JP2901048B2 - Image signal processing method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an ultrasonic CT (co
The present invention relates to an image signal processing method used for mputer tomography and a device for implementing the method.
In particular, the present invention relates to a technique for generating a distribution image of a medium boundary between a fluid and an elastic body.

[0002]

2. Description of the Related Art In recent years, an internal tissue such as a living body or wood has been observed from the outside using an ultrasonic signal, and a signal obtained by the observation has been obtained as one signal.
Various techniques for converting into a dimensional or multidimensional image signal for visualization have been proposed. Such techniques are commonly referred to as ultrasonic CT
It has been demonstrated that the living body is safer than X-ray CT or the like. A device that actually generates and displays a tomographic image of an internal tissue based on the image signal is called a CT scanner. The CT scanner includes a transmitter that transmits an ultrasonic signal to a sound field (a region where a sound wave acts, the same applies hereinafter) and a plurality of receivers that receive scattered wave signals scattered in the sound field. After generating a one-dimensional signal based on the attribute of the scattered wave signal received at each angle, for example, sound pressure, a waveform set of these one-dimensional signals is converted into a two-dimensional signal by a Fourier transform method, a convolution integration method, or the like. The conversion is performed, and a two-dimensional tomographic image is generated based on the conversion. Recently, a large number of two-dimensional tomographic images are generated while the CT scanner is gradually moved in parallel along the axis of a living body or the like, and these are three-dimensionally stacked to construct a three-dimensional tomographic image of the internal tissue. It has also been done.

[0003]

By the way, since the wavelength of an ultrasonic signal is several orders of magnitude longer than that of an X-ray or the like, wave characteristics appear more remarkably, and diffraction and Susceptible to scattering. Therefore, the finally generated tomographic image may be distorted or blurred and unclear. To cope with this, a high-frequency ultrasonic signal having excellent straightness may be used. However, there is a drawback in that as the frequency increases, attenuation in internal tissues becomes remarkable and it is difficult to reach a deep part. Further, since a single pulse (impulse) is usually used, a good S /
In order to obtain an N ratio (signal-to-noise ratio), it is necessary to increase the peak sound pressure. However, it has been pointed out that an excessively high sound pressure of an ultrasonic signal adversely affects tissues such as a living body. I have.

[0004] Therefore, in the conventional ultrasonic CT or CT scanner, there is a limit in increasing the frequency and the sound pressure.
Improvement was desired. In view of the background, an object of the present invention is to provide an image signal processing method capable of obtaining a clear tomographic image free of distortion and blur while using an ultrasonic signal at a frequency and a sound pressure that does not adversely affect living tissue at both frequencies and sound pressures. It is to provide a device.

[0005]

According to the image signal processing method provided by the present invention, an ultrasonic signal is transmitted to a sound field where an object to be observed exists, and a plurality of scattered wave signals scattered at a medium boundary of the object to be observed. The distribution of the medium boundary
A method performed in an apparatus for generating an image, the method comprising :
It generates a difference wave signal with attributes of the scattered wave signals if the attribute and the sound field of each scattered wave signal is homogeneous received, of the signal generated differential wave signal by back propagation using TLM method composite amplitude detects a point where the maximum, and wherein the benzalkonium to generate a pre-Symbol distribution image based on the position information of the detected point.

Here, the TLM method expresses a sound field by an equivalent grid-like transmission line network composed of a coil and a capacitor, and expresses a wave phenomenon based on the Huygens principle on a pulse wave transmitted on the transmission line network. This is a method of tracking as a transmission / scattering problem, and has a feature that a time domain solution can be directly obtained by simple calculation without requiring time integration. The specific contents of this TLM method are described in Fujii, Tsuchiya, Kagawa, Journal of the Japan Society for Simulation Technology, 11th Simulation Technology Conference (1992.06), pp. 135-138, “Transmission of sound waves using the transmission line network (TLM) method”. Transient analysis "and Yukio Kagawa, Ultrasonic TECHN
O12 issue "Numerical Analysis Method Overview" (Nippon Kogyo Publishing Co., Ltd., 1
993, 12).

