JP6632912B2 - X-ray CT system - Google Patents

Description

Embodiments of the present invention relate to an X-ray CT (Computed Tomography) apparatus.

  In the X-ray CT apparatus, the rotation speed of a rotating frame holding an X-ray tube, an X-ray detector, and the like has been increased for the purpose of shortening an imaging time by high-speed scanning. When the rotation speed is increased, an unwanted displacement may occur. The undesired displacement is, for example, displacement caused by vibration, distortion, torsion, uneven rotation speed, or the like due to vibration. The undesired displacement causes, for example, degradation of the resolution of the output image and generation of artifacts.

  Conventional X-ray CT systems have been working to mechanically suppress unwanted displacements such as increasing the rigidity of the rotating mechanism and firmly fixing it to the installation location, for example, in order to prevent degradation in image resolution. There is something. Further, for example, a measuring device for measuring the displacement of a rotating system such as a rotating frame or an X-ray tube or an X-ray detector installed on the rotating frame is provided, and image reconstruction is performed based on the displacement amount acquired by the measuring device. Attempts are sometimes made to make corrections.

  However, in such a conventional X-ray CT apparatus, there is a problem that the mechanical suppression of undesired displacement involves an increase in the weight and size of the apparatus, and therefore, the manufacturing cost increases and the installation work becomes complicated. There is a risk. In addition, when measuring the displacement of the rotating system by the measuring instrument, it is necessary to secure an environment in which the measuring instrument is not affected by vibration of the rotating system and the like, and providing the measuring instrument may increase the cost. .

JP 2014-68717 A

  The problem to be solved by the present invention is to provide an X-ray CT apparatus that can easily prevent the image quality from deteriorating due to undesired displacement.

The X-ray CT apparatus according to the embodiment includes an X-ray detector, a correction unit, a reconstruction unit, an image quality evaluation unit, and a determination unit. The X-ray detector detects X-rays emitted from the X-ray tube and outputs projection data. The correction unit changes the geometry information indicating the position information of the device regarding the collection of the projection data. The reconstructing unit generates a plurality of reconstructed images corresponding to different pieces of geometry information based on the projection data and the geometry information changed by the correction unit. The image quality evaluation unit obtains an image quality evaluation value indicating the image quality of a plurality of reconstructed images corresponding to different pieces of geometry information. The deciding unit decides geometry information for improving the image quality based on the image quality evaluating unit and the image quality evaluation value. The geometry information is a distance from the focal position of the X-ray tube to the X-ray detector.

FIG. 1 is a block diagram of an X-ray CT apparatus 1 according to a first embodiment. FIG. 4 is a diagram illustrating a CT value distribution along a certain pixel column on a reconstructed image according to the first embodiment. FIG. 4 is a diagram illustrating a change in geometry information by a correction function along a radial direction of a rotation trajectory according to the first embodiment with respect to a focal position of an X-ray tube. 5 is a flowchart illustrating a flow from scanning a phantom to determining geometry information according to the first embodiment. FIG. 11 is a diagram illustrating a change in geometry information for each rotation angle by a correction function along a radial direction of a rotation trajectory according to the second embodiment with respect to a focal position of an X-ray tube. FIG. 9 is a diagram for describing a change in geometry information in a rotation direction by a correction function according to Modification Example 2 with respect to a focal position of an X-ray tube. FIG. 14 is a diagram illustrating an example of a relationship between a rotation angle and geometry information according to a third modification.

(First embodiment)
An X-ray CT apparatus according to the present embodiment will be described with reference to FIG. As shown in FIG. 1, the X-ray CT apparatus 1 includes an input interface circuit 10, a control circuit 20, a high-voltage generator 31, an X-ray tube 32, an X-ray detector 41, a data collection circuit 42, , A reconstruction circuit 50, a storage circuit 60, a gantry 70, a gantry driving unit 71, a bed 80, and a display 90. The control circuit 20 has, as functions, a gantry control function 201, an X-ray control function 202, a bed control function 203, an image quality evaluation function 204, a correction function 205, and a determination function 206. The couch 80 includes a couchtop 801 on which a subject is placed, and a couch driving unit 802 controlled by the couch control function 203 of the control circuit 20.

  The input interface circuit 10 includes a mouse, a trackball, a joystick, a push button, a touch panel, a touch pad, a keyboard, and the like, and receives an input by an operator. The input interface circuit 10 receives, for example, information required for examination reservation and patient registration, or information on imaging conditions.

  When the input interface circuit 10 is configured by a touch panel, the input interface circuit 10 may be configured to have the function of the display 90.

  The control circuit 20 is a processor. The term “processor” used in the following description is, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), an application specific integrated circuit (ASIC), a programmable logic device (for example, a simple programmable device). It means a circuit such as a logic device (Simple Programmable Logic Device: SPLD), a complex programmable logic device (Complex Programmable Logic Device: CPLD), or a field programmable gate array (FPGA). The processor is not limited to being configured as a single circuit for each processor, but may be configured as one processor by combining a plurality of independent circuits to realize its function.

  In the present embodiment, the gantry control function 201, the X-ray control function 202, the bed control function 203, the image quality evaluation function 204, the correction function 205, and the determination function 206 are forms of programs executable on a processor. And stored in the storage area or the storage circuit 60 incorporated in the circuit of the processor. The control circuit 20 in a state where the program corresponding to each function is read has each function shown in the control circuit 20 of FIG. In FIG. 1, one control circuit 20 as a processor realizes a gantry control function 201, an X-ray control function 202, a bed control function 203, an image quality evaluation function 204, a correction function 205, and a determination function 206. This is an example.

  The storage circuit 60 includes a circuit for reading and writing information from and to a magnetic disk (for example, a hard disk), a flash memory (for example, a solid state drive, a USB memory, a memory card), or an optical disk (for example, CD, DVD). In addition, the form in which the storage circuit 60 is connected to other components of the X-ray CT apparatus 1 is not limited to the form in which the storage circuit 60 is connected by an internal circuit, and may be externally connected. The externally connected storage circuit 60 is, for example, an external hard disk by USB connection, or NAS (Network Attached Storage) via a LAN (Local Area Network) cable. Note that the storage circuit 60 is an example of a storage unit in the claims.

