JP5535532B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus that performs dual energy imaging.

  Conventionally, image diagnosis by dual energy imaging using an X-ray CT apparatus is known.

  For example, the X-ray tube voltage is switched between a high X-ray tube voltage and a low X-ray tube voltage, and the subject is scanned by irradiating the subject with X-rays. Collect projection data with low x-ray tube voltage. Then, by performing weighted addition / subtraction processing and image reconstruction processing on these two types of projection data having different X-ray tube voltages, a clinically valuable image such as an image capable of substance discrimination is generated (for example, a patent) (Refer to Document 1, FIG. 4, FIG. 6, etc., and Patent Document 2).

JP 2009-095405 A US2009 / 0052612

  By the way, it is known that the size and position of the X-ray focal region formed on the target of the X-ray tube changes minutely according to the X-ray tube voltage.

  Therefore, in dual energy imaging, the size and position of the X-ray focal region changes by switching the X-ray tube voltage, and when projection data is collected with a high X-ray tube voltage and when projection data is collected with a low X-ray tube voltage. There is a shift in the positional relationship between the subject and the irradiated X-ray beam. Then, the accuracy of the target generated image is deteriorated, for example, adversely affects the weighting addition / subtraction processing between projection data having different X-ray tube voltages or reconstructed images, and an error occurs in the contour of the subject.

  In view of the above circumstances, the present invention provides an X-ray CT apparatus capable of compensating for an X-ray focal region shift caused by switching of an X-ray tube voltage in dual energy imaging and accurately obtaining an image by dual energy imaging. The purpose is to do.

  In Japanese Patent Application Laid-Open No. 2008-159317 and Japanese Patent Application Laid-Open No. 10-116579, an X-ray tube apparatus is proposed that corrects the position and dimensions of the X-ray focal point by controlling the deflection of the electron beam of the X-ray tube. However, the configuration is different from that of the present invention.

  In a first aspect, the present invention relates to an X-ray that includes an electron generation source and a target, and emits X-rays from an X-ray focal region formed by collision of electrons emitted from the electron generation source with the target. By switching the tube and the X-ray tube voltage to perform dual energy imaging of the subject, the first projection data based on the first X-ray tube voltage and the second X-ray different from the first X-ray tube voltage An X-ray CT apparatus comprising: data collection means for collecting second projection data based on a tube voltage; and image generation means for generating a predetermined image using the first and second projection data, Forming means for acting on the emitted electrons to form an electric field or magnetic field for determining the traveling direction of the electrons, and a size of the X-ray focal region when the X-ray tube voltage is the first X-ray tube voltage. And / or position is in a predetermined state A first control condition of the forming means for forming a field or a magnetic field, and a size and / or position of the X-ray focal region when the X-ray tube voltage is the second X-ray tube voltage; Storage means for storing a second control condition of the forming means for forming an electric field or a magnetic field that is substantially the same as a predetermined state; and when the X-ray tube voltage is the first X-ray tube voltage Control means for controlling the forming means in accordance with the first control condition, and for controlling the forming means in accordance with the second control condition when the X-ray tube voltage is the second X-ray tube voltage; Equipped with X-ray CT system.

  In a second aspect, the present invention relates to the size of the X-ray focal region when the X-ray tube voltage is the first X-ray tube voltage and when the X-ray tube voltage is the second X-ray tube voltage, and / or Alternatively, there is provided the X-ray CT apparatus according to the first aspect, further comprising specifying means for acquiring information for specifying a position and specifying the first and second control conditions based on the information.

  In a third aspect, the present invention provides the data acquisition means for a predetermined subject when the X-ray tube voltage is the first X-ray tube voltage and when the X-ray tube voltage is the second X-ray tube voltage. A second aspect of the X-ray CT apparatus is provided that collects projection data, and wherein the specifying unit acquires the information based on a profile of the collected projection data.

  Here, the “predetermined subject” is, for example, a spherical or cylindrical phantom, a collimator that shapes an X-ray beam, a pinhole made of an X-ray absorber, or the like. be able to.

  In a fourth aspect, the present invention relates to a plurality of electrodes arranged so that the forming unit sandwiches a predetermined space between the electron generation source and the target, and a voltage is applied to each of the plurality of electrodes. There is provided an X-ray CT apparatus according to any one of the first to third aspects, comprising voltage application means for applying and forming an electric field.

  In a fifth aspect, the present invention relates to any one of the first to fourth aspects, wherein the plurality of electrodes are focusing electrodes built in the X-ray tube for focusing the emitted electrons. An X-ray CT apparatus according to one aspect is provided.

  In a sixth aspect, the present invention relates to at least a π + X-ray beam fan while the data acquisition means switches the X-ray tube voltage between the first X-ray tube voltage and the second X-ray tube voltage alternately. An X-ray CT apparatus according to any one of the first to fifth aspects, which collects projection data for a predetermined view angle of a fan angle is provided.

  In a seventh aspect, according to the present invention, the data collection unit collects projection data corresponding to a predetermined view angle of at least a π + X-ray beam fan angle by setting an X-ray tube voltage to the first X-ray tube voltage. An X-ray CT apparatus according to any one of the first to fifth aspects, which collects projection data for the predetermined view angle by switching an X-ray tube voltage to the second X-ray tube voltage. To do.

