A CT system geometry parameter calibration apparatus and method

Abstract

The application provides a CT system geometry parameter calibration method and device, the method comprises the following steps: constructing a calibration phantom of the CT system; placing the calibration phantom on the imaging system platform for rotation projection; performing image fitting on all the steel ball tracks obtained by projection to obtain a series of elliptical tracks; removing part of the elliptical tracks that do not meet the requirements according to the solving requirements to obtain target elliptical tracks; and calibrating the geometry parameters of the CT system according to the target elliptical tracks. Based on the scheme provided in the application, the geometry parameters of the CT system can be more accurately extracted and calibrated. The high-precision calibration directly improves the reconstruction quality of the CT image, reduces the image distortion, artifacts and other problems caused by the geometry parameter error, and thus improves the diagnostic accuracy and reliability of the image.

Description

Technical Field

[0001] The present application relates to the technical field of CT system calibration, and in particular to a device and method for calibrating geometric parameters of a CT system. Background Art

[0002] The geometric parameter calibration method of existing CT systems (especially cone-beam CT systems, also known as Micro-CT or small animal CT) is mainly used to solve the problem that the system geometry cannot meet the standard model (such as the FDK model) due to production and installation errors.

[0003] The general method is to collect projection data of a phantom (such as a phantom containing multiple small balls or a specific geometric shape) at one or more projection angles. These data are the basis for subsequent analysis and calibration. During the data acquisition process, it is necessary to ensure that the rotating stage can rotate smoothly and stop accurately at each preset angle in order to collect high-quality projection images. The collected projection data is then processed and analyzed to identify image distortion or offset caused by system geometric errors. Existing methods include analytical methods and phantom calibration methods, among which:

[0004] The analytical method collects phantom projection data from multiple angles and processes the projection images using methods such as adaptive binarization to find the center of mass of each sphere's projection image. This allows the user to fit an ellipse and calculate the system's geometric parameters. The phantom calibration method designs a specialized calibration phantom (e.g., one containing multiple spheres or a specific geometric shape) and uses the characteristics of the phantom's projection image to estimate the system's geometric error parameters. The system hardware position is then adjusted accordingly. This method is intuitive and easy to use, but it requires high precision in the design and processing of the phantom.

[0005] However, the existing solutions have the following disadvantages:

[0006] 1. Limited calibration accuracy: The limited machining precision of specially designed calibration phantoms can lead to errors in the calibration results. While algorithm-based geometric calibration methods can directly process projection data or images, they often require complex or computationally intensive calculations and can only determine a limited number of calibration parameters.

[0007] 2. High operational complexity: Some methods require manual adjustment of the position and angle of system hardware (such as the radiation source, detector, and rotation stage). These operations are not only time-consuming and labor-intensive, but also prone to human error. Some calibration methods require additional equipment or devices to assist, such as positioning bodies and positioning blocks. These devices have high requirements for spatial distance and installation accuracy, increasing the complexity of the operation.

[0008] 3. Poor versatility and universality: Some calibration methods may only be applicable to specific cone-beam CT system models and lack versatility and universality. When the system is upgraded or components are replaced, the complex calibration process may need to be repeated.

[0009] Model limitations: Model-based calibration methods may be limited by the complexity and applicability of the model and may not be able to adapt to all types of scanned objects and imaging requirements.

[0010] 4. High cost: To obtain high-precision calibration results, you may need to purchase specially designed calibration phantoms or high-precision measuring equipment, which increases the cost of system construction and maintenance. The long calibration process not only affects the normal use of the system, but may also increase the investment in manpower and material resources. Summary of the Invention

[0011] The present application aims to solve one of the technical problems in the related art at least to a certain extent.

[0012] To this end, the purpose of this application is to propose a method and device for calibrating geometric parameters of a CT system, so as to more accurately extract and calibrate the geometric parameters of the CT system.

[0013] To achieve the above objectives, the first embodiment of the present application provides a method for calibrating geometric parameters of a CT system, comprising:

[0014] Construct a calibration phantom for the CT system;

[0015] The calibration phantom is placed on the imaging system stage for rotational projection, and all the steel ball trajectories obtained by projection are image-fitted to obtain a series of elliptical trajectories;

[0016] According to the solution requirements, some elliptical trajectories that do not meet the requirements are removed to obtain the target elliptical trajectory;

[0017] The geometric parameters of the CT system are calibrated according to the target elliptical trajectory.

