CT device, operation method and operation program of CT device, and image display device

Abstract

A processor of a CT device according to the present invention detects an air region (AA) in which a subject is not present in a channel direction of a detection face, in a state in which the subject is disposed facing the detection face before performing imaging for acquiring a tomographic image of the subject, and derives a usable drive condition that is a drive condition of an X-ray source collected by irradiating a detector with X-rays in a state in which the subject is not present, the usable drive condition being a drive condition of an X-ray source that is usable when performing imaging on the basis of reference data representing a threshold drive condition set as a threshold value for controlling an incident dose within a range of a linear region of the detector in which a relation between the incident dose and an output value indicates linearity, the usable drive condition being for controlling the incident dose of the air region (AA) that is detected to be within the range of the linear region.

Description

CT device, operation method and operation program for CT device, and image display device

[0001] The technology of the present disclosure relates to a CT device, an operating method and operating program for the CT device, and an image display device.

[0002] Japanese Patent Application Laid-Open Publication No. 2014-140707 describes a CT (Computed Tomography) device (hereinafter referred to as a PCCT (Photon Counting Computed Tomography) device) equipped with a photon counting detector. Photon counting detectors have nonlinear characteristics in which the relationship between the incident dose and the output value becomes nonlinear as the incident dose increases. This is because as the incident dose increases, pileup occurs, in which signals corresponding to photons overlap, resulting in photon count leakage. Japanese Patent Application Laid-Open Publication No. 2014-140707 describes the use of a threshold value that is an X-ray intensity to avoid pileup.

[0003] In PCCT devices, artifacts may occur near the subject's body surface in reconstructed tomographic images. It is believed that these artifacts are due to the nonlinear characteristics of the photon-counting detector described above. This is because the area near the subject's body surface is close to an air region (a so-called "pass-through region") where X-rays directly strike the detector. Because the incident dose is high in the air region, pileup is likely to occur, and this effect is thought to affect the area near the subject's body surface. Japanese Patent Application Laid-Open Publication No. 2014-140707 does not mention such artifacts near the subject's body surface.

[0004] The technology according to the present disclosure provides a CT device, an operating method for a CT device, and an operating program that can suppress artifacts that occur near the body surface of a subject in a tomographic image.

[0005] The CT device according to the disclosed technology is a CT device comprising an irradiation device including an X-ray source that irradiates a subject with X-rays, a detector that outputs an output value according to the incident dose of X-rays incident on the detection surface, and a processor, wherein before performing imaging to acquire a tomographic image of the subject, the processor detects an air region where no subject is present in the channel direction of the detection surface with the subject positioned opposite the detection surface, and derives usable driving conditions that are driving conditions of the X-ray source that can be used when performing imaging, based on reference data that represents threshold driving conditions set as a threshold for controlling the incident dose within a linear region of the detector in which the relationship between the incident dose and the output value is linear, and that control the incident dose of the detected air region within the linear region.

[0006] The threshold drive condition and the enable drive condition may be the tube current supplied to the x-ray source.

[0007] The irradiation conditions of the irradiation device include a combination of multiple items selected from the following items: a tube voltage supplied to the X-ray source; a tube current-time product, which is the product of the tube current and the irradiation time; and a filter, which is provided in the irradiation device and absorbs a portion of the X-rays generated by the X-ray source.When there are multiple irradiation conditions in which the value or type of at least one of these multiple items is different, the reference data may be tube current data collected as a threshold driving condition for each irradiation condition.

[0008] The filter may include at least one of a uniform filter having a uniform absorption rate of X-rays in the channel direction and a peripheral suppression filter that suppresses the amount of radiation exposure to the peripheral part of the subject in the channel direction.

[0009] The reference data represents a distribution of threshold drive conditions in the channel direction, and the processor may derive the threshold drive condition that results in the maximum incident dose from among multiple threshold drive conditions corresponding to the air region as the maximum usable drive condition, which is the upper limit of the usable drive conditions.

[0010] Before performing the imaging, the processor may detect the air region by detecting the position of the subject's body surface based on a scanogram image obtained by irradiating the subject with X-rays while the subject is positioned opposite the detection surface.

[0011] The processor may detect a plurality of air regions corresponding to a plurality of different positions in the body axis direction of the subject and a plurality of air regions corresponding to a plurality of different irradiation angles at which X-rays are irradiated toward the subject, derive a plurality of usable driving conditions corresponding to the detected plurality of air regions, and use the derived plurality of usable driving conditions to set the irradiation conditions of the irradiation device when performing imaging.

[0012] The reference data represents the distribution of threshold drive conditions in the channel direction, and the processor derives the threshold drive condition that results in the maximum incident dose from among the multiple threshold drive conditions that correspond to air regions in the distribution of threshold drive conditions as the maximum usable drive condition, which is the upper limit of the usable drive conditions.Furthermore, when multiple air regions are detected, the maximum usable drive condition that results in the maximum incident dose from among the maximum usable drive conditions for each air region may be used to set the irradiation conditions of the irradiation device.

[0013] The apparatus is equipped with a bed device having a tabletop on which the subject is placed and capable of adjusting the position of the subject relative to the irradiation device by moving the tabletop, and the processor may detect the air region while aligning the subject with the center of rotation of the irradiation device by moving the tabletop.

[0014] The irradiation device may have a plurality of different types of peripheral suppression filters that can be selectively used as peripheral suppression filters for suppressing radiation exposure around the subject in the channel direction.

[0015] The processor may be capable of changing at least one of a tube current, a tube voltage, a tube current-time product, and a filter condition included in the irradiation conditions of the irradiation device based on the available driving conditions.

[0016] The processor may determine noise correction conditions when reconstructing a tomographic image based on the available drive conditions.

[0017] When the usable driving condition usable in the air region is set as the first usable driving condition, the processor derives, in addition to the first usable driving condition, a second usable driving condition, which is a usable driving condition in the subject region where the subject is present, based on the scanogram image obtained by irradiating X-rays while the subject is positioned opposite the detection surface and the reference data before performing the imaging, and when the incident dose corresponding to the second usable driving condition is greater than the incident dose corresponding to the first usable driving condition, the second usable driving condition may be used to set the irradiation condition of the irradiation device when performing the imaging.

[0018] The detector may be of the photon counting type.

[0019] The operating method of a CT device is a method for operating a CT device that includes an irradiation device that includes an X-ray source and irradiates a subject with X-rays, a detector that outputs an output value according to the incident dose of X-rays incident on the detection surface, and a processor, in which, before performing imaging to obtain a tomographic image of the subject, the processor detects an air region where no subject is present in the channel direction of the detection surface with the subject positioned opposite the detection surface, and derives usable driving conditions that are driving conditions of the X-ray source that can be used when performing imaging, based on reference data that represents threshold driving conditions set as a threshold for controlling the incident dose within a linear region of the detector in which the relationship between the incident dose and the output value is linear, and that control the incident dose of the detected air region within the linear region.

[0020] The operating program for a CT device is an operating program for a CT device that includes an irradiation device that includes an X-ray source and irradiates a subject with X-rays, a detector that outputs an output value according to the incident dose of X-rays incident on the detection surface, and a processor, and causes the processor to execute the following steps before performing imaging to obtain a tomographic image of the subject: detecting an air region where no subject is present in the channel direction of the detection surface with the subject positioned opposite the detection surface; and deriving usable driving conditions that are driving conditions of the X-ray source that can be used when performing imaging, based on reference data that represents threshold driving conditions set as a threshold for controlling the incident dose within a linear region of the detector in which the relationship between the incident dose and the output value is linear, the usable driving conditions being driving conditions of the X-ray source that can be used when performing imaging, for controlling the incident dose of the detected air region within the linear region.

[0021] According to the technology of the present disclosure, it is possible to suppress artifacts that occur near the body surface of the subject in a tomographic image.

[0022] 17 is a diagram showing a CT device. FIG. 18 is a diagram showing an outline of a CT device. FIG. 19 is a diagram showing a photon counting detector. FIG. 20 is a diagram showing a conventional integral type detector. FIG. 21 is a diagram showing non-linear characteristics of a detector. FIG. 22 is a diagram showing artifacts in a tomographic image. FIG. 23 is a diagram showing the configuration of an irradiation device. FIG. 24 is a diagram showing a dose profile in the channel direction. FIG. 25 is a diagram showing incident dose profiles according to the type of bowtie filter. FIG. 26 is a diagram showing changes in body surface position when the irradiation angle is changed. FIG. 27 is a diagram showing the incident dose profile for the case of FIG. 11. FIG. 28 is a diagram showing functional blocks of a processor. FIG. 29 is a diagram showing threshold currents in the linear and non-linear regions of a detector. FIG. 30 is a diagram showing changes in the profile of output values ​​when the tube current is changed. FIG. 31 is a diagram showing the correspondence between incident dose profiles and reference data. FIG. 32 is a diagram showing reference data for each irradiation condition. FIG. 33 is a diagram showing types of reference data different from FIG. 17. FIG. 34 is a diagram showing an example of detecting a body surface position based on a scanogram image. FIG. 35 is a diagram showing an example of detecting a body surface position based on a scanogram image different from FIG. 19. FIG. 36 is a diagram showing a maximum usable drive current. FIG. 37 is a diagram showing a maximum usable drive current different from FIG. 35. FIG. 38 is a diagram showing multiple maximum usable drive currents. FIG. 39 is a flowchart showing a processing procedure for a CT examination. FIG. 39 is a flowchart showing a processing procedure for deriving usable drive conditions. FIG. 39 is a diagram showing a first modified example of changing irradiation conditions. 31 is a diagram showing a modified example 2 in which noise correction conditions are changed. FIG. 32 is a diagram showing a modified example 3 in which a threshold current of the object region is also taken into consideration. FIG. 33 is a diagram showing an example of output values ​​of the object region in a scanogram image. FIG. 34 is a diagram showing an example of calculating reference data for the object region. FIG. 35 is a diagram showing an example of comparing drive conditions for the air region and the object region. FIG. 36 is a diagram showing an example different from FIG. 31 in which drive conditions for the air region and the object region are compared.

