JP7488710B2 - Ultrasound Imaging Device - Google Patents

Description

The present invention relates to ultrasound imaging technology that uses ultrasound to image the inside of a subject.

Ultrasound imaging technology is a technique that uses ultrasound (sound waves that are not intended to be heard, generally high-frequency sound waves of 20 kHz or higher) to non-invasively image the inside of a subject, including the human body.

Ultrasound imaging is performed using a technique called beamforming, which involves forming transmit and receive beams.

Synthetic aperture imaging is widely used in ultrasound beamforming. In a typical synthetic aperture imaging, one element (transducer) in the array that transmits and receives ultrasound in the ultrasound probe transmits, and the reflected signal is received by one or more receiving elements. The positions of the transmitting and receiving elements are shifted in the direction of the array, and transmission and reception are repeated. Aperture synthesis is performed by synthesizing signals received by receiving elements in different transmission events. In particular, when a single element transmits and the same single element receives, it is called monostatic aperture synthesis (Monostatic SA), and when multiple receiving elements receive, it is called bistatic aperture synthesis (Bi-static SA). Other synthetic aperture imaging methods include focused beam aperture synthesis (SA with focused beam), which transmits a focused beam with a transmission aperture consisting of multiple elements instead of transmitting with a single element.

Patent Document 1 also discloses an aperture synthesis technology called the synthetic aperture sequential beamforming (SASB) method, in which the reflected signal from a focused beam is received by a receiving aperture consisting of multiple elements, and then receiving beamforming is performed in two stages.

In the first step, a single receiving line passing through the transmission focus point is set along the depth direction, and the receiving focus is set at the same position as the transmission focus. A delay amount is set for the receiving signal of each transducer to form a receiving beam focused on the receiving focus point, and the first acoustic line signal is obtained by delaying and adding the signals by this delay amount. The acoustic line signal thus obtained is a sum of reflected signals from many observation points located on concentric arcs centered on the transmission focus point, that is, a low-resolution acoustic line signal in which the signals from many observation points located on the arcs are mixed with the same S/N ratio (called LRI: Low Resolution Image, etc.). In the second step, the first acoustic line signal and another acoustic line signal obtained by shifting the positions of the transmission focus and the receiving focus are delayed by a predetermined delay amount, and then weighted and added to obtain a second acoustic line signal. This delay amount is set according to the distance between the observation point on the acoustic line signal and each transmission focus. As a result, the reflected signals from the observation points can be added with the same phase, so that the second acoustic line signal becomes a high-resolution acoustic line signal (called HRI: High Resolution Image, etc.). Through these processes, signal values for each observation point within the observation area are obtained.

In addition, Non-Patent Document 1 discloses a technique for setting multiple receiving foci at positions shifted from the transmitting focus in the SASB method. In the first stage, multiple first acoustic line signals are obtained by a single transmission through parallel delay and addition processing. In the second stage, a delay is applied to the acoustic line signals obtained by shifting the positions of the virtual transmitting element and the virtual receiving element, and weighted addition is performed to obtain an acoustic line signal at the observation point. In this technique, the transmitting focus is regarded as a virtual sound source (virtual transmitting element), and multiple receiving foci are regarded as virtual receiving elements. The delay amount in the second stage is applied according to the distance between the observation point and the virtual transmitting element and the virtual receiving element, so that the phases of the reflected signals from the observation point are aligned and added to obtain a high-resolution acoustic line signal. In this way, Non-Patent Document 1 performs aperture synthesis both between the virtual transmitting elements and between the virtual receiving elements through the first and second processing, and obtains signals at each observation point within the observation area. In other words, the SASB method not only combines acoustic line signals between transmissions, but also between multiple acoustic line signals received in parallel, which increases the signal-to-noise ratio of the signal at the observation point and improves resolution.

In the first stage of the SASB method, the received signals are bundled into an acoustic line signal, which can then be transmitted in the bundled acoustic line signal state, after which processing can be performed to obtain the signal value for each observation point in the second stage. This has the advantage in terms of implementation of being able to reduce the scale of the hardware. For example, it is expected to be installed in wireless probes and compact machines where the first stage is performed inside the probe.

In addition, the SASB method described in Non-Patent Document 1 allows for scalable calculation cost performance depending on the number of receiver focuses (= virtual receiver elements) set, which is expected to lead to higher image quality and adoption in a wide range of products.

