CN109512389B - Photoacoustic tomography reconstruction algorithm based on finite-size planar transducer - Google Patents

Photoacoustic tomography reconstruction algorithm based on finite-size planar transducer Download PDF

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CN109512389B
CN109512389B CN201811257621.2A CN201811257621A CN109512389B CN 109512389 B CN109512389 B CN 109512389B CN 201811257621 A CN201811257621 A CN 201811257621A CN 109512389 B CN109512389 B CN 109512389B
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肖嘉莹
王波
彭宽
朱自强
肖梦迪
苏天宁
魏宁宁
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Abstract

The invention discloses a photoacoustic tomography reconstruction algorithm based on a limited-size planar transducer. In existing photoacoustic tomography reconstruction algorithms, the planar ultrasound transducer used is usually treated as an ideal point transducer or an infinite size planar transducer, but since the planar size of most ultrasound transducers in photoacoustic tomography is limited, the off-center targets will blur the tangential direction of the two reconstruction model images. Compared with the existing algorithm, the photoacoustic tomography reconstruction algorithm based on the limited-size planar transducer provided by the invention matches the size of the probe by prolonging the back projection distance, so that the tangential distortion of a photoacoustic tomography image of the limited-size planar transducer is effectively and quickly recovered, the tangential resolution is improved, and an omnidirectional high-resolution photoacoustic tomography result is obtained.

Description

Photoacoustic tomography reconstruction algorithm based on finite-size planar transducer
Technical Field
The invention relates to the technical field of photoacoustic tomography, in particular to a photoacoustic tomography reconstruction algorithm based on a limited-size planar transducer.
Background
The photoacoustic computed tomography technology is a medical imaging technology which has high imaging contrast and little harm to human bodies and is developed rapidly in recent years. Two-dimensional ring scanning is one of the most common implementation modes in photoacoustic tomography, and has been widely applied to visualization of a small animal cerebrovascular network, tumor detection of a nude mouse and imaging of a human finger joint structure. In this scanning mode, a plan view sensor is typically employed to perform a circular scan around the imaging target (or a circular array is present to avoid a circular mechanical scan).
Many considerations in the design of photoacoustic tomography (CSPAT) systems include optical illumination, system miniaturization, lower cost, less acquisition time, geometric constraints on the imaged object, etc., and image quality and spatial resolution remain the most important priorities; on the one hand, the distribution and characteristics of the ultrasound transducers play a key role in the spatial resolution of the images obtained in the CSPAT, and in order to ensure that the transducers collect all the ultrasound sources in the image domain at the desired locations of the CSPAT, sensors with large apertures or wide acceptance angles are generally employed, with dimensions in the order of millimeters. On the other hand, most reconstruction algorithms in CSPAT, such as the conventional backprojection algorithm, assume that the ultrasound transducer is of ideal point or infinite size, in which case the model mismatch between the limited transducer size in the reconstruction algorithm and the two reconstruction model approximations described above will result in some rotational blurring in the tangential direction in the image, these spin-blurring artifacts caused by the limited transducer aperture being the result of spatial averaging of the acoustic wave over the transducer area, which results in temporal smearing in the detected signal, which will translate into spatial smearing in the reconstructed image. Theoretical analysis shows that for conventional annular or spherical scanning, the bandwidth of the transducer inherently limits the image resolution, and the tangential resolution is proportional to the distance from the center of rotation and is equal to the transducer detection face aperture size. Currently, research in photoacoustic tomography has developed many advanced algorithms to improve tangential resolution, most of which are model-based, by addressing the geometric and other acoustic properties of the transducer to improve tangential resolution; however, these methods are complex and require a large number of values to perform a large matrix calculation.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a photoacoustic tomography reconstruction algorithm based on a limited-size planar transducer, compared with the existing algorithm, the photoacoustic tomography reconstruction algorithm based on the limited-size planar transducer matches the size of a probe by prolonging the back projection distance, effectively and quickly recovers the tangential distortion of a CSPAT image of the limited-size planar transducer, improves the tangential resolution of the CSPAT image, and accordingly obtains an omnidirectional high-resolution photoacoustic computer tomography result.
