JP7250532B2 - X-ray CT device and imaging planning device - Google Patents

Description

An embodiment of the present invention relates to an X-ray CT apparatus and an imaging planning apparatus.

An X-ray CT apparatus generally uses an X-ray tube (X-ray tube device) as an X-ray source. An X-ray tube is based on the principle that electrons emitted from a cathode are accelerated by an electric field between them and an anode, and emit X-rays when they collide with the anode and decelerate.

Along with the evolution of the X-ray CT apparatus, the performance required for the X-ray tube has also changed. In order to shorten the scanning time, it is necessary to increase the amount of X-rays generated per hour. In order to expand the width in the body axis direction for data acquisition at the same time, it is necessary to generate wide X-rays in the body axis direction, so it is necessary to increase the target angle. In order to improve the resolution, the focal size should be reduced. In order to meet these demands, it is necessary to increase the heat capacity of the anode and increase the rotational speed of the anode. ing.

In addition, there is a technology (FFS: flying focal spot) that improves resolution and suppresses artifacts by moving the focus at high speed during scanning and increasing the position of data to be collected (increasing ray sampling). It has been put to practical use. In this FFS, by moving the focal point at high speed, the X-ray generation region on the anode target surface is widened, so that the effect of lowering the maximum temperature on the anode target surface can also be obtained.

In the X-ray tube, most of the energy when the colliding electrons slow down becomes heat, and a very small part becomes X-rays, so the anode is always used in a state just before melting. Attempting to reduce the focal spot size while maintaining power will increase the density of the electron beam and produce the same amount of X-rays in a smaller area on the anode, thus generating more heat per unit area. It will exceed the limit temperature that does not melt. The focus size and the maximum output are basically in a trade-off relationship, and if the focus size is reduced in order to improve the resolution, the output must be lowered.

In addition, when the maximum temperature is lowered by moving the focus, moving the focus during one data collection (one view) is the same as increasing the size of the focus, which degrades the resolution. As an FFS, it is ideal to fix the focal point during acquisition of one piece of data and move it at high speed at the switching point of data acquisition. or sinusoidal. For example, in the case of a trapezoidal waveform, it is necessary to either not collect data while the focal point is moving, or even if it is collected, it will not be used for reconstruction. Furthermore, in a device that uses a collection method called sequential collection, in which the collection time differs depending on the pixel, the range of the focal position shifts for each pixel. The focal size becomes large.

Japanese Unexamined Patent Application Publication No. 2011-19802 JP-A-5-217534

The problem to be solved by the present invention is to achieve a higher level trade-off relationship between the focal spot size and the maximum output of the X-ray tube.

An X-ray CT apparatus according to an embodiment includes a controller. The controller controls movement of the focus of the X-ray tube. The control unit calculates a first speed for continuously and periodically moving the focus from the viewpoint of not melting the anode of the X-ray tube, and continuously and periodically moves the focus from the viewpoint of satisfying the required resolution. A second moving speed is calculated, and a third speed for continuously and periodically moving the focus is determined from the lower limit speed and the upper limit speed according to the purpose of photographing.

FIG. 1 is a block diagram showing a configuration example of an X-ray CT apparatus according to the first embodiment. FIG. 2 is a diagram showing a configuration example of an X-ray tube. FIG. 3 is a diagram showing a configuration example of an X-ray high voltage device. FIG. 4 is a flow chart showing a processing example of the X-ray CT apparatus. FIG. 5 is a diagram showing an example of changes in temperature of the anode of an X-ray tube with respect to time. FIG. 6 is a diagram showing an example of temperature change with respect to the cycle of moving the focal point of the anode of the X-ray tube. FIG. 7A is a diagram showing a state where the focus is at the first position. FIG. 7B is a diagram showing a state where the focus is at the second position. FIG. 7C is a diagram showing a state in which the apparent focal size increases as the focal point moves from the first position to the second position during one data acquisition (1 view). FIG. 8 is a diagram showing an example of change in apparent focal spot size with respect to the cycle of moving the focal spot of the anode of the X-ray tube. FIG. 9A is a diagram showing an example of focal positions in the front state. FIG. 9B is a diagram showing an example of the focus position in the center state. FIG. 9C is a diagram showing an example of the position of the focus in the back state. FIG. 10 is a diagram showing an example of changes in focal position with respect to the rotation angle of the gantry device 10. FIG. FIG. 11A is a diagram showing an example of positions of rays in the front state. FIG. 11B is a diagram showing an example of ray positions in the back state. FIG. 12 is a diagram showing an example of data collection by changing the focal position and sequential collection. FIG. 13 is a diagram showing an example of change in focal position by FFS as a comparative example. FIG. 14 is a diagram showing an example of data acquisition by sequential acquisition and changes in focal position by FFS as a comparative example. FIG. 15A is a diagram (1) showing an example of positions of rays when an X-ray tube is supported by a support mechanism in the second embodiment; FIG. 15B is a diagram (2) showing an example of positions of rays when the X-ray tube is supported by the support mechanism in the second embodiment; FIG. 15C is a diagram (3) showing an example of positions of rays when the X-ray tube is supported by the support mechanism in the second embodiment; 15D is a diagram (4) showing an example of the positions of rays when the X-ray tube is supported by the support mechanism in the second embodiment; FIG. FIG. 16A is a diagram (1) showing an example of a focus position when there is a support mechanism. FIG. 16B is a diagram (2) showing an example of the position of the focal point when there is a support mechanism; FIG. 16C is a diagram (3) showing an example of the position of the focus when there is a support mechanism.

