X-ray tube control system and x-ray computed tomography apparatus

Abstract

To reduce the difference in X-ray dose between the application of a high tube voltage and the application of a low tube voltage.SOLUTION: An X-ray tube control system according to an embodiment includes a tube voltage control unit and a grid control unit. The tube voltage control unit alternately switches a tube voltage applied between a cathode that emits electrons and an anode that receives the electrons from the cathode to generate X-rays, between a first tube voltage and a second tube voltage higher than the first tube voltage. The grid control unit alternately switches a grid voltage applied between the cathode and a grid electrode that controls electrons traveling from the cathode toward the anode, between a first grid voltage and a second grid voltage higher than the first grid voltage. The grid control unit performs control to lower and hold the grid voltage at the first grid voltage only during a first transition period in which the tube voltage transitions from the second tube voltage to the first tube voltage and during a first holding period in which the tube voltage is held at the first tube voltage, so that the tube current does not exceed an upper limit tube current determined based on allowable power.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The embodiments disclosed in this specification and the drawings relate to an X-ray tube control system and an X-ray computed tomography apparatus. [Background technology]

[0002] Dual energy scans are used to perform X-ray CT (Computed Tomography) imaging while alternately switching the tube voltage between high and low. In dual energy scans, it is desirable to collect projection data at the same X-ray dose when applying high and low tube voltages in order to equalize the quality of the projection data collected when applying high and low tube voltages. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-73115 [Patent Document 2] Patent Publication No. 2021-83472 [Patent Document 3] Japanese Patent Application Publication No. 2020-64720 Summary of the Invention [Problem to be solved by the invention]

[0004] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to reduce the difference in X-ray dose between when a high tube voltage is applied and when a low tube voltage is applied. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problem. Problems corresponding to the effects of each configuration shown in the embodiments described below can also be positioned as other problems. [Means for solving the problem]

[0005] An X-ray tube control system according to an embodiment includes a tube voltage controller and a grid controller. The tube voltage controller alternately switches a tube voltage applied between a cathode that emits electrons and an anode that receives electrons from the cathode and generates X-rays between a first tube voltage and a second tube voltage higher than the first tube voltage. The grid controller alternately switches a grid voltage applied between the cathode and a grid electrode that controls electrons traveling from the cathode to the anode between a first grid voltage and a second grid voltage higher than the first grid voltage. The grid controller controls the grid voltage to drop to the first grid voltage and maintain the first grid voltage only during a first transition period during which the tube voltage transitions from the second tube voltage to the first tube voltage and a first hold period during which the tube voltage is maintained at the first tube voltage, so that the tube current does not exceed an upper limit tube current based on an allowable power. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a diagram showing an example of the arrangement of an X-ray computed tomography apparatus according to this embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of an X-ray tube control system according to this embodiment. [Figure 3] FIG. 3 is a diagram showing a timing chart of KV switching in the first comparative example. [Figure 4] FIG. 4 is a timing chart of KV switching in the second comparative example. [Figure 5] FIG. 5 is a timing chart of KV switching in the first embodiment. [Figure 6] FIG. 6 is a timing chart of KV switching in the second embodiment. [Figure 7] FIG. 7 is a diagram illustrating a configuration example of a grid power supply circuit according to the second embodiment. [Figure 8] FIG. 8 is a schematic circuit diagram of a grid power supply circuit for both series and parallel connection, showing the connection relationship of switches that form a series circuit when a high voltage is applied. [Figure 9]FIG. 9 is a schematic circuit diagram of a grid power supply circuit for both series and parallel connection, showing the connection relationship of switches that form a parallel circuit when a low voltage is applied. [Figure 10] FIG. 10 is a timing chart of KV switching in the third embodiment. [Figure 11] FIG. 11 is a timing chart of KV switching in the fourth embodiment. [Figure 12] FIG. 12 is a timing chart of KV switching in the fifth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, the X-ray tube control system and the X-ray computed tomography apparatus according to this embodiment will be described in detail with reference to the drawings.

[0008] There are various types of X-ray computed tomography apparatuses (CT apparatuses), such as third-generation CT and fourth-generation CT, and any of these types can be applied to this embodiment. Here, the third-generation CT is a rotate / rotate-type in which the X-ray tube and detector rotate together around the subject. The fourth-generation CT is a stationary / rotate-type in which a large number of X-ray detection elements arranged in a ring shape are fixed, and only the X-ray tube rotates around the subject.

[0009] FIG. 1 is a diagram showing an example of the configuration of an X-ray computed tomography apparatus 1 according to this embodiment. As shown in FIG. 1, the X-ray computed tomography apparatus 1 includes a gantry 10, a bed 30, and a console 40. Although FIG. 1 illustrates multiple gantry 10s for ease of explanation, the apparatus may actually include one or multiple gantry 10s. The gantry 10 is a scanning device configured to perform X-ray CT scans of a subject P. The bed 30 is a transport device on which the subject P to be scanned for X-ray CT scans is placed and which positions the subject P. The console 40 is a computer that controls the gantry 10. For example, the gantry 10 and the bed 30 are installed in a CT examination room, and the console 40 is installed in a control room adjacent to the CT examination room. The gantry 10, the bed 30, and the console 40 are connected to each other by wire or wirelessly so that they can communicate with each other. The console 40 does not necessarily have to be installed in the control room. For example, the console 40 may be installed in the same room as the gantry 10 and the bed 30. The console 40 may also be incorporated into the cradle 10 .

[0010] As shown in FIG. 1, the gantry 10 includes an X-ray tube 11, an X-ray detector 12, a rotating frame 13, an X-ray high voltage device 14, a control device 15, a wedge 16, a collimator 17, and a data acquisition system (DAS) 18.

[0011] The X-ray tube 11 irradiates the subject P with X-rays. Specifically, the X-ray tube 11 includes a cathode that generates thermoelectrons, an anode that generates X-rays upon receiving thermoelectrons flying from the cathode, and a vacuum tube that holds the cathode and anode. The X-ray tube 11 is connected to the X-ray high voltage device 14 via a high-voltage cable. A tube voltage is applied between the cathode and the anode by the X-ray high voltage device 14. The application of the tube voltage causes thermoelectrons to fly from the cathode toward the anode. A tube current flows as the thermoelectrons fly from the cathode toward the anode. X-rays are generated when the thermoelectrons collide with the anode.

[0012] The X-ray detector 12 detects X-rays emitted from the X-ray tube 11 and passing through the subject P, and outputs an electrical signal corresponding to the amount of detected X-rays to the data acquisition circuitry 18. The X-ray detector 12 has a structure in which a plurality of X-ray detection element rows, each of which has a plurality of X-ray detection elements arranged in the channel direction, are arranged in the slice direction (row direction). The X-ray detector 12 is, for example, an indirect conversion type detector having a grid, a scintillator array, and a photosensor array. The scintillator array has a plurality of scintillators. The scintillator outputs light with an amount of light corresponding to the amount of incident X-rays. The grid is arranged on the X-ray incident surface side of the scintillator array and has an X-ray shielding plate that absorbs scattered X-rays. The grid is sometimes called a collimator (one-dimensional collimator or two-dimensional collimator). The photosensor array converts the light from the scintillator into an electrical signal corresponding to the amount of light. For example, a photodiode is used as the photosensor. The X-ray detector 12 may also be a direct conversion type detector.

[0013] The rotating frame 13 is an annular frame that supports the X-ray tube 11 and the X-ray detector 12 rotatably around a rotation axis (Z-axis). Specifically, the rotating frame 13 supports the X-ray tube 11 and the X-ray detector 12 so that they face each other. The rotating frame 13 is supported on a fixed frame (not shown) so that it can rotate around the rotation axis. The rotating frame 13 is rotated around the rotation axis by the control device 15, thereby rotating the X-ray tube 11 and the X-ray detector 12 around the rotation axis. The rotating frame 13 receives power from a drive mechanism of the control device 15 and rotates around the rotation axis at a constant angular velocity. An image field of view (FOV) is set in an opening 19 of the rotating frame 13.

[0014] In this embodiment, the rotation axis of the rotating frame 13 in the non-tilted state or the longitudinal direction of the tabletop 33 of the bed 30 is defined as the Z-axis direction, the axis direction perpendicular to the Z-axis direction and horizontal to the floor surface is defined as the X-axis direction, and the axis direction perpendicular to the Z-axis direction and perpendicular to the floor surface is defined as the Y-axis direction.

