JP2016049253A - Ultrasonic diagnostic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure a value related to the speed of a shear wave in a biological tissue with high accuracy.SOLUTION: An ultrasonic diagnostic device irradiates each of a plurality of excitation points Si defined in a subject 14 with a pressurization ultrasonic wave individually and excites a shear wave from each excitation point Si. As well as irradiation of the ultrasonic wave to each excitation point Si, the ultrasonic diagnostic device sequentially acquires tomographic frame data expressing a tomographic image with the lapse of time with transmission and reception of the ultrasonic wave for tomographic image measurement. The ultrasonic diagnostic device also measures displacement based on the shear wave on the basis of the plurality of sequentially-acquired pieces of tomographic frame data for each of the plurality of measurement points Mj defined in the subject 14. The ultrasonic diagnostic device obtains response characteristics expressing the relation between the shear wave excited at each excitation point Si and the displacement measured at each measurement point Mj, and obtains the distribution of elastic moduli in the biological tissue of the subject 14 by using the response characteristics.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an ultrasonic diagnostic apparatus, and more particularly to an apparatus for measuring a value related to a shear wave velocity in a living tissue.

  2. Description of the Related Art Ultrasonic diagnostic apparatuses that measure the elastic modulus of living tissue are widely used. The elastic modulus is defined as a value obtained by dividing the stress applied to an elastically deformed object by the distortion of the object, and represents the difficulty of deformation of the object. In general, a tissue having a disease such as cancer, arteriosclerosis, or cirrhosis has an elastic modulus different from that of other tissues. Therefore, it is often possible to find a disease by measuring the elastic modulus.

  As described in Patent Documents 1 and 2, in an ultrasonic diagnostic apparatus that measures an elastic modulus, a shear wave (shear wave) is generated in a living tissue by a vibrating body, and at the tomographic plane by transmitting and receiving ultrasonic waves. Some obtain ultrasonic data. In this ultrasonic diagnostic apparatus, the shear wave velocity distribution on the tomographic plane is obtained based on the ultrasonic data, and the elastic modulus distribution on the tomographic plane is obtained.

  In addition, there is an ultrasonic diagnostic apparatus that generates shear waves in a living tissue by vibration by a vibrating body or a user's hand and measures displacement at a plurality of measurement points in the living tissue. In this ultrasonic diagnostic apparatus, displacement is measured at a plurality of measurement points based on a plurality of tomographic images observed in time series. Then, using the displacement measurement value at each measurement point, the shear wave is focused on each measurement point in the calculation, and the shear wave velocity is determined based on the spatial distribution (amplitude distribution) of the amplitude of the focused shear wave. Desired. Specifically, the wavelength of the shear wave is determined based on the peak characteristics of the amplitude distribution of the shear wave, for example, the half width thereof, and the velocity of the shear wave is determined at each measurement point based on the wavelength and frequency of the shear wave. It is done. Furthermore, the elastic modulus at each measurement point is obtained from the speed obtained at each measurement point.

  In Non-Patent Document 1, noise generated in a medium is measured at a plurality of measurement points, and noise is measured at each measurement point, and the noise is focused on each measurement point after calculation. The technique to analyze is described. However, the technique described in Non-Patent Document 1 is a technique related to earthquake analysis and not a technique related to diagnosis of living tissue.

International Publication No. 2011-004661 Specification International Publication No. 2011-001776 Specification International Publication No. 2013-073304 Specification

Thomas Gallot, Stefan Catheline, Philippe Roux, and Michel Campillo, “A passive inverse filter for Green ’s function retrieval“ JASA Express Letters, Acoustical Society of America, J. Acoust. Soc. Am. 131, EL21 (2012).

  In an ultrasonic diagnostic apparatus that generates a shear wave in a living tissue by vibration by a vibrating body or a user's hand and measures displacement at a plurality of measurement points in the living tissue, the shear wave is generated at an unspecified point in the living tissue. Will occur. Moreover, the dispersion | variation in the intensity | strength of the shear wave generated at each point is large. Therefore, sufficient accuracy may not be obtained for the measurement result of the shear wave velocity.

  An object of this invention is to measure the value regarding the speed of the shear wave in a biological tissue with high precision.

  The present invention relates to an excitation process for irradiating an ultrasonic wave to an excitation point defined in a biological tissue and a displacement based on a shear wave excited at the excitation point by transmitting and receiving ultrasonic waves in the biological tissue. A measurement unit that performs measurement at a plurality of measurement points; and the excitation process is performed individually for each of the plurality of excitation points, and the measurement process is performed for each of the measurement points. A response characteristic calculation unit that obtains a response characteristic at each measurement point with respect to each excitation point, and a speed calculation unit that obtains a value relating to the shear wave velocity in the living tissue based on the response characteristic. It is characterized by.

