JP4363629B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT apparatus having a filter that reduces the X-ray exposure dose of a subject.

  In recent years, a VCT (Volume CT) in which a cone-shaped X-ray beam is generated and a solid-state imaging device is two-dimensionally arranged at a position facing the X-ray source has come to be used. In imaging using this VCT, image information of a plurality of slices is acquired at once in the slice direction that forms the short direction of the two-dimensional array, and the image quality is improved along with the improvement of imaging efficiency.

Here, in imaging of a subject using transmitted X-rays, the X-ray dose required for each imaging region is different. For this reason, a filter made of a material that attenuates X-rays such as acrylic or copper is provided between the X-ray source and the subject, and the imaging site of the subject is protected from excessive X-ray exposure. . In particular, a filter that adjusts the dose in the channel direction that is the longitudinal direction of the solid-state imaging device is called a bowtie filter and is disposed between the X-ray source and the subject.
JP-A-52-110582, (pages 4-6, FIGS. 1 and 3)

  However, according to the background art described above, the X-ray dose of the subject could not be changed for each slice. That is, the bow-tie filter is a fixed-shaped filter having the same shape in the slice direction, and has exactly the same X-ray source weakening effect in this direction, and the exposure dose of the subject is the same.

  In particular, in imaging using VCT, since imaging of a multi-slice having a width in the slice direction is performed, there are different imaging parts in the imaging range having this width, and the optimal exposure dose can be reduced. The appropriate dose differs for each imaging region. In such a case, using a filter having the same shape in the slice direction may cause more than necessary X-rays to be irradiated depending on the imaging region. Furthermore, in imaging performed while moving the subject in the slice direction, which is performed in a helical scan, the imaging region changes dynamically in the slice direction, and when a fixed filter is used, It is difficult to maintain an optimal exposure dose for each imaging region.

  For these reasons, it is important how to realize an X-ray CT apparatus having a filter capable of dynamically changing the X-ray dose irradiated to the subject including the slice direction.

  The present invention has been made to solve the above-described problems caused by the background art. An X-ray CT apparatus having a filter capable of dynamically changing an X-ray dose irradiated to a subject including a slice direction. The purpose is to provide.

  In order to solve the above-described problems and achieve the object, an X-ray CT apparatus according to a first aspect of the invention includes an X-ray tube that irradiates a cone-shaped X-ray beam, an irradiation direction of the X-ray beam, and A detection element for detecting the X-ray beam, two-dimensionally arranged on a substantially orthogonal plane, and a plurality of blocks disposed near the detection element side of the X-ray tube for attenuating the X-ray beam A filter unit comprising: a moving unit that moves the block in a direction substantially orthogonal to the irradiation direction; and a control unit that controls a block position of the movement.

  According to the first aspect of the invention, the X-ray tube emits a cone-shaped X-ray beam, and the X-ray is detected by the detection elements that are two-dimensionally arranged on a plane substantially orthogonal to the X-ray beam irradiation direction. The beam is detected, and the X-ray beam is attenuated by a filter unit composed of a plurality of blocks arranged in the vicinity of the detection element side of the X-ray tube, and this block is moved in a direction substantially orthogonal to the irradiation direction. Therefore, the block position of the movement is controlled by the control means, so that the X-ray attenuation effect by the block is set to be the largest required for each imaging region of the subject, and the subject X-ray exposure dose for each imaging region of the subject is the lowest.

  The X-ray CT apparatus according to the invention of the second aspect is characterized in that a plurality of layers of the blocks are arranged in the irradiation direction.

  According to the invention of the second aspect, the block controls the X-ray dose irradiated to the subject by changing the thickness in the irradiation direction.

  The X-ray CT apparatus according to the invention of the third aspect is characterized in that the blocks are arranged in a plurality of layers in a slice direction forming a short length of the two-dimensional arrangement.

  According to the invention of the third aspect, the block controls the X-ray dose irradiated to the subject for each block position in the slice direction.

