JP7557742B2 - ULTRASONIC IMAGING APPARATUS AND ULTRASONIC IMAGING METHOD - Google Patents

Description

The present invention relates to a medical ultrasound imaging device, and in particular to a technique for determining the velocity of body tissue.

Cardiovascular disease is one of the leading causes of death in developed countries, and it is known that abnormalities in the cardiovascular system can cause blood flow that differs from that in a healthy state, such as valve regurgitation. Evaluating abnormal blood flow can help determine the severity of cardiovascular disease and make treatment decisions. Ultrasound imaging devices are widely used to diagnose cardiovascular disease, as they can grasp the state of blood flow within the body in real time.

One of the methods for measuring blood flow using ultrasound imaging devices is the color Doppler method. Color Doppler is a velocity measurement method that estimates the moving speed of a scatterer (mainly red blood cells) by utilizing the Doppler effect that occurs when ultrasound reflects off the scatterer. By transmitting and receiving ultrasound at various depths and positions within the body in a short period of time, it is possible to obtain time changes in velocity distribution over a wide range. Previously, only two-dimensional color Doppler was available, which transmits and receives ultrasound on a two-dimensional cross section, but in recent years, three-dimensional color Doppler, which transmits and receives ultrasound in three dimensions and captures volumetric images, has also become popular. Three-dimensional color Doppler is useful for understanding complex blood flows, for example, when you want to understand the direction of valvular regurgitation in three dimensions.

In color Doppler, in order to obtain a wide range of velocity distribution, it is necessary to transmit ultrasonic transmission beams sequentially at multiple angles (ultrasonic beam scanning). The image update frequency of color Doppler (called frame rate in 2D color Doppler and volume rate in 3D color Doppler) decreases as the number of possible transmission angles increases, so the image update frequency of 3D color Doppler, which requires two-dimensional scanning, tends to be lower than that of 2D color Doppler, which performs one-dimensional scanning. However, with an image update frequency that is significantly lower than 10 Hz, such as 5 to 6 Hz, it is not possible to accurately observe the time change in blood flow, and it is difficult to capture the moment when valve regurgitation shows its maximum flow velocity, which is observed in the diagnosis of valvular heart disease. On the other hand, if the number of possible transmission angles per space (hereinafter referred to as transmission beam density) is reduced, the image becomes sparse, making it difficult to correctly capture the velocity distribution of blood flow. Therefore, it is necessary to perform 3D color Doppler volume imaging with a small number of transmission angles, and parallel receiving phasing, which receives reflected signals from multiple nearby directions from a single transmission in one direction, is widely used. By using this phasing method, the number of receive beams per space (hereafter referred to as receive beam density) can be kept high even if the transmit beam density is low, making it possible to achieve both a high volume rate and the acquisition of dense images.

Parallel receive phasing is useful for improving the image update frequency of ultrasound images, not limited to color Doppler. However, it is known that block-shaped artifacts occur in ultrasound images in which the receive beam density is increased using parallel receive phasing. This artifact occurs because the spatial continuity of signal values is maintained between receive beams belonging to transmit beams with the same transmit angle, whereas the signal values become discontinuous between receive beams belonging to transmit beams with different transmit angles. As a result, the image becomes discontinuous at each boundary position where the transmit angle changes. In particular, 3D imaging scanning is generally performed by scanning the first slice one-dimensionally, then scanning the adjacent slice one-dimensionally, and so on, which is repeated. This means that the timing of imaging differs between slices, and the above-mentioned artifacts are likely to occur in slices between slices. In ultrasound images with such artifacts, the structure and blood flow pattern of organs are displayed in a distorted manner, making it difficult to evaluate their morphology. In particular, with color Doppler, the visibility of valve regurgitation, shunt blood flow, and vortices, which are necessary for diagnosing diseases and evaluating cardiac function, is reduced, and quantitative evaluation of their size and flow rate becomes inaccurate, resulting in reduced diagnostic accuracy.

As a technique for reducing such slice-like artifacts, Patent Document 1 discloses a technique in which ultrasound imaging is performed for the same imaging range by dividing the imaging range into multiple transmission beam groups that are divided in time, and ultrasound is transmitted so that spatially adjacent transmission beams belong to different transmission beam groups. This technique performs spatially sparse ultrasound imaging multiple times and later combines these images, thereby reducing the difference in imaging timing across the entire imaging range per imaging and suppressing the occurrence of slice-like artifacts.

JP 2020-18845 A

The technology described in Patent Document 1 suppresses the occurrence of slice-shaped artifacts by reducing the difference in imaging timing over the entire imaging range, but the time difference between adjacent transmission beams becomes larger locally. This widening of the local time difference has a significant effect especially on color Doppler imaging. This is because, in imaging of B-mode images, which are morphological images, a transmission beam in each direction is usually transmitted only once, whereas in color Doppler imaging, a transmission beam in each direction is repeatedly transmitted multiple times in succession to detect the velocity. Since the number of repeated transmissions is usually around 10 times, when the technology described in Patent Document 1 is applied to color Doppler imaging, the time difference between adjacent transmission beams is about 10 times greater than that in B-mode image imaging. As a result, although the occurrence of slice-shaped artifacts can be prevented, block-shaped artifacts occurring between individual transmission beams tend to be more noticeable.

As described above, the technology described in Patent Document 1 has the problem that it cannot suppress block-shaped artifacts that occur between transmitted beams, particularly in color Doppler imaging.

The present invention aims to provide an ultrasound imaging device that performs three-dimensional ultrasound imaging while reducing artifacts caused by local transmission time differences.

The present invention solves the above problem by adopting a transmission pattern in which the transmission time difference between adjacent transmission beams is approximately the same in the azimuth and elevation directions in three-dimensional imaging using parallel receive phasing.

That is, the ultrasonic imaging device of the present invention includes a transmitting section that transmits ultrasonic transmission beams to an imaging range at multiple transmission angles, a receiving section that receives a reception beam, which is a reflected wave from the imaging range, by phasing it for each transmission angle, a signal combining section that combines two or more sets of reception signals obtained by repeating a full scan of one imaging range two or more times to generate an ultrasonic image of one time phase, and a transmission control section that controls the transmitting section. The transmission control section controls the transmitting section using a transmission sequence in which transmission beams that are adjacent in time and transmitted at different transmission angles during each full scan are transmitted at transmission angles that are not spatially adjacent in both the azimuth direction and the elevation direction.

