JP2007215698A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To estimate automatically geometrical parameters of an X-ray CT apparatus without visual judgement of an image. <P>SOLUTION: The X-ray CT apparatus reads projection data acquired by imaging a wire phantom (S11), sets an initial value and a repetition range of a rotation center channel as geometrical parameters (S12 and S13), generates a reconstituted image of the wire phantom (S14), extracts a region containing the wire phantom from the reconstituted image (S15), and determines and stores the length of the region as a characteristic value (S16 and S17). The X-ray CT apparatus then updates the value of the rotation center channel and repeats S14-S18 to search a geometrical parameter which gives an optimum characteristic value (S110). The optimum characteristic value thus acquired is determined to be an adjustment parameter. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus, and more particularly to a technique that makes it possible to more easily determine geometric parameters of an apparatus necessary for image reconstruction.

  The cone-beam X-ray CT apparatus rotates the X-ray source and the two-dimensional X-ray detector disposed opposite to the X-ray source on the circular orbital plane having the same rotation center, while rotating on the rotation center axis. A three-dimensional X-ray CT image is obtained by taking an X-ray projection image of a subject located with a two-dimensional X-ray detector and performing an image reconstruction calculation based on the X-ray projection image.

  As an image reconstruction algorithm of a cone beam X-ray CT apparatus using such a two-dimensional X-ray detector, a Feldkamp image three-dimensional image reconstruction method is known as a typical one (Non-patent Document 1).

  In addition, in the cone beam X-ray CT apparatus, in order to obtain a three-dimensional X-ray CT image free from artifacts due to mechanical design errors, parameters such as the rotation center channel, the projection angle pitch, and the detector mounting angle (hereinafter referred to as the detector mounting angle). , Referred to as geometric parameters) and applied to the image reconstruction operation. As a method for obtaining such a geometric parameter, there are conventional techniques described in Patent Document 1 and Patent Document 2.

  In the prior art of Patent Document 1, a rotating track surface calculation phantom configured by arranging metal balls in a straight line is separated from the rotation center axis by a predetermined distance, and the rotation center axis and the metal ball arrangement direction of the phantom are parallel to each other. In this state, the rotational parameters of the rotation trajectory plane and the rotation center channel are obtained from the trajectory of the metal sphere reflected in the addition image of the multiple X-ray projection images obtained by imaging. It is a method to seek.

In the prior art of Patent Document 2, a phantom including a plurality of wires is arranged so that the wires are positioned in parallel to the rotation center axis, and is rotated and photographed in a range of 200 degrees or more. This is a method for obtaining a suitable projection angle pitch and detector mounting angle geometric parameters from the cross-sectional shape of the wire shown in the X-ray CT image generated by the reconstruction calculation from the projection image.
JP 2005-237752 JP 2005-058309 A "PraCTical Cone-Beam Algorithm"; LAFeldkamp, et al .; J. Optical Society of America. A / Vol.1 (6), (1984), pp.612-19 "

  The above technique repeats the procedure of reconstructing calculation while changing geometric parameters until the cross-sectional shape of the wire shown in the X-ray CT image becomes suitable. Further, whether or not the cross-sectional shape is suitable depends on the visual judgment of the adjuster. In this case, since there is no quantitative index in the visual judgment, the result may vary depending on the person viewing the image. Judgment requires knowledge and experience of the adjustment principle.

  The present invention has been made in view of such circumstances, and in the geometric parameter adjustment of a cone beam X-ray CT apparatus, an X-ray CT for automatically obtaining a suitable geometric parameter without visual judgment from an image. An object is to provide an apparatus.

