JP5572521B2 - X-ray CT apparatus and image reconstruction method for X-ray CT apparatus - Google Patents

Description

  The present invention relates to an X-ray CT apparatus that measures an X-ray CT image of a subject, and relates to a technique for measuring an X-ray CT image that is an accurate CT value even when the subject protrudes from the field of view of an X-ray detector. Is.

  In the X-ray CT apparatus, when the subject protrudes outside the end channel of the detector, the technique disclosed in Patent Document 1 solves the problem.

  Specifically, projection data (da) outside the end channel is first created based on the projection data (dn) of the end vicinity channel. Next, the projection data is extrapolated using the outer projection data (da). Finally, a CT image is generated based on the extrapolated projection data. As a result, a normal CT image can be created even when the subject protrudes outside the end channel of the detector.

  A technique for converting an X-ray beam from a cone into parallel projection is disclosed in Patent Document 2.

JP 2004-65706 JP Japanese Patent Laid-Open No. 10-243941

L.A.Feldkamp, et al.:Practical Cone-Beam Algorithm: J. Optical Society of America, A / Vol.1 (6) pp. 612-619 (1984)

  Patent Document 1 describes that when a part of the subject protrudes from the end of an X-ray detector that performs parallel projection from a cone-shaped X-ray beam, the projection data of the protruding part is not projected. Obtained by extrapolation. For this reason, when an X-ray CT image is reconstructed using the projection data of the protruding portion, the CT value may be inaccurate, and obtaining the CT value accurately has been an unsolved problem.

  Further, the above-mentioned Patent Document 2 converts the X-ray beam from a cone to a parallel projection, so that when a cone beam and a two-dimensional X-ray detector are used, the image reconstruction is complicated and enormous. It is only described that it solves the technical problem that it cannot be realized because it takes too much processing time for computers that are widely spread, and suggests the solution of the above unresolved problems The description to do is not seen.

  The present invention has been made as a means for solving the above problems, and provides an X-ray CT apparatus capable of obtaining an accurate CT value even if the subject protrudes from the field of view of the X-ray detector. Objective.

  It is another object of the present invention to provide an image reconstruction method for an X-ray CT apparatus capable of obtaining an accurate CT value even if the subject protrudes from the field of view of the X-ray detector.

  A feature of the present invention is to set a plurality of rotation centers for shooting, convert projection data shot at a plurality of rotation centers into parallel projection images, calculate overlapping regions between the parallel projection images for a plurality of rotations, For the overlap region, a parallel projection image for a plurality of rotations is synthesized based on the rotation center position on the setting and the rotation center position in actual photographing.

  Specific configurations of the X-ray CT apparatus and the image reconstruction method thereof according to the present invention are shown in the following items.

  (1) An X-ray source (11) that irradiates the subject (2) with X-rays, and the X-ray source (11) disposed opposite to the X-ray that has passed through the subject (2) to detect the X-ray source An X-ray detector (12) that outputs projection data of the subject (2), and a rotation that rotates the X-ray source (11) and the X-ray detector (12) around the rotation center of the subject (2) Unit (13), an image reconstruction calculation unit (20) for reconstructing an X-ray CT image of the subject (2) based on projection data of the subject (2), and displaying the X-ray CT image An X-ray CT apparatus comprising a display unit (80), and a rotation center setting unit (70) for setting a plurality of imaging rotation centers, wherein the image reconstruction calculation unit (20) includes a plurality of A parallel projection image conversion unit (220) that converts projection data photographed at the rotation center of the image into a parallel projection image, and calculates an overlapping area between the parallel projection images for a plurality of rotations, and rotates the setting for the overlapping area Based on the center position and the center position of rotation in actual shooting X-ray CT apparatus characterized by comprising projection image synthesizing unit for synthesizing the parallel projection image of the number revolutions (230), the.

  (2) An X-ray source that irradiates the subject with X-rays, and an X-ray detector that is arranged opposite to the X-ray source and detects X-rays transmitted through the subject and outputs projection data of the subject A rotation unit that rotates the X-ray source and the X-ray detector around the rotation center of the subject, and image reconstruction that reconstructs the X-ray CT image of the subject based on the projection data of the subject An image reconstruction method for an X-ray CT apparatus, comprising: a calculation unit; and a display unit that displays the X-ray CT image, the step of setting rotation centers of a plurality of imaging, and imaging at a plurality of rotation centers Converting the projected data into a parallel projection image, calculating an overlap area between the parallel projection images for a plurality of rotations, and based on the rotation center position on the setting and the rotation center position in actual photographing for the overlap area A step of synthesizing parallel projection images for a plurality of rotations. Device image reconstruction method.

  Since the feature of the present invention is configured as described above, when a part of the subject protrudes from the end of the X-ray detector, the projection data of the protruding part is not extracted from the projection data of the part that does not protrude. Since it is obtained from the parallel projection image, an accurate CT value is obtained when reconstructing the X-ray CT image using the projection data of the protruding part, so it is accurate even if the subject protrudes from the field of view of the X-ray detector CT value can be obtained.

  According to the present invention, an accurate CT value can be obtained even if the subject protrudes from the field of view of the X-ray detector.

