Ultrasound-based shear wave imaging with increased pulse repetition interval

Abstract

For shear wave imaging using ultrasound, the apparent pulse repetition frequency is increased by combining (14) displacements from different transverse positions. Different combinations of different shear wave velocities and corresponding time offsets and / or attenuations, as well as corresponding scaling, are tested to find a smooth displacement distribution for the combination. Once a smooth displacement distribution is found, the corresponding shear wave velocity is estimated or determined (18).

Description

[0001] This application is a divisional application of the application with the application date of May 17, 2019, application number 201910411423.5 and the invention name as “Ultrasound-based shear wave imaging with increased pulse repetition interval”. Technical Field

[0002] This embodiment relates to shear wave imaging using ultrasound. Background Art

[0003] This embodiment relates to shear wave imaging using ultrasound. In shear wave elastography (SWEI), images are formed by tracking tissue displacements caused by shear waves induced by acoustic radiation force impulses (ARFI). The maximum pulse repetition frequency (PRF) used to track shear waves is determined by the round-trip time of the tracking pulse. In highly rigid media, such as tendons, shear waves travel faster than in soft tissues. The maximum PRF may not be sufficient to track fast shear waves.

[0004] A higher PRF can be achieved by tracking the shear wave using a series of separable signals transmitted in rapid succession. For example, the sampling rate can be increased by using tracking pulses at different frequencies or coded transmit pulses. This approach can suffer from artifacts caused by clutter noise from the different separable tracking transmissions and errors in the displacement estimate caused by using transmit pulses with different frequencies or codes. Summary of the Invention

[0005] By way of introduction, the preferred embodiments described below include methods, instructions, computer-readable media, and systems for shear wave imaging using ultrasound. The apparent PRF is increased by combining displacements from different lateral positions. Different combinations of different shear wave velocities and corresponding time offsets and / or attenuations and corresponding scaling are tested to find a combined smooth displacement distribution. Once a smooth displacement distribution is found, the corresponding shear wave velocity is estimated or determined.

[0006] In a first aspect, a method for shear wave imaging by an ultrasound imaging system is provided. A transducer transmits a push pulse. The push pulse generates a shear wave in the tissue of a patient. The ultrasound imaging system tracks tissue displacement over time at a plurality of locations in a region of interest. The tissue displacement occurs in response to the shear wave and is sampled at a first sampling rate for each location. A time displacement distribution is generated based on a combination of tissue displacements over time at different locations. The combination uses a scaling and a time offset of at least one tissue displacement over time. The time displacement distribution has a second sampling rate that is greater than the first sampling rate. A shear wave velocity is estimated using the time displacement distribution. An image showing the estimated value of the shear wave velocity is displayed.

[0007] In a second aspect, a method for shear wave imaging by an ultrasound imaging system is provided. A transducer transmits a push pulse that generates a shear wave in the patient's tissue. The ultrasound imaging system determines a displacement distribution of tissue displacement over time at each of a plurality of locations, and the tissue displacement is responsive to the shear wave. A combined displacement distribution formed by the displacement distributions of the plurality of locations is identified. The combined displacement distribution is selected from the combination based on a plurality of tests of different shear wave velocities, wherein the shear wave velocity of the different shear wave velocities that results in the smoothest combination in the combination indicates the combined displacement distribution. The shear wave velocity is estimated using the identification of the combined displacement distribution. An image showing the estimated value of the shear wave velocity is displayed.

[0008] In a third aspect, a system for shear wave imaging using ultrasound is provided. An ultrasound scanner is configured to transmit an acoustic radiation force pulse from a transducer into tissue and to scan the tissue as the tissue responds to the shear waves generated by the acoustic radiation force pulse. An image processor is configured to generate different distributions, each formed based on displacement over time from a different location, to test the different distributions, and to select one of the different distributions. A display is configured to display an image showing shear wave velocity based on the selected distribution.

[0009] The present invention is defined by the following claims, and nothing in this section should be construed as limiting those claims. Other aspects and advantages of the present invention are discussed below in conjunction with preferred embodiments, and other aspects and advantages of the present invention may be claimed later, alone or in combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The components and figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

[0011] Figure 1 is a flow chart of one embodiment of a method for shear wave imaging by an ultrasound imaging system;

[0012] Figure 2 illustrates sparse sampling of displacements at different locations for a shear wave in a rigid medium;

[0013] Figure 3 and Figure 4 The diagram shows Figure 2 An example combined displacement distribution of displacements at different locations of Figure 4 Combined displacement distribution ratio Figure 3 The combined displacement distribution is smoother; and

[0014] Figure 5is a block diagram of one embodiment of a system for shear wave imaging using ultrasound. DETAILED DESCRIPTION

[0015] This method provides shear wave imaging using ultrasound in highly rigid media. Examples of such rigid media in medical imaging include musculoskeletal (MSK) (e.g., tendons), prostate, and focal liver lesions. By increasing the apparent PRF, shear wave velocity can be more accurately estimated, even if the ultrasound round-trip travel time results in undersampling of a given location. The same approach can be used in less rigid media where tracking frequency is less of an issue. This approach increases the effective PRF, resulting in potentially more accurate shear wave velocity estimates.

