JP5174604B2 - Ultrasonic signal processing apparatus and method - Google Patents

Description

  The present invention relates to an ultrasonic signal processing apparatus and method capable of accurately obtaining an optimum value of an assumed sound speed.

  2. Description of the Related Art An ultrasonic signal processing apparatus that transmits an ultrasonic wave toward a subject, receives an ultrasonic echo from the subject, and displays an image (for example, a B-mode tomogram) generated based on the received signal is known. Yes.

  For example, since the time (propagation time) from transmitting an ultrasonic wave to receiving an ultrasonic echo varies depending on the depth of the reflection position in the subject, the propagation time is associated with the depth of the reflection position in the subject. At the same time, an amplitude image is generated by associating the amplitude value of the received signal with the density (or color) of the pixel.

  At this time, assuming a typical sound velocity in the subject, the propagation time is associated with the depth of the reflection position. The sound speed assumed here is generally called “assumed sound speed” (or “set sound speed”). However, since the actual sound speed in the subject varies depending on the tissue properties in the subject, it is not easy to assume a sound speed equal to the actual sound speed. If the assumed sound speed is different from the actual sound speed, the propagation time and the depth cannot be accurately associated with each other, and there is a problem that the image quality deteriorates.

Therefore, various methods for obtaining the optimum sound speed have been proposed. For example, Patent Literature 1 obtains an ultrasonic sound speed value when the amplitude of an ultrasonic reception signal is maximum, or obtains an ultrasonic sound speed value when the beam width of the ultrasonic reception signal is minimum. Thus, a technique for correcting the set ultrasonic sound velocity value by obtaining the ultrasonic sound velocity value when the high frequency component or dispersion of the spatial frequency is maximized with respect to the amplitude of the ultrasonic reception signal is disclosed.
JP-A-8-317926

  Speckle noise is generated due to countless scattering of ultrasonic echoes, but the speckle noise generation pattern differs depending on the assumed sound velocity, and the echo level varies depending on the location. It was difficult to accurately obtain the optimum value.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an ultrasonic signal processing apparatus and method that can accurately obtain the optimum value of the assumed sound speed.

  In order to achieve the above object, the invention according to claim 1 transmits an ultrasonic wave toward the subject, receives an ultrasonic echo from the inside of the subject, and receives the ultrasonic echo. An ultrasonic transmission / reception means for generating a signal, and an optimum sound speed acquisition means for acquiring an optimum sound speed by obtaining an assumed sound speed at which a high frequency component of a phase change of the received signal is maximized. A sound wave signal processing apparatus is provided.

  According to this, since the optimum sound speed is obtained by obtaining the assumed sound speed at which the high-frequency component of the phase change of the received signal of the ultrasonic echo is maximized, even if the speckle noise generation pattern differs according to the assumed sound speed Moreover, even if the echo level varies depending on the location, the optimum value of the assumed sound speed can be obtained accurately.

  According to a second aspect of the present invention, in the first aspect of the invention, the optimum sound speed acquisition unit may include, as the optimum sound speed, an arrangement direction of elements of the ultrasonic transmission / reception unit and a depth direction of the subject. A hypothetical sound speed at which a high frequency component of a phase change of the received signal in at least one of them is maximized is obtained.

  The invention according to claim 3 is the invention according to claim 1 or 2, wherein the optimum sound speed acquisition unit switches the assumed sound speed so that the sum of the magnitudes of the phase differences of the received signals is maximized. The assumed sound speed is determined as the optimum sound speed.

  According to this, the optimum sound speed can be obtained accurately and easily by determining the assumed sound speed that maximizes the sum of the magnitudes of the phase differences of the received signals as the optimum sound speed.

  According to a fourth aspect of the present invention, in the first or second aspect of the invention, the optimum sound speed acquisition unit switches the assumed sound speed to obtain an assumed sound speed at which a spatial frequency of the phase difference of the received signal is maximized. The optimum sound speed is determined.

  According to this, it is possible to accurately and easily acquire the optimum sound speed by determining the assumed sound speed at which the spatial frequency of the phase difference of the received signal is maximum as the optimum sound speed.

  The invention according to claim 5 is the invention according to claim 1, wherein the optimum sound speed acquisition means switches the assumed sound speed to extract a speckle component from the received signal, and the density of the speckle component is The maximum assumed sound speed is determined as the optimum sound speed.

  According to this, the optimum sound speed can be obtained accurately and easily by determining the assumed sound speed at which the density of the speckle component is maximum as the optimum sound speed.

  The invention according to claim 6 is the invention according to claim 5, further comprising phase difference acquisition means for acquiring a phase difference from the received signal, wherein the optimum sound speed acquisition means is based on the phase difference. It is characterized by extracting components.

  According to a seventh aspect of the present invention, in the fifth aspect of the present invention, the apparatus according to the fifth aspect further comprises amplitude acquisition means for acquiring amplitude from the received signal, and the optimum sound speed acquisition means extracts the speckle component based on the amplitude. It is characterized by doing.

  The invention according to claim 8 is the invention according to any one of claims 2, 3, 4 and 6, further comprising amplitude acquisition means for acquiring amplitude from the received signal, wherein the optimum sound speed acquisition means is The optimum sound speed is determined based on the amplitude.

  Thereby, the accuracy of the optimum sound speed can be improved by determining the optimum sound speed based also on the amplitude of the received signal.

  The invention according to claim 9 is the invention according to any one of claims 2, 3, 4, 6, and 8, wherein the optimum sound speed acquisition means is based on a phase difference in at least two directions. The optimum sound speed is obtained.

  Thereby, the accuracy of the optimum sound speed can be improved by using the phase differences in a plurality of directions.

According to a tenth aspect of the present invention, in the invention according to any one of the first to ninth aspects, the optimum sound speed acquisition means has a phase resolution in the arrangement direction of the elements of the ultrasonic transmission / reception means. The received signal that is greater than or equal to the interval is used.

  Thereby, the optimum sound speed can be determined more accurately based on the phase information of high SN.

The invention according to claim 11 is the invention according to any one of claims 1 to 10, wherein the optimum sound speed acquisition means uses the received signal obtained by one ultrasonic transmission. It is characterized by obtaining the speed of sound.

The invention according to claim 12 is the invention according to any one of claims 1 to 11, wherein the ultrasonic transmission / reception means is arranged in an array direction of elements of the ultrasonic transmission / reception means in one ultrasonic transmission. Further, it is possible to generate the reception signal having two or more sound rays.

