JP2009279301A - Cone beam x-ray ct system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tomographic image the picture quality level of which is uniform while suppressing the generation of artifacts in a CT system equipped with a rectangular two-dimensional X-ray detector. <P>SOLUTION: The radius and the height of a desired cylindrical reconstruction region are input by a reconstruction region inputting means 310. A reconstruction-expanded-region calculating means 320 calculates the amount of expansion of the reconstruction region on the basis of the reconstruction region having been input and a region for which the projection data in a viewing direction are not complete. A mean for calculating the amount of expansion in a vertical direction of projection data 330 forms expanded projection data corresponding to the expanded amount of the reconstruction region by extrapolating or copying the projection data, and performs the reconstruction by using the expanded projection data and the projection data. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a technique effective for generating a three-dimensional CT image in a CT apparatus including a rectangular two-dimensional X-ray detector.

  Conventionally, it is known that when a three-dimensional cone beam CT image is generated using rotational imaging data of a circular two-dimensional X-ray detector such as an X-ray image intensifier, the reconstruction area becomes a spherical shape. Yes.

  On the other hand, when rotation data of a two-dimensional X-ray detector such as a flat panel detector (hereinafter referred to as “FPD”) (in this specification, a rectangular shape and a square shape are collectively referred to as a rectangular shape) is used. The reconstruction area has a cylindrical shape about the rotation center axis. Strictly speaking, since the X-ray beam is not a parallel beam, a detector in the view direction (direction of rotational movement) is detected at the upper and lower ends of the cylindrical area. Reconstruction area in which data is aligned, that is, a reconstruction area in which projection data captured for each view is aligned by 180 degrees + α (α: an opening angle in the fan direction of the X-ray beam (a direction substantially orthogonal to the body axis direction)) Has a shape in which both ends of the columnar shape are narrowed like a pencil core.

  When reconstructing the projection data of the reconstruction area with the cylindrical ends narrowed like a pencil lead, the axial image of the slice near the upper and lower ends is 180 ° + α projection data at the center. Although it is reconstructed using the image, the peripheral portion of the axial image is darkly displayed because of lack of projection data. In addition, in the sagittal image and the coronal image, the upper end portion and the lower end portion are displayed darkly or the image is missing.

  FIG. 8 shows an image of a phantom constructed by putting water in a cylindrical container made of acrylic having a diameter of 20 cm and an inner diameter of 19 cm (wall thickness: 5 mm) using a conventional cone beam X-ray CT apparatus. A sagittal image (FIG. 8 (a)), a coronal image (FIG. 8 (b)), an axial image (which is obtained by reconstructing the projection data of the reconstruction area having a shape narrowed at both ends like a pencil core ( FIG. 8 (c)) is shown.

  In the sagittal image of FIG. 8A, since the X-ray beam is not expanded in the source direction, projection data of 180 ° + α or 360 ° necessary for CT reconstruction is not prepared, and projection is performed in the view direction. There is not enough data. For this reason, the area where the projection data is insufficient is displayed darker than the area where the projection data is aligned, and the almost right half of the reconstruction area becomes narrower as it approaches the right end.

  The coronal image in FIG. 8B is obtained by reconstructing projection data from a position where the X-ray source is present in a direction perpendicular to the paper surface, and the reconstruction area is directed to the left and right ends. And narrowed.

  The axial image in FIG. 8C is displayed darker as the area is farther from the rotational orbital plane of the X-ray source 11, and the reconstruction area is not circular.

As a calibration method for matching the image level of the reconstruction area where the projection data in the view direction is not aligned as shown in FIG. 8 with the image level of the reconstruction area where the projection data in the view direction is aligned, for example, Patent Document 1 discloses each reconstruction A method is disclosed in which the number of added projections at the time of CT backprojection calculation of points is stored and divided by the number of added projections.
JP 2002-204796 A

  In a cone beam X-ray CT apparatus using a rectangular two-dimensional detector, it is generally expected that the reconstruction area is a cylindrical shape centered on the rotation center axis. Since it is not a parallel beam, the reconstruction area in which all projection data are aligned in the view direction is narrow like a pencil core at the upper and lower ends of the cylindrical area, and this portion is a CT cross-sectional image that cannot be used effectively.

  The present inventor tried to match the image level of the area where the projection data is not aligned in the view direction by using the method of Patent Document 1, and found the following problems.

