Ultrasonic CT device, ultrasonic image generation method, and image generation device

Abstract

Provided are an ultrasonic CT apparatus, an ultrasonic image generation method, and an image generation apparatus. In the FWI method, a high-resolution reconstructed image is obtained by simulating the deviation between the waveform of the sound wave generated by the simulated sound source and the sound wave irradiated by the transducer. Sound waves are transmitted to a subject from one or more transducers in a transducer array having a plurality of transducers arranged therein. Measured sound pressures are received from the plurality of transducers by measuring the sound wave that has passed through an imaging region of the subject. The measured sound pressures are processed to generate a transmitted wave image of the imaging region. A simulated sound wave with a temporally varying sound pressure is generated from the simulated sound source. The sound pressures when the simulated sound wave passes through the imaging region and reaches a plurality of simulated detectors are calculated as calculated sound pressures. The transmitted wave image is used as an initial image, and the calculated sound pressures are used to sequentially correct the transmitted wave image. A sequential updating unit uses the calculated sound pressures to perform computational processing to bring the waveform of the simulated sound wave generated by the simulated sound source closer to the waveform of the sound wave transmitted by the transducer.

Description

Technical Field

[0001] The present invention relates to an ultrasonic CT image reconstruction method. Background Art

[0002] A breast-specific ultrasonic CT (computed tomography) device is known as a medical diagnostic device that uses ultrasound measurement to detect breast cancer (Patent Document 1). In this ultrasonic CT device, an array of transducers, acting as transmitters and receivers, is placed around a breast submerged in water. Ultrasonic waves transmitted through the breast are received around the entire circumference, and an image representing the distribution of sound velocity and attenuation is reconstructed from the received signals. Generally, the speed of sound and attenuation of tissue are quantitative values. Since the speed of sound and attenuation in tumors are higher than those in surrounding normal tissues such as mammary glands and fat, tumors can be quantitatively detected from the transmitted wave image.

[0003] As a method for capturing a transmitted wave image, there is known a method of irradiating a subject with a diffuse wave that spreads at a predetermined angle from a sound source (transducer) to obtain a transmitted signal (Non-Patent Document 1).

[0004] Known image reconstruction methods for generating a transmitted wave image from a transmission signal include the straight-ray method and the bent-ray method. The straight-ray method reconstructs the image by approximating the ultrasound trajectory to a straight line. While computationally efficient, it has low spatial resolution. The bent-ray method reconstructs the image by taking into account the refraction of ultrasound waves, resulting in higher spatial resolution than the straight-ray method.

[0005] Patent Document 1 discloses the FWI (full waveform inversion) method as a method for reconstructing a sonic velocity image with higher spatial resolution than the straight ray method and the curved ray method, and the method is also applied to clinical data.

[0006] In the FWI method, a sonic velocity image is generated from the measured transmission signal using conventional reconstruction methods. This sonic velocity image is used as the initial image. The sound pressure distribution is calculated based on a sound source and the initial image through simulation. The sonic velocity image is continuously updated to minimize the error between the generated simulation data (sound pressure distribution) and the measured data. Patent Document 1 discloses the calculation algorithm for the FWI method.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: U.S. Patent Application Publication No. 2016 / 0030000

[0010] Non-patent document 1: Pratt, R., "Seismic waveform inversion in the frequency domain, Part 1: Theory and verification in a physical scale model." GEOPHYSICS, 1999, 64(3), 888-901.

[0011] In the FWI method, the sound source used to simulate the sound pressure distribution in the imaging area generates a wave whose sound pressure varies over time, similar to an actual sound source (vibrator). To ensure that the signal of the wave emitted by the simulated sound source matches the signal of the wave emitted by the actual sound source, a signal source scaling factor γ is calculated as described in paragraph 0045 of Patent Document 1. Specifically, Patent Document 1 discloses that the estimation of the signal source scaling factor γ is treated as a linear estimation problem. The complex scaling factor γ is calculated by multiplying the measured received signal by the signal of the simulated wave and dividing the result by the square of the simulated wave signal.

[0012] However, in the calculation of the above-mentioned scaling factor γ, whether the phase of the wave generated by the actual sound source and the phase of the wave emitted by the sound source during simulation are consistent is not considered.

[0013] If the phase of the wave generated by the simulated sound source differs from the phase of the wave generated by the actual sound source, the estimated arrival time of the received signal wave may deviate by up to one cycle. Therefore, to improve FWI image quality, it is important to accurately match the phases of the two.

[0014] However, it is actually difficult to precisely control the phase of a wave emitted from an actual transducer at a given moment, and it is necessary to accurately estimate the phase of the wave emitted by the simulated sound source to match the phase of the wave of the actual sound source. Summary of the Invention

[0015] An object of the present invention is to reduce the deviation between the waveforms of sound waves generated from a simulated sound source and sound waves irradiated by a transducer in the FWI method, thereby obtaining a high-resolution reconstructed image.

