Pre-calibration method for a full-length multi-height multi-SID dual-plane radiographic system

Abstract

The present invention relates to the field of medical equipment and discloses a pre-calibration method for a full-length, multi-height, multi-SID dual-plane radiation system, comprising: providing a spiral steel ball phantom and a method for arranging the steel balls thereon, and calibrating any spiral steel ball phantom provided in conjunction with a self-calibration algorithm based on bundle adjustment, in conjunction with automatic exposure software. The arrangement method uses steel balls of two sizes, large and small, and regularly arranges the large and small steel balls so that a unique positioning of the local steel balls of any length captured is achieved; the automatic exposure software is used to perform and control the shooting and exposure process, collect calibration image data, and perform image analysis and calibration calculations using a self-calibration algorithm based on bundle adjustment. The calibration calculation includes two stages: parameter estimation and parameter optimization. The self-calibration algorithm is used to expand the adaptability to different phantoms, while considering a strategy for global optimization adjustment of dual-plane projection images under different SIDs, different heights, and different phantom placement angles.

Description

Technical Field

[0001] The present invention relates to the field of medical equipment, and in particular to a pre-calibration method for a full-length multi-height multi-SID dual-plane ray system. Background Art

[0002] Geometric calibration of an X-ray system involves geometric modeling of the spatial relationship between the radiation source and the plate. The radiation source is modeled as a point light source, and the plate is modeled as a spatial plane defined by four corner points. By imaging a calibration phantom, a geometric calibration algorithm analyzes the characteristic points on the calibration phantom in the X-ray image and calculates the spatial position of the radiation source and plate. Many X-ray image-based analysis and reconstruction algorithms require input of the spatial position of the radiation source and plate. The accuracy of these positions directly impacts the precision of subsequent algorithms, making geometric calibration a crucial prerequisite.

[0003] Based on the requirements of subsequent algorithm scenarios, geometric calibration can be categorized, from simple to complex, into single-height calibration and multi-height calibration. For example, 6-DOF bone and joint motion analysis technology based on 2D-3D registration of biplane dynamic images only requires calibration results at a single height, as imaging is performed at a single height. However, slit-scan full-length X-rays and X-ray-based tomo reconstruction require calibration results at multiple heights, as patients may be imaged at any number of heights. Based on the timing of calibration, geometric calibration can be further categorized as pre-calibration and on-site calibration. Pre-calibration is performed in advance by algorithm engineers, and the results are stored. When the calibration results are needed by subsequent algorithms, they are retrieved and used directly or after interpolation. On-site calibration, on-site calibration involves imaging the calibration phantom before each image acquisition and calculating the results on-site. Generally speaking, if the same accuracy is maintained, pre-calibration is more efficient than on-site calibration due to its simplified workflow. Its ability to calibrate once and reuse multiple times also makes it more versatile. For example, with the aforementioned 6-DOF analysis of joint motion using 2D-3D registration of dual-plane dynamic images, if on-site calibration is used, the operator must first place the calibration phantom and capture the images before the patient actually takes the dynamic images to be analyzed, and then calculate the results on-site. If the patient requires analysis of multiple areas at different heights, recalibration is required for each height change. This undoubtedly degrades the user experience and contradicts the commonly accepted convenience of "just-in-time" CT and MRI scans.

[0004] In the paper "Calibration of Stereo Radiography System for Radiostereometric Analysis Application," a water bottle with tantalum beads attached to its inner wall was used as a calibration object. Recalibration was performed based on the design parameters of EOS, a French company. However, the improvement in the accuracy of 3D reconstruction of spatial points was limited. Tomo reconstruction also requires accurate calibration information. To expand the calibration range, enlarging the calibration phantom is a straightforward and readily conceivable solution. However, existing techniques simply elongate the commonly used cubic calibration box into a rectangular one, which presents two problems: the material required and the number of holes required increase exponentially with height, and the cost of a cubic calibration box is already high; and the imaging range of the flat panel is limited, allowing only a portion of the box to be captured at a time. Determining which part of the box the captured image corresponds to is a critical issue that urgently needs to be addressed. All calibration algorithms, without exception, require accurate 3D-2D correspondence. Without being able to determine which beads project the 2D projections in the image, the calibration algorithm will fail.