[0023] First Embodiment A CT apparatus 11 shown in FIG. 1 is an example of a PCCT apparatus having a photon-counting detector according to the technology of the present disclosure. As is well known, the CT apparatus 11 obtains a tomographic image TP of a subject H by imaging the subject H using X-rays, which are an example of radiation. The CT apparatus 11 is installed, for example, in a radiology room in a medical facility. The subject H is, for example, a patient undergoing a CT examination. The CT apparatus 11 includes a gantry 16 and a console 12. The console 12 functions as an operation terminal and a control device for operating the gantry 16. The console 12 is operated by an operator such as a diagnostic radiologist. The operator is an example of a user of the CT apparatus 11. The console 12 also functions as an image processing apparatus that generates a tomographic image TP of the subject H by performing image processing on data output from the gantry 16.

[0024] As shown in Fig. 2, the gantry 16 is a main part of the CT device 11 and includes a gantry 18 and a bed device 19. In Fig. 2, in addition to a front view of the gantry 16, a side view of the gantry 16 is also shown within a rectangular dashed-line frame. The bed device 19 has a tabletop 19A on which a subject H can be placed in a supine position. The subject H is placed in a position in which the body axis is aligned with the longitudinal direction of the tabletop 19A (the Z-axis direction of the gantry 16). The tabletop 19A can move in the Z-axis direction while remaining horizontal.

[0025] The gantry 18 has an overall annular shape, and a circular opening 18A having a diameter larger than the width of the tabletop 19A is formed in the center. During imaging, the tabletop 19A, on which the subject H is placed, moves in the Z-axis direction relative to the gantry 18 to enter the opening 18A. The actual scan, which is imaging to obtain the tomographic image TP, employs, for example, a helical scan method, and imaging is performed while the tabletop 19A is moved relative to the gantry 18. Furthermore, the tabletop 19A of the bed device 19 can be adjusted not only in height but also in width (corresponding to the X-axis direction), for example. This allows the subject H to be positioned relative to the gantry 18, for example, by aligning the subject H with the center of the opening 18A.

[0026] An irradiation device 20, a detector 22, and a frame 23 are arranged inside the gantry 18. The irradiation device 20 is an X-ray irradiation device that irradiates X-rays toward the subject H, and is an example of an "irradiation device" according to the technology of the present disclosure. The irradiation device 20 has an X-ray source 21 and a filter 24. The X-ray source 21 has an X-ray tube that generates X-rays by causing electrons emitted from a cathode to collide with an anode. The filter 24 attenuates a portion of the X-rays generated by the X-ray source 21.

[0027] The detector 22 is an X-ray detector that detects X-rays that have passed through the subject H. The X-rays that have passed through the subject H are attenuated by interactions with structures such as organs and bones within the subject H (e.g., X-ray absorption and scattering). Each structure has an attenuation coefficient for X-rays, and the X-rays that have passed through the structure carry information reflecting the physical properties of the structure. The detector 22 detects X-rays that reflect the physical properties of the structures within the subject H. The detector 22 has a detection surface 22A (see FIG. 7, etc.) on which pixels are arranged two-dimensionally. The pixels are composed of detection elements, and each pixel outputs a detection signal. This makes it possible to detect a detection signal for each transmission position of the X-rays that pass through the structure of the subject H. The detector 22 also has a substantially arc-shaped configuration in accordance with the curvature of the gantry 18, and the detection surface 22A is also curved. As will be described later, the detector 22 is a photon-counting detector and is an example of a "detector" and a "photon-counting X-ray detector" according to the technology of the present disclosure.

[0028] The irradiation device 20 and the detector 22 are disposed in opposing positions within the gantry 18 and rotate around the Z-axis while maintaining their opposing orientation. The frame 23 is annular and rotatably supports the irradiation device 20 and the detector 22. During imaging, the irradiation device 20 irradiates the subject H with X-rays while the irradiation device 20 and the detector 22 rotate around the subject H on the tabletop 19A. X-rays that have passed through the subject H are incident on the detector 22 at each irradiation angle in the circumferential direction around the body axis of the subject H, and the detector 22 outputs a detection signal corresponding to the incident dose of the X-rays. During imaging, the tabletop 19A also moves in the Z-axis direction in synchronization with the rotation of the irradiation device 20 and the detector 22. As a result, X-ray detection signals are acquired at each position in the body axis direction of the subject H.

[0029] The DAS (Data Acquisition System) 25 collects the detection signals output by the detector 22, generates projection data PD for each irradiation angle around the body axis and each position in the body axis direction based on the collected detection signals, and outputs the generated projection data PD to the console 12.

[0030] In addition to the filter 24, the irradiation device 20 is also provided with an irradiation field limiter (also called a collimator) that limits the X-ray irradiation field. Reference numeral 26 denotes a high-voltage generator that generates a tube voltage to be supplied to the X-ray source 21. The X-ray source 21 and the detector 22 are electrically connected to the frame 23 by a slip ring, for example, and power supply and data transmission / reception are performed via the slip ring. The slip ring connection enables helical scan imaging, in which the X-ray source 21 and the detector 22 are rotated in one direction without reversing the rotation direction.

[0031] The gantry 16 is provided with a gantry control unit 27. Based on instructions from the console 12, the gantry control unit 27 controls the rotation of the X-ray source 21 and the detector 22, the movement of the tabletop 19A, and other components of the gantry 16.

[0032] The imaging conditions of the CT device 11 are set through the gantry control unit 27 by operating the console 12. The imaging conditions include the X-ray irradiation conditions of the X-ray source 21, as well as the imaging region, imaging range, and slice thickness. The X-ray irradiation conditions include the driving conditions and filter conditions of the X-ray source 21. The driving conditions of the X-ray source 21 include the tube voltage (unit: kV), tube current (unit: mA), rotation speed, and tube current-time product applied to the X-ray source 21. The rotation speed is the time required for the devices in the gantry 18, such as the irradiation device 20 including the X-ray source 21 and the detector 22, to make one rotation. The tube current-time product is the product of the tube current and the irradiation time (unit: msec), and specifies the cumulative X-ray exposure dose, referred to as the mAs value. The irradiation time is the time required for the devices in the gantry 18, such as the irradiation device 20, to make one rotation, and is the same as the rotation speed. The filter conditions specify whether or not to use a filter 24 and the type of filter 24 to be used.

[0033] The console 12 includes a display 31, an input device 32, a storage 33, a communication unit 34, and a processor 36. The console 12 is configured, for example, based on a personal computer, and its hardware configuration is similar to that of a general computer. The display 31 is, for example, a liquid crystal display, and displays an operation screen and captured tomographic images TP. The input device 32 is a device through which an operator inputs operation instructions, and is configured, for example, by a keyboard, a mouse, etc.

[0034] The storage 33 is a data storage that stores various programs such as a control program that controls each part of the console 12. The various programs include an application program 37 that causes the processor 36 to function as a control device for the CT device 11 and the console 12 as an image processing device. Examples of the storage 33 include a hard disk drive (HDD) and a solid state drive (SSD). The storage 33 also temporarily stores tomographic images TP acquired from the gantry 16. The application program 37 is an example of an "operation program" according to the technology of the present disclosure.

[0035] The communication unit 34 is a communication interface for communicating with devices such as an image DB (not shown), etc. The communication unit 34 is connected to a network (not shown) such as a LAN (Local Area Network) and / or a WAN (Wide Area Network), and controls transmission in accordance with communication protocols defined by various wired or wireless communication standards.

[0036] The processor 36 functions as a control unit 36A that controls each unit of the console 12 and an image processing unit 36B that executes various types of image processing. The processor 36 is configured, for example, by a CPU (Central Processing Unit) and memory such as RAM (Random Access Memory). The CPU functions as the processor 36 by loading various programs, including an application program 37, from the storage 33 into the memory and executing the loaded programs. The application program 37 is an example of an "operating program" according to the technology of the present disclosure.

[0037] The control unit 36A controls the gantry 16 via the gantry control unit 27 in accordance with instructions from the operator inputted from the input device 32. The setting of imaging conditions including irradiation conditions is performed by the control unit 36A.

[0038] The image processing unit 36B executes image reconstruction processing. The image reconstruction processing is processing for generating a tomographic image TP by reconstructing the tomographic image TP based on the projection data PD acquired from the gantry 16. The reconstruction of the tomographic image TP based on the projection data PD is performed, for example, by a filtered back projection (FBP) method or an iterative reconstruction (IR) method. The processor 36 is an example of a "processor" according to the technology of the present disclosure.

[0039] The principle and function of the photon-counting detector 22 will be briefly described using Fig. 3. As an example, the photon-counting detector 22 shown in Fig. 3 is a detector capable of counting the number of incident X-ray photons Ptn. Fig. 4 shows a conventional energy-integrating detector 922 as a comparative example. The detector 22 will be described in comparison with the comparative example as necessary.

[0040] 3, the detector 22 includes a panel unit 41 and a readout circuit 42. The panel unit 41 has an X-ray detection surface 22A on which pixels for detecting X-rays are arranged two-dimensionally. The panel unit 41 includes an X-ray conversion layer 41A, a common electrode 41B, and individual electrodes 41C. The X-ray conversion layer 41A is a semiconductor layer that directly converts incident X-rays into an electrical signal. The semiconductor layer that constitutes the X-ray conversion layer 41A is made of, for example, CdTe (cadmium telluride) and CdZnTe (cadmium zinc telluride). The individual electrodes 41C are electrodes corresponding to each pixel, and the common electrode 41B is an electrode common to all pixels. When an X-ray photon Ptn is incident on the X-ray conversion layer 41A, the X-ray conversion layer 41A generates pairs of electrons e and holes h as electric charges in an amount corresponding to the energy of the photon Ptn. A bias voltage is applied between the common electrode 41B and the individual electrodes 41C by the power supply 43, creating an electric field within the X-ray conversion layer 41A. Therefore, for example, the electrons e generated within the X-ray conversion layer 41A move to the common electrode 41B, and the holes h move to the individual electrodes 41C. This generates a voltage V corresponding to the electric charges in each individual electrode 41C. The electric charges generated for each incident photon Ptn in the X-ray conversion layer 41A reach the individual electrodes 41C in the order of incidence.

[0041] The readout circuit 42 includes an amplifier 42A and a photon counting unit 42B for each individual electrode 41C, and reads out the voltage V generated at each individual electrode 41C. When one photon Ptn is incident, a pulse signal having a voltage value corresponding to the energy of one photon Ptn is generated at the individual electrode 41C. The amplifier 42A amplifies the pulse signal. The photon counting unit 42B has a counter and counts the number of pulse signals whose magnitude is equal to or greater than a preset threshold. A pulse signal is sequentially generated at the individual electrode 41C for each incident photon Ptn, and the pulse signal is amplified by the amplifier 42A and input to the photon counting unit 42B. The photon counting unit 42B counts the sequentially input pulse signals.

