Ultrasonic image acquisition device and method

Abstract

An ultrasonic image acquisition device comprises: a library creation unit (55) that acquires, as an index indicating the magnitude of strain of a base material, the time from the transmission time of an ultrasonic wave to the time at which the absolute value of the intensity of a reflected wave becomes maximum, and creates a library in which the index and a reference waveform are associated, the reference waveform being the waveform of the reflected wave at the position where the index was acquired; an acoustic impedance calculation unit (57) that calculates the acoustic impedance in the depth direction of a measurement object on the basis of the waveform of the reflected wave from the measurement object and the reference waveform; and a correction unit (58) that calculates the amount of deviation of the spot position in the axial direction of an ultrasonic beam, and corrects the signal used for calculating the acoustic impedance on the basis of the deviation amount and a Gaussian distribution curve of the intensity of the ultrasonic beam.

Description

Ultrasound image acquisition device and method

[0001] The present invention relates to an ultrasonic imaging apparatus and method for acquiring an acoustic impedance image of a measurement target such as biological tissue using ultrasonic waves.

[0002] Observation of biological tissues has traditionally been performed using optical microscopes and electron microscopes. In recent years, ultrasonic microscopes with resolution equivalent to that of optical microscopes have been put to practical use, making it possible to observe biological tissues at the cellular level. When using ultrasonic microscopes, ultrasonic images such as sound velocity images and acoustic impedance images can be obtained from measured acoustic parameters such as tissue sound velocity and acoustic impedance.

[0003] Development of ultrasonic microscopes as diagnostic devices for biological tissues is progressing, and it is now possible to observe biological tissues with a resolution approaching that of optical microscopes. Optical microscopes use a staining method to distinguish between different chemical properties in biological tissues by changing their optical properties using chemicals. On the other hand, ultrasonic microscopes can distinguish between different physical properties, such as sound speed and acoustic impedance, without the use of chemicals or staining, which minimizes the impact on the subject and allows for non-invasive observation of biological tissues.

[0004] An ultrasonic imaging inspection device has been proposed that displays acoustic impedance images of cells using a pulsed excitation ultrasonic microscope (Non-Patent Document 1). The configuration of the ultrasonic imaging inspection device is shown in Figure 17. In this ultrasonic imaging inspection device, a reference member 101 is provided at the peripheral position of biological tissue 100 within the range scanned by ultrasonic waves, and ultrasonic waves are irradiated onto the biological tissue 100 and the reference member 101 via a resin film 3. In Figure 17, 102 denotes an ultrasonic beam irradiated from a transducer 1. In Figure 17, 4 denotes a scanning stage that moves the transducer 1 to change the position at which the ultrasonic beam is irradiated, and 103 denotes water, which is the propagation medium of the ultrasonic waves.

[0005] The relationship between the ultrasonic wave S0 irradiated to the reference member 101 at an angle perpendicular to its surface and the reflected wave Sr from the reference member 101 is expressed by the following equation (1): Sr=(Zs-Zr) / (Zs+Zr)S0 (1)

[0006] Zs is the acoustic impedance of the resin film 3, and Zr is the acoustic impedance of the reference member 101. The relationship between the ultrasonic wave S0 irradiated to the biological tissue 100 at an angle perpendicular to its surface and the reflected wave St from the biological tissue 100 satisfies the following equation (2): St = (Zs - Zt) / (Zs + Zt) S0 (2)

[0007] Zt is the acoustic impedance of the biological tissue 100. Therefore, from equations (1) and (2), the acoustic impedance Zt of the biological tissue 100 can be calculated by equation (3).

[0008]

[0009] In the ultrasonic imaging inspection device shown in Figure 17, a two-dimensional acoustic impedance image can be obtained by measuring the acoustic impedance Zt while moving the transducer 1 using the scanning stage 4. The acoustic impedance Zt is a parameter related to the stiffness of the biological tissue 100. The properties of the biological tissue 100 can be observed using the acoustic impedance image. Furthermore, a method has been proposed for estimating the acoustic impedance distribution in the depth direction using the acoustic impedance of the resin film 3 as a reference (Non-Patent Document 2).

[0010] However, since the resin film 3 has a thickness of only a few tens of micrometers, it is difficult to create a flat surface like an optical component. Therefore, even if the observation area of ​​the biological tissue 100 is a few tens of micrometers square, distortion and tilt exist in the resin film 3. In an ultrasound imaging inspection device, the ultrasonic beam is assumed to be a plane wave that propagates uniformly in the irradiation axis direction. However, in reality, the ultrasonic beam is focused by the acoustic lens 2 to irradiate a minute area, and the ultrasonic beam has a shape with a Gaussian distribution. The profile of a Gaussian beam is shown in Equations (4) to (8).

[0011]

[0012] In equations (4) to (8), λ is the wavelength of the ultrasonic beam, b is the confocal parameter, z Ｒ is the Rayleigh region, Z Ｒ is the Rayleigh radius, z is the axial distance, R(z) is the radius of curvature, ω(z) is the beam radius, θ is the beam divergence radius, ω ０ is the beam waist. The beam width of a Gaussian beam narrows toward the focal position and widens after passing through the focal position. Therefore, if the irradiation position of the ultrasonic beam is shifted due to distortion of the resin film 3, the beam spot will widen and the shape of the reflected wave will also change. If the acoustic impedance is calculated without taking into account such changes in the shape of the reflected wave, accurate acoustic impedance cannot be obtained.

[0013] FIG. 18 is a diagram showing the relationship between the axial distance and the beam radius when the focal position is set to a distance of 0. FIG. 19 is a diagram showing the relationship between the axial distance and the beam intensity when the focal position is set to a distance of 0. Here, the characteristics are shown for a case where a transducer 1 having an ultrasonic center frequency of 320 MHz, a focal distance of 600 μm, and a beam waist of 2.5 μm is used. In FIGS. 18 and 19, the larger the distance value in the negative direction, the closer the transducer is, and the larger the distance value in the positive direction, the farther the transducer is. As shown in FIGS. 18 and 19, the beam radius and beam intensity change nonlinearly with respect to the axial distance near the focal position, making it difficult to correct the shape of the reflected wave.

