the counting information collected by the gantry device 10. As shown in FIG. 2, the console device 30 includes an input device 31, a display device 32, a scan control unit 33, a preprocessing unit 34, a projection data storage unit 35, an image reconstruction unit 36, and an image storage. Unit 37 and a system control unit 38.

  The input device 31 includes a mouse, a keyboard, and the like that are used by an operator of the X-ray CT apparatus to input various instructions and various settings, and transfers instructions and setting information received from the operator to the system control unit 38. For example, the input device 31 receives a reconstruction condition when reconstructing the X-ray CT image data, an image processing condition for the X-ray CT image data, and the like from the operator. For example, the input device 31 receives an instruction to perform calibration of the X-ray detection element from the operator. Then, the input device 31 instructs the scan control unit 33 to reconstruct X-ray CT image data or perform calibration via the system control unit 38.

  The display device 32 is a monitor that is referred to by the operator, displays X-ray CT image data to the operator under the control of the system control unit 38, and displays various instructions and the like from the operator via the input device 31. A GUI (Graphical User Interface) for receiving various settings and the like is displayed.

  The scan control unit 33 controls the operation of the high voltage generation unit 11, the detector 13, the gantry driving unit 16, the collection unit 14, and the bed driving device 21 under the control of the system control unit 38 described later. The collection processing of the count information in the apparatus 10 is controlled. Further, the scan control unit 33 according to the first embodiment includes a calculation unit 33a. Details of the calculation unit 33a will be described in detail with reference to FIGS.

  The preprocessing unit 34 performs projection processing for each energy discrimination region by performing correction processing such as logarithmic conversion processing, offset correction, sensitivity correction, and beam hardening correction on the count result transmitted from the collection unit 14. Generate.

  The projection data storage unit 35 stores the projection data generated by the preprocessing unit 34. That is, the projection data storage unit 35 stores projection data for reconstructing X-ray CT image data.

  The image reconstruction unit 36 reconstructs a CT image based on the detection signal from the detector 13. That is, the image reconstruction unit 36 reconstructs X-ray CT image data by performing, for example, back projection processing on the projection data stored in the projection data storage unit 35. As the back projection process, for example, a back projection process by an FBP (Filtered Back Projection) method can be cited. Note that the image reconstruction unit 36 may perform reconstruction processing by, for example, a successive approximation method. Further, the image reconstruction unit 36 generates image data by performing various kinds of image processing on the X-ray CT image data. The image reconstruction unit 36 stores the reconstructed X-ray CT image data and image data generated by various image processes in the image storage unit 37.

  Here, the projection data generated from the counting result obtained by the photon counting CT includes information on the energy of X-rays attenuated by passing through the subject P. Therefore, the image reconstruction unit 36 can reconstruct X-ray CT image data of a specific energy component, for example. In addition, the image reconstruction unit 36 can reconstruct X-ray CT image data of each of a plurality of energy components, for example.

  Further, the image reconstruction unit 36 assigns a color tone corresponding to the energy component to each pixel of the X-ray CT image data of each energy component and superimposes a plurality of X-ray CT image data color-coded according to the energy component, for example. Image data can be generated. Further, the image reconstruction unit 36 can generate image data that enables identification of the substance by using, for example, the K absorption edge unique to the substance. Other image data generated by the image reconstruction unit 36 includes monochromatic X-ray image data, density image data, effective atomic number image data, and the like.

  The system control unit 38 performs overall control of the X-ray CT apparatus by controlling operations of the gantry device 10, the couch device 20, and the console device 30. Specifically, the system control unit 38 controls CT scan performed by the gantry device 10 by controlling the scan control unit 33. Further, the system control unit 38 controls the image reconstruction processing and the image generation processing in the console device 30 by controlling the preprocessing unit 34 and the image reconstruction unit 36. In addition, the system control unit 38 controls the display device 32 to display various image data stored in the image storage unit 37.

  The overall configuration of the X-ray CT apparatus according to the first embodiment has been described above. Under such a configuration, the X-ray CT apparatus according to the first embodiment reconstructs X-ray CT image data using a photon counting detector.

  In photon counting CT, the amount of X-rays is measured by counting the number of photons. The more photons per unit time, the stronger the X-rays. In addition, although individual photons have different energies, the photon counting CT can obtain information on energy components of X-rays by measuring photon energy. That is, in the photon counting CT, data collected by irradiating X-rays with one type of tube voltage can be divided into a plurality of energy components and imaged. For example, in photon counting CT, it is possible to obtain image data that enables identification of a substance using the difference in the K absorption edge.

  By the way, in order to accurately count photons by energy using the photon counting detector 13 having the above-described configuration, a calibration process for the energy value of the photons measured by the detector 13 is necessary. . 4 and 5 are diagrams for explaining the calibration processing according to the related art.

  FIG. 4 illustrates a calibration method according to the prior art using an X-ray tube capable of emitting monochromatic X-rays having a specific energy. The upper diagram in FIG. 4 shows the X-ray spectrum of the X-ray tube, and the lower diagram in FIG. 4 shows the X-ray spectrum detected by the X-ray detection element. Here, for example, as shown in the upper diagram of FIG. 4, if the X-ray tube 12 provided in the X-ray CT apparatus can irradiate monochromatic X-rays having a known specific energy, as shown in the lower diagram of FIG. By detecting the peak of the detection signal of the X-ray photon incident on the X-ray detection element, it is possible to obtain the correspondence between the detection signal of the photon and the energy value.

  However, the X-ray tube 12 provided in the X-ray CT apparatus generates continuous X-rays having a normal energy distribution. For this reason, the calibration method using the X-ray tube which can irradiate the monochrome X-ray which has the known specific energy cannot be applied. FIG. 5 illustrates a calibration method according to the prior art using an X-ray tube that emits continuous X-rays having an energy distribution. The upper diagram in FIG. 5 shows the X-ray spectrum of the X-ray tube, and the lower diagram in FIG. 5 shows the X-ray spectra detected by the X-ray detection elements A to C in order from the left. As shown in the upper diagram of FIG. 5, the X-ray tube 12 emits continuous X-rays having an energy distribution. As shown in the lower diagram of FIG. 5, the spectrum of the detection signal of the X-ray photon incident on each X-ray detection element has a wide energy distribution. As described above, even when continuous X-rays are irradiated onto the photon counting X-ray detector, if the energy resolution of the detector 13 is insufficient, the peak of the photon detection signal can be detected with sufficient accuracy. It becomes difficult.

  In nuclear medicine imaging apparatuses, a calibration method is known in which a standard radiation source having a known energy value is arranged on each detection element. This method is used for calibration of a surface detector of an X-ray CT apparatus. When applied to, it becomes a long time work. For this reason, there is a problem that calibration for correcting variations in X-ray energy sensitivity of each X-ray detection element cannot be performed accurately and simply.