[0018] Optionally, also include:

[0019] The calibration phantom is cylindrical and has steel balls embedded in it;

[0020] The calibration phantom has a circle of steel balls on the upper and lower sides, the two circles of steel balls are separated by a preset distance, each circle has a first preset number of steel balls, and the steel balls on each circle are distributed at equal angles;

[0021] A second preset number of steel balls are distributed in a spiral parallel to each other between the upper and lower circles of steel balls and are distributed at equal angles, and the upper and lower steel balls are equally spaced vertically;

[0022] The two rows of spiral steel balls are tangent to the loop formed by the upper and lower circles of steel balls, and the starting positions of the two rows of spirals differ by 180°.

[0023] Optionally, the geometric parameters of the CT system include:

[0024] The distance D from the source to the detector;

[0025] The distance R from the source point to the center of rotation;

[0026] The horizontal coordinate u0 of the detector center;

[0027] The vertical coordinate v0 of the detector center;

[0028] The tilt angle η of the detector in the plane;

[0029] Detector's horizontal deflection angle

[0030] The pitch angle σ of the detector;

[0031] Angle sampling interval Δβ.

[0032] Optionally, also include:

[0033] The distance D from the source point S to the detector is defined by the length of the central ray beam, and the corresponding detector center is defined as the intersection of the central ray on the detector. The three deflection angles of the detector are defined by the polar coordinate system.

[0034] Optionally, the process of calibrating the angle sampling interval Δβ according to the target elliptical trajectory includes:

[0035] Define the similarity S between the i-th projection and the first projection in the target elliptical trajectory (i) as follows:

[0036]

[0037] Among them, P (i) (u,v) is the pixel value of the pixel at position (u,v) of the i-th projection;

[0038] Assuming that the number of samples in one circle in the design index is N0, set the search radius ΔN, and search N in [N0-ΔN, N0+ΔN] to make formula (2) valid, so as to preliminarily obtain the calibrated sampling number N. Formula (2) is:

[0039] S (N) =min S (i) ,i∈[N0-ΔN,N0+ΔN] (2)

[0040] To further improve the accuracy, the following quadratic fitting method is used:

[0041] Set the ratio threshold t, if equation (3) holds, then let the number of samples per circle N * =N, otherwise for three points (N-1,S (N -1) )、(N,S (N) ) and (N+1,S (N+1) ) Make a quadratic parabola fit and take the lowest point of the obtained parabola as the sampling number N of a circle * , formula (3) is:

[0042] |S (N) -S (N-1) | <t,|S (N) -S (N+1) | <t (3)

[0043] According to the sampling number N * , the angle sampling interval Δβ is calculated to be 2π / N * .

[0044] Optionally, the process of calibrating the inclination angle η of the detector in a plane according to the target elliptical trajectory includes:

[0045] Since the center of the fitting ellipse in the target elliptical trajectory (u k ,v k ) is located on the actual z-axis, and the line connecting the centers of the several sets of fitted ellipses is the actual z-axis, and passes through the center of the detector (u0, v0). The theoretical z-axis is a vertical line, and the angle between the two is the inclination angle η of the detector in its plane. Therefore, the following linear equation can be established:

[0046]

[0047] Among them, C0 is a constant;

[0048] Based on the least squares method, let the coordinates of the i-th point on the k-th ellipse be expressed as and the center of the corresponding ellipse Satisfies the following equation:

[0049]

[0050] Solve equations (4) and (5) together to obtain the tilt angle η of the detector in the plane.

