CN101477677B - Method for tubular object virtually out-turning based on central path - Google Patents
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Abstract
The invention relates to a method for everting and rotating pipeline-shaped objects, belongs to the technical field of image processing, and is based on center paths. The method comprises the following steps: preprocessing and segmenting the prior original image; drawing out a datum plane by utilizing the data of the segmented image; extracting the center path of a pipeline-shaped object on the basis of the data of the datum plane so as to construct a model of point charges; calculating electric field lines which are perpendicular to the center path and start from point charges, wherein, each electric field line starting from one point charge forms a curved surface which is cut by an inner wall to form a cut curved surface, and obtaining the initial vectors of the cut curved surfaces; carrying out pulling process; drawing out the datum plane of the pulled image; everting and rotating the original image at the same time; and displaying the data of the pipeline-shaped object being processed by everting and rotating. The method which rotates the everted data and integrates the advantages of the pulling processing method and the everting processing method has the advantage that the distortion of a part which is bent by a wider band is avoided. Therefore, the inner wall of the pipeline-shaped object can be seen at various visual angles from the external.
Description
Technical Field
The invention relates to a method in the technical field of image processing, in particular to a method for everting and rotating a tubular object based on a central path.
Background
At present, the virtual endoscopic technology is mainly adopted for carrying out three-dimensional visual reconstruction on the inner wall of a tubular object by utilizing imaging data such as CT, MRI, ICT and the like. This technique simulates the travel of a conventional optical endoscope body inside a tubular object to observe the inner wall of the tubular object from the inside in the view point of the optical endoscope. The technology has the advantages of overcoming the defect that the endoscope body needs to be inserted into the inner part of the tubular object in the traditional optical endoscope and being a completely non-contact detection method. The existing virtual endoscopic technology has the defects that the observation angle is still limited to endoscopic observation, the visual angle is limited, the integral shape of a pipeline-shaped object cannot be observed, and the positioning of an interested part is not intuitive enough.
Wang et al published on "academic radiology" (pages 398-410) in 7.1999, "illuminating the column with curved cross sections: an a proproach to CT chromatography article proposes a virtual straightening method based on a central path. The method can straighten the bent pipe-shaped object, and is convenient for observing the morphological structure of the bent part. "Digital Evaporation of aHollow Structure," published by Zhao Jun et al in 2008 in "International Journal of biological Imaging" (Article ID 763028): an Application in Virtual networking "article proposes a method for Virtual eversion of a tubular object, i.e., turning the inner wall of the tubular object from inside to outside on a computer, so that An observer can see the overall shape of the tubular object and can observe the fine structure of the inner wall. The straightening method is convenient for observing the morphological structure of the bending part, but the method changes the spatial structure of the pipeline-shaped object. The eversion method can change the observation mode of the inner wall from the inward vision to the appearance, but the part with larger bending causes great distortion due to eversion.
Disclosure of Invention
The invention provides a method for virtually turning up and rotating a pipeline-shaped object based on a central path, aiming at the defects of the prior art, the method rotates the turned-up data, combines the advantages of straightening and turning-up processing methods, avoids distortion of a large bending part, and can look outside the inner wall of the pipeline-shaped object from various visual angles.
The method is realized by the following technical scheme that the method comprises the steps of obtaining the data of the inner wall surface and the datum plane of an object by segmenting the obtained scanning image of the pipeline-shaped object, then carrying out virtual eversion on the data of the inner wall surface, and finally carrying out virtual rotation on the everted image.
The invention comprises the following steps:
firstly, preprocessing and segmenting an existing original image. The raw image may be raw data of a tubular object acquired with CT, MRI, ICT imaging modalities. Preprocessing such as filtering and denoising is carried out to enhance the image segmentation effect. The segmentation can adopt various suitable image segmentation methods to segment the original image to obtain the data of the pipeline-shaped object. For the case of easier segmentation, automatic segmentation can be achieved using an algorithm. If automatic segmentation cannot be achieved, segmentation of the image may be achieved in combination with manual or fully manual methods.
The image segmentation refers to two basic characteristics based on the brightness value of the image: discontinuity and similarity, a process of extracting a specific attribute from an input image.