Schematic configuration diagram of a cone-beam X-ray CT apparatus 1 using a C-arm system to which the present invention is applied Schematic configuration diagram showing a C-arm type cone beam X-ray CT apparatus 1a mounted on a mobile X-ray apparatus to which the present invention is applied The figure explaining a mode that X-ray CT image is reconstructed by combining projection data photographed at two different rotation centers by the X-ray CT apparatus of the present invention The figure explaining the geometry of a parallel projection image in the projection image synthetic | combination part by this invention The flowchart which shows operation | movement of the 1st Example of this invention. Flow chart showing shooting process 1 is a flowchart for explaining processing for generating a parallel projection image according to the first embodiment of the present invention; The flowchart which shows operation | movement of the 2nd Example of this invention. Flowchart for explaining processing for generating a parallel projection image according to the third embodiment of the present invention The figure which shows the reconstruction visual field of an X-ray CT image at the time of applying the present invention to composition of projection data photoed at three different rotation centers Figure showing the reconstructed field of view of the X-ray CT image, which has a different arrangement from that of Fig. 10. The figure which shows the reconstruction visual field of a X-ray CT image at the time of applying the present invention to composition of projection data photoed at four different rotation centers

  Hereinafter, embodiments of an X-ray CT apparatus according to the present invention will be described in detail with reference to the accompanying drawings. In all the drawings for explaining the embodiments of the present invention, those having the same function are denoted by the same reference numerals, and repeated explanation thereof is omitted.

(Outline configuration)
First, a schematic configuration of a cone beam X-ray CT apparatus to which the present invention is applied will be described based on FIG. 1 and FIG. FIG. 1 is a schematic configuration diagram showing a cone beam X-ray CT apparatus (C-arm system) 1 to which the present invention is applied. FIG. 2 is a schematic configuration diagram showing a C-arm type cone beam X-ray CT apparatus 1a mounted on a mobile X-ray apparatus to which the present invention is applied.

  A cone beam X-ray CT apparatus 1 shown in FIG. 1 irradiates a subject 2 with X-rays and captures an X-ray transmission image 111 of the subject 2 and each component of the imaging unit 10 And a control arithmetic unit 20 that controls and reconstructs a three-dimensional CT image of the subject 2 based on the X-ray transmission image 111. In addition, a display device 80 that displays an image and an information input device 70 that includes a mouse, a keyboard, a trackball, or the like for inputting the position and parameters of the image displayed on the display device 80 are provided.

  The C-arm type cone-beam X-ray CT apparatus 1a installed in the mobile X-ray apparatus shown in Fig. 2 controls the imaging unit 10a and each component of the imaging unit 10a, and reconstructs a three-dimensional CT image. A control arithmetic unit 20a. The cone beam X-ray CT apparatus 1a is equipped with wheels 5 so that it can move between the examination room and the operating room.

  Incidentally, in FIG. 1, the rotation center axis 4 is provided in a direction parallel to the paper surface, and the X-ray source 11 and the two-dimensional X-ray detector 12 are turned around the rotation center axis 4. On the other hand, FIG. 2 is drawn so that the rotation center axis 4 exists in a direction perpendicular to the paper surface, and the X-ray source 11 and the two-dimensional X-ray detector 12 slide and rotate in a plane parallel to the paper surface. In either the cone beam X-ray CT apparatus 1 in FIG. 1 or the cone beam X-ray CT apparatus 1a mounted on the mobile X-ray apparatus in FIG. 2, the X-ray source 11 and the X-ray detector 12 are the subject. If the X-ray CT image can be acquired by rotating around 2, even if the cone beam X-ray CT device 1 in FIG. 1 slides and rotates in a plane parallel to the paper surface, it is mounted on the mobile X-ray device in FIG. 2. The cone-beam X-ray CT apparatus 1a thus made may rotate.

  Hereinafter, each component shown in FIG. 1 will be mainly described, and the components shown in FIG. 2 will be described as necessary.

(Shooting unit 10)
The imaging unit 10 detects a bed 17 on which the subject 2 is lying, an X-ray source 11 that irradiates the subject 2 with X-rays, and an X-ray that is installed opposite to the X-ray source 11 and passes through the subject 2 The two-dimensional X-ray detector 12 that outputs the X-ray transmission image 111, the C-type arm 13 that mechanically supports the X-ray source 11 and the two-dimensional X-ray detector 12, and the C-type arm 13 C-type arm holding body 14, a ceiling support 15 for attaching the C-type arm holding body 14 to the ceiling of the X-ray imaging room, and a ceiling rail 16 for supporting the ceiling support 15 so as to be movable in the front-rear and left-right directions. And an injector 18 for injecting a contrast medium into the subject 2.

  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 to be a cone, a quadrangular pyramid, or a multi-sided pyramid.

  As the two-dimensional X-ray detector 12, for example, a flat panel detector (abbreviated as `` FPD '') in which thin film transistor (TFT) elements of a plurality of channels are two-dimensionally used is used, but an X-ray image intensifier and A CCD TV camera (abbreviated as “II + TV camera”) may be used. Here, the II + TV camera is formed by an X-ray image intensifier that converts an X-ray transmission image into a visible light image, an optical lens that forms an image of the X-ray image intensifier, and an optical lens. Consists of a combination of a CCD TV camera and the like that captures the visible light image of the X-ray image intensifier. The imaging field of view of the two-dimensional X-ray detector 12 is typically a square shape indicated by FPD or a circular shape indicated by an II + TV camera. However, using a collimator or a density compensation filter, It is good also as the arbitrary shape cut out.

  The C-arm 13 rotates around the rotation center axis 4 at every predetermined projection angle when the subject 2 is imaged. This rotational movement makes it possible to perform X-ray imaging while rotating the circular orbits on substantially the same plane while the X-ray source 11 and the two-dimensional X-ray detector 12 are arranged to face each other. For this rotational movement, there are imaging geometry parameters that are used for the image reconstruction calculation. The imaging geometry parameters include a rotational trajectory plane (midplane) 3 that includes a circular trajectory drawn by the X-ray source 11 as the C-arm 13 rotates and the body of the subject 2 passing through the center of rotation. There is a rotation center axis 4 defined as an axis parallel to the axial direction.