  Therefore, the X-ray CT apparatus according to the first embodiment performs photon counting type based on the center of gravity of the X-ray spectrum detected by the photon counting type detector 13 in order to perform calibration accurately and simply. The detection signal detected by the detector 13 is corrected for each X-ray detection element. For example, the X-ray CT apparatus according to the first embodiment generates a detection signal detected by the photon counting detector 13 for each X-ray detection element based on a correction value calculated for each X-ray detection element. to correct. Then, the X-ray CT apparatus according to the first embodiment reconstructs a CT image based on the corrected detection signal. Such a function of the X-ray CT apparatus is realized by the collection unit 14 and the calculation unit 33a. Hereinafter, the collection unit 14 and the calculation unit 33a according to the first embodiment will be described in detail with reference to FIGS. 6 to 8 are diagrams for explaining the processing operation of the calculation unit 33a according to the first embodiment, and FIG. 9 is a diagram illustrating a configuration example of the collection unit 14 according to the first embodiment. is there.

  First, correction value calculation processing by the calculation unit 33a will be described. The calculation unit 33a calculates, for each X-ray detection element, a correction value that matches the reference centroid, which is the centroid of the reference X-ray spectrum, with the centroid of the X-ray spectrum detected by the photon counting detector 13. . In the first embodiment, the value of a calibrated spectrum meter is used as the reference X-ray spectrum.

  Here, the process of calculating the center of gravity of the X-ray spectrum will be described first. In the X-ray spectrum shown in FIG. 6, the horizontal axis indicates the energy position (X) (unit: keV), and the vertical axis indicates the spectrum intensity (S (X)) at the energy position. Here, the “centroid of the X-ray spectrum” indicates a position where the area of the X-ray spectrum is equally divided. In other words, the center of gravity of the X-ray spectrum is an energy position where the area of the X-ray spectrum in the energy band higher than the center of gravity is equal to the area of the X-ray spectrum in the energy band lower than the center of gravity. For example, when the spectrum intensity (S (X)) is the number of X-ray photons, the center of gravity of the X-ray spectrum is the number of X-ray photons in the energy band higher than the center of gravity, and the energy band lower than the center of gravity. This is an energy position where the count number of X-ray photons is the same. Then, the calculation unit 33a, when the energy position is X and the spectrum intensity at the energy position X is S (X), the centroid (C) of the X-ray spectrum is expressed by the following equation “C = ∫X * S (X) dx / ∫S (X) dx ”. For example, in the example illustrated in FIG. 6, the calculation unit 33 a calculates the center of gravity of the X-ray spectrum as an energy position that is 40 keV.

  In FIG. 7, a process of matching the reference centroid that is the centroid of the reference X-ray spectrum and the centroid of the X-ray spectrum detected by the detector 13 will be described. The left diagram in FIG. 7 shows an X-ray spectrum detected by a calibrated spectrum meter, and the right diagram in FIG. 7 shows an X-ray spectrum detected by an X-ray detection element.

  The calculation unit 33a obtains a reference X-ray spectrum from a detection signal detected using a spectrum meter, obtains a reference centroid from the reference X-ray spectrum, and calculates a correction value for each X-ray detection element. For example, a reference X-ray spectrum is obtained by measuring the spectrum of the X-ray tube 12 of the X-ray CT apparatus with a calibrated spectrum meter, and the reference centroid is obtained by obtaining the centroid of the reference X-ray spectrum. Desired. In such a case, a calibrated spectrum meter is arranged on the detector 13. The calibrated spectrum meter measures the X-ray spectrum irradiated by the X-ray tube 12 under a predetermined irradiation condition. For example, the calculation unit 33a acquires a measurement result via the input device 31 and calculates a reference center of gravity. In the example shown in the left diagram of FIG. 7, the calculation unit 33a calculates 46 keV as the reference centroid.

  Subsequently, the detector 13 detects X-rays irradiated from the X-ray tube 12 under the same predetermined irradiation conditions as the X-ray spectrum measurement by the calibrated spectrum meter. Here, the horizontal axis of the X-ray spectrum obtained from the detection signal output from each X-ray detection element of the detector 13 is represented by the peak value of the electric signal (pulse) that is the detection signal. The vertical axis of the X-ray spectrum obtained from the detection signal output from each X-ray detection element of the detector 13 is the count value (Count) of the electric signal (pulse) that is the detection signal. In other words, the count value of the electric signal (pulse) indicates the intensity for each peak value. Then, by plotting the intensity for each peak value, an X-ray spectrum obtained from the detection signal output by each X-ray detection element is obtained as shown in the right diagram of FIG.

  Then, the calculation unit 33a calculates the center of gravity of the X-ray spectrum shown in the right diagram of FIG. For example, in the example shown in the right diagram of FIG. 7, the calculation unit 33a calculates the peak value A1 as the center of gravity of the X-ray spectrum. As shown in the right diagram of FIG. 7, when the energy resolution of the X-ray detection element constituting the detector 13 is low, the data becomes duller than the X-ray spectrum measured by the calibrated spectrum meter. In addition, you may use the count rate (CPS: Count Per Second) of the electrical signal (pulse) which is a detection signal as a value which shows the intensity | strength for every peak value.

  Further, the calculation unit 33a uses the fact that the centroid of the X-ray spectrum measured by the calibrated spectrum meter and the centroid of the X-ray spectrum obtained from the detector 13 correspond to each other to detect at the centroid position. The energy value at the peak value of the vessel 13 is obtained. In the example shown in FIG. 7, the calculation unit 33a associates the peak value A1 with 46 keV.

  The calculation unit 33a performs the same process on each X-ray detection element, and obtains a sensitivity correction value for the same energy in each element. The upper diagram in FIG. 8 shows the X-ray spectra detected by the X-ray detection elements A to C in order from the left. Here, the center of gravity of the X-ray spectrum detected by the X-ray detection element A is the peak value A1, the center of gravity of the X-ray spectrum detected by the X-ray detection element B is the peak value A2, and the X-ray detection element C The center of gravity of the X-ray spectrum detected by is the peak value A3.

  As shown in the upper diagram of FIG. 8, the output waveforms of the respective X-ray detection elements vary. As shown in the lower diagram of FIG. 8, the calculation unit 33a can obtain a correction value of the output value by matching the centers of gravity of the X-ray spectra. For example, when the X-ray detection element B is used as a reference, the calculation unit 33a calculates a correction value for the peak value of the X-ray detection element A as “A1-A2” or “A1 / A2”. Further, when the X-ray detection element B is used as a reference, the calculation unit 33a calculates a correction value for the peak value of the X-ray detection element C as “A3-A2” or “A3 / A2”.

  Next, the collection unit 14 will be described. FIG. 9 is a diagram illustrating a configuration example of the collection unit 14 according to the first embodiment. As illustrated in FIG. 9, the collection unit 14 includes a plurality of collection units 140. The collection unit 140 corresponds to each X-ray detection element. Therefore, as many collection units 140 as the number of X-ray detection elements are provided. Each collection unit 140 includes a charge amplifier 141, a correction value storage unit 142, a waveform shaping circuit 143, a waveform discrimination circuit 144, and a counter 145.

