JP2009131563A - X-ray CT system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide X-ray CT equipment photographing a plurality of sites different in body diameter size at one scanning and maintaining a picture quality obtained by the scanning excellent. <P>SOLUTION: The X-ray CT equipment is mounted with an amplifier which has many kinds of gains and amplifies respective signals outputted by an X-ray detector with any of the many kinds of gains. Then, each size in the body diameter direction at each position in the body axis direction of a subject is premeasured and a gain value of a signal at each position is predetermined on the basis of measured sizes. When exposure of an X-ray starts, the gain of the amplifier is switched for each signal according to the determined gain value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus that reconstructs an image in a subject based on X-ray exposure.

  There is an X-ray CT apparatus that reconstructs the inside of an object as an image by exposing the object from multiple directions and treating each X-ray transmitted through the object as projection data.

  In the X-ray CT apparatus, X-rays are exposed to a subject, the amount of X-ray transmission is measured, and the inside of the subject is imaged mainly using a reconstruction algorithm called the Feldkamp method. Comparing the X-ray dose transmitted through the position where the body diameter of the subject is different, the X-ray transmission amount varies depending on the body diameter. At a position where the body width of the body diameter is large, the X-ray transmission amount is smaller than the X-ray dose transmitted through a position where the body width of the body diameter is smaller than that. Accordingly, if the body width of the body diameter is large, the value of the electrical signal obtained from the X-ray detector by exposing the position is also reduced, and the S / N ratio of the signal is lowered, and therefore reconstructed. The S / N ratio of the image also decreases.

  Therefore, when imaging an object with an X-ray CT apparatus, the gain of an amplifier that amplifies the signal from the X-ray detector is set according to the size of the object to be imaged, that is, the size of an imaging area called FOV. It is adjusted. For example, when photographing the head, a gain corresponding to the size of the head is set in advance. When photographing the chest, a gain corresponding to the size of the chest is set in advance. When photographing the abdomen, a gain corresponding to the size of the abdomen is set in advance.

  In recent years, the number of X-ray detection elements arranged in an X-ray CT apparatus has increased, and a wide range of scans can be performed by one scan (see, for example, “Patent Document 1”). As a result, it has become possible to photograph a plurality of parts with different body diameters such as the head and chest in one scan.

JP 2004-305527 A

  Conventionally, even if a plurality of parts having different body diameter body widths are photographed in one scan, only one gain can be set for one scan. In other words, the gain has to be set in accordance with the width, in other words, the size of one of the shooting areas.

  Therefore, for example, if the gain is set in accordance with a part having a small body diameter body width, the S / N ratio of the image is lowered in a part having a large body diameter body width. Moreover, if the gain is set in accordance with a portion having a large body width, the signal may overflow if a signal obtained by exposing a portion having a small body width is amplified. That is, in the conventional apparatus, when a plurality of parts having different body diameter body widths are imaged by one scan, there is a possibility that the image quality is deteriorated.

  In order to prevent overflow and a decrease in the S / N ratio of the image, it is necessary to scan in multiple times according to the body width, in other words, according to the size of the imaging region, which increases the amount of exposure to the subject. Connected.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to capture a plurality of portions having different body diameter body widths in one scan and to perform the scan. An X-ray CT apparatus capable of maintaining good image quality is provided.

  In order to solve the above-described problem, an X-ray CT apparatus according to the present invention described in claim 1 irradiates X-rays at scan positions aligned in the body axis direction of a subject and transmits the scan positions. Scanning means for detecting the detected X-rays and outputting each signal reflecting the amount of X-ray transmission at each scanning position, and a plurality of types of signals each having a plurality of types of gain and outputted from the scanning means An amplifier that amplifies at any one of the gains, a measuring means that measures in advance the body width in the body radial direction at each scan position before scanning by the scanning means, the body width and the gain of the amplifier, Storage means for storing the association of the amplifier in advance, and gain control means for switching the gain of the amplifier for each signal at each scan position in accordance with the previously measured body width. Rukoto, characterized by.

  The storage means stores in advance the association between the body width and the size of the imaging region, and the second storage means stores in advance the association between the size of the imaging region and the gain value. And the correspondence between the body width and the gain value via the size of the imaging region may be stored (corresponding to the invention according to claim 2).

  The measuring unit may measure a body width in the body radial direction of each scan position based on a planar transmission image of a subject imaged in advance (corresponding to the invention according to claim 3).

