JP7765778B2 - Shear wave propagation velocity estimation device - Google Patents

Description

The present invention relates to a shear wave velocity estimation device that uses ultrasound to estimate the propagation velocity of shear waves inside a measurement object. The shear wave velocity estimation device of the present invention is suitable for diagnosing lung tissue.

Trauma or infections such as COVID-19 can cause acute, severe inflammation in lung tissue, leading to the development of acute respiratory distress syndrome (ARDS). The primary pathology of ARDS is non-cardiogenic pulmonary edema, and as the inflammation persists, lung tissue becomes fibrotic, preventing adequate ventilation and resulting in a mortality rate of 30-40%.

ARDS is a syndrome caused by a wide variety of factors, and as there are few established treatments, patients are given symptomatic treatment such as artificial respiration while waiting for the lungs to recover. The treatment period is not fixed, and there are cases where the condition lasts for more than several months, so the mortality rate is high.
Therefore, the development of diagnostic techniques that can lead to disease progression and prognosis assessment for ARDS is an urgent issue not only in peacetime but also in situations where medical resources are depleted, such as during the spread of COVID-19.

Chest X-ray CT is primarily used as a diagnostic method for acute lung diseases, including ARDS.
However, chest X-ray CT scans involve the use of radiation and must be performed in a dedicated examination room. This means that patients with poor breathing conditions must be moved to the examination room, and there is a risk of radiation exposure, so the scan cannot be performed frequently.
Furthermore, in ARDS, the patient's condition changes from moment to moment, so it is desirable to develop an evaluation method that can be performed quickly and easily at the bedside without having to move the patient.

Currently, one of the assessment methods available at the bedside is the oximeter (pulse oximeter) which measures blood oxygen saturation.
However, the oximeter evaluates the respiratory condition, which is a result of lung disease, and does not evaluate the progression or prognosis of lung disease. It is also difficult to evaluate the area where dysfunction is occurring.

The functional requirements for a diagnostic method for acute lung disease are: (1) since acute lung disease can occur in anyone, anywhere, at any time, the device must be easy to use, (2) there must be no problem with radiation exposure, and the method must be highly safe for the living body; and (3) the localization of the lesion must be assessable.

On the other hand, it has been reported that in acute lung diseases, particularly ARDS, the elasticity (stiffness) of lung tissue increases as the disease progresses.
Therefore, attempts have been made to quantitatively measure lung elasticity and use it to evaluate the progression and prognosis of lung diseases.

Among the methods for quantitatively measuring lung elasticity, one that can also be applied in vivo is a method in which vibrations of approximately 40 to 200 Hz are applied using a vibrator placed on the surface of the living body, and the mechanical vibration waves (shear waves) that are transmitted to the lung parenchyma are visualized using nuclear magnetic resonance imaging (MRI), and elasticity is measured from the propagation velocity of the shear waves (see, for example, Non-Patent Documents 1 and 2).
However, this method requires a large, non-portable MRI diagnostic device, does not satisfy the above-mentioned functional requirements, and is therefore hardly a desirable diagnostic method.

On the other hand, there have been attempts to apply vibrations to the surface of a living body and measure shear waves that propagate through the pleura on the surface of the lungs using an ultrasound examination device (see, for example, Non-Patent Documents 3 and 4).
However, because ultrasound cannot reach the air-rich lung parenchyma, this method can only measure the sum of the elasticity of multiple biological tissues in the immediate vicinity of the pleura, and is therefore not suitable for measuring the elasticity of the lung parenchyma in clinically relevant ways.

Ultrasound examinations do not involve radiation exposure and can evaluate the lungs by placing a probe on the chest. Recently, ultrasound examination devices have become smaller, allowing results to be obtained immediately at the bedside, enabling pathological conditions to be assessed and diagnosed. Therefore, point-of-care ultrasound (POCUS) is widely used in emergency intensive care, disaster sites, home medical care, etc. (see, for example, Non-Patent Document 5).
Acute lung diseases cause specific artifacts, such as "B-lines," to appear in normal ultrasound images, so ultrasound devices are also used to diagnose acute lung diseases.

