A frequency domain beam synthesis method of a dual pulse wave synthetic aperture of focused wave emission

Abstract

The application discloses a kind of focused wave emission's dual-pulse wave synthetic aperture's frequency domain beam synthesis method.Belongs to the field of ultrasonic imaging, for the dual-pulse mode presented by focused wave sound field, frequency domain alternative method of time domain beam synthesis method is designed.Specifically includes: using sub-aperture to carry out focused ultrasonic emission;Echo signal acquisition in effective frequency band;Regional frequency domain delay function calculation and prestorage;Regional frequency domain delay function weighting superposition;Limited number of direction space-time signal reconstruction;Two-dimensional grid image interpolation reconstruction and display.The disclosed scheme has the imaging effect consistent with the focused wave emission's dual-pulse synthetic aperture time domain beam synthesis method, can be imaged in effective bandwidth, reduces the requirement of acquisition and storage system of hardware system, and can be used in medical ultrasonic imaging, sonar, radar and seismic wave etc. Array signal processing research field.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ultrasonic imaging, and relates to a frequency domain beam synthesis method of a synthetic aperture. BACKGROUND

[0002] In the field of ultrasonic imaging, the traditional beam synthesis method uses a simple delay-and-sum method, and the delay calculation of the method has limitations, and can only realize dynamic focusing in the receiving process, so that the obtained ultrasonic image has many artifacts, low resolution and poor contrast, and it is difficult to meet the increasing demand for clinical imaging quality. In subsequent development, the virtual source model improves the calculation method of the transmission delay, and realizes dynamic focusing in the transmission-receiving process by superimposing the echo data between the sub-apertures, thereby improving the lateral resolution of the image in the near field and the far field. However, it has an angle limitation at the focusing depth, and has a certain imaging blind area. Further research shows that the double pulse wave can better describe the focused transmission sound field, and solve the angle limitation problem of the virtual source model at the focusing depth. In combination with the synthetic aperture method, high resolution and uniform imaging in the entire depth range can be finally realized. For example, Nguyen et al. in "High-resolution ultrasound imaging with unified pixel-based beamforming", IEEE Transactions on Medical Imaging, vol. 35, No 1, 2016, pp. 98-108, and "Ultrasound pixel-based beamforming with phase alignments of focused beams", IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 64, No 6, 2017, pp. 937-946, describe some example schemes, which are incorporated herein by reference. However, the double pulse wave synthetic aperture beam synthesis method relies on the synthesis of multiple transmission data, and therefore, a large amount of ultrasonic echo data needs to be collected and stored, and the demand for the hardware system is too high. At the same time, the pixel-based calculation feature also puts extremely high requirements on the operation capacity of the hardware system to meet the demand for image frame rate in the clinic.

[0003] The frequency domain beam synthesis method originates from radar target detection and seismology research, uses block calculation in the frequency domain instead of conventional point-by-point calculation in the time domain, thereby greatly shortening the operation time; meanwhile, according to the characteristic that the ultrasonic echo signal is a bandpass signal, only partial frequency band data needs to be collected and stored, thereby reducing the demand for the hardware system. However, the current frequency domain beam synthesis method is only applicable to single-array element transmission-multiple-array element reception, the focusing point is located at the rear of the probe, and the like, and is applicable to the case of a large field of view but weak penetration and low imaging signal-to-noise ratio, and in principle cannot be applied to the double-pulse wave synthetic aperture method with the focusing point located at the front of the probe.

[0004] Therefore, there is an urgent need to invent a new frequency domain beam synthesis method, which can be applied to the double-pulse wave synthetic aperture method, while maintaining the high-quality imaging effect of the latter, and reducing the demand for the storage and operation capacity of the hardware system. SUMMARY

[0005] The purpose of the present application is to solve the problems of large data collection and large operation amount faced by the focused wave synthetic aperture imaging method in ultrasonic imaging, to perform equivalent beam synthesis operation in the frequency domain through frequency domain of the double-pulse delay model, to select the signal in the effective bandwidth for frequency domain beam synthesis in combination with the bandpass characteristic of the ultrasonic echo signal itself, and to reduce the demand for data collection amount, storage amount and operation amount while maintaining high-quality imaging effect; the specific operation steps are as follows:

[0006] Step 1: transmitting focused wave signals to the target imaging area through a plurality of sub-apertures divided by the probe;

[0007] Specifically, selecting a suitable ultrasonic wave center frequency and bandwidth, and transmitting focused ultrasonic waves to the target imaging area using an ultrasonic transducer;

[0008] Step 2: recording the echo signals reflected by the target imaging area to the focused sound field;