  In the present invention, ultrasonic waves are individually applied to each of a plurality of excitation points determined on the living tissue of the subject, and a shear wave is excited from each excitation point. The number of excitation points, the position of each excitation point, etc. are determined so that sufficient measurement accuracy can be obtained, for example. Therefore, compared to conventional devices where it is difficult to set the point at which shear waves are actually generated, such as when shear waves are generated at every point of biological tissue by vibration by a vibrating body or user's hand, Measurement accuracy increases.

  Desirably, the said vibration process includes the process which irradiates the ultrasonic wave of a pulse waveform to the said vibration point, and the said response characteristic is a time response characteristic about a displacement.

  Thereby, the time waveform of the ultrasonic wave irradiated to the excitation point can be approximated to a delta function time waveform, and the response characteristic can be approximated to an impulse response.

  Desirably, an inverse characteristic for the response characteristic, wherein an inverse characteristic for obtaining a state of a shear wave at each excitation point with respect to a displacement at each measurement point is obtained based on the response characteristic. A calculation unit, and the speed calculation unit includes a focus calculation unit that virtually focuses the shear wave on the measurement point based on the response characteristic and the inverse characteristic, and is based on the virtually focused shear wave. To determine the value for shear wave velocity.

  According to the present invention, a process of focusing a shear wave on a measurement point (a process of collecting and converging a shear wave on a measurement point) can be virtually realized by calculation. The processing after the vibration processing and the measurement processing are once executed as the actual measurement processing on the subject is executed by calculation, and the measurement of the shear wave velocity is simplified.

  Desirably, the inverse characteristic has a characteristic of reducing or eliminating the contribution of noise in the living tissue, and is given the information about the extreme focus state at the measurement point, so that the extreme focus state The state of the shear wave at each excitation point for realizing

  Since the process of obtaining the shear wave speed based on the shear wave virtually focused on the measurement point is performed based on the extreme focus state, the measurement accuracy is increased.

  According to the present invention, it is possible to measure a value related to the velocity of a shear wave in a living tissue with high accuracy.

It is a figure which shows the structure of an ultrasonic diagnosing device. It is a figure which shows a probe and a test object typically. It is a figure which shows notionally several tomographic frame data produced | generated by the frame data production | generation part. It is a figure which shows the example of the amplitude distribution on the x-axis which makes the origin the measurement point Mq which focused the shear wave.

  FIG. 1 shows the configuration of an ultrasonic diagnostic apparatus according to an embodiment of the present invention. The ultrasonic diagnostic apparatus includes a probe 10, a measurement unit 20, a response characteristic calculation unit 34, an inverse characteristic calculation unit 38, a velocity calculation unit 36, an elastic image generation unit 44, an image synthesis unit 46, a tomographic image generation unit 48, and a display unit. 50. The measurement unit 20, the response characteristic calculation unit 34, the inverse characteristic calculation unit 38, the velocity calculation unit 36, the elastic image generation unit 44, the image synthesis unit 46, and the tomographic image generation unit 48 are configured by, for example, a processor. In this case, the processor constructs each component by reading the program for the ultrasonic diagnostic apparatus. The detailed configuration and operation of each component will be described later.

  An outline of processing executed by the ultrasonic diagnostic apparatus will be described. The ultrasonic diagnostic apparatus individually irradiates each of a plurality of excitation points defined in the subject 14 with ultrasonic waves for pressurization (ultrasonic waves for generating shear waves), and generates shear waves from the respective excitation points. Excited. The ultrasonic diagnostic apparatus sequentially acquires tomographic frame data representing a tomographic image as time elapses by transmitting and receiving ultrasonic waves for measuring a tomographic image along with irradiation of ultrasonic waves to each excitation point. Furthermore, the ultrasonic diagnostic apparatus measures displacement based on shear waves based on a plurality of tomographic frame data acquired in time series for each of a plurality of measurement points defined in the subject 14. The ultrasonic diagnostic apparatus obtains a response characteristic representing the relationship between the shear wave excited at each excitation point and the displacement measured at each measurement point, and uses this response characteristic to determine the elasticity of the subject 14 in the living tissue. Find the distribution of rates. The ultrasonic diagnostic apparatus displays a tomographic image on a tomographic plane from which the elastic modulus distribution is obtained and an elastic image representing the elastic modulus distribution in an overlapping manner. Alternatively, the ultrasonic diagnostic apparatus displays the tomographic image and the elastic modulus distribution side by side.

