JP4648355B2 - Tube current adjusting method and apparatus, and X-ray CT apparatus - Google Patents

Description

  The present invention relates to a tube current adjusting method and apparatus and an X-ray CT (computerized tomography) apparatus, and more particularly to a tube current adjusting method and apparatus for optimizing the tube current of an X-ray tube, and such a tube current adjustment. The present invention relates to an X-ray CT apparatus provided with the apparatus.

  In an X-ray CT apparatus, an X-ray irradiation apparatus including an X-ray tube irradiates an X-ray beam (beam) having a spread (width) including an imaging range and a thickness in a direction perpendicular thereto. The thickness of the X-ray beam can be changed by adjusting the opening degree of the X-ray passage aperture of the collimator, thereby adjusting the slice thickness of imaging.

  The X-ray detection apparatus has a multi-channel X-ray detector in which a large number (for example, about 1000) of X-ray detection elements are arranged in an array in the width direction of the X-ray beam. To detect X-rays.

  The X-ray irradiation / detection device is rotated around the object to be scanned, and a projection image (projection) of the object to be imaged by X-rays is obtained in each of a plurality of view directions around the object to be imaged. A tomogram is generated (reconstructed) by a computer based on the projection data.

  A standard deviation (SD) is used as one of the indexes representing the quality of the reconstructed image. Since SD has a strong correlation with the area of the projection to be imaged when the tube current of the X-ray tube is constant, the tube current is automatically adjusted according to the area of the projection in order to obtain a proper SD tomographic image. To be done.

  In the automatic adjustment of the tube current, a projection is obtained by seeing through the imaging object in advance with X-rays, and an appropriate tube current is obtained according to the area. At that time, the cross section of the human body or the like to be imaged is generally elliptical, and the amount of X-ray transmission differs between the major axis direction and the minor axis direction, so according to the oval ratio, that is, the ratio of the major axis to the minor axis, The obtained tube current is corrected.

  In order to obtain the length of the major axis and the minor axis, the imaging object is seen through X-rays from the front (0 ° direction) and the side surface (90 ° direction), and the value at the center of the projection in the 0 ° direction Let the length be the value of the central part of the projection in the 90 ° direction as the length of the major axis.

  The above-described tube current adjustment has a problem in that efficiency is poor because the imaging target must be seen through with X-rays from the 0 ° direction and the 90 ° direction. Further, since fluoroscopy must be performed twice, there has been a problem that the exposure dose of X-rays to be imaged increases.

  The present invention has been made to solve the above problems, and an object thereof is to realize an efficient tube current adjusting method and apparatus, and an X-ray CT apparatus including such a tube current adjusting apparatus. That is. It is another object of the present invention to realize a tube current adjusting method and apparatus with a small exposure amount of X-rays to be imaged, and an X-ray CT apparatus provided with such a tube current adjusting device.

  The invention according to a first aspect for solving the above-described problem is a tube current adjusting method for adjusting the tube current of an X-ray tube irradiated to an imaging target, and assuming that the cross section of the imaging target is an ellipse Using the projection obtained by seeing through the imaging object with X-rays in a direction parallel to either the major axis or the minor axis, the lengths of the major axis and the minor axis are obtained and obtained from the projection. The tube current adjusting method is characterized in that the tube current of the X-ray tube is adjusted based on the area of the projection and the ratio between the major axis and the minor axis.

  An invention according to a second aspect for solving the above-described problem is a tube current adjusting device that adjusts a tube current of an X-ray tube that irradiates an imaging target when the cross section of the imaging target is assumed to be an ellipse. A diameter determining means for determining the length of the major axis and the minor axis using a projection obtained by seeing the imaging object through X-rays in a direction parallel to either the major axis or the minor axis of An area calculation means for obtaining an area of the projection using a projection, and a tube current adjustment for adjusting the tube current of the X-ray tube based on the area of the projection obtained from the projection and the ratio of the major axis to the minor axis A tube current adjusting device.

  In another aspect of the invention for solving the above-described problem, an X-ray is based on a projection area of an imaging target by transmission X-rays and a ratio of a major axis to a minor axis when the section of the imaging target is assumed to be an ellipse. A tube current adjusting method for adjusting a tube current of a tube, wherein an area of a projection obtained by seeing through the imaging object with X-rays in either the major axis or minor axis direction is obtained, and the length of the diameter in the see-through direction Is calculated based on the value of the central portion of the projection, and the length of the diameter perpendicular to the determined diameter is calculated using the area of the obtained projection and the length of the obtained diameter. This is a current adjustment method.

