CN114699168A - Calibration system and calibration method of biplane ultrasonic probe - Google Patents
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Abstract
The invention discloses a calibration system based on a biplane ultrasonic probe, which comprises an orthogonal biplane ultrasonic probe, a magnetic field generator, a magnetic navigation positioning receiver, a calibration phantom and a container. The calibration imitation body is a cuboid, is fixed at the bottom of the container and is immersed by an ultrasonic imaging medium in the container; the calibration imitation body is divided into an upper part and a lower part, and N-shaped wire groups are respectively inserted in the upper part and the lower part; the two imaging planes of the biplane ultrasonic probe are mutually vertical and can be respectively intersected with the two groups of silk thread groups of the calibration phantom; the magnetic navigation positioning receiver is fixed in the middle of the biplane ultrasonic probe; the magnetic navigation positioning probe is used for pointing and selecting the surface points of the imitation body so as to position and calibrate the imitation body; the magnetic field generator is arranged on one side of the container, and the magnetic field range of the magnetic field generator can cover the magnetic navigation positioning receiver and the magnetic navigation positioning probe on the ultrasonic probe. The scheme is beneficial to the standardized calibration data acquisition and labeling process; the coordinate transformation matrix from the magnetic navigation positioning receiver to the ultrasonic image is calculated with high precision.
Description
Technical Field
The invention relates to a calibration system and a calibration method of an orthogonal biplane ultrasonic probe, belonging to the technical field of medical instruments and computer vision.
Background
Prostate aspiration biopsy, is the gold standard for prostate cancer diagnosis. Ultrasound images are often used to guide prostate puncture surgery because of their ease of acquisition and real-time image quality. At present, a prostate puncture biopsy system with preoperative MR and intraoperative ultrasound fusion is clinically used, and spatial position information of a two-dimensional ultrasound image needs to be acquired. By fixing the electromagnetic positioning sensor on the ultrasonic probe, the spatial pose information of the sensor can be acquired in real time. In addition, the ultrasonic probe needs to be calibrated, so that the conversion relationship from the sensor coordinate system to the ultrasonic image coordinate system is obtained, and further the spatial position information of the ultrasonic image is obtained.
At present, in a common ultrasonic image calibration method, an ultrasonic image is made to scan an N-line body membrane in a water tank to form a plurality of bright spots, the spatial position information of the bright spots is identified and calculated, and a calibration matrix is calculated by combining the pixel coordinate information of the bright spots in the ultrasonic image.
In the prior art, an ultrasonic calibration phantom is mainly designed for a single-plane ultrasonic probe. For a biplane ultrasonic probe, because two imaging planes of the biplane ultrasonic probe are orthogonal to each other, have limited visual fields, and cannot be completely immersed in a water tank, it is difficult to find a proper angle and position so that the biplane ultrasonic probe can scan all points of an N-line phantom. Due to the poor imaging quality of the ultrasonic image, the problems of reflection, artifact and the like, and the overlarge area of the bright spot when the ultrasonic cross section is not completely vertical to the N line, the identification precision of the characteristic point is poor. The above reasons lead to difficult acquisition of calibration images and lower accuracy of the calibration matrix obtained by calculation.
Disclosure of Invention
In view of the above problems, the present invention provides a convenient and easy-to-use bi-planar ultrasonic probe calibration system and a robust and high-precision calibration method.
In order to achieve the purpose, the invention adopts the following technical scheme: a biplane ultrasonic probe calibration system comprises an orthogonal biplane ultrasonic probe, a calibration module and a calibration module, wherein the orthogonal biplane ultrasonic probe is provided with two ultrasonic imaging sections, and the two sections are orthogonal to each other; a magnetic field generator for generating a magnetic field for electromagnetic localization; the rigid body of the magnetic navigation positioning receiver is fixed on the biplane ultrasonic probe, and in a magnetic field generated by the magnetic field generator, the spatial position and attitude parameters of the receiver can be acquired in real time, so that the receiver can be used for acquiring the spatial pose parameters of two ultrasonic sections after calibration; the magnetic navigation positioning probe can acquire the space position parameter of the needle tip in real time in a magnetic field generated by the magnetic field generator and is used for positioning and calibrating the phantom; calibrating the imitation body to be a cuboid; and the container and the fixing device are used for containing the ultrasonic imaging medium.