[0014] Yoshifumi Saijo, et al., “Ultrasound Speed ​​and Impedance Microscopy for in vivo Imaging”, Proceedings of the 29th Annual International Conference of the IEEE EMBS, 2007Edo Bagus Prastika, et al., “Three-dimensional acoustic impedance mapping of human skin by improved time-frequency domain analysis”, Japanese Journal of Applied Physics 60, SDDE22, 2021

[0015] The present invention has been made to solve the above-mentioned problems, and aims to provide an ultrasonic image acquisition device and method that can eliminate the effects of changes in beam diameter and beam intensity due to distortion of the substrate.

[0016] an acoustic lens configured to focus an ultrasonic beam emitted from the transducer onto a measurement target on a substrate and irradiate the measurement target through the substrate; a beam manipulation unit configured to scan the entire region of the measurement target with the ultrasonic beam; a reference region extraction unit configured to create a reflection intensity map in which the intensity of the reflected wave obtained by the scanning is plotted on a two-dimensional plane corresponding to a horizontal plane perpendicular to the axial direction of the ultrasonic beam, and to extract a region in which the measurement target is not present as a reference region based on the reflection intensity map; a library creation unit configured to perform a process of acquiring, for each horizontal position in the reference region, the time from the transmission time of the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximized as an index indicating the magnitude of strain in the substrate, and to create a library in which the waveform of the reflected wave at the position where the index was acquired is used as a reference waveform and the index is associated with the reference waveform; and a library creation unit configured to extract from the library the reference waveform corresponding to the index for positions of calculation targets in a region of the measurement target other than the reference region. a first acoustic impedance calculation unit configured to acquire from the library a reference waveform corresponding to the index for the position of the calculation target, and calculate the acoustic impedance of the measurement target for each horizontal position in the area of ​​the measurement target based on the waveform of the reflected wave at the position of the calculation target and the reference waveform acquired from the library; a second acoustic impedance calculation unit configured to acquire from the library a reference waveform corresponding to the index for the position of the calculation target, calculate a signal by performing a process of deconvolving the reference waveform acquired from the library with the waveform of the reflected wave at the position of the calculation target, and calculate the acoustic impedance in the depth direction of the measurement target at the position of the calculation target based on this signal; and a correction unit configured to calculate an amount of shift of a spot position in the axial direction of the ultrasonic beam based on a shift of a peak position of the reflected wave at the position of the calculation target by the second acoustic impedance calculation unit, generate a window function based on the amount of shift and a Gaussian distribution curve of the intensity of the ultrasonic beam, and correct the intensity of the signal calculated by the second acoustic impedance calculation unit using the window function,The acoustic impedance in the depth direction of the measurement object is calculated based on the corrected signal.

[0017] Further, the ultrasonic image acquisition method of the present invention includes a first step of focusing an ultrasonic beam emitted from a transducer onto a measurement object on a substrate and irradiating the measurement object through the substrate; a second step of scanning the entire area of ​​the measurement object with the ultrasonic beam; a third step of creating a reflection intensity map in which the intensity of the reflected wave obtained by the scanning is plotted on a two-dimensional plane corresponding to a horizontal plane perpendicular to an axial direction of the ultrasonic beam, and extracting an area where the measurement object is not present as a reference area based on the reflection intensity map; a fourth step of performing a process of acquiring, for each horizontal position in the reference area, the time from the transmission time of the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximized as an index indicating the magnitude of strain in the substrate, and creating a library in which the index and the reference waveform are associated with each other by using the waveform of the reflected wave at the position where the index was acquired as a reference waveform; and a fourth step of acquiring, from the library, a reference waveform corresponding to the index for a position of a calculation object in an area of ​​the measurement object other than the reference area, and comparing the waveform of the reflected wave at the position of the calculation object with the live waveform. a fifth step of calculating the acoustic impedance of the object to be measured for each horizontal position in the area of ​​the object to be measured based on a reference waveform acquired from the library; a sixth step of calculating a reference waveform corresponding to the index for the position of the object to be measured in the area of ​​the object to be measured from the library, calculating a signal by performing a process of deconvolving the reference waveform acquired from the library on the waveform of the reflected wave at the position of the object to be measured, and calculating the acoustic impedance in the depth direction of the object to be measured at the position of the object to be measured based on this signal; and a seventh step of calculating an amount of deviation of a spot position in the axial direction of the ultrasonic beam based on a deviation of a peak position of the reflected wave at the position of the object to be measured in the sixth step, generating a window function based on the amount of deviation and a Gaussian distribution curve of the intensity of the ultrasonic beam, and correcting the intensity of the signal calculated in the sixth step using the window function, wherein the sixth step includes a step of calculating the acoustic impedance in the depth direction of the object to be measured based on the corrected signal.

[0018] According to the present invention, by providing a beam manipulation unit, a reference region extraction unit, a library creation unit, a first acoustic impedance calculation unit, a second acoustic impedance calculation unit, and a correction unit, it is possible to eliminate the effects of changes in beam diameter and beam intensity due to distortion of the substrate.

[0019] FIG. 1 is a block diagram showing the configuration of an ultrasound imaging apparatus according to an embodiment of the present invention. FIG. 2 is a flowchart illustrating the operation of the ultrasound imaging apparatus according to an embodiment of the present invention. FIG. 3 is a flowchart illustrating the operation of the ultrasound imaging apparatus according to an embodiment of the present invention. FIG. 4 is a flowchart illustrating the operation of the ultrasound imaging apparatus according to an embodiment of the present invention. FIG. 5 is a diagram showing an example of the waveform of a reflected wave received by a transducer. FIG. 6 is a diagram showing an example of a reflection intensity map according to an embodiment of the present invention. FIG. 7 is a diagram visually illustrating the concept of a library according to an embodiment of the present invention using the waveform of a reflected wave. FIG. 8 is a flowchart illustrating the operation of an acoustic impedance calculation unit and a correction unit according to an embodiment of the present invention. FIG. 9 is a diagram illustrating a method for calculating three-dimensional acoustic impedance according to an embodiment of the present invention. FIG. 10 is a diagram showing the relationship between axial distance and beam intensity. FIG. 11 is a diagram illustrating the relationship between the position of the beam spot and the range of the Gaussian distribution curve on which the window function is based. FIG. 12 is a diagram showing an example of a window function. FIG. 13 is a diagram showing an example of a power spectrum obtained by Fourier transforming a reflected wave. FIG. 14A is a horizontal acoustic impedance image of cardiomyocytes calculated using a conventional method. Fig. 14B is an acoustic impedance image of cardiomyocytes in the depth direction calculated by a conventional method. Fig. 15A is an acoustic impedance image of cardiomyocytes in the horizontal direction calculated by an embodiment of the present invention. Fig. 15B is an acoustic impedance image of cardiomyocytes in the depth direction calculated by an embodiment of the present invention. Fig. 16 is a block diagram showing an example configuration of a computer that realizes an ultrasound image acquisition device according to an embodiment of the present invention. Fig. 17 is a diagram showing the configuration of a conventional ultrasound image acquisition device. Fig. 18 is a diagram showing the relationship between axial distance and beam radius. Fig. 19 is a diagram showing the relationship between axial distance and beam intensity.