[0016] To achieve the above-mentioned object, the present invention comprises: a transmitting unit that transmits sound waves to a subject from one or more transducers in a transducer array comprising a plurality of transducers; a receiving unit that receives, from the plurality of transducers, measured sound pressures obtained by measuring the sound waves that have passed through an imaging region of the subject; an image reconstruction unit that processes the measured sound pressures to generate a transmitted wave image of the imaging region; and a sequential updating unit that generates a simulated sound wave having a temporally varying sound pressure from a simulated sound source, calculates the sound pressures when the simulated sound wave passes through the imaging region and reaches a plurality of simulated detectors, and sequentially corrects the transmitted wave image using the calculated sound pressures, using the transmitted wave image as an initial image. The sequential updating unit uses the calculated sound pressures to perform computational processing to bring the waveform of the simulated sound wave generated by the simulated sound source closer to the waveform of the sound wave transmitted by the transducers.

[0017] Effects of the Invention

[0018] According to the present invention, in the FWI method, changes in the sound pressure of waves generated from a simulated sound source through simulation can be made close to changes in the sound pressure irradiated by an actual transducer, thereby obtaining a high-resolution reconstructed image. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a block diagram showing the configuration of the ultrasonic CT apparatus according to the first embodiment.

[0020] Figure 2 This is a perspective view of the transducer array 30 of the ultrasonic CT apparatus according to the first embodiment.

[0021] Figure 3 It is an explanatory diagram showing the irradiation wave irradiated from the transducer 3a.

[0022] Figure 4 (a) is an explanatory diagram showing a transmitted wave image (initial image) reconstructed by an ultrasonic CT apparatus, (b) is an explanatory diagram showing how a simulated sound source 53 is set for the transmitted wave image and simulated sound waves 50 are irradiated to obtain a calculated sound pressure at the position of a simulated detector 54, (c) is a graph showing the difference between the calculated sound pressure and the measured sound pressure, (d) is an image showing the correction coefficient Δc for the calculated sound pressure obtained based on the difference in the measured sound pressure, (e) is a diagram showing the corrected image after subtracting the correction coefficient Δc, and (f) is an explanatory diagram showing the final image.

[0023] Figure 5 This is a flowchart showing an overview of the operation of the ultrasonic CT apparatus according to the first embodiment.

[0024] Figure 6 This is a flowchart showing the detailed operation of the ultrasonic CT apparatus according to the first embodiment.

[0025] Figure 7This is a flowchart showing the calibration determination function of the ultrasonic CT apparatus according to the second embodiment.

[0026] Figure 8 This is an example of a display screen showing the determination result of the calibration determination function of the ultrasonic CT apparatus according to the second embodiment.

[0027] Figure 9 This is an example of a display screen showing the result of the element degradation determination by the calibration determination function of the ultrasonic CT apparatus according to the second embodiment.

[0028] Description of Reference Numerals

[0029] 1...Subject

[0030] 2...beds

[0031] 3...Vibrator

[0032] 4...sink

[0033] 5...Preparation tank

[0034] 6...Sending Department

[0035] 7...Receiving Department

[0036] 8...Image Reconstruction Department

[0037] 9...Updated

[0038] 10...Input receiving unit

[0039] 11...Storage

[0040] 12...Control Department

[0041] 13...Display unit DETAILED DESCRIPTION

[0042] The present invention will be described below with reference to the accompanying drawings in relation to one embodiment.

[0043] Implementation Method 1

[0044] <Summary>

[0045] First use Figure 1 The outline of the ultrasonic CT apparatus according to the first embodiment will be described below.

[0046] Figure 1 is a diagram showing the overall structure of an ultrasonic CT apparatus. Figure 2 This is a perspective view of the breast of the subject 1 and the transducer array 3 inserted into the water tank 4. Figure 3This figure shows ultrasound waves 40 being emitted from a transducer array 30. While this description focuses on an ultrasonic CT apparatus suitable for breast cancer diagnosis, the example of generating breast images is described. However, the imaging target is not limited to the breast. Furthermore, the present invention is not limited to ultrasonic CT apparatuses that acquire tomographic images of acoustic information from a living organism; for example, it can also be applied to acquiring tomographic images of acoustic information from strata, as described in Non-Patent Document 1. Therefore, in the present invention, an ultrasonic CT apparatus is defined as one that acquires tomographic images of acoustic information from a substance.

[0047] like Figure 1 As described above, the ultrasonic CT apparatus of the first embodiment includes a transmitter 6, a receiver 7, an image reconstruction unit 8, a sequential update unit 9, an input receiving unit 10, a storage unit 11, and a control unit 12 for controlling the overall operation. Figure 2 As shown, a transducer array 30 in which a plurality of transducers 3 are arranged in a predetermined shape (eg, a ring shape) is connected. A display unit 11 is connected to the sequential updating unit 9 .

