CN103110429B - The optical calibration method of ultrasound probe - Google Patents

Abstract

The invention discloses a kind of scaling method of ultrasound probe, there are following steps: 1. paste feature identification point on ultrasonic probe, 2. fixing stereo calibration template and optical positioning device, 3. hand-held ultrasound scanning probe stereo calibration template, optical positioning device is utilized to obtain the spatial attitude of ultrasonic probe while each collection ultrasonoscopy, obtain initial data, then utilize method of least square to demarcate ultrasonic probe.Compared to traditional electromagnetism scaling method, tool has the following advantages: calibration result is more accurate; In calculating process, only have a unknown number (unknown transformation matrix), reduce the complexity of calibrated and calculated; Can be applied on the ultrasonic probe of various model, simple equipments, applied widely; Adopt optical positioning device to obtain ultrasonic probe attitude, without the need to considering the impact of power line, what make to demarcate is wider; The problem of electromagnetic compatibility is there is not in optical positioning device in surgical navigational environment.

Description

Optical Calibration Method of Ultrasonic Probe

technical field

The invention relates to an optical calibration method, in particular to an optical calibration method of an ultrasonic probe.

Background technique

Existing manual three-dimensional ultrasonic image calibration method based on the magnetic locator: fix an electromagnetic receiver on the ultrasonic probe, utilize the electromagnetic locator to obtain the spatial information of the receiver relative to the transmitter, and establish the ultrasonic image coordinate system I, receiver Coordinate system R, emitter coordinate system T, template coordinate system C four coordinate systems, the calibration formula is P(C)＝ ^C T _T ^T T _R ^R T _I P(I), where ^T T _R represents electromagnetic The transformation matrix from the receiver to the transmitter, the ultrasonic image coordinates P(I) of the known space point, the corresponding template coordinates P(C) and the ^T T _R of each measurement, are obtained by using the least square method The spatial mapping relationship between ^C T _T and ^R T _I .

The manual 3D ultrasound image calibration method based on the magnetic locator has the following problems:

(1) The designed N-shaped calibration template is only a two-dimensional calibration template in space, and the obtained calibration template coordinates lack the third-dimensional spatial information;

(2) The calibration formula P(C)＝ ^C T _T * ^T T _R * ^R T _I *P(I) has two unknown quantities, ^C T _T and ^R T _I , the calculation is more complicated, and multiple calibrations need to be collected image;

(3) By default, the receiver coordinate system is used instead of the ultrasonic probe coordinate system, and there may be transformation errors;

(4) It is necessary to fix the electromagnetic receiver on the ultrasonic probe, the movement is limited, and different specifications of electromagnetic positioners need to be equipped for different types of ultrasonic probes, which makes the equipment complicated and the adaptability is not high;

(5) Electromagnetic positioning equipment has electromagnetic compatibility problems in surgical navigation applications.

Contents of the invention

The present invention aims at the proposal of above problem, and a kind of ultrasonic probe calibration method developed, has the following steps:

S1. Select two identical cameras whose optical axes are parallel to each other as an optical positioning device, take the optical center of one camera as the origin of the coordinate system, the connecting line of the optical centers of the two cameras is the X axis, and take the optical axis of the camera As the Z axis, establish a space Cartesian coordinate system as the optical positioning equipment coordinate system C;

S2. Select and fix two identical rectangular frames, so that the two rectangular frames constitute two opposing surfaces in a spatial virtual cuboid, and use multiple calibration straight lines to connect the endpoints of the two rectangular frames as a three-dimensional calibration Template, select a vertex in the two rectangular frames as the origin, establish a space Cartesian coordinate system, as the three-dimensional calibration template coordinate system M;

S3. Select a 2D ultrasonic probe, paste at least three feature points on the ultrasonic probe, and establish a spatial Cartesian coordinate system as the ultrasonic probe coordinate system T;

S4. Use the ultrasonic probe to scan the calibration template, so that the scanning plane of the ultrasonic wave passes through each of the multiple calibration straight lines, record the spatial points where each scanning plane intersects with the calibration straight lines, and calculate the distance between each spatial point and the calibration line. Coordinates in the three-dimensional calibration template coordinate system M;

