N-line calibration method for annular ultrasonic transducer

Abstract

The invention discloses an N-line calibration method for an annular ultrasonic transducer, and the method comprises the steps: shooting a model calibration plate through a camera, and determining the pose of the model calibration plate in a camera coordinate system through an optical tracking method; the relative positions of the N-line model and the camera are kept unchanged, the annular ultrasonic transducers are moved, two-dimensional ultrasonic images obtained by the multiple sets of annular ultrasonic transducers for N-line scanning are obtained, and the camera synchronously records the pose of the transducer calibration plate in a camera coordinate system at the corresponding moment; according to the two-dimensional ultrasonic image obtained by scanning the N line by each group of annular ultrasonic transducers and the pose of the model calibration plate in the camera coordinate system, calculating to obtain the position coordinate of the N line imaging point in the camera coordinate system; according to the position coordinates of the N-line imaging points in the camera coordinate system and the poses, recorded by the camera, of the corresponding transducer calibration plates in the camera coordinate system, a transformation matrix from the ultrasonic image coordinate system to the transducer calibration plate coordinate system is obtained through calculation. The calibration method is simple and easy to implement.

Description

Technical Field

[0001] The invention relates to a probe calibration method, in particular to an N-line calibration method for an annular ultrasonic transducer. Background Art

[0002] Ultrasound probe calibration is a crucial link in the field of ultrasound imaging, which ensures the accuracy, reliability and repeatability of ultrasound images. Ultrasound probe calibration is to determine the position, direction and sound beam characteristics of the probe in space so as to make necessary corrections during image reconstruction. In three-dimensional ultrasound imaging, the calibration accuracy of the probe is crucial to the imaging quality of three-dimensional ultrasound images, which directly affects the positioning accuracy of the three-dimensional positioning system. Only by accurately calibrating the ultrasound probe can we determine the exact position of the two-dimensional ultrasound image in three-dimensional space.

[0003] Calibration using the N-line model is a widely used ultrasonic transducer calibration technology, but the existing N-line model is mainly used to calibrate small-sized linear array or area array probes. For large-sized annular transducers, calibration is more difficult, so that most three-dimensional annular array ultrasonic imaging systems still use mechanical positioning methods for three-dimensional positioning. Summary of the invention

[0004] Purpose of the invention: The technical problem to be solved by the present invention is to provide an N-line calibration method for an annular ultrasonic transducer in view of the deficiencies in the prior art.

[0005] In order to solve the above technical problems, the present invention discloses an N-line calibration method for an annular ultrasonic transducer. The calibration method is applied to a calibration system. The calibration system includes an annular ultrasonic transducer, a transducer calibration plate installed on the annular ultrasonic transducer, a camera, an N-line model, and a model calibration plate installed on the N-line model. The calibration method includes the following steps:

[0006] Step 1, calibrating the relative positions between different arc-shaped ring parts in the annular ultrasonic transducer;

[0007] Step 2, the camera photographs the model calibration plate, and determines the position and posture of the model calibration plate in the camera coordinate system by an optical tracking method;

[0008] Step 3: Keep the relative position between the N-line model and the camera unchanged, move the annular ultrasonic transducer, obtain multiple sets of two-dimensional ultrasonic images obtained by scanning the N-line by the annular ultrasonic transducer, and the camera synchronously records the position P of the transducer calibration plate in the camera coordinate system at the corresponding moment. c ;

[0009] Step 4: according to the two-dimensional ultrasonic image obtained by each group of the annular ultrasonic transducer for N line scanning and the position P of the model calibration plate in the camera coordinate system c , calculate and obtain the position coordinates of the N-line imaging points in the camera coordinate system;

[0010] Step 5, iteratively calibrate the transformation matrix between the ultrasound image coordinate system and the transducer calibration plate coordinate system according to the position coordinates of the N-line imaging points in the camera coordinate system and the position and posture of the transducer calibration plate in the camera coordinate system at the corresponding moment recorded by the camera.

[0011] Specifically, the camera is located at one end of the annular ultrasonic transducer, and the transducer calibration plate is installed on the annular ultrasonic transducer and on the same side as the camera; the N-line model is located in the inner ring space of the annular ultrasonic transducer; the model calibration plate is installed on the N-line model and on the same side as the camera; the annular ultrasonic transducer, the N-line model, the transducer calibration plate and the model calibration plate are all located in the field of view of the camera.

[0012] Specifically, the annular ultrasonic transducer is an annular array composed of more than three arc-shaped annular portions, and each arc-shaped annular portion is composed of a plurality of transducer array elements arranged in sequence along the arc length.

[0013] Specifically, the transducer calibration plate and the model calibration plate are respectively ArUco calibration plates, and the ArUco calibration plate includes a calibration plate body and an asymmetric circular calibration plate pattern and four ArUco codes arranged on the calibration plate body, the four ArUco codes enclose a rectangle for determining the image of the ArUco calibration plate in the image taken by the camera, and the asymmetric circular calibration plate pattern is located in the rectangle enclosed by the four ArUco codes; the asymmetric circular calibration plate pattern includes a single-row array of multiple rows of circular patterns, and the single-row arrays of two adjacent rows of circular patterns are staggered.