Y. Mariappan, K. Glaser, R. Hubmayer, A. Manduca, R. Ehman and K. McGee, “MR Elastography of Human Lung Parenchyma: Technical Development, Theoretical Modeling and In vivo Validation”, J. Magn Reson Imaging, 33: 1351-1361 (2011). J. Marineli, D. Levin, R. Vassallo, R. Carter, R. Humbmayr, R. Ehman and K. McGee, “Quantitative assessment of lung stiffness in patients with interstitial lung disease using MR elastography”, J Magn Reason Imaging, 2017, 46(2); 365-374. X. Zhang, T. Osborn, B. Zhou, D. Meixner, R. Kinnick, B. Bartholmai, J. Greenleaf and S. Kalra, “Lung ultrasound surface wave elastography: a pilot clinical study”, IEEE Trans Ultrason Ferroelectr Freq Control. 2017 Sep; 64(9): 1298-1304. X. Zhang, B. Zhou, S. Karla, B. Bartholnai, J. Greenleaf, and T. Osborn, “An ultrasonic surface wave technique for assessing skin and lung diseases”, Ultrasound Med Biol. 2018, 44(2):321-331. Moore CL, Copel JA. Point-of-care ultrasonography. N Engl J Med. 2011 Feb 24;364(8):749-57.

However, the above-mentioned methods are limited to evaluating the shape or the elasticity of the pleura on the surface of the lung or its immediate vicinity, making it difficult to measure the elasticity of lung tissue, which is expected to lead to a functional diagnosis of the lung.
Therefore, it is desirable to establish as soon as possible a method for measuring the elasticity of lung parenchyma using equipment that can be used by anyone, anywhere, anytime.

To solve the above-mentioned problems, the present invention aims to provide a shear wave velocity estimation device that can easily and accurately measure the elasticity of lung parenchyma.

The shear wave propagation velocity estimation device of the present invention comprises an exciter for exciting shear waves in a measurement object; an ultrasound probe for outputting ultrasound pulses to the measurement object and receiving ultrasound signals within the measurement object; an ultrasound Doppler device for obtaining ultrasound Doppler signals from the ultrasound signals received by the ultrasound probe; and a velocity estimation device for estimating the propagation velocity of the shear wave from the ultrasound Doppler signals obtained by the ultrasound Doppler device. The velocity estimation device estimates the phase of the shear wave in multiple B-lines or a single wide B-line from the ultrasound Doppler signals, and estimates the propagation velocity of the shear wave from the phase of the shear wave in multiple B-lines or a single wide B-line.

According to the shear wave propagation velocity estimation device of the present invention described above, radiation is not used, so there is no need to provide a specialized examination room as in the case of a chest X-ray CT. Furthermore, since measurements can be performed by operating a vibrator and an ultrasonic probe, measurements can be performed easily.
Since the propagation velocity of the shear wave in multiple B-lines or a single wide B-line is estimated, the elasticity of the pulmonary parenchyma, which is the measurement target, can be estimated from the propagation velocity of the shear wave, thereby enabling the elasticity of the pulmonary parenchyma to be measured with high accuracy.
Therefore, the present invention makes it possible to measure the elasticity of the lung parenchyma simply and accurately.

1 is a diagram showing a schematic configuration of an embodiment of a shear wave propagation velocity estimation device of the present invention, together with a cross-sectional structure of the lung and surrounding body tissues. FIG. 1A is a schematic diagram of a reflected ultrasonic signal at an ultrasonic scanning line where a B line appears, and FIG. 1B is a schematic diagram of a reflected ultrasonic signal at an ultrasonic scanning line where a B line does not appear. FIG. 10 is a diagram showing a model of lung echoes when two B-lines are formed. 2 is a flowchart of signal processing in the shear wave propagation velocity estimation device of FIG. 1 . FIG. 1 is a diagram showing a schematic diagram of a phantom used in an experiment and an outline of an experimental device. A is a B-mode image of a normal sponge phantom, and B is a B-mode image of a hardened sponge phantom. This is a power Doppler image obtained when vibration was applied to a normal sponge phantom. FIG. 10 shows the frame-by-frame changes in brightness values of power Doppler images appearing on each B-line when a normal sponge phantom is vibrated. This is a power Doppler image obtained when vibration was applied to a hardened sponge phantom. FIG. 10 shows the frame-by-frame transition of the brightness value of a power Doppler image appearing on each B-line when a hardened sponge phantom is vibrated. FIG. 10 shows the results of estimating the propagation velocity of shear waves from the phase difference of shear waves observed between B-lines in a normal sponge phantom and a hardened sponge phantom.