In order to achieve the above object, the present invention provides a photoacoustic tomography reconstruction algorithm based on a finite-size planar transducer, comprising the steps of:
1) fixing an imaging target in an annular ultrasonic transducer array, wherein medium water is filled between the annular ultrasonic transducer array and the imaging target;
2) the laser outputs laser pulses, so that the laser pulses uniformly irradiate the imaging target, and the imaging target is excited to generate an ultrasonic pulse signal;
3) each actual ultrasonic transducer in the annular ultrasonic transducer array simultaneously starts to record ultrasonic pulse signals reaching the actual ultrasonic transducer, the ultrasonic pulse signals are converted into electric signals, the received electric signals are amplified through the signal generation receiver and transmitted to the data acquisition system for electric signal acquisition; processing the acquired electric signals to obtain a reconstruction graph of a photoacoustic tomography reconstruction algorithm based on the limited-size planar transducer;
4) calculating an imaging target image by using a back projection algorithm
Figure GDA0003017600400000021
The signal value Si (t) of the ith actual ultrasonic transducer Pi corresponding to the pixel point is imaged to form the target image
Figure GDA0003017600400000022
The image value of the pixel point is equal to the reception of each actual ultrasonic transducer in the annular ultrasonic transducer array
Figure GDA0003017600400000023
By superposition of signal values of pixels, i.e.
Figure GDA0003017600400000031
Wherein the actual ultrasoundThe distance between the transducer and an imaging target is R, the time required by the received ultrasonic pulse signal is t, and an imaging target image is formed
Figure GDA0003017600400000032
The distance between the pixel point and the ith actual ultrasonic transducer Pi is R (x, y, Pi), and the image of the imaging target is
Figure GDA0003017600400000033
The signal value Si (R (x, y, Pi)/v) of the ith actual ultrasonic transducer Pi corresponding to the pixel point is shown, and v is the propagation speed of the ultrasound in the medium;
5) setting the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer to be L, and imaging
Figure GDA0003017600400000034
Figure GDA0003017600400000035
The distance between the pixel point and the virtual ultrasonic transducer is R' ═ R + L, then
Figure GDA0003017600400000036
The image value of the pixel point is equal to the reception of each virtual ultrasonic transducer
Figure GDA0003017600400000037
By superposition of signal values of pixels, i.e.
Figure GDA0003017600400000038
Wherein the image
Figure GDA0003017600400000039
The pixel point is at a distance R ' (x, y, Pi ') from the ith virtual ultrasonic transducer Pi ', and the image of the imaging target is imaged
Figure GDA00030176004000000310
Actual ultrasonic transducer at pixel point correspondenceThe signal value of Pi becomes Si [ (R '(x, y, Pi') -L)/v]；
6) Image processing method
Figure GDA00030176004000000311
The polar coordinates of the distance of the pixel point at and the ith virtual ultrasonic transducer Pi' are converted into rectangular coordinates, and equation (2) becomes:
Figure GDA00030176004000000312
where R is the diameter of the actual ultrasound transducer scanning trajectory, N is the number of actual ultrasound transducers, Si(t) is the signal value received by the actual ultrasound transducer at the i-th position, θiIs the angular coordinate of the ith actual ultrasonic transducer position, v is the propagation velocity of the ultrasound in the medium, and R + L is the diameter of the virtual ultrasonic transducer scanning track;
7) according to the vector subtraction and distance formula, the image reconstruction algorithm formula of the imaging target is as follows:
Figure GDA0003017600400000041
wherein I is an image
Figure GDA0003017600400000042
The reconstructed image value is processed, n is the scanning position number of the annular ultrasonic transducer array, Si(t) is the signal value received by the ith actual ultrasound transducer,
Figure GDA0003017600400000043
and L is the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer, and v is the propagation speed of the ultrasonic in the medium.