[0014] Specifically, the N-line model includes a top plate, a bottom plate, and a plurality of support columns connecting the top plate and the bottom plate, the plurality of support columns, the top plate, and the bottom plate together form a polygonal column shape, and the number of the support columns is the same as the number of the arc-shaped ring parts; the bottom of the top plate and the top of the bottom plate are provided with threading holes, and by threading, M groups of N lines are formed on each side of the N-line model, wherein; the M groups of N lines are arranged at intervals along a direction toward the middle of the top plate; the top of the top plate is provided with a groove for installing the model calibration plate.

[0015] Specifically, the method for determining the position and posture of the model calibration plate in the camera coordinate system by optical tracking method described in step 2 comprises: using an ArUco calibration block detection algorithm to detect the ArUco code (23) in the image captured by the camera, and calculating the image coordinates of the four ArUco codes on the model calibration plate; determining the image of the model calibration plate in the image captured by the camera according to a rectangle formed by the four ArUco codes; using a contour detection algorithm to detect the contours of all circular patterns in the asymmetric circular calibration plate pattern on the model calibration plate, and calculating the center coordinates of all circular patterns; and calculating the position and posture of the model calibration plate in the camera coordinate system according to the center coordinates.

[0016] Specifically, the camera in step 3 synchronously records the position and posture of the transducer calibration plate in the camera coordinate system at the corresponding moment, including:

[0017] While collecting and generating a two-dimensional ultrasonic image, the camera emits a trigger signal each time a photo is taken; after receiving the trigger signal, the annular ultrasonic transducer starts collecting once to achieve synchronization between the two-dimensional ultrasonic image and the transducer calibration plate image taken by the camera; during the collection process, the model calibration plate installed on the N-line model is blocked or later cut off to avoid errors in identifying the transducer calibration plate.

[0018] Specifically, the specific process of step 4 is:

[0019] The transformation matrix Rt from the N-line model coordinate system to the camera coordinate system is calculated based on the pose of the model calibration plate in the camera coordinate system. m2c ;

[0020] According to the two-dimensional ultrasonic image obtained by each group of the annular ultrasonic transducer for N-line scanning, the position coordinates P of each group of N-line imaging points in the ultrasonic image coordinate system are obtained. i ;

[0021] According to the position coordinates P of each of the N-line imaging points in the ultrasound image coordinate system i and the proportional relationship of similar triangles, the position coordinates P of the N-line imaging point in the N-line model are calculated m ;

[0022] According to the position coordinates P of the N-line imaging point in the N-line model m And the transformation matrix Rt from the N-line model coordinate system to the camera coordinate system m2c , calculate the position coordinates P of the N-line imaging point in the camera coordinate system c .

[0023] Specifically, according to the two-dimensional ultrasonic image obtained by scanning N lines by each group of the annular ultrasonic transducer, the position coordinates P of each group of N line imaging points in the ultrasonic image coordinate system are obtained. i include:

[0024] The obtained two-dimensional ultrasonic image is subjected to Gaussian filtering, binarization and corrosion and dilation processing in sequence, and then the position coordinates of N-line imaging points in the two-dimensional ultrasonic image after corrosion and dilation processing are extracted, and finally the position coordinates of 3M groups of N-line imaging points in the two-dimensional ultrasonic image are obtained.

[0025] Specifically, the specific process of step 5 is:

[0026] The transformation matrix Rt from the transducer calibration plate coordinate system to the camera coordinate system is calculated according to the position and posture of the transducer calibration plate in the camera coordinate system: w2c ;

[0027] The transformation matrix Rt from the ultrasonic image coordinate system to the transducer calibration plate coordinate system is obtained by measuring the transducer calibration plate and the annular ultrasonic transducer. i2w The initial parameters of

[0028] The position coordinates P of each group of N line imaging points in the ultrasound image coordinate system are i The transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w And the transformation matrix Rt from the transducer calibration plate coordinate system to the camera coordinate system w2c After transformation, the corresponding N-line imaging point position coordinates P in the camera coordinate system are obtained c ′, the calculation formula is as follows:

[0029] P c ′=Rt w2c ·Rt i2w ·P i ;

[0030] The position coordinates P of the N-line imaging point in the camera coordinate system c The position coordinates P of the N-line imaging point obtained after the transformation in the camera coordinate system c The error between ′ is used as the objective function, and the objective function is optimized by the RMSprop algorithm to transform the transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w Perform iterative calibration and finally obtain the transformation matrix Rt i2w The best parameters.

[0031] Beneficial effects:

[0032] The present invention proposes an N-line calibration method for an annular ultrasonic transducer. For the annular ultrasonic transducer, an adaptive N-line model is designed and a corresponding calibration system is built. The designed N-line model is scanned, and the error between the position of the N-line imaging point in the two-dimensional ultrasonic image obtained by scanning and the actual position in the camera coordinate system is used as the objective function for iteration to obtain the optimal parameters of the transformation matrix from the ultrasonic image coordinate system to the transducer calibration plate coordinate system. Through the above steps, the present invention implements an N-line calibration method for an annular ultrasonic transducer array, solving the problem of difficulty in probe calibration when the current large-size annular ultrasonic transducer is used for three-dimensional ultrasonic imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the above and other advantages of the present invention will become more clear.

[0034] Figure 1 It is a schematic diagram of the structure of the calibration system provided by the first embodiment of the present invention.

[0035] Figure 2 It is a schematic diagram of the three-dimensional structure of the annular ultrasonic transducer provided in the first embodiment of the present invention, wherein α=110°, β=10°, and R=75 mm.