  The X-ray control function 202 of the control circuit 20 controls the high voltage generator 31 based on, for example, imaging conditions such as a tube voltage, a tube current, and an imaging time. As the imaging conditions, for example, the X-ray control function 202 can read scan plan information stored in the storage circuit 60 in advance. Further, the imaging conditions may be acquired from a medical information management system (not shown) such as a hospital information system (Hospital Information System: HIS) or a radiology information system (Radiology Information System: RIS) outside the X-ray CT apparatus 1. . Further, the X-ray control function 202 may be configured to control the high-voltage generator 31 using the input information received by the input interface circuit 10 from the operator as an imaging condition.

  The high voltage generator 31 generates a high voltage based on the control by the X-ray control function 202 and supplies the high voltage to the X-ray tube 32.

  The X-ray tube 32 emits X-rays toward the X-ray detector 41. Further, near the X-ray tube 32, an X-ray stop 321 for adjusting the irradiation range of the X-rays emitted from the X-ray tube 32 is provided. The X-ray aperture 321 is, for example, a plurality of plate-like members made of a material that blocks X-rays.

  The rotating unit 33 is a support member that rotatably supports the X-ray tube 32 and the X-ray detector 41. For example, when the X-ray tube 32 and the X-ray detector 41 face each other and adopt a Rotate / Rotate method in which the X-ray tube 32 and the X-ray detector 41 both rotate on a circular orbit around the body axis of the subject, the rotating unit 33 includes the X-ray The tube 32 and the X-ray detector 41 are supported. FIG. 1 shows a Rotate / Rotate X-ray CT apparatus 1 as an example. In the following description, the X-ray CT apparatus 1 is described on the premise of the Rotate / Rotate method, but may be the Stationary / Rotate method. In the X-ray CT apparatus 1 of the Stationary / Rotate system, an X-ray detector 41 is arranged so as to surround the body axis of the subject, and the X-ray tube 32 rotates on a circular orbit around the subject. The rotating unit 33 of the X-ray CT apparatus 1 of the Stationary / Rotate system supports the X-ray tube 32.

  The gantry 70 includes a high-voltage generator 31, an X-ray tube 32, an X-ray detector 41, and a data acquisition circuit 42. The components included in the gantry 70 are not limited to the high voltage generator 31, the X-ray tube 32, the X-ray detector 41, and the data acquisition circuit 42, and may include other components. The gantry 70 may be configured to be tilted in order to perform imaging by tilting the rotation axis of the rotation unit 33 at a predetermined angle. Hereinafter, the tilt angle of the rotation axis of the rotation unit 33 with respect to the horizontal direction, which is set when the subject is imaged by tilting the gantry 70, is referred to as a tilt angle.

  The gantry control function 201 of the control circuit 20 controls the gantry drive unit 71. The gantry control function 201 controls the gantry driving unit 71 based on, for example, the rotation speed and the tilt angle. The rotation speed is set, for example, by specifying the time required for the rotation unit 33 to make one rotation. The information on the rotation speed and the tilt angle may be received by, for example, the input interface circuit 10 or may be acquired from an external medical information management system. Further, a predetermined value of the rotation speed may be determined in advance and stored in the storage circuit 60, and may be automatically read when the imaging condition is set.

  The gantry drive unit 71 drives the rotation unit 33 to rotate at a predetermined rotation speed based on the control by the gantry control function 201. When the horizontal direction is the X axis and the vertical direction is the Y axis, the rotating unit 33 drives the XY plane to rotate using the Z axis orthogonal to the X axis and the Y axis as the rotation axis. When changing the tilt angle of the gantry, the gantry driving unit 71 tilts the gantry 70 at a predetermined tilt angle based on the control by the gantry control function 201. At this time, the rotating unit 33 rotates around a rotation axis inclined by a tilt angle with respect to the Z axis.

  The X-ray detector 41 detects X-rays emitted from the X-ray tube 32 and transmitted through the subject, or X-rays incident without passing through the subject. The X-ray detector 41 has a plurality of X-ray detection elements for detecting X-rays. The X-ray detector 41 has a plurality of channels of X-ray detection elements in a direction along the XY plane. Here, a direction along the XY plane of the X-ray detector 41 is referred to as a channel direction. Further, the X-ray detector 41 may be configured to have multiple rows of X-ray detection elements in the slice direction. The X-ray detection element outputs an analog signal corresponding to the X-ray intensity to the data collection circuit 42.

  The data collection circuit 42 includes, for example, an integrating amplifier and an A / D converter. The data collection circuit 42 time-divides the analog signal output from the X-ray detector 41 by an integration amplifier at a predetermined data collection timing. The data collection timing can be set, for example, based on the above-described imaging conditions. Subsequently, the data collection circuit 42 converts the analog signal time-divided by the A / D converter into digital projection data. Hereinafter, the digitized projection data is simply referred to as projection data.

  The reconstruction circuit 50 generates a reconstructed image based on the projection data output by the data acquisition circuit 42 and geometry information that is positional information of a device relating to the acquisition of projection data. Note that the reconfiguration circuit 50 is an example of a reconfiguration unit in the claims. Devices related to the collection of projection data include, for example, the X-ray tube 32, the X-ray detector 41, the rotating unit 33, and the like. The reconstruction circuit 50 generates a reconstructed image by performing, for example, a convolution backprojection process based on the projection data. In the backprojection processing for image reconstruction, the reconstruction circuit 50 determines data of an X-ray detection element to be backprojected to each pixel of the reconstructed image based on information on the focal position of the X-ray tube 32. The geometry information is information for specifying the focal position of the X-ray tube 32.

