JP6469876B2 - Ultrasonic imaging apparatus and ultrasonic signal processing method - Google Patents

Description

  The present invention relates to an ultrasonic imaging apparatus, and more particularly to an elastography technique for an ultrasonic imaging apparatus.

  An ultrasonic imaging apparatus is an apparatus that transmits ultrasonic waves into a living body, receives reflected waves (echo signals), and images forms of muscles, internal organs, and the like in the living body according to the intensity and time. In addition to this form imaging, the ultrasound imaging device has functional imaging such as Doppler imaging that performs blood flow imaging and velocity estimation from the frequency shift of the echo signal, and elastography that images differences in the hardness of the reflected wave in vivo. Is possible.

  There are two major methods for elastography. One is a method that generates local displacement in the living body by an external stimulus such as convergent ultrasound, measures the propagation velocity of shear waves propagating from it, and calculates the hardness under a certain assumption. . The other is a technique of estimating the distribution of hardness by applying a certain stress to the target part and distorting it, estimating the magnitude of the distortion from echo signals before and after applying the stress. The latter method (hereinafter, this method is referred to as elastography) is relatively easy to measure and is widely used clinically.

  In elastography, a composite autocorrelation method is known as a technique for calculating a deformation amount from echo signals obtained before and after deformation (Non-patent Document 1, Patent Document 1). This method is a method of roughly detecting a zone where no aliasing occurs between two consecutive echo signals before and after deformation, and then obtaining a fine displacement by a phase difference. The compound autocorrelation method is a method for obtaining a large motion in units of pixels to a fine motion less than a pixel by a relatively short time calculation, and can be said to be the most suitable method for elastography.

  The autocorrelation method, which is the basis of the composite autocorrelation method, is a method used in the Doppler method for viewing blood flow, and has an assumption that the amplitude and frequency are constant. However, in elastography on the assumption that there is scattering in the object, amplitude fluctuations due to scattering and signal broadening always occur, and the assumption of constant amplitude and frequency is not realistic. Therefore, the composite autocorrelation method based on the autocorrelation method is also easily influenced by amplitude fluctuations in the phase difference calculation step for obtaining a fine displacement. For this reason, even in a living tissue that should have a uniform hardness, there is a problem that a pattern as if it corresponds to a change in echo intensity may be seen.

  In order to solve this problem, Patent Document 2 pays attention to the fact that each correlation calculation has a constant window width, calculates the position of the center of gravity using information on the intensity signal within the window width, It is disclosed that the position of the center of gravity is used for correcting the position of the displacement obtained from the correlation calculation result. However, in this method, only the fluctuation of the amplitude value given when the phase correlation values are integrated within the window width is corrected, and the calculation of the displacement itself is not corrected. In addition, since the correction range varies depending on the window width, the effectiveness of correction during integration is limited to a case where the window width is widened.

JP 2008-136880 A JP 2012-130559 A

Akira Shiina et al., Real Time Tissue Elasticity Imaging, J. Med. Ultrasonics Vol 26. No. 2 (1999)

  As described above, in the conventional technology, the problem with the composite autocorrelation method used for elastography, that is, the problem that the reflected echo intensity information caused by the scattering in the tissue affects the phase difference calculation and the correct calculation result cannot be obtained. It has not been solved. In general, it is conceivable to use a spatial filter in order to solve the problem of signal broadening, but the spatial filter is used to visualize the difference in the hardness of a structure that is significantly larger than the size of a scatterer. It is only effective. When trying to visualize the difference in hardness of a structure having the same size as a scatterer, it cannot be handled by a spatial filter.

  An object of the present invention is to derive the correct calculation result by eliminating the influence of fluctuations caused by the scattering in the tissue on the calculation result even for a structure having the same size as the scatterer.

  An ultrasonic imaging apparatus of the present invention that solves the above-described problems includes an ultrasonic probe that contacts an inspection object and transmits and receives ultrasonic waves, an ultrasonic signal to the ultrasonic probe, and an ultrasonic probe. And a signal processor for processing the received signal, the signal processor using the plurality of received signals corresponding to a plurality of times of transmission. An elasticity information calculation unit that calculates the displacement and / or strain of the internal tissue to be inspected during transmission. The elasticity information calculation unit includes: a correlation calculation unit that calculates the correlation of envelopes of the plurality of reception signals; an intensity correction unit that corrects the intensity of envelopes for the plurality of reception signals; and the correlation calculation unit A phase difference calculation unit that calculates a phase difference between the plurality of reception signals using the calculation result and the plurality of reception signals corrected by the intensity correction unit. The elasticity information calculation unit calculates the displacement and / or strain using the phase difference calculated by the phase difference calculation unit.