[0042] The readout circuit 42 outputs the number of pulse signals counted for each individual electrode 41C to the DAS 25 as a detection signal for each pixel. The number of pulse signals counted for each individual electrode 41C corresponds to the number of photons Ptn, and as a result, corresponds to the amount of X-rays incident on each pixel. The DAS 25 generates projection data PD based on the detection signal for each pixel input from the readout circuit 42. In this way, the photon-counting detector 22 differs from the comparative integral-type detector 922 shown in FIG. 4 in that it can accurately count the number of X-ray photons Ptn incident on each pixel.

[0043] In contrast, the integral type detector 922 shown in FIG. 4 includes a panel unit 941 and a readout circuit 942. The panel unit 941 is an indirect conversion type that converts X-rays into visible light and then converts the visible light into an electrical signal. The panel unit 941 includes a scintillator 941B that converts X-rays into visible light and a photoelectric conversion layer 941A that converts the visible light into an electrical signal. The photoelectric conversion layer 941A is configured such that a plurality of photoelectric conversion elements, such as photodiodes, that convert visible light into an electrical signal are arranged two-dimensionally. Each photoelectric conversion element corresponds to a pixel. The scintillator 941B and the photoelectric conversion layer 941A are provided with partitions 941C for defining pixels.

[0044] When an X-ray photon Ptn enters the scintillator 941B, visible light is generated and diffused within the scintillator 941B. The diffusion direction is omnidirectional, and a portion of the light is mixed with visible light generated by subsequent photons Ptn. The diffused light thus diffused enters the photoelectric conversion layer 941A. The photoelectric conversion layer 941A generates electric charge corresponding to the amount of incident diffused light. The readout circuit 942 reads out the electric charge generated in each pixel of the photoelectric conversion layer 941A. The readout circuit 942 has, for each pixel, an integrating amplifier CA composed of an amplifier and a capacitor, and a reset switch SW. The integrating amplifier CA accumulates electric charge generated by the diffused light incident on each pixel of the photoelectric conversion layer 941A in the capacitor and outputs a voltage corresponding to the accumulated electric charge. The integrating amplifier CA accumulates electric charge for a fixed period of time, and a voltage corresponding to the electric charge accumulated for the fixed period of time is read out as a detection signal. After the readout is completed, the accumulated electric charge is reset by the reset switch SW.

[0045] In this way, in the integral detector 922, the incident photons Ptn become diffused light, and the charge generated by the diffused light is integrated for a certain period of time and then read out. Therefore, in the detector 922, the detection signal for each pixel is a value obtained by integrating the energies of multiple photons Ptn, and it is not possible to output a detection signal representing the energy of each photon Ptn.

[0046] Due to these differences in configuration and principle, the photon counting detector 22, firstly, is capable of achieving higher resolution than the conventional integral type, and secondly, is capable of detecting signals for each energy bin in the X-ray energy spectrum.

[0047] First, regarding the improvement of resolution, as shown in FIG. 4, the integral type detector 922 requires partitions 941C to prevent crosstalk between adjacent pixels. Therefore, in the integral type detector 922, the partitions 941C restrict the reduction of the pixel size and pixel pitch. In contrast, the detector 22 shown in FIG. 3 does not require the partitions 941C, and therefore the pixel size and pixel pitch can be made smaller. This enables the detector 22 to have a higher resolution than conventional detectors.

[0048] In addition, signal detection for each energy bin enables the realization of a material decomposition function that obtains a material-decomposed image that distinguishes and visualizes multiple materials with different X-ray attenuation coefficients. Furthermore, signal detection for each energy bin also makes it possible to obtain a virtual monochromatic X-ray image such as that generated by DECT (Dual-energy CT) technology. DECT technology is a technology that generates a virtual monochromatic X-ray image that appears to be captured at a single energy by combining projection data PD captured at two different tube voltages using various weightings.

[0049] <Nonlinear Characteristics of Photon-Counting Detectors> In such photon-counting detectors 22, pile-up of photons Ptn is likely to occur as the incident dose increases, and when pile-up occurs, photons Ptn are not counted. As a result, as described above, in the region where the incident dose is high, the relationship between the incident dose and the output value of the detector 22 becomes nonlinear. The output value corresponds to the count value of the photons Ptn.

[0050] FIG. 5 shows the nonlinear characteristics of the detector 22. In the region where the incident dose is low, each detection element of the detector 22 is less likely to experience counting errors due to pileup, and therefore the output value OP increases as the incident dose increases, resulting in a linear relationship between the incident dose and the output value OP. In contrast, as the incident dose increases, counting errors due to pileup occur, and the output value OP decreases even when the incident dose increases, resulting in a nonlinear relationship between the incident dose and the output value OP. Therefore, in the region where the incident dose is high, the relationship between the incident dose and the output value OP is not linear as shown by the dotted line, but rather nonlinear as shown by the solid line. The region where the output characteristic of the detector 22 is linear is called the linear region, and the region where the output characteristic is nonlinear is called the nonlinear region. Here, the incident dose that forms the boundary between the linear region and the nonlinear region is called the threshold dose Dth.

[0051] <Artifacts Near the Body Surface of the Subject> As shown in the tomographic image TP in Fig. 6, in the CT device 11, artifacts AR (shown by hatching in Fig. 6) may occur near the body surface of the subject H due to the nonlinear characteristics of the detector 22 shown in Fig. 5. The area near the body surface of the subject H is close to an air region where X-rays directly strike the detector 22. The air region is an area where the subject H does not exist, and is a so-called "blank area." Since the incident dose is high in the air region, pile-up is likely to occur, and this influence affects the area near the body surface of the subject H. It is thought that this causes artifacts AR near the body surface.

[0052] 7 to 11, the cause of the artifact AR will be described. The artifact AR is affected by the incident dose profile, which represents the distribution of the incident dose in the channel direction of the detector 22, and the exposure dose profile, which represents the distribution of the exposure dose in the channel direction of the irradiation device 20. Therefore, first, the exposure dose profile will be described based on the configuration of the irradiation device 20 and the detection surface 22A of the detector 22 shown in FIG.

[0053] As shown in FIG. 7 , the detection surface 22A of the detector 22 is a curved surface on which a plurality of detection elements are arranged two-dimensionally. The detector elements of the detection surface 22A are arranged in a channel direction and a row direction. The channel direction is a direction along the curvature of the detection surface 22A and a direction along the circumferential direction of the circular opening 18A of the gantry 18. The row direction is a direction parallel to the rotation axis around which the detector 22 rotates and a direction along the Z-axis direction. In this way, the CT device 11 is an MDCT (Multi Detector-row CT) in which the detector 22 has a plurality of rows of detection elements. In FIG. 7 , "i" indicates a position in the channel direction, and "j" indicates a position in the row direction.

[0054] The irradiation device 20 irradiates a cone beam of X-rays having a fan angle extending in the channel direction and a cone angle extending in the column direction, corresponding to a detector 22 having a plurality of channels. The irradiation device 20 has, as filters 24, for example, a bowtie filter 24A and a physical filter 24B.

[0055] The bowtie filter 24A is a filter for suppressing radiation exposure in the peripheral portion of the subject H in the channel direction of the detector 22, and is an example of a "periphery suppression filter" according to the technology of the present disclosure. The bowtie filter 24A is called a bowtie filter because it has a shape similar to a bowtie, with a thickness that is thin near the center in the channel direction and gradually increases toward both sides. Due to this shape, the bowtie filter 24A attenuates the radiation dose toward the air region surrounding the subject H while ensuring the radiation dose toward the subject H. This suppresses radiation exposure that does not contribute to the image quality of the subject H. A peripheral suppression filter is also called a compensation filter. In contrast, the physical filter 24B has a uniform thickness in the channel direction, and attenuates the radiation dose of X-rays uniformly in the channel direction. The physical filter 24B is an example of a "uniform filter" according to the technology of the present disclosure.

[0056] The CT device 11 has, for example, a plurality of different types of bowtie filters 24A and physical filters 24B that can be selectively used. It is also possible to select whether or not to use the physical filter 24B. In this way, the CT device 11 can change the filter conditions by selecting whether or not to use the filter 24 and the type of filter 24.

[0057] By using such a filter 24, the exposure dose profile of the X-rays emitted by the irradiation device 20 and the incident dose profile of the X-rays incident on the detector 22 become as shown in FIG.

[0058] As shown in FIG. 8 , in the irradiation device 20, the profile of X-rays generated by the X-ray source 21 is uniform in the channel direction, as shown by the dose T1 at position T1. Next, the profile of X-rays transmitted through the bowtie filter 24A is a mountain-shaped profile with a peak in the center, as shown by the dose T2 at position T2. ​​Then, the profile of X-rays transmitted through the physical filter 24B is a profile in which the dose is attenuated by a constant attenuation rate (e.g., 0.8) relative to the dose T2 over the entire region in the channel direction, as shown by the dose T3 at position T3. The profile of the dose T3 is the irradiation dose profile irradiated to the subject H by the irradiation device 20. Note that in FIG. 8 , the profile of the dose T3 is conceptually shown, and is displayed as if it is attenuated by a constant width relative to the dose T2 over the entire region in the channel direction. However, more accurately, the profile of the dose T3 is a value obtained by multiplying the dose T2 by a constant attenuation rate over the entire region in the channel direction. Therefore, the absolute value of the attenuated dose increases as the dose T2 increases.

[0059] The incident dose profile of X-rays incident on the detector 22 is a profile shown as dose T4 at position T4. When the subject H is not present, the incident dose profile is the same as the exposure dose profile; when the subject H is present, the incident dose profile is affected by attenuation in the subject H, resulting in a profile in which the dose is reduced near the center where the subject H is present, as shown by the dotted line. Here, symbol BA indicates the region on the detection surface 22A of the detector 22 where the subject H is present, i.e., the subject region onto which the X-rays that have passed through the subject H are incident. Symbol AA indicates the region without the subject H, i.e., the air region onto which the X-rays are directly incident. Because the bowtie filter 24A makes the exposure dose profile mountain-shaped, the incident dose in the air region AA is lower than that in the center.

