A method and device for calibrating geometric parameter errors of cone-beam CT system

Abstract

The present invention discloses a method and device for calibrating the geometric parameter error of a cone-beam CT system. The method includes the following steps: marking an original pattern on a calibration wafer, fixing the calibration wafer on the upper surface of a rotating table; fixing the ray source, rotating table and flat-panel detector of the cone-beam CT system at a preset position; collecting a first projection pattern of the original pattern projected on the flat-panel detector; adjusting the distance between the rotating table and the ray source along the ray direction of the ray source, collecting a second projection pattern of the original pattern projected on the flat-panel detector; analyzing the positions of the first projection pattern and the second projection pattern, and calibrating the geometric parameter error of the cone-beam CT system. Based on the projection patterns of two calibration wafers, the geometric parameter error of the cone-beam CT system is given according to the coordinates, line segment length, direction and proportion of the corresponding end points of the pattern, so as to obtain real and accurate geometric parameters and improve the restoration degree of image reconstruction.

Description

Technical Field

[0001] The invention belongs to the technical field of cone-beam CT systems, and in particular relates to a method and a device for calibrating geometric parameter errors of cone-beam CT systems. Background Art

[0002] Generally speaking, a cone-beam CT system consists of four parts: a radiation source, a rotating table, an object to be scanned, and a flat-panel detector (such as Figure 1 The working principle is that the ray source projects the object to be scanned onto the flat-panel detector to obtain a two-dimensional projection pattern. By rotating the object to be scanned, the projection pattern at each rotation angle can be obtained. Finally, the three-dimensional image of the object to be scanned is reconstructed based on the projection pattern using the geometric relationship. In this process, the following requirements are met: (1) The line connecting the center of the ray source and the center of the flat-panel detector is coplanar with the rotation axis of the rotating stage; (2) The line connecting the center of the ray source and the center of the flat-panel detector is perpendicular to the plane of the flat-panel detector; (3) The rotation axis of the rotating stage is parallel to the plane of the flat-panel detector.

[0003] However, due to factors such as installation operation errors and component processing accuracy, the above three requirements are often not met, resulting in geometric parameter errors in the cone-beam CT system. The existence of geometric parameter errors in the cone-beam CT system will lead to errors in the shape and size of the two-dimensional projection pattern, thereby affecting the accuracy of three-dimensional image reconstruction, causing distortion or artifacts of the three-dimensional image, etc.

[0004] The geometric parameter errors of the cone-beam CT system cannot be completely eliminated, but some studies have shown that the error values ​​of each geometric parameter can be calculated through calibration methods, and then the errors of the cone-beam CT system, projection pattern or reconstruction process can be corrected to improve the fidelity of image reconstruction.

[0005] Patent CN109875591A discloses a geometric calibration method for a cone-beam CT system. First, the geometric transformation matrix of each projection image relative to the first projection image in a single CT scan is obtained through rigid registration. This matrix is ​​applied to each projection image except the first one, which is equivalent to taking the first projection image as the benchmark to eliminate the influence of geometric errors between projection images. However, this scheme does not actually solve the error value of the cone-beam CT system, and the transformed projection image is not an ideal projection pattern. Patent CN105997126A discloses a geometric parameter correction model and method for a cone-beam CT system. This method collects projection data of a square model at a projection angle, and then solves the geometric parameter error based on the ratio of the line segment length, the side length and the intersection coordinates on the projection. This model regards the distance error between the rotating table and the ray source and the distance error between the flat-panel detector and the ray source as the same error, which reduces the accuracy of solving other related errors. In addition, trigonometric equations are used in the solution process, and the solution process is relatively complicated and the results are relatively unstable. Patent CN102743184A discloses a method for calibrating the geometric parameters of an X-ray cone-beam computer tomography system. The method uses camera calibration technology to reproduce the geometric position relationship of the cone-beam CT system and uses the relationship between the spatial transformation matrix, spatial coordinate points and projection pattern coordinate points to calculate the geometric error. The calculation process of this method is relatively complicated and requires multiple projection patterns to solve, which takes a long time.