In ultrasound imaging, artifacts (false images) caused by the physical characteristics of ultrasound propagation can hinder accurate examination and diagnosis. The main types of artifacts are known to be multiple artifacts caused by ultrasound reflecting multiple times during propagation, side lobe artifacts caused by side lobes (secondary poles) occurring to the side of the main lobe (principal pole), and grating lobe artifacts caused by strong beam intensity in a direction different from the main lobe. Among these, grating lobe artifacts create a strong virtual image far away from the real image, so arrays are designed to avoid artifacts.

The angle θ at which a grating lobe occurs is determined by the direction of the main beam, the wavelength λ of the ultrasound, and the element pitch d of the array. In the case of a monostatic array in which the same element transmits and receives, when the position of the transmitting and receiving elements is shifted by one element, a grating occurs in the direction of θ, which satisfies the following equation for an integer N.

2d sin θ=Nλ (Equation 1)

To avoid grating lobes, the element pitch d of the array is designed to be less than λ/2 for the wavelength λ.

Patent Document 2 describes a bistatic array in which the transmitting element array and the receiving element array are separated, and in an array in which the transmitting element array and the receiving element array are orthogonalized, a virtual element is assumed for each combination of transmitting element and receiving element. In the technology of Patent Document 2, in order to avoid grating lobes, the element arrangement of the transmitting element array and the receiving element array is designed so that the virtual elements are unevenly distributed and there are regions in which the pitch between the virtual elements is smaller than the pitch between the real elements.

It is also known that grating lobes occur not only due to real element arrangements, but also due to virtual element arrangements, such as the transmit and receive foci in SASB.

JP 2018-110784 A Patent No. 3567039

M. Bae, Nam Ouk Kim, Moon Jeong Kang and Sung Jae Kwon, "A new synthetic aperture imaging method using virtual elements on both transmit and receive," 2015 IEEE International Ultrasonics Symposium (IUS), Taipei, 2015, pp. 1-4.

If the element pitch is reduced to avoid grating lobes, it becomes more difficult to fabricate the array, and a larger number of elements are required to maintain the resolution and S/N ratio. This leads to problems such as increased costs for transducers, wiring, circuits, and signal processing.

It is known that grating lobes occur not only in real element arrays, but also in virtual element arrays. Grating lobes in virtual element arrays occur as a result of combining multiple transmit and receive beams through signal processing in aperture synthesis imaging. To avoid grating lobes in virtual element arrays, the pitch of the virtual elements must be made small, just as in the case of real element arrays.

In the SASB method described in Non-Patent Document 1, the positions of the virtual elements are the positions of the transmit focus and receive focus. If the virtual element pitch is reduced to avoid grating lobes, the interval between the transmit focus and receive focus will become narrower, resulting in a decrease in frame rate and an increase in signal processing costs. In addition, the number of virtual elements required to maintain resolution will increase, leading to increased costs.

The objective of the present invention is to increase the pitch of elements (real elements or virtual elements) while suppressing grating lobes.

The ultrasound imaging device of the present invention has a transmitting element that transmits ultrasound to a subject, a plurality of arranged receiving elements that receive echoes of ultrasound generated in the subject for each ultrasound transmission event, a movement control unit that moves the positions of the transmitting element and the plurality of receiving elements in the arrangement direction for each transmission event, and a receiving beamformer that synthesizes the received signals obtained at the plurality of receiving elements for each transmission event between the plurality of receiving elements and between transmission events. The movement control unit moves the positions of the transmitting element and the receiving element so that the difference between the movement vector representing the movement of the position of the transmitting element between a transmission event and the transmission event immediately preceding it and the movement vector representing the movement of the position of the receiving element is different for two consecutive transmission events.

According to the present invention, even if the pitch of the transmitting and receiving elements (real or virtual elements) is increased, grating lobes can be suppressed simply by controlling the amount of movement of the elements. Therefore, the aperture diameter of the aperture synthesis can be increased without increasing the number of transmitting and receiving elements. This makes it possible to achieve high resolution without increasing the implementation cost of the device.

