CN111772676A - Ultrafast ultrasonic Doppler spinal cord micro-blood flow imaging system - Google Patents
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Abstract
The invention belongs to the technical field of biomedical ultrasonic imaging, and particularly relates to an ultrafast ultrasonic Doppler spinal cord micro-blood flow imaging system. The system comprises two parts, namely hardware and software; the hardware part comprises an ultrasonic transmitting and receiving module and an experimental equipment module. The software part comprises modules of ultra-fast ultrasonic imaging, beam synthesis, motion calibration, clutter filtering, Doppler imaging, difference calculation, correlation analysis and the like. Firstly, compiling an ultrasonic plane wave transmitting and receiving control module based on an ultra-fast ultrasonic imaging technology and a multi-angle plane wave compound imaging theory, and controlling hardware equipment to transmit and receive ultrasonic waves by computer software; processing the received echo data to finally obtain a Doppler blood flow image; and analyzing parameters such as the speed, the direction and the like of the blood flow. The system also provides a mode for imaging the spinal cord micro blood flow under the conditions of spinal cord pressurization, injury and stimulation, and can be used for spinal cord function analysis and physiological pathological analysis.
Description
Technical Field
The invention belongs to the technical field of biomedical ultrasonic imaging, and particularly relates to a spinal cord micro blood flow imaging system.
Background
Spinal cord injury is often accompanied by functional nerve injury, and complete loss of blood flow in the central area of the wound and a decrease in the amount of peripheral blood perfusion may be indicative of a focal of spinal cord functional injury.
Currently, the standard diagnostic methods for spinal injuries in clinical practice include X-ray plain film, Computed Tomography (CT), and Magnetic Resonance Imaging (MRI). These methods still suffer from the following disadvantages: electromagnetic radiation harmful to human health exists, the cost is high, and the imaging time is long. Compared with the existing methods, the ultrasonic imaging has the advantages of high imaging speed, portable equipment, low cost, no ionizing radiation and the like. As a novel blood flow monitoring technology, the ultrafast ultrasonic Doppler blood flow imaging method based on multi-angle plane wave compound imaging has the potential of performing high-resolution imaging on microvasculature and related microvasculature in spinal cord. (Heqiong, Roujian. research progress of ultra-high-speed ultrasound imaging [ J ]. Chinese medical imaging technology, 2014,30(8): 1251-1255.).
Disclosure of Invention
The invention aims to provide a spinal cord micro-blood flow imaging system which is high in imaging speed, portable in equipment, low in cost and free of ionizing radiation.
The spinal cord micro blood flow imaging system provided by the invention is based on an ultrafast ultrasonic Doppler technology and is divided into two parts, namely hardware and software. Wherein, the hardware part can be divided into two major modules: an ultrasonic signal transmitting and receiving module and an experimental equipment module. The ultrasonic signal transmitting and receiving module comprises a waveform generator, an ultrasonic probe, an analog signal amplifier, an analog signal filter, an analog-to-digital converter and a storage unit; the experimental equipment module comprises a spinal cord fixing device, a physiological signal detection device, a spinal cord pressurizing device, an electrical stimulation device, an oscilloscope and an arbitrary waveform generator; the software part comprises an ultrasonic pulse transmitting and receiving sequence module, a beam forming module, a motion calibration module, a clutter filtering and extracting module, a blood flow Doppler imaging module and a difference calculating module. The system relates to technologies including ultra-fast ultrasonic imaging technology, ultrasonic blood flow Doppler imaging technology, spinal cord function imaging and the like.
Hardware part
1. Ultrasonic transmitting and receiving module
The ultrasonic wave transmitting and receiving module comprises a waveform generator, an ultrasonic probe, an analog signal amplifier, an analog signal filter, an analog-to-digital converter and a storage unit. The module is used for transmitting a receiving echo signal of an ultrasonic signal.
Preferably, the connection mode and the work flow of the hardware module are as shown in fig. 2, firstly, the ultrasonic wave transmitting and receiving sequence set by software controls the waveform generator to generate a transmission waveform, and the ultrasonic probe converts the electric energy into the acoustic energy to transmit the ultrasonic wave; the receiving of the echo signals is completed by the same or different ultrasonic probes, the echo signals are processed by an analog signal amplifier and an analog signal filter, and then analog-to-digital conversion is performed by an analog-to-digital converter and sent to a storage unit to complete data storage.