  An X-ray CT apparatus according to the invention of a fourth aspect is characterized in that two of the blocks are arranged in a channel direction that forms the length of the two-dimensional array.

  According to the fourth aspect of the invention, in the case of a generally symmetrical object whose attenuation is large in the vicinity of the center, the block is moved to the symmetrical position from the center to obtain an optimal X-ray exposure dose. .

  The X-ray CT apparatus according to the invention of the fifth aspect is characterized in that the block has a rectangular parallelepiped shape.

  According to the fifth aspect of the invention, the blocks are arranged without gaps, and are compact and efficient.

  The X-ray CT apparatus according to the sixth aspect of the invention is characterized in that the filter unit includes a cylindrical support frame that surrounds a block forming a layer in the slice direction and the irradiation direction.

  According to the sixth aspect of the invention, since the filter unit surrounds the block forming a layer in the slice direction and the irradiation direction by the cylindrical support frame, the block is fixed in a slidable state in the channel direction. To do.

  The X-ray CT apparatus according to the seventh aspect of the invention is characterized in that the moving means includes a shaft connected to the block.

  According to the seventh aspect of the invention, since the moving means is connected to the block by the shaft, the moving means moves the block to a target position in the channel direction.

  The X-ray CT apparatus according to the invention of the eighth aspect is characterized in that the support frame is fixed to an outer frame of a case containing the moving means.

  According to the eighth aspect of the invention, the moving means and the filter unit are in a predetermined positional relationship, and block position control is accurately performed.

  Further, in the X-ray CT apparatus according to the invention of the ninth aspect, the control means causes the absorbed X-ray dose of the subject from a scout scan of the subject arranged between the X-ray tube and the detection element. And a first calculating means for calculating a block position for reducing the X-ray exposure dose of the subject for each of different imaging sites.

  According to the ninth aspect of the invention, the control means differs in the absorbed X-ray dose of the subject from the scout scan of the subject arranged between the X-ray tube and the detection element by the first calculation means. Since the block position for reducing the X-ray exposure dose of the subject is calculated for each imaging region, an optimal block position that minimizes the X-ray exposure dose is obtained for each imaging region of the subject.

  In the X-ray CT apparatus according to the invention of the tenth aspect, the control means performs image reconstruction using projection data of a subject arranged between the X-ray tube and the detection element. And second calculation means for calculating a block position for reducing the X-ray exposure dose of the subject based on a contribution rate of the projection data in the slice direction to the image reconstruction.

  According to the tenth aspect of the invention, the control means uses the second calculation means to perform image reconstruction using the projection data of the subject arranged between the X-ray tube and the detection element. Based on the contribution rate of the projection data in the slice direction to the image reconstruction, the block position for reducing the X-ray exposure dose of the subject is calculated. Reduce the dose.

  In the X-ray CT apparatus according to the invention of the eleventh aspect, when the control means performs imaging while moving the position of the subject, the block position is set as the result of the calculation for each position. It is characterized by matching.

  According to the eleventh aspect of the present invention, even when the subject is moved during imaging, the block position where the X-ray exposure dose is reduced most is always set at the moving position.

  As described above, according to the present invention, the X-ray attenuation effect of the block is different for each imaging region where the X-ray absorbed dose of the subject is different, and as a result, the X-ray exposure dose of the subject is increased. It is possible to reduce the X-ray exposure dose of the subject and make it safer in imaging including a different imaging region or dynamic imaging in which the imaging region moves, while minimizing the image quality without deterioration.

Exemplary embodiments of an X-ray CT apparatus according to the present invention will be described below with reference to the accompanying drawings. Note that the present invention is not limited thereby.
(Embodiment 1)
First, the overall configuration of the X-ray CT apparatus according to the first embodiment will be described. FIG. 1 shows a block diagram of an X-ray CT apparatus. As shown in FIG. 1, the apparatus has a scanning gantry 2 and an operation console 6.

