CN111220700A - Ultrasonic cavitation bubble motion vector estimation method - Google Patents
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Abstract
The invention provides a method for estimating a motion vector of an ultrasonic cavitation bubble, which comprises the following steps that 1, a focused ultrasonic system is adopted as a cavitation energy source to generate the ultrasonic cavitation bubble; step 2, detecting ultrasonic cavitation bubbles by emitting array wide beams, and acquiring continuous sequence ultrasonic cavitation original radio frequency signals; step 3, obtaining continuous N frames of ultrasonic cavitation images by adopting a wide beam minimum variance beam synthesis algorithm based on compressed sensing and an ultrasonic imaging algorithm of radio frequency data; step 4, the ultrasonic cavitation image carries out motion vector estimation through a pyramid LK optical flow method to form a cavitation bubble motion vector diagram of continuous frames; and 5, changing parameters and media of a focused ultrasonic energy source for generating cavitation, and repeating the steps 1 to 4 to obtain cavitation and cavitation motion vector estimation images under different conditions. The invention provides a vector estimation method which has high space-time resolution and high signal-to-noise ratio, can carry out transient motion vector estimation on ultrasonic cavitation bubbles under various conditions and realizes ultrasonic cavitation bubble motion tracking.
Description
Technical Field
The invention relates to the technical field of acoustic cavitation physics and application and ultrasonic imaging, in particular to a method which is combined with an array wide beam cavitation imaging technology, a compressed sensing-based wide beam minimum variance beam synthesis algorithm and a pyramid LK optical flow method, can carry out transient motion vector estimation on ultrasonic cavitation bubbles under various conditions and realizes motion tracking of the ultrasonic cavitation bubbles.
Background
The acoustic cavitation refers to a series of dynamic processes of vibration, growth, contraction and even collapse of cavitation nuclei in a medium under the action of ultrasonic waves. The ultrasonic wave is widely applied to various fields, and mainly applies the cavitation and mechanical effect, thermal effect, chemical effect, biological effect and the like accompanying the cavitation. Sonocavitation is also a key physical mechanism in the biomedical field in the aspects of tumor thermal ablation, in vitro lithotripsy, ultrasonic thrombolysis, gene transfection, drug controlled release and the like. The need for cavitation varies in different biomedical applications. For example, in thermal tumor ablation, cavitation can cause lesion deformation, thereby reducing the accuracy of treatment, and thus, cavitation suppression is desirable. In ultrasonic thrombolysis, cavitation is the main mechanism of thrombolysis, and thus enhanced cavitation is required. At present, in order to better control and utilize cavitation, on one hand, an effective cavitation detection and imaging method needs to be researched, and on the other hand, the transient motion characteristic of the ultrasonic cavitation bubble needs to be researched urgently.
The dynamic behavior of acoustic cavitation includes cavitation nucleation, cavitation bubble linear and nonlinear vibration, cavitation bubble growth, cavitation rapid contraction to collapse and collapse, and cavitation bubble dissipation. Cavitation bubbles grow in the negative pressure region formed by the longitudinal propagation of ultrasonic waves, and rapidly close in the positive pressure region, thereby being compressed and stretched under alternating positive and negative pressures. At the moment when the cavitation bubbles are compressed to collapse, a large amount of sound field energy is absorbed and is released in a small area in a concentrated manner, and physical phenomena such as high temperature and high pressure, sonoluminescence, strong shock waves, high-speed jet flow and the like can be generated locally. In a liquid medium, the lowest sound intensity at which cavitation occurs is called the cavitation threshold, and its magnitude depends on the ultrasonic frequency, the static pressure of the medium liquid, the initial temperature, the structural state of the liquid itself, and the diversity of cavitation nuclei added to the liquid. According to the stability of cavitation bubbles, acoustic cavitation can be divided into stable cavitation characterized by non-inertial cavitation and transient cavitation characterized by inertial cavitation. Factors that affect acoustic cavitation intensity include ultrasonic intensity, frequency, surface tension of the liquid, viscosity coefficient, temperature, and the like.