[0016] Displacement profiles from multiple lateral locations are combined. Displacement profiles from different lateral locations are time-shifted and scaled based on the likely shear wave velocity and attenuation. The time-shifted and scaled profiles are combined to form a displacement profile with a higher effective PRF. The shear wave velocity is calculated by finding the time shift that produces the smoothest or sufficiently smooth displacement profile.

[0017] Figure 1 An embodiment of a method for shear wave imaging by an ultrasound imaging system is shown. For shear waves traveling at higher velocities, the sampling frequency provided by ultrasound for tracking may be relatively infrequent, resulting in undersampling and less accurate shear wave velocity estimates. To increase the effective sampling rate, displacements over time from different locations are combined. The combination with the closest displacement distribution provides displacements at a greater sampling frequency for a more accurate estimate of shear wave velocity.

[0018] Using ultrasound imaging systems (such as Figure 5 The ultrasound imaging system (described herein) performs the various actions. A transducer and / or beamformer is used to acquire data, and an image processor estimates displacement based on the data and estimates shear wave velocity and / or attenuation based on the displacement. The ultrasound imaging system outputs the shear wave velocity. Other equipment, such as a computer or a detector, may be used to perform any of the described actions.

[0019] Can be Figure 1 Additional, different, or fewer actions are provided in the method. For example, action 18 is not provided. As another example, action 16 is not provided. In yet another example, an action for configuring an ultrasound system to scan a patient is provided.

[0020] The actions are performed in the order described or shown (e.g., from top to bottom or by number). Other orders can be provided, such as by repeating the actions for another region of interest. In one example, actions 16 and 18 are performed as part of action 14 or in reverse order.

[0021] In actions 10 and 12, the ultrasound system generates shear waves and detects shear waves at different locations in the patient's tissue. Acoustic radiation pulsation excitation (ARFI or push pulse) or other stress sources generate shear waves in the tissue. As the shear waves propagate through the tissue, the tissue shifts. The shear waves are detected based on the tissue displacement caused by the passage of the shear waves. By scanning the tissue with ultrasound, data for calculating the displacement over time is obtained. Using correlation or other similarity metrics, the displacements represented by the scans obtained at different times are determined. The ultrasound system obtains tissue displacements over time (i.e., a displacement distribution) for different locations.

[0022] In act 10, the beamformer generates an electrical signal for focused ultrasound transmission, and the transducer converts the electrical signal into an acoustic signal for transmitting a push pulse from the transducer to the focal region. ARFI is used. The acoustic excitation is transmitted into the patient. The acoustic excitation acts as a pulsating excitation for inducing shear waves. For example, a 400 cycle transmission waveform having an energy or peak amplitude level similar to or lower than a B-mode transmission for imaging tissue is transmitted as the acoustic beam. In one embodiment, the transmission is a shear wave generation sequence applied to the field of view. Any ARFI or shear wave imaging sequence may be used. Other stress sources, such as a hammer (a mechanical shock or vibration source), may be used.

[0023] The transmission is configured by energy, amplitude, timing or other characteristics to induce stress on the tissue sufficient to displace the tissue at the focal location. For example, the transmission focus of the beam is set relative to the field of view or region of interest (ROI) so that the generated shear wave displacement is throughout the field of view or ROI. The pulsating excitation generates shear waves at spatial locations. When the excitation is strong enough, shear waves are generated. Shear waves propagate laterally through tissue more slowly than longitudinal waves along the direction of acoustic emission, so the type of wave can be distinguished by timing and / or direction. The tissue displacement caused by the shear wave is greater at locations closer to the focal location where the wave was generated. As the wave travels, the magnitude of the wave decays. The stiffness of the tissue affects the shear wave velocity and / or attenuation.

[0024] In act 12 , tissue displacement is tracked. The ultrasound system (such as the system's image processor) tracks the displacement induced in response to the push pulse. For each of a plurality of locations, the displacement induced by the propagating shear wave is tracked. Tracking is axial (i.e., displacement is tracked one-dimensionally along a scan line) but can be two-dimensional or three-dimensional. The tracked locations are distributed transversely (e.g., perpendicular to the scan line) but can be distributed in two or three dimensions.

[0025] Tracking is done over time. For each location, the tissue displacement is found for any number of time samples over the period during which the wave is expected to propagate past that location. Time samples may be undersampled due to insufficient PRF for fast-traveling shear waves. Tracking at multiple locations provides a distribution of tissue displacement over time for different locations.

[0026] The time period tracked can include the time before the push pulse and / or shear wave is delivered to each given location. Similarly, the time period tracked can include the time after the tissue relaxes or the entire shear wave has propagated through each location. The tissue is scanned as the shear wave propagates through these locations.