[0009] Specifically, collecting and recording a plurality of groups (N) of sub-aperture received ultrasonic echo data, and then performing filtering and time gain control and other preprocessing operations; the ultrasonic echo data of each group of sub-apertures can be represented as [r1(t),..., r j (t),..., r M (t)], wherein r j (t) represents the value of the echo data column vector received by the array element numbered j at time t, and M represents the number of array elements in a single sub-aperture;

[0010] Further, one-dimensional Fourier transform is performed on the time domain ultrasonic echo data of each channel, so that the time domain matrix [r1(t),..., r j (t),..., r M[(t)] is represented as a frequency domain matrix [R1(ω), ..., R...]. j (ω), ..., R M [(ω)], where R j (ω) represents the column vector of frequency domain echo data received by array element j; during implementation, for each channel's frequency domain data R j (ω) can selectively discard part of the frequency band and only use the effective bandwidth near the center frequency of the emitted sound wave;

[0011] Step 3: Calculate the frequency domain delay function based on the dual-pulse model in different regions;

[0012] Specifically, the time-domain delay formulas for each region under the dual-pulse wave synthetic aperture model are transformed into frequency-domain delay formulas, and the frequency-domain delay functions for each region are calculated according to imaging requirements. The calculation of the frequency-domain delay function can be completed in the preparation stage and stored in a table, thereby reducing redundant calculations and improving computational efficiency. The weighted superposition of data from each channel can use fixed weighting coefficients or adaptive weighted superposition based on the coherence between channels to suppress interference and noise.

[0013] Step 4: Perform frequency domain delay weighting on the echo signal based on the dual-pulse model, dividing the signal into regions;

[0014] Specifically: based on the frequency domain delay function calculated within the region of the dual-pulse model, the data matrix [R1(ω), ..., R] of each channel in the frequency domain is... j (ω), ..., R M Alignment and weighted superposition of [(ω)] are performed to obtain the frequency domain data B(ω) after beamforming; the weighted superposition of data between channels includes, but is not limited to, direct summation, weighted average, coherence weighted average and minimum variance weighted average, in order to suppress interference and noise;

[0015] Step 5: Reconstruct the signal in a limited number of directions within each region;

[0016] Specifically, the frequency domain signal after time-delay weighted superposition is converted to the time domain through a one-dimensional inverse Fourier transform. Therefore, the frequency domain signal B(ω) is represented as the time domain signal b(t), where b(t) represents the data on the straight line radiated from the emission focal point or both ends of the sub-aperture along a finite number of directions within the target imaging region.

[0017] Step 6: Generate a two-dimensional grid image within the target area through interpolation;

[0018] Specifically: after filtering each data column b(t) after beam synthesis, the b(t) is interpolated into the image grid of the two-dimensional pattern to be displayed in an interpolation manner, and the intersecting parts of the line coverage areas are superimposed; the interpolation method is not limited; after filtering, speckle suppression, logarithmic compression and other operations on each frame of image obtained by interpolation, the image is transmitted to the display for real-time refresh display;

[0019] Further, in step (1), the probe can be selected from various array probes such as a linear array and a convex array; the sub-aperture division form is not fixed and can be selected according to imaging requirements.

[0020] The focusing depth of the focused wave field can be set to a positive value (the direction in which the probe emits sound waves), that is, the focal point is in the target imaging area, or a negative value (the opposite direction of the probe emitting sound waves), that is, the focal point is outside the target imaging area.

[0021] Further, in step (2), the recorded echo information can be an echo signal containing full-band information, or can only contain part of the information in the effective frequency band.

[0022] Further, in step (3), the division of the double-pulse model is divided into four sub-regions (regions (I), (II), (III), and (IV)) with the focal point as the intersection point and the focusing angle as the opening angle; a pair of longitudinally opposite sub-regions (regions (I) and (III)) and a pair of transversely opposite sub-regions (regions (II) and (IV)) are calculated respectively; the frequency domain delay function can be calculated during imaging, or a pre-calculated and stored frequency domain delay function can be called. Figure 2

[0023] The frequency domain range of the frequency domain delay function calculation can be adjusted to cover the entire sampling bandwidth or only the effective bandwidth of the selected echo signal or other arbitrary preset bandwidth.

[0024] Further, in step (4), the weighting superposition method for frequency domain delay weighting superposition of the echo signal includes but is not limited to direct summation, weighted average, coherence weighted average, minimum variance weighted average and other similar array channel weighting methods.

[0025] Further, in step (5), the signal reconstruction in a limited number of directions in each region is specifically: the signal reconstruction direction in the longitudinally opposite pair of sub-regions is a tilted straight line passing through the transmission focal point, and the signal reconstruction direction in the transversely opposite pair of sub-regions is a tilted straight line passing through the left and right endpoints of the sub-aperture; the number and angle of the tilted straight lines can be adjusted arbitrarily according to imaging requirements.