  According to a thirteenth aspect of the present invention, in the invention according to any one of the first to twelfth aspects, an image generated at the optimum sound speed is displayed on a display means.

  According to a fourteenth aspect of the present invention, in the invention according to any one of the first to thirteenth aspects, the optimum sound speed acquisition unit is configured to perform, for each region of interest, a region of interest in a space corresponding to the subject. The optimum sound speed is obtained.

  Thereby, even when the optimum sound speed varies depending on the location (area) in the space, the optimum sound speed can be obtained for each region of interest.

  The invention according to claim 15 is the invention according to claim 14, wherein a plurality of images respectively generated at a plurality of the optimum sound speeds are displayed side by side or switched and displayed on the display means alone. And

  As a result, it is possible to easily view a plurality of images respectively generated at a plurality of optimum sound velocities, so that an appropriate diagnosis can be performed.

  A sixteenth aspect of the invention is characterized in that, in the invention of the fourteenth aspect, a plurality of images respectively generated at a plurality of the assumed sound velocities are combined and displayed on a display means.

  This makes it possible to make an appropriate diagnosis by looking at the composite image.

  The invention according to claim 17 is the optimum sound speed mode according to any one of claims 1 to 16, wherein an optimum sound speed is determined by the optimum sound speed acquisition means and an image is generated at the optimum sound speed. And mode switching means for switching between a normal assumed sound speed mode in which an image is generated at a normal assumed sound speed without determining the optimum sound speed.

  The invention according to claim 18 transmits an ultrasonic wave toward the subject, receives an ultrasonic echo from within the subject, generates a reception signal indicating the ultrasonic echo, and receives the reception signal. An ultrasonic signal processing method is provided in which an optimum sound speed of an assumed sound speed is obtained by obtaining an assumed sound speed at which the high-frequency component of the phase change becomes maximum.

  In the present specification, the “spatial phase difference” is not particularly limited to the phase difference between adjacent pixels, and may be a phase difference between pixels appropriately thinned out. Further, in the present invention, the “spatial phase difference” is not particularly limited to the phase difference between the unit pixels as described above. The phase difference in the unit length, the phase difference in the unit area, or As long as the phase difference is in unit volume, the phase difference in any unit may be used.

  According to the present invention, it is possible to accurately obtain the optimum value of the assumed sound speed (optimum sound speed).

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<Principle of the present invention>
First, the principle of the present invention will be described.

  In FIG. 1, the ultrasonic probe 20 transmits an ultrasonic wave toward the subject 90 and receives a reflected wave (hereinafter referred to as “ultrasonic echo”) from within the subject 90 to receive the ultrasonic wave. A plurality of ultrasonic transmitting / receiving elements that generate reception signals indicating echoes are arranged.

  Hereinafter, the depth direction of the subject 90 may be referred to as a “distance direction”. In addition, the direction in which the ultrasonic transmitting / receiving elements are arranged (element arrangement direction) may be referred to as “azimuth direction” or “scan direction”. In addition, the direction orthogonal to both the depth direction and the element arrangement direction, that is, the direction orthogonal to the tomographic plane may be referred to as “slice direction” or “frame direction”.

  For convenience of explanation, the ultrasonic probe 20 having one-dimensionally arranged ultrasonic transmission / reception elements will be described as an example, but the present invention can also be applied to a case where the ultrasonic transmission / reception elements are two-dimensionally arranged. . Further, the present invention is not limited to the case where the ultrasonic transmission / reception elements are arranged in a planar shape, but can also be applied to a case where the ultrasonic transmission / reception elements are arranged in an arbitrary curved shape.

  Next, the relationship between speckle noise and the phase of the received signal will be described.

  Speckle noise occurs due to random interference of ultrasonic echoes within the subject. Interference includes constructive interference and canceling interference. Constructive interference occurs when the phase difference between the waves is small, and destructive interference occurs when the phase difference is close to π.

  FIG. 2 shows an example of constructive interference. FIG. 2A shows the state before interference, and three waves with small phase differences of 0.2 (rad), 0.4 (rad), and 0.6 (rad) are used for the reference wave. To interfere. FIG. 2B shows the state after interference, and the interference wave is represented by a solid line with respect to the reference wave represented by a broken line. When the phase difference is small in this way, an intensifying interference wave is obtained. As can be seen from FIG. 2B, the peak of the peak of the interference wave is close to the peak of the peak of the reference wave, and the interference wave having a small phase difference has a small phase difference from the reference wave even after the interference.

  FIG. 3 shows an example of destructive interference. FIG. 3A shows the state before interference. In this case, a phase difference of 3.0 (rad) and a phase difference of 3.2 (rad) in addition to the phase difference of 0.2 (rad) with respect to the reference wave. Is close to π (rad), causing large waves to interfere. At this time, as shown in FIG. 3B, a destructive interference wave is obtained as represented by a solid line with respect to a reference wave represented by a broken line. When waves having such a large phase difference are caused to interfere with each other, destructive interference waves are obtained. As can be seen from FIG. 3B, the peak of the peak of the interference wave is separated from the peak of the peak of the reference wave, and the interference wave having a large phase difference has a large phase difference from the reference wave even after the interference. .

  Previously, speckle noise was said to occur due to random interference of ultrasonic echoes in the subject, but interference between waves with a large phase difference as shown in FIG. Noise will be generated. In order to determine whether or not speckle noise is generated due to interference between waves having a large phase difference, it is only necessary to look at a continuous phase change obtained from the received signal. When interference between waves having a small phase difference continues, the phase change in the received signal is small and gradual. On the other hand, when interference between waves having a large phase difference continues and speckle noise occurs, the phase change in the received signal becomes large and intense.

  Therefore, speckle noise can be discriminated based on the magnitude of the phase difference in the received signal of the ultrasonic probe 20 or the frequency of the phase difference.

  Next, a method for obtaining the optimum value of the assumed sound speed using the relationship between the speckle noise and the phase of the received signal will be described.

  Since the point spread function becomes higher as the assumed sound speed is closer to the optimum sound speed, the phase change due to interference in speckles generated as interference of innumerable point spread functions becomes larger as the assumed sound speed is closer to the optimum sound speed.