  The correction value storage unit 142 stores a correction value for calibrating the detection signal of each X-ray detection element. The correction value is calculated by a calculation unit 33a included in the scan control unit 33.

  The charge amplifier 141 integrates and amplifies the charge collected in response to the photons incident on the X-ray detection element, and outputs it as a pulse signal of an electric quantity. This pulse signal has a wave height and an area corresponding to the amount of energy of photons.

  The waveform shaping circuit 143 and the scan control unit 33 are connected to the output side of the charge amplifier 141. The charge amplifier 141 switches the output of the pulse signal to either the scan control unit 33 or the waveform shaping circuit 143 in accordance with an instruction from the scan control unit 33. For example, the charge amplifier 141 outputs a pulse signal to the calculation unit 33 a of the scan control unit 33 when performing a process of calculating a correction value for calibration. Thereby, the calculation unit 33a calculates a correction value (see FIGS. 7 and 8). On the other hand, the charge amplifier 141 outputs a pulse signal to the waveform shaping circuit 143 when reconstructing the X-ray CT image data.

  The waveform shaping circuit 143 corrects the detection signal detected by the photon counting detector 13 for each X-ray detection element based on the center of gravity of the X-ray spectrum detected by the photon counting detector 13. For example, the waveform shaping circuit 143 corrects the detection signal detected by the photon counting detector 13 for each X-ray detection element based on the correction value calculated for each X-ray detection element. More specifically, the waveform shaping circuit 143 adjusts the frequency characteristics of the pulse signal output from the charge amplifier 141, and shapes the waveform of the pulse signal by giving a gain and an offset. A waveform discrimination circuit 144 is connected to the output side of the waveform shaping circuit 143. The waveform shaping circuit 143 is also referred to as a “correction unit”.

  The waveform discriminating circuit 144 compares the wave height or area of the response pulse signal to the incident photon with a threshold set in advance corresponding to a plurality of energy bands to be discriminated, and the comparison result with the threshold is compared with the counter 145 at the subsequent stage. The circuit that outputs to

  The counter 145 counts the waveform discrimination result of the response pulse signal for each corresponding energy band, and outputs the photon count result to the preprocessing unit 34 of the console device 30 as digital data.

  Specifically, the counter 145 uses the X-ray photon incident position (detection position) obtained by discriminating and counting each pulse output from the X-ray detection element and the energy value of the X-ray photon as a counting result. Collected for each phase of the tube 12 (tube phase). For example, the counter 145 sets the position of the X-ray detection element that outputs the pulses used for counting as the incident position.

  For example, the counting result collected by the counter 145 is “in the tube phase“ α1 ”, the photon count value of the energy discrimination region“ E1 <E ≦ E2 ”is“ N1 in the X-ray detection element at the incident position “P11” ”. ", And the photon count value in the energy discrimination area" E2 <E≤E3 "is" N2 "". Alternatively, the counting result collected by the counter 145 is as follows: “In the tube phase“ α1 ”, in the X-ray detection element at the incident position“ P11 ”, the total per unit time of the photons in the energy discrimination area“ E1 <E ≦ E2 ”” The numerical value is “n1”, and the count value per unit time of photons in the energy discrimination region “E2 <E ≦ E3” is “n2” ”.

  Thus, from the X-ray detection element corresponding to one pixel of the detector 13, count results corresponding to a plurality of energy bands are output to the preprocessing unit 34 as X-ray detection data.

  FIG. 10 is a flowchart illustrating a procedure of processing for calculating a correction value by the X-ray CT apparatus according to the first embodiment. The process of calculating the correction value is performed, for example, at the time of factory shipment or during regular maintenance inspection. Here, a calibrated spectrum meter is arranged on the detector 13. As shown in FIG. 10, the scan control unit 33 controls the X-ray tube 12 to emit X-rays (step S101). Then, the calculation unit 33a calculates a reference X-ray spectrum (step S102). For example, the calculation unit 33a calculates a reference X-ray spectrum from a detection signal detected using a calibrated spectrum meter.

  Subsequently, the calculation unit 33a calculates a reference centroid (step S103). For example, the calculation unit 33a acquires a measurement result obtained by a calibrated spectrum meter, and calculates a reference centroid that is a centroid of a reference X-ray spectrum.

  After the calibrated spectrum meter is removed from the detector 13, the scan control unit 33 controls the X-ray tube 12 to emit X-rays (Step S104). Thereby, a photon is detected by each X-ray detection element of the detector 13 (step S105).

  Subsequently, the calculation unit 33a calculates the center of gravity of the X-ray spectrum from the output from the detector 13 (step S106). For example, the calculation unit 33a uses the pulse signal output from the charge amplifier 141 to calculate the center of gravity of the X-ray spectrum detected by the X-ray detection element. Then, the calculation unit 33a calculates a correction value for each X-ray detection element (step S107). The calculation unit 33a stores the correction value calculated for each X-ray detection element in the correction value storage unit 142 of the collection unit 140 corresponding to the X-ray detection element.

  FIG. 11 is a flowchart illustrating a procedure of processing for reconstructing an X-ray CT image by the X-ray CT apparatus according to the first embodiment. As shown in FIG. 11, the scan control unit 33 controls the X-ray tube 12 to emit X-rays (step S201). Thereby, a photon is detected by each X-ray detection element of the detector 13 (step S202). The waveform shaping circuit 143 of the collection unit 14 corrects the detection signal detected by the photon counting detector 13 for each X-ray detection element based on the correction value (step S203).

  The image reconstruction unit 36 reconstructs X-ray CT image data based on the corrected detection signal (step S204). Then, the system control unit 38 displays the reconstructed X-ray CT image on the display device 32 (step S205).

  As described above, according to the first embodiment, the amount of calculation is small and the statistical error is small by performing calibration using the centroid of the X-ray spectrum. That is, the calibration method according to the first embodiment is a calibration method having both simplicity and accuracy.

  For this reason, even when the area of the detector 13 is large and the number of X-ray detection elements is large, the calibration can be easily realized. Therefore, compared with the calibration method according to the conventional technique, the number of calibration work steps is significantly increased. It becomes possible to reduce.

  The detection detected by the detector 13 based on a correction value for matching the reference centroid that is the centroid of the reference X-ray spectrum and the centroid of the X-ray spectrum detected by the photon counting detector 13. The signal is corrected for each X-ray detection element. As a result, calibration can be easily realized even with an X-ray detection element formed of SiPM and scintillator having a lower resolution than the direct conversion type detection element.

  Further, according to the first embodiment, calibration can be performed by using a calibrated spectrum meter in a standard configuration of an existing X-ray CT apparatus. For this reason, periodic calibration can be easily performed, so that stable and accurate X-ray CT image data can always be generated.

  In addition, although the process which calculates a correction value was demonstrated as what is implemented at the time of factory shipment, a regular maintenance inspection, etc., embodiment is not limited to this. For example, the X-ray CT apparatus may perform a process of calculating a correction value every time an X-ray CT image is reconstructed. In such a case, the X-ray CT apparatus performs the process of reconstructing the X-ray CT image shown in FIG. 11 after performing the process of calculating the correction value shown in FIG.