  Further, the two-dimensional X-ray detector 12 is not necessarily installed in parallel to the rotation center axis 4. This is because the mounting angle of the two-dimensional X-ray detector 12 is shifted from the reference angle (0 °).

  The displacement of the mounting angle of the two-dimensional X-ray detector 12 is indicated by imaging geometric parameters. The imaging geometric parameters include the circular orbit of the X-ray source 11 by the rotational movement of the C-type arm 13. It is represented by the reference angle of the mounting angle of the rotation track surface 3 which is a surface, the rotation center axis 4 and the two-dimensional X-ray detector 12.

  Next, a three-dimensional coordinate system is defined based on the rotation center axis 4. In the three-dimensional coordinate system, the direction of the rotation center axis 4 is the Z axis, the direction orthogonal to the Z axis and parallel to the horizontal plane of the bed 17 is the X axis, and the direction orthogonal to the Z axis and the X axis is the Y axis. The rotation center is defined as the intersection of the rotation track surface 3 and the rotation center axis 4.

Then, as shown in FIG. 3, an angle formed by the rotational shooting direction and the Y axis is defined as a projection angle 24 (β). Different geometric parameters are used for multiple, here two, rotational shots. The projection data in the vertical direction of the drawing is β ₀ , the oblique direction is β ₀ + 45 °, and the horizontal direction β ₀ + 90 ° is shown. The projection data of the two rotational shootings cannot be combined with different geometric system parameters.

Therefore, the fan beam angle (XY plane) direction, two of the center of rotation _{(C 1 x, C 1 y} ), (C 2 x, C 2 y) parallel projection image centered at the midpoint (Cx, Cy) of Convert to 26A, 26B, and 26C and match the geometric system parameters of parallel projection images 26A, 26B, and 26C, then synthesize parallel projection images 26A, 26B, and 26C, and overlap parallel projection images 26A, 26B, and 26C A region is formed. The overlapping region of the parallel projection images varies depending on the projection angle. When β ₀ , the overlapping region is small, and when β ₀ + 90 °, the two parallel projection images completely overlap.

The reconstruction field of view of the X-ray CT image includes a circular area centered on the rotation center of the first imaging (C ₁ x, C ₁ y, C ₁ z) and the rotation center of the second imaging (C ₂ x, This is a region surrounded by a circular region centered at C ₂ y, C ₂ z) and a common tangent line connecting the two circular regions.

  Here, in order to simplify the explanation, the cone beam angle (Z axis) direction is not converted.

  On the other hand, since the two-dimensional X-ray detector 12 is configured by arranging TFT elements of multiple channels in a two-dimensional direction, those two-dimensional directions are defined with 27 (u) and 28 (v) as axes. To do. The distance between the TFT elements of the two-dimensional detector is Δu on the u axis and Δv on the v axis. The X-ray transmission image 111 or projection data 211 of rotational imaging is determined by two-dimensional coordinates (u, v) and a projection angle 24 (β).

(Control calculation unit 20)
The control calculation unit 20 includes an imaging unit control unit 100 that controls the imaging unit 10, an image collection unit 110 that collects and stores the X-ray transmission image 111 output by the imaging unit 10, and the collected X-ray transmission image 111. An image reconstruction unit 200 that reconstructs a 3D CT image based on the image, a rotation center position input unit 310, a rotation geometry calculation unit 320, and an image display unit that displays a 3D CT image generated by the reconstruction unit 200 280.

(Shooting unit control unit 100)
The imaging unit control unit 100 controls the position of the C-type arm 13 on the ceiling rail 16 and the imaging system rotation control unit 101 that controls the rotational movement of the C-arm 13 around the rotation center axis 4 to control the C-type arm 13. An imaging system position control unit 102 that two-dimensionally controls the position of the mold arm 13 with respect to the subject 2, an X-ray irradiation control unit 103 that controls ON / OFF of a tube current flowing through the X-ray tube 11t, and an injector 18 Injector control unit 104 that controls the injection amount and injection timing of contrast medium to be injected into subject 2, bed control unit 105 for adjusting the position of subject 2 by controlling the position of bed 17, and two-dimensional And a detection system control unit 107 that controls imaging of the X-ray transmission image 111 by the X-ray detector 12.

  The imaging system position control unit 102 mounted on the mobile X-ray device C-arm type cone beam X-ray CT device 1a moves the position of the C-type arm 13 three-dimensionally by a fixed width in the front-back, left-right, and vertical directions. It can be done. In addition, as described above, the rotation direction of the C-arm 13 has the rotation center axis 4 in a direction parallel to the paper surface, and the X-ray source 11 and the two-dimensional X-ray detector 12 rotate around the rotation center axis 4. (Fig. 1), the rotation center axis 4 exists in a direction perpendicular to the paper surface, and the X-ray source 11 and the two-dimensional X-ray detector 12 slide and rotate in a plane parallel to the paper surface. (FIG. 2) or both of them may be provided.

(Image reconstruction unit 200)
The image reconstruction unit 200 includes a preprocessing unit 210, a filtering unit 250, a back projection unit 260, a parallel projection image generation unit 220, and a projection image synthesis unit 230.

  The pre-processing unit 210 converts the X-ray transmission image 111 collected by the image collecting unit 110 into an X-ray absorption coefficient distribution image (hereinafter referred to as “projection data 211”).