  You may make it further provide the pre-processing means which correct | amends based on the switched gain with respect to the signal amplified with the different gain according to the said scanning position (equivalent to invention of Claim 4).

  In order to solve the above problems, an X-ray CT apparatus according to the present invention described in claim 5 irradiates X-rays to the scan positions arranged in the body axis direction of the subject and transmits the scan positions. Scanning means for detecting the detected X-ray and outputting each signal reflecting the X-ray transmission amount at each scanning position, and each of the signals output from the X-ray detector having a plurality of types of gains. Based on an amplifier that amplifies at any gain and a planar transmission image of the subject that has been imaged in advance, the body width in the body diameter direction at each scan position is measured in advance before scanning by the scanning means. Measuring means, storage means for storing in advance a gain table in which the body width and the gain of the amplifier are associated, and searching for a gain corresponding to the body width at each scan position from the gain table, Each A gain determination unit that generates a correlation between a can position and a gain value of a signal at the scan position in advance before scanning by the scanning unit, and the gain of the amplifier according to the correlation generated by the gain determination unit A gain control means for switching each signal at each scan position, and a preprocessing means for performing correction based on the switched gain on the signal amplified with a different gain according to the scan position; It is characterized by providing.

  According to the present invention, even if a region including a plurality of types of body widths is scanned at one time, it is possible to prevent a part of the projection data from overflowing or a reduction in the S / N ratio of the image. A good image with high continuity and contrast effect can be generated. In addition, in order to prevent overflow and a decrease in the S / N ratio of the image, it is not necessary to scan in multiple times according to body width, in other words, according to the size of the imaging region, and it is possible to reduce exposure.

  Hereinafter, preferred embodiments of an X-ray CT apparatus according to the present invention will be specifically described with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of an X-ray CT apparatus 10 according to the present embodiment.

  The X-ray CT apparatus 10 is configured to perform a whole body scan or the like by continuously or intermittently changing the X-ray exposure position around the body axis of the subject P and the body axis direction by a helical scan or a conventional scan. In addition, the imaging of a plurality of parts of the subject P is completed in one scan. In the X-ray CT apparatus 10, a stage for scanning a subject P with a scanogram, a stage for analyzing the scanogram to determine a gain value for each scan position, and amplifying an X-ray detection signal for each scan position with the determined gain value. An image inside the subject P is finally obtained through the main scanning steps in order.

  The scanogram is a plane transmission image of the subject P. The scan position is the position of a cross section that is imaged by reconstruction. The gain value is a value indicating the magnification for amplifying the X-ray transmission signal that reflects the transmission amount of the X-ray transmitted through the subject P.

  In the stage of determining the gain value, the body width in the body diameter direction of the subject P is measured for each scan position from the scanogram, and the size of the imaging region called FOV corresponding to the body width is obtained for each scan position. Then, the gain value is determined according to the size of the imaging area obtained for each scan position. In the main scan, the X-ray transmission signal obtained continuously is continuously switched to the determined gain value in units of scan positions and amplified.

  The X-ray CT apparatus 10 includes a gantry 12, a bed 14, and a processing unit 16. The gantry 12 is an apparatus that irradiates the subject P with X-rays from multiple directions, detects X-rays that have passed through the subject P, and outputs X-ray transmission data that reflects the amount of X-ray transmission. is there. The gantry 12 has an opening 18 into which the subject P is inserted. The bed 14 is a device that inserts the subject P into the opening 18. The processing unit 16 generates scan plan data for controlling the driving of the gantry 12 and the bed 14, performs integrated control of the gantry 12 and the bed 14, and performs image reconstruction processing on the X-ray transmission data, thereby subject to the subject. It is a device that generates and displays an image in P. The scan plan data is a scan sequence in which tube current, tube voltage, and the like and a gain value are determined for each scan position.

  The gantry 12 accommodates a gantry 20 and a rotation drive device 22 therein. The gantry 20 is a ring body that can rotate around the opening 18. The rotary drive device 22 is configured by a motor and a gear having a meshing relationship with the gantry 20, and rotates the gantry 20 around the opening 18.

  In the gantry 20, an X-ray tube 24 and an X-ray detector 26 are installed facing each other across the opening 18. The X-ray tube 24 and the X-ray detector 26 emit X-rays to positions arranged in the body axis direction of the subject P, detect X-rays transmitted through the positions, and transmit X-rays at the positions. Scanning means for outputting each X-ray transmission signal reflecting the quantity.

