JP2006255187A - X-ray computed tomography apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To generate a scanogram free from distortion or with little distortion about an X-ray computed tomography apparatus having a function of generating the scanogram. <P>SOLUTION: The X-ray computed tomography apparatus is provided with: an X-ray tube 11 which generates X-rays; a multichannel X-ray detector 13 which detects X-rays having passed through a subject and repeatedly generates projection data; and a scanogram generation part 27 which extracts specific projection data where a corresponding X-ray beam is parallel to or nearly parallel to a specific direction from the projection data to generate scanogram data. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an X-ray computed tomography apparatus having a function of generating a scanogram.

  As known in Patent Documents 1 and 2, as is well known, a scanogram is obtained by fixing an X-ray tube at a specific rotation angle in an X-ray computed tomography apparatus and subjecting the top plate to continuous or intermittent irradiation of X-rays. It is a plane transmission image from a certain direction such as an X-ray image obtained by repeating data collection at a constant period while moving at a constant speed. The scan position and scan range are set using this scanogram, and further, the scanogram itself may be interpreted.

  In practice, as shown in FIG. 11, the scanogram is generated by performing a curved surface / plane conversion process, a high-pass filter process, and a density conversion process on the collected projection data.

In the curved surface / plane conversion processing, as shown in FIG. 12, the coordinate conversion is performed on a straight line passing through the rotation center O along the X-ray beam diffusing radially from the X-ray focal point F. Therefore, as the distance from the rotation center O increases, the difference between the actual position and the position on the scanogram increases and the distortion increases.
JP-A-5-317305 JP-A-5-192327

  An object of the present invention is to produce a scanogram with no or little distortion.

  An X-ray computed tomography apparatus according to the present invention includes an X-ray tube that generates X-rays, a multi-channel X-ray detector that detects X-rays transmitted through a subject and repeatedly generates projection data, and the projection A scanogram generation unit configured to extract specific projection data in which the corresponding X-ray beam is parallel or substantially parallel to the specific direction from the data and generate scanogram data;

  According to the present invention, it is possible to generate a scanogram with no or little distortion.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. As is well known, the X-ray computed tomography apparatus has a rotation / rotation (ROTATE / ROTATE) type in which an X-ray tube and a radiation detector are rotated as one body, and a large number of detection elements in a ring shape. There are various types such as a fixed / rotation (STATIONARY / ROTATE) type in which only the X-ray tube rotates around the subject, and the present invention can be applied to any type. Here, the rotation / rotation type that currently occupies the mainstream will be described. In addition, to reconstruct one slice of tomographic image data, projection data for about 360 ° around the periphery of the subject is required, and projection data for 180 ° + view angle is also required in the half scan method. . The present invention can be applied to any reconstruction method. Here, the half scan method will be described as an example. 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.

  FIG. 1 shows the configuration of the main part of an X-ray computed tomography apparatus according to this embodiment. The X-ray tube 11 is mounted on a ring-shaped rotating frame (not shown) together with the multi-channel X-ray detector 13. The X-ray detector 13 faces the X-ray tube 11 with the imaging region FOV interposed therebetween. The X-ray detector 13 has a plurality of detection elements arranged in an arc shape around the X-ray focal point. The X-ray detector 13 may include a plurality of detection element arrays along the slice direction Z. At the time of imaging, the subject P placed on the couch top 17 is inserted into the imaging area FOV. An upper collimator 15 for limiting the X-ray irradiation field to the subject P is attached to the X-ray tube 11. The upper collimator 15 typically includes four collimator plates having the property of completely shielding XX rays. Two collimator plates arranged across the X axis are provided in order to limit the slice thickness. Two collimator plates arranged across the Z axis are provided to limit the fan angle. Under the control of the gantry control unit 31, the X-ray control unit 12 applies a tube voltage to the X-ray tube 11 and supplies a filament heating current. When the filament is heated by the filament heating current, thermoelectrons are emitted from the filament. Thermal electrons are pulled by the anode and collide with the target. Thereby, X-rays are generated. The collimator controller 16 moves each collimator plate of the upper collimator 15 under the control of the gantry controller 31. Under the control of the gantry control unit 31, the rotation control unit 14 rotates a rotating frame on which the X-ray tube 11 and the X-ray detector 13 are mounted. The couch controller 18 moves the table 17 under the control of the gantry controller 31.

  The data acquisition unit 21 is generally called a DAS (data acquisition system). The data collection unit 21 converts a signal output from the X-ray detector 13 for each channel into a voltage signal, amplifies it, converts it into a digital signal, and outputs data (raw data). The preprocessing unit 22 performs logarithmic conversion processing on the raw data and performs various correction processing such as reference correction and water correction. The storage unit 23 stores the corrected data (projection data) in association with the channel, the angle of the X-ray tube 11 at the time of detection, and the position of the top plate 17 at the time of detection.

