Improved non-uniform soft tissue composite mesh model based on finite element method

Abstract

The invention relates to the field of contactless medical technology, and specifically to an improved non-uniform soft tissue composite grid model based on the finite element method, comprising: generating a rough tetrahedral grid according to a CT image; calculating the flexibility of the tetrahedron by the soft tissue CT value in the CT image to obtain its edge length refinement threshold; refining the tetrahedron by using the tetrahedron dichotomy method with the edge length refinement threshold as a judgment condition; constructing a non-uniform soft tissue composite grid model; performing deformation calculation on the constructed non-uniform soft tissue composite grid model based on the finite element method and the spring mass method, calculating the deformation of control points in the soft tissue grid model by using the improved finite element method, and performing deformation calculation on non-control points by using the spring mass method based on the deformation of the control points; the invention combines SMS and FEM algorithms to further improve the fineness of the model surface, and provides an improvement on the FEM algorithm to improve the real-time performance while ensuring the accuracy of the deformation, so that the real-time performance and the accuracy reach a new balance.

Description

Technical Field

[0001] The invention relates to the technical field of non-contact medical treatment, in particular to a non-uniform soft tissue composite grid model improved based on a finite element method. Background Art

[0002] In order to minimize cross-infection between doctors and patients, most hospitals have suspended or changed the outpatient services of some departments, which has further aggravated people's medical difficulties. Therefore, strengthening the protection of medical staff has become a top priority in the prevention and control of infectious diseases. Contactless medical care has stood out with its unique advantages and provided protection for doctors and patients.

[0003] At present, realizing human-computer interaction in the field of contactless medical technology has always been a key research difficulty, especially in the simulation of human soft tissue deformation. It is crucial to simulate the soft tissue deformation effect with high real-time performance and extremely realistic for medical staff. Summary of the invention

[0004] The purpose of the present invention is to provide an improved non-uniform soft tissue composite grid model based on the finite element method to solve the problems raised in the above background technology.

[0005] The technical solution of the present invention is: a non-uniform soft tissue composite grid model improved based on the finite element method, comprising the following steps:

[0006] S1, generate a rough tetrahedral mesh according to the CT image;

[0007] S2, calculating the flexibility of the tetrahedron by the CT value of the soft tissue in the CT image to obtain the edge length thinning threshold;

[0008] S3, using the tetrahedron edge length thinning threshold as a judgment condition to thin it using the tetrahedron bisection method;

[0009] S4, constructing a non-uniform soft tissue composite mesh model;

[0010] S5. Based on the finite element method and spring-mass method, the deformation calculation of the constructed non-uniform soft tissue composite mesh model is performed.

[0011] Preferably, in S2, the specific step of calculating the flexibility of the tetrahedron by the soft tissue CT value in the CT image to obtain the edge length thinning threshold thereof comprises:

[0012] S21. Obtain the CT values ​​of the vertices of the tetrahedron. The CT value reflects the density of a local tissue or organ of the human body. The inverse of the CT average value of the four vertices of the tetrahedron is used as the flexibility of the tetrahedron, which is obtained by the following formula:

[0013]

[0014] Where S n is the flexibility of the nth tetrahedron, is the CT value of the ith vertex in the nth tetrahedron, C min It is the minimum CT value of soft tissue in CT images;

[0015] S22. Calculate the edge length thinning threshold according to the flexibility of the tetrahedron, which is obtained by the following formula:

[0016]

[0017] Among them l max is the edge length thinning threshold of the nth tetrahedron, L max k is the soft tissue model accuracy value set by the user. l For the customized maximum refinement ratio, S n is the flexibility of the nth tetrahedron, C max , C min They are the maximum and minimum CT values ​​of soft tissue in CT images, respectively.

[0018] Preferably, in S3, the specific step of using the tetrahedron bisection method to thin the tetrahedron using the tetrahedron edge length thinning threshold as a judgment condition includes: comparing the tetrahedron edge length thinning threshold with the longest edge of the tetrahedron, and if the longest edge is greater than the edge length thinning threshold, thinning the tetrahedron using the tetrahedron bisection method;

[0019] Preferably, the tetrahedron bisection method is a method similar to tetrahedron subdivision proposed with reference to the Loop subdivision method, which divides the tetrahedron into two by inserting a vertex on the longest side of the tetrahedron and connecting the other two vertices not on the side with the newly added vertex.