  Further, a collimator 28 is disposed in the gantry 20 so as to be interposed between the X-ray tube 24 and the X-ray detector 26. Inside the gantry 12, a high voltage generating device 30 is disposed in a pair with the X-ray tube 24, and a diaphragm driving device 32 is disposed in a pair with the collimator 28, and is paired with the X-ray detector 26. Thus, a data acquisition device 34 called a DAS (Data Acquisition System) is arranged.

  Further, the gantry 12 is provided with a gantry controller 36 inside. The gantry control device 36 controls the high voltage generation device 30, the aperture driving device 32, the data collection device 34, and the bed 14 in units of scan positions according to the scan plan data. In other words, the gantry control device 36 transmits a control signal corresponding to the scan position being photographed to the high voltage generation device 30, the aperture driving device 32, the data collection device 34, and the bed 14.

  The high voltage generator 30 supplies a current for heating the filament to the X-ray tube 24 and applies a high voltage. This high-voltage generator 30 is a high-frequency inverter system, that is, rectifies a 50/60 Hz AC power source into a direct current, converts it to a high frequency alternating current of several kHz or more, boosts it, and rectifies it again. The method of applying is applied. The X-ray tube 24 generates X-rays upon receiving a current supply and application of a high voltage.

  The collimator 28 is driven by the aperture driving device 32 to define an X-ray imaging region and intercepts X-rays other than the imaging region, so that the X-ray generated by the X-ray tube 24 is fan-shaped or cone-shaped. Focus on the beam.

  The X-ray detector 26 arranges multi-row multi-channel X-ray detection elements in an array in two orthogonal directions. The array shape is an arc shape centered on the focal point of the X-ray generated from the X-ray tube 24. The X-ray detection element is an indirect conversion type in which X-rays are converted into light by a phosphor such as a scintillator, and further converted into charges by a photoelectric conversion element such as a photodiode, or an electron-hole pair in a semiconductor by X-rays. The direct conversion type utilizing the generation and the transfer to the electrode, that is, the photoconductive phenomenon, is the mainstream. The X-ray detector 26 detects X-rays that have passed through the subject P, and outputs an X-ray transmission signal that reflects the detected X-ray transmission amount for each X-ray detection element.

  The data collection device 34 collects and integrates the X-ray transmission signals output from the respective X-ray detection elements in synchronization with the X-ray exposure cycle, amplifies the gain signals according to the scan positions, and converts them into digital signals. Convert. Each time a control signal is input from the gantry control device 36, an X-ray transmission signal is collected from each X-ray detection element. The control signal for acquiring the X-ray transmission signal necessary for the reconstruction of the next scan position includes a gain switching signal for switching the gain value.

  The data collection device 34 outputs the X-ray transmission signal converted into the digital signal to the processing unit 16. The X-ray transmission signal output from the data acquisition device 34 is digital data including an integral value of each absorption coefficient along the transmission length of X-rays sequentially transmitted through substances having different X-ray absorption coefficients. It becomes data.

  The couch 14 includes a top board 38 on which the subject P is placed and a couch driving unit 40. The top plate 38 slides in the longitudinal direction by the driving of the bed driving unit 40 and is inserted into the opening 18. The couch driving unit 40 outputs a couch position signal indicating the position of the top board 38 to the gantry controller 36. The gantry control device 36 analyzes the bed position signal, and when the top board 38 has reached the X-ray exposure position for reconstructing the next scan position, the gain value corresponding to the next imaging position A control signal for switching to is input to the data collection device 34.

  In the processing unit 16, a pre-processing unit 42, a projection data storage unit 44, a reconstruction processing unit 46, an image storage unit 48, an image processing unit 50, and a display device 52 are sequentially connected by signal lines and mounted. . The processing unit 16 is provided with a console control unit 54.

  The preprocessing unit 42 performs sensitivity correction on the raw data composed of the X-ray transmission signal. The pre-processing device 56 performs sensitivity correction on the X-ray transmission signal necessary for reconstructing the scan position according to the size of the imaging region at the scan position. The raw data subjected to the sensitivity correction is called projection data, and is input and stored in the projection data storage unit 44.

  The reconstruction processing unit 46 reads the corrected projection data from the projection data storage unit 44 and back-projects it mainly using a reconstruction algorithm called the Feldkamp method, and reconstructs the object P as image data. . The reconstructed image data is input and stored in the image storage unit 48.

  The image processing unit 50 performs various types of image processing such as scan conversion processing for converting the image data stored in the image storage unit 48 into a video format of an orthogonal coordinate system to generate a display image. The display device 52 is a monitor such as a liquid crystal display or a CRT display, and displays the display image generated by the image processing unit 50.