FIG. 1 is a block diagram showing a concept of an ultrasonic imaging apparatus according to a first embodiment; 1A to 1C are diagrams for explaining the arrangement of transmitting elements and receiving elements of an ultrasonic imaging apparatus according to a first embodiment and a movement vector; 1A to 1C are diagrams for explaining the arrangement and movement vectors of virtual transmitting elements and receiving elements in the SASB method of the ultrasonic imaging apparatus according to the first embodiment. FIG. 1A is a diagram showing the positions and movement vectors of transmitting elements and receiving elements for each transmission event of the ultrasound imaging device of the first embodiment; FIG. 1B is a diagram showing the positions and movement vectors of transmitting elements and receiving elements for each transmission event of a comparative example; 1A is a graph showing the position of the phase center for each transmission event in the first embodiment; FIG. 1B is a graph showing the position of the phase center for each transmission event in the comparative example; FIG. 1 is a block diagram showing a specific configuration of an ultrasonic imaging apparatus according to a first embodiment. FIG. 2 is a diagram for explaining a transmission beam and a reception beam of the ultrasound imaging apparatus according to the first embodiment; FIG. 1A is a diagram for explaining a method for calculating a delay amount used by a first receive beamformer in the first embodiment; FIG. 1B is a diagram for explaining that a signal value at an observation point p of a second receive beamformer is included in a signal value at a representative point Qnm of a receive line; FIG. 1 is a diagram for explaining that the midpoint between the center of the transmission aperture and the center of the reception aperture in the first embodiment is the phase center. (a) is a diagram for explaining a monostatic array; (b) is a diagram for explaining a method for calculating a phase center; and (c) is a diagram for explaining that the interval between phase centers can be regarded as an element pitch d. A flow chart showing a process in which a movement control unit of an ultrasonic imaging apparatus according to a first embodiment calculates a movement amount of a receiving element for each transmission event. Block diagram of an ultrasonic imaging apparatus according to a second embodiment.

An ultrasound imaging device according to one embodiment of the present invention will be described with reference to the drawings.

First Embodiment
＜Overview＞
An overview of an ultrasound imaging apparatus according to a first embodiment will be described with reference to FIG.

As shown in FIG. 1, the ultrasound imaging device of the first embodiment includes a transmitting element 16 that transmits ultrasound to the subject 4, a plurality of receiving elements 26-1 to 26-4 arranged to receive ultrasound echoes generated in the subject 4 for each ultrasound transmission event, a movement control unit 30, and a receiving beamformer 20.

The transmitting element 16 and the multiple receiving elements 26-1 to 26-4 referred to here may be actual elements (transducers 2a) as shown in Figures 2(a) to (c), or they may be virtual elements (for example, elements assumed to be at the transmitting and receiving foci in the SASB method as described in Non-Patent Document 1) as shown in Figures 3(a) to (c).

The movement control unit 30 moves the positions of the transmitting element 16 and the receiving elements 26-1 to 26-4 in the arrangement direction of the receiving elements 26-1 to 26-4 for each ultrasonic transmission event (transmission phenomenon), for example, as shown in Figures 2(a) to (c) or 3(a) to (c).

The receive beamformer 20 synthesizes the receive signals obtained by the multiple receive elements 26-1 to 26-4 for each transmit event between the multiple receive elements 26-1 to 26-4 and between transmit events. An ultrasound image is generated using the synthesized signals.

Here, as shown in Figures 2(a) to (c) or 3(a) to (c), the movement of the position of the transmitting element 16 between a certain transmission event and the transmission event immediately before it is represented by a movement vector t. Similarly, the movement of the positions of the receiving elements 26-1 to 26-4 is represented by a movement vector r. The movement control unit 30 moves the positions of the transmitting element 16 and the receiving elements 26-1 to 26-4 so that the difference between the movement vector t of the transmitting element 16 and the movement vector r of the receiving elements 26-1, etc. is different for two consecutive transmission events.

For example, as shown in FIG. 4(a), the movement control unit 30 is configured to change at least one of the movement amount and movement direction of the positions of the receiving elements 26-1 to 26-4 depending on the transmission event. In the example of FIG. 4(a), the movement control unit 30 alternately sets zero and a predetermined value (e.g., 2Δ) as the movement amount of the positions of the multiple receiving elements 26-1 to 26-4 for each transmission event. At this time, the movement control unit 30 moves the position of the transmitting element 16 for each transmission event in a fixed direction by a predetermined fixed movement amount Δ.

In this way, when phase centers 210-1 to 210-4, which are the midpoints between the transmitting element 16 and the receiving elements 26-1 to 26-4, are found by moving the transmitting element 16 and the receiving elements 26-1 to 26-4 so that the difference between the movement vectors t and r is different for two consecutive transmission events, the distance P1 (FIG. 5(a)) between the phase centers 210-1 to 210-4 between consecutive transmission events can be made smaller than the distance P2 in the comparative examples (FIGS. 4(b) and 5(b)) in which the movement vectors t and r of the transmitting element 16 and the receiving elements 26-1 to 26-4 are kept constant for each transmission event (i.e., the transmitting elements and receiving elements are moved a constant amount (movement amount Δ) for each transmission event).

As a result, grating lobes can be suppressed even when the pitch of the receiving elements 26-1 to 26-4 is made sparser than in the comparative examples of Figures 4(b) and 5(b). Therefore, it is possible to obtain a high-resolution image by aperture synthesis while suppressing the amount of data by using elements (real elements or virtual elements) with a small sparse pitch.