[0027] The transducer and beamformer acquire echo data at different times to determine the displacement of the tissue. The displacement is detected using ultrasound scanning. At least some of the ultrasound data is responsive to displacement caused by shear waves or pressure. An area such as a region of interest, an entire field of view, or a sub-area of ​​interest is scanned using ultrasound. The area is monitored to detect the waves. The echo data represents the tissue when it is subjected to different amounts of pressure at different times. The area can be of any size, such as 5x5 mm laterally and 10 mm axially. For example, a B-mode scan is performed to detect tissue displacement. Any sampling or beamformer resolution can be used, such as measuring on a linear grid with sample locations every 0.25 mm. Doppler, color flow angiography, or other ultrasound modes can be used to detect displacement.

[0028] For a given time, ultrasound is transmitted to the tissue or region of interest. Any displacement imaging technique now known or later developed may be used. For example, pulses having a duration of 1-5 cycles and an intensity of less than 720 mW / cm 2 (e.g., B-mode pulses). Pulses of other intensities may be used. Scanning is performed for any number of scan lines. For example, eight or sixteen receive beams distributed in two dimensions are formed in response to each transmission. After or while applying the stress, B-mode transmissions are repeated along a single transmit scan line and repeated along an adjacent receive scan line. In other embodiments, other numbers of receive beams are formed in response to each transmission. Additional transmit scan lines and corresponding one or more receive lines may be used. Any number of repetitions may be used, such as approximately 120 times or in 15 ms or less for faster shear waves.

[0029] B-mode intensity can vary due to tissue displacement over time. Displacement is detected continuously or over a series of displacements for each location. For each monitored scan line, a series of data is provided that represents the temporal distribution of tissue motion caused by stress. By performing transmission and reception multiple times, data representing the region at different times is received.

[0030] Displacement is detected for each of a plurality of spatial locations. Displacement can be measured for any number of sample locations, such as every quarter millimeter in a 10 x 5 mm region of interest. A displacement distribution is determined at each sample point, or data from two or more sample points is combined to give a displacement distribution for a sub-region. Displacement is measured for each sample point and each sample time.

[0031] Velocity, variance, shifts in intensity patterns (e.g., speckle tracking), or other information is detected from the received data as a displacement between two times. In one embodiment using B-mode data, data from different scans are axially correlated as a function of time. For each depth or spatial sampling position, a correlation is performed across multiple depth or spatial sampling positions (e.g., a kernel of 64 depths, where the center depth is the point for which the distribution is calculated). For example, the current data set is correlated multiple times with the reference data set. The location of a subset of the data centered at a given location in the reference set is identified in the current set. Different relative transformations between the two data sets are performed.

[0032] The reference is the first or another data set or data from another scan. The reference set is from before the stress, but may also be from after the stress. The same reference is used for the entire displacement measurement, or the reference data changes during the advancing or moving window.

[0033] The similarity or correlation level of the data at each of the different offset positions is calculated. The transition with the greatest correlation represents the displacement or offset associated with comparing the current data to the reference for that time.

[0034] Any now known or later developed correlation method can be used, such as cross-correlation, pattern matching, or minimum sum of absolute differences. Tissue structure and / or speckle are correlated. Using Doppler detection, a clutter filter conveys information related to moving tissue. Tissue velocity is derived from the multiple echoes. Velocity is used to determine displacement toward or away from the transducer. Alternatively, the relative or differential velocity at different locations can indicate strain or displacement.

[0035] The displacement of the tissue starts from a steady state before the shear wave arrives, then increases to a maximum value, after which the displacement decays back to the steady state. Other displacement distributions are possible. Figure 2An example displacement distribution is shown for 16 adjacent locations over time. Due to undersampling, the usual gradual curve is not provided. In this example, the tissue is scanned seven times for tracking. For each lateral position, the peak of the displacement caused by the shear wave should occur at a time between the peaks of adjacent positions. With undersampling, the shear wave appears to occur at one set of locations at one time (approximately 6 μs) and appears to occur at another set of locations at another time (approximately 9 μs). Greater or lesser amounts of undersampling of the sampling rate for ultrasound tracking at each location can be provided. The accuracy of estimating the shear wave from the undersampled displacement distribution is poor.

[0036] In act 14, the image processor generates a temporal displacement distribution based on a combination of tissue displacements over time at different locations. To increase the effective PRF, displacements from different locations are used to create a displacement distribution with a greater sampling rate. Because the peak of the shear wave is located at one location at any given sampling time and not at another, this combination provides a distribution with a greater sampling rate.

[0037] The combination has displacements or displacement distributions from two or more locations. Since the displacement distributions themselves cannot be correlated to detect offsets and corresponding shear velocities, displacements from three or more locations can be combined. For example, displacements from four, eight, sixteen, or thirty-two laterally spaced locations can be combined. A combination is provided for a region of interest to estimate the shear wave velocity for that region.