[0015] The X-ray high voltage device 14 has a high voltage generator and an X-ray control device. The high voltage generator has electrical circuits such as a transformer and a rectifier, and generates a high voltage to be applied to the X-ray tube 11 and a filament current to be supplied to the X-ray tube 11. The X-ray control device controls the output voltage according to the X-rays emitted by the X-ray tube 11. The high voltage generator may be of a transformer type or an inverter type. The X-ray high voltage device 14 may be provided on the rotating frame 13 in the gantry 10, or on a fixed frame (not shown) in the gantry 10.

[0016] The wedge 16 adjusts the amount of X-rays irradiated onto the subject P. Specifically, the wedge 16 attenuates the X-rays so that the amount of X-rays irradiated onto the subject P from the X-ray tube 11 has a predetermined distribution. For example, the wedge 16 is made of a metal plate such as aluminum, such as a wedge filter or a bow-tie filter.

[0017] The collimator 17 limits the irradiation range of the X-rays that have passed through the wedge 16. The collimator 17 slidably supports multiple lead plates that shield the X-rays, and adjusts the shape of the slits formed by the multiple lead plates. The collimator 17 is sometimes called an X-ray aperture.

[0018] The data acquisition circuitry 18 reads out from the X-ray detector 12 an electrical signal corresponding to the X-ray dose detected by the X-ray detector 12. The data acquisition circuitry 18 amplifies the read electrical signal and integrates the electrical signal over a view period to acquire detection data having a digital value corresponding to the X-ray dose over the view period. The detection data is called projection data. The data acquisition circuitry 18 is realized, for example, by an application specific integrated circuit (ASIC) equipped with circuit elements capable of generating projection data. The projection data is transmitted to the console 40 via a non-contact data transmission device or the like.

[0019] In this embodiment, an integral type X-ray detector 12 and an X-ray computed tomography apparatus 1 equipped with an integral type X-ray detector 12 are described as examples, but the technology according to this embodiment can also be applied to a photon counting type X-ray detector.

[0020] The control device 15 controls the X-ray high-voltage generator 14 and the data acquisition circuit 18 to perform X-ray CT imaging in accordance with the scan control function 51 of the processing circuit 45 of the console 40. The control device 15 includes a processing circuit having a central processing unit (CPU) or a microprocessing unit (MPU), etc., and a drive mechanism such as a motor and an actuator. The processing circuit includes, as hardware resources, a processor such as a CPU and memory such as a read-only memory (ROM) or a random-access memory (RAM). The control device 15 executes various functions using a processor that executes programs loaded in the memory. Note that various functions are not limited to being implemented by a single processing circuit. A processing circuit may be configured by combining multiple independent processors, and each processor may execute a program to implement each function. The control device 15 may also be implemented using an ASIC or a field programmable gate array (FPGA). The control device 15 may also be realized by another complex programmable logic device (CPLD) or simple programmable logic device (SPLD). The control device 15 has a function of receiving input signals from an input interface 43 (described later) attached to the console 40 or the gantry 10 and controlling the operation of the gantry 10 and the bed 30. For example, the control device 15 receives input signals and controls the rotation of the rotating frame 13, the tilt of the gantry 10, and the operation of the bed 30 and the tabletop 33. The control of tilting the gantry 10 is realized by the control device 15 rotating the rotating frame 13 around an axis parallel to the X-axis direction based on inclination angle (tilt angle) information input via an input interface attached to the gantry 10. The control device 15 may be provided in the gantry 10 or in the console 40.

[0021] The bed 30 includes a base 31, a support frame 32, a top plate 33, and a bed driving device 34. The base 31 is placed on the floor. The base 31 is a housing that supports the support frame 32 so that it can move vertically (in the Y-axis direction) relative to the floor. The support frame 32 is a frame provided on top of the base 31. The support frame 32 supports the top plate 33 so that it can slide along the rotation axis (Z-axis). The top plate 33 is a flexible plate on which the subject P is placed.

[0022] The bed driving device 34 is housed in the housing of the bed 30. The bed driving device 34 is a motor or actuator that generates power to move the support frame 32 on which the subject P is placed and the tabletop 33. The bed driving device 34 operates under the control of the console 40 or the like.

[0023] The console 40 has a memory 41, a display 42, an input interface 43, a communication interface 44, and a processing circuit 45. Data communication between the memory 41, the display 42, the input interface 43, the communication interface 44, and the processing circuit 45 is performed via a bus (BUS). Note that although the console 40 will be described as being separate from the gantry 10, the gantry 10 may include the console 40 or some of the components of the console 40.

[0024] The memory 41 is a storage device such as a hard disk drive (HDD), a solid state drive (SSD), or an integrated circuit storage device that stores various information. In addition to an HDD or SSD, the memory 41 may be a portable storage medium such as a compact disc (CD), a digital versatile disc (DVD), a Blu-ray (registered trademark) disc (BD), or a flash memory. The memory 41 may also be a drive device that reads and writes various information from and to a semiconductor memory element such as a flash memory or a RAM. The storage area of ​​the memory 41 may be located within the X-ray computed tomography apparatus 1 or in an external storage device connected via a network. The memory 41 stores, for example, projection data and reconstructed image data.

[0025] The display 42 displays various types of information. For example, the display 42 outputs CT images generated by the processing circuitry 45, a GUI (Graphical User Interface) for receiving various operations from the operator, and the like. Any of a variety of displays can be used as the display 42, as appropriate. For example, the display 42 can be a liquid crystal display (LCD), a cathode ray tube (CRT) display, an organic electroluminescence display (OLED), or a plasma display.

[0026] The display 42 may be installed anywhere in the control room. Alternatively, the display 42 may be installed on the pedestal 10. The display 42 may be a desktop type, or may be configured as a tablet terminal or the like capable of wireless communication with the main body of the console 40. Alternatively, one or more projectors may be used as the display 42.

[0027] The input interface 43 accepts various input operations from the operator, converts the accepted input operations into electrical signals, and outputs the electrical signals to the processing circuitry 45. For example, the input interface 43 accepts from the operator acquisition conditions for acquiring projection data, reconstruction conditions for reconstructing CT images, and image processing conditions for generating post-processed images from CT images. Examples of the input interface 43 that can be used include a mouse, keyboard, trackball, switch, button, joystick, touchpad, and touch panel display, as appropriate. Note that in this embodiment, the input interface 43 is not limited to a device equipped with physical operation components such as a mouse, keyboard, trackball, switch, button, joystick, touchpad, and touch panel display. For example, an electrical signal processing circuit that receives an electrical signal corresponding to an input operation from an external input device provided separately from the device and outputs the electrical signal to the processing circuitry 45 is also included as an example of the input interface 43. The input interface 43 may also be provided on the gantry 10. The input interface 43 may also be configured as a tablet terminal or the like capable of wireless communication with the console 40.

[0028] The communication interface 44 includes a network interface card (NIC) for communicating various data via a network with external devices such as a workstation, PACS (Picture Archiving and Communication Systems), RIS (Radiology Information System), and HIS (Hospital Information System).

[0029] The processing circuitry 45 controls the overall operation of the X-ray computed tomography apparatus 1 in response to electrical signals of input operations output from the input interface 43. The processing circuitry 45 generates image data based on electrical signals output from the X-ray detector 12. For example, the processing circuitry 45 has, as hardware resources, processors such as a CPU, MPU, or GPU, and memories such as ROM and RAM. The processing circuitry 45 executes a scan control function 51, a reconstruction function 52, an image processing function 53, a display control function 54, and the like, by using a processor that executes programs loaded in the memory.

[0030] It should be noted that each of the functions 51 to 54 does not necessarily have to be realized by a single processing circuit, but may be realized by combining a plurality of independent processors to form a processing circuit, and each processor may execute a program to realize each of the functions 51 to 54.

[0031] The processing circuitry 45 executes a dual energy scan by controlling the X-ray high voltage generator 14, the control device 15, and the data acquisition circuitry 18 according to preset scan conditions using the scan control function 51. The scan conditions include the tube voltage, grid voltage, tube current, whether or not to use tube current modulation, the rotation speed of the rotating frame 13, the scan range, and the scan region.