  FIG. 2 schematically shows the probe 10 and the subject 14. The subject 14 has a plurality of excitation points Si. Each excitation point Si is individually irradiated with pressurizing ultrasonic waves, and shear waves are excited from each excitation point Si. FIG. 2 shows an example in which 16 excitation points Si (i = 1 to 16) are determined so as to surround the region of interest 52 whose elastic characteristics are to be measured.

  The number of excitation points Si is determined according to measurement conditions such as the time required for measurement, the size of the region of interest 52, the shape of the region of interest 52, and the like. Further, the positions of the plurality of excitation points Si are not limited to those surrounding the region of interest 52. That is, the excitation point Si may be located in the region of interest 52, or a part of the plurality of excitation points Si may surround the region of interest 52 and the remaining part may be located in the region of interest 52. Good. Further, all the excitation points Si may be located outside the region of interest 52.

  The probe 10 includes a plurality of vibration elements 12. The plurality of vibration elements 12 are arranged in the x-axis direction along a surface that is in contact with the subject 14. When the pressurizing ultrasonic wave is transmitted, the timing and level of the transmission signal applied to each vibration element 12 are adjusted. As a result, the pressurized beam 16 with the excitation point Si for exciting the shear wave as a focal point is formed. By applying the focus of the pressurizing beam 16 to the respective excitation points Si in order, the ultrasonic waves for pressurization are sequentially applied to the respective excitation points Si.

  For the region of interest 52, tomographic image data representing a tomographic image is sequentially acquired as time passes by transmitting and receiving ultrasonic waves for measuring a tomographic image at the probe 10. In the region of interest 52, Nr measurement points Mj (j = 1 to Nr) are defined. For each measurement point Mj, the displacement of the living tissue based on the shear wave is obtained based on a plurality of tomographic frame data acquired in time series.

  The number of measurement points Mj is determined according to measurement conditions such as the time required for measurement, the size and shape of the region of interest 52, and the like. In FIG. 2, the interval between adjacent measurement points is drawn large for convenience of explanation. For example, the plurality of measurement points Mj are uniformly arranged in the region of interest 52. The plurality of measurement points Mj are not necessarily arranged in a grid pattern in the region of interest 52.

  In the ultrasonic diagnostic apparatus, response characteristics representing the relationship between the shear wave excited from each excitation point Si and the displacement measured at each measurement point Mj are obtained. Then, using this response characteristic, the amplitude of the shear wave at each measurement point Mj (j = 1 to Nr) when the shear wave is virtually focused on one measurement point Mq is calculated. As will be described later, the velocity of the shear wave at the measurement point Mq is obtained based on the peak characteristic of the amplitude distribution at and near the measurement point Mq where the shear wave is focused. Furthermore, based on the shear wave velocity at each measurement point Mj and the density of the living tissue, the elastic modulus distribution in the living tissue is obtained.

  The detailed configuration and operation of the ultrasonic diagnostic apparatus will be described with reference back to FIG. The measurement unit 20 includes a transmission unit 22, a reception unit 26, a beam control unit 24, a phasing addition unit 28, a frame data generation unit 30, and a displacement calculation unit 32. The measurement unit 20 executes an excitation process in which an ultrasonic wave is transmitted from the probe 10 to an excitation point determined in the living tissue of the subject 14. Further, the measurement unit 20 performs a measurement process for obtaining a displacement based on a shear wave excited at the excitation point for each of a plurality of measurement points in the living tissue of the subject 14. The displacement is measured by transmitting and receiving ultrasonic waves in the probe 10.

  The vibration process will be described. According to the control by the beam control unit 24, the transmission unit 22 forms a pressurizing beam 16 having a focal point on the excitation point on the probe 10, and pressurizing ultrasonic waves having a pulse waveform approximate to an impulse waveform (delta function waveform). The timing and level of the transmission signal to be output to each vibration element 12 are adjusted so as to be transmitted. Each vibration element 12 generates an ultrasonic wave according to the transmission signal output from the transmission unit 22. Thereby, the focus of the pressurizing beam 16 is adjusted to one excitation point, and the ultrasonic wave for pressurization is irradiated to the excitation point. A shear wave is excited in the living tissue from the excitation point irradiated with the ultrasonic waves for pressurization. As will be described later, when the pressurizing beam 16 is focused on each of the plurality of excitation points in order, the ultrasonic waves for pressurization are sequentially applied to the respective excitation points, and shear waves are sequentially generated from the respective excitation points. Excited.

  Next, the measurement process will be described. In the following description, following the notation of FIG. 2, the symbol “Si” (i = 1 to Ns) is used for the Ns number of excitation points determined for the subject 14 as necessary, and the subject 14 is determined. The sign of “Mj” (j = 1 to Nr) is used as necessary for the Nr measurement points obtained.