The shear wave propagation velocity estimation device of the present invention includes an exciter for exciting shear waves in a measurement object, an ultrasonic probe for outputting ultrasonic pulses to the measurement object and receiving ultrasonic signals within the measurement object, an ultrasonic Doppler device for obtaining ultrasonic Doppler signals from the ultrasonic signals received by the ultrasonic probe, and a velocity estimation device for estimating the propagation velocity of the shear wave from the ultrasonic Doppler signals obtained by the ultrasonic Doppler device.
In the shear wave propagation velocity estimation device of the present invention, the velocity estimation device estimates the phase of the shear wave in multiple or one wide B-line from the ultrasonic Doppler signal, and estimates the propagation velocity of the shear wave from the phase of the shear wave in multiple or one wide B-line.

The shear wave velocity estimation device of the present invention focuses on B-lines, which are prominent findings that appear in ultrasound B-mode images of acute pulmonary diseases, and estimates the phase and propagation velocity of shear waves propagating through the pulmonary parenchyma from Doppler signals obtained on the B-lines.
The shear wave velocity estimation device of the present invention uses ultrasound to estimate the phase and propagation velocity of shear waves propagating through the pulmonary parenchyma, making it possible to measure the elasticity of the pulmonary parenchyma from the propagation velocity of the shear waves.

B-lines are artifacts that occur when ultrasound waves are reflected multiple times within the alveoli, which are filled with fluid.
Conversely, if a Doppler signal is detected on an ultrasound scanning line where a B-line appears, the signal obtained is the Doppler signal itself from the lung parenchyma. For lung diseases where multiple B-lines appear, if the phase of the shear wave is measured on the B-line, the elasticity of the lung parenchyma can be measured from the phase difference between the multiple B-lines.
Furthermore, even when multiple B-lines are connected to form a single wide B-line, if there is a phase difference in the shear waves within that single wide B-line, the elasticity of the lung parenchyma can be measured from the phase difference in the shear waves of that single wide B-line.
In other words, this measurement method focuses on the B-line, which is an artifact of ultrasound images but contains Doppler information about the lung parenchyma due to its generation process, and is a method for measuring the elasticity of the lung parenchyma, which could not be measured using ultrasound until now.
This measurement method is currently the only method that can measure the elasticity of lung parenchyma, which changes due to acute lung disease, using ultrasound, which has previously been thought to be difficult to measure.

The propagation velocity of the shear waves in the multiple or wide B-lines is estimated from the phase difference of the shear waves in the multiple or wide B-lines.
The following two methods, for example, can be used to estimate the propagation velocity of shear waves from the phase difference of shear waves in multiple B-lines or a single wide B-line.

The first method is to perform Fourier analysis on the Doppler signal.
Specifically, for example, the phase of a complex quadrature detection signal (complex Doppler signal) is determined by Fourier analysis at the excitation frequency (the frequency of the shear wave from the exciter). However, the phase may also be determined from the time at which the phase reaches a predetermined phase value (e.g., zero) without using Fourier analysis.

The second method is a method using the C-SWE method developed by the inventors of the present invention.
C-SWE (Continuous Shear Wave Elastography) is a shear wave imaging technique that utilizes the fact that when continuous shear waves are excited inside a living body from a small vibrator placed on the surface of the living body, shear wave images appear in color Doppler images for blood imaging, which are a standard function of general-purpose ultrasound echo devices (see, for example, Japanese Patent No. 649183). Note that power Doppler images may be used instead of color Doppler images.

In the C-SWE method, as described in Japanese Patent No. 649183, for example, vibrations having a center frequency that is n/4 times (where n is an odd number greater than or equal to 1) the burst frequency, which corresponds to the repetition period of the ultrasonic pulse, are applied to the object to be measured.
Specifically, the method for estimating the propagation velocity of a shear wave using the C-SWE method involves setting the frequency of the shear wave relative to the repetition frequency of the ultrasound so that it meets the frequency conditions of the C-SWE method described above (n/4 times the repetition frequency of the ultrasound (where n is an odd number greater than or equal to 1)), and then performing Fourier analysis in the frame direction on the color Doppler image or power Doppler image obtained at this time.

Compared to the first method, which uses Fourier analysis of Doppler signals, the second method, which uses C-SWE, can be used with conventional ultrasound diagnostic equipment without modification, and has the advantage of being easily incorporated into small, portable ultrasound diagnostic equipment.