  There are innumerable parameters that can be handled as geometry information for each rotation angle of the rotation unit 33, but a dominant element can be selected and used. The geometry information is, for example, the distance from the focal position of the X-ray tube 32 to the X-ray detection element of the X-ray detector 41. Further, the geometry information may be a distance from the focal position of the X-ray tube 32 to the rotation center of the rotating unit 33. Furthermore, the tilt information, the displacement in the vertical and horizontal directions on the slice plane, and the displacement in the Z-axis direction may be included in the geometry information. What kind of information is used as the geometry information depends on whether the X-ray CT apparatus 1 having a mount capable of setting a tilting angle or a driving method of a device related to collection of projection data (for example, a belt drive, a direct drive, or the like). It can be determined based on information such as whether the rotation of the device related to the collection of projection data is performed on the vertical plane or the horizontal plane.

  The reconstructed image generated by the reconstructing circuit 50 is stored in the storage circuit 60. Alternatively, the reconstructed image is transmitted via a network interface to an image storage communication system (PACS: Picture Archiving and Communication System) that acquires and stores medical image data of DICOM (Digital Imaging and Communication in Medicine) standard, not shown. And may be stored.

  The display 90 is configured by an arbitrary display device such as a liquid crystal display panel, a plasma display panel, and an organic electroluminescence panel. The display 90 displays, for example, a GUI (Graphical User Interface) that supports input by the input interface circuit 10. The display 90 displays, for example, a reconstructed image generated by the reconstructing circuit 50 or an image obtained by performing image processing on the reconstructed image.

  The couch 80 is a device that moves a subject to an imaging area of the X-ray CT apparatus 1. The couch 80 has a top plate 801 on which an object is placed on an upper surface. The couchtop 801 is moved in the longitudinal direction of the couchtop 801 by the bed driving unit 802. In addition, the couch 80 may be configured to be vertically moved by driving the couch driving unit 802.

  The couch control function 203 of the control circuit 20 controls the couch driving unit 802. The bed control function 203 determines, for example, the amount of movement that determines how much the table top 801 is moved up and down with respect to the current position, and the height of a reference point such as a floor to be changed. The bed driving unit 802 is controlled based on the information on the determined target position. The information on the movement amount and the target position may be received by, for example, the input interface circuit 10 or may be acquired from an external medical information management system. Further, a predetermined value of the movement amount and the target position may be determined in advance and stored in the storage circuit 60, and may be automatically read out when the imaging condition is set.

  The image quality evaluation function 204 of the control circuit 20 outputs, for example, an image quality evaluation value representing the image quality, quantified for evaluating the image quality, with respect to the reconstructed image obtained by scanning the evaluation phantom. The phantom has a structure such as a spherical bead. The beads are preferably composed of a substance having a high X-ray absorption or a low X-ray absorption so as to be identifiable on a reconstructed image than a substance such as water or polypropylene filled around the beads. . The substance having a high X-ray absorption amount is, for example, aluminum or plastic. The substance having a low X-ray absorption is, for example, air. In addition, the size and shape of the phantom, the position of attachment to the bed 80, and the position of the beads inside the phantom can be stored and read as known information in the storage circuit 60, for example. The image quality evaluation function 204 is an example of an image quality evaluation unit in the claims. Hereinafter, spherical beads will be described as an example, but it is not intended to limit the shape and size of the structure provided inside the phantom.

  The image quality evaluation function 204 uses, for example, the sharpness of the reconstructed image as the image quality evaluation value. The sharpness of the reconstructed image is a value indicating how clear the contour of the image shown in the reconstructed image is.

  FIG. 2 is a diagram for explaining the sharpness of the reconstructed image. The horizontal axis indicates a certain pixel column of the reconstructed image. The vertical axis is the CT value corresponding to each pixel. Since the CT value is determined for each pixel, the distribution of the CT value in the pixel column direction is originally discrete, but is shown in FIG. 2 as continuously changing.

2, the edge P _E point, CT value distribution D ₁ of CT value changes steeply is determined from the known bead position in phantom, a CT value distribution of the theoretical. On the other hand, as compared to CT value distribution D _1, CT value distribution D ₂ the CT value gradually changes in P _E near the point is the CT value distribution which corresponds to the reconstructed image actually obtained by scanning the phantom . And CT value distribution _{D 2} is compared with the CT value distribution _{D 1,} the gently CT value _{P E} point is changed, for example, rotating part X-ray tube 32 or the X-ray detector 41, or their support This is because undesired displacements of the device relating to the collection of projection data, such as 33, occur. Undesirable displacements include, for example, shake due to vibration, distortion, torsion, and uneven rotation speed.

To handle sharpness numerically, for example, the lower end of the CT value distribution _{D 2 B 1,} define the upper and _{B 2.} Further, the inclination angle of the tangential line G in _{P E} point CT value distribution _{D 2} defined as phi. The tangent line G is shown by a dashed line in FIG. Determining the intersection of the tangent line G and B = B _1, a 2 intersection of the intersection of the tangent line G and B = B _2, the width of the pixel column direction and the change region U. The change area U is an area where the CT value changes. Expressed as a mathematical expression, U = (B ₂ −B ₁ ) / tan φ. The change area U indicates how sharply the CT value distribution changes at the point where the CT value changes. The smaller the value of the change area U, the steeper the CT value distribution. That is, the smaller the change area U, the higher the sharpness. By using the value of the change area U as the sharpness, the sharpness can be treated numerically. Note that the sharpness is not limited to the value of the above-mentioned change area U, and an arbitrary index may be used.

  The image quality evaluation value output by the image quality evaluation function 204 according to the above-described procedure is stored in the storage circuit 60 and can be read out as appropriate.

  The correction function 205 of the control circuit 20 changes the geometry information used when the reconstruction circuit 50 generates a reconstructed image. Note that the correction function 205 is an example of a correction unit in the claims. In the present embodiment, the distance from the focal position of the X-ray tube 32 to the detection element of the X-ray detector 41 will be described as geometry information.