  According to the present invention, the displacement or distortion is calculated by taking into account the phase difference obtained from the intensity-corrected signal, so that the calculation error depending on the signal intensity in the autocorrelation calculation can be eliminated, and an accurate calculation result can be obtained. can get.

  As a result, it is possible to realize an elastography that is not affected by fluctuations in echo intensity, that is, prevents information deterioration depending on the structure. Further, since the intensity correction can be performed regardless of the window width, processing such as widening the window width for correction is unnecessary, and high spatial resolution can be maintained.

1 is a diagram showing an overall outline of an ultrasonic imaging apparatus to which the present invention is applied. The functional block diagram of the signal processing part of 1st embodiment. The flowchart which shows the process of the signal processing part of 1st embodiment. The flowchart which shows the process of a correlation calculating part. (A), (b) is a figure which shows the example of an input part, respectively. The functional block diagram of the elasticity information calculating part of 2nd embodiment. The flowchart which shows the process of the elasticity information calculating part of 2nd embodiment. The figure which shows the distortion calculation result of an Example and a comparative example.

Hereinafter, embodiments of the ultrasonic imaging apparatus of the present invention will be described.
The ultrasonic imaging apparatus of the present embodiment is an ultrasonic imaging apparatus having an elastography function, and an ultrasonic probe that contacts an inspection object and transmits / receives ultrasonic waves, and an ultrasonic wave to the ultrasonic probe. A transmission / reception unit that transmits a signal and receives a reception signal from an ultrasound probe; and a signal processing unit that processes the reception signal, and the signal processing unit includes a plurality of signals corresponding to a plurality of transmissions. An elastic information calculation unit that calculates a displacement and / or strain of the internal tissue to be examined during the plurality of transmissions using a received signal;

  The elasticity information calculation unit includes a correlation calculation unit that calculates the correlation of the envelopes of the plurality of received signals, an intensity correction unit that corrects the strength of the envelope for the plurality of received signals, and a calculation of the correlation calculation unit A phase difference calculation unit that calculates a phase difference between the plurality of reception signals using the result and the plurality of reception signals corrected by the intensity correction unit, and calculating the phase difference calculated by the phase difference calculation unit. And calculating the displacement and / or distortion.

  Hereinafter, the configuration of the ultrasonic imaging apparatus of the present embodiment will be described with reference to FIG. The ultrasonic imaging apparatus shown in FIG. 1 includes, as main elements, an ultrasonic probe (hereinafter simply referred to as a probe) 10 that transmits and receives ultrasonic waves while contacting the surface of an inspection object (for example, a human body) 100, a probe. The transmitter / receiver 20 that transmits an ultrasonic signal to the probe 10 and receives an ultrasonic signal (received signal) output from the probe 10 that has received the ultrasonic wave from the subject, processes the received signal, and B A signal processing unit 30 that creates an image to be inspected, such as a mode and an M mode, and calculates elasticity information of the internal tissue to be inspected, and information necessary for processing and control in the transmission / reception unit 20 and the signal processing unit 30 Input unit 40, a display unit 50 for displaying images and other information created by the signal processing unit 30, and a control unit 60 for controlling the entire apparatus.

  The probe 10 has a transducer element that converts an electric signal and an acoustic signal, such as a ceramic element, and irradiates an ultrasonic pulse to a two-dimensional or three-dimensional region in the inspection target, and from the region. An echo signal that is a reflected wave is received. As a scanning method for irradiating a two-dimensional or three-dimensional region with ultrasonic waves, there are a mechanical scanning method and an electronic scanning method, and either of them may be adopted. Here, a plurality of transducer elements are arranged in a two-dimensional direction. An electronic scanning probe 10 arranged in the above will be described as an example. A signal received by each transducer element of the probe 10 as a reflected wave of one ultrasonic pulse is handled as one frame data. That is, one frame data is composed of a plurality of lines of RF signals along the ultrasonic transmission / reception direction.