  In order to achieve the above object, an X-ray CT apparatus according to the present invention detects an X-ray source that irradiates a subject with X-rays and an X-ray that is disposed opposite to the X-ray source and passes through the subject. The X-ray detector that outputs the projection data of the subject, the X-ray source and the X-ray detector are rotated by a predetermined angle, and the X-ray source is rotated by the rotating means. An image reconstruction means for reconstructing a tomographic image of the subject using the projection data output by the X-ray detector in a state where the image reconstruction means comprises the image reconstruction means The geometry used for the reconstruction process is based on projection data of the phantom obtained by installing at least one phantom between the X-ray source and the X-ray detector and performing X-ray imaging. Multiple phantoms with varying academic parameters Generate a reconstructed image, calculate a feature amount of the phantom reconstructed image for each of the plurality of phantom reconstructed images, select a desired phantom reconstructed image based on the plurality of feature amounts, and select the selection Adjustment parameter determining means for determining a geometric parameter corresponding to the reconstructed phantom reconstruction image as an adjustment parameter, a feature amount of the phantom reconstructed image reconstructed by changing the geometric parameter, and a geometry corresponding to the feature amount Display means for displaying related information in association with a parameter, or a confirmation reconstructed image in which the geometric parameter is associated with a phantom reconstructed image reconstructed using the geometric parameter, To do.

  According to the present invention, in the geometric parameter adjustment of the cone beam X-ray CT apparatus, a suitable geometric parameter can be automatically obtained without visual judgment from an image.

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

  FIG. 1 is a conceptual diagram showing a configuration of an X-ray CT apparatus 1 according to an embodiment of the present invention. FIG. 2 is a block diagram showing a hardware configuration of the X-ray CT apparatus 1 of FIG.

  The X-ray CT apparatus 1 in FIG. 1 includes a scanner rotation unit 10 that outputs projection data, an image processing device 20 that performs an image reconstruction process based on the projection data, and generates a reconstructed image, and displays a reconstructed image. The image display device 30 is provided.

  As shown in FIG. 2, the scanner rotating unit 10 includes an X-ray source including an X-ray tube 11 that irradiates an X-ray beam that spreads in a cone or pyramid shape, and a detector 12 that detects transmitted X-rays. . The scanner rotating unit 10 includes a rotating disk 13 that rotates around the subject 50 placed on the bed 40 with the X-ray source 11 and the detector 12 facing each other.

  The detector 12 detects transmitted X-rays that have passed through the subject 50 and outputs projection data via a data acquisition device (hereinafter referred to as “DAS”) 14. The DAS 14 is connected to the preamplifier 15. The preamplifier 15 amplifies the projection data received from the DAS 14 and transfers the amplified projection data to the image processing apparatus 20. Currently, the rotation per turn of the turntable 13 equipped with the X-ray tube 11 and the detector 12 is increased to 0.5 seconds or less. As the detector 12, a multi-slice detector having a structure in which a plurality of rows are arranged in the body axis direction so that a plurality of tomographic images can be obtained by one scan is widely used.

  The image processing apparatus 20 includes a CPU 21, a main memory 22 that stores a control program and an image processing program for the image processing apparatus 20, a magnetic disk 23 that stores projection data and an image processing program, and parameters such as an effective visual field range. A keyboard 24 for performing the setting, a pointing device including a mouse 25, a trackball, a joystick, etc. and its controller 26, a display memory 27 for temporarily storing image data to be displayed on the image display device 30, and a scanner rotation And an input / output interface 28 for acquiring projection data from the unit 10. The above components are connected to each other by a common bus 29. The image processing apparatus 20 includes a main memory 22 and a magnetic disk 23 as storage devices, but may include other storage devices such as an FDD, a CD-RW drive, an MO (optical disk) drive, and a ZIP drive.

  An image processing program for reconstructing projection data and generating a reconstructed image is installed in the image processing apparatus 20 of the X-ray CT apparatus 1. Further, the image processing apparatus 20 is installed with a geometric parameter automatic determination program for detecting a geometric parameter unique to the X-ray CT apparatus and determining an adjustment parameter. When the CPU 21 loads and executes these programs on the main memory 22 as appropriate, the automatic determination program performs the processing described later and automatically determines the adjustment parameters. Then, the projection data of the subject is reconstructed using the automatically determined adjustment parameters.

  The image display device 30 is configured by a CRT device, a liquid crystal display device, or the like.

<First embodiment>
The first embodiment is an embodiment that detects and adjusts a rotation center channel indicating a channel position on an X-ray detector that projects the rotation center axis.

  FIG. 3 is a schematic diagram for explaining the rotation center channel and the midplane slice. The rotation center channel 71 is a channel on the detector 12 on which the rotation center axis 61 in the rotational movement of the turntable 13 is projected onto the detector 12. Further, the projection position on the detector 12 of the rotational orbital plane (midplane) 60 that is a plane including a circular orbit drawn by the X-ray source 11 and the two-dimensional X-ray detector 12 is referred to as a midplane slice 72.