Furthermore, the calibration imitation body is made of resin materials, is integrally a cuboid, is divided into an upper area and a lower area, is provided with a square hole respectively, is provided with a plurality of inverted triangular through holes on the side wall of the hole, and is inserted with nylon wires.
Furthermore, the holes of the upper area and the lower area of the calibration phantom are punched on the side wall and penetrate through the lines to form 5 groups of N shapes. And for the upper and lower two areas of the phantom, 5 groups of N-shaped lines formed by respectively inserting nylon lines have the same shape and size and are vertical to each other. The upper half area is used for acquiring data required by the calibration of the sagittal plane of the ultrasonic probe, and the lower half area is used for acquiring data required by the calibration of the cross section of the ultrasonic probe.
Furthermore, the lines penetrating through the triangular through holes of the calibration phantom are made of nylon materials, and the diameter of the lines is close to the ultrasonic wavelength adopted by the ultrasonic probe, so that the intersection points of the lines and the nylon lines in the ultrasonic image are clearer.
Further, the container and the fixing device are high enough to contain the calibration imitation body and the fixing device. The fixing device can fix the calibration imitation body on the bottom of the container. The container can be used for containing hot water at the temperature of human body temperature and immersing a calibration phantom. Wave-absorbing materials are arranged on the four walls of the container and used for reducing ultrasonic reflection.
A calibration method based on a biplane ultrasonic probe calibration system comprises the following steps:
step (1) calibration data acquisition: acquiring original data required by calibration by using the biplane ultrasonic probe calibration system;
marking calibration data: automatically or non-automatically, identifying and marking coordinates of a plurality of points formed by intersecting the ultrasonic cross section and the N-shaped line of the calibration phantom according to a specific geometric relationship;
step (3) calibration data preprocessing: calculating a plurality of characteristic points intersected with the bevel edges of all the groups of N lines and coordinates under a simulator coordinate system according to a distance geometric relationship between a plurality of points formed by the intersection of the ultrasonic cross section and the N-shaped lines of the calibration simulator;
and (4) performing iterative optimization by using a least square method, removing outliers and calculating a calibration matrix: calculating a coordinate transformation matrix from a magnetic navigation receiver coordinate system to an ultrasonic image coordinate system;
step (5) the biplane calibration matrix is averaged: averaging calibration matrixes respectively obtained by the two probes according to prior information of the two imaging planes of the ultrasonic probe which are orthogonal to each other;
further, the specific process of the step (1) is as follows:
vertically fixing a calibration imitation in a container, and enabling an imaging medium to immerse the imitation;
step (1.2) establishing a magnetic field generator coordinate system by taking the magnetic field generator as a reference;
step (1.3) acquiring the positioning data of the imitation body by using a needle-type probe, wherein the positioning data respectively comprise coordinates of a plurality of points on the right surface, the upper surface and the front surface of the cuboid calibration imitation body;
step (1.4) ensuring that the magnetic navigation positioning receiver is fixed in the middle of the ultrasonic probe;
step (1.5) collecting cross section calibration data: the ultrasonic cross section is intersected with the N line of the lower half part of the calibration phantom.
Respectively adjusting 3 translational degrees of freedom and 3 rotational degrees of freedom of an ultrasonic probe and collecting a plurality of images on the premise of ensuring that the ultrasonic image is intersected with all the N-shaped lines and the intersection points are clear and visible;
step (1.6) of collecting sagittal plane calibration data: the sagittal plane of the ultrasound is intersected with the N line of the upper half of the calibration phantom.