  In another aspect of the invention for solving the above-described problem, an X-ray is based on a projection area of an imaging target by transmission X-rays and a ratio of a major axis to a minor axis when the section of the imaging target is assumed to be an ellipse. An apparatus for adjusting a tube current of a tube, the tube current adjusting device for adjusting a tube current of the tube, an area calculating means for obtaining an area of a projection obtained by fluoroscopying the imaging target in either the major axis or minor axis direction; A diameter determining means for determining the length of the diameter based on the value of the central portion of the projection, and the length of the diameter perpendicular to the determined diameter using the area of the calculated projection and the length of the determined diameter A tube current adjusting device comprising a diameter calculating means for calculating.

  In another aspect of the invention for solving the above problems, an X-ray irradiation means including an X-ray tube, a tube current adjusting means for adjusting a tube current of the X-ray tube, an X-ray detection means, An X-ray CT apparatus having a tomographic image generating means for generating a tomographic image based on X-ray detection signals of a plurality of views detected by the X-ray detecting means, wherein the tube according to (2) is used as the tube current adjusting means. An X-ray CT apparatus characterized by using a current adjusting device.

  In another aspect of the invention for solving the above problems, the tube current of the X-ray tube is adjusted, the X-ray is irradiated from the X-ray tube, the X-ray is detected, and the detected plurality of views are displayed. In generating a tomographic image based on an X-ray detection signal, an X-ray tomographic imaging method is characterized in that the method described in (1) is used to adjust the tube current.

  In the present invention, using the projection obtained by seeing through the imaging object with X-rays in a direction parallel to either the major axis or the minor axis when the cross section of the imaging object is assumed to be an ellipse, the major axis And the tube current of the X-ray tube is adjusted based on the area of the projection obtained from the projection and the ratio of the major axis to the minor axis, so that the fluoroscopy is only from one direction. Just do it.

  Therefore, according to the present invention, it is possible to realize an efficient tube current adjusting method and apparatus, and an X-ray CT apparatus including such a tube current adjusting apparatus. In addition, it is possible to realize a tube current adjusting method and apparatus with a small amount of X-ray exposure to be imaged, and an X-ray CT apparatus provided with such a tube current adjusting device.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment. FIG. 1 shows a block diagram of an X-ray CT apparatus. This apparatus is an example of an embodiment of the present invention. An example of an embodiment relating to the apparatus of the present invention is shown by the configuration of the apparatus. An example of an embodiment related to the method of the present invention is shown by the operation of the apparatus.

  As shown in FIG. 1, the apparatus includes a scanning gantry 2, an imaging table 4, and an operation console 6. The scanning gantry 2 has an X-ray tube 20. X-rays (not shown) radiated from the X-ray tube 20 are shaped by the collimator 22 into, for example, a fan-shaped X-ray beam, that is, a fan beam, and irradiated to the detector array 24. The detector array 24 has a plurality of X-ray detection elements arranged in an array in the direction of the width of the fan-shaped X-ray beam. The configuration of the detector array 24 will be described later.

  The X-ray tube 20 and the collimator 22 are an example of an embodiment of the X-ray irradiation means in the present invention. The detector array 24 is an example of an embodiment of the X-ray detection means in the present invention. The X-ray tube 20, the collimator 22, and the detector array 24 constitute an X-ray irradiation / detection device. The X-ray irradiation / detection apparatus will be described later. A data collection unit 26 is connected to the detector array 24. The data collection unit 26 collects detection data of individual X-ray detection elements of the detector array 24.

  X-ray irradiation from the X-ray tube 20 is controlled by an X-ray controller 28. The connection relationship between the X-ray tube 20 and the X-ray controller 28 is not shown. The collimator 22 is controlled by a collimator controller 30. The connection relationship between the collimator 22 and the collimator controller 30 is not shown.

  The components from the X-ray tube 20 to the collimator controller 30 described above are mounted on the rotating unit 34 of the scanning gantry 2. The rotation of the rotating unit 34 is controlled by the rotation controller 36. The connection relationship between the rotating unit 34 and the rotation controller 36 is not shown.

  The imaging table 4 carries an imaging target (not shown) into and out of the X-ray irradiation space of the scanning gantry 2. The relationship between the imaging target and the X-ray irradiation space will be described later.

  The operation console 6 has a central processing unit 60. The central processing unit 60 is configured by, for example, a computer. A control interface 62 is connected to the central processing unit 60. The scanning gantry 2 and the imaging table 4 are connected to the control interface 62. The central processing unit 60 controls the scanning gantry 2 and the imaging table 4 through the control interface 62.

  The data acquisition unit 26, the X-ray controller 28, the collimator controller 30 and the rotation controller 36 in the scanning gantry 2 are controlled through the control interface 62. Note that illustration of individual connections between these units and the control interface 62 is omitted.

