CN113643428A - Full-parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT - Google Patents
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Abstract
The invention discloses a full-parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT, which comprises the following steps: establishing a space coordinate system by taking the rotation center as a coordinate origin, and expressing the position of the flat panel detector and the position of the X-ray source by using geometric parameters; placing the geometric model body at a rotation center, carrying out three-dimensional scanning on the geometric model body to obtain a system two-dimensional projection image, and obtaining a dome position I1 in the system two-dimensional projection; acquiring a small sphere position I3 on a geometric die body in a space coordinate system, constructing a projection matrix by using current geometric parameters, acquiring a projection relation P according to the projection matrix, and calculating a small sphere position I2 in a virtual projection drawing by using P and I3; and constructing a difference function between the I2 and the corresponding I1, and solving a minimum value of the difference function through fitting, wherein parameters in a projection matrix corresponding to the minimum value are optimal geometric parameters, so that the calibration of the geometric parameters is completed. The invention is used for providing geometric correction for the medical angiography X-ray machine so as to obtain an accurate three-dimensional image.
Description
Technical Field
The invention relates to the technical field of medical equipment, in particular to a full-parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT.
Background
The Digital Subtraction Angiography (DSA) equipment performs subtraction processing on digital images before and after the contrast agent is injected, so that a two-dimensional image of a blood vessel is obtained, the image definition is high, the resolution is good, real and reliable image information is provided for observing angiopathy and positioning measurement, diagnosis and interventional therapy of angiostenosis, and necessary conditions are provided for various interventional therapies. However, the two-dimensional image has the problem of structural superposition, and if a three-dimensional blood vessel image can be provided, the problem can be thoroughly solved.
The method for acquiring the three-dimensional image needs to carry out cone-beam CT reconstruction, the reconstruction needs an X-ray source, a rotation center and a flat panel detector as input, for a digital subtraction angiography device, in order to meet the clinical shooting requirement, a mechanical structure design with 6 shafts and more degrees of freedom is usually adopted, namely, the flat panel detector can rotate on the plane of the flat panel detector, the distance between the flat panel detector and the rotation center and the distance between the flat panel detector and the X-ray source can be adjusted, the digital subtraction angiography device is different from a C-shaped arm on radiotherapy equipment and a C-shaped arm (only one rotation shaft) of dentistry, the C-shaped arm of DSA equipment has two orthogonal rotation shafts, of course, both the flat panel detector and the X-ray source have installation errors, and simultaneously, because the flat panel detector and the X-ray source are positioned at two ends of the C-shaped arm, the weight is larger, and the digital subtraction angiography device is influenced by gravity, the C-shaped arms at different positions have deformation with different sizes, all the factors influence the relative position relationship of the X-ray source, the rotation center and the flat panel detector, and any geometric factor is not considered in the calibration link and can introduce image distortion into an image to generate an artifact, so that the diagnosis is interfered. The conventional CBCT calibration method is usually established on the basis of a simple geometric relation, cannot comprehensively correct related geometric parameters and has poor effect.
Therefore, how to provide a full-parameter geometric calibration method suitable for multi-degree-of-freedom cone-beam CT that fully considers the influence of the above factors and can effectively improve the three-dimensional reconstruction accuracy is a problem that needs to be solved by those skilled in the art.
Disclosure of Invention
In view of this, the present invention provides a full-parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT, which provides geometric calibration for a medical angiography X-ray machine to obtain an accurate three-dimensional image.
In order to achieve the purpose, the invention adopts the following technical scheme:
a full-parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT is disclosed, wherein a geometric mould body is provided with a small ball, and the method comprises the following steps:
s1, establishing a space coordinate system by taking a rotation center as a coordinate origin, and expressing the position of a flat panel detector and the position of an X-ray source by using geometric parameters;
s2, placing a geometric die body at a rotation center, carrying out three-dimensional scanning on the geometric die body to obtain a system two-dimensional projection image, identifying the circle center corresponding to each small ball in the system two-dimensional projection image by adopting a trained neural network model, and taking the circle center position as a small ball position I1 in the system two-dimensional projection;
s3, obtaining the position I3(x, y, z) of each small ball on the geometric die body in the space coordinate system, constructing a projection matrix by using the geometric parameters S1, obtaining a projection relation P according to the projection matrix, and calculating the position I2(u, v) of the small ball in a virtual projection graph by using the projection relation P and the position I3(x, y, z) of the small ball;
s4, constructing a difference function between the I2(u, v) and the corresponding I1, solving a minimum value of the difference function through fitting, and when the difference function is minimum, taking parameters in a corresponding projection matrix as optimal geometric parameters to finish the calibration of the geometric parameters.