[0038] In another embodiment, a window of defined positions is used. The window is centered at one position for one combination. The window is spatially offset so that it is centered at a different position for another combination. The tracking sample positions within the window for each set of window positions are combined. As a result, different combinations are provided for different groups of positions, thereby allowing spatial velocity estimation (i.e., different shear wave velocities for different positions (where the window is centered at different positions)).

[0039] Different possible combinations are tested. A plurality of test combinations are formed. The possible combinations are iterated over shear wave velocities and / or attenuation. Given a shear wave velocity, a time offset is calculated for each displacement distribution. The final shear wave velocity is the velocity that produces the smoothest combined distribution. Similarly, the possible combinations are iterated over shear wave attenuation coefficients. Given an attenuation coefficient, a scaling factor is calculated for each displacement distribution. The final attenuation coefficient is the attenuation coefficient that produces the smoothest combined distribution. A possible combination is selected to provide velocity and / or attenuation based on the time offset and / or scaling that results in a sufficiently smooth combined displacement distribution.

[0040] Each combination is formed using time offsets and / or scaling. Different time offsets and scalings corresponding to different velocities and attenuations are tested. One displacement profile is used as a reference, and the other displacement profiles in the combination are shifted and / or scaled in time based on the time offset and / or scaling provided by the velocity and / or attenuation being tested. Different combinations corresponding to different offsets and scalings are tested.

[0041] For example, each combination is formed with a time offset and / or scaling. The time offset and scaling of different individual distributions are different based on different positions. Four displacement distributions are used to form the combination, the first of which is a reference. The other three distributions are time offset and scaled. The amount by which each of the three distributions is offset and scaled is different. Assuming that there is 1 mm between any two adjacent positions in the four lateral positions, a time offset is applied based on the velocity being tested. In the first iteration, it is checked whether the shear wave velocity is 4m / s=4mm / ms. Distribution 1 has no offset; Distribution 2 is offset by t=d / vs=1mm / 4mm / ms=0.25ms; Distribution 3 is offset by t=d / vs=2mm / 4mm / ms=0.5ms; and Distribution 4 is offset by t=d / vs=3mm / 4mm / ms=0.75ms. The resulting combined displacement distribution obtained by combining all four distributions is the distribution tested for smoothness. When there are only two positions, a single time offset and / or single scaling is provided for each velocity and / or attenuation.

[0042] Any search pattern can be used in which a range of time offsets and scalings are tested based on speed and attenuation. Test different iterations. In one embodiment, different speeds and corresponding time offsets are tested. Once a speed is selected, different attenuations and corresponding scalings are tested. Attenuation can be tested, then speed. In another embodiment, only attenuation or only speed is tested. In yet another embodiment, both attenuation and speed are varied with each iteration. In another embodiment, a loop iteration is used. Different speeds are tested, then different attenuations are tested using the selected speed, then different speeds are tested using the selected attenuation and centered around the previously selected speed. Any number of loops throughout this process can be used.

[0043] The initial time offset and / or scaling applied can be based on an expected offset and / or scaling (i.e., expected velocity and / or attenuation). The type of tissue being imaged or for which shear wave velocity is being measured can be indicated by user input. The initial offset and / or scaling is based on the indication of the tissue. Other offsets and / or scalings different from the initial offset and / or scaling are also tested.

[0044] Scaling can be a given value or a linear scaling with respect to position. Positions far from a reference are scaled by an amount given by the attenuation and distance from the reference position. For time offset, the time offset is cumulative. The farther the position is from the reference position, the greater the time offset. The amount of time offset as a function of distance varies with velocity.

[0045] After applying a time offset and / or scaling for a given velocity and / or attenuation, the displacements resulting from the different positions are combined into a single displacement distribution. Figure 3 and 4 Shown from Figure 2 Two such combinations of displacements. Figure 3 Represents one possible combination according to a speed and a corresponding time offset, which are applied across various locations based on the location interval. Figure 4 represents another possible combination according to another speed and corresponding time offset.

[0046] The combination that provides the smoothest or sufficiently smooth displacement distribution is selected. The displacements caused by the propagating shear wave have a standard (i.e., typical) or expected distribution shape over time. Each possible combination is tested to determine the level of smoothness or matching the standard. The combination with the smoothest or most standard distribution is selected or identified. In other embodiments, a threshold level of smoothness or standardization is applied. Once a combination with sufficient smoothness or standardization is found, the search ends.

[0047] The level of smoothness may be measured in any manner. Figure 4 Shows a ratio Figure 3 Combinations with greater smoothness. The size of the time and / or amplitude gaps can be measured, where smaller gaps indicate greater smoothness. In another approach, a combination of displacements from different locations is transformed into the frequency domain. A Fourier transform is applied to the displacement distribution formed by the combination. The amount of high-frequency content is quantified. Lower high-frequency content indicates a smoother distribution. In yet another approach, the combined displacement distribution is matched to a template. The template or a default representation of the standard. The level of similarity (e.g., a correlation coefficient) between the combined displacement distribution and the template indicates the level of smoothness. Other measures of smoothness or standardization can be used.