Hereinafter, each embodiment of an X-ray CT apparatus and an imaging planning apparatus will be described with reference to the drawings. In addition, embodiment is not restricted to the following contents. In principle, the contents described in one embodiment and modification are similarly applied to other embodiments and modifications.

(First embodiment)
The configuration of an X-ray CT apparatus 1 according to the first embodiment will be described with reference to FIG. FIG. 1 is a block diagram showing a configuration example of an X-ray CT apparatus 1 according to the first embodiment. The X-ray CT apparatus 1 has a gantry device 10, a bed device 30, and a console device 40, as shown in FIG. For convenience of explanation, a plurality of gantry devices 10 are drawn in FIG. 1, but basically there is one gantry device 10 in the actual configuration. In FIG. 1, the longitudinal direction of the rotational axis of the rotating frame 16 or the top plate 33 of the bed device 30 in the non-tilted state of the gantry device 10 is defined as the Z-axis direction. Further, the axial direction perpendicular to the Z-axis direction and horizontal to the floor surface is defined as the X-axis direction. Also, the axial direction perpendicular to the Z-axis direction and perpendicular to the floor surface is defined as the Y-axis direction.

The gantry device 10 includes an X-ray tube 11, an X-ray detector 15, a rotating frame 16, an X-ray high voltage device 17, a control device 18, a wedge 19, a collimator 20, and a DAS (Data Acquisition System). 21.

The X-ray tube 11 is a vacuum tube that generates X-rays by irradiating thermal electrons from a cathode (filament) toward an anode (target) with a high voltage supplied from an X-ray high voltage device 17 . For example, the X-ray tube 11 is a rotating anode type X-ray tube that generates X-rays by irradiating a rotating anode with thermal electrons. This embodiment can be applied to both a single-tube type X-ray CT apparatus and a so-called multi-tube type X-ray CT apparatus in which a plurality of pairs of X-ray tubes and detectors are mounted on a rotating ring. is. The X-ray high-voltage device 17 not only generates high voltage, but also supplies power to the filament, power to drive the rotary anode, and drives a thermoelectron adjustment mechanism, which will be described later. Details will be described later.

FIG. 2 is a diagram showing a configuration example of the X-ray tube 11. As shown in FIG. In FIG. 2, the X-ray tube 11 includes a housing (tube housing) 111, a cathode 113, a thermoelectron adjustment mechanism 114, a rotating shaft 115, and an anode . The housing 111 is made of metal, for example, and has an X-ray window 112 through which X-rays generated inside pass. The cathode 113 generates thermal electrons. Thermal electrons are electrons emitted from a filament or a heated member excited by heat generated by a current flowing through a filament.

The anode 116 generates X-rays upon collision with thermoelectrons emitted from the cathode 113 . Specifically, a large potential difference is provided between the cathode 113 and the anode 116 . For example, a potential difference is provided between the cathode 113 and the anode 116 by grounding the anode 116 and making the potential of the cathode 113 negative. Due to this potential difference, thermoelectrons emitted from the cathode 113 are accelerated and collide with the anode 116 to generate X-rays. Also, the anode 116 is a rotating body rotated by the rotating shaft 115 and has a circular outer periphery when viewed from the axial direction of the rotating shaft 115 . The anode 116 has an umbrella shape, and the tip side of the umbrella faces the cathode 113 side. The side of the anode 116 facing the cathode 113 has a tapered surface, forming a surface inclined toward the X-ray window 112 by a slight angle with respect to the cathode 113 side. The anode 116 rotates to disperse the locations where heat is generated by the collision of thermoelectrons, thereby avoiding melting of the surface of the anode 116 due to heat generation. The rotary shaft 115 is supported by a bearing (not shown) or the like, and is rotationally driven by a rotating magnetic field generated by a stator coil (not shown) or the like. In the drawing, the thermionic trajectory (indicated by a dashed line) directed from the cathode 113 to the anode 116 is drawn in parallel with the rotating shaft 115 of the anode 116, but the present invention is not limited to this.

The thermoelectron adjustment mechanism 114 is provided along the trajectory of the thermoelectrons between the cathode 113 and the anode 116 so as to sandwich the trajectory, and changes the trajectory of the thermoelectrons emitted from the cathode 113 by an electric field or a magnetic field. to move the focal point on the anode 116 . The thermionic adjustment mechanism 114 includes adjustment poles 114a and 114b facing each other with a distance in the Y-axis direction, corresponding to the case where an electric field is used. When an electric field is used to adjust the trajectory, the adjustment electrodes 114a and 114b are, for example, plate-like electrodes, to which different negative potentials than the potential of the cathode 113 are applied to generate negatively charged thermoelectrons. The acting repulsive force changes the trajectory of the thermal electrons to the side with relatively higher potential. When a magnetic field is used to adjust the trajectory, the direction of force acting on the thermoelectrons is perpendicular to the direction of force acting on the thermoelectrons when an electric field is used to adjust the trajectory, and the adjustment poles 114a, 114b are: They are spaced apart in the X-axis direction. When a magnetic field is used to adjust the trajectory, the adjustment poles 114a and 114b are, for example, the magnetic poles of an electromagnet, and the trajectory of the thermoelectron changes due to the force that the flying thermoelectron receives from the magnetic field. Note that the thermoelectron adjustment mechanism 114 is not limited to being provided inside the housing 111 , and the thermoelectron adjustment mechanism 114 may be provided outside the housing 111 .