  In Patent Document 1, a two-dimensional X-ray detector is shifted so that the image of the rotation center axis deviates from the center, and the image level of the three-dimensional CT image when the image is rotated 360 degrees is made uniform. It is an effective technology. The method of Patent Document 1 is effective when a region where 360-degree detector data is aligned and a reconstruction point where projection data of 180 degrees or more necessary for the CT reconstruction calculation exists are mixed. The upper and lower ends of the cylindrical reconstruction area are areas where projection data in the 180-degree direction necessary for the CT reconstruction calculation is not prepared. Therefore, when applied, the image level becomes uniform, but there is a problem that artifacts occur. It was.

  The present invention has been made in view of the above-described problems, and suppresses the occurrence of artifacts, while the image level of a reconstructed image in a region where projection data in the view direction is not aligned and the region in which projection data in the view direction are aligned. The purpose is to make the reconstructed image uniform.

  The present invention extrapolates the projection data in the vertical direction and enlarges it, thereby enlarging the region where the projection data in the direction of 180 degrees exists, and without generating artifacts, the image level is uniform, and the complete cylindrical field of view 3 A cone beam X-ray CT apparatus for generating a two-dimensional cone beam CT image is realized.

  More specifically, a cone beam X-ray CT apparatus according to the present invention includes an X-ray source that generates cone-beam-shaped X-rays, and the X-ray that is disposed facing the X-ray source and transmitted through a subject. X-ray detector that detects and outputs projection data of the subject, rotation means for rotating the X-ray source and the X-ray detector, and extended projection data by extrapolating or copying the projection data And the expanded projection data is used to complement the data in the region near the upper and lower ends along the body axis direction of the subject in the X-ray detector, where the projection data is not aligned. , A projection data extending means for forming a cylindrical reconstruction region composed of the projection data and the extended projection data, and performing a reconstruction calculation based on the projection data and the extended projection data, and the X of the subject Line C Characterized in that it comprises a reconstruction means for generating an image, and a display means for displaying the X-ray CT images, the.

  According to the present invention, in a cone beam X-ray CT apparatus using projection data of a two-dimensional X-ray detector, an image level of a reconstructed image of a region where projection data in the view direction is not aligned while suppressing occurrence of artifacts. And a reconstructed image of a region where projection data in the view direction are aligned can be made uniform.

  Hereinafter, preferred embodiments of a cone beam X-ray CT apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

<First embodiment>
FIG. 1 is a block diagram showing a schematic configuration of a C-arm type cone beam X-ray CT apparatus 1 to which the present invention is applied. An X-ray transmission image of the subject 2 is irradiated by irradiating the subject 2 with X-rays. An imaging unit 10 that obtains projection data by imaging the control unit 20, and a control calculation unit 20 that controls each component of the imaging unit 10 and reconstructs a three-dimensional X-ray CT image of the subject 2 based on the projection data. With.

  FIG. 1A is a block diagram showing a schematic configuration of a sitting-type cone beam X-ray CT apparatus 1a to which the present invention is applied. The imaging unit 10a and each component of the imaging unit 10a are controlled, or a three-dimensional X-ray And a control arithmetic unit 20a for reconstructing a line CT image.

  FIG. 1 b shows an apparatus configuration similar to FIG. 1 a, but instead of the X-ray source 11 and the two-dimensional X-ray detector 12 rotating around the subject 2, the X-ray source 11, two-dimensional X-ray detection is performed. FIG. 2 is a block diagram showing a schematic configuration of a subject rotation type cone beam X-ray CT apparatus 1b, in which the subject 12 is fixed on the floor and the subject 2 placed on the turntable 19 rotates. Since the apparatus configuration of the cone beam X-ray CT apparatus of FIG. 1b is the same as that of the cone beam X-ray CT apparatus of FIGS.

  Further, the cone beam X-ray CT apparatus 1 (and 1a and 1b) of FIG. 1 (and FIGS. 1a and 1b) moves the display device 80 for displaying an image and the position of the image displayed on the display device 80. And an information input device 70 composed of a mouse, a keyboard, a trackball, and the like.