[0005] In the prior art, the closest to the present invention is the spiral phantom proposed in "Geometrical calibration for 3D x-ray imaging", such as Figure 1 As shown, its height is about the height of a detector plate. By using a spiral line to arrange the steel balls, the problem of overlapping steel ball projections can be effectively solved. At the same time, the correspondence between the projection and the steel balls is also very easy to determine: the height of the steel ball and the vertical coordinate of its projection on the image are one-to-one corresponding. However, due to the periodicity of the spiral line, a simple image Figure 1 The same method uses steel balls of the same size, but it is still impossible to determine which section the steel ball in the captured local image of the phantom corresponds to. Summary of the Invention

[0006] The purpose of the present invention is to address the shortcomings of the above-mentioned prior art and to provide a pre-calibration method for a full-length, multi-height, multi-SID dual-plane radiographic system. This method can meet all algorithm requirements, such as single-height and multi-height calibration, adaptive motion registration, slit scanning, tomo reconstruction, and CBCT reconstruction. Compared with general calibration algorithms that require the three coordinates of the calibration object's feature points, this calibration scheme can achieve high-precision calibration without the need for three coordinates.

[0007] A pre-calibration method for a full-length, multi-height, multi-SID dual-plane ray system, comprising:

[0008] A spiral steel ball phantom and a method for arranging the steel balls thereon are provided, wherein the arrangement method uses large and small steel balls, and the large and small steel balls are arranged regularly so that the steel balls are uniquely positioned for any length of the captured local area. The spiral steel ball phantom is placed at the center of the imaging space of a dual-plane X-ray system, and the dual-plane X-ray system includes an anterior and lateral X-ray source and an anteroposterior and lateral flat panels.

[0009] Calibrate any spiral steel ball phantom using a bundle adjustment-based self-calibration algorithm and automatic exposure software, including:

[0010] S100: controlling the shooting exposure process through the automatic exposure software, collecting calibration image data, and arranging the collected calibration image data into a 4-dimensional multidimensional array;

[0011] S200: Input the image of the 4-dimensional multidimensional array, identify and classify each image using an image recognition algorithm and a K means algorithm, and output an image of a multidimensional array, wherein the content of the multidimensional array is a combination of the serial ID of the steel ball and the two-dimensional coordinates of its projection point in the image;

[0012] S300: Calibrate the multi-dimensional array image output in step S200 using the self-calibration algorithm, and output the spatial positions of several sets of X-ray sources and flat panels, all in the same global coordinate system.

[0013] Furthermore, in the method for setting up the spiral steel ball phantom and the arrangement of the steel balls thereon, a virtual projection computer simulation program is constructed to determine whether the arrangement scheme of the spiral steel ball phantom and the steel balls thereon is feasible by checking the virtual simulation image, specifically including:

[0014] After receiving the key manufacturing parameters of the spiral steel ball phantom, the virtual projection computer simulation program generates a virtual phantom model in three-dimensional space and uses a ray projection GPU parallel algorithm to render virtual images of the phantom at different SIDs and different heights;

[0015] The key manufacturing parameters of the spiral steel ball mold include the cylinder height of the mold, the radius of the cylinder bottom surface, the number of steel balls, the height interval of the steel balls, the angle between adjacent steel balls and the radius of the large and small steel balls.

[0016] Furthermore, the arbitrary spiral steel ball phantom provided with the self-calibration algorithm based on the bundle adjustment further includes:

[0017] The bundle adjustment strategy simultaneously considers the global optimization adjustment of the dual-plane projection images under different SIDs, different heights, and different phantom placement angles. Based on the mechanical and physical characteristics of the machine, constraints are freely added. At the same time, after calibration is completed by the self-calibration algorithm, the three-dimensional coordinates of the steel ball on the phantom are output to calibrate the phantom itself.

[0018] Furthermore, in step S100, collecting calibration image data, and arranging the collected calibration image data into a 4-dimensional multidimensional array further includes:

[0019] S101: The spiral steel ball phantom is placed at the center of the imaging space. First, the SID is set to SID_1 using the automatic exposure software, the gantry height is adjusted to 0, and static double-shots are taken at fixed intervals d. The final gantry height is recorded as h. The imaging SIDs are then adjusted to SID_2 and SID_3, and the aforementioned static double-shots are performed at intervals d at SID_2 and SID_3, respectively.