Note that it is desirable that the distance P1 between the phase centers 210-1 to 210-4 between successive transmission events be equal to or less than 1/2 the wavelength λ of the ultrasound transmitted by the transmitting element 16.

The movement control unit 30 sets the movement vector r of the multiple receiving elements 26-1 to 26-4 to be the same. In other words, the receiving elements 26-1 to 26-4 are moved while maintaining the spacing between the receiving elements 26-1 to 26-4.

Furthermore, it is desirable for the movement control unit 30 to set the movement vectors t and r of the transmitting element 16 and the receiving elements 26-1 to 26-4 so that the multiple phase centers 210-1 to 210-4 are set at positions where they overlap the same number of times due to the repetition of the transmission event.

<Specific Configuration>
The ultrasonic imaging device 1 of the first embodiment will be described in further detail.

The ultrasound imaging device 1 described below performs receive beamforming and aperture synthesis using the SASB method to obtain signal values at multiple observation points set within the subject (see FIG. 3(b)). Therefore, the transmitting element 16 and multiple receiving elements 26-1 to 26-4 are virtual elements assumed to be at the positions of the transmitting focus and receiving focus in the SASB method, as shown in FIGS. 3(a) to (c).

However, as already mentioned, this embodiment is not limited to the SASB method, and any method that calculates the signal value of the observation point from multiple received signals by performing bi-static synthetic aperture (bi-static synthetic aperture) that performs aperture synthesis on both the transmitting aperture and the receiving aperture (see, for example, FIG. 3(a)) may be used.

As shown in FIG. 6, the ultrasound imaging device 1 includes a transmission beamformer 10, a reception beamformer 20, a movement control unit 30, a transmission/reception separation unit 40, an image processing unit 50, a control unit 60, and a console 70.

The ultrasonic probe 2 is connected to the transmit beamformer 10 and the receive beamformer 20 via a transmit/receive separator 40. The ultrasonic probe 2 incorporates a transducer array 200 in which transducers 2a capable of transmitting and receiving ultrasonic waves are arranged in a row.

The receive beamformer 20 includes a memory 201, a first receive beamformer 202, and a second receive beamformer 203, and performs receive beamforming and aperture synthesis using the SASB method to obtain signal values at multiple observation points set within the subject.

As shown in FIG. 7, the transmit beamformer 10 sets a transmit aperture 11 in the transducer array 200 of the ultrasound probe 2, and outputs a transmit signal to the transducer 2a in the transmit aperture 11. The transmit signal is converted into ultrasound by each transducer 2a in the transmit aperture 11, and is sent to the subject 4 as a transmit beam 15. At this time, the transmit beamformer 11 sets a delay time for each transmit signal sent to each transducer 2a, thereby setting the position of the transmit focus. The position of this transmit focus becomes the position of the virtual transmit element 16.

The transmit beamformer 10 can transmit a focused beam that forms a transmit focal point 16 within the subject 4 as the transmit beam 15, as shown in FIG. 7, or it can set a virtual transmit focal point (transmitting element) 16 in front of the transducer array 200 and transmit a beam that spreads within the subject 4 as the transmit beam 15.

A part of the transmitted beam 15 is reflected or scattered by a reflector or the like in the subject 4, becomes an echo, reaches the transducer array 200 of the ultrasound probe 2, and is received by each transducer 2a. The received signal output by each transducer 2a is temporarily stored in the element signal area 201a in the memory 201 via the transmission/reception separator 40.

The receive beamformer 20 sets multiple receive apertures 21-1 to 21-4 in the transducer array 200 for each transmission. In the example of FIG. 7, four receive apertures 21-1 to 21-4 are set. The receive beamformer 20 reads the receive signals of the multiple transducers 2a in the receive aperture 21-1 from the element signal area 201a of the memory 201, and performs a predetermined process such as adding up the receive signals after applying a delay amount to each receive signal, thereby forming a receive beam 25-1 and calculating a receive line signal for the receive line 27-1, which is then stored in the receive line area 201b in the memory 201. The method of calculating the receive line signal will be explained in detail later.

Similarly, for the other receiving apertures 21-2 to 21-4, receiving beams 25-2 to 25-4 are formed in the same manner, and receiving line signals for receiving lines 27-2 to 27-4 are calculated and stored in receiving line area 201b. As a result, receiving line signals for receiving lines 27-1 to 27-4 for one transmission event are stored in receiving line area 201b.