According to the configuration of the shear wave propagation velocity estimation device of the present invention described above, it includes an exciter for exciting shear waves in the measurement object, an ultrasonic probe for outputting ultrasonic pulses to the measurement object and receiving ultrasonic signals from within the measurement object, an ultrasonic Doppler device for obtaining ultrasonic Doppler signals from the ultrasonic signals received by the ultrasonic probe, and a velocity estimation device for estimating the propagation velocity of the shear wave from the ultrasonic Doppler signals obtained by the ultrasonic Doppler device.
This makes it possible to estimate the propagation velocity of the shear wave in the object to be measured by using the shear wave excited in the object to be measured from the vibrator and the ultrasonic pulse output to the object to be measured from the ultrasonic probe.
Since it uses shear waves and ultrasonic pulses, it does not use radiation, so there is no need to set up a specialized examination room like in chest X-ray CT.In addition, since measurements can be made by operating the vibrator and ultrasonic probe, measurements can be made easily.

In the shear wave propagation velocity estimation device of the present invention, the velocity estimation device estimates the phase of the shear wave in multiple or one wide B-line from the ultrasonic Doppler signal, and estimates the propagation velocity of the shear wave from the phase of the shear wave in multiple or one wide B-line.
This makes it possible to estimate the elasticity of the pulmonary parenchyma, which is the measurement target, from the propagation velocity of the shear wave estimated by the velocity estimation device, thereby enabling the elasticity of the pulmonary parenchyma to be measured with high accuracy.
Therefore, the shear wave propagation velocity estimation device of the present invention makes it possible to measure the elasticity of the lung parenchyma simply and accurately.

Furthermore, the shear wave propagation velocity estimation device of the present invention may be configured to estimate the propagation velocity of the shear wave by performing Fourier analysis on the phase of a complex quadrature detection signal obtained from an ultrasonic Doppler signal at the frequency of the shear wave. That is, the velocity estimation device is configured to estimate the propagation velocity of the shear wave by the first method described above.
With this configuration, the propagation velocity of the shear wave can be estimated from the ultrasonic Doppler signal.

Furthermore, in the shear wave velocity estimation device of the present invention, the frequency of the shear wave can be set to n/4 times the repetition frequency of the ultrasound (where n is an odd number equal to or greater than 1), and the velocity estimation device can estimate the phase of the shear wave in each B-line by performing Fourier analysis in the frame direction on a color Doppler image or a power Doppler image obtained from the ultrasound Doppler signal. That is, the velocity estimation device is configured to estimate the propagation velocity of the shear wave by the second method described above.
With this configuration, the propagation velocity of the shear wave can be estimated from a color Doppler image or a power Doppler image obtained from the ultrasonic Doppler signal.
Furthermore, as mentioned above, since it can be used without modifying conventional ultrasonic diagnostic equipment, there is an advantage that it can be easily incorporated into small, portable ultrasonic diagnostic equipment.

Specific embodiments of the shear wave propagation velocity estimation device of the present invention are described below.

FIG. 1 shows a schematic configuration diagram of an embodiment of a shear wave propagation velocity estimation device according to the present invention.
As shown in FIG. 1, the shear wave propagation velocity estimation device 10 of this embodiment includes an oscillator 1, a small vibrator 2, an ultrasonic probe 3, an ultrasonic Doppler device 4, a velocity estimation device 5, and an output device 6.
The functions of the components 1 to 6 of the shear wave propagation velocity estimation device 10 will be described later.

FIG. 1 also shows the cross-sectional structure of the lungs and surrounding body tissues, which are the objects to be examined by the shear wave propagation velocity estimation device 10 .
1, the cross-sectional structure of the lungs comprises, from the outside in, a body surface 11, soft tissue 12, pleura 13, and lung parenchyma 14. Although not shown, the lung parenchyma 14 contains many tiny alveoli that contain air inside.

When ultrasound is transmitted from the body surface 11 to the lung structure, the cross-sectional structure of which is shown in Figure 1, and a lung echo (chest echo) is obtained, a linear echo, not shown, extending in a lateral direction (horizontal direction) with high reflection strength due to soft tissue 12, known as a reflected echo or pleural echo, is observed from a position close to the body surface 11.

In acute lung diseases, if fluid accumulates in some of the alveoli in the lung parenchyma 14 and the alveoli with the accumulated fluid (two alveoli 15 shown in Figure 1) are in contact with the pleura 13, an artifact called a "B-line" that extends in a long line in the depth direction appears in the ultrasound echo image.
This is thought to be because the ultrasonic waves penetrate into the alveoli 15 where liquid has accumulated, and cause multiple reflections inside the alveoli 15 .
In FIG. 1, on the ultrasound imaging plane 7 surrounded by the dashed line, B-lines 1 and 2 are generated in the depth direction from two alveoli 15 where liquid has accumulated.