S5. Set the ultrasound image coordinate system I, and use the ultrasound image processing technology to obtain the coordinates of each space point in the ultrasound image, denoted as P(I)=(u _i , v _i ,0,1) ^T ;

S6. By the optical positioning equipment coordinate system C, the three-dimensional calibration template coordinate system M, the ultrasonic probe coordinate system T and the ultrasonic image coordinate system I, the formula is obtained:

P(M)＝ ^M T _C · ^C T _T (i) · ^T T _I · P(I),

In the formula: P (M) is the coordinate of the spatial point in the three-dimensional calibration template coordinate system M; P (I) is the image coordinate of the corresponding point in the ultrasonic image;

The matrix ^M T _C with a dimension of 4×4 is the transformation matrix from the coordinate system C to the coordinate system M, and the matrix is a fixed value;

The matrix ^C T _T (i) with a dimension of 4×4 is the transformation matrix from the coordinate system T to the coordinate system C, and each calibration image acquisition corresponds to a matrix ^C T _T (i);

The matrix ^T T _I whose dimension is 4×4 is the transformation matrix to be sought;

Let the objective function f(i)=|P _M (i)- ^M T _C ^C T _T (i) ^T T _I P _I (i)| ² , so that ^T T _I when f(i)=0 That is, for the purpose of calibration, multiple calibration images are collected, and after stacking the data, ^TTI is optimized using the method of least _squares to complete the calibration of the ultrasonic probe.

The distance between the optical centers of the two cameras, that is, the baseline distance, can be adjusted, and the angle between the optical axes can be adjusted. The number of the connecting lines is at least 7.

In the step S3, before scanning, the calibration template is immersed in water at 50°C. In the step S6, at least two images are collected.

Due to the above-mentioned technical scheme, the optical calibration method of the ultrasonic probe provided by the present invention has the following advantages:

1. A three-dimensional calibration template is designed to extend the coordinates of the calibration template from two-dimensional to three-dimensional, which can strengthen the constraints in the calibration calculation process and make the calibration results more accurate;

2. The unknown quantity in the calibration formula P(M)＝ ^M T _C ^C T _T (i) ^T T _I P (I) is only ^T T _I , which reduces the complexity of calibration calculations;

3. Directly use the coordinate system of the ultrasonic probe to transform with other coordinate systems, the physical meaning is clear, and the three characteristic identification points pasted on the probe are highly adaptable, and can be applied to various types of ultrasonic probes. The equipment is simple and the scope of application is wide;

4. Using optical positioning equipment to obtain the attitude of the ultrasonic probe, without considering the influence of the power line, making the calibration range wider;

5. Optical positioning equipment does not have the problem of electromagnetic compatibility in the surgical navigation environment.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description These are only some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to these drawings without any creative effort.

Fig. 1 is the schematic diagram of the transformation relationship of the spatial coordinate system of the present invention;

Fig. 2 is the schematic diagram of stereo calibration template of the present invention;

Fig. 3 is a schematic diagram of the characteristic mark point and its coordinate system setting on the ultrasonic probe of the present invention;

Fig. 4 is a schematic diagram of setting the stereo calibration template coordinate system of the present invention;

Fig. 5 is a schematic diagram of the coordinate calculation method of the space point E in the stereo calibration template coordinate system according to the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are Some, but not all, embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

Example 1:

As shown in Figure 1-5: an ultrasonic probe calibration method:

one. Setting of the coordinate system C of the optical positioning device: select two identical cameras whose optical axes are parallel to each other as the optical positioning device. As shown in Fig. 1, two identical cameras, equipped with the same type of lens, are preferably fixed on a horizontally arranged fixing frame.

Further, in order to meet different calibration ranges, the two cameras are movably connected to the fixed frame, that is, the distance between the optical centers of the two cameras, that is, the baseline distance, is set to be adjustable, and the optical axes of the two cameras are clamped The angle can also be adjusted to meet the calibration needs of different orientations.