In addition, although the case where the trajectory of thermoelectrons is changed in the Y-axis direction has been described in FIG. 2, the trajectory of thermoelectrons may be changed in other directions. For example, by providing another pair of adjustment poles in a direction perpendicular to the pair of adjustment poles 114a and 114b of the thermoelectron adjustment mechanism 114 and adjusting the magnitude of the electric field or magnetic field by the two pairs of adjustment poles perpendicular to each other, It is possible to change the trajectory of thermoelectrons in any direction.

FIG. 3 is a diagram showing a configuration example of the X-ray high voltage device 17. As shown in FIG. In FIG. 3, the X-ray high voltage device 17 includes an X-ray high voltage supply circuit 171, a filament power supply circuit 172, an anode rotation drive power supply circuit 173, and a thermoelectron adjustment mechanism drive circuit 174. The X-ray high voltage supply circuit 171 generates and supplies a high voltage applied between the cathode 113 and the anode 116 of the X-ray tube 11 . The X-ray high voltage supply circuit 171 has electric circuits such as a transformer and a rectifier, and includes a high voltage generation circuit that generates a high voltage to be applied to the X-ray tube 11 and an X-ray source that the X-ray tube 11 irradiates. and an x-ray control circuit for controlling the output voltage according to the line. The high voltage generation circuit may be of a transformer type or an inverter type. A filament power supply circuit 172 supplies power to the filament of the X-ray tube 11 . An anode rotation driving power supply circuit 173 supplies power for rotating the anode 116 .

The thermoelectron adjustment mechanism drive circuit 174 controls the electric field or magnetic field acting on the trajectory of thermoelectrons by means of voltage or current supplied to the thermoelectron adjustment mechanism 114 of the X-ray tube 11 . The X-ray high voltage device 17 may be provided on the rotating frame 16 or may be provided on a fixed frame (not shown). In addition, a configuration example in which the X-ray high-voltage device 17 includes the X-ray high-voltage supply circuit 171, the filament power supply circuit 172, the anode rotation drive power supply circuit 173, and the thermoelectron adjustment mechanism drive circuit 174 has been described. A part may be provided as another device.

Returning to FIG. 1 , the X-ray detector 15 detects X-rays emitted from the X-ray tube 11 and passing through the subject P, and outputs a signal corresponding to the detected X-ray dose to the DAS 21 . The X-ray detector 15 has, for example, a plurality of X-ray detection element arrays in which a plurality of X-ray detection elements are arranged in the channel direction (circumferential direction) along one circular arc centered on the focal point of the X-ray tube 11. have. The X-ray detector 15 has, for example, a structure in which a plurality of X-ray detection element arrays each having a plurality of X-ray detection elements arranged in the channel direction are arranged in the slice direction (column direction, row direction).

Also, the X-ray detector 15 is, for example, an indirect conversion type detector having a grid, a scintillator array, and a photosensor array. The scintillator array has a plurality of scintillators. The scintillator has a scintillator crystal that outputs a photon amount of light corresponding to the amount of incident X-rays. The grid is arranged on the surface of the scintillator array on the X-ray incident side and has an X-ray shielding plate that absorbs scattered X-rays. Note that the grid may also be called a collimator (one-dimensional collimator or two-dimensional collimator). The photosensor array has a function of converting the amount of light from the scintillator into an electrical signal, and includes photosensors such as photodiodes and photomultipliers (PMTs). The X-ray detector 15 may be a direct conversion type detector having a semiconductor element that converts incident X-rays into electrical signals.

The rotating frame 16 (mounting base) is an annular frame that supports the X-ray tube 11 and the X-ray detector 15 so as to face each other and rotates the X-ray tube 11 and the X-ray detector 15 by the control device 18 . For example, the rotating frame 16 is a casting made of aluminum. In addition to the X-ray tube 11 and the X-ray detector 15, the rotating frame 16 can also support the X-ray high voltage device 17 and the DAS 21. FIG. Additionally, rotating frame 16 may further support various configurations not shown in FIG. Below, in the gantry device 10, the portion that rotates together with the rotating frame 16 and the rotating frame 16 are also referred to as rotating portions. Although the Rotate/Rotate-Type (third-generation CT) in which the X-ray tube 11 and the X-ray detector 15 are integrally rotated around the subject P has been described, in addition to the above, a ring-shaped array may be used. There are various types such as Stationary/Rotate-Type (fourth generation CT) in which a large number of X-ray detection elements are fixed and only the X-ray tube 11 rotates around the subject P, and any type can be applied to the present embodiment. Applicable.