[0020]

[0016] An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 is a block diagram showing the configuration of an ultrasound imaging device according to an embodiment of the present invention. The ultrasound imaging device includes a transducer 1 for transmitting and receiving ultrasound, an acoustic lens 2 for focusing the ultrasound beam emitted from the transducer 1 and irradiating it onto biological tissue 100 to be measured, a resin film 3 (substrate) such as polystyrene for supporting the biological tissue 100, a scanning stage 4 capable of moving the transducer 1 in the axial direction of the ultrasound beam and in a horizontal direction perpendicular to the axial direction, and a processing device 5.

[0021] The processing device 5 comprises a transmitting unit 50 , a receiving unit 51 , a control unit 52 , a storage unit 53 , a reference region extracting unit 54 , a library creating unit 55 , acoustic impedance calculating units 56 and 57 , and a correcting unit 58 .

[0022] The scanning stage 4 may be a linear stage using a stepping motor or a piezo stage. The scanning stage 4 and the control unit 52 constitute a beam manipulation unit 6. Although not shown in FIG. 1 , distilled water is used as a propagation medium for ultrasonic waves between the bottom of the resin film 3 and the transducer 1.

[0023] First, the process of fixing the position of the transducer 1 in the axial direction of the ultrasonic beam before acquiring an ultrasonic image will be described with reference to Fig. 2. The transmitter 50 supplies a driving transmission pulse to the transducer 1, causing the transducer 1 to transmit an ultrasonic pulse (step S100 in Fig. 2). The acoustic lens 2 focuses the ultrasonic beam emitted from the transducer 1 and irradiates it onto the biological tissue 100 through the resin film 3, and causes the reflected wave from the biological tissue 100 to be incident on the transducer 1.

[0024] The transducer 1 receives the reflected wave from the biological tissue 100. The receiving unit 51 stores the signal of the reflected wave output from the transducer 1 in the storage unit 53 (step S101 in FIG. 2).

[0025] The control unit 52 controls the scanning stage 4 to move the transducer 1 along the axial direction of the ultrasonic beam irradiated onto the biological tissue 100, and simultaneously move the transducer 1 along a direction perpendicular to the axial direction (step S102 in FIG. 2). The control unit 52 adjusts the position of the transducer 1 so that the absolute value of the intensity of the reflected wave is maximized (step S103 in FIG. 2). Hereinafter, the direction perpendicular to the axial direction of the ultrasonic beam irradiated onto the biological tissue 100 will be referred to as the horizontal direction.

[0026] Next, the process of acquiring an ultrasound image will be described with reference to Figures 3 to 12. As described above, the transmitter 50 supplies a driving transmission pulse to the transducer 1, causing the transducer 1 to transmit an ultrasound pulse (step S200 in Figure 3). The transducer 1 receives a reflected wave from the biological tissue 100 (step S201 in Figure 3). The receiver 51 stores the waveform of the reflected wave output from the transducer 1 in the memory unit 53 (step S202 in Figure 3).

[0027] The control unit 52 controls the scanning stage 4 to move the transducer 1 in the horizontal direction (step S203 in FIG. 3). In this way, the processes of steps S200 to S202 are performed for each horizontal position while moving the transducer 1. The position of the transducer 1 in the axial direction of the ultrasonic beam is fixed at the position adjusted by the processes of steps S102 and S103.

[0028] After scanning the entire area of ​​the biological tissue 100 with the ultrasound beam (YES in step S204 in FIG. 3 ), the reference region extraction unit 54 may interpolate the waveform of the reflected wave (step S205 in FIG. 3 ). If the sampling rate is insufficient (if the acquisition period of the reflected wave is long), it is not possible to acquire the reflected wave with sufficiently fine positional resolution. Therefore, the reference region extraction unit 54 may estimate the waveform of the reflected wave for horizontal positions where the reflected wave has not been measured using a model-based estimation method such as a Kalman filter or an interpolation method such as machine learning. However, the processing of step S205 is not essential to the present invention.

[0029] 5 is a diagram showing an example of the waveform of a reflected wave received by the transducer 1. In FIG. 5, the time when the transducer 1 transmits the ultrasonic wave is set to 0. The reference area extraction unit 54 obtains the maximum absolute value of the intensity of the waveform of the reflected wave stored in the storage unit 53 for each horizontal position (step S206 in FIG. 3). For horizontal positions where the maximum absolute value of the intensity cannot be obtained, the reference area extraction unit 54 may calculate the maximum value using a statistical method (step S207 in FIG. 3). However, the processing of step S207 is not essential to the present invention.

[0030] The reference region extraction unit 54 creates a reflection intensity map by plotting the maximum absolute values ​​of the intensity acquired in steps S206 and S207 on a two-dimensional plane corresponding to a horizontal plane perpendicular to the axial direction of the ultrasound beam irradiated onto the biological tissue 100 (step S208 in Figure 3). An example of a reflection intensity map is shown in Figure 6. The horizontal and vertical axes in Figure 6 indicate horizontal position. In the example of Figure 6, the maximum absolute value of the intensity of the waveform of the reflected wave is expressed by color, with whiter colors indicating higher intensity. Note that the waveform of the reflected wave may be Fourier transformed, and the reflection intensity map may be created by using the power spectrum in the same frequency band as the irradiated ultrasound as the intensity distribution.

[0031] The reference region extraction unit 54 extracts, as a reference region, a region where no biological tissue 100 is present, based on the generated reflection intensity map (step S209 in FIG. 3 . Specifically, the reference region extraction unit 54 extracts, as a reference region, a region on the reflection intensity map where the maximum absolute value of intensity is equal to or greater than a predetermined threshold. In the example in FIG. 6 , the reference region is designated A1, and the region corresponding to the biological tissue 100 is designated A2.

