An ultrasonic probe calibration method

Abstract

This invention proposes an ultrasonic probe calibration method, the steps of which are: constructing a calibration system; acquiring parameters of the calibration object and a first infrared tracking ball and fitting the calibration object parameters; acquiring multiple ultrasonic images and performing ellipse fitting on them; calculating the coordinates of the cone vertex in the three-dimensional image coordinate system; constructing the transformation matrix of the two infrared tracking balls; and obtaining the ultrasonic probe calibration result. The target point coordinates in the three-dimensional image coordinate system of this invention are calculated through geometric relationships, eliminating the need for a specific calibration device to establish the correspondence between the image and spatial position, and not relying on control scanning surface imaging. Compared with existing technologies, this method reduces operational difficulty and effectively improves calibration efficiency while ensuring calibration accuracy.

Description

Technical Field

[0001] This invention belongs to the field of ultrasound imaging technology and relates to an ultrasound probe calibration method that can be applied to two-dimensional ultrasound image reconstruction. Background Technology

[0002] In the medical field, ultrasound probes are often used to scan the human body using ultrasound beams, and the reflected signals are received and processed to obtain ultrasound images of internal organs. Ultrasound probe calibration first involves fixing a rigid body with a known spatial position on the ultrasound probe, then calculating the transformation matrix of the ultrasound image coordinate system relative to the rigid body coordinate system, thereby determining the specific spatial position of the ultrasound image plane. The key lies in scanning a model with known dimensions and accurate geometric features to establish the correspondence between the image and the actual spatial position.

[0003] Using the center of the sphere as the target point is a common calibration method. However, obtaining the coordinates of the center of the sphere in the image coordinate system usually requires precise control of the scanning surface to align with the center of the sphere. This requires experienced personnel and increases the complexity of the calibration. For example, patent application CN117420215A, entitled "An Ultrasonic Probe Calibration Method and Device Based on Ellipse Fitting," discloses an ultrasonic probe calibration method. This method acquires the position of the center of a spherical calibration model in a two-dimensional ultrasonic image coordinate system using an ultrasonic probe; it transforms the two-dimensional ultrasonic image coordinate system to a three-dimensional world coordinate system through coordinate transformation to obtain the coordinates of the center of the spherical calibration model in the three-dimensional world coordinate system; and it solves the calibration equation to obtain the calibration transformation matrix, including rotation and translation matrices. This method overcomes the artifact errors generated when directly detecting ultrasonic image points. However, the process of acquiring the position of the center of the spherical calibration model in the two-dimensional ultrasonic image coordinate system requires a specific calibration device, making the operation relatively complex. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the existing technology and to propose an ultrasonic probe calibration method that aims to reduce the complexity of calibration while ensuring calibration accuracy.

[0005] To achieve the above objectives, the technical solution adopted by the present invention includes the following steps:

[0006] (1) Constructing a calibration system:

[0007] A calibration system is constructed, comprising a stylus, a conical calibration object fixed in a pool, a first infrared tracking ball fixed on the pool wall, an ultrasonic probe mounted on a second infrared tracking ball above the pool, and an infrared camera for capturing images of the infrared tracking ball.

[0008] (2) Collect parameters of the calibration object and the first infrared tracking ball, and fit the calibration object parameters:

[0009] In the world coordinate system, N points are uniformly sampled on the cone surface of the calibration object using a stylus. Simultaneously, the three-dimensional coordinates (x′1, y′1, z′1) and quaternions (q0′1, q1′1, q2′1, q3′1) of the centroid of the first infrared tracking sphere are collected using an infrared camera. The N sampled points are then fitted, and the coordinates X of the cone vertex in the world coordinate system are calculated based on the fitting results. W And the cone angle α of the cone, where N≥5;

[0010] (3) Acquire multiple ultrasound images and fit them to an ellipse:

[0011] The cone surface of the calibration object is scanned under M different poses of the ultrasound probe, and ellipse fitting is performed on the M scanned ultrasound images to obtain the coordinates of the center position of the ellipse in the two-dimensional image coordinate system. The angle γ between the major axis of the ellipse and the x-axis of the image coordinate system m Length of the semi-major axis of the ellipse, a m and the semi-minor axis length b m ;

[0012] (4) Calculate the coordinates of the cone vertex in the three-dimensional image coordinate system:

[0013] The length of the semi-major axis a of the ellipse m and semi-minor axis length b m Calculate the coordinates of the cone vertex in the 3D image coordinate system after centering. And through the coordinates of the center position of the ellipse The angle γ between the major axis of the ellipse and the x-axis of the image coordinate system m Construct a centrally positive matrix Then through and Calculate the coordinates of the cone vertex in a 3D image coordinate system

[0014] (5) Construct the transformation matrix for the two infrared tracking spheres:

[0015] The three-dimensional coordinates of the centroid of the first infrared tracking sphere were acquired by an infrared camera in each pose of the ultrasonic probe in the world coordinate system. and quaternions and the three-dimensional coordinates of the centroid of the second infrared tracking sphere. and quaternions Construct the transformation matrix from the rigid body coordinate system of the water pool to the world coordinate system for the first infrared tracking sphere. And the transformation matrix of the second infrared tracking sphere from the probe's rigid body coordinate system to the world coordinate system.

[0016] (6) Obtain the ultrasound probe calibration results:

[0017] The transformation matrix T' of the first infrared tracking sphere from the rigid body coordinate system of the pool to the world coordinate system is calculated using the three-dimensional coordinates (x′1, y′1, z′1) of the centroid of the first infrared tracking sphere in the world coordinate system and the quaternions (q0′1, q1′1, q2′1, q3′1). W←F and through T' W←F The coordinates of the cone's vertex in the world coordinate system X W and coordinates in a 3D image coordinate system and transformation matrix and Calculate the calibration matrix T P←I .

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] The target point coordinates in the three-dimensional image coordinate system of this invention are calculated through geometric relationships. There is no need for a specific calibration device to establish the correspondence between the image and the spatial position. It does not rely on controlling the scanning surface imaging. Compared with the prior art, it reduces the operation difficulty and effectively improves the calibration efficiency while ensuring calibration accuracy. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the implementation of the present invention.

[0021] Figure 2 This is a schematic diagram illustrating how the present invention solves for the vertex of a cone in an image coordinate system.

[0022] Figure 3 This is a schematic diagram of coordinate system transformation for the ultrasonic probe calibration method of the present invention. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0024] Reference Figure 1 The present invention includes the following steps:

[0025] Step 1) Construct the calibration system:

[0026] A calibration system is constructed, comprising a stylus, a conical calibration object fixed in a pool, a first infrared tracking ball fixed on the pool wall, an ultrasonic probe mounted on a second infrared tracking ball above the pool, and an infrared camera for capturing images of the infrared tracking ball.

[0027] In this step, the calibration cone is first fixed in the pool, ensuring that the relative position of the cone and the pool remains unchanged. Then, infrared tracking balls are fixed on the pool wall and the ultrasonic probe, respectively. The infrared tracking balls are installed asymmetrically, and more than three are installed. The infrared tracking balls are established as rigid body assets in the motion capture software of the infrared camera, and the rigid body coordinate system {F} of the pool and the rigid body coordinate system {P} of the probe are registered respectively. The pen tip coordinates in the world coordinate system {W} can be obtained through a stylus.