  In an eighth aspect, the present invention provides the image generating means, which performs image reconstruction processing on the first projection data to reconstruct the first image, and image reconstruction processing on the second projection data. And reconstructing means for reconstructing the second image, weighting and subtracting the first image and the second image with a first weighting to obtain a third image, and the first image A weighted subtracting means for obtaining a fourth image by weighting and subtracting the image and the second image with a second weighting different from the first weighting; and the third image and the fourth image. There is provided an X-ray CT apparatus according to any one of the first to seventh aspects, comprising weighted addition means for performing weighted addition processing to generate the predetermined image.

  Here, the “third image” and the “fourth image” are images in which a component representing the first substance is suppressed and a component representing the second substance different from the first substance is emphasized, An image in which the component representing the second substance is suppressed and the component representing the first substance is emphasized can be considered. These images are also called substance enhancement images, substance density images, substance separation images, or substance discrimination images.

  In a ninth aspect, according to the present invention, the image generation unit obtains third projection data by performing a weighted subtraction process on the first projection data and the second projection data with a first weight, Weighted subtracting means for obtaining fourth projection data by weighting and subtracting the first projection data and the second projection data with a second weight different from the first weight; and the third projection Weight addition means for obtaining fifth projection data by weighted addition processing of data and the fourth projection data, and reconstruction for reconstructing the predetermined image by image reconstruction processing of the fifth projection data And an X-ray CT apparatus according to any one of the first to seventh aspects.

  The weighting addition process and the weighting subtraction process in the eighth and ninth aspects may be addition processing and subtraction processing by higher-order weighting.

  According to the present invention, there is provided a forming means for forming an electric field or a magnetic field that acts on electrons emitted from an electron generation source to determine the traveling direction thereof, and the X-ray focal region has an X-ray focal region under a first X-ray tube voltage. The first control condition of the forming means for forming the electric field or the magnetic field so that the size and position are in a predetermined state and the predetermined X-ray focal region size and position are the same under the second X-ray tube voltage. The second control condition of the forming means for forming the electric field or magnetic field to be in a state is stored, and when the X-ray tube voltage is the first X-ray tube voltage, the first control condition is set. Therefore, since the forming means is controlled and the forming means is controlled according to the second control condition when the X-ray tube voltage is the second X-ray tube voltage, the X-ray focal region is changed even if the X-ray tube voltage is switched. The size and position of the To compensate for deviation of the X-ray focal region caused by switching the X-ray tube voltage in the energy photographing, it is possible to realize an X-ray CT apparatus in which the image can be obtained accurately by dual energy imaging.

It is a figure which shows the structure of the X-ray CT apparatus which concerns on 1st embodiment. It is a figure which shows roughly the structure and positional relationship of an X-ray tube, a collimator, and an X-ray detector. It is a figure which shows the principal part of an X-ray tube. It is a figure which shows an example of a mode that an X-ray focus area | region changes according to an X-ray tube voltage. It is a flowchart (flowchart) which shows the flow of a process in the X-ray CT apparatus of 1st embodiment. It is a flowchart which shows an example of a X-ray focus control setting calibration process. It is a figure for demonstrating the projection data collection of a phantom. It is a figure for demonstrating the 1st method of dual energy imaging | photography. It is a figure for demonstrating the 2nd method of dual energy imaging | photography. It is a figure which shows an example of the change with respect to the view angle (view angle) of the X-ray tube voltage in the dual energy imaging | photography by the said 1st method, and the voltage applied to the 1st and 2nd focusing electrode. It is a figure which shows an example of the change with respect to the view angle of the X-ray tube voltage in the dual energy imaging | photography by the said 2nd method, and the voltage applied to the 1st and 2nd focusing electrode. It is a flowchart which shows an example of an image generation process. It is a figure which shows an image generation process notionally. It is a flowchart which shows another example of an image generation process. It is a figure which shows the X-ray focal area | region from which only a position changes according to an X-ray tube voltage. It is a figure which shows the X-ray focus area | region from which only a magnitude | size changes according to an X-ray tube voltage.

  Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited thereby.

(First embodiment)
FIG. 1 is a diagram schematically showing the configuration of the X-ray CT apparatus according to the first embodiment.

  The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.

  The operation console 1 is acquired by an input device 2 that receives input from an operator, a central processing unit 3 that performs control of each unit for performing dual energy imaging, data processing for generating an image, and the like, and a scanning gantry 20. A data collection buffer (buffer) 5 that collects the data, a monitor 6 that displays an image, and a storage device 7 that stores a program, data, and the like.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and put into and out of the opening B of the scanning gantry 20. The cradle 12 is moved up and down and horizontally moved by a motor built in the imaging table 10. Here, the body axis direction of the subject 40, that is, the linear movement direction of the cradle 12 is the z direction, the vertical direction is the y direction, and the horizontal direction perpendicular to the z direction and the y direction is the x direction.

  The scanning gantry 20 includes a rotating unit 15 and a main body 20a that rotatably supports the rotating unit 15. The rotating unit 15 collimates the X-ray tube 21, the X-ray controller 22 that controls the X-ray tube 21, and the X-ray beam 81 generated from the X-ray tube 21, and a predetermined fan A collimator 23 for forming an X-ray beam 81 having an angle and a cone angle; an X-ray detector 24 for detecting the X-ray beam 81 transmitted through the subject 40; and an output of the X-ray detector 24 is converted into projection data. A DAS (Data Acquisition System) 25 (also referred to as a data acquisition device) that collects the data, an X-ray controller 22, a collimator 23, and a rotating unit controller 26 that controls the DAS 25 are mounted. The main body 20 a includes a control controller 29 that communicates control signals and the like with the operation console 1 and the imaging table 10. The rotating part 15 and the main body part 20 a are electrically connected via a slip ring 30.