The movement control unit 30 moves the position of the transmission aperture 11 and the positions of the reception apertures 21-1 to 21-4 in the row direction of the transducer array 200 for each transmission event, thereby moving the transmission focus (virtual transmission element) 16 and the reception focus (virtual reception elements) 26-1 to 26-4. At this time, the movement control unit 30 moves the positions of the transmission element 16 and the reception elements 26-1 to 26-4 so that the difference between the movement vector t of the transmission element 16 and the movement vector r of the reception elements 26-1, etc. is different for two consecutive transmission events (see Figures 3(a) to (c) and Figure 4(a)).

The receive beamformer 20 performs aperture synthesis processing on the receive line signals of receive lines 27-1 to 27-4 obtained by multiple transmit events, both within the same transmit event and between transmit events. This calculates the signal strength reflected, etc. at an observation point within an observation area set within the subject 4.

<Delay amount of receive beamforming>
Here, the amount of delay caused by the first receive beamformer 202 will be described in detail.

As the first stage of receive beamforming, the first receive beamformer 202 sets receive apertures 21-1 to 21-4 at predetermined intervals as shown in FIG. 7. It also sets the receive focal points 26-1 to 26-4 of the respective receive beams 25-2 to 25-4 to a predetermined depth (here, the same depth as the transmit focal point 16).

The first receive beamformer 202 adds the receive signals of each transducer 2a in the receive apertures 21-1 to 21-4 after delaying them by a predetermined amount. This calculates the receive line signals of the receive lines 27-1 to 27-4.

The delay amount for each reception signal of the transducer 2a is given by (Equation 2). For example, the delay amount Di given to the i-th transducer 2a in the reception aperture 21-1 is calculated from the distance Li between the reception focal point 26-1 and the transducer 2a and the sound speed c shown in Fig. 8(a). In (Equation 2), max(Li) is the maximum value of the distance between the transducer 2a in the reception aperture 21-1 and the reception focal point 26-1.

Di = (max (Li) - Li) / c ... (Equation 2)

The delay amount Di is constant for each transducer 2a, regardless of the position of the point (called the representative point) on the reception line 27-1, i.e., regardless of the reception time of the reception signal.

In this way, the delay and sum processing performed by the first receive beamformer 202 is a process of adding a fixed amount of delay to the receive signal output by each transducer 2a, and therefore can be realized by a low-cost, compact analog or digital circuit.

Next, in the second stage, the second receive beamformer 203 performs aperture synthesis between the receive beams and the transmit beams on the receive line signals calculated for each of the receive lines 27-1 to 27-4 during multiple transmit events. This calculates the signal strength reflected, etc. at an observation point within an observation region set within the subject 4.

8B, for example, a receiving line signal obtained for a receiving line I _nm passing through an m-th (m = 1 to M) receiving focal point R _nm in an n-th (n = 1 to N) transmission event includes a signal value reflected at an observation point p as a signal value of a representative point Q _nm of the receiving line I _nm . The representative point Q _nm is an intersection point between an elliptical curve having two foci, the transmitting focal point T _n and the receiving focal point R _nm , and the receiving line I _nm .

Therefore, the second receive beamformer 203 obtains the signal at the observation point p by performing aperture synthesis to synthesize receive line signals obtained for receive lines I _nm passing through the first to Mth receive foci R _nm for each of the first to Nth transmit events using (Equation 3).

In (Equation 3), s represents the position of the representative point Q _nm on the receiving line I _nm , and w _nm is a weight that is assigned by the second receiving beamformer 203.

For example, the weight w _nm can be given using the angle between the central axis of the transmission beam and the observation point p, and the angle between the central axis of the reception beam (reception lines 27-1 to 27-4) and the reception beam.

In this way, the second receive beamformer 203 performs aperture synthesis over the transmit interval n and aperture synthesis over the receive interval m. This makes it possible to calculate high-resolution signals for each observation point from the low-resolution signals bundled as receive line signals on the receive lines in the first stage.

The image processing unit 50 generates an ultrasound image by converting the signal value of each observation point p in the observation region generated by the receive beamformer 20 into the pixel value of the pixel at the position corresponding to the observation point p. The generated image is displayed on the display unit 3 connected to the image processing unit 50.

Note that the console 70 in Figure 6 accepts imaging conditions from the user.

<Movement of transmitting and receiving elements>
The movement control unit 30 controls the movement amount of the transmitting element (transmission focal point) 16 and the movement amount of the receiving elements 26-1 to 26-4 for each transmission event, thereby suppressing the occurrence of grating lobes. This will be described in detail below.

