JP4643332B2 - X-ray computed tomography system - Google Patents

Description

  The present invention relates to an X-ray computed tomography apparatus that rotates an X-ray tube at high speed together with an X-ray detector.

  The rotation of the gantry of the X-ray computed tomography apparatus is steadily increasing. Along with this, the shake of the rotation center has become a problem. As is well known, the components constituting the gantry of the X-ray computed tomography apparatus are very heavy. For example, X-ray tubes, high-voltage generators, and X-ray detectors each have several tens of kilograms. When a rotating frame on which such a heavy object is mounted is rotated at a high speed of, for example, 0.5 seconds or less, the center of rotation is shaken as the rotating frame is unbalanced or deformed.

  The path (ray) in the reconstruction back projection calculation is set on the assumption that the center of rotation is fixed. Therefore, the back projection line (path) in the back projection operation does not coincide with the X-ray path at the time of actual data collection. Due to the mismatch, the resolution of the image decreases. In addition, artifacts may occur in the image.

The reduction in image resolution and the occurrence of artifacts due to this blurring are the biggest obstacles to further increasing the speed of the gantry and further increasing the resolution.
JP 60-126143 A

  An object of the present invention is to reduce image resolution degradation and artifacts caused by shaking due to gantry rotation.

In the first aspect of the present invention, an X-ray tube that generates X-rays supported so as to be rotatable about a rotation axis, and a multi-rotator that rotates together with the X-ray tube for detecting X-rays transmitted through a subject. a channel-type X-ray detector, stores information about the deviation of the center channel of the X-ray detector due to rotational camera shake of the X-ray tube and the X-ray detector for each rotation angle of the previous SL X-ray tube And reconstructing the image of the subject under a back projection reconstruction method based on the output of the X-ray detector, the center channel according to the stored shift of the center channel Are arranged for each rotation angle, and a reconstruction processing unit is provided for setting the path of the back projection reconstruction method accordingly .
In the second aspect of the present invention, a method for determining a shift of a central channel of an X-ray computed tomography apparatus includes a wire disposed substantially parallel to a rotation axis between an X-ray tube and a multi-channel X-ray detector. Scanning, generating a sinogram based on the output of the X-ray detector, approximating the shadow of the wire on the sinogram with a sine curve, and the deviation of the shadow of the wire with respect to the approximated sine curve for each rotation angle Ask.

  According to the present invention, it is possible to reduce image resolution reduction and artifact generation due to shaking of the gantry rotation.

  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 adopted 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 X-ray tubes and X-ray detectors 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 that reduces the low-energy component of X-rays and shapes 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, 1000 X-ray detection elements are arranged in the channel direction (approximate to the X axis). Each channel is identified by a channel number from 1 to 1000. For example, 64 rows are arranged in parallel in the slice direction (Z-axis). That is, the multi-slice X-ray detector 8 is formed by arranging multi-channel detectors along a plurality of Z axes.

  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 number (for example, 1 to 1000) 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 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.

  Further, the operation console 23 includes a deviation calculation unit 33 and a deviation storage unit 34. The deviation calculation unit 33 specifies the actual central channel of the X-ray detector 8 from the sinogram acquired by scanning at least 360 degrees in a state where the wire is installed substantially parallel to the rotation axis RA in the imaging region, The wire shadow on the sinogram is approximated by a sine curve with the identified center channel as the center line, and the deviation between the sine curve and the wire shadow is calculated for each rotation angle (view number). Here, the deviation is expressed by the number of channels. Displacement of back projection line (path) of back projection calculation (back projection calculation) according to this shift eliminates inconsistency between back projection line on back projection calculation and X-ray path at the time of actual data collection. Or reduced. As a result, image resolution degradation and artifacts can be eliminated or reduced. A plurality of data sets relating to a shift of 360 degrees with different rotational speeds of the X-ray tube 3 are stored in the shift storage unit 34.

  The input device 24 is provided with a deviation measurement mode switch. When the deviation measurement mode switch is pressed, a deviation measurement program for causing the console control unit 25 as a computer to perform a deviation measurement operation described later is started. Accordingly, the console control unit 25 performs a control operation under a deviation measurement mode for supporting deviation measurement. Note that the deviation measurement program may be stored in a computer-readable storage medium.