Acoustic cavitation has transient characteristics and is discrete in time and space, as well as a microscopic phenomenon, requiring two-dimensional imaging of the spatial-temporal distribution of cavitation bubbles. The existing acoustic cavitation detection and imaging methods are mainly optical methods and acoustic methods. The optical method is mainly used for recording the behavior and the space-time dynamic distribution of cavitation bubbles visually through high-speed/ultrahigh-speed photography, acoustooptic luminescence, acoustooptic chemiluminescence and the like so as to know the motion characteristics and the physical process of the cavitation bubbles. However, on one hand, the optical method is not suitable for non-transparent media and in-situ research, and on the other hand, optical images, especially sonoluminescence and sonochemiluminescence images, are the superposition of optical information in shooting direction, and cannot give cavitation distribution of a certain fault plane. Acoustic methods include passive cavitation imaging and active cavitation imaging. The passive cavitation imaging is to obtain the two-dimensional spatial distribution of cavitation bubbles through passive receiving of the array transducer and channel signal source reconstruction, but accurate spatial position information cannot be provided and a reconstruction algorithm is complex because no acoustic detection signal is emitted. The active cavitation imaging comprises standard B mode cavitation imaging and ultra-fast cavitation imaging methods. The standard B-mode cavitation imaging adopts a line-by-line scanning mode, time difference exists between different scanning lines of the same frame of image, and the time resolution cannot meet the research on the transient characteristics of ultrasonic cavitation; due to the fact that plane waves are emitted in ultra-fast active cavitation imaging, the frame frequency can reach more than tens of thousands of Hz, and the sensitivity and the image quality of the ultra-fast active cavitation imaging need to be improved.
The transient motion characteristic of cavitation bubbles needs to be further researched on the basis of cavitation imaging. At present, a cavitation bubble transient motion characteristic research method mainly adopts high-speed photography to shoot cavitation bubbles, obtain cavitation bubble behavior sequence optical images and analyze motion characteristics of the cavitation bubbles. Firstly, the optical method cannot realize the motion characteristic research of the cavitation bubbles in the non-transparent medium, and particularly needs to monitor the transient motion characteristic of the cavitation bubbles in real time in the clinical application of ultrasonic therapy such as focused ultrasonic therapy. Secondly, there are many requirements for the position of the target object and the angle of obtaining light, and meanwhile, because the cavitation bubble size is small, high-speed photography and microscope are generally needed to be used in cooperation, and the randomness of the spatial position is difficult to ensure that the observed cavitation bubble is always on the focal plane of the microscope due to the influence of the field of view and the focal plane of the microscope.
Disclosure of Invention
The invention aims to provide an ultrasonic cavitation bubble motion vector estimation method, which aims to solve the problem that the optical method cannot realize the motion characteristic research of cavitation bubbles in a non-transparent medium, and particularly needs to monitor the transient motion characteristic of the cavitation bubbles in real time in the ultrasonic treatment clinical application such as focused ultrasonic treatment; the device has the advantages that requirements on the position of a target object and the angle of obtaining light rays are high, meanwhile, due to the fact that the cavitation bubbles are small in size, high-speed photography and a microscope are generally needed to be matched for use, the influence of the visual field and the focal plane of the microscope is caused, and the randomness of the spatial positions is difficult to guarantee that the observed cavitation bubbles are always located on the focal plane of the microscope.
The invention provides a vector estimation method which can carry out transient motion vector estimation on ultrasonic cavitation bubbles under various conditions and realize motion tracking of the ultrasonic cavitation bubbles.
The invention provides the following technical scheme, and an ultrasonic cavitation bubble motion vector estimation method comprises the following steps:
step 4, carrying out motion vector estimation on the ultrasonic cavitation image obtained in the step 3 by a pyramid LK optical flow method to form a cavitation bubble motion vector diagram of a continuous frame;
and 5, changing parameters and media of a focused ultrasonic energy source for generating cavitation, and repeating the steps 1 to 4 to obtain cavitation and cavitation motion vector estimation images under different conditions.
Further, the focused ultrasound system in step 1 includes a focused ultrasound transducer, a power amplifier and a dual-channel waveform generator for controlling timing, wherein a first channel of the dual-channel waveform generator drives the power amplifier to excite the focused ultrasound transducer to emit focused ultrasound waves.
Further, the focused ultrasonic transducer is fixed on one side of the water tank, the surface of the focused ultrasonic transducer is immersed in water, and sound absorption materials are placed on the bottom and the side wall of the water tank.
Further, the cavitation detection device for detecting the ultrasonic cavitation bubbles in the step 2 comprises: the second channel of the dual-channel waveform generator drives the array transducer to emit a wide beam and receive cavitation echo signals, and continuously acquired ultrasonic cavitation original radio frequency signals are acquired in a parallel channel data acquisition and storage mode through the parallel channel data acquisition and storage unit.
Furthermore, the open programmable ultrasonic system platform comprises an ultrasonic array transducer, a plane wave hardware receiving system and a host, wherein the ultrasonic array transducer is fixed right above an experimental sample in the water tank, and sound absorption materials are placed on the side wall of the water tank and the bottom of the water tank.
The focal region of the focused ultrasonic probe is positioned in the center of the interior of the sample, and the energy source device generates continuous energy to excite the generation of cavitation bubbles.