[0026] ​Further, in step (6), the two-dimensional grid image in the target region formed by interpolation is specifically: the signal after frequency domain reconstruction is converted to the time domain by one-dimensional inverse Fourier transform for display; the converted signal is still in different limited numbers of preset directions, and various data interpolation methods can be used to fill the two-dimensional grid image; the interpolation methods include but are not limited to: nearest neighbor interpolation, bilinear interpolation, cubic interpolation, spline interpolation and the like.

[0027] The application also provides an apparatus, namely, an ultrasonic frequency domain imaging apparatus for focused wave emission, which is used for realizing the frequency domain ultrasonic beam synthesis method of the double-pulse wave synthetic aperture of the focused wave emission by means of a hardware circuit.

[0028] The device comprises a mutual connection of an ultrasonic signal excitation and focusing emission module, an echo signal acquisition module, a frequency domain delay function calculation module under a double pulse model, an echo signal frequency domain delay weighting superposition module, a signal reconstruction module in a limited number of directions in each region, a two-dimensional image grid interpolation module, a data storage module, an image display module and a system control module; the ultrasonic signal excitation and focusing emission module comprises exciting the ultrasonic probe using arbitrary short pulse excitation, and controlling the preset sub-aperture emission to focus on the focused ultrasonic wave at a certain depth; the echo signal acquisition module comprises but is not limited to using a signal constant rate acquisition scheme (Nyquist sampling) and a signal low speed acquisition scheme (band pass sampling, or subspace sampling technology, or Fourier folding sampling technology), supplemented by signal filtering and time gain control and other signal preprocessing schemes; the frequency domain delay function calculation module under the double pulse model comprises partition calculation of the required frequency domain delay function based on the mathematical principle of the double pulse model in the frequency domain, and the calculation result is stored in the data storage module; the echo signal frequency domain delay weighting superposition module comprises firstly performing one-dimensional Fourier transform (1D-FFT) on the preprocessed acquisition signal, then performing weighted synthesis on the frequency domain acquisition signal of each channel and the corresponding frequency domain delay function on a plurality of radial lines passing through the focus point of the emission sound field, to obtain the frequency domain signal after beam synthesis; the signal reconstruction module in a limited number of directions in each region comprises one-dimensional inverse Fourier transform (1D-IFFT) on the frequency domain signal after beam synthesis in a limited number of directions in each region, to obtain the time domain beam synthesis result on the lines in a limited number of directions in each region; the two-dimensional image grid interpolation module comprises filling the signal reconstruction result in a limited number of directions in each region into the pre-defined two-dimensional image grid by an interpolation method, as initial image data, the interpolation method is not limited, and the initial image data is subjected to image post-processing in a manner not limited to longitudinal band pass filtering, speckle suppression, envelope detection, logarithmic compression, and then the compressed image is dynamically and continuously transmitted to a display device and a storage module for display and storage; the data storage module is composed of a storage chip and a peripheral circuit, and is responsible for storing the pre-calculated frequency domain delay function and backing up the frame data of real-time beam synthesis; the image display module can be composed of a related circuit and a display screen, or a display device of a computer itself can be selected; the system control module is responsible for controlling the signal excitation, the frequency domain weighting superposition method and the image post-processing process, but the control objects are not limited to these processes.

[0029] The application has the advantages that the application can completely replace the time domain delay beam synthesis method in the traditional focused wave ultrasonic imaging, meanwhile, with the band pass characteristics of the ultrasonic echo signal in the frequency domain, the data acquisition amount and the operation amount can be greatly reduced, the working speed requirement of the analog / digital (A / D) sampling chip is lowered, the complexity of the hardware system is simplified, the performance requirement of the system is lowered, and the application has high engineering application value. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 A schematic diagram of an ultrasound imaging device system according to an embodiment of the present application;

[0031] Figure 2 A spatial diagram of an ultrasound sensor structure and frequency domain beamforming in a target acoustic field region according to an embodiment of the present application;

[0032] Figure 3 A flow chart of a dual pulse wave synthetic aperture frequency domain beamforming procedure of an ultrasound imaging device according to an embodiment of the present application; Figure 1 A flow chart of a dual pulse wave synthetic aperture frequency domain beamforming procedure of an ultrasound imaging device according to an embodiment of the present application;

[0033] Figure 4 A qualitative comparison diagram of an embodiment of the present application and a conventional time domain beamforming in a phantom experiment;

[0034] Figure 5 A point target response curve and quantitative analysis comparison diagram of an embodiment of the present application and a conventional time domain beamforming in a phantom experiment. DETAILED DESCRIPTION

[0035] The present application will be further described below in conjunction with embodiments, it should be noted that the scope of protection of the present application is not limited to the following embodiments, these examples are listed only for illustrative purposes and do not limit the present application in any way.