  The console-side control unit 54 generates scan plan data as scan plan means and outputs it to the gantry control device 36. Further, the console side control unit 54 outputs a control signal for selecting correction data for sensitivity correction to the preprocessing unit 42.

  In such an X-ray CT apparatus 10, first, scan plan data for creating a scanogram is generated by the console side control unit 54 and output to the gantry control apparatus 36.

  The gantry control device 36 to which the scan plan data for creating the scanogram is input does not output a rotation control signal to the rotation drive device 22 that rotates the gantry 20, and the high voltage generation device 30 and the aperture drive device 32. And a drive signal is output to the data collecting device 34 and the bed driving unit 40. Therefore, in the X-ray CT apparatus 10, when driving for scanogram creation, the X-ray is emitted by moving the top plate 38 while the X-ray tube 24 and the X-ray detector 26 are fixed to one surface side of the subject P. After exposure, the subject P is scanned in the body axis direction, and a scanogram, which is a plane transmission image of the subject P, is acquired.

  When the scanogram is obtained, the console-side control unit 54 measures the body width in the body diameter direction of each scan position from the scanogram, and scan plan data in which the gain value corresponding to the measured body width is associated with each scan position. Generate. This scan plan data becomes control data for the main scan. When this scan plan data is generated, the console side control unit 54 outputs this scan plan data to the gantry control device 36.

  When the gantry control device 36 receives the scan plan data for the main scan, the gantry control device 36 sends the high voltage generator 30, the aperture drive device 32, the rotation drive device 22, the data collection device 34, and the bed drive unit 40 according to the scan plan data. In response, a drive signal is output. Therefore, in the X-ray CT apparatus 10, when driving for the main scan, the top plate 38 is moved while the gantry 20 rotates and the X-ray detector 26 rotates around the subject P between the X-rays. X-rays are then exposed, and the subject P is scanned by conventional scanning or helical scanning.

  The gantry control device 36 corresponds to the next scan position with respect to the data collection device 34 by referring to the scan plan data when the X-ray exposure position moves to the next scan position during the scan by the main scan. A gain switching signal for amplifying the transmitted X-ray transmission signal is output. When the data collection device 34 receives the gain switching signal, the data collection device 34 performs amplification processing by switching the gain with respect to the next input X-ray transmission signal.

  Then, the preprocessing unit 42 corrects the sensitivity characteristic caused by the difference in amplification accuracy of each gain with the correction data corresponding to the scan position to which the input X-ray transmission signal belongs, and then the projection data storage unit 44. The reconstructed image in the subject P is displayed on the display device 52 through the reconstruction processing unit 46, the image storage unit 48, and the image processing unit 50 in order.

  FIG. 2 is a block diagram showing a more detailed configuration relating to gain switching of the console-side control unit 54, the gantry control device 36, and the data collection device 34 in the X-ray CT apparatus 10 as described above.

  The data acquisition device 34 has an integrator 58, an amplifier 60, and an AD converter 62 that are electrically connected in that order for each output of each X-ray detection element. The amplifier 60 is connected to a plurality of resistors R1, R2, R3... Arranged in parallel and a switching circuit 64 connected to the resistors R1, R2, R3. These resistors R1, R2, R3,... Are resistors that determine the gain.

  The integrator 58 periodically accumulates the X-ray transmission signal, converts it into a voltage signal, and outputs it to the amplifier 60. The amplifier 60 is arranged in parallel with a plurality of resistors R1, R2, R3..., And the resistors are switched to any one. The switching circuit 64 includes a switch for selecting one of the resistors R1, R2, R3,..., And switches the resistor by support from the gantry control device 36. The AD converter 62 converts the analog signal output from the amplifier 60 into a digital signal.

  In the data collection device 34, the amplifier 60 amplifies the X-ray transmission signal with a gain value corresponding to the resistance value of the resistor switched by the switching circuit 64.

  The console side control unit 54, the gantry control device 36, and the switching circuit 64 are gain control means for controlling the gain of the amplifier 60.

  The gantry control device 36 includes a switching control unit 66 and a scan plan data storage unit 68.

  The switching control unit 66 identifies a scan position corresponding to the position where the X-rays are being exposed, and switches a gain switching signal for switching to a resistor that realizes a gain value corresponding to the identified scan position. 64. For the gain value for the identified scan position, the scan plan data stored in the scan plan data storage unit 68 is referred to.

