3D ultrasound imaging at high display frame rates using a wide focused transmit beam

Abstract

Ultrasound systems produce 3D images at high display frame rates. A volumetric region is scanned using a plane wave or diverging transmit beam to insonify most or even the entire volumetric region with each transmit event. To avoid collecting clutter signals in the azimuth and elevation dimensions, the plane wave or diverging beam is transmitted at an angle intermediate the elevation and azimuth directions. By transmitting the plane wave or diverging beam at multiple different angles that are a combination of both elevation and azimuth directions, sidelobe clutter is reduced in the resulting composite image.

Description

Technical Field

[0001] The present invention relates to ultrasound imaging systems and, in particular, to three-dimensional (3D) ultrasound imaging at high display frame rates using a wide focused or unfocused transmit beam. Background Art

[0002] Two-dimensional (2D) ultrasound imaging is usually performed by scanning a planar image field using a one-dimensional (1D) array transducer. A beam is emitted within the image field, and echoes are collected in response to each emission. The received echoes are beamformed by a delay and sum beamformer to form scan lines of coherent echo signals across the image field. A typical number of scan lines for an image may be 128-196 scan lines. The scan lines are processed by B-mode or Doppler processing to form a planar image of tissue and / or flow in the planar image field.

[0003] A similar approach can be used to scan a volumetric image field to produce a three-dimensional (3D) image of a volumetric region. Again a beam is transmitted and echoes are received, but this time over the entire volume and not just the plane. Therefore, scanning the volume takes much longer for 3D imaging. If, for example, the volume has the same elevation and azimuth dimensions as the azimuth dimension of the planar image described above, an image of equivalent quality would require 128x128 scan lines, for a total of over 16,000 scan lines. Since the echo acquisition time is governed by the fixed speed of sound in the object, the time required to acquire the entire volumetric image is long, and therefore the display frame rate will be slow.

[0004] The solution to the slow frame rate problem is to transmit beams that insonify a larger area of ​​the volume and return echoes from a larger area of ​​the volume, thereby requiring fewer transmit beams to scan the entire volume and produce a 3D image. The ultimate extension of this concept is to transmit beams that insonify most or even all of the volume area. However, the tradeoff is poor image resolution because there is little, if any, transmit beam focusing. Measures that can be taken to overcome this problem are to scan the volume area multiple times and then combine the structures, the combined scans resulting in an improvement in resolution throughout the image.

[0005] But this measure can still result in a 3D image with significant image clutter, since the sidelobe levels of a greatly unfocused transmit beam pattern will generally be very high. High sidelobe levels capture off-axis energy that will appear as image clutter in the final image.

[0006] Jean Provost et al., “3D ultrafast imaging in vivo” (Phys. Med. Biol. 59 (2014)) discloses a 3D ultrafast ultrasound imaging system and method based on the use of defocused waves that penetrate the entire volume of interest. Pedro Santos et al., “Diverging wave volumetric imaging using subaperture beamforming” (IEEE transactions on ultrasonics, ferroelectrics and frequency control, vol. 63, no. 12 (2016)) discloses another high frame rate 3D imaging system using subaperture beamforming. Summary of the invention

[0007] The invention is defined by the claims. It advantageously enables imaging of volumetric regions with only a few wide beams providing an improvement in display frame rate but without the development of excessive clutter in the resulting 3D image.

[0008] According to the principles of the present invention, an ultrasound imaging system for producing 3D images at a high display frame rate is described. A volumetric region is scanned using a plane wave or diverging transmit beam to insonify most or even the entire volumetric region with each transmit event. To avoid collecting clutter signals in the azimuth and elevation dimensions, the plane wave or diverging beam is transmitted at an angle intermediate the elevation and azimuth directions. By transmitting the plane wave or diverging beam at a plurality of different angles that are a combination of both the elevation and azimuth dimensions, sidelobe clutter is reduced in the resulting composite image.