  The correction function 205 changes the geometry information indicating the position information of the device regarding the collection of the projection data. The reconstruction circuit 50 generates a plurality of reconstructed images corresponding to different pieces of geometry information changed by the correction function 205. The correction function 205 changes, for example, geometry information for generating a reconstructed image of the correction phantom. The correction function 205 changes geometry information for generating a reconstructed image of the correction phantom, and a determination function 206 described later determines geometry information that improves image quality. The determined geometry information is used when generating a reconstructed image of the subject to be inspected.

  In the present embodiment, the finally determined geometry information is obtained by uniformly changing the geometry information over the entire circumference of the rotation trajectory of the device related to the collection of projection data. In other words, with respect to projection data in all view directions obtained by scanning, the geometry information in each view direction is changed to the same extent. The angle range over which projection data is acquired on the rotation trajectory is, for example, from 0 degrees to 360 degrees in the case of a full scan in which the subject is irradiated with X-rays from all directions. In the case of a half scan, the angle is from 0 degrees to (180 degrees + fan angle). In the following, a full scan is assumed.

  FIG. 3 is a diagram illustrating a change in geometry information by the correction function 205 along the radial direction of the rotation trajectory with respect to the focal position of the X-ray tube 32. The focal position of the X-ray tube 32 is indicated by a circle. The center of rotation of the X-ray tube 32, X-ray detector 41, and rotating unit 33 is indicated by crosses. Note that the X-ray aperture 321 is illustrated in FIG. 3 as not changing its position, so that the X-ray irradiation range indicated by the broken line changes when the focal position of the X-ray tube 32 changes. are doing.

  The correction function 205 changes the distance from the focal position of the X-ray tube 32 to the detection element of the X-ray detector 41 along the radial direction of the rotation trajectory. The focal position of the X-ray tube 32 is, for example, a position P, which is the currently set focal position of the X-ray tube 32, at a position a1, which is the focal position of the X-ray tube 32 farthest from the rotation center O, and the rotation center. The position is changed between the closest focal position of the X-ray tube 32 and the position b1. In other words, in the geometry information, the distance from the position a1 to the detection element of the X-ray detector 41 is the maximum value, and the distance from the position b1 to the detection element of the X-ray detector 41 is the minimum value. In other words, the distance R between the position a1 and the position b1 shown in FIG. 3 is a predetermined range, and the geometry information is changed by the correction function 205 within the range of the distance R. The distance R, which is the range in which the geometry information is changed, is determined by, for example, how much the theoretical position of the projection data collection device such as the X-ray tube 32, the X-ray detector 41, and the rotator 33 during rotation. It can be set based on the maximum displacement amount indicating whether or not to displace. In other words, the geometry information is changed within a range equal to or less than the maximum displacement amount. For example, the maximum displacement amount of the focal position of the X-ray tube 32 can be measured in advance based on the result of simulation or actual measurement when the rotating unit 33 is distorted by centrifugal force during rotation. In the case of performing a simulation, for example, information on the tilt angle of the gantry 70 may be used.

  When the image quality evaluation value of the reconstructed image obtained by changing the geometry information by the correction function 205 is better than before changing the geometry information, the determination function 206 stores the changed geometry information in, for example, a storage circuit. 60. The reconfiguration circuit 50 repeats the change of the geometry information, and uses the geometry information finally determined by the determination function 206 to reconstruct projection data of the subject to be actually inspected. The change of the geometry information is repeated, for example, until the image quality evaluation value of the reconstructed image becomes maximum. Whether the image quality evaluation value of the reconstructed image is maximal is determined by outputting the image quality evaluation value by the image quality evaluation function 204 again to the reconstructed image generated using the geometry information changed by the correction function 205. It is determined by repeating the procedure. The update of the geometry information may be terminated when the image quality evaluation value of the reconstructed image becomes equal to or larger than a predetermined threshold. Further, for example, it may be determined whether to end the update of the geometry information by combining both the search for the maximum point of the image quality evaluation value and the threshold processing for the image quality index value. When changing the geometry information, how finely the value of the geometry information is changed is arbitrary. The determining function 206 is an example of a determining unit in the claims.

  In FIG. 3, the focal position of the X-ray tube 32 with respect to the rotation center O is changed by changing the geometry information. However, FIG. 3 is merely an example, and changing the geometry information may be regarded as changing the position of the X-ray detector 41. Further, it may be considered that both the focal position of the X-ray tube 32 and the position of the X-ray detector 41 are changed. Further, even when a peripheral device of the X-ray tube 32 such as a wedge (not shown) provided on the X-ray irradiation surface side of the X-ray tube 32 is displaced, by substantially changing the position of the X-ray tube 32, the position of the X-ray tube 32 is changed. It is possible to correct the deviation of the geometry information.

  Although the two-dimensional image reconstruction has been described above, it is also possible to correct the geometry information in the three-dimensional image reconstruction in consideration of the spread of the X-ray in the Z-axis direction. The three-dimensional image reconstruction is, for example, cone beam reconstruction. In the correction of the geometry information in the cone beam reconstruction, for example, among the multi-row X-ray detection elements in the slice direction, the above-described change of the geometry information is repeated with respect to the geometry information of a certain row. The geometry information finally determined in one column may be applied to another column. That is, if the positions in the channel direction are the same, the geometry information of another column that has already been finally determined may be diverted. In addition, for each column in the channel direction, geometry information that improves image quality may be determined.

  Note that the above-described determination of the geometry information that improves the image quality is not necessarily required every time the subject to be inspected is scanned. For example, after the installation work of the X-ray CT apparatus 1 is completed, once the correction position coordinates are acquired under a plurality of imaging conditions including the rotational speed, the use is continued even if the subject changes. Can be. However, when a relatively heavy structure such as the X-ray tube 32 or the X-ray detector 41 is replaced, it is necessary to try again to improve the image quality of the reconstructed image by changing the geometry information. Is preferred.

  FIG. 4 is a flowchart showing a flow from scanning of a phantom to determination of geometry information.