[0007] Another image signal processing method provided by the present invention is to transmit an ultrasonic signal to a sound field where an object to be observed exists.
Both generate a distribution image of the medium boundary by receiving a plurality of scattered wave signals scattered at the medium boundary of the object to be observed.
A method performed in an apparatus for, each scattered wave signals received to detect the respective signal intensity distribution by back propagation using TLM method is homogeneous the signal intensity distribution and the sound field detected the difference distribution is derived in the signal intensity distribution of the scattered wave signal when, characterized by the Turkey to generate a pre-Symbol distribution image based on the difference distribution.

In the image signal processing method, the ultrasonic signal and the scattered wave signal are formed to include a burst-like signal, and the burst-like signal is used to detect the object to be observed.
It is also possible to generate a distribution image considering the moving speed.
No. The attribute of the scattered wave signal includes, for example, at least one of the sound pressure, the displacement, the velocity, and the density of the scattered wave signal. The acoustic impedance can also be determined based on speed and density.

Further, the image signal processing apparatus provided by the present invention is different from a transmitter for transmitting an ultrasonic signal to a sound field where an object to be observed exists, and a scattered wave signal scattered at a medium boundary of the object to be observed. And a differential wave for generating a differential wave signal between the attribute of the scattered wave signal received by each receiver and the attribute of the scattered wave signal when the sound field is homogeneous. Signal generation means, medium difference detection means for receiving the differential wave signal, performing back propagation control based on the TLM method, and detecting a point where the combined amplitude of each back propagation wave signal is maximum; Image generation means for generating the distribution image of the medium boundary based on the position information.

According to another aspect of the present invention, there is provided an image signal processing apparatus comprising: a transmitter for transmitting an ultrasonic signal to a sound field in which an object to be observed exists; and a scatterer scattered at a medium boundary of the object to be observed. A plurality of receivers for receiving wave signals at different positions, and scattered wave signals received by the respective receivers are input and backpropagation control is performed based on the TLM method, and the signal intensity distribution of each backpropagation wave signal Signal intensity distribution detecting means for detecting, and a difference distribution deriving means for deriving a difference distribution between the detected signal intensity distribution and the signal intensity distribution of the scattered wave signal when the sound field is homogeneous,
Image generating means for generating a distribution image of the medium boundary based on the derived difference distribution.

[0011]

When an ultrasonic signal is transmitted from a transmitter to a sound field in which an object to be observed is present , the ultrasonic signal is transmitted or reflected in accordance with a change in the medium of the object, and a plurality of ultrasonic signals spread at the medium boundary as a secondary sound source. Scattered wave signal. In the present invention,
These scattered wave signals are received by a plurality of receivers provided at different positions and guided to a differential wave signal generating means or a signal intensity distribution detecting means.

The differential wave signal generating means generates a differential wave signal between the attribute of the scattered wave signal and the attribute of the scattered wave signal when the sound field is homogeneous, and guides it to the medium boundary detecting means. In this way, each differential wave signal represents only the change in the acoustic impedance in the sound field, and the S / N ratio (signal-to-noise ratio) in the subsequent processing is significantly improved. In the medium boundary detection means, the generated differential wave signal is input, and a back propagation control based on the TLM method is performed, for example, a back propagation wave signal in which the time axis of propagation is inverted, and the combined amplitude of each back propagation wave signal is maximized. Is detected. Since each scattered wave signal has a medium boundary spread as a secondary sound source, the maximum point of the combined amplitude is the secondary sound source, that is, the medium boundary of the object. The position information of the detected point is generated for each of the differential wave signals, and is guided to the image generating means at the subsequent stage to obtain a boundary distribution image.

On the other hand, the signal intensity distribution detecting means receives the scattered wave signal, performs back propagation control based on the TLM method, detects the signal intensity distribution of each back propagated wave signal, and guides it to the difference distribution deriving means. These signal intensity distributions directly represent changes in acoustic impedance in the sound field. The difference distribution deriving means derives a difference distribution between each detected signal intensity distribution and the signal intensity distribution of the scattered wave signal when the sound field is homogeneous. As a result, the change point of the acoustic impedance, that is, the boundary of the medium in the object is known, and this is guided to the image generation means in the subsequent stage to obtain a boundary distribution image.