[0028] Step 2) Collect parameters of the calibration object and the first infrared tracking ball, and fit the calibration object parameters:

[0029] In the world coordinate system, N points are uniformly sampled on the cone surface of the calibration object using a stylus. Simultaneously, the three-dimensional coordinates (x′1, y′1, z′1) and quaternions (q0′1, q1′1, q2′1, q3′1) of the centroid of the first infrared tracking sphere are collected using an infrared camera. The N sampled points are then fitted, and the coordinates X of the cone vertex in the world coordinate system are calculated based on the fitting results. W And the cone angle α of the cone, where N≥5; in this embodiment N=1065, X W =[-83.6689,-91.7604,22.8297,1], α=17.4509;

[0030] The coordinates X of the cone vertex in the world coordinate system W The formulas for calculating the cone angle α of a cone are as follows:

[0031]

[0032]

[0033]

[0034]

[0035] L=ZD(D T Z)=UΛV T

[0036] d = V[:,3]

[0037] Among them, (v x ,v y ,v z (x) represents the fitting result of N sampling points. n ,y n ,z nLet represent the 3D coordinates of the nth sampling point, Z represent the vector from the vertex coordinates to each point in the sampling points, D represent the distance from the vertex to each point in the sampling points, L represent the residual matrix, and U, Λ, and V represent the left singular vector, singular value matrix, and right singular vector obtained by performing SVD decomposition on L, respectively. T V T Let D and V represent the transposes of D and V respectively, and d represent the direction of the cone axis.

[0038] The purpose of using an infrared camera to obtain the three-dimensional coordinates (x′1, y′1, z′1) of the centroid of the first infrared tracking ball and the quaternions (q0′1, q1′1, q2′1, q3′1) representing rotation in space is to calculate the coordinates of the cone vertex in the pool coordinate system. This ensures that even if the position of the pool changes during subsequent operations, the accurate position coordinates of the cone vertex can still be obtained.

[0039] Step 3) The cone surface of the calibration object is scanned in M ​​different poses of the ultrasound probe, and ellipse fitting is performed on the M scanned ultrasound images to obtain the coordinates of the center position of the ellipse in the two-dimensional image coordinate system. The angle γ between the major axis of the ellipse and the x-axis of the image coordinate system m Length of the semi-major axis of the ellipse, a m and the semi-minor axis length b m In this embodiment, M = 50;

[0040] The specific steps for ellipse fitting are as follows:

[0041] (3a) Set a trapezoidal mask to eliminate interference from additional information, determine a fixed scanning center and a fan-shaped area scanning range in the image, and uniformly discretize the angles of the fan-shaped area. Each angle represents a potential detection direction to ensure full coverage of the target area. Then calculate the ray corresponding to each discrete angle and perform pixel-by-pixel scanning along these rays. During the scanning process, find the first point whose brightness value exceeds the set threshold. This point is considered to be the upper surface of the cone and is added to the point set. Then the search for the current ray is terminated. Through this process, the fitted point set can be accurately and efficiently extracted from the ultrasound image.

[0042] (3b) The least squares method and RANSAC are used to accurately fit the fitted point set to an elliptical shape. RANSAC is used to remove outliers, and the geometric distance is used to define the loss when calculating the interior points in RANSAC.

[0043]

[0044] Where Q represents the matrix form of the ellipse, and X represents the homogeneous coordinates of the fitted point set.

[0045] Step 4) Calculate the coordinates of the cone's vertex in the 3D image coordinate system:

[0046] The length of the semi-major axis a of the ellipse m and semi-minor axis length b m Calculate the coordinates of the cone vertex in the 3D image coordinate system after centering. And through the coordinates of the center position of the ellipse The angle γ between the major axis of the ellipse and the x-axis of the image coordinate system m Construct a centrally positive matrix Then through and Calculate the coordinates of the cone vertex in a 3D image coordinate system

[0047] Figure 2 This is a schematic diagram of the present invention for solving the vertex of a cone in the image coordinate system. On the plane ρ perpendicular to the major axis of the ellipse ε obtained by scanning the cone, the vertex of the cone is determined by the intersection of the hyperbola H and the circle C.