  The measurement process is executed every time the ultrasonic wave for pressurization is applied to one excitation point Si. The z-axis direction displacement is obtained for each of the plurality of measurement points Mj by one measurement process. After irradiating one excitation point with pressurizing ultrasonic waves, the transmission unit 22 transmits to each vibration element 12 so that the plane wave 18 is transmitted from the probe 10 to the subject 14 according to control by the beam control unit 24. Output a signal. Each vibration element 12 generates an ultrasonic wave according to the transmission signal output from the transmission unit 22. For example, a plane wave 18 having a wavefront parallel to the contact surface of the probe 10 is generated by causing each vibration element 12 to generate ultrasonic waves having substantially the same intensity almost simultaneously. In addition, when the plurality of vibration elements 12 are not arranged in a straight line, the intensity of ultrasonic waves generated in each vibration element 12 and the generation of ultrasonic waves in each vibration element according to the position of each vibration element 12. The timing may be adjusted.

  The ultrasonic wave transmitted from the probe 10 is reflected in the subject 14 and received by each vibration element 12. Each vibration element 12 converts the received ultrasonic wave into a reception signal that is an electrical signal and outputs the reception signal to the reception unit 26. For example, the reception unit 26 is configured to acquire a reception signal output from each vibration element 12 according to control by the beam control unit 24 and perform processing such as amplification and quadrature detection. Thereby, the receiving unit 26 generates reception baseband data of a plurality of channels corresponding to the plurality of vibration elements 12 and temporarily stores each reception baseband data for phasing addition.

  The phasing / adding unit 28 phasing / adding the reception baseband data of the plurality of channels stored in the receiving unit 26 to generate a plurality of z-axis direction reception beam data. The plurality of z-axis direction reception beam data correspond to a plurality of reception beams that are directed in the depth direction (z-axis direction) of the subject 14 and arranged in the x-axis direction. The phasing addition unit 28 outputs each z-axis direction reception beam data to the frame data generation unit 30 and the tomographic image generation unit 48. Note that such phasing addition is described in, for example, Patent Document 3 described above.

  The frame data generation unit 30 generates one tomographic frame data corresponding to one transmission / reception of an ultrasonic wave based on each z-axis direction reception beam data output from the phasing addition unit 28. The beam control unit 24, the transmission unit 22, the probe 10, and the reception unit 26 repeatedly transmit and receive ultrasonic waves to and from the subject 14. The phasing addition unit 28 and the frame data generation unit 30 generate a plurality of tomographic frame data with the passage of time in response to a plurality of times of ultrasonic transmission / reception repeatedly performed with the passage of time. The frame data generation unit 30 outputs each tomographic frame data to the displacement calculation unit 32.

FIG. 3 conceptually shows a plurality of tomographic frame data generated by the frame data generation unit 30. In this figure, an ultrasonic wave for pressurization is irradiated to one excitation point at the excitation time Tpsh, and there are n pieces of time intervals between time t ₀ and time t ₀ + (n−1) · δ. An example in which the tomographic frame data F0 to Fn-1 is generated is shown. Each tomographic frame data includes a plurality of z-axis direction reception beam data 54 corresponding to a plurality of reception beams arranged in the x-axis direction.

  Each tomographic frame data is associated with xz coordinates. The data value BD (x, t ′) of the z-axis direction received beam data is associated with the point (x, z) on the tomographic plane from which the tomographic frame data is acquired. One z-axis direction received beam data 54 is associated with the x coordinate of the corresponding received beam, and the time parameter t ′ of one z-axis direction received beam data 54 is associated with the z coordinate. Here, if the speed of the ultrasonic wave that reciprocates through the biological tissue in the z-axis direction is v, there is a relationship of t ′ = 2z / v. Therefore, the point (x, z) corresponds to the data value BD (x, t ′ = 2z / v).

The displacement calculation unit 32 in FIG. 1 obtains the z-axis direction displacement at each measurement point Mj defined on the xz tomographic plane of the subject 14 based on the plurality of tomographic frame data output from the frame data generation unit 30. That is, the displacement calculation unit 32 obtains the movement amount in the z-axis direction as the displacement in the z-axis direction for the biological tissue particles corresponding to each measurement point Mj. The z-axis direction displacement at one measurement point Mj is the z-axis direction displacement dji (t) with respect to the straight line 56 passing through the measurement point Mj and perpendicular to the xz plane. However, the z-axis direction displacement dji (t) is a discrete value at times t = t ₀ , t ₀ + δ, t ₀ + 2δ,... T ₀ + (n−1) · δ. In FIG. 3, discrete z-axis direction displacements dji (t) are represented by black dots.