[0006] Therefore, how to overcome the existing problems in the geometric calibration of the cone-beam CT system to achieve a geometric error calibration calculation that is simple to operate and has strong universality, and can ensure that the results are accurate and stable is an urgent problem to be solved by technical personnel in this field. Summary of the invention

[0007] In view of the defects existing in the above-mentioned prior art, the present invention provides a method and device for calibrating the geometric parameter error of a cone-beam CT system. Based on the projection pattern generated by the calibration chip under different geometric parameters, the geometric parameter error of the cone-beam CT system is calibrated according to the length and direction ratio of the line segment on the projection pattern and the coordinates of the corresponding point on the pattern. Compared with the existing conventional cone-beam CT system calibration method, the present invention is simple to operate, has strong universality, and the calculation results of the geometric parameter error are accurate and stable.

[0008] In a first aspect, the present invention provides a method for calibrating geometric parameter errors of a cone-beam CT system, comprising the following steps:

[0009] marking an original pattern on a calibration wafer, and fixing the calibration wafer on an upper surface of a rotating stage;

[0010] Fix the radiation source, rotating table and flat panel detector of the cone beam CT system at the preset position;

[0011] Collect a first projection pattern of the original pattern projected onto the flat panel detector;

[0012] Along the direction of the ray of the ray source, the distance between the rotating stage and the ray source is adjusted to collect a second projection pattern of the original pattern projected on the flat panel detector;

[0013] The positions of the first projection pattern and the second projection pattern are analyzed to calibrate the geometric parameter errors of the cone-beam CT system.

[0014] Furthermore, the plane of the calibration chip where the original pattern is located is perpendicular to the upper surface of the turntable and passes through the rotation axis of the turntable. The center of the original pattern is collinear with the center of the radiation source and the center of the flat-panel detector, and the collinear direction is the normal to the plane of the calibration chip where the original pattern is located and the plane where the flat-panel detector is located.

[0015] Further, marking the original pattern on the calibration wafer specifically includes the following steps:

[0016] Setting a first coordinate system on the plane of the calibration wafer, wherein the lateral direction of the plane is the x-axis direction in the first coordinate system, and the longitudinal direction of the plane is the y-axis direction in the first coordinate system;

[0017] A cross-shaped original pattern is marked on the plane of the calibration wafer to set the first coordinate system. The distances between the center of the original pattern and the left, right, upper and lower end points are equal. The intersection of the cross is the center point of the original pattern. The horizontal and vertical lines of the cross correspond to the x-axis and y-axis directions in the first coordinate system, respectively, and the coordinate values ​​of each end point of the cross are given.

[0018] Further, the positions of the first projection pattern and the second projection pattern are analyzed to calibrate the geometric parameter error of the cone-beam CT system, which specifically includes the following steps:

[0019] A second coordinate system is set on the flat panel detector, and coordinate values ​​of each endpoint in the first projection pattern and the second projection pattern are given, wherein the horizontal direction of the plane where the flat panel detector is located is the x-axis direction in the second coordinate system, the vertical direction of the plane where the flat panel detector is located is the y-axis direction in the second coordinate system, and the center point of the flat panel detector is the origin of the second coordinate system;

[0020] By comparing and analyzing the coordinate values ​​of the corresponding endpoints in the first projection pattern and the original pattern, the translation error △x of the flat panel detector along the x-axis, the translation error △y of the flat panel detector along the y-axis and the rotation error η of the flat panel detector along the normal direction are obtained;

[0021] By comparing and analyzing the coordinate values ​​of the corresponding endpoints in the first projection pattern, the second projection pattern and the original pattern, the distance error △R between the rotating stage and the ray source, the rotation error Φ of the flat-panel detector along the y-axis, the rotation error θ of the flat-panel detector along the x-axis and the distance error △D between the flat-panel detector and the ray source are obtained.

[0022] Furthermore, the translation error △x of the flat panel detector along the x-axis is specifically expressed as:

[0023] Δx=x O

[0024] Among them, x O The x-axis coordinate of the center point of the original pattern projected in the second coordinate system,

[0025] The translation error △y of the flat panel detector along the y-axis is specifically expressed as:

[0026] Δy=y O

[0027] Among them, y O The y-axis coordinate of the center point of the original pattern projected in the second coordinate system,

[0028] The rotation error η of the flat-panel detector along the normal direction is specifically expressed as:

[0029]

[0030] Among them, x c ,y c They are respectively the x-axis coordinate and y-axis coordinate of the left end point of the original pattern projected in the second coordinate system.