  FIG. 4 is a flowchart showing an outline of the process of automatically determining the rotation center channel. The rotation center channel automatic determination process will be described below with reference to the flowchart of FIG.

  In step S11, a projection image is read (S11).

  A medical image data set of the subject imaged by the X-ray CT apparatus 1 is read from the scanner measurement unit 10 or an image recording apparatus or an image database (not shown) and developed in the main memory. Here, the image data set refers to several to several hundred projection images obtained by imaging the subject.

  In step S12, the rotation center channel initial value is set (S12).

  Sets the initial value of the rotation center channel parameter. Since the rotation center channel parameter is ideally the center channel in the column direction of the projection image, the initial value is 1/2 of the number of pixels in the column direction of the projection image. For example, if the number of pixels in the column direction of the projected image is 512 pixels, it is 256 channels, and if it is 1024 pixels, it is 512 channels.

  Alternatively, the rotation center channel parameter value determined from the addition image of the projection image obtained by photographing the wire may be used as described in Patent Document 1, even if it is not a design value.

  In step S13, a repeat range is set (S13).

  Sets the range and step value of the rotation center channel parameter when repeating the reconstruction calculation. The range of the rotation center channel has a constant width centered on the initial value set in 1.2. The value of the rotation center parameter that is repeatedly changed within the set width is set as the step value.

  For example, if the initial value of the rotation center channel parameter is 256, the repeat range is ± 3, and the step value is 0.5, the rotation center channel is 251, 251.5, 252, 252.5,..., 256,. The processing after 1.4 is repeated while changing the values to 5, 260, 260.5, and 261.

  In step S14, reconstruction calculation is performed (S14).

  Using the set rotation center channel parameter, a projection image obtained by photographing the wire is reconstructed, and a reconstructed image of the wire is created. At the time of reconstruction, reconstruction is performed by reducing the reconstruction field of view so that the wire is back-projected as much as possible in the image.

  For example, in an image reconstructed with the reconstruction field of view enlarged so that the entire imaging range is back-projected, the wire region 100 is imaged as a small point in the image as shown in FIG. In this case, an error occurs in wire region extraction and wire length calculation shown in step S15 and subsequent steps. In the reconstructed image with the reconstructed field of view being reduced, the wire region 101 is enlarged as shown in FIG. 6, so that the wire region extraction and wire length calculation shown in step S15 and subsequent steps are facilitated, and an error occurs. Becomes smaller.

  In step S15, wire region extraction is performed (S15).

  The wire region shown in the reconstructed image is extracted by the following procedure.

  (1) The image is raster-scanned to search for a pixel having a wire pixel value.

  (2) The wire region is extracted by the region expansion method using the pixel searched in (1) as the starting point. In extraction using the region expansion method, parameters are set so that backgrounds other than wires are not extracted.

(3) When a phantom in which a plurality of wires are arranged is photographed, the reconstructed image may include a plurality of wire regions 102 and 103 as shown in FIG. In this case, only the wire region closer to the center of the reconstructed image (for example, (x, y) = (256, 256) for a reconstructed image of 512 × 512 pixels) is extracted. To determine whether the region is closer to the center O of the reconstructed image, as shown in FIG. 8, a circle O ₁ with the center of the reconstructed image as the center point is set, and the radius of the circle O ₁ is increased. Check if there are wire pixels on the circumference. As shown in FIG. 9, the wire region including the wire pixel that first appears on the circumference is defined as a wire region close to the center. In FIG. 9, the wire region 102 is extracted.

  In step S16, the wire area length is calculated (S16).

  The artifact length of the wire region 102 extracted in step S15 is calculated. When the center of rotation channel is suitable, the wire that appears in the reconstructed image is imaged as a point and there is little artifact. However, if the parameters of the rotation center channel are not suitable, the wire area shown in the reconstructed image is an area including artifacts due to the rotation center channel shift. The wire region artifacts become longer as the center of rotation channel moves away from the preferred value. Therefore, the length of this artifact is used as an index for determining the rotation center channel parameter.