Preferably, in the image reconstruction process, a spherical scan is performed by using the annular ultrasound transducer array, and in a three-dimensional space, the spherical scan is formed by m two-dimensional plane scanning processes, that is, each actual ultrasound transducer has n × m scanning positions in total, then equation (4) becomes:
Figure GDA0003017600400000044
wherein I is an image
Figure GDA0003017600400000045
Processing the reconstructed image value, Si,j(t) is the signal received by the actual ultrasound transducer at the (i, j) position,
Figure GDA0003017600400000046
and (3) the position of the (i, j) th actual ultrasonic transducer, L is the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer, v is the propagation speed of the ultrasonic wave in the medium, and n multiplied by m is the number of scanning points of the actual ultrasonic transducer.
Preferably, the laser in step 2) is a pulse laser, and the laser pulse wavelength is 532 nm.
Preferably, the angular spacing between adjacent actual ultrasonic transducers is 1 degree, and 360 actual ultrasonic transducers are mounted on the annular ultrasonic transducer array.
Preferably, the processing of the acquired electrical signals in step 3) is to perform signal processing for image reconstruction by applying an image reconstruction algorithm of formula (4) in combination with a MATLAB software program in a computer.
Preferably, the processing of the acquired electrical signals is an image reconstruction by applying an image reconstruction algorithm of formula (5) in combination with a MATLAB software program in a computer.
Through the technical scheme, the invention has the beneficial effects that:
(1) in the existing photoacoustic tomography based on annular scanning, most models of ideal point transducers or infinite-size plane transducers are adopted for imaging, and imaging blurring of imaging targets which are deviated from the center in the tangential direction is easily caused. The invention successfully overcomes the problem by prolonging the distance of the virtual transducer, so that the tangential resolution is greatly improved;
(2) the method is based on the delay superposition algorithm, and is an algorithm with simple model, high degree of freedom, high algorithm stability and simple calculation;
(3) the invention can adapt to all ultrasonic transducer sizes without changing the structure of the existing photoacoustic tomography probe.
Drawings
FIG. 1a is a schematic diagram of a back projection algorithm for photoacoustic tomography using an ideal point transducer model;
FIG. 1b is a schematic diagram of a back projection algorithm for photoacoustic tomography using an infinite size transducer model;
FIG. 1c is a schematic diagram of a backprojection algorithm for photoacoustic tomography using a finite size transducer model;
FIG. 1d is a schematic diagram of an imaging process for photoacoustic tomography using a finite size transducer model;
FIG. 2a is an image of photoacoustic tomography using an ideal point transducer model reconstruction algorithm;
FIG. 2b is an image of photoacoustic tomography using an infinite size transducer model reconstruction algorithm;
FIG. 2c is an image of photoacoustic tomography using a finite size transducer model reconstruction algorithm;
FIG. 3a is a diagram of transverse profile data of three model reconstruction algorithms of photoacoustic tomography at a distance of 2mm from the center of an imaging target;
FIG. 3b is a diagram of the transverse profile data of the three model reconstruction algorithms of photoacoustic tomography at a distance of 4mm from the center of the imaging target;
FIG. 3c is a diagram of the transverse profile data of the three model reconstruction algorithms of photoacoustic tomography at a distance of 6mm from the center of the imaging target;
fig. 4 is a schematic diagram of a photoacoustic tomography system.
Description of the reference numerals
1 ideal point transducer 2 infinite size transducer 3 finite size transducer
4 virtual ultrasound transducer 5 projection line 6 imaging object
61 point target
Detailed Description
The technical solutions of the present invention are further described in detail with reference to the accompanying drawings and specific embodiments, which are only illustrative of the present invention and are not intended to limit the present invention.
