A fan-beam CT scanning plane coplanar calibration method based on double-column phantom

Abstract

The application relates to a fan-beam CT scanning plane coplanar calibration method based on a double-column model, belonging to the CT imaging technical field. Two equal-height cylinders are fixed at the edges of a rotating platform at intervals, a space three-dimensional coordinate system is established with the rotating platform and an X-ray source as reference objects, the initial position of the rotating platform is set so that the two cylinders are both separated from a scanning plane, the X-ray source is turned on to perform line scanning, the rotating platform is uniformly lifted and lowered, and a line array detector collects line scanning projection data; the relative height of the X-ray source and the line array detector and the relative height of two end portions of the line array detector are respectively adjusted until the two cylinders appear simultaneously in the line scanning projection data, which is regarded as the fan-beam CT scanning plane coplanar. The application can solve the technical problem that the scanning plane deviates from a theoretical model to affect the fan-beam CT imaging precision in the prior art.

Description

Technical Field

[0001] This invention belongs to the field of CT imaging technology, and specifically relates to a coplanar calibration method for fan-beam CT scanning planes based on a dual-column phantom. Background Technology

[0002] Industrial CT, as an important non-destructive testing technology, is a powerful analytical tool for modern industrial production and scientific research. The application potential of industrial CT is constantly being explored, not only for defect detection but also for geometric measurements such as dimensions, angles, and eccentricities.

[0003] Depending on the detector type, industrial CT is divided into cone-beam CT based on area array detectors and sector-beam CT based on linear array detectors. Industrial sector-beam CT consists of an X-ray source, a rotating platform, a linear array detector, and an electromechanical control system, such as... Figure 1 As shown, the object to be examined is fixed on a rotating platform, which drives its rotation. X-rays pass through the object and are received by the linear array detector, converted into photoelectric digital signals, forming projection data. After computer reconstruction, CT tomographic images are obtained. The X-ray source and linear array detector are fixed, while the rotating platform can be vertically raised and lowered (or the rotating platform can only rotate, while the X-ray source and linear array detector can be vertically raised and lowered synchronously), facilitating fan-beam CT imaging of different parts of the object.

[0004] Given projection angle β i The focal point S of the X-ray source and the linear array detector CD form a scanning plane SCD, and the plane where this scanning plane intersects the object being scanned is P. i A schematic diagram of the scanning plane is shown below. Figure 2 As shown, the projection angle β is traversed. i i = 0, ..., N-1, where N represents the total number of projection angles, and all planes P i The theoretical model for sector-beam CT imaging is that i = 0, ..., N-1 are coplanar and perpendicular to the rotation axis. However, in practical engineering applications, the scanning plane formed by the X-ray source focal point S and the linear array detector exhibits two dimensional variations: 1. The linear array detector is parallel to the rotating platform plane, but its position along the rotation axis is either too high or too low, meaning the distances from the linear array detector and the X-ray source to the rotating platform are inconsistent. 2. The linear array detector deflects around the SO axis, meaning the distances from the endpoints C and D of the linear array detector to the rotating platform are inconsistent. These dimensional variations violate the theoretical model of sector-beam CT imaging, thus affecting its accuracy. Summary of the Invention

[0005] To address the shortcomings of existing technologies, a coplanar calibration method for fan-beam CT scanning planes based on a dual-column phantom is proposed to solve the technical problem that the deviation of the scanning plane from the theoretical model affects the imaging accuracy of fan-beam CT.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for coplanar calibration of fan-beam CT scan planes based on a dual-column phantom includes the following steps:

[0008] Two cylinders of equal height are fixed at intervals along the edge of the rotating platform. A three-dimensional coordinate system is established with the rotating platform and the X-ray source as references.

[0009] Set the initial position of the rotating platform so that both cylinders are detached from the scanning plane, turn on the X-ray source to perform line scanning, the rotating platform moves up and down at a constant speed, and the linear array detector collects the line scanning projection data.