  The length of the artifact in the wire region is the distance from the pixel 102m having the maximum pixel value in the extracted wire region 102 to the end point pixel 102e in the wire region as shown in FIG. The reason why the pixel 102m having the maximum value in the wire region 102 is used as a reference is that the pixel having a high value in the reconstructed image is assumed to be the pixel that projects the wire of the projection image most in the back projection operation. . The procedure of the wire area length calculation process is shown below.

  (1) Search for the pixel 102m having the maximum pixel value in the wire region 102 extracted in step S15.

  (2) A circle centering on the pixel 102m having the maximum pixel value searched in (1) is set, and the radius of the circle is gradually increased until there are no pixels in the wire region 102 on the circumference of the circle. .

  (3) The radius of the circle when there is no pixel in the wire region on the circumference of the circle is defined as the wire artifact length (wire region length l).

  In step S17, the wire area length l is stored (S17).

  The wire area length l (artifact length) calculated in step S16 is stored in correspondence with the rotation center channel parameter at that time.

  In step S18, the repetitive range is determined (S18).

  It is determined whether or not the rotation center channel parameter is within the repetition range set in step S13. If the rotation center channel parameter is within the range, the process proceeds to step S19. If it is out of range, the process proceeds to step S110.

  In step S19, the rotation center channel is changed (S19).

  The rotation center channel parameter is changed based on the range and step value set in step S13, and the processing from step S14 is repeated.

  For example, when the rotation center channel parameter before the change is 253 channels and the step value set in step S13 is 0.5 channels, 253 + 0.5 = 253.5 channels is set as the change value.

  In step S110, the minimum value of the wire area length l is searched (S110).

  For each rotation center channel parameter set in the range and step value of step S13, the minimum value is retrieved from the wire region length l calculated by performing the processing of steps S14 to S19.

  The rotation center channel parameter corresponding to the searched minimum value is set as a suitable value.

  FIG. 11 is a graph showing the wire region length l with respect to the rotation center channel parameter when the image is actually taken using the apparatus (projection image size: 1024 × 1024 pixels). FIG. 12 shows an image at that time (a reconstructed image for confirmation including a wire and its artifact). In FIG. 11, when the rotation center channel parameter is 493.5 channels, the wire region length l is the minimum. Further, as shown in FIG. 12, when the rotation center channel parameter is 493.5 channels, the wire has the most symmetrical shape, and the image is well imaged with few artifacts. From these, it can be said that the determination of the rotation center channel parameter based on the wire region length l is appropriate.

  The graph of FIG. 11 and / or the reconstructed image of FIG. 12 may be displayed on the image display device 30 so that the user can confirm the effect of automatic determination.

<Second embodiment>
In the second embodiment, the projection angle pitch is automatically determined. The projection angle pitch refers to an angular interval at which the turntable 13 rotates every predetermined angle. This embodiment is a process for automatically determining an actual angular pitch because each X-ray CT apparatus may have a unique angular pitch and is not necessarily the same as a design value.

  FIG. 13 is a flowchart showing an outline of the processing for automatically determining the projection angle pitch.

  The projection angle pitch automatic determination process will be described below with reference to the flowchart of FIG.

  In step S21, the projection image is read (S21).

  Hundreds of projection images obtained by photographing the wire phantom are read out from the scanner measurement unit 10 or an image recording device or an image database (not shown) and developed in the main memory.

  In step S22, the initial value of the projection angle pitch is set (S22).

  Sets the initial value of the projection angle pitch parameter. Ideally, the projection angle pitch parameter should be a design value. The design value of the projection angle pitch is obtained by dividing the rotation angle at which the X-ray source 11 or the detector 12 rotates while irradiating X-rays by the number of projection images collected during that time. For example, when the rotation angle is 200 ° and the number of projection images collected during the rotation angle is 150, 200 ° / 150 images≈1.330 °.

  In step S23, a repetition range is set (S23).

  The range and step value of the projection angle pitch parameter when the reconstruction calculation is repeated are set. The range of the projection angle pitch has a constant width centered on the initial value set in step S22. The value of the projection angle pitch that is repeatedly changed within the set width is set as the step value.