And secondly, drawing a reference plane by using the segmented image data. The reference surface must be smooth and represent the actual shape of the tubular object, so different extraction methods should be adopted according to the actual situation. For a pipeline-shaped object with a thin outer wall and easy to divide, the surface of the outer wall can be extracted and used as an eversion datum plane; for a pipeline-shaped object with an irregular outer wall surface shape or difficult segmentation, the data of the inner wall surface obtained by surface drawing by using a cube with a larger scale can be used as a reference surface; for a pipe-shaped object with a relatively thick wall, the surface of the outer wall can be subjected to appropriate morphological corrosion to obtain a reference surface.
And thirdly, taking the reference surface in the second step as a reference to extract a central path of the pipeline-shaped object, then performing interpolation processing on the central path, and constructing a point charge model on the central path.
The central path is as follows: in a middle axis track inside the pipeline-shaped object, each point on the track meets the optimal central point criterion. General methods for generating a center path include a topology refinement method, a distance conversion method, and the like. For the case that the center path is difficult to automatically generate, the center path can also be obtained by using a manual calibration method. The center path must have a good centering degree and be smooth enough to avoid sharp bends and corners.
The point charge model refers to: to calculate the electric field line trajectories, a distributed point charge is simulated on the central path, while other data regions in the tunnel-like object have no charge. It is generally desirable to distribute the point charges uniformly and densely on the central path to ensure that the electric field lines pass through every data point on the inner wall of the tubular object after the electric field lines are generated.
And fourthly, calculating electric field lines which start from each point of charge and are vertical to the central path. The electric field lines from one point form a curved surface, the curved surface is cut by the inner wall to obtain a cross section surface, and an initial vector of the cross section surface is obtained.
The electric field lines refer to: under basic electromagnetic field theory, the electric field lines in an electric field generated by multiple point charge interactions. In practical application, the number of charges at the point of the whole central path is large, and electric field lines in all directions can be calculated by starting from each charge at each point on the central path and combining the interaction of a plurality of charges at local areas nearby the point.
The initial vector of the cross-section curved surface refers to: according to the tangential direction of each point on the central path, three conditions are passed: a. perpendicular to the tangential direction; b. the included angle between the initial vector and a central path point is as small as possible; c. the vector length is 1. Thus solving the profile initial vector of the point. The initial vector of the first cross-section is taken along a ray perpendicular to the tangent of the central path point.
Step five, straightening treatment, namely: and flattening all the obtained section curved surfaces from the starting point of the central path, and sequentially arranging. Ensuring that the central path points form a straight line during arrangement; the initial vector directions of all the section surfaces are consistent.
The flattening of each section surface means that: and taking the initial vector of the profile surface as a reference, and recording the included angle between the electric field line and the initial vector on the profile surface (the anticlockwise direction is a positive direction) and the length of the electric field line. And mapping each electric field line to a straight section corresponding to the central path point in the straightened image: mapping the electric field lines corresponding to the initial vector to a Y axis on a straight section with the central path point as an origin, and keeping the length unchanged; the electric field lines on the section surface, which form an angle with the initial vector, are mapped to the rays passing through the original point on the straight section, which form a corresponding angle with the Y axis (the anticlockwise direction is the positive direction), and the length is kept unchanged. This allows the curves to be mapped to straight lines and the profile to be mapped to a plane.
And sixthly, drawing a reference plane of the straightened image as an eversion reference plane by adopting the method in the second step.
And seventhly, turning the original image outwards and rotating the original image. Everting and rotating the image are performed simultaneously to minimize distortion.
a. And calculating a straight section (a section which is perpendicular to the central path through the central path point) corresponding to each central path point according to the straightened image.
b. And on each straight section, rays are emitted in all directions by taking the central path point as a starting point, the rays have an intersection point with the inner wall and the reference plane respectively, and the distance between the two intersection points is recorded. And taking the intersection point of the ray and the reference plane as a starting point to advance along the ray direction for the recorded distance, and contracting to obtain a point, namely the point obtained after the inner wall point is turned outwards.
c. And rotating the straight section after eversion by a certain angle anticlockwise around the straightened central path, recording the direction vector of the rotated Y axis, and recording the direction vector as a reference vector.