Respectively adjusting 3 translational degrees of freedom and 3 rotational degrees of freedom of an ultrasonic probe and collecting a plurality of images on the premise of ensuring that the ultrasonic image is intersected with all the N-shaped lines and the intersection points are clear and visible;
further, the specific process of the step (2) is to consider that for foreground points in a single frame image, there are points ABC collinear, DEF collinear, GHI collinear, JKL collinear, MNO collinear, ADG collinear, BEH collinear, CFI collinear, and there are AC/'GI, and AG/' CI, so that the labeling can exclude the interference of other noise points not meeting the above rule, and more accurately identify foreground points according to the collinear and parallel rules
Further, the specific process of the step (4) is as follows:
step (4.1), establishing a magnetic field generator coordinate system t (transmitter) by taking the magnetic field transmitter as a reference, establishing an analog reconstruction coordinate system c (recovery) by taking the calibration analog as a reference, establishing a receiver coordinate system r (receiver) by taking the magnetic navigation positioning receiver as a reference, and establishing an image coordinate system i (image) by taking the cross-section ultrasonic image and the sagittal ultrasonic image as references, wherein for any characteristic point, coordinate transformation is performed: p is a radical ofc＝Tct*Ttr*Tri*pi；
Step (4.2), knowing parameter information of the calibration phantom, and calculating a transformation matrix Tct from a transmitter (transmitter) coordinate system to a phantom reconstruction coordinate system (reconstruction) by using coordinates of the magnetic field generator of 30 points on the right surface, the upper surface and the front surface of the cuboid phantom;
and (4.3) screening outliers according to the 5-point coplanarity. For 5 feature points in the calibration picture, the feature points should belong to the same plane. The 5-point magnetic field generator coordinate system is used to coordinate fit the planes. If the distance between any point and the plane is larger than a specific threshold value, discarding the frame of picture data;
step (4.4), using iterative least square fitting to convert a matrix Tri from an image (image) coordinate system to a sensor/receiver (receiver) coordinate system;
step (4.5), using the calibration matrix T obtained in the step 6riPerforming coordinate transformation on the existing key points, and reconstructing the coordinates of a simulation coordinate system of the existing key points;
step (4.6), regarding the calculated coordinates of the imitation coordinate system, points with larger difference with the true values are taken as outliers to be screened out;
and (4.7) repeating the steps (4.4) to (4.6) until no outlier is screened out, and obtaining a calibration matrix T of the cross section or the sagittal planeri。
Further, the specific process of the step (5) is
Step (5.1), regarding the cross section and the sagittal plane, the form of the calibration matrix is 4 x 4 matrix
Wherein
The physical meanings of the ultrasonic probe are direction vectors of the ultrasonic probe in the axis direction, right above and right to the receiver coordinate system respectively;
step (5.2), order
Construct new cross section, sagittal plane calibration matrix
Due to the adoption of the technical scheme, the invention has the following advantages:
1. the calibration phantom is standardized in design and low in manufacturing cost, and the accuracy of the coordinate of the inserted nylon line is ensured by the design of the inverted triangular hole on the side wall;
2. the N-shaped line of the calibration phantom is divided into an upper area and a lower area which are vertical to each other, so that the calibration phantom is suitable for the condition that the cross section and the sagittal plane of the biplane calibration phantom are vertical to each other. After the calibration phantom is fixed, the ultrasonic probe is vertically inserted into an imaging medium, N-shaped lines of an upper area and a lower area of the calibration phantom can be used for respectively acquiring calibration images required by two planes, and the ultrasonic imaging plane is approximately vertical to the N-shaped lines, so that characteristic points in the images are clearer and easier to identify;
3. the calibration image acquisition process systematically stipulates the movement mode of the probe in the image acquisition process, and respectively adjusts 3 translational degrees of freedom and 3 rotational degrees of freedom, so that a large amount of ultrasonic data under different poses are acquired, and the accuracy of calibration matrix solving is facilitated;
4. according to the calibration phantom positioning method, the operation difficulty of manually positioning the specific point is avoided and the stability of the phantom position calculation is improved by using a mode of respectively performing least square fitting on 3 planes of the rectangular phantom by using multi-point coordinates;
5. the specified threading mode of 5 groups of N-shaped lines of the calibration phantom provides vertical prior information of the N-shaped lines, and is beneficial to identifying key points during manual marking of a calibration image;
6. the calibration image preprocessing mode provided by the invention improves the calculation precision of the calibration matrix by screening outliers;
7. according to the biplane vertical calibration post-processing method, the calculation error of the calibration matrix is reduced by introducing the biplane vertical information of the ultrasonic probe.