  For example, if the initial value of the projection angle pitch parameter is 1.330, the repetition range is ± 0.02, and the step value is 0.001, the rotation center channel is 1.310, 1.311, 1.312,. The process after step S24 is repeated while changing to 348, 1.349, 1.350.

  In step S24, reconstruction calculation is performed (S24).

  A projection image obtained by photographing the wire is reconstructed using the set projection angle pitch parameter, and a reconstructed image of the wire is created. At this time, the image to be reconstructed is a reconstructed image of the midplane slice 72 corresponding to the projection position when the midplane 60 is projected onto the detector 12. At the time of reconstruction, the reconstruction field is enlarged and reconstruction is performed so that all the wires arranged on the grid are back-projected as much as possible in the image. An example of the reconstructed image at this time is shown in FIG. FIG. 14 shows that a plurality of wire regions 104 are imaged.

  In step S25, wire area extraction is performed (S25).

  The wire region shown in the reconstructed image is extracted by the following procedure.

  (1) The image is raster-scanned to search for pixels having wire pixel values.

  (2) The wire region is extracted by the region expansion method using the pixel searched in (1) as the starting point. In extraction using the region expansion method, parameters are set so that backgrounds other than wires are not extracted.

  (3) Repeat steps (1) and (2) to extract all the wires that appear in the reconstructed image.

  In step S26, the circularity of the wire region is calculated (S26).

  The circularity of the wire region extracted in step S25 is calculated. When the projection angle pitch is suitably set, each wire appearing in the reconstructed image is imaged as a point, and its shape is close to a circle. However, if the projection angle pitch parameter is not suitable, the wire in the reconstructed image produces artifacts and the shape deviates from the circle. Utilizing such properties, the circularity of each wire is obtained, and the average value of the circularity of each wire is used as a determination index for the projection angle pitch. The procedure of wire area circularity calculation processing is shown below.

  (1) The number of pixels in the wire area extracted in step S25 is obtained.

  (2) Obtain the perimeter of the wire region extracted in step S25.

(3) The number of pixels of the wire region obtained in (1) and (2) is N, the perimeter is S, and the circularity C of the wire region is calculated by the equation (1).
[Formula 1] C = S ² / 4πN
(4) The processing from (1) to (3) is performed on each extracted wire, and the circularity is obtained.

  In step S27, the average value of the circularity of the wire region is calculated (S27).

  First, the average value of the circularity of each wire region obtained in step S26 is obtained. If the projection angle pitch parameter is not suitable, the wire becomes more prominent as the distance from the center of the reconstructed image increases, and the circularity increases. Therefore, the average value of circularity becomes large. On the other hand, when the projection angle pitch parameter is suitable, the shape of the wire is uniformly a circle at the center and the periphery of the reconstructed image, and the average value of the circularity is small.

  In step S28, the average value of the circularity of the wire region is stored (S28).

  The average circularity value of the wire area calculated in step S27 is stored in correspondence with the projection angle pitch parameter at that time.

  In step S29, repetitive range determination is performed (S29).

  It is determined whether or not the projection angle pitch parameter is within the repetition range set in step S23. If the projection angle pitch parameter is within the range, the process proceeds to step S210. If it is out of range, the process proceeds to step S211.

  In step S210, the projection angle pitch is changed (S210).

  The projection angle pitch parameter is changed based on the range and step value set in 2.3, and the processing from step S24 is repeated.

  For example, when the projection angle pitch parameter before the change is 1.311 ° and the step value set in 2.3 is 0.001 °, 1.311 + 0.001 = 1.322 ° is set as the change value. To do.

In step S211, the minimum value of the average value of the wire region circularity is searched (S211). The wire region calculated by performing the processing of S24 to S211 for each projection angle pitch parameter set in the range and step value of step S23. The minimum value is searched from the average circularity value.

  The projection angle pitch parameter corresponding to the searched minimum value is set as a suitable value.

  FIG. 15 is a graph showing the average value of the wire region circularity with respect to the projection angle pitch parameter when the image is actually taken using the apparatus. FIG. 16 shows an image at that time. In FIG. 15, when the projection angle pitch parameter is 1.332 °, the average value of the wire region circularity is the minimum. Further, as shown in FIG. 16, when the projection angle pitch parameter is 1.332 °, a wire is imaged as a point at the center and the periphery of the image, and the shape is also close to a circle. From these, it can be said that the determination of the projection angle pitch parameter based on the average value of the wire region circularity is appropriate.

