WO2018061025A1

WO2018061025A1 - Method and system for ultrasound beamforming using limited number of active transducer elements and diverging beams

Info

Publication number: WO2018061025A1
Application number: PCT/IN2017/050427
Authority: WO
Inventors: Arun K THITTAI; Lokesh B
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2016-09-27
Filing date: 2017-09-26
Publication date: 2018-04-05

Abstract

The present invention discloses a novel technique that combines Diverging Beam with Synthetic Aperture Technique (DB-SAT). This technique aims at reducing the system's complexity (only 8 or 16 active transmit 10 elements) without compromising the image quality, and yet yield frame rates comparable to or higher than that obtained from Conventional Focused Beamforming with Linear Array (CFB-LA).

Description

METHOD AND SYSTEM FOR ULTRASOUND BEAMFORMING USING LIMITED NUMBER OF ACTIVE TRANSDUCER ELEMENTS AND

DIVERGING BEAMS FIELD OF THE INVENTION

[0001] The present invention relates to synthetic aperture based ultrasound beamforming where diverging wave is transmitted by manipulating only limited number of active transmit elements and receiving echoes from full or partial aperture;

BACKGROUND OF THE INVENTION

[0002] Ultrasound (US) imaging is a widely used modality due to its non-invasive, non-ionizing, and real-time imaging capabilities. The US systems typically utilize a transducer with multiple elements arranged in a linear array fashion that act as both transmitter and receiver. Pressure waves are induced by activating the transducer elements. These pressure waves propagate through the tissue and get reflected, scattered, and attenuated depending on the mechanical properties of the tissue. The backscattered echoes are usually received by the same transducer elements and then processed in a step known as beamforming. The beamformed echo signals are demodulated to obtain the final image called B-mode or sonogram.

[0003] The two most popular approaches in US imaging are Phased Array (PA) and Conventional Linear Array (CLA) techniques. These approaches are routinely used in most standard scanners. Both techniques employ a focused beam during transmit and perform dynamic focusing on receive. [0004] PA (Macovski et al . 1979) imaging is a widely used technique because it typically yields best quality images. This technique involves transmission of pressure waves by exciting all the transducer elements, each with appropriate time-delay to account for desired focusing and steering of the beam. The echoes received by all the transducer elements are then appropriately delayed and added to obtain a single A-line. This transmit-receive process is repeated to acquire new A-lines from different spatial locations by steering the focused beam in different directions. All these A-lines are then populated next to each other to render a final image. In PA imaging, distance between the array elements must be half of the wavelength (λ/2) corresponding to the center frequency to avoid grating lobes. Given that a PA system utilizes all the elements (typically 64 or 128) during transmission and reception, it requires complex hardware resulting in an expensive system. [0005] In CLA technique, limited group of transducer elements (i.e. active aperture consisting of 32-elements or higher) are utilized both in transmission and reception to obtain a single A-line. The active aperture is electronically swept over the entire transducer aperture to generate an image. In comparison to PA imaging, CLA uses the transducer array having distance between the elements equal to wavelength (λ) as it is not involved in steering of the US beam. CLA is comparatively less complex than PA system at the expense of compromising the image quality in the process, but it is still not affordable for routine scanning in resource- poor settings. To overcome the system complexity, the notion of Synthetic Aperture (SA) radar was adapted to medical US imaging (Burckhardt et al . 1974) . [0006] SA technique (Ylitalo et al . 1994) makes use of only one channel to acquire data and form an image. Here, a single transducer element is activated at a time to transmit and backscattered echoes are received by the same element. All the transducer elements are activated sequentially using a multiplexer to transmit and receive the signal. The data from transmission and reception of all the elements are stored in the memory, which is then beamformed and processed to obtain the final image. The system is less complex, but has accompanying disadvantages like (a) poor image quality, (b) limited depth of penetration, (c) sensitivity to moving objects. The above-mentioned disadvantages made this technique impractical for biomedical applications. [0007] The various research efforts for adapting SA approach to biomedical US imaging can be broadly grouped into two themes discussed below.

[0008] Numerous techniques have been investigated to reduce the number of parallel receive channels (i.e. hardware complexity) of a PA system, where a tradeoff with frame rate is tolerated. Some of them are mentioned below : Transmit from the active elements and receive from non- redundant elements to obtain a single A-line (Karaman et al. 1995) .

Nock and Tray (1992) proposed a technique where few centre elements (i.e. 32 centre elements out of 128 transducer elements) are used during transmission and reception to obtain a single A-line.

Active aperture is used during transmission and reception, and then the active aperture is electronically swept over an entire transducer aperture to collect the full aperture data. These data are stitched together to obtain a single A-line (Johnson et al . 2005) .