  The scanning gantry 2 has an X-ray tube 20. X-rays radiated from the X-ray tube 20 are shaped by a collimator 22 into, for example, a cone-shaped X-ray beam (beam), and pass through the subject 1 to detect X-rays. The device 24 is irradiated. Note that the opening of the collimator 22 is controlled by the collimator controller 30 to adjust the width of the cone beam.

  The filter unit 21 is disposed between the collimator 22 and the subject 1 and is configured using a material that attenuates X-rays such as acrylic or copper. And the intensity | strength of the X-ray beam which permeate | transmits these materials is changed by controlling the material thickness of a permeation | transmission direction, and the exposure dose of the subject 1 is reduced. In addition, the filter part 21 controls the thickness of the material which comprises the filter part 21 by the filter controller 31 which makes | forms a control part.

  The X-ray detector 24 has a plurality of X-ray detection elements that are two-dimensionally arranged in the spreading direction of the cone beam X-rays orthogonal to the irradiation direction. The X-ray detector 24 has a multi-channel configuration in which a plurality of X-ray detection elements are arranged in a matrix, and, for example, 912 X-ray detection elements are arranged in a channel direction that forms the length of the matrix. Moreover, for example, 16 X-ray detection elements are arranged in a slice direction which is a short direction of the matrix.

  The X-ray detector 24 as a whole forms an X-ray incident surface curved in a cylindrical concave shape. As the X-ray detector 24, for example, a semiconductor X-ray detection element using cadmium tellurium (CdTe) or the like is used.

  A data collection unit 26 is connected to the X-ray detector 24. The data collection unit 26 collects detection data of individual X-ray detection elements of the X-ray detector 24. X-ray irradiation from the X-ray tube 20 is controlled by an X-ray controller 28. Note that the connection relationship between the X-ray tube 20 and the X-ray controller 28, the connection relationship between the collimator 22 and the collimator controller 30, and the connection relationship between the filter unit 21 and the filter controller 31 are not shown.

  The above-described components from the X-ray tube 20 to the filter controller 31 are mounted on the rotating unit 34 of the scanning gantry 2. Here, the subject 1 or the phantom is placed on a table in a bore 29 located at the center of the rotating unit 34. The rotating unit 34 rotates while being controlled by the rotation controller 36, and bombards X-rays from the X-ray tube 20, and the X-ray detector 24 detects the transmitted X-rays of the subject 1 or the phantom as projection information. The connection relationship between the rotating unit 34 and the rotation controller 36 is not shown.

  Moreover, the xyz coordinate axes described in FIG. 1 form a coordinate axis common to the xyz coordinate axes in the drawings described later, and illustrate the mutual positional relationship. Here, the subject 1 placed on the table is transported into the bore 29 by movement in the z-axis direction. The rotating unit 34 rotates in a plane formed by the xy axes, but rotates with an inclination from the xy plane when the scanning gantry 2 is tilted.

  The operation console 6 has a control processing device 60. The control processing device 60 is configured by a computer or the like, for example. A control interface (interface) 62 is connected to the control processing device 60. The scanning gantry 2 is connected to the control interface 62. The control processing device 60 controls the scanning gantry 2 through the control interface 62.

  The data collection unit 26, the X-ray controller 28, the collimator controller 30, the filter controller 31 and the rotation controller 36 in the scanning gantry 2 are controlled through the control interface 62. The individual connections between these units and the control interface 62 are not shown. In addition, the control processing device 60 forms a control unit for the filter unit 21 together with the filter controller 30.

  The control processing device 60 is also connected to a data collection buffer 64. 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 control processing device 60 through the data collection buffer 64.

  The control processing device 60 performs image reconstruction using transmission X-ray signals of a plurality of slices acquired through the data acquisition buffer 64, that is, projection information. A storage device 66 is also connected to the control processing device 60. The storage device 66 stores projection information collected in the data collection buffer 64, reconstructed tomographic image information, a program (program) for realizing the functions of the present device, and the like.