2. Experimental equipment module
The experimental equipment module is used for fixing the spinal cord, imaging the spinal cord micro blood flow under different conditions and monitoring related physiological signals.
Preferably, the module comprises a spinal cord fixation device, a physiological signal monitoring device, a spinal cord compression device, an electrical stimulation device, an oscilloscope and an arbitrary waveform generator. The spinal cord fixing device is used for fixing the spinal cord, the spinal cord can be quantitatively pressurized by using the spinal cord pressurizing device, and the change of the hemodynamics under different pressure conditions is observed by an oscilloscope; the electrical stimulation device can also be used for applying electrical stimulation to the observation object, and the relation between the spinal nerve function and the blood flow change is explored through spinal cord micro blood flow analysis, so that the related physiological pathological analysis is carried out; and a physiological signal monitoring device can be used for measuring physiological signals such as respiration, heartbeat and the like in real time, and carrying out motion calibration related to respiration and heartbeat and correlation analysis of other physiological signals and blood flow signals.
(II) software module
The software module comprises an ultrasonic pulse transmitting and receiving sequence module, a beam forming module, a motion calibration module, a clutter filtering module, a blood flow Doppler imaging module and a difference calculating module.
The software flow of the spinal cord blood flow doppler imaging system is shown in fig. 1. Firstly, an ultrasonic plane wave transmitting and receiving sequence is compiled by an ultrasonic pulse transmitting and receiving sequence module, and a hardware module is controlled to transmit ultrasonic waves and receive echo signals. And the beam synthesis module performs beam synthesis on the received echo signals to obtain a B-mode image, and performs coherent compounding on data of the B-mode image obtained by transmitting a plurality of angle plane waves to obtain a high-quality B-mode image. The position between the images is calibrated by a motion calibration module (breathing causes a shift in the position of the spinal cord and therefore requires calibration). And then, a clutter filtering module filters clutter, knowing that all received signals are y, that is, y is s + t + n, that s is a blood flow signal, that t is a tissue signal, and that n is a noise signal, tissue signals and noise signals in original signals need to be filtered, so that a clear blood flow signal diagram can be obtained. And then the Doppler blood flow diagram is obtained through further processing by a blood flow Doppler imaging module. If the spinal cord is pressurized or the observation object is electrically stimulated, the difference calculation is carried out on the image data obtained under different states by the difference calculation module so as to carry out spinal cord function analysis and related physiological and pathological analysis.
(1) Ultrasonic plane wave transmitting and receiving module
The transmitting and receiving sequence of the ultrasonic plane wave is determined, and the requirements of high frame frequency and high quality imaging are met based on an ultra-fast ultrasonic imaging technology.
Specifically, an efficient signal transmitting and receiving sequence (including transmitting of ultrasonic plane wave pulses and receiving of echo data) for performing ultrasonic Doppler imaging on spinal cord micro blood flow by using the ultra-fast ultrasonic plane wave imaging method is determined, a complete sequence comprises a plurality of subsequences, in each subsequence, a group of N inclined plane waves with different angles are transmitted, and B-mode images obtained by the N times of transmission of the inclined plane waves can be synthesized into a frame of high-quality B-mode image. The subsequence is repeated K times within 1 second, namely K frames of composite B-mode images with high quality can be obtained every second. And setting the sampling time length as t seconds, and analyzing the echo data obtained within the time t seconds.