d. Mapping each rotated straight section to the section surface where the corresponding point of the original central path is located: and recording the included angle between the ray and the reference vector on the straight section (the anticlockwise direction is the positive direction) and the length of the ray (the distance between the central path point and the eversion point in the straight section) by taking the reference vector as a reference. And mapping each ray to a section surface corresponding to the central path point in the original image: and defining the vector of the rotation angle between the profile surface and the initial vector as the initial vector. Mapping the ray corresponding to the reference vector to the electric field line corresponding to the initial vector on the section surface, and keeping the length unchanged; the ray of the straight section, which forms an angle with the reference vector, is mapped to the electric field line of the section surface, which forms a corresponding angle with the initial vector (the counterclockwise direction is the positive direction), and the length is kept unchanged. This allows straight lines to be mapped to curved lines and flat to curved surfaces.
And eighthly, three-dimensionally displaying the data after the eversion processing in an interactive mode.
The invention has the advantages that: 1. compared with virtual straightening, virtual rotation does not change the spatial form, and is more beneficial to determining the position. 2. Compared with virtual eversion, virtual rotation can increase the visual angle of an observer and is more beneficial to observing the inner walls where the bending is larger and even the inner walls are tightly attached together; the eversion datum planes are calculated after straightening in the virtual rotation, so that the condition that the datum planes are intersected due to overlarge curvature when the datum planes are calculated can be prevented. The invention can be applied to the field of utilizing CT, MRI, ICT and other imaging data.
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FIG. 1: mapping from electric field line profile to straight cross-section
FIG. 2: schematic diagram of inner wall point eversion in straight section
Detailed Description
The embodiments of the present invention will be described in detail below with reference to the accompanying drawings: the present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.
In this embodiment, U-shaped pipeline data is used as a sample of virtual eversion rotation, and the method is implemented according to the following steps:
1. and carrying out filtering processing on the obtained pipeline data, wherein the filtering processing comprises Gaussian filtering and median filtering, and removing noise. Using a region growing algorithm, data of the lumen portion of the tube is segmented from the image and used to map the inner wall surface.
2. For the data of the divided pipe inner wall, the morphological expansion corrosion method (expansion element radius 10, corrosion element radius 8) was also used to obtain a relatively smooth surface of the pipe outer wall as a reference plane.
3. And performing three-dimensional distance transformation on the pipeline data by taking the reference surface as a reference to obtain the shortest distance from each point in the pipeline to the reference surface. Generating a center path by adopting a Dijkstra shortest path method: that is, the starting point and the end point of the central path are selected in the cavity, and then the shortest path search is performed by taking the reciprocal of the distance value of each point in the cavity as a weight. Each point virtually distributes a unit of positive point charge over the center path.
4. And calculating the profile of each central path point, and obtaining an initial vector of the profile.
The section surface is a curved surface obtained by cutting the inner wall of the pipeline from a curved surface formed by electric field lines which are emitted from the point charge of the central path and are vertical to the directions of the central path. In the example, each profile is formed with 400 electric field lines uniformly distributed in the original direction.
a. Calculating an initial vector of the profile from the initial point of the electric field lines: calculating a plane passing through the central path point and perpendicular to the central path, emitting 400 rays (the included angle of every two adjacent rays is 0.9 degrees) on the plane at equal angle by taking the central path point as a center, and taking a point on the ray, which is 0.5 away from the central point, as an initial point; and calculating the initial vector of the profile surface.