[0032] Using the reconstruction function 52, the processing circuitry 45 performs preprocessing on the projection data output from the data acquisition circuitry 18, such as logarithmic conversion, offset correction, inter-channel sensitivity correction, beam hardening correction, and interpolation for data loss due to tube voltage switching. The processing circuitry 45 performs material decomposition on the preprocessed projection data and performs reconstruction on the material-decomposed projection data to generate a CT image (hereinafter referred to as a reference material image). The reconstruction process can use filtered back projection, iterative reconstruction, or reconstruction processes that apply machine learning. The processing circuitry 45 may also perform reconstruction on projection data without material decomposition to generate a CT image (also called an integral image).

[0033] Using the image processing function 53, the processing circuitry 45 converts the CT image generated by the reconstruction function 52 into a cross-sectional image of an arbitrary cross section or a rendering image of an arbitrary viewpoint direction. The conversion is performed based on an input operation received from an operator via the input interface 43. For example, the processing circuitry 45 performs three-dimensional image processing such as volume rendering, surface volume rendering, image value projection processing, MPR (Multi-Planer Reconstruction) processing, and CPR (Curved MPR) processing on the CT image to generate a rendering image of an arbitrary viewpoint direction. Note that the generation of a rendering image of an arbitrary viewpoint direction may be performed directly by the reconstruction function 52.

[0034] The display control function 54 causes the processing circuitry 45 to display various pieces of information on the display 42. As an example, the processing circuitry 45 displays various images generated by the image processing function 53 on the display 42.

[0035] Although the console 40 has been described as a single console that executes multiple functions, multiple functions may be executed by separate consoles. The processing circuitry 45 is not limited to being included in the console 40, but may also be included in an integrated server that collectively processes projection data acquired by multiple medical image diagnostic devices. Post-processing may be performed by either the console 40 or an external workstation. Furthermore, processing may be performed simultaneously by both the console 40 and the workstation.

[0036] Next, an explanation will be given of an X-ray tube control system 100 according to this embodiment, which includes the X-ray tube 11 and the X-ray high voltage device 14. The X-ray tube control system 100 according to this embodiment is assumed to be installed in an X-ray computed tomography apparatus 1.

[0037] 2 is a diagram showing an example of the configuration of an X-ray tube control system 100 according to this embodiment. As shown in FIG. 2, the X-ray tube control system 100 includes an X-ray tube 11 and an X-ray high voltage device 14.

[0038] The X-ray tube 11 is a vacuum vessel that houses a cathode 61, an anode 63, and a grid electrode 65. The cathode 61 emits electrons. Specifically, the cathode 61 has a filament 62 formed of a thin wire-like metal such as tungsten or nickel. The cathode 61 is connected to the X-ray high voltage device 14 via a cable or the like. The filament 62 receives a current for heating (hereinafter, filament current) from the X-ray high voltage device 14, generates heat, and emits electrons (thermions).

[0039] The anode 63 is a disk-shaped electrode made of a heavy metal such as tungsten or molybdenum. The anode 63 rotates as the rotor (not shown) rotates around its axis. A high tube voltage is applied between the cathode 61 and the anode 63 by the X-ray high voltage device 14. Electrons emitted from the cathode 61 fly toward and collide with the anode 63 due to the action of the tube voltage. A tube current flows as the electrons flow from the cathode 61 to the anode 63. The anode 63 receives the electrons and generates X-rays. The area on the anode 63 where the electrons collide forms a focal spot.

[0040] The grid electrode 65 is an electrode disposed between the cathode 61 and the anode 63. The grid electrode 65 controls electrons traveling from the cathode 61 to the anode 63. Specifically, a grid voltage with respect to the cathode potential is applied to the grid electrode 65 by the X-ray high voltage device 14.

[0041] 2, the X-ray high voltage device 14 has a tube voltage power supply circuit 71, a filament heating circuit 72, a grid power supply circuit 73, and a control circuit 74. The tube voltage power supply circuit 71, the filament heating circuit 72, and the grid power supply circuit 73 are each connected to the control circuit 74 via signal lines or the like.

[0042] The tube voltage power supply circuit 71 generates a tube voltage to be applied between the cathode 61 and the anode 63 under the control of the control circuit 74. For example, in the case of an inverter-type X-ray high voltage device, the tube voltage power supply circuit 71 includes an AC / DC converter that converts AC voltage from a commercial power source into DC voltage, an inverter that converts the DC voltage of the AC / DC converter into AC voltage, a transformer that boosts the AC voltage from the inverter, and a high-voltage rectifying and smoothing circuit that rectifies and smoothes the AC voltage boosted by the transformer to generate a high-DC voltage. The high-DC voltage from the high-voltage rectifying and smoothing circuit is applied between the cathode 61 and the anode 63 as the tube voltage.

[0043] The filament heating circuit 72 supplies a filament current to the filament 62 under the control of the control circuit 74. The supply of the filament current to the filament 62 heats the filament 62, causing the filament 62 to emit a number of electrons according to the temperature of the filament 62. Therefore, it is possible to control the tube current by controlling the filament current. The filament heating circuit 72 may be realized by a step-down circuit that steps down the voltage generated by the tube voltage power supply circuit 71, or may be realized by a power supply system independent of the tube voltage power supply circuit 71.

[0044] The grid power supply circuit 73 applies a grid voltage between the cathode 61 and the grid electrode 135 under the control of the control circuit 74. The application of the grid voltage adjusts the amount of electrons reaching the anode 63 from the filament 62, and therefore it is possible to control the tube current by controlling the grid voltage. The grid power supply circuit 73 may be realized by a step-down circuit that steps down the voltage generated by the tube voltage power supply circuit 71, or may be realized by a power supply system independent of the tube voltage power supply circuit 71.

[0045] The control circuit 74 is a processor that controls the tube voltage power supply circuit 71, the filament heating circuit 72, and the grid power supply circuit 73. Specifically, the control circuit 74 has a tube voltage control circuit 81, a filament control circuit 82, a grid control circuit 83, a tube current control circuit 84, and an X-ray control circuit 85.

[0046] The tube voltage control circuit 81 controls the tube voltage applied between the cathode 61 and the anode 63 under control of the X-ray control circuit 85. More specifically, the tube voltage control circuit 81 alternately switches the tube voltage between a first tube voltage and a second tube voltage higher than the first tube voltage. Hereinafter, the first tube voltage will be referred to as a low tube voltage or simply a low voltage, and the second tube voltage will be referred to as a high tube voltage or simply a high voltage. The alternate switching between the low tube voltage and the high tube voltage is also called KV switching.

[0047] Specifically, the tube voltage control circuit 81 controls the tube voltage power supply circuit 71 based on the difference between the measured tube voltage and the set tube voltage, thereby performing feedback control of the tube voltage. The measured tube voltage is detected by a tube voltage detector (not shown). The tube voltage detector detects the voltage applied between the cathode 61 and the anode 63 as the measured tube voltage. Data on the peak value of the detected measured tube voltage is supplied to the control circuit 74. The tube voltage detector is connected between the tube voltage power supply circuit 71 and the anode 63. The set tube voltage refers to a target value of the tube voltage designated by a user or determined by the processing circuitry 45. Data on the set tube voltage is supplied from the X-ray control circuit 85. In the case of dual energy scanning, a set tube voltage for the low tube voltage (hereinafter referred to as the set low tube voltage) and a set tube voltage for the high tube voltage (hereinafter referred to as the set high tube voltage) are designated. The tube voltage control circuit 81 instructs the tube voltage power supply circuit 71 to increase or decrease the tube voltage so that the difference between the measured tube voltage and the set low tube voltage becomes zero when a low tube voltage is applied, and so that the difference between the measured tube voltage and the set high tube voltage becomes zero when a high tube voltage is applied. The tube voltage power supply circuit 71 adjusts the tube voltage in accordance with the instruction from the tube voltage control circuit 81. This performs feedback control of the tube voltage.

[0048] The filament control circuit 82 controls the filament current that heats the filament 62 of the cathode 61. There are two types of filament current control methods: a control method that supplies a constant filament current (hereinafter referred to as the constant If method), and a tube current feedback control method that controls the tube current to a desired value. In the constant If method, the filament control circuit 82 instructs the filament heating circuit 72 to supply the set filament current to the filament 62. The set filament current refers to the set value of the filament current required to flow the target tube current. The set filament current is specified by a user or the like, or is determined by the processing circuit 45. The filament heating circuit 72 supplies the filament current to the filament 62 in accordance with the instruction from the filament control circuit 82.