In order to observe slight blood flow changes among continuous multiframe images and improve the imaging frame rate as much as possible, the shortest achievable time interval between two times of ultrasonic plane wave transmission needs to be calculated, the theoretical highest imaging frame rate needs to be calculated, and the plane wave transmitting and receiving sequence is designed according to the shortest time interval. Since the propagation time of ultrasonic waves in a certain distance is limited by the propagation speed of the acoustic waves in a specific medium, a minimum physical limit exists, and the analysis process is as follows:
the linear array probe is utilized to carry out compound plane wave scanning on the interested area, so that a rectangular imaging area can be observed. In order to obtain clear image information of the imaging area, the time interval between the previous plane wave pulse emission and the next plane wave pulse emission is more than or equal to the time required by the ultrasonic wave to reciprocate in the diagonal length of the rectangular area. Knowing the depth of the imaging region d, the total length of the probe array L, and the propagation velocity of the ultrasound waves in the soft tissue c, the shortest time interval between two ultrasound plane waves is calculated as follows:
the number of a group of oblique plane waves is N, then theoretically the highest frame frequency is:
(2) a beam synthesis module: and performing beam synthesis on the echo data received in a period of time to obtain data of a B-mode image of an imaging area. Namely, the wave beam synthesis is carried out on each group of echo data with N angles received by the ultrasonic probe within the time t seconds, and continuous multi-frame high-quality composite B mode images are obtained.
Preferably, after beam forming, coherent compound imaging is performed on image data obtained by emitting the multi-angle oblique plane waves of each group, so that the signal-to-noise ratio and the resolution of the image can be effectively improved, and a high-quality B-mode image is obtained.
(3) A motion calibration module: respiration causes the spinal cord to shift in position, requiring positional calibration between images.
(4) Clutter filtering module: the received echo data includes three components, echo signals of static tissue, echo signals of blood flow, and noise. Therefore, the image data after the motion calibration needs to be filtered to remove noise and data of static tissue signals.
Preferably, in the present invention, the filtering adopts a method of eigenvalue decomposition. The method mainly comprises the following three steps: and decomposing the characteristic value, filtering out the characteristic vector and the characteristic value corresponding to the clutter component, and reconstructing a matrix.
(4.1) eigenvalue decomposition: firstly, image data of continuous multiframes are constructed into a two-dimensional matrix Am*n. The eigenvalue decomposition process is as follows:
E(A*AT)＝λ*U*UT；
wherein, U is an eigenvector matrix of m, λ is a diagonal matrix of m, and diagonal elements are matrix eigenvalues.
And (4.2) filtering the characteristic vector and the characteristic value corresponding to the clutter component, wherein the known static tissue clutter component corresponds to a signal component with a larger characteristic value, and the known dynamic blood flow signal corresponds to a signal component with a smaller characteristic value. Setting the first k maximum eigenvalues and the corresponding eigenvectors to zero to obtain a new eigenvector matrix of Uk。
(4.3) reconstructing an image matrix as shown in the following formula,
Ym*nis a matrix of extracted dynamic blood flow components.
Further preferably, k is selected from the following methods: 1. the direct method comprises the following steps: the k value is directly determined, and k is recommended to be 5% -25% of m (total number of characteristic values). 2. An indirect method: and determining the characteristic value subset corresponding to the clutter component according to the relative distribution among the characteristic values. Firstly, the eigenvalues are arranged according to descending order, and the difference lambda of two adjacent eigenvalues isi-λi+1Down to a certain threshold value sigma or a ratio lambdai/λi+1And when the value is reduced to a certain threshold value, the serial number i of the corresponding characteristic value is taken as the value of k. The suggested value of sigma is 0.15-0.2, and the suggested value is 1.0015-1.002. 3. Doppler frequency shift analysis: the doppler shift of the blood flow signal is higher and the doppler shift of the soft tissue signal is lower. By calculating the Doppler shift f of each eigenvectoriAnd determining the eigenvectors and eigenvalues needing to be filtered. The Doppler frequency shift range after normalization is 0-0.5, one value of 0.03-0.04 is recommended to be taken as the cut-off frequency of the soft tissue signal, and the characteristic vector of the Doppler frequency shift below the cut-off frequency and the corresponding characteristic value are set to be zero, so that clutter components can be filtered. The subscript of the eigenvector corresponding to the cutoff frequency is k. The calculation formula of the doppler shift is as follows:
where PRF is the pulse transmit frequency, m is the total length of a feature vector, ei(j) Is the jth element corresponding to the ith feature vector.
(5) A Doppler imaging module: and further processing the extracted dynamic blood flow signal to obtain a Doppler imaging result.