The detection data generated by the DAS 21 is transmitted, for example, by optical communication from a transmitter having a light emitting diode (LED) provided on the rotating frame 16. It is sent to a receiver with a photodiode and forwarded to the console device 40 . Here, the non-rotating portion is, for example, a fixed frame (not shown in FIG. 1) that rotatably supports the rotating frame 16, or the like. The method of transmitting the detected data from the rotating frame 16 to the non-rotating portion of the gantry device 10 is not limited to optical communication, and any method can be adopted as long as data can be transmitted between the rotating portion and the non-rotating portion. I don't mind.

Controller 18 includes drive mechanisms, such as motors and actuators, and circuitry for controlling the mechanisms. The control device 18 receives input signals from the input interface 43 , an input interface provided in the gantry device 10 , and the like, and controls the operations of the gantry device 10 and the bed device 30 . For example, the control device 18 controls the rotation of the rotating frame 16, the tilt of the gantry device 10, the motions of the bed device 30 and the tabletop 33, and the like. As an example, the control device 18 rotates the rotating frame 16 about an axis parallel to the X-axis direction based on input inclination angle (tilt angle) information as control for tilting the gantry device 10 . Note that the control device 18 may be provided in the gantry device 10 or may be provided in the console device 40 .

Wedge 19 is a filter for adjusting the dose of X-rays emitted from X-ray tube 11 . Specifically, the wedge 19 transmits and attenuates the X-rays emitted from the X-ray tube 11 so that the X-rays emitted from the X-ray tube 11 to the subject P have a predetermined distribution. It is a filter that For example, the wedge 19 is a wedge filter or a bow-tie filter, and is constructed by processing aluminum or the like so as to have a predetermined target angle and a predetermined thickness.

The collimator 20 is a lead plate or the like for narrowing down the irradiation range of the X-rays transmitted through the wedge 19, and a slit is formed by combining a plurality of lead plates or the like. Note that the collimator 20 may also be called an X-ray diaphragm. The aperture and position of the collimator 20 are adjusted by a collimator adjustment circuit (not shown). Thereby, the irradiation range of the X-rays generated by the X-ray tube 11 is adjusted.

The DAS 21 has an amplifier that amplifies the electrical signal output from each X-ray detection element of the X-ray detector 15 and an A/D converter that converts the electrical signal into a digital signal. to generate The DAS 21 is implemented by, for example, a processor. Detection data generated by the DAS 21 is transferred to the console device 40 .

The bed device 30 is a device for placing and moving a subject P to be scanned, and has a base 31 , a bed driving device 32 , a top board 33 and a support frame 34 . The base 31 is a housing that supports the support frame 34 so as to be vertically movable. The bed drive device 32 is a drive mechanism that moves the table 33 on which the subject P is placed in the longitudinal direction of the table 33, and includes a motor, an actuator, and the like. A top plate 33 provided on the upper surface of the support frame 34 is a plate on which the subject P is placed. Note that the bed driving device 32 may move the support frame 34 in the longitudinal direction of the top plate 33 in addition to the top plate 33 . Only the top plate 33 may be moved, or a system in which the entire support frame of the bed device 30 is moved may be used. When applied to standing CT, a method of moving a patient support mechanism corresponding to the top plate 33 may be used. When performing a scan (such as a helical scan or a positioning scan) that involves a relative change in the positional relationship of the top plate 33 of the gantry device 10 , the relative change in the positional relationship may be performed by driving the top plate 33 . However, it may be performed by running the gantry device 10, or may be performed by combining them. When applied to dental CT, the couch device 30 and the like are not required.

The console device 40 has a memory 41 , a display 42 , an input interface 43 and a processing circuit 44 . Although the console device 40 will be described as being separate from the gantry device 10 , the console device 40 or a part of each component of the console device 40 may be included in the gantry device 10 .

The memory 41 is implemented by, for example, a RAM (Random Access Memory), a semiconductor memory device such as a flash memory, a hard disk, an optical disk, or the like. For example, the memory 41 stores projection data and reconstructed image data. Also, for example, the memory 41 stores a program for the circuit included in the X-ray CT apparatus 1 to realize its function. The memory 41 is also used as a non-transitory storage medium by hardware. The storage of projection data and reconstructed image data is not limited to the case where the memory 41 of the console device 40 stores the data. 1 may store projection data and reconstructed image data.

The display 42 displays various information. For example, the display 42 outputs a medical image (CT image) generated by the processing circuit 44, a GUI (Graphical User Interface) for accepting various operations from the operator, and the like. For example, the display 42 is a liquid crystal display or a CRT (Cathode Ray Tube) display. Also, the display 42 may be provided on the gantry device 10 . The display 42 may be of a desktop type, or may be configured by a tablet terminal or the like capable of wireless communication with the main body of the console device 40 .