[0020] S102: Rotate the spiral steel ball phantom three times, each time by 90°, and execute step S101 again after each rotation to capture images at four angles of 0°, 90°, 180°, and 270° respectively;

[0021] S103: The collected image data is organized into a 4-dimensional multidimensional array, 4×3×(h / d+1)×2, where 4 corresponds to 4 rotation angles, 3 corresponds to 3 SIDs, (h / d+1) is the number of exposure height positions, and 2 corresponds to two images in the frontal and lateral positions.

[0022] Furthermore, step S200 specifically includes:

[0023] Receive the collected 4-dimensional multidimensional array of images as input, and use image recognition algorithm to identify the center and radius of the projected steel ball for each image;

[0024] The K-means algorithm is used to perform binary classification on the steel balls identified in each image, and the classification results are arranged into a sequence. This sequence is unique in the entire steel ball arrangement sequence. The shape of the output multidimensional array is the same as the input 4-dimensional multidimensional array. The content of the array becomes a combination of the sequence ID of the steel ball and the two-dimensional coordinates of its projection point in the image.

[0025] Furthermore, in step S300, the multidimensional array image output in step S200 is calibrated by the self-calibration algorithm. The calibration calculation includes a parameter estimation stage and a parameter optimization stage. The final output result is 3×(h / d+1)×2 sets of spatial positions of X-ray sources and flat panels, where:

[0026] The parameter estimation stage includes estimating the three coordinates of the steel ball on the spiral steel ball phantom, the actual posture transformation of the spiral steel ball phantom at different placement angles, and the transformation in the SID direction, and using the above estimates as the initial values of the subsequent parameter optimization stage;

[0027] The parameter optimization stage uses an optimizer to optimize the estimation results of the parameter estimation stage.

[0028] Furthermore, in the parameter estimation stage, the coordinates of the reconstructed steel ball in each SID global coordinate system are first calculated, specifically including:

[0029] S301: Assuming the lead screw is completely vertical, for each SID, use the system design parameters and the shooting height preset by the automatic exposure software to initialize an ideal multi-SID, multi-height calibration environment;

[0030] S302: For each SID, the coordinates of the double-plane steel ball photographed at each height in the global coordinate system of the SID multi-height calibration environment are calculated using the triangulation method. Since the steel ball in the middle section of the spiral steel ball phantom will be photographed at multiple adjacent heights, there are multiple coordinates of the steel ball in the calculation result. The average value of these coordinates is taken as the reconstructed coordinate of the steel ball. Since the spiral steel ball phantom is placed at a total of 4 rotation angles, a set of steel ball coordinates will be calculated for each angle and each SID, and the reconstructed steel ball coordinates in the global coordinate system of each SID are stored.

[0031] The parameter estimation stage further comprises:

[0032] S303: Calculating the transformation of the spiral steel ball phantom from each rotation angle of 90°, 180°, and 270° to 0° under the same SID, and then averaging the same rotation angles under different SIDs to obtain an estimate of the transformation of the spiral steel ball phantom;

[0033] S304: Set the coordinates of the steel ball with an initial angle of 0° under SID_1 as the local coordinates, align the coordinates of each other SID and rotation angle to the local coordinates, and take the average after alignment. The average result is considered to be an initial estimate of the local coordinate system;

[0034] S305: aligning the steel ball coordinates of different SIDs at the same rotation angle with the steel ball coordinates of SID_1 at the same rotation angle, taking the average and obtaining the relationship between the global coordinate system and the local coordinate system at each SID.

[0035] Furthermore, the parameter optimization stage specifically includes:

[0036] The optimized parameters include the aspect ratio and skew of the anteroposterior and lateral plates, the distance of the anteroposterior light source from the plate at different SIDs and heights, the coordinates of the light source's vertical projection on the plate, the transformation between the principal coordinate system and the local coordinate system of the dual-plane system at different SIDs and heights, the pose transformation of the spiral steel ball phantom, and the globally unique anteroposterior and lateral plate transformation.