(6) A difference module: under the application of different external actions and normal state, the change of the micro blood flow images of the spinal cord is processed, and the spinal cord function analysis and the related physiological pathological analysis are carried out.
Features and effects of the system of the invention
In the invention, an ultrasonic plane wave transmitting and receiving control module is compiled based on an ultra-fast ultrasonic imaging technology and a multi-angle plane wave compound imaging theory, and a hardware device is controlled by computer software to transmit and receive ultrasonic waves; processing the received echo data to finally obtain a Doppler blood flow image; and analyzing parameters such as the speed, the direction and the like of the blood flow. The system also provides a mode for imaging the spinal cord micro blood flow under the conditions of spinal cord pressurization, injury and stimulation, and can be used for spinal cord function analysis and physiological pathological analysis.
The system can perform high-frame-frequency and high-quality imaging on spinal cord micro blood flow; continuous multi-frame high-resolution images of the rapid blood flow can be obtained, and a dynamic change map of the blood flow is generated; compared with the traditional micro blood flow imaging method, the system does not need to inject an ultrasonic contrast agent into the blood vessel, and improves the resolution and the signal-to-noise ratio of the blood flow image through high frame frequency imaging and multi-angle plane wave composite imaging. The imaging system can be used for hemodynamic analysis of the spinal cord and spinal cord functional analysis.
Drawings
Fig. 1 is a schematic diagram of a system architecture.
Fig. 2 is a schematic flow chart of the operation of the ultrasonic wave transmitting and receiving hardware module.
Fig. 3 is a schematic diagram illustrating the farthest propagation distance of the ultrasonic wave in the imaging region.
Fig. 4 is a schematic diagram of the ultrafast ultrasound plane wave transmitting and receiving sequence in the present embodiment.
Fig. 5 shows the result of image reconstruction of the echo data obtained by rat spinal cord imaging in the present embodiment.
Fig. 6 is a scattergram in which eigenvalues obtained by decomposing eigenvalues of data after beam synthesis are arranged in descending order in the embodiment.
FIG. 7 is a graph showing the difference (λ) between two adjacent eigenvalues after the eigenvalues are sorted in descending order in the examplei-λi+1) The scatter plot of (a).
FIG. 8 is a graph showing the ratio (λ) of two adjacent eigenvalues after the eigenvalues are sorted in descending order in the examplei/λi+1) The scatter plot of (a).
Fig. 9 is a graph showing the results of doppler shift analysis performed on each feature vector.
FIG. 10 is a dynamic blood flow graph extracted by eigenvalue decomposition after position calibration of multiple frames of images in the example.
FIG. 11 is a static anatomical map separated by eigenvalue decomposition after position calibration of multiple frames of images according to the example.
Figure 12 is a Doppler blood flow graph obtained by imaging the spinal cord of a rat in an example.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is specifically described below with reference to the embodiments and the accompanying drawings. The specific embodiments described herein are merely illustrative of the invention and do not delimit the invention.
Fig. 1 is a spinal cord micro blood flow ultrasonic imaging system proposed by the present invention, and the block diagram of the system is also applicable to the embodiment. This example is to image rat spinal cord using the spinal cord micro-blood flow imaging system proposed by the present invention, but is not limited to imaging rat spinal cord.
1. Designing ultrasonic transmitting and receiving sequence
In the invention, the spinal cord micro blood flow is imaged with high frame frequency and high quality by using the ultra-fast ultrasonic plane wave imaging technology. Therefore, the shortest time interval required for two adjacent oblique plane waves is determined, in one imaging the ultrasonic wave travels the farthest distance as shown in fig. 3, and the depth d of the imaging area is 6.308 × 10-3m, total length L of probe array 1.27 x 10-2m, the propagation speed c of the ultrasound in the soft tissue is 1.54 x 103m/s, calculated as the shortest time interval:
fig. 4 is an ultrasonic plane wave transmitting and receiving sequence of this embodiment, where the sequence includes a plurality of subsequences, each subsequence transmits a group of 27-angle oblique plane waves uniformly distributed at equal intervals between-10 ° and 10 °, the number of subsequences transmitted per second is set to K520, the number of oblique plane waves included in each subsequence is set to N27, and the time interval between each subsequence is t2，t2＝1.2*10-3(s)。
2. Transmitting plane wave and receiving echo signal
The spinal cord is gently punctured with a fine needle, causing local injury. The ultrasonic probe is fixed right above the spinal cord, the acquisition time length is set to be 1s, and echo data are acquired and stored. The working flow diagram of the ultrasonic signal transmitting and receiving module is shown in fig. 2, the ultrasonic signal is transmitted by firstly generating a transmission waveform by a waveform generator and converting electric energy into sound energy by a probe to transmit ultrasonic waves; the receiving of the echo signals is completed by the same or different ultrasonic probes, and the echo signals are processed by an analog signal amplifier and a filter, and then analog-to-digital conversion and data storage are carried out.