  By the way, recent software-based ultrasonic transmission / reception means stores a reception signal obtained by the ultrasonic transmission / reception element of the ultrasonic probe 20 in a memory as data (reception data), for example, the same transmission (one transmission). ), It is possible to generate reception data of two or more sound rays in the azimuth direction. In this example, the received data stored in the memory is phase-matched using the assumed sound speed, thereby generating a plurality of reception-focused ray data (received data). Thereby, adjacent sound ray data or frame reception data (also referred to as “RF data”) can be obtained at high speed.

  By performing quadrature detection on such received data, the amplitude and phase can be obtained. After quadrature detection, the temporal amplitude and phase in the distance direction are converted into a spatial amplitude and phase difference corresponding to each reflection position in the subject using the assumed sound speed. The amplitude and phase in the azimuth direction (element arrangement direction) are obtained as spatial information at the stage of orthogonal detection. The spatial amplitude and phase difference can be represented as an amplitude image and a phase difference image, respectively.

  By switching the assumed sound speed and obtaining the assumed sound speed at which the total sum of the phase differences or the spatial frequency of the phase difference is maximized, the optimum value of the assumed sound speed can be obtained. For example, it is possible to obtain the assumed sound speed by obtaining the sum of spatial phase differences or the spatial frequency from the above-described phase difference image. The kernel (attention area) for comparison is set so as to be the same area for the received data.

  In addition to the method of obtaining the optimum sound speed directly based on the magnitude of the phase difference or the spatial frequency, temporal or spatial speckle noise (speckle component) is extracted from the image, and the density of this speckle component is maximum. The optimum value of the assumed sound speed may be obtained by obtaining the assumed sound speed. For example, the assumed sound speed can be obtained based on the density of the spatial speckle component using at least one of the phase difference image and the amplitude image. In the present specification, “temporal phase change” (or “time domain phase change”) is a phase change per unit time. The “spatial phase change” (or “spatial domain phase change”) is a phase change per unit length, unit area, or unit volume. You may use the phase change of a pixel unit. The “temporal speckle component density” is the number of speckle components per unit time. “Spatial speckle component density” is the number of speckle components per unit length or unit area or unit volume. The number of speckle components in pixel units may be used.

  In short, in the present invention, the optimum sound speed is acquired by obtaining the assumed sound speed at which the high frequency component of the phase change of the received signal of the ultrasonic echo is maximized.

<Ultrasonic image processing device>
FIG. 4 is a block diagram illustrating a configuration example of an ultrasonic image processing apparatus including the ultrasonic signal processing apparatus according to the first embodiment.

  4, the ultrasonic image processing apparatus 10 mainly includes an operation unit 12, a display unit 14, an ultrasonic probe 20, a transmission / reception unit 22, a time domain signal processing unit 24, an amplitude image generation unit 26, and a phase difference image. The generation unit 28, the assumed sound speed setting unit 30, the optimum sound speed determination unit 32, the assumed sound speed storage unit 34, the image processing unit 40, and the mode switching unit 44 are configured.

  The operation unit 12 is an instruction input device for inputting a user instruction. For example, it is composed of a keyboard, a mouse and the like.

  The display unit 14 is a display device that can display an image. For example, it is configured by an LCD (liquid crystal display) or the like.

  The ultrasonic probe 20 transmits ultrasonic waves toward the subject and receives ultrasonic echoes reflected in the subject. The ultrasonic probe 20 includes, for example, a plurality of ultrasonic transducers constituting a one-dimensional ultrasonic transducer array (linear array probe), and each ultrasonic transducer is provided at both ends of a piezoelectric element such as PZT, for example. It is comprised by the vibrator | oscillator which formed the electrode.

  In addition to a linear array probe in which a plurality of ultrasonic transducers are arranged one-dimensionally, a sector probe that scans the inside of a subject in a fan shape, a convex array probe in which a plurality of ultrasonic transducers are arranged on a convex surface, or A two-dimensional array probe in which a plurality of ultrasonic transducers are two-dimensionally arranged may be used. Alternatively, a mechanical radial probe that performs radial scanning in an ultrasonic endoscope may be used.

  The transmission / reception unit 22 gives an ultrasonic transmission signal such as a Gaussian pulse to the ultrasonic probe 20 and causes the ultrasonic probe 20 to generate an ultrasonic wave.

  The ultrasonic probe 20 transmits an ultrasonic beam into the subject by driving the transmission / reception unit 22, and the subject is detected by a scanning method such as linear scanning, sector scanning, convex scanning, or radial scanning. Scan. The ultrasonic wave generated by the ultrasonic probe 20 is reflected by a reflector existing in the body of the subject, and the reflected ultrasonic wave is received by the ultrasonic probe 20. When the ultrasonic echo reflected in the subject is received by the ultrasonic probe 20, the ultrasonic probe 20 outputs a reception signal indicating the ultrasonic echo. After amplification and A (analog) / D (digital) conversion of the signal, reception focus is applied and input to the time domain signal processing unit 24. With reception focus, reception data (sound ray data) of two or more sound rays in the element arrangement direction is generated by one ultrasonic transmission. The reception focus is described in, for example, Japanese Patent Application Laid-Open No. 2008-167985. In this example, reception data obtained by one ultrasonic transmission is stored in a memory, and then the assumed sound speed is switched to generate a plurality of reception-focused sound ray data (reception data after reception focus).

  In this example, the resolution in the element arrangement direction of the reception data generated by the transmission / reception unit 22 is equal to or greater than the interval between the elements of the ultrasonic probe 20 (for example, ultrasonic transducers). As a result, the phase difference in the azimuth direction does not wrap around, and erroneous calculation of the speckle index can be prevented.

  The time domain signal processing unit 24 performs processing to acquire time domain amplitude information and time domain phase information from the received signal of the ultrasonic echo.

  The signal processing unit 24 of this example includes an orthogonal detection unit 241, an amplitude information calculation unit 242, and a phase information calculation unit 243.

  The quadrature detection unit 241 performs quadrature detection on the received signal indicating the ultrasonic echo.

  The received signal s (t) is expressed by the following equation.

[Equation 1]
s (t) = u (t) × cos (ωt + φ (t))
Here, u (t) indicates the amplitude, and φ (t) indicates the phase.

  The received signal s (t) is separated into a real component I (cos component) and an imaginary component Q (sin component) by quadrature detection, as shown in the following equation.

[Equation 2]
I = u (t) cos (φ (t))
Q = u (t) sin (φ (t))
The real component is also called I component and the imaginary component is also called Q component.