(Shooting unit 10)
The imaging unit 10 of the C-arm type cone beam X-ray CT apparatus 1 includes a bed 17 on which the subject 2 is placed, an X-ray source 11 that irradiates the subject 2 placed on the bed 17 with X-rays, and an X-ray source. A two-dimensional X-ray detector 12 which is installed at a position opposite to the object 11 and outputs projection data by detecting X-rays transmitted through the subject 2, and an X-ray source 11 and a two-dimensional X-ray detector 12 And a C-shaped arm 13 to be connected. In addition, the photographing unit 10 includes a C-type arm holding body 14 that holds the C-type arm 13, a ceiling support 15 that attaches the C-type arm holding body 14 to the ceiling, and the ceiling support 15 in the state of FIG. The ceiling rail 16 is supported so as to be movable in the two-dimensional direction, and the injector 18 injects a contrast medium into the subject 2.

(Shooting unit 10a)
The imaging unit 10 a of the sitting-type cone beam X-ray CT apparatus 1 a includes a chair 7 on which the subject 2 is placed, an X-ray source 11 that irradiates the subject 2 placed on the chair 7 with X-rays, and an X-ray source 11. The two-dimensional X-ray detector 12 that outputs the projection data by detecting the X-rays transmitted through the subject 2, the X-ray source 11, and the two-dimensional X-ray detector 12 are mechanically installed. And a swivel arm 5 connected to. The photographing unit 10 includes a support column 6 that holds the swing arm 5 and a fixed base 8 that holds the support column 6 and the chair 7.

  The X-ray source 11 includes an X-ray tube 11t that generates X-rays, and a collimator 11c that controls the direction of X-ray irradiation from the X-ray tube 11t in a cone or quadrangular pyramid shape.

  As the two-dimensional X-ray detector 12, a flat panel detector “FPD” using a TFT element is used. The two-dimensional X-ray detector 12 includes an X-ray image intensifier 12i (not shown) that converts an X-ray transmission image into a visible light image, and a television that captures a visible light image by the X-ray image intensifier 12i. A two-dimensional X-ray detector including a camera 12c (not shown) or the like may be used.

  When the subject 2 is imaged, the swivel arm 5 and the C-arm 13 rotate around the rotation center axis 4 at every predetermined projection angle (also referred to as a view angle). Thereby, the X-ray source 11 and the two-dimensional X-ray detector 12 perform X-ray imaging while rotating and moving on substantially the same circular orbit. For this rotational movement, there are geometric parameters used for the image reconstruction operation. As a geometric parameter, a rotational trajectory plane (midplane) 3 which is a plane including a circular trajectory drawn by the X-ray source 11 and the two-dimensional X-ray detector 12 by the rotational movement of the swivel arm 5 or the C-shaped arm 13. And the rotation center axis 4.

(Control operation unit 20)
The control calculation unit 20 (20a) includes an imaging unit control unit 100 (100a) that controls the imaging unit 10 (10a), and an image collection unit 110 that collects and stores projection data output from the imaging unit 10 (10a). Reconstructing means 200 for reconstructing a three-dimensional X-ray CT image based on the collected X-ray image data, and projection data expansion for complementing the cylindrical cone beam reconstruction region which is a feature of the present invention Means 300. Furthermore, the image display means 120 which displays the three-dimensional X-ray CT image produced | generated by the reconstruction means 200 is provided.

(Shooting unit control means 100)
The imaging unit control means 100 of the C-arm type cone beam X-ray CT apparatus 1 is an imaging system that controls the rotational movement of the C-arm 13 around the rotation center axis 4 (hereinafter referred to as “propeller rotation”). A rotation control means 101 and an imaging system position control means 102 for controlling the position of the ceiling support 15 on the ceiling rail 16 and controlling the position of the C-arm 13 relative to the subject 2 in two dimensions. Further, the imaging unit control means 100 determines the X-ray irradiation control means 103 that controls ON / OFF of the tube current that flows through the X-ray tube 11t, and the injection amount and injection timing of the contrast medium that the injector 18 injects into the subject 2. Injector control means 104 to be controlled, bed control means 106 for adjusting the position of the subject 2 by controlling the position of the bed 17, and detection for controlling radiographic image capturing by the two-dimensional X-ray detector 12 System control means 107.