  The process for obtaining the displacement in the z-axis direction is, for example, based on the two tomographic frame data acquired successively before and after the time, and the amount of movement of the biological tissue particles in the z-axis direction per one frame interval is expressed in the z-axis direction. Obtained as the direction velocity, and the integrated amount of the z-axis direction velocity is obtained as the z-axis direction displacement.

  As described above, the time waveform of the pressurizing ultrasonic wave approximates the impulse waveform. Accordingly, the z-axis direction displacement dji (t) at the measurement point Mj approximates the impulse response hji (t). Here, the impulse response hji (t) refers to a time response waveform of the displacement in the z-axis direction measured at the measurement point Mj when the excitation ultrasonic wave having an ideal impulse waveform is irradiated to the excitation point Si. The displacement calculation unit 32 regards the z-axis direction displacement dji (t) at the measurement point Mj as the impulse response hji (t) and outputs it to the response characteristic calculation unit 34.

  The measurement unit 20 performs one measurement process for each pressurizing process, and obtains an impulse response for each of the Nr measurement points Mj (j = 1 to Nr) for each excitation point. Then, Nr × Ns impulse responses hji (t) (i = 1 to Ns, j = 1 to Nr) are output to the response characteristic calculator 34. The impulse response hji (t) has significance as a response characteristic that represents the relationship between the shear wave excited at each excitation point and the displacement measured at each measurement point in the time domain.

  Here, the example in which the plane wave 18 having the wavefront parallel to the contact surface of the probe 10 is generated has been described. In addition to such a configuration, the beam control unit 24 controls the transmission unit 22 and the reception unit 26 to form one or a plurality of ultrasonic beams in the probe 10, and scans this ultrasonic beam on the xz tomographic plane. May be. In this case, a transmission signal whose delay time is adjusted so that an ultrasonic beam is formed is input to each vibration element 12. Each time the ultrasonic beam scans the xz tomographic plane once, one tomographic frame data is generated by the frame data generation unit 30, and one tomographic image data for one image is generated by the tomographic image generation unit 48 described later. The

  The response characteristic calculation unit 34 performs a Fourier transform process on each of the Nr × Ns impulse responses hji (t) to obtain Nr × Ns frequency responses Hji. Then, a response matrix H of Nr rows and Ns columns using each frequency response Hji as a matrix component is generated, and the response matrix H is output to the speed calculator 36 and the inverse characteristic calculator 38. The response matrix H has significance as a response characteristic that represents the relationship between the shear wave excited at each excitation point and the displacement measured at each measurement point in the frequency domain.

  The inverse characteristic calculation unit 38 decomposes the response matrix H as shown in the following (Equation 1) by singular value decomposition.

Here, U is a matrix of Nr rows and Nr columns. Σ is a matrix of Nr rows and Ns columns. V is a matrix of Ns rows and Ns columns. The superscript * indicates a conjugate transpose matrix. UU ^* , U ^* U, VV ^* , and V ^* V are all unit matrices. Σ is expressed by the following (Equation 2).

Here, σ _{1 to} σ _r have a relationship of σ ₁ ≧ σ ₂ ≧... ≧ σ _Nr ≧ 0. The diagonal component of Σ indicates the degree to which the shear wave excited at each excitation point contributes to the displacement measured at each measurement point.

Note that an inverse matrix H ⁻¹ of the response matrix H is expressed by (Equation 3) to (Equation 5).

That is, σ _n ⁺ is 1 / σ _n when σ _n is not 0, and 0 when σ _n is 0.

The inverse characteristic calculation unit 38 does not obtain the inverse matrix H ⁻¹ of the response matrix expressed by (Equation 3) to (Equation 5), but noise with noise components reduced or removed from the inverse matrix H ^−1. The filter pseudo inverse matrix H ⁻¹ is obtained. That is, the inverse characteristic calculation unit 38 obtains the noise filter pseudo inverse matrix H ¹ to ⁻¹ according to (Equation 6) to (Equation 8).

As shown in equation (8), sigma ^~ _n is, sigma _n is the 1 / sigma _n when the threshold sigma _th greater, are 0 when sigma _n is equal to or less than the threshold sigma _th. Sigma ^~ is the shear wave excited by the excitation point is, those below the threshold sigma _th among the components that contributed to the displacement to be measured at each measuring point Mj (j = 1~Nr), pseudo-inverse matrix Is a matrix that is forcibly invalidated when obtaining. That, sigma ^~ is a matrix to reduce or eliminate the noise component included in the noise filter pseudo-inverse matrix H ^~-1. The inverse characteristic calculation unit 38 outputs the noise filter pseudo inverse matrix H ^{to −1} thus obtained to the speed calculation unit 36. The threshold σ _th may be defined as a times the maximum value σ ₁ of the diagonal component (a is a positive value less than 1). Further, the inverse characteristic calculation unit 38 validates only a predetermined number L (σ _{1 to} σ _L ) from the larger one of the diagonal components σ _{1 to} σ _Nr of Σ and replaces them with the reciprocal, and the remaining σ the _{L + 1 ~σ Nr} may execute processing for obtaining the ^~ sigma replaced with 0.