[0051] Optionally, the process of calibrating the detector center coordinates (u0, v0) and the distance D from the source point to the detector according to the target elliptical trajectory includes:

[0052] For the fitted ellipse within the target elliptical trajectory, without considering When σ and , the equation is:

[0053]

[0054] Among them, a, b, are the parameters of the fitted ellipse;

[0055] Moreover, the spatial motion coordinates of the steel ball are η is just a rotation quantity that does not affect the shape of the ellipse and does not need to be considered, so we have:

[0056]

[0057] Then z 2 U 2 +(R 2 -r 2 )V 2 -2DRzV+z 2 D 2 =0

[0058] Right now:

[0059] Let ρ = R / r, ζ = z / R, then the above formula can be written in the form of (10), that is:

[0060]

[0061] in:

[0062] Among them, U, V, ρ, and ζ are all intermediate variables, x is the z coordinate value of the spatial motion coordinate of the steel ball, and r is the rotation radius of the steel ball;

[0063] Joint And the expressions of a and b are:

[0064]

[0065] Considering the i-th and j-th ellipses, we get:

[0066]

[0067] Subtracting (9) and (10) and simplifying them yields:

[0068]

[0069] Convert equation (11) into the following form:

[0070] Y ij =v0+D 2 X ij (12)

[0071] Among them, Yij and X ij All are intermediate variables;

[0072] Combined with the fitted ball trajectory ellipse equation, multiple ellipse equations are substituted and the vertical coordinate v0 of the detector center and the distance D from the source point to the detector are fitted using the linear regression method.

[0073] because For all parameters of the fitted ellipse equations Take the average value and get the horizontal coordinate u0 of the center of the detector.

[0074] Optionally, the process of calibrating the distance R from the source point to the rotation center according to the target elliptical trajectory includes:

[0075] For each point in the target elliptical trajectory, find ρ and ζ, and the expression is:

[0076]

[0077] Take any two points i and j, according to the cosine theorem, the expression of their distance is

[0078]

[0079] Among them, (α i -α j ) is the angle difference between the two steel balls in the horizontal direction, d ij is the distance between the two steel balls, both of which are known. Combined with the parameters of the corresponding fitted ellipse equation, the distance R from the source point to the rotation center is obtained.

[0080] Optionally, according to the target elliptical trajectory, the detector's horizontal deflection angle The calibration process includes:

[0081] For a fitted ellipse within the target elliptical trajectory, if S is the ray source, the positions of the two steel balls A and B differ by 180°, O is its symmetry center, A1 and B1 are ideal projections, A2 and B2 are actual projections, and the deflection angle of the detector in the horizontal direction is Let ∠SA2B2 be ∠1, then we have Established;

[0082] Furthermore, the following formula can be obtained:

[0083]

[0084] Also know:

[0085]

[0086] Solve equations (15) and (16) together to obtain the detector's horizontal deflection angle: The expression is:

[0087]

[0088] Here, arcsin represents the inverse sine function.

[0089] Optionally, the calibration process of the detector's pitch angle σ is related to the detector's horizontal deflection angle The calibration process is similar.

[0090] To achieve the above-mentioned purpose, a second embodiment of the present application provides a CT system geometric parameter calibration device, comprising:

[0091] A construction module for constructing a calibration phantom for a CT system;

[0092] A fitting module is used to place the calibration phantom on the imaging system platform for rotational projection, and perform image fitting on all the steel ball trajectories obtained by the projection to obtain a series of elliptical trajectories;

[0093] The removal module is used to remove some elliptical trajectories that do not meet the requirements according to the solution requirements to obtain the target elliptical trajectory;

[0094] The calibration module is used to calibrate the geometric parameters of the CT system according to the target elliptical trajectory.

[0095] The technical solutions provided by the embodiments of this application bring at least the following beneficial effects:

[0096] 1. High-Precision Calibration: This application significantly improves the calibration accuracy of CT system geometric parameters through innovative phantom design and more rigorous and accurate calculation methods. This high-precision calibration is a key factor in ensuring CT image quality and reducing artifacts and errors, and is of great significance in fields such as medical diagnosis and materials testing.

[0097] 2. Ingenious phantom design: Traditional phantom designs may have certain limitations, making it difficult to fully capture or accurately simulate the geometric characteristics of CT systems in actual use. However, the phantom design of this application is more ingenious, more accurately simulating the geometric relationships during CT scanning, thus providing a more reliable foundation for subsequent calibration work.

[0098] 3. Rigorous Calculation Methods: This application uses more rigorous and accurate calculation methods to calibrate geometric parameters. These methods may involve more advanced mathematical models, more sophisticated numerical analysis, or more optimized algorithm design to ensure the accuracy and reliability of the calculation results.

