JP2006271723A - X-ray computed tomography apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the alignment accuracy of an X-ray detector in an X-ray computed tomography apparatus and the alignment method of the X-ray detector. <P>SOLUTION: The X-ray computed tomography apparatus comprises: an X-ray tube 3 for generating X-rays which is supported so as to be freely rotated around a rotating shaft; the X-ray detector 8 having a plurality of detection element columns for detecting the X-rays transmitted through a subject; an image reconstitution part 28 for reconstituting the image of the subject on the basis of the output of the X-ray detector; a display part 32 for displaying the image; and a control part 25 for controlling the X-ray tube and the X-ray detector so that the X-ray tube photographs a transmission image at the respective positions of the top stage and the bottom stage. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an X-ray computed tomography apparatus having a multi-slice X-ray detector.

  The multi-slice X-ray detector is attached to the frame with great care so that the channel direction is orthogonal to the rotation axis. When the direction of the multi-slice X-ray detector is deviated and the channel direction is inclined with respect to the rotation axis, blurring and artifacts occur in the image. This is because the back projection operation is performed on the assumption that the channel direction of the multi-slice X-ray detector is exactly orthogonal to the rotation axis. That is, under the assumption, there is a discrepancy between the path (ray) in the back projection operation and the X-ray path at the time of actual data collection. Due to the mismatch, blur and artifacts occur in the image. This blur and artifact becomes more pronounced at higher resolutions. In other words, the higher the resolution, the higher the mounting accuracy of the multi-slice X-ray detector is required.

  An object of the present invention is to improve the alignment accuracy of an X-ray detector in an X-ray computed tomography apparatus and an alignment method of the X-ray detector.

In the first aspect of the present invention, an X-ray computed tomography apparatus includes an X-ray generator that generates X-rays that are rotatably supported around a rotation axis, and a plurality of detections that detect X-rays transmitted through a subject. An X-ray detector having an element array; a generation unit that generates an image of the subject based on an output of the X-ray detector; a display unit that displays the image; and the X-ray generation unit having a predetermined angle A control unit that controls the X-ray generation unit and the X-ray detector in order to capture a transmission image at each of the opposing angles;
In the second aspect of the present invention, an X-ray detector alignment method for an X-ray computed tomography apparatus includes a metal wire line interposed between an X-ray generator and an X-ray detector having a plurality of detection element arrays. A first transmission image is taken from a predetermined angle, a second transmission image is taken from an angle opposite to the predetermined angle, and the first transmission image and the second transmission image are displayed on the same screen.

  According to the present invention, the alignment accuracy of the X-ray detector can be improved.

  Embodiments of an X-ray computed tomography apparatus according to the present invention will be described below with reference to the drawings. In order to reconstruct one-slice tomographic image data, projection data for about 360 ° around the subject, and projection data for 180 ° + α (α: fan angle) are obtained by the half scan method. Needed. In the present embodiment, a half scan method that is effective for photographing a fast moving heart or the like is employed. In addition, the mechanism for converting incident X-rays into electric charges is based on an indirect conversion type in which X-rays are converted into light by a phosphor such as a scintillator and the light is further converted into electric charges by a photoelectric conversion element such as a photodiode. The generation of electron-hole pairs in semiconductors and their transfer to the electrode, that is, the direct conversion type utilizing a photoconductive phenomenon, is the mainstream. Any of these methods may be employed as the X-ray detection element, but here, the former indirect conversion type will be described. In recent years, the so-called multi-tube X-ray computed tomography apparatus in which a plurality of pairs of an X-ray tube and an X-ray detector are mounted on a rotating ring has been commercialized, and the development of peripheral technologies has progressed. Yes. The present invention can be applied to both a conventional single-tube X-ray computed tomography apparatus and a multi-tube X-ray computed tomography apparatus. Here, a single tube type will be described.

  FIG. 1 shows the internal configuration of the gantry of the X-ray computed tomography apparatus according to this embodiment. FIG. 2A shows a side structure of the gantry device, and FIG. 2B shows a front structure of the gantry device. The gantry device 1 is provided with an annular rotating frame 2 that is rotatably supported around a rotation center axis RA (Z axis). The vicinity of the center of the rotating frame 2 is opened to secure a photographing area. An X-ray tube 101 is mounted on the rotating frame 2.