[0009] According to another aspect, the present invention provides a method for generating a three-dimensional image. In one embodiment, the method includes transmitting a plane wave or a diverging wave to the target volume and collecting ultrasound echo signals returned from the target volume. A plurality of such waves are transmitted to the target volume at different angles. The echo signals are received from the transmissions and then processed on a spatial basis. Image data generated in response to each transmission can be compounded on a spatial basis. A volume image is generated from the compounded image data. The volume image is displayed. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the attached picture:

[0011] Figure 1a , 1b and 1c illustrate the sidelobe patterns of a two-dimensional transducer array aperture.

[0012] Figure 2The diagram illustrates the sidelobe improvement obtained by scanning a volumetric region with a diverging beam at an angle intermediate between the azimuth and elevation directions.

[0013] Figure 3a and 3b Two different diverging beam scan patterns are illustrated, both at angles that encompass both azimuth and elevation directions.

[0014] Figure 4 The diagram shows Figure 3a The vertices of the two divergent swept volumes.

[0015] Figure 5 The diagram shows Figure 3a The usage of a divergent beam scanning pattern causes sidelobe improvements.

[0016] Figure 6 The diagram shows Figure 3b The usage of a divergent beam scanning pattern causes sidelobe improvements.

[0017] Figure 7 An ultrasound imaging system constructed in accordance with the principles of the present invention is illustrated in block diagram form.

[0018] Figure 8 A second ultrasound imaging system constructed in accordance with the principles of the present invention is illustrated in block diagram form. DETAILED DESCRIPTION

[0019] Figure 1a is a perspective view of the aperture of a two-dimensional array 12 of transducer elements having rows and columns of elements extending in the azimuth (Az) and elevation (El) dimensions. The beam pattern of such an array is the Fourier complement of its aperture, Figure 1b As the beam pattern illustrates, the dominant lobe of the beam is aligned in the elevation direction of the columns of elements and in the azimuth direction of the rows of elements. Figure 1c A cross section taken through one of these dominant directions is shown in FIG. The graph illustrates a central main lobe 50 flanked on either side by a descending pattern of side lobes 52. The energy of the desired main lobe is considered to be accompanied by a considerable amount of off-axis energy captured by many side lobes 52 of significant amplitude. It is desirable to reduce the level of these side lobes to reduce clutter in ultrasound images.

[0020] When the transmission angle of a plane wave or diverging beam is neither azimuth nor elevation but somewhere in between (such as diagonally with respect to two reference dimensions), the sidelobe level can be reduced. The resulting beam pattern will thus span Figure 1b The transmit beam pattern is diagonal. Figure 2This effect is illustrated with reference to an ultrasound phantom 60 containing 9 point target reflectors in a central horizontal plane 62. When the phantom is scanned with a 9x9 sequence of divergent beams, all 81 shots from 81 separate and evenly spaced shot volume vertices, an image is formed of the central azimuth plane 64 of the phantom, as illustrated by ultrasound image 70a on the left side of image panel 70. The bright spots in the image are the central row of 3 reflectors in the phantom, and are believed to have an appreciable amount of clutter between the targets due to high sidelobe levels. A beam plot for this azimuth plane is shown in the left side illustration 80a of beam plot panel 80, which shows 3 peaks of target reflectors with intermediate sidelobe levels of approximately -30 dB. Ultrasound image 70b and beam plot 80b show similar results for images of the three target reflectors in the central elevation plane 66 of the phantom.

[0021] But when the image is formed by the diagonal plane 68 of the phantom aligned diagonally across the array aperture, the resulting sidelobes are significantly lower, with levels below -50 dB in the right beam profile 80 c. As a result, the three point targets in the diagonal plane 68 have much lower clutter levels, as shown by the rightmost ultrasound image 70 c in the image panel 70.