[0036] Figure 3 It is a schematic diagram of the three-dimensional structure of an N-line model of an annular ultrasonic transducer provided in the first embodiment of the present invention.

[0037] Figure 4 Schematic diagram of the ArUco calibration plate provided in the first embodiment of the present invention.

[0038] Figure 5 It is a flowchart of the N-line calibration method provided by the first embodiment of the present invention.

[0039] Figure 6 1 is a schematic diagram of N-line imaging point position calculation involved in the present invention, wherein the dotted line represents the ultrasound plane.

[0040] Figure 7 This is a physical photo of an N-line model and a ring-shaped ultrasonic transducer provided in one embodiment of the present invention.

[0041] Figure 8 yes Figure 7 Actual photo of the N-line model shown in Figure 1.

[0042] Fig. 9 yes Figure 7 Actual photo of the N line model shown in 2.

[0043] The description of the reference numerals in the accompanying drawings is: 1. annular ultrasonic transducer; 11. arc-shaped ring part; 111. transducer array element; 2. transducer calibration plate; 21. calibration plate body; 22. asymmetric circular calibration plate pattern; 23. ArUco code; 3. camera; 4. N-line model; 41. top plate; 42. bottom plate; 43. support column; 44. threading hole; 45. N-line; 46. groove; 5. model calibration plate. DETAILED DESCRIPTION

[0044] Example 1

[0045] Embodiment 11 of the present invention provides an N-line calibration method for an annular ultrasonic transducer, and the calibration method is applied to a calibration system.

[0046] See also Figures 1 to 7 The calibration system includes a ring-shaped ultrasonic transducer 1, a transducer calibration plate 2 installed on the ring-shaped ultrasonic transducer 1, an N-line model 4, a model calibration plate 5 installed on the N-line model 4, and a camera 3. Figure 7 This is a physical photo of an N-line model and a ring-shaped ultrasonic transducer provided in one embodiment of the present invention. Figure 8 and Fig. 9 They are Figure 7 Actual photos 1 and 2 of the N line model are shown in .

[0047] See now Figure 2 and Figure 7 The annular ultrasonic transducer 1 is an annular array composed of three arc-shaped ring parts 11. Each arc-shaped ring part 11 is composed of 64 transducer array elements 111 arranged in sequence along the arc length. The radius R of the annular ultrasonic transducer 1 is 7.5cm, the center frequency is 2MHz, and the bandwidth is 400KHz. The size of a single transducer array element is 12mm×2.048mm, and the material is PZT. According to the design when the transducer is manufactured, ideally, when there is no internal assembly error, the distance between each two adjacent transducer array elements 111 is 1.5mm. The included angle, that is, the central angle α corresponding to each arc-shaped ring portion 11 is 110°, and the interval β between the edges of the two arc-shaped ring portions 11 is 10°.

[0048] See also Figure 3 , Figure 8 and Fig. 9The N-line model 4 includes a top plate 41, a bottom plate 42, and three support columns 43 connecting the top plate 41 and the bottom plate 42. The three support columns 43, the top plate 41, and the bottom plate 42 together form a triangular prism shape. The number of the support columns 43 is the same as the number of the arc-shaped ring portion 11. Threading holes 44 are provided at the bottom of the top plate 41 and the top of the bottom plate 42. By threading, M groups of N lines 45 are formed on each side of the N-line model, where M≥3. For the M groups of N lines located on the same side, the plane where any N line is located is arranged parallel to the plane where any other N line is located. A total of 3M groups of N lines can be obtained for the entire N-line model. For each side of the N-line model, the M groups of N lines on that side are arranged at intervals in the direction toward the middle of the top plate 41. For example, Figure 3 Not all N lines are shown, only one layer of N lines is shown on each side, and there should be M layers. A groove for mounting the model calibration plate 5 is provided on the top of the top plate 41, and the model calibration plate 5 is mounted in the groove 46. The model calibration plate 5 is used to determine the pose of the N line model in the camera coordinate system, and then calculate the position coordinates of the N line imaging point in the camera coordinate system.

[0049] It should be understood that Figure 3 The enclosed part 14 forms a triangular prism, but the polygonal prism formed by the N-line model is not limited to a triangular prism, but can also be other polygonal prisms, as long as the polygonal prism has a number of sides that matches the number of arc-shaped ring parts 11, and each side is correspondingly provided with M groups of N lines.

[0050] In a specific embodiment, see Figure 1 , the axis of the polygonal prism formed by the N-line model can be set parallel to the axis of the annular ultrasonic transducer 1 to facilitate scanning. It should be understood that in N-line calibration, the design of the N-line is to calculate the position of the imaging point even if it is not parallel. Therefore, when simplified calculation is not considered, the axis of the polygonal prism formed by the N-line model and the axis of the annular ultrasonic transducer 1 may not be parallel.

[0051] It should be understood that the number of arc-shaped ring parts 11 and the number of transducer array elements 111 in the annular ultrasonic transducer 1 can be increased or decreased as required.

[0052] See also Figure 1, the camera 3 is located at one end of the annular ultrasonic transducer 1, and the transducer calibration plate 2 is installed on the annular ultrasonic transducer 1 and on the same side as the camera 3. The N-line model 4 is located in the inner ring space of the annular ultrasonic transducer 1. The model calibration plate 5 is installed in the groove of the N-line model 4 and on the same side as the camera 3. The annular ultrasonic transducer 1, the N-line model 4, the transducer calibration plate 2 and the model calibration plate 5 are all located in the field of view of the camera 3. Each side of the N-line model is arranged in a one-to-one correspondence with each arc-shaped ring portion 11 of the annular ultrasonic transducer 1.