  In the present embodiment, first, logarithmic conversion 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 the X-ray transmission image 111 taken with the subject 2 placed on the bed 17. Then, the projection data 211 of the subject 2 and the bed 17 is obtained by taking the difference between the X-ray transmission images subjected to the two natural logarithmic transformation operations.

  The parallel projection image generation unit 220 and the projection image synthesis unit 230 are features that characterize the present invention. First, the rotation geometry calculation unit 320 calculates projection data obtained by two rotational shootings using the parallel projection image generation unit 220. Convert to composite parallel projection geometry. Next, the projection image synthesizing unit 230 synthesizes projection data having two different rotation centers.

  The filtering unit 250 performs filtering processing in X-ray CT image reconstruction on the combined projection image 237 combined by the projection image combining unit 230.

  The backprojection unit 260 generates a three-dimensional CT image 270 by using, for example, a Feldkamp reconstruction method for the filtered composite projection image. The Feldkamp reconstruction method is a cone beam image reconstruction method and is disclosed in Non-Patent Document 1, for example.

(Rotation center position input unit 310)
The rotation center position input unit 310 inputs the rotation center positions of the two rotational shootings from the information input device 70. Here, the rotation center position causes a position error of several mm between the input rotation center position and the actual rotation center position. The parallel projection image alignment process for compensating for this position error will be described in detail in step S231 of FIG.

(Rotational geometry calculator 320)
The rotational geometry calculation unit 320 includes a geometric system of the projection data 211 (rotation center (C ₁ x, C ₁ y, C ₁ z)) and a geometric system of the projection data 212 (rotation center (C ₂ x, C ₂ From y, C ₂ z)), the rotation center (Cx, Cy, Cz), projection angle β, and distance from the rotation center of the combined parallel projection image defined as follows are set.

Rotation center: (Cx, Cy, Cz) = ((C 1 x + C 2 x) / 2, (C 1 y + C 2 y) / 2, (C 1 z + C 2 z) / 2)
Projection angle: β ₀ −90 ° ≦ β ≦ β ₀ + 90 °
Distance from rotation center (Cx, Cy, Cz): XY plane direction R, Z direction v
Next, various setting conditions of the cone beam X-ray CT apparatus 1 will be described as follows.

  The distance between the X-ray source 11 and the rotation center axis 4 is 800 mm, the distance between the rotation center axis 4 and the X-ray incident surface of the two-dimensional X-ray detector 12 is 400 mm, and the X-ray incident surface of the two-dimensional X-ray detector 12 Is 256 mm × 256 mm, the TFT elements are arranged in 1024 × 1024, and the element interval is 0.25 mm. The X-ray source 11 irradiates the subject 2 with X-rays. X-rays from the X-ray source 11 are incident on the X-ray incident surface of the two-dimensional X-ray detector 12. X-rays from the X-ray source 11 include transmitted X-rays that have passed through the subject 2 and direct X-rays that directly reach the X-ray incident surface of the two-dimensional X-ray detector 12, and the two-dimensional X-ray detector 12 includes Both transmitted X-rays and direct X-rays are incident. The two-dimensional X-ray detector 12 is formed by laminating an X-ray fluorescent layer, a photodiode layer, and a charge storage layer in order from the X-ray incident direction. An X-ray fluorescent layer (such as CsI) converts X-rays into light. The photodiode layer converts the converted light into an electric charge. A charge storage layer (such as a capacitor) stores the converted charge. The accumulated electric charges are read out at a constant frame rate.

  In rotation shooting mode, 1024 x 1024 TFT elements are not read out, and in order to save time, 2 x 2 elements are output together as a single pixel, resulting in an image size of 512 x 512, a pixel pitch of 0.5 mm, and every second. The X-ray transmission image 111 is read out in 30 frames.

  The imaging system rotation control unit 101 passes the two-dimensional X-ray detector 12 from the direction of the left hand of the subject 2 lying on the bed 17 (-100 degrees) to the ceiling direction (0 degrees), Rotate it to the right (+100 degrees). This rotational movement indicates that projection data having an angle of 200 degrees can be acquired. Here, the X-ray transmission image 111 of the subject 2 is imaged for 200 degrees to obtain projection data that is the basis of the X-ray CT image.

  Further, the scan time of the X-ray CT apparatus depends on the rotational speed of the C-arm 13, and a typical example is 40 degrees per second, and the scan time is, for example, 5 seconds.

  First, the imaging system rotation control unit 101 starts the turning of the C-arm 13 around the rotation center axis 4.

The X-ray irradiation control unit 103 irradiates X-rays from the X-ray tube 11t after the rotation speed becomes constant after the rotation acceleration period of the C-shaped arm 13, and the detection system control unit 107 is a two-dimensional X-ray detector. 12 starts imaging.
X-rays irradiated from the X-ray tube 11t pass through the subject 2, and are then detected by the two-dimensional X-ray detector 12 and converted into electrical signals. The electrical signal from the two-dimensional X-ray detector 12 undergoes A / D conversion (not shown), and is then collected by the image collection unit 110 as an X-ray transmission image 111 composed of a digital signal. The two-dimensional X-ray detector 12 collects 150 X-ray transmission images 111 in 5 seconds at a projection angle interval of 1.33 degrees in rotational imaging at 30 frames per second. 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 image reconstruction unit 200 reads the X-ray transmission image 111 from the image acquisition unit 110 during the rotation imaging operation or immediately after the end of the rotation imaging, performs a reconstruction operation on the X-ray transmission image 111, and performs 3 of the subject 2. Generate a dimensional CT image. The image display unit 280 displays a three-dimensional CT image on a display device 80 including a CRT device, a liquid crystal display device, or the like. The image display unit 280 is also used to display the X-ray transmission image 111 stored in the image collection unit 110.