[0032] Next, the library creation unit 55 performs a process of acquiring, for each horizontal position in the reference area, the time from when the transducer 1 transmits the ultrasonic wave to when the absolute value of the intensity of the reflected wave is maximum (tmax in FIG. 5 ), as an index indicating the magnitude of strain in the resin film 3. Then, the library creation unit 55 uses the waveform of the reflected wave at the position where the index is acquired as a reference waveform, and creates a library in which the reference waveform at the position where the index is acquired is associated with the index (step S210 in FIG. 3 ).

[0033] The library creation method will be described with reference to Fig. 4. The library creation unit 55 acquires waveform data of reflected waves stored in the storage unit 53 for the reference region (step S300 in Fig. 4). The library creation unit 55 acquires indices from the acquired waveform data of reflected waves (step S301 in Fig. 4). The library creation unit 55 associates the reference waveform at the position where the indices were acquired with the indices and registers them in the library (step S302 in Fig. 4). The library creation unit 55 performs the processes of steps S300 to S302 for each horizontal position in the reference region until the entire reference region has been processed (YES in step S303 in Fig. 4).

[0034] To visually explain the concept of a library using the waveform of a reflected wave, the library associates an index with a reference waveform as shown in Figure 7. However, since the index values ​​that can be obtained from waveform data are discrete values, it is not possible to create a library for all possible ranges of index values.

[0035] Therefore, the library creation unit 55 performs statistical processing to estimate the reference waveform corresponding to the index value for which the reference waveform cannot be obtained within the predetermined index numerical range, and to remove noise from the reference waveform (step S304 in FIG. 4). The numerical range of the index to be registered in the library can be determined in advance, for example, based on the magnitude of strain that may occur in the resin film 3. This completes the creation of the library. The created library is stored in the storage unit 53.

[0036] The acoustic impedance calculation unit 56 calculates the acoustic impedance Zt of the biological tissue 100 for each horizontal position (step S211 in FIG. 3 ). Specifically, for a calculation target position in a region other than the reference region (region of the biological tissue 100), the acoustic impedance calculation unit 56 determines the maximum absolute value of the intensity of the waveform of the reflected wave as the intensity St of the reflected wave obtained from the interface between the resin film 3 and the biological tissue 100. Furthermore, for the calculation target position, the acoustic impedance calculation unit 56 obtains, as an index, the time from the time the transducer 1 transmits ultrasound to the time when the absolute value of the intensity of the reflected wave reaches a maximum, and obtains a reference waveform corresponding to this index from the library. The acoustic impedance calculation unit 56 determines the maximum absolute value of the intensity of the reference waveform obtained from the library as the intensity Sr of the reflected wave obtained from the interface between the resin film 3 and the reference member 101.

[0037] The acoustic impedance calculation unit 56 can calculate the acoustic impedance Zt of the biological tissue 100 by substituting the intensities St and Sr of the reflected waves into equation (3). The acoustic impedance Zs of the resin film 3 is a known value, and the acoustic impedance Zr of the reference member 101 such as water or a culture medium is also a known value.

[0038] In this way, the acoustic impedance Zt of the biological tissue 100 can be calculated for each horizontal position in the region of the biological tissue 100, and a two-dimensional acoustic impedance image can be generated. Note that the acoustic impedance of the reference region is the known acoustic impedance Zr of the reference member 101.

[0039] Next, the acoustic impedance calculation unit 57 calculates the acoustic impedance in the depth direction of the biological tissue 100 using the method reported in Non-Patent Document 2 (step S212 in FIG. 3). The correction unit 58 corrects the signal calculated by the acoustic impedance calculation unit 57 through deconvolution, as will be described later. FIG. 8 is a flowchart explaining the operation of the acoustic impedance calculation unit 57 and the correction unit 58. FIG. 9 is a diagram explaining a method for calculating three-dimensional acoustic impedance.

[0040] The biological tissue 100 is composed of a plurality of thin layers. Therefore, as can be seen from (a) of FIG. 9, the incident wave S ０ On the other hand, the reflected wave includes reflections from the interfaces of multiple layers of the biological tissue 100. The acoustic impedance Zs of the resin film 3, which is the base material, is a known value. In this example, the acoustic impedance Zs of each layer of the biological tissue 100 shown in FIG. １ , Z ２ , ... Z ｉ－１ , Z ｉ (i is an integer of 2 or more) are calculated sequentially as follows:

[0041] First, the acoustic impedance calculation unit 57 acquires waveform data of the reflected wave stored in the storage unit 53 for the calculation target region (region of the biological tissue 100) other than the reference region (step S400 in FIG. 8 ). From the waveform of the acquired reflected wave, the acoustic impedance calculation unit 57 acquires, as an index, the time from the time when the transducer 1 transmits ultrasound to the time when the absolute value of the intensity of the reflected wave becomes maximum (step S401 in FIG. 8 ). Then, the acoustic impedance calculation unit 57 acquires reference waveform data corresponding to the index from the library (step S402 in FIG. 8 ).

[0042] Next, the acoustic impedance calculation unit 57 calculates a signal q obtained by performing a process of deconvolving the reference waveform obtained in step S402 with the reflected wave obtained in step S400 (step S403 in FIG. 8).

[0043] As shown in FIG. 9(c), the component q(t ０ ) corresponds to the reflected wave from the interface between the resin film 3 and the biological tissue 100. １ ) corresponds to the reflected wave from the interface between the layer closest to the resin film 3 and the second closest layer of the biological tissue 100. ２ ) corresponds to the reflected wave from the interface between the second closest layer and the third closest layer to the resin film 3 in the biological tissue 100. In this way, a signal q corresponding to the reflected wave from the interface of each layer of the biological tissue 100 can be obtained.

[0044] The correction unit 58 calculates the amount of deviation L of the spot position in the axial direction of the ultrasonic beam in order to correct the signal q calculated by the acoustic impedance calculation unit 57 (step S404 in FIG. 8). Specifically, the correction unit 58 acquires from the storage unit 53 the waveform of the reflected wave at the time when the adjustment of the position of the transducer 1 is completed by the processes of steps S102 and S103. Based on the waveform of the reflected wave at the time when the adjustment is completed, the correction unit 58 calculates the propagation time M of the ultrasonic wave as the time from the time when the transducer 1 transmits the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximum. ０ The correction unit 58 calculates the propagation time M ０ By multiplying 1 / 2 of this by the known sound velocity, the distance D between the transducer 1 and the ultrasonic reflection point at the time of adjustment completion is calculated. ０ Calculate.