3. Beam synthesis
And (3) carrying out image reconstruction on the echo data by using a beam synthesis algorithm, and carrying out coherent superposition on the 27B-mode images obtained from each group to synthesize a frame of high-quality B-mode image. This subsequence is repeated 520 times within 1s, i.e. 520 frames per second of high quality composite B-mode images can be obtained. In this embodiment, the sampling duration is set to 1s, and 520 frames of composite B-mode images can be obtained after data processing. Fig. 5 is an imaging result of a composite B-mode image of one frame obtained by beam combining in the present embodiment.
4. Motion calibration
Other factors such as heartbeat and electrical stimulation cause the spinal cord to shift in position, and therefore position calibration between multiple frames of images is performed.
5. Clutter filtering
(1) Constructing the data of continuous 520 frames of composite B mode images after motion calibration into a two-dimensional matrix Am*nThe eigenvalue decomposition process is as follows: e (A)T)＝λ*U*UTAnd U is a feature vector matrix of m × m, S is a diagonal matrix of m × m, and diagonal elements are feature values. In this embodiment, 520 eigenvalues obtained through eigenvalue decomposition are arranged in descending order as shown in fig. 6.
(2) Because the clutter components correspond to signal components with large eigenvalues, the first k maximum eigenvalues are set to zero, and then matrix reconstruction is carried out to realize clutter filtering. k can be selected by the following methods:
I. direct value taking method: k is directly selected according to the percentage p of the total number of the characteristic values, and the value of p is 5-20%, namely 26-130%. In the embodiment, the total number m of the characteristic values is 520, and the suggested value of k is 26-104.
II, indirect value taking method: after the eigenvalues are arranged in descending order, the difference lambda of two adjacent eigenvaluesi-λi+1Down to a certain threshold value sigma or a ratio lambdai/λi+1And taking the serial number of the corresponding characteristic value as the value of k when the serial number is reduced to a certain threshold value. The proposed value of sigma is 0.15-0.2, and FIG. 7 shows the difference lambda between two adjacent eigenvalues after the eigenvalues are arranged in descending order in the embodimenti-λi+1The suggested value of k in the embodiment is 30-102 according to the scatter diagram; the suggested value is 1.0015 to 1.002, and FIG. 8 is the ratio λ of two adjacent eigenvalues after the eigenvalues are arranged in descending order in the embodimenti/λi+1The k obtained from the scatter diagram is 30-102.
Frequency spectrum analysis method: for each feature vector, a doppler shift analysis is performed, and the result is shown in fig. 9. As shown in the figure, the signal component with lower doppler shift has larger amplitude and corresponds to the soft tissue region; in order to not filter blood flow signal components mixed in clutter signals, k is taken as a serial number of a feature vector corresponding to Doppler frequency shift of 0.03-0.04, and k can be taken as a value of 42-102.
The calculation formula of the doppler shift is as follows:
where PRF is the pulse transmit frequency, N is the sample length, ei(j) Is the jth element corresponding to the ith feature vector.
(3) Reconstructing the image matrix as shown in the following formula:
Ym*nis a matrix of extracted dynamic blood flow components. Fig. 10 is a dynamic blood flow graph extracted by the eigenvalue decomposition algorithm, and fig. 11 is a static organization graph separated by the eigenvalue decomposition method.