  The image display unit 80 displays an orthogonal cross-sectional image, a central projection image, and a three-dimensional CT image on the display device 80. The image display unit 280 is also used to display the X-ray transmission image 111 stored in the image collection unit 110.

  FIG. 4 is a conceptual diagram for explaining the combined display of CT images of multiple imaging according to the present invention.

  Here, the projection data projected onto the two-dimensional X-ray detector 12a from the rotation center 31 and the projection data projected onto the two-dimensional X-ray detector 12b from the rotation center 32 are synthesized.

  First, when synthesizing projection data, the geometrical parameters of the two rotational shootings are different, so projection data cannot be synthesized as it is, so the two rotation centers 31, 32 are taken in the fan beam angle (XY plane) direction. A line segment 33 passing through is drawn, converted into a parallel projection image centered on the midpoint 34 of the two rotation centers 31 and 32 on the line segment 33, and after combining the geometric system, the projection data is synthesized.

  Patent Document 2 discloses a method of converting a cone beam into a parallel beam in an X-ray beam.

  Then, an overlapping area between the parallel projection images for a plurality of rotations is calculated, and a parallel projection image for a plurality of rotations is synthesized based on the rotation center position set for the overlapping area and the rotation center position in actual photographing. The protrusion of the specimen 2 from the visual field of the two-dimensional detector 12 is eliminated, and an X-ray CT image with an accurate CT value can be generated.

  Next, details of each embodiment of the present invention will be described below.

  FIG. 5 is a flowchart showing the operation of the first embodiment of the present invention, FIG. 6 is a flowchart for explaining the photographing process of FIG. 5, and FIG. 7 is a flowchart for explaining the operation of generating a parallel projection image.

  Details of the processing will be described below with reference to FIGS.

(Step S310)
The operator uses the information input device 70 to input the rotation center (C ₁ x, C ₁ y, C ₁ z) of the first shooting and the rotation center (C ₂ of the second shooting) to the rotation center position input unit 310. x, C ₂ y, C ₂ z). The rotational geometry calculation unit 320 outputs the rotation center (C ₁ x, C ₁ y, C ₁ z) of the first shooting to the rotation center position input unit 310 and the rotation center (C ₂ x, C ₂ y, C ₂ z), and the rotation center (C ₁ x, C ₁ y, C ₁ z) of the first shooting and the rotation center (C ₂ x, C ₂ z) of the second shooting Store y, C ₂ z).

(Step S320)
The rotational geometry calculation unit 320 determines whether there are overlapping regions in the two X-ray CT images when the reconstruction calculation is separately performed from the first rotational imaging data and the second rotational imaging data.

  Specifically, if there is an overlapping area in the X-ray CT image, the projection data also has an overlapping area. If there is no overlapping area in the X-ray CT image, it is determined that there is no overlapping area in the projection data.

  Here, the distance between the X-ray source 11 and the rotation center axis 4 is SO, and the distance between the X-ray source 11 and the two-dimensional X-ray detector 12 is

If the effective visual field in the horizontal direction of the SD and two-dimensional X-ray detector 12 is 2R, the radius r is expressed by {Equation 1}.

X線CT像の再構成視野は半径ｒの球形または円柱形状となる。ステップS310で入力した、第1の撮影の回転中心と第2の撮影の回転中心の、XY平面における距離Lは｛数2｝で表される。 {Equation 1}

The reconstruction visual field of the X-ray CT image has a spherical or cylindrical shape with a radius r. The distance L in the XY plane between the rotation center of the first shooting and the rotation center of the second shooting input in step S310 is expressed by {Equation 2}.

ここで、｛数3｝のような関係式になれば重複領域が存在しない(Nｏ)のでステップS310に戻り、回転中心位置を再入力することになる。 {Equation 2}

Here, if a relational expression such as {Equation 3} is obtained, there is no overlapping region (No), so the process returns to step S310 and the rotation center position is re-input.

重複領域が存在する(Yes)の場合、回転幾何学計算部320は、合成平行投影像の幾何学(回転中心(Cx=(C₁x＋C₂x)/2，Cy=(C₁y＋C₂y)/2，Cz=(C₁z＋C₂z)/2)と、投影角度β₀が｛数4｝のとおり計算される。ステップS331に進む。 {Equation 3}

When the overlapping region exists (Yes), the rotational geometry calculation unit 320 calculates the geometry of the synthesized parallel projection image (rotation center (Cx = (C ₁ x + C ₂ x) / 2, Cy = (C ₁ y + C ₂ y ) / 2, Cz = (C ₁ z + C ₂ z) / 2), and the projection angle β ₀ is calculated as {Equation 4}, and the process proceeds to step S331.

(β₀−90°≦β≦β₀＋90°)
(ステップS331)
制御演算部20は、第1の撮影の回転中心において第1の回転撮影を行う。画像収集部110が収集したX線透過像111は、前処理手段210により前処理が施され、第1の投影データ211を生成する。 {Equation 4}

(β ₀ −90 ° ≦ β ≦ β ₀ + 90 °)
(Step S331)
The control calculation unit 20 performs the first rotation shooting at the rotation center of the first shooting. The X-ray transmission image 111 collected by the image collection unit 110 is pre-processed by the pre-processing unit 210 to generate first projection data 211.

