JP2019005493A - X-ray CT apparatus - Google Patents

Abstract

To provide an X-ray CT apparatus having high image quality and high object discrimination performance.SOLUTION: An X-ray CT apparatus according to an embodiment includes a photon counting-type detector and a setting part. The photon counting-type detector includes a plurality of detecting elements for detecting an X-ray photon, and a plurality of detecting parts comprising a counting circuit for counting the X-ray photon detected by the detecting elements. At the time of imaging, the setting part sets a time constant according to the position of each of the detecting elements in the photon counting-type detector in the counting circuit connected to each of the detecting elements.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to an X-ray CT apparatus.

  In medical imaging systems such as photon counting CT (Computed Tomography), X-rays are photon counted under a high dose. In these medical image systems, the number of channels of the X-ray detector is large, and a highly integrated element such as an ASIC (Application Specific Integrated Circuit) is generally used.

  In photon counting CT, the amount of X-rays absorbed by the subject is small or absent at the peripheral edge in the channel direction of the X-ray detector, so that the X-ray dose incident on the detection element increases. Here, it is necessary to shorten the time constant of the ASIC in order to reduce pileup. However, if the time constant is shortened in order to suppress the effect of pileup, the noise increases and the S / N ratio decreases. In addition, the amount of X-ray absorption by the subject is large at the center in the channel direction of the X-ray detector. For this reason, in the center part of the channel direction of an X-ray detector, since the X-ray dose which injects into a detection element is low, substance discrimination ability falls.

  On the other hand, if the time constant of the ASIC is lengthened, the material discrimination ability is improved due to noise reduction in the central part of the X-ray detector in the channel direction. Deteriorates.

US Patent Application Publication No. 2016/0033654 US Pat. No. 5,568,530 US Patent Application Publication No. 2009/0114826 US Pat. No. 861,0081

  The problem to be solved by the present invention is to provide an X-ray CT apparatus having high image quality and high substance discrimination ability.

  The X-ray CT apparatus according to the embodiment includes a photon counting detector and a setting unit. The photon counting detector includes a plurality of detection units each including a detection element that detects an X-ray photon and a counting circuit that is connected to the detection element and counts the X-ray photons detected by the detection element. The setting unit sets a time constant corresponding to the position of each detection element in the photon counting detector to the counting circuit connected to each detection element at the time of photographing.

FIG. 1 is a diagram illustrating a configuration example of an X-ray CT apparatus according to the first embodiment. FIG. 2 is a diagram for explaining the X-ray detector according to the first embodiment. FIG. 3 is a diagram for explaining the data collection circuit according to the first embodiment. FIG. 4 is a diagram for explaining a comparative example. FIG. 5 is a diagram for explaining a comparative example. FIG. 6 is a flowchart showing a processing procedure performed by the X-ray CT apparatus according to the first embodiment. FIG. 7 is a diagram for explaining the first embodiment. FIG. 8 is a diagram for explaining the first embodiment. FIG. 9 is a diagram for explaining the first embodiment. FIG. 10 is a diagram for explaining a modification of the first embodiment. FIG. 11 is a diagram for explaining the second embodiment. FIG. 12 is a diagram for explaining the second embodiment. FIG. 13 is a flowchart illustrating a processing procedure performed by the FPGA according to the second embodiment. FIG. 14 is a diagram for explaining the third embodiment. FIG. 15 is a flowchart illustrating a processing procedure performed by the FPGA 14a according to the third embodiment. FIG. 16 is a diagram for explaining the third embodiment. FIG. 17 is a diagram for explaining a modification of the third embodiment.

  Hereinafter, an X-ray CT apparatus according to an embodiment will be described with reference to the drawings.

  An X-ray CT (Computed Tomography) 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 apparatus can reconstruct X-ray CT image data having a high S / N ratio. In addition, the content described in one embodiment is similarly applied to other embodiments in principle.

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

  The gantry 10 is an apparatus that irradiates the subject P with X-rays and collects data related to the X-rays transmitted through the subject P. The gantry 10 includes an X-ray high-voltage device 11, an X-ray generator 12, and an X-ray detector. 13, a data collection circuit 14, a rotating frame 15, and a gantry control device 16. Further, in the gantry 10, as shown in FIG. 1, an orthogonal coordinate system including an X axis, a Y axis, and a Z axis is defined. That is, the X axis indicates the horizontal direction, the Y axis indicates the vertical direction, and the Z axis indicates the direction of the rotation center axis of the rotating frame 15 when the gantry 10 is not tilted.

  The rotating frame 15 supports the X-ray generator 12 and the X-ray detector 13 so as to face each other with the subject P interposed therebetween, and is fast at a circular orbit centered on the subject P by a gantry control device 16 described later. It is an annular frame that rotates in a circle.

  The X-ray generator 12 is an apparatus that generates X-rays and irradiates the subject P with the generated X-rays. The X-ray generator 12 includes an X-ray tube 12a, a wedge 12b, and a collimator 12c.

  The X-ray tube 12a is a vacuum tube that receives a high voltage from the X-ray high voltage device 11 and irradiates thermoelectrons from a cathode (sometimes referred to as a filament) to an anode (target). The X-ray beam is irradiated to the subject P with the rotation of. That is, the X-ray tube 12 a generates X-rays using the high voltage supplied from the X-ray high voltage device 11.

  The X-ray tube 12a generates an X-ray beam that spreads with a fan angle and a cone angle. For example, the X-ray tube 12a controls the X-ray high-voltage apparatus 11 to continuously expose X-rays around the subject P for full reconstruction or exposure that can be reconfigured for half reconstruction. It is possible to continuously expose X-rays in the irradiation range (180 degrees + fan angle). Further, the X-ray tube 12a can intermittently emit X-rays (pulse X-rays) at a preset position (tube position) under the control of the X-ray high voltage apparatus 11. Further, the X-ray high voltage apparatus 11 can also modulate the intensity of X-rays exposed from the X-ray tube 12a. For example, the X-ray high voltage apparatus 11 increases the intensity of X-rays emitted from the X-ray tube 12a at a specific tube position, and exposes from the X-ray tube 12a in a range other than the specific tube position. Reduce the intensity of the emitted X-rays.

  The wedge 12b is an X-ray filter for adjusting the X-ray dose of X-rays emitted from the X-ray tube 12a. Specifically, the wedge 12b transmits the X-rays exposed from the X-ray tube 12a so that the X-rays irradiated from the X-ray tube 12a to the subject P have a predetermined distribution. Attenuating filter. For example, the wedge 12b is a filter obtained by processing aluminum so as to have a predetermined target angle or a predetermined thickness. The wedge is also called a wedge filter or a bow-tie filter.

  The collimator 12c is composed of a lead plate or the like and has a slit in part. For example, the collimator 12c narrows down the X-ray irradiation range in which the X-ray dose is adjusted by the wedge 12b with the slits under the control of the X-ray high voltage apparatus 11 described later.

  The X-ray source of the X-ray generator 12 is not limited to the X-ray tube 12a. For example, in place of the X-ray tube 12a, the X-ray generator 12 collides with a focus coil that focuses an electron beam generated from an electron gun, a deflection coil that electromagnetically deflects, and an electron beam that deflects around a half circumference of the subject P. And a target ring that generates X-rays.

  The X-ray high voltage device 11 is composed of an electric circuit such as a transformer and a rectifier, and has a function of generating a high voltage to be applied to the X-ray tube 12a, and the X-ray tube 12a emits light. An X-ray control device that controls the output voltage according to the X-ray to be performed. The high voltage generator may be a transformer system or an inverter system. For example, the X-ray high voltage apparatus 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 12a. Further, the X-ray high voltage apparatus 11 receives control from the processing circuit 37 of the console 30.

  The gantry control device 16 includes a processing circuit configured by a CPU (Central Processing Unit) and the like and a driving mechanism such as a motor and an actuator. The gantry control device 16 has a function of controlling the operation of the gantry 10 by receiving an input signal from the input interface 31 attached to the console 30 or the input interface attached to the gantry 10. For example, the gantry control device 16 receives the input signal and rotates the rotary frame 15 to rotate the X-ray tube 12a and the X-ray detector 13 on a circular orbit around the subject P. Control for tilting the gantry 10 and control for operating the bed 20 and the top plate 22 are performed. The gantry control device 16 receives control from the processing circuit 37 of the console 30.

  Further, the gantry control device 16 monitors the position of the X-ray tube 12a, and when the X-ray tube 12a reaches a predetermined rotation angle (imaging angle), a timing at which data acquisition to the data acquisition circuit 14 is started. A view trigger signal indicating is output. For example, when the total number of views in rotational imaging is 2460 views, the gantry control device 16 outputs a view trigger signal every time the X-ray tube 12a moves about 0.15 degrees (= 360/2460) on a circular orbit. To do.