[0049] In the tube current feedback control method, the filament control circuit 82 controls the filament heating circuit 72 based on the difference between the measured tube current and the target tube current to perform tube current feedback control. The measured tube current is detected by a tube current detector (not shown). The tube current detector detects the current flowing due to thermions flowing from the cathode 61 to the anode 63 as the tube current. Data on the peak value of the detected measured tube current is supplied to the control circuit 74. The tube current detector is connected between the filament heating circuit 72 and the filament 62. The target tube current refers to the tube current setting value designated by a user or determined by the processing circuitry 45. Data on the target tube current is supplied from the X-ray control circuit 85. The filament control circuit 82 instructs the filament heating circuit 72 to decrease or increase the filament current so that the difference between the measured tube current and the target tube current becomes zero. The filament heating circuit 72 adjusts the filament current in accordance with the instruction from the filament control circuit 82. This performs tube current feedback control.

[0050] The grid control circuit 83 controls the grid voltage applied between the cathode 61 and the grid electrode 65. For example, the grid control circuit 83 alternately switches the grid voltage between a first grid voltage and a second grid voltage higher than the first grid voltage. Hereinafter, the first grid voltage will be referred to as a low grid voltage or simply a low voltage, and the second grid voltage will be referred to as a high grid voltage or simply a high voltage. Note that the grid voltage is not limited to two levels and may be set to three or more levels. The grid control circuit 83 controls the grid power supply circuit 73 to apply a high grid voltage when a low tube voltage is applied and a low grid voltage when a high tube voltage is applied, in synchronization with the switching between the low tube voltage and the high tube voltage by the tube voltage control circuit 81. Specifically, the grid control circuit 83 controls the grid voltage to drop to the low grid voltage and maintain the low grid voltage only during a first transition period during which the tube voltage transitions from the high tube voltage to the low tube voltage and a first hold period during which the tube voltage is maintained at the low tube voltage, so that the tube current does not exceed an upper limit tube current based on the allowable power. From another point of view, the grid control circuit 83 performs control to hold the grid voltage at a low grid voltage only during the first holding period.

[0051] Specifically, the grid power supply circuit 73 controls the grid power supply circuit 73 based on the difference between the measured tube current and the target tube current to perform tube current feedback control. As a result of the tube current feedback control, the grid voltage can be alternately switched between a low voltage and a high voltage. In the case of dual energy scanning, a first tube current and a second tube current greater than the first tube current are set as target tube currents. Hereinafter, the first tube current is referred to as a low tube current or simply a constant current, the second tube current is referred to as a high tube current or simply a high current, and the target tube current for a low tube current is referred to as a target low tube current, and the target tube current for a high tube current is referred to as a target high tube current. The grid control circuit 83 instructs the grid power supply circuit 73 to lower or raise the grid voltage so that the difference between the measured tube current and the target high tube current becomes zero when a low tube voltage is applied, and so that the difference between the measured tube voltage and the target low tube current becomes zero when a high tube voltage is applied. The grid power supply circuit 73 adjusts the grid voltage in accordance with the instruction from the grid control circuit 83. It becomes possible to alternately switch the grid voltage between a high voltage and a low tube voltage.

[0052] The tube current control circuit 84 controls the filament control circuit 82 and the grid control circuit 83 in accordance with control by the X-ray control circuit 85. As one example, the tube current control circuit 84 can switch the control method of the filament current by the filament control circuit 82 between a constant If method and a tube current feedback control method in accordance with a command from the X-ray control circuit 85. As another example, the tube current control circuit 84 can switch between the tube current control by the filament control circuit 82 and the tube current control by the grid control circuit 83 when a high tube voltage is applied and when a low tube voltage is applied.

[0053] The X-ray control circuit 85 functions as the core of the X-ray tube control system 100. For example, the X-ray control circuit 85 notifies the tube voltage control circuit 81 and the tube current control circuit 84 of various scan conditions, KV switching methods, and various signals from outside the X-ray tube control system 100.

[0054] An example of the operation of the X-ray tube control system 100 will now be described.

[0055] First, the problems with KV switching in Comparative Example 1 will be described in detail with reference to Fig. 3. Fig. 3 is a diagram showing a timing chart of KV switching in Comparative Example 1. In Comparative Example 1, it is assumed that the X-ray tube 11 is not equipped with a grid electrode 65.

[0056] 3, the set tube voltage is instantaneously switched alternately between high and low tube voltages each time the rotating frame 13 rotates a certain angle. Specifically, the tube voltage control circuit 81 receives a trigger signal from the control device 15, which is generated each time the rotating frame 13 rotates a small angle, and supplies alternate tube voltage increase and decrease instructions to the tube voltage power supply circuit 71 each time a predetermined number of trigger signals are received. The tube voltage power supply circuit 71 instantaneously switches the set tube voltage to high tube voltage at time THs when an increase instruction is issued, and instantaneously switches the set tube voltage to low tube voltage at time TLs when a decrease instruction is issued.

[0057] The target tube current in Comparative Example 1 represents an ideal tube current. As the target tube current indicates, ideally, the tube current alternates instantaneously between low and high in synchronization with the switching of the tube voltage, with a low tube current flowing when a high tube voltage is applied and a high tube current flowing when a low tube voltage is applied so that the X-ray dose is the same when a high tube voltage is applied and when a low tube voltage is applied.

[0058] The bottom part of Figure 3 shows graphs that schematically illustrate the trends in measured tube voltage and measured tube current. As shown in the bottom part of Figure 3, the measured tube voltage follows the set tube voltage with a delay. Specifically, the measured tube voltage begins to rise toward a high tube voltage from time point THs, and reaches the high tube voltage at time point TH1. The measured tube voltage also begins to fall toward a low tube voltage from time point TLs, and reaches the low tube voltage at time point TL1. Thereafter, the measured tube voltage alternates between the high tube voltage and the low tube voltage in a similar manner. Here, the period during which the tube voltage transitions from a low voltage to a high voltage (second transition period), in other words, the period from time THs to time TH1, will be called the rising period D1; the period during which the tube voltage is maintained at a high voltage (second holding period), in other words, the period from time TH1 to time TLs, will be called the high KV period D2; the period during which the tube voltage transitions from a high voltage to a low voltage (first transition period), in other words, the period from time TLs to time TL1, will be called the falling period D3; and the period during which the tube voltage is maintained at a low voltage (first holding period), in other words, the period from time TL1 to time THs, will be called the low KV period D4.

[0059] The left vertical axis of the graph of the measured tube current waveform represents the measured tube current [mA], and the right vertical axis represents mA equivalent to If. "mA equivalent to If" refers to the tube current that flows in accordance with the emission characteristics at a certain tube voltage and filament temperature. Ihigh_If refers to the tube current corresponding to the filament temperature when a high tube voltage is applied, and Ilow_If refers to the tube current corresponding to the filament temperature when a low tube voltage is applied.

[0060] In the case of Comparative Example 1, where no grid electrode is provided and feedback control of the filament current is not performed, the measured tube current fluctuates according to the filament temperature, but the acceleration of thermoelectrons fluctuates according to the measured tube voltage, so that, for example, a drop in the measured tube voltage results in a decrease in the measured tube current. As a result, it is not possible to maintain uniformity in the X-ray dose between the high KV period D2 and the low KV period D4, and it is therefore difficult to maintain uniformity in quality between the projection data collected in the high KV period D2 and the projection data collected in the low KV period D4.

[0061] Next, the problems with KV switching in Comparative Example 2 will be described in detail with reference to FIG. 4. FIG. 4 is a diagram showing a timing chart of KV switching in Comparative Example 2. The lower part of FIG. 4 shows graphs that schematically represent the transitions of the measured tube voltage, measured grid voltage, and measured tube current. In Comparative Example 2, the X-ray tube 11 is equipped with a grid electrode 65. The grid control circuit 83 in Comparative Example 2 switches the set grid voltage at the same timing as switching the set tube voltage. The behavior of the set tube voltage, target tube current, and measured tube voltage in Comparative Example 2 is the same as in Comparative Example 1. Note that the target tube current alternates between a high tube current I1 and a low tube current I2.