[0037] The optimization objective function is to minimize the square of the Euclidean distance between the two-dimensional point reprojected by the algorithm and the two-dimensional point detected on the real image at all angles, all SIDs, all heights and each viewing angle of the front and side.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The method of the present invention is combined with automatic exposure software, and engineers do not need to perform any other operations except placing the calibration phantom, thereby improving image acquisition efficiency;

[0040] (2) The present invention can cope with all algorithm requirements such as single-height and multi-height calibration, adaptive motion registration, slit scanning, tomo reconstruction, CBCT reconstruction, etc. through the bundle adjustment strategy, and has wide applicability;

[0041] (3) By setting up a self-calibration algorithm, the present invention does not require the three coordinates of the characteristic points of the calibration object compared to the general calibration algorithm. On the contrary, the three coordinates of the steel ball on the calibration model are output, thereby completing high-precision calibration and expanding the adaptability to different models.

[0042] (4) The algorithm needs to input the three coordinates of the calibration object. Since it relies on the three coordinates of the object, if the thermal stability and rigidity of the object's material are not good, its true coordinates will deviate from the three coordinates measured at a certain moment, which will cause the accuracy of the calibration algorithm to decrease. Whether using more stable materials or regularly measuring the three coordinates, it will increase manufacturing or later costs. This algorithm will calculate the three coordinates itself, completely eliminating the input of the object's three coordinate state, greatly reducing R&D costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0044] In the attached figure:

[0045] Figure 1 A schematic diagram of a spiral phantom calibration structure in the prior art;

[0046] Figure 2 This is a flow chart of a pre-calibration method for a full-length, multi-height, multi-SID dual-plane ray system according to the present invention;

[0047] Figure 3 This is a schematic diagram of the spiral steel ball mold structure of the present invention. DETAILED DESCRIPTION

[0048] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0049] Those skilled in the art will appreciate that, unless otherwise stated, the singular forms "a," "an," "said," and "the" used herein may also include plural forms. It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the stated features, integers, steps, operations, elements, and / or components, but does not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0050] The specific implementation of the present invention is described below with reference to the accompanying drawings and embodiments.

[0051] Example 1

[0052] like Figure 2 As shown, this embodiment provides a pre-calibration method for a full-length, multi-height, multi-SID dual-plane ray system, and the technical solution includes:

[0053] First, a spiral steel ball phantom and a method for arranging the steel balls thereon are provided. The arrangement method uses large and small steel balls, which are arranged regularly to achieve unique positioning of the steel balls for any length of the captured image. The spiral steel ball phantom is placed at the center of the imaging space of a biplane X-ray system, which includes an X-ray source for both frontal and lateral positions and two flat panels for both frontal and lateral positions.

[0054] At the same time, the self-calibration algorithm based on bundle adjustment can be used to calibrate any spiral steel ball phantom set up in conjunction with the automatic exposure software, including:

[0055] S100: controlling the shooting exposure process through the automatic exposure software, collecting calibration image data, and arranging the collected calibration image data into a 4-dimensional multidimensional array;

[0056] S200: Input the image of the 4-dimensional multidimensional array, identify and classify each image using an image recognition algorithm and a K means algorithm, and output an image of a multidimensional array, wherein the content of the multidimensional array is a combination of the serial ID of the steel ball and the two-dimensional coordinates of its projection point in the image;

[0057] S300: Calibrate the multi-dimensional array image output in step S200 using the self-calibration algorithm, and output the spatial positions of several sets of X-ray sources and flat panels, all in the same global coordinate system.

[0058] In the method for setting up the spiral steel ball phantom and the arrangement of the steel balls thereon, a virtual projection computer simulation program is constructed to quickly determine whether the arrangement scheme of the spiral steel ball phantom and the steel balls thereon is feasible by checking the virtual simulation image, specifically including:

[0059] After receiving the key manufacturing parameters of the spiral steel ball phantom, the virtual projection computer simulation program generates a virtual phantom model in three-dimensional space and uses a ray projection GPU parallel algorithm to quickly render virtual images of the phantom at different SIDs and different heights;

[0060] The key manufacturing parameters of the spiral steel ball mold include the cylinder height of the mold, the radius of the cylinder bottom surface, the number of steel balls, the height interval of the steel balls, the angle between adjacent steel balls and the radius of the large and small steel balls.