[0010] Adjust the relative height of the X-ray source and the linear array detector until both cylinders appear simultaneously in the online scanning projection data. This is considered as the X-ray source and the linear array detector having the same height. Adjust the relative height of the two ends of the linear array detector until both cylinders appear simultaneously in the online scanning projection data. This is considered as the two ends of the linear array detector having the same height, thus achieving coplanarity of the fan-beam CT scanning plane.

[0011] This technical solution is further configured such that the method for establishing the spatial three-dimensional coordinate system is as follows:

[0012] The origin is taken as the center of rotation of the rotating platform;

[0013] The rotation axis of the rotating platform is taken as the Z-axis, and the vertical upward direction is taken as the positive direction;

[0014] The X-axis is defined by the line connecting the rotation center of the rotating platform and the X-ray source, with the direction pointing towards the X-ray source being the positive direction.

[0015] The technical solution is further configured such that the initial position of the rotating platform is set so that both cylinders are below the scanning plane, the X-ray source is turned on to perform line scanning, the rotating platform rises at a constant speed, and the linear array detector collects the line scanning projection data.

[0016] Alternatively, set the initial position of the rotating platform so that both cylinders are above the scanning plane, turn on the X-ray source to perform line scanning, and lower the rotating platform at a constant speed while the linear array detector collects the line scan projection data.

[0017] The technical solution is further configured such that the two cylinders are a first cylinder and a second cylinder, the first cylinder is positioned close to the X-ray source and the second cylinder is positioned close to the linear array detector, the first cylinder and the second cylinder are positioned 180° apart, and the projection length of the two cylinders in the X-axis direction is L.

[0018] The technical solution is further configured such that the relative height between the X-ray source and the linear array detector is set to ΔH1, then: ΔH1=SDD×tan(γ);

[0019] γ = arctan(Δh / L);

[0020] Δh=(r2-r1)×τ×υ;

[0021] Where SDD represents the distance between the X-ray source and the linear array detector, the integration time of the linear array detector is τ, the lifting speed of the rotating platform is υ, and the row coordinates of the first and second cylinders appearing in the projection data are r1 and r2, respectively.

[0022] The technical solution is further configured such that if the first cylinder appears first in the projection data, it indicates that the position of the linear array detector is higher than the position of the X-ray source; conversely, it indicates that the position of the X-ray source is higher than the position of the linear array detector.

[0023] The technical solution is further configured such that, if the relative height between the two ends of the linear array detector is set to ΔH2, then: ΔH2=|r1-r2|×τ×υ;

[0024] The linear array detector has an integration time of τ, the rotating platform has a lifting speed of υ, and the row coordinates of the first and second cylinders in the projection data are r1 and r2, respectively.

[0025] This technical solution is further configured such that, before adjusting the relative height of the two ends of the linear array detector, it also includes:

[0026] Rotate the rotating platform so that the projection of the line connecting the first cylinder and the second cylinder is collinear with the Y-axis. The first cylinder and the second cylinder correspond to the two ends of the linear array detector, respectively.

[0027] The technical solution is further configured such that if the first cylinder appears first in the projected data, it indicates that the position of the end of the linear array detector corresponding to the first cylinder is lower than the position of the end of the linear array detector corresponding to the second cylinder; conversely, it indicates that the position of the end of the linear array detector corresponding to the second cylinder is lower than the position of the end of the linear array detector corresponding to the first cylinder.

[0028] This technical solution is further configured such that when two cylinders appear simultaneously in the online scanning projection data, 0≤|r1-r2|≤σ, where σ is a relaxation factor.