Drawings
FIG. 1 is a schematic diagram of the placement positions of all hardware of the calibration system described in the present invention;
FIG. 2 is a diagram of the acquisition of cross-sectional calibration data of an ultrasonic probe (left drawing) using an N-shaped line at the lower half part of a calibration phantom; collecting the ultrasonic probe sagittal plane calibration data (right picture) by using the N-shaped line of the calibration phantom mulberry half part
FIG. 3 is a schematic diagram of several coordinate systems involved in the calibration process;
FIG. 4 is a perspective view of a calibration phantom described herein, excluding a set of wires interposed therebetween;
FIG. 5 is a front view of a calibration phantom described herein;
FIG. 6 is a right side view of the calibration phantom described herein;
FIG. 7 is a schematic view of a phantom with all sets of filaments interspersed;
FIG. 8 is a schematic diagram of a set of N-shaped lines interspersed in a phantom;
FIG. 9 is an example of a single frame sagittal plane calibration image acquired;
FIG. 10 is a flow chart of the overall calibration process.
In the figure: 1. the ultrasonic probe comprises an orthogonal biplane ultrasonic probe body, 2 magnetic field generators, 3 magnetic navigation positioning receivers, 4 ultrasonic scanning cross sections, 5 calibration imitations, 6 containers.
Detailed Description
The present invention is described in detail below with reference to the attached drawings. However, the drawings are only provided for a better understanding of the invention and they should not be construed as limiting the invention.
Example 1: referring to fig. 1, 4, 5, 6, and 7, a biplane ultrasound probe calibration system includes an orthogonal biplane ultrasound probe 1 having two ultrasound imaging sections 4, which are orthogonal to each other; a magnetic field generator 2 for generating a magnetic field for electromagnetic positioning; the rigid body of the magnetic navigation positioning receiver 3 is fixed on the biplane ultrasonic probe, and in a magnetic field generated by the magnetic field generator, the spatial position and attitude parameters of the receiver can be acquired in real time, so that the spatial position and attitude parameters of two ultrasonic sections can be acquired after calibration; the magnetic navigation positioning probe can acquire the space position parameter of the needle tip in real time in a magnetic field generated by the magnetic field generator and is used for positioning and calibrating the phantom; calibrating a dummy 5 which is a cuboid; container 6 and fixing device for holding ultrasonic imaging medium.
The calibration imitation body 5 is made of resin materials, as shown in fig. 4, 5, 6 and 7, the whole body is a cuboid, is divided into an upper area and a lower area, is provided with a square hole respectively, and the side wall of each hole is provided with a plurality of inverted triangular through holes which can be used for inserting silk threads.
As shown in FIG. 7, holes in the upper and lower regions of the calibration phantom 5 are punched in the side wall, and the wires are inserted to form 5 sets of N-shaped wires. And 5 groups of N-shaped lines which are respectively formed by inserting nylon lines in the upper area and the lower area of the dummy are the same in shape and size and are vertical to each other.
In the calibration phantom 5, as shown in fig. 7, the nylon threads are inserted to form 5 groups of N-shaped threads, wherein 3 groups of N-shaped threads are parallel to each other, that is, C, D, E3 groups of N-shaped threads in fig. 7; the other 2 groups of N-shaped lines are opposite to the direction of the oblique sides of the N-shaped lines, namely A, B2 groups of N-shaped lines in FIG. 7. From the inside out, there are A, B, C, D, E5 sets of N-shaped lines as shown in fig. 7.
The line passing through the triangular through holes of the calibration imitation body 5 is made of nylon materials, and the diameter of the line is close to the ultrasonic wavelength adopted by the ultrasonic probe.
The container 6 and the fixing device are high enough to contain the calibration imitation body and the fixing device. The fixing device can fix the calibration imitation body on the bottom of the container. The container can be used for containing hot water at the temperature of human body and immersing the calibration phantom. Wave-absorbing materials are arranged on the four walls of the container and used for reducing ultrasonic reflection.