[0036] Medical ultrasound imaging systems often use focused wave ultrasound imaging mode to assist in human disease diagnosis, by linear array or convex array ultrasonic transducer to emit ultrasonic wave of preset center frequency to target detection area, using the same group of transducers to receive the reflection echo of human tissue at different depths, ultrasonic beamforming is performed inside the imaging device, and a two-dimensional image of the target area is reconstructed; the present application mainly includes a dual pulse wave synthetic aperture ultrasonic frequency domain beamforming method for focused wave transmission, and in some examples, an ultrasonic imaging device system as shown in Figure 1

[0037] Further, an ultrasonic frequency domain imaging device for focused wave transmission includes an ultrasonic signal excitation and focused transmission module, an echo signal acquisition module, a frequency domain delay function calculation module under a dual pulse model, an echo signal frequency domain delay weighting superposition module, a signal reconstruction module in a limited number of directions in each region, a two-dimensional image grid interpolation module, a data storage module, an image display module, a system control module, and the like.

[0038] The ultrasonic signal excitation and focused transmission module is configured to use any short pulse to excite the sub-aperture of various array probes such as linear array, convex array, etc. to transmit focused acoustic field; the division form of the sub-aperture is not fixed, and can be selected according to imaging requirements. ​

[0039] Wherein, the focusing emission module and the focusing sound field are configured to be adjustable in focusing depth, and the focusing depth can be set to a positive value (the direction in which the probe emits sound waves), that is, the focal point is in the target imaging area, or set to a negative value (the opposite direction of the probe emitting sound waves), that is, the focal point is outside the target imaging area.

[0040] Wherein, the echo signal acquisition module is configured in multiple rate acquisition modes, including not only the Nyquist sampling mode of normal rate, but also low rate sampling modes (including but not limited to bandpass sampling, subspace sampling, and Fourier folding sampling technologies); the recorded echo information can be an echo signal containing full-band information, or only information in a preset part of the effective frequency band.

[0041] Wherein, the frequency domain delay function calculation module under the dual pulse model is configured to be calculated in regions, and the regions are divided with the focal point as the intersection point and the focusing angle as the opening angle, so that a single sub-aperture imaging region is divided into four sub-regions as shown in the accompanying Figure 2 The frequency domain delay function is calculated in a pair of longitudinally opposite sub-regions and a pair of transversely opposite sub-regions.

[0042] Wherein, the frequency domain delay function calculation module under the dual pulse model is configured to be calculated in the imaging process, or to call the pre-calculated and stored frequency domain delay function.

[0043] Wherein, the echo signal frequency domain delay weighting superposition module is of multiple weighting methods, including but not limited to direct summation, weighted average, coherence weighted average, minimum variance weighted average, and similar array channel weighting methods.

[0044] Wherein, the signal reconstruction module in a limited number of directions in each region is configured to reconstruct in regions, and the signal reconstruction direction in a pair of longitudinally opposite sub-regions is a tilted straight line passing through the emission focal point, and the signal reconstruction direction in a pair of transversely opposite sub-regions is a tilted straight line passing through the left and right endpoints of the sub-aperture; the number and angle of the tilted straight lines can be adjusted arbitrarily according to imaging requirements.

[0045] Wherein, the two-dimensional image grid interpolation module is of multiple interpolation methods, and the time-space signal obtained after one-dimensional inverse Fourier transform and still in different preset directions of a limited number can be filled into a two-dimensional grid image using multiple data interpolation methods; the interpolation methods include but are not limited to: nearest neighbor interpolation, bilinear interpolation, cubic interpolation, spline interpolation, etc.

[0046] Wherein, the data storage module is composed of a storage chip and peripheral circuits, and is responsible for storing the pre-calculated frequency domain delay function and backing up the frame data of real-time beam synthesis.

[0047] The image display module is composed of a related circuit and a display screen, and can also use a display device of a computer to display the ultrasonic imaging result in real time and perform human-computer interaction.

[0048] The system control module is responsible for controlling signal excitation, frequency domain weighting mode and image post-processing.

[0049] The frequency domain delay function calculation, storage part and echo signal frequency domain delay weighting superposition part of the device are configured to be adjustable in the frequency domain range, so that the entire sampling bandwidth can be covered, or only the effective bandwidth of the selected echo signal or other arbitrary preset bandwidth can be covered.

[0050] The specific imaging process and data processing steps are referred to Figure 2 and Figure 3 , and are described in detail as follows:

[0051] Step 1: According to the preset transmission parameters (waveform center frequency f0, focusing depth D, and effective element number M), a focused ultrasonic wave is transmitted to the detection target area, and the formed acoustic field region is divided as shown in Figure 2 , and then the transducer receives analog signals [r1(t),..., r j (t),..., r M (t)] of reflected ultrasonic echoes at different depths and directions, wherein r j (t) represents the value of the echo data column vector received by the element numbered j at time t, and M represents the number of elements in a single sub-aperture.