The input interface 43 receives various input operations from the operator, converts the received input operations into electrical signals, and outputs the electrical signals to the processing circuit 44 . For example, the input interface 43 receives acquisition conditions for acquiring projection data, reconstruction conditions for reconstructing CT images, image processing conditions for generating post-processed images from CT images, and the like from the operator. . For example, the input interface 43 is implemented by a mouse, keyboard, trackball, switch, button, joystick, touch panel, or the like. Also, the input interface 43 may be provided in the gantry device 10 . Also, the input interface 43 may be composed of a tablet terminal or the like capable of wireless communication with the main body of the console device 40 .

A processing circuit 44 controls the operation of the entire X-ray CT apparatus 1 . For example, the processing circuit 44 has a scan control function 441 , an image generation function 442 , a display control function 443 and a control function 444 . The processing circuit 44 is implemented by, for example, a processor. The processing circuit 44 is an example of a control section.

For example, the processing circuit 44 reads a program corresponding to the scan control function 441 from the memory 41 and executes it, thereby controlling the X-ray CT apparatus 1 to perform scanning. Here, the scan control function 441 can execute scans in various methods such as conventional scan, helical scan, and step-and-shoot method.

Specifically, the scan control function 441 moves the subject P into the imaging opening of the gantry device 10 by controlling the bed driving device 32 . Also, the scan control function 441 supplies a high voltage to the X-ray tube 11 by controlling the X-ray high voltage device 17 . The scan control function 441 also controls movement of the focal point of the X-ray tube 11 via the thermoelectron adjustment mechanism drive circuit 174 of the X-ray high voltage device 17 . Also, the scan control function 441 adjusts the aperture and position of the collimator 20 . Also, the scan control function 441 rotates the rotating part including the rotating frame 16 by controlling the control device 18 . The scan control function 441 also causes the DAS 21 to collect projection data. In order to reconstruct a CT image, projection data for 360° around the object P is required, and projection data for 180°+fan angle are required even in the half-scan method. This embodiment can be applied to any reconstruction method.

Further, for example, the processing circuit 44 reads out a program corresponding to the image generation function 442 from the memory 41 and executes it to perform logarithmic conversion processing, offset correction processing, and inter-channel sensitivity processing on the detection data output from the DAS 21 . Data is generated after preprocessing such as correction processing and beam hardening correction. Note that data before preprocessing (detection data) and data after preprocessing may be collectively referred to as projection data. Also, for example, the image generation function 442 generates CT image data. Specifically, the image generation function 442 performs reconstruction processing using a filtered back projection method, an iterative reconstruction method, or the like on the preprocessed projection data to generate CT image data. The image generation function 442 performs reconstruction based on representative focal positions in one data acquisition (view) during the reconstruction process. The image generation function 442 also converts CT image data into tomographic image data or three-dimensional image data of an arbitrary cross section based on an input operation received from an operator via the input interface 43 .

Also, for example, the processing circuit 44 displays a CT image on the display 42 by reading out and executing a program corresponding to the display control function 443 from the memory 41 . Further, for example, the processing circuit 44 reads out a program corresponding to the control function 444 from the memory 41 and executes it, thereby controlling various functions of the processing circuit 44 based on an input operation received from the operator via the input interface 43 . to control.

Note that FIG. 1 shows a case where each processing function of the scan control function 441, the image generation function 442, the display control function 443, and the control function 444 is realized by a single processing circuit 44, but the embodiment is not limited to For example, the processing circuit 44 may be configured by combining a plurality of independent processors, and each processor may implement each processing function by executing each program. Further, each processing function of the processing circuit 44 may be appropriately distributed or integrated in a single or a plurality of processing circuits and implemented. The processing circuit 44 is not limited to being included in the console device 40, and may be included in an integrated server that collectively processes detection data acquired by a plurality of medical image diagnostic apparatuses. Although console device 40 has been described as performing multiple functions with a single console, multiple functions may be performed by separate consoles. Post-processing may be performed by either the console device 40 or an external workstation. Alternatively, the processing may be performed by both the console device 40 and the workstation.

Also, the portion of the scan control function 441 of the processing circuit 44 that performs the functions up to immediately before the execution of the scan can be called an imaging planning device, and can be cut out as a separate device.

The term "processor" used in the above description includes, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), an application specific integrated circuit (ASIC), a programmable logic device (e.g., Circuits such as Simple Programmable Logic Device (SPLD), Complex Programmable Logic Device (CPLD), or Field Programmable Gate Array (FPGA). The processor implements functions by reading and executing programs stored in the memory 41 . Instead of storing the program in the memory 41, the program may be directly incorporated into the circuit of the processor. In this case, the processor implements its functions by reading and executing the program embedded in the circuit. Note that each processor of the present embodiment is not limited to being configured as a single circuit for each processor, and may be configured as one processor by combining a plurality of independent circuits to realize its function. good.

FIG. 4 is a flowchart showing an example of processing of the X-ray CT apparatus 1, which is an example of processing for determining the moving speed (amplitude, period) of the focal point of the X-ray tube 11 prior to imaging (scanning). Note that the speed of movement of the focal point is determined by the amplitude and period of movement. In FIG. 4, the scan control function 441 of the processing circuit 44 acquires imaging conditions and the like (step S1). Scanning conditions, etc. include information such as scan protocol, high-definition mode/normal mode, conventional scan/helical scan, imaging site, and operation history. Also, operation history up to the last minute, requirements for high definition, requirements for low noise, requirements for low radiation exposure, etc. are determined. Furthermore, based on any or a combination of scan continuity, scan area, subject size, operation history up to the last minute, high definition requirement, low noise requirement, low exposure requirement, It is determined whether the goal should be resolution or maximum output.

