CN107753058B - Shear wave dynamic filtering method - Google Patents

Abstract

The invention discloses a shear wave dynamic filtering method, which applies a dynamic filtering technology to shear wave velocity estimation, calculates a frequency range with the maximum contribution of shear waves to displacement at a detection position by carrying out spectrum analysis on deformation-time curves at different detection positions, filters the deformation-time curves according to the range, and carries out shear wave velocity estimation by using a filtered deformation-time result. The method improves the influence of factors such as large attenuation of shear waves in tissue propagation on wave velocity estimation, so that the shear wave velocity estimation result is more accurate, and the effectiveness of the result is improved.

Description

Shear wave dynamic filtering method

Technical Field

The invention relates to the field of shear wave elastography, in particular to a shear wave dynamic filtering method.

Background

Shear waves are waves whose propagation direction is perpendicular to the vibration direction of the mass points of the medium, also called transverse waves, S-waves.

The shear wave elastography technology can realize real-time hardness quantitative detection of biological tissues and provide a basis for clinically judging the pathological changes of the tissues. Its basic principle is as follows: the acoustic radiation force focusing impact energy can generate shear waves in tissues, and due to the fact that the propagation speeds of the shear waves in the tissues with different hardness are different, the hardness and softness of the positions can be indirectly reflected by detecting the propagation speeds of the shear waves in the different positions. And performing pseudo-color mapping according to the wave velocity of the shear wave to obtain shear wave elastic imaging. Therefore, the accuracy of shear wave velocity estimation is crucial to the determination of tissue elasticity.

The shear wave can cause the displacement deformation of the tissue in the tissue propagation process, the displacement deformation of a target area is observed and detected by using an ultrafast frame frequency imaging technology, and a shear wave vibration curve of an observed position is reconstructed through the displacement deformation. The conventional method for estimating the wave velocity of the shear wave is a Time-of-five (tof) method, in which a peak matching is performed on the shear wave vibration curves reconstructed at different positions of an observation position to find a peak difference Time interval, which can be understood as the Time taken for the shear wave to propagate from the two detection positions. And the distance between the two sensing locations is known, and the average velocity of the shear wave passing between the two sensing locations is found by dividing the distance by the time.

The shear waves impacted by the acoustic radiation force decay very rapidly as the tissue propagates, and the farther away from the source of the impact, the lower the signal-to-noise ratio of the displacement estimate. And different from the shear wave generated by mechanical vibration in vibration elastography, the shear wave generated by focusing impact of the acoustic radiation force is not the shear wave of a single frequency, but contains a signal with a certain bandwidth. These factors directly affect the accuracy of shear wave velocity estimation.

Disclosure of Invention

In order to make the estimation of the wave speed of the shear wave more accurate, the invention provides a shear wave dynamic filtering method.

The technical scheme adopted by the invention for realizing the purpose is as follows: a shear wave dynamic filtering method comprises the steps of selecting an elastic observation area in the process of measuring the elasticity of biological tissues, exciting shear waves to propagate through the area through the impact of acoustic radiation force, collecting echo signals of the area by using an ultrahigh frame frequency imaging device to perform displacement estimation operation, and obtaining a deformation estimation result; and the displacement estimation result is subjected to the following steps:

step 1, determining the position of the depth to be observed, selecting at least two transverse detection positions, and making a deformation-time curve corresponding to the selected position according to a deformation estimation result matrix;

step 1, carrying out Fourier transform on a deformation-time curve corresponding to the selected depth and each transverse detection position to obtain a corresponding frequency spectrum curve;

step 3, respectively carrying out spectrum analysis on the deformation-time spectrum curve of each transverse detection position, and determining the cut-off frequency range of the spectrum curve of each corresponding detection position;

step 4, generating filter coefficients according to the cut-off frequency corresponding to each detection position;

step 5, filtering the deformation-time curve in the delay time direction according to the filter corresponding to each position;

step 6, searching the peak value of the deformation-time curve after filtering of each detection position, and recording the corresponding time;

step 7, performing linear fitting according to each transverse detection position and the corresponding time point of the wave peak value to obtain a time-distance linear line;

and 8, solving the slope of the time-distance straight line to obtain the shear wave velocity value.

In the invention, a dynamic filtering technology is applied to shear wave velocity estimation, a frequency range with the maximum contribution of shear waves to displacement at a detection position is calculated by performing spectrum analysis on deformation-time curves at different detection positions, the deformation-time curves are filtered according to the frequency range, and the shear wave velocity estimation is performed by using the filtered deformation-time result. The method improves the influence of factors such as large attenuation of shear waves in tissue propagation on wave velocity estimation, so that the shear wave velocity estimation result is more accurate, and the effectiveness of the result is improved.

Further, in the above shear wave dynamic filtering method: when generating the filter coefficients, the filter coefficient generation method selects the FIR filter window function method.