(Shooting unit control means 100a)
The imaging unit control unit 100 of the sitting-type cone beam X-ray CT apparatus 1a includes an imaging system rotation control unit 101 that controls the rotational movement of the revolving arm 6 about the rotation center axis 4, and two-dimensional X-ray detection. And a detection system control means 107 for controlling radiographic image capturing by the instrument 12. Further, the imaging unit control unit 100a adjusts the position of the subject 2 by controlling the position of the chair 7 and the X-ray irradiation control unit 103 that controls ON / OFF of the tube current flowing through the X-ray tube 11t. The chair position control means 105 is provided.

(Reconstruction means 200)
The reconstruction unit 200 includes a preprocessing unit 210, a filtering unit 220, and a back projection unit 230.

  The preprocessing unit 210 is a unit for converting the projection data collected by the image collection unit 110 into a distribution image of X-ray absorption coefficients. In the present embodiment, first, natural logarithmic conversion calculation is performed on each pixel data of an X-ray transmission image of air that has been imaged in advance without the subject 2 and the bed 17 being placed in the field of view. Next, a natural logarithmic conversion operation is performed on each pixel data of an X-ray transmission image taken with the subject 2 placed on the bed 17. By taking the difference between the two X-ray transmission images, an X-ray absorption coefficient distribution image of the subject 2 and the bed 17 is obtained.

The filtering unit 220 performs a filtering process in X-ray CT image reconstruction.
The backprojection means 230 performs a backprojection operation on the projection data after filtering processing based on, for example, the Feldkamp method, and generates a three-dimensional reconstructed CT image 240 (hereinafter referred to as “reconstructed CT image 240”). Generate. The output of the reconstructed CT image 240 is normally performed as a stack of axial cross-sectional images.

(Projection expansion means 300)
The projection data expansion unit 300 includes a reconstruction area input unit 310, a reconstruction expansion area calculation unit 320, and a projection data vertical direction expansion amount calculation unit 330.

  The reconstruction area input means 310 accepts input of the reconstruction radius and cylinder height of the cylinder reconstruction area.

  The reconstruction extension area calculation means 320 compares the reconstruction area (reconstruction radius, column height) input by the reconstruction area input means 310 with the reconstruction area where the projection data is aligned, The presence or absence of is calculated.

  The projection data vertical expansion amount calculation unit 330 converts the reconstruction area expansion amount set by the reconstruction expansion area calculation unit 320 into the vertical expansion amount of the projection data. If necessary, the image and the extension area are displayed on the display device 80, and the projection data longitudinal extension amount is determined. By extrapolating the projection data or copying the projection data, the projection data is extended in the vertical direction to generate vertical extension projection data 340.

  The longitudinally expanded projection data 340 is sent to the filtering unit 220, and the back projection unit 230 generates a three-dimensional cone beam CT image expanded in the body axis direction.

  A specification example of the cone beam X-ray CT apparatus 1 is as follows. The distance between the X-ray tube 11t and the rotation center axis 4 is 800 mm, the distance between the rotation center axis 4 and the X-ray incidence surface of the two-dimensional X-ray detector FPD is 400 mm, and the size of the FPD X-ray incidence surface is 400 mm × It has a rectangular shape of 300 mm, the image size is 2048 × 1536, and the pixel pitch is 0.2 mm. When X-rays enter the FPD, the light is first converted into light by a light emitter such as CsI on the X-ray incident surface, and the optical signal is converted into electric charge by a photodiode. The accumulated charge is converted into a digital signal by a TFT element at a constant frame rate and read out. In the rotation shooting mode, projection data is read out at 30 frames per second and an image size of 1024 × 768. The imaging system rotation control means 101 passes through the two-dimensional X-ray detector 12 from the direction of the left hand of the subject 2 (−100 degrees) to the ceiling direction (0 degrees) and to the right hand direction of the subject 2 (+100 degrees). That is, by moving the X-ray tube 11t from the right hand direction (+100 degrees) of the subject 2 to the ceiling direction (0 degrees) and moving to the left hand direction (−100 degrees) of the subject 2, Two-dimensional projection data of the subject 2 is imaged over a projection angle of degrees. A typical example of the rotational speed of the C-arm 13 is 40 degrees per second, and the scan time is 5 seconds.

  An example of the specification of rotation of the cone beam X-ray CT apparatus 1a is as follows. The imaging system rotation control unit 101 rotates the X-ray source 11 and the two-dimensional X-ray detector 12 around the subject 2, and the detection system control unit 107 captures 360 ° projection data of the subject 2. To do. A typical example of the rotation speed of the swivel arm 5 is 37.5 degrees per second, and the scan time is 9.6 seconds.