(Step S332)
The control calculation unit 20 performs the second rotation shooting at the rotation center of the second shooting. The X-ray transmission image 111 collected by the image collection unit 110 is pre-processed by the pre-processing unit 210 to generate second projection data 212.

(Step S220)
The parallel projection image conversion unit 220 sets a parallel projection angle β for generating a composite parallel projection image. The parallel projection angle β is calculated in step S320: an angle range of β ₀ −90 ° ≦ β ≦ β ₀ + 90 ° is taken and is changed at an equal angular interval Δβ. For example, a value obtained by dividing 180 ° by the number of shots is used as Δβ.

(Step S221)
The parallel projection image conversion unit 220 converts the first projection data 211 output from the first imaging process (step S331) into the geometry of the composite parallel projection image, and the parallel projection image at the first parallel projection angle β. (221p) is generated. Parallel projection conversion is performed from the subroutine shown in FIG.

(Step S225)
The pixel on the combined parallel projection image is defined as (R, v). R is the distance from the rotation center (Cx, Cy, Cz) of the combined parallel projection image in the detector surface left-right direction (XY plane), and v is the detector vertical direction (Z-axis) distance. The parallel projection image conversion unit 220 performs an operation of drawing a straight line from the pixel (R, v) on the combined parallel projection image in the direction of the parallel projection angle β set in step S220.

(Step S226)
The parallel projection image conversion unit 220 calculates whether the pixel has an intersection between (R, v) and the reconstruction area 1 of the first projection data 211. If there is an intersection (YES), the next step S227 is performed. Proceed to If there is no intersection (NO), the process proceeds to step S228.

(Step S227)
The parallel projection image conversion unit 220 reads from the two-dimensional X-ray detector 12 the corresponding projection values defined by the first projection data 211 (projection angle β ₁ and detector coordinates (u ₁ , v ₁ )). .

(Step S228)
The parallel projection image conversion unit 220 checks whether or not the intersections in step S226 have been calculated for all the parallel projection image pixels (R, v) for the current parallel projection angle β, and all the parallel projection image pixels ( If (NO) is not calculated for R, v), the processing from step S225 to step S227 is repeated. In the case of (YES) calculated for the pixels (R, v) of all the parallel projection images, the parallel projection image (221p) at the first parallel projection angle β is output, and the process proceeds to step S222.

(Step S222)
The parallel projection image conversion unit 220 generates a parallel projection image (222p) at the second parallel projection angle β from the second projection data 212 output by the second imaging process (step S332).

(Step S231)
The parallel projection image conversion unit 220 has an accuracy of a fraction of the detector pitch (about 0.1 mm) in the Δu direction (lateral direction) and a detector pitch (0.5 mm) in the Δv direction (vertical direction). Align it.

  The rotation center position input unit 310 corrects the error of the input rotation center position using the correlation between the overlapping regions of the first parallel projection image (221p) and the second parallel projection image (222p). In the area for calculating the correlation value, a horizontally long ROI is taken in the vicinity of the rotation track surface (midplane) 3 in order to suppress the influence of the cone angle (Z-axis) direction. The second parallel projection image is shifted by a fraction of the detector pitch (about 0.1 mm) in the Δu direction (lateral direction) and by the detector pitch (about 0.5 mm) in the Δv direction (vertical direction). While moving in parallel, a correlation value with the first parallel projection image is obtained, and (ΔR, Δv) having the largest correlation value is calculated. The expression for calculating the correlation value uses {Equation 5}.

(ステップS233)
投影像合成部230は、ステップS231が計算した補正量(ΔR，Δv)を用いて、第1の平行投影像(221p)と第2の平行投影像(222p)を合成し、平行投影角度βの合成投影像233を生成する。 {Equation 5}

(Step S233)
The projection image synthesis unit 230 synthesizes the first parallel projection image (221p) and the second parallel projection image (222p) using the correction amounts (ΔR, Δv) calculated in step S231, and produces a parallel projection angle β A composite projection image 233 is generated.

(Step S250)
The filtering unit 250 performs a filtering process on the composite projection image 233 having the parallel projection angle β.

(Step S260)
The back projection means 260 performs back projection processing of the combined projection image with the parallel projection angle β subjected to the filtering processing.

(Step S270)
The image reconstruction unit 200 confirms whether or not all parallel projection angles β have been processed, and if not processed for all parallel projection angles β (NO), the parallel projection angle β is incremented by Δβ, and step S220 ~ Repeat the process of step S260. If all parallel projection angles β have been processed (YES), the process proceeds to step S280.

(Step S280)
The image reconstruction unit 200 converts the reconstructed image back-projected for all parallel projection angles β into CT values defined as so-called Hounsfield values, and outputs an X-ray CT image.

  As described above, according to the first embodiment, a plurality of rotation centers for shooting are set, projection data shot at a plurality of rotation centers are converted into parallel projection images, and the parallel projection images for a plurality of rotations are combined. Since the parallel projection images for multiple rotations are synthesized based on the set rotation center position and the actual rotation center position for the overlap area, the overlapping area is calculated from the end of the two-dimensional X-ray detector 12. When reconstructing an X-ray CT image using the projection data of the protruding part, when the part of the specimen protrudes, the projection data of the protruding part is obtained from the parallel projection image from the projection data of the part that does not protrude Therefore, an accurate CT value can be obtained even if the subject protrudes from the field of view of the X-ray detector.

  In addition, the unique effect of the first embodiment is that the radius of the effective field of view of the two-dimensional X-ray detector 12 and the distance L between the two rotation centers are determined in the first step. Can be guided to perform the setting operation.