[0048] The vibrator array 30 is as follows Figure 2 As shown, the water tank 4 is arranged inside or outside the cylindrical water tank 4. The water tank 4 is arranged at the lower part of the opening provided in the bed 2 on which the subject 1 is placed (refer to Figure 1 ). Subject 1 lies prone on bed 2 and inserts her breast into the opening of bed 2. Although not shown, a drive mechanism capable of parallel movement of the transducer array in the axial direction of water tank 4 may also be provided. Transducer 3 is an ultrasonic transceiver, using, for example, a piezoelectric element. Water tank 4 is filled with warm water and connected to a preparatory tank 5. Preparatory tank 5 purifies, superheats, and degasses the warm water in water tank 4.

[0049] like Figure 3 As shown, the transmitter 6 outputs a transmission signal (electrical signal) to one or more transducers 3a of the transducer array 30, transmitting sound waves (here, ultrasound waves) 40 from the one or more transducers 3a. The sound waves 40, which pass through the imaging area of ​​the subject 1, reach the multiple transducers of the transducer array 30, and their sound pressure is measured. The receiver 7 receives the measured sound pressure (electrical signal) measured by the multiple transducers 3. In the case of a ring-shaped transducer array 30, the imaging area is the area within the ring of the subject 1. The receiver 7 receives the measured sound pressure and performs A / D conversion.

[0050] The image reconstruction unit 8 processes the measured sound pressure to reconstruct a transmitted wave image of the imaging region. As the transmitted wave image, an image showing the distribution of physical property values ​​(eg, sound velocity distribution and / or attenuation distribution) in the imaging region is generated by a known method.

[0051] The successive updating unit 9 uses the transmitted wave image generated by the image reconstruction unit 8 as the initial image (reference Figure 4 (a)), the transmitted wave image is sequentially corrected using the sound pressure obtained by calculation. That is, the sequential updating unit 9 generates a simulated sound wave (hereinafter referred to as a simulated sound wave) 50 (sound velocity c) (refer to the reference) whose sound pressure changes over time from a simulated sound source (hereinafter referred to as a simulated sound source) 53. Figure 4 (b)) The sound pressure (hereinafter referred to as calculated sound pressure) when the simulated sound wave 50 passes through the imaging area and reaches a plurality of simulated detectors (referred to as simulated detectors) 54 is obtained by calculation. The successive updating unit 9 is, for example, as shown in FIG. Figure 4 (c) The difference between the calculated sound pressure and the measured sound pressure is obtained, and the correction coefficient Δc of the sound velocity that minimizes the difference is obtained (refer to Figure 4 (d)), the correction coefficient Δc of the speed of sound is used to correct the transmitted wave image successively (refer to Figure 4 (e), (f)).

[0052] At this time, in the first embodiment, the successive updating unit 9 uses the calculated sound pressure to perform processing to bring the waveform of the pseudo sound wave 50 generated by the pseudo sound source 53 used in the successive correction closer to the waveform of the sound wave 40 transmitted by the transducer 3 a .

[0053] This allows the waveform of the sound pressure of the sound wave 50 generated from the simulated sound source 53 in the FWI method to be approximated to the waveform of the sound pressure of the sound wave 40 actually irradiated by the transducer 3 a , thereby obtaining a high-definition reconstructed image.

[0054] For example, the successive updating unit 9 makes the waveform of the temporal change in sound pressure of the simulated sound wave 50 become a plurality of different waveforms, calculates the sound pressure for each of the plurality of waveforms, and selects the waveform of the simulated sound wave 50 generated by the simulated sound source 53 used in the successive correction of the transmitted wave image based on the difference between the measured sound pressure and the calculated sound pressure.

[0055] The selection of the waveform of the pseudo acoustic wave 50 may be performed only once before the above-mentioned successive correction of the transmitted wave image, or may be performed every time the image is updated in the successive correction process.

[0056] In addition, the waveform of the simulated sound wave 50 mentioned here includes any waveform as long as it is a shape that represents the time change of sound pressure. For example, in addition to the phase, maximum amplitude, and frequency, the waveform whose time change is a straight line (rectangular wave, triangular wave, etc.) or a curve (sine wave and other functions) is set as a different waveform. In the case of a curve, different waveforms such as the curve shape are set as different waveforms.

[0057] For example, the sequential updating unit 9 is configured to prepare waveforms having different phases as a plurality of waveforms of the pseudo sound wave 50 and select a waveform having a specific phase from among the waveforms.

[0058] As a selection method, for example, the sequential updating unit 9 calculates the value of a predetermined function representing the difference between the measured sound pressure and the calculated sound pressure for a plurality of waveforms and selects the waveform for which the value of this function is minimized. For example, a function that calculates the square or sum of the squares of the difference between the measured sound pressure and the calculated sound pressure can be used.

[0059] Furthermore, the sequential update unit 9 desirably arranges the pseudo sound source 53 at the position of the transducer 3 a that transmits sound waves in the transmitter 6 , and arranges the pseudo detector 54 at the position of the transducer 3 that receives and measures sound pressure in the receiver 7 .

[0060] The following uses Figure 5 The operation of each part of the ultrasonic CT apparatus is described using the flow chart below.