  The X-ray detector 13 is an example of a photon counting detector that includes a plurality of detection elements and outputs a signal corresponding to the counted number of photons. In the X-ray detector 13, for example, a plurality of X-ray detection elements (also referred to as “sensors” or simply “detection elements”) are arranged in the channel direction along one circular arc with the focal point of the X-ray tube 12a as the center. It is composed of a plurality of X-ray detection element arrays. The X-ray detector 13 has a structure in which a plurality of X-ray detection element arrays in which a plurality of X-ray detection elements are arranged in the channel direction are arranged in the slice direction. Each X-ray detection element of the X-ray detector 13 detects X-rays irradiated from the X-ray generator 12 and passed through the subject P, and outputs an electric signal (pulse) corresponding to the X-ray dose to the data collection circuit 14. To output. The electric signal output by each X-ray detection element is also referred to as a detection signal. FIG. 2 is a diagram for explaining the X-ray detector 13 according to the first embodiment.

  As shown in FIG. 2, the X-ray detector 13 includes a detection element 130 that detects X-ray photons, and an ASIC (Application Specific Integrated) that is connected to the detection element 130 and counts X-ray photons detected by the detection element 130. Circuit) 134 is a photon counting detector having a plurality of detection units. In the example of FIG. 2, one detection unit among the plurality of detection units is illustrated. Hereinafter, a case where the X-ray detector 13 is a direct conversion type detector will be described.

  Each detection element 130 includes a semiconductor 131, a cathode electrode 132, and a plurality of anode electrodes 133. Here, the semiconductor 131 is a semiconductor such as cadmium telluride (CdTe) or zinc cadmium telluride (CZT). Each of the anode electrodes 133 corresponds to an individual detection pixel (also referred to as “pixel”). When the X-ray photon is incident, the detection element 130 directly converts the X-ray incident on the detection element 130 into an electric charge and outputs it to the ASIC 134.

  In the following, the case where the X-ray detector 13 is a direct conversion type semiconductor detector will be described. However, the present invention is not limited to this. For example, as the X-ray detector 13, for example, a grid, a scintillator array, and an optical sensor An indirect conversion type detector composed of an array can also be used. The scintillator array is composed of a plurality of scintillators, and the scintillator is composed of a scintillator crystal that outputs a number of lights according to incident X-ray energy. The grid is an X-ray shielding plate that is disposed on the surface on the X-ray incident side of the scintillator array and has a function of absorbing scattered X-rays. The photosensor array has a function of converting into an electrical signal corresponding to the amount of light from the scintillator, and is composed of a photosensor such as a photomultiplier tube, for example. Here, the optical sensor is, for example, PD (Photodiode), APD (Avalanche Photodiode), SiPM (Silicon photomultipliers), or the like.

  The ASIC 134 counts the number of X-ray photons incident on the detection element 130 by discriminating individual charges output from the detection element 130. Further, the ASIC 134 measures the energy of the counted X-ray photons by performing arithmetic processing based on the magnitude of each charge. The ASIC 134 includes, for example, a capacitor 134a, an amplifier circuit 134b, a waveform shaping circuit 134c, a comparator circuit 134d, and a counter 134e. The ASIC 134 is an example of a counting circuit.

  The capacitor 134a accumulates the charge output from the detection element 130. The amplifier circuit 134b is a circuit that integrates and amplifies the electric charge collected in the capacitor 134a in response to the X-ray photons incident on the detection element 130, and outputs the electric charge as a pulse signal. The pulse height or area of the pulse signal has a correlation with the photon energy.

  Here, in the ASIC 134, a time constant (τ (ns)) is set in the amplifier circuit 134b. Here, the time constant indicates the integration time in the amplifier circuit 134b. In other words, the time constant of each ASIC 134 is determined by the integration time in each ASIC 134. Further, white noise is generated in the ASIC 134. When the time constant is larger than the white noise frequency, the white noise is canceled, so that the frequency characteristics are improved and the noise is reduced. As a result, when the time constant is larger than the frequency of white noise, the substance discrimination ability is improved.

  The waveform shaping circuit 134c is a circuit that shapes the waveform of the pulse signal by adjusting the frequency characteristics of the pulse signal output from the amplifier circuit 134b and giving a gain and an offset.

  The comparator circuit 134d compares the pulse height or area of the response pulse signal to the incident photon with a preset threshold value corresponding to a plurality of energy bands to be discriminated, and the comparison result with the threshold value is sent to the counter 134e at the subsequent stage. It is a circuit to output.

  The counter 134e counts the waveform discrimination result of the response pulse signal for each corresponding energy band, and outputs the photon count result to the data collection circuit 14 as digital data.

  The data acquisition circuit 14 (DAS: Data Acquisition System) is a circuit that collects the result of the counting process from each detection element 130 of the X-ray detector 13 and generates detection data. In other words, the data collection circuit 14 collects the count results obtained by the X-ray detector 13. Here, the detection data is, for example, a sinogram. The sinogram is data in which the results of the counting process incident on each detection element 130 at each position of the X-ray tube 12a are arranged. The data acquisition circuit 14 collects the result of the counting process at each view angle from the X-ray detector 13 in synchronization with the view trigger signal, and generates a sinogram. The data collection circuit 14 outputs the data for the necessary circumference by repeating the process of outputting or saving the result of the counting process at regular intervals (views) or storing it in the storage circuit 35 and the process of resetting the result of the counting process. get.

  The data acquisition circuit 14 transmits various control signals to the X-ray detector 13. FIG. 3 is a diagram for explaining the data collection circuit 14 according to the first embodiment. As shown in FIG. 3, the data collection circuit 14 includes an FPGA (Field-Programmable Gate Array) 14a. As shown in FIG. 3, the data collection circuit 14 is connected to the X-ray detector 13 by, for example, a rigid flexible board 17. The rigid flexible substrate 17 is a substrate in which a rigid wiring board portion having hardness and strength for mounting components and a flexible wiring board that can be bent are integrated. For example, the FPGA 14a receives a view trigger signal from the gantry control device 16, and controls the X-ray detector 13 based on the received view trigger signal. The data collection circuit 14 is an example of a collection unit, and the FPGA 14a is an example of a setting unit.

  The data output from the data collection circuit 14 is referred to as detection data, and logarithmic conversion processing, offset correction processing, sensitivity correction processing between channels, gain correction processing between channels, pile-up correction processing, response to the detection data. Data subjected to preprocessing such as function correction processing and beam hardening correction is referred to as raw data. Further, the detection data and the raw data are collectively referred to as projection data.

  The bed 20 is a device for placing and moving the subject P to be scanned, and includes a bed driving device 21, a top plate 22, a base 23, and a base (support frame) 24.

  The top plate 22 is a plate on which the subject P is placed. The base 24 supports the top plate 22. The base 23 is a housing that supports the base 24 so as to be movable in the vertical direction. The couch driving device 21 is a motor or an actuator that moves the subject P into the rotary frame 15 by moving the top 22 on which the subject P is placed in the major axis direction of the top 22. The couch driving device 21 can move the top plate 22 also in the X-axis direction.

  The top plate moving method may be a method of moving only the top plate 22 or a method of moving the base 24 of the bed 20 together. In the case of standing CT, a method of moving a patient moving mechanism corresponding to the top board 22 may be used.

  For example, the gantry 10 executes 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 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. In the following embodiment, a change in the relative position between the gantry 10 and the top plate 22 will be described as being realized by controlling the top plate 22, but the embodiment is not limited to this. For example, when the gantry 10 is self-propelled, a change in the relative position between the gantry 10 and the top plate 22 may be realized by controlling the traveling of the gantry 10. Further, the relative position of the gantry 10 and the top plate 22 may be changed by controlling the traveling of the gantry 10 and the top plate 22.

  The console 30 is a device that accepts an operation of the X-ray CT apparatus 1 by an operator and reconstructs X-ray CT image data using the counting results collected by the gantry 10. As shown in FIG. 1, the console 30 includes an input interface 31, a display 32, a storage circuit 35, and a processing circuit 37.

  The input interface 31 receives various input operations from the operator, converts the received input operations into electrical signals, and outputs them to the processing circuit 37. For example, the input interface 31 receives from the operator collection conditions when collecting projection data, reconstruction conditions when reconstructing a CT image, image processing conditions when generating a post-processed image from a CT image, and the like. . For example, the input interface 31 is realized by a mouse, a keyboard, a trackball, a switch, a button, a joystick, or the like.

  The display 32 displays various information. For example, the display 32 outputs a medical image (CT image) generated by the processing circuit 37, a GUI (Graphical User Interface) for receiving various operations from the operator, and the like. For example, the display 32 is configured by a liquid crystal display, a CRT (Cathode Ray Tube) display, or the like.

  The storage circuit 35 is realized by, for example, a RAM (Random Access Memory), a semiconductor memory element such as a flash memory, a hard disk, an optical disk, or the like. The storage circuit 35 stores, for example, projection data and reconstructed image data.

  The processing circuit 37 executes, for example, a system control function 371, a preprocessing function 372, a reconstruction processing function 373, an image processing function 374, a scan control function 375, a display control function 376, and a determination function 377. Here, for example, the system control function 371, the pre-processing function 372, the reconstruction processing function 373, the image processing function 374, the scan control function 375, the display control function 376, and the determination, which are components of the processing circuit 37 shown in FIG. Each processing function executed by the function 377 is recorded in the storage circuit 35 in the form of a program executable by a computer. The processing circuit 37 is, for example, a processor, and realizes a function corresponding to each read program by reading and executing each program from the storage circuit 35. In other words, the processing circuit 37 in a state where each program is read has each function shown in the processing circuit 37 of FIG.