After fixing the optical instrument, usually, choose the optical center of the left camera as the axis, the baseline of the two cameras as the X axis, and the optical axis of the left camera as the Z axis, and establish a Cartesian coordinate system as the optical positioning The device coordinate system C.

two. The three-dimensional calibration template and its coordinate system setting: as shown in Figure 2: select two identical rectangular frames or rectangular panels, in this embodiment, select two identical rectangular frames, and make the two frames vertically opposite each other It is set so that the orthographic projection of the rectangular frame A on another rectangular frame B completely coincides with the rectangular frame B, which can also be expressed as that the two rectangular frames form a virtual cuboid in space in the coordinate system of the optical positioning device. two sides. Multiple calibration lines are selected to connect vertices of the two rectangular panels. Due to the needs of calculation, at least seven calibration straight lines are needed to connect all the vertices of the two rectangular frames in the form of polyline N-shaped connection. The specific connection method is shown in Figure 2: Among the six faces of the virtual space cuboid, except for the faces where the two rectangular frames are located, among the remaining four rectangular faces, there are at least three faces with three calibration lines Connect the vertices of each surface, and the three straight lines are N-shaped. Taking point O as the origin of coordinates, establish a spatial rectangular coordinate system as the coordinate system M of the three-dimensional calibration template.

three. The feature mark points and coordinate system setting on the ultrasonic probe, as shown in Figure 3, select a 2D ultrasonic probe, paste at least three feature points on the ultrasonic probe, preferably, the feature points can be visible light feature points and infrared LED feature points, etc., establish a space Cartesian coordinate system as the ultrasonic probe coordinate system T.

Four. Each ultrasonic detector has its own coordinates on its scanning surface, which is set as the ultrasonic image coordinate system I in the present invention.

five. Use an ultrasonic probe to scan the calibration template, put the calibration template into water with a water temperature of about 50°C during work, scan the template with a 2D ultrasonic probe, the scanning plane intersects the calibration line at D, E, F, G, H, M , N seven space points, as shown in Figure 4.

The coordinates of each space point in the ultrasonic image coordinate system I are obtained by using ultrasonic image processing technology, which is denoted as P(I)=(u _i ,v _i ,0,1) ^T .

The obtained seven space points D, E, F, G, H, M, N, the coordinates of each point in the three-dimensional template coordinate system M are set as P(M)=( _xi , y, _{zi ,} 1 ) ^T , taking D, E, and F coplanar three points as an example, similar to triangle EFC, according to the principle of similar triangles, the coordinates of point E in the stereo calibration template are (x _E ,y _E ,z _E ,1) ^T ,

in $x_{\mathrm{E.}}=\mathrm{OC}\·\frac{\mathrm{DE}}{\mathrm{DF}},{\mathrm{the\; y}}_{\mathrm{E.}}=\mathrm{OA}\·\frac{\mathrm{DE}}{\mathrm{DF}},z_{\mathrm{E.}}=0,$

Knowing the geometric size information of the stereo calibration template OA, OB, OC, DE, DF can be measured in the ultrasound image, so that the coordinates of point E in the stereo calibration template coordinate system can be obtained, and each calibration image can be obtained as The coordinates of the three points E, G, and M are shown in Figure 4.

Assuming that the image coordinates of the corresponding points of the spatial points in the ultrasonic image are P(I)=( _{u i} , vi ,0,1) ^T , the image coordinates can be obtained by using ultrasonic image processing technology.

Since the three-dimensional calibration template is stationary relative to the optical positioning device during the calibration process, the transformation matrix ^M T _C remains unchanged, and it can be obtained by measuring the coordinates of the three space points O, A, and C in the coordinate system of the optical positioning device transformation matrix ^M T _C ;

While collecting ultrasonic calibration images each time, the coordinates of three feature points on the ultrasonic probe can be obtained by using optical positioning equipment, and the transformation matrix ^C T _T (i) can be obtained, and each calibration image corresponds to a transformation matrix.

Obtain calibration formula P (M)= ^M T _C ^C T _T (i) ^T T _I P (I) by above-mentioned condition, in the formula, P (M) is the coordinate of space point in the three-dimensional calibration template coordinate system; P(I) is the image coordinate of the corresponding point in the ultrasound image; the matrix ^M T _C with a dimension of 4×4 is the transformation matrix from the coordinate system C to the coordinate system M, and the matrix is a fixed value; the dimension is 4×4 The matrix ^C T _T (i) is the transformation matrix from the coordinate system T to the coordinate system C, and each calibration image acquisition corresponds to a matrix ^C T _T (i); the matrix ^T T _I with a dimension of 4×4 is the desired transformation matrix.