  The transmission / reception unit 20 is transmitted by the probe 10 that has received a transmission beamformer (BF) 21 that transmits electrical signals for transmission pulses to the probe 10 and an ultrasonic wave (acoustic signal) that is a reflected wave from the inspection target. A reception beamformer (BF) 23 that receives electrical signals to be generated and generates RF data, and a transmission / reception control unit 25 that switches between transmission and reception.

  The operation of the transmission / reception unit 20 is the same as that of a normal ultrasonic imaging apparatus, and is briefly described as follows. First, an electrical signal for transmission pulse is transmitted from the transmission BF 21 to a probe 10 via a digital / analog (D / A) converter (not shown), and ultrasonic waves (acoustic signals) from the probe 10 toward an inspection target. Is sent. The acoustic signal reflected in the process of propagating through the inside of the inspection object is received by the probe 10, converted into an electric signal, and sent to the reception BF 23 as reception data through an A / D converter (not shown). The reception BF 23 performs addition processing in consideration of the time delay applied by the transmission BF during transmission on the signals received by the plurality of elements. The received signal after the addition processing is then subjected to processing such as attenuation correction by a correction unit (not shown), and then sent to the signal processing unit 30 as RF data. The operation of the transmission / reception unit 20 is controlled by the transmission / reception control unit 25.

The signal processing unit 30 includes a quadrature detection unit 31 that generates complex RF data from the RF data output from the reception BF 23, a memory unit 32 that stores frame data (here, complex RF data) as a buffer, and an amplitude from the RF data. Amplitude calculation unit 33 for calculating A, an elastic information calculation unit 34 for performing autocorrelation calculation and distortion calculation using complex RF data for two frames continuous in time, and irradiation of ultrasonic waves using RF data A display image to be displayed on the display unit 50 using the tomographic image generation unit 35 that generates the tomographic image of the image, the elasticity information such as strain calculated by the elasticity information calculation unit 34, the tomographic image created by the tomographic image generation unit 35, and the like. And a display image creation unit 36 to create. Although not shown in FIG. 1, a Doppler calculation unit that calculates a Doppler speed or the like from a frequency shift of continuously collected RF signals may be provided.

  The quadrature detection unit 31 generates complex RF data from the RF data based on transmission / reception parameters preset in the transmission / reception control unit 25 or values arbitrarily set via the input unit 40.

  The memory unit 32 temporarily stores frame data. When elastography is performed by the ultrasonic imaging apparatus of the present embodiment, the memory unit 32 accumulates two temporally continuous frame data. After accumulating data for two frames that are temporally continuous, a specific line of data along the transmission / reception direction of the ultrasonic waves is transferred to a subsequent calculation unit or the like as a set of two frames.

  The amplitude calculator 33 calculates the amplitude value of the signal from the complex RF data and sends it to the elasticity information calculator 34 and the tomographic image generator 35. The elasticity information calculation unit 34 calculates the displacement and strain of a desired structure inside the inspection target by a modified method based on a known complex autocorrelation calculation. Details of the elasticity information calculation unit 34 will be described later.

  Although not shown, the tomographic image generation unit 35 includes a processing unit that performs post-process processing such as gain control and logarithmic compression generally used in popular ultrasonic imaging apparatuses, and an amplitude calculation unit. Appropriate preprocessing is performed on the amplitude information sent from 33 to generate a tomographic image showing the form inside the examination object.

The display image creation unit 36 includes a scan converter and a cine memory (not shown), and adjusts the color scale based on the tomographic image created by the tomographic image generation unit 35 and the elasticity information output from the elasticity information calculation unit 34. performs such adjustment of the superimposed image, to create a video data, sent to the display unit 50 on the display unit 50.

  The control unit 60 controls the data flow in the apparatus main body and the operation of each element. The transmission / reception control unit 25 that controls transmission / reception can also serve as the control unit.

  The input unit 40 is a device for an operator to input numerical values such as commands and parameters necessary for processing by the signal processing unit 30 and the control unit 60, and includes one or a plurality of input devices such as a keyboard, a mouse, and a touch panel. . The display unit 50 is a display device for displaying an ultrasound image, elasticity information, and other incidental information, such as a patient ID and an imaging date. The display unit 50 can also function as the input unit 40 by displaying a GUI.