[0099] 4. Strong versatility: Traditional calibration methods may only be applicable to specific specifications or types of CT systems. However, through ingenious design and optimization, this application makes it applicable to CT systems of different specifications. This versatility not only reduces the complexity and cost of calibration work, but also improves the efficiency and flexibility of calibration, allowing different users to select the appropriate CT system for calibration according to their needs.

[0100] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0101] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0102] Figure 1 is a flow chart of a method for calibrating geometric parameters of a CT system according to an embodiment of the present application;

[0103] FIG2( a) is a front view of a calibration phantom according to an embodiment of the present application;

[0104] FIG2( b ) is a side view of a calibration phantom according to an embodiment of the present application;

[0105] FIG2( c ) is a top view of a calibration phantom according to an embodiment of the present application;

[0106] Figure 3 is a schematic diagram illustrating rotational projection of a calibration phantom according to an embodiment of the present application;

[0107] Figure 4 is a schematic diagram of elliptical trajectories obtained by image fitting of all steel ball trajectories obtained by projection according to an embodiment of the present application;

[0108] Figure 5 is a schematic diagram of a target elliptical trajectory according to an embodiment of the present application;

[0109] Figure 6 is a schematic diagram of geometric parameters according to an embodiment of the present application;

[0110] Figure 7 is a schematic diagram of multiple fitted ellipses according to an embodiment of the present application;

[0111] Figure 8 is a schematic diagram of the projection of a steel ball symmetrical about a rotation axis in the horizontal direction according to an embodiment of the present application;

[0112] Figure 9 This is a block diagram of a CT system geometric parameter calibration device according to an embodiment of the present application. DETAILED DESCRIPTION

[0113] The following describes in detail embodiments of the present application. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0114] A method and apparatus for calibrating geometric parameters of a CT system according to an embodiment of the present application will be described below with reference to the accompanying drawings.

[0115] Figure 1 FIG. 1 is a flow chart of a method for calibrating geometric parameters of a CT system according to an embodiment of the present application. Figure 1 As shown, the method includes the following steps:

[0116] Step 101: construct a calibration phantom for the CT system.

[0117] Traditional phantom designs may have certain limitations and are difficult to fully cover or accurately simulate the geometric characteristics of CT systems in actual use. Therefore, this application proposes a calibration phantom that can more accurately simulate the geometric relationships during CT scanning, thereby providing a more reliable foundation for subsequent calibration work.

[0118] The front view, side view and top view of the calibration phantom constructed in the embodiment of the present application are respectively as follows: Figure 2(a) 、 2(b) and as shown in Figure 2(c).

[0119] Among them, the calibration phantom is cylindrical and has steel balls embedded in it; there is a circle of steel balls on the upper and lower parts of the calibration phantom, and the two circles of steel balls are at a preset distance apart. Each circle has a first preset number of steel balls, and the steel balls on each circle are distributed at equal angles; a second preset number of steel balls are distributed in a spiral between the upper and lower circles of steel balls and are distributed at equal angles, and the upper and lower steel balls are equidistant vertically; the two columns of spiral steel balls are tangent to the loop formed by the upper and lower circles of steel balls, and the starting positions of the two spiral columns differ by 180°.

[0120] In a possible embodiment, the motif parameters are as follows:

[0121] 1. The mold body is cylindrical, 15cm high, with an outer diameter of about 10.5cm and an inner diameter of about 9.5cm. It contains a steel ball with a diameter of 0.5mm.

[0122] 2. There is a circle of steel balls on the upper and lower parts of the phantom, and the two circles of steel balls are 10 cm apart (as shown by the dotted line in the front view). There are 16 steel balls on each circle, and they are distributed at an equal angle of 22.5° (as shown in the top view);

[0123] 3. Between the upper and lower circles of steel balls, 30 steel balls are distributed in parallel spirals, that is, 15 steel balls are distributed on each spiral line, and the same angular spacing is 22.5° (as shown in the top view). At the same time, the upper and lower steel balls are equally spaced vertically (as shown in the front view);

[0124] 4. The two rows of spiral steel balls are tangent to the ring line formed by the upper and lower circles of steel balls, and the starting positions of the two rows of spirals differ by 180°.

[0125] It is understandable that due to the variability of actual scenarios, the overall size of the phantom, the diameter of the steel balls, and the number of steel balls distributed in the upper and lower circles and the two spiral lines can be flexibly adjusted according to actual needs. This application does not limit this.

