CN117158911B - A multi-sound velocity adaptive photoacoustic tomography image reconstruction method - Google Patents

Abstract

The invention belongs to the technical field of photoacoustic image reconstruction and discloses a multi-sonic velocity adaptive photoacoustic tomography image reconstruction method, which includes step 1: using an ultrasonic transducer to emit ultrasonic waves to irradiate biological tissue in an ultrasonic tomography equipment system; Step two, the ultrasonic transducer receives the ultrasonic signal emitted by the biological tissue; step three, use the iterative reconstruction algorithm of the bending ray model to reconstruct the sound velocity distribution of the biological tissue; step four, divide the biological tissue into several regions with different sound speeds; Step 5: Use a laser to emit pulsed laser light in the photoacoustic tomography equipment system to irradiate the tissue; Step 6: The ultrasonic transducer receives the photoacoustic signal emitted by the light absorption point in the tissue; Step 7: Calculate the propagation of the photoacoustic signal to the ultrasonic transducer. The multi-sound velocities involved in the energizer process; step eight, the multi-sound velocities are adaptively applied to the delayed summation algorithm to reconstruct the photoacoustic tomography image. The invention effectively improves the quality of photoacoustic imaging reconstructed images.

Description

A multi-sound velocity adaptive photoacoustic tomography image reconstruction method

Technical field

The invention belongs to the technical field of photoacoustic image reconstruction, and in particular relates to a multi-sound velocity adaptive photoacoustic tomographic image reconstruction system and method.

Background technique

Photoacoustic tomography is a non-destructive medical imaging method that has been developed in recent years. It combines the high contrast characteristics of pure optical imaging and the high penetration depth characteristics of pure ultrasound imaging, and can provide high-resolution and high-contrast in vivo imaging. Tissue imaging provides an important means for studying the structural morphology, physiological characteristics, metabolic functions, case characteristics, etc. of biological tissues. It has broad application prospects in the field of biomedical clinical diagnosis and in vivo tissue structure and function imaging. The main principle of photoacoustic tomography is the conversion of light energy into sound energy, that is, the photoacoustic effect. When a pulsed laser irradiates biological tissue, the light absorber inside the tissue absorbs the laser energy, undergoes instantaneous thermoelastic expansion, and excites ultrasonic waves, that is, photoacoustic signals. The photoacoustic signal propagates toward the tissue surface and is eventually received by the ultrasound transducer. According to the delay, amplitude and preset system sound speed of the received photoacoustic signal, the distribution of optical properties inside the biological tissue, especially the distribution of light absorption coefficient, is reconstructed through computer processing. These information items can be used to quantitatively measure specific substances in biological tissues, such as oxyhemoglobin and deoxygenated hemoglobin in blood, and based on this, clinical research on breast cancer can be carried out.

Considering the complexity of biological tissues, in order to simplify the problem during photoacoustic tomography, existing methods usually assume that the propagation speed of photoacoustic signals in biological tissues is constant, ignoring the uneven distribution of sound speed in tissues. coming impact. Research shows that biological tissues with different compositions often have different acoustic properties, and the difference in sound speed within soft tissues can reach 10%. Therefore, the inaccuracy of the preset sound velocity in the photoacoustic tomography image reconstruction algorithm is one of the main reasons for the low quality of the reconstructed image.

Contents of the invention

In order to solve the above technical problems, the specific technical solution of a multi-sound velocity adaptive photoacoustic tomography image reconstruction method of the present invention is as follows:

A multi-sound velocity adaptive photoacoustic tomography image reconstruction method, including the following steps:

Step 1: The ultrasonic transducer emits ultrasonic waves to irradiate the biological tissue;

Step 2: The ultrasonic transducer receives the ultrasonic signal emitted by the biological tissue;

Step 3: Use the iterative reconstruction algorithm of the bending ray model to reconstruct the sound velocity distribution of the biological tissue;

Step 4: Divide the biological tissue into several regions with different sound speeds;

Step 5: The laser emits pulsed laser light to irradiate the tissue;

Step 6: The ultrasonic transducer receives the photoacoustic signal emitted by the light absorption point in the tissue;

Step 7: Calculate the multiple sound velocities involved in the process of propagating the photoacoustic signal to the ultrasonic transducer;

Step 8: Multi-sound velocity adaptation is applied to the delayed summation algorithm to reconstruct the photoacoustic tomography image.