  The system control function 371 controls various functions of the processing circuit 37 based on the input operation received from the operator via the input interface 31.

  The pre-processing function 372 performs logarithmic conversion processing, offset correction processing, sensitivity correction processing between channels, gain correction processing between channels, pile-up correction processing, response function correction processing on the detection data output from the data acquisition circuit 14. The raw data is generated by performing pre-processing such as beam hardening correction. The preprocessing function 372 is an example of a correction unit.

  The reconstruction processing function 373 generates X-ray CT image data by performing reconstruction processing using a filter-corrected back projection method, successive approximation reconstruction method, or the like on the projection data generated by the preprocessing function 372. To do. The reconstruction processing function 373 stores the reconstructed X-ray CT image data in the storage circuit 35. Note that X-ray CT image data reconstructed from data including all energy information by adding all bin information for each pixel is also referred to as a “base image”.

  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. For this reason, the reconstruction processing function 373 can reconstruct X-ray CT image data of a specific energy component, for example. The reconstruction processing function 373 can reconstruct X-ray CT image data of each of a plurality of energy components, for example.

  Further, the reconstruction processing function 373 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. Generated image data is generated. In addition, the reconstruction processing function 373 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 reconstruction processing function 373 includes monochromatic X-ray image data, density image data, effective atomic number image data, and the like.

  In addition, as an application of X-ray CT, there is a technique for discriminating the type, abundance, density, and the like of a substance contained in the subject P using the fact that the X-ray absorption characteristics differ for each substance. This is called substance discrimination. For example, the reconstruction processing function 373 performs substance discrimination on the projection data and obtains substance discrimination information. Then, the reconstruction processing function 373 reconstructs a substance discrimination image using the substance discrimination information that is a result of substance discrimination.

  The reconstruction processing function 373 can apply a full scan reconstruction method and a half scan reconstruction method to reconstruct a CT image. For example, the reconstruction processing function 373 requires 360 degrees of projection data around the subject in the full scan reconstruction method. Also, the reconstruction processing function 373 requires projection data for 180 degrees + fan angle in the half-scan reconstruction method. Hereinafter, in order to simplify the description, it is assumed that the reconstruction processing function 373 uses a full-scan reconstruction method in which reconstruction is performed using projection data for 360 degrees around the subject. The reconstruction processing function 373 is an example of a reconstruction processing unit.

  The image processing function 374 is based on the input operation received from the operator via the input interface 31, and the X-ray CT image data generated by the reconstruction processing function 373 is generated by a known method, such as a tomographic image of any section or rendering. It is converted into image data such as a three-dimensional image by processing. The image processing function 374 stores the converted image data in the storage circuit 35.

  The scan control function 375 controls the CT scan performed on the gantry 10. For example, the scan control function 375 controls the operations of the X-ray high voltage device 11, the X-ray detector 13, the gantry control device 16, the data collection circuit 14, and the couch driving device 21, thereby starting scanning on the gantry 10, Control execution of scan and end of scan. Specifically, the scan control function 375 controls the collection processing of projection data in photographing for collecting positioning images (scanograms, scanograms) and main photographing (scanning) for collecting images used for diagnosis.

  Here, the scan control function 375 can capture a two-dimensional scano image and a three-dimensional scano image. For example, the scan control function 375 performs continuous imaging while fixing the X-ray tube 12a at a position of 0 degree (a position in the front direction with respect to the subject P) and moving the top plate 22 at a constant speed. Take a two-dimensional scano image with. Alternatively, the scan control function 375 may repeat the imaging intermittently in synchronization with the movement of the top plate 22 while the X-ray tube 12a is fixed at the 0 degree position and the top plate 22 is moved intermittently. Take a two-dimensional scano image. The scan control function 375 can capture a positioning image not only from the front direction but also from any direction (for example, the side surface direction) with respect to the subject. For example, when the X-ray tube 12a is imaged at a position of 90 degrees (position in the lateral direction with respect to the subject P), imaging from the side surface of the subject P is performed and a two-dimensional scan image is obtained. Note that the position of the X-ray tube 12a can be taken from a plurality of arbitrary positions if necessary.

  The scan control function 375 captures a three-dimensional scanogram by collecting projection data for the entire circumference of the subject in scanogram capture. For example, the scan control function 375 collects projection data for the entire circumference of the subject by helical scanning or non-helical scanning. Here, the scan control function 375 executes a helical scan or a non-helical scan with a lower dose than the main imaging over a wide range such as the entire chest, abdomen, the entire upper body, and the whole body of the subject. As the non-helical scan, for example, a step-and-shoot scan is executed.

  The display control function 376 controls the display 32 to display various image data stored in the storage circuit 35.

  The determination function 377 determines a time constant corresponding to the position of each detection element 130 in the X-ray detector 13. Details of the determination function 377 will be described later.

  The configuration of the X-ray CT apparatus 1 according to the first embodiment has been described above. With this configuration, the X-ray CT apparatus 1 according to the first embodiment integrates the output signal (charge) of the X-ray detector 13 and shapes the waveform, and then divides the signal into a plurality of windows according to the signal level. Then, the number of incident X-rays in each window is counted with a counter. The X-ray CT apparatus 1 acquires data for a necessary circumference and acquires CT images with a plurality of energy windows. Here, the processing time of the output signal (charge) is determined by the time constant of the integration time set in the ASIC 134.

  Here, as a comparative example, the case where the time constant of each ASIC 134 is set to a constant value in the X-ray detector 13 will be described. That is, in the comparative example, it is assumed that the same time constant is set in all ASICs 134. The problem in the comparative example will be described with reference to FIGS. 4 and 5. 4 and 5 are diagrams for explaining a comparative example.

  In the X-ray detector 13, the X-ray dose incident on each detection element 130 varies depending on the position in the X-ray detector 13. For example, as shown in FIG. 4, since the amount of absorption by the subject P is large at the center of the X-ray detector 13, the X-ray dose incident on the detection element 130 is low. Here, for convenience of explanation, the X-ray dose is assumed to be n1 (c / s). On the other hand, since the amount of absorption by the subject P is small or absent at the peripheral portion of the X-ray detector 13, the X-ray dose incident on the detection element 130 increases. Here, for convenience of explanation, the X-ray dose is n2 (c / s). The central portion of the X-ray detector 13 indicates a region near the center in the channel direction of the X-ray detector 13, and the peripheral portion of the X-ray detector 13 is the center in the channel direction of the X-ray detector 13. The peripheral area away from.

  The upper diagram in FIG. 5 shows the X-ray incidence timing when the X-ray dose is n1, and the lower diagram in FIG. 5 shows the X-ray incidence timing when the X-ray dose is n2. FIG. 5 shows a case where the time constant is τ (ns). As shown in the upper diagram of FIG. 5, when the incident X-ray dose is low, the number of X-rays incident during each time constant is 1 or less. Thus, when the incident X-ray dose is low, pile-up hardly occurs.

  On the other hand, as shown in the lower diagram of FIG. 5, when the incident X-ray dose is high, the number of incident X-rays during each time constant may be two or more. That is, when the number of X-ray photons incident on the X-ray detector 13 is large, a phenomenon called pile-up in which another X-ray enters during a time constant continuously occurs. When the pile-up occurs, the number of X-ray photons to be counted and the detected energy value of the X-ray are not correct values, and the finally obtained image quality deteriorates. For example, when one 60 KeV X-ray photon and one 80 KeV X-ray photon are incident during the time constant, the X-ray detector 13 measures that one 140 KeV X-ray photon is incident. To do.

  Therefore, it is conceivable to obtain a high image quality by shortening the time constant as much as possible to suppress the effect of pileup. However, if the time constant is shortened, white noise having a frequency lower than that of the time constant cannot be removed, so that the amount of noise increases. That is, if the time constant is shortened, white noise that cannot be removed increases, and the frequency characteristics deteriorate. As a result, noise increases and the S / N ratio decreases. If the time constant is shortened, the substance discrimination ability of the detection element 130 with a low X-ray dose incident on the central portion in the channel direction in the X-ray detector 13 is lowered.

  On the contrary, if the time constant is lengthened, the substance discrimination ability is improved due to noise reduction in the detection element 130 in the center in the channel direction in the X-ray detector 13, but the peripheral edge in the channel direction is detected in the X-ray detector 13. In the element 130, image quality deteriorates due to pile-up. Thus, in the comparative example, it was difficult to achieve both high image quality and high substance discrimination ability.

  For this reason, the X-ray CT apparatus 1 according to the first embodiment captures the time constant corresponding to the position of each detection element 130 in the photon counting detector on the ASIC 134 connected to each detection element 130. Set at time. Hereinafter, the first embodiment will be described with reference to FIGS. 6 to 9. FIG. 6 is a flowchart showing a processing procedure performed by the X-ray CT apparatus 1 according to the first embodiment, and FIGS. 7 to 9 are diagrams for explaining the first embodiment.