[0048] Choose the combination of time shifting and / or scaling that results in the smoothest (or most normalized) or sufficiently smooth (or normalized) displacement distribution. Test different possible combinations. Figure 3 and 4 In the example, the selection that results in Figure 4 The selection identifies the time shift and / or scaling that results in the combination.

[0049] Since displacements from different locations are combined into one distribution, the effective sampling rate increases. For example, Figure 4 The effective sampling rate of the displacement distribution is Figure 2 The sampling rate of any displacement distribution for a single location is 16 times higher than that of the original data. Thus, a more complete or better sampled displacement distribution is provided, allowing the shear wave velocity to be estimated based on a greater amount of information (i.e., more samples).

[0050] In act 16, the image processor estimates the shear wave velocity using the temporal displacement profile. The shear wave velocity in the patient tissue is determined. The shear wave velocity is identified using the selected or identified combined displacement profile. The shear wave velocity is identified as a relatively or sufficiently smooth or normalized combined displacement profile having a corresponding larger PRF relative to the PRF of the displacement over time for the location.

[0051] In one embodiment, the offset resulting in the combination indicates shear wave velocity. The time offset for the selected combination is based on the shear wave velocity. The selection of the combination indicates the position-based time offset resulting from a given velocity, and the position-based time offset provides an estimate of the velocity. In an alternative embodiment, different combinations are selected for different spatial windows or sub-regions. The resulting combinations can be correlated with each other to provide phase offsets between the combinations. The distances between the window centers and the phase offsets from the maximum correlation for the different combinations indicate the shear wave velocity.

[0052] In act 18, the image processor estimates the attenuation. The attenuation is estimated using the selected combined displacement distribution. The scaling of the selected combination indicates the rate of change of the amplitude with distance. The scaling applied to the displacement maps to the attenuation of the shear wave. The attenuation is an exponential function: exp (α*d) , where α is the attenuation coefficient and d is the lateral position or distance from a reference position. Scaling that results in a smooth or normalized displacement distribution indicates shear wave attenuation in the tissue. The attenuation can be determined as a function of position (i.e., spatial window placement).

[0053] In act 19, the image processor transmits the estimated value of shear wave velocity, the estimated value of shear wave attenuation, and / or both. The transmission is to a display, a memory, or a network. For example, the transmission is output from or within the ultrasound imaging system.

[0054] In one embodiment, an image showing estimated values ​​of velocity and / or attenuation is output. The values ​​for a region of interest are provided on a B-mode image or other ultrasound image. One and / or multiple values ​​of velocity and / or attenuation are output as text, numbers, or encoded in a graphic. For example, a user selects a location on the B-mode image. In response, the ultrasound system calculates a value for shear wave velocity of a region centered on the selected location. The numerical, textual, and / or graphical representation of the calculated values ​​is superimposed on the B-mode image, displayed independently, or otherwise communicated to the user (e.g., added to a report).

[0055] An image of the spatial distribution of shear velocity is generated while estimating the velocity at different locations. For example, a shear wave velocity image is color modulated based on the shear velocity as a function of position in the region of interest. The shear wave velocity image is superimposed on a B-mode image that covers a larger field of view than the region of interest or the shear wave velocity image. Alternatively, the brightness, tent, hue, or color map is based on velocity. In other embodiments, other types of elastic imaging, shear-free or elastic images, and / or different types of ultrasound imaging are provided.

[0056] Figure 5 One embodiment of a system 50 for shear wave imaging using ultrasound is shown. The system 50 implements Figure 1 The displacements as a function of time from different locations are combined into a displacement distribution with a larger effective PRF to estimate the shear wave properties in the patient's tissue.

[0057] System 50 includes a transmit beamformer 52, a transducer 54, a receive beamformer 55, an image processor 56, a display 58, and a memory 57. Additional, different, or fewer components may be provided. For example, a user input may be provided for a user to interact with the system, such as for selecting a location at which measurements are to be taken or for specifying the placement of a region of interest.

[0058] System 50 is a medical diagnostic ultrasound imaging system or ultrasound scanner. System 50 is configured to transmit acoustic radiation force pulsations from a transducer 54 into tissue and scan the tissue at multiple locations as the tissue responds to shear waves created by the acoustic radiation force pulsations. The ultrasound scanner tracks the response to the shear waves. In alternative embodiments, system 50 includes a front-end scanner and a back-end processor, such as a personal computer, workstation, PACS station, or other arrangement for real-time or post-acquisition imaging, in the same location or distributed over a network. The scanning components (e.g., transmit beamformer 52, transducer 54, and receive beamformer 55) are part of a device that is distinct from memory 57, image processor 56, and / or display 58. The back-end can retrieve data from memory or from transmission over a network. The front-end provides the data to memory or the network.