Next, the scan control function 441 calculates the lower limit speed (amplitude, period) for continuous and periodic movement of the focus from the viewpoint of not melting the anode 116 of the X-ray tube 11 (step S2). The lower limit speed is an example of a first speed.

FIG. 5 is a diagram showing an example of changes in temperature of the anode 116 of the X-ray tube 11 with respect to time. In FIG. 5, curve a is the change when the focus position of the X-ray tube 11 is not moved, curve b is the change when the focus is moved in the first period, and curve c is the change when the focus is moved in the first period. Curve d shows the change when the focus is moved in the third period, which is shorter than the second period. Curve e shows the change when the focus is moved in the fourth period, which is shorter than the third period. A curve f shows an example of change when the focal point is moved in a fifth period shorter than the fourth period. That is, when the focus position of the X-ray tube 11 is not moved (curve a), the relative temperature reaches 100% corresponding to melting in a predetermined time. can be suppressed. FIG. 6 is a diagram showing an example of temperature change with respect to the cycle of moving the focal point of the anode 116 of the X-ray tube 11. When the cycle is small (when the focus is moved at high speed), the temperature of the anode 116 decreases. can be made

For example, the scan control function 441 applies a predetermined relative temperature (eg, 60%) to the mathematical expression or numerical data indicating the relationship shown in FIG. Get the amplitude and period. If there are multiple sets of relevant movement amplitudes and periods, the scan control function 441 may obtain those multiple sets. In addition, it is desirable that the amplitude of the focal point movement is equal to or larger than the focal point size. This is because in the periodic movement of the focal point, if the same position is exposed to thermoelectrons, it is likely to melt.

Next, returning to FIG. 4, the scan control function 441 calculates the upper limit speed (amplitude, period) for continuously and periodically moving the focal point of the X-ray tube 11 from the viewpoint of satisfying the required resolution (step S3 ). The upper limit speed is an example of a second speed. If the focal point moves during one data acquisition (one view), it becomes equivalent to an increase in the focal size, which causes a decrease in resolution. shift the focus over a long period of time. For example, it is desirable to move the focal point at a period longer than 10 times the data collection sampling. Moreover, it is desirable to synchronize the movement of the focal point with the rotation of the gantry device 10 . For example, it is desirable to move the focal point at a long period of 1/10 or more of the rotation of the gantry. As an example, two cycles of focal point movement can be performed with one rotation of the gantry.

FIG. 7A is a diagram showing a state where the focus is at the first position, FIG. 7B is a diagram showing the state where the focus is at the second position, and FIG. FIG. 10 is a diagram showing a state in which the apparent focal size is widened by moving from the first position to the second position; That is, when the focus moves from the first position as shown in FIG. 7A to the second position as shown in FIG. 7B during one data acquisition (1 view), the apparent focus size is as shown in FIG. It spreads out and the resolution decreases. FIG. 8 is a diagram showing an example of change in apparent focus size with respect to the cycle of moving the focus of the anode 116 of the X-ray tube 11. When the cycle is large (moving slowly), the apparent focal size can be reduced and the resolution can be increased.

For example, the scan control function 441 applies a predetermined relative resolution (eg, 1.2) to the mathematical expression or numerical data indicating the relationship shown in FIG. Obtain the amplitude and period of the movement. If there are multiple sets of relevant movement amplitudes and periods, the scan control function 441 may obtain those multiple sets.

Next, returning to FIG. 4, the scan control function 441 determines the speed (amplitude, period) is determined (step S4). An example of the third speed is the speed at which the focal point is continuously and periodically moved according to the purpose of photographing. At this time, when the scan control function 441 determines that the purpose of photographing should prioritize the resolution, the speed of continuously and periodically moving the focus is brought closer to the lower limit speed, and the purpose of photographing is to maximize the output. If it is determined that priority should be given, the speed at which the focus is continuously and periodically moved is brought closer to the upper limit speed. Thereafter, the scan control function 441 controls the thermoelectron adjustment mechanism drive circuit 174 to move the focal point of the X-ray tube 11 at the determined speed (amplitude, period), and executes imaging (scanning).

Steps S 1 to S 4 shown in FIG. 4 are steps corresponding to the scan control function 441 . Steps S1 to S4 are steps in which the scan control function 441 is realized by the processing circuit 44 reading out a program corresponding to the scan control function 441 from the memory 41 and executing it. The order of processing in the flow chart illustrated in FIG. 4 may be changed within a range that does not essentially affect the results. In addition, processing may be performed in parallel to the extent that the results are not essentially affected.