  The amplitude information calculation unit 242 calculates the amplitude u (t) by the following equation based on the I component and Q component obtained by the quadrature detection unit 241.

[Equation 3]
u (t) = √ (I ² + Q ² )
The phase information calculation unit 243 calculates the phase φ (t) by the following equation based on the I component and Q component obtained by the quadrature detection unit 241.

[Equation 4]
φ (t) = tan ^-1 I / Q
The quadrature detection unit 241 and the amplitude information calculation unit 242 constitute temporal amplitude acquisition means for acquiring u (t) as amplitude information in the time domain. Further, the quadrature detection unit 241 and the phase information calculation unit 243 constitute temporal phase acquisition means for acquiring φ (t) as phase information in the time domain.

  The amplitude image generation unit 26 generates an amplitude image (B-mode tomographic image) indicating the amplitude of the ultrasonic echo from each reflection position in the subject. In the distance direction, the amplitude information in the time domain is converted into amplitude information in the spatial domain using the assumed sound speed. Specifically, since the time (propagation time) from the transmission of an ultrasonic pulse to the reception of an ultrasonic echo pulse varies depending on the depth of the reflection position, the propagation time is calculated using the assumed sound velocity. An amplitude image is generated by associating the value of the reflection position with the pixel value (indicating density or color) and associating the value with the amplitude.

  The phase difference image generation unit 28 generates a phase difference image indicating the phase difference of the ultrasonic echo from each reflection position in the subject. In the distance direction, the phase information in the time domain is converted into phase difference information in the spatial domain using the assumed sound velocity. Specifically, since the time (propagation time) from the transmission of an ultrasonic pulse to the reception of an ultrasonic echo pulse varies depending on the depth of the reflection position, the propagation time is calculated using the assumed sound velocity. A phase difference image is generated by associating the value of the phase difference with the pixel value (indicating density or color).

  Note that when speckle noise exists parallel to either the azimuth direction or the distance direction, the phase change is small in that direction, but the phase change is large in the direction orthogonal thereto, so the phase difference in the azimuth direction and the distance direction It is preferable to obtain both of the phase differences. Two-dimensional phase difference information may be obtained.

  The amplitude image generation unit 26 constitutes a spatial amplitude acquisition unit that acquires amplitude information in the spatial region. In addition, the phase difference image generation unit 28 constitutes a spatial phase difference acquisition unit that acquires phase difference information in the spatial domain.

  The assumed sound speed setting unit 30 sets the assumed sound speed by giving the assumed sound speed to the transmission / reception unit 22, the amplitude image generation unit 26, and the phase difference image generation unit 28.

  The optimum sound speed determination unit 32 switches the assumed sound speed to be given to the transmission / reception unit 22, the amplitude image generation unit 26, and the phase difference image generation unit 28 via the assumed sound speed setting unit 30, and assumes that the high frequency component of the phase change is maximized. The optimum sound speed is obtained by obtaining the sound speed.

  Although it is possible to determine the optimum sound speed based only on the phase difference or the optimum sound speed based only on the amplitude, it is preferable to determine the optimum sound speed based on both the phase and the amplitude. . By using both the phase and amplitude, a more accurate determination can be made.

  The assumed sound speed storage unit 34 is a non-volatile memory that stores the assumed sound speed.

  The image processing unit 40 performs various image processes on the amplitude image and the phase difference image. The image processing unit 40 has a function of synthesizing the amplitude image and the phase difference image. For example, the amplitude image and the phase difference image are superimposed by modulating the luminance or color of the amplitude image with the phase difference image.

  The display control unit 42 selects an arbitrary image from an amplitude image, a phase difference image, and a composite image thereof according to an instruction from a mode switching unit 44 described later, performs enlargement / reduction processing and layout processing, and displays the display unit. 14 There are various combinations of images to be displayed. For example, the amplitude image and the phase difference image are arranged and displayed on the display unit 14. Along with the amplitude image, a composite image of the amplitude image and the phase difference image may be displayed side by side. Only the amplitude image or only the composite image may be displayed.

  The mode switching unit 44 has a function of switching the optimum sound speed acquisition mode in addition to the function of switching the display mode. Specifically, the optimum sound speed is determined by the optimum sound speed determination unit 32 and the phase difference image and the amplitude image are generated based on the optimum sound speed, and the assumed sound speed storage unit 34 is determined without determining the optimum sound speed. The normal assumed sound speed mode for generating the phase difference image and the amplitude image is switched according to the normal assumed sound speed stored in advance.

  In the optimum sound speed mode of this example, the same received signal obtained from the transmission of the same ultrasonic wave (one transmission) by the ultrasonic probe 20 is used, and the assumed sound speed is sequentially switched from among a plurality of assumed sound speeds. The optimum sound speed is obtained from a plurality of assumed sound speeds. Specifically, the optimum sound speed determination unit 32 switches the assumed sound speed to be given to the transmission / reception unit 22, the phase difference image generation unit 28, and the amplitude image generation unit 26 via the assumed sound speed setting unit 30.

  In this example, the resolution of the phase in the arrangement direction of the elements of the ultrasonic probe 20 is equal to or greater than the interval of the elements of the ultrasonic probe 20. That is, the resolution in the element arrangement direction of the phase difference image generated by the phase difference image generation unit 28 is equal to or greater than the interval of the elements (for example, ultrasonic transducers) of the ultrasonic probe 20.

  The time domain signal processing unit 24, the amplitude image generation unit 26, the phase difference image generation unit 28, the assumed sound speed setting unit 30, the optimum sound speed determination unit 32, the image processing unit 40, the display control unit 42, and the mode switching unit 44 are For example, it includes a CPU (Central Processing Unit). Some of these may be configured by a circuit.

  Although the case where the phase difference image is generated by the phase difference image generation unit 28 as information indicating the phase difference corresponding to each reflection position in the subject has been described as an example, the present invention generates a phase difference image. In this case, there is no particular limitation, and invisible information may be generated instead of a visible image. Similarly, the case where an amplitude image is generated by the amplitude image generation unit 26 as information indicating the spatial amplitude corresponding to each reflection position in the subject has been described as an example, but the present invention generates an amplitude image. In this case, there is no particular limitation, and invisible information may be generated instead of a visible image.

  Hereinafter, the optimum sound speed acquisition will be described in various embodiments.