[0062] As shown in the lower part of Figure 4, the grid control circuit 83 switches the grid voltage in synchronization with the switching of the target tube current. Specifically, the tube current control circuit 84 supplies a tube current increase instruction to the grid control circuit 83 at the time TLs when the target tube current switches from a low current I2 to a high current I1 (the time when the target tube voltage switches from a high voltage to a low voltage). Upon receiving this instruction, the grid control circuit 83 sets the target tube current to the target high tube current I1 and applies a grid voltage that results in the high tube current I1 at low voltage. As a result, the measured grid voltage begins to drop, and accordingly the measured tube current begins to increase. From TL1 onwards, the measured grid voltage is maintained at a low voltage so that the measured tube current maintains the high tube current I1.

[0063] Meanwhile, at THs, the time point when the target tube current switches from high to low (the time point when the target tube voltage switches from low to high), the tube current control circuit 84 supplies a tube current decrease instruction to the grid control circuit 83. Upon receiving this instruction, the grid control circuit 83 sets the target tube current to the target low tube current I2 and applies a grid voltage that results in the low tube current I2 at high voltage. As a result, the measured grid voltage begins to rise, and accordingly, the measured tube current begins to decrease. From TH1 onwards, the measured grid voltage is maintained at a high voltage so that the measured tube current maintains the low tube current I2.

[0064] The target low tube current I2 and the target high tube current I1 are preferably determined to values ​​that result in the same X-ray dose in the high KV period D2 and the low KV period D4. The high grid voltage corresponds to the grid voltage required to pass the target low tube current I2 under the high tube voltage, and the low grid voltage corresponds to the grid voltage required to pass the target high tube current I1 under the low tube voltage. The target high tube current I1 is equal to the tube current Ilow_If.

[0065] Because the tube voltage is several orders of magnitude larger than the grid voltage, the rising period D1 and falling period D3 of the measured tube voltage tend to be longer than the rising period and falling period of the measured grid voltage, respectively, as shown in Figure 4. For example, if the tube voltage and grid voltage starts to decrease simultaneously, the grid voltage may drop to a low grid voltage during the falling period D3, when the measured tube voltage is not maintained at a low tube voltage. As a result, there is a risk that a measured tube current exceeding the high tube current I1 may temporarily flow. Similarly, if the tube voltage and grid voltage are increased simultaneously, there is a risk that a measured tube current below the low tube current I2 may flow during the rising period D1.

[0066] If the measured tube current jumps up and exceeds the upper limit tube current Ilim during the falling period D3, the allowable power of the X-ray tube 11 may be exceeded, potentially damaging various devices in the X-ray tube control system 100. The upper limit tube current Ilim refers to a tube current determined based on the measured tube voltage and the allowable power. As an example, the upper limit tube current Ilim can be obtained by dividing the allowable power by the measured tube voltage. The allowable power is the upper limit of power that will not damage the X-ray tube 11, etc., and is determined by factors such as the focal spot size, the output of the X-ray high voltage generator 14, and the power supply power of the tube voltage power supply circuit 71. The upper limit tube current Ilim shown in FIG. 4 is a schematic representation of the upper limit tube current obtained by dividing the allowable power by the measured tube voltage. The upper limit tube current Ilim has an antiphase relationship with the measured tube voltage.

[0067] This embodiment provides an X-ray tube control system 100 that can maintain uniformity of the X-ray dose in the high KV period D2 and the low KV period D4 while avoiding the flow of a measured tube current exceeding the upper limit tube current in the falling period D3. Hereinafter, this embodiment will be described in Examples 1 to 5. First, Example 1, which is the basic form of KV switching according to this embodiment, will be described.

[0068] Example 1 FIG. 5 is a timing chart of KV switching in the first embodiment. As shown in FIG. 5 , the grid control circuit 83 controls the measured grid voltage to decrease to and maintain the low grid voltage only during the falling period D3 and the low KV period D4 so that the measured tube current does not exceed the upper limit tube current Ilim during the low KV period D4 in which the tube voltage is maintained at the low tube voltage. The phrase "controlling the measured grid voltage to decrease to and maintain the low grid voltage only during the falling period D3 and the low KV period D4" means that, unlike Comparative Example 2 in which the measured grid voltage reaches the low grid voltage during the falling period D3 immediately before the low KV period D4 and is maintained at the low grid voltage until the low KV period D4, the measured grid voltage is not maintained at the low grid voltage during the falling period D3 and is maintained at the low grid voltage only during the low KV period D4. As will be described later, the start of the decrease of the measured grid voltage is not limited to the falling period D3 and may also occur during the falling period D4. In this case, both the control to lower the measured grid voltage to a low grid voltage and the control to maintain the low grid voltage are performed only during the low KV period D4. In other words, the grid control circuit 83 performs control to maintain the measured grid voltage at a low grid voltage only during the low KV period D4.

[0069] The grid voltage drop will be described in detail. The grid control circuit 83 performs tube current feedback control by adjusting the grid voltage. Even if a tube current increase command is supplied from the tube current control circuit 84 at time TLs when the target tube current switches from low current I2 to high current I1 (time when the target tube voltage switches from high voltage to low voltage), the grid control circuit 83 according to the first embodiment does not immediately execute tube current feedback based on the difference between the target high tube current I1 and the measured tube current. Even if a tube current increase command is supplied, the grid control circuit 83 maintains the high grid voltage, which is the measured grid voltage at the immediately preceding time TLs. Therefore, except for the final portion of the period D3, the measured tube current falls below the low current I2. The start time of the drop in the measured grid voltage according to the first embodiment is shifted backward in time relative to time TLs by up to time TL1 when the target tube voltage reaches the low voltage.

[0070] As an example, the grid control circuit 83 starts lowering the grid voltage when the measured tube voltage drops from the high voltage to the reference value V1 after time point TLs has passed. Specifically, the tube voltage control circuit 81 monitors the measured tube voltage. When it detects that the measured tube voltage has dropped to the reference value V1, it supplies a signal indicating this to the grid control circuit 83 via the X-ray control circuit 85 and the tube current control circuit 84. Upon receiving this signal, the grid control circuit 83 sets the target tube current to the target high tube current I1 and controls the grid voltage in accordance with tube current feedback based on the difference between the target high tube current I1 and the measured tube current. As a result, the measured grid voltage begins to drop when it reaches the reference value V1, and when the measured grid voltage reaches the low voltage, the low voltage is maintained. By maintaining the measured grid voltage at the low voltage, the measured tube current is maintained at the high current I1 or the tube current Ilow_If.

[0071] The reference value V1 refers to the tube voltage value at which grid voltage reduction begins, and is set to a value between the high and low tube voltages. More specifically, the reference value V1 can be calculated by measuring in advance the period during which the measured grid voltage reduces from the high grid voltage to the low grid voltage and adding the estimated amount of tube voltage that can be reduced during that period to the low tube voltage. The grid control circuit 83 may also start reducing the grid voltage when the measured tube voltage reduces from the high tube voltage to the low tube voltage. In this case, both the control to reduce the measured grid voltage to the low grid voltage and the control to maintain the grid voltage are performed only during the low KV period D4.

[0072] According to the first embodiment, the measured grid voltage is decreased to and maintained at a low grid voltage only during the low KV period D4 when the measured tube voltage is stable at a low tube voltage. This prevents or reduces the measured tube current from exceeding the upper limit tube current Ilim during the decrease period D3, thereby preventing or reducing damage to devices installed in the X-ray tube control system 100. This allows a low tube current to flow when a high tube voltage is applied and a high tube current to flow when a low tube voltage is applied, thereby reducing the difference in X-ray dose between when a high tube voltage is applied and when a low tube voltage is applied. This in turn allows for improved uniformity in the quality of projection data collected when a high tube voltage is applied and when a low tube voltage is applied.

[0073] The grid voltage rise is the same as in Comparative Example 2. Even if a command to decrease the tube current is issued, the grid control circuit 83 maintains the low grid voltage, which is the measured grid voltage at the immediately preceding time point THs. Therefore, except for the final portion of period D1, the measured tube current falls below the low current I2. That is, the grid control circuit 83 immediately changes the grid voltage to one that provides a high tube voltage and a low tube current at time point THs when the command to decrease the tube current is issued. As a result, the measured grid voltage begins to rise from time point THs, and when the grid voltage reaches the high voltage, the high voltage is maintained. By maintaining the measured grid voltage at a high voltage, the measured tube current is maintained at the low current I2.