  The calculation unit 33a may detect the X-ray signal by the X-ray detection element of the detector 13 a plurality of times to minimize the measurement error of the center of gravity of the X-ray spectrum. Thereby, the calculation part 33a can raise the precision of the correction value calculated for every X-ray detection element more.

(Modification of the first embodiment)
In the first embodiment described above, the centroid of the X-ray spectrum detected by the detector 13 is obtained by measuring the X-ray spectrum irradiated by the X-ray tube 12 under a predetermined irradiation condition with a calibrated spectrum meter. However, the embodiment is not limited to this. For example, the sensitivity of each X-ray detection element varies depending on the maximum energy generated according to the tube voltage supplied to the X-ray tube 12. Therefore, in order to perform more accurate measurement, it is preferable to perform calibration for each tube voltage used at the time of photographing.

  Therefore, in the modification of the first embodiment, calibration according to the tube voltage of the X-ray tube 12 is performed, and calibration according to each tube voltage of the X-ray tube 12 is performed for each X-ray detection element. A case where data is calculated will be described. FIG. 12 is a diagram for explaining the processing operation of the X-ray CT apparatus according to the modification of the first embodiment.

  FIG. 12 shows the case where the tube voltage of the X-ray tube 12 is 60 kV, 80 kV, and 100 kV. The upper left figure in FIG. 12 shows the X-ray spectrum measured with a calibrated spectrum meter when the tube voltage is 60 kV, and the upper right figure in FIG. 12 shows the detection when the tube voltage is 60 kV. 7 shows an X-ray spectrum detected by the X-ray detection element of the vessel 13. Further, as shown in the upper left diagram of FIG. 12, when the tube voltage is 60 kV, the maximum energy of the X-ray spectrum measured by the calibrated spectrum meter is 60 keV. In such a case, the calculation unit 33a calculates the reference centroid as an energy position that is 32 keV. Then, the calculation unit 33a associates the peak value of the center of gravity of the X-ray spectrum detected by the X-ray detection element of the detector 13 with the reference center of gravity.

  The middle left figure of FIG. 12 shows the X-ray spectrum measured by a calibrated spectrum meter when the tube voltage is 80 kV, and the middle right figure of FIG. 12 shows the detection when the tube voltage is 80 kV. 7 shows an X-ray spectrum detected by the X-ray detection element of the vessel 13. Also, as shown in the left diagram in the middle of FIG. 12, when the tube voltage is 80 kV, the maximum energy of the X-ray spectrum measured by the calibrated spectrum meter is 80 keV. In such a case, the calculation unit 33a calculates the reference gravity center as an energy position that is 40 keV. Then, the calculation unit 33a associates the peak value of the center of gravity of the X-ray spectrum detected by the X-ray detection element of the detector 13 with the reference center of gravity.

  12 shows the X-ray spectrum measured with a calibrated spectrum meter when the tube voltage is 100 kV, and the lower right diagram in FIG. 12 shows the tube voltage when the tube voltage is 100 kV. The X-ray spectrum detected by the X-ray detection element of the detector 13 is shown. Further, as shown in the lower left diagram of FIG. 12, when the tube voltage is 100 kV, the maximum energy of the X-ray spectrum measured by the calibrated spectrum meter is 100 keV. In such a case, the calculation unit 33a calculates the reference centroid as an energy position that is 43 keV. Then, the calculation unit 33a associates the peak value of the center of gravity of the X-ray spectrum detected by the X-ray detection element of the detector 13 with the reference center of gravity.

  Note that the calculation unit 33a may detect the X-ray signal by the X-ray detection element of the detector 13 a plurality of times at each tube voltage to minimize the measurement error of the center of gravity of the X-ray spectrum. Thereby, the calculation part 33a can raise the precision of the correction value calculated for every X-ray detection element more.

(Second Embodiment)
In the first embodiment, the X-ray spectrum serving as a reference is measured using a calibrated spectrum meter. By the way, instead of using an individual measuring instrument such as a calibrated spectrum meter, a reference X-ray spectrum is detected using a reference detection element having an energy resolution higher than that of the X-ray detection element of the detector 13, Each X-ray detection element may be calibrated. For this reason, in the second embodiment, a case where a reference detection element with high energy resolution is used in combination in the detector 13 will be described.

  The configuration of the X-ray CT apparatus according to the second embodiment is different except that the configuration of the X-ray detection element included in the detector 13 and the configuration of the collection unit 14 are different, and the configuration of the scan control unit 33 is different. The configuration is the same as that of the X-ray CT apparatus shown in FIG. Therefore, hereinafter, only the configuration of the detector 13 according to the second embodiment, the configuration of the collection unit 14 according to the second embodiment, and the configuration of the scan control unit 33 according to the second embodiment will be described. . In the X-ray CT apparatus according to the second embodiment, it is assumed that only the detector 13 can be moved on the rotating frame 15.

  FIG. 13 is a diagram for explaining an example of the detector 13 according to the second embodiment. FIG. 13 shows a case where the detector 13 is viewed from the Y-axis side. In the detector 13, X-ray detection elements are two-dimensionally arranged on the surface. For example, a plurality of X-ray detection element arrays arranged in the channel direction (X-axis direction in FIG. 13) are arranged along the body axis direction (Z-axis direction shown in FIG. 13) of the subject P. In the example of FIG. 13, a plurality of X-ray detection elements arranged in the body axis direction are shown as one X-ray detection element group.

  As shown in FIG. 13, the detector 13 according to the second embodiment has one X-ray detection element group 13a and a plurality of X-ray detection element groups 13b. In the example shown in FIG. 13, the X-ray detection element group 13 a is arranged at one end of the detector 13 in the channel direction. Each X-ray detection element of the X-ray detection element group 13b is an indirect conversion type detector composed of a scintillator and an optical sensor. Here, the optical sensor is, for example, SiPM. In addition, each X-ray detection element of the X-ray detection element group 13a is a reference detection element having a higher energy resolution than each X-ray detection element of the X-ray detection element group 13b. Each X-ray detection element of the X-ray detection element group 13a can be constituted by, for example, a cadmium telluride (CdTe: cadmium telluride) semiconductor or a cadmium zinc telluride (CdZnTe: cadmium zinc telluride) semiconductor. Detector.

  FIG. 14 is a diagram illustrating a configuration example of the collection unit 14 according to the second embodiment. As illustrated in FIG. 14, the collection unit 14 includes a collection unit 140 and a collection unit 150.

  The collection unit 140 corresponds to each X-ray detection element of the X-ray detection element group 13b. Therefore, as many collection units 140 as the number of X-ray detection elements in the X-ray detection element group 13b are provided. Each collection unit 140 includes a charge amplifier 141, a correction value storage unit 142, a waveform shaping circuit 143, a waveform discrimination circuit 144, and a counter 145. The functions of the units included in the collection unit 140 according to the second embodiment are the same as the functions of the units included in the collection unit 140 according to the first embodiment.