<First Embodiment>
In the first embodiment, the optimum sound speed determination unit 32 in FIG. 4 switches the assumed sound speed among a plurality of assumed sound speeds, and assumes the assumed sound speed that maximizes the sum of the magnitudes of the phase differences as the optimum sound speed. judge. FIG. 5 is a flowchart showing an exemplary flow of the optimum sound speed acquisition process in the first embodiment. This process is mainly executed by the CPU constituting the optimum sound speed determination unit 32 in FIG. 4 according to a program.

  In step S10, an initial value of assumed sound speed is set. For example, the assumed sound speed setting unit 30 reads the initial value of the assumed sound speed from the assumed sound speed storage unit 34, and sets the initial value of the assumed sound speed in the transmission / reception unit 22, the amplitude image generation unit 26, and the phase difference image generation unit 28.

  In step S11, the transmission / reception unit 22 performs reception focus, and the amplitude image generation unit 26 generates an amplitude image based on the amplitude information obtained from the reception data. Further, the phase difference image generation unit 28 generates a phase difference image based on the phase difference information obtained from the received data. For the distance direction, spatial amplitude information is generated from temporal amplitude information, and spatial phase difference information is generated from temporal phase information.

  In step S12, the sum of the magnitudes (absolute values) of the temporal or spatial phase differences is calculated. Here, the sum of the spatial phase differences may be calculated for the entire phase difference image, or the sum of the spatial phase differences may be calculated for the region of interest in the phase difference image. . That is, the sum total may be obtained for the entire space area, or the sum may be obtained for the region of interest in the entire space area.

  In step S14, it is determined whether or not all assumed sound speeds have been completed. For example, the assumed sound speed setting unit 30 reads the end value of the assumed sound speed from the assumed sound speed storage unit 34, and compares the current assumed sound speed with the end value. If not completed, the process proceeds to step S15. If completed, the process proceeds to step S16.

  In step S15, the assumed sound speed is changed by one step, and the process returns to step S11.

  When all the assumed sound velocities are completed, the optimum sound speed is determined from the change in the sum of the magnitudes of the phase differences due to the switching of the assumed sound velocities in step S16. Specifically, as shown in FIG. 6, it is determined that the assumed sound speed at which the sum of the magnitudes of the phase differences is maximum is the optimum sound speed.

  In this example, the optimum sound speed is determined based on the sum of absolute values of spatial phase differences in the element arrangement direction. Depending on the types of the ultrasound probe 20 and the transmission / reception unit 22, the optimum sound speed may be determined based on the sum of absolute values of the phase difference in the depth direction of the object, or the depth direction and the element. The optimum sound speed may be determined based on the sum of the square roots of the square sums of the phase differences in both directions of the arrangement direction. Alternatively, the optimum sound speed may be determined based on the sum of absolute values of two-dimensional phase differences.

Second Embodiment
In the second embodiment, the optimum sound speed determination unit 32 in FIG. 4 switches the assumed sound speed among a plurality of assumed sound speeds, and the assumed sound speed at which the spatial frequency of the phase difference of the received data is maximized is the optimum sound speed. Is determined. FIG. 7 is a flowchart showing an exemplary flow of the optimum sound speed acquisition process in the second embodiment. This process is mainly executed by the CPU constituting the optimum sound speed determination unit 32 in FIG. 4 according to a program.

  Steps S20 and S21 are the same as steps S10 and S11 in the first embodiment shown in FIG.

  In step S22, the spatial frequency of the phase difference of the received data is calculated. Here, the spatial frequency may be obtained from the entire phase difference image, or the spatial frequency may be obtained from the region of interest in the phase difference image. That is, the spatial frequency may be obtained for the entire spatial region, or the spatial frequency may be obtained for the region of interest in the entire spatial region. In this example, the phase difference spatial frequency indicates the frequency (number of waves) of change in phase difference per unit area (or unit length).

  In step S24, it is determined whether or not all assumed sound speeds have been completed. If not completed, the process proceeds to step S25. If completed, the process proceeds to step S26.

  In step S25, the assumed sound speed is changed by one step, and the process returns to step S21.

  When all the assumed sound velocities are completed, the optimum sound speed is determined from the change in the spatial frequency of the phase difference due to the switching of the assumed sound speed in step S26. Specifically, as shown in FIG. 8, the assumed sound speed at which the spatial frequency of the phase difference is maximum is determined as the optimum sound speed.

  Note that the optimum sound speed may be determined based on one of the spatial frequency of the phase difference in the depth direction of the subject and the spatial frequency of the phase difference in the arrangement direction of the elements of the ultrasonic transmission / reception means. Further, the optimum sound speed may be determined based on the spatial frequency of the two-dimensional phase difference.

<Third Embodiment>
In the third embodiment, the optimum sound speed determination unit 32 in FIG. 8 switches the assumed sound speed among a plurality of assumed sound speeds, extracts the speckle component from the received data of the ultrasonic echo, and the density of the speckle component Is assumed to be the optimum sound speed. There are various modes for extracting speckle components from received data of ultrasonic echoes.

First, there is an aspect in which a spatial position where speckle is generated is determined based on a spatial phase difference of received data. In this aspect, the position where the phase suddenly changes spatially is determined as the speckle position. For example, in the phase difference image, pay attention to each pixel sequentially, and the vertical phase difference Δφ (y) corresponding to the depth direction (distance direction) and the horizontal direction position corresponding to the element arrangement direction (azimuth direction). A pixel position (x, y) at which the square root of the sum of squares with the phase difference Δφ (x) (√ (Δφ (y) ² + Δφ (x) ² )) exceeds the threshold is determined as a speckle position. Instead of using the phase difference of each pixel as it is, a determination is made using a representative value (for example, an average value or a phase difference with the highest frequency) of the phase difference in a fixed size area centered on each pixel. May be. Further, the speckle position may be determined based on the phase difference of one of the vertical direction and the horizontal direction, or based on the larger phase difference of the vertical direction and the horizontal direction, The speckle position may be determined.

  Second, there is an aspect in which the spatial position where speckle is generated is determined based on the spatial amplitude of the received data. For example, in the amplitude image, attention is sequentially paid to each pixel, and a pixel position having an amplitude difference with respect to surrounding pixels and having a smaller amplitude difference than a threshold is determined as a speckle position. Instead of using the amplitude of each pixel as it is, the determination may be made by using a representative value (for example, an average value or an amplitude having the maximum frequency) of the amplitude in a region of interest having a fixed size centered on each pixel. . The speckle position may be determined based on the amplitude of one of the vertical direction and the horizontal direction, and the speckle position may be determined based on the larger amplitude of the vertical direction and the horizontal direction. May be determined.