[0060] However, as shown in FIG. 9 , the incident dose profile differs depending on the type of bowtie filter 24A. For example, when a bowtie filter 24A of type BF1 shown in FIG. 9A is used, the incident dose profile has a mountain-shaped peak with a relatively narrow width. The incident dose profile when the subject H is not present is similar to the exposure dose profile. In the incident dose profile shown in FIG. 9A , the region indicated by the dotted line is the profile when the subject H is present. When a bowtie filter 24A of type BF2 shown in FIG. 9B is used, the incident dose profile has a mountain-shaped peak with a relatively wide width, similar to a top hat shape. The incident dose profile when the subject H is not present is similar to the exposure dose profile. In the incident dose profile shown in FIG. 9B , the region indicated by the dotted line is the profile when the subject H is present. The same applies hereinafter.

[0061] When the irradiation dose profile differs in this way, the incident dose near the body surface of the subject H located at the boundary between the subject region BA and the air region AA differs even for the same subject H. In Fig. 9, the dose D1 near the body surface in Fig. 9(A) is lower than the dose D2 near the body surface in Fig. 9(B).

[0062] Furthermore, as shown in FIG. 10 , even when the same bowtie filter 24A is used for the same subject H, if the irradiation angle is different, the dose near the body surface will also be different. The example shown in FIG. 10 uses the same bowtie filter 24A and subject H as in FIG. 8 , with only the irradiation angle indicated by θ being different. While θ is 0° in FIG. 8 , FIG. 10 shows a state in which the irradiation angle is rotated 90° clockwise from the position in FIG. 8 , and θ is 90°. In the case of FIG. 8 , the body surface position corresponds to the body width of the subject H, whereas in the case of FIG. 10 , the body surface position corresponds to the body thickness of the subject H. The body width of the subject H is the lateral width when viewed from the front, and corresponds to the width in the coronal plane. The body thickness of the subject H is the thickness of the subject H in the anterior-posterior direction when viewed from the side, and corresponds to the width in the sagittal plane. Generally, the body thickness of the subject H is narrower than the body width, so the body surface position is closer to the center in the case of FIG. 10 where θ is 90° compared to the case of FIG. 8 where θ is 0°.

[0063] FIG. 11A shows an incident dose profile corresponding to the case of FIG. 10 where θ is 90°. The solid line is a profile when the subject H is not present, and is the same as the exposure dose profile according to the type of bowtie filter 24A of BF1, while the central dotted line is a profile when the subject H is present. The type of bowtie filter 24A is BF1, as in FIG. 9A. FIG. 11B also shows an incident dose profile corresponding to the case of FIG. 10 where θ is 90°. However, in FIG. 11B, the type of bowtie filter 24A is BF2, as in FIG. 9B. In FIG. 11B, the solid line is a profile when the subject H is not present, and is the exposure dose profile according to the type of bowtie filter 24A of BF2, while the central dotted line is a profile when the subject H is present.

[0064] 11(A) and 9(A) only differ in the irradiation angle, and the incident dose profiles when the subject H is not present are the same, but the body surface positions are different because the body widths and body thicknesses of the subjects H are different. The difference between FIG. 11(B) and FIG. 9(B) also only differs in the irradiation angle, and the incident dose profiles when the subject H is not present are the same, but the body surface positions are different because the body widths and body thicknesses of the subjects H are different. As shown in FIG. 11 , when θ is 90°, the body surface position is a position corresponding to the body thickness, which is narrower than the body width, and therefore the body surface position is closer to the center than in the case of FIG. 9 where θ is 0°. Therefore, the doses D3 and D4 near the body surface positions in FIG. 11(A) and FIG. 11(B) are greater than the doses D1 and D2 near the body surface positions in FIG. 9(A) and FIG. 9(B).

[0065] <Artifact Suppression by Processor> As described above, the incident dose is high near the body surface of the subject H, and when the incident dose exceeds the threshold dose Dth shown in FIG. 5 , artifacts AR may occur. The bowtie filter 24A suppresses the incident dose in the air area AA and near the body surface by forming a mountain-shaped exposure dose profile. However, when the body surface position in the channel direction of the detection surface 22A is closer to the center due to factors such as the irradiation angle and individual differences between subjects H, the incident dose near the body surface is often high. As a measure to suppress the incident dose near the body surface, for example, a method of providing multiple types of bowtie filters 24A and selectively using the bowtie filters 24A depending on the individual differences between subjects H can be considered. However, there is a limit to the number of bowtie filters 24A, making it difficult to adapt to all subjects H. In addition, switching the bowtie filter 24A cannot accommodate changes in the body surface position when the irradiation angle changes. Therefore, in the CT device 11, the processor 36 controls the irradiation dose of the irradiation device 20 according to the position on the body surface of the subject H, thereby suppressing the artifact AR.

[0066] 12 , the processor 36 executes a reference data collection process, an irradiation condition reception process, a usable drive condition derivation process, and an irradiation condition setting process in the control unit 36A. The irradiation condition reception process is a process of receiving imaging conditions including irradiation conditions input by an operator from the input device 32. In the irradiation condition reception process, the processor 36 stores the received imaging conditions including the irradiation conditions in the storage 33.

[0067] The reference data collection process is a process for collecting reference data to be referred to in the usable drive condition derivation process, which will be described later. The processor 36 collects the reference data by driving the irradiation device 20 and the detector 22 in advance in a state where the subject H is not present. The reference data is stored in the storage 33, for example. The reference data will be described later.

[0068] The usable driving condition derivation process is a process of deriving usable driving conditions based on reference data. The usable driving conditions are driving conditions of the X-ray source 21 that can be used when performing imaging to acquire a tomographic image TP of the subject H. The imaging to acquire the tomographic image TP of the subject H is a main scan to acquire the tomographic image TP used for diagnosis, as opposed to the scanogram imaging described below that is performed to adjust the imaging range. The usable driving conditions are usable driving conditions that make it possible to suppress artifacts AR that occur near the body surface shown in FIG. 6. An example of the usable driving condition is the tube current supplied to the X-ray source 21. The derived usable driving conditions are stored in the storage 33, for example.

[0069] The irradiation condition setting process is a process for setting irradiation conditions for performing imaging. The processor 36 uses available driving conditions to set irradiation conditions for performing imaging. For example, the processor 36 determines the suitability of irradiation conditions input by the operator based on the available driving conditions and automatically resets the irradiation conditions. Alternatively, the processor 36 prompts the operator to reset the irradiation conditions by displaying the irradiation conditions to be reset on the display 31. When resetting the irradiation conditions, the processor 36 can change at least one of the tube current, tube voltage, tube current-time product, and filter conditions. Regarding the filter conditions, for example, the processor 36 switches the bowtie filter 24A depending on the width of the subject H in the channel direction. Alternatively, the processor 36 can switch the presence or absence or type of the physical filter 24B. This can also adjust the incident dose. The processor 36 may automatically switch the filter conditions, or may present the filter conditions to the operator, who can then switch the filter conditions.

[0070] 13 to 18, the reference data collection process will be described. The reference data is the driving conditions of the X-ray source 21 collected by irradiating the detector 22 with X-rays in the absence of the subject H, and is data representing threshold driving conditions set as thresholds for controlling the incident dose within the linear region of the detector 22, where the relationship between the incident dose and the output value is linear. As described above, the driving conditions are, for example, tube current, and the threshold driving conditions are threshold current.

[0071] In Fig. 13, the horizontal axis represents the tube current TC, and the vertical axis represents the output value OP of the detector element of the detector 22. As shown in Fig. 5, the nonlinear characteristics of the detector 22 indicate that, in the linear region, increasing the tube current also increases the output value OP, whereas in the nonlinear region, increasing the tube current TC does not increase the output value OP. When collecting reference data, the change in the output value OP as shown in Fig. 13 is monitored for each detector element while changing the tube current TC, and a threshold current TCth, which is the boundary between the linear region and the nonlinear region, is collected. The threshold current TCth is set, for example, to a tube current TC corresponding to a threshold th that is 90% of the peak value pk of the output value OP. Therefore, the threshold current TCth is a value smaller than the tube current TCpk corresponding to the peak value pk.

[0072] The processor 36 collects such threshold currents TCth for each detector element defined by coordinates in the channel direction (i) and column direction (j). The threshold currents TCth corresponding to each detector element are shown as threshold currents TCth(i,j) in FIG.

[0073] 14A and 14B show the incident dose profile in the channel direction and the change in the output value OP when the tube current TC is increased. Fig. 14A shows the case where the tube current is changed from TC0 to TC1. In this case, the incident dose profile in the channel direction is less than the threshold dose Dth over the entire range, and the profile of the output value OP of the detector 22 is also less than the threshold th, and the entire range is within the linear region.

[0074] FIG. 14(B) shows the case where the tube current is changed from TC1 to TC2. At TC2, the incident dose exceeds the threshold dose Dth at the center in the channel direction and reaches the nonlinear region. In the profile of the output value OP, the portion that reaches the non-dosimeter region does not rise above the threshold th corresponding to the threshold current TCth. FIG. 14(C) shows the case where the tube current is changed from TC2 to TC3. At TC3, the incident dose exceeds the threshold dose Dth at the center in the channel direction as well as at a portion of the periphery and reaches the nonlinear region. In the profile of the output value OP, the portion that reaches the non-dosimeter region corresponding to the threshold current TCth does not rise above the threshold th. In this way, the threshold current TCth in the channel direction also changes depending on the shape of the incident dose profile in the channel direction.

[0075] 15, the difference ΔD until the threshold dose Dth is reached varies in the incident dose profile in the channel direction. At the center where the incident dose is high, the difference from the threshold dose Dth is the smallest at ΔD(1), and as the incident dose decreases, the difference increases in the order of ΔD(2) and Δ(3).

[0076] The reference data is collected as a distribution of the threshold currents TCth of the detection elements arranged in the channel direction, as shown by RFD in FIG. 15 . The reference data RFD has values ​​corresponding to the differences ΔD(1) to ΔD(3), and therefore has a shape similar to an inverted incident dose profile. If the incident dose profile is mountain-shaped with a peak in the center, the reference data RFD will have an inverted mountain-shaped shape with a drop in the center and a bottom value.

[0077] 16, the reference data RFD is collected for each irradiation condition of the irradiation device 20. The plurality of reference data RFD for each irradiation condition is stored, for example, as table data. As an example of the irradiation conditions when collecting the reference data RFD, there are a plurality of irradiation conditions with different tube voltage values ​​and filter conditions. For example, the irradiation conditions (80-1 to 80-4) have a tube voltage of 80 kV, and the irradiation conditions (120-1 to 120-4) have a tube voltage of 120 kV.