  The scan plan data is generated by the console side control unit 54 and stored in the scan plan data storage unit 68. In this scan plan data, a scan position and a gain value are associated with each other. A gain value for each scan position is determined in accordance with the body width in the body diameter direction of the scan position.

  The console side control unit 54 includes a body width measurement unit 70, an imaging region determination unit 72, an imaging region table storage unit, a gain determination unit 74, a gain table storage unit 76, and a scan plan data transmission unit 78. For convenience of explanation, the imaging area determination unit 72, the imaging area table storage unit, the gain determination unit 74, the gain table storage unit 76, and the scan plan data transmission unit 78 will be described separately. The imaging area determination unit 72, the imaging area table storage unit, the gain table storage unit 76, and the scan plan data transmission unit 78 may be included.

  The console-side control unit 54 measures the body width in the body diameter direction at each scan position, determines the size of the imaging region at each scan position from the body width, and determines the gain at each scan position from the size of the imaging region. The value is determined. That is, the gain value is determined through the size of the imaging region based on the body width in the body diameter direction at each scan position.

  The body width measuring unit 70 analyzes the scanogram obtained by imaging before the main scan for each scan position, and measures the body width in the body diameter direction at each scan position. The scanogram is read from the image storage unit 48. Each scan position is classified according to the slice thickness input using the man-machine interface.

  The imaging region determination unit 72 determines the size of the imaging region corresponding to the body width in the body radial direction for each scan position. As for the imaging region, a plurality of sizes such as S (Small), M (Middle), and L (Large) are determined in advance. The imaging region determination unit 72 refers to the imaging region table stored in the imaging region table storage unit, and determines the size of the imaging region for each scan position from the body width in the body diameter direction measured by the body width measurement unit 70. To decide.

  The gain determination unit 74 determines a gain value corresponding to the size of the imaging region for each scan position. The gain value is determined in advance in a plurality of magnitudes such as 1 times, 8 times, 16 times, etc. according to the resistors R1, R2, R3,. The gain determination unit 74 refers to the gain table stored in the gain table storage unit 76 and determines a gain value for each scan position from the size of the imaging region determined by the imaging region determination unit 72.

  The scan plan data transmitting unit 78 collects the association of the determined tube current, tube voltage, and the like with the gain value for each scan position in the scan plan data and outputs it to the gantry controller 36. That is, the scan plan data includes an association between the gain value determined by the gain determination unit 74 and information indicating the scan position where the gain value is selected.

  FIG. 3 is a flowchart showing processing for creating scan plan data of the console-side control unit 54.

  First, when a scanogram is generated by pre-scanning, the body width measurement unit 70 reads the scanogram from the image storage unit 48 (S01). When the scanogram is read, the scanogram is divided at each scan position (S02), and the body width in the body diameter direction at each scan position is measured (S03).

  FIG. 4 is a schematic diagram for explaining the process of measuring the body width in the body radial direction at each scan position using the scanogram generated by the pre-scan.

  The body width measurement unit 70 divides the scanogram in the body axis direction at intervals of the slice thickness input using the man-machine interface, and attaches scan position information PI that identifies the scan position in order from the head direction to each region. Each area is set as a scan position. Then, each scan position is scanned in the body diameter direction.

  The body width measuring unit 70 stores a threshold value in advance. The threshold value is a boundary value between a pixel value of a pixel representing the image of the subject P and a pixel value of a pixel represented by the background image. The body width measurement unit 70 sequentially compares the pixel values of the pixels constituting each scan position with a threshold value, and counts the number of pixels having a pixel value exceeding the threshold value for each scan position. This count value Cv is the body width in the body diameter direction.

  When the body width measurement unit 70 measures the body width in the body diameter direction, the body width measurement unit 70 associates the measured body width with the scan position information PI that identifies the scan position having the body width.

  When the body width in the body diameter direction at each scan position is measured, the imaging region determination unit 72 reads the imaging region table stored in the imaging region table storage unit (S04), and refers to the read imaging region table. Then, information indicating the size of the imaging region corresponding to the measured body width is associated with each scan position (S05).

  FIG. 5 is a data configuration diagram showing an imaging area table stored in the imaging area table storage unit.

  In the imaging area table, information indicating the size of the imaging area is associated with each body width category in the body diameter direction. The body width in the body diameter direction is a physical quantity representing a count value Cv or a distance obtained by converting the count value into an actual size value such as millimeters. The information indicating the size of the shooting area is, for example, S (Small), M (Middle), or L (Large). As the body width in the body diameter direction is larger, information indicating a larger imaging region is associated.