A photoacoustic tomography reconstruction algorithm based on a finite size planar transducer, comprising the steps of:
fixing an imaging target in an annular ultrasonic transducer array, wherein medium water is filled between the annular ultrasonic transducer array and the imaging target;
the laser outputs a laser beam and sends out an ultrasonic trigger signal;
after the laser pulse uniformly irradiates an imaging target, the imaging target absorbs light, is rapidly heated and expanded and then contracts, so that an ultrasonic pulse signal is generated, and the ultrasonic pulse signal starts to propagate to the periphery at a fixed sound velocity v in medium water;
at the moment when the laser pulse is emitted, simultaneously starting to record ultrasonic pulse signals reaching the actual ultrasonic transducers by each actual ultrasonic transducer in the annular ultrasonic transducer array; after the collected ultrasonic pulse signals are converted into electric signals by an actual ultrasonic transducer, the received ultrasonic electric signals are amplified by a signal generation receiver and transmitted to a data acquisition system for ultrasonic signal acquisition;
combined with software programs in the computer (use)
Figure GDA0003017600400000061
Executing signal processing) and using a back projection algorithm to reconstruct images, setting the distance between an ultrasonic transducer and an imaging target as R, setting the time required by a received ultrasonic pulse signal as t, and calculating the imaging target images according to the relation t between the time t and the distance R as R/v, and the v as 1.495 mm/mu s by using the back projection algorithm
Figure GDA0003017600400000062
The signal value Si (t) of the ith actual ultrasonic transducer Pi corresponding to the pixel point is imaged to form a target image
Figure GDA0003017600400000063
The distance between the pixel point and the ith actual ultrasonic transducer Pi is R (x, y, Pi), and the image of the imaging target is
Figure GDA0003017600400000064
The signal value Si (R (x, y, Pi)/v) of the ith actual ultrasonic transducer Pi corresponding to the pixel point at (b), the target image is imaged
Figure GDA0003017600400000065
The image value of the pixel point is equal to the reception of each actual ultrasonic transducer in the annular ultrasonic transducer array
Figure GDA0003017600400000066
Figure GDA0003017600400000071
By superposition of signal values of pixels, i.e.
Figure GDA0003017600400000072
Setting the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer to be L, and then imaging
Figure GDA0003017600400000073
Figure GDA0003017600400000074
The distance between the pixel point and the virtual ultrasonic transducer is R' ═ R + L, and the image is
Figure GDA0003017600400000075
The distance between the pixel point and the ith virtual ultrasonic transducer Pi ' is R ' (x, y, Pi '); imaging a target image
Figure GDA0003017600400000076
The signal value of the actual ultrasonic transducer Pi corresponding to the pixel point at becomes Si [ (R' (x,y,Pi′)-L)/v](ii) a Then
Figure GDA0003017600400000077
The image value of the pixel point is equal to the reception of each virtual ultrasonic transducer
Figure GDA0003017600400000078
By superposition of signal values of pixels, i.e.
Figure GDA0003017600400000079
Image processing method
Figure GDA00030176004000000710
The polar coordinates of the distance of the pixel point at and the ith virtual ultrasonic transducer Pi' are converted into rectangular coordinates, and equation (2) becomes:
Figure GDA00030176004000000711
where R is the diameter of the actual ultrasound transducer scanning trajectory, N is the number of actual ultrasound transducers, Si(t) is the signal value received by the actual ultrasound transducer at the i-th position, θiIs the angular coordinate of the ith actual ultrasonic transducer position, v is the propagation velocity of the ultrasound in the medium, and R + L is the diameter of the virtual ultrasonic transducer scanning track;
according to the vector subtraction and distance formula, the image reconstruction algorithm formula of the imaging target is as follows:
Figure GDA00030176004000000712
wherein I is an image
Figure GDA00030176004000000713
The reconstructed image value is processed, n is the scanning position number of the annular ultrasonic transducer array, Si(t) is the ith actual ultrasonic transducerThe value of the received signal is determined,
Figure GDA00030176004000000714
and L is the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer, and v is the propagation speed of the ultrasonic in the medium.