[0078] For example, in the irradiation condition (80-1), the physical filter 24B is not used in the filter conditions, and F2-1 is selected as the bow tie filter 24A. The reference data RFD collected under this irradiation condition (80-1) is RFD1-1. In the irradiation condition (80-3) for the reference data RFD1-3, the bow tie filter 24A of F2-1, which is the same as that of the reference data RFD1, and the physical filter 24B of F1-1 are used. As shown in FIG. 17, the reference data RFD1-1 and the reference data RFD1-3 have similar shapes in the channel direction. However, since the reference data RFD1-1 does not have the physical filter 24B, the incident dose is increased, and conversely, the threshold current TCth shifts downward.

[0079] Furthermore, the reference data RFD1-2 uses a bowtie filter 24A of type F2-2 different from that of the reference data RFD1, and is collected under irradiation conditions (80-2) in which the physical filter 24B is not used. As shown in Fig. 18, the reference data RFD1-1 and the reference data RFD1-2 use different types of bowtie filters 24A, and therefore the incident dose profiles are different, and as a result, the shapes of the distributions of the threshold current TCth in the channel direction are also different.

[0080] In this way, the reference data RFD is collected for each irradiation condition, and the processor 36 uses these reference data RFD depending on the irradiation condition selected for imaging.

[0081] 19 to 23, the process of deriving usable driving conditions will be described. First, before performing imaging (corresponding to a main scan) for acquiring a tomographic image TP of the subject H, the processor 36 detects an air area AA where no subject is present in the channel direction of the detection surface 22A with the subject H placed opposite the detection surface 22A.

[0082] 19 and 20 show scanogram images SP of the subject H. Before performing imaging equivalent to the main scan, the processor 36 acquires the scanogram images SP by irradiating the subject H with X-rays while the subject H is positioned opposite the detection surface 22A. The X-ray dose irradiated in this case is equal to or less than the threshold dose Dth, which is lower than the dose of the main scan. The scanogram images SP are acquired for multiple irradiation angles. The scanogram image SP shown in FIG. 19 is a scanogram image when the irradiation angle θ is 0°, and the scanogram image SP shown in FIG. 20 is a scanogram image when the irradiation angle θ is 90°. The scanogram image SP is, for example, a two-dimensional projection image of the subject H acquired at each irradiation angle. This scanogram image SP is used to adjust the imaging range depending on the imaging region of the subject H.

[0083] As an example, the processor 36 detects an air area AA based on the scanogram image SP used for such positioning. The processor 36 extracts the contour of the subject H by performing image analysis on the scanogram image SP and detects the body surface position HS of the subject H, indicated by the symbol HS in FIGS. 19 and 20 . The processor 36 then detects the position of the subject H in the body width direction, as indicated by the body surface positions HS1 and HS2 in FIG. 19 . Here, the symbol LR indicates the imaging range LR in the body axis direction, and in the example of FIGS. 19 and 20 , the imaging range LR is the chest and abdomen. The body surface position HS is detected at each position in the body axis direction. The widths W1 and W2 indicate the body widths of the body surface positions HS1 and HS2, respectively. The processor 36 also detects the position of the subject H in the body thickness direction, as indicated by the body surface positions HS3 and HS4 in FIG. 20 . Widths W3 and W4 shown in FIG. 20 indicate the body thickness of the subject H at body surface positions HS3 and HS4, respectively.

[0084] The processor 36 detects the subject region BA and the air region AA based on the detected body surface position HS. Then, as shown in Figures 19 and 20, the processor 36 detects a plurality of air regions AA corresponding to a plurality of different positions in the body axis direction of the subject H and a plurality of air regions AA corresponding to a plurality of different irradiation angles at which X-rays are irradiated toward the subject H.

[0085] Then, as shown in FIGS. 21 and 22, the processor 36 reads out the reference data RFD corresponding to the irradiation conditions, and reads out the threshold currents TCth corresponding to the plurality of air areas AA detected from the reference data RFD.

[0086] For example, as shown in FIG. 21 , the processor 36 reads the threshold current TCth for the portion of the air region AA corresponding to body surface position HS1 from the reference data RFD. Furthermore, as shown in FIG. 22 , the processor 36 reads the threshold current TCth for the portion of the air region AA corresponding to body surface position HS3 from the same reference data RFD. The processor 36 then derives the threshold current TCth at which the incident dose is maximized from among the multiple threshold currents TCth corresponding to the air region AA as the maximum usable drive current TCmu, which is the upper limit of the usable drive current. As shown in FIGS. 21 and 22 , the threshold current TCth at which the incident dose is maximized, derived as the maximum usable drive current TCmu, is the minimum value in the reference data RFD representing the distribution of the threshold current TCth in the channel direction. In this example, when the incident dose profile is mountain-shaped, decreasing from the center to the periphery, the position at which the incident dose is maximized in the air region AA is considered to be the body surface position HS closest to the center.

[0087] Then, as shown in Fig. 23, the processor 36 selects the maximum usable drive current TCmu that maximizes the incident dose from the maximum usable drive currents TCmu for each of the multiple air regions AA at each irradiation angle and position in the body axis direction, and uses this to set the irradiation conditions. In the example shown in Fig. 23, the irradiation angles are selected at 30° intervals from the entire range of 0° to 360°. Then, the maximum usable drive currents TCmu for each position P1, P2, P3, P4, etc. in the body axis direction at each irradiation angle are derived. From these maximum usable drive currents TCmu, the minimum value is selected, and the selected minimum value is used to set the irradiation conditions. In Fig. 23, for example, the maximum usable drive current TCmu (P3, 90) indicated by hatching is selected.

[0088] Since the tube current of the irradiation device 20 must be set to a single value during imaging, the processor 36 uses the minimum value among the multiple maximum usable drive currents TCmu as the upper limit of the tube current TC used in the irradiation conditions. This makes it possible to keep the incident dose within the linear region for all air regions in the imaging range LR.

[0089] The operation of the above configuration will be described below with reference to the flowcharts shown in FIGS. 24 and 25. First, before performing imaging using the CT device 11, reference data RFD is collected. In step S1100, the processor 36 waits for input of imaging conditions, including irradiation conditions. When imaging is to be performed, the operator inputs the imaging conditions. When the imaging conditions are input, the processor 36 accepts the input imaging conditions, including the irradiation conditions, and stores them in the storage 33. Then, the operator inputs an instruction to move the top 19A in accordance with the imaging conditions so that the center of the imaging region of the subject H is aligned with the center of the opening 18A of the gantry 18, i.e., the rotation center of the irradiation device 20 and the detector 22. In step S1200, the processor 36 moves the position of the top 19A based on the operator's instruction, thereby aligning the subject H with the rotation center.

[0090] Next, when the operator inputs an instruction for scanogram imaging, in step S1300, the processor 36 executes scanogram imaging to obtain a scanogram image SP. As a result, the processor 36 obtains a scanogram image SP as shown in FIGS. 19 and 20. In step S1400, the processor 36 determines an imaging range according to the imaging region based on the scanogram image SP. In step S1500, the processor 36 sets the irradiation conditions accepted by the operator. Next, in step S1600, the processor 36 derives usable driving conditions to determine the validity of the accepted irradiation conditions.

[0091] As shown in FIG. 25 , in the usable drive condition derivation process, in step S1610, the processor 36 detects an air area AA at each position in the body axis direction for each irradiation angle based on the scanogram image SP as shown in FIGS. 19 and 20 . In step S1620, the processor 36 reads reference data RFD that matches the irradiation conditions. In step S1630, the processor 36 reads the threshold current TCth corresponding to the air area AA from the reference data RFD as shown in FIGS. 21 and 22 . Then, in step S1640, the processor 36 selects the minimum threshold current TCth from the threshold currents TCth within the read air area AA. Then, the processor 36 derives the selected threshold current TCth as the maximum usable drive current TCmu. As shown in FIG. 15 , the minimum threshold current TCth in the air area AA corresponds to the position where the incident dose in the air area AA is maximum. Furthermore, in step S1640, the processor 36 uses the maximum usable driving current TCmu that maximizes the incident dose, among the maximum usable driving currents TCmu of the multiple air areas AA, as shown in FIG. 23, to set the irradiation conditions.

[0092] Returning to FIG. 24 , in step S1700, the processor 36 determines whether the received irradiation conditions are appropriate based on the derived maximum usable drive current TCmu, thereby determining whether the irradiation conditions need to be reset. As an example, the processor 36 determines whether the tube current received as an irradiation condition is within the range of the derived maximum usable drive current TCmu. If the received tube current exceeds the maximum usable drive current TCmu, the processor 36 determines that the irradiation conditions need to be reset (Y in step S1700). On the other hand, if the received tube current is within the range of the maximum usable drive current TCmu, the processor 36 determines that the irradiation conditions do not need to be reset (N in step S1700). If the irradiation conditions need to be reset, the processor 36 resets the irradiation conditions in step S1800. For example, if the tube current TC of the received irradiation conditions exceeds the maximum usable drive current TCmu, the processor 36 changes the tube current TC to the maximum usable drive current TCmu.

[0093] In step S1900, the processor 36 performs imaging equivalent to a main scan under irradiation conditions that have been reset as necessary. In step S2000, the processor 36 performs image reconstruction based on the projection data PD acquired in the imaging, and generates a tomographic image TP.

[0094] As described above, the CT apparatus 11 according to the technique of the present disclosure includes the irradiation device 20 including the X-ray source 21 and irradiating the subject H with X-rays, the photon counting detector 22 that outputs an output value according to the incident dose of X-rays incident on the detection surface 22A, and the processor 36. Before performing imaging to obtain a tomographic image TP of the subject H, the processor 36 detects an air region AA where the subject H is not present in the channel direction of the detection surface 22A with the subject H placed opposite the detection surface 22A. Furthermore, the processor 36 derives usable driving conditions (for example, a maximum usable driving current TCmu) that are driving conditions of the X-ray source 21 that can be used when performing imaging, and that are usable driving conditions for controlling the detected incident dose in the air region within the linear region, based on reference data that represents threshold driving conditions (for example, a threshold current TCth) that are driving conditions of the X-ray source 21 collected by irradiating X-rays onto the detector 22 in the absence of the subject H, and that are set as a threshold for controlling the incident dose within the linear region of the detector 22 in which the relationship between the incident dose and the output value is linear.