  In the configuration of the ultrasonic imaging apparatus described above, some elements, for example, the transmission / reception control unit 25, the control unit 60 (which may include the transmission / reception control unit 25), and the signal processing unit 30 (especially for various calculations and image generation). Function) can be realized by a computer system including a central processing unit (CPU), a GPU (Graphics Processing Unit) and a memory, and the function is software incorporated in the CPU or GPU or read from the outside by the CPU or GPU. realizable. Some of the functions of these elements can be replaced with hardware such as ASIC and FPGA.

  When performing elastography with an ultrasonic imaging apparatus, pressure in a predetermined direction, for example, a direction parallel to the irradiation direction of ultrasonic waves, is applied to the probe 10 brought into contact with the surface of the inspection object. Then, the two frame data before and after applying pressure are acquired, and the displacement of the structure inside the inspection object caused by the pressure is calculated by calculating between these data. The quantity is calculated as elasticity information. The ultrasonic imaging apparatus according to the present embodiment employs a new technique based on the composite autocorrelation method as a technique for calculating displacement and distortion. The configuration of the signal processing unit 30, particularly the configuration of the elasticity information calculation unit 34, for realizing this technique will be described below.

  As shown in the functional block diagram of FIG. 2, the elasticity information calculation unit 34 includes an intensity correction unit 341, a correlation calculation unit 343, a strain calculation unit 345, and a post-processing unit 347. The distortion calculation unit 345 includes a phase difference calculation unit 345a and a distortion calculation unit 345b. Although not essential, a band limiting unit 349 that limits the band of the RF signal to the vicinity of the center frequency can be provided.

  The intensity correction unit 341 performs correction using the amplitude calculated by the amplitude calculation unit 33 on the two pieces of RF data input to the elasticity information calculation unit 34. The two pieces of RF data are reception data acquired before and after pressure application. The intensity correction unit 341 corrects these two RF data to RF data including phase information that is not affected by the amplitude (intensity).

  The correlation calculation unit 343 performs a first-stage process of the composite autocorrelation method, that is, a process of determining a range in which aliasing does not occur using the correlation of the envelope. The distortion calculation unit 345 calculates the phase difference, which is the second stage of the composite autocorrelation method, using the calculation result of the correlation calculation unit 343 and the RF data corrected by the intensity correction unit 341 (phase difference calculation). Function of the unit 345a) and various amounts such as displacement and strain are calculated using the phase difference (function of the strain calculation unit 345b). The post-processing unit 347 performs distortion gradation conversion, color mapping, and the like.

式中、Ａは振幅値、ｆ_０は基本周波数、φは位相を表す。Next, details of the operation of the signal processing unit 30 will be described with reference to the flow of FIG.
The signal input to the signal processing unit 30, that is, the RF signal 300 phased and added by the reception BF 23 can be expressed as the following equation (1).

In the formula, A represents an amplitude value, f ₀ represents a fundamental frequency, and φ represents a phase.

The quadrature detection unit 31 converts the RF signal input to the signal processing unit 30 into a complex RF signal (S301). The converted complex RF signals (also referred to as QI signals) are expressed by equations (2) and (3), respectively.

The amplitude calculator 32 calculates the amplitude value A from the complex RF signals I and Q by the following equation (4) (S302).

  The calculated amplitude value A is used for generating a tomographic image in the tomographic image generating unit 35 and also used for correcting the intensity of the RF signal in the intensity correcting unit 341 of the elasticity information calculating unit 34 described later.

  Next, the elasticity information calculation unit 34 performs a calculation corresponding to the first stage of the composite autocorrelation method by performing the calculation by the correlation calculation unit 343 (S303), and performs aliasing from the correlation between the envelopes of the two RF signals. A range that does not occur, that is, a rough phase difference range is determined. On the other hand, after the intensity correction unit 341 performs intensity correction on the complex RF signal, the phase difference of the corrected complex RF signal is calculated (S304 to S306).

  Since the calculation (S303) performed by the correlation calculation unit 343 is described in detail in Non-Patent Document 1, the outline thereof will be described here with reference to FIG.