  Next, an outline of operations in imaging by the cone beam X-ray CT apparatus 1 (1a) of the present embodiment will be described.

  In the C-arm Cone Beam X-ray CT apparatus 1, first, the imaging system rotation control means 101 starts the propeller rotation of the C-arm 13. After passing through the rotation acceleration period, the X-ray irradiation control means 103 irradiates the X-ray tube 11t with X-rays, and the detection system control means 107 starts imaging by the two-dimensional X-ray detector 12. X-rays irradiated from the X-ray tube 11t pass through the subject 2 and are taken into the two-dimensional X-ray detector 12. The signal from the two-dimensional X-ray detector 12 undergoes A / D conversion and is then recorded in the image acquisition unit 110 as two-dimensional projection data composed of a digital signal. The standard scanning mode of the two-dimensional X-ray detector FPD is 30 frames per second. The projection angle interval by imaging is 1.33 degrees, and 150 X-ray transmission images are acquired in 5 seconds. When the rotation imaging at 200 degrees is completed, the X-ray irradiation control unit 103 ends the X-ray irradiation of the X-ray tube 11t, and the imaging system rotation control unit 101 stops the rotation after a rotation deceleration period.

  The imaging operation by the sitting-type cone beam X-ray CT apparatus 1a is as follows. First, the imaging system rotation control means 101 starts to rotate the revolving arm 5. After passing through the rotation acceleration period of 22.5 degrees, the X-ray irradiation control means 103 emits X-rays from the X-ray tube 11t, and the detection system control means 107 starts imaging by the two-dimensional X-ray detector 12. The projection angle interval by photographing is 1.25 degrees, and 288 projection data are acquired in 9.6 seconds. When the 360-degree rotation imaging is completed, the X-ray irradiation control means 103 ends the X-ray irradiation from the X-ray tube 11t, and the imaging system rotation control means 101 stops the rotation after passing through a rotation decelerating period of 22.5 degrees. To do.

  The reconstruction unit 200 reads the two-dimensional projection data from the image collection unit 110 in parallel with the above imaging or after the imaging is completed, performs an image reconstruction operation based on the projection data, and performs 3 of the subject 2. Reconstruction calculation of a dimensional X-ray CT image is performed. The image display unit 120 displays a three-dimensional X-ray CT image on a display device 80 including a CRT device or a liquid crystal display device. The image display unit 120 is also used to display two-dimensional projection data recorded in the image collection unit 110 and to set or display an area to be expanded in the projection data expansion unit 300.

  FIG. 2 shows a state of a cylindrical reconstruction field of view of a cone beam X-ray CT apparatus equipped with a rectangular two-dimensional detector. The X-ray source 11 and the two-dimensional X-ray detector 12 rotate 180 + α degrees or 360 degrees around the rotation center axis 4 while maintaining the facing position.

  The cylindrical region 31 represents an ideal cylindrical reconstruction region determined from the lengths of the two-dimensional X-ray detector 12 in the horizontal direction and the vertical direction. The region 32 is a reconstruction region when the X-ray beam is not a parallel beam, and is a reconstruction region where projection data is aligned at the upper and lower ends of the cylindrical region 31, that is, 180 + α degrees or 360 degrees at a certain vertical position. The reconstructed area where all the projection data of FIG. The present invention complements the projection data at the upper and lower ends of the cylindrical reconstruction area where the projection data in the view direction required for the CT reconstruction calculation is slightly insufficient by the vertical extension of the projection data, thereby completely reconstructing the cylindrical area. In order to generate a region, a full circular output slice is beneficial in the axial section display.

  However, for cross-sectional display in the coronal / sagittal direction, expansion by extrapolation or copying of the end portion appears as a straight body surface and does not necessarily lead to improved visibility, so the user can turn on / off the coronal / sagittal expansion display. Provide functions that can be set.

  Next, the contents of each process of the projection data expansion means 300, which is a feature of the present invention, will be described using the flowchart of FIG. 3 and the display screen diagrams of FIGS.