[0074] Example 2 FIG. 6 is a timing chart of KV switching in the second embodiment. The second embodiment is an improved version of the first embodiment. As shown in FIG. 6 , during a fall period D3 in which the tube voltage transitions from a high tube voltage to a low tube voltage, the grid control circuit 83 lowers the measured grid voltage so as to allow a tube current equal to or greater than the high tube current I1 to flow, with the upper limit of the upper tube current Ilim. The upper limit tube current Ilim is defined as the upper limit of the tube current that does not exceed the allowable power at the current measured tube voltage. Specifically, the upper limit tube current Ilim is set to the value obtained by dividing the allowable power by the measured tube voltage. Note that, in the second embodiment as well, the grid control circuit 83 controls the measured grid voltage to be lowered to and maintained at the low grid voltage only during the fall period D3 and the low KV period D4 so that the measured tube current does not exceed the upper limit tube current Ilim. Furthermore, the grid control circuit 83 controls the measured grid voltage to be maintained at the low grid voltage only during the low KV period D4. Note that the "control to lower the grid voltage to a low grid voltage" according to the second embodiment differs from the first embodiment in that the measured grid voltage is controlled to be lowered from a high grid voltage toward a low grid voltage, and includes control to actively lower the measured grid voltage from a high grid voltage to a voltage V2 that is lower than the low grid voltage, and then control to raise the measured grid voltage from the voltage V2 toward the low grid voltage.

[0075] The reduction of the measured grid voltage will now be described in detail. At time TLs when an instruction to increase the tube current is issued, the grid control circuit 83 instantly sets the target tube current to the upper limit tube current Ilim or a value not exceeding it, and controls the grid voltage in accordance with tube current feedback based on the difference between the upper limit tube current Ilim or a value not exceeding it and the measured tube current. During the fall period D3, the grid control circuit 83 reduces the measured grid voltage to voltage V2, which is lower than the low grid voltage, and then increases it to the low grid voltage by the low KV period D4. The reduction of the measured grid voltage to voltage V2 can be achieved by, for example, amplifying the difference between the upper limit tube current or a value not exceeding it and the measured tube current. As the grid voltage changes, the measured tube current increases from the low tube current to near the upper limit tube current Ilim during the fall period D3, and then decreases to the high tube current I1 by the low KV period D4. By rapidly reducing the grid voltage to voltage V2 during the fall period D3, the tube current can be increased beyond the high tube current I1 within the upper limit tube current Ilim.

[0076] An increase in the tube current promotes the release of charge accumulated in the tube voltage power supply circuit 71, thereby enabling the rate at which the tube voltage decreases to be increased. As a result, the period D3 from time TLs (the time at which the measured tube voltage starts to decrease) to time TL2 (the time at which the measured tube voltage reaches the low tube voltage) in Example 2 is shorter than the period D3 from time TLs to time TL1 in Example 1.

[0077] Next, the increase in the measured grid voltage will be described in detail. At time THs when a command to decrease the tube current is issued, the grid control circuit 83 instantly sets the target tube current to the target low tube current I2 as a preset lower limit, and controls the grid voltage according to tube current feedback based on the difference between the target tube current and the measured tube current. The preset lower limit may be any value below the target low tube current I2, and the extent to which it falls below this limit is not important. For example, the preset lower limit may be a value slightly below the target low tube current I2, zero, or a value slightly above zero. During the rising period D1, the grid control circuit 83 increases the measured grid voltage to a voltage V3 higher than the high grid voltage, and then decreases it to the high grid voltage by the high KV period D2. The increase in the measured grid voltage to voltage V3 can be achieved by, for example, amplifying the difference between the target tube current and the measured tube current. As the grid voltage changes, the measured tube current decreases from a high tube current I1 to zero or near zero during the rising period D1, and then increases to a low tube current I2 by the high KV period D2. By rapidly increasing the grid voltage to voltage V3 during the rising period D1, it is possible to decrease the tube current to zero or near zero.

[0078] A decrease in the tube current suppresses the release of charge accumulated in the tube voltage power supply circuit 71, thereby enabling the rate at which the tube voltage increases to be increased. As a result, the period D1 from time THs (the time at which the measured tube voltage starts to increase) to time TH2 (the time at which the measured tube voltage reaches the high tube voltage) in Example 2 is shorter than the period D1 from time THs to time TH1 in Example 1.

[0079] According to the second embodiment, during the fall period D3, grid control is used to increase the tube current as much as possible up to the upper limit of the tube current lim, thereby promoting a decrease in the measured tube voltage. During the rise period D1, grid control is used to reduce the tube current to the set lower limit, thereby promoting an increase in the measured tube voltage. This shortens the fall period D3 and the rise period D1, thereby making it possible to relatively extend the low-KV period D4 and the high-KV period D2. While projection data collected during the fall period D3 and the rise period D1 cannot be used for image reconstruction, and only projection data collected during the low-KV period D4 and the high-KV period D2 can be used for image reconstruction, according to the second embodiment, shortening the fall period D3 and the rise period D1 makes it possible to improve the collection efficiency of projection data usable for image reconstruction.

[0080] The method for lowering the grid voltage during the falling period D3 is not limited to the above method. As an example, the grid control circuit 83 may lower the grid voltage according to a table recording the gradient of the grid voltage drop (hereinafter referred to as the gradient table). Since a sudden drop in the grid voltage may damage the equipment, it is preferable to set a gradient in the gradient table that will not cause damage to the equipment. It is preferable to prepare a gradient table for each set tube voltage. A gradient table created for the increase may also be used for the increase in the grid voltage during the rising period D1.

[0081] Here, a description will be given of the configuration of a grid power supply circuit 73 according to a second embodiment, which is suitable for switching the grid voltage between a low grid voltage and a high grid voltage. The grid power supply circuit 73 has an arrangement of power storage elements that can be switched between a parallel circuit having power storage elements connected in parallel and a series circuit having power storage elements connected in series. The grid power supply circuit 73 operates as a parallel circuit in the low KV period D4 and as a series circuit in the high KV period D2. A detailed description will be given below with reference to FIG. 7.

[0082] Fig. 7 is a diagram showing an example of the configuration of a grid power supply circuit 73 according to Example 2. As shown in Fig. 7, the grid power supply circuit 73 includes a circuit 91, a circuit 92, and a switch 93. The circuits 91 and 92 are connected to the grid electrode 65 via the switch 93. The circuit 91 includes a power supply 94 and a high-voltage power supply circuit 95. The power supply 94 has a power supply capacity for a high grid voltage.

[0083] The circuit 92 includes a power supply 96 and a low-voltage power supply circuit 97. The power supply 96 has a power supply capacity for a low grid voltage. The power supply capacity of the power supply 96 can be smaller than the power supply capacity of the power supply 94 for a high grid voltage. This makes it possible to reduce the costs of the power supplies 94 and 96.

[0084] The switch 93 is a circuit element that switches the conduction of the circuit 91 and the circuit 92 to the grid electrode 65. The switch 93 is switched by the grid control circuit 83. The grid control circuit 83 connects the switch 93 to the circuit 91 in response to an instruction to decrease the tube current (THs), uses the circuit 91 to increase the grid voltage to a high voltage during the rising period D1, and maintains the grid voltage at a high voltage during the high KV period D2. The grid control circuit 83 connects the switch 93 to the circuit 92 in response to an instruction to increase the tube current (TLs), uses the circuit 92 to decrease the grid voltage to a low voltage during the falling period D3, and maintains the grid voltage at a low voltage during the low KV period D4. By switching the circuit 91 and the circuit 92 in this way, the grid voltage can be appropriately switched between a high voltage and a low voltage.

[0085] 7, it is assumed that the grid circuit 73 is configured with separate circuits 91 and 92. However, the circuits 91 and 92 may be configured with a single circuit that can switch between series and parallel connection of the power storage elements. The grid power supply circuit for both series and parallel use differs from the grid power supply circuit 73 only in circuit configuration and has the same functions.

[0086] FIG. 8 is a schematic circuit diagram of a grid power supply circuit 99 for both series and parallel use, showing the connection relationship of switches that form a series circuit when a high grid voltage is applied. FIG. 9 is a schematic circuit diagram of a grid power supply circuit 99 for both series and parallel use, showing the connection relationship of switches that form a parallel circuit when a low grid voltage is applied. The inverters in FIGS. 8 and 9 correspond to the power supplies 94 and 96 in FIG. 7. As shown in FIGS. 8 and 9, the grid power supply circuit 99 can be switched between series and parallel by switching the switches. The grid power supply circuit 99 can have a smaller circuit scale than the case where the series circuit 91 and the parallel circuit 92 shown in FIG. 7 are separate. The grid power supply circuits 73 and 99 shown in FIGS. 7, 8, and 9 can also be used in Example 1.