  FIG. 2 is a diagram schematically showing the configuration and positional relationship of the X-ray tube, collimator, and X-ray detector.

  The X-ray tube 21, the collimator 23, and the X-ray detector 24 are supported by a predetermined base portion of the rotating unit 15 and maintain a positional relationship as illustrated. That is, the X-ray tube 21 and the X-ray detector 24 are disposed to face each other with the opening B of the scanning gantry 20 interposed therebetween, and the collimator 23 is disposed between the X-ray tube 21 and the opening B. Has been.

  As shown in FIG. 2, the X-ray tube 21 has a structure in which a cathode sleeve (cathode sleeve) 21 s and a target electrode 21 t that is rotatably supported around an axis in the z direction are accommodated in a housing 21 h. have.

FIG. 3 is a view showing a main part of the X-ray tube. The cathode sleeve 21s has a cathode filament (cathode
filament) 21c, first focusing electrode 21a and second focusing electrode 21b. The cathode filament 21c and the target electrode 21t are arranged at a predetermined interval so as to face each other in the z direction. Further, the first and second focusing electrodes 21a and 21b are respectively arranged so as to sandwich a predetermined space between the cathode filament 21c and the target electrode 21t in the x direction.

  The cathode filament 21c, the first focusing electrode 21a, the second focusing electrode 21b, and the target electrode 21t are connected to the X-ray controller 22. The voltage Ec is applied to the cathode filament 21c and the voltage Et is applied to the target electrode 21t so that the X-ray tube voltage V is applied between the cathode filament 21c and the target electrode 21t. Voltages Ea and Eb are applied to the first and second focusing electrodes 21a and 21b, respectively. The thermoelectrons emitted from the cathode filament 21c are accelerated by the X-ray tube voltage V toward the target electrode 21t, and are formed by the first and second focusing electrodes 21a and 21b to which the voltages Ea and Eb are applied. 3 is focused into a beam extending in the z direction under the action of the electric field, resulting in an electron beam 70 as shown in FIG. The electron beam 70 collides with the rotating target electrode 21t to form an X-ray focal region f, and X-rays are emitted from the X-ray focal region f.

  The X-ray tube voltage V is variable. Here, when performing dual energy imaging, the X-ray tube voltage V is set to a relatively low first X-ray tube voltage V1 and a relatively low second X-ray. Switch to tube voltage V2. The first and second X-ray tube voltages V1 and V2 are, for example, 140 [kV] and 80 [kV], respectively.

  Even if there is no change in the voltages Ea and Eb applied to the first and second focusing electrodes 21a and 21b, the size and position of the X-ray focal region f may change slightly when the X-ray tube voltage V changes. Are known. Here, it is assumed that the size and position of the X-ray focal region f change in the x direction according to the X-ray tube voltage V.

  FIG. 4 is a diagram showing an example of how the X-ray focal region changes in accordance with the X-ray tube voltage, and FIG. 4A is a diagram in which the main part of the X-ray tube 21 is viewed in the y direction. 4 (b) is a view of the main part when viewed in the z direction. For example, as shown in FIG. 4, when the X-ray tube voltage V is the first X-ray tube voltage V1, the x-direction width d1 is formed on the target electrode 21t by the collision of the electron beam 71 formed at this time. An X-ray focal region f1 is formed. On the other hand, when the X-ray tube voltage V is the second X-ray tube voltage V2, due to the collision of the electron beam 72 formed at this time, the width in the x direction is d2 larger than d1 on the target electrode 21t. An X-ray focal region f2 whose position is shifted from the X-ray focal region f1 in the x direction by a distance d3 is formed.

  As shown in FIG. 2, the X-ray detector 24 is a multi-row X formed by arranging a plurality of detection element arrays in the z direction by arranging a plurality of X-ray detection elements 24a in the channel direction CH. It is a line detector. Here, the number of channels is 1000, the number of columns is 64, and channel numbers 1 to 1000 and column numbers 1 to 64 are given as shown in the figure.

  As shown in FIG. 2, the collimator 23 includes two cylindrical shielding bars 23a and 23b and two rectangular shielding plates 23c and 23d. The shielding rods 23a and 23b are arranged with a predetermined interval in the z direction so that the longitudinal direction thereof coincides with the x direction. Further, the shielding plates 23c and 23d are arranged at a predetermined interval in the x direction with the longitudinal direction thereof coinciding with the z direction. Here, the shielding rod on the negative side in the z direction (front side in FIG. 2) is 23a, the shielding rod on the positive side in the z direction (far side in FIG. 2) is 23b, and the negative side in the x direction (left side in FIG. 2). ) Is 23c, and the positive (right side in FIG. 2) shielding plate in the x direction is 23d. A slit S for allowing X-rays to pass through is formed by the shielding rods 23a and 23b and the shielding plates 23c and 23d having such a positional relationship.