[0029] The beneficial effects of this invention are:

[0030] By performing line scan imaging on two cylinders of equal height, the changes in the scanning plane deviating from the theoretical model of fan-beam CT can be identified and measured. By adjusting the orientation of the linear array detector, the scanning plane can be made as close as possible to the theoretical model, thereby improving the imaging accuracy of fan-beam CT. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of an industrial fan-beam CT scanner in the current technology;

[0032] Figure 2 This is a schematic diagram of the industrial fan-beam CT scanning plane in existing technology;

[0033] Figure 3 This is a flowchart of the fan-beam CT scanning planar coplanar calibration method based on a dual-column phantom in an embodiment of the present invention;

[0034] Figure 4 This is a schematic diagram illustrating the relative height adjustment between the X-ray source and the linear array detector in an embodiment of the present invention;

[0035] Figure 5 This is a schematic diagram illustrating the relative height adjustment of the two ends of the linear array detector in an embodiment of the present invention;

[0036] Figure 6 This is a line scan image of the dual-column phantom before the relative height of the X-ray source and the linear array detector is adjusted in an embodiment of the present invention.

[0037] Figure 7 This is a line scan image of the dual-column phantom after the relative height of the X-ray source and the linear array detector is adjusted in an embodiment of the present invention.

[0038] Figure 8 This is a line scan image of the dual-column phantom before the relative height of the two ends of the linear array detector is adjusted in an embodiment of the present invention.

[0039] Figure 9 This is a line scan image of the dual-column phantom after the relative height of the two ends of the linear array detector is adjusted in an embodiment of the present invention. Detailed Implementation

[0040] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments in this application, other similar embodiments obtained by those skilled in the art without creative effort should all fall within the scope of protection of this application. Furthermore, directional terms mentioned in the following embodiments, such as "up," "down," "left," and "right," are only for reference to the directions in the accompanying drawings; therefore, the directional terms used are for illustrative purposes and not for limiting the invention.

[0041] The essence of CT imaging is to construct and solve a system of linear equations. Assuming a two-dimensional image is represented as f... s,t Let s be the row index of the image, where 0 ≤ s ≤ H-1, and H represents the height of the 2D image. Let t be the column index of the 2D image, where 0 ≤ t ≤ W-1, and W represents the width of the 2D image. For ease of description, we use an equivalent one-dimensional array f. jTo represent two-dimensional image data, where j = s·W + t, the projection data can be represented as:

[0042]

[0043] Among them, a i,j Let be the weighting coefficient for the i-th projection line passing through the j-th image pixel, and N represent the total number of projection angles. It is usually represented in matrix form:

[0044]

[0045] The coefficient matrix M is related to the scanning geometry pose, and the unknowns are... They are distributed within a fixed two-dimensional plane.

[0046] The scanning object rotates in a circular motion relative to the X-ray source-linear array detector to obtain projection data along different paths at different projection angles. i Projection data The set construction is about The linear equations are such that the key to fan-beam CT imaging can be summarized in two points:

[0047] 1. Unknown Distributed within a fixed two-dimensional plane;

[0048] 2. Constructing a framework for the unknown. linear equation system

[0049] Among them, "unknown" The concept of "distributed within a fixed two-dimensional plane" is a theoretical model determined by the fan-beam CT reconstruction algorithm. Combined with the specific scanning process, this theoretical model can be expressed as: given a projection angle β... i The focal point S of the X-ray source and the linear array detector CD form a scanning plane SCD, and the plane where this scanning plane intersects the object being scanned is P. i A schematic diagram of the scanning plane is shown below. Figure 2 As shown, the projection angle β is traversed. i i = 0, ..., N-1, where N represents the total number of projection angles, and all planes P i The elements i = 0, ..., N-1 are coplanar and perpendicular to the axis of rotation, as shown below. Figure 2As shown. In practical engineering applications, the scanning plane formed by the X-ray source focal point S and the linear array detector exhibits changes in two dimensions (see Background Art). These two changes violate the theoretical model of fan-beam CT imaging, thus affecting the accuracy of fan-beam CT imaging. Therefore, the inventors propose a coplanar calibration method for the fan-beam CT scanning plane based on a dual-cylinder phantom. By performing linear scanning imaging on the dual-cylinder phantom, the method identifies and measures the changes in the scanning plane's deviation from the theoretical model of fan-beam CT, and adjusts the orientation of the linear array detector to make the scanning plane as close as possible to the theoretical model.