[0045] Furthermore, the correction unit 58 calculates the time from the time when the transducer 1 transmits the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximum as the propagation time M of the ultrasonic wave, based on the waveform of the reflected wave acquired by the acoustic impedance calculation unit 57 in step S400. １ The correction unit 58 calculates the propagation time M １ By multiplying 1 / 2 of this by the known sound velocity, the distance D between the transducer 1 and the ultrasonic reflection point at the position to be calculated for acoustic impedance is calculated. １ Although not shown in FIG. 1, if a propagation medium such as water exists between the transducer 1 and the resin film 3 as in the conventional case, the known sound velocity of the ultrasonic wave passing through the propagation medium is used to calculate the distance D ０ , D １ The correction unit 58 calculates the distance D ０ and the distance D at the horizontal position to be calculated １ The difference between these is defined as the deviation amount L of the spot position in the axial direction of the ultrasonic beam.

[0046] Next, the correction unit 58 creates a window function for correcting the signal q based on the shift amount L of the spot position (step S405 in FIG. 8). The relationship between the axial distance and the beam intensity when the focal position is set to a distance of 0 is as shown in FIG. 10. The profile I(r) of the ultrasonic beam shown in FIG. 10 can be expressed as in equation (9).

[0047]

[0048] In formula (9), I ０ is the peak radiant intensity at the beam center, r is the radial distance from the axis, and w(z) is the distance from the radiant intensity I ０ 1 / e ２ (13.5%), z is the propagation distance from the plane where the wavefront becomes flat, and P is the total power of the ultrasonic beam. A window function can be created based on a Gaussian distribution curve in the direction in which the distance from the transducer 1 increases (depth direction of the biological tissue 100) from a start position that is a shift amount L away from the peak of the Gaussian distribution curve of intensity. In the example of Figure 10, the range of the Gaussian distribution curve that is the basis of the window function is indicated by a dashed line 200.

[0049] The position that is shifted by the amount L from the peak of the Gaussian distribution curve corresponds to the position of the interface between the resin film 3 and the biological tissue 100. The range of the Gaussian distribution curve that is the basis of the window function varies depending on the amount of shift L of the spot position. When the beam spot is located at the interface between the resin film 3 and the biological tissue 100 as shown in (a) of Figure 11, the amount of shift L is 0, so the range 200 of the Gaussian distribution curve that is the basis of the window function is as shown in (b) of Figure 11. The start position of the window function is the peak position of the Gaussian distribution curve.

[0050] As shown in (c) of FIG. 11, when the beam spot is located at a position on the deeper side of the biological tissue 100 than the interface between the resin film 3 and the biological tissue 100, the amount of deviation L (=D ０ -D １ ) becomes a negative value. In this case, the start position of the window function moves from the peak position of the Gaussian distribution curve by the amount of deviation L in the direction approaching the transducer 1, so the range 200 of the Gaussian distribution curve that is the basis of the window function becomes as shown in FIG. 11(d).

[0051] 11(e), the shift amount L takes a positive value when the beam spot is located closer to the transducer 1 than the interface between the resin film 3 and the biological tissue 100. In this case, the start position of the window function moves away from the peak position of the Gaussian distribution curve by the shift amount L, so the range 200 of the Gaussian distribution curve that is the basis of the window function becomes as shown in FIG.

[0052] In this way, the correction unit 58 creates a window function from the Gaussian distribution curve in the range determined by the deviation amount L. However, when creating the window function, the correction unit 58 normalizes the intensity value of the Gaussian distribution curve to a value where the peak intensity is 1.

[0053] Furthermore, since the signal q shown in (c) of Figure 9 is time-dependent, it is necessary to convert the horizontal axis of the Gaussian distribution curve into a time axis. As described above, the sound speed of the ultrasonic waves passing through the propagation medium between the transducer 1 and the resin film 3 is a known value. The correction unit 58 can convert the distance value on the horizontal axis of the Gaussian distribution curve shown in Figure 10 into time by dividing it by the sound speed. The time value at this time is a value that makes the peak position of the Gaussian distribution curve 0. Therefore, the correction unit 58 converts the converted time value into time by dividing the start position of the window function by t ０ In this way, a window function W such as that shown in FIG. 12 can be created.

[0054] Finally, the corrector 58 corrects the intensity of the signal q calculated by the acoustic impedance calculator 57 in step S403 by multiplying the signal q by a window function W (step S406 in FIG. 8).

[0055] Next, the acoustic impedance calculation unit 57 calculates the component q(t ０ ) and the known acoustic impedance Zs of the resin film 3, the acoustic impedance Z of the layer of the biological tissue 100 closest to the resin film 3 is calculated. １ is calculated by equation (10) (step S407 in FIG. 8).

[0056]

[0057] The acoustic impedance calculation unit 57 calculates the acoustic impedance Z １and the acoustic impedance Zs of the resin film 3, the transmittance T when the ultrasound travels back and forth through each layer from the resin film 3 to the layer of the biological tissue 100 closest to the resin film 3 is calculated. ０ is calculated by equation (11) (step S408 in FIG. 8).

[0058]

[0059] The acoustic impedance calculation unit 57 calculates the component q(t １ ) and transmittance T ０ Based on this, the reflection coefficient Γ of the interface between the layer closest to the resin film 3 and the layer second closest to the resin film 3 in the biological tissue 100 is calculated. １ is calculated by equation (12) (step S409 in FIG. 8).

[0060]

[0061] Next, the acoustic impedance calculation unit 57 calculates the acoustic impedance Z of the layer of the biological tissue 100 closest to the resin film 3. １ and the reflection coefficient Γ １ Based on this, the acoustic impedance Z of the layer of the biological tissue 100 that is second closest to the resin film 3 is ２ is calculated by equation (13) (step S407).

[0062]

[0063] The acoustic impedance calculation unit 57 calculates the acoustic impedance Z １ , Z ２ Based on this, the transmittance T １ is calculated by equation (14) (step S408).