  The imaging region determination unit 72 reads the body width in the body diameter direction associated with the information for each scan position information PI. Then, referring to the imaging region table, information indicating the size of the imaging region corresponding to the section to which the read body width in the body radial direction belongs is set to the scan position associated with the body width in the read body radial direction. Further associated with the identified scan position information PI.

  Thereby, as shown in FIG. 6, for example, information indicating the size of the imaging region indicated by S (Small) is associated with each scan position included in the region corresponding to the head of the subject P, and For example, information indicating the size of the imaging region indicated by L (Large) is associated with each scan position included in the region corresponding to the body of the subject P.

  When the information indicating the size of the imaging region is associated with each scan position, the gain determination unit 74 reads the gain table stored in the gain table storage unit 76 (S06), and refers to the read gain table. A gain value corresponding to information indicating the size of the imaging region is associated with each scan position (S07).

  FIG. 7 is a data configuration diagram showing a gain table stored in the gain table storage unit 76.

  In the gain table, a gain value is associated with each piece of information indicating the size of the shooting area. The gain value is information representing an amplification factor such as 1 ×, 8 ×, or 16 ×, for example. This gain value matches the gain of the amplifier 60 when each of the resistors R1, R2, R3 is selected. In this gain table, a gain value with a larger magnification is associated with an increase in the size of the imaging region.

  For each scan position information PI, the gain determination unit 74 reads information indicating the size of the imaging region associated with the information. Then, referring to the gain table, the gain value corresponding to the information indicating the size of the read imaging region is further associated with the scan position information PI associated with the information indicating the size of the read imaging region.

  As a result, as shown in FIG. 8, each scan position included in the region corresponding to the head of the subject P is associated with, for example, a gain value indicated by 1 × and corresponds to the body of the subject P. For example, a gain value indicated by 16 times is associated with each scan position included in the region.

  When the gain value is associated with each scan position, the scan plan data transmission unit 78 generates scan plan data including each association between the scan position and the gain value (S08). When the scan plan data is generated, the scan plan data transmission unit 78 outputs the scan plan data to the gantry control device 36 (S09), and ends this process.

  Next, processing for switching gains according to scan plan data including associations between scan positions and gain values will be described with reference to FIG. FIG. 9 is a flowchart illustrating a process of switching the gain.

  First, from the couch driving unit 40, couch information indicating the amount of movement of the top board 38 is intermittently input to the switching control unit 66. When receiving the couch information (S11, Yes), the switching control unit 66 determines whether the X-ray exposure position has moved to the next scan position from the amount of movement of the top plate 38 indicated by the couch information (S12).

  When the X-ray exposure position moves to the next scan position (S12, Yes), the scan plan data is referred to from the scan plan data storage unit 68.

  FIG. 10 is a data configuration diagram showing a data configuration of scan plan data. In the scan plan data, scan position information PI is arranged, and corresponding gain values are associated with these pieces of information.

  The switching control unit 66 refers to the scan plan data and sets a gain value associated with the scan position information PI that identifies the next scan position, that is, the scan position including the position where the X-ray exposure is started. Read (S13). Then, the switching control unit 66 compares the read gain value with the gain value read in the previous routine (S14). Note that at the time when X-ray exposure is started, there is no previous routine, so this comparison is not performed.

  If the read gain value is different from the gain value read in the previous routine (S14, Yes), a switching circuit for switching a gain switching signal to switch to a resistor corresponding to the gain represented by the read gain value. 64 (S15). This gain switching signal is a control signal for switching to a new gain corresponding to a new scan position including the position where X-ray exposure is started.

  When receiving the gain switching signal (S16), the switching circuit 64 switches the circuit connection to a resistor corresponding to the gain switching signal (S17). By switching the circuit, a gain corresponding to a new scan position including the position where the X-ray exposure is started is set in the amplifier 60.

  When the X-ray transmission signal is input (S18), the amplifier 60 uses the X-ray transmission signal with a gain corresponding to a new scan position including the next scan position, that is, the position where the X-ray exposure is started. Amplify (S19).

  In addition, when the X-ray exposure position moves to the next scan position (Yes in S12), the switching control unit 66 refers to the scan plan data after the process of S13 to S15 is completed, and newly moves. It is determined whether or not the scanned position is the last area to be scanned (S20). If the newly moved scan position is not the final region (S20, No), the routine of S11 to S20 is repeated. On the other hand, if the newly moved scan position is the final region (S20, Yes), the gain switching process is terminated.