[0022] exist Figure 3a and 3b The method used to generate Figure 2 Experimental results for a grid of 90 transmit beam positions. Figure 2 The result is obtained by emitting a sequence of 81 diverging beams from the 2D array aperture 12, wherein the vertex of each diverging beam is a virtual vertex located behind the surface of the array, so that the resulting diverging beam has the form of a truncated pyramid. The 81 diverging beams have their vertices located at each intersection of the horizontal and vertical lines of the grid 90. Figure 4 The shapes of two of the beam volumes bounded by large dots AV1 and AV2 are shown in FIG. 1 . The vertex AV1 of one diverging beam volume is located on the grid 90 behind the 2D array aperture 12, as shown in the figure. This point is centrally located relative to the aperture as Figure 3a The point AV1 shown, which causes the truncated pyramid of divergent beam energy to be located symmetrically relative to the aperture, is Figure 4 The solid line 92 marks the edge of the pyramid beam volume. If a centerline is drawn downward from the pyramid vertex AV1, it will extend from the center of the 2D array 12 and perpendicular to the surface of the array. Figure 4The pyramid of divergent beam energy for the AV2 divergent beam on the diagonal of the left rear corner of the grid shown in 9 results in a beam that is angled relative to the AV1 beam, as seen by the dashed line 94 marking the edge of the AV2 pyramid beam volume. The entire AV2 divergent beam is thus steered in a different direction and at an angle relative to the AV1 divergent beam. Although the centerline of the AV2 pyramid is directed toward the center of the volumetric image field, it nevertheless extends from a different point on the array surface than that of AV1 and at a different (non-orthogonal) angle. When the echoes received from the divergent beam transmissions are compounded, these angular differences in the divergent transmit beams result in lower sidelobe levels for the resulting image.

[0023] exist Figure 3a , the 17 grid intersections in the diagonal direction across the grid 90 define the virtual vertices of the 17 diverging plane waves emitted from the corresponding 2D array apertures 12. Figure 4 As shown, 17 plane waves will be transmitted at 17 different angles relative to the surface of the hole. When 17 such transmit plane wave beams are transmitted and the echoes caused by them are collected by the array and coherently combined on a volumetric basis, images of the corresponding azimuth 64, elevation 66 and diagonal 68 planes of the phantom 60 are generated, as shown by Figure 5 The corresponding beam plots for the three images are shown in panel 170 of FIG. 182 , wherein the beam plot 180 c for the diagonal plane shows sidelobe levels around −40 dB, which is circled by 182 in the figure.

[0024] Figure 3b The grid 90 of FIG. 9A shows an intermediate sequence of 41 transmit events evenly distributed across the grid and in a diagonal relationship to each other, resulting in a plane wave diverging beam with 41 different transmit angles. When the phantom 60 is scanned using this scan sequence and the same three reference planes 64, 66 and 68 are imaged, the image is as follows: Figure 6 is presented as shown in panel 270. Using 41 different emission volume angles, the sidelobe level in the diagonal plane is around -50 dB, as circled at 282 in panel 280c, close to Figure 2 The results for a sequence of 81 emission events are shown in .

[0025] Reference now Figure 7, an ultrasonic diagnostic imaging system constructed according to the principles of the present invention is shown in block diagram form. A two-dimensional array of transducer elements is provided in an ultrasound probe 10 for transmitting ultrasound waves and receiving echo information. The transducer array 12 is capable of scanning in three dimensions, wherein the beam is steered in both elevation and azimuth. The transducer array 12 is coupled to a microwave beamformer 14 in the probe, which controls the transmission and reception of signals through the array elements. The microwave beamformer is a probe integrated circuit capable of transmitting beam steering and at least partially beamforming signals received by a group or "patch" of transducer elements, as described in U.S. Patents US 5997479 (Savord et al.), US 6013032 (Savord), US 6623432 (Powers et al.), and US 8177718 (Savord). The microbeamformer is coupled by the probe cable to a transmit / receive (T / R) switch 16 which switches between transmit and receive and protects the system beamformer from high energy transmit signals. Transmission of a plane wave or diverging ultrasound beam from the transducer array 12 under the control of the microbeamformer 14 is directed by a beamformer controller 18 coupled to the T / R switch and the main beamformer 20, which receives user operated input from a user interface or control panel 38. Among the transmit characteristics controlled by the transmit controller are the focus, number, spacing, amplitude, shape, phase, frequency, polarity and diversity of the transmit waveform. The beam formed in the direction of beam transmission can be steered straight ahead from the transducer array or at different angles on either side of the unsteered beam for a wider fan-shaped field of view. For the 3D imaging techniques described above, an unfocused plane wave or diverging beam is used for transmission.