  The control processing device 60 is also connected to a display device 68 and an operation device 70. The display device 68 displays tomographic image information or scan information output from the control processing device 60. The operation device 70 is operated by an operator and inputs various instructions and information to the control processing device 60. The operator uses the display device 68 and the operation device 70 to operate the device.

  Next, the X-ray irradiation and detection part centering on the filter unit 21 will be described in detail with reference to FIG. FIG. 2 is a diagram showing the filter unit 21 and related X-ray irradiation and detection portions. The filter unit 21 includes a primary filter 23, a block unit 130, a support frame 120, and moving means 121. Here, the moving means 121 is controlled by the filter controller 31. Further, the X-ray tube 20, the collimator 22, the filter unit 21, the X-ray detector 24, and the like shown in FIG. 2 form a part of the rotating unit 34, and in the scanning gantry 2 with their mutual positions fixed. Rotate.

  The primary filter 23 is made of a thin plate having a thickness of several millimeters such as aluminum (Al) or copper (Cu) disposed in the vicinity of the collimator 22 and mainly prevents excessive X-ray exposure of the subject 1 due to unforeseen circumstances. To prevent.

  The block unit 130 is composed of a plurality of blocks such as acrylic or copper. These blocks have a rectangular parallelepiped shape and are fitted into the support frame 120 without a gap. And it can slide only in the channel direction of the X-ray detector 24. Further, these blocks exist in a pair symmetrical with respect to the channel direction around the X-ray beam irradiated from the X-ray tube 20. Similarly, the support frame 120 and the moving means 121 that support these blocks also exist in pairs on the left and right. This portion is conventionally called a bowtie filter.

  In addition, the support frame 120 is fixed to the moving unit 121 so as to surround the case of the moving unit 121, similarly to the block unit 130. Inside the moving means 121, the same number of motors as the blocks are arranged, and these motors are connected to each block by a shaft. Then, by driving these motors, the block is moved to an arbitrary position in the channel direction. The motors are individually controlled by the filter controller 31. Moreover, since the block part 130 and the moving means 121 have the common support frame 120, when changing a block position, vibrations, such as a shift | offset | difference shifted from the channel direction, are prevented.

  Further, the beam width of the X-ray beam generated from the X-ray tube 20 is controlled in the channel direction and also in the slice direction orthogonal to the rotation surface of the rotation unit 34 by controlling the opening of the collimator 22. After that, an X-ray beam having a predetermined width in the slice direction is transmitted through the block 130 and partially attenuated, and then transmitted through the subject 1 (not shown) to be detected two-dimensionally in the channel direction and the slice direction. It is detected by an X-ray detector 24 in which elements are arranged.

  In FIG. 2, the block unit 130 is composed of 4 × 4 blocks in the slice direction and the irradiation direction. This is an example for easy understanding of the drawing. For example, the X-ray detector 24 includes In the case of having 16 rows of detection elements in the slice direction, it is preferable to provide 16 blocks in the slice direction. Furthermore, it is possible to stack more blocks in the irradiation direction of the block unit 130 and optimize the exposure dose of the subject 1 more precisely.

  Next, the operation of the block unit 130 will be described with reference to FIGS. The block unit 130 dynamically changes the block position according to the imaging region of the subject 1. As an example, a case where the subject 1 at the position shown in FIG. 3 is imaged is shown.

  FIG. 3 is a diagram illustrating a positional relationship among the subject 1, the X-ray tube 20, and the filter unit 21 when performing imaging. The X-ray beam generated from the X-ray tube 20 expands in the slice direction, and images a boundary region including both the chest and abdomen of the subject 1. Here, the chest region has a chest cavity inside and X-ray attenuation is small, and the abdominal region has various organs inside and X-ray attenuation is large. Therefore, irradiating the chest region with the same dose of X-rays as the abdominal region results in excessive X-ray exposure for the subject 1.