  Between the X-ray tube 101 and the imaging region, a wedge filter 4 for reducing the low energy component of X-rays and shaping the X-ray intensity distribution, and an X-ray for shaping the X-rays into a cone beam here. A line collimator 5 is arranged. The wedge filter 4 and the X-ray collimator 5 are mounted on the rotating frame 2 via the mounting base 6. A multi-slice type (multi-row type) X-ray detector 8 is arranged with respect to the X-ray tube 101 via an imaging region. The multi-slice X-ray detector 8 includes a plurality of X-ray detection elements each having a square light receiving surface of 0.5 mm × 0.5 mm, for example. For example, 916 X-ray detection elements are arranged in the channel direction (approximate to the X axis). For example, 64 rows are arranged in parallel in the slice direction (Z-axis). A data acquisition device 9 generally called a DAS (data acquisition system) converts a signal output from the detector 8 for each channel into a voltage signal, amplifies it, and further converts it into a digital signal. The X-ray detector 8 is mounted on the mounting frame 7 together with the data collection device 9. The mounting frame 7 is aligned with respect to the X-axis and Z-axis directions by positioning pins 11 and 13, and fixed to the rotating frame 2 by fixing screws 10, 12, 14, and 15.

  FIG. 2 shows functional blocks of the X-ray computed tomography apparatus according to this embodiment. In addition to the above-described X-ray tube 3 and the like, the gantry device 1 is provided with a high-pressure generator 16, an aperture driving device 17, a rotation driving device 18, and a gantry controller 19. The high voltage generator 16 applies a tube voltage (high voltage) between the cathode and anode of the X-ray tube 3 and supplies a filament current to the filament of the X-ray tube 3 under the control of the gantry controller 19. X-rays are generated from the focal point of the X-ray tube 3 by applying a tube voltage and supplying a filament current. The aperture driving device 17 changes the opening of the X-ray collimator 5 according to the control of the gantry control unit 19. The rotation drive device 18 drives the rotation frame 2 to rotate according to the control of the gantry control unit 19. The couch device 21 is movably supported along the Z axis (body axis) for placing the subject P, and the couchtop 22 is moved and driven in accordance with the control of the gantry controller 19. A top plate driving unit 35.

  The preprocessing unit 26 of the operation console 23 performs correction processing such as sensitivity correction on the data (raw data) output from the data collection device 9. The preprocessed raw data is generally referred to as projection data. The projection data is associated with a view angle representing the rotation angle of the X-ray tube 3, a channel number, a column number, and each code representing the position of the top plate 22, and is stored in the projection data storage unit 27. The operation console 23, together with the preprocessing unit 26 and the projection data storage unit 27, reconstructs the tomographic image data, for example, by a half reconstruction method based on the input device 24, the console control unit 25, and the stored projection data. The processing unit 28, the image storage unit 29 that stores the reconstructed tomographic image data, the image processing unit 31 that generates, for example, MPR image (arbitrary slice conversion image) data from the stored tomographic image data, the tomographic image data and the MPR image And the like. The input device 24 is provided with an alignment mode switch. When the alignment mode switch is pressed, an alignment program for causing the console control unit 25 as a computer to perform an alignment operation described later is started. Accordingly, the console control unit 25 performs a control operation under an alignment mode for supporting the alignment of the X-ray detector 8. Note that the alignment program may be stored in a computer-readable storage medium.

  FIG. 3 shows an alignment operation of the X-ray detector 8 by the console control unit 25. The alignment of the X-ray detector 8 is not constantly performed, but is performed at the time of product shipment, installation, periodic inspection, and the like. As shown in FIG. 4, it is assumed that the X-ray detector 8 initially has its center line inclined with respect to the Z axis (rotation axis RA). In other words, the X-ray detector 8 initially assumes that the center line intersects the rotation plane (XY plane). The alignment of the X-ray detector 8 is to align the X-ray detector 8 so that the center line of the X-ray detector 8 is orthogonal to the Z axis (rotation axis RA).

  As a preparatory work for alignment of the X-ray detector 8, the console control unit 25 places, for example, a lead alloy metal wire 42 having X-ray absorption at a predetermined position on the top plate 22 as shown in FIG. 5. It is guided through the display device 32 that it is placed in a direction substantially perpendicular to the Z axis. The operator arranges the wire 42 at a predetermined position on the top plate 22 in a direction substantially orthogonal to the Z axis according to the guide. The top plate 22 is formed with an auxiliary line 41 for assisting the position and orientation of the wire 42. At this stage, the wire 42 does not have to be completely exactly perpendicular to the Z axis. When the completion of installation of the wire 42 is input via the input device 24, the console control unit 25 moves the top plate 22 to the reference position. As a result, the wire 42 is positioned substantially at the center of the cone beam X-ray.