  The shielding rods 23a and 23b are supported by a predetermined base so as to be rotatable with an eccentric shaft extending in the longitudinal direction of the shielding rod as a rotation axis, and the rotation shafts of the shielding rods 23a and 23b are not illustrated. The output shaft of the collimator control motor is connected. With this configuration, the collimator control motor is driven independently and the shielding rods 23a and 23b are rotated to control the position of the slit S and the width in the z direction while the shielding rods 23a and 23b remain parallel. Can do.

  The above-described configuration of the collimator 23 is an example, and any configuration may be used as long as the position and width of the slit S can be adjusted.

  With the above-described configuration, the X-ray beam 81 emitted from the X-ray focal region f of the X-ray tube 21 passes through the slit S formed by the collimator 23, so that a predetermined cone angle and a fan are set. A fan-shaped X-ray beam 81 having a corner is formed. The X-ray detector 81 detects the X-ray beam 81 transmitted through the subject 40 placed on the cradle 12 and transported to the opening B.

  The scanning gantry 20 is an example of data collection means in the present invention. The first and second focusing electrodes 21a and 21b of the X-ray tube 21 and the X-ray controller 22 are examples of forming means in the present invention. The central processing unit 3 is an example of an image generation unit, a control unit, a specifying unit, a reconstruction unit, a weighting subtraction unit, and a weighting addition unit in the present invention, and these units are executed by executing a predetermined program. Function as.

  Hereafter, the flow of processing in the X-ray CT apparatus of the first embodiment will be described.

  FIG. 5 is a flowchart showing the flow of processing in the X-ray CT apparatus of the first embodiment.

  In step S1, the X-ray focus control setting is calibrated. (Hereinafter, this processing is referred to as X-ray focus control setting calibration processing). Details of the X-ray focus control setting calibration process will be described later with reference to FIGS.

  In step S2, dual energy imaging by X-ray focus control is performed. Details of the dual energy imaging by the X-ray focus control will be described later with reference to FIGS.

  In step S3, an image generation process is performed to generate a predetermined image using the projection data obtained by the dual energy imaging in step S2. Details of this image generation processing will be described later with reference to FIGS.

  In step S4, the image generated in step S3 is displayed on the monitor.

  The details of the X-ray focus control setting calibration process (S1) will now be described. Here, when the X-ray tube voltage V is the first X-ray tube voltage V1, the setting voltage Ea1 to be applied to the first focusing electrode 21a and the setting voltage to be applied to the second focusing electrode 21b. Each of Eb1 is preset with a predetermined voltage value. Similarly, when the X-ray tube voltage is the second X-ray tube voltage V2, the setting voltage Ea2 to be applied to the first focusing electrode 21a and the setting voltage Eb2 to be applied to the second focusing electrode 21b are: Each of the predetermined voltage values is preset. These set voltages are stored in the memory 7 or the memory of the X-ray controller 22.

  FIG. 6 is a flowchart illustrating an example of the X-ray focus control setting calibration process. FIG. 7 is a diagram for explaining the projection data collection of the phantom.

  In step S11, as shown in FIG. 7A, the spherical phantom 41 is placed on the cradle 12 so as to be placed in the photographing space in the opening B. At this time, the center of the phantom 41 is positioned on the iso-center IC. When the direction in which the size or position of the X-ray focal region f changes by switching the X-ray tube voltage V is to be detected in one direction, the cylindrical phantom is attached to the column axis. May be placed perpendicular to the direction to be detected.

  In step S12, the X-ray tube voltage V is changed to the first X-ray tube voltage V1 in a state where the rotating unit 15 on which the X-ray tube 21 is mounted is stopped, and the first and second focusing electrodes 21a and 21b are A set voltage Ea1 and a set voltage Eb1 are applied. In this state, as shown in FIG. 7A, the phantom 41 is irradiated with the X-ray beam 81 from the X-ray focal region f1, the transmitted X-ray is detected by the X-ray detector 24, and projection data for one view. Are collected for one slice (one detector row).

  In step S13, as shown in FIG. 7B, the edge channel numbers a1 and b1 corresponding to the edges are obtained in the profile p1 of the signal intensity D with respect to the channel direction CH of the projection data collected in step S12. . Here, the edge channel number a1 is a channel number corresponding to the edge closer to the channel number 1, and the edge channel number b1 is a channel number corresponding to the edge closer to the channel number 1000. In this case, as shown in FIG. 7A, the edge channel number a1 is the channel number 1000 in the x direction of the X-ray focal region f1 when the X-ray tube voltage V is the first X-ray tube voltage V1. It corresponds to the intersection of the X-ray detector 24 and a tangent line that passes through one end of the side and circumscribes on the channel number 1 side of the phantom 41. The edge channel number b1 passes through the other end of the X-ray focal region f1 near the channel number 1 in the x direction, and is at the intersection of the tangent line circumscribed on the channel number 1000 side of the phantom 41 and the X-ray detector 24. Correspond. Therefore, the edge channel numbers a1 and b1 reflect the positions of both ends in the x direction of the X-ray focal region f1.

  In step S14, the X-ray tube voltage V is changed to the second X-ray tube voltage V2 in a state in which the rotating unit 15 on which the X-ray tube 21 is mounted is stopped, and the first and second focusing electrodes 21a and 21b are A set voltage Ea2 and a set voltage Eb2 are applied. In this state, as shown in FIG. 7A, the phantom 41 is irradiated with the X-ray beam 81 from the X-ray focal region f2, the transmitted X-ray is detected by the X-ray detector 24, and projection data for one view. For example, one slice is collected.