Another major research theme using SA approach has been to develop strategies to improve frame rate (orders of magnitude improvement compared to CLA technique) and/or lateral resolution of an image without worrying about the cost and complexity of the system. Some popular approaches are listed below.

[0009] Similar to SA approach, single element is used during transmission, but all elements are used during reception. This process is repeated until all the transducer elements are used for excitation (Jensen et al . 2006) . This technique improves the lateral resolution and it is maintained throughout the depth. In order to improve frame rate, only few emissions are used during transmission (Sparse emission) followed by receiving with all transducer elements (Lockwood et al . 1998) . Multiple (active) elements are excited simultaneously during transmission (MSTA) to send out unfocused US beam while all elements are used during reception and the process is repeated with an aperture overlap in the transmission (Trots et al . 2010) .

Coherent plane wave compounding technique provides magnitude order of increased frame rate (-1000 fps), but it is a complex system since it needs to activate all the elements during transmission and reception. (Montaldo et al . 2009)

[0010] The prior art methods and devices were not able to reduce the system complexity without sacrificing the frame rate and image quality. One has to use multiple elements during transmission to improve the depth of penetration and Signal to Noise Ratio (SNR) . Therefore, current SA approaches use multiple elements during transmit (with aperture overlap) and utilize data received with all elements. All of these methods however excite multiple elements simultaneously (i.e., unfocussed US beam) .

OBJECTS OF THE INVENTION

[0011] To meet the objects of the invention and to overcome the drawbacks of the prior art it is disclosed herein an apparatus for synthetic aperture based ultrasound beamforming resulting in high frame rate and improved image quality by emitting diverging beam using few transmit elements and receiving echoes from full aperture or partial aperture. [0012] It is yet another object of the invention to disclose a method of synthetic aperture based ultrasound beamforming to reduce the system complexity of linear array imaging by using limited number of active elements at a time during transmission.

SUMMARY OF THE INVENTION

[0013] To meet the objects of the invention and to overcome the drawbacks of the prior art it is disclosed herein A method of ultrasound imaging, the method comprising: exciting a set of limited number of active transmit elements from an array of transducer elements with appropriate delay values to induce diverging waves; receiving backscattered echoes at the transducer elements; translating electronically the active transmit elements along the array of transducer elements by repeating sequentially the said steps of exciting and receiving; reconstructing the received echoes in a receive beamformer and processing it to form an image; and displaying the image in a display device.

[0014] It is disclosed here a system for ultrasound imaging comprising of: an array of transducer elements; means to excite a set of limited number of active transmit elements from the array of transducer elements with appropriate delay values to induce diverging waves, the said transducer elements further receives backscattered echoes; an electronic translating means to electronically translate the active transmit elements along the array of transducer elements; a receive beamformer to reconstruct the received echoes and processing it to form an image; and a display device to display the image.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig.l shows an embodiment of the invention proposed with "diverging" beam and full or partial receive aperture.

Fig.2 is a block diagram of an embodiment of the invention

Fig.3 is a schematic representation of an embodiment of the invention.

Fig.4 (a) shows the experimentally obtained ultrasound image of a medium containing point scatterrers at various positions obtained from the CLA imaging with NaTx= NaRx= 64 elements and number of transmits per image =128.

Fig.4(b) shows the experimentally obtained ultrasound image of a medium containing point scatterrers at various positions obtained from an embodiment of the invention with NaTx= 8 and number of transmits per image =16 and after filtering.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The invention and its various embodiments is better understood by reading the description along with the accompanying drawings which appear herein for purpose of illustration only and does not limit the invention in any way.

[0017] Referring to FIG. 2, the block diagram shows a system for ultrasound imaging comprises of an array of transducer elements . The transducer elements act as both transmitter and receiver. A transmit beamformer excites a set of limited number of active transmit elements from the array of transducer elements with appropriate delay values to induce diverging waves . Backscattered echoes are received at the transducer elements. The process of transmitting and receiving is repeated throughout the transducer elements by electronically translating the active transmit elements along the array of transducer elements. A receive beamformer reconstructs the received echoes and processing it to form an image and a display device to display the image. The image may also be plotted on a plotting media.

[0018] The working of an embodiment of the invention is shown in Fig.l and FIG.3. During transmit, a diverging beam is sent out from the active elements, NaTX (= 8 or 16), of the transducer array by supplying appropriate delay values and backscattered echoes are received by the full or partial transducer elements. This transmit- receive process is continued by electronically translating the active transmit sub-aperture with/ without any overlap, until all the transducer elements are used during transmission. The raw RF data for each transmit sub-aperture are stored separately.

[0019] In the disclosed method, NaTX elements will be excited with appropriate delay values to send out diverging beam. As opposed to focused beam, where the center element of an active transmit aperture is excited last and its end elements are excited first, here it is done in reverse order. The data from NRX for each transmit is used in beam forming.