[0048] The coordinates of the cone vertex in the 3D image coordinate system after centering are correct The equations are calculated simultaneously in a plane passing through the major axis of an ellipse and perpendicular to the ultrasound image, with the vertices of the ellipse's major axis and the ellipse's foci as vertices, and the equation of a circle with the major axis of the ellipse as the chord length and the cone angle as the circumference angle. The expressions for the hyperbola equation and the circle equation are as follows:

[0049]

[0050]

[0051] in, and The x-axis and y-axis coordinates of the cone vertex in the 3D image coordinate system after centering are correct.

[0052] The coordinates of the cone vertex in the 3D image coordinate system after centering are correct Centrally positive matrix and the coordinates of the cone vertex in the 3D image coordinate system The construction and calculation formulas are as follows:

[0053]

[0054]

[0055]

[0056] in, express The inverse transform.

[0057] (5) Construct the transformation matrix for the two infrared tracking spheres:

[0058] The three-dimensional coordinates of the centroid of the first infrared tracking sphere were acquired by an infrared camera in each pose of the ultrasonic probe in the world coordinate system. and quaternions and the three-dimensional coordinates of the centroid of the second infrared tracking sphere. and quaternions Construct the transformation matrix from the rigid body coordinate system of the water pool to the world coordinate system for the first infrared tracking sphere. And the transformation matrix of the second infrared tracking sphere from the probe's rigid body coordinate system to the world coordinate system.

[0059] Transformation matrix The calculation formula is:

[0060]

[0061]

[0062]

[0063] in, Represents the rotation matrix. This represents the translation vector.

[0064] Calculation formula and same.

[0065] (6) Obtain the ultrasound probe calibration results:

[0066] The transformation matrix T' of the first infrared tracking sphere from the rigid body coordinate system of the pool to the world coordinate system is calculated using the three-dimensional coordinates (x′1, y′1, z′1) of the centroid of the first infrared tracking sphere in the world coordinate system and the quaternions (q0′1, q1′1, q2′1, q3′1). W←F and through T' W←F The coordinates of the cone's vertex in the world coordinate system X W and coordinates X in the three-dimensional image coordinate system I and the transformation matrix T W←F and T W←P Calculate the calibration matrix T P←I .

[0067] Figure 3 This explains the relationships between the various coordinate systems required to solve the calibration matrix, including the transformation matrix T from the ultrasound image coordinate system {I} to the probe rigid body coordinate system {P}. P←I The transformation matrix from the probe's rigid body coordinate system {P} to the world coordinate system {W} Transformation matrix from world coordinate system {W} to rigid body coordinate system {F} of the pool

[0068] This embodiment uses the ICP algorithm to solve the problem, and the calibration equation is given by utilizing the transformation relationship between coordinate systems:

[0069]

[0070] Where T scale This represents the scale matrix composed of the parameters of the ultrasound instrument.