  In step S15, as shown in FIG. 7B, edge channel numbers a2 and b2 corresponding to the edges are obtained in the profile p2 of the signal intensity D with respect to the channel direction CH of the projection data collected in step S14. Here, the edge channel number a2 is a channel number corresponding to the edge closer to the channel number 1, and the edge channel number b2 is a channel corresponding to the edge closer to the channel number 1000. In this case, as shown in FIG. 7A, the edge channel number a2 is the channel number 1000 in the x direction of the X-ray focal region f2 when the X-ray tube voltage V is the second X-ray tube voltage V2. It corresponds to the intersection of the X-ray detector 24 and a tangent line that passes through one end of the side and circumscribes on the channel number 1 side of the phantom 41. The edge channel number b2 passes through the other end of the X-ray focal region f2 near the channel number 1 in the x direction, and intersects the tangent line circumscribed on the channel number 1000 side of the phantom 41 and the X-ray detector 24. Correspond. Therefore, the edge channel numbers a2 and b2 reflect the positions of both ends in the x direction of the X-ray focal region f2.

  In step S16, it is determined whether the difference between the edge channel numbers a1 and a2 is smaller than a predetermined reference value ε1, and whether the difference between the edge channel numbers b1 and b2 is smaller than a predetermined reference value ε2. When it is determined that the condition is satisfied, the size and position deviation between the X-ray focal region f1 and the X-ray focal region f2 is considered to be within an allowable range, and the X-ray focal point control setting calibration process is performed. finish. On the other hand, if it is determined that the condition is not satisfied, the process proceeds to step S17, and the set voltages Ea2 and Eb2 are set so that the difference between the edge channel numbers a1 and a2 or the difference between the edge channel numbers b1 and b2 becomes small. The set voltages Ea2 and Eb2 are updated by adding / subtracting the minute voltage ΔE to / from at least one of them. Then, the process returns to step S14.

The set voltages Ea1 and Eb1 at the time when the X-ray focus control setting calibration process is completed are an example of the first control condition of the present invention, and the set voltages Ea2 and Eb2 at the same point are the second control condition of the present invention. It is an example.

  Instead of placing the phantom 41, the width between the shielding plates 23c and 23d of the collimator 23 may be narrowed to detect channel numbers corresponding to the edges of these shielding plates. Alternatively, instead of placing the phantom 41, a plate-shaped shielding plate whose cross section in the xy plane has a longitudinal direction in the x direction is placed, and a channel number corresponding to the edge of the shielding plate is detected. You may make it do. Further, in order to make the displacement of the size and position between the X-ray focal region f1 and the X-ray focal region f2 within an allowable range, an X-ray tube is used instead of the set voltages Ea2 and Eb2 at the X-ray tube voltage V2. The set voltages Ea1 and Eb1 at the voltage V1 may be adjusted, or both the set voltages of the X-ray tube voltages V1 and V2 may be adjusted.

  Next, details of dual energy imaging (S2) by X-ray focus control will be described.

  FIG. 8 is a diagram for explaining a first method of dual energy imaging, and FIG. 9 is a diagram for explaining a second method of dual energy imaging.

  In the dual energy imaging, for example, as shown in FIG. 8, the X-ray tube 21 is rotated around the subject 40, and the X-ray tube voltage V is relatively higher than the first X-ray tube voltage V <b> 1. The X-ray is irradiated onto the subject 40 while switching to the low second X-ray tube voltage V2 at a high speed in units of one or several views, and the transmitted X-ray is detected and projection data for a predetermined view angle is detected. By the method of collecting

  For example, as shown in FIG. 9, the X-ray tube 21 is rotated around the subject 40, and the X-ray tube voltage V is set to one X-ray tube voltage (for example, the first X-ray tube voltage V1). The object 40 is irradiated with a line, the transmitted X-ray is detected, projection data for a predetermined view angle is collected, and then the X-ray tube voltage V is set to the other X-ray tube voltage (for example, the second X-ray tube voltage). V2) is used to irradiate the subject 40 with X-rays, detect the transmitted X-rays, and collect projection data for a predetermined view angle.

  The predetermined view angle is, for example, π + X-ray beam corresponding to half scan (X-ray beam) fan angle α (rad) to 2π [full scan]. rad].

  In this example, when performing such dual energy imaging, the voltages Ea and Eb applied to the first and second focusing electrodes 21a and 21b are switched according to the X-ray tube voltage V, respectively. That is, when the X-ray tube voltage V is the first X-ray tube voltage V1, the X-ray controller 22 applies the setting voltage Ea1 to the first focusing electrode 21a and sets it to the second focusing electrode 21b. A voltage Eb1 is applied. The X-ray controller 22 applies the set voltage Ea2 to the first focusing electrode 21a and the voltage to the second focusing electrode 21b when the X-ray tube voltage V is the second X-ray tube voltage V2. Eb2 is applied.

  FIG. 10 is a diagram showing an example of changes in the X-ray tube voltage and the voltage applied to the first and second focusing electrodes with respect to the view angle (projection direction) in dual energy imaging by the first method. FIG. 11 is a diagram showing an example of changes in the X-ray tube voltage and the voltage applied to the first and second focusing electrodes with respect to the view angle in dual energy imaging by the second method.