9A is a diagram showing an example of the focus position in the front state, FIG. 9B is a diagram showing an example of the focus position in the center state, and FIG. 9C is a diagram showing an example of the focus position in the back state, In this example, the focus position is moved continuously and periodically in synchronization with the rotation of the gantry device 10 . In addition, when the focus is on the front (FIG. 9A) and when it is on the back (FIG. 9C), the areas on the anode 116 that are the focus are not overlapped. However, it is not essential that the focal regions do not overlap. Considering the cooling of the anode 116, it is ideal that the focal regions do not overlap, but if the focal regions overlap, the effect of preventing melting will not be lost. FIG. 10 is a diagram showing an example of a change in the position of the focal point with respect to the rotation angle of the gantry 10. When the gantry 10 is directly above, the front is the front, and when the gantry 10 is rotated clockwise by 45° in the figure, Center (Center), Rotate another 45° to Back, Rotate another 45° to Center, Rotate another 45° to Front, Rotate another 45° to Center, Rotate another 45° It becomes the back in the state, and it becomes the center in the state where it is further rotated 45 degrees.

FIG. 11A is a diagram showing an example of ray positions in the front state, and FIG. 11B is a diagram showing an example of ray positions in the back state. That is, when the focal position changes, the position of the ray connecting the focal position and the detection pixel changes. Therefore, the image generation function 442 calculates a representative position of the focus for each view (for example, an average position of the focus that changes slightly) according to the position of the moving focus, and calculates the position of the focus and the detection pixel. Then, the image is reconstructed by back-projecting along the calculated ray position. Note that the positions of rays used for reconstruction may not only be calculated individually for each piece of data, but may also be grouped into several groups and the same value may be used within each group.

FIG. 12 is a diagram showing an example of data collection by changing the focal position and sequential collection. In FIG. 12, the focal position is continuously and periodically between the front (F) and the back (B), for example, in a sinusoidal manner, at half the rotation period of the gantry, as described in FIG. It is subject to change. In this case, since the speed of movement of the focal point takes into consideration the speed of continuous and periodic movement of the focal point from the viewpoint of satisfying the required resolution, the change in the focal position within the short period ΔT is very small. , there is almost no reduction in resolution due to differences in focal positions. Therefore, as shown in the order of pixels (columns) on the left side of the drawing, in sequential acquisition in which pixel values are acquired column by column from the X-ray detector 15, the views view1 and view2 are first acquired in consecutive views as in S101. Even if the pixels in the first column are acquired and the pixels in the last column are acquired with a shift as in S102, there is almost no reduction in resolution due to the difference in focal position. In addition, the speed of movement of the focal point takes into consideration the speed of continuous and periodic movement of the focal point from the viewpoint of not melting the anode 116 of the X-ray tube 11. Therefore, the melting of the anode 116 is also prevented, and the X-rays A higher level of trade-off between focal size and maximum output of the tube 11 is realized. That is, it is possible to obtain a larger output with the focal size equivalent to the conventional one, or to maintain the output while reducing the focal size.

FIG. 13 is a diagram showing an example of changes in focal position by FFS as a comparative example. shows another realistic sinusoidal focus position transition. FIG. 14 is a diagram showing an example of focal position change by FFS and data acquisition by sequential acquisition as a comparative example, and shows the case of trapezoidal wave-like focal position transition in the middle of FIG. 13 . In these cases, one data acquisition period (one view) is half of the movement period of the focal position.

In FIG. 14, among the transitions of the focal position shown in the upper row, the transitional states between front (F) and back (B) cannot determine the focal position and should be excluded from the data. Therefore, the X-ray irradiation that does not contribute to the image increases, which is not preferable. Also, as shown in the order of pixels (columns) on the left side of the drawing, in sequential acquisition in which pixel values are acquired for each column from the X-ray detector 15, the views view1 and view2 are first When the pixels in the first column are acquired and the pixels in the last column are acquired with a shift as in S202, the values of pixels at different focal positions are mixed, and the effective focal size cannot be aligned. However, the actual focal spot size becomes large. As described above, in the present embodiment, such inconvenience does not occur.

The case where the focus is moved over a relatively long period of time with respect to the interval of one data collection (one view) has been described, but when a predetermined instruction is given by the operator or when the purpose of photographing is predetermined, If it is determined that a higher resolution is required than the resolution of , the focus may be shifted to a mode in which continuous and periodic movement of the focal point is not performed. In addition, when a predetermined instruction is given by the operator, the focus is moved in the FFS mode in which the focus is discontinuously changed to a plurality of positions (the focus is moved in approximately the same cycle as the data acquisition sampling). may

Also, when subtraction is performed in an orbit-synchronous scan, changing the position of the focal point tends to affect the image quality, so in principle, the operation of moving the focal point is stopped. However, by synchronizing the period of focal position change between multiple scans in which subtraction is performed, it is possible to move the focal point even when subtraction is performed in orbit-synchronous scans. The same is true for dynamic scanning.

Also, with regard to the calibration data acquired for calibrating the X-ray tube 11, the calibration data can be acquired in advance according to the focal position, and the calibration data can be selectively used according to the focal position during scanning. .

(Second embodiment)
In the above-described first embodiment, reconstruction is performed in consideration of the positions of rays (paths) according to the position of the moving focal point. However, in the second embodiment, even if the position of the focal point on the anode 116 of the X-ray tube 11 moves, the position of the focal point viewed from the X-ray detector 15 side does not change, and reconstruction processing is performed. to make it easier.