Suppose the current point of the central path is P0A dot, a vertical plane, may be represented by a dot-normal. The point is P0Point, normal vector
Can be-by P1-P-1And obtaining the product through normalization. Can order
Has the coordinates of (n)1，n2，n3)，P0Has the coordinates of (p)1，p2，p3). The plane can then be expressed as: n is1(x-p1)+n2(y-p2)+n3(z-p3)＝0。
The obtained profile surface initial vector is the positive direction (Y-axis direction) of the vertical plane. The initial vector of the first section surface is arbitrarily perpendicular to the normal vector
The unit vector of (2). Let the required profile initial vector of the current center path point be
The unit tangent vector pointing to the next center path point (i.e., the approximate center path tangent vector) is
The profile surface initial vector of the last central path point is
Initial vector of profile surface
The following three constraints are satisfied:
condition 1: perpendicular to the unit tangent vector, i.e.: <math><mrow><mover><mi>N</mi><mo>→</mo></mover><mo>·</mo><mover><mi>Y</mi><mo>→</mo></mover><mo>=</mo><mn>0</mn></mrow></math>
condition 2: the included angle with the initial vector of a profile is as small as possible (which can be equivalently coplanar with the tangent vector and the initial vector of the previous profile). Namely:
condition 3: the length of the profile initial vector is 1. Namely:
at this time, the process of the present invention,must have <math><mrow><mover><mi>N</mi><mo>→</mo></mover><mo>·</mo><mover><mi>Y</mi><mo>→</mo></mover><mo>=</mo><mn>0</mn><mo>,</mo></mrow></math> That is to sayAnd
and is vertical. Then, take P0Is the origin of coordinates and the positive direction in the plane
Is a normal vector in the Y-axis direction
Is the direction of the Z axis, and the Z axis is the Z axis,for the X-axis direction, a new coordinate system may be defined. In the new coordinate system, the unit vector of each coordinate axis direction has the following coordinates in the original coordinate system:
in the new coordinate system, the initial positions of the electric field lines are simply positioned, all in the X-Y plane, and can be represented by polar coordinates (rho, theta) in the planei) To indicate. Where ρ is the step length parameter and θiThe parameter n (number of electric field lines taken per section) can be obtained by the following equation:
the coordinates of the initial position of the electric field lines in the new coordinate system are then as follows:
converting the initial position of the electric field line in the new coordinate system into the original coordinate system can be implemented by the following formula:
in the example where p, n are 0.5 and 400, respectively, the i-th field line is the field line rotated counterclockwise from the initial vector by (i-1) × (n/360).
b. Calculating the direction of the electric field lines and obtaining the next point in the electric field lines: assuming a current center pathCurrent point is P0When the current point on the electric field line is Q, P is calculated0Point charge in the neighborhood of point k (2 k +1 point charge: P is considered together)-k，P-(k-1)，…P0，…P(k-1)，Pk) The direction of the field strength generated at the point Q. The electric field lines are advanced one step from the point Q along the direction (the smaller the step is, the more accurate the calculation is, but the calculation time is increased, in this case, 0.1) to get the next point of the electric field lines.
For the current position Q (Q)1，q2，q3) Calculating all points P-k，P-(k-1)，…P0，…P(k-1)，PkAssuming that all are charged with a unit positive charge, the electric field strength direction for the Q point. By using the superposition principle of electric field force, each P can be calculatediAnd accumulating the electric field intensity to the Q point.
The electric field strength generated by each point charge at one point in space is given by the formula:
wherein,
is the vector of the point charge to that point, i.e. <math><mrow><mover><msub><mi>r</mi><mi>i</mi></msub><mo>→</mo></mover><mo>=</mo><msub><mi>P</mi><mi>i</mi></msub><mo>-</mo><mi>Q</mi><mo>.</mo></mrow></math>
Because each spot charges into the electric field strength of the spot,the values are the same and may be omitted. The direction of the electric field strength can then be obtained using the following equation:
then, make a pair again
After normalization, the direction of the electric field lines, i.e. the direction of the electric field lines, can be obtained
According to the current position Q and the current normalized electric field direction
The point coordinate Q' can be obtained after advancing by a distance ρ in the direction of the electric field lines. Namely:
the method can be used for obtaining the section surface formed by 400 electric field lines emitted by each point of charge on the central path.
5. And (6) straightening treatment. The ith electric field line (the electric field line that starts from the initial vector and turns counterclockwise (i-1) × (n/360)) of the kth profile and the distance l of the electric field line from the center path point to the inner wall of the pipe are recorded: (k, i, l); mapping the ith electric field line of the kth section surface to a ray which is rotated counterclockwise by (i-1) × (n/360) degrees around the origin from the Y axis in a section plane where z is k, wherein the length of the ray is l; the planes are coordinate transformed so that the resulting image is in the first quadrant. This ensures that the initial vector of the profile coincides with the Y-axis of the section plane (see fig. 1). The section planes are sequentially arranged to obtain the straightened pipeline inner wall data.
