CN113842158A - Photoacoustic/ultrasonic endoscopic probe based on fixed-focus sound field and dynamic focus reconstruction algorithm - Google Patents

Abstract

The invention relates to the field of endoscopic imaging, and discloses a photoacoustic/ultrasonic endoscopic probe based on a fixed-focus sound field and a dynamic focus reconstruction algorithm, which comprises the following steps of 1) determining an equivalent focus sound field and an equivalent focus of a transducer according to inherent parameters of the transducer; 2) determining the length of a path, which is F, of a signal fed back by the equivalent focus received by the transducer by taking the equivalent focus as a reference point, and determining the equivalent beam waist radius omega of a sound field focused by the transducer₀And equivalent half depth of focus Z₀(ii) a 3) And calculating the pixel value of a single pixel point obtained by scanning the transducer according to the parameters, and processing the pixel point by a delay superposition algorithm to obtain a final image. The invention optimizes the conventional delay superposition algorithm to obtain a novel delay superposition algorithm, and adopts a novel endoscopic probe which is combined with the algorithm for use, so that under the premise of ensuring the imaging speed,the imaging definition and accuracy of the final imaging are improved.

Description

Photoacoustic/Ultrasonic Endoscopic Probe and Dynamic Focus Reconstruction Algorithm Based on Fixed-focus Sound Field

technical field

The invention relates to the field of endoscopic imaging, in particular to a photoacoustic/ultrasonic endoscopic probe based on a fixed-focus sound field and a dynamic focus reconstruction algorithm.

Background technique

Acoustic focused photoacoustic endoscopic imaging is an emerging biomedical endoscopic imaging method that integrates high resolution, high penetration depth, and high optical contrast. In many acoustic focusing photoacoustic endoscopic imaging systems, since a piezoelectric effect-based transducer can be used to receive ultrasound, and this transducer can also transmit ultrasound, the photoacoustic/ultrasonic imaging uses the same transducer. Therefore, the devices can be easily integrated together to form an acoustically focused photoacoustic/ultrasonic imaging system. Photoacoustic/ultrasound imaging has good application prospects in specific occasions.

In the conventional photoacoustic/ultrasonic endoscopic imaging probe, its basic structure enables it to perform 360-degree circular scanning imaging. In addition, a planar ultrasonic transducer can also be used, and then a parabolic ultrasonic and laser mirror can be used to form a focused sound field. In the imaging of such systems, the photoacoustic and endoscopic ultrasound imaging algorithms are relatively similar. Generally, they use a linear propagation model similar to the ultrasonic beam commonly used in B-mode ultrasound, and are calculated directly according to the distance of the pixel relative to the transducer and the speed of sound. The delay time, and the rotation angle of the transducer during endoscopic scanning are used to fan out the resulting data for imaging. Among them, since ultrasound involves the round-trip of the signal, the sound speed needs to be set to half of the actual sound speed during ultrasound imaging.

This imaging method is simple and straightforward, but due to the application of the fixed-focus probe, it can only obtain the best lateral resolution at the focal position, including the best resolution in the tangential direction and along the axial direction of the probe, and the best resolution in the far-focus position. area, its lateral resolution will decrease rapidly. When the numerical aperture of the ultrasonic sensor is larger, its optimal lateral resolution is the smallest, but the imaging depth of field is also smaller in this case, and vice versa. This situation is mainly caused by the fact that the current acoustic focusing photoacoustic/ultrasonic endoscopic imaging algorithm does not consider the focused sound field of the actual transducer, so a new algorithm is urgently needed to consider the sound field of the transducer, so that The out-of-focus area can also have a higher lateral resolution, thereby ensuring the clarity and accuracy of the overall image.

SUMMARY OF THE INVENTION

The problem to be solved by the present invention is to provide a photoacoustic/ultrasonic endoscopic dynamic focusing reconstruction algorithm based on a fixed-focus sound field, which can improve the lateral resolution of the out-of-focus region and make the imaging clearer and more accurate.

The further problem to be solved by the present invention is to provide a photoacoustic/ultrasonic endoscopic probe based on a fixed-focus sound field, which can improve the lateral resolution of the out-of-focus region and make the imaging clearer and more accurate.