[0126] In step 102 , the calibration phantom is placed on the imaging system stage for rotational projection, and all the steel ball trajectories obtained by the projection are image-fitted to obtain a series of elliptical trajectories.

[0127] In this step, the image acquisition step is performed. First, the calibration phantom is placed on the imaging system platform for rotation projection. The projection result is as follows: Figure 3 As shown, the projection trajectories of most small steel balls are ellipses.

[0128] Then, the calibration phantom is imaged, and a series of elliptical trajectories are obtained by image fitting of all the projected steel ball trajectories.

[0129] Ideally, you would get Figure 4 The image shown is a series of elliptical trajectories.

[0130] It is understandable that in order to make the image layers clear, Figure 4 The images shown are for illustration only and do not depict the entire elliptical trajectory.

[0131] Step 103: According to the solution requirements, some elliptical trajectories that do not meet the requirements are removed to obtain the target elliptical trajectory.

[0132] In the embodiment of the present application, according to the solution requirements, some fitting elliptical trajectories need to be removed, and the target elliptical trajectory obtained after removal is as follows: Figure 5 shown.

[0133] Step 104 : calibrate the geometric parameters of the CT system according to the target elliptical trajectory.

[0134] For common spiral or circular orbit CT, the quality of reconstruction results is mainly affected by the following important geometric parameters:

[0135] The distance D from the source to the detector;

[0136] The distance R from the source point to the rotation center;

[0137] The horizontal coordinate u0 of the detector center;

[0138] The vertical coordinate v0 of the detector center;

[0139] The tilt angle η of the detector in the plane;

[0140] Detector's horizontal deflection angle

[0141] The pitch angle σ of the detector;

[0142] Angle sampling interval Δβ.

[0143] The schematic diagram of the geometric parameters is as follows: Figure 6 As shown, the distance D from the source point S to the detector is defined by the length of the central ray beam, and the corresponding detector center is defined as the intersection of the central ray on the detector, and the three deflection angles of the detector are defined by the polar coordinate system.

[0144] Specifically, the present application is directed to the following geometric parameter calibration process:

[0145] 1. Angle sampling interval Δβ

[0146] In the embodiment of the present application, the similarity S between the i-th projection and the first projection in the target elliptical trajectory is first defined. (i) as follows:

[0147]

[0148] Among them, P (i) (u,v) is the pixel value of the pixel at position (u,v) of the i-th projection.

[0149] In another embodiment of the present application, in order to improve calculation accuracy, the similarity between two projections can also be defined as follows:

[0150]

[0151] Among them, P (i) (n) is the coordinate value of the projection center of the nth steel ball in the i-th projection.

[0152] Then, assuming that the number of samples in one circle in the design index is N0, set the search radius ΔN, and search N in [N0-ΔN, N0+ΔN] to make formula (2) valid, so as to preliminarily obtain the calibrated sampling number N. Formula (2) is:

[0153] S (N) =min S (i) ,i∈[N0-ΔN,N0+ΔN] (2)

[0154] In a possible embodiment, the search radius ΔN may be set to the number of samples rotated by 10°.

[0155] To further improve the accuracy, the following quadratic fitting method can be used:

[0156] Set the ratio threshold t (can be taken as 0.1), if formula (3) holds, then let the number of samples in one circle N * =N, otherwise for three points (N-1,S (N-1) )、(N,S (N) ) and (N+1,S (N+1) ) Make a quadratic parabola fit and take the lowest point of the obtained parabola as the sampling number N of a circle * , formula (3) is:

[0157] |S (N) -S (N-1) | <t,|S (N) -S (N+1) | <t (3)

[0158] Finally, according to the sampling number N * , the angle sampling interval Δβ=2π / N can be calculated * .

[0159] 2. The tilt angle η of the detector in the plane

[0160] like Figure 7 As shown, the center of the fitted ellipse in the target elliptical trajectory (u k ,v k ) is on the actual z-axis, and the line connecting the centers of the several sets of fitted ellipses is the actual z-axis, passing through the center of the detector (u0, v0). The theoretical z-axis is a vertical line, and the angle between the two is the inclination angle η of the detector in its plane. Therefore, the following linear equation can be established:

[0161]

[0162] Where C0 is a constant.