[0061] In addition, in this embodiment, the image reconstruction unit 8 and the sequential update unit 9 are composed of a computer having a processor such as a CPU (Central Processing Unit) and a GPU (Graphics Processing Unit) and a memory. The CPU reads and executes a program stored in the memory, thereby realizing the functions of the image reconstruction unit 8 and the sequential update unit 9 by software. The image reconstruction unit 8 and the sequential update unit 8 can also be partially or entirely realized by hardware. For example, the image reconstruction unit 8 and the sequential update unit 8 can be constructed using a custom IC such as an ASIC (Application Specific Integrated Circuit) or a programmable IC such as an FPGA (Field-Programmable Gate Array), and their operations can be realized by performing circuit design.

[0062] use Figure 5 The following flow is used to describe the outline of the operation of the ultrasonic CT apparatus.

[0063] First, in step 501, the transmitter 6 transmits the sound wave 40 from the transducer 3a to the subject 1. The sound wave 40 that has passed through the imaging area of ​​the subject 1 is received by the transducer 3. The receiver 7 receives the measured sound pressure u from the transducer 3. m .

[0064] In step 502, the image reconstruction unit 8 uses the measured sound pressure u m Here, an example of generating a sound velocity distribution image (hereinafter referred to as a sound velocity image) as a transmitted wave image will be described below.

[0065] In step 503, the successive updating unit 9 calculates the calculated sound pressure u detected by the analog detector 54 when the analog sound wave 50 is irradiated from the analog sound source 53 to the subject 1. s, use the calculated sound pressure u s A process is performed to make the waveform of the pseudo sound wave 50 close to the sound wave 40 .

[0066] In step 504 , when the value of the predetermined objective function is equal to or greater than the threshold, the sequential updating unit 9 proceeds to step 505 and corrects the sonic velocity image using the pseudo sound wave 50 that has been brought close to the sound wave 40 in step 503 .

[0067] The sequential updating unit 9 repeats steps 503 to 505 until the value of the objective function becomes smaller than the threshold value in step 504 .

[0068] As a result, the sonic velocity image can be corrected using the pseudo acoustic wave 50 whose waveform is close to the acoustic wave 40 , and thus a high-resolution reconstructed image can be obtained with high accuracy.

[0069] use Figure 6 To illustrate the process Figure 5 The details of the processing of steps 501 to 505 are shown.

[0070] <Step 501>

[0071] pass Figure 6 The processing of step 501 is described in detail with reference to steps 511 to 513.

[0072] In step 511, in the ultrasonic CT apparatus of this embodiment, when the input receiving unit 10 receives an instruction to start imaging from the user, the transmitting unit 6 specifies the transducer 3a stored in advance in the storage unit 11 for each view and outputs a transmission signal of a predetermined frequency (approximately several MHz) to the transducer 3a. The transducer 3a, having received the transmission signal, transmits the acoustic wave 40 to the subject 1.

[0073] At step 512, a portion of the sound wave 40 irradiated to the subject 1 is transmitted through the subject 1, received by the transducer 3, and converted into an electrical signal (measurement of the sound pressure u m The receiving unit 7 receives the measured sound pressure u from the vibrator 3. m , perform A / D conversion and other processing.

[0074] The transmission and reception of ultrasonic waves in steps 511 and 512 are performed from all views (step 513 ).

[0075] <Step 502>

[0076] pass Figure 6 Steps 514 and 515 explain the processing of step 502 in detail.

[0077] In step 514, the image reconstruction unit 8 uses the measured sound pressure u mPerform image reconstruction processing to generate the sound velocity image within the subject 1. In step 515, the sound velocity image becomes the initial image c n (n = 0).

[0078] A specific example of image reconstruction processing will be briefly described. For example, the image reconstruction unit 8 can obtain the sound velocity image by the straight ray method. That is, the image reconstruction unit 8 performs a Hilbert transform on the measured sound pressure output by each oscillator 3 in each view in the time direction to obtain the reception timing of the maximum amplitude of the received wave. The image reconstruction unit 8 also obtains the reception timing of the maximum amplitude in the same manner for the measured sound pressure of each oscillator 3 received in advance before the insertion of the subject 1. The image reconstruction unit 8 calculates the difference in reception timing before and after the insertion of the subject 1 for each view and each reception channel, and obtains a set of these data, that is, a sinogram. The image reconstruction unit 8 reconstructs a tomographic image by processing the sinogram of the difference in reception timing using a filtered back projection method (Filtered Back Projection, FBP), etc. This tomographic image is a distribution image of the difference in the "slowness" of ultrasonic waves before and after the insertion of the subject 1. "Slowness" is the reciprocal of the sound velocity. The image reconstruction unit 8 uses the sound velocity value (estimated value) of water to generate the distribution image (sound velocity image) of the sound velocity of the subject 1 from the distribution image of the difference in "slowness".

[0079] <Steps 503 - 505>

[0080] In step 503, through Figure 6 Steps 516 - 520 are performed to process the waveform of the simulated acoustic wave 50 used in the sequential update process of the sound velocity image in steps 504 and 505 to make it close to the waveform of the actually transmitted acoustic wave 40. For the sake of explanation, first, the principle of the sequential update process based on the FWI method will be described.