Preferably, the geometric parameters include:
the X-ray detector comprises a distance SDD from an X-ray source to a flat panel detector, a distance SID from the X-ray source to a rotation center, an offset delta 1 of the center position of the flat panel detector in the X direction, an offset delta 2 of the center position of the flat panel detector in the Y direction, an offset delta 3 of the position of the X-ray source in the X direction, an offset delta 4 of the position of the X-ray source in the Y direction, a rotation angle beta of a frame around a rotation shaft of a sickbed, a rotation angle gamma of the frame around the rotation shaft perpendicular to the spatial vertical direction of the sickbed and a rotation angle alpha of the flat panel detector in a flat panel detector plane.
Preferably, the geometric mould body is a cylinder, the small balls are embedded on the outer wall of the cylinder, and all the small balls are uniformly distributed in a spiral shape.
Preferably, the position I3(x, y, z) of the small sphere in S3 is the coordinates of the center point of the small sphere, and is obtained by a segmentation algorithm and circle center fitting.
Preferably, the same projection matrix as the cone beam CT back projection is constructed using all the geometrical parameters of the system, the projection matrix comprising a rotation matrix, a translation matrix and a scaling matrix; wherein:
the rotation matrix is:
the translation matrix:
the scaling matrix is:
preferably, the method for calculating the projection relation P in S3 includes:
P＝A4A6A5A1A2A3。
the technical scheme shows that compared with the prior art, the invention discloses a full-parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT, which mainly solves the problem of solving the geometric parameters of a multi-degree-of-freedom CBCT system, considers all degrees of freedom of the system, establishes a space coordinate system by taking a rotation center as a coordinate origin, completely expresses the position of a flat panel detector and any position information of an X-ray source by using the geometric parameters, constructs a projection matrix consistent with the reconstruction process for the geometric phantom in the space coordinate system by using the geometric parameters, acquires the association between a two-dimensional image and a three-dimensional image, calculates and acquires a virtual projection image of the geometric phantom by the relationship, compares the positions of small balls in the projection image acquired by the virtual projection image and the real projection of the system, optimizes and calibrates the geometric parameters according to the corresponding parameters when the difference is minimum, accurate geometric information is provided for CBCT when three-dimensional reconstruction is carried out, the accuracy of a reconstruction result is ensured, and artifacts caused by system geometric deviation are removed.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 is a schematic flow chart of a full-parameter geometric calibration method for multi-degree-of-freedom cone-beam CT according to the present invention;
FIG. 2 is a two-dimensional projection image of a system in a full-parameter geometric calibration method for multi-degree-of-freedom cone-beam CT according to the present invention;
fig. 3 is a schematic structural diagram of a geometric phantom in a full-parameter geometric calibration method for multi-degree-of-freedom cone beam CT according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The embodiment of the invention discloses a full-parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT, wherein a geometric mould body is provided with a small ball, and as shown in figure 1, the method comprises the following steps:
s1, establishing a space coordinate system by taking a rotation center as a coordinate origin, and expressing the position of a flat panel detector and the position of an X-ray source by using geometric parameters;
s2, placing the geometric mould body at a rotation center, carrying out three-dimensional scanning on the geometric mould body to obtain a system two-dimensional projection image, as shown in FIG. 2, identifying the circle center corresponding to each small ball in the system two-dimensional projection image by adopting a trained neural network model, and taking the circle center position as a small ball position I1 in the system two-dimensional projection;
s3, obtaining each small ball position I3(x, y, z) on the geometric die body in the space coordinate system, constructing a projection matrix by using current geometric parameters through an RTK technology, obtaining a projection relation P according to the projection matrix, and calculating a small ball position I2(u, v) in the virtual projection drawing by using the projection relation P and the small ball position I3(x, y, z);
s4, constructing a difference function between the I2(u, v) and the corresponding I1, solving a minimum value of the difference function through fitting, and when the difference function is minimum, taking parameters in a corresponding projection matrix as optimal geometric parameters to finish the calibration of the geometric parameters.
It should be noted that:
in this embodiment, in S2, the U-net model is used to train the projection data of the ball, the trained U-net model is used to identify the center of circle corresponding to the ball in the projection, and further obtain and correct the position of the center of circle corresponding to the ball in the projection, and the corrected position of the center of circle is used as the ball position I1.