[0095] The maximum usable drive current TCmu is derived as the operating condition for the air area AA where the incident dose is high. Therefore, by using the maximum usable drive current TCmu to set the irradiation conditions, it is possible to keep the incident dose near the body surface of the subject H within the linear region of the detector 22. The photon-counting detector 22 has a nonlinear characteristic in which photons Ptn are likely to be missed when the incident dose is high, resulting in a nonlinear relationship between the incident dose and the output value. According to the technology disclosed herein, since the operating condition for the air area AA is derived, it is possible to suppress artifacts AR that occur near the body surface of the subject H in the tomographic image TP due to the nonlinear characteristic of the photon-counting detector 22.

[0096] In the above embodiment, the threshold drive condition (for example, the threshold current TCth) and the usable drive condition (for example, the maximum usable drive current TCmu) are the tube current supplied to the X-ray source 21. The tube current is a major item of the irradiation conditions that defines the dose. Therefore, using the tube current as the drive condition is convenient.

[0097] The irradiation conditions of the irradiation device 20 include a combination of multiple items selected from the following: the tube voltage supplied to the X-ray source 21; the tube current-time product, which is the product of the tube current and the irradiation time; and the filter 24, which is provided in the irradiation device 20 and absorbs a portion of the X-rays generated by the X-ray source 21. As shown in FIG. 16 as an example, when there are multiple irradiation conditions in which at least one of these multiple items has a different value or type, the reference data RFD is data on the tube current (corresponding to the threshold current TCth) collected as a threshold drive condition for each irradiation condition. The example shown in FIG. 16 shows irradiation conditions composed of a combination of tube voltage and filter conditions. Various irradiation conditions are used in actual imaging. Therefore, collecting the tube current as a threshold drive condition for each irradiation condition is convenient.

[0098] The filter 24 also includes at least one of a uniform filter (for example, the physical filter 24B) that has a uniform absorption rate of X-rays in the channel direction, and a peripheral suppression filter (for example, the bowtie filter 24A) that suppresses the amount of radiation exposure in the peripheral parts of the subject H in the channel direction. The uniform filter and the peripheral suppression filter are important elements that define the incident dose profile that affects the artifact AR, and therefore, by collecting threshold drive conditions for each irradiation condition that includes these filter conditions, it becomes possible to control the incident dose appropriately to suppress the artifact AR.

[0099] Furthermore, the reference data RFD represents the distribution of threshold drive conditions (e.g., threshold current TCth) in the channel direction, and the processor 36 derives the threshold drive condition that maximizes the incident dose from among the multiple threshold drive conditions corresponding to the air area AA as the maximum usable drive current TCmu, which is the upper limit of the usable drive conditions. This makes it easy to set irradiation conditions that maximize the incident dose within a range that does not deviate from the linear region of the detector 22. The higher the incident dose, the better the S / N ratio and contrast, and thus the better the image quality can be expected. Therefore, it is possible to acquire a tomographic image TP with good image quality while suppressing artifacts AR.

[0100] Furthermore, before performing imaging, the processor 36 detects the air region AA by detecting the body surface position HS of the subject H based on a scanogram image SP obtained by irradiating the subject H with X-rays while the subject H is positioned opposite the detection surface 22A. By using the scanogram image SP, the body surface position HS can be detected more accurately than with visible light. If the body surface position HS can be detected accurately, artifacts can be effectively reduced. Furthermore, the scanogram image SP is an image used for adjusting the imaging range, etc. Therefore, by using this image to detect the air region AA, radiation exposure can be reduced compared to when a scanogram image SP is acquired specifically for detecting the air region AA. Note that an optical image may be used to detect the air region AA instead of the scanogram image SP.

[0101] 23 as an example, the processor 36 detects a plurality of air regions AA corresponding to a plurality of different positions in the body axis direction of the subject H and a plurality of air regions AA corresponding to a plurality of different irradiation angles at which X-rays are irradiated toward the subject H, derives a plurality of usable drive conditions (for example, threshold current TCth) corresponding to the detected plurality of air regions AA, and uses the derived plurality of usable drive conditions to set the irradiation conditions of the irradiation device when performing imaging. Since a plurality of usable drive conditions corresponding to a plurality of air regions with different irradiation angles and positions in the body axis direction are used to set the irradiation conditions, more appropriate irradiation conditions can be set than when there is only one usable drive condition.

[0102] The reference data also represents the distribution of threshold drive conditions (e.g., threshold current TCth) in the channel direction. The processor 36 derives the threshold drive condition, among the plurality of threshold drive conditions corresponding to the air area AA in the distribution of threshold drive conditions, that which maximizes the incident dose as the maximum usable drive condition (e.g., maximum usable drive current TCmu), which is the upper limit of the usable drive conditions. Furthermore, when the plurality of air areas AA are detected, the maximum usable drive condition, among the maximum usable drive conditions for each air area AA, that which maximizes the incident dose is used to set the irradiation conditions. Because the maximum usable drive condition, among the plurality of maximum usable drive conditions, that which maximizes the incident dose is used to set the irradiation conditions, the incident dose in the imaging range LR is controlled to be within the linear region overall. This allows for more appropriate irradiation conditions to be set.

[0103] The CT apparatus 11 according to the technique of the present disclosure includes a table 19 having a top 19A on which a subject H is placed, and a bed device 19 capable of adjusting the position of the subject H relative to the irradiation device 20 by moving the top 19A. As shown in Fig. 24 , in the positioning process, the processor 36 detects an air area AA in a state in which the subject H is aligned with the center of rotation of the irradiation device 20 by moving the top 19A. As shown in Fig. 8 as an example, the incident dose profile in the channel direction often peaks at the center of rotation, so the dose irradiated to the subject H can be increased by aligning the subject H with the center of rotation. This makes it easier to obtain good image quality.

[0104] (Variation 1) Furthermore, as shown in Fig. 26 , the processor 36 may change at least one of the tube current, tube voltage, tube current-time product, and filter condition included in the irradiation conditions based on an available drive condition (for example, the maximum available drive current TCmu). In the example shown in Fig. 26 , the processor 36 changes at least one of the tube voltage, tube current-time product, and filter condition other than the tube current so that the incident dose falls within a range whose upper limit is defined by the maximum available drive current TCmu. In this way, the incident dose may be controlled by adjusting items other than the tube current.

[0105] (Variation 2) The processor 36 may also determine noise correction conditions for reconstructing a tomographic image TP based on usable drive conditions (for example, the maximum usable drive current TCmu). For example, when it is desired to increase the incident dose to ensure image quality, it may be impossible to control the tube current of the irradiation conditions within the range of the maximum usable drive current TCmu. In this case, the processor 36 may determine noise correction conditions for the image reconstruction process executed by the image processing unit 36B to suppress artifacts AR. The noise correction conditions may be, for example, the correction strength of the noise correction process in the iterative image reconstruction method.

[0106] (Variation 3) Furthermore, as shown in FIGS. 28 to 32, usable drive conditions for the object region BA may be used in setting the irradiation conditions in addition to usable drive conditions for the air region AA. The processor 36 derives usable drive conditions, for example, using the procedure shown in step S1600B in FIG. 28. Step S1600B in FIG. 28 is based on step S1600 in FIG. 25 but differs in some respects. The following mainly describes the differences. In step S1610B in FIG. 28, the processor 36 detects the object region BA in addition to the air region AA based on the scanogram image SP. While step S1610 in FIG. 25 detects the air region AA, step S1610B in FIG. 28 also detects the object region BA. However, since the object region BA can also be detected by detecting the air region AA, the substantial processing of step S1610B in FIG. 28 is the same as step S1610 in FIG. 25.

[0107] Next, steps S1620B, S1630B, and S1640B shown in Fig. 28 are similar to steps S1620, S1630, and S1640 shown in Fig. 25. However, in Modification 3, the maximum usable drive current TCmu derived in the air area AA is set as the first usable drive current.

[0108] Steps S1650B to S1670B shown in Fig. 28 are processes not present in Fig. 25. In step S1650B, the processor 36 calculates the threshold current TCth of the subject area BA.

[0109] 29 shows the distribution of output values ​​OP in the scanogram image SP. In the subject area BA, when the imaging site is the chest, a large amount of radiation passes through the lung field area LG corresponding to the lung field, resulting in a relatively large incident radiation dose. Therefore, the output value OP is relatively high.

[0110] 30 , the processor 36 derives reference data RFD_H, which is the distribution of threshold currents TCth in the subject region BA indicated by the dotted line in FIG. 30 , based on the reference data RFD in which the subject H is not present. Specifically, if the threshold current TCth at a certain pixel (i, j) in the subject region BA is TCth_Body, the processor 36 calculates TCth_Body based on the following equation, for example: In the equation, first, TCth_air(i, j) is the threshold current TCth at a certain pixel (i, j) in the air region AA extracted from the reference data RFD. Then, OP_SCN(i, j) is the output value OP of a certain pixel (i, j) in the scanogram image SP, and OP_RFD(i, j) is the output value OP of a certain pixel (i, j) when the reference data RFD is acquired. TC_SCN is the tube current when the scanogram image SP is acquired, and TC_RFD is the tube current when the reference data RFD is acquired.

[0111]

[0112] The second term on the right side of the equation is the reciprocal of the ratio of the output value OP of the detector 22 when the reference data RFD is acquired to the output value OP when the scanogram image SP is acquired. This is because, as shown in FIG. 15 , the threshold current TCth is inversely proportional to the output value OP, which has a positive correlation with the incident dose. The third term on the right side of the equation is the ratio of the tube current when the reference data RFD is acquired to the tube current when the scanogram image SP is acquired. Because the tube current is not necessarily the same when the reference data RFD is acquired and when the scanogram image SP is acquired, it is corrected by this third term.

[0113] For example, consider an object region BA at a certain pixel (i, j) where the incident dose is half that of the air region AA. In this case, the output value OP of the object region BA is half that of the air region AA, so the second term on the right side of the equation is "2," since the ratio of OP_SCN(i, j) to OP_RFD(i, j) is half. If TC_SCN and TC_RFD are both the same tube current, the third term on the right side of the equation is "1." In this case, the threshold current TCth_Body(i, j) of a certain pixel (i, j) in the object region BA is twice the threshold current TCth_AIR(i, j) of the air region AA. This means that in the object region BA where the output value OP is half that of the air region AA, the margin until the threshold current TCth is reached is twice that of the air region AA. Similarly, in a region where the X-ray attenuation rate is high and the incident dose is one-third, the threshold current TCth_Body(i,j) is three times that of the air region AA. That is, the threshold current TCth_Body(i,j) in a region where the X-ray attenuation rate is low and the incident dose is high, such as the lung field, exhibits a value relatively close to that of the air region AA. In contrast, the threshold current TCth_Body(i,j) in a region where the X-ray attenuation rate is high and the incident dose is low, such as bone, is relatively far from the value of the air region AA.