Let the objective function f(i)=|P _M (i)- ^M T _C ^C T _T (i) ^T T _I P _I (i)| ² , so that ^T T _I when f(i)=0 That is, for the purpose of calibration, multiple calibration images (at least 2 images) are collected, and after stacking the data, the least square method can be used to optimize and obtain ^T _TI in engineering.

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto, any person familiar with the technical field within the technical scope disclosed in the present invention, according to the technical solution of the present invention Any equivalent replacement or change of the inventive concepts thereof shall fall within the protection scope of the present invention.

Claims (4)

1. an optical calibration method for ultrasound probe, is characterized in that having following steps:

S1. two pieces of identical cameras that optical axis is parallel to each other are chosen, as optical positioning device, using the photocentre of side camera as coordinate origin, the photocentre connecting line of two pieces of cameras is X-axis, using the optical axis of this camera as Z axis, set up rectangular coordinate system in space, as optical positioning device coordinate system C;

S2. choose and fix two identical rectangular frames, make two faces relative in 2 described virtual cuboids of rectangular frame Special composition, use each end points that many are demarcated straight line and connect described two rectangular frames, as stereo calibration template, choose a summit in described two rectangular frames as initial point, set up rectangular coordinate system in space, as stereo calibration template coordinate system M;

S3. choose the ultrasound probe of a 2D, this ultrasound probe is pasted at least three characteristic points, sets up rectangular coordinate system in space, as ultrasound probe coordinate system T;

S4. ultrasound probe scanning calibrating template is used, hyperacoustic plane of scanning motion is made to demarcate each demarcation straight line of straight line through described many, record each plane of scanning motion and the spatial point of demarcating straight line intersection, calculate the coordinate of each spatial point in described stereo calibration template coordinate system M;

S5. set ultrasound image coordinates system I, utilize the coordinate of each spatial point in ultrasonography described in the acquisition of ultrasonoscopy treatment technology, be denoted as P (I)=(u _i, v _i, 0,1) ^t;

S6. by described optical positioning device coordinate system C, stereo calibration template coordinate system M, ultrasound probe coordinate system T and ultrasound image coordinates system I, formula is obtained:

P(M)＝ ^MT _C· ^CT _T(i)· ^TT _I·P(I)，

In formula: P (M) is the coordinate of spatial point in stereo calibration template coordinate system M; P (I) is the image coordinate of corresponding point in ultrasonoscopy;

Dimension is the matrix of 4 × 4 ^mt _cfor the transformation matrix of coordinate system C to coordinate system M, this matrix is fixed value;

Dimension is the matrix of 4 × 4 ^ct _ti () is the transformation matrix of coordinate system T to coordinate system C, each uncalibrated image gathers a corresponding matrix ^ct _t(i);

Dimension is the matrix of 4 × 4 ^tt _ifor required transformation matrix;

If object function f (i)=| P _m(i)- ^mt _c. ^ct _t(i) ^tt _ip _i(i) | ², when making f (i)=0 ^tt _ibe required by demarcation, gather several uncalibrated images, after stacking data, use the method optimization of method of least square to obtain ^tt _i, complete the demarcation of ultrasound probe; P _mi () represents the coordinate of target at coordinate system M; P _ii () represents the coordinate of target at coordinate system I;

The number of described connecting line is at least 7.

2. the optical calibration method of a kind of ultrasound probe according to claim 1, is further characterized in that: the photocentre distance of described two pieces of cameras and parallax range scalable, optical axis included angle scalable.

3. the optical calibration method of a kind of ultrasound probe according to claim 1, is further characterized in that: in described step S3, before scanning, is immersed by described calibrating template in the water of 50 DEG C.

4. the optical calibration method of a kind of ultrasound probe according to claim 1, is further characterized in that: in described step S6, gathers at least 2 width images.