The ultrasound imaging method of the present invention also includes a step of repeating a full scan of the imaging range two or more times by transmitting and receiving ultrasound, and a step of synthesizing two or more sets of signals obtained by the two or more full scans to generate an ultrasound image of one time phase, and uses a transmission pattern in which the transmission time difference between adjacent transmission beams is approximately the same in the azimuth direction and the elevation direction in each full scan.

The present invention makes it possible to reduce artifacts caused by local transmission time differences in ultrasound imaging.

FIG. 1 is a block diagram showing an example of the configuration of an ultrasound imaging apparatus according to a first embodiment. FIG. 4 is a flowchart showing an operation process flow of a control unit according to the first embodiment. FIG. 2 is a diagram showing a spatial representation of a typical transmission order for obtaining a color Doppler image. FIG. 1 is a timeline showing a typical transmission switching order. FIG. 4 is a diagram showing an example of a transmission pattern according to the first embodiment. FIG. 11 is a diagram showing another example of a transmission pattern according to the first embodiment. FIG. 11 is a diagram showing yet another example of a transmission pattern according to the first embodiment. FIG. 4 is a diagram showing an example of a transmission sequence according to the first embodiment. FIG. 9 is a diagram showing an example of a time change in Doppler velocity obtained in the transmission sequence of FIG. 8 . FIG. 13 is a diagram showing an example of applying a fitting curve to the time change of Doppler velocity. FIG. 11 is a diagram showing an example of an image display presented to a user. FIG. 11 is a diagram showing another example of an image display presented to a user. FIG. 13 is a diagram showing yet another example of an image display presented to a user. FIG. 11 is a flowchart showing an operation process flow of a control unit according to the second embodiment. FIG. 11 is a diagram showing an example of a transmission sequence according to the second embodiment. FIG. 4 is a diagram showing an example of dividing an imaging range of a B-mode image. FIG. 13 is a diagram showing another example of dividing the imaging range of a B-mode image. FIG. 13 is a diagram showing yet another example of dividing the imaging range of a B-mode image. FIG. 13 is a diagram showing an example in which the imaging range of a B-mode image is divided so as to overlap with a part of the imaging range;

An embodiment of the present invention will be described below with reference to the drawings.

<Embodiment 1>
The ultrasonic imaging device of this embodiment has a common configuration with a general ultrasonic imaging device, but is characterized by the addition of a means for reducing artifacts caused by local transmission time differences. In this embodiment, three-dimensional color Doppler imaging, which is most likely to cause artifacts due to local transmission time differences, is taken as an example, but the present invention may also be applied to imaging of B-mode images.

The ultrasound imaging device of this embodiment will be described below with reference to FIG.

The ultrasound imaging device 1 generates ultrasound images while controlling the ultrasound probe 2, and includes an input unit 10, a control unit 11, a transmission unit 12 that transmits ultrasound signals (transmission beams), a reception unit 13 that receives echo signals, a display unit 14, and a memory (storage unit) 15. The control unit 11 includes a signal processing unit 41 that calculates a tomographic image called a B-mode image or an image of blood flow velocity distribution called a color Doppler image based on the acquired echo signals, and a transmission sequence control unit (transmission control unit) 42 that controls the transmission angle and time of the transmission beam transmitted by the transmission unit 12. Here, a pattern that spatially determines the scanning order of the transmission angle is called a transmission pattern, and a time-based transmission order including the transmission pattern is called a transmission sequence.

The ultrasound probe 2 comes into contact with the living body 3 to be examined, and irradiates ultrasonic waves to the cardiovascular system 30 in the living body 3 according to the signal generated by the transmitting unit 12, and the receiving unit 13 receives the echo signal of the cardiovascular system 30. The ultrasound probe 2 generates continuous waves or pulse waves according to the scanning method, and captures a two-dimensional cross-sectional image or a three-dimensional image of the imaging range 21.

The input unit 10 includes a keyboard and a pointing device that allow a doctor or technician (hereinafter collectively referred to as a user) who operates the ultrasound imaging device to set the operating conditions of the ultrasound imaging device in the control unit 11. It also includes a function for importing information from external devices, such as an electrocardiogram, when such information is used in the examination.

The control unit 11 controls the transmission unit 12, reception unit 13, display unit 14, and signal processing unit 41 based on the operating conditions of the ultrasound imaging device set by the input unit 10, and can be constructed, for example, in the CPU (Central Processing Unit) of a computer system.

The transmitter 12 has an oscillator that generates a signal of a predetermined frequency and sends a drive signal to the ultrasound probe 2. The transmitter 12 also has a beamformer (not shown) for changing the transmission angle and depth of the ultrasound (transmission beam) emitted from the ultrasound probe 2. In three-dimensional color Doppler imaging, the transmission angle is changed in two dimensional directions, the azimuth direction and the elevation direction, and transmission and reception are performed multiple times at each transmission angle. The transmission sequence that determines the transmission angle and transmission timing of this transmission beam is instructed to the transmitter 12 by the transmission sequence control unit 42.

Although not shown, the receiving unit 13 includes a receiving circuit and an A/D (Analog-to-Digital) converter with a sampling frequency of typically 10 MHz to 50 MHz, and also performs signal processing such as phasing addition, detection, and amplification on the echo signals received by the ultrasonic probe 2. However, the A/D converter may be provided in front of the signal processing unit 41 instead of the receiving unit 13, in which case the signal processing unit 41 performs signal processing such as phasing addition, detection, and amplification. Furthermore, the receiving unit 13 may have a received data memory (not shown) that temporarily stores echo signals for each receiving element of the ultrasonic probe 2 or for each opening grouping the elements.
Furthermore, the receiving unit 13 of this embodiment performs parallel receiving phasing for receiving reflected waves in a plurality of directions for a transmission beam with one transmission angle.

The signal processing unit 41 forms an ultrasound image, for example, by software executed by the CPU. It includes a B-mode image calculation unit 411 that forms a B-mode image by visualizing the reflection intensity distribution of the echo signal as a brightness distribution, a color Doppler calculation unit 412 that forms a color Doppler image by estimating the motion information of a moving object based on the Doppler effect from the phase information of the echo signal, and a signal synthesis unit 414 that synthesizes multiple sets of signal data obtained by scanning the imaging range multiple times to generate an ultrasound image of one time phase. It also includes a display image formation unit 13 that displays the B-mode image and the color Doppler image as a display image on the display unit 14.