[0050] According to an embodiment of the present invention, a method for coplanar calibration of the fan-beam CT scanning plane based on a dual-column phantom is provided. Please refer to [link to relevant documentation]. Figure 3 This includes the following steps:

[0051] S100. Two cylinders of equal height are fixed at intervals along the edge of the rotating platform. A three-dimensional coordinate system is established with the rotating platform and the X-ray source as references.

[0052] S200. Set the initial position of the rotating platform so that both cylinders are detached from the scanning plane. Turn on the X-ray source to perform line scanning. The rotating platform moves up and down at a constant speed. The linear array detector collects the line scanning projection data.

[0053] S300. Adjust the relative height of the X-ray source and the linear array detector until both cylinders appear simultaneously in the online scanning projection data. This is considered as the X-ray source and the linear array detector having the same height. Adjust the relative height of the two ends of the linear array detector until both cylinders appear simultaneously in the online scanning projection data. This is considered as the two ends of the linear array detector having the same height.

[0054] It should be noted that two independent cylinders of equal height form a dual-cylinder phantom. By performing line scan imaging on these two cylinders, the deviation of the scanning plane from the theoretical model of fan-beam CT can be identified and measured. Furthermore, by adjusting the orientation of the linear array detector, the scanning plane is made as close as possible to the theoretical model (i.e., the fan-beam CT scanning plane is coplanar), thereby improving the imaging accuracy of fan-beam CT. Specifically, the orientation adjustment of the linear array detector includes adjusting the overall height of the linear array detector and adjusting the relative height of the two ends of the linear array detector.

[0055] In the coplanar calibration method for the fan-beam CT scanning plane of the dual-cylinder phantom in this embodiment, the method for establishing the spatial three-dimensional coordinate system is as follows:

[0056] The origin is taken as the center of rotation of the rotating platform;

[0057] The rotation axis of the rotating platform is taken as the Z-axis, and the vertical upward direction is taken as the positive direction;

[0058] The X-axis is defined by the line connecting the rotation center of the rotating platform and the X-ray source, with the direction pointing towards the X-ray source being the positive direction.

[0059] In the fan-beam CT scanning plane coplanar calibration method of the dual-cylinder phantom in this embodiment, the initial position of the rotating platform is set so that both cylinders are lower than the scanning plane, the X-ray source is turned on to perform line scanning, the rotating platform rises at a constant speed, and the linear array detector collects the line scanning projection data.

[0060] Alternatively, set the initial position of the rotating platform so that both cylinders are above the scanning plane, turn on the X-ray source to perform line scanning, and lower the rotating platform at a constant speed while the linear array detector collects the line scan projection data.

[0061] In the fan-beam CT scanning coplanar calibration method of the dual-cylinder phantom in this embodiment, the two cylinders are a first cylinder and a second cylinder, respectively. The first cylinder is set close to the X-ray source, and the second cylinder is set close to the linear array detector. The first cylinder and the second cylinder are set at a distance of 180°, and the projection length of the two in the X-axis direction is L.

[0062] The coplanar calibration method for the fan-beam CT scanning plane of the dual-column phantom in this embodiment is as follows: Figure 4 As shown, if the relative height between the X-ray source and the linear array detector is set to ΔH1, then: ΔH1=SDD×tan(γ);

[0063] γ = arctan(Δh / L);

[0064] Δh=(r2-r1)×τ×υ;

[0065] Where SDD represents the distance between the X-ray source and the linear array detector, the integration time of the linear array detector is τ, the lifting speed of the rotating platform is υ, and the row coordinates of the first and second cylinders appearing in the projection data are r1 and r2, respectively.

[0066] It should be noted that when the linear array detector is higher than the X-ray source, the first cylinder appears in the projection data first, followed by the second cylinder. When the linear array detector is lower than the X-ray source, the second cylinder appears in the projection data first, followed by the first cylinder.