First, one of the complex RF signals x (t) and y (t) of two frames, for example, the signal y (t) is only a half integer multiple (n / 2 times) of the period T of the reference signal (basic frequency signal). A cross-correlation function Rxy between the shifted signal y (t−nT / 2) and the signal x is obtained (S401). This cross-correlation function can be regarded as an autocorrelation function of an envelope (that is, A (t)), and its phase (phase difference) Ψ is expressed by the following equation (5).
[Equation 5]
Ψ (t; n) = f ₀ (τ−nT / 2)
= Ψ (t; 0) −nπ (5)

In the equation, τ is a time transition due to compression between two frames. When the amount of displacement is small, Ψ (t; 0) is equal to the phase Φ (t; 0) obtained from the real part Qxy and imaginary part Ixy of the autocorrelation function of the envelope by the following equation (6).
[Equation 6]
Φ (t; 0) = arc (Ixy / Qxy) (6)
Ψ (t; 0) = Φ (t; 0)

However, when the phase difference is large (| Ψ | ≧ π), aliasing occurs, and Ψ cannot be determined uniquely from Qxy and Ixy. Therefore, among the integers n, an integer m that does not cause aliasing is obtained. For this reason, first, the absolute value C (t; n) of the correlation function of x (t) and y (t−nT / 2) is calculated by equation (7) (S402), and n where C is maximized is represented by m. (S403).

  Thus, when n = m, Ψ (t; m) = Φ (t; m) is established, and the phase Ψ (that is, the phase difference between the signals x and y) is obtained.

  On the other hand, the intensity correction unit 341 corrects the RF signal before quadrature detection using the amplitude value A obtained in step S302 of FIG. 3 for correction (S304). Specifically, an inverse number (1 / A) of the amplitude value A is used as an intensity correction coefficient (correction value), and the RF signal before quadrature detection is multiplied by the inverse number (1 / A) of A. Instead of using the reciprocal of the amplitude as the intensity correction coefficient, the maximum value Amax of one amplitude may be used as a reference point, and the reciprocal of the magnitude of the amplitude with respect to the reference point (Amax / A) may be used. As the intensity correction coefficient, (1 / A) or (Amax / A) described above may be set in advance, or the operator can select either via the input unit 40 (for example, a GUI as shown in FIG. 5). Alternatively, a value obtained by multiplying these by an appropriate coefficient may be input. FIG. 5A shows an example of inputting an intensity correction coefficient, and FIG. 5B shows an example of selecting a specified value.

  In addition, when intensity correction is performed using (1 / A), noise enlargement becomes significant in a portion where the S / N of the signal is low. In order to prevent this, arithmetic processing using a narrow band frequency filter is performed in advance on the RF signal before correction, and components other than the central frequency are removed or components other than the center frequency are removed by a weighting filter or the like. It is preferable to perform processing such as reduction. Arithmetic processing using such a filter is realized by the band limiting unit 349 described above.

式中、ＱＩ信号の下付文字ｘ、ｙは、それぞれ第一フレーム、第二フレームのデータであることを示す。Next, the corrected RF signal is subjected to quadrature detection by the quadrature detection unit 31 to obtain a corrected complex RF signal (S305). The corrected complex RF signal (QI signal) after quadrature detection is expressed by equations (8) and (9).

In the expression, subscripts x and y of the QI signal indicate data of the first frame and the second frame, respectively.

  As can be seen from equations (8) and (9), the corrected complex signal is not affected by the amplitude value. The phase difference calculation unit 345a obtains Qxy and Ixy in Equation (6) described above from the complex product of the corrected complex RF signals obtained for the RF signals of the two frames, and uses the “arctan” to determine the phase (position). The phase difference Φ is calculated (S306).

式中、λは参照信号の波長である。Next, the distortion calculation unit 345 (distortion calculation unit 345b) uses the “m” obtained by the correlation calculation unit 343 in step S303 and the phase difference Φ obtained in step S306 to calculate the displacement generated between the two frames. Calculate (S307). The displacement δ (t) is obtained by the following equation (10).

Where λ is the wavelength of the reference signal.

Next, a local tissue strain amount at the target location is calculated using the displacement δ described above (S308). The strain S can be obtained by differentiating the displacement in the depth direction (ultrasonic irradiation direction). That is, the distortion S is given by the following equation (11).

  As a specific calculation method, for example, the distortion of each point along the depth direction can be calculated by dividing the displacement difference of each sampling point by the sampling point interval.

The post-processing unit 347 performs gradation conversion or color mapping of the distortion value from the representative value and the variation of the distortion value obtained from two consecutive frames (S309). For example, the gradation value or the color mapped distortion value can be superimposed on the tomographic image generated by the tomographic image generation unit 35 and displayed on the display unit 50 .