The threshold σ _th is determined, for example, according to the measurement accuracy of the shear wave velocity obtained as a measurement result. This measurement accuracy is, for example, the degree of change in the measurement result when the same measurement is performed a plurality of times while changing the threshold σ _th , the measurement result obtained by this embodiment, and the measurement obtained by another measurement method. It can be evaluated by comparison with the result.

The speed calculation unit 36 includes a focus calculation unit 40 and a speed distribution calculation unit 42. The focus calculation unit 40 uses the response matrix H output from the response characteristic calculation unit 34 and the noise filter pseudo inverse matrix H ¹ to ^-1 output from the inverse characteristic calculation unit 38 to obtain the following (Equation 9) and ( Based on equation (10), a focus vector Ψq representing a displacement in the z-axis direction at each measurement point Mj (j = 1 to Nr) when the shear wave is focused on the measurement point Mq is calculated.

  Here, Ψq and Rq are Nr rows and 1 column matrix (vector). As shown in (Expression 10), the value of the qth row of Rq is 1, and the values of the other rows are all 0. Rq is a matrix representing a state in which the shear wave is focused in the narrowest range at the measurement point Mq, that is, an extreme focus state. ψj (ω) (j = 1 to Nr) indicates a shear wave component of the angular frequency ω at each measurement point Mj. In order to explain the physical significance of (Equation 9), the following (Equation 11) will be explained.

G ^to q represented by (Expression 11) are expressions in which the response matrix H on the right side of (Expression 9) is not multiplied. G ^to q are matrices of Ns rows and 1 column. G ^to q indicate ideal shear waves to be excited from the respective excitation points Si (i = 1 to Ns) in order to focus the shear waves in the narrowest range in the vicinity of the measurement point Mq. That is, (Equation 11) uses the noise filter pseudo inverse matrix H ^{to −1 in} which the noise component is reduced or removed, so that the excitation sources G ^to q that realize an extreme focus state at the measurement point Mq are inversely calculated. Means that it is required.

As described above, the noise filter pseudo inverse matrix H ^{to −1} represents the inverse characteristic (inverse characteristic) of the response characteristic represented by the response matrix H, and the shear wave at each excitation point with respect to the displacement at each measurement point. Represents a state. Furthermore, the noise filter pseudo inverse matrix H ^{1 to −1} has a characteristic of reducing or removing the contribution of noise in the living tissue, and is given a vector Rq as information about the extreme focus state at the measurement point, The state of the shear wave at each excitation point for realizing the extreme focus state is represented.

(Equation 9) is an equation for obtaining the frequency response to the ideal shear waves G ^to q represented by (Equation 11) for each measurement point Mj (j = 1 to Nr) using the response matrix H. I can say that. That is, the focus vector Ψq is observed at each measurement point Mj (j = 1 to Nr) when the ideal shear wave G ^to q is focused on the measurement point Mq based on the actually measured response matrix H. Z-axis direction amplitude of the shear wave.

  The focus calculation unit 40 obtains a total of Nr focus vectors Ψq for each of q = 1 to q = Nr, and outputs them to the velocity distribution calculation unit 42.

  The velocity distribution calculation unit 42 obtains the velocity of the shear wave corresponding to each measurement point based on the Nr focus vectors Ψq. In order to explain the principle of the processing executed by the velocity distribution calculation unit 42, the characteristics of the focus vector Ψq will be explained here.

  FIG. 4A shows an example of an amplitude distribution on the x-axis with the measurement point Mq focused on the shear wave as the origin. This amplitude distribution associates the amplitude of each component of the focus vector Ψq with the x-axis. However, the measurement points Mq-3 to Mq + 3 are arranged in the x-axis direction. The amplitude at the measurement point Mq that is the focus point is the largest, and the amplitude decreases in the order of the measurement points Mq, Mq + 1, Mq + 2, and Mq + 3 as the distance from the measurement point Mq increases. Each amplitude is indicated by an absolute value of ψq + 1, ψq + 2, and ψq + 3, respectively. Similarly, as the distance from the measurement point Mq increases, the amplitude decreases in the order of the measurement points Mq, Mq-1, Mq-2, and Mq-3. Each amplitude is indicated by an absolute value of ψq-1, ψq-2, and ψq-3, respectively.