  Subsequently, the console control unit 25 controls the rotation driving device 18 to rotate the rotating frame 2 and set the X-ray tube 3 to a predetermined position as shown in FIGS. 6 (a) and 6 (b). Typically, the X-ray detector 8 is disposed at a position on the uppermost stage as a position shifted by 180 degrees from the predetermined position, typically at a position on the lowermost stage. This arrangement is referred to as a top view. One shot is taken under the control of the console control unit 25 in this top view (S1). That is, the filament current is supplied from the high voltage generator 16 to the X-ray tube 3 and the tube voltage is applied for a short time. Thereby, a pulse X-ray is generated and detected by the X-ray detector 8. The X-ray transmission image (top view image) data is stored in the image storage unit 29.

  Similarly, the console control unit 25 controls the rotation driving device 18 to rotate the rotating frame 2 and move the X-ray tube 3 to the position of S1 as shown in FIGS. 7 (a) and 7 (b). The X-ray detector 8 is typically disposed at the lowermost position as a position shifted by 180 degrees from the uppermost position. This arrangement is referred to as a bottom view. In this bottom view, one-shot shooting is performed under the control of the console control unit 25 (S2). The X-ray transmission image (bottom view image) data is stored in the image storage unit 29.

  FIG. 8 shows the arrangement of the detector 8 and the wire 20 viewed from the top view and the bottom view in the initial state of FIG. The console control unit 25 causes the display device 32 to display the top view image and the bottom view image simultaneously in parallel. Due to the displacement of the detector 8, as shown in FIG. 9A, on the top view image, the wire image is not parallel to the image center line (X axis) but is inclined. This angle is defined as an angle α1. Similarly, as shown in FIG. 9B, on the bottom view image, the wire image is inclined at an angle α2 with respect to the image center line (X axis).

  Under the control of the console control unit 25, a message for inquiring whether or not the angle α1 and the angle α2 are equal is displayed on the display device 32. In addition, it is not left to the operator to determine whether or not the angle α1 and the angle α2 are equal, but the wire image is extracted from the transmission image by image processing and the angles α1 and α2 are automatically calculated to obtain the angle α1 and the angle α2. It may be automatically determined whether or not α2 is equal. That is, the console control unit 25 specifies an approximate straight line that is close to the wire image from the top view image, for example, by binarization processing using a threshold value as image recognition processing, and calculates an inclination angle α1 of the approximate straight line with respect to the horizontal axis (X axis). To do. Similarly, the console control unit 25 specifies an approximate straight line that is close to the wire image from the bottom view image, for example, by binarization processing using a threshold value as image recognition processing, and sets the inclination angle α2 of the approximate straight line with respect to the horizontal axis (X axis) of the image. calculate. The calculated angles α1 and α2 are numerically displayed on the display device 32 under the control of the console control unit 25.

  Further, it is possible to manually perform the operation until the operator aligns the guide line with the wire image and to automatically determine whether the angle α1 and the angle α2 are equal.

  The difference between the angle α1 and the angle α2 means that the wire 20 is not exactly orthogonal to the Z axis, in other words, the wire 20 is inclined with respect to the XY plane. Yes. The operator slightly adjusts the direction of the wire 20 so that the angle α1 and the angle α2 are equivalent, and then presses a “retry” button on the display screen. Accordingly, the processes of S1, S2, and S3 are executed again under the control of the console control unit 25. The processes of S1, S2, S3, and S4 are repeated until the angle α1 and the angle α2 are equivalent.

  After the wire 20 serving as a reference for alignment of the detector 8 is set to be orthogonal to the Z axis, the alignment operation of the detector 8 is actually started.

  Adjustment of the mounting position of the X-ray detector 8 is performed by a shim 30 as a thin metal adjustment plate having a thickness of 0.5 mm, for example, as shown in FIG. 11 (S5). The detector / DAS mounting frame 7 is fixed to the rotating frame 2 with fixing screws 10 and 12 with the shim 30 interposed therebetween. The shim 30 has a U-shaped notch so that the shim 30 is engaged with the fixing screws 10 and 12 so as not to be displaced. An arbitrary number of shims 30 are overlapped and sandwiched only on one of the left fixing screw 10 and the right fixing screw 12, or the number of shims 30 sandwiched between the left fixing screw 10 and the shim 30 sandwiched between the right fixing screws 12. The difference in the direction of the center line of the X-ray detector 8 with respect to the Z axis (rotation axis RA) can be corrected by making the number of the two different. The number of shims 30 sandwiched only between one of the left fixing screw 10 and the right fixing screw 12 or the difference between the number of shims 30 sandwiched between the left fixing screw 10 and the number of shims 30 sandwiched between the right fixing screws 12 is calculated. It can be determined to some extent according to the shift angle α1 (= α2). The number or the difference between the numbers may be determined by the console control unit 25 based on the shift angle α1 (= α2) and displayed on the display device 32.