  According to the present embodiment, in the calculation for obtaining the phase difference of the composite autocorrelation method, the influence of amplitude fluctuation due to scattering can be eliminated and the phase difference can be accurately calculated by using the signal subjected to intensity correction. . The signal intensity correction in this embodiment is not affected by the window width. Therefore, processing such as widening the window width for correction is not required, and correction can be performed without sacrificing spatial resolution.

<Second embodiment>
The ultrasonic imaging apparatus of the present embodiment is characterized in that a function for performing a signal thinning process and interpolation is added to the elasticity information calculation unit 34 of the signal processing unit 30 of the ultrasonic imaging apparatus of the first embodiment described above. is there. That is, the elasticity information calculation unit of the present embodiment includes a thinning unit that thins out data of sample points whose amplitude values exceed a threshold value among a plurality of sample point data constituting the received signal, and the calculated displacement and / or Or an interpolation unit that complements data having a missing value in the distortion.
Since other configurations and operations are the same as those of the first embodiment, the characteristic parts of the second embodiment will be described below.

  FIG. 6 shows a functional block diagram of the elasticity information calculation unit 34 of the ultrasonic imaging apparatus of the present embodiment. In FIG. 6, the same elements as those in FIG. As illustrated, the elasticity information calculation unit 34 of the present embodiment includes a thinning-out unit 342, an intensity correction unit 341, a correlation calculation unit 343, a distortion calculation unit 345, an interpolation unit 346, and a post-processing unit 347. . The elasticity information calculation unit 34 of the present embodiment has a thinning unit 342 and an interpolation unit 346 added to the elasticity information calculation unit 34 of the first embodiment. Also in the present embodiment, a band limiting unit 349 that limits the band of the RF signal to the vicinity of the center frequency may be provided.

  Prior to the correction by the intensity correction unit 341, the thinning unit 342 performs a process of thinning out data whose amplitude exceeds a predetermined threshold from the received signal (RF signal) input to the signal processing unit 30. The received signal input to the signal processing unit 30 is a digital signal sampled at a predetermined sampling frequency by an A / D converter, and each sample point data of the digital signal has an amplitude value. The thinning unit compares the amplitude value of the data at each sample point with a threshold value, and when the threshold value is exceeded, sets the amplitude value at that sample point to blank or zero.

  There is a deficiency in data used for calculation by the thinning process in the thinning unit 342. Due to this, data loss occurs in the subsequent calculation results. The interpolation unit 346 interpolates such missing data as the calculation result. For example, the displacement or strain value calculated by the strain calculation unit 345 includes a missing data point corresponding to the sample point thinned out by the thinning unit 342. The interpolation unit 346 interpolates the missing data points.

  Hereinafter, details of the processing of the elasticity information calculation unit 34 in the present embodiment will be described with reference to FIG. 7, the same processes as those in FIG. 4 are denoted by the same reference numerals, and detailed description thereof is omitted.

  First, the amplitude calculator 33 calculates the amplitude using the complex RF signals Q and I obtained by quadrature detection on the received RF signal (S301, S302). Next, the correlation calculation unit 343 performs the first-stage envelope correlation calculation of the composite autocorrelation method (S303). In parallel, the intensity correction unit 341 performs intensity correction using the RF signal before quadrature detection (S310).

  In the intensity correction (S310), as shown in FIG. 6, first, high-frequency noise and the like are removed by statistical processing in advance for the amplitude (amplitude of each sample point) obtained in step S302. Specifically, first, an average value and a standard deviation of amplitude values are calculated for a certain range of sample points (S311). Next, it is determined whether the difference (absolute value) between the amplitude value and the average value of each sample point is within a certain threshold (S312). For example, a value that is an integral multiple of the standard deviation is used as the threshold value. If the difference between the amplitude value and the average value exceeds a predetermined threshold, the thinning unit 342 thins the sample point and removes it from the subsequent correction processing (S313). After performing this thinning process, the intensity correction unit 341 performs intensity correction by multiplying the RF signal by a reciprocal of the amplitude value, for example, as a correction value (S314).