The scattered wave signal when the sound field is homogeneous is, for example, a transmitted ultrasonic wave when the internal tissue of the object to be observed is made of a medium having the same acoustic impedance or when no object exists in the sound field. Signal, and the sound field is obtained in advance by an experiment. For example , if the acoustic impedance of each tissue is known , such as a living body , the sound of each tissue
Read the impedance data into ROM (read only memor
It is convenient to store it in y) or the like and read it out at any time. Depending on the sound field or the object, there is a case where it is better to actually measure each time, or a case where a simulation can be performed based on existing data. Also, since the TLM algorithm uses a physical model called Huygens' principle, when modeling the propagation of a scattered wave signal or a counter-propagating wave signal, it is necessary to use a numerical method such as the finite element method or the boundary element method. There is no difficulty in the arithmetic processing required, and in principle, the Born approximation, Rh
Since it is not necessary to consider the ytov approximation or the like, it is possible to strictly consider the wave property. Therefore, straightness of the ultrasonic signal or the scattered wave signal is not always required. For example, even in the case of a long wave ultrasonic signal, distortion and blur of the boundary distribution image are reduced.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a configuration example of a main part when the present invention is applied to an ultrasonic CT. In the figure, 1 is an observed object, 2 is an ultrasonic sensor, 3 is an image preprocessing unit, 4 is a tomographic image generation unit, 5
Indicates a display control unit. The object to be observed 1 may be of any type as long as it can form a sound field, such as a living body, wood, fluid, elastic body, and the like.

The ultrasonic sensor 2 includes a transmitter and a receiver. For example, after transmitting an ultrasonic signal to the object 1 to be observed,
The scattered wave signal from the observed object 1 is received for a certain period. A configuration in which one transmitter is fixedly arranged and one or more receivers are rotated, and a plurality of pairs of transmitters and receivers are arranged around the axis of the object 1 to be observed, and each of them is rotated. The configuration may be such that only one sensor is used as the transmitter and the receiver. The received plurality of scattered wave signals are guided to the image preprocessing unit 3.

The image preprocessing unit 3 includes a signal attribute file 3
1. Propagation algorithm storage unit 32, scattered wave signal input unit 3
3, a signal attribute comparing unit 34, a back propagation processing unit 35, a secondary sound source position detecting unit 36, a working memory 37, and a control unit 38 for controlling the operation of each unit. In the signal attribute file 31, data including at least one of the attributes of the scattered wave signal when the object to be observed 1 is homogeneous, for example, at least one of sound pressure, displacement, and velocity is obtained in advance or obtained by simulation. The propagation algorithm storage unit 32
Stores a numerical algorithm of time-reversed sound wave propagation based on the TLM method. This will be further described later.

The scattered wave signal input unit 33 is connected to the ultrasonic sensor 2
The signal attribute comparison unit 34 inputs the attribute of each input scattered wave signal and the attribute of the corresponding signal stored in the signal attribute file 31, such as sound pressure, displacement amount, and the like. A difference wave signal is generated by comparing at least one of the speed and the density. The acoustic impedance based on the speed and the density may be compared (above, the differential wave signal generating means). This differential wave signal is temporarily stored in, for example, the work area 37 and is read out at the time of processing. When the real-time processing is performed, the processing is directly guided to the back propagation processing unit 35 without passing through the work area 37. The back propagation processing unit 35 reads the numerical algorithm from the propagation algorithm storage unit 32 and performs back propagation control on the differential wave signal based on the TLM method. The secondary sound source position detection unit 36 Is to detect a point at which the combined amplitude becomes maximum and generate position information of the point (above, medium boundary detecting means). This position information is guided to a tomographic image generation unit (image generation means) 4, where a distribution image of a medium boundary is generated, and then visualized by a display control unit 5.

Next, prior to the description of the operation of the ultrasonic CT having the above configuration, the principle of the present embodiment will be briefly described with reference to FIG. Now, as shown in FIG. 2A, assuming that a single-pulse ultrasonic signal p is incident on a propagation path including a medium A and a medium B having different acoustic impedances, the ultrasonic signal p As shown in (b), it is reflected at the boundary of the medium and becomes scattered sound signals p1 and p2. This is as if the medium boundary is a sound source (secondary sound source), and the scattered wave signals p1 and p2 are observed to be emitted therefrom. The scattered wave signals p1 and p2 are observed at observation points M1 and M2 at different positions, and their waveforms are input to the image preprocessing unit 3. Then, the difference between the observed waveform and the waveform of the medium A1 alone is obtained, and these are used as input waves S1 and S2 of the observation point M1 and the observation point M2, respectively (FIG. 2 (c)). Back propagation is performed as shown in FIG. 2D according to the numerical algorithm of propagation. When the time is reversed for an appropriate time, a point where the amplitude of the combined wave S12 in which the counter-propagating waves overlap is maximized as shown in FIG. 2D. The point of the maximum amplitude is the medium boundary to be determined. Although FIG. 2 shows an example of a one-dimensional case, a medium boundary can be derived based on the same principle in a case of two-dimensional or more.