  FIG. 3 shows a deviation measurement operation by the console control unit 25. The deviation measurement is not performed on a regular basis, but is performed at the time of product shipment, installation, periodic inspection, and the like. As a preparatory work for deviation measurement, the console control unit 25 places, for example, a lead alloy metal wire 20 having X-ray absorptivity substantially parallel to the Z axis at a specified position on the top plate 22 as shown in FIG. The placement is guided through the display device 32. The operator places the wire 20 in a prescribed position on the top plate 22 in the YZ plane substantially parallel to the Z axis according to the guide. The top plate 22 is formed with auxiliary lines 37 and 38 for assisting the position and orientation of the wire 20. 5A and 5B, the wire 20 is shown from two directions together with the X-ray tube 3, the X-ray detector 8, and the like. The wire 20 does not need to coincide with the Z axis (rotation center axis RA). The wire 20 may be displaced by an arbitrary distance Δd from the rotation center axis (RA) coinciding with the Z axis, typically in the height direction (Y axis). This contributes to significantly improving the installation work efficiency of the wire 20 together with the auxiliary wires 37 and 38. As will be described later, the sine curves alternate with an amplitude equivalent to this distance Δd.

After the installation of the wire 20 is completed, a scan of at least 360 degrees is performed under the control of the console control unit 25 (S1). This scan is performed using, for example, one or two rows of the multi-slice detectors 8. The projection data for 360 degrees collected by scanning is stored in the projection data storage unit 27 in association with the channel number and the view number representing the rotation angle. The deviation calculation unit 33 generates a sinogram illustrated in FIG. 6A based on the stored 360-degree projection data. As is well known, a sinogram is generated by arranging projection data for 360 degrees with the channel number on the horizontal axis and the view number on the vertical axis. Typically, the deviation calculation unit 33 binarizes the sinogram so that only the shadow N _{WIR (VIEW)} of the wire 20 remains as a trajectory. N _{WIR (VIEW)} represents the channel number where the shadow of the wire 20 exists in each view.

The deviation calculation unit 33 integrates the sinogram along the view number axis or averages the sinogram along the view number axis when not binarizing (S2). Thereby, as illustrated in FIG. 6B, a wire shadow appearance frequency distribution in the channel direction is generated. Deviation calculation unit 33 identifies the channel number N _C corresponding to the center of gravity of the distribution (S3). The channel number N _C is a counterpart to the practical center channel ,, X-ray detector 8, and should not stop the integer is calculated with the precision of decimal point first place.

Next, as shown in FIG. 7, the deviation calculation unit 33 approximates the sine curve N _{SINF (VIEW)} centered on the center channel number Nc to the wire shadow N _{WIR (VIEW)} by, for example, the least square method. (S4). This sine curve N _{SINF (VIEW)} represents the channel number where the shadow of the wire 20 exists in each view when it is assumed that there is no blurring of the rotating frame 2 due to deformation or the like.

Next, the view number N is initialized to 1 (S5), and the deviation N _{dev (N)} between the shadow N _{WIR (N)} of the wire 20 and the sine curve N _{SINF (N) in} order from the view number “1”. The
N _{dev (N)} = N _{SINF (N)} -N _{WIR (N)}
(S6).

Then, the channel number N _C of the actual center channel specified in S3 is shifted according to the deviation N _{dev (N)} (S7). The channel number N _{CREC (N)} of the shifted actual center channel in the view number N is
N _{CREC (N)} = N _C + N _{dev (N)}
Is required.

The variable N is incremented by 1 (S8), and S6 to S8 are repeated until N = 1000 is reached (S9). As a result, the channel number N _{CREC (N)} of the actual central channel is obtained for each view.

The data of the channel number _{NCREC (N)} of the actual central channel of each view obtained in this way is related to the rotational speed data of the X-ray tube 3 at the scan S1 for the _pass measurement. It is stored in the storage unit 34. The data of the channel number N _{CREC (N)} of the actual center channel of each view is measured at various rotation speeds and stored in the deviation storage unit 34.