[0061] By checking virtual simulation images, you can quickly determine whether a design solution is feasible or compare the pros and cons of different solutions.

[0062] Specifically, in this embodiment, a spiral steel ball mold is shown, such as Figure 3 As shown, the manufacturing parameters corresponding to the phantom are as follows:

[0063] Calibration column height: 1760;

[0064] Calibration column bottom radius: 150;

[0065] Total number of calibration column steel balls: 89;

[0066] Radius of small steel ball of calibration column: 2;

[0067] Radius of the large steel ball of the calibration column: 3.25;

[0068] Ball radius sequence (from low to high): 2 2 2 2 2 2 2 3.25 2 2 2 2 2 3.25 3.25 2 2 2 2 3.25 2 3.25 2 2 2 2 3.25 3.25 3.25 2 2 2 3.25 2 2 3.25 2 2 2 3.25 2 3.25 3.25 2 2 2 3.25 3.25 2 3.25 2 2 2 3.25 3.25 3.25 3.25 2 2 3.25 2 2 3.25 3.25 2 2 3.25 2 3.25 2 3.25 2 2 3.25 2 3.25 3.25 3.25 2 2 3.25 3.25 2 3.25 3.25 2 2 3.25 3.25;

[0075] The difference between the adjacent steel ball angles (°): 33.4884;

[0076] The distance between adjacent steel balls in the z direction: 20;

[0077] The minimum length of the steel ball sequence that can be uniquely positioned by the calibration column is 7; the theoretical coordinates of the steel balls (from low to high):

[0078]

[0079]

[0080]

[0081]

[0082] Furthermore, the arbitrary spiral steel ball phantom provided with the self-calibration algorithm based on the bundle adjustment further includes:

[0083] The bundle adjustment strategy (Bundle Adjustment) simultaneously considers the global optimization adjustment of the dual-plane projection images under different SIDs, different heights, and different phantom placement angles, fully considers the mechanical and physical characteristics of the machine, and freely adds constraints. At the same time, after the calibration is completed by the self-calibration algorithm, the three-dimensional coordinates of the steel ball on the phantom are output to calibrate the phantom itself.

[0084] In this embodiment, the algorithm does not require the precise 3D coordinates of the calibration phantom as input. Instead, after calibration, it outputs the 3D coordinates of the steel balls on the calibration phantom. This is why it is called a "self-calibration" algorithm, named because it also calibrates the phantom itself. Compared to many existing calibration algorithms that require precise 3D coordinates of the steel balls, self-calibration algorithms offer significant advantages. This is because as the calibration phantom grows, the spatial range of the steel balls distributed on it also increases. However, the measurable space of typical 3D coordinate measurement machines is limited, making 3D coordinate measurement of steel balls on large phantoms extremely difficult and costly. Using a self-calibration algorithm undoubtedly expands its adaptability to different phantoms.

[0085] The bundle adjustment strategy, in essence, simultaneously considers and globally optimizes dual-plane projection images at different SIDs, heights, and phantom placement angles. This strategy also fully considers the mechanical and physical characteristics of the machine, allowing for the flexible addition of constraints to achieve even more accurate calibration results.

[0086] In step S100, collecting calibration image data, and organizing the collected calibration image data into a 4-dimensional multidimensional array further includes:

[0087] S101: Place the spiral steel ball phantom at the center of the imaging space. First, set the SID to SID_1 using the automatic exposure software, adjust the gantry height to h, and perform static anteroposterior and lateral double-shots at fixed intervals d. Then, adjust the imaging SIDs to SID_2 and SID_3, and perform the aforementioned static double-shot image acquisition at intervals d at SID_2 and SID_3, respectively.

[0088] S102: Rotate the spiral steel ball phantom three times, each time by 90°, and execute step S101 again after each rotation to capture images at four angles of 0°, 90°, 180°, and 270° respectively;

[0089] S103: The collected image data is organized into a 4-dimensional multidimensional array, 4×3×(h / d+1)×2, where 4 corresponds to 4 rotation angles, 3 corresponds to 3 SIDs, (h / d+1) is the number of exposure height positions, and 2 corresponds to two images in the frontal and lateral positions.