15A to 15D are diagrams showing examples of positions of rays when the X-ray tube 11 is supported by the support mechanism 12 in the second embodiment. 15A to 15D, X-ray tube 11 is attached to rotating frame 16 via support mechanism 12. In FIGS. The support mechanism 12 moves the X-ray tube 11 in a direction opposite to the moving direction of the focal point of the X-ray tube 11 as viewed from the X-ray detector 15 side, and adjusts the focal position as viewed from the X-ray detector 15 side. do not change. For example, the support mechanism 12 is composed of a piezoelectric element such as a piezoelectric element, an actuator such as a motor, or the like. Other configurations are the same as those shown in FIG. 1 and the like.

FIG. 15A shows the case where the focus of the X-ray tube 11 is moved to the front position (FIG. 9A) and the X-ray tube 11 itself remains at its original position without moving, and FIG. position (FIG. 9A), and the X-ray tube 11 is moved in the back direction (to the right in the drawing). FIG. 15C shows the case where the focus of the X-ray tube 11 is moved to the back position (FIG. 9C) and the X-ray tube 11 itself is not moved and remains at the original position, and FIG. position (FIG. 9C), and the X-ray tube 11 is moved in the front direction (to the left in the drawing). 15A to 15D, the dashed-dotted line extending from the X-ray tube 11 toward the X-ray detector 15 indicates the same position, but as shown in FIGS. By moving, the position of the focal point seen from the X-ray detector 15 side becomes the same at all positions from the front to the back.

The positions of the focal points without the support mechanism 12 are shown in FIGS. 9A to 9C. FIG. 9A shows the focal point at the front position, and FIG. 9B shows the focal point at the center position. 9C is the case where the focus is in the back position. 16A to 16C are diagrams showing an example of the position of the focal point when the support mechanism 12 is present. FIG. 16A shows the case where the focal point is in the front position and the entire X-ray tube 11 moves in the back direction. 16B shows the case where the focus is at the center position and the entire X-ray tube 11 does not move, and FIG. It shows the case of moving to . In FIGS. 9A to 9C and FIGS. 16A to 16C, dashed lines indicate the same position, and the support mechanism 12 moves the X-ray tube 11 so that the X-ray detector is shown in FIGS. 16A to 16C. The position of the focus seen from the 15 side becomes the same.

According to at least one embodiment described above, the trade-off relationship between the focus size and the maximum output of the X-ray tube can be realized at a higher level.

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

REFERENCE SIGNS LIST 1 X-ray CT apparatus 10 gantry device 11 X-ray tube 113 cathode 116 anode 12 support mechanism 174 thermoelectric adjustment mechanism drive circuit 18 control device 40 console device 41 memory 42 display 43 input interface 44 processing circuit 441 scan control function

Claims (10)

A control unit for controlling the movement of the focus of the X-ray tube,
The control unit
calculating a first speed for continuously and periodically moving the focus from the viewpoint of not melting the anode , which is the rotating body of the X-ray tube;
calculating a second speed for continuously and periodically moving the focus from the viewpoint of satisfying the required resolution;
Determining a third speed for continuously and periodically moving the focus according to the purpose of photographing from the first speed and the second speed;
X-ray CT equipment. wherein the first velocity, the second velocity and the third velocity are determined by the amplitude and period of movement of the focal spot;
The X-ray CT apparatus according to claim 1. said amplitude is a width exceeding the size of the focal spot,
The X-ray CT apparatus according to claim 2. The control unit
If it is determined that the purpose of the photographing is to give priority to resolution, the third speed is brought close to the first speed,
If it is determined that the purpose of the shooting is to prioritize the maximum output, the third speed is brought closer to the second speed.
The X-ray CT apparatus according to any one of claims 1-3. The control unit
The purpose of imaging based on any one or combination of continuity of scan, area of scan, size of subject, operation history up to the last minute, requirement for high definition, requirement for low noise, requirement for low exposure determines whether resolution should be prioritized or whether the shooting objective should prioritize maximum output;
The X-ray CT apparatus according to claim 4. The control unit does not move the focus when a predetermined instruction is given or when it is determined that the purpose of shooting requires a resolution higher than a predetermined resolution.
The X-ray CT apparatus according to any one of claims 1-5. The control unit discontinuously changes the focus to a plurality of positions when a predetermined instruction is given,
The X-ray CT apparatus according to any one of claims 1-6. A support mechanism for moving the X-ray tube in a direction opposite to the direction in which the focus of the X-ray tube is moved,
The X-ray CT apparatus according to any one of claims 1-7. The controller performs reconstruction based on a representative focus position during one data acquisition.
The X-ray CT apparatus according to any one of claims 1-8. A control unit for controlling the movement of the focus of the X-ray tube,
The control unit
calculating a first speed for continuously and periodically moving the focus from the viewpoint of not melting the anode , which is the rotating body of the X-ray tube;
calculating a second speed for continuously and periodically moving the focus from the viewpoint of satisfying the required resolution;
Determining a third speed for continuously and periodically moving the focus according to the purpose of photographing from the first speed and the second speed;
Shooting planning device.

Images