In which, the experimental process can be simply changed, even if the single actual ultrasonic transducer is driven by the stepping motor to make a 360-degree rotation instead of the annular ultrasonic transducer array, as shown in fig. 1d (only the case when N is 1 is shown in the figure), when the actual ultrasonic transducer is at the P1 position, at the time t1, because the distance between the point on the curve of X1 and the actual ultrasonic transducer is equal, the signal received by the actual ultrasonic transducer at this time is the superposition of the ultrasonic pulse signals generated by all points on X1 at the actual ultrasonic transducer position P1; in a similar way, at the time t2, the signal received by the actual ultrasonic transducer is the superposition of ultrasonic pulse signals generated by all points on the X2 curve; therefore, according to the different time of the ultrasonic pulse signals at different points reaching the actual ultrasonic transducer, the actual ultrasonic transducer collects a time sequence of signals after passing through a laser pulse; then the actual ultrasonic transducer is rotated to the next position specified by the experimental requirement through the stepping motor, namely P2 in the figure continues to collect ultrasonic signals, and the process is repeated until the ultrasonic transducer rotates for one circle, so that the function of the annular ultrasonic transducer array can be realized.
In case of applying the whole circular scan of the circular ultrasound transducer array, the angle phi between two adjacent actual ultrasound transducers is the same, and the total number of actual ultrasound transducers is N.
In the reconstruction process, a photoacoustic tomography technology based on spherical scanning can also be adopted, and in a three-dimensional space, the photoacoustic tomography technology is formed by m two-dimensional acquisition plane processes, namely, an annular ultrasonic transducer array is used for continuously moving a certain distance along the axial direction to repeatedly acquire ultrasonic pulse signals until axial scanning is completed, namely, the actual ultrasonic transducer has n multiplied by m scanning positions in total, and then the formula (4) is changed into:
Figure GDA0003017600400000081
wherein I is an image
Figure GDA0003017600400000082
Processing the reconstructed image value, Si,j(t) is the signal received by the actual ultrasound transducer at the (i, j) position,
Figure GDA0003017600400000083
and (3) the position of the (i, j) th actual ultrasonic transducer, L is the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer, v is the propagation speed of the ultrasonic wave in the medium, and n multiplied by m is the number of scanning points of the actual ultrasonic transducer.
The imaging capability of the photoacoustic tomography reconstruction algorithm based on the limited-size planar transducer is mainly compared with an ideal-point transducer model reconstruction algorithm and an infinite-size transducer model reconstruction algorithm; the imaging target 6 sample adopts a phantom model made of agar powder, wherein 2g of agar powder is added into 100mL of water, heated and dissolved, poured into a corresponding mold for standing, and cooled to form cylindrical agar block gel; and then the lead core is taken as a point target 61 and is respectively inserted into the positions 0mm, 2mm, 4mm and 6mm away from the center of the phantom, so as to obtain a sample of the imaging target 6. The peripheral diameter of the imaging target 6 is 40mm, and 4 point targets 61 are uniformly distributed at positions 0mm to 6mm away from the center of the imaging target 6 in the transverse x direction; the laser adopts Nimma-600 type Q-switched Nd, YAG, nanosecond laser, and outputs laser wavelength 532nm, pulse width 8ns, and repetition frequency 10 Hz; the diameter of the planar transducer used for acquisition is 5mm, the central frequency is 5MHz, and the bandwidth is 71%; the distance R between the center of the imaging target 6 and the planar transducer is 22 mm; the annular ultrasonic transducer array is provided with 360 planar transducers, the angle interval between the planar transducers is 1 degree, and the imaging target 6 is positioned at the rotation center of annular scanning; the lateral direction may be represented by the x-direction and the tangential direction may be represented by the y-direction.
The invention adopts the plane transducer 3 with limited size and finds a simple form of back projection algorithm, thereby greatly improving the tangential resolution of the image.