[0114] Thus, the above formula is a formula for calculating the threshold current TCth of the subject area BA based on the reference data RFD of the air area AA, using the ratio between the output value OP of the reference data RFD and the output value OP of the subject area BA of the scanogram image SP.

[0115] The processor 36 calculates the threshold current TCth_Body(i,j) of each pixel (i,j) in the subject area BA using the above equation and derives reference data RFD_H, which is the distribution of the threshold current TCth_Body(i,j) in the subject area BA shown by the dotted line in Figure 30.

[0116] Returning to FIG. 28, in step S1660B, the processor 36 selects the minimum threshold current TCth in the reference data RFD_H in the object region BA, and derives it as the second usable drive current.

[0117] Then, in step S1670B, the processor 36 selects the smaller of the first available drive current for the air area AA and the second available drive current for the object area BA, and uses the selected available drive condition to set the irradiation condition.

[0118] For example, as shown in FIG. 31, when the first usable drive current TCair in the air area AA is smaller than the second usable drive current TClg in the subject area BA (i.e., when the incident dose is larger), the processor 36 selects the first usable drive current TCair as the maximum usable drive current TCmu to be used for setting the irradiation conditions.

[0119] On the other hand, as shown in FIG. 32, when the second usable drive current TClg in the subject area BA is smaller than the first usable drive current TCair in the air area AA (i.e., when the incident dose is larger), the processor 36 selects the second usable drive current TClg as the maximum usable drive current TCmu to be used for setting the irradiation conditions.

[0120] As described above, in the third modification, when the usable drive condition usable in the air area AA is set to the first usable drive condition (for example, the first usable drive current TCair), the processor 36 derives, in addition to the first usable drive condition, a second usable drive condition (for example, the second usable drive current TC1g) that is a usable drive condition in the object area BA where the object H is present, based on the scanogram image SP obtained by irradiating X-rays while the object H is positioned opposite the detection surface 22A and the reference data RFD before performing imaging. Then, when the incident dose corresponding to the second usable drive condition is greater than the incident dose corresponding to the first usable drive condition, the second usable drive condition is used to set the irradiation conditions of the irradiation device when performing imaging. This makes it possible to set appropriate irradiation conditions that suppress artifacts AR, including in the object area BA.

[0121] In the above embodiment, the processor 36 resets the irradiation conditions as an example of using the available driving conditions to set the irradiation conditions, but other methods are also possible. For example, the processor 36 may simply present the available driving conditions to the operator by displaying them on the display 31. This can prompt the operator to reset the irradiation conditions.

[0122] In the above embodiment, an example has been described in which the maximum usable tube current TCmu is derived as the usable drive condition. However, since the usable drive condition is a drive condition for controlling the incident dose, it may be something other than the tube current, such as the tube voltage.

[0123] Although the example has been described in which the maximum usable driving condition at which the incident dose is maximized is derived as the usable driving condition, it does not have to be the maximum as long as it is within the usable range. Even if it is not the maximum, it is possible to suppress artifacts AR, although the image quality will be somewhat degraded.

[0124] Furthermore, in the above embodiment, a two-dimensional scanogram image, which is a two-dimensional projection image of the subject H acquired at each irradiation angle, has been described as an example of the scanogram image SP for detecting the body surface position of the subject H. However, a three-dimensional scanogram image may be used instead. The three-dimensional scanogram image is a tomographic image acquired by performing a helical scan in which the irradiation device 20 and the detector 22 are rotated in a spiral, as in the main scan, and reconstructing the data collected by the helical scan. However, the radiation dose when acquiring a three-dimensional scanogram image is lower than that in the main scan. Because the three-dimensional scanogram image is a tomographic image similar to the main scan, it is possible to detect the body surface position of the subject H with greater accuracy.

[0125] In the above embodiment, the photon-counting detector 22 has been described as an example of the detector, but the technology of the present disclosure may also be applied to the integral detector 922 shown in Figure 4 if a nonlinear region such as that shown in Figure 5 occurs. This is because the occurrence of a nonlinear region causes the artifact AR shown in Figure 6. Of course, the nonlinear region shown in Figure 5 and the resulting artifact AR in Figure 6 tend to be more pronounced in photon-counting detectors, so the technology of the present disclosure is particularly effective when using a photon-counting detector.

[0126] From the above description, the technology described in the following supplementary paragraphs can be understood.

[0127] [Supplementary Item 1] A CT apparatus comprising: an irradiation device including an X-ray source that irradiates a subject with X-rays, a detector that outputs an output value according to an incident dose of X-rays incident on a detection surface, and a processor, wherein the processor, before performing imaging to obtain a tomographic image of the subject, detects an air region where no subject is present in a channel direction of the detection surface with the subject positioned opposite the detection surface, and derives usable driving conditions that are driving conditions of the X-ray source that can be used when performing imaging, the usable driving conditions being for controlling the incident dose in the detected air region within the linear region, based on reference data representing threshold driving conditions collected by irradiating X-rays to the detector in the absence of the subject, the threshold driving conditions being set as a threshold for controlling the incident dose within the linear region of the detector in which the relationship between the incident dose and the output value is linear. [Supplementary Item 2] The CT apparatus according to Supplementary Item 1, wherein the threshold driving conditions and the usable driving conditions are tube currents supplied to the X-ray source. [Supplementary Item 3] The CT apparatus according to Supplementary Item 2, wherein the irradiation conditions of the irradiation device include a combination of a plurality of items selected from the following: a tube voltage supplied to the X-ray source, a tube current-time product which is the product of the tube current and the irradiation time, and a filter provided in the irradiation device and which absorbs a portion of the X-rays generated by the X-ray source; and when there are a plurality of irradiation conditions in which at least one of these plurality of items differs in value or type, the reference data is tube current data collected as a threshold driving condition for each irradiation condition. [Supplementary Item 4] The CT apparatus according to Supplementary Item 3, wherein the filters include at least one of a uniform filter having a uniform X-ray absorption rate in the channel direction and a peripheral suppression filter which suppresses radiation exposure to peripheral portions of the subject in the channel direction. [Supplementary Item 5] The CT apparatus according to any one of Supplementary Items 1 to 4, wherein the reference data represents a distribution of threshold driving conditions in the channel direction, and the processor derives, from a plurality of threshold driving conditions corresponding to air regions, a threshold driving condition which maximizes the incident dose as a maximum usable driving condition which is the upper limit of usable driving conditions.[Supplementary Item 6] The CT device according to any one of Supplementary Items 1 to 5, wherein, before performing imaging, the processor detects an air region by detecting a body surface position of the subject based on a scanogram image obtained by irradiating X-rays on the subject with the subject positioned opposite the detection surface. [Supplementary Item 7] The CT device according to any one of Supplementary Items 1 to 6, wherein the processor detects a plurality of air regions corresponding to a plurality of different positions in the body axis direction of the subject and a plurality of air regions corresponding to a plurality of different irradiation angles at which X-rays are irradiated towards the subject, derives a plurality of usable drive conditions corresponding to the detected plurality of air regions, and uses the derived plurality of usable drive conditions to set irradiation conditions of an irradiation device when performing imaging. [Supplementary Item 8] The CT apparatus according to Supplementary Item 6 or Supplementary Item 7, wherein the reference data represents a distribution of threshold drive conditions in the channel direction, and the processor derives, from among a plurality of threshold drive conditions corresponding to air regions in the distribution of threshold drive conditions, a threshold drive condition at which the incident dose is maximum as a maximum usable drive condition that is an upper limit of usable drive conditions, and further, when a plurality of air regions are detected, the maximum usable drive condition at which the incident dose is maximum is used to set the irradiation conditions of the irradiation apparatus. [Supplementary Item 9] The CT apparatus according to any one of Supplementary Items 1 to 8, wherein the CT apparatus comprises a bed device having a top plate on which a subject is placed and capable of adjusting the position of the subject with respect to the irradiation apparatus by moving the top plate, and the processor detects the air regions while aligning the subject with the center of rotation of the irradiation apparatus by moving the top plate. [Supplementary Item 10] The CT apparatus according to any one of Supplementary Items 1 to 9, wherein the irradiation device has a plurality of different types of peripheral suppression filters that can be selectively used as peripheral suppression filters that suppress radiation exposure around the subject in the channel direction. [Supplementary Item 11] The CT apparatus according to any one of Supplementary Items 1 to 10, wherein the processor is capable of changing at least one of the tube current, tube voltage, tube current time product, and filter condition included in the irradiation conditions of the irradiation device based on the available drive conditions.[Supplementary Item 12] The CT device according to any one of Supplementary Items 1 to 11, wherein the processor determines, based on the usable drive conditions, noise correction conditions for reconstructing a tomographic image. [Supplementary Item 13] When the usable drive conditions usable in the air region are set as first usable drive conditions, the processor derives, in addition to the first usable drive conditions, a second usable drive condition that is a usable drive condition in an object region where the object is present, based on the reference data and a scanogram image obtained by irradiating the object with X-rays while the object is placed opposite the detection surface, before performing imaging, and when the incident dose corresponding to the second usable drive condition is higher than the incident dose corresponding to the first usable drive condition, uses the second usable drive condition to set the irradiation condition of the irradiation device for performing imaging. [Supplementary Item 14] The detector is a photon counting type. A CT device according to any one of Supplementary Items 1 to 13. [Supplementary Item 15] A method for operating a CT device comprising: an irradiation device including an X-ray source and irradiating X-rays onto a subject; a detector that outputs an output value according to an incident dose of X-rays incident on a detection surface; and a processor, wherein the processor, before performing imaging to acquire a tomographic image of the subject, detects an air region where no subject is present in the channel direction of the detection surface with the subject positioned opposite the detection surface; and derives usable driving conditions that are driving conditions of the X-ray source that can be used when performing imaging, for controlling the incident dose in the detected air region within the linear region, based on reference data that represents threshold driving conditions that are collected by irradiating X-rays onto the detector in the absence of the subject, and that are threshold driving conditions set as a threshold for controlling the incident dose within the linear region of the detector in which the relationship between the incident dose and the output value is linear.[Supplementary Item 16] An operating program for a CT apparatus including an irradiation device that includes an X-ray source and irradiates a subject with X-rays, a detector that outputs an output value according to an incident dose of X-rays incident on a detection surface, and a processor, the operating program causing the processor to execute the following steps: before performing imaging to obtain a tomographic image of the subject, a process of detecting an air region where no subject is present in the channel direction of the detection surface with the subject positioned opposite the detection surface; and a process of deriving usable driving conditions that are driving conditions of the X-ray source that can be used when performing imaging, based on reference data that represent threshold driving conditions set as a threshold for controlling the incident dose within a linear region of the detector where the relationship between the incident dose and the output value is linear, the usable driving conditions being driving conditions of the X-ray source that can be used when performing imaging, for controlling the incident dose in the detected air region within the linear region.