[0052] Step 2: The analog signals of the ultrasonic echoes are sampled by using low-rate methods such as bandpass sampling, subspace undersampling and Fourier folding sampling, the frequency band range of the collected signals is as narrow as possible to the effective frequency band near the center frequency of the ultrasonic wave, and a low-rate ADC chip is used to convert the analog signals of the ultrasonic echoes into digital signals, and then one-dimensional discrete Fourier transform (FFT) is used to obtain frequency domain echo digital signals [R1(ω),..., R j (ω),..., R M (ω)]; in some other examples, the analog signals of the ultrasonic echoes can also be directly converted into digital signals by using Nyquist sampling at a high rate, and then one-dimensional discrete Fourier transform (1D-FFT) is used to obtain frequency domain echo digital signals, and the effective frequency band near the center frequency of the ultrasonic wave is selected to obtain frequency domain echo digital signals [R1(ω),..., R j (ω),..., R M (ω)] of each channel in the effective frequency band.

[0053] Step 3: In Figure 2The provided sound field model, the sound wave propagation delay to each point in the imaging area is calculated in the process of ultrasonic wave emission-receiving; in the time domain imaging model, according to the sound field mode of focused wave ultrasonic emission, the sound energy is mainly concentrated in the (I), (III) area, and there is a certain energy leakage near the focus point F in the (II), (IV) area; according to the double pulse model of focused emission mentioned by Nghia et al. in the article Ultrasound Pixel-Based Beamforming With Phase Alignments of Focused Beams (IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 39, No 6, 1992, pages 716-721) incorporated by reference herein. The emission sound field mode h t (x P , t) is shown in formula (1):

[0054]

[0055] Wherein, c represents the sound speed, t represents the propagation time, R1 and R2 represent the propagation distance of the two pulses, β1 and β2 represent the angular coefficient of the two pulses; therefore, the formula of the double pulse synthetic aperture time domain beam synthesis of the focused wave emission can be expressed as formula (2):

[0056]

[0057] Wherein, c1 and c2 represent the weighted synthesis of the two pulses, which need to satisfy the relationship c1+c2=1; the emission sound field mode of such double pulse makes the delay calculation of time domain imaging proceed in different regions, as shown in Figure 2 , divided into four imaging regions (I)-(IV); the pulse form of the emission sound field is dominated by a single pulse of the double pulse in the (I), (III) region, and the delay τ P，I and τ P，III are calculated as shown in formulas (3) and (4), the weight c1 or c2 of the dominant pulse is 1, and the weight of the non-dominant pulse is 0; the pulse form of the emission sound field is dominated by two pulses in the (II), (IV) region, and the delay τ P，L and τ P，R are calculated, respectively, corresponding to the delay of the propagation path from the target imaging point P to the left and right endpoints of the sub-aperture, and the delay τ P，L and τ P，R are calculated as shown in formulas (5) and (6), respectively, c1 or c2 is H L / (H L +H R) and H R / (H L +H R ), H L and H R are defined as Figure 2 ;

[0058]

[0059]

[0060]

[0061]

[0062] where P represents the imaging target point, O represents the center of the transducer sub-aperture array and the coordinate origin, Z P represents the longitudinal coordinate position of point P, X m represents the transverse coordinate position of receiving element m relative to point O, X m，L and X m，R represent the length of the line relative to the left end point of the sub-aperture and the right end point of the sub-aperture, θ D represents the angle between the line connecting P and F and the Z axis, θ L and θ R respectively represent the angle between the line connecting P and the left end point and the right end point of the sub-aperture and the Z axis;

[0063] The above is the time delay calculation method of time domain beam synthesis in dual-pulse wave synthetic aperture imaging, and the time domain function after time delay is represented as r m (τ P (Z P , θ)) It should be noted that although the time domain time delay calculation functions of (I) to (IV) are different, they mainly exist two dependent variables: depth Z P and angle θ (θ D , θ L and θ R ), wherein the depth Z P and the propagation time maintain a linear positive correlation relationship, which can be replaced by Z P = ct / 2;

[0064] Step 4, the calculation of the frequency domain time delay function. The present application considers converting the time domain time delay function into the frequency domain time delay function, which must involve one-dimensional Fourier transform of the time dimension, therefore, the first variable Z P of the above time domain time delay calculation will be changed to the frequency domain, and cannot be represented by the geometric structure in Figure 2 ; but the angle argument θ will still appear in the frequency domain time delay formula of the echo signal (formula (7)):

[0065]

[0066] Using variable substitution, t in the above formula is replaced by τ, and the frequency domain delay formula of echo signals in regions (I), (III) is obtained as formula (8):

[0067]