[0026] The echoes received by adjacent groups of transducer elements ("patches") are beamformed by appropriately delaying them and then combining them in the microbeamformer 14. The partial beamformed signals produced by the microbeamformer 14 from each patch are coupled to a receiver in the form of a main beamformer 20, wherein the partial beamformed signals from the individual patches of transducer elements are combined into a received scanline of fully beamformed coherent echo signals over the entire scanned target volume. Preferably, the beamformer 20 is a multiple beamformer that produces multiple received scanlines from the echoes received after the transmit event. For example, the main beamformer 20 may produce hundreds or even thousands of appropriately steered and spaced received scanlines from the insonified target volume.

[0027] The coherent echo signals of the scan lines received from each plane wave or diverging beam scan are stored in the scan compound memory 22, where they are combined on a spatial basis with the echo signals received from the previous scan of the target volume. When the received scan lines of each transmit volume are in a common spatial distribution relative to the dimensions of the pyramid volume through which it is insonified, beamformer programming is facilitated, the scan lines from different scans are actually all at different spatial angles from each other and the echoes from the intersection points are combined on a spatial basis. Since the flight time of each echo determines its spatial position in the volume, the echoes with the same x, y, z coordinates in the target volume are added together and stored in the corresponding x, y, z storage locations of the scan compound memory 22. When the echoes from each different scan volume are received, they are added to the echo data previously received from the same x, y, z position of the target volume and stored in the memory. In this way, the echoes received from all 81 (or 17 or 41) volume scans of the previous example are coherently compounded in the memory 22.

[0028] The coherent echo signals undergo signal processing through a signal processor 26, which includes filtering through digital filters and noise or speckle reduction, such as through frequency compounding. The filtered echo signals also undergo orthogonal bandpass filtering in the signal processor 26. This operation performs three functions: band limiting the RF echo signal data, generating in-phase and orthogonal pairs (I and Q) of the echo signal data, and decimating the digital sampling rate. The signal processor can also shift the frequency band to a lower or baseband frequency range. For example, the digital filter of the signal processor 26 can be a filter of the type disclosed in U.S. Pat. No. 5,833,613 (Averkiou et al.).

[0029] The composite and processed coherent echo signals are coupled to a B-mode processor 30, which generates a signal for a B-mode image (such as a tissue image) of a structure in the object. The B-mode processor calculates (I 2 +Q 2 ) 1 / 2The amplitude (envelope) detection of the I and Q signal components of the orthogonal demodulation is performed using the amplitude of the echo signals in the form of a quadrature signal. The quadrature echo signal components are also coupled to the Doppler processor 34. The Doppler processor 34 stores an ensemble of echo signals from discrete points in the image field, which is then used to estimate the Doppler shift at the point in the image using a fast Fourier transform (FFT) processor. The rate at which the ensemble is acquired determines the range of velocities of motion that the system can accurately measure and depict in the image. The Doppler shift is proportional to the motion (e.g., blood flow and tissue motion) at the point in the image field. For color Doppler images, the estimated Doppler flow value at each point in the vessel is wall filtered and converted to a color value using a lookup table. The wall filter has an adjustable cutoff frequency, above or below which motion, such as low-frequency motion of the vessel wall, will be rejected when imaging flowing blood. The B-mode image signals and Doppler flow values ​​are coupled to a multi-planar reformatter 32, which extracts the image signals of the desired planes of the 3D image data set when a planar image of the scanned volume is desired. The extraction is performed on the basis of the x, y, z coordinates of the 3D data set of tissue and flow signals, and the extracted signals are then formatted for display in a desired display format (e.g., a linear display format or a sector display format). The B-mode image or the Doppler image can be displayed separately, or both can be shown together in an anatomically registered manner, wherein a color Doppler overlay shows the tissue in the blood vessels and the blood flow in the vessels of the B-mode tissue image. Another display possibility is to display images of the same anatomical structure that have been processed differently side by side. This display format is useful when comparing images.