As shown in fig. 4, the signal generating receiver adopts Olympus ultrasonic pulse generating/receiving device (model 5072PR, 5073PR), and the data acquisition system adopts a data acquisition system which is manufactured by beijing di yang shi science and technology ltd and has a signal of LDI400 VSE; the laser emits laser pulses to the imaging target 6, the imaging target 6 is heated to generate ultrasonic pulse signals, the ultrasonic pulse signals are received by each ultrasonic transducer in the annular ultrasonic transducer array and converted into electric signals, and the electric signals are amplified by the signal generating receiver and then acquired by the data acquisition system; if the ultrasonic transducer is used for scanning, the ultrasonic transducer needs to rotate around a shaft under the action of the stepping motor to realize annular scanning, if the ultrasonic transducer is used for spherical scanning, the annular ultrasonic transducer array moves along the axial direction under the action of the stepping motor to realize spherical scanning, or the ultrasonic transducer can rotate around the shaft and then move along the axial direction to realize spherical scanning, wherein a driver drives the stepping motor to work under the control of a computer; and then combining with an MATLAB software program in a computer to apply a back projection algorithm to carry out image reconstruction on the acquired signal data, and finally obtaining an image of the imaging target.
As shown in fig. 1c, in the present invention, a virtual ultrasound transducer 4 far from the actual ultrasound transducer is created first, and the distance value between the virtual ultrasound transducer 4 and the actual ultrasound transducer is set to L, then the signal received by the actual ultrasound transducer can be approximated to the signal received by the virtual ultrasound transducer 4, but with a time delay of L/v; will be provided with
Figure GDA0003017600400000101
Substituting the rectangular coordinates, the reconstruction value at the image point (x, y) obtained by the invention is determined by the signal of the position of the actual ultrasonic transducer and the scanning radius of the virtual ultrasonic transducer:
Figure GDA0003017600400000102
where R is the diameter of the actual ultrasound transducer scan trajectory and N is the actualNumber of ultrasonic transducers, Si(t) is the signal received by the actual ultrasound transducer at the i-th place, θiIs the angular coordinate of the ith actual ultrasound transducer position, v is the propagation velocity of ultrasound in the medium, and R + L is the diameter of the virtual ultrasound transducer scanning trajectory.
A general schematic diagram of a back projection algorithm is shown in fig. 1a, 1b, the back projection algorithm first measuring the time delay between a pixel and each ultrasound transducer and then deriving the final pixel value from the sum of the respective time delayed ultrasound transducer signals.
Fig. 1a shows a schematic image reconstruction when the transducer is considered as an ideal point transducer 1, which is also the principle of most current CSPAT reconstruction algorithms, where the projection line 5 (or equal time delay line) is a set of concentric curves centered around the actual ultrasound transducer position, and where the reconstruction value at (x, y) on the image is determined by the signal of the transducer position:
Figure GDA0003017600400000103
wherein R is the diameter of the scanning track of the ultrasonic transducer, N is the number of the total ultrasonic transducers, Si(t) is the signal received by the ultrasonic transducer at the ith, θiIs the angular coordinate of the ith ultrasonic transducer, and v is the propagation velocity of the ultrasound in the medium; this equation can provide an ideal point transducer 1 with uniform resolution, but for a planar transducer with limited size, the image tangential resolution will deteriorate when the point target 61 is far from the central position of the imaging target 6.
A second model of the general backprojection algorithm is based on a large size line transducer, i.e. a transducer 2 considered to be of infinite size, in which case the schematic diagram is shown in fig. 1b, the projection line 5 is parallel to the plane of the infinite size transducer 2; in this algorithm, the reconstructed values at (x, y) on the image are:
Figure GDA0003017600400000111
wherein R is the diameter of the scanning track of the ultrasonic transducer, N is the number of the total ultrasonic transducers, Si(t) is the signal received by the ultrasonic transducer at the ith, θiIs the angular coordinate of the ith ultrasonic transducer, and v is the propagation velocity of the ultrasound in the medium; however, for a smaller ultrasonic transducer, when the point target 61 is away from the central position of the imaging target 6, the image tangential resolution will deteriorate.
Fig. 2a is an image derived using the ideal point transducer 1 model imaging algorithm, equation (6), and fig. 2b is an image derived using the infinite size transducer 2 model imaging algorithm, equation (7). It can be seen that the two point targets 61 which are 4mm and 6mm away from the center of the imaging target 6 have significantly longer transverse profiles than the two point targets 61 which are 0mm and 2mm away from the center of the imaging target 6, the farther away from the center of the imaging target 6, the more significant the tangential y-direction distortion of the target, and the poorer the tangential y-direction resolution of the system.