[0031] Furthermore, by comparing and analyzing the coordinate values ​​of corresponding endpoints in the first projection pattern, the second projection pattern and the original pattern, the distance error △R between the rotating stage and the ray source, the rotation error Φ of the flat panel detector along the y-axis and the rotation error θ of the flat panel detector along the x-axis are obtained, which specifically includes the following steps:

[0032] By comparing and analyzing the coordinate values ​​of corresponding endpoints in the first projection pattern, the second projection pattern and the original pattern, the distance error △R between the rotating stage and the ray source is obtained;

[0033] Based on the obtained distance error △R between the rotating stage and the radiation source, and combined with the coordinate values ​​of the corresponding endpoints in the first projection pattern and the original pattern, the rotation error Φ of the flat-panel detector along the y-axis, the rotation error θ of the flat-panel detector along the x-axis, and the distance error △D between the flat-panel detector and the radiation source are obtained.

[0034] Furthermore, by comparing and analyzing the coordinate values ​​of corresponding endpoints in the first projection pattern, the second projection pattern and the original pattern, the distance error △R between the rotating stage and the ray source is obtained, which specifically includes the following steps:

[0035] Determine the coordinate points C, C', E, and E' where the left end point and the right end point of the original pattern are projected onto the first projection pattern and the second projection pattern respectively;

[0036] Respectively obtain the distances OC, OC', OE, OE' between the origin O of the second coordinate system and the coordinate points C, C', E, E', the distance R between the rotating stage and the ray source, and the distance △r between the rotating stage and the ray source adjusted when the second projection pattern is collected compared with the first projection pattern;

[0037] The distance error △R between the rotating table and the radiation source is obtained by comprehensive analysis and calculation of OC, OC', OE, OE', R and △r.

[0038] Furthermore, OC, OC', OE, OE', R and △r are comprehensively analyzed and calculated to obtain the distance error △R between the rotating stage and the radiation source. If the rotation error Φ of the flat-panel detector along the y-axis is not 0, it can be specifically expressed as:

[0039]

[0040] If the rotation error Φ of the flat panel detector along the y-axis is 0, then △R is specifically expressed as:

[0041]

[0042] Where R is the distance between the rotating table and the radiation source.

[0043] Furthermore, the rotation error Φ of the flat panel detector along the y-axis is specifically expressed as:

[0044]

[0045] Where l is the distance between the center of the original pattern and the left, right, top, and bottom endpoints;

[0046] Obtain the coordinate points F and F' projected from the left and right endpoints of the original pattern to the first projection pattern, respectively, obtain the distances OF and OF' between the origin O of the second coordinate system and F and F', and the rotation error θ of the flat panel detector along the x-axis, which can be specifically expressed as:

[0047]

[0048] The distance error △D between the flat panel detector and the radiation source is specifically expressed as:

[0049]

[0050] Where D is the distance between the flat panel detector and the radiation source.

[0051] In a second aspect, the present invention further provides a device for calibrating geometric parameter errors of a cone-beam CT system, using the method for calibrating geometric parameter errors of a cone-beam CT system as described above, comprising:

[0052] A collection unit, used to collect a first projection pattern of the original pattern projected on the flat panel detector, and to collect a second projection pattern of the original pattern projected on the flat panel detector after adjusting the distance between the rotating stage and the ray source along the ray direction of the ray source;

[0053] an analysis and processing unit, configured to analyze positions of the first projection pattern and the second projection pattern;

[0054] The result output unit is used to calibrate the geometric parameter errors of the cone-beam CT system.

[0055] The present invention provides a method and device for calibrating geometric parameter errors of a cone-beam CT system, which at least have the following beneficial effects:

[0056] (1) Compared with the existing conventional cone-beam CT system geometric parameter error calibration method, the present invention is simple to operate, has strong universality, and the calculation results of the geometric parameter error are accurate and stable.

[0057] (2) The calibration material (i.e., calibration wafer) required by the present invention is easy to obtain, the original pattern of the mark is also easy to process, and the calibration method is highly practical.

[0058] (3) The present invention only needs to horizontally move the rotating table in the direction of the ray to project two projection patterns, and analyze and calculate the two projection patterns. The operation is simple and the error caused by manual operation is minimized.