[0087] Example 3 The kV switching according to Examples 3 to 5 is combined with tube current modulation (AEC: Auto Exposure Control). When kV switching is combined with tube current modulation, the X-ray control circuit 85 controls the tube voltage control circuit 81 to alternately switch between high and low tube voltages, while controlling the tube current control circuit 84 in conjunction with the rotation angle of the X-ray tube 11 or the rotating frame 13 to modulate the tube current. Note that in Examples 3 to 5 as well, the grid control circuit 83 controls the measured grid voltage to drop to a low grid voltage and maintain the voltage only during the fall period and low KV period so that the measured tube current does not exceed the upper limit tube current Ilim. Also in Examples 3 to 5 as well, the grid control circuit 83 may control the measured grid voltage to drop to a low grid voltage and maintain the voltage only during the low KV period so that the measured tube current does not exceed the upper limit tube current Ilim. For example, the "control to lower the grid voltage to a low grid voltage" according to the third to fifth embodiments may be control to lower the measured grid voltage from a high grid voltage toward a low grid voltage as in the first embodiment, or may include control to actively lower the measured grid voltage from the high grid voltage to a voltage V2 that is lower than the low grid voltage, and then control to raise the measured grid voltage from the voltage V2 toward the low grid voltage as in the second embodiment.

[0088] FIG. 10 is a timing chart of KV switching in Example 3. Unlike FIGS. 3 to 6, FIG. 10 omits illustrations of the rising period, high KV period, falling period, and low kV period, as well as the behavior of the measured grid voltage and measured tube current during the rising and falling periods of the tube voltage, for ease of explanation. For tube current modulation, two types of target tube currents are set: one for high tube voltage and one for low tube voltage. Each target tube current is set to gradually increase as the body thickness or X-ray transmission path length of the subject P increases, and to gradually decrease as the body thickness or X-ray transmission path length decreases. As the target tube current is modulated, the tube current Ihigh_If corresponding to If when a high tube voltage is applied and the tube current Ilow_If corresponding to If when a low tube voltage is applied are also modulated. FIG. 10 illustrates only a portion of a 360-degree angular range in which the target tube current gradually increases.

[0089] As the tube voltage is switched between high and low, the grid voltage is also switched between high and low. In the third embodiment, the low grid voltage is set to zero, and the high grid voltage is set to a grid voltage determined based on the current target tube current and allowable power.

[0090] 10, in the low KV period, the tube current control circuit 84 instructs the filament control circuit 82 to execute tube current feedback control. In response to this instruction, the filament control circuit 82 executes tube current feedback control by adjusting the filament current in order to reduce the difference between the target tube current corresponding to the low KV period and the measured tube current. In the high KV period D2, the tube current control circuit 84 instructs the grid control circuit 83 to execute tube current feedback control. In response to this instruction, the grid control circuit 83 executes tube current feedback control by adjusting the grid voltage in order to reduce the difference between the target tube current corresponding to the high KV period D2 and the measured tube current.

[0091] According to the third embodiment, tube current feedback control based on the difference between the set tube current and the measured tube current is executed by the filament control circuit 82 in the low KV period, and by the grid control circuit 83 in the high KV period, based on the grid voltage. By using both tube current feedback control by adjusting the filament current and tube current feedback control by adjusting the grid voltage, it is possible to reduce wear of the filament 62 compared to when tube current feedback control by adjusting the filament current is always executed.

[0092] Example 4 Fig. 11 is a timing chart of KV switching in Example 4. In Fig. 11, similar to Fig. 10, the rising period, high KV period, falling period, and low kV period are not shown, and the behavior of the measured grid voltage and the measured tube current during the rising and falling periods of the tube voltage are not shown. As shown in Fig. 11, the low grid voltage in Example 4 is set to a constant value higher than zero, unlike Example 3.

[0093] 11 , the tube current control circuit 84 instructs the filament control circuit 82 to execute feedback control during the low KV period. In response to this instruction, the filament control circuit 82 executes feedback control based on the filament current in order to reduce the difference between the target tube current corresponding to the low KV period and the measured tube current. The tube current control circuit 84 instructs the grid control circuit 83 to execute feedback control during the high KV period. In response to this instruction, the grid control circuit 83 executes feedback control based on the grid voltage in order to reduce the difference between the target tube current corresponding to the high KV period and the measured tube current.

[0094] As described above, the low grid voltage is set to a constant value greater than 0. Therefore, according to the fourth embodiment, as in the second embodiment, the measured grid voltage is temporarily reduced to a voltage V2 (see FIG. 6) lower than the low grid voltage during the falling period 3, thereby shortening the falling period 3.

[0095] In Example 3, the measured tube current changes closer to the tube current Ilow_If than in Example 4. Therefore, the tube current suppression amount C1 (see FIG. 10) by grid control in the high KV period is reduced compared to the suppression amount C3 (see FIG. 11) according to Example 4. Furthermore, the tube current suppression amount C2 (see FIG. 10) by grid control in the low KV period is reduced compared to the suppression amount C4 (see FIG. 11) according to Example 4. Therefore, Example 3 can reduce wear of the filament 62 compared to Example 4.

[0096] Example 5 Fig. 12 is a diagram showing a timing chart of KV switching in Example 5. In Fig. 12, similar to Figs. 10 and 11, the rising period, high KV period, falling period, and low kV period, as well as the behavior of the measured grid voltage and the measured tube current during the rising and falling periods of the tube voltage are omitted.

[0097] In the fifth embodiment, the constant If method is adopted as the control method for the filament current. In this case, the low grid voltage changes according to the target high X-ray tube current, and the high grid voltage changes according to the target low X-ray tube current, due to the tube current feedback control performed by adjusting the grid voltage using the grid control circuit 83. Furthermore, due to the adoption of the constant If method for the filament current, the X-ray tube current Ihigh_If corresponding to If when a high X-ray tube voltage is applied and the X-ray tube current Ilow_If corresponding to If when a low X-ray tube voltage is applied remain constant.

[0098] As shown in FIG. 12 , the tube current control circuit 84 instructs the filament control circuit 82 to execute constant If filament current control and instructs the grid control circuit 83 to execute feedback control based on the grid voltage during both the high KV period and the low KV period. In response to this instruction, the filament control circuit 82 supplies a constant filament current to the filament 62 during the high KV period and the low KV period. The filament current is set to a current value that does not exceed the maximum power even when the maximum tube current at a low tube voltage is applied. During the low KV period, the grid control circuit 83 executes tube current feedback control by adjusting the grid voltage to reduce the difference between the set high tube current and the measured tube current. During the high KV period, the grid control circuit 83 executes tube current feedback control by adjusting the grid voltage to reduce the difference between the set low tube current and the measured tube current.

[0099] According to the fifth embodiment, unlike the third and fourth embodiments, there is no need to perform tube current feedback control by adjusting the filament current. Since the filament 62 has a relatively large thermal inertia, it can be said that the responsiveness of the tube current to changes in the filament current is relatively low. Therefore, by performing all tube current feedback control by adjusting the grid voltage, it becomes possible to modulate the tube current at high speed.

[0100] In Examples 3 and 4, the suppression amounts C1 and C3 of the tube current due to grid control in the high KV period (see FIGS. 10 and 11) are reduced compared to the suppression amount C5 (see FIG. 12) in Example 5. Also, the suppression amounts C2 and C4 of the tube current due to grid control in the low KV period (see FIGS. 10 and 11) are reduced compared to the suppression amount C6 (see FIG. 12) in Example 5. Therefore, in Examples 3 and 4, it is possible to reduce wear of the filament 62 and the grid electrode 65 compared to Example 5.

[0101] (Variation 1) Although the kV switching according to Examples 3 and 4 is combined with tube current modulation, this embodiment is not limited thereto. A tube current control circuit 84 according to Modification 1 combines the kV switching according to Examples 3 and 4 with tube current non-modulation in which the target high tube current and the target low tube current are constant. In this case, the filament control circuit 82 performs feedback control based on the filament current during the low KV period to reduce the difference between the set high tube current corresponding to the low KV period and the measured tube current, and the grid control circuit 83 performs feedback control based on the grid voltage during the high KV period to reduce the difference between the set low tube current corresponding to the high KV period and the measured tube current. The low grid voltage may be set to zero as in Example 3, or may be set to a constant value higher than zero as in Example 4.