  In the above description, the average value and standard deviation of amplitude values were used as the statistical processing method when performing decimation processing. However, a method that calculates outliers of amplitude signals using the Mahalanobis distance is used. May be. Prior to statistical processing, high-frequency noise that is twice or more the center frequency may be removed using a low-pass filter. Note that the threshold used by the thinning unit 342 for determination of thinning may be set in advance according to a predetermined statistical processing method, or the operator may input via the input unit 40 (for example, a GUI as shown in FIG. 5). Alternatively, a statistical processing method may be selected. FIG. 5A shows an example of inputting a thinning threshold, and FIG. 5B shows an example of setting a specified value or selecting a specified value.

  The RF signal after the intensity correction is orthogonally detected to obtain a complex RF signal (S305), and the phase difference is calculated (S306) as in the first embodiment. However, the phase difference obtained in step S306 includes data points having no value.

  Next, using the calculation result of step S303 and the phase difference obtained in step S306, the displacement is calculated by the above-described equation (10) (S307), and further the distortion value is calculated (S308). Since the calculated distortion value uses data obtained by thinning out the signal value of a predetermined data point in the correction processing step, it includes data points having no value. Therefore, the interpolation unit 346 interpolates the distortion value of the data point without this value (S315). As the interpolation method, a known method such as linear interpolation or spline interpolation can be used.

Thereafter, the gradation value is color-graded or color-mapped to obtain image data displayed on the display unit 50 (S309), as in the first embodiment.
7 illustrates the case where the interpolation process (S315) is performed after the distortion value calculation process (S308), but in principle, the interpolation process can be performed after intensity correction and before phase difference detection. However, in order not to make the error manifest in the distortion value calculation step, it is preferably performed after that.

  In the RF signal intensity correction (S304) in the first embodiment, when the reciprocal of the amplitude value of the signal is simply multiplied, noise enlargement becomes remarkable in the portion where the S / N is low. Therefore, by thinning out data in which noise is expanded in advance and interpolating data that is insufficient after the distortion value calculation, elasticity information that has continuity and eliminates the influence of noise can be obtained. In the present embodiment, similarly to the first embodiment, prior to intensity correction, arithmetic processing using a narrow-band frequency filter is performed, and components other than the central frequency are removed, or a center is obtained by a weighting filter or the like. It goes without saying that processing such as reducing components other than the frequency (function of the band limiting unit 349) may be performed.

  The distortion calculation was simulated using the RF signals of two frames. As a RF signal, a sine wave is used, a carrier waveform having a frequency of 1/8 of the sampling frequency is modulated with a frequency of 1/42 and a waveform with 1% distortion applied to the waveform is created. These two waveforms were used as signals of two consecutive frames. The distortion value was calculated from the displacement calculated by performing the calculation of performing the cross-correlation between the envelopes of the two RF signals and the process of calculating the phase difference after correcting the intensity of the RF signal according to the method of the first embodiment. . As a comparative example, a distortion value was calculated by a conventional composite autocorrelation method without correcting the intensity of the RF signal.

  The results are shown in FIG. In FIG. 8, the upper left waveform is the created RF signal, and the lower left waveform is the waveform after signal intensity correction. The right side shows the calculated distortion value (distortion distribution). The upper side shows a case where no signal intensity correction is performed (comparative example), and the lower side shows a case where signal intensity correction is performed (example). Each correct value is indicated by a two-dot chain line.

  As can be seen from the results of FIG. 8, the distortion calculated from the RF signal without intensity correction has a large deviation from the correct value, and fluctuation corresponding to the modulation fluctuation is seen, whereas the RF signal with intensity correction performed. Thus, there is no significant deviation from the calculated distortion.

  ADVANTAGE OF THE INVENTION According to this invention, the ultrasonic imaging apparatus which can obtain accurate elastic information and can provide effective diagnostic information is provided. As an example of a specific effect, it becomes possible to diagnose the characteristics of fine structures that require a resolution of about 1 mm, such as visualization of the internal properties of intravascular plaques, and to detect small tumors in the initial stage.

DESCRIPTION OF SYMBOLS 10 ... Ultrasonic probe, 20 ... Transmission / reception part, 21 ... Transmission beam former, 23 ... Reception beam former, 25 ... Transmission / reception control part, 30 ... Signal processing part, 31 ... Quadrature detection unit, 32 ... Memory unit, 33 ... Amplitude calculation unit, 34 ... Elasticity information calculation unit, 35 ... Tomographic image generation unit, 36 ... Display image generation unit, 50 ... Display unit, 40 ... Input unit, 341 ... Intensity correction unit, 342 ... Thinning unit, 343 ... Correlation calculation unit, 345 ... Distortion calculation unit, 345a ... Phase difference Calculation unit, 345b ... distortion calculation unit, 346 ... interpolation unit, 347 ... post-processing unit, 349 ... band limiting unit.