[0059] (4) The calibration process of the geometric parameter error of the cone-beam CT system of the present invention does not involve solving complex equations and functions, and the calculation method is simple and reasonable. Therefore, the error calculation result is accurate and stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a structural diagram of a cone beam CT system in the prior art;

[0061] FIG2(a) is a schematic diagram of the effect of Φ not being 0 on △R in a certain embodiment provided by the present invention, and FIG2(b) is a schematic diagram of the effect of Φ not being 0 on △D in a certain embodiment provided by the present invention;

[0062] Figure 3 A structural diagram of a cone-beam CT system geometric parameter calibration system provided by the present invention;

[0063] FIG. 4( a ) is a schematic diagram of an original pattern of a calibration wafer according to an embodiment of the present invention, and FIG. 4( b ) is a schematic diagram of a projection pattern according to an embodiment of the present invention;

[0064] Figure 5 A schematic flow chart of a method for calibrating geometric parameter errors of a cone-beam CT system provided by the present invention;

[0065] FIG6(a) is a structural diagram of a geometric parameter calibration system of a cone-beam CT system when acquiring a first projection pattern provided by the present invention, and FIG6(b) is a structural diagram of a geometric parameter calibration system of a cone-beam CT system when acquiring a second projection pattern provided by the present invention;

[0066] FIG. 7( a ) is a schematic diagram of a first projection pattern provided by the present invention, and FIG. 7( b ) is a schematic diagram of a second projection pattern provided by the present invention;

[0067] FIG8(a) is a top view of the geometric relationship between the ray source and the plane where the line segment aa' is located in the first projection pattern provided by the present invention, and FIG8(b) is a top view of the geometric relationship between the ray source and the plane where the line segment aa' is located in the second projection pattern provided by the present invention;

[0068] Fig. 9 A top view of the geometric relationship between the ray source and the plane where the line segment aa' is located when Φ=0 provided by the present invention;

[0069] 10( a ) is a schematic diagram of the plane geometric relationship between the ray source and the line segment aa' in the first projection pattern provided by the present invention, and FIG. 10( b ) is a schematic diagram of the plane geometric relationship between the ray source and the line segment bb' in the first projection pattern provided by the present invention. DETAILED DESCRIPTION

[0070] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0071] The terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The singular forms "a", "said" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings, and "multiple" generally includes at least two.

[0072] It should also be noted that the term "includes", "comprising" or any other variation thereof is intended to cover non-exclusive inclusion, so that a commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprising a ..." do not exclude the existence of other identical elements in the commodity or device including the elements.

[0073] like Figure 1 As shown in Figure 1, the geometric parameter errors of the cone-beam CT system include the translation and rotation errors of the ray source, the rotating stage, and the flat-panel detector along the three-dimensional coordinate system. Except for the distance error between the rotating stage and the ray source, the above errors can be decomposed into the geometric errors of the flat-panel detector in space, that is, there are 7 types of errors, namely:

[0074] 1. The distance error △R between the rotating table and the radiation source;

[0075] 2. The distance error △D between the flat panel detector and the radiation source;

[0076] 3. Translation error △x of the flat panel detector in the x-axis direction;

[0077] 4. Translation error △y of the flat panel detector in the y-axis direction;

[0078] 5. Rotation error Φ of the flat panel detector along the y-axis;

[0079] 6. Rotation error θ of the flat panel detector along the x-axis;

[0080] 7. Rotation error η of the flat-panel detector along the normal direction.

[0081] The geometric parameter error calibration of existing cone-beam CT systems usually combines the distance error △R between the rotating stage and the radiation source and the distance error △D between the flat-panel detector and the radiation source into one error. In fact, this approach is not scientific and will reduce the accuracy of solving the errors related to it.

[0082] As shown in Figure 2(a) and Figure 2(b), when considering the effect of parameters on the projection pattern, the difference between the distance error △R between the rotating stage and the radiation source and the distance error △D between the flat panel detector and the radiation source is shown under the influence of θ and Φ. For example, when Φ is not 0, when △R appears, the pattern is not just a geometric scaling of the coordinate value. At this time, △R not only affects the size of the line segment length, but also affects the ratio of the line segment length. As for the appearance of △D, the effect of △D is the geometric scaling of the coordinate value.

[0083] Therefore, the present invention provides a cone beam CT system geometric parameter calibration system, such as Figure 3 As shown, it consists of a radiation source, a rotating stage, a calibration wafer and a flat panel detector. A square calibration wafer is selected, and a "cross" shaped original pattern is engraved on it, as shown in Figure 4 (a). The distances from the four endpoints a, a', b, b' of the original pattern to the center o of the original pattern are equal, that is, oa = oa' = ob = ob' = l, and aa' is parallel to the bottom surface of the calibration wafer, and bb' is perpendicular to the bottom surface of the calibration wafer. The calibration wafer is fixed on the rotating stage.