The color Doppler calculation unit 412 estimates, for example, the average velocity, average variance, average power, etc., as the motion information of the moving object for each measurement point with a different azimuth angle and depth. The moving object includes, for example, tissue such as the heart wall, blood flow, and contrast agent. In this embodiment, unless otherwise specified, the color Doppler is treated as the velocity distribution of blood flow. In addition, although not shown in FIG. 1, the signal processing unit 41 may apply a clutter filter to the motion information of the moving object at a stage subsequent to the color Doppler calculation unit 412 or at a stage subsequent to the signal synthesis unit 414 to eliminate or suppress signal components (called clutter signals) originating from stationary tissue or slow-moving tissue. In general, echo signals originating from blood flow are weaker than echo signals originating from surrounding tissues, so the blood flow can be emphasized and displayed by using a clutter filter.

The transmission sequence control unit 42 instructs the transmission unit 12 to transmit ultrasound with a transmission pattern in which the transmission time difference between spatially adjacent transmission beams is approximately the same in the azimuth direction and the elevation direction. This transmission sequence may be generated each time imaging is performed based on imaging conditions such as the angle of view and depth range, or a pattern prepared in advance may be applied. Details of the transmission sequence will be described later.

Some or all of the functions of the components of the signal processing unit 41 and the transmission sequence control unit 42 may be realized by software executed by the same CPU as that constituting the control unit 11 or a different CPU, or may be realized by hardware such as an ASIC (Application Specific Integrated Circuit), FPGA (Field-Programmable Gate Array), or GPU (Graphics Processing Unit).

The memory 15 stores echo signals, information necessary for calculations in the signal processing unit 41 and the transmission sequence control unit 42 (such as information instructed by the user via the input unit 10), and the processing results of the signal processing unit 41 and the transmission sequence control unit 42 (B-mode images, color Doppler images, etc.).

The operation of the ultrasound imaging device of this embodiment in the above configuration will be described. Figure 2 shows the flow of operations when performing three-dimensional color Doppler imaging.

[Step S1]
When imaging is started, the transmission sequence control unit 42 instructs the transmission unit 12 to execute a transmission sequence (S1). The transmission sequence instructed by the transmission sequence control unit 42 is a predetermined scanning order of transmission beams in three-dimensional color Doppler imaging, and is stored in the memory 15. In the present embodiment, a transmission sequence that suppresses slice-like artifacts and block-like artifacts in a color Doppler image is set.

The transmission sequence is explained in detail below.

As a premise, in 3D color Doppler imaging, transmission is performed with different transmission angles in the azimuth and elevation directions, and multiple reception beams are obtained from a transmission beam with the same transmission angle. In the following explanation, the range in which multiple reception beams are obtained from a transmission beam with one transmission angle is called a transmission block. Reception beams belonging to the same transmission block have velocity information of the same time phase. Furthermore, since it is necessary to obtain velocity information from signals with two or more different time phases in transmission to obtain a color Doppler image, transmission is performed at least twice in the same direction, usually 5 to 15 times. At this time, transmission is performed with an interval until the ultrasound reaches the deepest part of the imaging depth and returns to the probe element surface. This time interval is called PRT (pulse repetition time).

Under these assumptions, if the time difference between transmission blocks is relatively short and there is no large difference in either the azimuth or elevation directions, artifacts are suppressed, but if the time difference between transmission blocks is large, artifacts are likely to occur. This will be explained in detail using the conventional transmission sequence shown in Figures 3 and 4. Figure 3 is a diagram that represents the order in which the transmission angle is switched, i.e., the scanning order, spatially by arranging it on a surface at a certain depth, and shows the case of scanning a 6 x 6 transmission block as an example.

In the scanning sequence shown in Figure 3, the transmission angle is switched sequentially in the azimuth direction while scanning one-dimensionally to acquire signals from the first stage of transmission block groups. Next, the transmission angle is shifted in the elevation direction, and the transmission angle is again switched sequentially in the azimuth direction while scanning one-dimensionally to acquire signals from the second stage of transmission block groups. By repeating this process, signals from the entire imaging range are acquired.

Figure 4 is a diagram showing the timing of transmission switching during a series of volume imaging, in which transmission 400 for obtaining a B-mode image and transmission 410 for obtaining a color Doppler image are repeated as a set, and transmission 410 for obtaining a color Doppler image scans the transmission block groups from the first to sixth stages, and shows a case in which nine repeated transmissions are performed at one transmission angle as an example.

In the scanning order shown in Figure 3, when nine repeated transmissions are performed at one transmission angle as shown in Figure 4, the scanning time difference ΔTa between adjacent transmission blocks in the azimuth direction is 9 x PRT, whereas the time difference ΔTe between adjacent transmission blocks in the elevation direction is 6 x 9 x PRT (54 x PRT), resulting in a large difference of six times the time. This time difference causes slice-shaped artifacts to occur between the first and second stages.

For example, assuming imaging of the heart and a PRT of 0.2 milliseconds, in the scanning sequence of FIG. 3, the time difference ΔTa between adjacent transmission blocks in the azimuth direction is 1.8 milliseconds, which is sufficiently short compared to the time it takes for the mitral valve, aortic valve, etc. to open and the inflow velocity to reach its maximum (usually about 100 to 150 milliseconds, hereinafter referred to as the maximum flow velocity arrival time), but the time difference ΔTe between adjacent transmission blocks in the elevation direction is 54 times the PRT, that is, 10.8 milliseconds. This corresponds to more than 10% of the maximum flow velocity arrival time, and if it is assumed for simplicity that the inflow blood flow increases in speed at a constant acceleration after the valve opens, it is estimated that a speed difference of more than 10% of the maximum flow velocity may occur between adjacent transmission blocks in the elevation direction. In other words, when the scanning sequence is arranged in stages as shown in FIG. 3, there is a six-fold difference between the time difference between adjacent transmission blocks in the azimuth direction and the time difference between adjacent transmission blocks in the elevation direction, and slice-shaped artifacts may occur between the first and second stages.

The technology described in Patent Document 1 divides the transmission blocks into two or three transmission block groups and sets the scanning order so that adjacent transmission blocks in the azimuth or elevation direction do not belong to the same transmission block group. Although this technology is effective in suppressing slice artifacts in B-mode images, it cannot suppress block artifacts in color Doppler images, which are transmitted multiple times at the same transmission angle, because the time difference between adjacent transmission blocks is large and a speed difference of 10% or more of the maximum flow velocity can occur.