[0020] In S4, the construction of the non-uniform soft tissue composite grid model includes the following specific steps:

[0021] S41, iteratively performing a determination refinement operation on the tetrahedrons in the coarse tetrahedral mesh model until the tetrahedrons no longer meet the refinement condition;

[0022] S42, after the above process, all tetrahedrons in the model are refined to complete the construction of the internal model of the non-uniform soft tissue, and all the current vertices are control vertices;

[0023] S43, further refine the external mesh of the model based on the Loop method until the maximum side length of the triangle block in the mesh is less than k l L max So far, the surface refinement of the non-uniform soft tissue has been completed, the newly added vertices are non-control vertices, and the construction of the non-uniform soft tissue composite grid model has been completed.

[0024] Preferably, in S5, the deformation of the control points in the soft tissue mesh model is calculated using an improved finite element method, and the deformation of the non-control points is calculated using a spring-mass method based on the deformation of the control points.

[0025] Preferably, the calculation of the control point deformation in the soft tissue mesh model based on the improved finite element method includes the following steps: according to the Saint-Venant principle, the soft tissue model is divided into a strong deformation area and a micro-deformation area; for the strong deformation area, the finite element method is used for calculation; for the micro-deformation area, the physical variable is obtained by the direct attenuation method;

[0026] Preferably, the division of the strong deformation zone and the micro deformation zone is obtained by calculating the strong deformation radius, and the area within the strong deformation radius with the force application point as the center is defined as the strong deformation zone, and the other areas are defined as the micro deformation zone. The strong deformation radius is obtained by the following formula:

[0027]

[0028] Where R q is the strong deformation radius, F in is the external force exerted by the instrument on the model, C min is the minimum CT value of soft tissue in CT images, k R is the deformation factor obtained through experiments;

[0029] Preferably, the direct attenuation method for physical variables includes calculating the displacement information of the current vertex by searching for the displacement information of the outer adjacent vertices of the current vertex, until the displacement of the outer adjacent vertex is less than the set displacement threshold and is deemed to no longer affect the farther vertices and the calculation is stopped; wherein the outer adjacent vertex refers to the vertex that is closest to the current vertex and closer to the force application point of the instrument relative to the current vertex, and the displacement information of the current vertex is obtained by the following formula:

[0030]

[0031] Where T p is the current vertex displacement, T adj is the displacement of the outer adjacent vertex, d p is the distance between the current vertex and the force application point of the instrument, d adj C is the distance between the outer adjacent vertex and the force application point of the instrument, p is the CT value of the current vertex.

[0032] The present invention provides an improved non-uniform soft tissue composite grid model based on the finite element method, which has the following improvements and advantages compared with the prior art:

[0033] Based on the idea that the softer the tissue, the greater the deformation, the present invention generates a non-uniform grid model using tetrahedron dichotomy according to the flexibility of soft tissue. The model can depict the deformation of soft parts more finely with the same number of tetrahedrons, thereby improving the real-time performance to a certain extent while taking into account the simulation of the real system. A composite model combining SMS and FEM is proposed. SMS is used based on the vertex displacement calculated by FEM to further improve the fineness of the model surface without affecting the real-time performance of the system, thereby bringing a better visual experience to users. The FEM algorithm is optimized based on the Saint-Venant principle, and the soft tissue model is divided into a strong deformation zone and a micro deformation zone. The finite element method and the direct attenuation method are used for deformation calculation respectively, thereby improving the real-time performance of the model. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be further explained below in conjunction with the accompanying drawings and embodiments:

[0035] Figure 1 is a flow chart of an improved non-uniform soft tissue composite grid model based on the finite element method provided by an embodiment of the present invention;

[0036] Figure 2 It is a schematic diagram of the loop refinement method and the tetrahedron bisection method provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The present invention is described in detail below, and the technical solutions in the embodiments of the present invention are clearly and completely described. 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.

[0038] The present invention provides an improved non-uniform soft tissue composite grid model based on the finite element method. The technical solution of the present invention is:

[0039] like Figure 1 As shown, the non-uniform soft tissue composite mesh model improved based on the finite element method includes the following steps:

[0040] S1. Generate a rough tetrahedral mesh based on the CT image, and save the maximum and minimum CT values ​​C of the simulated organ in the CT image for the convenience of subsequent calculations. max and C min ;

[0041] S2. Calculate the flexibility of the tetrahedron by the CT value of the soft tissue in the CT image to obtain the edge length thinning threshold.