  FIG. 4 is a diagram illustrating the position of each block of the block unit 130 when the subject 1 is disposed at the position illustrated in FIG. 3. Each of these blocks is moved to the illustrated position by the moving means 121. At this time, first, imaging part information is input from the operation console 6 by the operator. On the other hand, in the control processing device 60, block position information for reducing the exposure dose of the subject 1 is registered in advance for each imaging region. Then, the control processing device 60 specifies an imaging region for each block position in the slice direction from the imaging region information and the registered block position information, determines each block position, and performs setting via the filter controller 31. .

  FIG. 4A is a diagram showing the block position of the entire block unit 130. The position in the channel direction is set for each block. FIG. 4B shows only the block arrangement on the S4 plane in the slice direction. As shown in FIG. 3, the X-rays irradiated to the chest are transmitted through the surfaces S1 and S2, and the X-rays applied to the abdomen are transmitted through the surfaces S3 and S4. Accordingly, the S1 and S2 surfaces have the same block cross section as the same part, and the S3 and S4 surfaces are the same.

  In addition, since the X-rays irradiated to the chest are transmitted through the S1 and S2 planes, as shown in FIG. 4A, the irradiation direction is thick and the attenuation of the X-rays is large. The S3 and S4 planes are As shown in FIG. 4B, the X-rays are thin in the irradiation direction and the attenuation of the X-rays is small.

  FIG. 4C illustrates a cross section in the slice direction at the center position of the block unit 130 as an example. The S1 and S2 planes corresponding to the chest region are composed of two blocks in the irradiation direction, so the attenuation of X-rays is large. The S3 and S4 planes corresponding to the abdominal region are composed of one block in the irradiation direction, so The attenuation is small.

  As described above, in the first embodiment, the block unit 130 of the filter unit 21 is configured by a plurality of rectangular parallelepiped blocks that are divided into two in the channel direction and stacked in the slice direction and the irradiation direction in which X-rays are incident. By moving the block in the channel direction, the transmitted X-ray dose of the block unit 130 is particularly different in the slice direction. Therefore, when the imaging range of the subject 1 includes a plurality of parts having different absorbed X-ray doses The exposure dose can be reduced for each part, and the total exposure dose of the subject 1 can be reduced without significant deterioration in image quality.

In the first embodiment, the block unit 130 has a stacked structure of a large number of blocks having the same shape. However, by using a plurality of blocks having different shapes in the slicing direction and these moving means, the same is achieved. The exposure dose reduction filter can be easily configured.
(Embodiment 2)
By the way, in Embodiment 1 described above, the block position optimized in the slice direction is determined based on the difference in absorbed X-ray dose between the chest region and the abdominal region, and the total exposure dose is reduced. The block position in the slice direction can be determined from the difference in the contribution rate when performing the measurement, and the exposure dose of the subject 1 can be reduced. Therefore, the second embodiment shows a case where the block position in the slice direction is determined from the difference in contribution rate when performing image reconstruction.

  Here, the X-ray CT apparatus includes the scanning gantry 2 and the operation console 6 shown in FIG. 1, and the filter unit 21 is exactly the same as that shown in FIG. .

  Here, the case where helical scanning is performed is shown as an example. In the helical scan, the rotating unit 34 continuously rotates, and simultaneously moves the subject 1 on the cradle at a predetermined speed to perform imaging. At this time, the image reconstruction performed by the control processing device 60 is schematically shown in FIG.

  FIG. 5 is a diagram illustrating image reconstruction performed by the control processing device 60. In addition, the X-ray detector 24 has illustrated the case where it has an X-ray detection element of 4 slices in the slice direction. In this helical scan, helical data D1 to D4 for each slice are acquired. Then, the control processing device 60 generates the sinogram S1 at the specific position from the helical data D1 to D4 by cutting out the helical data D1 to D4 in the vicinity of the specific position and using the interpolation function f or the like.