[0068] where A = (D + (cτ - D)cosθ D ) 2 - (Dsinθ D - X m cosθ D ) 2 , B = c(cτ - D)cosθ D + cDcos 2 θ D + cX m cosθ D sinθ D , R m denotes the frequency domain signal after delay in the mth channel, and T represents the maximum value of the support range of delay τ; since the double pulses in regions (II), (IV) are dominant, the frequency domain delay formula of echo signals needs to be calculated respectively, as shown in formula (9):

[0069]

[0070] where θ represents θ L and θ R respectively when calculating the left end point and the right end point of the sub-aperture corresponding to the pulse, and X represents X m，L and X m，R respectively; the frequency domain delay formula of echo signals derived in regions (I)-(IV) can be unified as formula (10) by using the following formula:

[0071]

[0072] where Q θ (n) is the Fourier transform form of q(k, θ, τ), which is called a frequency domain delay function, and θ represents θ L , θ R and θ D respectively. According to the analysis, the frequency domain delay function can be mainly concentrated in the direct current and low frequency components, so the frequency domain delay formula of echo signals can be approximately represented as formula (11) using a limited number of points (N2-N1+1):

[0073]

[0074] Step 5, process the frequency domain echo digital signals [R1(ω), ..., R] acquired from each channel. j (ω), ..., R M (ω)] Beamforming is performed in the frequency domain, and the beams are superimposed according to a certain weighting method (in some instances, window functions such as rectangular windows and Hamming windows can be used, or some adaptive weighting factors such as coherence factors and minimum variance factors can be used), as shown in formula (12):

[0075]

[0076] Step 6, for the sampling frequency f s The determined frequency band [-f s / 2,f s For each frequency point k within [ / 2], repeat the beamforming and superposition process in step 5 until beamforming is completed in the θ direction for the entire frequency band, resulting in the beamformed frequency domain signal R. S (k, θ); In some instances, based on the narrowband characteristics of ultrasonic echo signals, the effective single-sideband [f] can be defined only near the center frequency of the waveform. L f H For each frequency domain point k within the range, repeat the beamforming and superposition process in step 5.

[0077] Step 7, process the frequency domain signal R after beamforming. S Performing a one-dimensional inverse Fourier transform on (k, θ) yields the synthesized time-domain signal r. S (t, θ); in regions (I), (III), r S (t, θ) is unified into a radial straight line passing through the focal point, where θ is equal to the angle θ between this line and the z-axis (vertical direction). D In regions (II) and (IV), r S (t, θ) represent two straight lines passing through the left and right ends of the sub-aperture, respectively, corresponding to two pulse waves. θ is equal to the angle θ between the straight line passing through the left end of the sub-aperture and the z-axis. L Or the angle θ between the straight line passing through the right end of the sub-aperture and the z-axis direction. R ;

[0078] Step 8: Within the angular range that can cover the target imaging area, set a limited number of included angles θ. D And θ, their quantities are N respectively. θD and N θ In some examples, N θD Take 41, N θ Take 11; repeat steps 5, 6, and 7 to iterate through these angles and obtain N respectively. θD and N θ The time-domain signal r after beamformingS (t, θ);

[0079] Step 9, using the signals r on the various angular lines generated by beamforming in step 7. S (t, θ) is a standard value. Interpolation is performed on the pixels near it to fill the entire two-dimensional grid of the target area in the coordinate system, so as to obtain the two-dimensional signal distribution s(x, z) in the imaging area of ​​a single sub-aperture. The interpolation methods include, but are not limited to: nearest neighbor interpolation, bilinear interpolation, cubic interpolation, spline interpolation, etc.

[0080] Step 10: Linearly adjust the position of the transducer excitation sub-aperture, repeat steps 1-9 to complete the convergence imaging of multiple sub-apertures, and obtain the two-dimensional signal distribution s in the imaging regions of N sub-apertures. n (x, z) Finally, the overlapping regions between the N sub-apertures are superimposed and averaged to obtain the complete two-dimensional image signal S(x, z) of the synthesized aperture. The frequency domain delay function in step 4 does not need to be calculated repeatedly; it only needs to call the pre-stored value in memory.

[0081] Step 11: Perform bandpass filtering, speckle suppression, and envelope extraction on the initial two-dimensional image data S(x, z) along the z-axis to obtain the carrier information. The envelope extraction scheme can be Hilbert transform or orthogonal demodulation to obtain the two-dimensional image envelope data E(x, z).

[0082] Step 12: Logarithmically compress the two-dimensional image envelope data E(x, z) and adjust it to a preset dynamic display range to obtain the two-dimensional image I(x, z);

[0083] Step 13: Transmit each frame image I(x, z) to the display module for refresh display, and store each frame image in the data storage unit.

[0084] The above steps 1-13 detail the principle and process of the dual-pulse wave synthesized aperture frequency domain beamforming method.