The shear wave propagation velocity estimation device 10 of this embodiment utilizes B-lines generated from the alveoli 15 where fluid accumulates, obtains information about shear waves propagating through the lung parenchyma 14 from the Doppler signals obtained on the B-lines, and measures the elasticity of the lung parenchyma using ultrasound.

The functions of the components 1 to 6 of the shear wave propagation velocity estimation device 10 are as follows.
The oscillator 1 determines the frequency of the shear wave SW that is propagated inside the lung parenchyma 14 shown in FIG.
The small-sized vibrator 2 excites shear waves SW inside the lung parenchyma 14. When the small-sized vibrator 2 is brought into contact with the body surface 11 and shear waves SW are generated from the small-sized vibrator 2, the shear waves SW can be excited inside the lung parenchyma 14 as shown in FIG.
The ultrasonic probe 3 outputs ultrasonic pulses to the lung parenchyma 14 and receives the resulting ultrasonic signal SU.
The ultrasonic Doppler device 4 obtains an ultrasonic Doppler signal SUD on the B line from the ultrasonic signal SU received by the ultrasonic probe 3 .
The velocity estimation device 5 estimates the phase of the shear wave SW from the ultrasonic Doppler signal SUD, and estimates the propagation velocity v of the shear wave SW propagating inside the lung parenchyma 14 from the phase of the shear wave SW.
The output device 6 outputs the value of the propagation velocity v of the shear wave SW estimated by the velocity estimation unit 5 .

The ultrasonic probe 3 generally has a width of about 5 to 7 cm.
The small vibrator 2 and the ultrasonic probe 3 are configured as small devices.
Furthermore, if the ultrasonic Doppler device 4, the velocity estimation device 5, and the output device 6 are each constructed as small devices, the overall device can be made smaller.
Furthermore, by making the entire device smaller and configuring it so that it can be transported on casters or the like, it will be possible to transport the device to a hospital room or to load it into an ambulance vehicle.

In the shear wave propagation velocity estimation device 10 shown in Figure 1, the ultrasound Doppler device 4 and the velocity estimation device 5 are separate devices, but the ultrasound Doppler device and the velocity estimation device may also be configured as an integrated device.

Using the shear wave propagation velocity estimation device 10 of this embodiment, the propagation velocity v of the shear wave SW can be measured, for example, by the method described below.

First, a continuous shear wave SW is propagated into the lung parenchyma 14 by a small vibrator 2 placed on the body surface 11 .
At this time, at the position where the alveoli 15 containing the liquid are present, an artifact called a B-line occurs, in which continuous ultrasonic waves appear to be reflected from a position deeper than the depth Z _A of the alveoli 15.
The reflected ultrasonic signal y(x,z,t) generated at this time can be modeled by the following equation (1).

Here, y ₀ (x,z) is the amplitude of the reflected ultrasound signal on the B line, and θ(x,z,t) is the phase change of the ultrasound due to the shear wave SW, and is given by the following equation (2).

Here, f ₀ is the center frequency of the ultrasonic wave, c is the speed of sound, and ξ(x, z _L , t) is the displacement caused by the shear wave SW and is given by the following equation (3).

In equation (3), ξ ₀ (x,z _L ,t) is the amplitude of the shear wave SW, φ _b (x,z _L ) is the phase of the shear wave SW, and ω _b is the angular frequency of the shear wave SW.
In equation (3), it should be noted that, due to the mechanism by which the B-line appears, the amplitude and phase of the shear wave SW are the amplitude and phase at the position of the depth Z _A of the alveolus 15 .

Figure 2A shows a schematic diagram of the reflected ultrasound signal from an ultrasound scan line where a B-line appears. Figure 2B shows a schematic diagram of the reflected ultrasound signal from an ultrasound scan line where a B-line does not appear.

In ultrasound scan lines where B-lines appear, multiple reflections occur due to the fluid-filled alveoli 15, as shown in Figure 2A. In contrast, in ultrasound scan lines where B-lines do not appear, multiple reflections do not occur, as shown in Figure 2B.