  FIG. 6 shows a flowchart for explaining the operation of the X-ray CT apparatus 1 and describes which step in the flowchart corresponds to each component. Step S <b> 1 is a step corresponding to the scan control function 375. This is a step in which the scan control function 375 is realized by the processing circuit 37 calling and executing a predetermined program corresponding to the scan control function 375 from the storage circuit 35. In step S1, the scan control function 375 captures a scanogram. For example, the scan control function 375 captures a two-dimensional scan image while fixing the X-ray tube 12a at a position of 0 degree (a position in the front direction with respect to the subject P).

  Step S2 is a step corresponding to the determination function 377. This is a step in which the determination function 377 is realized by the processing circuit 37 calling and executing a predetermined program corresponding to the determination function 377 from the storage circuit 35. In step S2, the determination function 377 determines a time constant.

  Here, the determination function 377 determines a time constant based on the X-ray dose incident on each detection element 130 and generates correspondence information. For example, the determination function 377 estimates the X-ray dose incident on each detection element 130 from the count result of each detection element 130 and determines a time constant based on the estimated X-ray dose. As an example, the determination function 377 determines the time constant by estimating the X-ray dose incident on the detection element 130 from the scanogram count result taken in step S1.

  The determination function 377 performs a threshold determination process shown below on the scanogram count result to determine a time constant. For example, the determination function 377 estimates an area where the scanogram count result is equal to or greater than the threshold in the channel direction, and selects the X-ray dose incident on the detection element 130 as a high dose area. Then, the determination function 377 shortens the time constant of the ASIC 134 connected to the detection element corresponding to the high dose region. On the other hand, the determination function 377 estimates an area where the scanogram count result is less than the threshold in the channel direction as the X-ray dose incident on the detection element 130 is a low dose, and selects the low dose area. The determination function 377 increases the time constant of the ASIC 134 connected to the detection element corresponding to the low dose region.

  More specifically, since the absorption by the subject P is large in the central part in the channel direction, the dose of incident X-rays is low. For this reason, the determination function 377 selects the central portion in the channel direction as the low-dose region and increases the time constant of the central portion in the channel direction as shown in FIG. Further, since the absorption by the subject P is small or absent at the peripheral edge in the channel direction, the dose of incident X-rays becomes high. For this reason, the decision function 377 selects the peripheral portion in the channel direction as the high-dose region, and shortens the time constant of the peripheral portion in the channel direction as shown in FIG.

  Note that the determination function 377 may determine a time constant by performing threshold determination processing on a representative slice, and apply the determined time constant to other slices in the same manner. Each process may be performed to determine the time constant. The representative slice is, for example, a central slice in the slice direction.

  Then, the determination function 377 generates correspondence information in which each detection element is associated with the time constant of the ASIC 134 connected to the detection element. More specifically, the determination function 377 generates correspondence information in which an ID is associated with a time constant as illustrated in FIG.

  Here, “ID” in the correspondence information indicates an identifier for uniquely identifying the detection element, and “time constant” indicates a time constant of the ASIC 134 connected to the detection element identified by the ID. In the example shown in FIG. 8, the detection element 130 with time constant T1 <time constant T2 and “ID” “yyy” and “zzz” is a high dose at the peripheral edge in the channel direction in the X-ray detector 13. It is assumed that the detection element 130 that is arranged in the region and whose “ID” is “xxxx” is arranged in the X-ray detector 13 in the low-dose region at the center in the channel direction. As an example, the determination function 377 determines that the time constant of the ASIC 134 connected to the detection elements whose “ID” is “yyy” and “zzz” is “T1”, and the “ID” is “xxxx”. It is determined that the time constant of the ASIC 134 connected to the detection element is “T2”.

  Step S3 is a step realized by the FPGA 14a. In step S4, the FPGA 14a sets a time constant. Here, the FPGA 14 a sets a time constant corresponding to the position of each detection element 130 in the photon counting detector to the ASIC 134 connected to each detection element 130 at the time of photographing. For example, the FPGA 14 a sets a time constant based on the X-ray dose incident on each detection element 130 in each ASIC 134.

  For example, the FPGA 14a sets a time constant in each ASIC 134 with reference to the correspondence information shown in FIG. That is, the FPGA 14 a sets a time constant based on the X-ray dose incident on each detection element 130 estimated from the counting result of each detection element 130 in each ASIC 134. More specifically, the FPGA 14a sets the time constant “T1” in the ASIC 134 connected to the detection element 130 whose IDs are “yyyy” and “zzz”. Further, the FPGA 14a sets a time constant “T2” in the ASIC 134 connected to the detection element 130 whose ID is “xxxx”. In this way, the FPGA 14a sets a larger time constant for the ASIC 134 connected to the detection element whose incident X-ray dose is less than the threshold value than the ASIC 134 connected to the detection element whose incident X-ray dose is greater than or equal to the threshold value. . In the first embodiment, the time constant set in each ASIC 134 at the start of shooting remains fixed during shooting.

  Step S4 is a step corresponding to the scan control function 375. This is a step in which the scan control function 375 is realized by the processing circuit 37 calling and executing a predetermined program corresponding to the scan control function 375 from the storage circuit 35. In step S4, the scan control function 375 executes a main scan.

  Step S <b> 5 is a step corresponding to the preprocessing function 372. This is a step in which the preprocessing function 372 is realized by the processing circuit 37 calling and executing a predetermined program corresponding to the preprocessing function 372 from the storage circuit 35. By the way, the detection data obtained in the main scan in step S4 is a signal obtained as a result of processing with different time constants in each ASIC 134. Here, as the time constant changes, the pile-up occurrence rate also changes. By changing the occurrence rate of pile-up, for example, the number of counts decreases, and the rate at which the spectrum shifts higher changes. That is, by setting different time constants, the detection data changes in the count number and spectrum. For this reason, in step S5, the preprocessing function 372 corrects the detection data. In other words, the preprocessing function 372 corrects the counting result by the data collection circuit 14.

  FIG. 9 illustrates a case where the count number of detection data is corrected. In FIG. 9, the horizontal axis represents the tube current value (mA), and the vertical axis represents the number of detected X-rays. Here, the number of detected X-rays should theoretically have a linear relationship with the dose of X-rays to be irradiated. For example, as indicated by T0 in FIG. 9, the number of detected X-rays increases linearly as the dose of X-rays to be irradiated increases. However, in actual measurement, the number of detected X-rays reaches a peak as the irradiation dose increases, resulting in a non-linear relationship. Here, the dose that reaches the peak is determined by the time constant. For example, the larger the time constant, the lower the number of detected X-rays. For this reason, the preprocessing function 372 identifies the time constant set in each ASIC 134 and corrects the number of X-rays that are the counting result in each ASIC 134. In the following, it is assumed that time constant T1 <time constant T2 and that the number of X-rays detected with respect to the dose of X-rays to be irradiated is measured in advance at time constant T1 and time constant T2.

  For example, in the example shown in FIG. 9, when the number of detected X-rays is C1 and the time constant is T1, the preprocessing function 372 corrects the detected number of X-rays from C1 to C2. In the example shown in FIG. 9, when the number of detected X-rays is C1 and the time constant is T2, the preprocessing function 372 corrects the number of detected X-rays from C1 to C3.

  Step S 6 is a step corresponding to the reconstruction processing function 373. This is a step in which the reconfiguration processing function 373 is realized by the processing circuit 37 calling and executing a predetermined program corresponding to the reconfiguration processing function 373 from the storage circuit 35. In step S6, the reconstruction processing function 373 reconstructs an image. For example, the reconstruction processing function 373 generates a base image based on projection data obtained by performing preprocessing on the detection data corrected by the preprocessing function 372 in step S5. That is, the reconstruction processing function 373 reconstructs an image based on the corrected count result.

  Step S <b> 7 is a step corresponding to the display control function 376. This is a step in which the display control function 376 is realized by the processing circuit 37 calling and executing a predetermined program corresponding to the display control function 376 from the storage circuit 35. In step S <b> 7, the display control function 376 displays an image on the display 32.

  As described above, in the first embodiment, in the X-ray CT apparatus 1, the time constant corresponding to the position of each detection element 130 in the X-ray detector 13 is set in the ASIC 134 connected to each detection element at the time of imaging. To do. For example, the ASIC 134 connected to the detection element 130 disposed in the low-dose region which is the central portion in the channel direction of the X-ray detector 13 is connected to the detection element disposed in the high-dose region which is the peripheral portion in the channel direction. A time constant larger than that of the ASIC 134 connected to 130 is set.

  Thereby, for example, in the central portion in the channel direction of the X-ray detector 13, the time constant is long, so the S / N ratio is improved and the substance discrimination ability is increased. On the other hand, the peripheral portion in the channel direction of the X-ray detector 13 has a short time constant, so that it is hardly affected by pile-up even under a high dose, and there is little deformation of the histogram, and an almost correct count rate can be obtained. Further, in the high-dose region at the peripheral portion in the channel direction of the X-ray detector 13, the necessity for substance discrimination is low, and the substance discrimination ability is required in the low-dose region at the center in the channel direction of the X-ray detector 13. is there. For this reason, according to the X-ray CT apparatus 1 according to the first embodiment, it is possible to obtain high image quality and high substance discrimination ability.