[0090] Specifically, in this embodiment, the spiral steel ball phantom is placed at the center of the imaging space. The automatic exposure software first controls the SID to 1350mm. The frame is then raised to 1760mm, and static dual-lens images are captured every 40mm. The SIDs are then adjusted to 1500mm and 1800mm, with the aforementioned dual-lens image acquisition at 40mm intervals at each SID. The spiral column is then rotated 90°, and the aforementioned three SIDs are performed, with image exposures spaced 40mm apart at each SID. A total of three rotations are performed, capturing images at four angles: 0°, 90°, 180°, and 270°. The collected image data is organized into a 4-dimensional multidimensional array: 4x3x45x2, where 4 corresponds to the four angles, 3 corresponds to the three SIDs, 45 corresponds to (1760 / 40+1) exposure height positions, and 2 corresponds to two images in the two SIDs.

[0091] Step S200 specifically includes:

[0092] Receive the collected 4-dimensional multidimensional array of images as input, and use image recognition algorithm to identify the center and radius of the projected steel ball for each image;

[0093] The K-means algorithm is used to perform binary classification on the steel balls identified in each image, and the classification results are arranged into a sequence. This sequence is unique in the entire steel ball arrangement sequence. The shape of the output multidimensional array is the same as the input 4-dimensional multidimensional array. The content of the array becomes a combination of the sequence ID of the steel ball and the two-dimensional coordinates of its projection point in the image.

[0094] Furthermore, in step S300, the multidimensional array image output in step S200 is calibrated by the self-calibration algorithm. The calibration calculation includes a parameter estimation stage and a parameter optimization stage. The final output result is 3×(h / d+1)×2 sets of X-ray system light sources and the spatial positions of the flat panel. In this embodiment, the result is 3×45×2 sets of X-ray system light sources and the spatial positions of the flat panel, where 3 corresponds to 3 SIDs, 45 corresponds to 45 heights, and 2 corresponds to two sets of X-ray systems in the frontal and lateral positions.

[0095] The parameter estimation stage includes estimating the three coordinates of the steel ball on the spiral steel ball phantom, the actual posture transformation of the spiral steel ball phantom at different placement angles, and the transformation in the SID direction, and using the above estimates as the initial values of the subsequent parameter optimization stage;

[0096] The parameter optimization stage uses an optimizer to optimize the estimation results of the parameter estimation stage.

[0097] In the parameter estimation stage, the coordinates of the reconstructed steel ball in the global coordinate system of each SID are first calculated, specifically including:

[0098] S301: Assuming that the lead screw of the frame is completely vertical, for each SID, using the system design parameters and according to the shooting height preset by the automatic exposure software, initialize an ideal multi-SID, multi-height calibration environment;

[0099] S302: For each SID, the coordinates of the double-plane steel ball photographed at each height in the global coordinate system of the SID multi-height calibration environment are calculated using the triangulation method. Since the steel ball in the middle section of the spiral steel ball phantom will be photographed at multiple adjacent heights, there are multiple coordinates of the steel ball in the calculation result. The average value of these coordinates is taken as the reconstructed coordinate of the steel ball. Since the spiral steel ball phantom is placed at a total of 4 rotation angles, a set of steel ball coordinates will be calculated for each angle and each SID, and the reconstructed steel ball coordinates in the global coordinate system of each SID are stored.

[0100] Specifically, we organize the calculation results of S302 into the following two-dimensional table to facilitate subsequent algorithm description and writing:

[0101] Deg_1(0°) Deg_2(90°) Deg_3(180°) Deg_4(270°) SID_1(1350) SID_2(1500) SID_3(1800)

[0102] Each cell in the table stores the SID and the steel ball coordinates reconstructed in the SID global coordinate system under the rotation angle combination of the spiral steel ball phantom.

[0103] The parameter estimation stage further comprises:

[0104] S303: Calculating the transformation of each rotation angle of 90° (Deg_2), 180° (Deg_3), and 270° (Deg_4) of the spiral steel ball phantom to 0° under the same SID, and then averaging the same rotation angles of different SIDs to obtain an estimate of the transformation of the spiral steel ball phantom;

[0105] S304: Set the coordinates of the steel ball with an initial angle of 0° under SID_1 (SID_1×Deg_1) as the local coordinates, align the coordinates of each other SID and rotation angle to the local coordinates, and take the average after alignment. The average result is considered to be an initial estimate of the local coordinate system;

[0106] S305: aligning the steel ball coordinates of different SIDs at the same rotation angle with the steel ball coordinates of SID_1 at the same rotation angle, taking the average and obtaining the relationship between the global coordinate system and the local coordinate system at each SID.