  A portion including the central processing unit 60, the control interface 62, and the X-ray controller 28 is an example of the embodiment of the tube current adjusting device of the present invention. An example of an embodiment relating to the apparatus of the present invention is shown by the configuration of the apparatus. An example of an embodiment related to the method of the present invention is shown by the operation of the apparatus. The portion comprising the central processing unit 60, the control interface 62, and the X-ray controller 28 is also an example of the embodiment of the tube current adjusting means in the present invention.

  A data collection buffer 64 is also connected to the central processing unit 60. A data collection unit 26 of the scanning gantry 2 is connected to the data collection buffer 64. Data collected by the data collection unit 26 is input to the data collection buffer 64. The data collection buffer 64 temporarily stores input data.

  The central processing unit 60 performs image reconstruction based on a plurality of views of projections collected through the data collection buffer 64. The central processing unit 60 is an example of an embodiment of tomographic image generation means in the present invention. For the image reconstruction, for example, a filtered back projection method or the like is used. A storage device 66 is also connected to the central processing unit 60. The storage device 66 stores various data, reconstructed images, programs, and the like.

  Further, a display device 68 and an operation device 70 are connected to the central processing unit 60, respectively. The display device 68 displays a reconstructed image and other information output from the central processing unit 60. The operation device 70 is operated by an operator and inputs various instructions and information to the central processing device 60.

  FIG. 2 shows a schematic configuration of the detector array 24. The detector array 24 is a multi-channel X-ray detector in which a large number of X-ray detection elements 24 (i) are arranged. A large number of X-ray detection elements 24 (i) form an X-ray incident surface curved in a cylindrical concave shape as a whole. i is a channel number, for example, i = 1 to 1000.

  The X-ray detection element 24 (i) is configured by, for example, a combination of a scintillator and a photodiode. However, the present invention is not limited to this. For example, a semiconductor X-ray detection element using cadmium tellurium (CdTe) or the like, or an ionization chamber type X-ray detection element using xenon (Xe) gas may be used.

  FIG. 3 shows the interrelationship among the X-ray tube 20, the collimator 22, and the detector array 24 in the X-ray irradiation / detection apparatus. 3A is a diagram showing a state seen from the front of the scanning gantry 2, and FIG. 3B is a diagram showing a state seen from the side. As shown in the figure, the X-rays radiated from the X-ray tube 20 are shaped by the collimator 22 into a fan-shaped X-ray beam 400 and irradiated to the detector array 24.

  FIG. 3A shows the spread of the fan-shaped X-ray beam 400, that is, the width of the X-ray beam 400. The width direction of the X-ray beam 400 coincides with the channel arrangement direction in the detector array 24. In (b), the thickness of the X-ray beam 400 is shown.

  With the body axis intersecting such a fan surface of the X-ray beam 400, for example, as shown in FIG. 4, the imaging target 8 placed on the imaging table 4 is carried into the X-ray irradiation space. The scanning gantry 2 has a cylindrical structure including an X-ray irradiation / detection device inside.

  The X-ray irradiation space is formed in the inner space of the cylindrical structure of the scanning gantry 2. An image of the imaging target 8 sliced by the X-ray beam 400 is projected onto the detector array 24. X-rays that have passed through the imaging target 8 are detected by the detector array 24. The thickness th of the X-ray beam 400 applied to the imaging target 8 is set by adjusting the aperture of the collimator 22.

  FIG. 5 shows a block diagram of the central processing unit 60 relating to tube current adjustment. The function of each block in the figure is realized by a computer program, for example. As shown in the figure, the central processing unit 60 has a projection area calculation unit (unit) 602. The projection area calculation unit 602 is an example of an embodiment of the area calculation means in the present invention. The projection area calculation unit 602 calculates the area of a projection obtained by seeing through the imaging target 8 with X-rays.

  The projection area calculation will be described with reference to FIG. As shown in the figure, assuming that a projection in the direction of 0 °, for example, is obtained for the imaging target 8 by the X-ray beam 400, the projection area calculation unit 602 obtains the sum of the projection data of each channel input from the data collection buffer 64. This is the projection area s1.

  Based on the projection data input from the data collection buffer 64, the first diameter determination unit 604 determines a first diameter when the cross section of the imaging target 8 is assumed to be an ellipse, that is, a length parallel to the projection direction. Determine. The first diameter determining unit 604 is an example of an embodiment of a diameter determining means in the present invention.