In order to solve the above technical problems, the present invention provides a photoacoustic/ultrasonic endoscope probe based on a fixed-focus sound field and a dynamic focus reconstruction algorithm, including the following steps: 1) According to the inherent parameters of the transducer, determine the transducer 2) Taking the equivalent focus as the reference point, determine the length of the path traversed by the signal fed back by the equivalent focus received by the transducer as F, and determine the transducer The equivalent beam waist radius ω ₀ and the equivalent semi-focal depth Z ₀ of the focused sound field of the transducer; 3) According to the above parameters, calculate the pixel value of a single pixel point scanned by the transducer, and superimpose the original time delay The algorithm is optimized to obtain a new type of delay stacking algorithm. Through this new type of delay stacking algorithm, the scanned image pixels are processed, and the time length of the ultrasonic signal corresponding to the pixel received by the transducer is determined. sort to get the final image.

Preferably, in step 3, the final image is obtained by the following formula:

为需要重建的像素坐标，S(i,t)为换能器在第i个位置时换能器接收到的信号，t为时间，

为第i个像素点所在位置的相位系数，

为第i个像素点所在位置的权重系数。通过该优选技术方案，将传统的延时叠加算法进行优化，得到新的延时叠加算法，延时叠加算法能够通过简单的计算推导，从而得到最终的图像数据，并且通过该叠加算法的计算，所需要的计算时间更短，效率更高，并且该算法，在保证了计算效率的同时，提高了成像的清晰度，使得最终的成像更加清晰准确。Among them, Nd is the number of directional positions scanned by the transducer,

is the pixel coordinate to be reconstructed, S(i,t) is the signal received by the transducer when the transducer is at the i-th position, t is the time,

is the phase coefficient at the position of the i-th pixel,

is the weight coefficient of the position of the i-th pixel. Through this preferred technical solution, the traditional time delay superposition algorithm is optimized to obtain a new time delay superposition algorithm. The time delay superposition algorithm can be deduced through simple calculation, so as to obtain the final image data, and through the calculation of the superposition algorithm, The required calculation time is shorter and the efficiency is higher, and the algorithm, while ensuring the calculation efficiency, improves the clarity of the imaging, making the final imaging clearer and more accurate.

Further preferably, the phase coefficient is determined by the relative position of the pixel point and the focus of the transducer. In photoacoustic imaging, the phase coefficient is:

In ultrasound imaging, the phase coefficient is:

Wherein, v is the propagation velocity of ultrasonic waves in the medium, a is the axial distance of the pixel point relative to the transducer focus, and b is the radial distance of the pixel point relative to the transducer focus. Through this preferred technical solution, by defining the phase coefficients of photoacoustic imaging and ultrasonic imaging, the device can more accurately locate the relative positional relationship between the pixel point and the transducer focus when performing photoacoustic imaging and ultrasonic imaging. , thereby improving the accuracy of imaging.

Preferably, the expression of the axial distance a between the pixel point and the focus of the transducer is as follows:

为换能器在第i个位置时的焦点，

为换能器第i个位置时的旋转中心。通过该优选技术方案，确定了像素点与换能器焦点之间的轴向距离，从而能够提高图像像素点在轴向距离的方向上成像的准确性。in,

is the focus of the transducer at the i-th position,

is the rotation center of the transducer at the i-th position. With this preferred technical solution, the axial distance between the pixel point and the transducer focus is determined, so that the imaging accuracy of the image pixel point in the direction of the axial distance can be improved.

Further preferably, the expression of the radial distance b of the pixel point from the transducer focus is as follows:

为换能器在第i个位置时的焦点，

为换能器第i个位置时的旋转中心。通过该优选技术方案，确定了像素点与换能器焦点之间的径向距离，从而能够提高图像像素点在径向距离的方向上成像的准确性。in,

is the focus of the transducer at the i-th position,

is the rotation center of the transducer at the i-th position. Through this preferred technical solution, the radial distance between the pixel point and the focus of the transducer is determined, so that the imaging accuracy of the image pixel point in the direction of the radial distance can be improved.