[0081] <Principle of sequential update of the FWI method>

[0082] The FWI method includes a sound pressure calculation algorithm for calculating the calculated sound pressure u s and an algorithm for solving the inverse problem of correcting the transmitted wave image to minimize the objective function.

[0083] In the sound pressure calculation algorithm, a simulated sound source 53 is arranged at the position of the oscillator 3a in the space represented by the sound velocity image, a simulated acoustic wave 50 is generated from the simulated sound source 53, and the sound pressure (calculated sound pressure u s ) of the simulated acoustic wave 50 that reaches the simulated detector 54 arranged at the position of the oscillator 3 through the space represented by the transmitted sound velocity image is obtained through simulation.

[0084] When FWI is performed in the frequency domain, the pseudo sound source 53 is represented by a complex number as in the following equation (1).

[0085]

Mathematical formula 1

[0086] f=A·exp{i·θ} (1)

[0087] Here, A is a complex coefficient for adjusting the intensity and phase of the pseudo sound source, i is an imaginary number, and θ is the phase. The adjustment coefficient A of the pseudo sound source can be obtained by, for example, the well-known Pratt signal estimation method (Non-Patent Document 1).

[0088] The sound pressure can be determined by, for example, solving the Helmholtz equation in the frequency domain using the finite difference method. The Helmholtz equation in the frequency domain is represented by the following equation.

[0089]

Mathematical formula 2

[0090]

[0091] Here, ω is the angular frequency, and r is the position. c(r) is the pixel value (sound velocity) of the pixel at position r in the sound velocity image (imaging area). u(r, ω) is a vector representing the sound pressure at position r in the sound velocity image and is a matrix. f(r) is the simulated sound source represented by the function of equation (1), and r is the position of the sound source.

[0092] In formula (2),

[0093]

Mathematical formula 3

[0094]

[0095] is the Laplace operator.

[0096] When the space is discretized and the equation (2) is represented by a matrix expression based on the finite difference method, it becomes as follows.

[0097]

Mathematical formula 4

[0098] S(r)u(r)=f(r) or u(r)=S(r) -1 f(r) (4)

[0099] In Equation (4), S(r) is called the impedance matrix and represents the coefficient matrix of the sound pressure u(r) in the finite difference method. Specifically, S(r) represents the matrix within the parentheses on the left side of the sound pressure u(r) in Equation (2), as shown in Equation (5). In Equation (4), the frequency ω is assumed to be fixed and is omitted.

[0100]

Mathematical formula 5

[0101]

[0102] As is clear from the second equation on the right side of equation (4), the sound source f(r) represented by equation (1) and the inverse matrix S(r) of the impedance matrix S(r) can be obtained. -1 The sound pressure u(r) at the position r of the sound velocity image is obtained by calculation. The sound pressure at the position r obtained by calculation is represented by a matrix as the calculated sound pressure u s (r).

[0103] Next, as an algorithmic solution to the inverse problem, the calculated sound pressure u obtained in the calculation of equation (4) is obtained according to equation (6): s (r) and the sound pressure u measured in step 512 m The residual sum of squares is the objective function E(c), and it is minimized.

[0104]

Mathematical formula 6

[0105]

[0106] In formula (6), H represents the Hermitian transpose.

[0107] In order to minimize the objective function E(c) of the equation (6), the pixel value c of the sonic velocity image can be successively corrected by, for example, the steepest descent method represented by the following equation (7).

[0108]

Mathematical formula 7

[0109]

[0110] In formula (7), n is the number of repetitions, c is n is the pixel value (sound speed or attenuation) on the sound speed image before correction (nth time), c n+1 is the pixel value on the corrected (n+1th) transmitted wave image, and α is a parameter for adjusting the correction amount called the step size.

[0111] Here, the second term on the right side of equation (7) is represented by Δc as shown in equation (8).

[0112]

Mathematical formula 8

[0113]

[0114] In the above description, the sonic velocity image is corrected using the steepest descent method, but other algorithms such as the conjugate gradient method can also be used. Furthermore, the FWI of this embodiment can be performed in either the frequency domain or the time domain.

[0115] <Step 503>

[0116] Using the above principle, Figure 6 The processing of making the waveform of the simulated sound wave 50 close to the waveform of the sound wave 40 in step 503 will be specifically described with reference to steps 516 to 520.

[0117] First, in step 516, the successive updating unit 9 calculates the impedance matrix S(r) based on the sound velocity c(r) (r is the position of the pixel) represented by the pixel value of the sound velocity image in step 515 using equation (5), and calculates its inverse matrix S(r) -1 .

[0118] In step 517, the simulated sound source 53 is represented by f using equation (1). The successive updating unit 9 calculates S(r) -1 , f as the simulated sound source 53 and the second equation on the right side of equation (4) to calculate the calculated sound pressure u at the position of the simulated detector 54 s .