The received ultrasound is subjected to quadrature detection using a reference signal R(t) given by the following equation (4). This quadrature detection is performed by the ultrasound Doppler device 4 of the shear wave propagation velocity estimation device 10 in Figure 1.

In this case, the complex Doppler signal after quadrature detection is given by the following equations (5) to (7).

The phase φ _b (x,z _L ) of the shear wave can be obtained from the quadrature detection signal (the above-mentioned complex Doppler signal) by the following two methods.
The first method is to obtain the phase of a complex quadrature detection signal (complex Doppler signal) by Fourier analysis at the excitation frequency. However, the phase may also be obtained from the time when the phase becomes a predetermined phase value (e.g., zero) without using Fourier analysis.
The second method is to set the frequency of the shear waves relative to the repetition frequency of the ultrasound waves so that it meets the frequency conditions of the C-SWE method described above (n/4 times the repetition frequency of the ultrasound waves (where n is an odd number greater than or equal to 1)), and then perform Fourier analysis in the frame direction on the color Doppler image or power Doppler image obtained at this time.

While either of the above two methods can be used, the second method is based on the C-SWE method, and can be said to be easily applied to small, portable ultrasound diagnostic equipment, in the sense that it allows the phase of shear waves to be obtained without any modification to the ultrasound echo device.

In an actual shear wave propagation velocity estimation device, there is a delay in the timing of ultrasonic transmission depending on the position of the ultrasonic scanning line, and the phase φ _M (x,z _L ) of the shear wave obtained from the quadrature detection signal can be written as the sum of the phase difference due to the propagation of the shear wave and the phase difference due to the delay in the ultrasonic transmission timing, as shown in the following equation (8).

Here, φ _US (x) is the phase difference due to the delay in the ultrasonic transmission timing. φ _US (x) is determined by the ultrasonic transmission/reception method of the ultrasonic diagnostic device, and this value is known. Therefore, using φ _US (x), the phase due to the propagation of shear waves can be calculated using the following equation (9).

Next, a method for determining the propagation velocity v of the shear wave SW from the phase of the shear wave SW when a plurality of B lines are formed will be described.
FIG. 3 shows, as an example, a model of a lung echo when two B-lines are formed.
In this case, the phases of the shear waves SW measured on the B-line 1 and B-line 2 can be expressed by the following equations (10) and (11), respectively.

Here, assuming that the shear wave SW is propagating in the x-axis direction (azimuth direction) as shown by arrow 8 in Figure 3, the propagation velocity v of the shear wave SW can be estimated using the following equation (12):

Here, _fb is the frequency of the shear wave SW.
From this, the propagation velocity v of the shear wave SW between the two B lines can be estimated by the following equation (13).

Here, x ₁ and x ₂ are the positions of B line 1 and B line 2 in the x-axis direction, respectively, as shown in FIG.
The above equation (13) is the basic equation for obtaining the propagation velocity v of the shear wave SW in the pulmonary parenchyma in the present invention.

In reality, there may be three or more B lines.
This method is also applicable to cases where lung disease has progressed and multiple B-lines are connected in the lateral direction (horizontal direction) to form a single wide B-line.
If there is a phase difference of the shear wave SW in the single wide B-line, the propagation velocity v of the shear wave SW can be obtained from the phase difference of the shear wave SW in the single wide B-line.
Specifically, the phase of the shear wave SW is measured at two different points on the wide line B. From the phase difference between the two points, the propagation velocity v of the shear wave SW can be obtained.

Next, signal processing in the shear wave propagation velocity estimation device 10 of this embodiment will be described.
FIG. 4 shows a flowchart of signal processing in the shear wave propagation velocity estimation device 10 of this embodiment.

In the flowchart shown in Figure 4, first, in step S11, vibration is applied from the biological surface (body surface) 11 using the small vibrator 2, and shear waves SW are propagated to the lung parenchyma 14.

Next, in step S12, the ultrasound Doppler device 4 performs quadrature detection on the ultrasound signal SU received from the ultrasound probe 3 to obtain a complex quadrature detection signal (IQ signal).

Next, in step S13, the phase of the shear wave SW is obtained from the IQ signal. As a method for obtaining the phase of the shear wave SW, the following two methods can be used, as described above.
1. Method using the phase of the complex quadrature detection signal 2. Method using the C-SWE method

Thereafter, in step S14, the propagation velocity v of the shear wave SW is measured using the above equation (13) from the phase of the shear wave SW estimated from the two B lines.
In this way, the shear wave velocity estimation device 10 of this embodiment can obtain the propagation velocity v and estimate the stiffness of the lung parenchyma 14.