  Since the length of the diameter parallel to the projection direction is represented by the height of the central portion of the projection, that is, projection data at the central portion of the projection, for example, projection data based on data measured in the central channel of the detector array 24 is extracted. This is the length r0 of the first diameter. Here, since the first diameter is obtained from the projection in the 0 ° direction, it is an elliptical minor diameter. In obtaining the first diameter, it is preferable to use an average value of projection data of the central channel and a plurality of channels in the vicinity thereof in order to obtain a highly accurate value.

  When the second diameter calculation unit 606 uses the projection area input from the projection area calculation unit 602 and the first diameter input from the first diameter determination unit 604 and assumes that the cross section of the imaging target 8 is an ellipse. , That is, a diameter perpendicular to the first diameter (here, the major axis) is calculated. The second diameter calculation unit 606 is an example of an embodiment of a diameter calculation means in the present invention.

  The calculation of the second diameter is performed based on the formula for the area of the ellipse. The area of the ellipse is determined by the product of the major axis, the minor axis, and the circumference, and since the area s1 and the minor axis r0 are known, the major axis r90 can be obtained by calculation.

  The tube current calculation unit 608 calculates the tube current based on the projection area s1 input from the projection area calculation unit 602. In calculating the tube current, first, SD corresponding to the projection area s1 is obtained. The relationship between the projection areas S and SD is obtained in advance by phantom measurement or the like under a standard tube current, and for example, the correspondence as shown in FIG. 7 is stored in the memory.

  Therefore, SDδa corresponding to the projection area s1 is obtained from such a relationship. The relationship between the tube current mAa that provides SDδa and the tube current mAb that provides SDδb required for the reconstructed image has the following relationship.

  So from this relationship,

To obtain the desired tube current.
The oval ratio calculation unit 610 uses the first diameter r0 input from the first diameter determination unit 604 and the second diameter r90 input from the second diameter calculation unit 606 to use the oval ratio.

Ask for.
The tube current correction unit 612 corrects the tube current mAb input from the tube current calculation unit 608 according to the oval ratio R input from the oval ratio calculation unit 610. The relationship between the oval ratio R and the correction coefficient K is given in advance as shown in FIG. 8, for example, and the tube current is corrected as shown in the following equation using the correction coefficient K obtained from this relationship.

A signal representing the corrected tube current mAb ′ is supplied to the X-ray controller 28 through the control interface 62.
The operation of this apparatus will be described. In operation of this apparatus, first, tube current adjustment according to an imaging target is performed. FIG. 9 shows a flow diagram of the tube current adjustment operation. Hereinafter, the tube current adjustment will be described with reference to FIG.

  First, in step 902, scout imaging is performed. In scout imaging, fluoroscopic imaging is performed from a predetermined direction on a part of the imaging object 8 from which a tomographic image is to be captured. The scout shooting direction is, for example, the 0 ° direction, that is, from the front of the imaging target 8. Instead of the 0 ° direction, the 90 ° direction, that is, the side surface of the imaging target 8 may be used. Hereinafter, although an example of the 0 ° direction will be described, the same applies to the case of the 90 ° direction.

  Next, in step 904, the projection area s1 is calculated. The calculation of the projection area s1 is performed by the projection area calculation unit 602 as described above.

Next, in step 906, the first diameter is determined. The determination of the first diameter is performed by the first diameter determination unit 604 as described above.
Next, in step 908, the second diameter is calculated. The calculation of the second diameter is performed by the second diameter calculation unit 606 as described above.

  Next, in step 910, the tube current is calculated. The calculation of the tube current is performed by the tube current calculation unit 608, the oval ratio calculation unit 610, and the tube current correction unit 612 as described above.

  In this way, it is possible to obtain a tube current corrected by the oval ratio only by performing fluoroscopic imaging from one direction. Therefore, the efficiency of adjusting the tube current can be increased, and the exposure amount of the imaging object 8 can be reduced to half of the conventional amount.

  FIG. 10 shows a flowchart of the operation at the time of imaging of this apparatus. As shown in the figure, in step 912, the operator inputs a scan plan through the operation device 70. The scan plan includes X-ray irradiation conditions, slice thickness, slice position, and the like. Here, among the X-ray irradiation conditions, the tube current is automatically adjusted as described above. Hereinafter, the apparatus operates under the operation of the operator and control by the central processing unit 60 according to the input scan plan.

  In step 914, scan positioning is performed. That is, the operator operates the operation device 70 to move the imaging table 4 so that the center of the imaging region of the imaging target 8 coincides with the rotation center (isocenter) of the X-ray irradiation / detection device.

  After such scan positioning is performed, scanning is performed in step 916. That is, the X-ray irradiation / detection device is rotated around the imaging target 8 to collect, for example, 1000 views of projections in the data collection buffer 64.