Example 2: referring to fig. 2, 3, 8, 9 and 10, based on the above mentioned biplane ultrasound probe calibration system, the present invention also provides a calibration method based on the biplane ultrasound probe calibration system, which includes the following steps:
(1) and (3) calibration data acquisition: the original data required by calibration is collected by using the biplane ultrasonic probe calibration system, and concretely,
(1.1) vertically fixing the calibrated imitation body in a container, and immersing the imitation body in clean water. Heating water to reach the human body temperature of 37 ℃;
(1.2) establishing a magnetic field generator coordinate system by taking the magnetic field generator as a reference;
and (1.3) acquiring the phantom positioning data by using a needle probe. Namely, the coordinates of the magnetic field generator coordinate system of 30 points on the right surface, the upper surface and the front surface of the cuboid phantom are collected randomly;
(1.4) fixing the magnetic navigation positioning receiver in the middle of the ultrasonic probe;
(1.5) collecting cross section calibration data, as shown in figure 2 (left);
(1.5.1) vertically inserting the ultrasonic probe into the water, and immersing the first half section in the water. The position of the ultrasonic probe is adjusted, so that the visual field of the cross section of the ultrasonic probe is intersected with 5 groups of N lines on the lower half part of the phantom, and 15 bright spots corresponding to the 5 groups of N lines can be clearly seen on the cross section imaging;
(1.5.2) acquiring 360 images at the frequency of 0.3s per frame, and acquiring the pose data of the magnetic navigation positioning receiver. During the period, the ultrasonic probes are respectively translated in the x direction, the y direction and the N direction of a calibrated imitation coordinate system, the ultrasonic probe is rotated by taking the axial direction of the ultrasonic probe as an axis, the imaging plane of the ultrasonic probe faces the imitation, the pitching ultrasonic probe and the imaging plane of the ultrasonic probe face the imitation, the ultrasonic probe is rotated left and right, and 15 bright spots are ensured to be clearly visible;
(1.6) sagittal plane calibration data were collected as shown in FIG. 2 (right):
(1.6.1) vertically inserting the ultrasonic probe into the water, and immersing the first half section in the water. Adjusting the position of the ultrasonic probe, so that the sagittal view field of the ultrasonic probe is intersected with 5 groups of N lines on the lower upper half part of the phantom, and 15 bright spots corresponding to the 5 groups of N lines can be clearly seen on the cross section imaging;
(1.6.2) acquiring 360 images at the frequency of 0.3s per frame, and acquiring the pose data of the magnetic navigation positioning receiver. During the period, the ultrasonic probes are respectively translated in the x direction, the y direction and the N direction of a calibrated imitation coordinate system, the ultrasonic probe is rotated by taking the axial direction of the ultrasonic probe as an axis, the imaging plane of the ultrasonic probe faces the imitation, the pitching ultrasonic probe and the imaging plane of the ultrasonic probe face the imitation, the ultrasonic probe is rotated left and right, and 15 bright spots are ensured to be clearly visible;
(2) marking calibration data: after the calibration data is acquired, the image needs to be manually marked. As shown in fig. 9, taking the cross-sectional ultrasound cross-section calibration image as an example, the ultrasound cross-section intersects with 5 groups of N lines to form 15 bright points, i.e., a to O, and the pixel coordinates of the 15 points in the image coordinate system are manually marked. Further, following the 5 sets of N line interleaving pattern specified in FIG. 7, for the foreground points in the sagittal plot shown in FIG. 9, there are points ABC collinear, DEF collinear, GHI collinear, JKL collinear, MNO collinear, ADG collinear, BEH collinear, CFI collinear, and AC/DF/GI, and AG/BH/CI. When manual marking is carried out, the interference of other noise points which do not accord with the rule can be eliminated, and foreground points can be more accurately identified according to the collinear and parallel rules;
(3) calibration data preprocessing: and (3) calculating coordinates of 5 points formed by intersecting oblique sides of each group of N lines among 15 points formed by intersecting the ultrasonic cross section and 5 groups of N lines under a calibrated phantom coordinate system. As shown in FIG. 6, a set of N lines, for example, includes 3 sides, AC, EG, IK, where point A, C, E, G, I, K is the intersection of the N lines with the inner wall of the calibration phantom. During the calibration image acquisition, the ultrasonic cross-section BFJ intersects the set of N lines at B, F, J three points. When AC and EG extension lines intersect at a point D and EG and IK extension lines intersect at a point H, there are Δ BDF to Δ JHF, and there are