Further, the method for synthesizing the wide-beam minimum variance beam based on the compressed sensing in step 3 includes the following steps:
3.1, Fourier transform is carried out on the two-dimensional ultrasonic cavitation original radio frequency data, namely, the two-dimensional ultrasonic cavitation original radio frequency data are converted into frequency domain signals from time domain signals;
3.2 selecting frequency points in the effective bandwidth from the frequency domain information distribution of the ultrasonic cavitation radio frequency data, and randomly sampling and extracting the frequency points in the effective bandwidth according to the ratio of 40%;
3.3 adopting a space smoothing method and a diagonal loading method to construct a steady covariance matrix of the extracted frequency points, and calculating the optimal weighting coefficient of the minimum variance adaptive beam synthesis corresponding to the frequency points, thereby obtaining the frequency domain output of the frequency points under the optimal beam synthesis;
3.4 traversing all the extracted frequency points according to 3.3, and rebuilding frequency domain information in an effective bandwidth by a regularized multi-focus underdetermined system (RM _ FOCUSS) algorithm, and then rebuilding the whole frequency domain information;
and 3.5, transforming the whole frequency domain information to a time domain through inverse Fourier transform to obtain a cavitation radio frequency signal after beam synthesis.
Further, in the step 3, the ultrasonic imaging algorithm based on the radio frequency data images the cavitation radio frequency signal after the beam synthesis: and (3) carrying out filtering processing, time gain compensation, envelope detection, secondary sampling, logarithmic compression and coordinate scanning transformation on the sequence two-dimensional cavitation radio-frequency signal subjected to beam synthesis to obtain a continuous sequence two-dimensional cavitation image with high resolution and high signal-to-noise ratio.
Further, the motion vector estimation by the pyramid LK optical flow method adopted in step 4 includes:
carrying out Gaussian filtering pretreatment on two continuous frames of cavitation images;
performing down-sampling on the preprocessed cavitation image by adopting a pyramid technology to obtain an L +1 layer image;
and (3) realizing motion estimation of an image of a certain layer of the pyramid by adopting a calculation formula of an LK optical flow method, expanding the motion information of the layer to the next layer, and so on to realize motion estimation of the original image of the bottommost layer.
The pyramid LK optical flow method comprises the following specific steps:
4.1 each time, two continuous frames of cavitation images are selected from the sequence two-dimensional cavitation images: the ith frame and the (i + 1) th frame of cavitation image;
4.2, carrying out Gaussian filtering pretreatment on the cavitation images of the ith frame and the (i + 1) th frame;
4.3, down-sampling the image to form an image pyramid model: from layer 0 (original image) to layer L (coarse image);
4.4 setting the optical flow initial value of the highest layer of the pyramid to be 0, starting to calculate the optical flow of the image of the highest layer of the pyramid by using an LK optical flow method, taking the obtained optical flow result as the optical flow initial value of the L-1 layer, calculating the optical flow value of the L-1 layer, and obtaining the optical flow estimation of the L-1 layer by the sum of the optical flow result and the optical flow initial value of the L-1 layer, repeating the steps until the original image of the bottom layer is obtained;
4.5 repeating 4.1 to 4.4 obtains the motion vector estimation map of cavitation bubbles at different continuous time.
Further, in the step 5, changing parameters and media of a focused ultrasonic energy source for generating cavitation, and repeating the steps 1 to 5 to obtain cavitation and cavitation motion vector estimation images under different conditions; changing the action time of the focused ultrasound, wherein the time resolution reaches several microseconds, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation bubble motion vector images formed along with time change; changing the time delay between the stop moment of the focused ultrasound and the emission of the wide beam by the array transducer, and repeating the steps 1 to 5 to obtain cavitation bubbles dissipated along with the change of time and a cavitation bubble motion vector image; changing the duty ratio of the focused ultrasound, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation bubble motion vector images formed under the condition of changing with different focused ultrasound duty ratios; changing the size of the focused ultrasonic energy, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation bubble motion vector images which change along with the focused ultrasonic energy; changing the medium under the action of the focused ultrasound, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation motion vector images under different media.