  In the low-dose region, the time constant is long, but because the dose is low, the influence of the pile-up on histogram deformation and counting rate can be ignored. In the high dose region, since the time constant is short, the S / N ratio is small, but the count rate is not affected and the influence on the image quality may be eliminated. For this reason, in the X-ray CT apparatus 1 according to the first embodiment, the correction process by the preprocessing function 372 described above may be omitted.

  In addition, the reconstruction processing function 373 may perform substance discrimination on the projection data and reconstruct a substance discrimination image using substance discrimination information that is a result of substance discrimination. Here, when reconstructing a substance discrimination image, in addition to the process of correcting the count number of detection data, a process of correcting the spectrum of detection data is required. Below, the process which correct | amends the spectrum of the detection data by the reconstruction process function 373 is demonstrated.

The relationship between the X-ray spectrum S (E) detected by the X-ray detector 13 and the X-ray spectrum S ₀ (E) incident on the X-ray detector 13 uses a response function R (E, nτ). Is represented by the following formula 1.

Here, S ₀ (E) is an X-ray spectrum irradiated to the subject P, μ is an average attenuation coefficient of the path, and L is a projection length of the path. Further, the response function R (E, nτ) includes an X-ray dose n and a time constant τ. A plurality of response functions R are set by the X-ray dose n and the time constant τ. Here, the X-ray spectrum S (E), the X-ray spectrum S ₀ (E) irradiated to the subject P, and the response function R (E, nτ) are known. For this reason, the reconstruction processing function 373 can calculate the absorption amount (−μL) from Equation 1. That is, the reconstruction processing function 373 can perform substance discrimination in consideration of the change in the spectrum of the detection data due to the difference in time constant. When generating the base image, it is only necessary to correct the count number of the detection data, and it is not necessary to correct the spectrum difference generated in the detection data.

(Modification of the first embodiment)
In addition, although embodiment mentioned above demonstrated the case where material discrimination was performed using the response function which considered the time constant, embodiment is not limited to this. For example, substance discrimination may be performed using a lookup table. In such a case, the look-up table stores a detection spectrum that is determined in advance according to the projection length of each substance to be distinguished. Also, a plurality of lookup tables are created in advance under imaging conditions in which the tube current and tube voltage are variously changed. Then, the reconstruction processing function 373 performs material discrimination by searching a detection spectrum corresponding to the detection data from a lookup table. FIG. 10 is a diagram for explaining a modification of the first embodiment.

FIG. 10 shows a case where water and calcium are distinguished from each other. As shown in FIG. 10, the look-up table shows the case where the thickness of calcium is changed to 1, 2, 3, 4 (mm) with respect to the thickness of water 5, 10, 15, 20 (cm). The histogram is stored. As an example, S (E) ₁₁ shows a histogram when water is 5 cm and calcium is 1 mm, and S (E) ₂₁ shows a histogram when water is 5 cm and calcium is 2 mm. Here, the reconstruction processing function 373 performs material discrimination by specifying a histogram similar to the detection data using a lookup table created under imaging conditions that are likely to cause pileup, for example.

(Second Embodiment)
In the first embodiment described above, the case has been described in which the time constant corresponding to the position of each detection element is set in the ASIC 134 connected to each detection element during imaging. Here, in the first embodiment, the time constant set in each ASIC 134 at the start of shooting remains fixed during shooting.

  By the way, the cross section of the subject P is often not circular but elliptical. For this reason, as shown in FIG. 7, when the region having a short time constant and the region having a long time constant are fixed, the X-ray tube 12a makes one rotation around the subject P. There is a case where a high dose of X-rays is incident on a region where a constant is set. In such a case, the image quality deteriorates as a result of the count number being reduced or the spectrum being shifted to a higher one due to pileup.

  For this reason, the time constant set according to the position of each detection element 130 may be dynamically changed according to the imaging angle of the X-ray tube 12a without being fixed during imaging. . Therefore, in the second embodiment, a case will be described in which a time constant corresponding to the shooting angle is set in each ASIC 134.

  The configuration of the X-ray CT apparatus 1 according to the second embodiment is the same as the configuration of the X-ray CT apparatus 1 shown in FIG. 1 except that the determination function 377 and a part of the FPGA 14a are different. For this reason, in the second embodiment, only the function executed by the determination function 377 and the FPGA 14a will be described.

  The determination function 377 determines a time constant corresponding to the imaging angle based on the X-ray dose incident on each detection element 130, and generates correspondence information. For example, the determination function 377 estimates the X-ray dose incident on each detection element 130 from the count result of each detection element 130 and determines a time constant based on the estimated X-ray dose. For example, the determination function 377 estimates the X-ray dose incident on the detection element 130 from the scanogram count result and determines the time constant. In the second embodiment, the scan control function 375 captures a two-dimensional scano image from two directions. For example, the scan control function 375 fixes the X-ray tube 12a at a position of 0 degree (a position in the front direction with respect to the subject P) and captures a two-dimensional scan image (0 degree scanogram). The X-ray tube 12a is fixed at a position of 90 degrees (position in the lateral direction with respect to the subject P) and a two-dimensional scan image (90 degrees scanogram) is taken.

  The determination function 377 determines the time constant by performing the following threshold determination process on the count result of the two-dimensional scano image from two directions. In other words, the determination function 377 estimates the X-ray dose incident on each detection element using a two-dimensional scanogram from two directions, and determines a time constant. FIG. 11 is a diagram for explaining the second embodiment.

  For example, in the channel direction, the determination function 377 estimates that the X-ray dose incident on the detection element 130 is a high dose in an area where the count result of the 0-degree scanogram is greater than or equal to the threshold, and as illustrated in FIG. Select the dose region H1. Then, the determination function 377 shortens the time constant of the ASIC 134 connected to the detection element corresponding to the high dose region H1, as shown in FIG. On the other hand, the decision function 377 estimates that the X-ray dose incident on the detection element 130 is a low dose in an area where the count result of the 0-degree scanogram is less than the threshold in the channel direction, as shown in FIG. Select the dose region L1. And the determination function 377 lengthens the time constant of the ASIC 134 connected to the detection element corresponding to the low dose region L1, as shown in FIG.

  Further, for example, the determination function 377 estimates an area where the 90-degree scanogram count result is equal to or greater than the threshold in the channel direction, and the X-ray dose incident on the detection element 130 is a high dose, as shown in FIG. The high dose region H2 is selected. Then, the determination function 377 shortens the time constant of the ASIC 134 connected to the detection element corresponding to the high dose region H2, as shown in FIG. On the other hand, the determination function 377 estimates an area where the 90-degree scanogram count result is less than the threshold value in the channel direction, and the X-ray dose incident on the detection element 130 is low, as shown in FIG. The dose region L2 is selected. Then, the determination function 377 increases the time constant of the ASIC 134 connected to the detection element corresponding to the low dose region L2, as shown in FIG.

  As described above, when the imaging angle is 90 degrees, the determination function 377 selects a range narrower than the low-dose area L1 when the imaging angle is 0 degrees as the low-dose area L2. Further, when the imaging angle is 90 degrees, the determination function 377 selects a range wider than the high dose area H1 when the imaging angle is 0 degrees as the high dose area H2.

  Note that the determination function 377 may determine a time constant by performing threshold determination processing on a representative slice, and apply the determined time constant to other slices in the same manner. Each process may be performed to determine the time constant. The representative slice is, for example, a central slice in the slice direction.

  Then, the determination function 377 generates correspondence information in which each detection element 130 is associated with the imaging angle and the time constant of the ASIC 134 connected to the detection element 130. FIG. 12 is a diagram for explaining the second embodiment. For example, as illustrated in FIG. 12, the determination function 377 generates correspondence information in which an ID, the number of views, and a time constant are associated with each other.

  Here, “ID” in the correspondence information indicates an identifier for uniquely identifying the detection element 130. “Number of views” indicates the integrated value of the view trigger signal received from the gantry control device 16. For example, the number of views includes “N1 << N <N2” indicating that the range of the view number is N1 or more and less than N2, and “N2 << N <N3” indicating that the range of the view number is N2 or more and less than N3. Etc. are stored. The number of views is information corresponding to the imaging angle of the X-ray tube 12a, and is reset every time the X-ray tube 12a rotates once. Further, here, the position where the X-ray tube 12a is 0 degrees (the position in the front direction with respect to the subject P) is set to the view number 0, and the X-ray tube 12a starts imaging from the position of 0 degrees when imaging starts. Assume that the number of views is accumulated each time the X-ray tube 12a moves on a circular orbit. “Time constant” indicates the time constant of the ASIC 134 connected to the detection element identified by the ID in the corresponding number of views. In the example shown in FIG. 12, the detection element 130 in which the time constant T1 <time constant T2 and “ID” is “yyyy” is arranged in the high dose region in the peripheral portion in the channel direction in the X-ray detector 13. The detection element 130 whose “ID” is “xxxx” is arranged in the X-ray detector 13 in the low dose region in the center in the channel direction. In addition, the detection element 130 whose “ID” is “yyyyx” is arranged in a region between the central portion and the peripheral portion in the channel direction in the X-ray detector 13, and is low even in a high dose region according to the imaging angle. It can also be a dose region.