[0067] The coplanar calibration method for the fan-beam CT scanning plane of the dual-column phantom in this embodiment is as follows: Figure 5 As shown, if the relative height between the two ends of the linear array detector is set to ΔH2, then: ΔH2=|r1-r2|×τ×υ;

[0068] The linear array detector has an integration time of τ, the rotating platform has a lifting speed of υ, and the row coordinates of the first and second cylinders in the projection data are r1 and r2, respectively.

[0069] In the fan-beam CT scanning plane coplanar calibration method of the dual-column phantom in this embodiment, before adjusting the relative height of the two ends of the linear array detector, the method further includes:

[0070] Rotate the rotating platform so that the projection of the line connecting the first cylinder and the second cylinder is collinear with the Y-axis.

[0071] In the coplanar calibration method of the fan-beam CT scanning plane of the dual-cylinder phantom in this embodiment, when the two cylinders appear simultaneously in the online scanning projection data, 0≤|r1-r2|≤σ, where σ is the relaxation factor.

[0072] To verify the effectiveness of the above method, a dual-cylinder phantom was used to perform coplanar calibration of the scanning plane of a sector-beam CT system. This sector-beam CT system has a focal length of 3500 mm and a linear array detector element size of 0.4 mm. The cylinders were designed with a diameter of 8 mm and were made of steel. The designed height h of the cylinders was rigorously calibrated, and the height difference between the two cylinders was less than 10 μm.

[0073] 1. Adjust the relative height of the X-ray source and the linear array detector.

[0074] like Figure 4 As shown, the dual-cylinder phantom is fixed to the edge of the rotating platform, positioned front-to-back along the ray direction, with a distance of approximately 700 mm between them. The integration time of the linear array detector is set to 40 ms, and the lifting speed of the rotating platform is set to 0.5 mm / s. The rotating platform rises at a uniform speed, and the line scan image of the dual-cylinder phantom is as follows. Figure 6 As shown. Due to the difference in magnification, the wider cylinder in the image corresponds to the cylinder at the X-ray source end (i.e., the first cylinder), which is easily identifiable. The first cylinder enters the scanning field of view first, indicating that the linear array detector is higher than the X-ray source. Measurements show that the first-row coordinates of the two cylinders in the height direction differ by approximately 8 pixels in the line scan image. Therefore:

[0075] Δh=(r2-r1)×τ×υ=8×0.04s×0.5mm / s=0.16mm;

[0076]

[0077] Lower the linear array detector height by approximately 0.8 mm and repeat the above linear scanning process. At this point, the two cylinders enter the scanning field of view almost simultaneously, as shown... Figure 7 As shown, at this point, it can be assumed that the X-ray source and the linear array detector are at almost the same height from the plane of the rotating platform.

[0078] 2. Adjust the relative height of the two ends of the linear array detector.

[0079] like Figure 5As shown, the rotating platform rotates 90°, positioning the two cylinders side-by-side. At this point, the first cylinder corresponds to end D of the linear array detector, and the second cylinder corresponds to end C. The integration time of the linear array detector is set to 40 ms, and the lifting speed of the rotating platform is set to 0.5 mm / s. The rotating platform rises at a uniform speed, and the line scan image of the dual-cylinder phantom is shown below. Figure 8 As shown. Observation reveals that the first cylinder enters the scanning field of view first, indicating that the D-end of the linear array detector corresponding to the first cylinder is lower than the C-end of the linear array detector corresponding to the second cylinder. Measurements show that the first row coordinates of the two cylinders differ by approximately 10 pixels along their height direction. Therefore:

[0080] ΔH2=10×0.04s×0.5mm / s=0.2mm;

[0081] The D-end of the linear array detector is raised by approximately 0.2 mm, and the above linear scanning process is repeated. At this time, the two cylinders enter the scanning field of view almost simultaneously, as shown... Figure 9 As shown, at this point, it can be assumed that the heights of both ends of the linear array detector from the plane of the rotating platform are almost the same.