[0059] The transmit beamformer 52 is an ultrasound transmitter, memory, pulse generator, analog circuitry, digital circuitry, or a combination thereof. The transmit beamformer 52 is configured to generate waveforms with varying or relative amplitudes, delays, and / or phasing for multiple channels. When acoustic waves are transmitted from the transducer 54 in response to the generated electrical waveforms, one or more acoustic beams are formed. A series of transmit beams is generated to scan an area. Sector, Vector®, linear, or other scanning formats may be used. In alternative embodiments, the transmit beamformer 52 generates plane waves or diverging waves for faster scanning. The same area is scanned multiple times. For shear imaging, a series of scans along the same line is used.

[0060] The same transmit beamformer 52 can generate both the pulsed excitation (ARFI or push pulse) and the acoustic beam for tracking. An electrical waveform is generated for ARFI, and then another for tracking. In alternative embodiments, a different transmit beamformer is used for ARFI generation than for tracking. The transmit beamformer 52 causes the transducer 54 to generate acoustic energy. Using a delay profile across the channels, the transmit beamformer 52 directs the push pulse to one or more desired focal locations and scans to track displacement.

[0061] The transducer 54 is an array for generating acoustic energy from an electrical waveform. For the array, relative delays focus the acoustic energy. A given transmission event corresponds to substantially simultaneous transmission of acoustic energy by different elements at a given focusing delay. A transmission event can provide a pulse of ultrasonic energy to displace tissue. The pulse is either a pulsating excitation or a tracking pulse. A pulsating excitation includes a waveform that has many cycles (e.g., 500 cycles) but occurs in a relatively short time due to shear wave propagation to cause tissue displacement over a longer time. A tracking pulse can be a B-mode transmission, such as using a 1-5 cycle waveform. Tracking pulses are used to scan areas of the patient experiencing stress changes.

[0062] The transducer 54 is a 1-, 1.25-, 1.5-, 1.75-, or 2-dimensional array of piezoelectric or capacitive diaphragm elements. A wobbler array may be used. The transducer 54 includes multiple elements for transducing between acoustic and electrical energy. A receive signal is generated in response to ultrasonic energy (echoes) striking the elements of the transducer 54. These elements are connected to the channels of the transmit and receive beamformers 52 and 55.

[0063] The receive beamformer 55 includes multiple channels having amplifiers, delay and / or phase rotators, and one or more adders. Each channel is connected to one or more transducer elements. The receive beamformer 55 is configured by hardware or software to apply relative delay, phase, and / or apodization to form one or more receive beams in response to each imaging or tracking transmission. Receiving may not occur for echoes from the pulsating excitation used to displace tissue. The receive beamformer 55 uses the received signals to output data representing spatial position. The relative delay and / or phasing and summation of the signals from the different elements provide beamforming. In an alternative embodiment, the receive beamformer 55 is a processor that generates samples using Fourier or other transforms.

[0064] In coordination with the transmit beamformer 52, the receive beamformer 55 generates data representing the region at different times. Following ARFI, the receive beamformer 55 generates beams representing positions along one or more lines at different times. Data (e.g., beamformed samples) are generated by scanning the region with ultrasound. By repeatedly performing the scan, ultrasound data representing the region at different times after the pulsed excitation is acquired.

[0065] The receive beamformer 55 outputs beam summation data representing different sample positions. Dynamic focusing can be provided. The data may be used for different purposes. For example, a different scan is performed for B-mode or tissue data than for shear ultrasound imaging. Alternatively, B-mode data is also used to track shear waves. As another example, data for shear imaging is acquired using a series of shared scans, and B-mode or Doppler scans are performed separately or using some of the same data. The ultrasound or echo data comes from any processing stage, such as beamformed data before detection or data after detection.

[0066] The memory 57 is a non-transitory computer-readable storage medium. For example, the memory 57 is a cache, a buffer, RAM, a removable medium, a hard drive, or other non-transitory computer-readable storage medium. Computer-readable storage media include various types of volatile and non-volatile storage media.

[0067] The memory 57 is configured by the image processor 56, the controller, or the memory processor to store and provide data. The memory 57 stores any data used to estimate shear wave characteristics. For example, the memory 57 stores ultrasound data (beamformed data and / or detected data), displacement, displacement distribution, time offset, scaling, smoothness level, velocity, and / or attenuation.

[0068] The image processor 56 operates according to instructions stored in the memory 57 or another memory to estimate the attenuation and / or shear wave velocity of the patient's tissue. Instructions for implementing the processes, methods and / or techniques discussed herein are provided on a computer-readable storage medium or memory. In response to one or more instruction sets stored in or on a computer-readable storage medium, the functions, actions or tasks illustrated in the accompanying drawings or described herein are performed. The functions, actions or tasks are independent of the specific type of instruction set, storage medium, processor or processing strategy and can be performed by software, hardware, integrated circuits, firmware, microcode, etc. operating alone or in combination. Similarly, processing strategies can include multi-processing, multi-tasking, parallel processing, etc. In one embodiment, the instructions are stored on a removable media device for reading by a local or remote system. In other embodiments, the instructions are stored at a remote location for transmission over a computer network or over a telephone line. In yet other embodiments, the instructions are stored within a given computer, CPU, GPU or system.