[0030] The image data is coupled to an image memory 36, wherein the image data is stored in a memory location addressable according to the spatial location from which the image value was acquired. The image data from the 3D scan can be accessed by a volume renderer 42, which converts the echo signals of the 3D data set into a 3D image as a projection viewed from a given reference point, as described in U.S. Pat. No. 6,530,885 (Entrekin et al.). The 3D image produced by the volume renderer 42 and the 2D images produced by the multiplanar reformatter 32 from the planes of the scanned volume are coupled to a display processor 48 for further enhancement, buffering and temporary storage for display on the image display 40.

[0031] exist Figure 8 A second embodiment of the ultrasound imaging system of the present invention is illustrated in block diagram form in FIG. Figure 8 In the implementation mode Figure 71. However, instead of controlling the main system beamformer, the beamformer controller 118 now controls the addressing of receivers in the form of microchannel memories 120, in addition to its control of the microbeamformer. The microchannel memories are 3D data memories that receive and store the signals generated by the patches of elements of the 2D array transducer, storing them corresponding to their positions in the target volume being scanned. After all echo signals have been received from the target volume in accordance with the transmission of the plane wave or diverging beam, the 3D volume of data is combined on a spatial basis with the 3D data received from the previous transmission event by a synthetic focusing processor 122. Adding all the echoes received from the plane wave or diverging transmission events on a spatial basis achieves synthetic focusing, whereby image data at points throughout the volume are fully focused. See, for example, U.S. Patent No. 4,604,697 (Luthra et al.) for a description of synthetic focusing. Similar to the previous embodiments, the combination of data by the synthetic focusing processor provides a composite of 3D data sets from multiple plane wave or diverging scans of the target volume.

[0032] It should be noted that the ultrasound system suitable for use in embodiments of the present invention and in particular Figure 7 and 8 The component structure of the ultrasound system can be implemented in hardware, software or a combination thereof. Various embodiments of the ultrasound system and / or components and their controllers, or components and controllers therein, can also be implemented as part of one or more computers or microprocessors. The computer or processor may include a computing device, an input device, a display unit and an interface, for example, for accessing the Internet. The computer or processor may include a microprocessor. The microprocessor may be connected to a communication bus, for example, to access a PACS system or a data network for importing training images. The computer or processor may also include a memory. Memory devices (such as scanning composite memory 22, image memory 36 and multi-channel memory 120) may include random access memory (RAM) and read-only memory (ROM). The computer or processor may also include a storage device, which may be a hard drive or a removable storage drive, such as a floppy disk drive, an optical drive, a solid-state thumb drive, etc. The storage device may also be other similar modules for loading computer programs or other instructions into a computer or processor.

[0033] As used herein, the term "computer" or "module" or "processor" or "workstation" may include any processor-based or microprocessor-based system, including systems using microcontrollers, reduced instruction set computers (RISC), ASICs, logic circuits, and any other circuits or processors capable of performing the functions described herein. The above examples are exemplary only, and are therefore not intended to limit the definition and / or meaning of these terms in any way.

[0034] The computer or processor executes an instruction set stored in one or more storage elements to process input data. The storage element may also store data or other information as desired or required. The storage element may be in the form of an information source or physical memory element within a processing machine. The instruction set of an ultrasound system including instructions for controlling the acquisition, processing and display of ultrasound images as described above may include various commands that instruct a computer or processor as a processing machine to perform specific operations, such as the methods and processes of various embodiments of the present invention. The instruction set may be in the form of a software program. The software may take various forms, such as system software or application software, and may be embodied as a tangible and non-transient computer-readable medium. The operation of the scanning composite memory and synthetic focusing processor is typically performed by or under the direction of a software routine. In addition, the software may take the form of a collection of separate programs or modules within a larger program or a portion of a program module. The software may also include modular programming in the form of object-oriented programming. The processing of input data by the processing machine may be in response to an operator command, or in response to the results of a previous process, or in response to a request from another processing machine.