  In step S1, the operator scans the phantom placed on the top plate 801 of the bed 80. The projection data obtained by scanning the phantom is sent to the reconstruction circuit 50.

  In step S2, the reconstruction circuit 50 generates a reconstructed image based on the projection data. At this time, as the geometry information used when the reconstructing circuit 50 generates the reconstructed image, for example, an initial value stored in the storage circuit 60 in advance is used.

  In step S3, the image quality evaluation function 204 evaluates the image quality of the reconstructed image generated by the reconstruction circuit 50, and outputs an image quality evaluation value.

  In step S4, the determination function 206 determines whether the image quality of the reconstructed image is better than before changing the geometry information. If the image quality is sufficient, the process proceeds to step S6. When the geometry information having the maximum image quality evaluation value is determined as the final geometry, for example, when the image quality evaluation value obtained by changing the geometry information changes from an increasing tendency to a decrease, the image quality of the reconstructed image is changed. Is determined to be sufficient. Further, when the geometry information whose image quality evaluation value exceeds a predetermined threshold value is determined as the final geometry, the image quality becomes insufficient when the image quality evaluation value obtained by changing the geometry information exceeds the threshold value. Is determined. If the determination function 206 determines that the image quality is sufficient, the process proceeds to step S6, and if not, the process proceeds to step S5.

  In step S5, the correction function 205 changes the geometry information for generating a reconstructed image. After the change of the geometry information by the correction function 205, the process returns to step S2.

  In step S6, geometry information is determined. In step S4, when the case where the image quality evaluation value is maximum is considered to be sufficient image quality, the geometry information having the maximum image quality evaluation value is finally reconstructed for the subject to be inspected. Determined as geometry information used to generate an image. In step S4, when it is considered that the image quality is sufficient when the image quality evaluation value exceeds a predetermined threshold, the geometry information at the time when the image quality was last evaluated is finally determined as the object to be inspected. It is determined as geometry information used for generating a reconstructed image of the sample. The determined geometry information is stored in the storage circuit 60, and a series of flows ends.

  In steps S2 to S5 of the flow described above, the repetition processing is performed. Instead of performing the repetition processing, image quality evaluation values are obtained for a plurality of pieces of geometry information at a time by, for example, matrix calculation, and whether the image quality is sufficient is determined. May be evaluated.

  Basically, in order to determine the geometry information, projection data obtained during one scan, that is, one rotation of the X-ray tube 32 may be obtained. However, if it is determined that noise is included in the reconstructed image, a flow for determining the geometry information may be executed again. Further, the geometry information described in the flowchart may be determined based on projection data obtained by two or more scans.

  Also, in step S4, if the image quality of the reconstructed image is not sufficient, the flow proceeds to step S5 to change the geometry information. However, even if all pieces of geometry information that can be set within a predetermined range are tried, the If the image quality of the constituent image is not evaluated to be sufficient, the geometry information having the best image quality of the reconstructed image may be set as the final geometry information among the values of the tried geometry.

  In the X-ray CT apparatus 1 described above, the correction function 205 changes the geometry information of the projection data obtained by scanning the phantom for evaluation to obtain the geometry information that improves the image quality. However, projection data used by the correction function 205 is not limited to data obtained by scanning a phantom for evaluation. For example, projection data obtained by previously scanning a patient as a subject with the same X-ray CT apparatus 1 may be used. In this case, unlike the evaluation phantom, the position of an object such as a bead used as a reference in determining the image quality of the reconstructed image is not known in advance. Then, a reference area is specified on the reconstructed image. As a result, the same effect as when the evaluation phantom is used can be obtained.

  According to the X-ray CT apparatus 1 according to the first embodiment, the image quality evaluation function 204 evaluates the image quality of the reconstructed image based on the projection data obtained by scanning the phantom, and outputs the image quality evaluation value. . Further, the correction function 205 determines geometry information based on the image quality evaluation value output by the image quality evaluation function 204. The finally determined geometry information is used when the reconstruction circuit 50 generates a reconstructed image of the subject to be inspected.

  Thus, in the X-ray CT apparatus 1, even if an undesired displacement occurs in a device related to collection of projection data, such as the X-ray tube 32, the X-ray detector 41, and the rotating unit 33, the image quality of the reconstructed image due to the displacement is reduced. Reduction can be reduced. In addition, since it is not necessary to provide a measuring device or the like for detecting an undesired displacement and the image quality can be improved by the calculation processing, the image quality evaluation function 204 and the image quality evaluation function 204 according to the present embodiment can be applied to the conventional X-ray CT apparatus. The correction function 205 can be realized.

  The X-ray CT apparatus 1 changes the geometry information within a predetermined range when the geometry information is determined by the image quality evaluation function 204 and the correction function 205. The range in which the geometry information is changed is set, for example, based on the maximum structural displacement that can be moved during rotation of devices related to projection data collection, such as the X-ray tube 32, the X-ray detector 41, and the rotating unit 33. . This makes it possible to suppress an increase in the amount of calculation required to determine the geometry information, unlike the case where parameters are randomly changed to search for a point at which image quality is improved.

(Second embodiment)
In the X-ray CT apparatus 1 according to the second embodiment, the correction function 205 changes the geometry information for each position of the focal position of the X-ray tube 32 on the rotation trajectory. Hereinafter, the configuration different from the first embodiment will be mainly described, and the contents overlapping with the first embodiment will be omitted. Also, the reference numerals of the drawings will be described by assigning the same reference numerals to the parts common to the first embodiment.

  The correction function 205 changes the geometry information within a predetermined range. In the first embodiment, the geometry information is changed uniformly over the entire circumference of the rotation trajectory of the device related to the acquisition of the projection data. In the present embodiment, however, the focal position of the X-ray tube 32 on the rotation trajectory is changed. The geometry information is changed for each position, that is, for each rotation angle. The image quality evaluation function 204 evaluates whether or not the image quality evaluation value of the reconstructed image based on the changed geometry information is higher than before the change, as in the first embodiment.