  Similarly to the first embodiment, the graph of FIG. 15 and / or the reconstructed image for confirmation of FIG. 16 may be displayed on the image display device 30 so that the user can confirm the effect of automatic determination.

<Third embodiment>
In the third embodiment, the detector mounting angle is automatically determined. FIG. 17 is a flowchart showing an outline of processing for automatically determining the detector mounting angle. The detector mounting angle is described below with reference to the flowchart of FIG.

  In step S31, the projection image is read (S31).

  Hundreds of projection images obtained by photographing the wire phantom are read from the scanner measurement unit 10 or an image recording device or an image database (not shown) and developed in the main memory.

  In step S32, an initial value of the detector mounting angle is set (S32).

  Set the initial value of the detector mounting angle parameter. The detector mounting angle α is an angle formed by the rotation center of the rotation mechanism and the detector 12 as shown in FIG. It is desirable that the detector 12 is mounted perpendicularly and parallel to the rotation center axis 61, and the mounting angle is 0.0 °. Therefore, the initial value of the detector mounting angle parameter is 0.0 °. A rotation center axis 61 in FIG. 18 indicates a direction of a projection line of the rotation center axis 61 to the detector 12. In FIG. 18, a line 12CL on the detector 12 that forms an angle α with the rotation center axis 61 indicates a projected line on the design of the detector 12.

  In step S33, a repeat range is set (S33).

  The range and step value of the detector mounting angle parameter when the reconstruction calculation is repeated are set. The range of the detector mounting angle has a certain width centered on the initial value set in step S32. The value of the detector mounting angle that is repeatedly changed within the set width is taken as the step value.

  For example, if the initial value of the detector mounting angle parameter is 0.0, the repeat range is ± 1.0, and the step value is 0.1, the detector mounting angle is -1.0, -0.9, -0.8. ,..., 0.8, 0.9, 1.0 and the like, and the processes after step S34 are repeated.

  In step S34, reconstruction calculation is performed (S34).

  Using the set detector mounting angle parameter, a projection image obtained by photographing the wire is reconstructed, and a reconstructed image of the wire is created. At the time of reconstruction, the reconstruction field of view is reduced and reconstruction is performed so that the wire is backprojected as much as possible in the image as in the process of step S14. As shown in FIG. 19, the images to be reconstructed here are three reconstructed images of a cross section of the midplane 60 and a cross section (A and B in the figure) separated from the midplane 60 cross section to some extent.

  In step S35, the wire region is extracted (S35).

  The wire region shown in the reconstructed image is extracted by the following procedure.

  (1) The image is raster-scanned to search for a pixel having a wire pixel value.

  (2) The wire region is extracted by the region expansion method using the pixel searched in (1) as the starting point. In extraction using the region expansion method, parameters are set so that backgrounds other than wires are not extracted.

  (3) When a phantom in which a plurality of wires are arranged is photographed, the reconstructed image may include a plurality of wires as shown in FIG. In this case, only the wire region closer to the center of the reconstructed image (for example, (x, y) = (256, 256) for a reconstructed image of 512 × 512 pixels) is extracted. To determine whether the area is closer to the center of the reconstructed image, set a circle centered on the center of the reconstructed image as shown in FIG. Check if there is a wire pixel. As shown in FIG. 9, the wire region including the wire pixel that first appears on the circumference is defined as a wire region close to the center.

  In step S36, the circularity of the wire region is calculated (S36).

  The circularity of the wire region extracted in step S35 is calculated for each of the three reconstructed images. When the detector mounting angle is suitably set, each wire appearing in the three reconstructed images is imaged as a point, and its shape is close to a circle. However, if the detector mounting angle parameter is not suitable, the wire appearing in the reconstructed image located away from the midplane section will cause artifacts and the shape will depart from the circle. Utilizing such a property, the circularity of the wire reflected in each reconstructed image is obtained, and the average value of the circularity of the three reconstructed images is used as an indicator for determining the detector mounting angle. The procedure of wire area circularity calculation processing is shown below.