On the premise that the shape of the calibration phantom is known, the coordinates of the points D and H are easy to calculate. Then there is
(4) And (3) performing iterative optimization by a least square method, removing outliers and calculating a calibration matrix: calculating a coordinate transformation matrix from a magnetic navigation receiver coordinate system to an ultrasonic image coordinate system;
(4.1) establishing a magnetic field generator coordinate system T (transmitter) by taking a magnetic field transmitter as a reference, establishing a simulated body reconstruction coordinate system C (recovery) by taking a calibrated simulated body as a reference, establishing a receiver coordinate system R (receiver) by taking a magnetic navigation positioning receiver as a reference, and establishing an ultrasonic image coordinate system I (image) by taking a transverse-section ultrasonic image and a sagittal-section ultrasonic image as references, wherein for any characteristic point, coordinate transformation is performed: p is a radical ofc＝Tct*Ttr*Tri*pi；
(4.2) knowing the parameter information of the calibration phantom, calculating a transformation matrix T from a transmitter coordinate system to a phantom reconstruction coordinate system (reconstruction) by using the coordinates of the magnetic field generator coordinate system of 30 points on the right surface, the upper surface and the front surface of the rectangular parallelepiped phantom which are acquiredct；
(4.2.1) for each 30 points of the right, top and front surfaces of the phantom, respectively, a least squares fit plane equation a x + B y + C N-1 ═ 0 was calculated for each of the 3 planes, and 3 sets of plane parameters a1, B1, C1, a2, B2, C2, A3, B3, C3 were calculated for each of the 3 planes;
(4.2.2) using 3 groups of plane parameters to calculate coordinates of a magnetic field generator coordinate system of the right upper corner point of the front face of the cuboid calibration phantom in a joint manner;
(4.2.3) respectively calculating coordinates of the magnetic field generator in the positive directions of the x, y and z axes of the cuboid calibration phantom by using 3 groups of plane parameters;
(4.2.4) calculating a conversion matrix Tct from a magnetic field generator T coordinate system to a phantom reconstruction coordinate system C by using 1 point coordinate and 3 direction vectors shown in 2.2 and 2.3, and fitting 4 homogeneous coordinates by a least square method;
and (4.3) screening the outliers according to the 5-point coplanarity. For 5 points in each picture, they should belong to the same plane. The plane was fitted using 5-point coordinates:
(4.3.1) for each calibration image, calculating coordinates of a magnetic field generator coordinate system of 5N-line bevel edge intersection points, and fitting a plane;
(4.3.2) respectively calculating the distance between 5 points and the plane;
(4.3.3) if the distance between any point and the plane is greater than a specific threshold value, discarding all data of the frame of calibration image and not participating in calculation any more;
(4.4) transformation matrix T from image (image) to sensor/receiver (receiver) coordinate system using iterative least squares fittingri：
(4.4.1) in the step of preprocessing the calibration data, calculating the coordinate p of the magnetic field generator coordinate system by the coordinate of the simulated reconstruction coordinate system of all the marked feature pointst＝inverse(Tct)*pc；
(4.4.2) calculating the coordinates p of the coordinate system of the receiver for the coordinates of the coordinate system of each magnetic field generator in the above stepr＝inverse(Ttr)*ptWherein, TtrObtaining a conversion matrix from a magnetic navigation positioning receiver to a transmitter by magnetic navigation positioning equipment;
(4.4.3) randomly selecting a group of 6 parameters to construct an initial rigid body transformation matrix, wherein the group of 6 parameters comprises rotation of gamma angle along an x axis, rotation of beta angle along a y axis, rotation of alpha angle along an N axis and translation (x, y, z);
(4.4.4), modifying matrix transformation parameters alpha, beta, gamma, x, y and N, and reconstructing a transformation matrix;
(4.4.5), calculation
(4.4.6) calculating the difference
(4.4.7), if the difference is smaller, keeping the iterative modification of the step;
and (4.4.8) repeating the steps (4.4.4) to (4.4.7) until the error is converged.