[0042] The vertex CT values ​​of the tetrahedron are obtained. According to the density of a local tissue or organ of the human body measured by the CT value, the inverse of the average CT value of the four vertices of the tetrahedron is taken as the flexibility of the tetrahedron, which is obtained by the following formula:

[0043]

[0044] Where S n is the flexibility of the nth tetrahedron, is the CT value of the ith vertex in the nth tetrahedron, C min It is the minimum CT value of soft tissue in CT images.

[0045] The edge length thinning threshold is calculated based on the flexibility of the tetrahedron and is obtained by the following formula:

[0046]

[0047] Among them l max is the edge length thinning threshold of the nth tetrahedron, L max The soft tissue model accuracy value set for the user is temporarily set to 20mm, k l The maximum refinement ratio is temporarily set to 1 / 4 for customization. n is the flexibility of the nth tetrahedron, C max , C min are the maximum and minimum values ​​of soft tissue CT values ​​in CT images, respectively;

[0048] S3, using the tetrahedron edge length thinning threshold as a judgment condition to thin it using the tetrahedron bisection method;

[0049] Compare the edge length thinning threshold of the tetrahedron with the longest edge in the tetrahedron. If the longest edge is greater than the edge length thinning threshold, the tetrahedron is thinned using tetrahedron bisection.

[0050] The so-called tetrahedron bisection method is a method similar to the tetrahedron subdivision method proposed with reference to the Loop subdivision method. A vertex is inserted on the longest side of the tetrahedron, and then the other two vertices that are not on this side are connected to the newly added vertex, thereby dividing the tetrahedron into two. Figure 2 As shown;

[0051] S4. Constructing a non-uniform soft tissue composite mesh model:

[0052] The tetrahedrons in the rough tetrahedral mesh model are iteratively judged to be refined until they no longer meet the refinement conditions; after the above process, all tetrahedrons in the model are refined to complete the construction of the internal model of the non-uniform soft tissue, and all the current vertices are control vertices;

[0053] Then, the external mesh of the model is further refined based on the Loop method until the maximum side length of the triangle block in the mesh is less than k. l L max Until now, the surface refinement of the non-uniform soft tissue has been completed, and the newly added vertices are non-control vertices;

[0054] Thus, the construction of the composite mesh model of non-uniform soft tissue was completed;

[0055] S5. The deformation of the constructed non-uniform soft tissue composite mesh model is calculated based on the finite element method and the spring-mass method. The deformation of the control points in the soft tissue mesh model is calculated using the improved finite element method, and the deformation of the non-control points is calculated based on the control point deformation using the spring-mass method.

[0056] According to Saint-Venant's principle, when soft tissue is subjected to external force, only significant stress will be generated near it, while the stress far away can be ignored. Therefore, we divide the soft tissue model into a strong deformation zone and a micro-deformation zone. For the strong deformation zone, the finite element method is used for calculation; for the micro-deformation zone, the direct attenuation method of physical variables is used to obtain the stress.

[0057] The division of the above strong deformation zone and micro deformation zone is obtained by calculating the strong deformation radius. The area within the strong deformation radius with the force application point as the center is defined as the strong deformation zone, and the other areas are defined as the micro deformation zone. The strong deformation radius is obtained by the following formula:

[0058]

[0059] Where R q is the strong deformation radius, F in is the external force exerted by the instrument on the model, C min is the minimum CT value of soft tissue in CT images, k R is the deformation factor obtained through experiments;

[0060] The above-mentioned direct attenuation method of physical variables refers to calculating the displacement information of the current vertex by searching for the displacement information of the outer adjacent vertices of the current vertex, until the displacement of the outer adjacent vertex is less than the set displacement threshold and is considered to no longer affect the farther vertices and the calculation is stopped; the outer adjacent vertex refers to the vertex that is closest to the current vertex and closer to the force application point of the instrument relative to the current vertex. The displacement information of the current vertex is obtained by the following formula:

[0061]

[0062] Where T p is the current vertex displacement, T adj is the displacement of the outer adjacent vertex, d p is the distance between the current vertex and the force application point of the instrument, d adjC is the distance between the outer adjacent vertex and the force application point of the instrument, p is the CT value of the current vertex.