  As an example, the determination function 377 determines that the time constant of the ASIC 134 connected to the detection element 130 whose “ID” is “yyyy” is “T1” regardless of the number of views. The determination function 377 determines that the time constant of the ASIC 134 connected to the detection element 130 whose “ID” is “xxxx” is “T2” regardless of the number of views. The determination function 377 indicates that the time constant of the ASIC 134 connected to the detection element 130 whose “ID” is “yyyyx” is “T2” when the view number “N1 << N <N2”, and the view number “N2 << N <N3 ”is“ T1 ”, the view number“ N3 << N <N4 ”is“ T2 ”, and the view number“ N4 << N <N1 ”is“ T1 ”.

  The FPGA 14a sets a time constant corresponding to the position of each detection element 130 in the photon counting detector to the ASIC 134 connected to each detection element 130 at the time of photographing. For example, the FPGA 14 a sets a time constant based on the X-ray dose incident on each detection element 130 in each ASIC 134.

  For example, the FPGA 14a sets a time constant in each ASIC 134 with reference to the correspondence information shown in FIG. Here, the FPGA 14 a sets, for example, a time constant corresponding to the number of views at the start of shooting in each ASIC 134. Here, it is assumed that the number of views at the start of shooting is N1. The FPGA 14a sets a time constant T1 in the ASIC 134 connected to the detection element 130 whose ID is “yyyy”. Further, the FPGA 14a sets a time constant T2 in the ASIC 134 connected to the detection element 130 having IDs “yyyyx” and “xxxx”. That is, the FPGA 14 a sets a time constant based on the X-ray dose incident on each detection element 130 estimated from the counting result of each detection element 130 in each ASIC 134.

  Furthermore, the FPGA 14a sets a time constant corresponding to the number of views in each ASIC 134 during shooting. In other words, the FPGA 14a dynamically changes the time constant set before the start of shooting according to the shooting angle without being fixed during shooting. FIG. 13 is a flowchart illustrating a processing procedure performed by the FPGA 14a according to the second embodiment.

  Steps S101 to S106 are steps realized by the FPGA 14a. In step S <b> 101, the FPGA 14 a determines whether the start of imaging has been received from the gantry control device 16. Here, when the FPGA 14a determines that the start of imaging has been received from the gantry control device 16 (Yes in Step S101), the FPGA 14a proceeds to Step S102. On the other hand, if the FPGA 14a does not determine that the start of imaging has been received from the gantry control device 16 (No in step S101), the determination process in step S101 is repeated.

  In step S <b> 102, the FPGA 14 a determines whether a view trigger signal has been received from the gantry control device 16. If the FPGA 14a determines that a view trigger signal has been received from the gantry control device 16 (Yes in step S102), the process proceeds to step S103. On the other hand, if the FPGA 14a does not determine that the view trigger signal has been received from the gantry control device 16 (No in step S102), the determination process in step S102 is repeated.

  In step S103, the FPGA 14a reads the correspondence information. For example, the FPGA 14 a reads the correspondence information illustrated in FIG. 12 from the storage circuit 35. In step S104, the FPGA 14a determines whether to change the time constant. For example, the FPGA 14a updates the current view number every time a view trigger signal is received. Then, the FPGA 14a refers to the correspondence information, and compares the time constant corresponding to the updated number of views with the time constant of the updated number of views for each detection element. Here, the FPGA 14a determines to change the time constant when the compared time constants are different. On the other hand, the FPGA 14a determines not to change the time constant when the compared time constants are the same.

  If the FPGA 14a determines to change the time constant (step S104, Yes), the FPGA 14a proceeds to step S105. On the other hand, if the FPGA 14a does not determine to change the time constant (No at Step S104), the FPGA 14a proceeds to Step S106.

  In step S105, the FPGA 14a sets a time constant corresponding to the updated number of views. In step S <b> 106, the FPGA 14 a determines whether or not the end of photographing has been received from the gantry control device 16. Here, if the FPGA 14a determines that the end of shooting has been received from the gantry control device 16 (step S106, Yes), the shooting ends. On the other hand, if the FPGA 14a does not determine that the end of imaging has been received from the gantry control device 16 (No in step S106), the process proceeds to step S102 and executes the determination process.

  As described above, in the second embodiment, the FPGA 14a sets a time constant corresponding to the shooting angle in each ASIC 134 during shooting. Thus, according to the second embodiment, an optimal time constant can be set for all regions and angles to be imaged while the X-ray tube 12a and the X-ray detector 13 rotate once. For this reason, it becomes possible to obtain higher image quality and material discrimination ability.

  In the second embodiment described above, the case where the X-ray dose incident on each detection element is estimated and the time constant is determined using a two-dimensional scano image from two directions has been described. It is not limited to this. For example, to determine the time constant according to the imaging angle, the time constant may be determined by estimating the X-ray dose incident on each detection element based on a three-dimensional scano image. When estimating the X-ray dose incident on each detection element based on the three-dimensional scanogram and determining the time constant, it is possible to set the time constant with a finer range of imaging angles.

(Third embodiment)
In the third embodiment, a case where the X-ray dose incident on each detection element 130 is calculated in real time and a time constant is set based on the calculated X-ray dose will be described. The configuration of the X-ray CT apparatus 1 according to the third embodiment is the same as the configuration of the X-ray CT apparatus 1 shown in FIG. 1 except that the determination function 377 and a part of the FPGA 14a are different. For this reason, in the third embodiment, only the function executed by the determination function 377 and the FPGA 14a will be described.

  The determination function 377 determines a time constant based on the X-ray dose incident on each detection element 130 and generates correspondence information. For example, the determination function 377 generates correspondence information in which each detection element 130, the X-ray dose incident on each detection element 130, and the time constant of the ASIC 134 connected to each detection element 130 are associated with each other. FIG. 14 is a diagram for explaining the third embodiment. For example, as shown in FIG. 14, the determination function 377 generates correspondence information in which an ID, a count number, and a time constant are associated with each other.

  Here, “ID” in the correspondence information indicates an identifier that uniquely identifies the detection element 130. “Count” indicates the X-ray dose in view units incident on each detection element 130. For example, “C <C11” indicating that the count value is less than C11 is stored in the “count number” corresponding to “yyy” in “ID” as the X-ray dose in view units corresponding to the low-dose region. The Further, for example, in “count value” corresponding to “yyy” in “ID”, “C11 << indicating that the count value is C11 or more and less than C12 as an X-ray dose in view units corresponding to a slightly low dose region. C <C12 ”is stored. Further, for example, “count value” corresponding to “yyy” in “ID” indicates “C12 << indicating that the count value is C12 or more and less than C13 as an X-ray dose in view units corresponding to a slightly high dose region. C <C13 ”is stored. Further, for example, “C13 << C” indicating that the count value is C13 or more as the X-ray dose in view units corresponding to the high-dose region is included in the “count value” corresponding to “yyy” in “ID”. Stored.

  Each detection element 130 has variations due to individual differences. For this reason, it is desirable to set different counts between IDs. For example, “C <C21” indicating that the count value is less than C21 is stored in the “count number” corresponding to “yyyx” as “X dose in view units corresponding to the low dose region”. The In addition, for example, in “count value” corresponding to “yyyx” in “ID”, “C21 << indicating that the count value is C21 or more and less than C22 as an X-ray dose in view units corresponding to a slightly low dose region. C <C22 ”is stored. Further, for example, in “count value” corresponding to “ID” of “yyyx”, “C22 << indicating that the count value is C22 or more and less than C23 as an X-ray dose in view units corresponding to a slightly high dose region. C <C23 ”is stored. Further, for example, “C23 << C” indicating that the count value is C23 or more as the X-ray dose in the view unit corresponding to the high-dose region is included in the “count value” corresponding to “yyyx” in “ID”. Stored.

  Similarly, for example, in “count number” corresponding to “xxxx” in “ID”, “C <C31” indicating that the count value is less than C31 as the X-ray dose in view units corresponding to the low-dose region. Is stored. Further, for example, “count value” corresponding to “xxxx” in “ID” indicates “C31 << indicating that the count value is C31 or more and less than C32 as an X-ray dose in view units corresponding to a slightly low dose region. C <C32 ”is stored. Further, for example, in “count value” corresponding to “xxxx” in “ID”, “C32 << indicating that the count value is C32 or more and less than C33 as an X-ray dose in view units corresponding to a slightly high dose region. C <C33 ”is stored. Further, for example, in “count value” corresponding to “xxxx” in “ID”, “C33 << C” indicating that the count value is equal to or greater than C33 as the X dose in view units corresponding to the high dose region. Stored.

  “Time constant” indicates the time constant of the ASIC 134 connected to the detection element 130 identified by the ID in the corresponding count number of the view unit. In the example shown in FIG. 14, it is assumed that time constant V1 <time constant V2 <time constant V3 <time constant V4.

  As an example, the determination function 377 indicates that the time constant of the ASIC 134 connected to the detection element 130 whose “ID” is “yyy” is “X <C11” in which the count number corresponds to the low-dose region. "T4" and the X-ray dose "C11 << C <C12" corresponding to the slightly low dose region is "T3" and the X-ray dose "C12 << C <C13" corresponding to the slightly high-dose region Is “T2”, and the X-ray dose “C13 << C” corresponding to the high-dose region is determined to be “T1”.