  The collection unit 150 corresponds to each X-ray detection element of the X-ray detection element group 13a. Therefore, as many collection units 150 as the number of X-ray detection elements in the X-ray detection element group 13a are provided. Each collection unit 150 includes a charge amplifier 151, a waveform shaping circuit 153, a waveform discrimination circuit 154, and a counter 155.

  The charge amplifier 151 integrates and amplifies the charge collected in response to photons incident on each X-ray detection element of the X-ray detection element group 13a, and outputs it as a pulse signal of an electric quantity. A waveform shaping circuit 153 and a scan control unit 33 are connected to the output side of the charge amplifier 151. The charge amplifier 151 outputs a pulse signal to the calculation unit 33 a of the scan control unit 33 when performing the process of calculating the correction value. Thereby, the calculation unit 33a calculates a correction value. On the other hand, the charge amplifier 151 outputs a pulse signal to the waveform shaping circuit 153 when a pixel corresponding to a reference detection element arranged at one end in the channel direction is used for reconstruction of an X-ray CT image.

  The waveform shaping circuit 153 adjusts the frequency characteristics of the pulse signal output from the charge amplifier 151 and shapes the waveform of the pulse signal by giving a gain and an offset.

  The waveform discrimination circuit 154 compares the wave height or area of the response pulse signal to the incident photon with a threshold value set in advance corresponding to a plurality of energy bands to be discriminated, and compares the result of comparison with the threshold value to the counter 155 at the subsequent stage. The circuit that outputs to

  The counter 155 counts the waveform discrimination result of the response pulse signal for each corresponding energy band, and outputs the photon count result to the preprocessing unit 34 of the console device 30 as digital data.

  FIG. 15 is a diagram illustrating a configuration example of the scan control unit 33 according to the second embodiment. As illustrated in FIG. 15, the scan control unit 33 according to the second embodiment includes a calculation unit 33a and a movement control unit 33b.

  The movement control unit 33 b controls the movement of the detector 13 in the channel direction independently of the X-ray tube 12. In other words, the movement control unit 33b moves only the detector 13 on the rotating frame 15 in the channel direction. FIG. 16 is a diagram for explaining the processing operation of the movement control unit 33b according to the second embodiment. 16 shows a case where the reference detection element is positioned at a position facing the X-ray tube 12. In this state, the X-ray tube 12 emits X-rays, and the reference detection element detects the X-ray spectrum. That is, the movement control unit 33b positions the reference detection element at a position facing the X-ray tube 12 when obtaining the reference center of gravity.

  Subsequently, the movement control unit 33b moves the detector 13 in the channel direction. As a result, the X-ray detection element of the X-ray detection element group 13b arranged next to the reference detection element is positioned at a position facing the X-ray tube 12 as shown in FIG. In this state, the X-ray tube 12 emits X-rays, and the X-ray detection elements of the X-ray detection element group 13b detect the X-ray spectrum. That is, when the movement control unit 33b obtains the center of gravity of the X-ray spectrum from the detection signal detected by the correction target X-ray detection element, the movement control unit 33b faces the correction target X-ray detection element to the X-ray tube 12. Position.

  Similarly, the movement control unit 33b moves the detector 13 in the channel direction. As a result, the X-ray detection elements of the X-ray detection element group 13b arranged next to the reference detection elements are positioned at positions facing the X-ray tube 12 as shown in the right diagram of FIG. In this state, the X-ray tube 12 emits X-rays, and the X-ray detection elements of the X-ray detection element group 13b detect the X-ray spectrum. That is, when the movement control unit 33b obtains the centroid of the X-ray spectrum from the detection signal detected by the X-ray detection element to be corrected, as in the case shown in FIG. Is positioned at a position facing the X-ray tube 12. Thus, the movement control unit 33b moves the position of the detector 13 in the channel direction without moving the position of the X-ray tube 12. Then, the movement control unit 33b detects the X-ray detection when obtaining the center of gravity of the X-ray spectrum from the position of the reference detection element in the channel direction when obtaining the reference center of gravity and the detection signal detected by the X-ray detection element to be corrected. Control is performed so that the position of the element in the channel direction matches.

  The calculation unit 33a according to the second embodiment obtains a reference X-ray spectrum from a detection signal detected using the reference detection element, and obtains a reference centroid based on a characteristic X-ray of the reference X-ray spectrum. Thus, a correction value is calculated for each X-ray detection element. FIG. 17 is a diagram for explaining the processing operation of the calculation unit 33a according to the second embodiment.

  The left figure of FIG. 17 shows the X-ray spectrum measured by the reference detection element, and the right figure of FIG. 17 shows the X-ray spectrum detected by the X-ray detection element of the X-ray detection element group 13b. Note that the X-ray spectrum measured by the reference detection element is indicated by a peak value and a count. In other words, the X-ray spectrum measured by the reference detection element is not measured as an energy value. For this reason, the calculation unit 33a according to the second embodiment associates the peak value of the X-ray spectrum measured by the reference detection element with the energy value.

  Here, it is assumed that the characteristic X-ray energy value of the X-ray tube 12 is known. For example, the calculation unit 33a identifies the characteristic X-ray of the reference X-ray spectrum measured by the reference detection element, and associates the peak value of the identified characteristic X-ray with the energy value. Here, a case where the characteristic X-ray of the X-ray spectrum is 60 keV is shown. Further, the calculation unit 33a obtains a reference centroid that is a centroid of a reference X-ray spectrum. And the calculation part 33a calculates the energy value corresponding to the peak value of a reference | standard gravity center based on the energy value of the specified characteristic X-ray. In the example illustrated in the left diagram of FIG. 17, the calculation unit 33a calculates 46 keV as the reference centroid.

  Subsequently, the calculation unit 33a calculates the center of gravity of the X-ray spectrum detected by the X-ray detection element of the X-ray detection element group 13b. In the example shown in the right diagram of FIG. 17, the calculation unit 33a calculates the peak value A1 as the center of gravity of the X-ray spectrum. Then, the calculation unit 33a associates the peak value A1 with 46 keV.

  The processing procedure for reconstructing an X-ray CT image by the X-ray CT apparatus according to the second embodiment is the same as the processing procedure shown in FIG. 10 except that the details of the processing for calculating the correction value in step S102 are different. It is the same. For this reason, only the procedure of the process of calculating the correction value by the X-ray CT apparatus according to the second embodiment will be described below with reference to FIG.

  FIG. 18 is a flowchart showing a procedure of processing for calculating a correction value by the X-ray CT apparatus according to the second embodiment. This process corresponds to the process in step S102 shown in FIG. In the example of FIG. 18, a case will be described in which the X-ray detection element with high energy resolution is positioned at a position facing the X-ray tube 12 at the start of the process of calculating the correction value.