As described above, the shear wave velocity estimation device 10 of this embodiment includes a small vibrator 2 that excites shear waves SW in the lung parenchyma 14 that is the measurement target; an ultrasound probe 3 that outputs ultrasound pulses to the lung parenchyma 14 and receives ultrasound signals SU within the lung parenchyma 14; an ultrasound Doppler device 4 that obtains an ultrasound Doppler signal SUD from the received ultrasound signal SU; and a velocity estimation device 5 that estimates the propagation velocity of the shear wave SW from the ultrasound Doppler signal SUD.

As a result, according to the shear wave propagation velocity estimation device 10 of this embodiment, the propagation velocity of the shear wave SW in the lung parenchyma 14 can be estimated by the velocity estimation device 5 using the shear wave SW excited from the small vibrator 2 and the ultrasonic pulse output from the ultrasonic probe 3.
Furthermore, since the shear wave propagation velocity estimation device 10 of this embodiment uses shear waves SW and ultrasonic pulses, it does not use radiation, so there is no need to provide a specialized examination room like that used for chest X-ray CT.In addition, since measurements can be made by operating the small vibrator 2 and the ultrasonic probe 3, measurements can be made easily.

In the shear wave propagation velocity estimation device 10 of this embodiment, the velocity estimation device 5 estimates the phase of the shear wave SW in multiple or one wide B-line from the ultrasonic Doppler signal SUD, and estimates the propagation velocity of the shear wave SW from the phase of the shear wave SW in multiple or one wide B-line.
This allows the elasticity of the lung parenchyma 14, which is the measurement target, to be estimated from the propagation velocity of the shear wave SW estimated by the velocity estimation device 5, so that the elasticity of the lung parenchyma 14 can be measured with high accuracy.
Therefore, the shear wave velocity estimation device 10 of this embodiment can measure the elasticity of the lung parenchyma 14 easily and accurately.

[experiment]
In order to confirm the effect of the shear wave propagation velocity estimation device of the present invention, an experiment was carried out using a phantom that mimics the structure of the lungs.

(Experimental equipment)
A schematic diagram of the phantom and an outline of the experimental apparatus used in the experiment are shown in FIG.

As shown in Figure 5, the phantom used in the experiment has a two-layer structure. Starting from the layer closest to the ultrasound probe 3, it consists of a konjac layer 31 with a thickness of approximately 10 mm, which simulates the soft tissue interposed between the body surface and lung tissue, and a coarse sponge layer 32 located below the konjac layer 31, which simulates the lungs containing air.
As shown in Figure 5, three tiny glass beads 33 with a diameter of approximately 2 mm were embedded at intervals of several mm into the sponge layer 32 just below the interface between the sponge layer 32 and the konjac layer 31 to simulate alveoli filled with liquid.
Furthermore, a thin layer of ultrasonic gel was applied to the boundary between the konjac layer 31 and the sponge layer 32 so that ultrasonic waves could reach the glass beads 33 .

The shear wave propagation velocity estimation device used was the shear wave propagation velocity estimation device 10 shown in Figure 1, which is equipped with a small vibrator 2, an ultrasonic probe 3, an ultrasonic Doppler device 4, etc.

The small vibrator 2 was placed about 10 mm away from the ultrasonic probe 3 so that shear waves would propagate from the right side to the left side in FIG.
The shear wave velocity estimation device 10 was set to a power Doppler mode, and the excitation frequency was set to 72.8 Hz, which was selected so that it was within the frequency range of the C-SWE method and so that the wavefront of the shear wave could be easily visualized on the power Doppler image.

The experiments were conducted using a regular sponge (hereafter referred to as a regular sponge phantom) as the sponge layer 32 within the phantom, and a sponge that had been thoroughly sprayed with acrylic paint and hardened to simulate hardened lungs (hereafter referred to as a hardened sponge phantom).

(Experimental results)
B-mode images, which are ultrasound echo images of the two sponge phantoms, are shown in Figures 6A and 6B. Figure 6A shows a B-mode image of the normal sponge phantom, and Figure 6B shows a B-mode image of the hardened sponge phantom.

Figures 6A and 6B show that in both sponge phantoms, a B-line, in which a bright echo trails vertically (in the depth direction), is formed in the area where the glass beads 33 are embedded.