In the transmission sequence of this embodiment, in order to suppress the above-mentioned slice-like artifacts and block-like artifacts in the color Doppler image, a transmission sequence is used that satisfies the following conditions for transmission to obtain a color Doppler image: the time difference between adjacent transmission blocks in the azimuth direction is approximately the same as the time difference between adjacent transmission blocks in the elevation direction (condition 1), and the representative value of the time difference between adjacent transmission blocks is sufficiently short compared to the maximum flow velocity arrival time (condition 2). Here, "approximately the same time difference" preferably means, for example, that the standard deviation of the time difference in four directions (up, down, left, right in FIG. 3) is within about 2 on average, and "sufficiently short compared to the maximum flow velocity arrival time" preferably means, for example, within about 5% of the maximum flow velocity arrival time. These reference values may be determined in advance based on the overall appearance of the image, the visibility of valve regurgitation, shunt blood flow, and vortex, and the evaluation accuracy of their size and flow rate, or may be specified by the user.

The above-mentioned condition 1 can be expressed by formula (1) where the up, down, left and right directions on a surface at the same depth are represented by U, D, L and R, the representative value of the scan order difference between adjacent transmission blocks is Ni (i = any one of U, D, L and R), and the average value of the scan order difference between transmission blocks in four local directions is μ. _R1 is the upper limit of tolerance for the standard deviation of the scan order difference between adjacent transmission blocks, and in the above example, _R1 = 2.

Moreover, the above-mentioned condition 2 can be expressed by the following formula (2), where M is the number of repeated transmissions continuously transmitted in the same direction, PRT is the pulse repetition period, and T is the maximum flow velocity arrival time. N ₀ represents the average value of all N.

式（２）において、Ｒ_２は最大流速到達時間に対する送信ブロック間の時間差の許容上限を意味し、前掲の例では、Ｒ_２＝５％となる。

In equation (2), _R2 means the allowable upper limit of the time difference between transmission blocks relative to the maximum flow velocity arrival time, and in the above example, _R2 =5%.

Examples of scanning orders (transmission patterns) that satisfy condition 1 (equation (1)) are shown in Figures 5, 6, and 7. In Figures 5 to 7, T stands for a transmission block, and the numbers attached to T indicate the scanning order.

Figure 5 shows a scan order in which the transmission blocks in the azimuth direction are grouped into block groups, each consisting of two rows from the top, and in each block group, the rows are alternated from left to right, and the scan order is in a zigzag pattern. In this case, the difference in scan order (N) between most transmission blocks is 5 to 7, and an almost uniform time difference can be obtained, but between transmission blocks belonging to different block groups, a large time difference occurs for some transmission blocks (for example, between the transmission block in scan order 2 and the transmission block in scan order 20). Although continuity is locally reduced in such areas, both the transmission block in scan order 2 and the transmission block in scan order 20 maintain continuity in the remaining three directions, and each of the neighboring transmission blocks maintains continuity in four directions (up, down, left, right, etc.), so slice-like artifacts and block-like artifacts are unlikely to occur.

Figure 6 shows a scan order for scanning in a diagonal direction. In this case, the difference in scan order between all adjacent transmission blocks is within 6, and the difference in scan order is almost uniform in the four directions (up, down, left, and right) regardless of which transmission block is focused on. Figure 7 shows a scan order for dividing the 36 transmission blocks into 9 areas (sections) consisting of 4 transmission blocks each, and scanning each area in a circular fashion. Although there is a large variation in the difference in scan order compared to the other two methods, it is highly random and has no directional specificity, so at least the occurrence of slice-like artifacts can be suppressed.

While none of the transmission sequences shown in Figures 5 to 7 caused the difference in scanning order to be large in a specific direction, the typical value of the difference in scanning order was roughly 6, which is not an improvement over the method in Figure 3, and it is highly likely that condition 2: the typical value of the time difference between adjacent transmission blocks being sufficiently short compared to the time to reach the maximum flow velocity, is not met.

In order to obtain a transmission sequence that satisfies condition 2, in this embodiment, repeated transmissions that are normally transmitted continuously in the same direction are divided in time and performed multiple times. A specific example is shown in FIG. 8. In FIG. 8, a transmission 800 for obtaining a B-mode image and a transmission 810 for obtaining a color Doppler image (hereinafter abbreviated as color Doppler transmission) are set and the set is repeated as in FIG. 3, but the color Doppler transmission 810 performs transmission (full scan) to scan all transmission blocks that constitute the imaging range multiple times. In one full scan, for example, scanning of transmission blocks 1 to 36 is performed in one round in any of the scan orders shown in FIG. 5 to FIG. 7. The scan order of FIG. 5 to FIG. 7 to be selected can be appropriately determined according to the position of the imaging target (region of interest) in the imaging range. Such a full scan is repeated. In this case, the number of transmissions M (or the number of full scans that determine it) in one transmission angle direction in one full scan is determined so that the time difference between the transmission blocks in each full scan satisfies formula (2). That is, the product (PRT×M×N ₀ ) of PRT, the number of transmissions M, and the average scan order difference N ₀ between adjacent transmission blocks is set to be equal to or less than the allowable time difference (T×R ₂ ) between transmission blocks relative to the maximum flow velocity arrival time T.

Here, the number of transmissions M in one transmission angular direction is expressed by the following formula (3), where the number of full scans is _M2 and the number of speed estimations to be averaged is _M1 . Here, the number of speed estimations to be averaged is the total number of speed estimations performed during multiple full scans (here, _M2 times) when a speed estimation performed between two consecutive transmissions in one transmission angular direction is considered to be one. When the full scan is performed once, all transmissions are performed consecutively, so M-1 speed estimations can be performed. On the other hand, when the full scan is divided into multiple scans, even if the number of transmissions M is the same, some transmissions are discontinuous, so the number of speed estimations is reduced accordingly.

At this time, a minimum value M _min is set for the number of transmissions M so as not to make it too small. This minimum value M _min is the allowable number of repeated transmissions required for filtering by the clutter filter, and an appropriate value is set according to the characteristics of the clutter filter.

Generally, one of the purposes of repeatedly transmitting in the same direction about 10 times in color Doppler imaging is to obtain a more reliable numerical value by averaging the velocity estimates multiple times; the more averaging, the more stable the numerical value becomes. Another purpose is to obtain the effect of a filter called a clutter filter, which cuts out low-velocity components; this effect is difficult to obtain if the number of repeated transmissions is small. The minimum number of repeated transmissions required for clutter filtering differs depending on the filter design, but for example, it will not function properly if the number of repeated transmissions is less than four.

In the transmission sequence of the present embodiment, even if the number of repeated transmissions in one whole transmission is reduced, the number of speed estimations _M1 can be secured by performing multiple whole transmissions, thereby achieving the first effect of stabilizing speed estimation. Furthermore, by setting a limit to the number of transmissions M in one whole transmission to the minimum value _Mmin as described above, the second effect can be guaranteed.