  The image reconstruction unit 25 reconstructs tomographic image data based on the projection data collected at the time of scanning and stored in the storage unit 23. The image is displayed on the screen of the display unit 29. The scanogram generation unit 27 extracts specific projection data in which the corresponding X-ray beam is parallel or substantially parallel to a specific direction from the projection data collected at the time of scanogram imaging and stored in the storage unit 23, and the scanogram Generate data. The system control unit 24 is provided to control the operation of the entire apparatus according to scanogram imaging and scanning.

  Next, the operation of this embodiment will be described. As shown in FIG. 2, the scanogram generation process includes data acquisition, parallel beam extraction process, curved surface / plane conversion process, high-pass filtering process, and density conversion process in order.

  In data collection, as shown in FIGS. 3, 4, and 5, the X-ray tube 11 continuously rotates together with the X-ray detector 13 at a constant speed. The X-ray is irradiated continuously or intermittently on the subject limited to a specific angular range during the rotation period, here, a range of α ° centered on zero ° (zenith position), and the projection data is Collected and stored. In principle, the angle range α coincides with the fan angle that accommodates the imaging region FOV.

  While the X-ray tube 11 is moving in the angular range, the top plate 17 is stopped, and during the period in which the X-ray tube 11 is moving in a range other than the angular range, the top plate 17 is detected by X-rays. A distance that is a fraction of the detection element width of the detector 13, a distance equivalent to the detection element width, or a distance that is an integer multiple of 2 or more of the detection element width is moved. Considering the improvement of the scanogram resolution and the reduction of the imaging time, the top plate 17 preferably moves a distance equivalent to the detection element width of the X-ray detector 13. Focusing only on the movement of the top plate 17, the top plate 17 moves intermittently in synchronization with the rotation of the X-ray tube 11. That is, the top plate 17 repeats the movement and stop of the detection element width of the X-ray detector 13.

  As described above, the X-ray tube 11 is irradiated to the subject only during the period in which the X-ray tube 11 is moving within the angular range α. The tube voltage is applied only during the period in which the X-ray is moving, and during the period in which the X-ray tube 11 is moving in a range other than the angle range α, the application of the tube voltage is stopped, This is realized by intermittent generation. However, X-rays may be generated continuously and the same irradiation may be realized by the shutter mechanism.

  During the period in which the X-ray tube 11 is moving within the angle range α, projection data is repeatedly collected for all channels. Here, it is assumed that projection data for all channels collected at the same time, that is, projection data for all channels having the same rotational position of the X-ray tube 11 and the same top plate position, is handled in units of one set. During the period in which the X-ray tube 11 is moving within the angle range α, a plurality of projection data sets are collected with different rotational positions of the X-ray tube 11 and the same top plate position. A plurality of projection data sets are repeatedly collected by continuous rotation and intermittent movement of the top board.

  After the data collection is completed, projection data for a predetermined number of adjacent channels collected as X-ray beams in the center position of the angle range α, in this example, a direction of zero degrees and a direction approximate thereto, is obtained for each projection data set. Extracted by the scanogram generation unit 27. That is, projection data corresponding to a predetermined number of X-ray beams in a specific direction and a direction approximate thereto is extracted from the projection data set for each view for which data has been collected. The same projection data extraction process is repeated in the same way for all the top plate positions.

  Here, the projection data is extracted not only in a specific direction from each projection data set but also in a direction approximate to it, but the projection data is limited to only a specific direction from each projection data set. You may make it extract.

  Projection data in which the corresponding X-ray beams are parallel or substantially parallel to each other is extracted from all projection data collected at the time of scanogram imaging, and based on the extracted projection data, general curved surface / plane conversion processing, high-pass filter processing, and Through the density conversion process, scanogram data is generated.

  By generating a scanogram from projection data of parallel or substantially parallel beams in this way, it is possible to eliminate or reduce the scanogram distortion up to the edge portion as well as the central portion.

  In the above description, data collection is repeated while the top plate 17 is moved intermittently. However, data collection may be repeated while continuously moving the top plate 17 as in helical scanning. In this case, the collected data is rearranged in parallel as shown in FIG. The moving distance of the top plate 17 per rotation of the X-ray tube 11 is set to a distance that is a fraction of the detection element width, a distance equivalent to the detection element width, or a distance that is an integer multiple of 2 or more. . Considering the improvement of the scanogram resolution and the reduction of the imaging time contrary to this, the movement distance of the top plate 17 per rotation of the X-ray tube 11 is preferably set to a distance equivalent to the detection element width.