(4.5) using the calibration matrix T obtained in the above step 6riPerforming coordinate transformation on the existing key points, and reconstructing the coordinates of a simulation coordinate system of the existing key points;
(4.6) regarding the points with larger difference with the true value of the simulated body reconstruction coordinate system coordinates obtained by calculation, screening out the points as outliers;
(4.7) repeating (4.4) to (4.6) until no outliers are screened out, resulting in a calibration matrix T for the transverse or sagittal planeri；
(5) And averaging a biplane calibration matrix: averaging calibration matrixes respectively obtained by the two probes according to prior information of the two imaging planes of the ultrasonic probe which are orthogonal to each other;
(5.1) for the transverse and sagittal planes, the calibration matrix is 4 x 4 matrix
Wherein
The physical meanings of the ultrasonic probe are direction vectors of the ultrasonic probe in the axis direction, right above and right to the receiver coordinate system respectively;
(5.2) order
Construct new cross section, sagittal plane marking matrix
The embodiments of the present invention have been described in detail with reference to the drawings, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the gist of the present invention.
Claims (10)
1. A biplane ultrasonic probe calibration system is characterized in that the calibration system comprises
The orthogonal biplane ultrasonic probe is provided with two ultrasonic imaging sections which are vertical to each other and can be respectively intersected with the two groups of silk thread groups of the calibration phantom;
the magnetic field generator is used for generating a magnetic field for electromagnetic positioning, is arranged on one side of the container and ensures that the magnetic field range can cover a magnetic navigation positioning receiver and a magnetic navigation positioning probe on the ultrasonic probe;
the magnetic navigation positioning receiver is fixed in the middle of the biplane ultrasonic probe through a rigid body, and can acquire self spatial position and attitude parameters in real time in a magnetic field generated by the magnetic field generator;
the magnetic navigation positioning probe can acquire the space position parameter of the needle tip in real time in a magnetic field generated by the magnetic field generator and is used for positioning and calibrating the phantom;
the calibration imitation body is a cuboid, is fixed at the bottom of the container and is immersed by the ultrasonic imaging medium in the container; the calibration imitation body is divided into an upper part and a lower part, and N-shaped wire groups are respectively inserted in the upper part and the lower part;
and the container and the fixing device are used for containing the ultrasonic imaging medium.
2. The bi-planar ultrasonic probe calibration system of claim 1, wherein the calibration phantom 5 is made of resin, and is a rectangular solid divided into an upper region and a lower region, each region has a square hole, the side wall of each hole has an inverted triangular through hole, and a nylon wire is inserted in the through holes.
3. The biplane ultrasonic probe calibration system according to claim 2, wherein holes in the upper and lower regions of the calibration phantom 5 are perforated in the side wall and inserted with lines to form 5 groups of N-shaped lines, and the 5 groups of N-shaped lines formed by respectively inserting nylon lines in the upper and lower regions of the phantom are identical in shape and size and perpendicular to each other.
4. The system for calibrating a biplane ultrasound probe according to claim 3, wherein the nylon threads are interlaced to form 5 sets of N-shaped threads, wherein 3 sets of N-shaped threads are parallel to each other, and the other 2 sets of N-shaped threads are opposite to the hypotenuse of the N-shaped threads.
5. The bi-planar ultrasound probe calibration system of claim 2, wherein the lines passing through the triangular through holes are made of nylon and have a diameter similar to the ultrasound wavelength used by the ultrasound probe.
6. A calibration method based on any biplane ultrasonic probe calibration system in claims 1-5, characterized by comprising the following steps:
step (1) calibration data acquisition: acquiring original data required by calibration by using a biplane ultrasonic probe calibration system;
marking calibration data: automatically or non-automatically, identifying and marking coordinates of a plurality of points formed by intersecting the ultrasonic cross section and the N-shaped line of the calibration phantom according to a specific geometric relationship;
step (3) calibration data preprocessing: according to the distance geometric relationship of a plurality of points formed by the intersection of the ultrasonic section and the N-shaped line of the calibration phantom, calculating a plurality of characteristic points formed by the intersection of the ultrasonic section and the bevel edge of each group of N lines and the coordinates of the characteristic points in the phantom coordinate system;
and (4) performing iterative optimization by using a least square method, removing outliers and calculating a calibration matrix: calculating a coordinate transformation matrix from a magnetic navigation receiver coordinate system to an ultrasonic image coordinate system;
step (5), averaging a biplane calibration matrix: and averaging the calibration matrixes respectively obtained by the two probes according to the prior information of the mutual orthogonality of the two imaging planes of the ultrasonic probe.