When it comes to the PRT, if we focus on one scan, the time interval between transmissions is the PRT itself, so the speed range defined by the PRT does not change.

In the example of Fig. 8, by setting _Mmin = 5, equation (3) is satisfied and the number of speed estimations to be averaged is made almost the same as the original, so the number of speed estimations to be averaged _M1 is set to 12, the number of full scans _M2 is set to 3, and the number of repeated transmissions M is set to 5. However, Fig. 8 is an example of a transmission sequence that satisfies the above-mentioned conditions, and this embodiment is not limited to this. In addition, the scanning order in the full scan is not limited to the scanning order of Figs. 5 to 7 as long as it satisfies equation (1).

The transmission sequence determined to satisfy the above-mentioned conditions may be automatically generated to satisfy formulas (1), (2), and (3) based on the imaging conditions such as the angle of view and depth range specified by the user via the input unit 10, or may be selected from pre-prepared patterns that satisfy these formulas and applied. Also, transmission sequences corresponding to possible imaging conditions may be prepared in advance. In general, since the user rarely significantly changes the imaging conditions used in 3D color Doppler imaging, it is advantageous in terms of improving the response of the device operation to prepare a transmission sequence corresponding to the imaging conditions in advance.

[Step S2]
The transmitter 12 transmits a drive signal to the ultrasound probe 2 in the designated transmission order, transmission angle, and number of repeated transmissions based on the transmission sequence instructed by the transmission sequence control unit 42. As a result, for example, as shown in Fig. 8, transmission 800 for obtaining a B-mode image and transmission 810 for obtaining a color Doppler image (multiple full scans) are alternately repeated. Here, the transmission 800 for obtaining a B-mode image and the transmission 810 for obtaining a color Doppler image do not necessarily need to be performed in the same spatial range. In addition, the number of possible transmission angles, the transmission angle, and the order of scanning may be the same or different between the transmission 800 for obtaining a B-mode image and the transmission 810 for obtaining a color Doppler image.

At this time, other transmission conditions such as the drive frequency, wave number, and convergence of the transmission beam are the same as those for conventional 3D color Doppler imaging. In general, the transmission angle and convergence of the transmission beam are converted into a transmission wavefront that realizes them, and a drive delay time that forms the transmission wavefront is given to each drive element of the ultrasound probe 2.

[Step S3]
The ultrasound probe 2 transmits ultrasound waves in accordance with the drive signal sent from the transmission unit 12. The ultrasound probe 2 also receives echo signals returned from inside the body. This process is the same as that of normal 3D color Doppler imaging.

[Step S4]
The receiving unit 13 performs detection processing on the echo signal received by the ultrasonic probe 2. This detection processing includes detection, amplification, phasing and addition, as in normal 3D color Doppler imaging. As a result of the detection processing, the reception signal used to form a B-mode image and the reception signal used to form a color Doppler image are sent to the signal processing unit 41, respectively. At this time, the reception signal is divided into information for each depth by appropriately providing a window in the reception time. The reception signal used to form the color Doppler image sent to the signal processing unit 41 may have undergone parallel reception phasing at this point, or may be transmitted to the signal processing unit 41 as the reception signal for each channel, and the signal processing unit 41 may perform parallel reception phasing. The effect of the parallel reception phasing performed by the receiving unit 13 is expected to be an effect of reducing the amount of information transmitted by the receiving unit 13 to the signal processing unit 41, thereby suppressing the time required for transmission.

[Step S5]
The signal processing unit 41 forms a B-mode image and a color Doppler image based on the signal sent from the receiving unit 13. Since the formation of a B-mode image is the same as in normal cases, only the formation of a color Doppler image will be described here.

<Step S51>
The color Doppler calculation unit 412 performs color Doppler calculation for each transmission angle in the same manner as in normal 3D color Doppler imaging, and sends the calculation results to the signal synthesis unit 414. Here, color Doppler calculation refers to calculating the Doppler velocity for each depth based on the phase components of the received signals of two or more time phases obtained by repeated transmission. For example, when the transmission sequence of Fig. 8 is used, three sets of Doppler velocities of four consecutive time phases obtained by repeated transmission are obtained for each transmission angle and each depth, each set having different timings for the entire scan.

<Step S52>
The signal synthesis unit 414 synthesizes a plurality of sets of Doppler velocities with different timings of the overall scan based on the calculation results sent from the color Doppler calculation unit 412, thereby forming a color Doppler image of one volume, and sends the image to the display image formation unit 413. At this time, the synthesis method may be a simple arithmetic average, as in normal 3D color Doppler imaging, but in this embodiment, since the Doppler velocities are obtained by performing the overall scan multiple times, and a simple average of the Doppler velocities obtained in different overall scans may not correctly capture the change in blood flow, a synthesis method that solves this problem is adopted. Specifically, a method of adding the Doppler velocities after weighting them according to the timing at which they were obtained, or a method of fitting the Doppler velocities obtained in each scan is adopted.

First, we will explain weighted addition using an example in which the Doppler velocities obtained using the transmission sequence in Figure 8 are synthesized.

Figure 9 shows the distribution of Doppler velocities along the time axis obtained by three full scans, with crosses indicating the Doppler velocities obtained at each time phase. When the transmission sequence of Figure 8 is used, the four consecutive time phases obtained by repeated transmission have a sufficiently short time interval (= PRT) relative to the time to reach the maximum flow velocity, but the time phase that returns after a full cycle of scans 1 to 36 has a long time interval (= PRT x 5 x 36). For this reason, for example, if the timing of the central full scan 812 of the three full scans 811 to 813 coincides with the time phase of the peak blood flow velocity, simply arithmetically averaging the 12 Doppler velocities obtained by the first to third full scans 811 to 813 will result in the maximum Doppler velocity that can be obtained being rounded down to the Doppler velocities of the surrounding time phases, and the true maximum Doppler velocity will be missed.

In this embodiment, the maximum Doppler velocity can be measured by combining the first to third overall scans 811-811 with a larger weighting coefficient the closer they are to the central time phase. Weighted addition is performed by weighting each overall scan and averaging, for example, as shown in equation (4). In this case, the weighting coefficient is increased for overall scans closer to the central time phase.

ただし、Ｖ₀は推定した中央時相のドプラ速度、Ｖ_i,jはi回目の全体走査におけるj回目の繰り返し送信を意味し、α_iは全体走査毎の重み係数を表す。

where V ₀ is the estimated Doppler velocity in the central phase, V _i,j is the j-th repeated transmission in the i-th full scan, and α _i is the weighting coefficient for each full scan.