  FIG. 4 shows a setting screen in the projection data expansion unit 300. There is a two-dimensional X-ray projection image display area 40 on the upper side of the screen, a vertical extension amount setting field 81 on the lower side of the screen, and a coronal / sagittal extension button 82 for setting ON / OFF of coronal / sagittal extension display, There is. The vertical expansion amount is 0 (no projection expansion processing), and the vertical expansion amount necessary for reconstructing a complete cylindrical field of view calculated geometrically from the detector size and the X-ray source-detector distance. = A numerical value up to a MAX value (50 mm in the figure) can be set, and input is performed using an information input device 70 such as a mouse, a keyboard, or a trackball. Reference numeral 41 denotes a CT reconstruction area when there is no projection extension process, and 42 denotes a CT reconstruction area when calculated with the set vertical extension amount.

FIG. 5 shows a display screen of a three-dimensional X-ray CT image reconstructed from the projection data generated by the projection data expansion means 300 when the coronal / sagittal expansion display ON is selected in FIG. Screen 50 is an axial reconstruction image display screen,
A screen 60 shows a display screen of coronal and sagittal longitudinal sectional images.

  The boundary lines 51 and 61 are the boundary lines of the CT reconstruction area when there is no projection expansion process, and the boundary lines 52 and 62 are the boundary lines and lines of the CT reconstruction area when backprojected with the set vertical expansion amount. 53 and 63 are the reconstruction radius and height of the cylinder input by the reconstruction area input means 310, that is, the boundary line when the maximum amount is expanded in the vertical direction (when expansion of MAX = 50 mm in FIG. 4 is performed). Show.

  FIG. 6 shows a display screen of a three-dimensional X-ray CT image reconstructed from the projection data generated by the projection data extension means 300 when the coronal / sagittal extension display OFF is selected in FIG. A screen 50 shows an axially reconstructed image display screen, and a screen 60 shows a display screen for coronal and sagittal longitudinal cross-sectional images. The boundary line 51 is the boundary line of the axial cross-section display area when there is no projection expansion process, the boundary line 52 is the boundary line of the axial cross-section display area expanded by the set vertical extension amount, and the lines 53 and 63 are the reconstruction area The reconstruction radius and height of the cylinder input by the input means 310 are shown.

  In FIG. 6, the axial image includes vertical extension projection data 340 including the extension projection data extended in the vertical direction by the projection data vertical extension amount calculation unit 330 and the projection data detected from the two-dimensional X-ray detector 12. Is a reconstructed image obtained by back projecting, and the area inside the boundary lines 51 and 52 is displayed as a cross-sectional image. On the other hand, the longitudinal sectional images such as coronal and sagittal are sectional images obtained by reconstructing only the projection data detected from the two-dimensional X-ray detector 12 without including the extended projection data. A cross-sectional image consisting only of the area inside is displayed.

  Hereinafter, the procedure of the projection data expansion process will be described with reference to FIG.

(Step S310)
The reconstruction radius and the column height of the cylinder reconstruction area are input from the information input device 70 such as a mouse, a keyboard, or a trackball, and the reconstruction area input unit 310 acquires the input value.

  Also, instead of the user inputting the reconstruction radius and cylinder height, a function for automatically setting the maximum cylindrical reconstruction field of view calculated geometrically from the detector size and the X-ray source-detector distance may be provided. Good.

(Step S320)
The reconstruction extension area calculation means 320 compares the reconstruction area (reconstruction radius, column height) acquired by the reconstruction area input means 310 in step S310 with the reconstruction area where the projection data is aligned, and the reconstruction area extension amount The presence or absence of expansion is calculated. The calculated reconstruction extended area can be displayed as an extended area boundary when a three-dimensional X-ray CT image is displayed as shown by the boundary lines 52 and 62 in FIG.

(Step S321)
If the reconstruction area (reconstruction radius, cylinder height) set in step S310 is larger than the reconstruction area where the projection data is aligned, and is determined to be larger (YES), the projection data after step S330 Perform extension processing.

  On the other hand, if it is determined to be small (NO), the projection data need not be expanded, and the process ends (step S322).

(Step S330)
The reconstruction extension area calculation means 320 converts the reconstruction area extension amount set in step S320 into an extension amount in the vertical direction of the projection data, and is geometrically calculated from the detector size and the X-ray source-detector distance. The longitudinal expansion amount necessary for reconstructing the complete cylindrical field of view = MAX value (50 mm in FIG. 4) is calculated.