[0102] (Variation 2) The X-ray tube control system 100 according to the above embodiment includes the X-ray tube 11, the tube voltage power supply circuit 71, the filament heating circuit 72, the grid power supply circuit 73, and the control circuit 74. However, the X-ray tube control system 100 does not have to include the X-ray tube 11, the tube voltage power supply circuit 71, the filament heating circuit 72, and the grid power supply circuit 73; in other words, it may include only the control circuit 74. In this case, the X-ray tube control system 100 may be provided in the console 40 of the X-ray computed tomography apparatus 1, or may be provided in a server computer connected to the X-ray computed tomography apparatus 1 via a network. The control circuit 74 is capable of controlling the tube voltage power supply circuit 71, the filament heating circuit 72, and the grid power supply circuit 73 via the network.

[0103] Furthermore, the X-ray tube control system 100 does not necessarily have to include the X-ray tube 11, the tube voltage power supply circuit 71, the filament heating circuit 72, and the grid power supply circuit 73; in other words, it may have only the X-ray high voltage device 14.

[0104] According to at least one of the embodiments described above, the difference in X-ray dose between when a high tube voltage is applied and when a low tube voltage is applied can be reduced.

[0105] The term "processor" used in the above description refers to a circuit such as a CPU, a GPU, an application specific integrated circuit (ASIC), a programmable logic device (e.g., a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA)). A processor realizes its function by reading and executing a program stored in a memory circuit. Note that instead of storing a program in a memory circuit, the program may be directly embedded in the processor circuit. In this case, the processor realizes its function by reading and executing the program embedded in the circuit. On the other hand, if the processor is, for example, an ASIC, the function is directly embedded in the processor circuit as a logic circuit instead of storing the program in a memory circuit. Note that each processor in this embodiment is not limited to being configured as a single circuit for each processor, but may be configured as a single processor by combining multiple independent circuits to realize its function. Furthermore, multiple components in FIGS. 1 and 2 may be integrated into a single processor to realize its function.

[0106] Although several embodiments have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations of embodiments can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0107] 1 X-ray computed tomography equipment 10 Mounting stand 11 X-ray tube 12 X-ray detector 13 Rotating Frame 14 X-ray high voltage device 15 Control device 16 Wedge 17 Collimator 18 Data Collection Circuit 19 Opening 30 berths 31 Foundation 32 Support frame 33 Top plate 34 Bed drive unit 40 Console 41 memory 42 Display 43 Input Interface 44 Communication Interface 45 Processing circuit 51 Scan control function 52 Reconfiguration function 53 Image processing function 54 Display control function 61 Cathode 62 filaments 63 Anode 65 Grid electrode 71 Tube voltage power supply circuit 72 Filament heating circuit 73 Grid Power Circuit 74 Control Circuit 81 Tube voltage control circuit 82 Filament control circuit 83 Grid Control Circuit 84 Tube current control circuit 85 X-ray control circuit

Claims

1. a tube voltage control unit that alternately switches a tube voltage applied between a cathode that emits electrons and an anode that receives electrons from the cathode and generates X-rays between a first tube voltage and a second tube voltage that is higher than the first tube voltage; a grid control unit that alternately switches a grid voltage applied between the cathode and a grid electrode that controls electrons moving from the cathode to the anode between a first grid voltage and a second grid voltage that is higher than the first grid voltage, the grid control unit performs control to reduce the grid voltage to the first grid voltage and control to maintain the grid voltage only during a first transition period in which the tube voltage transitions from the second tube voltage to the first tube voltage and a first maintenance period in which the tube voltage is maintained at the first tube voltage, so that the tube current does not exceed an upper limit tube current based on an allowable power. X-ray tube control system.

2. 2. The X-ray tube control system according to claim 1, wherein the grid control unit starts lowering the grid voltage when the tube voltage drops to a reference value between the first tube voltage and the second tube voltage or to the second tube voltage.

3. 2. The X-ray tube control system according to claim 1, wherein the grid control unit reduces the grid voltage during the first transition period so as to pass a tube current equal to or greater than a first target tube current when the first tube voltage is applied, with the upper limit of the tube current being the limit.

4. 4. The X-ray tube control system according to claim 3, wherein the grid control unit drops the grid voltage to a voltage lower than the first grid voltage during the first transition period, and then raises the grid voltage to the first grid voltage by the time the first holding period begins.

5. 2. The X-ray tube control system according to claim 1, wherein the grid control unit increases the grid voltage to reduce the tube current to a set lower limit value during a second transition period in which the tube voltage transitions from the first tube voltage to the second tube voltage.

6. 6. The X-ray tube control system according to claim 5, wherein the grid control unit increases the grid voltage to a voltage higher than the second grid voltage during the second transition period, and then decreases the grid voltage to the second grid voltage by the time a second holding period in which the tube voltage is maintained at the second tube voltage begins.

7. further comprising a grid power supply circuit that applies a grid voltage to the grid electrode; The grid power supply circuit includes: a storage element arrangement that can be switched between a parallel circuit having storage elements connected in parallel and a series circuit having storage elements connected in series, the parallel circuit operates during the first holding period, and operates as the series circuit during a second holding period in which a tube voltage is maintained at the second tube voltage; 6. The X-ray tube control system according to claim 3 or 5.

8. a filament control unit that controls a filament current supplied to the filament of the cathode; the filament control unit executes, during the first holding period, feedback control based on a filament current to reduce a difference between a first target tube current corresponding to the first holding period and an actually measured tube current; the grid control unit performs feedback control based on a grid voltage in a second hold period in which the tube voltage is maintained at the second tube voltage, to reduce a difference between a second target tube current corresponding to the second hold period and an actual measured tube current.

2. The X-ray tube control system of claim 1.

9. 9. The x-ray tube control system of claim 8, wherein the first grid voltage is zero.

10. 9. The x-ray tube control system of claim 8, wherein the first grid voltage is a constant value greater than zero.

11. a filament control unit for controlling a predetermined heating current for heating a filament of the cathode; the grid control unit executes feedback control based on a grid voltage during the first holding period to reduce a difference between a first target tube current corresponding to the first holding period and an actual measured tube current, and executes feedback control based on a grid voltage during a second holding period in which the tube voltage is maintained at the second tube voltage to reduce a difference between a second target tube current corresponding to the second holding period and an actual measured tube current.

2. The X-ray tube control system of claim 1.

12. 2. The X-ray tube control system according to claim 1, wherein the grid control unit controls the grid voltage so that the measured tube current follows the modulated target tube current.

13. an X-ray tube having the cathode, the anode, and the grid electrode; a tube voltage power supply circuit that applies a tube voltage between the cathode and the anode under control of the tube voltage control unit; a grid power supply circuit that applies a grid voltage between the cathode and the grid electrode under control of the grid control unit.

2. The X-ray tube control system of claim 1.

14. a tube voltage control unit that alternately switches a tube voltage applied between a cathode that emits electrons and an anode that receives electrons from the cathode and generates X-rays between a first tube voltage and a second tube voltage that is higher than the first tube voltage; a grid control unit that alternately switches a grid voltage applied between the cathode and a grid electrode that controls electrons moving from the cathode to the anode between a first grid voltage and a second grid voltage that is higher than the first grid voltage, the grid control unit performs control to hold the grid voltage at the first grid voltage only during a first holding period in which the tube voltage is held at the first tube voltage. X-ray tube control system.

15. an X-ray tube having a cathode that emits electrons, an anode that receives electrons from the cathode and generates X-rays, and a grid electrode that controls electrons traveling from the cathode to the anode; an X-ray detector that detects X-rays generated from the anode and transmitted through the subject; a data acquisition circuit for acquiring projection data via the X-ray detector; a tube voltage control unit that alternately switches a tube voltage applied between the cathode and the anode between a first tube voltage and a second tube voltage higher than the first tube voltage; a grid control unit that alternately switches a grid voltage applied between the cathode and the grid electrode between a first grid voltage and a second grid voltage that is higher than the first grid voltage, the grid control unit performs control to reduce the grid voltage to the first grid voltage and control to maintain the grid voltage only during a first transition period in which the tube voltage transitions from the second tube voltage to the first tube voltage and a first maintenance period in which the tube voltage is maintained at the first tube voltage, so that the tube current does not exceed an upper limit tube current based on an allowable power. An X-ray computed tomography apparatus comprising:

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