Claims

1. An ultrasound imaging system for producing a three-dimensional image of a target volume, comprising: An ultrasound probe (10) comprising a two-dimensional array (12) of transducer elements, the two-dimensional array being adapted to transmit a plane wave or a diverging wave to the target volume and to collect ultrasound echo signals returned from the target volume, a receiver coupled to receive the echo signal from each transmission and adapted to process the echo signals returned from the target volume on a spatial basis; an image data compositer coupled to the receiver and adapted to composite the image data generated in response to each transmission on a spatial basis; an image processor (30, 32, 34) coupled to receive the composite image data and adapted to generate a volumetric image; and a display (40) adapted to display the volume image, characterised in that the two-dimensional array is further adapted to emit a plurality of such waves to the target volume at different angles, the different angles being diagonal in azimuth and elevation relative to a grid (90) behind the two-dimensional array, and Each of the plurality of such waves is a diverging beam having a vertex that is a virtual vertex located on the grid (90) behind the two-dimensional array, so that the diverging beam has the form of a truncated pyramid, wherein a plurality of the virtual vertices form the grid (90).

2. The ultrasound imaging system according to claim 1, wherein: The two-dimensional array is further adapted to transmit a plurality of such waves at angles including both azimuth and elevation dimensions.

3. The ultrasound imaging system according to claim 1, wherein: The receiver comprises a beamformer (20) adapted to process received echo signals by beamforming.

4. The ultrasound imaging system according to claim 3, wherein: The ultrasound probe (10) further comprises a microwave beamformer (14) coupled to the elements of the two-dimensional array, the microwave beamformer being adapted to perform partial beamforming of echo signals received by patches of the array elements.

5. The ultrasound imaging system according to claim 4, wherein: The beamformer (20) is further adapted to beamform the partially beamformed echo signals generated by the microbeamformer (14).

6. The ultrasound imaging system according to claim 3, wherein: The image data complexer is adapted to store the echo signals on a spatial basis.

7. The ultrasound imaging system according to claim 1, wherein: The image data composite includes one of a scanning composite memory (22) and a synthetic focus processor (122).

8. The ultrasound imaging system according to claim 1, wherein: The receiver includes a microchannel memory (120) adapted to store echo signals acquired by the two-dimensional array on a spatial basis.

9. The ultrasound imaging system according to claim 7, wherein: The synthetic focusing processor (122) is adapted to combine echo signals received from the target volume from multiple transmissions on a spatial basis.

10. The ultrasound imaging system according to claim 1, wherein: The image processor also includes a B-mode processor (30).

11. The ultrasound imaging system according to claim 1, wherein: The image processor also includes a Doppler processor (34).

12. The ultrasound imaging system according to claim 1, wherein: The image processor further comprises a multi-planar reformatter (32) adapted to extract image data of image planes from the 3D data set.

13. The ultrasound imaging system according to claim 1, wherein: The image processor further comprises a volume renderer (42) adapted to generate a projection image from the 3D image data set.

14. A method for generating a three-dimensional image of a target volume, comprising: Using an ultrasound probe (10) including a two-dimensional array (12) of transducer elements to transmit a plane wave or a diverging wave to the target volume and to collect ultrasound echo signals returned from the target volume; receiving said echo signal from each transmission; processing the echo signals returned from the target volume on a spatial basis; compounding the image data generated in response to each of the shots on a spatial basis; generating a volumetric image based on the composited image data; and displaying the volume image, characterised in that the method is adapted to use the two-dimensional array to transmit a plurality of such waves to the target volume at different angles, the different angles being diagonal in azimuth and elevation relative to a grid (90) behind the two-dimensional array, and Each of the plurality of such waves is a diverging beam having a vertex that is a virtual vertex located on the grid (90) behind the two-dimensional array, so that the diverging beam has the form of a truncated pyramid, wherein a plurality of the virtual vertices form the grid (90).

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