Third, there is an aspect in which extraction is performed based on both the spatial phase difference and the spatial amplitude of the received data. Specifically, for example, the phase difference is multiplied by a coefficient corresponding to the amplitude difference, and the speckle position is determined based on the result.
As speckle discrimination, binary discrimination as to whether speckle noise is present or multi-level discrimination as to how much speckle noise is included may be performed, and the result may be output. The phase change amount itself may be used as data indicating the speckle-likeness.

  Further, speckle discrimination may be performed using a discriminant function for discriminating whether or not it is speckle noise. FIG. 9 is a flowchart showing an exemplary flow of the optimum sound speed acquisition process in the third embodiment. This process is mainly executed by the CPU constituting the optimum sound speed determination unit 32 in FIG. 4 according to a program.

  Steps S30 and S31 are the same as steps S10 and S11 in the first embodiment shown in FIG.

  In step S32, speckle noise is extracted. For example, pay attention to each pixel sequentially in the phase difference image, multiply the value of the target pixel (spatial phase difference) by a coefficient corresponding to the amplitude difference, and determine the position of the target pixel whose result is larger than the threshold value. Determined as speckle position. In this case, it is determined that the speckle degree is higher as the spatial phase difference is larger and the speckle degree is higher as the amplitude difference is smaller. Here, the amplitude difference is the value (spatial amplitude) and amplitude of the pixel of interest in the amplitude image when attention is paid to the pixel in the amplitude image at a position corresponding to the position of the pixel of interest in the phase difference image. This is the difference from the values of surrounding pixels in the image.

  In step S33, the density of speckle noise in the kernel of a predetermined size is calculated. Specifically, the density of speckle noise per unit area in a fixed area of a certain size in the phase difference image is calculated. For example, the number of pixels (speckle pixels) determined to be speckle positions is counted. There is also a method of taking the sum of speckle degrees (speckle-likeness). The kernels used for comparison are set so that the received data is in the same area.

  In step S34, it is determined whether or not all assumed sound speeds have been completed. If not completed, the process proceeds to step S35, and if completed, the process proceeds to step S36.

  In step S35, the assumed sound speed is changed by one step, and the process returns to step S31.

When all the assumed sound velocities are completed, the optimum sound speed is determined from the change in the density of speckle noise due to the switching of the assumed sound speed in step S36. Specifically, as shown in FIG. 10, it is determined that the assumed sound speed at which the density of speckle noise is maximum is the optimum sound speed.
Note that the optimum sound speed determination unit 32 may extract a speckle component based only on the phase difference. Further, the speckle position may be determined based on one of the phase difference in the depth direction of the subject and the phase difference in the arrangement direction of the elements of the ultrasonic transmission / reception means. Further, the optimum sound speed may be determined based on a two-dimensional phase difference.

  Further, the optimum sound speed determination unit 32 may determine the speckle position based only on the amplitude. In addition, the case where the optimal sound speed is acquired using the total sum of the magnitudes of the phase differences in the first embodiment will be described, and the case where the optimal sound speed is acquired using the high frequency components of the phase differences in the second embodiment will be described. In the third embodiment, the case where the optimum sound speed is acquired using the density of the speckle component has been described. However, these (“the sum of the magnitudes of the phase difference”, “the high frequency component of the phase difference”, “the speckle”). The optimum sound speed may be acquired by a combination of “component density”).

<Fourth embodiment>
In the fourth embodiment, the optimum sound speed is obtained for each ROI for a plurality of ROIs (also referred to as “regions of interest”) in the space corresponding to the subject, and a plurality of images respectively generated by the plurality of optimum sound speeds are synthesized. Then, a composite image is displayed.

  FIG. 11 is a flowchart illustrating an exemplary flow of the image generation process. This processing is executed according to a program mainly by the CPU that constitutes the optimum sound speed determination unit 32, the amplitude image generation unit 26, the phase difference image generation unit 28, and the image processing unit 40 of FIG.

  In step S41, the optimum sound speed in each ROI is acquired mainly by the optimum sound speed determination unit 32.

  For example, as shown in FIG. 12A, it is assumed that a plurality of ROIs (401 to 404) are set in a space corresponding to the subject (hereinafter referred to as “image space”). Then, the assumed sound speed is switched for each ROI (401 to 404), and the optimum sound speed for each ROI (401 to 404) is acquired. In addition, since the process which calculates | requires the optimal sound speed for every ROI should just use either of the processes demonstrated in 1st Embodiment-3rd Embodiment, the description is abbreviate | omitted.

  In step S42, the amplitude image generation unit 26 acquires an amplitude image corresponding to each optimum sound speed. Specifically, the amplitude image is generated by converting temporal amplitude information into spatial amplitude information for each optimum sound speed.

  For example, as shown in FIG. 12B, amplitude images 411 to 414 (overall images) respectively corresponding to the optimum sound speeds of the ROIs (401 to 404) are acquired. Only the partial images 421 to 424 of each ROI may be generated.

  In step S43, the image processing unit 40 synthesizes an amplitude image corresponding to each optimum sound speed value around each ROI position.

  For example, as illustrated in FIG. 12C, the composite image 430 is generated by combining the partial images 421 to 424 of each ROI. Alternatively, the weights of the partial images 421 to 424 of each ROI may be increased and the weights of the non-ROI portions 431 to 434 may be decreased in the entire images 411 to 411 to perform the synthesis.

  In step S44, various types of image processing (for example, logarithmic compression, gain, DR, STC, gray map adjustment, scan conversion, etc.) are performed on the composite image 430.

  For convenience of explanation, the case where the space is equally divided into a plurality of ROIs has been described as an example. However, as shown in FIG. 13, the ROIs (501 to 504) can be freely set. For example, an amplitude image generated at a normal assumed sound speed is displayed on the display unit 14, the user sets an ROI of reference numerals 501 to 503 by the operation unit 12 while viewing the display unit 14, and the remaining area is set as an area of reference numeral 504. To do.