[0119] Next, in step 518, the successive updating unit 9 calculates the sound pressure u of the simulated sound source 53. s The N coefficients a(k) (k = 0, 1, ..., N-1) represented by formula (9) are multiplied to generate N calculated sound pressures a(k)u s As is clear from equation (9), the N coefficients a(k) are complex numbers of different angles θk, which are used to calculate the sound pressure u s The coefficient of the phase shift effect.

[0120]

Mathematical formula 9

[0121]

[0122] Next, at step 519, the successive updating unit 9 calculates the sound pressure a(k)u by s The calculated sound pressure u is s (c) The sound pressure a(k)u is calculated based on the N types of calculations in step 518 by using the equation (6') obtained by substitution. s and the sound pressure u measured in step 512 m Calculate the objective function E(k). In this way, N kinds of objective functions E(k) can be obtained.

[0123]

Mathematical formula 10

[0124]

[0125] In step 520, the successive updating unit 9 selects the objective function E(k′) that becomes the minimum value among the N types of objective functions E(k) (k′ is the value of k when the objective function E(k) becomes the minimum value), and selects the coefficient a(k′) of the objective function E(k′) that can obtain the minimum value.

[0126] According to the second equation on the right side of the above equation (4), when the impedance matrix S(r) is the same, that is, when the subject 1 is the same, even if both sides of the second equation of equation (4) are multiplied by a(k'), equation (4) still holds. Therefore, it can be seen that the sound source a(k')f obtained by multiplying the simulated sound source 53 represented by f by a(k') can be obtained to obtain the sound source a(k')f in the calculation of the sound pressure u s a(k′)u obtained by multiplying a(k′) s That is, the sound pressure a(k′)u that minimizes the objective function E(k′) can be obtained by adjusting the phase a(k′)f of the pseudo sound source 53. s .

[0127] Thus, through steps 516 to 520 , a(k′)f can be obtained by adjusting the phase of the pseudo sound source 53 so that the sound pressure waveform 50 generated from the pseudo sound source 53 is closest to the phase of the sound wave 40 generated by the transducer 3 a .

[0128] <Step 504>

[0129] exist Figure 6 Step 521 of Figure 5 The processing of the objective function of step 504.

[0130] In step 521, the successive updating unit 9 calculates the sound pressure a(k′)u s The measured sound pressure u obtained in step 512 m The objective function E(k') is calculated by using the formula (6). When the calculated objective function E(k') is smaller than a predetermined threshold, the successive updating unit 9 ends the process and the speed of sound image c in step 515 is n Become the final image. If the objective function E(k′) is greater than the predetermined threshold, proceed to step 505 ( Figure 6 In steps 522 to 523), the sonic velocity image is corrected.

[0131] <Step 505>

[0132] exist Figure 6 Steps 522 to 523 of Figure 5 Step 505 is the correction process of the sonic velocity image.

[0133] In step 522, the successive updating unit 9 calculates the sound speed c that minimizes E(c) by, for example, the steepest descent method. Specifically, the correction coefficient Δc is calculated for each pixel of the sound speed image by equation (8).

[0134] In step 523, the successive updating unit 9 updates the pixel value c as shown in equation (7). nSubtract Δc from the pixel to calculate the corrected pixel value c n+1 Thus, the corrected sound velocity image c is obtained. n+1 .

[0135] The successive updating unit 9 returns to step 515 and updates the speed of sound image c n Replaced with the corrected sonic velocity image c calculated in step 523 n+1 Until the objective function E(k′) becomes smaller than the threshold th in step 521, the speed of sound image is corrected successively by repeating steps 515 to 523.

[0136] When the objective function E(k′) becomes smaller than the threshold value th in step 521 , the successive updating unit 9 determines that the successive correction has converged, and causes the display unit 13 to display the transmitted wave image.

[0137] As described above, in this embodiment, the waveform (eg, phase) of the sound wave 50 generated by the pseudo sound source 53 in the FWI method can be gradually brought close to the sound wave 40 generated by the transducer 3 a , thereby generating a sonic image with high spatial resolution.

[0138] In addition, Figure 5 as well as Figure 6 In the process, the structure of performing step 503 (steps 516 to 520) to obtain the coefficient a(k′) each time a correction is made is described, but it can also be constructed so that the coefficient a(k′) is calculated only for the first time, and the successive corrections are performed in the same way as in the past after the second time.

[0139] In the above embodiment, in step 521, the successive correction is judged to have converged when the objective function E(c) becomes less than a predetermined threshold value th. However, other criteria can be used. For example, the successive correction can be judged to have converged when steps 515 to 523 are repeated a predetermined number of times.

[0140] Furthermore, in the above embodiment, the image reconstruction unit 8 has been described as generating a sonic velocity image as a transmitted wave image and sequentially correcting the sonic velocity image. However, an attenuation distribution image (attenuation image) may also be generated and sequentially corrected in the same manner.

[0141] Furthermore, by moving the transducer array 30 up and down (in the axial direction of the water tank 4 ) and collecting the measured sound pressure by the receiving unit 7 , the image reconstruction unit 8 can obtain a transmitted wave image of the subject 1 as a three-dimensional image.