  (1) The number of pixels in the wire area extracted in step S35 is obtained.

  (2) Obtain the perimeter of the wire region extracted in step S35.

  (3) The number of pixels of the wire region obtained in (1) and (2) is N, the perimeter is S, and the circularity C of the wire region is calculated by Equation (2).

[Formula 2] C = S ² / 4πN
(4) The processing from (1) to (3) is performed on the three reconstructed images, and the circularity of each wire region is obtained.

  In step S37, the average value of the circularity of the wire region is calculated (S37).

  The average value of the circularity of each wire region obtained in step S36 is obtained. If the detector mounting angle parameter is not suitable, the wire becomes more artifact as it moves away from the midplane cross section and the circularity increases. Therefore, the average value of circularity becomes large. On the other hand, when the parameter of the detector mounting angle is suitable, the shape of the wire is uniformly a circle even in the reconstructed image at a position away from the midplane cross section, and the average value of the circularity is small.

  In step S38, the average value of the circularity of the wire region is stored (S38).

  The average circularity value of the wire region calculated in step S37 is stored in correspondence with the detector mounting angle parameter at that time.

  In step S39, repeated range discrimination is performed (S39).

  It is determined whether or not the detector mounting angle parameter is within the repetition range set in step S33. If the detector mounting angle parameter is within the range, the process proceeds to step S310. If it is out of range, the process proceeds to step S311.

  In step S310, the detector mounting angle is changed (S310).

  The detector mounting angle parameter is changed based on the range and step value set in step S33, and the processing from step S34 is repeated.

  For example, if the detector mounting angle parameter before change is 0.1 ° and the step value set in 3.3 is 0.1 °, then 0.1 + 0.1 = 0.2 ° And

  In step S311, the minimum value of the average value of the circularity of the wire region is searched (S311).

  For each detector mounting angle parameter set in step S33 and step, the minimum value is retrieved from the average values of the wire region circularity values calculated by performing the processing in steps S34 to S311.

  The detector mounting angle parameter corresponding to the searched minimum value is set as a suitable value.

  FIG. 20 is a graph showing the average value of the wire region circularity with respect to the detector mounting angle parameter when photographing is actually performed using the apparatus. FIG. 21 shows a wire image when the circularity average value is minimized. In FIG. 20, when the detector mounting angle parameter is 0.6 °, the average value of the wire region circularity is the minimum. Further, as shown in FIG. 21, the wire image at that time is substantially circular with the three reconstructed images, and no artifacts due to mounting angle deviation are observed. From these, it can be said that the determination of the detector mounting angle parameter based on the average value of the wire region circularity is appropriate.

  Similarly to the first and second embodiments, the graph of FIG. 20 and / or the confirmation reconstructed image of FIG. 21 may be displayed on the image display device 30 to allow the user to confirm the effect of the automatic determination process.

  In the confirmation display, FIGS. 11, 15, and 20 are displayed as graphs. However, the relationship between the geometric parameter and the feature amount of the phantom reconstruction image may be displayed using a table format.

  In the above embodiment, the adjustment parameter is automatically determined using the wire phantom. However, in the aspect in which the circularity is obtained as the feature amount, the cross-sectional shape is not limited to the wire phantom.

The conceptual diagram which shows the structure of the X-ray CT apparatus concerning one embodiment of this invention. The block diagram which shows the hardware constitutions of the X-ray CT apparatus 1 of FIG. Schematic diagram for explaining the rotation center channel and midplane slice Flow chart showing rotation center channel automatic determination processing Reconstruction image when reconstructing the wire with a wide reconstruction field of view Reconstructed image when wire is reconstructed with a small reconstruction field of view Example when multiple wires are shown in the reconstructed image Schematic showing the process of searching for a wire region close to the center of the image Schematic view when searching for a wire region close to the center of the image Schematic explaining wire area length l A graph showing the wire area length l with respect to the rotation center channel parameter when the image is actually taken using the apparatus Reconstructed image when the rotation center channel parameter is changed The flowchart which shows the process of the projection angle pitch automatic determination of this invention Schematic showing a reconstructed image when photographing wires arranged in a grid Graph showing the average value of the circularity of the wire area with respect to the projection angle pitch parameter when actually shooting with the device Reconstructed image when changing projection angle pitch parameter The flowchart which shows the process of the detector attachment angle automatic determination of this invention Schematic explaining the detector mounting angle Schematic showing the cross-sectional position of the reconstructed image used for automatic detector mounting angle determination Graph showing the average value of the circularity of the wire area with respect to the detector mounting angle parameter when the image was actually taken using the device Reconstructed image when the wire area circularity average value is minimum