6. And performing morphological expansion corrosion on the straightened pipeline data (the radius of an expansion element is 10, and the radius of a corrosion element is 8) to obtain a datum plane of the straightened pipeline data.
7. The original image is subjected to eversion rotation.
The straightened image is first processed:
a. taking the ith ray of the kth section plane as an object, taking the intersection point of the ray and the reference plane as a center, and symmetrically enabling the intersection point of the ray and the inner wall to be out of the reference plane along the ray and shrinking by a length (shown in figure 2). Recording the distance l from the central path point of the ith ray of the kth section plane to the eversion point1：(k，i，l1). Thus each ray has a scalar to describe.
b. Rotate the straightened pipe data counterclockwise by an angle theta: the ith ray of the kth section plane is rotated to the position of the (i + theta/(2 pi/n) (n is the number of rays per section) ray, and the description scalar quantity of the ray is (k, i + theta/(2 pi/n), l)1)。
Then mapping the processed data information into the original image:
a. setting the image data to 0, and calculating the electric field line curved surface of the k point of the central path of the original image.
b. The distance path point l from the center on the i + theta/(2 pi/n) th electric field line1As the boundary point of the electric field lines, 255.
c. And traversing each central path point in the steps to obtain a binary image of the original image after being everted and rotated by an angle theta.
d. And (3) carrying out interpolation processing on the image: the points on the electric field line between the center path point to the boundary points are placed 255.
8. And (3) redrawing the data of the inner wall surface of the pipeline, which is mapped out of the reference surface and rotated, by using a VTK three-dimensional visualization tool kit, and displaying the data in a user-controlled manner. The user can change the visual angle at will, and the inner wall surface is observed from the outside.
The implementation mode not only keeps various advantages of the original virtual eversion technology, for example, the inner wall of the pipeline can be displayed on the outer surface virtually, and a user can flexibly change different visual angles, but also overcomes the defect that the visual angle is limited when the position with overlarge curvature is observed, the position to be observed can be rotated, so that the observation is more flexible and convenient, the observation of the bent position is more delicate, and simultaneously, distortion problems caused by the eversion method are avoided, and more reliable data are provided for observing the inner wall of the pipeline.
Claims (6)
1. A method for virtual eversion rotation of a tubular object based on a central path, comprising the steps of:
firstly, preprocessing and segmenting an existing original image;
secondly, drawing a reference surface by using the segmented image data, wherein the reference surface is smooth and can represent the actual form of the object;
thirdly, taking the reference surface in the second step as a reference to extract a central path of the pipeline-shaped object, then performing interpolation processing on the central path, and constructing a point charge model on the central path;
fourthly, calculating electric field lines which start from each point of charge and are vertical to the central path, forming a curved surface by the electric field lines which start from one point, cutting the curved surface by the inner wall to obtain a cross-sectional surface, and obtaining an initial vector of the cross-sectional surface;
step five, straightening treatment, namely: flattening all the obtained section surfaces from the starting point of the central path, and arranging the section surfaces in sequence, wherein the central path points are ensured to form a straight line during arrangement, and the initial vector directions of all the section surfaces are consistent;
sixthly, drawing a reference plane of the straightened image as an eversion reference plane by adopting the method in the second step;
seventhly, turning the original image out and rotating the original image, wherein the turning out and the rotating of the original image are carried out simultaneously;
eighthly, three-dimensionally displaying the data after the eversion treatment in an interactive mode;
the initial vector of the profile surface refers to: from the tangential direction of each point on the central path, three conditions are obtained:
a. perpendicular to the tangential direction;
b. the included angle between the initial vector and a central path point is as small as possible;
c. the vector length is 1;
thus solving the initial vector of the section surface of the point, and selecting a ray passing through the central path point and perpendicular to the tangential direction from the initial vector of the first cross section;
