JP5390549B2 - X-ray computed tomography system - Google Patents

Description

  The present invention relates to an X-ray computed tomography apparatus.

  An X-ray computed tomography apparatus provides information about a subject based on the intensity of X-rays that have passed through the subject, and includes many images including diagnosis, treatment, and surgical planning of diseases. It plays an important role in medical practice. Therefore, the reduction of the exposure dose per time has become an important issue as the exposure opportunities increase.

  For example, in Patent Documents 1 and 2, data is collected by irradiating X-rays only in the central region, and data in peripheral regions not irradiated with X-rays is data collected using weak X-rays or previously collected data. Replenishment by means of reducing the exposure dose.

  However, in many cases, in many cases, since the site of interest such as the heart is off the center of rotation, the technique for reducing the exposure dose cannot be used, or the effect of reducing the exposure dose is limited.

JP 2002-17716 A Japanese Patent Laid-Open No. 10-248835

  An object of the present invention is to make effective the effect of reducing the exposure dose by the filter.

  An X-ray computed tomography apparatus according to the present invention includes an X-ray tube rotatable around a subject, a gantry having an X-ray detector, a rotation axis of the X-ray tube, and two axes orthogonal to the rotation axis. A bed having a top plate that is movable with respect to three axes, a control unit that controls movement of the top plate based on a deviation about three axes between a position of interest with respect to the subject and the rotation center of the X-ray tube, A filter is provided between the X-ray tube and the subject in order to increase the dose at the rotation axis and lower at the periphery.

  According to the present invention, the effect of reducing the exposure dose by the filter can be made effective.

The figure which shows the structure of the X-ray computed tomography apparatus by embodiment of this invention. The flowchart which shows operation | movement of this embodiment. The figure which shows the example of the scan plan screen constructed | assembled by the expert system of FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The X-ray computed tomography apparatus includes a rotation / rotation type in which an X-ray tube and a radiation detector are rotated as one body, and a large number of detection elements are arrayed in a ring shape. There are various types such as a fixed / rotation type in which only the subject 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 object and projection data for about 360 ° is obtained, and projection data for 180 ° + α (α: fan angle) is also obtained by the half scan method. Needed. The present invention can be applied to any reconstruction method. Here, the former example will be described. 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. 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 configuration of an X-ray computed tomography apparatus according to this embodiment. This X-ray computed tomography apparatus has a gantry 1 configured to collect projection data relating to a subject. The gantry 1 includes an X-ray tube 10 and an X-ray detector 23. The X-ray tube 10 and the X-ray detector 23 are mounted on a ring-shaped rotating frame 12 that is rotationally driven by a gantry driving device 25. For convenience of explanation, the rotation center axis of the rotary frame 12 is set as the Z axis, the X axis is set in the vertical direction, and the Y axis is set in the horizontal direction. The central portion of the rotating frame 12 is opened, and the subject P placed on the top 2a of the bed 2 is inserted into the opening. The bed 2 is equipped with a top board driving device 2b. The top plate driving device 2b is provided to drive the movement of the top plate 2a in each of the three directions XYZ. A wedge filter 11 is disposed between the X-ray tube 10 and the subject in order to increase the dose of X-rays on the rotation axis and lower on the periphery.

  A tube voltage is applied between the cathode and anode of the X-ray tube 10 from the high voltage generator 21, and a filament current is supplied to the filament of the X-ray tube 10 from the high voltage generator 21. X-rays are generated by applying a tube voltage and supplying a filament current. As the X-ray detector 23, either a one-dimensional array type detector or a two-dimensional array type detector may be adopted. The X-ray detection element has 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. A two-dimensional array type detector has 40 rows arranged in the slice direction, for example. What consists of a single column is a one-dimensional array type detector.

  A data acquisition device 26 generally called a DAS (data acquisition system) converts a signal output from the detector 23 for each channel into a voltage signal, amplifies it, and converts it into a digital signal. This data (raw data) is supplied to the computer unit 3 outside the gantry. The preprocessing unit 34 of the computer unit 3 performs correction processing such as sensitivity correction on the data (raw data) output from the data collection device 26 and outputs projection data. This projection data is sent to and stored in the storage device 41 of the computer system 3.

  The computer system 3 includes a system controller 29, a scan controller 30, a reconstruction unit 36 for reconstructing image data from projection data, a display 38, an input device 39, a scan expert system, together with the preprocessing unit 34 and the storage device 41. 43. The scan expert system 43 is a system constructed mainly to support setting of a scan plan. A procedure for setting a scan plan by the scan expert system 43 will be described.