In this embodiment, the numerical algorithm of the time-reversed sound wave propagation is stored in the propagation algorithm storage unit 32. Hereinafter, the specific contents will be described in detail. For example, FIG. 3 shows a two-dimensional TLM basic element model, that is, a unit cell, and FIG. 4 shows a reflection / scattering state when an impulse is incident on this unit cell.

Referring to FIG. 3, this unit cell has four terminals (pairs). Here, if an ultrasonic signal p (referred to as an incident pulse in this description) having a unit sound pressure is incident on one terminal (node) as shown in FIG. 4A, three other branches are included in this branch. Since the connection is made, the connection point is terminated with a standardized impedance of 1/3. Therefore, the reflection coefficient Γ is given by the well-known formula-
As shown in FIG. 4 (b), -1/2 is reflected by the first input terminal, and a pulse having a sound pressure of [1/2] is transmitted through the other terminals. Go. Such reflected and transmitted pulses are directed to adjacent nodes (FIG. 4).
(C)), it becomes a new incident pulse to each cell and scatters in a chain in four directions (FIGS. 4 (d) and (e)). The way it spreads is exactly what Huygens' principle shows. Therefore, if the input and reflecting the sound pressure pulses each _{^{k p i n, k p r}} n to the terminal n, from the superposition of the steps when the pulses in all four terminals is incident, Figure 4
The sound pressure of the pulse returning to the first input terminal in (a) is
It can be expressed as the following equation (1).

[0022]

(Equation 1)

In the equation (1), i represents incidence, r represents reflection (scattering), and k and k + 1 represent time. Expanding when a pulse is incident from all lines of the element, its propagation state becomes
It is represented by a general formula such as the formula (2).

[0024]

(Equation 2)

The expression (2) becomes the expression (3) when the scattering matrix is further expressed by [S].

[0026]

(Equation 3)

The back propagation control is equivalent to obtaining the input wave from the reflected or scattered wave by performing the reverse procedure to this propagation procedure, and can be realized by performing the calculation of the equation (4).

[0028]

(Equation 4)

The above-mentioned principle is embodied by storing the numerical algorithm of the formula (3) or (4) in the propagation algorithm storage unit 32, reading it out at the time of inputting the differential wave signal, and back-propagating it. is there.

Next, the operation of the ultrasonic CT having the configuration shown in FIG. 1 will be described with reference to FIGS. First, the ultrasonic sensor 2
Is driven (step 101), an ultrasonic signal is transmitted toward the observed object 1, and a plurality of scattered wave signals from the observed object 1 are received by receivers at different positions (step 102). The image preprocessing unit 3 receives these scattered wave signals, calculates the difference between the scattered wave signal and the attribute of the scattered wave signal when the observed object 1 is homogeneous, and generates a plurality of differential wave signals (step 103). .

Thereafter, each differential wave signal is back-propagated to detect a medium boundary point (step 104). The details of the processing here are as shown in FIG. 6, and upon input of one differential wave signal, the numerical algorithm of the above-described time axis inversion is read from the propagation algorithm storage unit 32 (step 20).
1), back propagation control of each differential wave signal is performed according to this numerical algorithm (step 202). This is repeated until the combined amplitude of the differential wave signal becomes maximum (step 2).
03), when the maximum amplitude point is detected, the position information at that time is generated and output to the tomographic image generation unit 4 (step 2).
04). Usually, a plurality of medium boundaries exist, and since each medium boundary is continuous in a predetermined direction, the tomographic image generation unit 4 combines the position information of the maximum amplitude point in each differential wave signal to obtain image information. Is generated (step 105 in FIG. 5) and displayed on the display control unit 5.