[0085] Figure 3 The flowchart shown provides Figure 1The flowchart illustrates the process of dual-pulse wave synthetic aperture frequency domain beamforming in an ultrasonic imaging device with focused wave emission. First, the ultrasonic center frequency and focusing depth information are preset. Second, ultrasonic waves are emitted from a sub-aperture radial target area using an ultrasonic transducer. Third, the echo signals received by each channel of the transducer are recorded and converted to the frequency domain. Then, frequency domain beamforming is performed based on a pre-calculated frequency domain delay function to obtain frequency domain signals along several lines covering the target imaging area. Next, the frequency domain signals from the beamforming are converted to time domain signals via inverse Fourier transform. Finally, the time domain signals along the several lines are interpolated into a two-dimensional grid image, which undergoes post-processing steps such as bandpass filtering, speckle suppression, envelope detection, and logarithmic compression for display.

[0086] The following is an example of dual-pulse synthetic aperture frequency domain beamforming tested on the Phantom platform. This example uses a multi-target phantom, Model 040 (CIRS Inc.), as the scanning target. The scanning range is set to 20mm wide laterally and 35mm deep longitudinally (10mm–45mm). Five linear point targets and one hyperechoic circular cyst are present within the scanning area. The ultrasound sensor used in this example is an L11-5V linear array probe with a total of 128 independent elements, and the selected ultrasound center frequency (f0) is 5.2MHz. This example was conducted on the Vantage programmable fast ultrasound imaging platform. 256. The sampling frequency of each channel was set to 20.8MHz on a Verasonics instrument. The scanning mode was set to sub-aperture sliding scan, with each consecutive 65 array elements forming a sub-aperture, used to independently transmit focused ultrasound waves to the target area. After each transmit-receive event, the sub-aperture position was moved to enter the next transmit-receive event. In this example, the sub-aperture movement interval was one array element size, and a total of 128 transmit-receive operations were performed. In this example, the sampling frequency of each channel of the programmable fast ultrasound imaging platform was set to 4 times the center frequency of the ultrasound waves. In post-processing, frequency domain information with an effective bandwidth of 3.5MHz to 6MHz was obtained through digital bandpass sampling and subspace sampling, which was used for the frequency domain beamforming imaging proposed in this invention. However, the sampling implementation method of this invention is not limited to this method and also includes more sampling methods, such as acquiring frequency domain information within the effective bandwidth through hardware analog circuits for bandpass sampling, subspace sampling, and Fourier folding sampling.

[0087] In this example, assuming a sound speed of 1540 m / s, the time-domain (Coherent) aperture beamforming of a dual-pulse wave emitted by a focused wave is used. Imaging was performed using both the time-domain (PB) and frequency-domain (FDBF) methods, with the dynamic display range of the image set to 70 dB. The synthetic aperture range of the imaging was set to be within the overlapping area covered by 64 adjacent sub-apertures, where the coverage area of ​​regions (II) and (IV) was set to W1 = λf0 / L and W1 = 2λf0 / L, respectively, where λ represents the wavelength of the emitted ultrasound and L represents the length of the sub-aperture. In time-domain imaging, 128 longitudinal scan lines were set with 128 array elements as the center, and beamforming was performed only on the scan lines. In frequency-domain imaging, in regions (I) and (III), beamforming was performed on 81 straight lines passing through the focal point, uniformly distributed in radians of [-4.21, 4.21] radians. In regions (II) and (IV), beamforming was performed on 10 straight lines passing through the left and right endpoints and the coverage area, respectively. It was best to interpolate the images onto the same two-dimensional grid as the time-domain imaging for display.

[0088] Figure 4 The image compares the results of the aforementioned Coherent Beamforming (Coherent PB) and Frequency Beamforming (FDBF) imaging methods, and demonstrates the imaging effect of FDBF using echo signals from a portion of the effective frequency band (3.5MHz–6MHz). Qualitative analysis of these image results shows that the overall effect of FDBF imaging is very close to that of Coherent PB. The FDBF imaging results using a portion of the effective frequency band are slightly different. The difference lies in that the lateral resolution of point targets within the depth of focus is slightly worse than that of Coherent PB, but the lateral resolution of point targets outside the depth of focus is slightly better for FDBF than for Coherent PB. In terms of the longitudinal resolution of point targets, the FDBF imaging results are slightly worse than those of Coherent PB. The imaging effect of hyperechoic cyst targets is similar, and the speckle distribution characteristics and texture of the images are also quite similar.