  As shown in FIG. 18, the scan control unit 33 controls the X-ray tube 12 to emit X-rays (step S301). Then, the calculation unit 33a calculates a reference X-ray spectrum (step S302). For example, the calculation unit 33a calculates a reference X-ray spectrum from a detection signal detected using an X-ray detection element with high energy resolution.

  Subsequently, the calculation unit 33a identifies characteristic X-rays of the reference X-ray spectrum (step S303). For example, the calculation unit 33a acquires a measurement result by an X-ray detection element with high energy resolution, and associates the peak value of the characteristic X-ray with the known energy value of the characteristic X-ray. Further, the calculation unit 33a calculates a reference center of gravity (step S304). For example, the calculation unit 33a acquires a measurement result by an X-ray detection element with high energy resolution, and calculates a reference centroid. Then, the calculation unit 33a calculates the energy value of the reference centroid based on the identified characteristic X-ray energy value (step S305).

  Subsequently, the movement control unit 33b moves the detector 13 by one detection element group (step S306). Then, the scan control unit 33 controls the X-ray tube 12 to emit X-rays (Step S307). Thereby, a photon is detected by the X-ray detection element located in the position facing the X-ray tube 12 among the X-ray detection elements of the detector 13 (step S308).

  Subsequently, the calculation unit 33a calculates the center of gravity of the X-ray spectrum from the output from the X-ray detection element positioned at the irradiation position among the X-ray detection elements of the detector 13 (step S309). For example, the calculation unit 33a calculates the center of gravity of the X-ray spectrum detected by the X-ray detection element using the pulse signal output from the charge amplifier 141 corresponding to the X-ray detection element positioned at the irradiation position. . Then, the calculation unit 33a calculates the correction value of the X-ray detection element positioned at the irradiation position (step S310). The calculation unit 33a stores the correction value calculated for the X-ray detection element positioned at the irradiation position in the correction value storage unit 142 of the collection unit 140 corresponding to the X-ray detection element.

  Subsequently, the scan control unit 33 determines whether there is an X-ray detection element whose correction value has not been calculated (step S311). Here, when the scan control unit 33 determines that there is an X-ray detection element whose correction value has not been calculated (step S311, Yes), the scan control unit 33 proceeds to step S306. Thereby, the movement control unit 33b moves the detector 13 by one detection element group, and calculates the correction value of the X-ray detection element positioned at the irradiation position. On the other hand, if the scan control unit 33 does not determine that there is an X-ray detection element whose correction value has not been calculated (No in step S311), the scan control unit 33 ends the process of calculating the correction value.

  As described above, according to the second embodiment, the correction value for matching the reference centroid, which is the centroid of the reference X-ray spectrum, with the centroid of the X-ray spectrum detected by the photon counting detector 13. Based on the above, the detection signal detected by the detector 13 is corrected for each X-ray detection element. Further, calibration is performed using the center of gravity of the X-ray spectrum, so that the calculation amount is small and the statistical error is small. That is, the calibration method according to the second embodiment is a calibration method having both simplicity and accuracy.

  Here, there is an X-ray detection element with high energy resolution such as CdTe, but it is expensive and difficult to mass-produce. For this reason, CdTe has not become widespread until it is used in a large-scale surface detector. On the other hand, reconstruction of an actual X-ray CT image may not require as much energy resolution as CdTe. However, it was necessary to perform calibration properly even with an X-ray detection element having an energy resolution lower than that of CdTe. In the second embodiment, a reference detection element having a high energy resolution is combined in the detector 13, and the detection detected by the detector 13 based on the center of gravity of the X-ray spectrum measured by the reference detection element having a high energy resolution. The signal is corrected for each X-ray detection element. In the second embodiment, the X-ray CT image is reconstructed by correcting the detection signal detected by the X-ray detection element formed of inexpensive SiPM and scintillator for each X-ray detection element. As a result, according to the second embodiment, it is possible to achieve both reduction in cost and collection of accurate image data.

  In the second embodiment, the case where the detector 13 includes the reference detection element has been described. However, the embodiment is not limited to this. For example, the reference detection element may be arranged at a place other than the detector 13 as long as the X-ray spectrum irradiated by the X-ray tube 12 can be measured. In such a case, the calculation unit 33a calibrates the detection signal of the reference detection element to the detection signal when the reference detection element is positioned at a position facing the X-ray tube 12, and the X-ray Calculate the center of gravity of the spectrum.

  In the second embodiment, the reference detection element is described as being disposed at one end in the channel direction in the detector 13, but the embodiment is not limited to this. For example, the reference detection element may be arranged at an arbitrary position such as the center in the channel direction in the detector 13. When the reference detection element is arranged at one end in the channel direction in the detector 13, the detection signal from the reference detection element may not be used for the reconstruction of the X-ray CT image.

  FIG. 22 is a diagram (1) for explaining another example of the detector according to the second embodiment, and FIG. 23 is for explaining another example of the detector according to the second embodiment. (2) of FIG. In the example shown in FIG. 22, the X-ray detection element group 13 a is arranged at both ends in the channel direction in the detector 13. In the example shown in FIG. 22, the X-ray detection element groups 13 a are arranged at both ends of the detector 13 in the channel direction and at the center of the detector 13 in the channel direction.

(Modification of the second embodiment)
In the above-described embodiment, the case where the reference detection element is positioned at the position facing the X-ray tube 12 for calibration has been described. However, the embodiment is not limited to this. For example, in the X-ray detector 13, when a position facing the X-ray tube 12 is defined as a central part, and a position around the central part is defined as a peripheral part, the quality of X-rays detected at the central part And the quality of X-rays detected in the periphery are different. Further, the X-ray quality detected at each position in the peripheral portion is different. In other words, the quality of the detected X-ray differs depending on the relative positional relationship between the X-ray tube 12 and each X-ray detection element.

  For this reason, it is desirable to perform calibration while holding the relative positions of the X-ray detection elements of the X-ray detection element group 13b and the X-ray tube 12. Therefore, as a modification of the second embodiment, a case will be described in which calibration is performed while holding the relative positions of the X-ray detection elements and the X-ray tube 12. The configuration of the X-ray CT apparatus according to the modification of the second embodiment is the same as the configuration of the X-ray CT apparatus according to the second embodiment, except that some functions of the movement control unit 33b are different. It is the same. FIG. 24 is a diagram for explaining the processing operation of the movement control unit 33b according to the modification of the second embodiment.

FIG. 24 illustrates a case where a reference detection element is disposed at the left end as one end in the channel direction, similarly to the detector 13 illustrated in FIG. 13. Further, in FIG. 24, the case of calibrating the X-ray detector elements D _T disposed to the right of the channel direction of the reference detector element D _R. In the 24 left, the X-ray detecting elements D _C arranged at the center of the detector 13 are positioned at a position facing the X-ray tube 12. The state shown in the left diagram of FIG.

  The movement control unit 33 b controls the movement of the detector 13 in the channel direction independently of the X-ray tube 12. Then, the movement control unit 33b detects the X-ray detection when obtaining the center of gravity of the X-ray spectrum from the position of the reference detection element in the channel direction when obtaining the reference center of gravity and the detection signal detected by the X-ray detection element to be corrected. Control is performed so that the position of the element in the channel direction matches.