[0069] The image processor 56 includes a B-mode detector, a Doppler detector, a pulsed wave Doppler detector, a correlation processor, and / or a Fourier transform processor for detecting and processing information from the beamformed ultrasound samples for display.

[0070] In one embodiment, the image processor 56 includes one or more detectors and a separate processor. The separate processor is a control processor, a general purpose processor, a digital signal processor, an application specific integrated circuit, a field programmable gate array, a network, a server, a processor group, a graphics processing unit, a digital signal processor, an analog circuit, a digital circuit, a combination thereof, or other devices now known or later developed for estimating shear wave properties in tissue. For example, the separate processor is configured by hardware, firmware, and / or software to perform Figure 1 Any combination of one or more of actions 12-19 shown in .

[0071] Image processor 56 is configured to measure displacement profiles (i.e., displacement as a function of time) at a plurality of locations. For example, beamformed or detected data is correlated with a reference axis to determine the amount of tissue displacement at a given time at each of a plurality of scan lines. The location is a sample location or a subregion of a region of interest of the receive beamformer 55. A displacement profile is generated for each different location over time. For example, 16 displacement profiles are generated for 16 corresponding laterally spaced locations.

[0072] Image processor 56 is configured to generate different displacement distributions based on different combinations of distributions from different locations. Based on different velocities and / or attenuations, different time offsets and / or scaling are applied to form different combinations. The resulting displacement combinations from different locations (i.e., combined distributions) are tested. This testing determines the level of similarity or smoothness with a standard. Based on the testing, a combined distribution is selected. For example, the combination of displacements from different locations based on time offset for a given velocity and / or scaling for a given attenuation that has the greatest or sufficient level of smoothness is selected.

[0073] The image processor 56 is configured to estimate the shear wave velocity. By iterating through the shear wave velocity and the resulting time offset, the velocity for a selected one of the different distributions is the velocity of the shear wave in the patient. The distance between the sample locations is considered as part of the applied time offset. The combinations resulting from the various velocities and corresponding time offsets are tested, thereby indicating the velocity of the shear wave in the tissue based on the combination selected for the test. As a result, the test of the combination provides the shear wave velocity. Similarly, scaling provides attenuation.

[0074] The image processor 56 is configured to generate one or more images. The image includes color-modulated areas and / or alphanumeric text that represent or are based on velocity or attenuation values, such as annotations on an image of a 2D or 3D representation of the tissue. The velocity and / or attenuation values ​​are displayed as text, numbers, and / or in a graphic separate from any spatial representation of the tissue or in annotations on any spatial representation of the tissue. In the case of estimating the spatial distribution of velocity and / or attenuation, an image of the spatial distribution of velocity or attenuation can be generated. The image is presented as a region of interest or overlay within the B-mode image or presented separately.

[0075] Display 58 is a CRT, LCD, projector, plasma, or other display for displaying values, graphs, two-dimensional images, or three-dimensional representations. A two-dimensional image represents a spatial distribution in an area, such as a plane. A three-dimensional representation is presented based on data representing a spatial distribution in a volume. Display 58 is configured by image processor 56 or other device by inputting a signal to be displayed as an image. Display 58 displays an image representing an estimated value. For example, the image shows shear wave velocity based on a selected combined displacement distribution.

[0076] Due to the combined distribution, the shear wave velocity provided on the image is more accurate and / or is provided for rigid media that would otherwise be undersampled. As a result, doctors are provided with more accurate information for diagnosis, prognosis, and / or treatment. For example, doctors can be more confident in determining whether a lesion should be biopsied. This can avoid unnecessary surgical procedures.

[0077] Although the present invention has been described above with reference to various embodiments, it will be appreciated that many changes and modifications may be made without departing from the scope of the invention. Therefore, the foregoing detailed description is intended to be illustrative rather than restrictive, and it will be understood that the following claims, including all equivalents, are intended to define the spirit and scope of the invention.

Claims

1. An ultrasonic imaging system (50) for performing shear wave imaging, the ultrasonic imaging system (50) comprising: a transducer (54) configured to transmit a push pulse that generates a shear wave in the patient's tissue; The image processor (56) is configured to: tracking tissue displacement over time at a plurality of locations in a region of interest, the tissue displacement being responsive to the shear wave, the tissue displacement over time being provided at a first sampling rate for each location; generating a temporal displacement profile as a single sampling curve based on a combination of the tissue displacements over time at different ones of the plurality of locations, such that different tissue displacements from the different locations form the single sampling curve, the combination using a time offset and a scaling of at least one of the tissue displacements over time, the temporal displacement profile having a second sampling rate greater than a first sampling rate; as well as estimating shear wave velocity using the time displacement distribution; and A display (58) is configured to display an image showing the estimated value of the shear wave velocity.

2. The ultrasound imaging system (50) of claim 1, wherein the image processor (56) is further configured to iteratively test different combinations including the combination, wherein different time offsets and different scalings are provided for different ones of the iterations based on different velocities and attenuations.