  FIG. 2 shows a scan plan setting procedure. First, after the system main power is turned on, the scan expert system 43 is activated (S1), and an initial screen for setting a scan plan is displayed on the display 38. A plurality of imaging region candidates are presented on the initial screen. When an imaging region is selected by the operator through the operation of the input device 39, a list of a plurality of expert plans corresponding to the imaging region is displayed. Each expert plan includes a scan mode indicating a distinction between single scan / multi-slice scan / helical scan, scan start position, end position, CTDI (CT dose index), tube voltage, tube current, scan speed (X-ray tube 10). Time required for one rotation), the number of slices, imaging slice thickness, image slice thickness, helical pitch indicating the distance the top 2a moves during one rotation period of the X-ray tube 10, reconstruction mode, imaging field of view (imaging FOV) , Reconstruction field of view (reconstruction FOV) and the like. A desired expert plan is selected by the operator via the input device 39 (S2). When the pre-scan start button on the input device 39 is pressed by the operator, a pre-scan is performed under the control of the scan controller 30 in order to collect scanograms for detailed plan setting and data for tomographic images. Performed (S3).

  This pre-scan is a helical scan. However, the helical scan in the pre-scan is performed at a high speed with a helical pitch that is lower than the tube current of the main scan (low dose) and longer than the helical pitch of the main scan at the helical pitch. Data collected by the pre-scan is temporarily stored in the storage device 41. The scan expert system 43 uses the data collected in the pre-scan to scan the X-ray tube 10 at a zero degree (top position) as a frontal image of the subject and the X-ray tube 10 to 90 degrees. A scanogram as a side image of a subject at a certain time is generated, and the reconstruction unit 36 uses an image slice thickness that is larger than that at the time of the main scan at any slice position using the data collected in the pre-scan. In addition, a tomographic image having a lower resolution than that in the main scan is reconstructed (S4). This tomographic image is reconstructed with a reconstructed visual field (reconstructed FOV) that substantially coincides with a circular pre-scan photographing visual field (imaging FOV) centered on the rotation center axis.

  The scan expert system 43 constructs and displays the scan plan screen illustrated in FIG. 3 from the above two-direction scanogram, the tomographic image, and the plan selected in S2 (S5). A frame for designating a scan range in the Z direction is overlaid on the front scanogram, and a line for designating a tilt angle is overlaid on the side scanogram. The operator arbitrarily operates these frames and lines to designate the scan range and tilt angle in the Z direction.

  When enlargement reconstruction (zooming reconstruction) is selected in the reconstruction mode, the scan expert system 43 superimposes a circular region of interest ROI for designating the range of enlargement reconstruction on the tomographic image on the scan plan screen. To display. The operator operates the input device 39 to arbitrarily enlarge / reduce the region of interest ROI and move it so that the organ of interest such as the heart fits in the region of interest ROI (S6). When the setting of the region of interest ROI is completed, the scan expert system 43 changes the position of the center of the region of interest ROI, that is, the center of the enlarged reconstruction region, to the center of the tomographic image, that is, the position of the rotation center axis of the X-ray tube 10. On the other hand, the moving distance of the top plate 2a in the XY2 direction necessary for matching is calculated (S7).

  Further, the scan expert system 43 determines whether or not the top plate 2a can actually move the calculated moving distance (S8). In addition to determining whether the assumed X or Y position of the tabletop 2a after movement is within the range of the movement limit of the tabletop 2a by the tabletop driving device 2b, it is also possible to determine the outer contour of the subject. It is determined whether or not the entire area is within the shooting FOV. For example, when even a part of the chest of the subject is out of the imaging FOV, it is determined that the top plate 2a cannot move the calculated moving distance. For this purpose, the scan expert system 43 is equipped with an image processing function for extracting the skin contour of the subject from the tomographic image using threshold processing or the like. The position of the extracted contour is shifted according to the calculated movement distance, and the determination is performed by comparing the coordinates of the contour after the shift with the coordinates of the range of the imaging FOV.

  When it is determined that the top 2a can move the travel distance calculated in S8, the scan expert system 43 waits for the click of the button labeled “top travel” in the scan plan screen, and then calculates in S8. The data of the movement distance in each XY direction is supplied to the scan controller 30. The scan controller 30 controls the top board driving device 2b according to the supplied movement distance data in each XY direction, and moves the top board 2b in each XY direction by the distance calculated in S8 (S9). Thereby, the center of the enlarged reconstruction range coincides with the rotation center axis of the X-ray tube 10. The scan expert system 43 moves the tomographic image within the tomographic image display area in the scan plan screen according to the change in the position of the top 2b from a position sensor (not shown) (S10). The center of the tomographic image display area in the scan plan screen is fixed as the position of the rotation center axis. Thereby, the operator can confirm the coincidence between the center of the enlarged reconstruction range due to the movement of the top plate 2b and the rotation center axis of the X-ray tube 10.