For example, the movement control unit 33b moves the detector 13 to the right in the channel direction by a distance corresponding to the 1 X-ray detection element. Here, the movement control unit 33b moves the position of the detector 13 in the channel direction without moving the position of the X-ray tube 12 as in the second embodiment. Accordingly, the reference detector element D _R is the X-ray detector elements D _T moves to a position located before the move. The X-ray tube 12 in this state is irradiated with X-rays, the reference detector element D _R detects the X-ray spectrum. That is, the movement control unit 33b determines the X-ray detection element _DT and the X-ray tube 12 to be corrected in a state where the center of the photon counting detector 13 faces the X-ray tube 12 when obtaining the reference center of gravity. in a position matching the relative position of the position the reference detector element D _R.

Subsequently, the movement control unit 33b moves the detector 13 to the left in the channel direction by a distance corresponding to the 1 X-ray detection element. Also in this case, the movement control unit 33b moves the position of the detector 13 in the channel direction without moving the position of the X-ray tube 12 as in the second embodiment. Thereby, the X-ray detection element _DT is positioned at the same position as the initial state. In this state, the X-ray tube 12 emits X-rays, and the X-ray detection element _DT detects the X-ray spectrum. That is, when the movement control unit 33 b obtains the center of gravity of the X-ray spectrum from the detection signal detected by the X-ray detection element _{DT to} be corrected, the center of the photon counting detector 13 faces the X-ray tube 12. Position to position.

The calculation unit 33a according to the second embodiment determines the X-ray spectrum as a reference from the detected detection signal by using a reference detector element D _R, based on the characteristic X-rays of the X-ray spectrum as a reference A reference center of gravity is obtained, and a correction value of the X-ray detection element _DT is calculated. As a result, in the modification of the second embodiment, it is possible to perform calibration while holding the relative positions of the X-ray detection elements of the X-ray detection element group 13b and the X-ray tube 12.

In the modification of the second embodiment described above, by moving the reference detector element D _R is irradiated with X-rays at a position X-ray detector elements D _T of the calibration target is placed in the initial state, thereafter, Although it has been described that the X-ray detection element _DT to be calibrated is irradiated with X-rays after returning to the initial state, the embodiment is not limited to this. That holds the relative position between the calibration target of the X-ray detection element and the X-ray tube 12, is moved to the reference detector element D _R, the detector 13 may be moved in any order . For example, X-rays are emitted from the X-ray tube 12 in the initial state, and each X-ray detection element to be calibrated detects an X-ray spectrum. Then, to a position calibrated X-ray detecting elements are arranged to move the reference detector element D _R irradiated with X-rays by the X-ray tube 12 in the initial state, the reference detector element D _R is the X-ray spectrum To detect. Movement control unit 33b, at every position where calibrated X-ray detecting elements are arranged in the initial state, the process is repeated until the reference detector element D _R detects the X-ray spectrum in a predetermined order.

  Moreover, although the case where the detector 13 similar to FIG. 13 was used was described in the modification of the second embodiment, the embodiment is not limited to this. For example, the detector 13 as shown in FIGS. 22 and 23 may be used. In such a case, since calibration is possible using a plurality of reference detection elements, the number of movements of the detector 13 and the number of X-ray irradiations can be reduced.

(Third embodiment)
In the above-described embodiment, the calculation unit 33a is described as calculating the center of gravity using the calibrated spectrum meter or the X-ray spectrum detected by the reference detection element as it is, but the embodiment is not limited to this. It is not limited. For example, the calculation unit 33a may calculate the center of gravity of the X-ray spectrum after performing preprocessing such as noise removal. FIG. 19 is a diagram illustrating a configuration example of a scan control unit according to another embodiment.

  As illustrated in FIG. 19, the scan control unit 33 according to the third embodiment includes a calculation unit 33a and a spectrum processing unit 33c. The spectrum processing unit 33c preprocesses the reference X-ray spectrum. 20 and 21 are diagrams for explaining the processing operation of the spectrum processing unit 33c according to the third embodiment. 20 and 21, the case where the X-ray spectrum detected by the reference detection element is preprocessed will be described. However, the same applies to the case where the X-ray spectrum detected by the calibrated spectrum meter is preprocessed. is there.

  In FIG. 20, the process of removing the offset will be described. The left diagram in FIG. 20 shows an X-ray spectrum serving as a reference before performing preprocessing detected by the X-ray detection element A which is a reference detection element. The spectrum processing unit 33c generates the X-ray spectrum shown in the right diagram of FIG. 20 by removing the offset. The spectrum processing unit 33c determines the upper limit value of the band of the X-ray spectrum as follows. For example, the spectrum processing unit 33c estimates the upper limit of the energy value based on the tube voltage of the X-ray tube 12, and removes the X-ray spectrum having an energy value equal to or higher than the estimated upper limit value. The spectrum processing unit 33c determines the lower limit value of the band of the X-ray spectrum as follows. For example, the spectrum processing unit 33c removes an X-ray spectrum having an energy value equal to or lower than the energy value removed by a bow tie filter (not shown).

  Subsequently, the smoothing process will be described with reference to FIG. The left diagram in FIG. 21 shows an X-ray spectrum serving as a reference before performing preprocessing detected by the X-ray detection element A which is a reference detection element. The spectrum processing unit 33c generates an X-ray spectrum having a stable average value as shown in the right diagram of FIG. 21 by removing noise of the X-ray spectrum shown in the left diagram of FIG.

  Then, in the third embodiment, the calculation unit 33a calculates the center of gravity of the X-ray spectrum serving as a reference after preprocessing, and calculates a correction value for each X-ray detection element. Thus, by performing preprocessing on the X-ray spectrum, statistical errors can be further reduced.

  In the third embodiment, the case of preprocessing a reference X-ray spectrum has been described, but the embodiment is not limited to this. For example, the spectrum processing unit 33c may pre-process the reference X-ray spectrum and the X-ray spectrum detected by the photon counting detector 13. In such a case, the calculation unit 33a calculates the center of gravity of the X-ray spectrum serving as a reference after preprocessing and the center of gravity of the X-ray spectrum detected by the photon counting detector 13, and calculates the correction value as an X-ray detection element. Calculate for each. The spectrum processing unit 33c may preprocess only the X-ray spectrum detected by the photon counting detector 13 without preprocessing the reference X-ray spectrum.

(Other embodiments)
The embodiment is not limited to the above-described embodiment.

  In the first to third embodiments described above, when the process of calculating the correction value is performed, the charge amplifier 141 outputs the pulse signal to the calculation unit 33a of the scan control unit 33. Although described, the embodiment is not limited to this. For example, the waveform shaping circuit 143 may output a pulse signal to the calculation unit 33a of the scan control unit 33. In such a case, the waveform shaping circuit 143 switches the output of the pulse signal to either the scan control unit 33 or the waveform discrimination circuit 144 in accordance with an instruction from the scan control unit 33.