[ 0020 ] From diverging beam with synthetic aperture beamformer, the backscattered signal at a point 'p' (x

(azimuth) , z (depth) ) in the image can be reconstructed as follows:

N_t N_r

(p) =∑∑^RF^t-T)

i=l =1

[ 0021 ] Where, Nt and Nr denotes the number of emissions from transmit aperture and the number of receive elements in the transducer array, respectively. RFij (t) is the received signal for ith emission from transmit aperture and th element receive, 'wij' is the weighting function (apodization) applied to this signal. The round trip time τ( χ , ζ ) is comprised of the time taken to reach the given point from center of transmit emission and time for the echo to be received back by the transducer element. [ 0022 ] The time taken to reach the given point [x,z is computed as:

Where, xi is the location of the center of the transmit emission and Zd is the distance from transducer array to the virtual point source.

[ 0023 ] The time taken to reach the receive element xj from the point (x,z) is computed as:

Where, xj is the location of the receiving

element. The total round trip time to and from the point (x,z) is computed as: τ(x_rz) =Tt+T_r.

Preliminary Simulation Results

[0024] Figure 4a and 4b shows a comparison of images obtained using CLA imaging with typical settings and using proposed technique (i.e., diverging beam with synthetic aperture beamforming) with

The proposed technique results in better image quality in terms of spatial resolution than the CLA technique. The technique of sending diverging waves and using limited number of active elements not only reduces system complexity and cost, but also yields high frame rate.

Advantage

[0025] The diverging beam with synthetic aperture ultrasound beamforming method provides less complex US system by activating only 8 or 16 transducer elements at time during transmit.

[0026] This method sends out diverging beam from the active elements and then it is electronically translated with or without an overlap to achieve sparse emission, which leads to a high frame rate system.

[0027] For each transmit, radio frequency data collected from only partial receive aperture (e.g., three quarter or half the full aperture size) can be used to reconstruct the image, thereby reducing the complexity in terms of hardware and data handling. [0028] This method allows for dynamic focusing in both transmit and receive, which leads to better lateral resolution and it is maintained throughout the depth of imaging .

[0029] Typical CLA imaging uses under sampled linear array (i.e., pitch = λ) to achieve reduced complexity, but proposed method can use proper sampled array (i.e., pitch = λ/2) without increasing complexity of the system, but at the expense of frame rate (compared to λ-pitch array) .

[0030] It will be obvious to a person skilled in the art that with the advance of technology, the basic idea of the invention can be implemented in a plurality of ways. The invention and its embodiments are thus not restricted to the above examples but may vary within the scope of the claims.

[0031] Further the above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto

Claims

A method of ultrasound imaging, the method comprising :

exciting a set of limited number of active transmit elements from an array of transducer elements with appropriate delay values to induce diverging waves;

receiving backscattered echoes at the transducer elements;

translating electronically the active transmit elements along the array of transducer elements by repeating sequentially the said steps of exciting and receiving;

reconstructing the received echoes in a receive beamformer and processing it to form an image; and

displaying the image in a display device.

The method as claimed in claim 1 wherein the said translating step may have an overlap between two adjacent sets of active transmit elements.

The method as claimed in any of the preceding claims wherein, the received echoes data may be from only a partial receive aperture.

The method as claimed in any of the preceding claims wherein the said array may be a single array or two-dimensional array. The method as claimed in any of the preceding claims wherein the said array may be a sampled array of pitch less than or equal to λ/2.

The method as claimed in any of the preceding claims wherein the said array may be curved array.

The method as claimed in any of the preceding claims may further be realized by utilizing the spatial compounding approaches at each transmit sub- aperture .

A system for ultrasound imaging comprising of: an array of transducer elements;

means to excite a set of limited number of active transmit elements from the array of transducer elements with appropriate delay values to induce diverging waves, the said transducer elements further receive backscattered echoes;

an electronic translating means to electronically translate the active transmit elements along the array of transducer elements ;

a receive beamformer to reconstruct the received echoes and processing it to form an image; and

a display device to display the image.

The system as claimed in claim 8 wherein the said electronically translation may have an overlap between two adjacent sets of active transmit elements .

10. The system as claimed in claims 8 or 9 wherein, the received echoes data may be from only a partial receive aperture.

11. The system as claimed in claims 8, 9 or 10 wherein, the said array may be a single array or two- dimensional array.

12. The system as claimed in claims 8, 9, 10 or 11 wherein, the said array may be a sampled array of pitch less than or equal to λ/2.

13. The system as claimed in claims, 8, 9, 10, 11 or 12 wherein, the said array may be curved array.

14. The system as claimed in claims, 8, 9, 10, 11, 12 or 13 may further be realized by utilizing the spatial compounding approaches at each transmit sub-aperture .