Explanation of symbols

    1: X-ray CT apparatus, 10: scanner measurement unit, 20: image processing apparatus, 30: image display apparatus, 50: subject

Claims (4)

An X-ray source for irradiating the subject with X-rays;
An X-ray detector that is disposed opposite to the X-ray source, detects the X-ray transmitted through the subject, and outputs projection data of the subject;
Rotating means for rotating the X-ray source and the X-ray detector at predetermined angles;
Image reconstruction means for reconstructing a tomographic image of the subject using projection data output by the X-ray detector in a state where the X-ray source is rotated by the rotation means;
An X-ray CT apparatus comprising:
The image reconstruction unit is configured to perform reconstruction based on projection data of the phantom obtained by performing X-ray imaging by installing at least one phantom between the X-ray source and the X-ray detector. Generate multiple phantom reconstruction images with varying geometric parameters used for processing,
A feature amount of the phantom reconstruction image is calculated for each of the plurality of phantom reconstruction images, a desired phantom reconstruction image is selected based on the plurality of feature amounts, and the selected phantom reconstruction image is selected. An adjustment parameter determining means for determining a corresponding geometric parameter as an adjustment parameter;
Relevant information relating the feature quantity of the phantom reconstructed image reconstructed by changing the geometric parameter and the geometric parameter corresponding to the feature quantity, or reconstruction using the geometric parameter and the geometric parameter Display means for displaying a confirmation reconstruction image associated with the phantom reconstruction image;
An X-ray CT apparatus comprising: The geometric parameter is a rotation center channel parameter indicating a channel position on the X-ray detector on which a rotation center axis of rotation movement by the rotation means is projected,
The image reconstruction means generates a reconstructed image of a plurality of wire phantoms by changing the rotation center channel parameter,
The adjustment parameter determination unit includes a wire region extraction unit that extracts one wire region from each of the reconstructed images of the plurality of wires, and a wire region length that calculates the length of the wire region extracted by the wire region extraction unit. A calculation means; and a wire area length minimum value search means for searching for a minimum value of a plurality of wire area lengths determined for each reconstructed image, and the rotation center channel parameter at which the wire area length is a minimum value. Determine as adjustment parameter,
The X-ray CT apparatus according to claim 1. The geometric parameter is a projection angle pitch indicating a predetermined rotation angle interval in the rotation of the rotation means,
The image reconstruction means generates a reconstructed image of a plurality of wire phantoms by changing the projection angle pitch,
The adjustment parameter determining means includes a plurality of wire areas extracting means for extracting a plurality of wire areas reflected in a reconstructed image of the phantom including the wire, and a plurality of wire areas extracted by the plurality of wire area extracting means individually for circularity. A circularity average value calculating means for calculating an average value of the circularity of the obtained individual wire regions, and a circularity average minimum for searching for a minimum value of the plurality of circularity average values obtained for each reconstructed image Value search means, and determine the projection angle pitch at which the circularity average value is the minimum value as the adjustment parameter,
The X-ray CT apparatus according to claim 1. The geometric parameter is a detection indicating an angular difference between a direction of a projection line of the rotation center axis on the X-ray detector and a direction of a projection line on the design of the X-ray detector in the rotational movement of the rotating means. Mounting angle,
The adjustment parameter determining means is a wire area extracting means for extracting at least one wire area from the reconstructed image for each of the reconstructed image sets of at least two or more wire phantoms having different positions on the rotation center axis; Based on the circularity of the wire area of each reconstructed image extracted by the wire area extracting means, a circularity average value calculating means for obtaining an average value of the circularity of the wire area of the reconstructed image set; A circularity average minimum value search means for searching for a minimum average value, and determining a detector mounting angle at which the average value of the circularity is a minimum value as the adjustment parameter;
The X-ray CT apparatus according to claim 1.

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