  For example, the projection data corresponding to the view angle 2π is collected while the X-ray tube voltage V is switched between the first X-ray tube voltage V1 and the second X-ray tube voltage V2 every one or several views according to the view angle θ. When performing dual energy imaging, the voltages Ea and Eb applied to the first and second focusing electrodes 21a and 21b are changed as shown in FIG. Also, for example, after collecting projection data for a view angle of 2π while changing the view angle θ by changing the X-ray tube voltage V to the first X-ray tube voltage V1, the X-ray tube voltage V is changed to the second X-ray tube. In the case of performing dual energy imaging in which projection data for the view angle 2π is collected while changing the view angle θ to the voltage V2, the voltages Ea and Eb applied to the first and second focusing electrodes 21a and 21b are illustrated. 11 is changed.

  Thus, the first projection data PHV by the first X-ray tube voltage V1 and the second X-ray tube are suppressed while suppressing the displacement of the size and position of the X-ray focal region f caused by the switching of the X-ray tube voltage V. The second projection data PLV by the voltage V2 is collected via the slip ring 30.

  However, in the case of dual energy imaging by the first method, the first and second projection data PHV and PLV cannot collect the projection data of the view by the other X-ray tube voltage switched to each other. Therefore, projection data that could not be collected is obtained by performing weighted addition processing on the projection data of a view close to the view or filtering in the view direction.

  Next, details of the image generation process (S3) will be described.

  FIG. 12 is a flowchart illustrating an example of image generation processing. FIG. 13 is a diagram conceptually illustrating the image generation process. In this image generation process, an image is obtained using X-rays having a desired single X-ray energy based on projection data obtained by dual energy imaging, that is, two types of projection data having different X-ray tube voltages. An image representing the subject, a so-called monochromatic image is generated.

  In step Z1, as shown in FIG. 12, the first projection data PHV is subjected to image reconstruction processing to reconstruct the first X-ray tube voltage image GHV (first image), and the second The second X-ray tube voltage image GLV (second image) is reconstructed by performing image reconstruction processing on the projection data PLV.

  Specifically, first, predetermined pre-processing including logarithmic conversion, correction of radiation quality, correction of sensitivity of the X-ray detector, and the like is performed on the first and second projection data PHV and PLV. Next, a predetermined reconstruction function is superimposed on the preprocessed first and second projection data PHV and PLV. Thereafter, the first and second projection data PHV and PLV on which the reconstruction function is superimposed are respectively backprojected to obtain first and second X-ray tube voltage images GHV and GLV.

  In step Z2, as shown in FIG. 12, the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV are weighted and subtracted by the first weighting to obtain the density distribution of the first substance. A first material density image (third image) is obtained. Further, a second substance density image (first density) representing the density distribution of the second substance by weighting and subtracting the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV by the second weighting. 4 images). In this example, the first substance is water, the second substance is iodine, the first substance density image W representing the density distribution of water, and the second substance density image Io representing the density distribution of iodine (iodine). And get.

The first and second material density images W and Io can be obtained by performing weighted subtraction processing according to the following mathematical formula, for example.

  Here, kw is a conversion coefficient for expressing the pixel value W (x, y) of the first substance density image by the density [mg / ml] of water, and kio is the pixel value Io (x of the second substance density image. , y) is a conversion coefficient for representing iodine density [mg / ml], Rio is the dual energy ratio of iodine in the first X-ray tube voltage V1 and the second X-ray tube voltage V2, and Rw is the first The dual energy ratio of water in the X-ray tube voltage V1 and the second X-ray tube voltage V2.

  The dual energy ratio Rio is a pixel value (CT value) GCio, LV (x corresponding to iodine on the image GCLV (x, y) obtained when the X-ray is taken by the second X-ray tube voltage V2. , y) is a value obtained by dividing the pixel value GCio, HV corresponding to iodine on the image GCHV (x, y) obtained when the X-ray is taken with the first X-ray tube voltage V1 ( Pixel value ratio). Further, the dual energy ratio Rw is obtained by converting the pixel value GCw, LV (x, y) corresponding to water on the image GCLV (x, y) to the pixel value GCw, LV corresponding to water on the image GCHV (x, y). A value obtained by dividing by HV (x, y).

  In step Z3, as shown in FIG. 12, the first substance density image W and the second substance density image Io are weighted and added, and imaging is performed using X-rays having a predetermined single X-ray energy. An image GNV representing the processed subject is obtained.

  The image GNV can be obtained, for example, by performing a weighted addition process according to the following formula.

  Here, keV1 is the effective X-ray energy of the X-ray by the X-ray tube voltage NV, μw (keV1) is the X-ray absorption coefficient of water at the effective X-ray energy keV1, and μio (keV1) is the iodine of the effective X-ray energy keV1. An X-ray absorption coefficient, kc, is a conversion coefficient for converting the pixel value of the image by the X-ray tube voltage NV into a CT value. As is well known, the CT value is a value indicating the degree of X-ray absorption of a substance, the CT value of air is represented by -1000 [HU], and the CT value of water is represented by 0 [HU].

  The above is an example of the image generation process, but the image generation process is not limited to the above process, and for example, the following process may be used.

  FIG. 14 is a flowchart illustrating another example of the image generation process.