ただし、β_i,jは時相毎の重み係数を表す。 Alternatively, the arithmetic average may be calculated by weighting each phase as shown in Equation 5. In this case, the weighting coefficient is set to be larger for phases closer to the central phase.

Here, β _i,j represents a weighting coefficient for each phase.

Alternatively, instead of weighted addition, a fitting function may be calculated for 12 Doppler velocities, and the value of the central phase _t0 may be used as the combined Doppler velocity value, as shown in Fig. 10. For the fitting function, for example, a polynomial with an order equal to or less than the number of phases used in the calculation may be used. In particular, in order to ignore the effect of variations between close phases, a polynomial with an order of M ₂ -1, as shown in equation (6), may be used.

式中、a、ｂ、ｃはそれぞれ所定の係数を表す。

In the formula, a, b, and c each represent a predetermined coefficient.

This synthesis process can be performed simultaneously for all transmission angles after the color Doppler calculation unit 412 has completed color Doppler calculations for all transmission angles and sent the calculation results together to the signal synthesis unit 414, or it can be performed for each transmission angle by sending the calculation results to the signal synthesis unit 414 each time the color Doppler calculation unit 412 completes color Doppler calculations for each transmission angle. The method of sending the calculation results of the color Doppler calculations sequentially to the signal synthesis unit 414 is expected to have the effect of reducing the overall calculation time, since the calculations of the color Doppler calculation unit 412 and the signal synthesis unit 414 can be processed in parallel.

The calculations of the signal synthesis unit 414 may also include a process of synthesizing signals at the same position calculated from receive beams belonging to different transmit blocks (this synthesis process is called inter-transmit block synthesis to distinguish it from the synthesis of multiple transmit results described above). It is known that an offset error occurs in the Doppler velocity calculated based on receive beams at positions away from the center of the transmit beam among the receive beams obtained by parallel receive phasing, and the positive and negative of this offset error are reversed in adjacent transmit blocks. Therefore, inter-transmit block synthesis, which synthesizes signals at the same position originating from adjacent transmit blocks, has the effect of reducing such errors. Note that inter-transmit block synthesis may be performed on the echo signal itself or after the calculation of the Doppler velocity, and approximately the same effect can be obtained.

Generally, if the time difference between the combined transmission blocks is large, it becomes difficult to obtain such an error reduction effect. However, this embodiment employs a transmission sequence that makes the time difference between adjacent transmission blocks in the azimuth direction and the time difference between adjacent transmission blocks in the elevation direction sufficiently short compared to the maximum flow velocity arrival time, so it is also useful for obtaining an error reduction effect by combining transmission blocks in all directions.

[Step S6]
The display image forming unit 413 forms an image to be presented to the user based on the image signals sent from the B-mode image calculation unit 411 and the signal synthesis unit 414. The display form of the image is not particularly limited, but it is common to display a B-mode image and a color Doppler image in an overlapping manner.

Examples of images presented to the user are shown in Figures 11 to 13. Figure 11 is an example of a three-dimensional image, Figure 12 is an example of a two-dimensional cross-sectional image, and Figure 13 is an example of two orthogonal cross-sectional images displayed side by side. Doppler velocity can be displayed as a color distribution by preparing a color bar that corresponds to the flow velocity value and color tone, as in two-dimensional color Doppler. Figures 12 and 13 show Doppler velocity using this method. Alternatively, as shown in Figure 11, only areas where the Doppler velocity is above a certain level can be colored. This display method is effective for visually checking the presence or absence of flow velocity, and is particularly effective when one wishes to check only the presence or absence of valvular regurgitation in a display format where color coding is difficult to see, such as a three-dimensional display.

When the time difference between adjacent transmission blocks in the elevation direction is large, the three-dimensional color Doppler image is more likely to have blocking in this direction, and the cross-sectional color Doppler image in this direction is more likely to have blocking than the cross-sectional image in the scanning direction. In contrast, in this embodiment, the time difference between adjacent transmission blocks in the azimuth direction and the time difference between adjacent transmission blocks in the elevation direction are both made sufficiently short compared to the maximum flow velocity arrival time, which is expected to reduce the difference in the likelihood of blocking occurring depending on such directions.

As described above, according to this embodiment, slice-like and block-like artifacts can be suppressed in 3D color Doppler images that require repeated transmission by using a transmission sequence that satisfies the following two conditions. Condition 1: The scan order difference between transmission blocks is reduced and leveled in four directions, and Condition 2: The representative value of the time difference between adjacent transmission blocks is sufficiently shorter than the maximum flow velocity arrival time of the fluid to be measured. Condition 2 is achieved by dividing the entire scan into multiple entire scans in which the number of transmissions per transmission angle is divided in time, and setting the number of transmissions per transmission angle (M) in each entire scan to be equal to or less than the allowable value for the maximum flow velocity arrival time. Furthermore, the effect of the clutter filter can be ensured by setting the number of transmissions per transmission angle (M) to be equal to or greater than the allowable minimum value.

In addition, according to this embodiment, in the synthesis of Doppler signals obtained by calculation between adjacent echo signals from multiple transmissions, synthesis processing is performed taking into account the timing of multiple overall scans and the time phase of the echo signals, thereby making it possible to prevent inaccurate measurement of the true maximum Doppler velocity. Furthermore, in this embodiment, echo signals from the same position belonging to adjacent transmission blocks or Doppler signals calculated therefrom can be synthesized between blocks, and in this case, since the scan order difference between transmission blocks is smoothed in four directions, the effect of reducing errors in the block end regions expected by inter-block synthesis can be improved.

<Embodiment 2>
In this embodiment, control for aligning the time phases of a B-mode image and a color Doppler image is added to the control of multiple full scans and the scan order in transmission to obtain a color Doppler image in embodiment 1. For this reason, the transmission to obtain a B-mode image is divided and is performed between each full scan of a transmission group (multiple full scans) to obtain a color Doppler image.

In imaging using a transmission sequence in which the transmission 800 for obtaining a B-mode image and the transmission group 810 for obtaining a color Doppler image are completely separate as in the first embodiment, if the representative phase of the B-mode image is the central phase of the transmission for obtaining the B-mode image and the representative phase of the color Doppler image is the central phase of the transmission group for obtaining the color Doppler image, the representative phase of the B-mode image and the representative phase of the color Doppler image will always be separated in time. For example, if the total number of transmissions for obtaining a B-mode image is about one-tenth of the total number of transmissions for obtaining a color Doppler image and the volume rate of the entire transmission sequence is 10 Hz, the representative phase of the B-mode image and the representative phase of the color Doppler image will be separated by about 50 milliseconds. In such a case, for example, a situation may occur in which a B-mode image in a phase in which the valve is closed and a color Doppler image in which inflow blood flow is occurring are displayed overlapping each other. In this embodiment, by adding control to align the time phases of the B-mode image and the color Doppler image, the time phase discrepancy that may occur when both images are displayed overlapping each other is eliminated.