6. Doppler blood flow diagram
The dynamic blood flow map is further processed to obtain a doppler blood flow map, as shown in fig. 12. The spinal cord microvascular structure of the rat can be seen in the figure, and local micro blood flow loss caused by mechanical injury of penetrating a fine needle at the position of-2.5 mm in the horizontal direction and-1.5 mm to-2 mm in the vertical direction can be observed.
The function and effect of the embodiment are as follows:
in this embodiment, by applying the ultrasonic imaging system for spinal cord micro-blood flow provided by the present invention, a continuous multi-frame high-quality spinal cord blood flow map is obtained based on an ultrafast ultrasonic doppler blood flow imaging technique, spinal cord micro-blood flow can be observed in each frame of image, and dynamic change conditions of blood flow within a period of time can be observed, and a doppler image of blood flow can be obtained through further processing. In this embodiment, the lack of local micro blood flow in the damaged area after the spinal cord is mechanically injured can be observed, so as to visually reflect the injured condition of the spinal cord.
The above embodiments are preferred examples of the present invention, and are not intended to limit the scope of the present invention.
Claims (5)
1. An ultrafast ultrasonic Doppler spinal cord micro-blood flow imaging system is characterized by comprising two parts, namely hardware and software; the hardware part comprises an ultrasonic signal transmitting and receiving module and an experimental equipment module; the ultrasonic signal transmitting and receiving module comprises a waveform generator, an ultrasonic probe, an analog signal amplifier, an analog signal filter, an analog-to-digital converter and a storage unit; the experimental equipment module comprises a spinal cord fixing device, a physiological signal detection device, a spinal cord pressurizing device, an electrical stimulation device, an oscilloscope and an arbitrary waveform generator; the software part comprises an ultrasonic pulse transmitting and receiving sequence module, a beam forming module, a motion calibration module, a clutter filtering and extracting module, a blood flow Doppler imaging module and a difference calculating module;
the module work flow of the hardware part is as follows: firstly, controlling a waveform generator to generate a transmission waveform by an ultrasonic wave transmitting and receiving sequence set by software, and converting electric energy into sound energy by an ultrasonic probe to transmit ultrasonic waves; the receiving of the echo signals is completed by the same or different ultrasonic probes, the echo signals are processed by an analog signal amplifier and an analog signal filter, then analog-to-digital conversion is performed by an analog-to-digital converter, and the processed echo signals are sent to a storage unit to complete data storage;
in the experimental equipment module, the spinal cord fixing device is used for fixing the spinal cord; quantitatively pressurizing the spinal cord by using a spinal cord pressurizing device, and observing the change of hemodynamics under different pressure conditions by using an oscilloscope; or the electrical stimulation device is used for applying electrical stimulation to the observation object, and the relation between the spinal nerve function and the blood flow change is explored through spinal cord micro blood flow analysis, so that the related physiological pathological analysis is carried out; a physiological signal monitoring device is used for measuring physiological signals such as respiration and heartbeat in real time, and performing respiratory heartbeat related motion calibration and correlation analysis of other physiological signals and blood flow signals;
the work flow of the software part is as follows: firstly, an ultrasonic plane wave transmitting and receiving sequence is compiled by an ultrasonic pulse transmitting and receiving sequence module, and a hardware module is controlled to transmit ultrasonic waves and receive echo signals; the beam synthesis module carries out beam synthesis on the received echo signals to obtain a B mode image, and carries out coherent compounding on data of the B mode image obtained by transmitting a plurality of angle plane waves to obtain a high-quality B mode image; calibrating, by a motion calibration module, a position between the images; then, a clutter filtering module filters clutter, and a blood flow Doppler imaging module further processes the clutter to obtain a Doppler blood flow diagram; and (3) applying pressure to the spinal cord or applying electrical stimulation to the observation object, and carrying out difference calculation on the obtained image data in different states by a difference calculating module so as to carry out spinal cord function analysis and related physiological pathological analysis.