Compared with the prior art, the invention has the beneficial effects that:
1. the cavitation bubbles are detected by adopting the wide beam, so that the problem of time difference of the same frame of cavitation images is solved, and the transient research of the cavitation bubbles is realized by high frame frequency;
2. the method adopts a wide-beam minimum variance beam synthesis algorithm based on compressed sensing to carry out beam synthesis on original cavitation radio frequency data, so that a cavitation image can be ensured to have higher spatial resolution and signal-to-noise ratio, and the cavitation imaging time can be greatly shortened, thereby improving the imaging speed;
3. a pyramid LK optical flow method is adopted to carry out motion vector estimation on cavitation images of continuous frames, on one hand, the pyramid technology can solve the problem that an optical flow basic constraint equation is not established due to the fact that the displacement phase difference of two frames of images is large due to the fact that the motion speed of ultrasonic cavitation bubbles is high, on the other hand, the LK optical flow method is a sparse optical flow estimation method based on local parameterization of gradients, optical flow calculation is carried out on image feature points, and the operation speed is high. Therefore, the pyramid LK optical flow method is high in calculation speed and accurate in result, and can achieve ultrasonic cavitation bubble transient motion vector estimation;
4. changing the action time of the focused ultrasound to obtain cavitation bubbles and cavitation bubble motion vector images which change along with the action time, wherein the time resolution can reach several microseconds; changing the time delay between the stop moment of the focused ultrasound and the emission of the wide beam by the array transducer to obtain cavitation bubbles and cavitation bubble motion vector images dissipated along with the change of time; changing the duty ratio of the focused ultrasound to obtain cavitation bubbles and cavitation bubble motion vector images which change along with the duty ratio; changing the size of the focused ultrasonic energy to obtain cavitation bubbles and cavitation bubble motion vector images which change along with the energy; changing the medium of the focused ultrasound effect can obtain cavitation bubbles and cavitation motion vector images under different media.
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Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Also, like reference numerals are used to refer to like parts throughout the drawings. In the drawings:
FIG. 1 is a flow chart of the ultrasonic cavitation bubble motion vector estimation method of the present invention;
FIG. 2 is a schematic diagram of a focused ultrasonic cavitation source and an array wide beam cavitation detection device according to the present invention;
FIG. 3 is a flowchart of a compressed sensing-based wide-beam minimum-variance beam-forming algorithm in the present invention;
FIG. 4 is a flowchart of an algorithm for motion vector estimation based on pyramid LK optical flow in accordance with the present invention;
FIG. 5 is a schematic diagram of the timing control of the focused ultrasound cavitation generation and array wide beam cavitation detection in accordance with the present invention;
FIG. 6 is a two-dimensional ultrasonic cavitation image after beam synthesis with a simulated blood vessel gelatin agar phantom as an experimental sample provided in the present invention;
FIG. 7 is a two-dimensional ultrasonic cavitation image of two consecutive frames of regions of interest after beam-forming treatment with a simulated vascular gelatin agar phantom as an experimental sample provided in the present invention;
fig. 8 is a cavitation bubble motion vector diagram obtained by processing two continuous frames of two-dimensional cavitation images in fig. 7 by a pyramid LK optical flow method.
Wherein, 1, focusing ultrasonic transducer; 2. a power amplifier; 3. a dual channel waveform generator; 4. an open programmable ultrasound system platform; 5. an array transducer; 6. a computer; 7. a water tank; 8. testing the sample; 9. a sound absorbing material.
Detailed Description
Preferred embodiments of the invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only for explaining the technical principle of the invention, and do not limit the scope of the invention.
It should be noted that in the description of the present invention, the terms of direction or positional relationship indicated by the terms "upper", "lower", "left", "right", "inner", "outer", etc. are based on the direction or positional relationship shown in the drawings, which are only for convenience of description, and do not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention.
Furthermore, it should be noted that, in the description of the present invention, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be connected internally or indirectly. Specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
Referring to fig. 1, a method for estimating a motion vector of an ultrasonic cavitation bubble comprises the following steps: firstly, a focused ultrasound system is used as a cavitation energy source to generate ultrasonic cavitation bubbles, the cavitation bubbles are detected by emitting array wide beams, and continuous sequence ultrasonic cavitation original radio frequency signals are collected. Secondly, a wide-beam minimum variance beam synthesis algorithm and a radio frequency imaging algorithm based on compressed sensing are adopted to obtain continuous N frames of ultrasonic cavitation images. And thirdly, carrying out motion vector estimation by using a pyramid LK optical flow method to form a cavitation bubble motion vector diagram of continuous frames.
The ultrasonic cavitation bubble motion vector estimation method specifically comprises the following steps:
On the one hand, parameters of the emitted focused ultrasonic wave, including excitation time, duty ratio and pulse repetition frequency, can be controlled through waveform editing of the waveform generator, and the time resolution of the focused ultrasonic wave can reach microsecond, and on the other hand, the acoustic power of the focused ultrasonic wave can be set through a panel of the power amplifier 2.
Specifically, the focused ultrasound transducer 1 is fixed on one side of the water tank 7, the water tank 7 is in a full water state, the surface of the focused ultrasound transducer 1 is immersed in water, and the sound absorption material 9 is placed on the bottom and the side wall of the water tank 7. The experimental sample 8 is fixed in the water tank 7 by a holder, and the ultrasonic waves emitted from the focused ultrasonic transducer 1 are focused on the inner central region of the experimental sample 8.