In the reconstruction processing of the tomographic image in the reconstruction processing unit 28, the channel of the actual central channel of each view corresponding to the rotational speed of the X-ray tube 3 at the time of scanning under the control of the console control unit 34. Data of the number N _{CREC (N)} is provided from the deviation storage unit 34 to the reconstruction processing unit 28. The reconstruction processing unit 28 _arranges the position of the center channel for each view as shown in FIG. 8 according to the channel number N _{CREC (N)} of the actual center channel of each provided view, and backs up accordingly. Set the projection path.

  According to the present embodiment, the discrepancy between the path (ray) on the back projection operation in the reconstruction processing due to the rotation blur and the X-ray path at the time of actual data collection is eliminated or reduced, and the image resulting therefrom The occurrence of blurring and artifacts can be eliminated or 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 apparatus 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 determination procedure of the center channel of the detector by the console control part of FIG. The figure which shows the metal wire put on a top plate for the wire scan of FIG. The figure which shows the positional relationship of the wire of FIG. 4, and rotating shaft RA. The figure which illustrates the sinogram produced | generated by the deviation calculation part of FIG. The figure which shows the shift | offset | difference for every rotation angle calculated by the shift | offset | difference calculation part of FIG. FIG. 3 is a supplementary diagram of reconstruction processing using a shift for each rotation angle calculated by a shift calculation unit in FIG. 2.

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, 16 ... High pressure generator, 17 DESCRIPTION OF SYMBOLS ... Aperture drive device, 18 ... Rotation drive device, 19 ... Pedestal control unit, 21 ... Bed device, 22 ... Top plate, 23 ... Operation console, 26 ... Preprocessing unit, 27 ... Projection data storage unit, 24 ... Input device, DESCRIPTION OF SYMBOLS 25 ... Console control part, 28 ... Reconstruction process part, 29 ... Image memory | storage part, 31 ... Image processing part, 32 ... Display apparatus, 33 ... Deviation calculation part, 34 ... Deviation memory | storage part, 35 ... Top-plate drive part.

Claims (4)

An X-ray tube that generates X-rays supported rotatably around a rotation axis;
A multi-channel X-ray detector rotating together with the X-ray tube for detecting X-rays transmitted through the subject;
A storage unit for storing information about the deviation of the center channel of the X-ray detector due to rotational camera shake of the X-ray tube and the X-ray detector for each rotation angle of the previous SL X-ray tube,
An image of the subject is reconstructed based on a back projection reconstruction method based on an output of the X-ray detector, and the position of the center channel is determined according to the stored shift of the center channel. An X-ray computed tomography apparatus comprising: a reconstruction processing unit that is arranged for each and sets a path of the back-projection reconstruction method in accordance therewith . An X-ray tube that generates X-rays supported rotatably around a rotation axis;
A multi-channel X-ray detector rotating together with the X-ray tube for detecting X-rays transmitted through the subject;
A reconstruction processing unit for reconstructing the image of the subject based on the output of the X-ray detector;
In order to provide the reconstruction processing unit with information on the shift of the center channel of the X-ray detector accompanying the rotational shake of the X-ray tube and the X-ray detector for each rotation angle of the X-ray tube. to, said information relating to deviation of the center channel the X-ray tube rotation angle and stores each combination of rotational speed storage unit and the X-ray computed tomography apparatus you characterized by comprising a. Scan a wire placed between the X-ray tube and the multi-channel X-ray detector substantially parallel to the rotation axis,
Generating a sinogram based on the output of the X-ray detector;
Approximate the shadow of the wire on the sinogram with a sine curve,
A method of determining a shift of a center channel of an X-ray computed tomography apparatus, wherein a shift of a shadow of the wire with respect to the approximated sine curve is obtained for each rotation angle. Generating a one-dimensional distribution for the channel direction of the wire shadow by averaging the sinogram in the view direction;
Identify the position of the center of gravity of the generated one-dimensional distribution;
4. The method for determining a shift of a center channel of an X-ray computed tomography apparatus according to claim 3, wherein the sine curve is generated centering on the specified position of the center of gravity.

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