[0053] See also Figure 4 , the transducer calibration plate 2 and the model calibration plate 5 can both be ArUco calibration plates. Figure 4 , the ArUco calibration plate includes a calibration plate body 21 and an asymmetric circular calibration plate pattern 22 and four ArUco codes 23 arranged on the calibration plate body 21. The calibration plate body 21 can be composed of a glass substrate and an alumina panel, and the whole is rectangular. The asymmetric circular calibration plate pattern 22 and the four ArUco codes 23 can be set on the calibration plate body 21 by printing, wherein the four ArUco codes 23 are respectively set at the four corners of the calibration plate body 21. ArUco, the full name of Augmented Reality University of Cordoba. ArUco code is a binary square reference mark for camera 3 posture estimation. Its main advantages are simple and fast detection and strong robustness. The four ArUco codes 23 enclose a rectangle for determining the image of the ArUco calibration plate in the image taken by the camera 3, and the asymmetric circular calibration plate pattern 22 is located in the rectangle enclosed by the four ArUco codes 23.

[0054] The asymmetric circular calibration plate pattern 22 includes multiple rows of circular pattern single-row arrays, and two adjacent rows of circular pattern single-row arrays are arranged alternately. This embodiment uses the design of an asymmetric circular calibration plate pattern to facilitate detection of the calibration plate orientation and avoid misidentification as a mirror image or inversion.

[0055] In Example 1 of the present invention, the size of the ArUco calibration plate is 60mm×85mm and the thickness is 3mm. If the number of circular patterns in the asymmetric circular calibration plate pattern 22 is too small, the accuracy of the position calculation will be affected. If the number is too large, the amount of calculation will be too large, affecting the real-time performance of the system. Therefore, taking all factors into consideration, the asymmetric circular calibration plate pattern 22 includes 11 rows of single-row arrays of circular patterns, and each row of single-row arrays of circular patterns includes 4 circular patterns. The diameter of the circular pattern is 4mm. The center distance between two adjacent circular patterns in the same row is 12mm. The column distance between two adjacent rows of single-row arrays of circular patterns is also 12mm.

[0056] The annular ultrasonic transducer 1 can be moved manually or electrically. When the electric method is adopted, the calibration system can also include a driving transmission device and a calibration controller. The driving transmission device is used to drive the annular ultrasonic transducer 1 to move. The calibration controller is connected to the driving transmission device, the camera 3 and the annular ultrasonic transducer 1.

[0057] See also Figure 5 The calibration method for the annular ultrasonic transducer provided in Embodiment 1 of the present invention comprises the following steps:

[0058] Step 1, calibrating the relative positions of different arc-shaped ring parts 11 in the annular ultrasonic transducer 1;

[0059] Specifically, before the N-line calibration, the relative positions between the transducer array elements need to be calibrated first, so as to facilitate the subsequent ultrasonic imaging of the entire N-line model and improve the imaging quality. Since the relative positions of the transducer array elements inside each arc-shaped ring portion 11 are known, the relative positions between the different arc-shaped ring portions 11 in the annular ultrasonic transducer 1 need only be calibrated.

[0060] Step 2, the camera 3 photographs the model calibration plate 5, and determines the position and posture of the model calibration plate 5 in the camera coordinate system by an optical tracking method;

[0061] Step 3, keeping the relative position of the N-line model 4 and the camera 3 unchanged, moving the annular ultrasonic transducer 1, obtaining multiple groups of two-dimensional ultrasonic images obtained by the annular ultrasonic transducer 1 for N-line scanning, and the camera 3 synchronously records the position and posture of the transducer calibration plate 2 in the camera coordinate system at the corresponding moment.

[0062] Specifically, the N-line model 4 is located in the ring of the annular ultrasonic transducer 1. During the entire process of scanning the N-line, the position of the N-line model 4 is kept unchanged, and the annular ultrasonic transducer 1 is driven up and down by the driving transmission device to scan the N-line model 4.

[0063] Step 4, calculating the position coordinates of the N-line imaging points in the camera coordinate system according to the two-dimensional ultrasonic images obtained by each group of the annular ultrasonic transducer 1 for N-line scanning and the pose of the model calibration plate 5 in the camera coordinate system;

[0064] Specifically, the N-line imaging points are the intersections of the ultrasonic plane of the annular ultrasonic transducer 1 and the N lines. The ultrasonic plane of the annular ultrasonic transducer 1 and each group of N lines will produce three intersections, which are presented as bright spots in the two-dimensional ultrasonic image collected by the annular ultrasonic transducer 1. Therefore, for the annular ultrasonic transducer 1 in this embodiment, there are 9M bright spots in the two-dimensional ultrasonic image formed by it, which serve as subsequent extraction targets.

[0065] Step 5, iteratively calibrate the transformation matrix between the ultrasound image coordinate system and the transducer calibration plate coordinate system according to the position coordinates of the N-line imaging points in the camera coordinate system and the position and posture of the transducer calibration plate 2 in the camera coordinate system at the corresponding moment recorded by the camera 3.