In an imaging method that uses aperture synthesis, increasing the aperture synthesis aperture (the width of the array of real or virtual elements) can improve the image resolution. On the other hand, increasing the number of elements in the receiving aperture (real receiving elements or virtual receiving elements) increases the amount of data in the receiving signal (element signal or receiving line signal), and increases the amount of calculations in the receiving beamformer 20. If the receiving aperture is enlarged to avoid this, while the pitch of the receiving elements is made sparse so as not to increase the number of receiving elements, grating lobe artifacts may occur.

In this embodiment, as described above, the movement control unit 30 controls the movement amount of the transmitting element (virtual transmitting element) 16 and the movement amount of the receiving elements (virtual receiving elements) 26-1 to 26-4 for each transmission event to suppress the occurrence of grating lobes. Specifically, the movement control unit 30 moves the positions of the transmitting elements and receiving elements so that the difference between the movement vector r of the receiving element along the row direction of the transducer 2a and the movement vector t of the transmitting element is different for two consecutive transmission events (see FIG. 4(a)).

In this embodiment, the receiving elements 26-1 to 26-4 in the same transmission event are arranged at equal intervals, and the movement vector r is the same for the multiple receiving elements 26-1 to 26-4. In other words, the movement control unit 30 moves the receiving elements 26-1 to 26-4 while maintaining the intervals between them.

Furthermore, when the movement control unit 30 calculates the phase centers 210-1 to 210-4 as shown in FIG. 9, the arrangement interval P1 (FIG. 6(a)) of the phase centers resulting from synthesis between transmission events is set to be equal to or less than 1/2 the wavelength λ of the ultrasound received by the transducer array 200.

As shown in Figure 10(a), in general, when the same transducer 2a is used to repeatedly transmit and receive signals and perform aperture synthesis (monostatic SA), if the phase difference between adjacent elements in the round-trip propagation distance, which is the sum of the propagation distance of the transmitted wave and the propagation distance of the reflected wave, is a multiple of the wavelength, the echo signals of the transducers reinforce each other, causing grating lobes. For this reason, it is known that in order to avoid grating lobes in monostatic aperture synthesis, the element spacing d must be made smaller than λ/2.

As described in Non-Patent Document 1, the SASB method, which sets multiple receiving foci for each transmission, sets virtual receiving elements (receiving foci) 26-1 to 26-4 with the transmission focus 16 as a virtual transmitting element, and since the transmitting element and receiving element are different transducers 2a, it is called bistatic aperture synthesis.

Here, as shown in Figure 10 (b), if we assume a virtual element at the midpoint (phase center) between the transmitting element and the receiving element using the phase center method, and let the distance from the transmitting element to the reflection point be L1, the distance from the reflection point to the receiving element be L2, and the distance from the phase center possible element to the reflection point be L3, it is known that when L3 is sufficiently large relative to the distance σ between the phase center and the transmitting element or receiving element, the following approximation formula holds.
L1+L2=2*L3 ... (Equation 4)

This is called the phase center approximation, and the phase center approximation allows bistatic aperture synthesis to be treated as an approximation to monostatic aperture synthesis, in which transmission and reception are performed from the phase center for each transmission event. As a result, although the calculation of the grating lobe generation angle in bistatic aperture synthesis is more complicated than that in monostatic aperture synthesis, the phase center approximation makes it possible to calculate the grating lobe generation angle using a simple formula such as the above-mentioned (Formula 1). (Formula 1) is rewritten as follows:
2d sin θ=Nλ (Equation 1)

In other words, as shown in Figures 5(a) and (b), the interval between phase centers 210-1 to 210-4 between transmission events can be considered as the element pitch d in Figure 10(a), and by making this smaller than λ/2, grating lobes can be avoided (Figure 10(c)).

In this embodiment, as described above, the movement control unit 30 controls the movement vector t of the transmitting element 16 and the movement vector r of the receiving elements 26-1 to 26-4 for each transmission event, and moves the positions of the transmitting elements and receiving elements so that the difference between the movement vector t and the movement vector r is different for two consecutive transmission events (see FIG. 4(a)). For example, in the specific example shown in FIG. 4(a), the movement amount of the transmission movement vector t is the same movement amount Δ for each transmission event, while the movement amount of the reception movement vector r is set to alternate between 0 and 2Δ for each transmission event.

As a result, in the comparative example shown in FIG. 4(b) and FIG. 5(b), when the transmitting element 16 and the receiving elements 26-1 to 26-4 are moved by a constant amount of movement Δ for each transmission, the arrangement interval P2 of the phase centers between multiple transmission events is Δ, whereas in the present embodiment, when the amount of movement of the receiving elements 26-1 to 26-4 is set to alternate between 0 and 2Δ for each transmission event, as in FIG. 4(a) and FIG. 5(a), the arrangement interval P1 (FIG. 6(a)) of the phase centers between multiple transmission events becomes Δ/2, which can be made smaller than P2.