2. The system of claim 1, wherein the transmitting and receiving module of the ultrasonic plane waves determines a transmitting and receiving sequence of the ultrasonic plane waves to meet the requirements of high frame rate and high quality imaging; a complete sequence comprises a plurality of subsequences, in each subsequence, a group of N inclined plane waves with different angles are emitted, and B-mode images obtained by emitting the inclined plane waves for N times can be synthesized into a frame of high-quality B-mode image; repeating the subsequence K times within 1 second, namely obtaining K frames of high-quality composite B mode images per second; setting the sampling time as t seconds, and analyzing echo data obtained within the time t seconds;
firstly, the shortest time interval which can be reached between two times of ultrasonic plane waves is calculated, the theoretical highest imaging frame frequency is calculated, and accordingly, a plane wave transmitting and receiving sequence is designed, the depth of an imaging area is known to be d, the total length of a probe array is known to be L, the propagation speed of ultrasonic waves in soft tissues is known to be c, and then the shortest time interval between two times of ultrasonic plane waves is calculated as follows:
the number of a group of inclined plane waves is N, and the highest frame frequency is:
3. the ultrafast ultrasound doppler spinal cord micro-blood flow imaging system of claim 2, wherein the beam forming module performs beam forming on echo data received over a period of time to obtain data of a B-mode image of an imaging region; performing beam forming on each group of echo data with N angles received by the ultrasonic probe within t seconds to obtain continuous multi-frame high-quality composite B-mode images;
after beam forming, coherent compound imaging is carried out on image data obtained by transmitting each group of multi-angle inclined plane waves, so that the signal-to-noise ratio and the resolution of the image are effectively improved, and a high-quality B mode image is obtained.
4. The ultrafast ultrasound doppler spinal cord micro blood flow imaging system of claim 3, wherein in the clutter filtering module: because the received echo data comprises three parts, namely echo signals of static tissues, echo signals of blood flow and noise, the image data after motion calibration needs to be filtered, and the data of the noise and the static tissue signals are filtered;
the filtering adopts a characteristic value decomposition method, which comprises the following three steps of characteristic value decomposition, filtering out a characteristic vector and a characteristic value corresponding to clutter components, and matrix reconstruction;
(4.1) eigenvalue decomposition: firstly, image data of continuous multiframes are constructed into a two-dimensional matrix Am*nThe eigenvalue decomposition process is as follows:
E(A*AT)＝λ*U*UT；
wherein, U is a eigenvector matrix of m × m, λ is a diagonal matrix of m × m, and diagonal elements are matrix eigenvalues;
(4.2) filtering feature vectors and feature values corresponding to the clutter components, wherein the known static tissue clutter components correspond to signal components with large feature values, and the known dynamic blood flow signals correspond to signal components with small feature values; setting the first k maximum eigenvalues and the corresponding eigenvectors to zero to obtain a new eigenvector matrix of Uk；
(4.3) reconstructing an image matrix, wherein the calculation formula is as follows:
Ym*nis a matrix of extracted dynamic blood flow components.
5. The system of claim 4, wherein k in the clutter filtering module is selected from the following methods:
(1) the direct method comprises the following steps: directly determining the value of k, wherein k is 5-25% of the total number m of the characteristic values;
(2) an indirect method: determining a characteristic value subset corresponding to the clutter component according to the relative distribution among the characteristic values; firstly, the eigenvalues are arranged according to descending order, and the difference lambda of two adjacent eigenvalues isi-λi+1Down to a certain threshold value sigma or a ratio lambdai/λi+1When the value is reduced to a certain threshold value, the serial number i of the corresponding characteristic value is taken as the value of k; the value of sigma is 0.15-0.2, and the value of sigma is 1.0015-1.002;
(3) doppler frequency shift analysis: by calculating the Doppler shift f of each eigenvectoriDetermining a feature vector and a feature value which need to be filtered; the range of the Doppler frequency shift after normalization is 0-0.5, one value of 0.03-0.04 is taken as the cut-off frequency of the soft tissue signal, the characteristic vector of the Doppler frequency shift below the cut-off frequency and the corresponding characteristic value are set to zero, and then the filtered component can be filtered; the subscript of the eigenvector corresponding to the cut-off frequency is k; the calculation formula of the doppler shift is as follows:
where PRF is the pulse transmit frequency, m is the total length of a feature vector, ei(j) Is the jth element corresponding to the ith feature vector.
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