[0066] In one embodiment of the present invention, the relative positions of the three arc-shaped ring parts 11 are calibrated using the N-line model, and include the following steps:

[0067] Each arc ring portion 11 images the M groups of N lines facing it separately, and each arc ring portion can obtain 3M N-line imaging points. Since the model parameters of the N-line model are known, the positions of the N-line imaging points in the N-line model can also be calculated, and the method will be described in the subsequent step 4. The relative positions between the three arc ring portions 11 can be obtained by using the relative position relationship between the N-line imaging points formed by the three arc ring portions 11. Multiple scans can be performed to obtain multiple groups of images, and the calibration accuracy can be improved by averaging the calculated calibration results.

[0068] In Example 1 of the present invention, the method for determining the position and posture of the model calibration plate 5 in the camera coordinate system by optical tracking method described in step 2 includes: using the ArUco calibration block detection algorithm to detect the ArUco code 23 in the image taken by the camera 3, and obtaining the image coordinates of four ArUco codes 23 on the model calibration plate 5; determining the image of the model calibration plate 5 in the image taken by the camera 3 according to the rectangle formed by the four ArUco codes 23; then using the contour detection algorithm to detect all circular pattern contours in the asymmetric circular calibration plate pattern 22 in the model calibration plate 5, and calculating the center coordinates of all circular patterns; and calculating the position and posture of the model calibration plate 5 in the camera coordinate system according to the center coordinates.

[0069] The ArUco calibration block detection algorithm is described in S. Garrido-Jurado, R. Mu noz Salinas, FJ Madrid-Cuevas, and MJ Marín-Jiménez. Automatic generation and detection of highly reliable fiducial markers under occlusion. Pattern Recognition, 47(6): 2280-2292, 2014.

[0070] In Embodiment 1 of the present invention, the camera 3 in step 3 synchronously records the position and posture of the transducer calibration plate 2 in the camera coordinate system at the corresponding moment, including:

[0071] While collecting and generating a two-dimensional ultrasonic image, the calibration controller transmits a trigger signal every time the camera 3 takes a photo; after receiving the trigger signal, the annular ultrasonic transducer 1 starts collecting once, thereby achieving synchronization between the two-dimensional ultrasonic image and the image of the transducer calibration plate 2 taken by the camera 3.

[0072] It should be understood that, during the acquisition process, the model calibration plate 5 installed on the N-line model needs to be shielded or later cut off to avoid errors in identifying the transducer calibration plate 2.

[0073] In Example 1 of the present invention, the specific process of step (4) is as follows:

[0074] Step 4-1: Calculate the transformation matrix Rt from the N-line model coordinate system to the camera coordinate system based on the pose of the model calibration plate 5 in the camera coordinate system. m2c ;

[0075] Among them, Rt m2c Can be decomposed into the corresponding rotation matrix R m2c and the translation vector t m2c , respectively represent the corresponding rotation and translation, which are used for the subsequent calculation of the position coordinates of the N-line imaging point in the camera coordinate system.

[0076] Step 4-2, according to the two-dimensional ultrasonic image obtained by each group of the annular ultrasonic transducer 1 for N-line scanning, obtain the position coordinates P of each group of N-line imaging points in the ultrasonic image coordinate system i ;

[0077] Step 4-3, based on the position coordinates P of each of the N-line imaging points in the ultrasound image coordinate system i and the proportional relationship of similar triangles, the position coordinates P of the N-line imaging point in the N-line model are calculatedm ;

[0078] Step 4-4, according to the position coordinates P of the N-line imaging point in the N-line model m And the transformation matrix Rt from the N-line model coordinate system to the camera coordinate system m2c , calculate the position coordinates P of the N-line imaging point in the camera coordinate system c .

[0079] It should be understood that step 4-1 only needs to be performed before step 4-4, and is not limited to being performed before step 4-2.

[0080] More specifically, the specific process of step 4-2 is:

[0081] The obtained two-dimensional ultrasonic image is subjected to Gaussian filtering, binarization processing and corrosion and expansion processing in sequence, and then the position coordinates of N-line imaging points in the two-dimensional ultrasonic image after corrosion and expansion processing are extracted, and finally the position coordinates of 3M groups of N-line imaging points in the two-dimensional ultrasonic image are obtained.

[0082] Specifically, due to the presence of noise, bubbles and other reasons in the process of ultrasonic imaging, the original ultrasonic image is first smoothed and denoised by a Gaussian filter to remove high-frequency noise in the image. The image is then binarized to facilitate the determination of the specific position of the N-line imaging point. Due to the thickness effect of the ultrasonic imaging plane and the unsatisfactory transverse focusing of the ultrasound, the N-line imaging points in the ultrasonic image formed by the annular transducer will diffuse axially. Therefore, the image can be morphologically processed by first corroding and then expanding to make the N-line imaging points smoother and rounder. Finally, the specific coordinates of the center of the N-line imaging point in the two-dimensional ultrasonic image are calculated based on the obtained orientation of the N-line imaging point.

[0083] More specifically, the specific process of step 4-3 is:

[0084] Each group of N-line imaging points includes three N-line imaging points, which can be abstracted as the intersections of the ultrasound image plane and three of the N lines. Take a group of N lines as an example, Figure 6 As shown, Figure 6 Three intersection points are shown, namely points A, B and C. From the two-dimensional ultrasound image, the coordinates of points A, B and C in the ultrasound image coordinate system can be obtained respectively, denoted by P i . The values ​​of line segments AB and BC are calculated, and the ratio k between the two can be calculated by the following formula, namely:

[0085]

[0086] According to the similarity between ΔADB and ΔCBE, then:

[0087]

[0088] |BD|+|BE|=|DE|

[0089] In the formula, |BD| represents the length of the corresponding line segment BD, and the same applies to the other line segments.