Fig. 2c shows an image obtained by using the finite transducer 3 model imaging algorithm of the present invention, i.e., formula (3), where formula (3) can be converted into formula (4) according to the vector subtraction and distance formula, and where a spherical scan is performed, formula (4) can be converted into formula (5). Wherein the distance value between the selected virtual ultrasound transducer 4 and the actual ultrasound transducer 3 is 27 mm. Comparing fig. 2a and fig. 2b, it can be seen that, by adopting the algorithm of the present invention, the tangential resolutions of the other targets in the graph are remarkably improved and the tangential blurring artifact is effectively restored, except that the tangential resolutions of the targets at the positions of 0mm and 2mm are basically similar to those in fig. 2a and fig. 2 b.
Fig. 3a, 3b and 3c are transverse data profiles extracted from the point target 61 of fig. 2a, 2b and 2c at 2mm, 4mm and 6mm from the center of the imaging target 6, respectively.
Wherein the solid line is the lateral profile reconstructed with the algorithm of the present invention, the dotted line is the lateral profile reconstructed with the ideal point model algorithm, and the dotted line is the lateral profile reconstructed with the infinite model algorithm.
The ordinate is the normalized data value of the gray value of the pixel point at 2/4/6mm of the abscissa of fig. 2a/2b/2c, 0 is black, 1 is white; the abscissa is the ordinate of the pixel in fig. 2a/2b/2 c;
dividing the gray value of the pixel point at 2mm of the abscissa by the maximum gray value of the pixel point at the abscissa to obtain a normalized data value;
repeatedly substituting the image values of pixel points at positions 4mm and 6mm of the abscissa to obtain corresponding normalized data values; the profile data charts shown in fig. 3a, 3b and 3c can be obtained by using the normalized data values as the ordinate and the y-axis coordinate corresponding to the pixel point in fig. 2a/2b/2c as the abscissa.
By contrast, it can be seen that the CSPAT pattern tangential blurring artifacts of the photoacoustic tomography reconstruction algorithm employing the limited-size planar transducer of the present invention are effectively recovered.
The above embodiments are only examples of the present invention, but the present invention is not limited to the above embodiments, and the above embodiments are used for explaining the present invention, and any other changes, such as pre-processing and post-processing of data, which do not depart from the spirit and principle of the present invention, should be regarded as equivalent substitution ways, and all fall within the protection scope of the present invention.

Claims (6)

1. A photoacoustic tomography reconstruction algorithm based on a finite size planar transducer, comprising the steps of:
1) fixing an imaging target in an annular ultrasonic transducer array, wherein medium water is filled between the annular ultrasonic transducer array and the imaging target;
2) the laser outputs laser pulses, so that the laser pulses uniformly irradiate the imaging target, and the imaging target is excited to generate an ultrasonic pulse signal;
3) each actual ultrasonic transducer in the annular ultrasonic transducer array simultaneously starts to record ultrasonic pulse signals reaching the actual ultrasonic transducer, the ultrasonic pulse signals are converted into electric signals, the received electric signals are amplified through the signal generation receiver and transmitted to the data acquisition system for electric signal acquisition; processing the acquired electric signals to obtain a reconstruction graph of a photoacoustic tomography reconstruction algorithm based on the limited-size planar transducer;
4) calculating an imaging target image by using a back projection algorithm
Figure FDA0003017600390000011
The ith actual ultrasonic transducer P corresponding to the pixel point atiSignal value of (si), (t), then the target image is imaged
Figure FDA0003017600390000012
The image value of the pixel point is equal to the reception of each actual ultrasonic transducer in the annular ultrasonic transducer array
Figure FDA0003017600390000013
By superposition of signal values of pixels, i.e.