为第i个像素所在位置的权重系数，所述权重系数可通过如下的公式计算得出：Preferably, the

is the weight coefficient of the position of the i-th pixel, and the weight coefficient can be calculated by the following formula:

为一个由当前像素点与换能器的距离所决定的位置限制函数。通过该优选技术方案，将权重系数通过位置限制函数进行限制，能够进一步对像素点的位置进行限定，从而能够更加准确的确定像素点的相对位置，进而保证了叠加后的图像的准确性。in,

is a position limit function determined by the distance between the current pixel and the transducer. Through the preferred technical solution, the weight coefficient is limited by the position limit function, and the position of the pixel point can be further limited, so that the relative position of the pixel point can be more accurately determined, thereby ensuring the accuracy of the superimposed image.

Further preferably, the position limit function limits the range of the delay stacking in the following manner:

Wherein, a ₁ , a ₂ , b ₁ and b ₂ are the limit parameters of the position limit function, and the limit parameters are calculated by the following formula:

a ₂ =R, b ₁ =-R, b ₂ =R,

Among them, D is the diameter of the endoscopic probe, and R is the radius of the imaging area. Through this preferred technical solution, a limit parameter is introduced to further limit the range of time-lapse stacking, and the limit parameter is combined with the inherent parameters of the probe and the final imaging parameters, thereby improving the practicability of the limit function, making the limit function capable of The range of the time-lapse stacking is accurately limited, thereby improving the accuracy of the image after the time-lapse stacking.

A second aspect of the present invention provides a photoacoustic/ultrasonic endoscopic probe based on a fixed-focus sound field, comprising: a transparent casing, a calcium fluoride lens, a reflector, a protective structure and a transducer, the endoscopic probe is composed of a rotating motor Drive its movement to complete the lateral annular scanning imaging operation, the transparent shell wraps the outer surface of the endoscope probe, the sound wave emitted by the transducer is refracted and focused by the calcium fluoride mirror and the reflector, When irradiated on the probe, the endoscopic probe can implement the photoacoustic/ultrasonic endoscopic dynamic focus reconstruction algorithm based on the fixed-focus sound field provided in the first aspect of the present invention.

Preferably, the transducer is a flat-field ultrasonic transducer. Through this preferred technical solution, a flat-field ultrasonic transducer is used, and its sound field is more uniform, and its manufacturing difficulty is lower than that of a focused ultrasonic transducer, and the receiving surface is flat to facilitate the reception of sound wave signals fed back from various positions , thereby improving the clarity of the final image imaging.

Further preferably, the reflecting mirror can be a parabolic reflecting mirror, and the parabolic reflecting mirror takes the arc height as a reference line, and the reference line is perpendicular to the center of the calcium fluoride mirror. Through this preferred technical solution, the parabolic reflector is used to make the transducer and the focal point mirror-symmetrical with respect to the center of the parabolic reflector, and the reflector can realize photoacoustic and ultrasonic reflection, so that the device is generally applicable to various photoacoustic and ultrasound examinations.

Through the above technical solutions, the photoacoustic/ultrasonic endoscopic dynamic focusing reconstruction algorithm based on fixed-focus sound field provided by the present invention optimizes the conventional time delay stacking algorithm, optimizes and limits the parameters therein, and introduces the limiting parameters, thereby A new type of time-lapse stacking algorithm is obtained, through which the imaging clarity and accuracy of the final imaging can be improved on the premise of ensuring the imaging speed.

The present invention also provides a photoacoustic/ultrasonic endoscope probe based on a fixed-focus sound field. By using a plane ultrasonic transducer in combination with a parabolic mirror to work, the general applicability of the device is improved, and the device can receive signals from various Angle feedback sound wave signal, which increases the clarity and accuracy of its final image.

Other features and advantages of the present invention will be described in detail in the detailed description that follows.