[0090] Since the model parameters are known, that is, the total length of DE is known, and the position of the thin line in the N-line model coordinate system is known, the position coordinates P of each N-line imaging point in the image in the N-line model can be calculated. m ;

[0091] More specifically, the specific process of step 4-4 is:

[0092] Since the internal parameters of the N-line model are known, that is, the position of the N-line in the model coordinate system is known. For each group of N-line imaging points, according to the lengths of BD and DE, we can know the position of each N-line imaging point in the N-line, and then according to the position of the N-line in the model coordinate system, we can calculate the position coordinates P of each N-line imaging point in the model coordinate system. m .

[0093] Using the transformation matrix Rt from the N-line model coordinate system to the camera coordinate system m2c And the position coordinates P of the N-line imaging point in the N-line model m , we can calculate the position coordinates P of the N-line imaging point in the camera coordinate system c ,Right now:

[0094] P c =Rt m2c ·P m

[0095] More specifically, the specific process of step (5) is:

[0096] Step 5-1, obtain the transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w The initial parameters of .

[0097] The transformation matrix from the ultrasound image coordinate system to the transducer calibration plate coordinate system, that is, the calibration target, is denoted as Rt i2w . Rt i2w The initial parameters of Rt are determined by the approximate positions of the transducer calibration plate 2 and the annular ultrasonic transducer 1 in real space, and can be obtained by measuring the real relative positions of the two in the experiment. i2w The initial parameters of .

[0098] Specifically, the transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system is i2w Can be decomposed into the corresponding rotation matrix Ri2w and the translation vector t i2w , respectively represent the corresponding rotation and translation. Since in this example the transducer calibration plate 2 is parallel to the ultrasonic plane formed by the annular ultrasonic transducer 1, the rotation matrix R i2w The initial parameters are:

[0099]

[0100] R i2w The translation in 3D space is measured as [x0, y0, z0] T , then t i2w =[x0,y0,z0] T As Rt i2w The translation vector t i2w These two initial parameters will be used as the initial parameters of Rt in the subsequent iterative solution steps. i2w The initial parameters of .

[0101] Step 5-2: The position coordinates P of each group of N line imaging points obtained in step (4-2) in the ultrasound image coordinate system are i The transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w And the transformation matrix Rt from the transducer calibration plate coordinate system to the camera coordinate system w2c After transformation, the corresponding N-line imaging point position coordinates P in the camera coordinate system are obtained c ′, the calculation formula is as follows:

[0102] P c '=Rt w2c ·Rt i2w ·P i ;

[0103] Among them, Rt i2w ·P i The meaning is the transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w , the coordinate P in the ultrasound image coordinate system i Transform into the transducer calibration plate coordinate system in three-dimensional space.

[0104] The following is a detailed description of the process of converting the two-dimensional coordinates in the two-dimensional ultrasound coordinate system to the three-dimensional coordinates in the three-dimensional space. Specifically, the upper left corner pixel of the two-dimensional ultrasound image is the coordinate origin, the horizontal direction is the x-axis, the vertical direction is the y-axis, the two-dimensional ultrasound image plane is the xOy plane, and the vertical direction is the z-axis. A three-dimensional coordinate system is established, and the coordinates of each point on the two-dimensional ultrasound image are [u, v, 0] T , set the coordinates when transformed to the corresponding transducer calibration plate coordinate system as [x w,y w ,z w ] T , according to Rt i2w The corresponding rotation matrix R i2w and the translation vector t i2w , the positional relationship between the coordinates in the transducer calibration plate coordinate system and the coordinates in the ultrasound image coordinate system is:

[0105]

[0106] In this way, the coordinate P in the ultrasound image coordinate system is realized. i Transformation to the transducer calibration plate coordinate system in three-dimensional space.

[0107] Then multiply the transformation matrix Rt from the transducer calibration plate coordinate system to the camera coordinate system by w2c , transform the coordinates from the transducer calibration plate coordinate system to the camera coordinate system, and obtain the position coordinates P of the corresponding N-line imaging point in the camera coordinate system c ′.

[0108] So far, we have calculated the actual and theoretical positions of 3M groups of N-line imaging points in the camera coordinate system through the N-line model coordinate system and the transducer calibration plate coordinate system, that is, P c and P c ′.

[0109] Step 5-3, based on the Rt i2w The initial parameters are based on the actual position coordinates P of the 3M groups of N line imaging points in the camera coordinate system. c The theoretical position coordinates P of the N-line imaging point obtained after the transformation in the camera coordinate system c The error between ′ and the transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w Solve it, that is:

[0110]

[0111] Specifically, the actual position coordinates P of the 3M groups of N line imaging points in the camera coordinate system are c The theoretical position coordinates P of the N-line imaging point obtained after the transformation in the camera coordinate system c The error between ′ is used as the objective function, and the objective function is optimized by the RMSprop (Root Mean Square Propagation) algorithm to transform the transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w Perform iterative calibration and finally obtain the transformation matrix Rt i2wThe RMSprop algorithm is an improvement on the traditional gradient descent algorithm. For the specific process, please refer to Tieleman, T, & Hinton, G.. (2012). Lecture 6.5-rmsprop: Divide the gradient by a running average of its recent magnitude. COURSERA: Neural networks for machine learning. 4(2), 26-31.