Referring to fig. 2, in step 2, the ultrasound cavitation bubbles generated in step 1 are detected by emitting array wide beams, and continuous sequence ultrasound cavitation original radio frequency signals are acquired and obtained.
Specifically, the cavitation detection device for detecting the ultrasonic cavitation bubbles in the step 2 comprises an array transducer 5 capable of programmably emitting a wide beam, a parallel channel data acquisition and storage unit and a dual-channel waveform generator 3 for controlling a time sequence, wherein a second channel of the dual-channel waveform generator 3 generates a synchronous signal, the array transducer 5 is controlled to emit the wide beam to detect cavitation, and a continuously acquired ultrasonic cavitation original radio frequency signal is obtained in a manner of acquisition and storage by the parallel channel data acquisition and storage unit.
The transmitting and receiving parameters can be realized by programming the open type programmable ultrasonic system platform 4, and the continuously acquired ultrasonic cavitation original radio frequency signals can be stored in the computer 6.
The waveform generator realizes time synchronization between the cavitation generating device and the cavitation detecting device through the first channel and the second channel, time delay between the cavitation generating device and the cavitation detecting device can be set according to requirements, and the time resolution can reach microsecond.
Specifically, the array wide-beam cavitation detection device comprises an open type programmable ultrasonic system platform 4, the open type programmable ultrasonic system platform comprises an ultrasonic array transducer 5, a plane wave hardware receiving system and a host, the ultrasonic array transducer is fixed right above an experimental sample in a water tank, and sound absorption materials 9 are placed on the side wall of the water tank 7 and the bottom of the water tank 7.
The sound absorbing material 9 may be RTV silicone rubber based composite material or the like, but other sound absorbing materials that achieve the same or similar effects are within the scope of the present invention.
Referring to fig. 3, in step 3, a wide-beam minimum variance beam-forming algorithm based on compressed sensing and an ultrasonic imaging algorithm of radio frequency data are used to obtain consecutive N frames of ultrasonic cavitation images.
The compressed sensing-based wide-beam minimum variance beam forming method in the step 3 comprises the following steps:
(1) obtaining the number of the array elements required according to the dynamic aperture, and recording the number as M;
(2) calculating the delay amount of the radio frequency signals at different depth points to obtain delayed signals RF (t);
RF(t)＝[rf1(t) rf2(t) … rfM(t)]T
(3) performing Fourier transform on the RF (t) to obtain RF (w);
RF(w)＝[rf1(w) rf2(w) … rfM(w)]T
(4) selecting an effective bandwidth w according to the ultrasonic center frequency and the relative bandwidth thereofkAnd K is the number of frequency points in the effective bandwidth, the corresponding covariance matrix is used as the frequency domain signal RF (w)k),k＝1,2,…,K；
(5) In the effective bandwidth wkTaking a diagonal matrix T with an internal construction dimension of K multiplied by K as a decimation matrix, wherein elements on a diagonal corresponding to the decimated row are 1, and the rest are 0;
(6) for frequency domain signal RF (w)k) Random extraction is carried out according to rows: RF (w)ex)＝T×RF(wk) The extraction proportion is 40%;
(7) for decimated frequency points RF (w)ex) Dividing the array with the effective array element number of M into overlapping sub-arrays with the array element number of L, wherein the total number of the sub-arrays is M-L +1, and obtaining the covariance matrix after forward spatial smoothing by adopting the spatial smoothing principle
Wherein, RFl(wex) Is RF (w)ex) The subarray of (1), L is 1,2, 1, M-L +1, in sequence]HRepresents a conjugate transpose of the matrix;
(8) representing the backward spatial smoothing covariance matrix by the forward covariance matrixMatrix of
Wherein Q represents a transformation matrix, only the elements on the anti-diagonal of the matrix are 1, the rest are 0, and the superscript represents the conjugation of the matrix;
(9) smoothing matrix by forward space
And backward spatial smoothing matrix
Averaging to obtain a forward and backward spatial smoothing matrix
(10) A robust covariance matrix is constructed by the diagonal loading method as follows:
wherein,
i represents a diagonal identity matrix;
(11) by the formula
Calculating an optimum weighting coefficient, wherein]-1Representing the inverse of the matrix, a (θ)0) The expression direction vector is a one-dimensional unit vector with the length of L;
(12) the frequency is calculated byOptimal output of points y (w)ex)：
(13) Repeating the steps (7) to (12), and calculating the optimal output of all the extracted frequency points;
(14) constructing K multiplied by K Fourier basis F as sparse basis, substituting the sparse basis into regularized multipoint underdetermined system focusing (RM _ FOCUSS) algorithm to reconstruct optimal beam forming frequency domain output y (w) in all effective bandwidths of the array elementk)；
(15) Repeating the steps (1) to (14) to obtain the optimal frequency domain output of all array elements in the effective bandwidth; then, compensating the frequency points outside the effective bandwidth by 0 to obtain the optimal output y (w) of the whole frequency domain;
(16) and (5) performing inverse Fourier transform on the y (w) to obtain a radio frequency signal y (t) of the whole cavitation imaging time domain, namely the cavitation radio frequency signal after beam synthesis.