  FIG. 4B shows the amplitude distribution on the x-axis over a wider range than in FIG. When the amplitude at the measurement point Mq where the amplitude is maximum is A, the distance between the two points on the x-axis where the amplitude is half A / 2 is referred to as a half-value width D. When the shear wave is sufficiently focused, the peak characteristic at the focus point is wavelength-dependent, and for example, it is known that the half-value width D is half the wavelength λ of the shear wave. Therefore, for example, by obtaining the half width D from the peak characteristic of the amplitude distribution, the wavelength of the shear wave is obtained as λ = 2D. When the angular frequency of the shear wave is ω, the shear wave velocity C at the focus point is obtained as C = f · λ = ω · λ / (2π) = ω · D / π. Here, f is the frequency of the shear wave, and f = ω / (2π). As for the relationship between the peak characteristic and the wavelength of the shear wave, various relationships can be considered in addition to the relationship between the half width and the wavelength. Accordingly, the shear wave wavelength may be determined from the peak characteristics using other relationships known to those skilled in the art.

  Here, the principle of obtaining the shear wave velocity at the focus point with one measurement point as the focus point has been described, but the shear wave velocity is obtained in the same manner for the other measurement points.

  Here, an example has been described in which the measurement points are arranged in the x-axis direction and the shear wave velocity at the focus point is obtained based on the amplitude distribution on the x-axis. In the general case where multiple measurement points are arranged on a straight line extending in a direction different from the x-axis direction, focus is based on the amplitude distribution on the straight line (amplitude distribution on the measurement point arrangement axis). The speed of the shear wave at the point is determined.

  Returning to FIG. 1, the velocity distribution calculation unit 42 focuses each measurement point Mj (j = 1 to Nr) based on the Nr focus vectors Ψq (q = 1 to Nr) output from the focus calculation unit 40. The amplitude distribution on the measurement point array axis as a point is obtained. Then, the shear wave velocity at each measurement point is obtained based on each amplitude distribution. The velocity distribution calculation unit 42 associates the velocity of the shear wave with each measurement point, generates velocity distribution data on the xz plane, and outputs the velocity distribution data to the elastic image generation unit 44.

  The velocity distribution calculation unit 42 may obtain velocity distribution data for one predetermined angular frequency and output the velocity distribution data to the elastic image generation unit 44, or may obtain velocity distribution data for each of a plurality of different angular frequencies. Each velocity distribution data obtained for the angular frequency may be output to the elastic image generation unit 44.

  The elastic image generation unit 44 converts the shear wave velocity at each measurement point into an elastic modulus E, and obtains elastic modulus distribution data from the velocity distribution data. This processing is performed by obtaining the elastic modulus E based on the following (Equation 12) using the shear wave velocity C.

Here, ρ is the density of living tissue, and in the case of soft tissue, it is approximately 1000 kg / m ³ . The elastic image generation unit 44 generates elastic image data based on the elastic modulus distribution data and outputs it to the image composition unit 46. When the elastic modulus distribution data is obtained for each of a plurality of different angular frequencies, for example, the elastic image generation unit 44 generates elastic image data for the angular frequency selected by the user's operation, and the image composition unit 46. Output to.

  The elastic image indicated by the elastic image data represents, for example, a region having a large elastic modulus in blue, a region having a small elastic modulus in red, and a region in which the elastic modulus is an intermediate value thereof in green or yellow. The elastic modulus may be expressed by a paint pattern, a numerical value, a three-dimensional graph, or the like in addition to the color.

  Next, processing in which the tomographic image generation unit 48 generates tomographic image data on the xz tomographic plane based on each z-axis direction received beam data output from the phasing addition unit 28 will be described. The tomographic image generation unit 48 generates tomographic image data on the xz tomographic plane based on each z-axis direction reception beam data corresponding to a plurality of reception beams arranged in the x-axis direction, and outputs the tomographic image data to the image composition unit 46. The tomographic image generation unit 48 generates tomographic image data for one image corresponding to one transmission / reception of ultrasonic waves. The tomographic image generation unit 48 generates tomographic frame data for a plurality of images corresponding to a plurality of times of ultrasonic transmission / reception repeatedly performed with time, and outputs the tomographic frame data to the image synthesis unit 46.

  Based on the tomographic image data output from the tomographic image generation unit 48 and the elastic image data output from the elastic image generation unit 44, the image composition unit 46 shows, for example, an image obtained by superimposing the elastic image on the tomographic image. Tomographic / elastic image data is generated and output to the display unit 50. The display unit 50 displays an image based on the tomographic / elastic image data. Thus, the user can grasp the elastic modulus distribution together with the tomographic image, and can diagnose cancer, arteriosclerosis, liver fibrosis, cirrhosis, and the like. Note that the tomographic / elastic image data may be image data in which the tomographic image and the elastic image are individually shown side by side. Further, the image composition unit 46 may display the tomographic image and the elasticity image on the display unit 50 singly.