[0112] In a specific implementation, the present application provides a computer storage medium and a corresponding data processing unit, wherein the computer storage medium can store a computer program, and when the computer program is executed by the data processing unit, the invention content of the N-line calibration method for an annular ultrasonic transducer provided by the present invention and some or all of the steps in each embodiment can be executed. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0113] Example 2

[0114] Different from the first embodiment, in the present embodiment, the relative positions between the three arc-shaped ring parts 11 are calibrated without using the N-line model, and the specific process is as follows:

[0115] The three arc-shaped ring parts 11 are respectively the first arc-shaped ring part, the second arc-shaped ring part and the third arc-shaped ring part. For the first arc-shaped ring part, two transducer array elements in another second arc-shaped ring part are used as sound source points to respectively transmit ultrasonic signals, and the following formula can be obtained:

[0116]

[0117] Among them, (x1′, y1′, z1′) and (x2′, y2′, z2′) are the positions of the two transducer array elements as sound sources in the second arc ring part. Since the relative positions of these two transducer array elements are known, their coordinates are known. (x, y, z) is the position of the receiving transducer array element in the first arc ring part. i is the distance from the receiving transducer array element to the i-th sound source, and its value can be obtained from d i =ct i Find, t i is the flight time of the received signal, and c is the speed of sound. is the distance from the ith sound source to the coordinate origin, where i = 1 or 2. The coordinate origin is located at the center of the annular transducer.

[0118] Similarly, by replacing the two sound sources, we can get another equation. Repeating this process, we can get a set of linear equations. By solving the set of linear equations, we can get the relative position between the first arc ring portion and the second arc ring portion. Similarly, we can calibrate the relative position between the first arc ring portion and the third arc ring portion, and the second arc ring portion and the third arc ring portion.

[0119] Those skilled in the art can clearly understand that the technical solutions in the embodiments of the present invention can be implemented by means of computer programs and their corresponding general hardware platforms. Based on this understanding, the technical solutions in the embodiments of the present invention are essentially or partly contributed to the prior art can be embodied in the form of a computer program, i.e., a software product, which can be stored in a storage medium and includes several instructions for enabling a device including a data processing unit (which can be a personal computer, a server, a single-chip microcomputer, a MUU or a network device, etc.) to execute the methods described in various embodiments of the present invention or certain parts of the embodiments.

[0120] The present invention provides a concept and method for N-line calibration of an annular ultrasonic transducer. There are many methods and ways to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A method for N-line calibration of a ring ultrasonic transducer, characterized in that: The calibration method is applied to a calibration system, the calibration system comprising an annular ultrasonic transducer (1), a transducer calibration plate (2) mounted on the annular ultrasonic transducer (1), a camera (3), an N-line model (4), and a model calibration plate (5) mounted on the N-line model (4), and the calibration method comprises the following steps: Step (1), calibrating the relative positions between different arc-shaped ring parts (11) in the annular ultrasonic transducer (1); Step (2), the camera (3) photographs the model calibration plate (5), and determines the position and posture of the model calibration plate (5) in the camera coordinate system by an optical tracking method; Step (3), keeping the relative position of the N-line model (4) and the camera (3) unchanged, moving the annular ultrasonic transducer (1), acquiring a plurality of groups of two-dimensional ultrasonic images obtained by scanning the annular ultrasonic transducer (1) for N lines, and the camera (3) synchronously recording the position and posture of the transducer calibration plate (2) in the camera coordinate system at the corresponding moment; Step (4), based on the two-dimensional ultrasonic image obtained by each group of the annular ultrasonic transducer (1) for N-line scanning and the position of the model calibration plate (5) in the camera coordinate system, calculate the position coordinates P of the N-line imaging point in the camera coordinate system c ; Step (5), according to the position coordinates P of the N-line imaging point in the camera coordinate system c The camera (3) records the position of the transducer calibration plate (2) in the camera coordinate system at the corresponding moment, and iteratively calibrates the transformation matrix between the ultrasound image coordinate system and the transducer calibration plate coordinate system.

2. The N-line calibration method for a ring ultrasonic transducer according to claim 1, characterized in that: The camera (3) is located at one end of the annular ultrasonic transducer (1); the transducer calibration plate (2) is installed on the annular ultrasonic transducer (1) and on the same side as the camera (3); the N-line model (4) is located in the inner ring space of the annular ultrasonic transducer (1); the model calibration plate (5) is installed on the N-line model (4) and on the same side as the camera (3); the annular ultrasonic transducer (1), the N-line model (4), the transducer calibration plate (2) and the model calibration plate (5) are all located in the field of view of the camera (3).

3. The N-line calibration method for a ring ultrasonic transducer according to claim 2, characterized in that: The annular ultrasonic transducer (1) is an annular array composed of more than three segments of the arc-shaped annular portions (11), and each of the arc-shaped annular portions (11) is composed of a plurality of transducer array elements (111) arranged in sequence along the arc length.

4. The N-line calibration method for a ring ultrasonic transducer according to claim 3, characterized in that: The transducer calibration plate (2) and the model calibration plate (5) are respectively ArUco calibration plates, the ArUco calibration plate comprising a calibration plate body (21) and an asymmetric circular calibration plate pattern (22) and four ArUco codes (23) arranged on the calibration plate body (21), the four ArUco codes (23) enclosing a rectangle for determining an image of the ArUco calibration plate in an image captured by the camera (3), the asymmetric circular calibration plate pattern (22) being located within the rectangle enclosed by the four ArUco codes (23); the asymmetric circular calibration plate pattern (22) comprises a plurality of rows of circular pattern single-row arrays, and two adjacent rows of circular pattern single-row arrays are arranged in an alternating manner.