Figure FDA0003017600390000014
Wherein, the actual distance of the ultrasonic transducer from the imaging target is R, the time required by the received ultrasonic pulse signal is t, and the imaging target image is
Figure FDA0003017600390000015
Pixel point of (ii) and the ith actual ultrasonic transducer PiIs R (x, y, P)i) Imaging a target image
Figure FDA0003017600390000016
The ith actual ultrasonic transducer P corresponding to the pixel point atiSignal value S ofi(R(x,y,Pi) V), v is the propagation speed of the ultrasound in the medium;
5) setting the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer to be L, and imaging
Figure FDA0003017600390000017
Distance between pixel point at (x, y) position and virtual super positionThe distance of the acoustic transducer is R' ═ R + L, then
Figure FDA0003017600390000018
The image value of the pixel point is equal to the reception of each virtual ultrasonic transducer
Figure FDA0003017600390000019
By superposition of signal values of pixels, i.e.
Figure FDA0003017600390000021
Wherein the image
Figure FDA0003017600390000022
Is far away from the ith virtual ultrasonic transducer Pi'distance R' (x, y, P)i') imaging a target image
Figure FDA0003017600390000023
The actual ultrasonic transducer P corresponding to the pixel point atiBecomes Si[(R ′ (x,y,Pi ′ )-L)/v] ；
6) Image processing method
Figure FDA0003017600390000024
Pixel point of (ii) and the ith virtual ultrasonic transducer PiWhen the polar coordinates of the distance of' are converted into rectangular coordinates, equation (2) becomes:
Figure FDA0003017600390000025
where R is the diameter of the actual ultrasound transducer scanning trajectory, N is the number of actual ultrasound transducers, Si(t) is the signal value received by the actual ultrasound transducer at the i-th position, θiIs the angular coordinate of the ith actual ultrasonic transducer position, and v is the ultrasonic mediumThe propagation velocity in the matter, R + L is the diameter of the virtual ultrasonic transducer scanning track;
7) according to the vector subtraction and distance formula, the image reconstruction algorithm formula of the imaging target is as follows:
Figure FDA0003017600390000026
wherein I is an image
Figure FDA0003017600390000027
The reconstructed image value is processed, n is the scanning position number of the annular ultrasonic transducer array, Si(t) is the signal value received by the ith actual ultrasound transducer,
Figure FDA0003017600390000028
and L is the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer, and v is the propagation speed of the ultrasonic in the medium.
2. The limited-size planar transducer-based photoacoustic tomography reconstruction algorithm of claim 1, wherein in the image reconstruction process, a spherical scan is performed by using the annular ultrasound transducer array, and in a three-dimensional space, the spherical scan is composed of m two-dimensional planar scan processes, that is, each actual ultrasound transducer has n × m scan positions in total, then formula (4) becomes:
Figure FDA0003017600390000031
wherein I is an image
Figure FDA0003017600390000032
Processing the reconstructed image value, Si,j(t) is the signal received by the actual ultrasound transducer at the (i, j) position,
Figure FDA0003017600390000033
and (3) the position of the (i, j) th actual ultrasonic transducer, L is the distance value between the virtual ultrasonic transducer and the actual ultrasonic transducer, v is the propagation speed of the ultrasonic wave in the medium, and n multiplied by m is the number of scanning points of the actual ultrasonic transducer.
3. The limited size planar transducer based photoacoustic tomography reconstruction algorithm of claim 1 or 2, wherein the laser in step 2) is a pulsed laser, the laser pulse wavelength being 532 nm.
4. The limited size planar transducer based photoacoustic tomography reconstruction algorithm of claim 1 or 2, wherein the angular separation between adjacent actual ultrasound transducers is 1 degree, and the annular ultrasound transducer array is mounted with 360 actual ultrasound transducers.
5. The limited size planar transducer-based photoacoustic tomography reconstruction algorithm of claim 1, wherein the processing of the acquired electrical signals of step 3) is an image reconstruction by applying the image reconstruction algorithm of formula (4) in combination with a MATLAB software program in a computer.
6. The limited size planar transducer-based photoacoustic tomography reconstruction algorithm of claim 2, wherein the processing of the acquired electrical signals is an image reconstruction performed by applying the image reconstruction algorithm of equation (5) in conjunction with a MATLAB software program in a computer.
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