[0163] When solving the problem, the least square method can be used to reduce the error. Let the coordinates of the i-th point on the k-th ellipse be expressed as and the center of the corresponding ellipse Satisfies the following equation:

[0164]

[0165] By solving equations (4) and (5) simultaneously, we can obtain the tilt angle η of the detector in the plane.

[0166] 3. Detector center coordinates (u0, v0) and the distance D from the source to the detector

[0167] For the fitted ellipse within the target elliptical trajectory, without considering When σ and , the equation is:

[0168]

[0169] Among them, a, b, are the parameters of the fitted ellipse.

[0170] Moreover, the spatial motion coordinates of the steel ball are η is just a rotation quantity that does not affect the shape of the ellipse and does not need to be considered, so we have:

[0171]

[0172] Then z 2 U 2 +(R 2 -r 2 )V 2 -2DRzV+z 2 D 2 =0

[0173] Right now:

[0174] Let ρ = R / r, ζ = z / R, then the above formula can be written in the form of (10), that is:

[0175]

[0176] in:

[0177] Among them, U, V, ρ, and ζ are all intermediate variables, z is the z coordinate value of the spatial motion coordinate of the steel ball, and r is the rotation radius of the steel ball.

[0178] Joint And the expressions of a and b are:

[0179]

[0180] Considering the i-th and j-th ellipses, we get:

[0181]

[0182] Subtracting (9) and (10) and simplifying them yields:

[0183]

[0184] Convert equation (11) into the following form:

[0185] Y ij =v0+D2 X ij (12)

[0186] Among them, Y ij and X ij All are intermediate variables.

[0187] Combined with the fitted ball trajectory ellipse equation, multiple ellipse equations are substituted and the vertical coordinate v0 of the detector center and the distance D from the source point to the detector are fitted using the linear regression method.

[0188] Furthermore, due to For all parameters of the fitted ellipse equations By taking the average value, we can get the horizontal coordinate u0 of the center of the detector.

[0189] 4. The distance R from the source point to the rotation center

[0190] For each point in the target elliptical trajectory, first calculate ρ and ζ, and the expression is:

[0191]

[0192] Take any two points i and j, according to the cosine theorem, the expression of their distance is

[0193]

[0194] Among them, (α i -α j ) is the angle difference between the two steel balls in the horizontal direction, d ij is the distance between the two steel balls, which are all known. Combined with the parameters of the corresponding fitted ellipse equation, the distance R from the source point to the rotation center is obtained.

[0195] In addition, to reduce errors, multiple solutions can be performed to obtain the average value.

[0196] 5. Detector's horizontal deflection angle and pitch angle σ

[0197] After correction according to the calibrated parameters, the two plane out-of-plane rotation angles of the detector are calibrated.

[0198] The out-of-plane rotation angle of the detector will make the projection of the steel ball symmetrical about the rotation axis no longer symmetrical. Taking the horizontal direction as an example, Figure 8 As shown, the present application can calibrate the external rotation angle of the surface.

[0199] For a fitted ellipse within the target elliptical trajectory, if S is the ray source, the positions of the two steel balls A and B differ by 180°, O is its symmetry center, A1 and B1 are the projections under ideal conditions, A2 and B2 are the actual projections, and the deflection angle of the detector in the horizontal direction is Let ∠SA2B2 be ∠1, then we have Established.

[0200] Furthermore, the following formula can be obtained through analysis:

[0201]

[0202] Also know:

[0203]

[0204] Solve equations (15) and (16) together to obtain the detector's horizontal deflection angle: The expression is:

[0205]

[0206] Here, arcsin represents the inverse sine function.

[0207] Similar to the above calibration of the detector center, A and B are steel balls equidistant from the center of the phantom and 180° apart. Then the O' position can be calculated from the detector center, and AB are known design parameters, so the detector's horizontal deflection angle can be calculated.

[0208] Similarly, the pitch angle σ of the detector can be calculated in the vertical direction.

[0209] At this point, the geometric parameters of the CT system can be calibrated.

[0210] In order to implement the above embodiment, the present application also proposes a CT system geometric parameter calibration device.