[0089] Figure 5 Showing Figure 4 The horizontal and vertical cross-sectional response curves and quantitative index analysis of the images were performed. The horizontal and vertical cross-sectional response curves were displayed using regions containing point targets; the quantitative index analysis selected point target response half-width (FWHM), cyst contrast (CR), and structural similarity (SSIM). Overall, the horizontal resolution of FDBF point targets was slightly better than that of Coherent PB, but the vertical resolution of FDBF point targets was slightly worse than that of Coherent PB. Figure 4Image analysis of the hyperechoic cyst within the black circular region and the surrounding gray annular region yielded CR calculations. The results showed that FDBF and Coherent PB had similar CR indices, and the ultrasound images generated by both time-domain and frequency-domain imaging methods exhibited high structural similarity. These results demonstrate that, under focused wave emission conditions, the frequency-domain beamforming algorithm with dual-pulse synthetic aperture achieves almost identical beamforming and ultrasound imaging effects compared to the time-domain beamforming algorithm. It can serve as an alternative to the time-domain beamforming algorithm for focused wave imaging, and effectively addresses the challenges of large data acquisition and computational demands.

[0090] Finally, further explanation is needed. The specific implementation examples above are only used to illustrate the technical solutions involved in this invention, but the content involved in this invention is not limited to these examples (for example, the sampling frequency is not necessarily selected as 4 times the center frequency of the transmitted waveform). Any combination, equivalent substitution, modification and implementation of the technical solutions of this invention are covered within the scope of the claims of this invention. Documents incorporated in this patent application by reference should also be considered as part of the entire application. In addition, although the disclosed dual-pulse wave synthetic aperture frequency domain beamforming implementation mainly involves medical imaging of ultrasound, the disclosed beamforming technology can be applied to other similar beamforming fields, such as acoustic radar, sonar, array microphone processing and seismic wave research, as well as radio radar, radio communication and other fields.

[0091] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A frequency domain beamforming method for a dual-pulse wave synthesizing aperture of a focused wave emission, characterized in that: The specific operating steps are as follows: (1) A focused wave signal is emitted through a radial target imaging area divided by several sub-holes of the probe; the probe is one of various array probes such as linear array or convex array; The sub-aperture division method is selected according to the imaging requirements; (2) Record the echo signal reflected by the target imaging area to the focused sound field; the focusing depth of the focused sound field is a positive value and is towards the direction of the sound wave emitted by the probe, that is, the focal point is within the target imaging area; or it is set to a negative value and is towards the opposite direction of the sound wave emitted by the probe, that is, the focal point is outside the target imaging area; The recorded echo information is either an echo signal containing full-band information or only containing partial information within a portion of the effective frequency band. (3) Calculate the frequency domain delay function based on the dual-pulse model for regional division; the dual-pulse model divides the region into 4 partitions with the focal point as the intersection point and the focal angle as the sub-angle, and calculates the frequency domain delay function for a pair of vertically opposite partitions and a pair of horizontally opposite partitions respectively; at this time, the frequency domain delay function is calculated during the imaging process, or a pre-calculated and stored frequency domain delay function can be called. The signal reconstruction direction in a pair of longitudinally opposite partitions is an inclined straight line passing through the emission focal point, while the signal reconstruction direction in a pair of laterally opposite partitions is an inclined straight line passing through the left and right endpoints of the sub-aperture; the number and angle of the inclined straight lines can be adjusted arbitrarily according to imaging requirements. The frequency domain range calculated by the frequency domain delay function can cover the entire sampling bandwidth, or it can cover only the effective bandwidth of the selected echo signal or other arbitrary preset bandwidth. (4) Based on the dual-pulse model, the echo signal is divided into regions and then subjected to frequency domain delay weighted superposition; the weighting superposition method for frequency domain delay weighted superposition of the echo signal includes, but is not limited to, direct summation, weighted average, coherence weighted average and minimum variance weighted average array channel weighting method; (5) Reconstructing signals in a limited number of directions within each region; specifically, reconstructing signals in a limited number of directions within each region involves: The frequency domain delayed weighted superposition signal is located in a finite number of different preset directions. The signal is reconstructed in a finite number of directions by using a one-dimensional inverse Fourier transform. The resulting time domain signal is still located in a finite number of different preset directions. (6) A two-dimensional grid image of the target area is formed by interpolation; The interpolation process to form a two-dimensional grid image within the target area specifically involves: For the reconstructed time-domain signal in a limited number of directions, various data interpolation methods are used to fill the two-dimensional grid image; wherein, the interpolation methods include, but are not limited to, nearest neighbor interpolation, bilinear interpolation, cubic interpolation and spline interpolation.

2. An ultrasonic frequency domain imaging device for focused wave emission, characterized in that: The method for frequency domain beamforming of a dual-pulse wave synthetic aperture based on focused wave emission as described in claim 1 includes an ultrasonic signal excitation and focusing emission module, an echo signal acquisition module, a frequency domain delay function calculation module under a dual-pulse model, an echo signal frequency domain delay weighted superposition module, a signal reconstruction module in a limited number of directions within each region, a two-dimensional image grid interpolation module, a data storage module, an image display module, and a system control module, all interconnected.

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