Figure 7 shows the power Doppler image obtained when vibration was applied to a normal sponge phantom. In Figure 7, legends have been added to the image to make it easier to see the areas with high Doppler signal intensity.

Figure 7 shows that, over time, increasingly intense power Doppler images appear in the B-line 1 to B-line 3 sections formed in the sponge layer. This is because the shear wave front propagates from right to left. The time intervals at which Doppler signals appear on each B-line were measured and found to be approximately every 0.45 seconds.

Figure 8 shows the frame-by-frame changes in the brightness values of the power Doppler images appearing on each B-line when a normal sponge phantom is vibrated.

As can be seen from Figure 8, the wavefront of a shear wave appears as abrupt changes in brightness, and in Figure 8, the time progression of high-brightness areas is shown by the dashed line. Furthermore, the slope of the dashed line in Figure 8 indicates the elasticity of the sponge.

On the other hand, Figure 9 shows the power Doppler image obtained when vibration was applied to the hardened sponge phantom.

As can be seen from Figure 9, similar to observations with the normal sponge phantom, high-intensity power Doppler images appear only on the B-lines in the sponge layer. The time interval at which Doppler signals appear on each B-line is approximately 0.3 seconds. This time interval is shorter than that of the normal sponge, and this result indicates that the elasticity of the hardened sponge is higher than that of the normal sponge.

Figure 10 shows the frame-by-frame changes in brightness values of the power Doppler images appearing on each B-line when the hardened sponge phantom is vibrated. In Figure 10, as in Figure 8, the dashed lines show the results of connecting areas with high brightness.

Figure 10 shows that the slope of the dashed line is steeper than the results for the regular sponge (Figure 8). This indicates that the elasticity of the hardened sponge phantom is higher than that of the regular sponge phantom.

The propagation velocity v of the shear wave was estimated from the phase difference of the shear wave observed between the B lines using equation (13). Specifically, the propagation velocity v was estimated at three locations shown in Figures 6A and 6B: between B line 1 and B line 2, between B line 2 and B line 3, and between B line 1 and B line 3.
The results of estimating the propagation velocity v in the normal sponge phantom and the hardened sponge phantom are shown in Fig. 11. In Fig. 11, the mean value and standard deviation of the estimated propagation velocity v are shown by error bars.

As can be seen from FIG. 11, the propagation velocity of the hardened sponge phantom is 1.53 times faster than that of the normal sponge phantom.
This difference in propagation velocity reflects the difference in elasticity between the two sponge phantoms.
This result indicates that the shear wave propagation velocity estimation device of the present invention can be used to estimate the propagation velocity of shear waves propagating within a sponge.

1. Oscillator, 2. Small vibrator, 3. Ultrasound probe, 4. Ultrasound Doppler device, 5. Velocity estimation device, 6. Output device, 7. Ultrasound image plane, 10. Shear wave velocity estimation device, 11. Body surface, 12. Soft tissue, 13. Pleura, 14. Lung parenchyma, SW: Shear wave, SU: Ultrasound signal, SUD: Ultrasound Doppler signal, v: Shear wave velocity

Claims (3)

a vibrator for exciting shear waves in the measurement target;
an ultrasonic probe that outputs ultrasonic pulses to the measurement object and receives ultrasonic signals from within the measurement object;
an ultrasonic Doppler device for obtaining an ultrasonic Doppler signal from the ultrasonic signal received by the ultrasonic probe;
a velocity estimation device that estimates a propagation velocity of the shear wave from the ultrasonic Doppler signal obtained by the ultrasonic Doppler device,
The velocity estimation device estimates the phase of the shear wave in multiple or one wide B-line from the ultrasonic Doppler signal, and estimates the propagation velocity of the shear wave from the phase of the shear wave in the multiple or one wide B-line. The shear wave propagation velocity estimation device of claim 1, wherein the velocity estimation device estimates the propagation velocity of the shear wave by Fourier analysis of the phase of a complex quadrature detection signal obtained from the ultrasonic Doppler signal at the frequency of the shear wave. The frequency of the shear wave is set to n/4 times the repetition frequency of the ultrasonic wave (where n is an odd number equal to or greater than 1),
2. The shear wave propagation velocity estimation device according to claim 1, wherein the velocity estimation device estimates the phase of the shear wave in each of the B-lines by performing Fourier analysis in a frame direction on a color Doppler image or a power Doppler image obtained from the ultrasonic Doppler signal.