[0211] Figure 9 FIG. 1 is a block diagram of a CT system geometric parameter calibration device 10 according to an embodiment of the present application, comprising:

[0212] Construction module 100, for constructing a calibration phantom of a CT system;

[0213] The fitting module 200 is used to place the calibration phantom on the imaging system stage for rotational projection, and perform image fitting on all the steel ball trajectories obtained by the projection to obtain a series of elliptical trajectories;

[0214] The removal module 300 is used to remove some elliptical trajectories that do not meet the requirements according to the solution requirements to obtain the target elliptical trajectory;

[0215] The calibration module 400 is configured to calibrate the geometric parameters of the CT system according to the target elliptical trajectory.

[0216] As to the apparatus in the above-mentioned embodiments, the specific manner in which each module performs operations has been described in detail in the embodiments related to the method, and will not be described in detail here.

[0217] It should be understood that the various forms of flow shown above can be reordered, added to, or have steps deleted. For example, the steps described in this application can be performed in parallel, in series, or in a different order, as long as the desired results of the technical solutions of this application can be achieved, and this application does not limit here.

[0218] The above detailed description does not constitute a limitation on the protection scope of the present application. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for calibrating geometric parameters of a CT system, characterized in that: include: Construct a calibration phantom for the CT system; The calibration phantom is placed on the imaging system stage for rotational projection, and all the steel ball trajectories obtained by projection are image-fitted to obtain a series of elliptical trajectories; According to the solution requirements, some elliptical trajectories that do not meet the requirements are removed to obtain the target elliptical trajectory; calibrating geometric parameters of the CT system according to the target elliptical trajectory; The geometric parameters of the CT system include: The distance from the source to the detector ; The distance from the source point to the center of rotation ; Horizontal coordinate of the detector center ; The vertical coordinate of the detector center ; Tilt angle of the detector in the plane ; Detector's horizontal deflection angle ; Pitch angle of the detector ; Angle sampling interval ; Among them, the source Distance to detector It is defined by the length of the central ray beam, and the corresponding detector center is defined as the intersection of the central ray on the detector. The three deflection angles of the detector are defined by the polar coordinate system; According to the target elliptical trajectory, the angle sampling interval The calibration process includes: Define the similarity between the i-th projection and the first projection in the target elliptical trajectory as follows: (1) in, is the i-th projection The pixel value of the position pixel; Assume that the number of samples in one circle in the design index is , set the search radius ,exist Find N so that formula (2) is established, so as to preliminarily obtain the calibration sampling number N, formula (2) is: (2) To further improve the accuracy, the following quadratic fitting method is used: Set the ratio threshold , if formula (3) holds, then the number of samples in one circle is , otherwise for three points 、 as well as Fit a quadratic parabola and use the lowest point of the resulting parabola as the number of samples for one circle. , formula (3) is: (3) According to the number of samples , calculate the angle sampling interval .

2. The method according to claim 1, characterized in that Also includes: The calibration phantom is cylindrical and has steel balls embedded in it; The calibration phantom has a circle of steel balls on the upper and lower sides, the two circles of steel balls are separated by a preset distance, each circle has a first preset number of steel balls, and the steel balls on each circle are distributed at equal angles; A second preset number of steel balls are distributed in a spiral parallel to each other between the upper and lower circles of steel balls and are distributed at equal angles, and the upper and lower steel balls are equally spaced vertically; The two rows of spiral steel balls are tangent to the loop formed by the upper and lower circles of steel balls, and the starting positions of the two rows of spirals differ by 180°.

3. The method according to claim 1, characterized in that According to the target elliptical trajectory, the inclination angle of the detector in the plane The calibration process includes: Since the center of the fitted ellipse in the target elliptical trajectory Located on the actual z-axis, and the line connecting the centers of several sets of fitted ellipses is the actual z-axis, and passes through the center of the detector , and the theoretical z-axis is a vertical straight line, and the angle between the two is the inclination angle of the detector in its plane , so the following equation of the line can be established: (4) in, is a constant; Based on the least squares method, let the coordinates of the i-th point on the k-th ellipse be expressed as , and the center of the corresponding ellipse Satisfies the following equation: ，(5) Solve equations (4) and (5) together to obtain the tilt angle of the detector in the plane: .