[0064]

[0065] The acoustic impedance calculation unit 57 calculates the component q(t ２ ) and transmittance T ０ , T １ Based on this, the reflection coefficient Γ of the interface between the layer second closest to the resin film 3 and the layer third closest to the resin film 3 in the biological tissue 100 is calculated. ２is calculated by equation (15) (step S409).

[0066]

[0067] Similarly, steps S407 to S409 are performed for each interface of the layers of the biological tissue 100. The acoustic impedance Z of the layer of the biological tissue 100 closest to the resin film 3 is ｉ can be calculated as in equation (16).

[0068]

[0069] Gamma ｉ－１ is the reflection coefficient of the interface between the (i-1)th closest layer and the i-th closest layer to the resin film 3 in the biological tissue 100, Z ｉ－１ is the acoustic impedance of the (i-1)th layer of the biological tissue 100 closest to the resin film 3. The transmittance T ｉ－１ can be calculated as in equation (17).

[0070]

[0071] The reflection coefficient Γ of the interface between the layer of the biological tissue 100 closest to the resin film 3 and the layer of the biological tissue 100 closest to the resin film 3 is ｉ can be calculated as in equation (18).

[0072]

[0073] The acoustic impedance calculation unit 57 performs the processes of steps S407 to S409 for all components of the corrected signal q. After completing the processes for all components of the signal q (YES in step S410 in FIG. 8), the acoustic impedance calculation unit 57 determines whether or not the processes for the entire region of the calculation target (the entire region of the biological tissue 100) have been completed (step S411 in FIG. 8). If the processes have not been completed, the acoustic impedance calculation unit 57 returns to step S400.

[0074] In this way, by performing the process of FIG. 8 for each horizontal position in the region of the biological tissue 100, the acoustic impedance Z １, Z ２ , ... Z ｉ－１ , Z ｉ It is possible to calculate the acoustic impedance image in the depth direction, and generate the acoustic impedance image in the depth direction. Note that the acoustic impedance in the depth direction of the reference region is the known acoustic impedance Zr of the reference member 101.

[0075] The library may be created in Fourier space. An example of a power spectrum obtained by Fourier transforming the reflected wave is shown in FIG. 13. In the example of FIG. 13, the peak P １ In addition to this, there is a peak P ２ When creating a library in Fourier space, the peak P ２ The waveform of peak P １ The waveform of peak P １ , P ２ Alternatively, a waveform in a predetermined band B including the above may be used as the reference waveform.

[0076] Fig. 14A is an acoustic impedance image of cardiomyocytes in the horizontal direction calculated using a conventional method, and Fig. 14B is an acoustic impedance image in the depth direction taken at line I-I in Fig. 14A. Fig. 15A is an acoustic impedance image of cardiomyocytes in the horizontal direction calculated using this embodiment, and Fig. 15B is an acoustic impedance image in the depth direction taken at line II-II in Fig. 15A. Reference numeral 300 in Figs. 14B and 15B indicates the resin film 3. In the examples of Figs. 14A, 14B, 15A, and 15B, acoustic impedance is represented by color, with whiter colors indicating higher acoustic impedance.

[0077] 14B, image defects and spatial bias in impedance can be confirmed due to distortion, tilt, and vibration of the resin film 3. On the other hand, in this example, it can be seen that the acoustic impedance distribution in the depth direction can be clearly visualized.

[0078] It should be noted that a phased array transducer consisting of multiple transducer elements may be used instead of the general transducer 1. The focal position of the ultrasonic beam can be controlled by providing an appropriate delay to the transmission pulse supplied from the transmitting unit 50 to each transducer element. In this case, the phased array transducer and the transmitting unit 50 serve as a beam manipulation unit.

[0079] Furthermore, a beam steering mechanism such as a galvanometer mirror may be used instead of the scanning stage 4. The focal position of the ultrasonic beam can be controlled by changing the rotation angle and position of the beam steering mechanism that reflects the ultrasonic beam. In this case, the beam steering mechanism and the control unit 52 that controls the beam steering mechanism constitute the beam steering unit.

[0080] In this embodiment, no description is given of processing of the generated acoustic impedance image, but it goes without saying that appropriate processing such as image display, storage, data transmission, etc. may be performed as needed.

[0081] The processing device 5 described in this embodiment can be realized by a computer equipped with a CPU (Central Processing Unit), a storage device, and an interface, and a program that controls these hardware resources. An example of the configuration of this computer is shown in FIG.

[0082] The computer includes a CPU 500, a storage device 501, and an interface device (I / F) 502. The I / F 502 is connected to the transducer 1, a drive unit for the scanning stage 4, and the like. In such a computer, a program for realizing the ultrasonic image acquisition method of the present invention is stored in the storage device 501. The CPU 500 executes the processing described in this embodiment in accordance with the program stored in the storage device 501.

[0083] Some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes.

[0084] (Supplementary Note 1) The ultrasonic image acquisition device of the present invention includes a transducer configured to transmit and receive ultrasonic waves, an acoustic lens configured to focus an ultrasonic beam emitted from the transducer onto a measurement target on a substrate and irradiate the measurement target through the substrate, a beam manipulation unit configured to scan the entire region of the measurement target with the ultrasonic beam, a reference region extraction unit configured to create a reflection intensity map in which the intensity of a reflected wave obtained by the scanning is plotted on a two-dimensional plane corresponding to a horizontal plane perpendicular to an axial direction of the ultrasonic beam, and to extract a region in which the measurement target is not present as a reference region based on the reflection intensity map, a library creation unit configured to perform a process of acquiring, for each horizontal position in the reference region, the time from the transmission time of the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximized as an index indicating the magnitude of strain in the substrate, and to create a library in which the waveform of the reflected wave at the position where the index is acquired is used as a reference waveform and the index and the reference waveform are associated with each other, and a library creation unit configured to extract, for each horizontal position in the reference region, the reference waveform corresponding to the index for a position of a calculation target in a region of the measurement target other than the reference region, the reference waveform corresponding to the index, and the library creation unit configured to extract, for each horizontal position in the reference region, the time from the transmission time of the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximized as an index indicating the magnitude of strain in the substrate, the library creation unit configured to create a library in which the index and the reference waveform are associated with each other, a first acoustic impedance calculation unit configured to acquire from the library a reference waveform corresponding to the index for the position of the calculation target in the region of the measurement target, calculate a signal by performing a process of deconvolving the reference waveform acquired from the library with the waveform of the reflected wave at the position of the calculation target, and calculate the acoustic impedance in the depth direction of the measurement target at the position of the calculation target based on this signal; and a correction unit configured to calculate an amount of shift of a spot position in the axial direction of the ultrasonic beam based on a shift of a peak position of the reflected wave at the position of the calculation target by the second acoustic impedance calculation unit, generate a window function based on the amount of shift and a Gaussian distribution curve of the intensity of the ultrasonic beam, and correct the intensity of the signal calculated by the second acoustic impedance calculation unit using the window function, wherein the second acoustic impedance calculation unitCalculating the acoustic impedance of the measurement object in the depth direction based on the corrected signal.