  The FPGA 14a sets a time constant corresponding to the position of each detection element 130 in the photon counting detector to the ASIC 134 connected to each detection element 130 at the time of photographing. For example, the FPGA 14 a sets the time constant in the current view to the ASIC 134 connected to each detection element 130 based on the X-ray dose incident on each detection element 130 in the view before the current view. In the third embodiment, the FPGA 14a sets the same time constant as an initial value for all ASICs 134 at the first scan position (view number 1) at the start of imaging. Then, the FPGA 14a executes a process for setting each ASIC 134 with a time constant based on the X-ray dose incident on each detection element 130 during the photographing. In the third embodiment, a case is described in which the time constant is set using the count result of the previous view. However, the count result of the view before the current view is used, not limited to the count result of the previous view. You may set a time constant.

  FIG. 15 is a flowchart illustrating a processing procedure performed by the FPGA 14a according to the third embodiment. Note that the processing in step S201 and step S202 shown in FIG. 15 is the same as the processing in step S101 and step S102 shown in FIG.

  In step S203, the FPGA 14a acquires the count value from the counter 134e. For example, the FPGA 14 a sets the time constant in the current view to the ASIC 134 connected to each detection element 130 based on the X-ray dose incident on each detection element 130 in the view immediately before the current view.

  Here, first, a process of acquiring the count result of the immediately preceding view will be described. The FPGA 14a acquires the count result of the previous view from the counter 134e. FIG. 16 is a diagram for explaining the third embodiment. In FIG. 16, the output timing of the view trigger signal by the gantry control device 16 is shown. As shown in FIG. 16, one cycle of the view trigger signal is a period indicated by a double arrow 16a. This one-cycle view trigger signal consists of a positive pulse part in the first half and a negative pulse part in the second half. The FPGA 14a acquires the counting result counted during the period of the one-cycle view trigger signal from the counter 134e.

  In step S204, the FPGA 14a reads the correspondence information. For example, the FPGA 14 a reads the correspondence information illustrated in FIG. 14 from the storage circuit 35. In step S205, the FPGA 14a determines whether to change the time constant. For example, each time the FPGA 14a acquires the count result of the immediately preceding view, the FPGA 14a refers to the correspondence information and specifies the time constant corresponding to the count result of the immediately preceding view for each detection element 130. Then, the FPGA 14a compares the specified time constant with the currently set time constant. Here, the FPGA 14a determines to change the time constant when the compared time constants are different. On the other hand, the FPGA 14a determines not to change the time constant when the compared time constants are the same.

  If the FPGA 14a determines to change the time constant (step S205, Yes), the FPGA 14a proceeds to step S206. On the other hand, if the FPGA 14a does not determine to change the time constant (No at Step S205), the FPGA 14a proceeds to Step S207.

  In step S206, the FPGA 14a sets a time constant corresponding to the count result of the immediately preceding view. For example, when the count result acquired from the counter 134e that counts the output result of the detection element 130 identified by the ID “yyy” is a value that is greater than or equal to C11 and less than C12, the FPGA 14a has an ASIC 134 that corresponds to the ID “yyy”. Is set to a time constant T3. For example, the FPGA 14 a corresponds to “yyyx” when the count result acquired from the counter 134 e that counts the output result of the detection element 130 identified by ID “yyyyx” is a value less than or equal to C22 and less than C23. A time constant T2 is set in the ASIC 134 to be operated.

  In step S207, the FPGA 14a determines whether or not the end of photographing has been received from the gantry control device 16. Here, if the FPGA 14a determines that the end of shooting has been received from the gantry control device 16 (Yes in step S207), the shooting ends. On the other hand, when the FPGA 14a does not determine that the end of imaging has been received from the gantry control device 16 (No in Step S207), the FPGA 14a proceeds to Step S202 and executes the determination process.

  As described above, in the third embodiment, the FPGA 14a sets a time constant based on the count result of the immediately preceding view for each view in each ASIC 134 during shooting. As a result, according to the third embodiment, it is possible to set an optimal time constant for all regions and angles to be imaged while the X-ray tube 12a and the X-ray detector 13 rotate once. For this reason, it becomes possible to obtain higher image quality and material discrimination ability.

  Note that the reconstruction processing function 373 may perform substance discrimination on the projection data and reconstruct a substance discrimination image using substance discrimination information that is a result of substance discrimination. Here, the reconstruction processing function 373, in the case of reconstructing a substance discrimination image, in addition to the process of correcting the count number of detection data, in the first embodiment and the modification of the first embodiment The described processing for correcting the spectrum of the detection data is executed. For example, the correspondence information shown in FIG. 13 is also stored in the storage circuit 35 of the console 30. The composition processing function 373 selects an appropriate response function with reference to the set value of the time constant when reconstructing the substance discrimination image.

(Modification of the third embodiment)
In the third embodiment described above, the case where the count value is calculated in the time of one cycle of the view trigger signal has been described, but the embodiment is not limited to this. For example, the count value may be calculated in the first half or the second half of the view trigger signal for one cycle. In other words, the FPGA 14 a sets the time constant in the current view to the ASIC 134 connected to each detection element 130 based on the X-ray dose incident on each detection element 130 in a predetermined period of the past view period. FIG. 17 is a diagram for explaining a modification of the third embodiment. The upper part of FIG. 17 describes the case where the count value is calculated in the first half of the view trigger signal for one cycle, and the lower part of FIG. 17 is the case where the count value is calculated in the second half of the view trigger signal for one cycle. Will be described.

  More specifically, as shown in the upper part of FIG. 17, the FPGA 14a calculates the number of counts only from the first positive pulse part 17a of the view trigger signal and selects the time constant in the second negative pulse part 17b. A time constant is set at the change point 17c of the view trigger pulse. Alternatively, as shown in the lower part of FIG. 17, the FPGA 14a calculates the count from only the latter negative pulse portion 17d of the view trigger signal, selects the time constant in the first positive pulse portion 17e, and selects the next view trigger pulse. The time constant is set at the change point 17f. As described above, the FPGA 14a sets the time constant in the current view to the ASIC 134 connected to each detection element 130 based on the X-ray dose incident on each detection element 130 in the first half or the second half of the past view period. To do.

  Thereby, the FPGA 14a can eliminate the time lag in selecting the time constant and setting the time constant. In such a case, the counter 134e outputs a count result for setting a time constant to the FPGA 14a separately from the count result for one cycle of the view trigger signal for reconstructing the X-ray CT image data. In addition, the period for calculating the count value is not limited to the first half or the second half of the view trigger signal for one cycle, and is the half of the first half or the second half of the view trigger signal for one cycle. It may be a period.

  In the third embodiment described above, the FPGA 14a has been described as setting the same time constant as an initial value for all ASICs 134 at the first scan position (view number 1) at the start of imaging. The embodiment is not limited to this. For example, the FPGA 14a may set the time constant of the first scan position (view number 1) using the method described in the first embodiment. That is, the FPGA 14a is connected to each detection element 130 with a time constant based on the X-ray dose incident on each detection element 130, referring to the correspondence information shown in FIG. ASIC 134 is set.

  In the third embodiment described above, the case where the time constant is set for each view has been described. However, the embodiment is not limited to this. For example, the FPGA 14a may set a time constant for each predetermined view such as every five views.

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

  In the above-described embodiment, a case has been described in which the time constant is determined using a two-dimensional scan image or a three-dimensional scan image obtained by photographing a scanogram before photographing the main scan. The form is not limited to this. For example, the time constant may be determined using a past image of the same subject or an image captured by another medical image diagnostic apparatus.

  The determination function 377 may determine the time constant by estimating the X-ray dose incident on each detection element 130 from information related to the body type of the subject. For example, the determination function 377 acquires the body thickness of the subject as information related to the body shape from the reconstructed image obtained by reconstructing a three-dimensional scanogram, and estimates the X-ray dose incident on each detection element 130. More specifically, the determination function 377 acquires the vertical and horizontal lengths in the axial image obtained by reconstructing the three-dimensional scano image as the body thickness of the subject, and generates an X-ray according to the body thickness. The X-ray dose incident on each detection element 130 is estimated from the attenuation amount. Then, the determination function 377 performs a threshold determination process on the estimated X-ray dose and determines a time constant. In such a case, the FPGA 14 a sets a time constant based on the X-ray dose incident on each detection element 130 estimated from information related to the body shape of the subject in each ASIC 134.

  Further, the determination function 377 may determine the time constant by estimating the X-ray dose incident on each detection element 130 from information related to the body shape of the subject stored as patient information. For example, the determination function 377 acquires information such as the height, weight, chest circumference, and waist circumference of the subject P stored in the storage circuit 35 as patient information, and estimates the X-ray dose incident on each detection element 130. Then, the determination function 377 performs a threshold determination process on the estimated X-ray dose and determines a time constant. Further, the determination function 377 receives input of information such as the height, weight, chest circumference, and girth of the subject P from the operator via the input interface 31, and determines the X-ray dose incident on each detection element 130 from the received information. A time constant may be determined by estimation. In such a case, the FPGA 14a sets a time constant based on the X-ray dose incident on each detection element 130, which is estimated from information related to the body shape of the subject stored as patient information, in each ASIC 134.