The present invention will be described in more detail with reference to the following examples.

Drawings

FIG. 1 is a flow chart of example 1 of the present invention.

FIG. 2 shear wave displacement estimation results.

FIG. 3 shear wave displacement signal-to-noise ratio.

FIG. 4 is a plot of "deformation versus time" for each position.

FIG. 5 fits a "time-distance" straight line.

Figure 6 shows the elastic mass assessment.

Detailed Description

In this embodiment, before estimating the shear wave velocity, dynamic spectrum analysis is performed on the shear wave displacement estimation results at different detection positions, and the cut-off frequency range of the filter is determined according to the actual center frequency and the bandwidth, so that the shear wave frequency range in which the detection position contributes most to the tissue displacement can be effectively selected. And filtering the shear wave displacement estimation result according to the filter, and estimating the shear wave velocity by using the filtered result. The shear wave displacement estimation result after dynamic filtering can make the peak information more obvious, and improve the accuracy of the shear wave velocity estimation result, and the specific flow is shown in fig. 1.

In the process of measuring the elasticity of the biological tissue, an elastic observation area is selected, shear waves are excited by the impact of acoustic radiation force to propagate through the area, an echo signal of the area is collected by using an ultrahigh frame frequency imaging technology to carry out displacement estimation operation to obtain a deformation estimation result, and then the following steps are carried out on the displacement estimation result:

step 1: determining the position of the depth to be observed, selecting two or more transverse detection positions, and making a deformation-time curve corresponding to the selected position according to the deformation estimation result matrix, as shown in fig. 2. The figure shows the "deformation-time" curves for five different lateral test positions, although at different positions the waveforms are substantially similar, identical in frequency, but different at the moment of arrival of the peak, and it can be seen that the acoustic radiation force impact is at 50 ms.

Step 2: and carrying out Fourier transform on the deformation-time curve corresponding to each transverse detection position to obtain a corresponding frequency spectrum curve. As shown in fig. 3, the graph shows the spectrum of the curve for detection position one.

And step 3: the "deformation-time" spectrum curves of the respective lateral detection positions are subjected to spectrum analysis, and the cutoff frequency ranges of the spectrum curves of the respective corresponding detection positions are determined, as shown in fig. 4. The cut-off frequency method can be determined according to the spectrum amplitude, and the position which is different from the spectrum peak value by more than 30 dB-50 dB can be selected as the cut-off frequency range.

And 4, step 4: and generating a filter coefficient according to the cut-off frequency corresponding to each detection position. The filter coefficient generation method may select, for example, an FIR filter window function method.

And 5: and filtering the deformation-time curve in the delay time direction according to the filter corresponding to each position.

Step 6: the peak value of the "deformation-time" curve after filtering at each detection position is searched, and the corresponding time is recorded, as shown in fig. 5, the curve at position 1 is the filtered curve, and the basic shape is unchanged compared with the corresponding curve in fig. 2, which indicates that the carried information is not lost.

And 7: and performing linear fitting according to each transverse detection position and the time point corresponding to the wave peak value to obtain a time-distance linear line, as shown in fig. 6.

And 8: and (3) calculating the slope of the time-distance straight line to obtain the shear wave velocity value.

Claims (2)

1. A shear wave dynamic filtering method is characterized in that: in the process of measuring the elasticity of the biological tissue, selecting an elastic observation area, stimulating shear waves to propagate through the area through the impact of acoustic radiation force, and acquiring echo signals of the area by using an ultrahigh frame frequency imaging device to perform displacement estimation operation to obtain a deformation estimation result; and the displacement estimation result is subjected to the following steps:

step 1, determining the position of the depth to be observed, selecting at least two transverse detection positions, and making a deformation-time curve corresponding to the selected position according to a deformation estimation result matrix;

step 2, carrying out Fourier transform on the deformation-time curve corresponding to the selected depth and each transverse detection position to obtain a corresponding frequency spectrum curve;

step 3, respectively carrying out spectrum analysis on the deformation-time spectrum curve of each transverse detection position, and determining the cut-off frequency range of the spectrum curve of each corresponding detection position; in the step, a position which is different from a peak value of a frequency spectrum by more than 30 dB-50 dB is selected as a cut-off frequency range;

step 4, generating filter coefficients according to the cut-off frequency corresponding to each detection position;

step 5, filtering the deformation-time curve in the delay time direction according to the filter corresponding to each position;

step 6, searching the peak value of the deformation-time curve after filtering of each detection position, and recording the corresponding time;

step 7, performing linear fitting according to each transverse detection position and the corresponding time point of the wave peak value to obtain a time-distance linear line;

and 8, solving the slope of the time-distance straight line to obtain the shear wave velocity value.

2. A method of dynamic filtering of shear waves according to claim 1, characterized in that: when generating the filter coefficients, the filter coefficient generation method selects the FIR filter window function method.

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