3. The ultrasound imaging system (50) of claim 1, wherein the image processor (56) is further configured to determine the combination and the corresponding time offset and scaling based on a smoothness level of the time displacement distribution, a smoothness level measured in the frequency domain, or a match with a template.

4. The ultrasound imaging system (50) of claim 1, wherein the image processor (56) is further configured to identify a combination of time offset and scaling that matches a criterion.

5. The ultrasound imaging system (50) according to claim 1, wherein The image processor (56) is further configured to generate the temporal displacement distribution according to a combination of three or more tissue displacements over time at three or more locations.

6. The ultrasound imaging system (50) according to claim 1, wherein: The image processor (56) is further configured to estimate attenuation based on the scaling.

7. A non-transitory computer-readable storage medium comprising instructions stored thereon, the instructions, when executed by a processor, causing the processor to implement a method for performing shear wave imaging, the method comprising: transmitting (10) a push pulse that generates (14) a shear wave in patient tissue; tracking (12) tissue displacement over time at a plurality of locations in a region of interest, the tissue displacement being responsive to the shear wave, the tissue displacement over time being provided at a first sampling rate for each location; generating (14) a temporal displacement distribution as a single sampling curve based on a combination of the tissue displacements over time at different ones of the plurality of locations, such that different tissue displacements from the different locations form the single sampling curve, the combination using a time shift and scaling of at least one of the tissue displacements over time, the temporal displacement distribution having a second sampling rate greater than a first sampling rate; Estimate (16) the shear wave velocity using the time displacement distribution; and An image is displayed (19) showing the estimated value of the shear wave velocity.

8. The non-transitory computer-readable storage medium of claim 7, wherein the instructions further cause the processor to: iteratively test different combinations comprising the combination, wherein different time offsets and different scalings are provided based on different speeds and attenuations for different ones of the iterations.

9. The non-transitory computer-readable storage medium of claim 7, wherein the instructions further cause the processor to: determine the combination and corresponding time offset and scaling based on a smoothness level of the time displacement distribution, a smoothness level measured in the frequency domain, or a match with a template.

10. The non-transitory computer-readable storage medium of claim 7, wherein the instructions further cause the processor to: identify a combination of time offset and scaling that matches a criterion. 11 . The non-transitory computer-readable storage medium of claim 7 , wherein the instructions further cause the processor to generate the temporal displacement distribution based on a combination of three or more of the tissue displacements over time at three or more locations. 12 . The non-transitory computer-readable storage medium of claim 7 , wherein the instructions further cause the processor to: estimate attenuation based on the scaling.

13. A non-transitory computer-readable storage medium comprising instructions stored thereon, the instructions, when executed by a processor, causing the processor to implement a method for performing shear wave imaging, the method comprising: transmitting (10) a push pulse that generates (14) a shear wave in patient tissue; determining a displacement distribution of tissue displacement over time at each of a plurality of locations, the tissue displacement being responsive to the shear wave, the tissue displacement over time being provided at a first sampling rate for each location; iteratively testing different combined displacement distributions formed from the displacement distributions at the plurality of locations, wherein the different combined displacement distributions are formed by applying different time offsets to the tissue displacement distributions at the plurality of locations based on different given shear wave velocities, and each of the different combined displacement distributions is formed as a single sampling curve based on the displacement distributions at the plurality of locations; identifying and selecting the combined displacement profile from the different combined displacement profiles based on a plurality of tests of different shear wave velocities, wherein a shear wave velocity among the different shear wave velocities that results in a smoothest combination among the combinations is indicative of the combined displacement profile, the combined displacement profile having a second sampling rate greater than a first sampling rate; Estimate (16) the shear wave velocity using the signature of the combined displacement distribution; and An image is displayed (19) showing the estimated value of the shear wave velocity.

14. The non-transitory computer-readable storage medium of claim 13, wherein the identification comprises: The smoothness level of the combination is measured.

15. The non-transitory computer-readable storage medium of claim 13, wherein the identification comprises: Identification is performed based on the plurality of tests at different time offsets and different scalings.

16. The non-transitory computer-readable storage medium of claim 13, the method further comprising: The shear wave attenuation is estimated (18) according to the scaling that results in the smoothest combination among the combinations.

17. The non-transitory computer-readable storage medium of claim 13, wherein the identification comprises: The combined displacement distribution is identified from the displacement distributions for two or more locations.

18. The non-transitory computer-readable storage medium of claim 13, wherein estimating (16) the shear wave velocity comprises estimating (16) the shear wave velocity based on a time offset that results in a smoothest combination of the combinations.

19. The non-transitory computer-readable storage medium of claim 13, wherein: The instructions further cause the processor to form different combined displacement distributions based on different shear wave velocities and the attenuation of displacement over time at the different locations, and estimate the shear wave velocity based on the selected combined displacement distribution.

20. The non-transitory computer-readable storage medium of claim 13, wherein: The instructions further cause the processor to select the combined displacement profile based on the tested smoothness level.

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