  When it is determined that the top 2a cannot move the movement distance calculated in S8, the scan expert system 43 cannot make the center of the enlarged reconstruction range coincide with the rotation center axis of the X-ray tube 10 Is displayed in a pop-up on the screen (S12). At the same time, the scan expert system 43 uses the XY2 direction for the center of the enlarged reconstruction range to be closest to the rotation center axis of the X-ray tube 10 on the condition that the entire contour of the subject falls within the imaging FOV. Is calculated (S13). The moving distance for the closest approach is calculated so that the coordinates of one location (one pixel) on the contour of the subject after movement coincide with the coordinates of a certain position on the outer periphery of the imaging FOV.

  Similarly to the above, the scan expert system 43 waits for the click of the button labeled “top plate movement” in the scan plan screen, and supplies the scan controller 30 with the data of the movement distances in the XY directions calculated in S13. . The scan controller 30 controls the top board driving device 2b according to the supplied movement distance data in each XY direction, and moves the top board 2b in each XY direction by the distance calculated in S13 (S9). Thereby, the center of the enlarged reconstruction range is closest to the rotation center axis of the X-ray tube 10. The degree of approach can be confirmed by the movement of the tomographic image in the tomographic image display area in the scan plan screen in S10.

  Each parameter of the scan plan is arbitrarily modified as necessary. When the “OK” button in the screen is clicked, the main scan is executed under the control of the scan controller 30 (S11).

  As described above, the center of the enlarged reconstruction area can be automatically matched with the rotation center axis of the X-ray tube 10, or can be automatically brought closest when it cannot be matched. Thereby, the dose reduction effect by the wedge filter configured to increase the dose in the vicinity of the rotation center and suppress the dose at the end portion can be made effective.

  In the above description, the center of the enlarged reconstruction range is made to match or approach the center of rotation, but any position may be made to match or approach the center of rotation. Further, in the above description, the example in which the top plate moves so that the center of the enlarged reconstruction range coincides with or approaches the rotation center is described. However, the center of the enlarged reconstruction range coincides with or approaches the rotation center. In addition, the top plate may be fixed and the gantry may be moved, or the gantry may be moved together with the top plate.

(Modification)
The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the stage of implementation. Furthermore, the above embodiment includes various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, some constituent requirements may be deleted from all the constituent requirements shown in the embodiment.

DESCRIPTION OF SYMBOLS 1 ... Stand, 2 ... Bed, 2a ... Top plate, 2b ... Top plate drive device, 3 ... Computer unit, 10 ... X-ray tube, 11 ... Wedge filter, 12 ... Rotating frame, 21 ... High voltage generator, 23 ... X-ray detector, 25 ... gantry driving device, 26 ... data collection device, 29 ... system controller, 30 ... scan controller, 34 ... preprocessing unit, 36 ... reconstruction unit, 38 ... display, 39 ... input device, 41 ... Storage device, 43 ... Scan expert system.

Claims (2)

A gantry having an X-ray tube and an X-ray detector rotatable around the subject;
A bed having a top plate that is movable with respect to three axes of a rotation axis of the X-ray tube and two axes orthogonal to the rotation axis;
A control unit for controlling the movement of the top plate based on a shift with respect to the three axes between a position of interest with respect to the subject and a rotation center of the X-ray tube;
A filter disposed between the X-ray tube and the subject to increase the X-ray dose at the rotation axis and lower at the periphery;
Comprising
The controller determines whether or not a movement distance of the top plate for making the position of interest coincide with a position of a rotation axis of the X-ray tube is within a range of movement limits of the top plate. X-ray computed tomography apparatus. A gantry having an X-ray tube and an X-ray detector rotatable around the subject;
A bed having a top plate that is movable with respect to three axes of a rotation axis of the X-ray tube and two axes orthogonal to the rotation axis;
A control unit for controlling the movement of the top plate based on a shift with respect to the three axes between a position of interest with respect to the subject and a rotation center of the X-ray tube;
A filter disposed between the X-ray tube and the subject to increase the X-ray dose at the rotation axis and lower at the periphery;
Comprising
The X-ray computed tomography is characterized in that the control unit determines whether or not the entire area of the subject is within a field of view when the position of interest matches the position of the rotation axis of the X-ray tube. Shooting device.

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