[0084] Setting a first coordinate system on the plane of the calibration wafer, wherein the lateral direction of the plane is the x-axis direction in the first coordinate system, and the longitudinal direction of the plane is the y-axis direction in the first coordinate system;

[0085] A cross-shaped original pattern is marked on the plane of the calibration wafer in the set first coordinate system. The intersection of the cross is the center point of the original pattern. The horizontal and vertical lines of the cross correspond to the x-axis and y-axis directions in the first coordinate system respectively, and the coordinate values ​​of each end point of the cross are given.

[0086] A second coordinate system is set on the flat panel detector, wherein the horizontal direction of the plane where the flat panel detector is located is the x-axis direction in the second coordinate system, the vertical direction of the plane where the flat panel detector is located is the y-axis direction in the second coordinate system, and the center point of the flat panel detector is the origin of the second coordinate system.

[0087] As shown in FIG. 4( b ), the four endpoints a, a′, b, b′ of the original pattern and the center o of the original pattern correspond to the four endpoints A, A′, B, B′ and O of the projected pattern on the flat panel detector.

[0088] The analysis shows that the influence of the geometric parameter error of the cone-beam CT system on the projection pattern is as follows:

[0089] The rotation error Φ of the flat panel detector along the y-axis changes the ratio of line segments OA and OA' and the length of line segment AA' on the projection pattern;

[0090] The rotation error θ of the flat panel detector along the x-axis changes the ratio of line segment OB to line segment OB' and the length of BB' on the projection pattern.

[0091] As shown in Figure 2(a), the distance error △R between the rotating stage and the radiation source changes the length of line segments AA' and BB', and changes the ratio of line segments OA and OA' when Φ is not 0. Similarly, △R changes the ratio of line segments OB and OB' when θ is not 0. As shown in Figure 2(b), the distance error △D between the flat-panel detector and the radiation source changes the length of line segments AA' and BB', but does not change the ratio of line segments OA and OA' and line segments OB and OB'.

[0092] The rotation error η of the flat panel detector along the normal direction changes the directions of line segments BB' and AA' on the projection pattern;

[0093] The translation △x of the flat panel detector along the x direction and the translation △y along the y direction change the coordinates of point O on the projection pattern.

[0094] Combining the above analysis, we can get:

[0095] The effects of η, △x, △y, and △D on the projection pattern are independent and do not interfere with each other;

[0096] The effects of Φ and θ on the projection pattern do not interfere with each other, but the effects of Φ and θ on the projection pattern are respectively related to △R.

[0097] Φ, θ, △R, and η, △x, △y, △D are also unrelated to each other.

[0098] According to the influence of the above geometric parameters on the projection pattern. Figure 5 As shown, the present invention provides a method for calibrating geometric parameter errors of a cone-beam CT system, comprising the following steps:

[0099] marking an original pattern on a calibration wafer, and fixing the calibration wafer on an upper surface of a rotating stage;

[0100] Fix the radiation source, rotating table and flat panel detector of the cone beam CT system at the preset position;

[0101] Collect a first projection pattern of the original pattern projected onto the flat panel detector;

[0102] Along the direction of the ray of the ray source, the distance between the rotating stage and the ray source is adjusted to collect a second projection pattern of the original pattern projected on the flat panel detector;

[0103] The positions of the first projection pattern and the second projection pattern are analyzed to calibrate the geometric parameter errors of the cone-beam CT system.

[0104] As shown in FIG6(a), during installation, the plane with the original pattern on the calibration wafer is fixed vertically at the center of the rotating table, that is, the center of the "cross" pattern is located on the rotating axis of the rotating table, and the center of the "cross" pattern is collinear with the center of the radiation source and the center of the flat-panel detector. The installation is carried out in accordance with the requirements of "(1) the line connecting the center of the radiation source and the center of the flat-panel detector is coplanar with the rotating axis of the rotating table, (2) the line connecting the center of the radiation source and the center of the flat-panel detector is perpendicular to the plane of the flat-panel detector, and (3) the rotating axis of the rotating table is parallel to the plane of the flat-panel detector", and the distance R between the radiation source and the rotating table and the distance D between the radiation source and the flat-panel detector are measured.

[0105] Turn on the radiation source to obtain a first projection pattern, as shown in FIG7(a), where o, a, a', b, b' on the calibration wafer correspond to O, C, C', F, F' on the first projection pattern, respectively.

[0106] The radiation source is turned off, and the radiation source and the flat-panel detector remain at the same position as when the first projection pattern was obtained. The calibration wafer and the rotating stage are moved by △r along the direction of the central ray of the radiation source, as shown in FIG6(b).