  The range in which the correction function 205 changes the geometry information is, for example, the maximum value of the amount that can be displaced when the device related to the collection of projection data is distorted due to the effect of centrifugal force or gravity during rotation, as a result of simulation or actual measurement. It can be set by measuring in advance based on this. As a device relating to the collection of projection data, for example, when obtaining the maximum displacement amount of the rotating unit 33 by simulation, the weight of the X-ray tube 32 and the X-ray detector 41, the set rotation speed of the rotating unit 33, the tilt of the gantry 70 Arbitrary information such as an angle is used.

The range in which the correction function 205 changes the geometry information takes into account, for example, the radial force of the rotation trajectory of the device regarding the acquisition of projection data. For example, if a location where the distance from the focal position of the X-ray tube 32 to the X-ray detector 41 is longer than a theoretical value due to centrifugal force or the like in the rotational orbit is known in advance, the image quality is improved at that location. The range in which the geometry information is changed may be set so that the geometry information to be evaluated has a value larger than the theoretical value. Further, for example, if a location where the distance from the focal position of the X-ray tube 32 to the X-ray detector 41 is shorter than the theoretical value due to gravity or the like is known in advance, geometry information for evaluating whether the image quality is improved at that location. May be set to a value smaller than the theoretical value, so that the range in which the geometry information is changed may be set.
As described above, the range in which the geometry information is changed may be appropriately set based on empirical rules or physical laws.

  For example, the geometry information determined at a certain rotation angle may be referred to for determining the geometry information at a rotation angle near the rotation angle. Although there is a possibility that undesired displacement may occur in the equipment related to the acquisition of projection data, it is considered that the geometry information does not change extremely between geometries with neighboring rotation angles. The geometry information can be determined by referring to the angle geometry information.

  The range in which the correction function 205 changes the geometry information may be based on a result obtained by simulating the periodicity of the unevenness of the rotation speed of the device related to the collection of the projection data or an actually measured value, in addition to the example described above. The periodicity of the rotation speed unevenness is simulated or measured in advance, and a range in which the correction function 205 changes the geometry information is determined based on the result. Some set values of the rotation speed are stored in the storage circuit 60 in advance as imaging conditions. The period of the rotation speed unevenness, that is, an undesired displacement pattern can be stored in the storage circuit 60 in association with a rotation speed set value that can be set as an imaging condition. The periodicity of the rotation speed unevenness may be estimated using, for example, the tilt angle of the gantry 70.

  FIG. 5 is a diagram for explaining the change of the geometry information for each rotation angle by the correction function 205 along the radial direction of the rotation trajectory with respect to the focal position of the X-ray tube 32. The focal position of the X-ray tube 32 is indicated by a circle. The center of rotation of the X-ray tube 32, X-ray detector 41, and rotating unit 33 is indicated by crosses. Note that the X-ray stop 321 is illustrated in FIG. 5 as not changing its position, so that the X-ray irradiation range indicated by the broken line changes when the focal position of the X-ray tube 32 changes. are doing.

  In FIG. 5, the focal position set at the position P11 is changed to a position a11. On the other hand, the focal position set at the position P12 is changed to the position a12. The displacement of the focal position set at the position P11 is ΔR1, and the displacement of the focal position set at the position P12 is ΔR2. ΔR1 and ΔR2 illustrated in FIG. 5 are different. That is, the amount of change in the geometry information differs for each rotation angle. In other words, the distance from the focal position of the X-ray tube 32 to the detection element of the X-ray detector can be different for each rotation angle of the focal position of the X-ray tube 32 on the rotation trajectory.

  Although the flowchart shown in FIG. 4 showing the flow from scanning the phantom to determining the geometry information has also been described in the first embodiment, the processing in the following steps differs in the second embodiment.

  If it is determined in step S4 that the image quality of the reconstructed image is not sufficient, in step S5, the correction function 205 changes the geometry information for each rotation angle of the device related to the acquisition of the projection data on the rotation trajectory. When the geometry information is changed at the stage of entering step S5, the process returns to step S2.

  Note that among all the rotation angles on the rotation trajectory, the geometry information corresponding to a part of the rotation angles may be changed, and the process may return to step S2 to generate a reconstructed image.

  According to the X-ray CT apparatus 1 according to the second embodiment described above, the image quality evaluation function 204 and the correction function 205 determine the geometry information for generating a reconstructed image of the subject to be inspected. At this time, the geometry information is changed within a predetermined range for each rotation angle. The range in which the geometry information is changed is, for example, changed for each position on the rotation trajectory of the X-ray tube 32 and the X-ray detector 41, that is, for each rotation angle. Accordingly, it is possible to cope with a case where the geometry information for improving the image quality is different for each rotation angle due to the influence of centrifugal force, gravity, or the like.

  Further, the geometry information determined at a certain rotation angle may be referred to for determining the geometry information at a rotation angle near the rotation angle. Thus, it is possible to suppress an increase in the amount of calculation as compared with a case where the geometry information is independently determined for each rotation angle.

(Modification 1)
In the first and second embodiments described above, the correction of the geometry information in the channel direction on the XY plane of the device related to the collection of projection data, such as the X-ray tube 32 and the X-ray detector 41, has been described. The position of the couch top 801 in the Z-axis direction of the bed 80 may be considered when correcting the geometry information.

  Undesirable displacement in the Z-axis direction occurs, for example, when performing a helical scan that scans the subject while sending the subject to the opening of the gantry 70. The couchtop 801 of the couch 80 moves along the Z-axis direction with the subject placed thereon. For example, the feed speed at which the couch driving unit 802 supplies the couchtop 801 with a driving force to move the couchtop 801 is set. May differ from the theoretical value. For example, in the cone beam reconstruction, the correction function 205 changes the geometry information for each row of the X-ray detection elements of the X-ray detector 41 to determine the geometry information.