  FIG. 11 is a graph showing the relationship among the gain, the size of the imaging region, and the scan position by the X-ray CT apparatus 10. The horizontal axis indicates the X-ray transmission signal and the imaging region identified by the scan position information, and the vertical axis indicates the gain and the size of the imaging region.

  According to the X-ray CT apparatus 10, the X-ray exposure is performed while the body width in the body radial direction of the scan position where the X-ray exposure is started, that is, the gain corresponding to the size of the imaging region at the scan position is switched. The shooting position transitions to a new scan position.

  That is, the subject P is scanned while switching in units of scan positions to an optimum gain that matches the body width of the scan position. Therefore, for example, when a region including a plurality of types of body diameter body widths, such as a whole body scan, is scanned at a time, the scan position with an optimum gain that matches the body width of the scan position where the X-ray is exposed. X-ray transmission signal reflecting the amount of X-ray transmission can be amplified.

  Therefore, even if a region including a plurality of types of body diameter body widths is scanned at a time, it is possible to prevent a part of the projection data from overflowing or a decrease in the S / N ratio of the image. A good image with high continuity and contrast effect can be generated. In addition, in order to prevent overflow and a decrease in the S / N ratio of an image, it is not necessary to perform scanning for each body diameter body width, in other words, for each size of an imaging region, thereby reducing exposure. it can.

  Next, preprocessing for correcting the sensitivity of X-ray transmission signals having different gains input during one scan generated by the X-ray CT apparatus 10 will be described.

  FIG. 12 is a diagram illustrating a detailed configuration relating to sensitivity correction of the preprocessing unit 42 and the console side control unit 54. FIG. 13 is a data structure diagram showing how correction data is stored.

  As shown in FIG. 12, the pre-processing unit 42 has a correction data storage unit 80 constituted by an internal ROM that stores correction data. As shown in FIG. 13, the correction data storage unit 80 stores correction data for each size or gain of the imaging region. In the X-ray CT apparatus 10 according to the present embodiment, since the size and gain of the imaging area correspond one-to-one, if correction data is stored corresponding to either the size or gain of the imaging area. Good.

  In addition, the console side control unit 54 has an imaging region table storage unit 84 configured by a preprocessing control unit 82 and an internal RAM. The imaging region table storage unit 84 stores the association between each scan position generated by the imaging region determination unit 72 and the size of the imaging region at each scan position.

  Information indicating the scan position where the X-ray exposure is started is input to the console side control unit 54 from the gantry control device 36 based on the bed information indicating the movement amount of the bed 14. The console-side control unit 54 refers to the association stored in the imaging region table storage unit 84 and outputs information indicating the size of the imaging region corresponding to the information indicating the scan position to the preprocessing unit 42. .

  When the information indicating the size of the shooting area is input, the preprocessing unit 42 reads correction data associated with the input information indicating the large tree of the shooting area from the correction data storage unit 80. Then, the sensitivity characteristic generated due to the difference in amplification accuracy or the like is corrected for the input X-ray transmission signal using the read correction data. That is, the preprocessing unit 42 corrects the sensitivity characteristic using the correction data corresponding to the scan position.

  FIG. 14 is a flowchart showing preprocessing for correcting the sensitivity of X-ray transmission signals having different gains.

  First, when information indicating the scan position is input (S31), the preprocessing control unit 82 reads information indicating the size of the imaging area corresponding to the scan position from the imaging area table storage unit 84 (S32). The data is input to the processing unit 42 (S33).

  When the X-ray transmission signal is input, the preprocessing unit 42 reads correction data corresponding to information indicating the size of the imaging region input from the preprocessing control unit 82 from the correction data storage unit 80 (S34). Using the read correction data, sensitivity correction is performed on the X-ray transmission signal (S35).

  Thereby, even if X-ray transmission signals having different gains are input, optimum sensitivity correction can be performed on the respective X-ray transmission signals.

  In this embodiment, the gain is switched in correspondence with the scan position. However, the information corresponding to the scan position, such as the movement amount of the top board 38, the bed position, and the elapsed time after the scan is started. The gain may be switched according to the above. In this case, in the scan plan data, a gain value is described in correspondence with the amount of movement of the couch 38, the bed position, or the elapsed time. The bed position is calculated by converting the coordinate position on the image obtained by measuring the body width in the body diameter direction to the position of the bed 14. The movement amount and elapsed time of the top plate 38 are calculated from the coordinate position on the image obtained by measuring the body width in the body diameter direction and the moving speed of the top plate 38.