[0107] The ray source is turned on again to obtain a second projection pattern, as shown in FIG7( b ), where o, a, a′, b, b′ on the calibration wafer correspond to O, E, E′, G, G′ on the second projection pattern, respectively.

[0108] The points in the first projection pattern and the second projection pattern are placed in the second coordinate system, and the errors of various geometric parameters are calculated according to the coordinates of the points on the two projection patterns and the length, proportion and direction of the line segments, as shown in Figure 7(a) and Figure 7(b).

[0109] First, △x, △y and η are calculated, in which only the first projection pattern is needed.

[0110] According to the above analysis, the coordinates of point O on the first projection pattern are only related to the translation error △x of the flat panel detector along the x-axis and the translation error △y along the y-axis. Assuming that the flat panel detector translates upward and to the right as the positive direction, for the first projection pattern, the coordinates of point O are (x O ,y O ),but:

[0111] Δx=x O

[0112] Δy=y O

[0113] The direction of the line segment CO on the first projection pattern is only related to the rotation error η of the flat panel detector along the normal direction. Assuming that the clockwise rotation from the front to the back is the positive direction, the coordinates of point C are (x C ,y C ),but:

[0114]

[0115] Then, ΔR is calculated, in which process the first projection pattern and the second projection pattern need to be combined.

[0116] Take the plane formed by the ray source and the line aa' as a cross section to form a top view, as shown in Figure 8(a) and Figure 8(b). Assume that the rotation error Φ of the flat panel detector along the y-axis is clockwise from top to bottom, and assume that when Φ = 0, the projection length of the line segment oa on the flat panel detector is L, then

[0117] If the rotation error Φ of the flat panel detector along the y-axis is not 0:

[0118]

[0119] For the cross section corresponding to the first projection pattern, according to the triangle similarity theorem, we have:

[0120]

[0121]

[0122] Arranging the above two formulas, we can get:

[0123]

[0124]

[0125] so:

[0126]

[0127]

[0128] Dividing the above two formulas, we have:

[0129]

[0130] Similarly, for the cross section corresponding to the second projection pattern, we have:

[0131]

[0132] Dividing the above two formulas, we have:

[0133]

[0134] After finishing, we can get:

[0135]

[0136] At this point, we need to consider how to transform the formula when the denominator is 0, which can be divided into the following two cases:

[0137] The first case: If (OC-OC′)*(OE+OE′)≠0, (OC+OC′)*(OE-OE′)≠0 and (OC-OC′)*(OE+OE′)-(OC+OC′)*(OE-OE′)=0, the denominator of the above formula is 0 but the numerator is not 0, △R→∞, this situation cannot happen.

[0138] The second case: If (OC-OC′)*(OE+OE′)=0 and (OC+OC′)*(OE-OE′)=0, it means (OC-OC′)=0 and (OE-OE′)=0, then Φ=0. Fig. 9 As shown, according to the relationship between the sides of similar triangles:

[0139]

[0140] After rearrangement, if (OC-OC′)*(OE+OE′)=0 and (OC+OC′)*(OE-OE′)=0, that is, Φ=0, △R is calculated using the following formula:

[0141]

[0142] Next, Φ and θ are calculated, in which only the first projection pattern is required.

[0143] Cross sections are made along the plane formed by the ray source and the straight line aa' and along the plane formed by the ray source and the straight line bb' to form top views, as shown in FIG. 10( a ) and FIG. 10( b ).

[0144] For the plane formed by the ray source and the straight line aa', according to the formula You can get:

[0145]

[0146] Similarly, for the plane formed by the ray source and the straight line bb', we have:

[0147]

[0148] therefore:

[0149]

[0150] Finally, ΔD is calculated, in which only the first projection pattern is required.

[0151] The formula Substitute into the formula Among them, there are:

[0152]

[0153] therefore:

[0154]

[0155] In order to verify the feasibility and accuracy of the geometric error parameter solution algorithm proposed in this patent, 8 groups of simulation experiments with different parameter errors were carried out:

[0156] Assume that the lengths of the four sides of the "cross" pattern on the calibration chip are all l = 50 mm, the distance between the flat-panel detector and the radiation source is D = 900 mm, the distance between the rotating stage and the radiation source is R = 300 mm, the pixel resolution on the flat-panel detector is 0.1 mm / pixel, and the size of the flat-panel detector is 5000 pixels * 5000 pixels.