  With the configuration according to the first modification, it is possible to reduce the influence of image quality deterioration caused by unevenness in the feed speed of the top plate 801 of the bed 80 on the reconstructed image.

(Modification 2)
In the first embodiment and the second embodiment described above, the geometry information changed by the correction function 205 includes the distance from the focal position of the X-ray tube 32 to the X-ray detecting element 41 and the focal position of the X-ray tube 32. Although it has been attempted to improve the image quality of the reconstructed image by changing the distance information such as, for example, the X-ray CT apparatus 1 according to the present modified example has a rotation angle (angle) associated with the projection data of each view. The position information is corrected as geometry information.

  The reconstruction circuit 50 generates a reconstructed image using the projection data associated with the rotation angle. The correction function 205 attempts to improve the image quality evaluation value of the reconstructed image by changing the rotation angle associated with the projection data.

  The correction function 205 changes the rotation angle associated with the projection data within a predetermined range. As in the first embodiment, the image quality evaluation function 204 evaluates whether the image quality evaluation value of the reconstructed image based on the changed rotation angle is higher than before the change.

  FIG. 6 is a diagram for explaining a change in the rotation direction geometry information by the correction function 205 with respect to the focal position of the X-ray tube 32. The focal position of the X-ray tube 32 is indicated by a circle. The center of rotation of the X-ray tube 32, X-ray detector 41, and rotating unit 33 is indicated by crosses. Note that the X-ray aperture 321 is illustrated in FIG. 3 as not changing its position, so that the X-ray irradiation range indicated by the broken line changes when the focal position of the X-ray tube 32 changes. are doing.

  The correction function 205 changes the rotation angle of the focal position of the X-ray tube 32 with respect to the rotation center along the rotation direction of the rotation trajectory. The focal position of the X-ray tube 32 is, for example, a position a2 that is a focal position of the X-ray tube 32 obtained by rotating the currently set focal position of the X-ray tube 32 counterclockwise with respect to the rotation center O. And the position b2 which is the clockwise rotation of the X-ray tube 32 with respect to the rotation center O. In other words, the geometry information sets the angle θ formed by the position a2, the position b2, and the rotation center O to a predetermined range, and the geometry information is changed by the correction function 205 within the range of the angle θ.

  The range in which the correction function 205 changes the rotation angle as the geometry information may be, for example, the width of the range changed at each point on the rotation trajectory of the rotation unit 33.

  Since the rotation angle associated with the projection data can be changed by the configuration according to the second modification, the deterioration of the image quality of the reconstructed image due to the undesired displacement of the X-ray tube 32 or the X-ray detector 41 in the rotation direction can be prevented. Can be prevented.

(Modification 3)
In the second embodiment described above, the correction function 205 changes the geometry information for each position of the focal position of the X-ray tube 32 on the rotation trajectory. That is, the geometry information for each position on the rotation trajectory is individually changed for each rotation angle. On the other hand, in the third modification, the geometry information is changed for each frequency component, assuming that the actual geometry information has a periodic displacement pattern with respect to the theoretical geometry information. Hereinafter, the difference of the correction function 205 from the above-described second embodiment will be described.

  The correction function 205 changes the geometry information indicating the position information of the device regarding the collection of the projection data with the frequency component, and determines the final geometry information. FIG. 7 is a diagram illustrating an example of a relationship between a rotation angle and geometry information. In FIG. 7, a component e1 having a displacement pattern of one cycle during one rotation of a device related to collection of projection data, a component e2 having a displacement pattern of two cycles during one rotation, and four components during one rotation The three components e3 and e3 having the displacement pattern are illustrated as components that affect the geometry information. The bold line E is geometry information obtained as a result of combining the three components. That is, the bold line E represents a change according to the rotation angle of the actual geometry information that deviates from the theoretical geometry information. The geometry information Rt is theoretical geometry information, that is, a value of the geometry information when an undesired displacement does not occur. Note that the rotation angle may be replaced with a view.

Here, when the phase of the component e1 is represented by θ, and the phase shifts of the component e2 and the component e3 with respect to the component e1 are represented by γ ₂ and γ ₃ , the actual geometry information represented by the thick line E is expressed as follows: it can.
R = Rt + α ₁ sin θ + α ₂ sin (2θ + γ ₂ ) + α ₃ sin (4θ + γ ₃ )
Here, the alpha ₁ and alpha ₂ and alpha _3, is a coefficient corresponding to the component e1 and component e2 and component e3. These coefficients may be considered as the magnitude of the amplitude of the displacement pattern of each component.

Correction function 205, of the formula described above, the coefficient alpha ₁ and alpha ₂ and alpha _3, and the phase shift gamma _2, by varying at least one of the gamma _3, changing the geometry information. Then, similarly to the second embodiment, the determination function 206 determines geometry information that is obtained as a result of changing the correction function 205 and that improves the quality of the reconstructed image.

  In the above description, the correction function 205 focuses on three frequency components in order to express the displacement pattern of the geometry information. However, in practice, when a component that affects the geometry information is viewed as a frequency component, a displacement pattern of the actual geometry information with respect to the theoretical geometry information can be expressed only with the three frequency components described above. It may not be possible. In that case, the frequency information for expressing the displacement pattern may be increased to change the geometry information. Further, it is not always necessary to use a large number of frequency components for expressing the displacement pattern of the geometry information, and only a dominant frequency component may be used. Further, the number of frequency components for expressing the displacement pattern of the geometry information may be less than three. The number of frequency components viewed by the correction function 205 to change the geometry information may be set in each X-ray CT apparatus.

  With the configuration according to Modification 3 described above, for example, when an undesired displacement of a device related to collection of projection data occurs with periodicity, it is possible to efficiently obtain geometry information that improves the quality of a reconstructed image. it can. In addition, by changing the geometry information by looking at the unwanted displacement of the device related to the collection of projection data in the frequency component, it is more efficient with less computational complexity compared to changing the geometry information independently for each rotation angle. In addition, it is possible to obtain geometry information that improves the image quality.