Claims

1. A method for calibrating an ultrasonic probe, characterized in that, Includes the following steps: (1) Constructing a calibration system: A calibration system is constructed, comprising a stylus, a conical calibration object fixed in a pool, a first infrared tracking ball fixed on the pool wall, an ultrasonic probe mounted on a second infrared tracking ball above the pool, and an infrared camera for capturing images of the infrared tracking ball. (2) Collect parameters of the calibration object and the first infrared tracking ball, and fit the calibration object parameters: In the world coordinate system, N points are uniformly sampled on the cone surface of the calibration object using a stylus. Simultaneously, the three-dimensional coordinates (x′1, y′1, z′1) and quaternions (q0′1, q1′1, q2′1, q3′1) of the centroid of the first infrared tracking sphere are collected using an infrared camera. The N sampled points are then fitted, and the coordinates X of the cone vertex in the world coordinate system are calculated based on the fitting results. W And the cone angle α of the cone, where N≥5; (3) Acquire multiple ultrasound images and fit them to an ellipse: The cone surface of the calibration object is scanned under M different poses of the ultrasound probe, and ellipse fitting is performed on the M scanned ultrasound images to obtain the coordinates of the center position of the ellipse in the two-dimensional image coordinate system. The angle γ between the major axis of the ellipse and the x-axis of the image coordinate system m Length of the semi-major axis of the ellipse, a m and the semi-minor axis length b m ; (4) Calculate the coordinates of the cone vertex in the three-dimensional image coordinate system: The length of the semi-major axis of the ellipse is a m and semi-minor axis length b m Calculate the coordinates of the cone vertex in the 3D image coordinate system after centering. And through the coordinates of the center position of the ellipse The angle γ between the major axis of the ellipse and the x-axis of the image coordinate system m Construct a centrally positive matrix Then through and Calculate the coordinates of the cone vertex in a 3D image coordinate system (5) Construct the transformation matrix for the two infrared tracking spheres: The three-dimensional coordinates of the centroid of the first infrared tracking sphere were acquired by an infrared camera in each pose of the ultrasonic probe in the world coordinate system. and quaternions and the three-dimensional coordinates of the centroid of the second infrared tracking sphere. and quaternions Construct the transformation matrix from the rigid body coordinate system of the water pool to the world coordinate system for the first infrared tracking sphere. And the transformation matrix of the second infrared tracking sphere from the probe's rigid body coordinate system to the world coordinate system. (6) Obtain the ultrasound probe calibration results: The transformation matrix T' of the first infrared tracking sphere from the rigid body coordinate system of the pool to the world coordinate system is calculated using the three-dimensional coordinates (x′1, y′1, z′1) of the centroid of the first infrared tracking sphere in the world coordinate system and the quaternions (q0′1, q1′1, q2′1, q3′1). W←F and through T' W←F The coordinates of the cone's vertex in the world coordinate system X W and coordinates in a three-dimensional image coordinate system and transformation matrix and Calculate calibration matrix T P←I .

2. The method according to claim 1, characterized in that, The coordinates X of the cone vertex in the world coordinate system mentioned in step (2) W The formulas for calculating the cone angle α of the cone and the cone are as follows: L=Z-D(D T Z)=UΛV T d = V[:,3] Among them, (v x ,v y ,v z (x) represents the fitting result of N sampling points. n ,y n ,z n Let represent the 3D coordinates of the nth sampling point, Z represent the vector from the vertex coordinates to each point in the sampling points, D represent the distance from the vertex to each point in the sampling points, L represent the residual matrix, and U, Λ, and V represent the left singular vector, singular value matrix, and right singular vector obtained by performing SVD decomposition on L, respectively. T V T Let D and V represent the transposes of D and V respectively, and d represent the direction of the cone axis.

3. The method according to claim 1, characterized in that, The cone vertex coordinates in the three-dimensional image coordinate system after centering as described in step (4) The equations are calculated simultaneously in a plane passing through the major axis of an ellipse and perpendicular to the ultrasound image, with the vertices of the ellipse's major axis and the ellipse's foci as vertices, and the equation of a circle with the major axis of the ellipse as the chord length and the cone angle as the circumference angle. The expressions for the hyperbola equation and the circle equation are as follows: in, and The x-axis and y-axis coordinates of the cone vertex in the 3D image coordinate system after centering are correct.

4. The method according to claim 1, characterized in that, The cone vertex coordinates in the three-dimensional image coordinate system after centering as described in step (4) Centrally positive matrix and the coordinates of the cone vertex in the 3D image coordinate system The construction and calculation formulas are as follows: in, express The inverse transform of .

5. The method according to claim 1, characterized in that, The transformation matrix mentioned in step (5) The calculation formula is: in, Represents the rotation matrix. This represents the translation vector.

6. The method according to claim 1, characterized in that, The calculation of calibration matrix T in step (6) P←I The calculation formula is: Where T scale This represents the scale matrix composed of the parameters of the ultrasound instrument.

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