In the tomographic image obtained in this manner, the propagation state of the scattered wave signal is incorporated without approximation.
Irrespective of the frequency of the ultrasonic signal to be used, distortion and blur are reduced and become clearer. Therefore, it is not necessary to increase the frequency of the ultrasonic signal in order to obtain straightness. Further, since the difference between the attribute of the scattered wave signal and the attribute of the scattered wave signal when the observed object 1 is homogeneous is taken, the differential wave signal that is back-propagated is a change in the acoustic impedance of the observed object 1. Only the minutes are represented, and the S / N ratio is remarkably improved as compared with the conventional method. Therefore, it is not necessary to increase the peak sound pressure as in the related art. Accordingly, it is possible to realize an ultrasonic CT capable of obtaining a clear tomographic image free from distortion and blur while using an ultrasonic signal that does not adversely affect a living tissue or the like. Although the description has been made on the assumption that the ultrasonic signal and the scattered wave signal are single pulses, these signals are not necessarily limited to a single pulse, and may be a burst signal having a plurality of pulses or other waveforms. . By using the burst signal, the speed at which the medium boundary moves can be detected.

The above description is an example of the case where the medium boundary is detected based on the position information of the point where the combined amplitude of the differential wave signal is maximum. In addition, the signal intensity distribution of the scattered wave signal, that is, the acoustic energy It is also possible to detect a medium boundary based on the distribution. In this case, in the image pre-processing unit 3 in FIG. 1, instead of the part for processing the differential wave signal, the received scattered wave signal is subjected to the back propagation control based on the TLM method, and the signal intensity distribution of each back propagation wave signal is performed. And a difference distribution deriving means for deriving a difference distribution between the detected signal intensity distribution and the signal intensity distribution of the scattered wave signal when the sound field is homogeneous, and the derived difference is provided. The configuration is such that the medium boundary is specified by analyzing the distribution. The signal intensity distribution can be obtained by simple calculation at the time of back propagation, and the difference distribution can be easily obtained by taking the difference from the distribution obtained by actual measurement or the like in advance. The procedure for generating a tomographic image based on the specified medium boundary is almost the same as in the case described above. According to such a configuration, a portion having a large difference distribution is a portion having a large difference in acoustic impedance of the medium (medium boundary), so that the device configuration is simpler than in a case where position information of a maximum amplitude point is detected. . In addition, since the density distribution and the velocity distribution of the scattered wave signal can also be obtained by spatially integrating the signal intensity distribution or the difference distribution, it is possible to detect the medium boundary from multiple sides and generate a more accurate tomographic image. It is possible to do. Note that the present invention relates to an ultrasonic CT
In addition, the present invention is applicable to all kinds of applications that require this type of image processing.

[0034]

As is clear from the above description, according to the present invention, a difference wave signal between the attribute of each received scattered wave signal and the attribute of the scattered wave signal when the sound field is homogeneous is generated. Therefore, the received differential wave signal represents only the change in the acoustic impedance of the sound field, and the S / N ratio in the subsequent processing is significantly improved. In addition, since each differential wave signal is back-propagated in accordance with a back-propagation algorithm based on the TLM method, and the point where the combined amplitude is maximized is detected or specified, the wave nature of the sound wave is strictly taken into consideration and used. There is an effect that the frequency and the peak voltage of the generated ultrasonic signal can be set arbitrarily. Therefore, it is also possible to use a long-wave ultrasonic signal in which wave characteristics appear remarkably.

Further, the backscattering control based on the TLM method is performed on the received scattered wave signal to detect the signal strength distribution of each backpropagated wave signal, and the scattering when the detected signal strength distribution and the sound field are homogenous. The difference distribution from the signal intensity distribution of the wave signal is derived, and the medium boundary is specified by analyzing the difference distribution. Since the configuration of the image signal processing apparatus is simplified, and the density distribution and the velocity distribution of the scattered wave signal can be easily obtained, the method of detecting the boundary of the medium is multifaceted.

As described above, according to the present invention, the conventional problems can be solved at once, and distortion and blurring do not occur while using an ultrasonic signal at a level that does not adversely affect living tissue in both frequency and sound pressure. An image signal processing method and apparatus capable of obtaining a clear tomographic image can be provided.