Description of drawings

1 is a structural diagram of an acoustic focusing photoacoustic endoscope probe used in the prior art and a scanning equivalent structural diagram;

Fig. 2 is the structure diagram and scanning equivalent structure diagram of the acoustic focusing photoacoustic endoscope probe based on the parabolic reflector used in the present invention;

Fig. 3 is the mirror structure schematic diagram used in the present invention;

Fig. 4 is the structure diagram of the object scanned by the present invention;

Fig. 5 (a) and (b) are respectively the photoacoustic result graph that the algorithm of the present invention is compared with the original algorithm;

Figures 6(a) and (b) are respectively the ultrasound result graphs comparing the algorithm of the present invention with the original algorithm.

reference number

1 Transparent housing 2 Hollow acoustically focused ultrasound transducer

3 Multimode fiber 4 45 degree reflector on top

5 calcium fluoride lens 6 reflector

7 Protective structure 8 Flat-field ultrasonic transducer

Detailed ways

The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, but not to limit the present invention.

In the description of the present invention, it should be noted that, unless otherwise expressly specified and limited, the terms "installation", "arrangement" and "connection" should be understood in a broad sense. For example, the term "connection" may be a fixed connection or a It can be a detachable connection or an integral connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal communication of two elements or an interaction relationship between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

An embodiment of the photoacoustic/ultrasonic endoscope probe and dynamic focus reconstruction algorithm based on the fixed-focus sound field of the present invention includes the following steps: 1) According to the inherent parameters of the transducer, determine the equivalent focus of the transducer Sound field and equivalent focus; 2) Taking the equivalent focus as the reference point, determine the path length of the signal fed back by the equivalent focus received by the transducer as F, and determine the amount of sound field gathered by the transducer The equivalent beam waist radius ω ₀ and the equivalent half focal depth Z ₀ ; 3) According to the above parameters, calculate the pixel value of a single pixel point scanned by the transducer, and optimize the original delay stacking algorithm, A new type of delay stacking algorithm is obtained. Through the new type of delay stacking algorithm, the image pixels obtained by scanning are processed and sorted according to the time length of the ultrasonic signals corresponding to the pixels received by the transducer. Get the final image.

Compared with the existing algorithm, the present invention not only considers the actual shape of the focused sound field, but also realizes the dynamic focusing of the endoscopic probe in the tangential direction by delaying, superimposing and reconstructing the signals collected by multiple transducer positions. Effectively expand the depth of field of its imaging, so as to obtain a full range of high-resolution acoustic focusing photoacoustic endoscopic imaging results.

In some embodiments of the photoacoustic/ultrasonic endoscope probe and dynamic focus reconstruction algorithm based on the fixed-focus sound field of the present invention, in step 3, using the data parameters obtained in the above steps 1 and 2, the final result is obtained by the following formula image:

为需要重建的像素坐标，S(i,t)为换能器在第i个位置时换能器接收到的信号，t为时间，

为第i个像素点所在位置的相位系数，

为第i个像素点所在位置的权重系数。Among them, Nd is the number of directional positions scanned by the transducer,

is the pixel coordinate to be reconstructed, S(i,t) is the signal received by the transducer when the transducer is at the i-th position, t is the time,

is the phase coefficient at the position of the i-th pixel,

is the weight coefficient of the position of the i-th pixel.

Among them, the phase coefficient is determined by the relative position of the pixel point and the transducer focus. In photoacoustic imaging, the phase coefficient is:

In ultrasound imaging, the phase coefficient is:

Among them, v is the propagation speed of ultrasonic waves in the medium, a is the axial distance of the pixel point relative to the focal point of the transducer, b is the radial distance of the pixel point relative to the focal point of the transducer, and the pixel point distance The expressions for the axial distance a and the radial distance b of the energizer focus are as follows:

为换能器在第i个位置时的焦点，

为换能器第i个位置时的旋转中心，

为第i个像素所在位置的权重系数，所述权重系数可通过如下的公式计算得出：in,

is the focus of the transducer at the i-th position,

is the rotation center of the transducer at the i-th position,

is the weight coefficient of the position of the i-th pixel, and the weight coefficient can be calculated by the following formula:

为一个由当前像素点与换能器的距离所决定的位置限制函数，该位置限制函数通过以下方式来限制延时叠加的范围：in,

is a position limit function determined by the distance between the current pixel and the transducer. The position limit function limits the range of the delay stacking in the following ways:

Wherein, a ₁ , a ₂ , b ₁ and b ₂ are the limit parameters of the position limit function, and the limit parameters are calculated by the following formula:

a ₂ =R, b ₁ =-R, b ₂ =R,

Among them, D is the diameter of the endoscopic probe, and R is the radius of the imaging area. It can be seen from the above-mentioned endoscopic imaging algorithm that, compared with the existing imaging algorithms in use, the novel time-lapse stacking algorithm adopted in the present invention is optimized on the basis of the conventional time-lapse stacking algorithm, thereby obtaining a new type of delay stacking algorithm. The new time-lapse superposition algorithm not only retains the effects of the traditional time-lapse superposition algorithm in imaging with fast calculation speed and high imaging efficiency, but also improves the final imaging clarity, so that the algorithm of the present invention is adopted. In the field of endoscopic imaging, it can not only ensure the imaging efficiency, but also improve the clarity and accuracy of imaging.

As shown in Fig. 1, the endoscopic imaging probe used by the existing endoscopic imaging algorithm includes a transparent casing 1, a hollow acoustically focused ultrasonic sensor 2, a multi-mode optical fiber 3, and a top 45-degree reflector 4 components. to reflect light and ultrasound. During scanning, as the top mirror 6 rotates, it can perform lateral annular scanning imaging.

In Fig. 1, the equivalent beam waist radius ω ₀ and the equivalent semi-focal depth Z ₀ are obtained by the following formulas:

where f ₀ is the center frequency of the endoscopic probe, and NA is the numerical aperture of the transducer.

As shown in the figure, L1 represents the distance between the hollow focused ultrasonic sensor and the center of the mirror 6, and L2 represents the distance between the center of the mirror 6 and the focal point of the hollow focused ultrasonic sensor. In the ordinary acoustic focusing photoacoustic endoscopic reconstruction algorithm In this algorithm, the reconstruction value of any point (θ, r) represented by polar coordinates on the image is only determined by the signal of a single detector. :

I(θ,r)=S _θ (r/v))

where S _θ (t) represents the signal received by the transducer at the angle θ, and v is the propagation speed of ultrasound in the medium. However, since it does not consider the size and shape of the detection surface of the ultrasonic sensor, it only The best resolution results are obtained where the focus is.

As shown in FIG. 2 to FIG. 3, it is an endoscopic imaging probe used by the endoscopic imaging algorithm in the present invention, the probe includes a transparent casing 1, a calcium fluoride lens 5, a reflector 6, a protective structure 7 and a flat surface used for In the field ultrasonic transducer 8, the probe moves as a whole under the drive of the rotating motor, and completes the lateral annular scanning imaging operation, wherein, L3 in the figure represents the distance between the flat field ultrasonic sensor 8 and the center of the calcium fluoride lens 5, L4 represents the distance between the center of the calcium fluoride lens 5 and the center of the reflector 6, and L5 represents the focal length of the parabolic reflector 6. During scanning, due to the presence of the reflector 6, it is equivalent to the plan ultrasonic sensor 8 being located in the dotted line in the figure. Marked mirror position, at the same time, the reflector 6 realizes the function of the acoustic lens, so the sound field at the rear end of the reflector 6 is a focused sound field, as shown in Figure 2. Therefore, during the scanning process, the same Compared with the endoscopic imaging device, the hollow focused ultrasonic sensor 2 in the existing endoscopic imaging device performs rotational scanning on a scanning path with a radius of L3+L4 around the center of rotation.