(Step S331)
The user uses the information input device 70 to display 0 (no projection expansion processing) and the maximum vertical expansion amount calculated in step S330 = MAX value (50 mm in FIG. 4), and the vertical expansion amount to be actually applied. Enter.

(Step S332)
The projection data vertical extension amount calculation means 330 extends the projection data in the vertical direction by the extrapolation processing method or the copy of the vertical boundary value by the vertical extension amount input in step S331, and the vertical extension. Projection data 340 is generated.
When attempting to reconstruct the upper and lower ends of the cylinder, as can be seen from FIG. 2, the projection data is insufficient in the vertical direction in the view direction closer to the X-ray source 11. Extrapolation of projection data in the vertical direction compensates for this lack of projection data. Since the ratio of the view direction in which the projection data is insufficient increases toward the upper and lower ends and the outer periphery of the cylinder, the ratio at which the extended projection data is used for the reconstruction calculation increases.

  In FIG. 4 described above, when the extended display is turned off for the sagittal image and the coronal image, the sagittal image and the coronal image are reconstructed only for the projection data as in the case where NO is determined in step S321. However, the axial image is generated using the vertical extension projection data 340.

  FIG. 7 shows a sagittal image obtained by imaging a phantom using a cone beam X-ray CT apparatus to which the present invention is applied and reconstructing it using projection data expanded in the vertical direction using the method of this embodiment. (FIG. 7A), a coronal image (FIG. 7B), and an axial image (FIG. 7C) are shown. The phantom was constructed by putting water in a cylindrical container made of acrylic having a diameter of 20 cm and an inner diameter of 19 cm (wall thickness: 5 mm). According to the present embodiment, as shown in FIG. 7C, the projection data in the projection angle direction (view direction) that could not be collected is complemented by the extrapolation of the projection data in the vertical direction. A uniform CT image with a circular field of view can be output even with an axial section far away from the center.

  As a result, for example, in a 64-slice multi-slice X-ray detector, an axial image of 1 to 4 slices near the upper end and an axial image of 61 to 64 slices near the lower end can be obtained. Even when the axial images are arranged and displayed, the axial images having the same image level can be arranged and displayed, and the visibility can be improved.

  Further, according to the present invention, as shown in the sagittal image of FIG. 7A and the coronal image of FIG. 7B, the boundary lines of the reconstruction visual fields at the upper end and the lower end can be displayed horizontally.

  According to the present invention, in a cone beam X-ray CT apparatus using a rectangular two-dimensional detector, an area where projection data in a 180-degree direction necessary for CT reconstruction calculation is present is enlarged to generate an artifact. 3D X-ray CT images with a uniform image level and a complete cylindrical field of view can be generated, thus eliminating CT cross-sectional images that cannot be effectively used for diagnosis, contrast imaging of the head, abdomen, etc. It can be expected to improve the diagnostic performance in the orthopedic field such as the tooth jaw, lumbar vertebra, and limbs.