The cavitation radio frequency signal after the beam synthesis is imaged by the wide beam minimum variance beam synthesis algorithm based on the compressed sensing and the ultrasonic imaging algorithm based on the radio frequency data: and filtering the sequence two-dimensional cavitation radio frequency signal after beam synthesis, time gain compensation, envelope detection, secondary sampling, logarithmic compression and coordinate scanning conversion to obtain a continuous sequence two-dimensional cavitation image with high resolution and high signal-to-noise ratio.
Referring to fig. 4, in step 4, motion vector estimation is performed on the ultrasonic cavitation image obtained in step 3 by a pyramid LK optical flow method, so as to form a cavitation bubble motion vector diagram of a continuous frame.
And (3) performing motion estimation by adopting a pyramid LK optical flow method: firstly, carrying out Gaussian filtering pretreatment on two continuous frame cavitation images; then, performing down-sampling on the preprocessed cavitation image by adopting a pyramid technology to obtain an L +1 layer image; and then, realizing motion estimation of an image of a certain layer of the pyramid by adopting a calculation formula of a specific LK optical flow method, expanding the motion information of the layer to the next layer, and so on to realize motion estimation of the original image of the bottommost layer.
The pyramid LK optical flow method comprises the following steps:
① preprocessing two continuous frames of cavitated images by Gaussian filtering;
② down-sampling the image to form a pyramid model from layer 0 (original image) to layer L (coarse image);
③ setting the initial value of the top-level image, i.e. the L-th layer optical flow, to 0
The optical flow value of the L-th layer is calculated as the minimum equation:
wherein, Ixu+Iyv+ItA typical optical flow constraint equation is denoted by 0,
Ix,Iypartial derivatives of the image brightness function in the x-direction and y-direction, respectively, ItRepresents the partial differential of the image brightness function to the frame interval t, omega represents the gray scale region in the image, W (x, y) represents the window weight function of the region, in n pixels in omega,
④ the LK optical flow values calculated in step ③ are sorted according to the least squares principle:
wherein,
W＝diag(W(x1,y1),…,W(xn,yn))，
⑤ the L-th layer is calculated to obtain the accurate light flow value d according to the step ④LThen the optical flow value of the L-th layer is gL+dL；
⑥ uses the optical flow value of the L-th layer as the initial value g of the optical flow of the L-1 layerL-1＝gL+dLThen, the accurate optical flow value d of the L-1 layer image is calculated by the LK optical flow value calculation formula in step ④L-1Then the optical flow value of the L-1 th layer is gL-1+dL-1；
⑦ the light flow value of the L-1 layer is used as the initial light flow value of the L-2 layer, i.e. gL-2＝gL-1+dL-1The exact optical flow value of layer L-2 is calculated in step ④, and so on, to obtain the optical flow value of layer 0.
And 5, changing parameters and media of a focused ultrasonic energy source for generating cavitation, and repeating the steps 1 to 4 to obtain cavitation and cavitation motion vector estimation images under different conditions.
Specifically, changing parameters and media of a focused ultrasonic energy source for generating cavitation, and repeating the steps 1 to 5 to obtain cavitation and cavitation motion vector estimation images under different conditions; changing the action time of the focused ultrasound, wherein the time resolution reaches several microseconds, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation bubble motion vector images formed along with the change of time; changing the time delay between the stop moment of the focused ultrasound and the emission of the wide beam by the array transducer 5, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation bubble motion vector images which are dissipated along with the change of time; changing the duty ratio of the focused ultrasound, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation bubble motion vector images formed under the condition of changing with different focused ultrasound duty ratios; changing the size of the focused ultrasonic energy, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation bubble motion vector images which change along with the focused ultrasonic energy; changing the medium under the action of the focused ultrasound, and repeating the steps 1 to 5 to obtain cavitation bubbles and cavitation motion vector images under different media.
Examples
The present invention will be described in detail by taking a non-transparent gelatin agar replica as an example, but the specific examples described are only for explaining the present invention and are not intended to limit the present invention.
Fig. 6 to 8 are diagrams showing the vector diagrams of the focusing ultrasonic cavitation and the cavitation motion using the blood vessel-like gelatin agar phantom as the experimental sample 8 according to the present invention.
FIG. 6 shows that a is deaerated water, b is gelatin agar membrane, c is cavitation micro-bubble group, and the dotted line part is pipeline.