  There are various types of display control by the display control unit 42, and a plurality of images (for example, an amplitude image and a phase difference image) respectively generated at a plurality of optimum sound velocities are arranged side by side or switched to be displayed independently. May be. For example, the amplitude images 411 to 414 generated at the optimum sound speeds shown in FIG. 12B are displayed side by side on the screen of the display unit 14 or switched and displayed alone.

<Determination of speckle>
Below, the specific example of speckle discrimination is demonstrated.

  The discriminant function is created based on a feature quantity created from the phase change amount (phase difference), for example, by preparing spatial phase information in which the position of speckle or non-speckle is known in advance. That is, a predetermined feature amount is calculated from the phase change data that is known to be speckle and the phase change data that is known to be non-speckle, and a speckle discriminant function is created therefrom. The

  Here, the feature amount may be a single pixel of phase change amount data. However, in the case of a single pixel, when the phase change amount is obtained by taking the difference, a slight pixel shift occurs. When the phase change amount is obtained from the vertical and horizontal directions, the phase of the center of the crossing portion is obtained. Since there is a problem that the change does not increase, it is desirable to use a plurality of data such as the value of the neighboring pixel of the target pixel and the calculation result with the neighboring pixel. At this time, since a plurality of data are multidimensional, the dimension may be lowered by performing PCA (principal component analysis) or the like so that the threshold value can be easily designed.

  FIG. 14 shows an example of the speckle discriminant function.

  Here, the vertical direction phase change amount and the horizontal direction phase change amount are defined as a feature amount (1) and a feature amount (2), respectively, non-speckle noise is represented by ◯, and speckle noise is represented by x. In the example shown in FIG. 14, the discriminant function is set as a straight line that separates the non-speckle noise O and speckle noise x regions. A function (or threshold) for discriminating between speckle noise and non-speckle noise is designed based on the result converted into the feature quantity in this way. The discriminant function is not limited to such a linear function.

  The discriminant function setting method is not particularly limited. For example, a statistical method using known data (learning data) such as SVM (support vector machine) (for example, Nero Christianini as a reference). And a linear or non-linear discriminant function used for known classification such as “Introduction to Support Vector Machine” by John Taylor, etc.). Of course, speckle may be determined only by the threshold value if it can be determined only by giving a threshold value for each feature amount. In addition, a discrimination function may be set with data representing a phase change, such as pixels of continuous phase data, as a feature quantity without being converted into a phase change amount.

  FIG. 15 shows an example of discriminant function generation processing for speckle extraction using SVM (support vector machine).

  As shown in FIG. 15, first, speckle portions and non-speckle portions are manually labeled from the phantom image, and known data for creating a speckle discriminant function is created. Note that when labeling speckles and non-speckle parts, labeling is not performed for ambiguous parts. Next, a feature amount used for discrimination of speckle is extracted from the speckle portion and the non-speckle portion of the known data.

  Here, as a feature amount, as shown in FIG. 16, a central pixel c of 3 × 3 pixels is set as a target pixel, and the target pixel c and four neighboring pixels a, b, d, and e above, below, left, and right are vertically A plurality of feature quantities of (distance) direction phase change and lateral (azimuth) direction phase change are used.

  Next, an SVM (support vector machine) is applied using a plurality of phase change amounts of the labeled pixels (the target pixel and its vicinity) in the vertical direction and the horizontal direction as feature amounts, and a discriminant function (speckle discriminator) is applied. ) Is generated. Of course, the data and feature quantities used for labeling may be changed so that the discrimination result is optimal.

  In this way, a speckle discriminant function is created by extracting feature amounts from phase data whose speckle or non-speckle positions are known in advance. Then, speckle extraction is performed based on the spatial phase information.

  For example, when the phase difference in the azimuth (transverse) direction and the phase difference in the distance (vertical) direction are input, the optimum sound speed determination unit 32 converts the phase difference into a feature amount used for speckle discrimination and is created in advance. It is discriminated whether it is speckle using a speckle discriminant function, and speckle extraction is performed.

  The discrimination result may not only indicate whether the speckle is binary, but also output the difference from the threshold as speckle-likeness indicating how much speckle noise is included in multiple values. . In the case of multi-value output, the value may be further adjusted using a LUT (Look Up Table) or the like.

  Note that the speckle discriminant function changes depending on the condition of ultrasonic transmission / reception, and therefore, in the case of an actual apparatus, it is desirable to set the discriminant function for each condition.

  In the example described above, speckle components are extracted using phase information instead of amplitude information. Depending on the shape of the speckle and the echo level, it may be difficult to distinguish the speckle because the amplitude may not be able to be extracted accurately, but by using the phase information, the speckle shape and surrounding It is possible to extract speckle components independent of the echo level.

  In this way, speckle components can be extracted using only phase information. However, speckle components can be extracted by combining amplitude information (amplitude component or its envelope component) and phase information, such as adding amplitude information to feature quantities. You may make it do. In this case, phase information (the phase difference in the azimuth direction and the phase difference in the distance direction are used, but spatial amplitude information is also used together with the spatial phase difference information, and the amplitude information (pixel value) is added to the feature amount. Dimension (2 dimensions in the example shown in FIG. 16) may be increased to perform speckle extraction.

  Thus, speckle extraction using both amplitude information and phase information enables more accurate speckle extraction.

  17A shows an example of a phase difference image when the assumed sound speed is slower than the optimum sound speed, FIG. 17B shows an example of a phase difference image when the assumed sound speed is the optimum sound speed, and FIG. c) shows an example of a phase difference image when the assumed sound speed is faster than the optimum sound speed. In any of the first to third embodiments described above, a phase difference image as shown in FIG. 17B is obtained at the optimum sound speed. An appropriate diagnosis can be performed by displaying the phase difference image obtained at such an optimum sound speed alone or by composing or switching with another image (for example, an amplitude image). As described above, for convenience of explanation, the ultrasonic probe 20 having the one-dimensionally arranged ultrasonic transmitting / receiving elements has been described as an example. However, the present invention can also be applied to the case where the ultrasonic transmitting / receiving elements are two-dimensionally arranged. .

  The present invention is not limited to the examples described in the present specification and the examples illustrated in the drawings, and various design changes and improvements may be made without departing from the spirit of the present invention.