Further, the ultrasonic transducer described in step 1 both receives ultrasonic waves and emits ultrasonic waves. The ultrasonic transducer array is annular, and multiple ultrasonic transducers are evenly selected in the ultrasonic transducer array to emit ultrasonic waves one after another.

Further, in step two, the ultrasonic transducer successively receives the ultrasonic waves emitted by the biological tissue being measured in step one. The ultrasonic waves pass through the acoustic lens in the ultrasonic probe and reach the ultrasonic transducer. The ultrasonic transducer records the sound pressure.

Further, the iterative reconstruction algorithm of the bending ray model in step three is as follows:

Bending rays are used to characterize an array element in a ring-shaped ultrasonic transducer array that emits an ultrasonic signal. The ultrasonic wave is refracted when passing through areas with different sound speeds, and is finally received by the ultrasonic transducer. The formula is as follows:

,

in, is the first arrival time vector of the ultrasonic signal reaching the ultrasonic transducer;/> is the slowness vector of the biological tissue being measured;/> It is the coefficient matrix that connects the two, the spatial distribution of the ultrasonic transducer and the slowness distribution of the tissue/> Directly related, during the process of sound velocity reconstruction, the theoretical sound wave arrival time based on the bending ray model and the experimental observation arrival time are continuously compared until the deviation between the two is less than the set threshold.

Further, the fourth step includes the following specific steps:

For the sound velocity distribution reconstructed in step 3, each sound source point is traversed, and other sound source points whose sound speed differs from this point within a set range are classified into the same area. Finally, each area is traversed, and the values within the area are The average sound speed of all sound source points is used as the sound speed of the area, and then a biological tissue is divided into several areas with different sound speeds.

Further, the irradiated biological tissue in step five produces a photoacoustic effect that satisfies the equation:

, (1)

in It is the sound pressure generated by the thermal expansion of biological tissues,/> is the specific heat capacity,/> is the isobaric volume thermal expansion coefficient,/> is the empirical sound speed,/> is the heat source function excited by the incident pulse laser,/> ,/> is the spatial light absorption function of biological tissue,/> As a function of light irradiation, the laser hits the absorption point and the ultrasonic transducer starts to receive signals at the same time.

Further, the pulse laser irradiates the absorption point below or on the side of the biological tissue to be measured, and the plane where the ultrasonic transducer array is located is perpendicular to the plane where the laser is located.

Further, the ultrasonic transducer used in step 6 is consistent with the ultrasonic transducer used in step 2. The ultrasonic transducer receives the ultrasonic wave generated by the photoacoustic effect in step 5 and reaches the ultrasonic transducer through the acoustic lens in the ultrasonic probe. transducer to record the sound pressure intensity. Suppose the number of ultrasonic transducers is , record number/> The sound pressure signal generated by an ultrasonic transducer due to the photoacoustic effect in formula (1) is/> , /> .

Furthermore, the photoacoustic signal in step seven passes through multiple areas with different sound speeds in the process of propagating from the light absorption point to the tissue surface, and is finally received by the ultrasonic transducer. In the several different sound speed areas drawn in step four Based on , of which/> Represents the light absorption point,/> Indicates ultrasonic transducer, while/> is the starting point of the propagation path,/> is the end point of the propagation path. When calculating the propagation path of the photoacoustic signal in each area, select the propagation vector/> The most consistent path is used to calculate each propagation path and the sound speed corresponding to the propagation path, and finally obtain the multi-sound velocity information on the entire propagation path.