(1) Preparing a simulated blood vessel tissue phantom: mixing 12% gelatin (g), 3% agar (g) and 85% deionized water (ml), heating and stirring with a magnetic stirrer to obtain a prepared imitation solution, pouring into a mold, cooling and solidifying, and slowly extracting out the tube to form the vascular tissue-imitated pipeline phantom.
(2) System construction: referring to fig. 2, the system mainly comprises a focused ultrasound transducer 1, a power amplifier 2, a dual-channel waveform generator 3, an open programmable ultrasound system platform 4 and a computer 6. The open programmable ultrasound system platform includes an ultrasound array transducer 5, a plane wave hardware receiving system, and a mainframe. The focusing ultrasonic transducer is fixed on the side wall of the transparent organic glass water tank 7, the water tank 7 is filled with deaerated water, and the room temperature is kept at (20 +/-2 ℃). Sound absorbing material 9 is placed on the walls and bottom of the tank 7 to reduce interference from multiple reflections of the focused ultrasound beam. The experimental blood vessel-imitating body model is fixed at the focus of the focused ultrasound. The vessel tissue-imitating pipeline is filled with physiological saline with the concentration of 0.9 percent. Directly above the phantom is an ultrasonic array transducer 5 for acquiring ultrasonic cavitation signals.
(3) And (3) time sequence control: fig. 5 shows a time sequence control of the generation and detection of the focused ultrasound cavitation, wherein the action time T of the focused ultrasound, the time delay D between the stop time of the focused ultrasound and the emission of the wide beam by the ultrasound array transducer 5, and T and D can be adjusted and controlled by the dual-channel waveform generator 3, and the adjustment range is from μ s to s. Here, the focused ultrasound action time T is set to 20ms, the time delay D of the first channel and the second channel of the two-channel waveform generator 3 is set to 8ms, and the power amplifier 2 is set to electric power of 100W.
(4) Signal acquisition: a focused ultrasonic system capable of setting sound pressure, excitation time and duty ratio is used as a cavitation energy source to generate ultrasonic cavitation bubbles. After the action of the focused ultrasound is stopped, a second channel transmitting signal of the dual-channel waveform generator 3 is triggered to be input to the open type ultrasonic system platform, the ultrasonic array transducer is driven to transmit plane waves to the cavitation bubbles, and a parallel channel data acquisition and storage unit acquires and stores multi-frame radio frequency cavitation echo signals.
(5) Signal processing: and processing the acquired radio frequency cavitation signals by adopting a wide-beam minimum variance beam synthesis method based on compressed sensing, and realizing continuous multi-frame sequence cavitation imaging on the processed data by adopting a radio frequency ultrasonic imaging algorithm.
(6) And (3) motion estimation: and selecting two continuous frames in the sequence cavitation image, and performing motion vector estimation by adopting a pyramid LK optical flow method to obtain a motion vector image of the ultrasonic cavitation bubbles.
Due to the long integral evanescence process of ultrasonic cavitation, as shown in (a) and (b) of fig. 7, which show two-dimensional cavitation images of the region of interest (ROI) of the 20 th and 21 st frames in succession in the sequence of two-dimensional cavitation images, the focal-region cavitation bubble group can be clearly seen by naked eyes from the two cavitation images, however, the change of the motion track of the cavitation bubbles in a short time is difficult to observe.
However, in the invention, the ultrasonic wide beam is used for detecting cavitation, and the pyramid LK optical flow algorithm is used for carrying out motion estimation on the ultrasonic cavitation under the premise of ensuring the high resolution of the image by combining the wide beam minimum variance beam synthesis algorithm based on compressed sensing, so that the motion track change of the cavitation bubble group can be seen in a short time. That is, fig. 8 is a cavitation bubble motion vector diagram obtained by subjecting two consecutive frames of two-dimensional cavitation images in fig. 7 to pyramid LK optical flow processing, and it can be seen that the ultrasonic cavitation bubbles are in a dissipation motion process.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (9)
1. An ultrasonic cavitation bubble motion vector estimation method is characterized in that: the method comprises the following steps:
step 1, a focused ultrasound system is adopted as a cavitation energy source to generate ultrasonic cavitation bubbles;
step 2, detecting the ultrasonic cavitation bubbles generated in the step 1 by emitting array wide beams, and acquiring continuous sequence ultrasonic cavitation original radio frequency signals;
step 3, obtaining continuous N frames of ultrasonic cavitation images by adopting a wide beam minimum variance beam synthesis algorithm based on compressed sensing and an ultrasonic imaging algorithm of radio frequency data;
step 4, carrying out motion vector estimation on the ultrasonic cavitation image obtained in the step 3 by a pyramid LK optical flow method to form a cavitation bubble motion vector diagram of a continuous frame;
and 5, changing parameters and media of a focused ultrasonic energy source for generating cavitation, and repeating the steps 1 to 4 to obtain cavitation and cavitation motion vector estimation images under different conditions.