  According to the X-ray CT apparatus 1 of at least one of the above-described embodiments or the modified examples, the image quality evaluation function 204 for evaluating the image quality of the reconstructed image and the correction function 205 for correcting the geometry information are provided. It is possible to prevent an undesired displacement of a device related to the collection of projection data, which is caused by blurring, distortion, torsion, uneven rotation speed, and the like, from affecting the reconstructed image, such as deterioration of image quality.

  Although several embodiments of the present invention have been described, these embodiments are provided by way of example and are not intended to limit the scope of the invention. These new embodiments can be implemented in other various forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and their equivalents.

1 X-ray CT apparatus 32 X-ray tube 33 Rotating section 41 X-ray detector 20 Control circuit 204 Image quality evaluation function 205 Correction function

Claims (11)

An X-ray detector that detects X-rays emitted from the X-ray tube and outputs a signal related to projection data;
A correction unit that changes geometry information representing position information of a device regarding the collection of the projection data,
Said projection data and said on the basis of said geometric data of varying the correction unit, and a reconstruction unit for generating a plurality of reconstructed images corresponding to the different geometry information,
An image quality evaluation unit that obtains an image quality evaluation value representing the image quality of the reconstructed image corresponding to the plurality of different pieces of geometry information,
Based on the image quality evaluation value, of the different geometry information, a determination unit that determines the geometry information that improves the image quality,
Equipped with a,
The geometry information is a distance from a focal position of the X-ray tube to the X-ray detector.
X-ray CT device. An X-ray detector that detects X-rays emitted from the X-ray tube and outputs a signal related to projection data;
A correction unit that changes geometry information representing position information of a device regarding the collection of the projection data,
A reconstruction unit that generates a plurality of reconstructed images corresponding to different geometry information based on the projection data and the geometry information changed by the correction unit,
An image quality evaluation unit that obtains an image quality evaluation value representing the image quality of the reconstructed image corresponding to the plurality of different pieces of geometry information,
Based on the image quality evaluation value, of the different geometry information, a determination unit that determines the geometry information that improves the image quality,
With
The geometry information is a rotation angle on a rotation orbit of the device,
X-ray CT device. An X-ray detector that detects X-rays emitted from the X-ray tube and outputs a signal related to projection data;
A correction unit that changes geometry information representing position information of a device regarding the collection of the projection data,
A reconstruction unit that generates a plurality of reconstructed images corresponding to different geometry information based on the projection data and the geometry information changed by the correction unit,
An image quality evaluation unit that obtains an image quality evaluation value representing the image quality of the reconstructed image corresponding to the plurality of different pieces of geometry information,
Based on the image quality evaluation value, of the different geometry information, a determination unit that determines the geometry information that improves the image quality,
With
The correction unit sets a range in which the geometry information is changed based on a maximum displacement amount of the device.
X-ray CT device. An X-ray detector that detects X-rays emitted from the X-ray tube and outputs a signal related to projection data;
A correction unit that changes geometry information representing position information of a device regarding the collection of the projection data,
A reconstruction unit that generates a plurality of reconstructed images corresponding to different geometry information based on the projection data and the geometry information changed by the correction unit,
An image quality evaluation unit that obtains an image quality evaluation value representing the image quality of the reconstructed image corresponding to the plurality of different pieces of geometry information,
Based on the image quality evaluation value, of the different geometry information, a determination unit that determines the geometry information that improves the image quality,
With
The correction unit sets a range in which the geometry information is changed based on a rotation speed of the device.
X-ray CT device. An X-ray detector that detects X-rays emitted from the X-ray tube and outputs a signal related to projection data;
A correction unit that changes geometry information representing position information of a device regarding the collection of the projection data,
A reconstruction unit that generates a plurality of reconstructed images corresponding to different geometry information based on the projection data and the geometry information changed by the correction unit,
An image quality evaluation unit that obtains an image quality evaluation value representing the image quality of the reconstructed image corresponding to the plurality of different pieces of geometry information,
Based on the image quality evaluation value, of the different geometry information, a determination unit that determines the geometry information that improves the image quality,
With
The correction unit changes a periodic displacement pattern of actual geometry information with respect to theoretical geometry information by at least one frequency in a rotation of the device, with a frequency component.
X-ray CT device. An X-ray detector that detects X-rays emitted from the X-ray tube and outputs a signal related to projection data;
A correction unit that changes geometry information representing position information of a device related to collection of the projection data,
A reconstruction unit that generates a plurality of reconstructed images corresponding to different geometry information based on the projection data and the geometry information changed by the correction unit,
An image quality evaluation unit that obtains an image quality evaluation value representing the image quality of the reconstructed image corresponding to the plurality of different pieces of geometry information,
Based on the image quality evaluation value, of the different geometry information, a determination unit that determines the geometry information that improves the image quality,
With
The determining unit determines geometry information for improving the image quality for each column of the X-ray detection elements of the X-ray detector.
X-ray CT device. The device related to the collection of the projection data is at least one of the X-ray tube, the X-ray detector, and a rotating unit that supports the X-ray tube or the X-ray detector,
The X-ray CT apparatus according to claim 1. The image quality evaluation unit is to improve the image quality that the image quality evaluation value is maximum,
The X-ray CT apparatus according to claim 1. The image quality evaluation unit is that the image quality is improved when it is determined that the image quality evaluation value satisfies a predetermined criterion by threshold processing.
X-ray CT apparatus according to any one of claims 1 to 8. The geometry information is obtained based on a reconstructed image generated using projection data obtained by scanning a phantom,
The reconstruction unit uses the geometry information when generating a reconstructed image of the subject to be examined.
The X-ray CT apparatus according to claim 1. The geometry information is obtained based on a reconstructed image generated using projection data obtained by scanning the first subject,
The reconstruction unit, when generating a reconstructed image of the second subject, using the geometry information obtained using the projection data,
The X-ray CT apparatus according to any one of claims 1 to 10.

Images