Claims (12)

An ultrasonic probe that contacts an inspection object and transmits / receives ultrasonic waves, a transmission / reception unit that transmits an ultrasonic signal to the ultrasonic probe and receives a reception signal from the ultrasonic probe, and the reception signal A signal processing unit for processing
The signal processing unit includes an elasticity information calculation unit that calculates a displacement and / or strain of the internal tissue to be examined during the plurality of transmissions using a plurality of reception signals corresponding to the plurality of transmissions. ,
The elasticity information calculation unit includes: a correlation calculation unit that calculates the correlation of envelopes of the plurality of reception signals; an intensity correction unit that corrects the intensity of envelopes for the plurality of reception signals; and the correlation calculation unit A phase difference calculation unit that calculates a phase difference between the plurality of reception signals using a calculation result and the plurality of reception signals corrected by the intensity correction unit, and the phase difference calculated by the phase difference calculation unit An ultrasonic imaging apparatus, wherein the displacement and / or distortion is calculated by using a computer. The ultrasonic imaging apparatus according to claim 1,
The ultrasonic imaging apparatus, wherein the intensity correction unit uses a reciprocal of the amplitude of the reception signal as a correction value. The ultrasonic imaging apparatus according to claim 1,
The intensity correction unit uses, as a correction value, a value obtained by dividing the maximum value of the amplitude of the received signal by the amplitude of the received signal. The ultrasonic imaging apparatus according to claim 1,
The ultrasonic imaging apparatus further comprising an input unit that receives an input of a correction value used by the intensity correction unit. The ultrasonic imaging apparatus according to claim 1,
The elasticity information calculation unit includes: a thinning unit that thins out data of sample points whose amplitude value exceeds a threshold value among the data of a plurality of sample points constituting the reception signal; and the calculated displacement and / or distortion An ultrasonic imaging apparatus, further comprising: an interpolation unit that interpolates data with missing values. The ultrasonic imaging apparatus according to claim 5, wherein
The ultrasonic imaging apparatus, wherein the thinning unit determines whether or not a numerical value obtained by performing statistical processing on sample point data of the received signal exceeds a threshold value. The ultrasonic imaging apparatus according to claim 6,
The ultrasonic imaging apparatus, wherein the thinning unit determines whether or not a difference (absolute value) between an amplitude value of each sample point of the received signal and an average value of all sample points exceeds a threshold value. The ultrasonic imaging apparatus according to claim 5, wherein
The ultrasonic imaging apparatus further comprising an input unit for setting a threshold used by the thinning unit. The ultrasonic imaging apparatus according to claim 1, wherein:
The ultrasonic imaging apparatus, wherein the signal processing unit further includes a band limiting unit that removes or reduces a component other than a central frequency of the received signal. An ultrasonic signal processing method for processing the ultrasonic reception signals of a plurality of temporally continuous frames acquired from an inspection object and acquiring elasticity information of the inspection object,
Calculating the amplitude of each of the plurality of frames of ultrasonic reception signals;
Obtaining a correlation function of the ultrasonic reception signals of the plurality of frames, calculating a peak shift of an envelope of the ultrasonic reception signals;
Correcting the intensity of the ultrasonic reception signals of the plurality of frames using respective amplitudes;
Calculating a phase difference between the ultrasonic reception signals of the plurality of frames using the peak deviation of the envelope and the ultrasonic reception signals of the plurality of frames after intensity correction;
Using the phase difference, calculating a displacement of the inspection object in a time transition at the time of ultrasonic reception signal acquisition of the plurality of frames;
An ultrasonic signal processing method including: The ultrasonic signal processing method according to claim 10,
In the ultrasonic signal processing method, the intensity correction step includes correcting the intensity by applying a reciprocal of an amplitude value to the ultrasonic reception signals of the plurality of frames. The ultrasonic signal processing method according to claim 10,
Before the step of correcting the intensity, the method includes a step of thinning out data points whose amplitude value exceeds a predetermined threshold value from the ultrasonic reception signal;
An ultrasonic signal processing method comprising the step of interpolating data points whose values are missing before or after the step of calculating the displacement.

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