  In the ultrasonic diagnostic apparatus according to the present invention, the ultrasonic waves for pressurization are individually applied to each of the plurality of excitation points determined on the living tissue of the subject, and shear waves are excited from each excitation point. The ultrasonic waves for pressurization are sequentially applied to the plurality of excitation points. Each time an excitation point is irradiated with ultrasonic waves for pressurization and a shear wave is excited from one excitation point, the displacement at each measurement point in the living tissue is measured. Thereby, response characteristics at each measurement point with respect to each excitation point are obtained. In the ultrasonic diagnostic apparatus, the shear wave velocity and the elastic modulus distribution in the living tissue are obtained based on the response characteristics.

  The number of excitation points, the position of each excitation point, etc. are determined so that sufficient measurement accuracy can be obtained for the velocity or elastic modulus of the shear wave. Therefore, the measurement accuracy is higher than in the case where shear waves are generated at all points of the living tissue by vibration by the vibrating body or the user's hand.

  Further, when a plane wave is used as the ultrasonic wave transmitted from the probe when measuring the displacement at each measurement point, the tomographic frame data and the tomographic image data for one frame corresponding to one transmission / reception of the ultrasonic wave are obtained. can get. As a result, the displacement at each measurement point is quickly measured.

  In the ultrasonic diagnostic apparatus according to the present invention, the excitation source for realizing the extreme focus state at the measurement point is virtually realized by using the noise filter pseudo inverse matrix. Then, the shear wave excited from such an ideal excitation source is virtually focused on the measurement point based on the response matrix based on the actual measurement, and the amplitude distribution around the measurement point that becomes the focus point is obtained. Furthermore, the velocity of the shear wave is determined based on the amplitude distribution. Therefore, since the amplitude distribution is obtained using an ideal excitation source, the measurement accuracy of the shear wave wavelength is increased, the measurement accuracy of the shear wave velocity, and further the measurement accuracy of the elastic modulus is increased.

DESCRIPTION OF SYMBOLS 10 Probe, 12 Vibration element, 14 Subject, 16 Pressurization beam, 18 Plane wave, 20 Measurement part, 22 Transmission part, 24 Beam control part, 26 Reception part, 28 Phased addition part, 30 Frame data generation part, 32 Displacement Calculation unit, 34 Response characteristic calculation unit, 36 Speed calculation unit, 38 Inverse characteristic calculation unit, 40 Focus calculation unit, 42 Speed distribution calculation unit, 44 Elastic image generation unit, 46 Image composition unit, 48 Tomographic image generation unit, 50 display Part, region of interest, Si excitation point, Mj measurement point.

Claims (4)

Excitation processing that irradiates ultrasonic waves to the excitation point determined in the living tissue,
A measurement unit that performs a measurement process of measuring displacement based on a shear wave excited at the excitation point at a plurality of measurement points in the living tissue by transmitting and receiving ultrasonic waves;
The excitation process is individually executed for each of the plurality of excitation points, and the measurement process is executed for each of the measurement points, thereby obtaining a response characteristic at each of the measurement points with respect to each of the excitation points. A response characteristic calculator,
An ultrasonic diagnostic apparatus comprising: a speed calculation unit that obtains a value related to a shear wave speed in the living tissue based on the response characteristics. The ultrasonic diagnostic apparatus according to claim 1,
The excitation process includes a process of irradiating the excitation point with ultrasonic waves of a pulse waveform,
The ultrasonic diagnostic apparatus, wherein the response characteristic is a time response characteristic with respect to displacement. The ultrasonic diagnostic apparatus according to claim 1 or 2,
An inverse characteristic for the response characteristic, wherein an inverse characteristic for obtaining a state of a shear wave at each excitation point with respect to a displacement at each measurement point is obtained based on the response characteristic. Prepared,
The speed calculator is
Based on the response characteristic and the inverse characteristic, a focus calculation unit that virtually focuses the shear wave on the measurement point, and obtaining a value related to the speed of the shear wave based on the virtually focused shear wave. A characteristic ultrasonic diagnostic apparatus. The ultrasonic diagnostic apparatus according to claim 3.
The inverse characteristic has a characteristic to reduce or eliminate the contribution of noise in the living tissue, and is provided with information on the extreme focus state at the measurement point, thereby realizing the extreme focus state. An ultrasonic diagnostic apparatus characterized by expressing a state of a shear wave at each excitation point for the purpose.

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