The processing contents of this embodiment will be described below with reference to Figs. 14 and 15, focusing on the differences from embodiment 1. Fig. 14 is a diagram showing the operation processing flow, and Fig. 15 is a diagram showing transmission switching along the time axis. In Fig. 14, the same processes as in Fig. 2 are indicated with the same reference numerals, and duplicated explanations will be omitted. Also, in this embodiment, the device configuration is common to the configuration of embodiment 1 shown in Fig. 1, so Fig. 1 will be referred to as appropriate.

[Step S11]
When imaging is started, the transmission sequence control unit 42 instructs the transmission unit 12 to execute a transmission sequence.

The transmission sequence includes a transmission for obtaining a B-mode image and a transmission for obtaining a color Doppler image, and the transmission for obtaining a color Doppler image is a transmission sequence that satisfies the same conditions (condition 1, condition 2, and condition 3 if necessary) as in step S1 of embodiment 1. The transmission for obtaining a B-mode image is divided into multiple transmission groups and performed between transmissions (each full scan) for obtaining a color Doppler image. This means that a B-mode image of one volume is obtained in parts. In the example shown in FIG. 15, the entire transmission 800 for obtaining a B-mode image is equally divided by the number of full scans (811, 812, 813) for obtaining a color Doppler image to create three scan groups 801 to 803, and each scan group is performed between one full scan for obtaining a color Doppler image and the next full scan.

For transmission to obtain a B-mode image, the division of the transmission 800 for obtaining a B-mode image corresponds to the division of the imaging range, and each transmission scanning group scans the corresponding divided area of the imaging range. Examples of the method of dividing the imaging range are shown in Figs. 16 to 18. Figs. 16 to 18 are each a diagram that represents the range in which divided transmission is performed, with respect to transmission to obtain a B-mode image, by arranging it on a surface at a certain depth.

The example shown in FIG. 16 is a method in which the imaging range is simply divided into three in the elevation direction, and the first area (small area) to the third area are scanned from the top with scan groups 801 to 803. The example shown in FIG. 17 is an example in which the imaging range shown in FIG. 16 is further divided into smaller areas, and the spatially separated small areas (such as the three first areas) are scanned sequentially with scan groups 801 to 803, so that the time phase of the B-mode image is aligned throughout the imaging range. The example shown in FIG. 18 is an example in which transmissions belonging to three transmission groups are performed in a mosaic pattern, and adjacent areas in the azimuth and elevation directions are scanned by different transmission groups, i.e., at different times. Such mosaic-pattern transmissions have the effect of preventing a large time difference in imaging timing only between specific slices, i.e., at the boundaries between areas.

Furthermore, it is possible to scan the divided imaging ranges in each scanning group so that they overlap partially. An example of scanning with some overlap is shown in FIG. 19. This example shows the case of the area division in FIG. 16, where the first area scanned by the first scanning group and the second area scanned by the second scanning group overlap partially in the elevation direction, and the second area scanned by the second scanning group and the third area scanned by the third scanning group overlap partially in the elevation direction. By overlapping the small areas to be scanned in this way, it is possible to prevent a large time difference in imaging timing at the boundary between areas as in FIG. 18 with simpler control than FIG. 18. FIG. 19 shows an example of scanning with some overlap in the case of FIG. 16, but it can be similarly applied to area division other than that of FIG. 16.

Note that the range of the B-mode images shown in Figures 16 to 19 does not necessarily have to be the same as the range of the color Doppler images.

In this embodiment, by using a transmission sequence in which a transmission to obtain a B-mode image is performed between transmissions to obtain a color Doppler image as shown in FIG. 15, the representative phase of the B-mode image and the representative phase of the color Doppler image can be brought closer together. If the ratio of the total number of transmissions of the B-mode image and the color Doppler image and the volume rate of the entire transmission sequence are the same as in the above example, the transmission sequence in FIG. 15 can keep the representative phase of the B-mode image and the representative phase of the color Doppler image to about 17 milliseconds.

[Step S5]
The signal processing unit 41 forms a B-mode image and a color Doppler image based on the reception signal sent from the receiving unit 13. The formation (combination method) of the color Doppler image is the same as in the first embodiment. Here, only the formation of the B-mode image will be described.

<Step S50>
The B-mode image calculation unit 411 performs calculation processing for each transmission angle in the same manner as in normal B-mode imaging, and sends the calculation results to the signal synthesis unit 414 .

<Step S53>
The signal synthesis unit 414 arranges the calculation results sent from the B-mode image calculation unit 411 at a predetermined position and reconstructs a three-dimensional B-mode image. At this time, when imaging is performed for a transmission group such as that shown in Figs. 16 to 18, slice-shaped or block-shaped artifacts are likely to occur in the B-mode image at the boundaries of small regions with different imaging timings. In order to avoid this, a smoothing filter may be applied to the boundaries of the small regions. Also, as shown in Fig. 19, when imaging is performed in advance so that the small regions partially overlap each other, the signals of the overlapping parts are synthesised. Synthesis processing at the signal stage has the effect of preventing a decrease in the spatial resolution of the image compared to later filter processing.

As described above, according to this embodiment, by dividing the transmission to obtain a B-mode image in time, it is possible to eliminate the time lag with the color Doppler image on average, and when the B-mode image and the color Doppler image are superimposed, it is possible to eliminate the lag between the images, such as the velocity information at a certain point in time being presented on a morphological image of a different time phase. Furthermore, according to this embodiment, when dividing the transmission to obtain a B-mode image, it is possible to prevent the occurrence of slice-like or block-like artifacts due to division by overlapping the division pattern or applying a smoothing filter to the received data.

The above describes an embodiment of the present invention using an example of performing 3D color Doppler imaging, but the present invention is characterized by performing full scans multiple times to synthesize images of one time phase, and by using a specific transmission sequence for each full scan, and can be similarly applied not only to color Doppler imaging, but also to the imaging of other ultrasound images, including B-mode images.