  In step Z11, the first projection data PHV and the second projection data PLV are weighted and subtracted by the first weighting, and the first material density image representing the density distribution of the first material is obtained. 3 projection data PW is obtained. Further, a second material density image representing a density distribution of a second substance different from the first substance by performing a weighted subtraction process on the first projection data PHV and the second projection data PLV with a second weighting. To obtain fourth projection data PIO.

  In step Z12, the third projection data PW and the fourth projection data PIO are weighted and added, and an image GNV representing the subject imaged using X-rays having a predetermined single X-ray energy is displayed. The fifth projection data PNV that is the basis is obtained.

  In step Z13, the fifth projection data PNV is subjected to image reconstruction processing to reconstruct an image GNV representing the subject imaged using X-rays having a predetermined single X-ray energy.

  In this example, as the first and second substance density images, an image representing the water density distribution and an image representing the iodine density distribution are used, but the present invention is not limited to this combination. For example, it may be a combination of an image representing the density distribution of water and an image representing the density distribution of calcium, and a combination of an image representing the density distribution of iodine and an image representing the density distribution of calcium.

  For details of the above-described processing for generating a monochrome image, refer to Patent Document US2009 / 0052612.

  Note that an image different from the present example may be generated in the image generation processing in step S3. For example, the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV are weighted and subtracted so as to suppress or enhance the image portion of the bone or soft portion, so-called energy subtraction image (energy subtraction image). image) may be generated. Further, for example, a ratio image may be generated in which the pixel value ratio between the first X-ray tube voltage image GHV and the second X-ray tube voltage image GLV is a new pixel value.

  As described above, according to the first embodiment, when the X-ray tube voltage V is the first X-ray tube voltage V1, the electric field for forming the electric field where the size and position of the X-ray focal region f are in a predetermined state is formed. When the setting voltages Ea1 and Eb1 applied to the first and second focusing electrodes 21a and 21b, respectively, and the X-ray tube voltage V is the second X-ray tube voltage V2, the size and position of the X-ray focal region f are The setting voltages Ea2 and Eb2 to be applied to the first and second focusing electrodes 21a and 21b, respectively, for forming the electric field to be in the predetermined state are stored. In dual energy imaging, when the X-ray tube voltage V is the first X-ray tube voltage V1, the setting voltages Ea1 and Eb1 are applied to the first and second focusing electrodes 21a and 21b, respectively, and the X-ray tube When the voltage V is the second X-ray tube voltage V2, set voltages Ea2 and Eb2 are applied to the first and second focusing electrodes 21a and 21b, respectively. Thereby, the size and position shift of the X-ray focal region f caused by the change of the X-ray tube voltage V can be compensated, and an image obtained by dual energy imaging can be obtained with high accuracy, that is, with few artifacts.

  In addition, according to the first embodiment, since feedback control is not used, the voltages Ea and Eb applied to the first and second focusing electrodes 21a and 21b can be switched at high speed. . Therefore, it is particularly effective for dual energy imaging that collects projection data for a predetermined view angle while alternately switching the X-ray tube voltage V to the first X-ray tube voltage V1 and the second X-ray tube voltage V2. Is big.

  In addition, according to the first embodiment, since the focusing electrode normally provided in the X-ray tube is used as the plurality of electrodes for forming the electric field that determines the traveling direction of the electron beam of the X-ray tube 21, There is no need to provide an electrode or a means for applying a voltage to the electrode, and the cost, time, material cost, etc. required for the design can be suppressed.

  In this embodiment, for example, as shown in FIG. 15, the x-direction width d1 of the X-ray focal region f1 is the same as the x-direction width d2 of the X-ray focal region f2, and the position is in the x direction. A case where only the position of the X-ray focal region f changes according to the X-ray tube voltage V, such as a shift of the distance d3, can be dealt with. Further, for example, as shown in FIG. 16, the x-ray width f1 of the X-ray focal region f1 is different from the x-direction width d2 of the X-ray focal region f2, and the center position thereof does not change. It is also possible to cope with the case where only the size of the X-ray focal region f changes according to the above.

(Second embodiment)
A pair of focusing electrodes similar to those of the first and second focusing electrodes 21a and 21b are provided in the y direction, and the state of the electric field formed by these focusing electrodes is switched according to the X-ray tube voltage V. Deviations in the size and position of the X-ray focal region f caused by the switching of the tube voltage V may be compensated in the x direction and the y direction.

(Third embodiment)
By providing means for forming a magnetic field that acts on the electron beam emitted from the cathode filament 21c to determine the traveling direction of the electron beam and switching the state of the magnetic field to be formed according to the X-ray tube voltage V, You may make it compensate the shift | offset | difference of the magnitude | size and position of the X-ray focus area | region f which arises by switching of tube voltage V. FIG.

(Fourth embodiment)
The relationship between the displacement amount of the voltages Ea and Eb applied to the first and second focusing electrodes 21a and 21b and the displacement amount of the size and position of the X-ray focal region f is obtained and stored in advance.