[0082] The present invention has been described in detail above. The above description is only a preferred embodiment of the present invention and should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the scope of this application should still fall within the scope of the present invention.

Claims

1. A method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom, characterized in that, Includes the following steps: Two cylinders of equal height are fixed at intervals along the edge of the rotating platform. A three-dimensional coordinate system is established with the rotating platform and the X-ray source as references. Set the initial position of the rotating platform so that both cylinders are detached from the scanning plane, turn on the X-ray source to perform line scanning, the rotating platform moves up and down at a constant speed, and the linear array detector collects the line scanning projection data. Adjust the relative height of the X-ray source and the linear array detector until both cylinders appear simultaneously in the online scanning projection data; this is considered as the X-ray source and the linear array detector being at the same height. Adjust the relative height of the two ends of the linear array detector until the two cylinders appear simultaneously in the online scanning projection data, which is considered to be the case that the two ends of the linear array detector are at the same height.

2. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 1, characterized in that, The method for establishing the three-dimensional coordinate system is as follows: The origin is taken as the center of rotation of the rotating platform; The rotation axis of the rotating platform is taken as the Z-axis, and the vertical upward direction is taken as the positive direction; The X-axis is defined by the line connecting the rotation center of the rotating platform and the X-ray source, with the direction pointing towards the X-ray source being the positive direction.

3. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 1, characterized in that, Set the initial position of the rotating platform so that both cylinders are below the scanning plane, turn on the X-ray source to perform line scanning, the rotating platform rises at a constant speed, and the linear array detector collects the line scan projection data. Alternatively, set the initial position of the rotating platform so that both cylinders are above the scanning plane, turn on the X-ray source to perform line scanning, and lower the rotating platform at a constant speed while the linear array detector collects the line scan projection data.

4. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 2, characterized in that, The two cylinders are a first cylinder and a second cylinder, respectively. The first cylinder is positioned close to the X-ray source, and the second cylinder is positioned close to the linear array detector. The first cylinder and the second cylinder are spaced 180° apart, and their projected length in the X-axis direction is L.

5. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 4, characterized in that, Let the relative height between the X-ray source and the linear array detector be ΔH1, then: ΔH1=SDD×tan(γ); γ = arctan(Δh / L); Δh=(r2-r1)×τ×υ; Where SDD represents the distance between the X-ray source and the linear array detector, the integration time of the linear array detector is τ, the lifting speed of the rotating platform is υ, and the row coordinates of the first and second cylinders appearing in the projection data are r1 and r2, respectively.

6. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 5, characterized in that, If the first cylinder appears in the projection data first, it means that the linear array detector is located higher than the X-ray source; conversely, it means that the X-ray source is located higher than the linear array detector.

7. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 4, characterized in that, If the relative height between the two ends of the linear array detector is set to ΔH2, then: ΔH2=|r1-r2|×τ×υ; The linear array detector has an integration time of τ, the rotating platform has a lifting speed of υ, and the row coordinates of the first and second cylinders in the projection data are r1 and r2, respectively.

8. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 7, characterized in that, Before adjusting the relative height of the two ends of the linear array detector, the following steps are also included: Rotate the rotating platform so that the projection of the line connecting the first cylinder and the second cylinder is collinear with the Y-axis. The first cylinder and the second cylinder correspond to the two ends of the linear array detector, respectively.

9. The method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to claim 8, characterized in that, If the first cylinder appears first in the projected data, it means that the position of the end of the linear array detector corresponding to the first cylinder is lower than the position of the end of the linear array detector corresponding to the second cylinder. Conversely, it means that the position of the end of the linear array detector corresponding to the second cylinder is lower than the position of the end of the linear array detector corresponding to the first cylinder.

10. A method for coplanar calibration of fan-beam CT scanning planes based on a dual-column phantom according to any one of claims 5-9, characterized in that, When two cylinders appear simultaneously in the online scanning projection data, 0≤|r1-r2|≤σ, where σ is the relaxation factor.

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