5. The N-line calibration method for a ring ultrasonic transducer according to claim 4, characterized in that: The N-line model comprises a top plate (41), a bottom plate (42) and a plurality of support columns (43) connecting the top plate (41) and the bottom plate (42); the plurality of support columns (43), the top plate (41) and the bottom plate (42) are combined to form a polygonal column shape, and the number of the support columns (43) is the same as the number of the arc-shaped ring portion (11); the bottom of the top plate (41) and the top of the bottom plate (42) are provided with threading holes (44); by threading, each side of the N-line model forms M groups of N lines, wherein M≥3; the M groups of N lines are arranged at intervals along a direction toward the middle of the top plate (41); and the top of the top plate (41) is provided with a groove for mounting the model calibration plate (5).

6. The N-line calibration method for a ring ultrasonic transducer according to claim 5, characterized in that: The method for determining the position and posture of the model calibration plate (5) in the camera coordinate system by optical tracking in step (2) comprises: using an ArUco calibration block detection algorithm to detect the ArUco code (23) in the image captured by the camera (3), and calculating the image coordinates of four ArUco codes (23) on the model calibration plate (5); determining the image of the model calibration plate (5) in the image captured by the camera (3) according to a rectangle formed by the four ArUco codes (23); using a contour detection algorithm to detect the contours of all circular patterns in the asymmetric circular calibration plate pattern (22) on the model calibration plate (5), and calculating the center coordinates of all circular patterns; and calculating the position and posture of the model calibration plate (5) in the camera coordinate system according to the center coordinates.

7. The N-line calibration method for a ring ultrasonic transducer according to claim 1, characterized in that: The camera (3) in step (3) synchronously records the position and posture of the transducer calibration plate (2) in the camera coordinate system at the corresponding moment, including: While collecting and generating a two-dimensional ultrasonic image, the camera (3) is caused to emit a trigger signal each time a photo is taken; after receiving the trigger signal, the annular ultrasonic transducer (1) starts a collection to achieve synchronization between the two-dimensional ultrasonic image and the image of the transducer calibration plate (2) taken by the camera (3); during the collection process, the model calibration plate (5) installed on the N-line model is blocked or later cut off to avoid errors in identifying the transducer calibration plate (2).

8. The N-line calibration method for a ring ultrasonic transducer according to claim 1, characterized in that: The step (4) comprises: According to the pose of the model calibration plate (5) in the camera coordinate system, the transformation matrix Rt from the N-line model coordinate system to the camera coordinate system is calculated: m2c ; According to the two-dimensional ultrasonic image obtained by each group of the annular ultrasonic transducer (1) for N-line scanning, the position coordinates P of each group of N-line imaging points in the ultrasonic image coordinate system are obtained. i ; According to the position coordinates P of each of the N-line imaging points in the ultrasound image coordinate system i and the proportional relationship of similar triangles, the position coordinates P of the N-line imaging point in the N-line model are calculated m ; According to the position coordinates P of the N-line imaging point in the N-line model m And the transformation matrix Rt from the N-line model coordinate system to the camera coordinate system m2c , calculate the position coordinates P of the N-line imaging point in the camera coordinate system c .

9. The N-line calibration method for a ring ultrasonic transducer according to claim 8, characterized in that: According to the two-dimensional ultrasonic image obtained by each group of the annular ultrasonic transducer (1) for N-line scanning, the position coordinates P of each group of N-line imaging points in the ultrasonic image coordinate system are obtained. i include: The obtained two-dimensional ultrasonic image is subjected to Gaussian filtering, binarization and corrosion and dilation processing in sequence, and then the position coordinates of N-line imaging points in the two-dimensional ultrasonic image after corrosion and dilation processing are extracted, and finally the position coordinates of 3M groups of N-line imaging points in the two-dimensional ultrasonic image are obtained.

10. The N-line calibration method for a ring ultrasonic transducer according to claim 8, characterized in that: The step (5) comprises: The transformation matrix Rt from the transducer calibration plate coordinate system to the camera coordinate system is calculated based on the position and posture of the transducer calibration plate (2) in the camera coordinate system: w2c ; The transformation matrix Rt from the ultrasonic image coordinate system to the transducer calibration plate coordinate system is obtained by measuring the transducer calibration plate (2) and the annular ultrasonic transducer (1). i2w The initial parameters of The coordinates P of each group of N line imaging points in the ultrasound image coordinate system are i The transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w And the transformation matrix Rt from the transducer calibration plate coordinate system to the camera coordinate system w2c After transformation, the corresponding N-line imaging point position coordinates P in the camera coordinate system are obtained c ′, the calculation formula is as follows: P c ′=Rt w2c ·Rt i2w ·P i ; The position coordinates P of the N-line imaging point in the camera coordinate system c The position coordinates P of the N-line imaging point obtained after the transformation in the camera coordinate system c The error between ′ is used as the objective function, and the objective function is optimized by the RMSprop algorithm to transform the transformation matrix Rt from the ultrasound image coordinate system to the transducer calibration plate coordinate system i2w Perform iterative calibration and finally obtain the transformation matrix Rt i2w The best parameters.

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