[0142] Implementation Method 2

[0143] The ultrasonic CT apparatus according to Embodiment 2 will be described. The ultrasonic CT apparatus according to Embodiment 2 has the same configuration as that of Embodiment 1, and in addition to the same functions as those of Embodiment 1, it also includes a calibration determination function.

[0144] In FWI simulation, it is necessary to determine in advance the position (coordinates) of each of the multiple transducers 3 in the transducer array 30 and the delay time of each transducer 3, which is used to correct the transmission and reception timing of sound waves 40 caused by variations in the functions of the components that make up the transducers 3. These transducer positions and delay times are called calibration information and are determined in advance through calibration measurements. When the calibration information (transducer positions and delay times) is correct, the measured and calculated sound pressures match with high accuracy.

[0145] The ultrasonic CT apparatus of Embodiment 2 uses a calibration determination function to transmit and receive sound waves 40 to and from water without placing a subject 1, thereby measuring the measured sound pressure and comparing the measured sound pressure with the calculated sound pressure. In this case, similar to Embodiment 1, processing is performed to bring the waveform of the simulated sound wave 50 generated by the simulated sound source 53 closer to the sound wave 40, enabling accurate determination of the accuracy of the calibration information.

[0146] use Figure 7 The operation of each part of the ultrasonic CT apparatus when determining the calibration information is described in the following flow. Figure 7 In the process of Figure 6 The same steps in the process are marked with the same numbers and explained briefly.

[0147] In step 511, the subject 1 is not placed in the water tank 4, and the sound wave 40 is transmitted from the vibrator 3 to the water. In step 712, the measured sound pressure u is obtained. m And record the water temperature. Perform steps 511 and 712 for all views.

[0148] In step 714 , the speed of sound of water is calculated using the Del Gross equation or the like based on the water temperature measured in step 712 , and a speed of sound image having the calculated speed of sound values ​​is generated.

[0149] By performing steps 517 to 520 in the same manner as in the first embodiment using the generated water sound velocity image, a coefficient a(k′) is selected that makes the waveform of the sound wave 50 generated from the pseudo sound source 53 close to the sound wave 40 .

[0150] In step 721, the sound pressure a(k′)u is calculated. s The sound pressure u measured when measuring water in step 712 mThe closer the phase difference is to 0, the higher the accuracy of the calibration information can be determined, and the larger the phase difference is, the lower the accuracy of the calibration information can be determined. Therefore, if the phase difference is predetermined and is greater than the threshold value th', the process proceeds to step 722 and the calculated sound pressure a(k')u is calculated. s and measured sound pressure u m The phase difference and amplitude difference (sound pressure difference) of Figure 8 The display unit 13 displays the information in this manner, and also displays a message prompting the execution of calibration.

[0151] Thus, in the device of embodiment 2, water can be measured and the accuracy of the calibration information can be determined as described above. By performing this process regularly (for example, once a month), the user can be informed whether recalibration is required due to aging.

[0152] In the second embodiment, since it is possible to determine and calculate the sound pressure a(k′)u s and measured sound pressure u m The phase difference can also determine whether the amplitude difference (sound pressure difference) is above the threshold, so the vibrator 3 with a phase difference and / or amplitude difference above the threshold can be regarded as a degraded defective element that outputs an abnormal value, such as Figure 9 This is done to prompt the display of replacement components.

[0153] Furthermore, the output of the vibrator 3 (measured sound pressure u) of the defective component can be monitored until the component is replaced. m ) mask and not use it in image generation. Specifically, Figure 6 In the process of measuring the sound pressure u m In steps 514, 519 and 521, the sound pressure u of the vibrator 3 of the defective element is measured. m Mask it and do not use it in the calculation.

[0154] Furthermore, by performing the calibration measurement after the ultrasonic CT apparatus is installed, the accuracy of the calibration information can be determined.

Claims

1. An ultrasonic CT device, characterized in that: have: a transmitting unit configured to transmit sound waves to a subject from one or more transducers in a transducer array in which a plurality of transducers are arranged; a receiving unit that receives, from a plurality of transducers, measured sound pressures of the sound waves that have passed through an imaging region of the subject, obtained by the plurality of transducers; an image reconstruction unit that processes the measured sound pressure to generate a transmitted wave image of the imaging area; and a sequential updating unit that generates a simulated sound wave having a temporally varying sound pressure from a simulated sound source, calculates the sound pressure when the simulated sound wave passes through the imaging area and reaches a plurality of simulated detectors, and sequentially corrects the transmitted wave image using the calculated sound pressure, using the transmitted wave image as an initial image. The successive updating unit uses the calculated sound pressure to perform a calculation process to make the waveform of the simulated sound wave generated by the simulated sound source close to the waveform of the sound wave transmitted by the vibrator. As the calculation processing, the successive updating unit makes the waveform of the temporal sound pressure change of the simulated sound wave generated by the simulated sound source become a plurality of different waveforms, obtains the calculated sound pressure for each of the plurality of waveforms, and selects the waveform of the simulated sound wave used in the successive correction of the transmitted wave image from the plurality of waveforms based on the difference between the measured sound pressure and the calculated sound pressure.