  In the first to third embodiments, the X-ray CT apparatus calculates the correction value. However, the embodiment is not limited to this. For example, in the image processing methods described in the first to third embodiments, an image processing apparatus including a calculation unit having the same function as the calculation unit 33a may calculate a correction value. . In such a case, the calculation unit of the image processing apparatus acquires the detection signal detected by the calibrated spectrum meter and the reference detection element from the X-ray CT apparatus. The calculation unit of the image processing apparatus matches the centroid of the X-ray spectrum detected by each X-ray detection element with the centroid of the reference X-ray spectrum detected by the calibrated spectrum meter or the reference detection element. The correction value to be calculated is calculated.

  The image processing apparatus may further include a spectrum processing unit having the same function as the spectrum processing unit 33c according to the third embodiment. In such a case, the calculation unit of the image processing apparatus calculates the center of gravity of the X-ray spectrum serving as a reference after the preprocessing, and calculates a correction value for each X-ray detection element.

  In the first to third embodiments, the case where the X-ray CT apparatus reconstructs an X-ray CT image has been described. However, the embodiment is not limited to this. For example, the reconstruction of the X-ray CT image may be executed by an apparatus other than the X-ray CT apparatus as long as the raw data collected by the X-ray CT apparatus can be acquired. For example, the image processing apparatus corrects the detection signal collected by the X-ray CT apparatus based on the correction value for each X-ray detection element, and reconstructs an X-ray CT image using the corrected detection signal.

  The functions of the waveform shaping circuit 143 and the image reconstruction unit 36 described in the first to third embodiments can also be realized by software. For example, the functions of the waveform shaping circuit 143 and the image reconstruction unit 36 are executed by a computer executing an image processing program that defines the processing procedure described as being performed by the waveform shaping circuit 143 and the image reconstruction unit 36 in the above embodiment. This is realized. This image processing program is stored in, for example, a hard disk or a semiconductor memory device, and is read and executed by a processor such as a CPU or MPU. The image processing program can be recorded and distributed on a computer-readable recording medium such as a CD-ROM (Compact Disc-Read Only Memory), an MO (Magnetic Optical disk), or a DVD (Digital Versatile Disc). Note that the image processing program may cause the computer to further execute the processing procedure described as being performed by the calculation unit 33a according to the first to third embodiments. Similarly, the functions of the movement control unit 33b described in the second embodiment and the spectrum processing unit 33c described in the third embodiment can be realized by software.

  According to at least one embodiment described above, calibration can be performed accurately and simply.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

13 Detector 14 Collection Unit 143 Waveform Shaping Circuit 36 Image Reconstruction Unit

Claims (14)

A photon counting type detector having a plurality of X-ray detection elements for detecting X-ray photons emitted from an X-ray tube;
A correction unit that corrects the detection signal detected by the photon counting detector for each X-ray detection element based on the center of gravity of the X-ray spectrum detected by the photon counting detector;
An X-ray CT apparatus comprising: a reconstruction unit that reconstructs a CT image based on the corrected detection signal. A calculation unit that calculates, for each X-ray detection element, a correction value that matches a reference centroid that is a centroid of a reference X-ray spectrum and a centroid of the X-ray spectrum detected by the photon counting detector; Prepared,
The correction unit corrects the detection signal detected by the photon counting detector for each X-ray detection element based on a correction value calculated for each X-ray detection element. The X-ray CT apparatus described.   The calculation unit obtains the reference X-ray spectrum from a detection signal detected using a spectrum meter, obtains the reference centroid from the reference X-ray spectrum, and determines the correction value as the X-ray detection element. The X-ray CT apparatus according to claim 2, wherein the X-ray CT apparatus is calculated for each. The calculation unit calculates the correction value corresponding to the tube voltage of the X-ray tube for each X-ray detection element,
The said correction | amendment part correct | amends the said detection signal detected by the said photon counting type detector for every said X-ray detection element based on the said correction value according to the tube voltage of the said X-ray tube. X-ray CT apparatus described in 1. A reference detection element having a higher energy resolution than the X-ray detection element of the photon counting type detector;
The calculation unit obtains the reference X-ray spectrum from the detection signal detected using the reference detection element, obtains the reference centroid based on the characteristic X-ray of the reference X-ray spectrum, and The X-ray CT apparatus according to claim 2, wherein a correction value is calculated for each X-ray detection element.   The movement of the photon counting type detector in the channel direction is controlled independently of the X-ray tube, the position of the reference detection element in the channel direction when the reference centroid is obtained, and the X-ray detection element to be corrected The X-ray CT apparatus according to claim 5, further comprising a movement control unit that performs control so that a position in the channel direction of the X-ray detection element when the center of gravity of the X-ray spectrum is obtained from a detection signal detected by the X-ray spectrum. .   The movement control unit positions the reference detection element at a position facing the X-ray tube when obtaining the reference center of gravity, and detects an X-ray spectrum from a detection signal detected by the correction target X-ray detection element. The X-ray CT apparatus according to claim 6, wherein the X-ray detection element to be corrected is positioned at a position facing the X-ray tube when determining the center of gravity.   When the movement control unit obtains the reference centroid, the relative position between the X-ray detection element to be corrected and the X-ray tube in a state where the center of the photon counting detector faces the X-ray tube. When the reference detection element is positioned at a position that coincides with a specific position and the center of the X-ray spectrum is obtained from the detection signal detected by the X-ray detection element to be corrected, the center of the photon counting detector is The X-ray CT apparatus according to claim 6, wherein the X-ray CT apparatus is positioned at a position facing the X-ray tube.   The X-ray CT apparatus according to claim 5, wherein the photon counting detector includes the reference detection element in at least a part of the X-ray detection element.   The X-ray CT apparatus according to claim 1, wherein the photon counting detector is a surface detector. A spectrum processing unit for preprocessing the reference X-ray spectrum;
The said calculation part calculates the said reference gravity center from the X-ray spectrum used as the said reference after a pre-processing, and calculates the said correction value for every said X-ray detection element. X-ray CT system.   The center of gravity of the X-ray spectrum is an energy position where the area of the X-ray spectrum in an energy band higher than the center of gravity is equal to the area of the X-ray spectrum in an energy band lower than the center of gravity. X-ray CT apparatus as described in any one of these. A detection signal detected by a photon counting detector having a plurality of X-ray detection elements for detecting X-ray photons emitted from an X-ray tube is used as a center of gravity of the X-ray spectrum detected by the photon counting detector. Based on the correction unit for correcting for each X-ray detection element,
An image processing apparatus comprising: a reconstruction unit configured to reconstruct a CT image based on the corrected detection signal. A detection signal detected by a photon counting detector having a plurality of X-ray detection elements for detecting X-ray photons emitted from an X-ray tube is used as a center of gravity of the X-ray spectrum detected by the photon counting detector. On the basis of the correction for each X-ray detection element,
An image processing program for causing a computer to execute each process for reconstructing a CT image based on the corrected detection signal.

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