  The measurement of the body width is based on a scanogram. However, the subject P may be actually measured with a measure or a projector and the result may be input.

It is a block diagram which shows the structure of the X-ray CT apparatus which concerns on this embodiment. It is a block diagram which shows the further detailed structure which concerns on gain switching. It is a flowchart which shows the process which produces scan plan data. It is a schematic diagram for demonstrating the process which measures the body width of the body diameter direction of each scanning position using a scanogram. It is a data block diagram which shows an imaging | photography area | region table. It is a schematic diagram for demonstrating the process which associates the magnitude | size of an imaging region with each scanning position. It is a data block diagram which shows a gain table. It is a schematic diagram for demonstrating the process which associates a gain value with each scanning position. It is a flowchart which shows the process which switches a gain. It is a figure which shows the data structure of scan plan data. 6 is a graph showing a relationship among a gain, a size of an imaging region, and a scan position. It is a figure which shows the detailed structure which concerns on the sensitivity correction of a pre-processing part and a console side control part. It is a data structure figure which shows the memory | storage aspect of correction data. It is a flowchart which shows the pre-process which carries out a sensitivity correction | amendment of the X-ray transmissive signal from which a gain differs.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 X-ray CT apparatus 12 Stand 14 Bed 16 Processing unit 18 Opening 20 Gantry 22 Rotation drive device 24 X-ray tube 26 X-ray detector 28 Collimator 30 High voltage generator 32 Diaphragm drive device 34 Data acquisition device 36 Stand control device 38 Top Plate 40 Bed drive unit 42 Preprocessing unit 44 Projection data storage unit 46 Reconstruction processing unit 48 Image storage unit 50 Image processing unit 52 Display device 54 Console side control unit 56 Preprocessing device 58 Integrator 60 Amplifier 62 AD converter 64 Switching Circuit 66 Switching control unit 68 Scan plan data storage unit 70 Body width measurement unit 72 Imaging region determination unit 74 Gain determination unit 76 Gain table storage unit 78 Scan plan data transmission unit 80 Correction data storage unit 82 Preprocessing control unit 84 Imaging region table Storage unit P Subject PI Scan position information Cv count value

Claims (5)

X-rays are exposed to each scan position aligned in the body axis direction of the subject, X-rays transmitted through each scan position are detected, and each signal reflecting the X-ray transmission amount at each scan position is output. Scanning means to
An amplifier having a plurality of types of gain, and amplifying each of the signals output from the scanning means by any of a plurality of types of gains;
Measuring means for measuring in advance the body width in the body diameter direction of each scanning position before scanning by the scanning means;
Storage means for storing in advance the association between the body width and the gain of the amplifier;
Gain control means for switching the gain of the amplifier according to the pre-measured body width for each signal at each scan position;
Providing
X-ray CT apparatus characterized by this. The storage means
First storage means for storing in advance the association between the body width and the size of the imaging region;
Second storage means for storing in advance a correspondence between the size of the imaging region and the gain value;
Including
Storing the association between the body width and the gain value via the size of the imaging region;
The X-ray CT apparatus according to claim 1. The measuring means includes
Measuring the body width in the body diameter direction of each scan position based on a planar transmission image of the subject imaged in advance;
The X-ray CT apparatus according to claim 1. Pre-processing means for performing correction based on the switched gain for a signal amplified with a different gain according to the scan position;
The X-ray CT apparatus according to claim 1. X-rays are exposed to each scan position aligned in the body axis direction of the subject, X-rays transmitted through each scan position are detected, and each signal reflecting the X-ray transmission amount at each scan position is output. Scanning means to
An amplifier having a plurality of types of gain, and amplifying each signal output from the X-ray detector with any gain;
Based on a plane transmission image of the subject imaged in advance, a measuring unit that measures in advance the body width in the body radial direction of each scan position before scanning by the scanning unit;
Storage means for storing in advance a gain table in which the body width and the gain of the amplifier are associated;
The gain corresponding to the body width at each scan position is searched from the gain table, and the association between each scan position and the gain value of the signal at the scan position is generated in advance before the scan by the scanning means. Gain determining means;
Gain control means for switching the gain of the amplifier for each signal at each scan position according to the association generated by the gain determination means;
Preprocessing means for performing correction based on the switched gain on the signal amplified with a different gain according to the scan position;
Providing
X-ray CT apparatus characterized by this.

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