The movement control unit 30 controls the amount of movement so that this distance P1 is less than or equal to 1/2 the wavelength λ of the ultrasonic wave, thereby suppressing grating lobes.

In general, the movement amount Δ of the transmitting element 16 is equal to the pitch of the transducer 2a (actual element), and the pitch of the transducer 2a (actual element) is often equal to or smaller than the wavelength λ. In this case, the array spacing P1 of the phase centers between the transmitting events is λ/2 or less, and the occurrence of grating lobe artifacts can be avoided.

According to this embodiment, a large diameter receiving aperture can be set while suppressing the amount of data by using a small number of receiving elements 26-1 to 26-4 at a sparse pitch. As a result, a high resolution image can be obtained by aperture synthesis.

In addition, in this embodiment, the spacing between the receiving elements 26-1 to 26-4 and the number of receiving elements 26-1 to 26-4 per transmission are preset so that the phase centers 210-1 to 210-4 overlap the same number of times at each position as a result of combining multiple transmission events. For example, in FIG. 6(a), there are four receiving elements per transmission, and the phase centers 210-1 to 210-4 overlap two by two at the same position.

However, when the number of receiving elements 26-1 to 26-4 is an odd number, such as five, the number of overlapping phase centers between the transmitting vents varies from two to three, two to three, and so on, and even if the arrangement interval P1 of the phase centers is less than or equal to half the wavelength λ, grating lobes will occur due to the spacing between positions where the number of overlapping phase centers is large. To avoid this, in this embodiment, the number of receiving elements is set in advance so that the number of overlapping phase centers is uniform.

<Processing for Determining Movement Vector>
Here, the process in which the movement amount control unit 30 determines the movement vectors t and r will be further explained with reference to the flow chart of FIG.

In this embodiment, when the movement vector t of the transmitting element 16 is constant, grating lobes are avoided by varying the movement vector r of the receiving element for each transmission event.

The movement amount control unit 30 reads necessary settings from the control unit 60 in advance, such as the wavelength (λ), the movement amount (Δt) of the transmitting element 16, the number (M) of receiving elements 26-1 to 26-M per transmission, and the spacing (Δr) between receiving elements 26-1 to 26-M during one transmission event.

First, the pitch d (see FIG. 10(a)) required to avoid grating lobes is determined to be, for example, 1/2 the wavelength λ (step 1101).

Next, the position of the phase center of the current transmission event is calculated. For example, the positions of the transmitting element 16 and receiving elements 26-1 to 26-M of the first transmission event are determined by arranging them at a predetermined interval from one end of the array of transducers 2a. From the determined positions of the transmitting element 16 and receiving elements 26-1 to 26-M, the phase centers 210-1 to 210-M of the first transmission event are calculated, for example, as shown in FIG. 9. The calculated positions of the phase centers 210-1 to 210-M of the first transmission event are shifted in the direction of the array of transducers 2a by the previously calculated pitch d, and the positions of the phase centers 210-1 to 210-M of the second transmission event are set (step 1102).

Next, the movement vector r of the receiving elements 26-1 to 26-M is calculated from the second position of the transmitting element 16, which is moved by the movement vector t (movement amount Δt) from the position of the first transmitting element 16, and the positions of the phase centers 210-1 to 210-M of the second transmission event set in step 1102 (step 1103).

Steps 1102 and 1103 are repeated until the movement vectors r of the receiving elements 26-1 to 26-M for all transmission events are calculated.

This makes it possible to determine the arrangement of the transmitting element 16 and receiving elements 26-1 to 26-M for each transmission event. The movement control unit 30 moves the receiving elements 26-1 to 26-M with the calculated movement vector r, and moves the transmitting element 16 with a constant movement vector t, resulting in an arrangement interval d between the phase centers of the transmission events, making it possible to avoid grating lobes.

The process of the flow in FIG. 11 in which the movement amount control unit 30 determines the movement vectors t and r may be performed for each transmission event, and the movement vectors t and r for moving the transmitting element 16 and the receiving elements 26-1 to 26-M may be determined before the next transmission event. Alternatively, the movement vectors t and r may be determined in advance for all transmission events and stored in the memory 201, and the movement control unit 30 may read and set the movement vectors t and r from the memory 201 for each transmission event.

As already explained, in this embodiment, the first receive beamformer 202 performs processing that adds a fixed amount of delay to the received signal, and therefore can be realized by hardware such as a low-cost and compact analog circuit or digital circuit, but can also be realized by software by having the CPU execute a program stored in the built-in memory.