[0085] (Appendix 2) In the ultrasonic image acquisition device described in Appendix 1, the correction unit calculates the amount of deviation of the spot position in the axial direction of the ultrasonic beam based on the peak position of the reflected wave when the absolute value of the intensity of the reflected wave is maximum when the entire area of ​​the measurement object is scanned with the ultrasonic beam and the peak position of the reflected wave at the position to be calculated by the second acoustic impedance calculation unit.

[0086] (Supplementary Note 3) In the ultrasonic image acquisition device described in Supplementary Note 2, the correction unit sets a position that is away from the peak of the Gaussian distribution curve by the deviation amount as the start position of the window function, creates the window function based on the Gaussian distribution curve on the side where the distance from the transducer increases from the start position, and multiplies the signal calculated by the second acoustic impedance calculation unit by the window function to correct the signal intensity.

[0087] (Supplementary Note 4) In the ultrasound image acquisition device according to Supplementary Note 1, the library creation unit estimates a reference waveform corresponding to a value of an index for which the reference waveform cannot be acquired by statistical processing.

[0088] (Appendix 5) In the ultrasound image acquisition device described in Appendix 1, the first acoustic impedance calculation unit calculates the acoustic impedance of the measurement object based on a reference waveform acquired from the library, a waveform of a reflected wave at the position of the calculation object, the known acoustic impedance of the substrate, and the known acoustic impedance of a reference member arranged to cover the measurement object.

[0089] (Appendix 6) The ultrasonic image acquisition method of the present invention includes a first step of focusing an ultrasonic beam emitted from a transducer onto a measurement object on a substrate and irradiating the measurement object through the substrate; a second step of scanning the entire area of ​​the measurement object with the ultrasonic beam; a third step of creating a reflection intensity map in which the intensity of the reflected wave obtained by the scanning is plotted on a two-dimensional plane corresponding to a horizontal plane perpendicular to the axial direction of the ultrasonic beam, and extracting an area where the measurement object is not present as a reference area based on the reflection intensity map; a fourth step of performing a process of acquiring, for each horizontal position in the reference area, the time from the transmission time of the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximized as an index indicating the magnitude of strain in the substrate, and creating a library in which the index and the reference waveform are associated with each other by using the waveform of the reflected wave at the position where the index was acquired as a reference waveform; and a fourth step of acquiring, from the library, a reference waveform corresponding to the index for a position of a calculation object in an area of ​​the measurement object other than the reference area, and extracting the reflected wave at the position of the calculation object. a fifth step of calculating the acoustic impedance of the object to be measured for each horizontal position in the area of ​​the object to be measured based on the waveform and a reference waveform acquired from the library; a sixth step of calculating a reference waveform corresponding to the index for the position of the object to be measured in the area of ​​the object to be measured from the library, calculating a signal by performing a process of deconvolving the reference waveform acquired from the library on the waveform of the reflected wave at the position of the object to be measured, and calculating the acoustic impedance in the depth direction of the object to be measured at the position of the object to be measured based on this signal; and a seventh step of calculating an amount of shift of a spot position in the axial direction of the ultrasonic beam based on a shift of a peak position of the reflected wave at the position of the object to be measured in the sixth step, generating a window function based on the amount of shift and a Gaussian distribution curve of the intensity of the ultrasonic beam, and correcting the intensity of the signal calculated in the sixth step using the window function, wherein the sixth step includes a step of calculating the acoustic impedance in the depth direction of the object to be measured based on the corrected signal.

[0090] (Appendix 7) In the ultrasonic image acquisition method described in Appendix 6, the seventh step includes a step of calculating the amount of deviation of the spot position in the axial direction of the ultrasonic beam based on the peak position of the reflected wave when the absolute value of the intensity of the reflected wave is maximum when the entire area of ​​the measurement object is scanned with the ultrasonic beam and the peak position of the reflected wave at the position to be calculated in the sixth step.

[0091] (Supplementary Note 8) In the ultrasonic image acquisition method described in Supplementary Note 7, the seventh step includes a step of setting a position that is away from the peak of the Gaussian distribution curve by the deviation amount as a start position of the window function, creating the window function based on the Gaussian distribution curve on the side where the distance from the start position to the transducer is greater, and multiplying the signal calculated in the sixth step by the window function to correct signal intensity.

[0092] (Supplementary Note 9) In the ultrasound image acquisition method described in Supplementary Note 6, the fourth step includes a step of estimating a reference waveform corresponding to a value of an index for which the reference waveform cannot be acquired by statistical processing.

[0093] (Supplementary Note 10) In the ultrasonic image acquisition method described in Supplementary Note 6, the fifth step includes a step of calculating the acoustic impedance of the measurement object based on a reference waveform acquired from the library, a waveform of a reflected wave at the position of the calculation object, the known acoustic impedance of the substrate, and the known acoustic impedance of a reference member arranged to cover the measurement object.

[0094] The present invention can be applied to techniques for generating ultrasound images.