  In the above description, the opening in the X direction of the upper collimator 15 is fully opened so that the fan angle according to the imaging region FOV is obtained. However, the data actually used for the scanogram is only data of several channels that are the same as or close to a specific direction in the projection data set. Therefore, in order to reduce exposure, the collimator controller 16 may control the collimator 15 in the X direction (opening around the Z axis) to be narrowed down to correspond to the several channels. In that case, the collimator plate of the collimator 15 is dynamically controlled so that the center of the narrow opening moves from the rear end to the front end of the fan angle while the X-ray tube 11 moves in the angle range α.

  Further, as shown in FIG. 7, the opening degree of the upper collimator 15 in the X direction may be adjusted to half of the fan angle α corresponding to the imaging region FOV. In this case, as shown in FIGS. 8 and 9, opposite data is used for the data for one column of the scanogram. In data collection, as shown in FIG. 10, the X-ray tube 11 rotates continuously with the X-ray detector 13 at a constant speed. A specific angle range during the rotation period, here a range from α / 2 ° before zero ° (zenith position) to zero °, and 180 ° (bottom part) with a half-rotation point symmetry The X-ray is irradiated on the subject continuously or intermittently within a range from the angle α / 2 ° before the position) to 180 °, and the projection data is collected and stored.

  In this data collection method, the collimator plate 15 of the collimator 15 is dynamically moved as the X-ray tube 11 is rotated, so that high-precision and high-precision control is not required, and the collimator 15 is opened halfway. It can be fixed and the exposure dose can be substantially reduced to half.

  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.

1 is a configuration diagram of an X-ray computed tomography apparatus according to an embodiment of the present invention. The figure which shows the scanogram production | generation processing procedure of this embodiment. The time chart which shows the projection data collection operation | movement of FIG. FIG. 4 is a supplementary diagram of FIG. 3. FIG. 4 is a supplementary diagram of FIG. 3. The figure which shows the data arrangement | sequence of the scanogram at the time of a helical scan in this embodiment. In this embodiment, the figure which shows the state which opened the opening degree of the collimator for exposure reduction. Supplementary drawing of FIG. The figure which shows the data array of the scanogram corresponding to FIG. The time chart corresponding to FIG. The figure which shows the conventional scanogram production | generation processing procedure. The schematic diagram which shows the generation | occurrence | production principle of the distortion of the conventional scanogram.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... X-ray tube, 12 ... X-ray control part, 13 ... X-ray detector, 14 ... Rotation control part, 15 ... Upper collimator, 16 ... Collimator control part, 17 ... Bed top plate, 18 ... Bed control part, 21 Data collection unit, 22 Pre-processing unit, 23 Data storage unit, 24 System control unit, 25 Image reconstruction unit, 27 Scanogram generation unit, 29 Display unit, 31 Base control unit

Claims (11)

An X-ray tube that generates X-rays;
A multi-channel X-ray detector that detects X-rays transmitted through a subject and repeatedly generates projection data;
An X-ray computer comprising: a scanogram generation unit that extracts specific projection data in which a corresponding X-ray beam is parallel or substantially parallel to a specific direction from the projection data, and generates scanogram data. Tomography equipment.   The X-ray is irradiated on the subject continuously or intermittently limited to a specific angle range corresponding to the specific direction during a period in which the X-ray tube continuously rotates around the subject. The X-ray computed tomography apparatus according to claim 1.   The X-ray computed tomography apparatus according to claim 2, wherein the specific angle range corresponds to a fan angle of the X-ray.   The X-ray computed tomography apparatus according to claim 2, wherein a top plate on which the subject is placed is moved intermittently in synchronization with the specific angle range.   5. The X-ray computed tomography apparatus according to claim 4, wherein the top plate repeatedly moves and stops the detection element width of the X-ray detector.   The X-ray computed tomography apparatus according to claim 2, wherein a top plate on which the subject is placed continuously moves with the rotation of the X-ray tube.   The X-ray computed tomography apparatus according to claim 6, wherein a moving distance of the top plate per rotation of the X-ray tube is set to a detection element width of the X-ray detector.   2. The X-ray computed tomography according to claim 1, further comprising a collimator for limiting an X-ray fan angle from the X-ray tube to the subject to approximately half of a fan angle corresponding to an imaging region. apparatus. An X-ray tube that generates X-rays;
An X-ray detector that detects X-rays transmitted through the subject and repeatedly generates a projection data set;
A scanogram generator for extracting scan data for a predetermined number of channels in which the corresponding X-ray beams are parallel or substantially parallel from each of the projection data sets, and generating scanogram data. X-ray computed tomography apparatus characterized. An X-ray tube that generates X-rays;
An X-ray detector that detects X-rays transmitted through the subject and repeatedly generates a projection data set;
An X-ray computer comprising: a scanogram generator for extracting scangram data by extracting one channel of projection data in which a corresponding X-ray beam is parallel to a specific direction from each of the projection data sets. Tomography equipment.   The X-ray computed tomography apparatus according to claim 9 or 10, wherein the top plate on which the subject is placed moves intermittently or continuously.

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