  Image reconstruction is performed in step 918 after scanning or in parallel with scanning. That is, based on the projections of a plurality of views collected in the data collection buffer 64, the central processing unit 60 reconstructs an image by, for example, a filtered back projection method and generates a tomographic image.

  The reconstructed tomographic image is displayed on the display device 68 in step 920. Since the tube current is automatically adjusted according to the projection area and oval ratio of the imaging target, a high-quality tomographic image can be obtained.

It is a block diagram of an example of an apparatus of an embodiment of the invention. It is a schematic diagram of the detector array in the apparatus shown in FIG. It is a schematic diagram of the X-ray irradiation / detection apparatus in the apparatus shown in FIG. It is a schematic diagram of the X-ray irradiation / detection apparatus in the apparatus shown in FIG. It is a block diagram of the central processing unit regarding a tube current adjustment function. It is a conceptual diagram of the projection of imaging object. It is a graph which shows the relationship between a projection area and SD. It is a graph which shows the relationship between an oval ratio and a tube current correction coefficient. It is a flowchart of operation | movement of the apparatus shown in FIG. It is a flowchart of operation | movement of the apparatus shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 Scanning gantry 4 Imaging table 6 Operation console 8 Imaging object 20 X-ray tube 22 Collimator 24 Detector array 26 Data acquisition part 28 X-ray controller 30 Collimator controller 34 Rotation part 36 Rotation controller 60 Central processing unit 62 Control interface 64 Data acquisition buffer 66 Storage Device 68 Display Device 70 Operating Device 602 Projection Area Calculation Unit 604 First Diameter Determination Unit 606 Second Diameter Calculation Unit 608 Tube Current Calculation Unit 610 Oval Ratio Calculation Unit 612 Tube Current Correction Unit

Claims (7)

A tube current adjusting method for adjusting a tube current of an X-ray tube irradiated to an imaging target which is a human body ,
Using the projection obtained by seeing through the imaging object with X-rays in a direction parallel to either the major axis or the minor axis when the cross section of the imaging object is assumed to be an ellipse, the area of the projection, Find the length of either the major axis or the minor axis,
The length of the other diameter is calculated using the area of the projection obtained from the projection and the length of the determined diameter,
A tube current adjusting method, wherein the tube current of the X-ray tube is adjusted based on a projection area obtained from the projection and a ratio between the major axis and the minor axis. 2. The tube current adjustment method according to claim 1, wherein the area of the projection is obtained by a sum of values of the projection in the see-through direction. The tube current of the X-ray tube is required for the obtained projection area and the reconstructed image using a relationship between the projection area stored in advance and the index representing the quality of the reconstructed image and the tube current giving the quality. The tube current which gives the said quality is calculated | required from the parameter | index showing the quality of a reconstructed image, The said tube current calculated | required using the ratio of the said major axis and the said minor axis is calculated | required, or it calculates | requires. 3. The tube current adjusting method according to 2. A tube current adjusting device that adjusts a tube current of an X-ray tube that irradiates an imaging target that is a human body ,
Using the projection obtained by seeing through the imaging object with X-rays in a direction parallel to either the major axis or the minor axis when the cross section of the imaging object is assumed to be an ellipse, the area of the projection is calculated. An area calculation means to be obtained;
Using the projection, the length of one of the major axis and the minor axis is determined, and the length of the other diameter is calculated using the area of the projection and the determined diameter. Diameter determining means to be obtained by
Tube current adjusting means for adjusting the tube current of the X-ray tube based on the area of the projection obtained from the projection and the ratio of the major axis to the minor axis;
A tube current adjusting device comprising: 5. The tube current adjusting device according to claim 4, wherein the area calculating unit obtains the area of the projection by a sum of the values of the projection in the see-through direction. The tube current adjusting means uses a relationship between a projection area stored in advance and an index representing the quality of the reconstructed image and a tube current giving the quality, and the reconstruction required for the calculated projection area and the reconstructed image. A means for obtaining a tube current that gives the quality from an index representing the quality of the image, correcting the obtained tube current using a ratio of the major axis to the minor axis, and obtaining a tube current of the X-ray tube; The tube current adjusting device according to claim 4 or 5, characterized by the above-mentioned. Based on X-ray irradiation means including an X-ray tube, tube current adjusting means for adjusting the tube current of the X-ray tube, X-ray detection means, and X-ray detection signals of a plurality of views detected by the X-ray detection means. An X-ray CT apparatus comprising a tomographic image generating means for generating a tomographic image,
An X-ray CT apparatus using the tube current adjusting device according to any one of claims 4 to 6 as the tube current adjusting means.

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