1 is a block diagram showing a schematic configuration of a C-arm type cone beam X-ray CT apparatus 1 to which the present invention is applied. FIG. It is a block diagram which shows schematic structure of the cone-beam X-ray CT apparatus 1a of the sitting position to which this invention is applied. It is a block diagram which shows schematic structure of the cone rotation X-ray CT apparatus 1b of the subject rotation system to which this invention is applied. It is a figure explaining the mode of the cylindrical shape reconstruction visual field of the cone beam X-ray CT apparatus provided with the rectangular-shaped two-dimensional detector. It is a flowchart for demonstrating the process in the projection data expansion means of this invention. It is a figure which shows the setting screen in the projection data expansion means 300 of this invention. It is a figure which shows the display screen of the three-dimensional X-ray CT image reconstructed from the projection data produced | generated by the projection data expansion means 300 of this invention. It is a figure which shows the display screen of the three-dimensional X-ray CT image reconstructed from the projection data produced | generated by the projection data expansion means 300 of this invention. It is a figure which shows each cross section of the three-dimensional X-ray CT image reconstructed from the projection data produced | generated by the projection data expansion means 300 of this invention. It is a figure which shows the example of each cross-sectional image by the conventional cone beam X-ray CT image.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Cone beam X-ray CT apparatus, 1a ... Cone beam X-ray CT apparatus of a sitting position, 1b ... Cone beam X-ray CT apparatus of a subject rotation type, 2 ... Subject, 3 ... Rotating track surface (midplane), DESCRIPTION OF SYMBOLS 4 ... Rotation center axis | shaft, 5 ... Turning arm, 6 ... Support | pillar, 7 ... Chair, 8 ... Fixed mount, 10 ... Imaging | photography part, 10a ... Imaging | photography part of cone beam X-ray CT1a of a sitting position, 11 ... X-ray source, 11t DESCRIPTION OF SYMBOLS ... X-ray tube, 11c ... Collimator, 12 ... Two-dimensional X-ray detector, 13 ... C-type arm, 14 ... C-type arm holder, 15 ... Ceiling support, 16 ... Ceiling rail, 17 ... Sleeper, 18 ... Injector DESCRIPTION OF SYMBOLS 19 ... Rotary table, 20 ... Control calculating part, 20a ... Control calculating part of cone-beam X-ray CT1a of sitting position, 21 ... Projection of rotation center axis to two-dimensional X-ray detector 12, 30 ... Axial reconstructed image Display area, 31 ... ideal Column reconstruction area, 32... Reconstruction area when the X-ray beam is not a parallel beam, 40... X-ray projection image display area, 41... CT reconstruction area when there is no projection expansion processing, 42. CT reconstruction area when calculated by the amount, 50 ... axial reconstruction image display screen, 51 ... boundary line of the axial reconstruction area when there is no projection extension processing, 52 ... when back projection is performed with the set vertical extension amount , 53 ... boundary line of the input reconstruction radius of the cylinder, 60 ... longitudinal section image display screen, 61 ... boundary line of the vertical direction reconstruction without projection expansion processing, 62 ... Boundary line of vertical reconstruction area when backprojected with set vertical expansion amount, 63... Reconstructed height of input cylinder, 70... Information input device, 80... Display device, 81. Column, 82 ... Coronal Sagittal expansion button, 100 ... Imaging unit control means, 100a ... Imaging unit control means for sitting-type cone beam X-ray CT1a, 101 ... Imaging system rotation control means, 100b ... Imaging unit for subject rotation-type cone beam X-ray CT1b Control means 102 ... Imaging system position control means 103 ... X-ray irradiation control means 104 ... Injector control means 105 ... Chair position control means 106 ... Bed control means 107: Detection system control means 110 ... Image collection means , 120 ... Image display means, 200 ... Reconstruction means, 210 ... Preprocessing means, 220 ... Filtering means, 230 ... Back projection means, 240 ... Reconstructed CT image, 300 ... Projection data expansion means, 310 ... Reconstruction area input Means 320... Reconstruction expansion area calculation means 330. Projection data vertical direction extension amount calculation means 340... Vertical extension projection data T

Claims (4)

An X-ray source for generating cone-beam X-rays;
An X-ray detector disposed opposite to the X-ray source, detecting the X-ray transmitted through the subject and outputting projection data of the subject;
Rotating means for rotating the X-ray source and the X-ray detector;
Extended projection data is generated by extrapolating or copying the projection data, and using the extended projection data, an area near the upper and lower ends along the body axis direction of the subject in the X-ray detector. Projection data expansion means for forming a cylindrical reconstruction area composed of the projection data and the extended projection data by complementing the data of the area where the projection data is not aligned;
Reconstructing means for performing a reconstruction operation based on the projection data and the extended projection data, and generating an X-ray CT image of the subject;
Display means for displaying the X-ray CT image;
A cone beam X-ray CT apparatus comprising: Further comprising setting means for setting the height of the cylindrical reconstruction region to a desired value;
The projection data extending means generates the extended projection data according to the set value;
The cone beam X-ray CT apparatus according to claim 1. The reconstruction unit generates at least one of an axial image, a sagittal image, and a coronal image of the subject;
The cone beam X-ray CT apparatus according to claim 1 or 2, And further comprising selection means capable of selecting whether to reconstruct the sagittal image and the coronal image based only on the projection data or based on the projection data and the extended projection data.
The reconstruction unit reconstructs the sagittal image and the coronal image based on a selection result by the selection unit, and reconstructs the axial image based on the projection data and the extended projection data.
The cone beam X-ray CT apparatus according to claim 3.