S1 = f (k1 * D1, k2 * D2,..., K4 * D4)
Here, k1,..., K4 are contribution rates of the helical data D1 to D4. Thereafter, the image data is reconstructed from the sinogram S1 using a filtered back projection (Filtered Back Projection) or the like.

  FIG. 6A shows the physical meaning of the contribution ratios k1,..., K4. FIG. 6A shows the relative positions of the X-ray tube 20 and the X-ray detector 24 in the slice direction. Here, the X-ray tube 20 is positioned approximately above the center of the X-ray detector 24, and the imaging cross section obtained by interpolation is positioned between the slices 2 and 3. Accordingly, the data D1 and 4 of the slices 1 and 4 located at the end of the X-ray detector 24 in the slice direction include image information of a cross section inclined from the imaging cross section obtained by interpolation, and an error increases.

  As a result, when an imaging section is obtained using the interpolation function f, weighting is performed according to the inclination of the section for each slice, and image information with higher accuracy is acquired. In the example of FIG. 6, slices 2 and 3 have the highest contribution because they are closest to the interpolated tomographic plane, and slices 1 and 4 are tilted from the most interpolated tomographic plane, so the lowest contribution is achieved. Have Note that the optimum contribution rate is experimentally selected in consideration of the interpolation method, the number of slices, the magnitude of the inclination, and the like.

  Here, in a slice with a small contribution rate, even if the X-ray dose to be irradiated is reduced, the degree of image quality deterioration is small, and therefore the exposure dose can be effectively reduced. The control processing device 60 sets the block position so that the thickness of the block in the irradiation direction is substantially proportional to the reciprocal of the contribution rate. FIG. 6B shows an example of a cross section in the slice direction at the center of the block portion 130 when the number of slices is four. In the vicinity of the center corresponding to the slices 3 and 4 of the X-ray detector 24, the block thickness in the irradiation direction is thin, the transmitted X-ray dose increases, and at the end corresponding to the slices 1 and 4, the block thickness in the irradiation direction is Thicker, the transmitted X-ray dose is reduced.

As described above, in the second embodiment, the block thickness at the end portion in the slice direction of the block portion 130 with a small contribution rate is thinned, so that the X-ray that passes through the end portion in the slice direction with a small contribution rate is used. The exposure dose of the subject 1 can be reduced without causing large deterioration in image quality.
(Embodiment 3)
By the way, in the first embodiment, a block position for each imaging region such as a chest region or abdominal region that is set in advance is selected from imaging information input by the operator, and the X-ray dose is determined for each imaging region in the slice direction. Although the minimized block position is set and the total radiation dose is reduced, the projection position of the subject 1 obtained by a scout scan and the projection information is obtained for the block position for each imaging region. Can be automatically determined based on the projection length information included in. Therefore, the third embodiment shows a case where the block position is determined from the image information acquired by the scout scan.

  Here, the X-ray CT apparatus has the scanning gantry 2 and the operation console 6 shown in FIG. 1, and the filter unit 21 is exactly the same as that shown in FIG. To do.

  FIG. 7 is a flowchart showing the operation of the control processing device 60 according to the third embodiment. First, the operator places the subject 1 on the table (step S701). Then, a scout scan of the subject 1 is performed (step S702). FIG. 8A shows this scout scan. The subject 1 placed on the imaging table 4 is moved in the z-axis direction, and the X-ray tube 20 remains fixed without rotating, and the subject 1 is projected within a range where the subject 1 is scanned. Get information.

  Thereafter, the operator selects a shooting mode (step S703). In this selection, a high sensitivity mode in which the blocks of the filter unit 21 are fixedly arranged, or an exposure reduction mode in which the block positions are rearranged in synchronization with the table positions are selected. In particular, in the case of a helical scan or the like in which the table moves continuously, the imaging region moves during the scan, so the exposure reduction mode is preferable.