7. The calibration method based on the biplane ultrasonic probe calibration system as claimed in claim 6, wherein the specific process of the step (1) is as follows:
vertically fixing a calibration imitation in a container, and enabling an imaging medium to immerse the imitation;
step (1.2) establishing a magnetic field generator coordinate system by taking the magnetic field generator as a reference;
step (1.3) acquiring the positioning data of the imitation body by using a needle-type probe, wherein the positioning data respectively comprise coordinates of a plurality of points on the right surface, the upper surface and the front surface of the cuboid calibration imitation body;
step (1.4) ensuring that the magnetic navigation positioning receiver is fixed in the middle of the ultrasonic probe;
step (1.5) collecting cross section calibration data: intersecting the ultrasonic cross section with the N line of the lower half part of the calibration phantom; respectively adjusting 3 translational degrees of freedom and 3 rotational degrees of freedom of an ultrasonic probe and collecting a plurality of images on the premise of ensuring that the ultrasonic image is intersected with all the N-shaped lines and the intersection points are clear and visible;
step (1.6) of collecting sagittal plane calibration data: the ultrasonic sagittal plane is intersected with the N lines of the upper half part of the calibration phantom, 3 translational degrees of freedom and 3 rotational degrees of freedom of the ultrasonic probe are respectively adjusted on the premise of ensuring that the ultrasonic image is intersected with all the N lines and the intersection points are clearly visible, and a plurality of images are acquired.
8. The bi-planar ultrasound probe calibration system-based calibration method according to claim 6, wherein the specific process of step (2) is to consider that for foreground points in a single frame image, there are points ABC collinear, DEF collinear, GHI collinear, JKL collinear, MNO collinear, ADG collinear, BEH collinear, CFI collinear, and there are AC | | | DF | | | GI, and AG | | | | BH | | | CI, then eliminate the interference of other noise points that do not meet the above rules during labeling, and identify foreground points more accurately according to the collinear and parallel rules.
9. The calibration method based on the biplane ultrasonic probe calibration system as claimed in claim 6, wherein the specific process of the step (4) is as follows:
step (4.1) establishing a magnetic field generator coordinate system T (transmitter) by taking a magnetic field transmitter as a reference, establishing a simulated body reconstruction coordinate system C (recovery) by taking a calibrated simulated body as a reference, establishing a receiver coordinate system R (recovery) by taking a magnetic navigation positioning receiver as a reference, establishing an image coordinate system I (image) by taking a cross section ultrasonic image and a sagittal plane ultrasonic image as references, and carrying out coordinate transformation on any characteristic point: p is a radical ofc＝Tct*Ttr*Tri*pi；
Step (4.2) knowing parameter information of the calibration phantom, and calculating a transformation matrix Tct from a transmitter (transmitter) coordinate system to a phantom reconstruction coordinate system (reconstruction) by using coordinates of the magnetic field generator of 30 points on the right surface, the upper surface and the front surface of the cuboid phantom;
step (4.3) screening outliers according to 5-point coplanarity, wherein 5 characteristic points in the calibrated picture belong to the same plane, fitting the plane by using the coordinates of the magnetic field generator coordinate system of the 5 points, and discarding the picture data of the frame if the distance between any one point and the plane is calculated to be larger than a specific threshold value;
step (4.4) using iterative least squares fitting to transform matrix Tri from image (image) coordinate system to sensor/receiver (receiver) coordinate system;
step (4.5) using the calibration matrix T _ { ri } obtained in the step 6 to perform coordinate transformation on the existing key points and reconstruct the coordinates of the simulated coordinate system of the key points;
step (4.6) for the calculated coordinates of the imitation coordinate system, points with larger difference with the true values are taken as outliers to be screened out;
step (4.7) repeating steps (4.4) - (4.6) until no outlier is screened out, and obtaining the calibration matrix T of the cross section or the sagittal planeri。
10. The calibration method based on the biplane ultrasonic probe calibration system as claimed in claim 6, wherein the specific process of the step (5) is as follows:
step (5.1), regarding the cross section and the sagittal plane, the calibration matrix form is 4-4 matrix
Wherein
The physical meanings of the ultrasonic probe are direction vectors of the ultrasonic probe in the axis direction, right above and right to the receiver coordinate system respectively;
step (5.2), order
Construct new cross section, sagittal plane calibration matrix
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