Reference Signs List 1: Ultrasonic imaging device 10: Input unit 11: Control unit 12: Transmitter 13: Receiver 14: Display unit 15: Memory 2: Ultrasonic probe 21: Imaging range 3: Living body 30: Cardiovascular system 41: Signal processing unit 411: B-mode image calculation unit 412: Color Doppler calculation unit 413: Display image formation unit 414: Signal synthesis unit 42: Transmission sequence control unit

Claims (18)

a transmitting unit that transmits an ultrasonic transmission beam to an imaging range at a plurality of transmission angles;
a receiving unit that receives a reception beam, which is a reflected wave from the imaging range, by phasing the reception beam for each transmission angle;
a signal synthesis unit that synthesizes two or more sets of received signals obtained by repeating a full scan that scans one entire imaging range two or more times, and generates an ultrasound image of one time phase;
and a transmission control unit that controls the transmission unit, wherein the transmission control unit controls the transmission unit using a transmission sequence in which, in each full scan , transmission beams that are adjacent in time to each other in different transmission angles are at transmission angles that are not adjacent to each other in the azimuth direction and the elevation direction, and the transmission time difference between adjacent transmission beams in the azimuth direction and the elevation direction is equal to or less than a predetermined time difference in both the azimuth direction and the elevation direction . 2. The ultrasound imaging device according to claim 1,
An ultrasonic imaging apparatus, wherein the transmission sequence is characterized in that a standard deviation of a transmission time difference between adjacent transmission beams in an azimuth direction and a transmission time difference between adjacent transmission beams in an elevation direction is within an allowable upper limit . 2. The ultrasound imaging device according to claim 1,
the transmitting at the plurality of transmit angles includes transmitting at the same transmit angle a plurality of times;
The ultrasonic imaging apparatus according to claim 1, wherein the transmission sequence is a transmission sequence in which a plurality of transmissions at the same transmission angle are divided into full scans for each transmission. 4. The ultrasonic imaging apparatus according to claim 3,
The ultrasonic imaging device is characterized in that the transmission sequence is a transmission sequence used for color Doppler imaging, and the number of transmissions in each full scan is set within an allowable value determined by the time it takes for the fluid contained in the imaging range to reach its maximum flow velocity. 2. The ultrasound imaging device according to claim 1,
The ultrasonic imaging device is characterized in that the transmission sequence is a transmission sequence in which the transmission angle of the transmission beam is sequentially switched to either the azimuth direction or the elevation direction, and at that time, the transmission angle of the adjacent transmission angle is also switched to the other direction. 2. The ultrasound imaging device according to claim 1,
The ultrasonic imaging apparatus according to claim 1, wherein the transmission sequence is a transmission sequence in which the transmission angle of the transmission beam is switched to an angle shifted in an azimuth direction and an elevation direction, and the imaging range is scanned in an oblique direction. 2. The ultrasound imaging device according to claim 1,
The ultrasound imaging device is characterized in that the transmission sequence is a transmission sequence that divides the imaging range into a plurality of sections in which the transmission beam has different transmission angles, and scans all sections by switching the transmission angle so that temporally adjacent transmission beams have transmission angles included in different sections. 2. The ultrasound imaging device according to claim 1,
A storage unit for storing the transmission sequence,
The ultrasonic imaging apparatus according to claim 1, wherein the transmission control unit controls the transmission unit using a transmission sequence stored in the storage unit. 4. The ultrasonic imaging apparatus according to claim 3,
The signal synthesis unit calculates a representative value using the received signals obtained from multiple full scans when synthesizing received signals obtained by multiple transmissions at the same transmission angle. 10. The ultrasound imaging device according to claim 9,
The ultrasonic imaging apparatus according to claim 1, wherein the signal synthesis unit calculates the representative value using a different weight for each of a plurality of full scans. 10. The ultrasound imaging device according to claim 9,
The ultrasonic imaging apparatus according to claim 1, wherein the signal synthesis unit calculates a representative value by applying a different weight to each of the reception signals obtained in a plurality of full scans for each time phase in which the reception signals were obtained. 10. The ultrasound imaging device according to claim 9,
The ultrasonic imaging device according to claim 1, wherein the signal synthesis unit calculates a fitting curve by arranging received signals obtained in each of a plurality of full scans on a time axis, and determines a representative value from the fitting curve. 10. The ultrasound imaging device according to claim 9,
The ultrasonic imaging device is characterized in that, when combining reception signals obtained by multiple transmissions at the same transmission angle, the signal combining unit combines reception signals obtained from the same position by transmissions at different transmission angles. 2. The ultrasound imaging device according to claim 1,
the transmission sequence includes a transmission for obtaining a B-mode image and a transmission for obtaining a color Doppler image;
An ultrasonic imaging apparatus, comprising: a first transmitting section for receiving a first signal from a first receiving portion of the first ultrasonic wave; 15. The ultrasound imaging device according to claim 14,
The transmission for obtaining the B-mode image includes a plurality of transmission groups each scanning a plurality of small regions included in the imaging range, and the transmission unit performs at least one of the plurality of transmission groups between one full scan and the next full scan. 16. The ultrasound imaging device according to claim 15,
The signal synthesis unit applies a spatial filter to the vicinity of the boundaries of the small regions for different reception signals of the transmission groups when generating a B-mode image of the imaging range using reception signals obtained from each small region by scanning the multiple transmission groups. 16. The ultrasound imaging device according to claim 15,
the range scanned by the plurality of transmission groups includes one small area and an area near a boundary of another small area adjacent thereto;
The signal combining unit combines overlapping received signals obtained from the same position by scanning the multiple transmission groups when generating a B-mode image of the imaging range using received signals obtained from each small region by scanning the multiple transmission groups. a step of repeating a full scan of scanning an imaging range two or more times by transmitting and receiving ultrasonic waves;
and generating an ultrasound image of one time phase by combining two or more sets of signals obtained by two or more full scans.
Each of the multiple full scans includes a transmitting step of transmitting a transmission beam multiple times at a same transmission angle with a predetermined PRT;
In each full scan, a transmission pattern is used in which the transmission time difference between adjacent transmission beams is approximately the same in the azimuth direction and the elevation direction;
The ultrasonic imaging method according to the present invention, wherein each of the multiple full scans includes a transmission step of transmitting a transmission beam multiple times at the same transmission angle with a predetermined PRT, and the number of transmissions M at the same transmission angle satisfies the following formula :

In the formula, N ₀ represents the average value of the transmission time difference between the two azimuth directions and the transmission time difference between the two elevation directions, T represents the time to reach the maximum flow velocity of the fluid contained in the imaging range, and R ₂ represents the tolerance of the time difference.

Images