  In the second embodiment, a case will be described in which the output level of the X-ray transmission image 111 changes in two rotation imaging.

  In particular, in the first imaging, the environmental temperature of the X-ray tube 11t and the acquisition circuit of the X-ray detector 12 may be in a low state, and the X-ray is compared with the case where the temperature is acquired in a stable state. In some cases, the output value of the transmission image 111 becomes small, and the output levels of the two differ.

  In addition, due to the influence of the radiation angle of the X-ray source 11 incident on the junction pixel of the two projection data being different (so-called heel effect), a difference occurs in the value of the junction of the projection data, and a false image of the X-ray CT image It may cause.

  FIG. 8 is a flowchart showing the operation of the second exemplary embodiment of the present invention. The difference between the second embodiment and the first embodiment is that step S232 is added between step S231 and step S233.

と

を使用する。
各ROI平均値の中間値を求め、第1の平行投影像(221p)を各ROI平均値の加算値を前者のROI平均値の2倍で除した数を乗じ、第2の平行投影像(222p)を各ROI平均値の加算値を後者のROI平均値の2倍で除した数を乗じて、2つの投影データの輝度を等しくしてから、ステップS233に進む。
(Step S232)
The parallel projection image conversion unit 220 calculates the ROI average value calculated when calculating the correlation value in (Equation 4) in step S231.

When

Is used .
The intermediate value of each ROI average value is obtained, and the first parallel projection image (221p) is multiplied by the sum of each ROI average value divided by twice the former ROI average value to obtain the second parallel projection image ( 222p) is multiplied by a value obtained by dividing the added value of each ROI average value by twice the latter ROI average value to make the luminances of the two projection data equal, and then the process proceeds to step S233.

  As described above, according to the second embodiment, a plurality of rotation centers for shooting are set, projection data shot at a plurality of rotation centers are converted into parallel projection images, and the parallel projection images corresponding to a plurality of rotations are combined. Since the parallel projection images for multiple rotations are synthesized based on the set rotation center position and the actual rotation center position for the overlap area, the overlapping area is calculated from the end of the two-dimensional X-ray detector 12. When reconstructing an X-ray CT image using the projection data of the protruding part, when the part of the specimen protrudes, the projection data of the protruding part is obtained from the parallel projection image from the projection data of the part that does not protrude Therefore, an accurate CT value can be obtained even if the subject protrudes from the field of view of the X-ray detector.

  In addition, a unique effect of the second embodiment is that an accurate CT value can be obtained even when data is collected in different temperature environments or the subject protrudes. Therefore, it becomes possible to reduce the ineffective X-ray exposure to the subject 2.

  In the third embodiment, a case will be described in which the rotation center axis and the detector tilt angle are shifted in the Z-axis direction in two rotation shootings. The cause of the deviation between the rotation center axis and the detector tilt angle is the mechanical accuracy and mechanical distortion of the C-type arm 13, the C-type arm holding body 14, and the ceiling support body 15. It changes depending on the 3D position and rotation angle. Although the difference in the three-dimensional position of the C-arm 13 between the first rotation and the second rotation is slight, the rotation center axis and detector tilt angle are slightly different, and the CT image is connected. It cannot be said that there is no influence on the occurrence of images and artifacts. Correction of deviation from the Z axis is performed in parallel projection conversion processing.

  In the third embodiment, one or both of the error based on the tilt displacement of the X-ray detector and the error based on the tilt displacement of the rotation center axis are corrected.

FIG. 9 shows a flowchart for explaining processing for generating a parallel projection image according to the third embodiment of the present invention. Advance, as a function of the three-dimensional position and the rotation angle of the C-arm 13, the displacement of the rotation center axis, the direction vector _{(e x, e y, e} z) and the detector tilt angle θ is measured, the table is ready Has been.

(Step S224)
The parallel projection image conversion unit 220 refers to the rotation center axis displacement / detector tilt angle displacement table, and the first imaging rotation center axis direction vector (e _1x , e _1y , e _1z ), detector tilt angle θ _1. Then, the rotation center axis direction vector (e _2x , e _2y , e _2z ) and detector inclination angle θ ₂ of the _second imaging are calculated.

(Step S225)
The parallel projection image conversion unit 220 performs an operation of drawing a straight line from the pixel (R, v) on the combined parallel projection image in the direction of the parallel projection angle β set in step S220.

(Step S226)
The parallel projection image conversion unit 220 calculates whether the pixel (R, v) intersects the reconstruction area of the first projection data 211, and the pixel (R, v) reconstructs the first projection data 211. If there is an intersection with the area (YES), the process proceeds to the next step S227. If the pixel (R, v) has no intersection with the reconstruction area of the first projection data 211 (NO), the process proceeds to step S228.

(Step S227)
The parallel projection image conversion unit 220 includes a rotation center axis direction vector (e _1x , e _1y , e _1z ), a detector plane having a parallel projection angle β, and a unit vector (cos β, −sin β, 0) in the rotation orbit plane direction. Computes the inner product t of The first projection data 211 is rotated by −θ ₁ −arctan (t), and the projection value corresponding to the pixel (R, v) of the combined parallel projection image is read out.

(Step S228)
The parallel projection image conversion unit 220 confirms whether or not all (R, v) have been scanned for the current parallel projection angle β, and in the case of NO, repeats the processing from step S225 to step S227. If YES, the parallel projection image (221p) at the first parallel projection angle β is output, and the process proceeds to step S222.