The second receive beamformer 203 and the movement control unit 30 can of course be realized by software, with the CPU executing a program previously stored in the built-in memory, or they can be configured in part or in whole by hardware. For example, the second receive beamformer 203 and the movement control unit 30 can be configured using a custom IC such as an ASIC (Application Specific Integrated Circuit) or a programmable IC such as an FPGA (Field-Programmable Gate Array), and the circuit can be designed to realize the function.

<<Second embodiment>>
An ultrasonic imaging apparatus according to the second embodiment will be described with reference to Fig. 12. The ultrasonic imaging apparatus according to the second embodiment differs from the first embodiment in that a first receive beamformer 202 is mounted on the probe 2.

The rest of the configuration is the same as that of the device in the first embodiment, so a description will be omitted.

The first receive beamformer 202 performs a calculation to bundle receive signals into receive line signals using the SASB method, but since this calculation requires a small amount of calculation, the scale of the required calculation circuit is also small. Therefore, it can be installed in the probe 2.

In addition, since the number of bundled receive line signals is the number of receive apertures, the probe 2 only needs to transmit a smaller number of receive line signals than the transducer 2a to the main device 1. This reduces the amount of data transmitted between the probe 2 and the main device 1, and reduces the size of the transmission line. It also enables wireless transmission of a small amount of data.

1...Ultrasound imaging device (main body), 10...Transmit beamformer, 20...Receive beamformer, 30...Aperture movement control unit, 201...Memory, 202...First receive beamformer, 203...Second receive beamformer

Claims (6)

a transmission element that transmits ultrasonic waves to a subject; a plurality of arranged reception elements that receive echoes of the ultrasonic waves generated in the subject for each transmission event of the ultrasonic waves; a movement control unit that moves the positions of the transmission elements and the plurality of reception elements in the direction of the arrangement for each transmission event; and a reception beamformer that synthesizes reception signals obtained in the plurality of reception elements for each transmission event between the plurality of reception elements and between the transmission events;
the movement control unit moves positions of the transmitting element and the receiving element such that a difference between a movement vector representing a movement of a position of the transmitting element and a movement vector representing a movement of a position of the receiving element between the transmitting event and the immediately preceding transmitting event is different between two consecutive transmitting events;
The movement control unit sets the movement vector of the transmitting element and the movement vector of the multiple receiving elements so that the distance between successive transmission events of multiple phase centers, which are the midpoints between the transmitting element and each of the multiple receiving elements, is smaller than the distance between the transmitting element and the multiple receiving elements when the transmitting element and the multiple receiving elements are moved by a constant movement vector for each transmission event. 2. The ultrasonic imaging apparatus according to claim 1 , wherein a distance between successive transmission events of the phase center is equal to or less than 1/2 of a wavelength λ of the ultrasonic wave transmitted by the transmission element. 2. The ultrasonic imaging device according to claim 1 , wherein the movement control unit sets movement vectors of the transmitting elements and the receiving elements so that the multiple phase centers are set at positions where they overlap the same number of times due to the repetition of the transmission event. 2. The ultrasonic imaging device according to claim 1, further comprising a transducer array in which transducers that actually transmit and receive ultrasonic waves are arranged,
the transmitting element and the plurality of receiving elements are virtual elements assumed in a synthetic aperture sequential beamforming (SASB) method,
The transmitting element is assumed to be at a position of a transmission focus of a transmission beam transmitted by the transducer array to the subject,
The receiving element is assumed to be at the position of a focus of a receiving beam formed by processing receiving signals output from the plurality of transducers that receive echoes of a transmitting beam transmitted by the transducer array,
The movement control unit moves the transmitting elements by moving the transmission focal point of the transmission beam transmitted by the transducer array, and moves the receiving elements by moving the position of the receiving focal point formed by the receiving beamformer. 5. The ultrasonic imaging device according to claim 4 , wherein the movement control unit forms a plurality of the receive beams for each transmit event of the transmit beam by the SASB method, and performs aperture synthesis on the receive beams between receive beams within the same transmit event and/or between transmit events. 2. The ultrasonic imaging device according to claim 1, further comprising an ultrasonic probe having a built-in transducer array in which transducers that actually transmit and receive ultrasonic waves are arranged,
the receive beamformer includes a first receive beamformer that forms the plurality of receive beams, and a second receive beamformer that performs aperture synthesis on the receive beams;
The ultrasonic imaging apparatus according to claim 1, wherein the first receive beamformer is mounted within the ultrasonic probe.

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