4. The method according to claim 3, characterized in that According to the target elliptical trajectory, the center coordinates of the detector are and the distance from the source to the detector The calibration process includes: For the fitted ellipse within the target elliptical trajectory, without considering and When , the equation is: (6) in, 、 、 、 are the parameters of the fitted ellipse; Moreover, the spatial motion coordinates of the steel ball are ( , , ), It is just a rotation amount, which does not affect the shape of the ellipse and does not need to be considered, so we have: (7) (8) but Right now: make , , then the above formula can be written in the form of (10), that is: in: , , , in, 、 、 、 are intermediate variables. is the z coordinate value of the steel ball’s spatial motion coordinates, is the rotation radius of the steel ball; Joint as well as 、 The expression of is: (9) Considering the i-th and j-th ellipses, we get: (10) Subtracting (9) and (10) and simplifying them yields: (11) Convert Equation (11) into the following form: (12) in, and All are intermediate variables; Combined with the fitted ball trajectory ellipse equation, multiple ellipse equations are substituted and the vertical coordinate of the detector center is fitted using the linear regression method. and the distance from the source to the detector ; because , for all the parameters of the fitted ellipse equations Take the average value to get the horizontal coordinate of the center of the detector .

5. The method according to claim 4, characterized in that According to the target elliptical trajectory, the distance from the source point to the rotation center The calibration process includes: For each point in the target elliptical trajectory, find 、 , the expression is: (13) Take any two points i and j, according to the cosine theorem, the expression of their distance is (14) in, is the angle difference between the two steel balls in the horizontal direction, is the distance between the two steel balls, both of which are known. Combined with the parameters of the corresponding fitted ellipse equation, the distance from the source point to the rotation center is obtained .

6. The method according to claim 5, characterized in that According to the target elliptical trajectory, the deflection angle of the detector in the horizontal direction The calibration process includes: For a fitted ellipse within the target elliptical trajectory, if S is the ray source, the positions of the two steel balls A and B differ by 180°, O is its symmetry center, A1 and B1 are ideal projections, A2 and B2 are actual projections, and the deflection angle of the detector in the horizontal direction is ,remember for , then Established; Furthermore, the following formula can be obtained: (15) Also know: (16) Solve equations (15) and (16) together to obtain the detector's horizontal deflection angle: , the expression is: in, Represents the inverse sine function.

7. The method according to claim 6, characterized in that Pitch angle of the detector The calibration process and the detector's horizontal deflection angle The calibration process is similar.

8. A CT system geometric parameter calibration device, characterized in that: include: A construction module for constructing a calibration phantom for a CT system; A fitting module is used to place the calibration phantom on the imaging system platform for rotational projection, and perform image fitting on all the steel ball trajectories obtained by the projection to obtain a series of elliptical trajectories; The removal module is used to remove some elliptical trajectories that do not meet the requirements according to the solution requirements to obtain the target elliptical trajectory; a calibration module, configured to calibrate geometric parameters of the CT system according to the target elliptical trajectory; The geometric parameters of the CT system include: The distance from the source to the detector ; The distance from the source point to the center of rotation ; Horizontal coordinate of the detector center ; The vertical coordinate of the detector center ; Tilt angle of the detector in the plane ; Detector's horizontal deflection angle ; Pitch angle of the detector ; Angle sampling interval ; Among them, the source Distance to detector It is defined by the length of the central ray beam, and the corresponding detector center is defined as the intersection of the central ray on the detector. The three deflection angles of the detector are defined by the polar coordinate system; According to the target elliptical trajectory, the angle sampling interval The calibration process includes: Define the similarity between the i-th projection and the first projection in the target elliptical trajectory as follows: (1) in, is the i-th projection The pixel value of the position pixel; Assume that the number of samples in one circle in the design index is , set the search radius ,exist Find N so that formula (2) is established, so as to preliminarily obtain the calibration sampling number N, formula (2) is: (2) To further improve the accuracy, the following quadratic fitting method is used: Set the ratio threshold , if formula (3) holds, then the number of samples in one circle is , otherwise for three points 、 as well as Fit a quadratic parabola and use the lowest point of the resulting parabola as the number of samples for one circle. , formula (3) is: (3) According to the number of samples , calculate the angle sampling interval .

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