In order to further implement the above technical solution, the geometric parameters include:
the X-ray detector comprises a distance SDD from an X-ray source to a flat panel detector, a distance SID from the X-ray source to a rotation center, an offset delta 1 of the center position of the flat panel detector in the X direction, an offset delta 2 of the center position of the flat panel detector in the Y direction, an offset delta 3 of the position of the X-ray source in the X direction, an offset delta 4 of the position of the X-ray source in the Y direction, a rotation angle beta of a frame around a rotation shaft of a sickbed, a rotation angle gamma of the frame around the rotation shaft perpendicular to the spatial vertical direction of the sickbed and a rotation angle alpha of the flat panel detector in a flat panel detector plane.
It should be noted that:
the present embodiment considers all the degrees of freedom of the system, involving nine parameters including three coordinates of the X-ray point source, and six coordinates of the planar detector. Meanwhile, the projection process of the CBCT system is accurately modeled, parameters and matrixes in the geometric calibration and three-dimensional reconstruction processes are completely consistent, and information of the nine parameters is accurately obtained through fitting.
In order to further implement the above technical solution, as shown in fig. 3, the geometric mold body is a cylinder, the beads are embedded on the outer wall of the cylinder, and all the beads are uniformly distributed in a spiral shape.
In order to further implement the above technical solution, the position I3(x, y, z) of the small sphere in S3 is the coordinate of the center point of the small sphere, and is obtained by a segmentation algorithm and circle center fitting.
In order to further implement the technical scheme, a projection matrix which is the same as the cone beam CT back projection is constructed by using all geometrical parameters of the system, wherein the projection matrix comprises a rotation matrix, a translation matrix and a scaling matrix; wherein:
the rotation matrix is:
translation matrix:
scaling the matrix:
in order to further implement the above technical solution, the method for calculating the projection relationship P in S3 includes:
P＝A4A6A5A1A2A3。
the embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. The device disclosed by the embodiment corresponds to the method disclosed by the embodiment, so that the description is simple, and the relevant points can be referred to the method part for description.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (5)
1. A full parameter geometric calibration method suitable for multi-degree-of-freedom cone beam CT is provided, wherein a small ball is arranged on a geometric mould body, and the method is characterized by comprising the following steps:
s1, establishing a space coordinate system by taking a rotation center as a coordinate origin, and expressing the position of a flat panel detector and the position of an X-ray source by using geometric parameters;
s2, placing a geometric die body at a rotation center, carrying out three-dimensional scanning on the geometric die body to obtain a system two-dimensional projection image, adopting a trained neural network model to identify the circle center corresponding to each small ball in the system two-dimensional projection image, and taking the circle center position as a small ball position I1 in the system two-dimensional projection;
s3, obtaining each small ball position I3(x, y, z) on the geometric mould body in the space coordinate system, constructing a projection matrix by using the current geometric parameters through an RTK technology, obtaining a projection relation P according to the projection matrix, and calculating a small ball position I2(u, v) in a virtual projection graph by using the projection relation P and the small ball position I3(x, y, z);
s4, constructing a difference function between the I2(u, v) and the corresponding I1, solving a minimum value of the difference function through fitting, and when the difference function is minimum, taking parameters in a corresponding projection matrix as optimal geometric parameters to finish the calibration of the geometric parameters.
2. The method of claim 1, wherein the geometric parameters include:
the X-ray detector comprises a distance SDD from an X-ray source to a flat panel detector, a distance SID from the X-ray source to a rotation center, an offset delta 1 of the center position of the flat panel detector in the X direction, an offset delta 2 of the center position of the flat panel detector in the Y direction, an offset delta 3 of the position of the X-ray source in the X direction, an offset delta 4 of the position of the X-ray source in the Y direction, a rotation angle beta of a machine frame around a rotation shaft of a sickbed, a rotation angle gamma of the machine frame around the rotation shaft perpendicular to the spatial vertical direction of the sickbed and a rotation angle alpha of the flat panel detector in the plane of the flat panel detector.
3. The method as claimed in claim 1, wherein the geometric phantom is a cylinder, the beads are embedded in an outer wall of the cylinder, and all the beads are uniformly distributed in a spiral shape.
4. The method of claim 1, wherein the position of the hemisphere I3(x, y, z) in S3 is the coordinate of the center point of the hemisphere, and is obtained by a segmentation algorithm and circle center fitting.
5. The method of claim 2, wherein the projection matrix is constructed by using all the geometrical parameters of the system, and comprises a rotation matrix, a translation matrix and a scaling matrix.
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