The calcium fluoride lens 5 can transmit light and reflect sound. When performing photoacoustic detection, when a pulsed laser is used to scan the tissue for detection, the pulsed laser can pass through the calcium fluoride lens 5 and irradiate on the reflecting mirror 6. The reflecting mirror 6 is a customized parabolic reflector 6, which can reflect photoacoustic and ultrasonic waves. When the pulsed laser passes through the calcium fluoride lens 5 and irradiates the parabolic reflector 6, the pulsed laser passes through the parabolic reflector 6. Focusing, the pulsed laser is focused on the equivalent focus of the focused sound field, and the equivalent beam waist radius ω ₀ and the equivalent half focal depth Z ₀ of the focused sound field are determined accordingly. When the pulsed laser acts on the tissue, the irradiated tissue Respond quickly, and the feedback ultrasonic signal is received by the flat-field ultrasonic transducer 8, so as to specify the data parameters of the feedback information.

When performing ultrasonic detection, when the flat-field ultrasonic transducer 8 emits ultrasonic waves for detection, the emitted ultrasonic waves are refracted by the light-transmitting and reflective calcium fluoride lens 5, and the refracted ultrasonic waves are again focused by the parabolic reflector 6, and finally Acting on the tissue, the tissue is subjected to the action of ultrasound to feed back an ultrasound signal, which is finally received by the flat-field ultrasound transducer 8, and specific feedback information parameters are obtained.

Taking the situation shown in Figure 4 as an example, the diameter of the hollow area of the detection background is 5mm, and the overall outer diameter is 23mm. In the x-direction, there are 9 point targets evenly distributed at positions of 3mm to 11mm. The diameter of the transducer used for acquisition is 10mm, the focal length is 20mm, and the distance L5 from the focal point to the center of rotation is 7mm. The focused ultrasound sensor has a center frequency of 15MHz and a bandwidth of 75%.

Figure 5(a) is the photoacoustic image obtained by the original acoustic focusing photoacoustic endoscopic algorithm. The lateral resolution of the target at the 7mm position is the highest. The depth of field is very limited.

Fig. 5(b) is a photoacoustic image obtained by using the photoacoustic endoscopic dynamic focusing reconstruction algorithm based on the fixed-focus sound field in the present invention. Compared with Fig. 5(a), it can be seen that the algorithm in the present invention is used to divide the 7mm position target. Except that the lateral resolution is not much different from Figure 5(a), the lateral resolutions of other targets in the figure have been significantly improved. It can be seen that the algorithm used in the present invention can produce relatively obvious effects in photoacoustic imaging. improve.

Figure 6(a) is the ultrasound image obtained by the original acoustic focusing photoacoustic endoscopic algorithm. The lateral resolution of the target at the 7mm position is the highest. Very limited.

Fig. 6(b) is an ultrasonic image obtained by using the ultrasonic endoscopic dynamic focusing reconstruction algorithm based on the fixed-focus sound field in the present invention. Compared with Fig. 6(a), it can be seen that the algorithm in the present invention is used to divide the lateral resolution of the 7mm position target. 6(a), the lateral resolutions of other targets in the figure have been significantly improved. It can be seen that the algorithm used in the present invention can significantly improve the field of ultrasound imaging.

Through the above-mentioned dynamic focus reconstruction algorithm of photoacoustic/ultrasonic endoscopy based on fixed-focus sound field, it can be clearly seen that the algorithm has universal applicability in different system structures, and in the photoacoustic and endoscopic ultrasound experiments, the On the premise of the lateral resolution of the focal area, the lateral resolution of the out-of-focus area is significantly improved, resulting in a clearer and more accurate final image.

In the description of the present invention, reference to the description of the terms "one embodiment", "some embodiments", "one embodiment", etc. means that a particular feature, structure, material or characteristic described in connection with the embodiment or example is contained in in at least one embodiment or example of the present invention. In the present invention, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

The preferred embodiments of the present invention have been described above in detail with reference to the accompanying drawings, however, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, a variety of simple modifications can be made to the technical solutions of the present invention, including combining various specific technical features in any suitable manner. No further explanation is required. However, these simple modifications and combinations should also be regarded as the contents disclosed in the present invention, and all belong to the protection scope of the present invention.