  Thereafter, the control processing device 60 determines whether or not the exposure reduction mode is selected (step S704), and when the exposure reduction mode is not selected (No at step S704), the block of the filter unit 21 is initialized (step S704). Step S705). In this initial setting, for example, the block arrangement determined by the chest or abdomen is set by the filter controller 31. Then, scanning is performed with the block arrangement fixed (step S706), and data is collected.

  Further, when the exposure reduction mode is selected (Yes at Step S704), the control processing device 60 obtains projection length information of the imaging range of the subject 1 from the projection image information acquired by the scout scan of Step S702. (Step S707). Here, the projection length information is an amount calculated by logarithmic conversion from the X-ray intensity detected by the X-ray detection element, assuming that the X-ray absorption coefficient of the tissue portion of the subject 1 is substantially constant. 1 represents the transmission length of X-rays in the internal tissue part.

  FIG. 8B is an example of projection length information acquired by the scout scan of the subject 1 shown in FIG. FIG. 8B shows the table position on the horizontal axis and the projection length on the vertical axis, and the table position on the horizontal axis corresponds to the position of the table on which the subject 1 is placed as shown in FIG. ing.

  The projection length acquired from the projection information shown in FIG. 8B includes the chest cavity at the table position corresponding to the chest of the subject 1, so the projection length is small, and the table corresponding to the abdomen of the subject 1. In position, the organs are densely packed, so the projection length is large.

  Returning to FIG. 7, the control processing device 60 determines a block arrangement for each table position from the projection length information obtained in step S707 (step S708). In this determination, the correlation table between the projection length and the filter thickness shown in FIG. 8C is used. This figure is obtained in advance by experiment, and the projection length and the filter thickness are in an inversely proportional relationship. When the projection length is short, the attenuation of X-rays in the subject 1 is small, so a thicker filter is used. The thick filter attenuates the irradiated X-rays and can reduce the exposure dose of the subject 1 without greatly degrading the image quality.

  FIG. 8D shows the optimum filter thickness for each table position on which the subject 1 is placed, which is obtained from FIGS. 8B and 8C. Here, the table position shown on the horizontal axis of FIG. 8D forms the same coordinate axis as the horizontal axis of FIG. In FIG. 8D, a thick filter is used at the table position corresponding to the chest of the subject 1 because the projection length is small, and a thin filter is used at the table position corresponding to the abdomen of the subject 1 because the projection length is large. Used.

  FIG. 8D shows the filter thickness in the slice direction corresponding to the table position. Similarly, the filter thickness in the channel direction of the X-ray detector 24 can be obtained from the projection length in the channel direction. .

  Returning to FIG. 7, the control processing device 60 performs initial setting for the block unit 130 (step S <b> 709). Here, based on the table position at the start of scanning and the filter thickness information for each table position obtained in step S708, the filter controller 31 moves the block in the channel direction, and performs predetermined processing over the slice direction and the channel direction. A block portion 130 having a filter thickness is formed.

  Thereafter, the control processing device 60 starts scanning (step S710). Then, the control processing device 60 determines whether or not the table has moved (step S711). If the table has not moved (No at step S711), data is collected as it is (step S713), and the table is moved. If so (Yes in step S711), the block unit 130 is rearranged so as to have a filter thickness corresponding to the moved position (step S712), and then data collection in step S713 is performed.

  Thereafter, the control processing device 60 determines whether or not the data collection has ended (step S714). If the data collection has not ended (No at step S714), the control processing device 60 proceeds to step S711 and confirms the table movement. Do. If the data collection is completed (Yes at step S714), the process is terminated.

  As described above, in Embodiment 3, the projection length information of the imaging range of the subject 1 is obtained by scout scanning, the filter thickness for each table position is determined from this projection length information, and at the time of scanning, Since the filter thickness is automatically controlled according to the change in the table position, a plurality of portions of the subject 1 having greatly different projection lengths as in the case of photographing the chest and abdomen using a helical scan or the like, When the table is continuously moved and imaged, the filter thickness can be dynamically changed to the lowest exposure dose, and the exposure dose during scanning can be reduced without significant image quality degradation.