the flattening of each section surface means that: using the profile surface initial vector as a reference, recording the included angle between the electric field lines and the initial vector on the profile surface and the length of the electric field lines, and mapping each electric field line to a straight section corresponding to the central path point in the straightened image: mapping the electric field lines corresponding to the initial vector to a Y axis on a straight section with the central path point as an origin, and keeping the length unchanged; the electric field lines on the section surface, which form an angle with the initial vector, are mapped to the rays passing through the original point on the straight section, which form a corresponding angle with the Y-axis clamp, the length is kept unchanged, so that the curve is mapped to the straight line, and the section surface is mapped to the plane;
the everting and rotating of the original image comprises the following steps:
a. calculating a straight section corresponding to each central path point according to the straightened image, namely a section which passes through the central path point and is perpendicular to the central path;
b. on each straight section, rays are emitted to all directions by taking a central path point as a starting point, the rays, the inner wall and the reference plane respectively have an intersection point, the distance between the two intersection points is recorded, the recorded distance is advanced along the ray direction by taking the intersection point of the rays and the reference plane as the starting point, and a point obtained by contraction is a point obtained after the inner wall point is turned outwards;
c. rotating the everted straight section anticlockwise around a straightened central path, recording a direction vector of the rotated Y axis, and recording the direction vector as a reference vector;
d. mapping each rotated straight section to a profile surface where a corresponding point of an original central path is located, recording the included angle between a ray and a reference vector on the straight section and the length of the ray by taking the reference vector as a reference, and mapping each ray to the profile surface corresponding to the central path point in an original image, wherein the specific steps are as follows: defining the vector of the rotation angle between the profile surface and the initial vector as an initial vector, mapping the ray corresponding to the reference vector to the electric field line corresponding to the initial vector on the profile surface, and keeping the length unchanged; the ray of the straight section, which forms an angle with the reference vector, is mapped to the electric field line of the section surface, which forms a corresponding angle with the initial vector, and the length is kept unchanged, so that the straight line is mapped to the curve, and the plane is mapped to the curved surface.
2. The method for virtual eversion rotation of a tubular object based on a central path as claimed in claim 1, wherein said raw images are raw data of the tubular object obtained by CT, MRI, ICT imaging means.
3. The method for virtual eversion rotation of a tubular object based on a central path as claimed in claim 1, wherein the preprocessing refers to filtering and denoising processing of an image.
4. The method for virtual eversion rotation of a tubular object based on a central path as claimed in claim 1, wherein said dividing is: and extracting the data of the tubular object from the input image data by adopting an automatic segmentation algorithm or combining a manual segmentation method or a completely manual segmentation method.
5. The method for virtual eversion rotation of a tubular object according to claim 1, wherein the central path is selected from the group consisting of: in a middle axis track in the pipeline-shaped object, each point on the track meets the optimal central point criterion, and the central path generation method comprises a topological refinement method, a distance transformation method or a manual calibration method to obtain the central path.
6. The method for virtual eversion rotation of a tubular object based on a central path as claimed in claim 1, wherein the point charge model is: the point charges are distributed uniformly and densely on the central path to ensure that after the electric field lines are generated, the electric field lines pass through each data point on the inner wall of the tubular object.
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| CN1711968A (en) * | 2005-05-26 | 2005-12-28 | 西安理工大学 | Rapid progressive three-dimensional reconstructing method of CT image from direct volume rendering |
| CN1850005A (en) * | 2006-05-18 | 2006-10-25 | 上海交通大学 | Invented eversion type 3-D external viewing method for cavity organ internal wall |
| CN101013506A (en) * | 2007-02-13 | 2007-08-08 | 上海交通大学 | Visual method for virtual incising tubular organ |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN1711968A (en) * | 2005-05-26 | 2005-12-28 | 西安理工大学 | Rapid progressive three-dimensional reconstructing method of CT image from direct volume rendering |
| CN1850005A (en) * | 2006-05-18 | 2006-10-25 | 上海交通大学 | Invented eversion type 3-D external viewing method for cavity organ internal wall |
| CN101013506A (en) * | 2007-02-13 | 2007-08-08 | 上海交通大学 | Visual method for virtual incising tubular organ |
Non-Patent Citations (1)
| Title |
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| JP特开平11-258189A 1999.09.24 |
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