Claims (10)

1. a photoacoustic/ultrasonic endoscopic dynamic focusing reconstruction algorithm based on fixed-focus sound field, is characterized in that, comprises the following steps: 1) According to the inherent parameters of the transducer, determine the equivalent focused sound field and equivalent focus of the transducer; 2) Taking the equivalent focus as the reference point, determine the path length of the signal fed back by the equivalent focus received by the transducer as F, and determine the equivalent beam waist radius ω of the focused sound field of the transducer ₀ and the equivalent semi-focal depth Z ₀ ; 3) According to the above parameters, calculate the pixel value of a single pixel point scanned by the transducer, and optimize the original delay stacking algorithm to obtain a new type of delay stacking algorithm. Through this new type of delay stacking algorithm, The image pixels obtained by scanning are processed and sorted according to the time length of the ultrasonic signals corresponding to the pixels received by the transducer, so as to obtain a final image. 2. The photoacoustic/ultrasonic endoscopic dynamic focusing reconstruction algorithm based on fixed-focus sound field according to claim 1, is characterized in that, in step 3, obtain the final image by following formula:

Among them, Nd is the number of directional positions scanned by the transducer,

is the pixel coordinate to be reconstructed, S(i,t) is the signal received by the transducer when the transducer is at the i-th position, t is the time,

is the phase coefficient at the position of the i-th pixel,

is the weight coefficient of the position of the i-th pixel. 3. The photoacoustic/ultrasonic endoscopy dynamic focusing reconstruction algorithm based on fixed-focus sound field according to claim 2, wherein the phase coefficient is determined by the relative position of the pixel point and the focus of the transducer. In imaging, the phase coefficient is:

In ultrasound imaging, the phase coefficient is:

Wherein, v is the propagation velocity of ultrasonic waves in the medium, a is the axial distance of the pixel point relative to the transducer focus, and b is the radial distance of the pixel point relative to the transducer focus. 4. The photoacoustic/ultrasonic endoscopy dynamic focus reconstruction algorithm based on fixed-focus sound field according to claim 3, wherein the expression of the axial distance a of the pixel point from the transducer focus is as follows:

in,

is the focus of the transducer at the i-th position,

is the rotation center of the transducer at the i-th position. 5. the photoacoustic/ultrasonic endoscopy dynamic focusing reconstruction algorithm based on fixed-focus sound field according to claim 3, is characterized in that, the expression of the radial distance b of described pixel point distance transducer focus is as follows:

in,

is the focus of the transducer at the i-th position,

is the rotation center of the transducer at the i-th position. 6. The photoacoustic/ultrasonic endoscopic dynamic focusing reconstruction algorithm based on fixed-focus sound field according to claim 3, wherein the

is the weight coefficient of the position of the i-th pixel, and the weight coefficient can be calculated by the following formula:

in,

is a position limit function determined by the distance between the current pixel and the transducer. 7. The photoacoustic/ultrasonic endoscopic dynamic focus reconstruction algorithm based on fixed-focus sound field according to claim 6, wherein the position limit function limits the range of time delay stacking in the following manner:

Wherein, a ₁ , a ₂ , b ₁ and b ₂ are the limit parameters of the position limit function, and the limit parameters are calculated by the following formula:

a ₂ =R, b ₁ =-R, b ₂ =R, Among them, D is the diameter of the endoscopic probe, and R is the radius of the imaging area. 8. A photoacoustic/ultrasonic endoscopic probe based on a fixed-focus sound field, characterized in that it comprises: a transparent casing, a calcium fluoride lens, a reflector, a protective structure and a transducer, and the endoscopic probe is driven by a rotating motor When it moves, the transparent shell is wrapped around the outer surface of the endoscope probe, and the signal wave emitted by the transducer is irradiated to the reflector through the calcium fluoride lens, and is focused on the probe by the reflector. , and the endoscopic probe can implement the algorithm of any one of claims 1 to 7 . 9 . The photoacoustic/ultrasonic endoscopic probe based on a fixed-focus sound field according to claim 8 , wherein the transducer is a flat-field ultrasonic transducer. 10 . 10. The photoacoustic/ultrasonic endoscope probe based on a fixed-focus sound field according to claim 8, wherein the reflector can be a parabolic reflector, and the parabolic reflector takes the arc height as a reference line, so The reference line is perpendicular to the center of the calcium fluoride lens.

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