[0063] The above description enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. The non-uniform soft tissue composite grid model improved based on the finite element method is characterized by: The following steps are involved: S1, generate a rough tetrahedral mesh according to the CT image; S2. Calculate the flexibility of the tetrahedron by the CT value of the soft tissue in the CT image to obtain the edge length thinning threshold. The specific steps include: S21. Obtain the CT values ​​of the vertices of the tetrahedron. The CT value reflects the density of a local tissue or organ of the human body. The inverse of the CT average value of the four vertices of the tetrahedron is used as the flexibility of the tetrahedron, which is obtained by the following formula: in For the The flexibility of a tetrahedron, For the The tetrahedron The CT value of each vertex, It is the minimum CT value of soft tissue in CT images; S22. Calculate the edge length thinning threshold according to the flexibility of the tetrahedron, which is obtained by the following formula: in For the The edge length thinning threshold of tetrahedrons, The soft tissue model accuracy value set by the user, To customize the maximum refinement ratio, For the The flexibility of a tetrahedron, are the maximum and minimum values ​​of soft tissue CT values ​​in CT images, respectively; S3, using the tetrahedron side length thinning threshold as a judgment condition to thin the tetrahedron using tetrahedron bisection method, including: comparing the tetrahedron side length thinning threshold with the longest side of the tetrahedron, if the longest side is greater than the side length thinning threshold, then using the tetrahedron bisection method to thin the tetrahedron; The tetrahedron bisection method is a method similar to the tetrahedron subdivision method proposed with reference to the Loop subdivision method. A vertex is inserted on the longest side of the tetrahedron, and the other two vertices not on the side are connected to the newly added vertex to divide the tetrahedron into two. S4, constructing a non-uniform soft tissue composite mesh model; S5. Based on the finite element method and spring-mass method, the deformation calculation of the constructed non-uniform soft tissue composite mesh model is performed.

2. The non-uniform soft tissue composite grid model improved based on the finite element method according to claim 1, characterized in that: In S4, the construction of the non-uniform soft tissue composite grid model includes the following specific steps: S41, iteratively performing a determination refinement operation on the tetrahedrons in the coarse tetrahedral mesh model until the tetrahedrons no longer meet the refinement condition; S42, after the above process, all tetrahedrons in the model are refined to complete the construction of the internal model of the non-uniform soft tissue, and all the current vertices are control vertices; S43, the external mesh of the model is further refined based on the Loop method until the maximum side length of the triangle block in the mesh is less than So far, the surface refinement of the non-uniform soft tissue has been completed, the newly added vertices are non-control vertices, and the construction of the non-uniform soft tissue composite grid model has been completed.

3. The non-uniform soft tissue composite grid model improved based on the finite element method according to claim 1, characterized in that: In S5, the deformation of the control points in the soft tissue mesh model is calculated using the improved finite element method, and the deformation of the non-control points is calculated using the spring-mass method based on the deformation of the control points.

4. The non-uniform soft tissue composite grid model improved based on the finite element method according to claim 3, characterized in that: The method for calculating the deformation of control points in a soft tissue mesh model based on an improved finite element method includes the following steps: according to the Saint-Venant principle, the soft tissue model is divided into a strong deformation zone and a micro-deformation zone; for the strong deformation zone, the finite element method is used for calculation; for the micro-deformation zone, the physical variables are obtained by a direct attenuation method.

5. The non-uniform soft tissue composite grid model improved based on the finite element method according to claim 4, characterized in that: The division of the strong deformation zone and the micro deformation zone is obtained by calculating the strong deformation radius. The area within the strong deformation radius with the force application point as the center is defined as the strong deformation zone, and the other areas are defined as the micro deformation zone. The strong deformation radius is obtained by the following formula: in is the strong deformation radius, is the external force exerted by the instrument on the model, is the minimum CT value of soft tissue in CT images, is the deformation factor obtained through experiments.

6. The improved non-uniform soft tissue composite grid model based on finite element method according to claim 4, characterized in that: The direct attenuation method of the physical variable includes calculating the displacement information of the current vertex by searching for the displacement information of the outer adjacent vertices of the current vertex, until the displacement of the outer adjacent vertex is less than the set displacement threshold and is considered to no longer affect the farther vertices and the calculation is stopped; wherein the outer adjacent vertex refers to the vertex that is closest to the current vertex and closer to the force application point of the instrument relative to the current vertex, and the displacement information of the current vertex is obtained by the following formula: in is the current vertex displacement, is the displacement of the outer adjacent vertices, is the distance between the current vertex and the force application point of the instrument, is the distance between the outer adjacent vertex and the force application point of the instrument, is the CT value of the current vertex.

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