2. The ultrasonic CT apparatus according to claim 1, wherein: The phases of the multiple waveforms of the simulated sound waves are different.

3. The ultrasonic CT apparatus according to claim 1, wherein: The sequential updating unit calculates a value of a predetermined function representing a difference between the measured sound pressure and the calculated sound pressure for the plurality of waveforms of the pseudo sound wave, and selects a waveform that minimizes the value of the function.

4. The ultrasonic CT apparatus according to claim 3, wherein: The sequential updating unit calculates a sum of squares of differences between the measured sound pressure and the calculated sound pressure as the predetermined function.

5. The ultrasonic CT apparatus according to claim 1, wherein: The sound pressure of the multiple waveforms of the simulated sound wave is calculated based on the sound source represented by a(k)f, wherein f is an arbitrary complex number and a(k) is a(k)=exp{i·θ k }, θ k =(2π / N)·k, k=0, 1, …, N-1, N is an arbitrary integer, i is an imaginary number, and θ is the phase.

6. The ultrasonic CT apparatus according to claim 1, characterized in that: The successive updating unit obtains a correction coefficient of sound velocity that minimizes the difference between the measured sound pressure and the calculated sound pressure when successively correcting the transmitted wave image, and successively corrects the transmitted wave image using the correction coefficient of sound velocity.

7. The ultrasonic CT apparatus according to claim 1, wherein: The sequential updating unit arranges the pseudo sound source at a position of the transducer at which the transmitting unit transmits sound waves, and arranges the pseudo detector at a position of the transducer at which the receiving unit receives the measured sound pressure.

8. The ultrasonic CT apparatus according to claim 1, wherein: The successive update unit calculates at least one of the phase difference and the sound pressure difference between the measured sound pressure measured for water without configuring the subject and the calculated sound pressure obtained for the simulated sound wave after the operation processing, wherein the waveform of the simulated sound wave is made close to the waveform of the sound wave sent by the vibrator in the operation processing.

9. The ultrasonic CT apparatus according to claim 8, characterized in that: The sequential updating unit causes a display unit to display a message prompting execution of calibration when at least one of the phase difference and the sound pressure difference is equal to or greater than a predetermined threshold value.

10. The ultrasonic CT apparatus according to claim 8, wherein: When at least one of the phase difference and the sound pressure difference is equal to or greater than a predetermined threshold, the successive updating unit masks the measured sound pressures of the phase difference and the sound pressure difference that are equal to or greater than the threshold, and does not use them in the successive correction of the transmitted wave image.

11. A method for generating an ultrasonic image, characterized in that: transmitting sound waves to the subject from one or more transducers in a transducer array in which a plurality of transducers are arranged; receiving a measured sound pressure obtained by measuring the sound pressure of the sound wave transmitted through the imaging region of the subject by a plurality of transducers, processing the measured sound pressure to generate a transmitted wave image of the imaging area, A simulated sound wave with temporally varying sound pressure is generated from a simulated sound source, and the sound pressure when the simulated sound wave passes through the imaging area and reaches the plurality of simulated detectors is calculated as a calculated sound pressure. Using the calculated sound pressure, the waveform of the simulated sound wave generated by the simulated sound source is made close to the waveform of the sound wave transmitted by the vibrator, using the calculated sound pressure of the simulated sound wave that is close to the waveform of the sound wave, and sequentially correcting the transmitted wave image with the transmitted wave image as an initial image; The waveform of the temporal sound pressure change of the simulated sound wave generated by the simulated sound source is made into a plurality of different waveforms, the calculated sound pressure is obtained for each of the plurality of waveforms, and the waveform of the simulated sound wave used in the successive corrections of the transmitted wave image is selected from the plurality of waveforms based on the difference between the measured sound pressure and the calculated sound pressure.

12. An ultrasonic image generating device, characterized in that: have: an image reconstruction unit that transmits an acoustic wave to a subject from one or more transducers of a transducer array having a plurality of transducers arranged therein, receives a measured acoustic pressure obtained by the plurality of transducers when the acoustic wave has passed through an imaging region of the subject, and processes the measured acoustic pressure to generate a transmitted wave image of the imaging region; and a sequential updating unit that generates a simulated sound wave having a temporally varying sound pressure from a simulated sound source, calculates the sound pressure when the simulated sound wave passes through the imaging area and reaches a plurality of simulated detectors, and sequentially corrects the transmitted wave image using the calculated sound pressure, using the transmitted wave image as an initial image. The successive updating unit uses the calculated sound pressure to perform a calculation process to make the waveform of the simulated sound wave generated by the simulated sound source close to the waveform of the sound wave transmitted by the vibrator. As the calculation processing, the successive updating unit makes the waveform of the temporal sound pressure change of the simulated sound wave generated by the simulated sound source become a plurality of different waveforms, obtains the calculated sound pressure for each of the plurality of waveforms, and selects the waveform of the simulated sound wave used in the successive correction of the transmitted wave image from the plurality of waveforms based on the difference between the measured sound pressure and the calculated sound pressure.

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