  As described above, according to the third embodiment, a plurality of rotation centers for shooting are set, projection data shot at a plurality of rotation centers are converted into parallel projection images, and the parallel projection images for a plurality of rotations The overlap projection area is calculated and a parallel projection image for a plurality of rotations is synthesized based on the set rotation center position and the actual rotation center position for the overlap area. When the part protrudes, the projection data of the protruding part is obtained from the parallel projection image from the projection data of the part that does not protrude, so it is accurate when reconstructing the X-ray CT image using the projection data of the protruding part Since the CT value is obtained, an accurate CT value can be obtained even if the subject protrudes from the field of view of the X-ray detector.

  Further, the unique effect of the third embodiment is that, in addition to the calculation of the rotation center position correction amount in the alignment of the parallel projection image in step S231, the calculation of the correlation value with respect to the rotation correction angle of the projection data 211, 222 is added, The accuracy of alignment can be improved.

<Other variations>
The configurations described in the first to third embodiments are examples, and the present invention can be modified as appropriate without departing from the technical idea.

  For example, the present invention can be applied to the synthesis of projection data photographed at three or more different rotation centers. Figures 10 and 11 show the case of applying to the synthesis of projection data taken at three different rotation centers. The reconstruction field of view of the X-ray CT image is surrounded by three circular areas and a common tangent line connecting them. It becomes an area to be. Further, as shown in FIG. 12, it can be applied to synthesis of projection data photographed at four different rotation centers. By applying the present invention, it is possible to obtain an X-ray CT image having a large field of view and an accurate CT value even when a small X-ray detector is used.

  1 Cone beam X-ray CT device, 1a C-arm type cone beam X-ray CT device mounted on a mobile X-ray device, 2 Subject, 3 Rotating orbital surface (midplane), 4 Rotating center axis, 5 Wheels, 10 Imaging unit, 10a Imaging unit of C-arm type cone-beam X-ray CT system 1a mounted on mobile X-ray device, 11 X-ray source, 11t X-ray tube, 11c collimator, 12 2-dimensional X-ray detector, 13 C Type arm, 14 C type arm holder, 15 ceiling support, 16 ceiling rail, 17 bed, 18 injector, 20 control calculation unit, 20a Control of C-arm type cone-beam X-ray CT1a mounted on mobile X-ray device Arithmetic unit, 70 information input device, 80 display device, 100 imaging unit control unit, 100a imaging unit control unit of C-arm type cone beam X-ray CT apparatus 1c mounted on mobile X-ray device, 101 imaging system rotation control unit , 102 Imaging system position control unit, 103 X-ray irradiation control unit, 104 Injector control unit, 105 Sleeper Control unit, 107 detection system control unit, 110 image acquisition unit, 111 X-ray transmission image, 200 image reconstruction unit, 210 preprocessing unit, 211 projection data, 250 filtering unit, 260 back projection unit, 270 3D CT image, 280 Image display unit, 310 rotation center position input unit, 320 rotation geometry calculation unit

Claims (6)

An X-ray source for irradiating the subject with X-rays;
An X-ray detector disposed opposite to the X-ray source and detecting X-rays transmitted through the subject and outputting projection data of the subject;
A rotation unit that rotates the X-ray source and the X-ray detector around the rotation center of the subject, and an image reconstruction calculation unit that reconstructs an X-ray CT image of the subject based on projection data of the subject An X-ray CT apparatus comprising: a display unit that displays the X-ray CT image;
A rotation center setting unit configured to set a rotation center of a plurality of shootings, and the image reconstruction calculation unit includes a parallel projection image conversion unit that converts projection data shot at a plurality of rotation centers into a parallel projection image, and a plurality of rotations A projection image combining unit that calculates an overlap area between the parallel projection images for minutes, and combines the parallel projection images for a plurality of rotations based on the rotation center position set for the overlap area and the rotation center position in actual shooting,
An X-ray CT apparatus comprising:   The projection image synthesis unit calculates an error value between the rotation center position on the setting and the rotation center position in actual photographing, and calculates an error between the rotation center position on the setting and the actual rotation center position based on the error value. The X-ray CT apparatus according to claim 1, wherein the X-ray CT apparatus corrects and synthesizes the parallel projection images for the plurality of rotations.   The X-ray CT apparatus according to claim 1, wherein the projection image synthesizing unit further synthesizes the parallel projection images for the plurality of rotations by matching an output level of the projection data of the overlapping region. The projected image combining unit, X-rays CT apparatus according to what Re one of claims 1 to 3, characterized in that to correct the error based on the inclination displacement of the X-ray detector. The projected image combining unit, X-rays CT apparatus according to what Re one of claims 1 to 4, characterized in that to correct the error based on the inclination displacement of the rotation center axes of rotation. An X-ray source that irradiates the subject with X-rays, an X-ray detector that is disposed opposite to the X-ray source, detects X-rays transmitted through the subject, and outputs projection data of the subject; A rotation unit that rotates the X-ray source and the X-ray detector around the rotation center of the subject; an image reconstruction calculation unit that reconstructs an X-ray CT image of the subject based on projection data of the subject; An X-ray CT apparatus image reconstruction method comprising: a display unit for displaying the X-ray CT image;
A step of setting a plurality of rotation centers of photographing; a step of converting projection data photographed at a plurality of rotation centers into parallel projection images; and a calculation of an overlapping region between the parallel projection images for a plurality of rotations. An image reconstruction method for an X-ray CT apparatus, comprising: combining a rotation center position set for an area and a parallel projection image for a plurality of rotations based on a rotation center position in actual imaging.

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