[0095] 1...transducer, 2...acoustic lens, 3...resin film, 4...scanning stage, 5...processing device, 6...beam manipulation unit, 50...transmitting unit, 51...receiving unit, 52...control unit, 53...storage unit, 54...reference area extraction unit, 55...library creation unit, 56, 57...acoustic impedance calculation unit, 58...correction unit

Claims

1. A transducer configured to transmit and receive ultrasonic waves; an acoustic lens configured to focus an ultrasonic beam emitted from the transducer onto a measurement target on a substrate and irradiate the measurement target through the substrate; a beam manipulation unit configured to scan the entire area of ​​the measurement target with the ultrasonic beam; a reference area extraction unit configured to create a reflection intensity map that depicts the intensity of the reflected wave obtained by the scanning on a two-dimensional plane corresponding to a horizontal plane perpendicular to the axial direction of the ultrasonic beam, and to extract an area where the measurement target is not present as a reference area based on the reflection intensity map; and a library creation unit configured to perform a process of obtaining, for each horizontal position in the reference area, the time from the transmission time of the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximized as an index indicating the magnitude of distortion of the substrate, and to create a library in which the index and the reference waveform are associated with each other, using the waveform of the reflected wave at the position where the index was obtained as a reference waveform; a first acoustic impedance calculation unit configured to acquire from the library a reference waveform corresponding to the index for a position of a calculation target in a region of the measurement target other than the reference region, and calculate an acoustic impedance of the measurement target for each horizontal position in the region of the measurement target based on a waveform of a reflected wave at the position of the calculation target and the reference waveform acquired from the library; a second acoustic impedance calculation unit configured to acquire from the library a reference waveform corresponding to the index for a position of a calculation target in the region of the measurement target, calculate a signal by performing a process of deconvolving the reference waveform acquired from the library with respect to the waveform of a reflected wave at the position of the calculation target, and calculate an acoustic impedance in the depth direction of the measurement target at the position of the calculation target based on this signal; and a correction unit configured to calculate an amount of deviation of a spot position in the axial direction of the ultrasonic beam based on a deviation of a peak position of a reflected wave at a position to be calculated by the second acoustic impedance calculation unit, generate a window function based on the amount of deviation and a Gaussian distribution curve of the intensity of the ultrasonic beam, and correct the intensity of the signal calculated by the second acoustic impedance calculation unit using the window function,The ultrasonic imaging apparatus according to claim 1, wherein the second acoustic impedance calculation unit calculates an acoustic impedance in a depth direction of the measurement object based on the corrected signal.

2. An ultrasonic image acquisition device as described in claim 1, characterized in that the correction unit calculates the amount of deviation of the spot position in the axial direction of the ultrasonic beam based on the peak position of the reflected wave when the absolute value of the intensity of the reflected wave is maximum when the entire area of ​​the measurement object is scanned with the ultrasonic beam, and the peak position of the reflected wave at the position to be calculated by the second acoustic impedance calculation unit.

3. An ultrasonic image acquisition device as described in claim 2, characterized in that the correction unit sets a position away from the peak of the Gaussian distribution curve by the shift amount as the start position of the window function, creates the window function based on the Gaussian distribution curve on the side where the distance from the transducer to the start position is greater, and multiplies the signal calculated by the second acoustic impedance calculation unit by the window function to correct the signal intensity.

4. An ultrasonic image acquisition device according to claim 1, wherein the library creation unit estimates a corresponding reference waveform for an index value for which a reference waveform cannot be acquired by statistical processing.

5. An ultrasonic image acquisition device as described in claim 1, characterized in that the first acoustic impedance calculation unit calculates the acoustic impedance of the measurement object based on a reference waveform acquired from the library, a waveform of a reflected wave at the position of the calculation object, a known acoustic impedance of the base material, and a known acoustic impedance of a reference member arranged to cover the measurement object.

6. A first step of focusing an ultrasonic beam emitted from a transducer onto a measurement target on a substrate and irradiating the measurement target through the substrate; a second step of scanning the entire area of ​​the measurement target with the ultrasonic beam; a third step of creating a reflection intensity map depicting the intensity of the reflected wave obtained by the scanning on a two-dimensional plane corresponding to a horizontal plane perpendicular to the axial direction of the ultrasonic beam, and extracting an area where the measurement target is not present as a reference area based on the reflection intensity map; and a fourth step of performing a process of obtaining, for each horizontal position in the reference area, the time from the transmission time of the ultrasonic wave to the time when the absolute value of the intensity of the reflected wave is maximized as an index indicating the magnitude of distortion of the substrate, and creating a library in which the index and the reference waveform are associated with each other by using the waveform of the reflected wave at the position where the index was obtained as a reference waveform. a fifth step of acquiring from the library a reference waveform corresponding to the index for a position of a calculation target in a region of the measurement target other than the reference region, and calculating an acoustic impedance of the measurement target for each horizontal position in the region of the measurement target based on the waveform of a reflected wave at the position of the calculation target and the reference waveform acquired from the library; a sixth step of acquiring from the library a reference waveform corresponding to the index for a position of a calculation target in the region of the measurement target, calculating a signal obtained by performing a process of deconvolving the reference waveform acquired from the library with the waveform of a reflected wave at the position of the calculation target, and calculating an acoustic impedance in the depth direction of the measurement target at the position of the calculation target based on this signal; and a seventh step of calculating an amount of shift of a spot position in the axial direction of the ultrasonic beam based on a shift of a peak position of a reflected wave at the position of the calculation target in the sixth step, generating a window function based on the amount of shift and a Gaussian distribution curve of the intensity of the ultrasonic beam, and correcting the intensity of the signal calculated in the sixth step by the window function, The ultrasonic image acquiring method according to claim 6, wherein the sixth step includes a step of calculating an acoustic impedance in a depth direction of the measurement object based on the corrected signal.

7. An ultrasonic image acquisition method as described in claim 6, characterized in that the seventh step includes a step of calculating the amount of deviation of the spot position in the axial direction of the ultrasonic beam based on the peak position of the reflected wave when the absolute value of the intensity of the reflected wave is maximum when the entire area of ​​the measurement object is scanned with the ultrasonic beam, and the peak position of the reflected wave at the position that is the subject of calculation in the sixth step.

8. An ultrasonic image acquisition method according to claim 7, characterized in that the seventh step includes a step of setting a position away from the peak of the Gaussian distribution curve by the shift amount as the start position of the window function, creating the window function based on the Gaussian distribution curve on the side where the distance from the transducer increases from the start position, and multiplying the signal calculated in the sixth step by the window function to correct signal intensity.

9. An ultrasound image acquisition method according to claim 6, characterized in that the fourth step includes a step of estimating a corresponding reference waveform for an index value for which a reference waveform cannot be acquired by statistical processing.

10. An ultrasonic image acquisition method as described in claim 6, characterized in that the fifth step includes a step of calculating the acoustic impedance of the measurement object based on a reference waveform acquired from the library, a waveform of a reflected wave at the position of the calculation object, a known acoustic impedance of the substrate, and a known acoustic impedance of a reference member arranged to cover the measurement object.

Images