2. The method according to claim 1, wherein the focused ultrasound system in step 1 comprises a focused ultrasound transducer, a power amplifier and a dual-channel waveform generator for controlling timing, wherein a first channel of the dual-channel waveform generator drives the power amplifier, and excites the focused ultrasound transducer to emit focused ultrasound waves.
3. The ultrasonic cavitation bubble motion vector estimation method according to claim 2, wherein the focused ultrasonic transducer is fixed to one side of the water tank, the surface of the focused ultrasonic transducer is immersed in the water, and sound absorption materials are placed on the bottom and the side wall of the water tank.
4. The method for estimating the motion vector of the ultrasonic cavitation bubbles according to claim 1, wherein the cavitation detection device for detecting the ultrasonic cavitation bubbles in the step 2 comprises an array transducer of an open programmable ultrasonic system platform, a parallel channel data acquisition and storage unit and a dual-channel waveform generator for controlling a time sequence, wherein a second channel of the dual-channel waveform generator drives the array transducer to transmit a wide beam and receive a cavitation echo signal, and a continuously acquired ultrasonic cavitation original radio frequency signal is obtained in a manner of acquisition and storage by the parallel channel data acquisition and storage unit.
5. The method according to claim 4, wherein the open-type programmable ultrasound system platform comprises an ultrasound array transducer, a plane wave hardware receiving system and a host, the ultrasound array transducer is fixed in the water tank right above the experimental sample, and sound-absorbing materials are placed on the side wall and the bottom of the water tank.
6. The method for estimating the motion vector of the ultrasound cavitation bubble according to claim 1, wherein the method for synthesizing the compressed sensing-based wide-beam minimum variance beam in step 3 comprises the following steps:
3.1, Fourier transform is carried out on the two-dimensional ultrasonic cavitation original radio frequency data, namely, the two-dimensional ultrasonic cavitation original radio frequency data are converted into frequency domain signals from time domain signals;
3.2 selecting frequency points in the effective bandwidth from the frequency domain information distribution of the ultrasonic cavitation radio frequency data, and randomly sampling and extracting the frequency points in the effective bandwidth according to the ratio of 40%;
3.3 adopting a space smoothing method and a diagonal loading method to construct a steady covariance matrix of the extracted frequency points, and calculating the optimal weighting coefficient of the minimum variance adaptive beam synthesis corresponding to the frequency points, thereby obtaining the frequency domain output of the frequency points under the optimal beam synthesis;
3.4 traversing all the extracted frequency points according to 3.3, and rebuilding frequency domain information in an effective bandwidth by a regularized multi-focus underdetermined system (RM _ FOCUSS) algorithm, and then rebuilding the whole frequency domain information;
and 3.5, transforming the whole frequency domain information to a time domain through inverse Fourier transform to obtain a cavitation radio frequency signal after beam synthesis.
7. The method for estimating the motion vector of the ultrasonic cavitation bubble according to claim 6, wherein the ultrasonic imaging algorithm based on the radio frequency data in the step 3 images the cavitation radio frequency signal after the beam synthesis: and (3) carrying out filtering processing, time gain compensation, envelope detection, secondary sampling, logarithmic compression and coordinate scanning transformation on the sequence two-dimensional cavitation radio-frequency signal subjected to beam synthesis to obtain a continuous sequence two-dimensional cavitation image with high resolution and high signal-to-noise ratio.
8. The method for estimating motion vectors of ultrasound cavitation bubbles according to claim 1, wherein the pyramid LK optical flow method adopted in the step 4 is used for motion vector estimation, and comprises the following steps:
carrying out Gaussian filtering pretreatment on two continuous frames of cavitation images;
performing down-sampling on the preprocessed cavitation image by adopting a pyramid technology to obtain an L +1 layer image;
and (3) realizing motion estimation of an image of a certain layer of the pyramid by adopting a calculation formula of an LK optical flow method, expanding the motion information of the layer to the next layer, and so on to realize motion estimation of the original image of the bottommost layer.
9. The method for estimating the motion vector of the ultrasonic cavitation bubbles according to claim 1, wherein in the step 5, parameters of a focused ultrasonic energy source generating cavitation and a medium are specifically changed: changing the action time of the focused ultrasound, wherein the time resolution reaches several microseconds; changing the time delay between the moment when the focused ultrasound stops and the wide beam emitted by the array transducer; changing the duty ratio of the focused ultrasound; changing the size of the focused ultrasonic energy; the medium of the focused ultrasound effect is changed.
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