Further, step eight applies multi-sound velocity adaptation to the calculation of the propagation time of the photoacoustic signal from the absorption point to the ultrasonic transducer. For each absorption point, the delay summation algorithm formula is as follows:

,

in, Represents the photoacoustic tomography image reconstructed based on the delayed summation algorithm,/> Indicates ultrasonic transducer/> at time/> Received photoacoustic signal,/> Represents the propagation of the photoacoustic signal from the absorption point to the ultrasonic transducer/> the required propagation time, is the weight,

Multisonic adaptation applied to propagation time In the calculation, the calculation formula is as follows:

,

in is the required propagation time,/> is the position of the absorption point,/> For ultrasonic transducers/> , at the speed of sound/> The propagation path in the area is/> ,/> is the boundary point of the area, vector/> Closest propagation vector/> ,Right now and/> The angle between the two vectors is the smallest; similarly, when the speed of sound is/> The propagation path in the area is/> ,/> is the boundary point of the area, and similarly the vector/> Closest propagation vector/> ;Until the end of the propagation path/> The area where the sound speed is/> , the propagation path is/> ;The weight formula is:

,

in, is the distance from the absorption point to the ultrasonic transducer,/> is the angle between the absorption point and the ultrasonic transducer. In the two-dimensional XY plane, with the center of the annular ultrasonic transducer array as the coordinate origin, assuming the absorption point/> The coordinates are , ultrasonic transducer/> The coordinates are/> , then/> The calculation formula is:

,

The calculation formula is:

.

A multi-sound velocity adaptive photoacoustic tomography image reconstruction method of the present invention has the following advantages:

The present invention uses ultrasonic tomography to reconstruct the sound velocity distribution of biological tissues, divides different sound velocity areas, and on this basis calculates the multi-sonic velocities involved in the propagation process of the photoacoustic signal, and applies the multi-sonic velocity adaptively to the photoacoustic In tomographic image reconstruction, the adverse effects of inaccuracies in the preset sound velocity on photoacoustic imaging are avoided, thereby effectively improving the quality of photoacoustic imaging reconstructed images.

Description of the drawings

Figure 1 is a flow chart of the method of the present invention;

Figure 2 is a flow chart of the iterative reconstruction algorithm based on the bending ray model in the method of the present invention;

Figure 3 is a schematic diagram of the propagation path of the photoacoustic signal calculated in different sound speed regions in the method of the present invention;

Figure 4 shows the experimental sample diagram using k-wave for simulation verification;

Figure 5(a) is a photoacoustic tomography image reconstruction effect diagram using preset sound velocity;

Figure 5(b) is a diagram showing the reconstruction effect of multi-sound velocity adaptive photoacoustic tomography image using the present invention.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

Considering that the photoacoustic tomography system and the ultrasonic tomography system have similar acoustic signal receiving and processing procedures, the fusion of the two systems can be achieved at low software and hardware costs. Therefore, in order to overcome the problem of inaccurate preset sound speed in the existing photoacoustic tomography technology, the present invention provides a light source based on ultrasonic sonic tomography that does not rely on the preset sound speed and has multiple sound velocities that can adapt to different biological tissues. Acoustic tomography image reconstruction algorithm. This algorithm first reconstructs the sound velocity distribution of biological tissues based on ultrasonic sonic tomography, divides different sound velocity areas, and then calculates the propagation of the photoacoustic signal to the ultrasonic transducer (ultrasonic sensor). The multi-sound velocity involved in the approach is finally applied to the photoacoustic tomography image reconstruction adaptively, thereby reconstructing a higher quality image.

As shown in Figure 1, the present invention discloses a multi-sound velocity adaptive photoacoustic tomography image reconstruction method, which includes the following steps:

Step 1: Use an ultrasonic transducer to emit ultrasonic waves in an ultrasonic tomography equipment system to irradiate biological tissue;

The ultrasonic tomography equipment system can complete the reconstruction of photoacoustic tomography images and the reconstruction of ultrasonic tomography images. The ultrasonic transducer used in the system can both receive ultrasonic waves and transmit ultrasonic waves. The ultrasonic transducer array in the system is ring-shaped. The ultrasonic transducer array surrounds the biological tissue to be measured, and heavy water is filled between them. Several ultrasonic transducers are evenly selected in the ultrasonic transducer array to emit ultrasonic waves one after another.

Step 2: The ultrasonic transducer receives the ultrasonic signal emitted by the biological tissue;

The ultrasonic transducer successively receives the ultrasonic waves emitted by the biological tissue being measured in step 1. The ultrasonic waves pass through the acoustic lens in the ultrasonic probe and reach the ultrasonic transducer, and the sound pressure is recorded. The greater the number of ultrasound transducers used, the higher the quality of the reconstructed image.

Step 3: Use the iterative reconstruction algorithm of the bending ray model to reconstruct the sound velocity distribution of the biological tissue;

The principle of the algorithm is shown in Figure 2. An array element in the annular ultrasonic transducer array emits an ultrasonic signal. The ultrasonic wave will be refracted when passing through areas with different sound speeds, and is finally received by the ultrasonic transducer. This refraction process can use bending rays. Characterization, as shown in the following formula:

,

in, is the first arrival time vector of the ultrasonic signal reaching the ultrasonic transducer;/> is the slowness (reciprocal of the speed of sound) vector of the biological tissue being measured;/> It is the coefficient matrix that connects the two, the spatial distribution of the ultrasonic transducer and the slowness distribution of the tissue/> Directly related, during the process of sound velocity reconstruction, the theoretical sound wave arrival time based on the bending ray model and the experimental observation arrival time are continuously compared until the deviation between the two is less than the set threshold.

Step 4: Divide the biological tissue into several regions with different sound speeds;

For the sound velocity distribution reconstructed in step 3, traverse each sound source point, and other sound source points whose sound velocity differs from that point within a certain range (such as <=10m/s) are classified into the same area. Finally, each area is traversed, and the average sound speed of all sound source points in the area is used as the sound speed of the area. Furthermore, a biological tissue is divided into several regions with different sound speeds.

Step 5: Use a laser in the ultrasonic tomography equipment system to emit pulsed laser light to illuminate the tissue;

In the equipment system of ultrasonic tomography, the irradiated biological tissue produces a photoacoustic effect, which satisfies the equation:

, (1)

in It is the sound pressure generated by the thermal expansion of biological tissues,/> is the specific heat capacity,/> is the isobaric volume thermal expansion coefficient,/> is the empirical sound speed,/> is the heat source function excited by the incident pulse laser,/> ,/> is the spatial light absorption function of biological tissue,/> As a function of light irradiation, the laser hits the absorption point and the ultrasonic transducer starts to receive signals at the same time.

In the ultrasonic tomography equipment system, the working wavelength of the laser pulse emitted by the laser is 1064nm, and the maximum power is about 2J/cm ² . The pulsed laser irradiates the absorption point below or on the side of the biological tissue being measured. The ultrasound transducer surrounds the biological tissue being tested, with heavy water filling between them. The plane of the ultrasonic transducer array is generally perpendicular to the plane of the laser.

Step 6: The ultrasonic transducer receives the photoacoustic signal emitted by the light absorption point in the tissue;

The ultrasonic transducer used in step six is the same as the ultrasonic transducer used in step two. It also surrounds the biological tissue to be measured, and is filled with heavy water between the ultrasonic transducer and the biological tissue. The ultrasonic transducer receives the ultrasonic wave generated by the photoacoustic effect in step 5. The ultrasonic wave passes through the acoustic lens in the ultrasonic probe and reaches the ultrasonic transducer, and the sound pressure intensity is recorded. The greater the number of ultrasound transducers used, the higher the quality of the reconstructed image. The ultrasonic transducers are arranged in a ring shape. Assume that the number of ultrasonic transducers is , record number/> The sound pressure signal generated by an ultrasonic transducer due to the photoacoustic effect in formula (1) is/> , /> .

Step 7: Calculate the multiple sound velocities involved in the process of propagating the photoacoustic signal to the ultrasonic transducer;

The photoacoustic signal in step seven passes through multiple areas with different sound speeds as it propagates from the light absorption point to the tissue surface, and is finally received by the ultrasound transducer. On the basis of several different sound speed areas drawn in step 4, the sound speed area of the tissue is restricted by the imaging area and is discretized into a grid of a certain size. Assuming that the photoacoustic signal propagates to the ultrasonic transducer along a straight line, the propagation vector is denoted by for , of which/> Represents the light absorption point,/> Indicates ultrasonic transducer, while/> is the starting point of the propagation path,/> is the end point of the propagation path. When calculating the propagation path of the photoacoustic signal in each area, select the propagation vector/> The path that is closest to consistency. As each propagation path is calculated, the sound speed corresponding to the propagation path is also calculated, and finally the multi-sound velocity information on the entire propagation path is obtained.

The propagation path of the photoacoustic signal in each sound speed region needs to be calculated separately. As shown in Figure 3, the photoacoustic signal starts from the light absorption point Propagate to ultrasound transducer/> , the propagation vector is/> , the propagation path of the photoacoustic signal in the sound speed region 2 is calculated as/> ,This is because the vector/> and propagation vector/> The angle is the smallest, that is, closest to the propagation direction. In the same way, the propagation path in sound speed region 3 is found to be/> , the propagation path in area 1 is/> . Finally, the total propagation path of the photoacoustic signal is/> , the corresponding polysonic velocity is/> .

Step 8: Multi-sound velocity adaptation is applied to the delayed summation algorithm to reconstruct the photoacoustic tomography image;

Multisonic velocity adaptation is applied to the calculation of the propagation time of the photoacoustic signal from the absorption point (sound source point) to the ultrasonic transducer. For each absorption point, the delayed summation algorithm formula is as follows:

,

in, Represents the photoacoustic tomography image reconstructed based on the delayed summation algorithm,/> Indicates ultrasonic transducer/> at time/> Received photoacoustic signal,/> Represents the propagation of the photoacoustic signal from the absorption point to the ultrasonic transducer/> the required propagation time, is the weight.

Multisonic adaptation applied to propagation time In the calculation, the calculation formula is as follows:

,

in is the required propagation time,/> is the position of the absorption point,/> For ultrasonic transducers/> , at the speed of sound/> The propagation path in the area is/> ,/> is the boundary point of the area, vector/> Closest propagation vector/> , that is/> and/> The angle between the two vectors is the smallest; similarly, when the speed of sound is/> The propagation path in the area is/> ,/> is the boundary point of the area, and similarly the vector/> Closest propagation vector/> ;Until the end of the propagation path/> The area where the sound speed is/> , the propagation path is/> . Considering that the signal strength received by the ultrasonic transducer is in a cosine relationship with the angle between the absorption point and the ultrasonic transducer, and that the signal strength is inversely proportional to the distance, the weight formula in the present invention is designed as:

,

in, is the distance from the absorption point to the ultrasonic transducer,/> is the angle between the absorption point and the ultrasonic transducer. In the two-dimensional XY plane, with the center of the annular ultrasonic transducer array as the coordinate origin, assuming the absorption point/> The coordinates are , ultrasonic transducer/> The coordinates are/> , then/> The calculation formula is:

,

According to the formula for calculating the angle between two vectors in analytical geometry, The calculation formula is:

.

The present invention will be further described below in conjunction with simulation implementation examples.

The simulation implementation example is as follows:

The k-wave simulation tool is used to generate the experimental sample as shown in Figure 4. The three hairs (marked 1, 2, and 3 in the figure) that form a blood vessel pattern buried in the sample constitute the sound source point and are also the light absorption points. And the sample is divided into three areas with different sound speeds. Area 1 has a sound speed of 1400m/s, area 2 has a sound speed of 1510m/s, and area 3 has a sound speed of 1590m/s. The three hairs are completely included in area 3. The k-wave simulation tool was used to generate a semi-ring-shaped ultrasonic transducer array, containing 512 ultrasonic transducers, evenly distributed on the semi-ring.

Similarly, the k-wave simulation tool is used to simulate ultrasonic emission. The ultrasonic transducer array receives the ultrasonic signal and sends it to the host computer to reconstruct the sound velocity distribution of the sample and divide the sound velocity area.

Similarly, the k-wave simulation tool is used to simulate that after the pulse laser irradiates the sample, the hair emits a photoacoustic signal and propagates outward. The ultrasonic transducer array receives the photoacoustic signal and sends it to the host computer for multi-sound velocity adaptive photoacoustic tomography images. Reconstruction,The reconstructed image is shown in Figure 5(a).

In order to compare the photoacoustic imaging effect between the method proposed in this invention and the method of preset sound speed, the preset sound speed is set to 1500m/s (the average sound speed of the three sound speed areas in the above sample), and is similarly substituted into the delay summation The photoacoustic tomography image is reconstructed in the algorithm, and the reconstructed image is shown in Figure 5(b).

It can be seen from Figure 5(a) and Figure 5(b) that compared with the method of preset sound velocity, the artifacts and distortion of the photoacoustic tomography image obtained by this method are significantly reduced. The three hairs in the picture are clearly visible, and the image quality is improved. Significantly improved.

The description of the above embodiments is only used to help understand the method and the core idea of the present invention. It should be pointed out that for those of ordinary skill in the art, several improvements and modifications can be made without departing from the present invention. , these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims (6)

1. A multi-sound velocity adaptive photoacoustic tomography image reconstruction method, characterized by comprising the following steps: Step 1: The ultrasonic transducer emits ultrasonic waves to irradiate the biological tissue; The ultrasonic transducer both receives ultrasonic waves and emits ultrasonic waves, the ultrasonic transducer array is annular, and multiple ultrasonic transducers are evenly selected in the ultrasonic transducer array to emit ultrasonic waves one after another; Step 2: The ultrasonic transducer receives the ultrasonic signal emitted by the biological tissue; Step 3: Use the iterative reconstruction algorithm of the bending ray model to reconstruct the sound velocity distribution of the biological tissue; Step 4: Divide the biological tissue into several regions with different sound speeds; For the sound velocity distribution reconstructed in step 3, each sound source point is traversed, and other sound source points whose sound speed differs from this point within a set range are classified into the same area. Finally, each area is traversed, and the values within the area are The average sound speed of all sound source points is used as the sound speed of the area, and then a biological tissue is divided into several areas with different sound speeds; Step 5: The laser emits pulsed laser light to irradiate the tissue; Step 6: The ultrasonic transducer receives the photoacoustic signal emitted by the light absorption point in the tissue; Step 7: Calculate the multiple sound velocities involved in the process of propagating the photoacoustic signal to the ultrasonic transducer; In the process of propagating from the light absorption point to the tissue surface, the photoacoustic signal passes through multiple areas with different sound speeds and is finally received by the ultrasonic transducer. Based on the several different sound speed areas drawn in step 4, the sound speed area of the tissue is affected by The imaging area is restricted and discretized into a grid. Assume that the photoacoustic signal propagates to the ultrasonic transducer along a straight line, and the propagation vector is recorded as , of which/> Represents the light absorption point,/> Indicates ultrasonic transducer, while/> is the starting point of the propagation path,/> is the end point of the propagation path. When calculating the propagation path of the photoacoustic signal in each area, select the propagation vector/> For the path that is most consistent, calculate each propagation path and the sound speed corresponding to the propagation path, and finally obtain the multi-sound velocity information on the entire propagation path; Step 8: Multi-sound velocity adaptation is applied to the delayed summation algorithm to reconstruct the photoacoustic tomography image; Multi-sonic velocity adaptation is applied to the calculation of the propagation time of the photoacoustic signal from the absorption point to the ultrasonic transducer. For each absorption point, the delay summation algorithm formula is as follows: , in, Represents the photoacoustic tomography image reconstructed based on the delayed summation algorithm,/> Indicates ultrasonic transducer/> at time/> Received photoacoustic signal,/> Represents the propagation of the photoacoustic signal from the absorption point to the ultrasonic transducer/> the required propagation time, is the weight, Multisonic adaptation applied to propagation time In the calculation, the calculation formula is as follows: , in is the required propagation time,/> is the position of the absorption point,/> For ultrasonic transducers/> , at the speed of sound/> The propagation path in the area is/> ,/> is the boundary point of the area, vector/> Closest propagation vector/> , that is/> and The angle between the two vectors is the smallest; similarly, when the speed of sound is/> The propagation path in the area is/> ,/> is the boundary point of the area, and similarly the vector/> Closest propagation vector/> ;Until the end of the propagation path/> The area where the sound speed is/> , the propagation path is/> ;The weight formula is: , in, is the distance from the absorption point to the ultrasonic transducer,/> is the angle between the absorption point and the ultrasonic transducer. In the two-dimensional XY plane, with the center of the annular ultrasonic transducer array as the coordinate origin, assuming the absorption point/> The coordinates are , ultrasonic transducer/> The coordinates are/> , then/> The calculation formula is: , The calculation formula is: . 2. The multi-sonic adaptive photoacoustic tomography image reconstruction method according to claim 1, characterized in that the ultrasonic transducer in step 2 successively receives the ultrasonic waves emitted by the biological tissue being measured in step 1, and the ultrasonic waves pass through the ultrasonic wave. The sound wave lens in the probe reaches the ultrasonic transducer, which records the sound pressure. 3. The multi-sonic adaptive photoacoustic tomography image reconstruction method according to claim 1, characterized in that the iterative reconstruction algorithm of the bending ray model in step three is as follows: Bending rays are used to characterize an array element in a ring-shaped ultrasonic transducer array that emits an ultrasonic signal. The ultrasonic wave is refracted when passing through areas with different sound speeds, and is finally received by the ultrasonic transducer. The formula is as follows: , in, is the first arrival time vector of the ultrasonic signal reaching the ultrasonic transducer;/> is the slowness vector of the biological tissue being measured;/> It is the coefficient matrix that connects the two, the spatial distribution of the ultrasonic transducer and the slowness distribution of the tissue/> Directly related, during the process of sound velocity reconstruction, the theoretical sound wave arrival time based on the bending ray model and the experimental observation arrival time are continuously compared until the deviation between the two is less than the set threshold. 4. The multi-sonic velocity adaptive photoacoustic tomography image reconstruction method according to claim 1, characterized in that the irradiated biological tissue in step five produces a photoacoustic effect that satisfies the equation: , (1) in It is the sound pressure generated by the thermal expansion of biological tissues,/> is the specific heat capacity,/> is the isobaric volume thermal expansion coefficient,/> is the empirical sound speed,/> is the heat source function excited by the incident pulse laser,/> ,/> is the spatial light absorption function of biological tissue,/> As a function of light irradiation, the laser hits the absorption point and the ultrasonic transducer starts to receive signals at the same time. 5. The multi-sonic velocity adaptive photoacoustic tomography image reconstruction method according to claim 4, characterized in that the pulsed laser irradiates the absorption point below or on the side of the biological tissue to be measured, and the plane where the ultrasonic transducer array is located and the laser is located. The plane is vertical. 6. The multi-sonic adaptive photoacoustic tomography image reconstruction method according to claim 4, characterized in that the ultrasonic transducer used in step six is consistent with the ultrasonic transducer used in step two, and the ultrasonic transducer is The transducer receives the ultrasonic waves generated by the photoacoustic effect in step 5 and reaches the ultrasonic transducer through the acoustic lens in the ultrasonic probe, and records the sound pressure intensity. Suppose the number of ultrasonic transducers is , record number/> The sound pressure signal generated by an ultrasonic transducer due to the photoacoustic effect in formula (1) is/> , .