[0107] Furthermore, the parameter optimization stage specifically includes:

[0108] The optimized parameters include the aspect ratio and skew of the anteroposterior and lateral plates, the distance of the anteroposterior light source from the plate at different SIDs and heights, the coordinates of the light source's vertical projection on the plate, the transformation between the principal coordinate system and the local coordinate system of the dual-plane system at different SIDs and heights, the pose transformation of the spiral steel ball phantom, and the globally unique anteroposterior and lateral plate transformation.

[0109] The optimization objective function is to minimize the square of the Euclidean distance between the two-dimensional points reprojected by the algorithm and the two-dimensional points detected on the real image at all angles, all SIDs, all heights and all viewing angles (front and side).

[0110] Obviously, those skilled in the art may make various changes and modifications to the invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the invention fall within the scope of the claims and their equivalents, the invention is intended to include such modifications and variations.

[0111] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A pre-calibration method for a full-length multi-height multi-SID dual-plane ray system, characterized in that: include: A spiral steel ball phantom and a method for arranging the steel balls thereon are provided, wherein the arrangement method uses large and small steel balls, and the large and small steel balls are arranged regularly so that the steel balls are uniquely positioned for any length of the captured local area. The spiral steel ball phantom is placed at the center of the imaging space of a dual-plane X-ray system, and the dual-plane X-ray system includes an anterior and lateral X-ray source and an anteroposterior and lateral flat panels. Calibrate any spiral steel ball phantom using a bundle adjustment-based self-calibration algorithm and automatic exposure software, including: S100: controlling the shooting exposure process through the automatic exposure software, collecting calibration image data, and arranging the collected calibration image data into a 4-dimensional multidimensional array; S200: Input the image of the 4-dimensional multidimensional array, identify and classify each image using an image recognition algorithm and a K means algorithm, and output an image of a multidimensional array, wherein the content of the multidimensional array is a combination of the serial ID of the steel ball and the two-dimensional coordinates of its projection point in the image; S300: Calibrate the multi-dimensional array image output in step S200 using the self-calibration algorithm, and output the spatial positions of several sets of X-ray sources and flat panels, all in the same global coordinate system.

2. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 1 is characterized in that: In the method for setting up the spiral steel ball phantom and the arrangement of the steel balls thereon, a virtual projection computer simulation program is constructed to determine whether the arrangement scheme of the spiral steel ball phantom and the steel balls thereon is feasible by checking the virtual simulation image, specifically including: After receiving the key manufacturing parameters of the spiral steel ball phantom, the virtual projection computer simulation program generates a virtual phantom model in three-dimensional space and uses a ray projection GPU parallel algorithm to render virtual images of the phantom at different SIDs and different heights; The key manufacturing parameters of the spiral steel ball mold include the cylinder height of the mold, the radius of the cylinder bottom surface, the number of steel balls, the height interval of the steel balls, the angle between adjacent steel balls and the radius of the large and small steel balls.

3. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 1 is characterized in that: The arbitrary spiral steel ball phantom configured with the bundle adjustment-based self-calibration algorithm further includes: The bundle adjustment strategy simultaneously considers the global optimization adjustment of the dual-plane projection images under different SIDs, different heights, and different phantom placement angles. Based on the mechanical and physical characteristics of the machine, constraints are freely added. At the same time, after calibration is completed by the self-calibration algorithm, the 3D coordinates of the steel ball on the phantom are output to calibrate the phantom itself.

4. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 1 is characterized in that: In step S100, collecting calibration image data, and organizing the collected calibration image data into a 4-dimensional multidimensional array further includes: S101: Place the spiral steel ball phantom at the center of the imaging space, first set the SID to SID_1 through the automatic exposure software, and adjust the rack height to , every fixed distance Take a static anteroposterior and lateral double shot once, record the final rack height as h, then adjust the SID of the photography to SID_2 and SID_3, and perform the above-mentioned fixed distance at SID_2 and SID_3 respectively Static dual-shot image acquisition; S102: Rotate the spiral steel ball phantom three times, each time by 90°, and execute step S101 again after each rotation to capture images at four angles of 0°, 90°, 180°, and 270° respectively; S103: Arrange the collected image data into a 4-dimensional multidimensional array. ,in, Corresponding to 4 rotation angles, Corresponding to 3 SIDs, is the number of exposure height positions, Two corresponding images in the frontal and lateral positions.

5. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 4 is characterized in that: Step S200 specifically includes: Receive the collected 4-dimensional multidimensional array of images as input, and use image recognition algorithm to identify the center and radius of the projected steel ball for each image; The K-means algorithm is used to perform binary classification on the steel balls identified in each image, and the classification results are arranged into a sequence. This sequence is unique in the entire steel ball arrangement sequence. The shape of the output multidimensional array is the same as the input 4-dimensional multidimensional array. The content of the array becomes a combination of the sequence ID of the steel ball and the two-dimensional coordinates of its projection point in the image.

6. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 5, characterized in that: In step S300, the self-calibration algorithm is used to calibrate the multidimensional array image output in step S200. The calibration calculation includes a parameter estimation stage and a parameter optimization stage. The final output result is The spatial position of the X-ray source and the flat panel, where The parameter estimation stage includes estimating the 3D coordinates of the steel ball on the spiral steel ball phantom, the actual position transformation of the spiral steel ball phantom at different placement angles, and the transformation in the SID direction, and using the above estimates as initial values for the subsequent parameter optimization stage; The parameter optimization stage uses an optimizer to optimize the estimation results of the parameter estimation stage.

7. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 6, characterized in that: In the parameter estimation stage, the coordinates of the reconstructed steel ball in the global coordinate system of each SID are first calculated, specifically including: S301: Assuming that the lead screw of the frame is completely vertical, for each SID, using the system design parameters and according to the shooting height preset by the automatic exposure software, initialize an ideal multi-SID, multi-height calibration environment; S302: For each SID, the coordinates of the double-plane steel ball photographed at each height in the global coordinate system of the SID multi-height calibration environment are calculated using the triangulation method. Since the steel ball in the middle section of the spiral steel ball phantom will be photographed at multiple adjacent heights, there are multiple coordinates of the steel ball in the calculation result. The average value of these coordinates is taken as the reconstructed coordinate of the steel ball. Since the spiral steel ball phantom is placed at a total of 4 rotation angles, a set of steel ball coordinates will be calculated for each angle and each SID, and the reconstructed steel ball coordinates in the global coordinate system of each SID are stored.

8. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 7, characterized in that: The parameter estimation stage further comprises: S303: Calculating the transformation of the spiral steel ball phantom from each rotation angle of 90°, 180°, and 270° to 0° under the same SID, and then averaging the same rotation angles under different SIDs to obtain an estimate of the transformation of the spiral steel ball phantom; S304: Set the coordinates of the steel ball with an initial angle of 0° under SID_1 as the local coordinates, align the coordinates of each other SID and rotation angle to the local coordinates, and take the average after alignment. The average result is considered to be an initial estimate of the local coordinate system; S305: aligning the coordinates of the steel balls of different SIDs at the same rotation angle with the coordinates of the steel ball of SID_1 at the same rotation angle, taking the average and obtaining the relationship between the global coordinate system and the local coordinate system at each SID.

9. The pre-calibration method of the full-length multi-height multi-SID dual-plane ray system according to claim 8, characterized in that: The parameter optimization stage specifically includes: The optimized parameters include the aspect ratio and skew of the anteroposterior and lateral plates, the distance of the anteroposterior light source from the plate at different SIDs and heights, the coordinates of the light source's vertical projection on the plate, the transformation between the principal coordinate system and the local coordinate system of the dual-plane system at different SIDs and heights, the pose transformation of the spiral steel ball phantom, and the globally unique anteroposterior and lateral plate transformation. The optimization objective function is to minimize the square of the Euclidean distance between the two-dimensional point reprojected by the algorithm and the two-dimensional point detected on the real image at all angles, all SIDs, all heights and each viewing angle of the front and side.

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