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram of an ultrasonic CT according to one embodiment of the present invention.

FIGS. 2A and 2B are one-dimensional waveform state transition diagrams for explaining the principle of the present embodiment. FIG.
Is at the time of scattering, (c) is at the time of incidence of the differential wave signal (incident wave),
(D) shows the state when the composite wave is specified.

FIG. 3 is an explanatory diagram of a TLM basic element model (unit cell) according to the embodiment.

FIG. 4 is an explanatory diagram of the impulse incident / reflected state and its spread state in the model.

FIG. 5 is a flowchart showing an operation procedure of the ultrasonic CT according to the embodiment.

FIG. 6 is a flowchart showing details of a medium boundary specifying process in FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Observed object (sound field) 2 Ultrasonic sensor (transmitter, receiver) 3 Image pre-processing part 31 Signal attribute file 32 Propagation algorithm storage part 33 Scattered wave signal input part 34 Signal attribute comparison part 35 Back propagation processing Unit 36 secondary sound source position detection unit 37 working memory 38 control unit 4 tomographic image generation unit (image generation means) 5 display control unit

Continuation of front page (72) Inventor Eiichi Ando 2-3-1 Shibasaki, Chofu-shi, Tokyo Shimada Rika Kogyo Co., Ltd. (56) References JP-A-60-80442 (JP, A) JP-A-60-80761 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) A61B 8/00-8/15 G01N 29/00-29/28

Claims (6)

(57) [Claims] An ultrasonic signal is transmitted to a sound field in which an object to be observed exists, and a plurality of scattered wave signals scattered at a medium boundary of the object to be observed are received to thereby obtain a distribution image of the medium boundary.
A method for generating a differential wave signal between an attribute of each received scattered wave signal and an attribute of a scattered wave signal when the sound field is homogeneous, wherein the generated differential wave Transmission Line Matri
x: hereinafter, by back propagation using TLM) method to detect the point where the composite amplitude is maximum of the signal, wherein the benzalkonium to generate a pre-Symbol distribution image based on the position information of the detected point Image signal processing method. 2. The distribution of the medium boundary by transmitting an ultrasonic signal to a sound field where the object to be observed exists and receiving a plurality of scattered wave signals scattered at the medium boundary of the object to be observed.
A method performed in an apparatus for generating an image, each scattered wave signals received to detect the respective signal intensity distribution by back propagation using TLM method, the sound field and the signal intensity distribution detected deriving a difference distribution between the signal intensity distribution of the scattered wave signal when a homogeneous, pre on the basis of the difference distribution Stories
Image signal processing method comprising the Turkey to generate the distribution image. Wherein said saw including an ultrasonic signal and the scattered wave signals governance over scan <br/> preparative like signal, of the object to be observed object using the signal
3. The image signal processing method according to claim 1, wherein a distribution image considering a moving speed is generated . Is wherein the attribute of the scattered wave signals, dissipating sound pressure Ranha signal, displacement, velocity, and an image signal processing method according to claim 1 Symbol mounting, characterized in that it comprises at least one density. 5. A wave transmitter for transmitting an ultrasonic signal to the sound field to be observed object exists, the scattered wave signals scattered by the medium boundary of the object to be observed object to one another
A plurality of receivers to receive at different positions, and a differential wave signal between an attribute of the scattered wave signal received by each receiver and an attribute of the scattered wave signal when the sound field is homogeneous. A differential wave signal generating unit, a medium boundary detecting unit that receives the differential wave signal, performs a back propagation control based on the TLM method, and detects a point where a combined amplitude of each back propagation wave signal is maximum; An image generation unit configured to generate a distribution image of the medium boundary based on the position information of the point. 6. A transmitter for transmitting an ultrasonic signal to a sound field in which an object to be observed exists, and a plurality of receivers for receiving scattered wave signals scattered at a boundary of a medium of the object to be observed at different positions. Signal intensity distribution detecting means for inputting the scattered wave signal received by each receiver and performing back propagation control based on the TLM method to detect the signal intensity distribution of each back propagated wave signal; Difference distribution deriving means for deriving a difference distribution between the distribution and the signal intensity distribution of the scattered wave signal when the sound field is homogeneous, and image generating means for generating a distribution image of the medium boundary based on the derived difference distribution An image signal processing device comprising: