Geometric parameter calibration device and method for CT (Computed Tomography) system

Abstract

The invention provides a CT system geometric parameter calibration method and device. The method comprises the steps that a calibration die body of a CT system is constructed; the calibration die body is placed on an imaging system object placing table for rotary projection, image fitting is carried out on all steel ball tracks obtained through projection, and a series of elliptical tracks are obtained; according to a solving demand, removing part of elliptical trajectories which do not meet requirements to obtain a target elliptical trajectory; and according to the target elliptical trajectory, calibrating geometric parameters of the CT system. Based on the scheme provided by the invention, the geometric parameters of the CT system can be extracted and calibrated more accurately. The high-precision calibration directly improves the reconstruction quality of the CT image, and reduces the problems of image distortion, artifacts and the like caused by geometric parameter errors, thereby improving the diagnosis 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 called Micro-CT or small animal CT) is mainly to solve the problem that the system geometry cannot meet the standard model (such as 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 collection 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. Then the collected projection data is 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 the projection data of the phantom at multiple angles, and uses methods such as adaptive binarization to process the projection image, find the center of mass of each ball projection image, and then fit the ellipse and calculate the geometric parameters of the system; the phantom calibration method estimates the geometric error parameters of the system through the characteristics of the phantom projection image by designing a special calibration phantom (such as a phantom containing multiple balls or a specific geometric shape), and adjusts the system hardware position accordingly. This method is intuitive and easy to operate, but it requires high requirements for the design and processing accuracy of the phantom.

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

[0006] 1. Limited calibration accuracy: The specially designed calibration phantom may cause errors in the calibration results due to its limited processing accuracy. At the same time, although the algorithm-based geometric calibration method can directly process the projection data or projection image, it usually requires complex calculations or has a large amount of calculations, and the calibration parameters determined are limited.

[0007] 2. High operational complexity: Some methods require manual adjustment of the position and angle of system hardware (such as radiation sources, detectors, rotating tables, etc.). These operations are not only time-consuming and laborious, but also prone to human errors. Some calibration methods require additional equipment or devices to assist in completion, such as positioning bodies and positioning blocks. These devices have high requirements for spatial distance and installation accuracy, which increases the complexity of the operation.

[0008] 3. Poor versatility and universality Some calibration methods may only be applicable to specific models of cone beam CT systems, lacking versatility and universality. When the system is upgraded or parts are replaced, a 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: In order to obtain high-precision calibration results, it may be necessary 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 some extent.

[0012] To this end, the purpose of the present 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-mentioned purpose, the first embodiment of the present application proposes 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] According to the target elliptical trajectory, the geometric parameters of the CT system are calibrated.

[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 parts, 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 arranged in parallel spirally 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 rotation center;

[0026] The horizontal coordinate u of the detector center 0 ;

[0027] The vertical coordinate v of the detector center 0 ;

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

[0029] The detector's horizontal deflection angle

[0030] The pitch angle σ of the detector;

[0031] Angular 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 the position of the i-th projection (u,v);

[0038] Assume that the number of samples in one circle in the design index is N 0 , set the search radius ΔN, in [N 0 -ΔN,N 0 +ΔN] to find N so that formula (2) holds true, so as to preliminarily obtain the calibrated sampling number N, formula (2) is:

[0039] S (N) =min S (i) ,i∈[N 0 -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 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) ) to fit a quadratic parabola, 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 the plane according to the target elliptical trajectory includes:

[0045] Since the center of the fitting ellipse in the target ellipse trajectory (u k ,v k ) is 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 (u 0 ,v 0 ), and the theoretical z-axis is a vertical straight line. The angle between the two is the inclination angle η of the detector in its plane, so the following linear equation can be established:

[0046]

[0047] Among them, C 0 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] By solving equations (4) and (5) simultaneously, we can obtain the inclination angle η of the detector in the plane.

[0051] Optionally, according to the target elliptical trajectory, the detector center coordinates (u 0 ,v 0 ) and the distance D from the source to the detector for calibration, including:

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

[0053]

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

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

[0056]

[0057] Then 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, 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] Lian Li And the expressions of a and b are:

[0064]

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

[0066]

[0067] Subtracting equation (9) and (10) and simplifying them, we get:

[0068]

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

[0070] Y ij =v 0 +D 2 X ij (12)

[0071] Among them, Y ij 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 v of the detector center is fitted using the linear regression method. 0 and the distance D from the source to the detector;

[0073] because For all the parameters of the fitted ellipse equations Take the average value and get the horizontal coordinate u of the center of the detector 0 .

[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, ρ and ζ are obtained, and the expressions are:

[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 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 deflection angle of the detector in the horizontal direction 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 projections under ideal conditions, A2 and B2 are actual projections, and the deflection angle of the detector in the horizontal direction is Note ∠SA 2 B2 is ∠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 deflection angle of the detector in the horizontal direction: The expression is:

[0087]

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

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

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

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

[0092] A fitting module is used to place the calibration phantom on the imaging system storage table 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 solution provided by the embodiments of the present application brings at least the following beneficial effects:

[0096] 1. High-precision calibration: This application significantly improves the calibration accuracy of the geometric parameters of the CT system 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 the fields of medical diagnosis, material testing, etc.

[0097] 2. Ingenious phantom design: Traditional phantom design may have certain limitations and it is difficult to fully cover or accurately simulate the geometric characteristics of the CT system in actual use. The phantom design of this application is more ingenious and can more accurately simulate the geometric relationship during the CT scanning process, thereby providing a more reliable basis for subsequent calibration work.

[0098] 3. Rigorous calculation method: 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 CT systems of specific specifications or types, but this application, through clever design and optimization, 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 work, allowing different users to choose the appropriate CT system for calibration according to their own 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 the 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 of performing rotational projection on a calibration phantom according to an embodiment of the present application;

[0107] Figure 4 is a schematic diagram of an elliptical trajectory after 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] Fig. 9 It is a block diagram of a CT system geometric parameter calibration device according to an embodiment of the present application. DETAILED DESCRIPTION

[0113] The embodiments of the present application are described in detail below, and 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 device 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 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 comprises the following steps:

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

[0117] Traditional phantom design may have certain limitations and it is difficult to fully cover or accurately simulate the geometric characteristics of the CT system in actual use. Therefore, this application proposes a calibration phantom that can more accurately simulate the geometric relationship during the CT scanning process, thereby providing a more reliable basis 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) As shown in Figure 2(c).

[0119] Among them, the calibration model is cylindrical and has embedded steel balls; there are a circle of steel balls on the upper and lower parts of the calibration model, 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 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 has a steel ball embedded in it, and the diameter of the steel ball is 0.5mm;

[0122] 2. There are two circles of steel balls on the upper and lower parts of the phantom, and the distance between the two circles of steel balls is 10 cm (as shown by the dotted line in the front view). There are 16 steel balls on each circle, and they are distributed at equal angles: 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), and 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 circular 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 model, the diameter of the steel balls, and the number of steel balls distributed in the upper and lower circles and the two columns of spiral lines can be flexibly adjusted according to actual needs, and this application does not limit this.

[0126] Step 102, placing the calibration phantom on the imaging system stage for rotational projection, performing image fitting on all the steel ball trajectories obtained by the projection, and obtaining 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 stage 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, remove some elliptical trajectories that do not meet the requirements 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 orbit 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 u of the detector center 0 ;

[0138] The vertical coordinate v of the detector center 0 ;

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

[0140] The detector's horizontal deflection angle

[0141] The pitch angle σ of the detector;

[0142] Angular 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 above-mentioned geometric parameter calibration process as follows:

[0145] 1. Angle sampling interval Δβ

[0146] In the embodiment of the present application, firstly, the similarity S between the i-th projection and the first projection in the target elliptical trajectory is 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 the 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 N 0 , set the search radius ΔN, in [N 0 -ΔN,N 0 +ΔN] to find N so that formula (2) holds true, so as to preliminarily obtain the calibrated sampling number N, formula (2) is:

[0153] S (N) =min S (i) ,i∈[N 0 -ΔN,N 0 +ΔN] (2)

[0154] In a possible embodiment, the search radius ΔN may be taken as 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 equation (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) ) to fit a quadratic parabola, 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 can be calculated as 2π / N * .

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

[0160] like Figure 7 As shown in the figure, the center of the fitted ellipse in the target ellipse trajectory (u k ,v k ) is 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 (u 0 ,v 0 ), and 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] Among them, C 0 Is a constant.

[0163] When solving the problem, the least square method can be used to reduce the error. Suppose the coordinates of the i-th point on the k-th ellipse are 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 inclination angle η of the detector in the plane.

[0166] 3. Detector center coordinates (u 0 ,v 0 ) and the distance D from the source to the detector

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

[0168]

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

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

[0171]

[0172] Then 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, ζ are all intermediate variables, z is the z coordinate value of the spatial motion coordinates of the steel ball, and r is the rotation radius of the steel ball.

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

[0179]

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

[0181]

[0182] Subtracting equation (9) and (10) and simplifying them, we get:

[0183]

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

[0185] Y ij =v 0 +D 2 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 v of the detector center is fitted using the linear regression method. 0 and the distance D from the source to the detector;

[0188] Furthermore, due to For all the parameters of the fitted ellipse equations Taking the average value, we can get the horizontal coordinate u of the center of the detector 0 .

[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, 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 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 used to obtain the average value.

[0196] 5. Detector deflection angle in the horizontal direction and the 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. Take the horizontal direction as an example. Figure 8 As shown, the present application can therefore 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 Note ∠SA 2 B 2 is ∠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 deflection angle of the detector in the horizontal direction: 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 on the phantom that are equidistant from the center and 180° apart. Then the O' position can be calculated from the detector center, and AB is a known design parameter, 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 calibration of the geometric parameters of the CT system can be achieved.

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

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

[0212] A construction module 100 is used to construct 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] A 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 used to calibrate the geometric parameters of the CT system according to the target elliptical trajectory.

[0216] Regarding the device in the above embodiment, the specific manner in which each module performs operations has been described in detail in the embodiment of the method, and will not be elaborated here.

[0217] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps recorded in this application can be executed in parallel, sequentially or in different orders, as long as the expected results of the technical solution of this application can be achieved, and this document is not limited here.

[0218] The above specific implementations do not constitute a limitation on the protection scope of this application. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principles of this application should be included in the protection scope of this 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; According to the target elliptical trajectory, the geometric parameters of the CT system are calibrated.

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 parts, 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 arranged in parallel spirally 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 The geometric parameters of the CT system include: The distance D from the source to the detector; The distance R from the source point to the rotation center; The horizontal coordinate u0 of the detector center; The vertical coordinate v0 of the detector center; The detector’s tilt angle η in the plane; The detector's horizontal deflection angle The pitch angle σ of the detector; Angle sampling interval Δβ; 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 with the detector, and the three deflection angles of the detector are defined by the polar coordinate system.

4. The method according to claim 3, characterized in that The process of calibrating the angle sampling interval Δβ according to the target elliptical trajectory includes: Define the similarity S between the i-th projection and the first projection in the target elliptical trajectory (i) as follows: Among them, P (i) (u,v) is the pixel value of the pixel at the position of the i-th projection (u,v); Assuming that the number of samples in one circle in the design index is N0, set the search radius ΔN, and find N in [N0-ΔN, N0+ΔN] so that formula (2) is valid, so as to preliminarily obtain the calibrated sampling number N, formula (2) is: S (N) =min S (i) ,i∈[N0-ΔN,N0+ΔN] (2) To further improve the accuracy, the following quadratic fitting method is used: Set the ratio threshold t. If equation (3) holds, then 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) ) to fit a quadratic parabola, and take the lowest point of the obtained parabola as the sampling number N of a circle * , formula (3) is: |S (N) -S (N-1) |<t,|S (N) -S (N+1) |<t (3) According to the sampling number N * , the angle sampling interval Δβ is calculated to be 2π / N * .

5. The method according to claim 4, characterized in that The process of calibrating the inclination angle η of the detector in the plane according to the target elliptical trajectory includes: Since the center of the fitting ellipse in the target ellipse trajectory (u k ,v k ) is 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 (u0, v0), while the theoretical z-axis is a vertical straight line. The angle between the two is the inclination angle η of the detector in its plane, so the following straight line equation can be established: Among them, C0 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: By solving equations (4) and (5) simultaneously, we can obtain the inclination angle η of the detector in the plane.

6. The method according to claim 5, characterized in that 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: For the fitted ellipse within the target elliptical trajectory, without considering When and σ, the equation is: Among them, a, b, These are the parameters of the fitted ellipse; And, the spatial motion coordinates of the steel ball are η is just a rotation quantity, which does not affect the shape of the ellipse and need not be considered, so we have: Then z 2 U 2 +(R 2 -r 2 )V 2 -2DRzV+z 2 D 2 = 0 Right now: Let ρ = R / r, ζ = z / R, then the above formula can be written in the form of (10), that is: in: Among them, U, V, ρ, 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; Lian Li And the expressions of a and b are: Considering the i-th and j-th ellipses, we get: Subtracting equation (9) and (10) and simplifying them, we get: Convert equation (11) into the following form: Y ij =v0+D 2 X ij (12) Among them, Y ij and X ij All are intermediate variables; 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. because For all the parameters of the fitted ellipse equations Take the average value and get the horizontal coordinate u0 of the center of the detector.

7. The method according to claim 6, characterized in that The process of calibrating the distance R from the source point to the rotation center according to the target elliptical trajectory includes: For each point in the target elliptical trajectory, ρ and ζ are obtained, and the expressions are: Take any two points i and j, according to the cosine theorem, the expression of their distance is Among them, (α i -α j ) is the angle difference between the two 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.

8. The method according to claim 7, 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 projections under ideal conditions, 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; Furthermore, the following formula can be obtained: Also know: Solve equations (15) and (16) together to obtain the deflection angle of the detector in the horizontal direction: The expression is: Here, arcsin represents the inverse sine function.

9. The method according to claim 8, characterized in that The calibration process of the detector's pitch angle σ is related to the detector's horizontal deflection angle The calibration process is similar.

10. A CT system geometric parameter calibration device, characterized in that: include: A construction module for constructing a calibration phantom of a CT system; A fitting module is used to place the calibration phantom on the imaging system storage table for rotational projection, and perform image fitting on all the steel ball trajectories obtained by 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; The calibration module is used to calibrate the geometric parameters of the CT system according to the target elliptical trajectory.

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