Explanatory drawing used to explain the positional relationship between the ultrasound probe and the subject It is a graph which shows the example of constructive interference, (a) is a graph showing before interference, (b) is a graph showing after interference. It is a graph which shows the example of destructive interference, (a) is a graph showing before interference, (b) is a graph showing after interference. The block diagram which shows the structural example of the ultrasonic image processing apparatus containing the ultrasonic signal processing apparatus which concerns on this invention The flowchart which shows the flow of an example of the optimal sound speed acquisition process in 1st Embodiment. Explanatory drawing used for explanation of determination of optimum sound speed in the first embodiment The flowchart which shows the flow of an example of the optimal sound speed acquisition process in 2nd Embodiment. Explanatory drawing used for explanation of determination of optimum sound speed in the second embodiment The flowchart which shows the flow of an example of the optimal sound speed acquisition process in 3rd Embodiment. Explanatory drawing used for explanation of determination of optimum sound speed in the third embodiment The flowchart which shows the flow of an example of the image generation process in 4th Embodiment. (A) is an explanatory diagram used to describe an example of an ROI, (b) is an explanatory diagram used to describe a plurality of images generated at the optimum sound speed for each ROI, and (c) is an explanatory diagram of a composite image Illustration used Explanatory drawing used to explain freely set ROI Explanatory drawing showing an example of speckle discriminant function Explanatory drawing which shows an example of the generation process of a speckle discriminant function Explanatory drawing used to explain feature quantities for speckle discrimination FIG. 17A is an explanatory diagram illustrating an example of a phase difference image when the assumed sound speed is slower than the optimum sound speed, and FIG. 17B is an explanatory diagram illustrating an example of a phase difference image when the assumed sound speed is the optimum sound speed. FIG. 17C is an explanatory diagram illustrating an example of a phase difference image when the assumed sound speed is higher than the optimum sound speed.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Ultrasonic image processing apparatus, 12 ... Operation part, 14 ... Display part, 20 ... Ultrasonic probe, 22 ... Transmission / reception part, 24 ... Time domain signal processing part (Temporal amplitude acquisition part, Temporal phase acquisition part) ), 26... Amplitude image generation unit (spatial amplitude acquisition unit), 28... Phase difference image generation unit (spatial phase difference acquisition unit), 30... Assumed sound speed setting unit, 32. Storage unit 40 ... Image processing unit 42 ... Display control unit 44 ... Mode switching unit

Claims (18)

An ultrasonic transmission / reception means for transmitting an ultrasonic wave toward the subject, receiving an ultrasonic echo from within the subject, and generating a reception signal indicating the ultrasonic echo;
Optimum sound speed acquisition means for acquiring the optimum sound speed by obtaining an assumed sound speed at which the high frequency component of the phase change of the received signal is maximized;
An ultrasonic signal processing apparatus comprising:   The optimum sound speed acquisition means assumes that the optimum sound speed has a maximum high-frequency component of the phase change of the received signal in at least one of the arrangement direction of the elements of the ultrasonic transmission / reception means and the depth direction of the subject. The ultrasonic signal processing apparatus according to claim 1, wherein a speed of sound is obtained.   The said optimal sound speed acquisition means switches the said assumed sound speed, and determines the assumed sound speed that maximizes the sum of the magnitudes of the phase differences of the received signals as the optimum sound speed. Ultrasonic signal processing device.   The ultrasonic wave according to claim 1 or 2, wherein the optimum sound speed acquisition means determines the assumed sound speed at which the spatial frequency of the phase difference of the received signal is maximum by switching the assumed sound speed as the optimum sound speed. Signal processing device.   The optimum sound speed acquisition means switches the assumed sound speed, extracts a speckle component from the received signal, and determines an assumed sound speed at which the density of the speckle component is maximum as an optimum sound speed. 2. The ultrasonic signal processing apparatus according to 1. Comprising phase difference acquisition means for acquiring a phase difference from the received signal;
The ultrasonic signal processing apparatus according to claim 5, wherein the optimum sound speed acquisition unit extracts the speckle component based on the phase difference. Amplitude acquisition means for acquiring amplitude from the received signal,
The ultrasonic signal processing apparatus according to claim 5, wherein the optimum sound speed acquisition unit extracts the speckle component based on the amplitude. Amplitude acquisition means for acquiring amplitude from the received signal,
The ultrasonic signal processing apparatus according to claim 2, wherein the optimal sound speed acquisition unit determines the optimal sound speed based on the amplitude.   The said optimal sound speed acquisition means calculates | requires the said optimal sound speed based on the phase difference in an at least 2 or more direction, The any one of Claim 2, 3, 4, 6 and 8 characterized by the above-mentioned. Ultrasonic signal processing device. The optimal sound speed acquisition unit uses the received signal whose phase resolution in the arrangement direction of the elements of the ultrasonic transmission / reception unit is equal to or greater than the interval between the elements. The ultrasonic signal processing device according to item. The ultrasonic processing apparatus according to claim 1, wherein the optimum sound speed acquisition unit obtains the optimum sound speed using the received signal obtained by one ultrasonic transmission. . The ultrasonic transmission / reception means can generate the reception signals of two or more sound rays in the arrangement direction of the elements of the ultrasonic transmission / reception means by one ultrasonic transmission. The ultrasonic signal processing apparatus of any one of Claims.   13. The ultrasonic signal processing apparatus according to claim 1, wherein an image generated at the optimum sound speed is displayed on a display unit.   The supersonic sound acquisition means obtains the optimal sound speed for each region of interest for a region of interest in a space corresponding to the inside of the subject. Sonic signal processing device.   The ultrasonic signal processing apparatus according to claim 14, wherein a plurality of images respectively generated at a plurality of the optimum sound speeds are displayed side by side or switched and displayed alone on a display unit.   The ultrasonic signal processing apparatus according to claim 14, wherein a plurality of images respectively generated by the plurality of assumed sound speeds are synthesized and displayed on a display unit.   An optimal sound speed mode in which an optimal sound speed is determined by the optimal sound speed acquisition means and an image is generated based on the optimal sound speed, and a normal assumed sound speed mode in which an image is generated based on a normal assumed sound speed without determining the optimal sound speed. The ultrasonic signal processing apparatus according to any one of claims 1 to 16, further comprising mode switching means for switching. While transmitting an ultrasonic wave toward the subject, receiving an ultrasonic echo from within the subject, and generating a reception signal indicating the ultrasonic echo,
An ultrasonic signal processing method, wherein an optimum sound speed of an assumed sound speed is obtained by obtaining an assumed sound speed at which a high frequency component of a phase change of the received signal is maximized.