  When the X-ray tube voltage V is the first X-ray tube voltage V1 and when the X-ray tube voltage V2 is the second X-ray tube voltage V2, projection data of a predetermined subject such as the phantom 41 or the collimator 23 is at least for one view. collect. Next, based on the collected projection data profile, the geometric positional relationship between the X-ray tube 21 and the X-ray detector 24, the size, shape, position, etc. of the predetermined subject, the X-ray focal region The size and position shift amount between f1 and the X-ray focal region f2 are specified. In order to cancel the specified shift amount, the first and second focusing electrodes 21a are used when the X-ray tube voltage V is the first X-ray tube voltage V1 and when the X-ray tube voltage V2 is the second X-ray tube voltage V2. , 21b, that is, the set voltages Ea1, Eb1 and the set voltages Ea2, Eb2 may be specified using the above relationship.

  In each of the above embodiments, an axial scan is assumed as a scanning method in dual energy imaging, but this may be a helical scan.

1 Operation Console 2 Input Device 3 Central Processing Unit 5 Data Collection Buffer 6 Monitor 7 Storage Device 10 Imaging Table 12 Cradle 15 Rotating Unit 20 Scanning Gantry 20a Main Body 21 X-ray Tube 21a First Focusing Electrode 21b Second Focusing Electrode 21c Cathode filament 21h Housing 21s Cathode sleeve 21t Target electrode 22 X-ray controller 23 Collimator 23a, 23b Shielding rod 23c, 23d Shielding plate 24 X-ray detector 24a X-ray detector 25 DAS
26 Rotating part controller 29 Control controller 30 Slip ring 40 Subject 41 Phantom 70, 71, 72 Electron beam 81 X-ray beam 100 X-ray CT apparatus B Opening f X-ray focal area f1 First X-ray focal area f2 Second X-ray focus area IC Isocenter S Slit

Claims (7)

An X-ray tube that includes an electron source and a target and emits X-rays from an X-ray focal region formed by electrons emitted from the electron source colliding with the target, and switching an X-ray tube voltage By performing dual energy imaging of the subject, the first projection data based on the first X-ray tube voltage and the second projection data based on the second X-ray tube voltage different from the first X-ray tube voltage are obtained. An X-ray CT apparatus comprising: data collection means for collecting; and image generation means for generating a predetermined image using the first and second projection data,

A plurality of forming means that act on the emitted electrons to form an electric field or a magnetic field that determines a traveling direction of the electrons, and are arranged so as to sandwich a predetermined space between the electron generation source and the target. Forming means, and a voltage applying means for applying a voltage to each of the plurality of focusing electrodes to form an electric field ;

When the X-ray tube voltage is the first X-ray tube voltage, the X-ray focal region is applied to each of the plurality of focusing electrodes for forming an electric field or a magnetic field in which the size and position of the X-ray focal region is in a predetermined state. A first control condition comprising a voltage, and an electric field or magnetic field in which the size and position of the X-ray focal region is substantially the same as the predetermined state when the X-ray tube voltage is the second X-ray tube voltage Storage means for storing a second control condition comprising a voltage applied to each of the plurality of focusing electrodes for forming

The forming means is controlled according to the first control condition when the X-ray tube voltage is the first X-ray tube voltage, and when the X-ray tube voltage is the second X-ray tube voltage, An X-ray CT apparatus comprising: control means for controlling the forming means according to a second control condition.
Information for specifying the size and position of the X-ray focal region when the X-ray tube voltage is the first X-ray tube voltage and when the X-ray tube voltage is the second X-ray tube voltage; The X-ray CT apparatus according to claim 1, further comprising: a specifying unit that specifies the first and second control conditions based on the first and second control conditions.
The data collection means collects projection data of a predetermined subject when the X-ray tube voltage is the first X-ray tube voltage and when the X-ray tube voltage is the second X-ray tube voltage,

The X-ray CT apparatus according to claim 2, wherein the specifying unit acquires the information based on a profile of the collected projection data. The data acquisition means switches projection data corresponding to a predetermined view angle of at least a π + X-ray beam fan angle while alternately switching an X-ray tube voltage between the first X-ray tube voltage and the second X-ray tube voltage. X-ray CT apparatus according to any one of claims 1 to 3 to be collected.
The data collection means collects projection data corresponding to a predetermined view angle of at least a π + X-ray beam fan angle by setting the X-ray tube voltage to the first X-ray tube voltage, and then converts the X-ray tube voltage to the second X-ray tube voltage. The X-ray CT apparatus according to any one of claims 1 to 4 , wherein projection data corresponding to the predetermined view angle is collected by switching to a tube voltage.
The image generating means includes
Reconstructing means for reconstructing the first image by reconstructing the first projection data to reconstruct the first image, and reconstructing the second image by reconstructing the second projection data;

The first image and the second image are weighted and subtracted by a first weight to obtain a third image, and the first image and the second image are converted to the first weight. Are weighted subtracting means for obtaining a fourth image by weighted subtraction with different second weightings;

The X-ray according to any one of claims 1 to 5 , further comprising weighted addition means for generating the predetermined image by performing weighted addition processing on the third image and the fourth image. CT device.
The image generating means includes
The first projection data and the second projection data are weighted and subtracted by a first weighting to obtain third projection data, and the first projection data and the second projection data are Weighted subtracting means for obtaining fourth projection data by performing weighted subtraction with a second weight different from the first weight;

Weighting and adding means for weighting and adding the third projection data and the fourth projection data to obtain fifth projection data;
X-ray CT apparatus according to any one of claims 1 to 5 having a reconstructing means the fifth projection data image reconstruction processing to the reconstructing the predetermined image.

Images