[0128] In the above embodiment, the processes performed by a processing unit such as the processor 36 of the console 12 are executed by any computer. Furthermore, any computer may execute these processes using a processor as hardware, a program as software, or a combination of both. In this case, the processor is configured to execute various processes in this embodiment in cooperation with the program, and may function as each unit or means in this embodiment. Furthermore, the order in which the processes are executed by the processor is not limited to the order described above and may be changed as appropriate.

[0129] The given computer may be a general-purpose computer, a computer for specific applications, a workstation, or any other system capable of executing each process. The processor may be configured with one or more pieces of hardware, and the type of hardware is not limited. For example, the processor may be configured with hardware such as a central processing unit (CPU), a micro processing unit (MPU), a programmable logic device such as a field programmable gate array (FPGA), a dedicated circuit for executing specific processes such as an application specific integrated circuit (ASIC), a graphics processing unit (GPU), or a neural processing unit (NPU). The type of hardware may also be a combination of different types of hardware. When multiple pieces of hardware are configured to execute one or more processes of a certain processor, the multiple pieces of hardware may be located in devices physically separated from each other, or may be located in the same device. Furthermore, in any embodiment, the order of the processes performed by the processor is not limited to the order described above and may be changed as appropriate. The hardware is configured by an electric circuit (circuitry) or the like that combines circuit elements such as semiconductor elements.

[0130] Furthermore, the program may be software, such as firmware or microcode. The program may also be, for example, a group of program modules, each function of which may be implemented by a processor configured to perform the respective function. The program may be program code and / or multiple code segments stored in one or more non-transitory computer-readable media (e.g., storage media and / or other storages). The program may be stored across multiple non-transitory computer-readable media that reside in physically separate devices. The program code or code segment may represent a procedure, a function, a subprogram, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. The program code or code segment may be connected to another code segment or a hardware circuit by sending or receiving information, data, arguments, parameters, or memory contents.

[0131] The technology of the present disclosure can also be appropriately combined with the various embodiments and / or various modified examples described above. Furthermore, the technology is not limited to the above embodiments, and various configurations can be adopted without departing from the spirit of the present disclosure. Furthermore, the technology of the present disclosure also covers, in addition to programs, storage media that non-temporarily store programs. The storage medium is, for example, a computer-readable non-temporary storage medium such as a USB (Universal Serial Bus) memory, a flexible disk, or a CD-ROM (Compact Disc Read Only Memory). The program may also be provided online via a network such as the Internet. The technology of the present disclosure also covers, in addition to programs, program products. A program product includes any type of product for providing a program. Like a program, a program product may be provided stored on a computer-readable non-temporary storage medium or provided online.

[0132] The above-described description and illustrations are a detailed explanation of the parts related to the technology of the present disclosure and are merely an example of the technology of the present disclosure. For example, the above description of the configuration, functions, actions, and effects is an explanation of an example of the configuration, functions, actions, and effects of the parts related to the technology of the present disclosure. Therefore, it goes without saying that unnecessary parts may be deleted, new elements may be added, or replacements may be made to the above-described description and illustrations within the scope of the gist of the technology of the present disclosure. Furthermore, to avoid confusion and facilitate understanding of the parts related to the technology of the present disclosure, the above-described description and illustrations omit explanations of common technical knowledge that do not require particular explanation to enable the implementation of the technology of the present disclosure.

[0133] In this specification, "A and / or B" is synonymous with "at least one of A and B." In other words, "A and / or B" means that it may be only A, only B, or a combination of A and B. Furthermore, in this specification, the same concept as "A and / or B" is also applied when three or more things are expressed by connecting them with "and / or."

[0134] The disclosure of Japanese Patent Application No. 2024-086571, filed on May 28, 2024, is incorporated herein by reference in its entirety. In addition, all documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A CT device comprising: an irradiation device including an X-ray source that irradiates a subject with X-rays; a detector that outputs an output value according to the incident dose of the X-rays incident on a detection surface; and a processor, wherein the processor, before performing an imaging operation to obtain a tomographic image of the subject, detects an air region where the subject is not present in the channel direction of the detection surface with the subject positioned opposite the detection surface; and derives usable driving conditions that are driving conditions of the X-ray source that can be used when performing the imaging operation, based on reference data that represent threshold driving conditions set as a threshold for controlling the incident dose within a linear region of the detector in which the relationship between the incident dose and the output value is linear. The usable driving conditions are driving conditions of the X-ray source that can be used when performing the imaging operation, and are used to control the incident dose of the detected air region within the linear region.

2. The CT apparatus according to claim 1, wherein the threshold drive condition and the enable drive condition are tube currents supplied to the X-ray source.

3. The CT device according to claim 2, wherein the irradiation conditions of the irradiation device include a combination of a plurality of items selected from the following items: a tube voltage supplied to the X-ray source; a tube current-time product which is the product of the tube current and irradiation time; and a filter which is provided in the irradiation device and absorbs a portion of the X-rays generated by the X-ray source; and when there are a plurality of irradiation conditions in which at least one of these plurality of items has a different value or type, the reference data is data on the tube current collected as the threshold drive condition for each of the irradiation conditions.

4. The CT device according to claim 3, wherein the filter includes at least one of a uniform filter having a uniform absorption rate of X-rays in the channel direction and a peripheral suppression filter that suppresses the amount of exposure to radiation in the peripheral areas of the subject in the channel direction.

5. The CT device described in claim 1, wherein the reference data represents a distribution of the threshold drive conditions in the channel direction, and the processor derives the threshold drive condition that maximizes the incident dose from among the multiple threshold drive conditions corresponding to the air region as the maximum usable drive condition, which is the upper limit of the usable drive conditions.

6. The CT device according to claim 1, wherein, before the imaging is performed, the processor detects the air region by detecting the position of the subject's body surface based on a scanogram image obtained by irradiating the subject with X-rays while the subject is positioned opposite the detection surface.

7. The CT device according to claim 1, wherein the processor detects a plurality of air regions corresponding to a plurality of different positions in the body axis direction of the subject and a plurality of air regions corresponding to a plurality of different irradiation angles at which X-rays are irradiated toward the subject, derives a plurality of usable driving conditions corresponding to the detected plurality of air regions, and uses the derived plurality of usable driving conditions to set the irradiation conditions of the irradiation device when performing the imaging.

8. The CT device according to claim 6, wherein the reference data represents a distribution of the threshold drive conditions in the channel direction, and the processor derives, from among a plurality of threshold drive conditions corresponding to the air regions in the distribution of the threshold drive conditions, the threshold drive condition at which the incident dose is maximum as a maximum usable drive condition that is the upper limit of the usable drive conditions, and further, when a plurality of the air regions are detected, the maximum usable drive condition at which the incident dose is maximum is used to set the irradiation conditions of the irradiation device.

9. A CT device as described in claim 1, comprising a bed device having a tabletop on which a subject is placed and capable of adjusting the position of the subject relative to the irradiation device by moving the tabletop, wherein the processor detects the air region while aligning the subject with the center of rotation of the irradiation device by moving the tabletop.

10. The CT apparatus according to claim 1, wherein the irradiation device has a plurality of different types of peripheral suppression filters that can be selectively used as peripheral suppression filters for suppressing radiation exposure around the subject in the channel direction.

11. The CT device according to claim 1, wherein the processor is capable of changing at least one of the tube current, tube voltage, tube current time product, and filter condition included in the irradiation conditions of the irradiation device based on the available driving conditions.

12. The CT apparatus according to claim 1, wherein the processor determines noise correction conditions for reconstructing a tomographic image based on the available drive conditions.

13. The CT device according to claim 1, wherein, when the usable driving condition usable in the air region is set as the first usable driving condition, the processor, before performing the imaging, derives, in addition to the first usable driving condition, a second usable driving condition which is a usable driving condition in the subject region where the subject is present, based on the reference data and a scanogram image obtained by irradiating the subject with X-rays while the subject is positioned opposite the detection surface, and when the incident dose corresponding to the second usable driving condition is greater than the incident dose corresponding to the first usable driving condition, uses the second usable driving condition to set the irradiation condition of the irradiation device when performing the imaging.

14. The CT apparatus according to claim 1, wherein the detector is of a photon counting type.

15. A method for operating a CT device comprising: an irradiation device including an X-ray source that irradiates a subject with X-rays; a detector that outputs an output value according to the incident dose of the X-rays incident on a detection surface; and a processor, wherein the processor, before performing imaging to obtain a tomographic image of the subject, detects an air region where the subject is not present in the channel direction of the detection surface with the subject positioned opposite the detection surface; and derives usable driving conditions that are driving conditions of the X-ray source that can be used when performing the imaging, based on reference data that represent threshold driving conditions set as a threshold for controlling the incident dose within a linear region of the detector in which the relationship between the incident dose and the output value is linear, the usable driving conditions being driving conditions of the X-ray source that can be used when performing the imaging, and for controlling the incident dose in the detected air region within the linear region.

16. An operating program for a CT apparatus comprising an irradiation device including an X-ray source and irradiating a subject with X-rays, a detector that outputs an output value according to the incident dose of the X-rays incident on the detection surface, and a processor, the operating program causing the processor to execute the following steps: before performing imaging to obtain a tomographic image of the subject, a process of detecting an air region where the subject is not present in the channel direction of the detection surface with the subject positioned opposite the detection surface; and a process of deriving usable driving conditions that are driving conditions of the X-ray source that can be used when performing the imaging, based on reference data representing threshold driving conditions set as a threshold for controlling the incident dose within a linear region of the detector where the relationship between the incident dose and the output value is linear.

Images