  In S5, after the shim 30 is sandwiched according to the shift angle α1 (= α2) and the detector / DAS mounting frame 7 is fixed to the rotating frame 2 with the fixing screws 10 and 12, although not shown, as in S1 and S2, Top view shooting and bottom view shooting are performed to obtain a top view image and a bottom view image. The operator visually recognizes the top view image and the bottom view image displayed on the display device 32, and determines whether or not the deviation angle is zero, that is, “α1 = α2 = 0” (S6). When the deviation angle is zero, the wire 20 substantially overlaps the center line of the detector 8 as shown in FIG. Also, as shown in FIGS. 10B and 10C, the wire image substantially overlaps the center line on the top view image and the bottom view image.

  When the angle of deviation between the top view image and the bottom view image displayed on the display device 32 is zero, that is, when “α1 = α2 = 0” is not established, the process returns to S5, and the adjustment of the detector 8 by the shim 30 is performed again. . Finally, the adjustment of the detector 8, the top view shooting, the bottom view shooting, and the determination of S6 by the shim 30 in S5 are repeated until the angle of deviation between the top view image and the bottom view image becomes zero.

  As described above, according to the present embodiment, since the position of the X-ray detector can be objectively adjusted, the alignment accuracy can be improved. If the X-ray detector misalignment is eliminated or reduced, the path (ray) on the back projection operation in the reconstruction process matches the X-ray path at the time of actual data collection. The occurrence of blurring and artifacts caused by money can be suppressed.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The figure which shows the internal structure of the mount part of the X-ray computed tomography apparatus which concerns on embodiment of this invention. 1 is a functional block diagram of an X-ray computed tomography apparatus according to the present embodiment. The figure which shows the operation | movement procedure at the time of the alignment (position alignment) mode of the detector by the console control part of FIG. The figure which shows the initial state of the X-ray detector of FIG. The figure which shows the metal wire set | placed on a top plate for the position alignment of the X-ray detector 8 in this embodiment. FIG. 4 is a supplementary diagram of S1 in FIG. 3. FIG. 4 is a supplementary diagram of S2 of FIG. The figure which shows the positional relationship of the wire and detector in the initial state of FIG. The figure which shows the top view image image | photographed by S1 of FIG. 3, and the bottom view image image | photographed by S2. The figure which shows the top view image and bottom view image after the completion of adjustment of FIG. The figure which shows the shim for alignment adjustment of S5 of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Mount, 2 ... Rotating frame, 3 ... X-ray tube, 4 ... Wedge filter, 5 ... X-ray collimator, 6 ... X-ray collimator mounting plate, 7 ... Detector / DAS mounting frame, 8 ... Multi-slice type X Line detector, 9 ... Data collection device (DAS), 10 ... Fixing screw, 11 ... Positioning pin, 12 ... Fixing screw, 13 ... Positioning pin, 14 ... Fixing screw, 15 ... Positioning pin.

Claims (6)

An X-ray generator that generates X-rays that are rotatably supported around a rotation axis;
An X-ray detector having a plurality of detection element arrays for detecting X-rays transmitted through the subject;
A generating unit that generates an image of the subject based on an output of the X-ray detector;
A display unit for displaying the image;
The X-ray generation unit includes a control unit that controls the X-ray generation unit and the X-ray detector in order to capture a transmission image at a predetermined angle and an opposite angle. Line computed tomography equipment. The control unit causes the display unit to display a transmission image captured when the X-ray generation unit is positioned at a predetermined angle and a transmission image captured when the X-ray generation unit is positioned at an opposing angle. The X-ray computed tomography apparatus according to claim 1. The X-ray computed tomography apparatus according to claim 1, further comprising a structure for adjusting an attachment angle of the X-ray detector with respect to the rotation axis. 4. The X-ray computed tomography apparatus according to claim 3, wherein the structure has a shim sandwiched between a frame for mounting the X-ray detector and the X-ray detector. The transmission image includes an image of a metal wire line, the metal wire line image is extracted from the transmission image at the predetermined angle and the opposite angle, and the metal wire with respect to the horizontal axis of the transmission image. 2. The X-ray computed tomography apparatus according to claim 1, further comprising means for calculating an inclination angle of the metal wire line image of the line image and means for displaying the calculated inclination angle. Taking a first transmission image from a predetermined angle with a metal wire interposed between the X-ray generator and an X-ray detector having a plurality of detection element rows,
Taking a second transmission image from an angle facing the predetermined angle,
An X-ray detector alignment method for an X-ray computed tomography apparatus, wherein the first transmission image and the second transmission image are displayed on the same screen.

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