  Moreover, although embodiment mentioned above demonstrated the case where one threshold value was set and two types of time constants were set, embodiment is not limited to this. For example, a plurality of threshold values may be set and three or more types of time constants may be determined. In such a case, the FPGA 14 a sets the determined three or more types of time constants in each ASIC 134.

  In the above-described embodiment, the time constant is determined based on the X-ray dose incident on the detection element 130. However, the embodiment is not limited to this. For example, the time constant may be set differently in the center and the edge in the channel direction without estimating the X-ray dose incident on each detection element. For example, the FPGA 14a is disposed in the ASIC 134 connected to the detection element 130 disposed in the first region that is the central portion in the channel direction of the X-ray detector 13 and in the second region that is the peripheral portion in the channel direction. A time constant larger than that of the ASIC 134 connected to the detection element 130 is set. Further, the FPGA 14a may set a time constant that gradually decreases from the center to the periphery in the channel direction of the X-ray detector 13.

  Further, the FPGA 14a may set each ASIC 134 with a time constant corresponding to the imaging angle without estimating the X-ray dose incident on each detection element. For example, the FPGA 14a changes the range of the first area and the range of the second area according to the shooting angle, and connects the ASIC 134 connected to the detection element 130 arranged in the first area after the change. A time constant larger than that of the ASIC 134 connected to the detection element 130 arranged in the changed second region may be set.

  In the above-described embodiment, the ASIC 134 is described as being disposed in the X-ray detector 13, but the embodiment is not limited to this. For example, the ASIC 134 may be disposed in the data collection circuit 14.

  In the above-described embodiment, the reconstruction processing function 373 has been described as performing material discrimination on projection data to obtain material discrimination information, but the embodiment is not limited thereto. For example, the reconstruction processing function 373 may perform substance discrimination on the image data to obtain substance discrimination information.

  In the above-described embodiment, the X-ray CT apparatus 1 of the Rotate / Rotate-Type (third generation CT) that rotates around the subject with the X-ray tube 12a and the X-ray detector 13 integrated is described. The form is not limited to this. For example, in the X-ray CT apparatus, in addition to the third generation CT, X-ray detectors having a plurality of X-ray detection elements are dispersed and fixed in a ring shape, and only the X-ray tube rotates around the subject. There is a Stationary / Rotate-Type (fourth generation CT). The embodiment described above can also be applied to the fourth generation CT. The above-described embodiment can also be applied to a hybrid X-ray CT apparatus that combines the third generation CT and the fourth generation CT.

  The above-described embodiment can also be applied to a conventional one-tube type X-ray CT apparatus, and a so-called multi-tube bulb in which a plurality of pairs of X-ray tubes and detectors are mounted on a rotating ring. The present invention is also applicable to a type of X-ray CT apparatus.

  In the above-described embodiment, the processing circuit 37 has been described as executing a plurality of functions. However, the embodiment is not limited to this. For example, a plurality of functions may be provided in the console 30 as independent circuits, and each circuit may execute each function. For example, a determination function 377 executed by the processing circuit 37 may be provided as a determination circuit, and the determination circuit may execute the determination function. Further, the preprocessing function 372 executed by the processing circuit 37 may be provided as a preprocessing circuit, and the preprocessing circuit may execute the preprocessing function. Further, the reconstruction processing function 373 executed by the processing circuit 37 may be provided as a reconstruction processing circuit, and the reconstruction processing circuit may execute the reconstruction processing function.

  In the above-described embodiment, the preprocessing function 372, the determination function 377, and the reconstruction processing function 373 are described as being executed in the console 30, but the embodiment is not limited to this. For example, the pre-processing function 372, the determination function 377, and the reconfiguration processing function 373 may be executed in an external workstation.

  Further, the time constant determination process and the substance discrimination process described in the above-described embodiment can be realized by software. For example, the time constant determination process is realized by causing a computer to execute a time constant determination program that defines the processing procedure described as being performed by the determination function 377 in the above embodiment. Further, for example, the substance discrimination processing is realized by causing a computer to execute a substance discrimination program that defines the processing procedure described as being performed by the preprocessing function 372 and the reconstruction processing function 373 in the above embodiment. These time constant determination program and substance discrimination program are stored in, for example, a hard disk or a semiconductor memory device, and read and executed by a processor such as a CPU or MPU. The substance discrimination 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).

  The term “processor” used in the above description is, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), an application specific integrated circuit (ASIC), a programmable logic device (for example, It means circuits such as a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA). The processor implements a function by reading and executing a program incorporated in the processor circuit. Instead of incorporating the program into the processor circuit, the program may be stored in the storage circuit 35 of the console 30. In this case, the processor implements the function by reading and executing the program stored in the storage circuit 35. Note that each processor of the present embodiment is not limited to being configured as a single circuit for each processor, but may be configured as a single processor by combining a plurality of independent circuits to realize the function. Good. Furthermore, a plurality of components in FIG. 1 may be integrated into one processor to realize the function.

  In the description of the above embodiment, each component of each illustrated apparatus is functionally conceptual and does not necessarily need to be physically configured as illustrated. In other words, the specific form of distribution / integration of each device is not limited to the one shown in the figure, and all or a part of the distribution / integration is functionally or physically distributed in arbitrary units according to various loads or usage conditions. Can be integrated and configured. Further, all or a part of each processing function performed in each device may be realized by a CPU and a program that is analyzed and executed by the CPU, or may be realized as hardware by wired logic.

  Moreover, the control method demonstrated by said embodiment is realizable by executing the control program prepared beforehand by computers, such as a personal computer and a workstation. This control program can be distributed via a network such as the Internet. The control program can also be executed by being recorded on a computer-readable recording medium such as a hard disk, a flexible disk (FD), a CD-ROM, an MO, and a DVD and being read from the recording medium by the computer.

  According to at least one embodiment described above, high substance discrimination ability can be realized.

  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.

10 frame 13 X-ray detector 14 data acquisition circuit 14a FPGA
30 Console 37 Processing circuit 372 Preprocessing function 373 Reconfiguration processing function 377 Determination function

Claims (14)

A photon counting detector having a plurality of detection units each including a detection element that detects an X-ray photon and a counting circuit that is connected to the detection element and counts the X-ray photons detected by the detection element;
A setting unit for setting a time constant according to the position of each detection element in the photon counting detector to a counting circuit connected to each detection element at the time of photographing;
An X-ray CT apparatus comprising:   The X-ray CT apparatus according to claim 1, wherein the setting unit sets a time constant based on an X-ray dose incident on each detection element in each counting circuit.   The X-ray CT apparatus according to claim 2, wherein the setting unit sets, in each counting circuit, a time constant based on an X-ray dose incident on each detecting element, which is estimated from a counting result of each detecting element.   The X-ray CT apparatus according to claim 2, wherein the setting unit sets, in each counting circuit, a time constant based on an X-ray dose incident on each detection element, which is estimated from information related to a body shape of the subject.   The setting unit sets a larger time constant in a counting circuit connected to a detection element whose incident X-ray dose is less than a threshold value than a counting circuit connected to a detection element whose incident X-ray dose is equal to or more than the threshold value. The X-ray CT apparatus according to any one of claims 2 to 4.   The X-ray CT apparatus according to claim 1, wherein the setting unit sets a time constant corresponding to an imaging angle in each counting circuit.   The setting unit is connected to a counting circuit connected to a detection element disposed in a first region which is a central portion in the channel direction of the photon counting detector, and a second region which is a peripheral portion in the channel direction. The X-ray CT apparatus according to claim 1, wherein a time constant larger than that of a counting circuit connected to a detection element arranged in the circuit is set.   The setting unit changes the range of the first region and the range of the second region according to the shooting angle, and counts connected to the detection elements arranged in the changed first region. The X-ray CT apparatus according to claim 7, wherein a time constant larger than that of the counting circuit connected to the detection element arranged in the second region after the change is set in the circuit.   The setting unit sets a time constant in a current view in a counting circuit connected to each detection element based on an X-ray dose incident on each detection element in a past view before the current view. The X-ray CT apparatus described.   The setting unit uses a view immediately before the current view as the past view, and a counting circuit in which a time constant in the current view is connected to each detection element based on an X-ray dose incident on each detection element in the past view The X-ray CT apparatus according to claim 9, wherein   The setting unit sets a time constant in a current view in a counting circuit connected to each detection element based on an X-ray dose incident on each detection element in a predetermined period of the past view period. The X-ray CT apparatus according to 9 or 10.   The setting unit sets a time constant in the current view in a counting circuit connected to each detection element based on an X-ray dose incident on each detection element in the first half or second half of the period of the past view. The X-ray CT apparatus according to claim 11. A collection unit for collecting the counting results by the photon counting detector;
A correction unit for correcting the counting result;
A reconstruction processing unit that reconstructs an image based on the corrected count result;
The X-ray CT apparatus according to claim 1, further comprising:   The X-ray CT apparatus according to claim 1, wherein a time constant of each counting circuit is determined by an integration time in each counting circuit.

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