[0157] Assume that the geometric parameter errors of the simulated cone-beam CT system are as shown in Table 1:

[0158] Table 1 System geometric parameter error setting values ​​for simulation experiments

[0159] parameter △D(mm) △R(mm) △x(mm) △y(mm) Φ(°) θ(°) η(°) Experiment 1 10 5 2 3 8 5 3 Experiment 2 10 0 0 0 0 0 0 Experiment 3 0 5 0 0 0 0 0 Experiment 4 0 0 2 3 0 0 0 Experiment 5 0 0 0 0 8 0 0 Experiment 6 0 0 0 0 0 5 0 Experiment 7 0 0 0 0 0 0 3 Experiment 8 0 0 0 0 0 0 0

[0160] The method to obtain the simulated real projection pattern is:

[0161] For Experiment 1, a 5000 pixels*5000 pixels black background image is created as the simulated real projection pattern 1. The coordinates of a point on the pattern (x error ,y error ), then ignoring △r, for each (x error ,y error ), find the coordinates (x correct ,y correct ), let the simulated real projection pattern coordinates (x error ,y error ) is (x correct ,y correct ) value. At this time, if x correct and correct is a decimal, we can use the linear interpolation method to find (x correct ,y correct ) value.

[0162] By traversing each coordinate on the simulated real projection pattern and performing the above operation, the simulated real projection pattern 1 corresponding to the simulated ideal projection pattern due to the error can be obtained.

[0163] Create a 5000 pixels * 5000 pixels black background image as the simulated real projection pattern 2. The coordinates of a point on the pattern (x error ,y error ), assuming that the projection pattern is obtained by moving the rotating stage by △r=5mm, then let △R=△R+△r, and obtain the simulated real projection pattern 2 according to the method of obtaining the simulated real projection pattern 1.

[0164] For the other seven groups of experiments, the initial values ​​of the geometric error parameters were set according to Table 1, and two simulated projection patterns were obtained for each group of experiments.

[0165] The geometric parameters calculated by the final eight groups of simulation experiments are shown in Table 2. The comparison between the experimental results and the error setting values ​​of the simulated projection pattern shows that the results of the geometric parameter errors of the patented algorithm are accurate.

[0166] Table 2 Calculation results of geometric parameter errors based on simulated projection patterns

[0167] parameter △D(mm) △R(mm) △x(mm) △y(mm) Φ(°) θ(°) η(°) Experiment 1 11.0959 5.3684 2.0000 3.0000 8.0670 4.9875 2.9924 Experiment 2 10.4843 -0.0611 0.0000 0.0000 0.0000 0.0000 0.0000 Experiment 3 0.4899 5.0630 0.000m 0.0000 0.0000 0.0000 0.0000 Experiment 4 0.2389 -0.0203 2.0000 3.0000 0.0000 0.0000 0.0000 Experiment 5 -0.0637 -0.1219 0.0000 0.0000 7.9985 0.0000 0.0000 Experiment 6 0.2389 -0.0203 0.0000 0.0000 0.0000 5.0010 0.0000 Experiment 7 0.1754 -0.0413 0.0000 0.0000 0.0063 -0.0063 2.9998 Experiment 8 0.2389 -0.0203 0.0000 0.0000 0.0000 0.0000 0.0000