  In the third embodiment, the filter unit 21 includes the block unit 130 and the moving unit 121 illustrated in FIG. 2, but the thickness of the primary filter 23 provided between the block unit 130 and the collimator 22 and The position can be made variable by the filter controller 31, and the exposure dose of the subject 1 can be reduced. At this time, automatic control using projection length information acquired by a scout scan similar to the above can also be performed.

It is a block diagram which shows the whole structure of a X-ray CT apparatus. FIG. 3 is a diagram illustrating a configuration of a filter unit according to the first embodiment. FIG. 3 is a diagram illustrating a subject imaging position according to the first embodiment. FIG. 4 is a diagram illustrating an operation of a block unit according to the first embodiment. 6 is a diagram schematically illustrating data processing according to Embodiment 2. FIG. FIG. 10 is a diagram illustrating an arrangement of block units according to the second embodiment. 10 is a flowchart illustrating processing for determining a block position according to the third embodiment. FIG. 10 is a diagram schematically showing the processing of the third embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Subject 2 Scanning gantry 4 Imaging table 6 Operation console 20 X-ray tube 21 Filter part 22 Collimator 23 Primary filter 24 X-ray detector 26 Data acquisition part 28 X-ray controller 29 Bore 30 Collimator controller 31 Filter controller 34 Rotation part 36 Rotation Controller 60 Control processing device 62 Control interface 64 Data collection buffer 66 Storage device 68 Display device 70 Operation device 120 Support frame 121 Moving means 130 Block section

Claims (9)

An X-ray tube that emits a cone-shaped X-ray beam;
A detection element for detecting the X-ray beam, which is two-dimensionally arranged in a slice direction, which is a body axis direction of a subject to be imaged, and in a channel direction orthogonal thereto, on a plane substantially orthogonal to the irradiation direction of the X-ray beam; ,
A filter unit including a plurality of blocks disposed near the detection element side of the X-ray tube to attenuate the X-ray beams stacked in the irradiation direction and the slice direction ;
Moving means for moving each of the plurality of blocks in a direction substantially orthogonal to the irradiation direction;
Control means for controlling the movement of the block in the moving means so that the filter unit has an arrangement of the block so as to attenuate X-rays according to the imaging position in the body axis direction of the subject ;
An X-ray CT apparatus comprising: The block, X-rays CT apparatus according to claim 1, characterized in that the two sequences in the channel direction of the elongate said two-dimensional array. The block, X-rays CT apparatus according to claim 1 or 2, characterized in that it has a rectangular parallelepiped shape. The X-ray CT apparatus according to claim 1 , wherein the filter unit includes a cylindrical support frame that surrounds blocks stacked in the slice direction and the irradiation direction. It said moving means, X-rays CT apparatus according to claim 1, characterized in that it comprises a shaft connected to the block. The X-ray CT apparatus according to claim 4 , wherein the support frame is fixed to an outer frame of a case in which the moving unit is built. The control means calculates an X-ray exposure dose of the subject for each imaging region having a different absorbed X-ray dose of the subject from a scout scan of the subject disposed between the X-ray tube and the detection element. X-ray CT apparatus according to any one of claims 1 to 6, characterized in that it comprises a first calculating means for calculating the block position to reduce. The control means determines the contribution ratio of the projection data in the slice direction to the image reconstruction when performing image reconstruction using projection data of a subject arranged between the X-ray tube and the detection element. based on the X-ray CT apparatus according to any one of claims 1 to 6, characterized in that it comprises a second calculating means for calculating the block position to reduce the X-ray exposure dose of the subject. Said control means, said in performing imaging while moving the body axis direction of the position of the subject, according to claim 6 or 7 the block position, and wherein the match the result of the calculation for each of the positions X-ray CT apparatus described in 1.

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