[0168] Although preferred embodiments of the present invention have been described, additional changes and modifications may be made to these embodiments by those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention. Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for calibrating geometric parameter errors of a cone-beam CT system, characterized in that: The steps include: marking an original pattern on a calibration wafer, and fixing the calibration wafer on an upper surface of a rotating stage; Fix the radiation source, rotating stage and flat panel detector of the cone beam CT system at a preset position, wherein the plane of the calibration wafer where the original pattern is located is perpendicular to the upper surface of the rotating stage, and passes through the rotation axis of the rotating stage, the center of the original pattern is collinear with the center of the radiation source and the center of the flat panel detector, and the collinear direction is the normal direction of the plane of the calibration wafer where the original pattern is located and the plane where the flat panel detector is located; Collect a first projection pattern of the original pattern projected onto the flat panel detector; Along the direction of the ray of the ray source, the distance between the rotating stage and the ray source is adjusted to collect a second projection pattern of the original pattern projected on the flat panel detector; A second coordinate system is set on the flat panel detector, and coordinate values ​​of each endpoint in the first projection pattern and the second projection pattern are given, wherein the horizontal direction of the plane where the flat panel detector is located is the x-axis direction in the second coordinate system, the vertical direction of the plane where the flat panel detector is located is the y-axis direction in the second coordinate system, and the center point of the flat panel detector is the origin of the second coordinate system; By comparing and analyzing the coordinate values ​​of the corresponding endpoints in the first projection pattern and the original pattern, the translation error △x of the flat panel detector along the x-axis, the translation error △y of the flat panel detector along the y-axis and the rotation error η of the flat panel detector along the normal direction are obtained; Determine the coordinate points C, C', E, and E' where the left end point and the right end point of the original pattern are projected onto the first projection pattern and the second projection pattern respectively; Respectively obtain the distances OC, OC', OE, OE' between the origin O of the second coordinate system and the coordinate points C, C', E, E', the distance R between the rotating stage and the ray source, and the distance △r between the rotating stage and the ray source adjusted when the second projection pattern is collected compared with the first projection pattern; Comprehensively analyze and calculate OC, OC', OE, OE', R and △r to obtain the distance error △R between the rotating table and the radiation source; Based on the obtained distance error △R between the rotating stage and the ray source, and in combination with the coordinate values ​​of the corresponding endpoints in the first projection pattern and the original pattern, the rotation error Φ of the flat panel detector along the y-axis, the rotation error θ of the flat panel detector along the x-axis, and the distance error △D between the flat panel detector and the ray source are obtained, which can be specifically expressed as: Where D is the distance between the flat panel detector and the radiation source, and l is the distance between the center of the original pattern and the left, right, top, and bottom end points.

2. The method for calibrating geometric parameter errors of a cone-beam CT system according to claim 1, characterized in that: Marking the original pattern on the calibration wafer includes the following steps: Setting a first coordinate system on the plane of the calibration wafer, wherein the lateral direction of the plane is the x-axis direction in the first coordinate system, and the longitudinal direction of the plane is the y-axis direction in the first coordinate system; A cross-shaped original pattern is marked on the plane of the calibration wafer to set the first coordinate system. The distances between the center of the original pattern and the left, right, upper and lower end points are equal. The intersection of the cross is the center point of the original pattern. The horizontal and vertical lines of the cross correspond to the x-axis and y-axis directions in the first coordinate system, respectively, and the coordinate values ​​of each end point of the cross are given.

3. The method for calibrating geometric parameter errors of a cone-beam CT system according to claim 2, characterized in that: The translation error △x of the flat panel detector along the x-axis is specifically expressed as: Δx=x O Among them, x O The x-axis coordinate of the center point of the original pattern projected in the second coordinate system, The translation error △y of the flat panel detector along the y-axis is specifically expressed as: Δy=y O Among them, y O The y-axis coordinate of the center point of the original pattern projected in the second coordinate system, The rotation error η of the flat-panel detector along the normal direction is specifically expressed as: Among them, x c ,y c They are respectively the x-axis coordinate and y-axis coordinate of the left end point of the original pattern projected in the second coordinate system.

4. The method for calibrating geometric parameter errors of a cone-beam CT system according to claim 2, characterized in that: Comprehensively analyze and calculate OC, OC', OE, OE', R and △r to obtain the distance error △R between the rotating stage and the radiation source. If the rotation error Φ of the flat-panel detector along the y-axis is not 0, it can be specifically expressed as: If the rotation error Φ of the flat panel detector along the y-axis is 0, it can be specifically expressed as: Where R is the distance between the rotating table and the radiation source.

5. The method for calibrating geometric parameter errors of a cone-beam CT system according to claim 2, characterized in that: The rotation error Φ of the flat panel detector along the y-axis is specifically expressed as: Where l is the distance between the center of the original pattern and the left, right, top, and bottom endpoints; Obtain the coordinate points F and F' projected from the left and right endpoints of the original pattern to the first projection pattern, respectively, obtain the distances OF and OF' between the origin O of the second coordinate system and F and F', and the rotation error θ of the flat panel detector along the x-axis, which can be specifically expressed as:

6. A device for calibrating geometric parameter errors of a cone-beam CT system, characterized in that: The method for calibrating geometric parameter errors of a cone-beam CT system according to any one of claims 1 to 5 comprises: A collection unit, used to collect a first projection pattern of the original pattern projected on the flat panel detector, and to collect a second projection pattern of the original pattern projected on the flat panel detector after adjusting the distance between the rotating stage and the ray source along the ray direction of the ray source; an analysis and processing unit, configured to analyze positions of the first projection pattern and the second projection pattern; The result output unit is used to calibrate the geometric parameter errors of the cone-beam CT system.

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