Asymmetry of the acoustic radiation force pulse

Abstract

Providing asymmetry to a push pulse in acoustic radiation force impulse (ARFI) imaging (32). MI is based on negative pressure. By increasing the positive pressure (30) more than the negative pressure, the magnitude of the displacement can be increased without exceeding the MI limit. Similarly, negative voltages depolarize while positive voltages do not depolarize, so using (30) ARFI or push pulses with asymmetric positive-negative peak pressures or voltages allows for the generation of larger magnitude displacements without damaging the transducer (14).

Description

Background Art

[0001] This embodiment relates to acoustic radiation force impulse (ARFI) or elastic imaging. By delivering ARFI as a push pulse, ultrasound can be used to displace tissue directly or by generating shear waves or longitudinal waves. The displacement caused by the push pulse can be measured using further ultrasound scanning. Elastic, shear or other types of parametric imaging measure tissue properties based on the displacement caused by ARFI.

[0002] ARFI-based ultrasound imaging is limited by the low displacement signal. The induced displacement is directly related to the applied force and, therefore, to the local acoustic intensity. This intensity is often constrained by regulatory limits. One regulatory limit, the Mechanical Index (MI), is defined as:

[0003]

[0004] in is the peak rarefaction (negative) pressure, and is the center frequency of the transmission. The MI is constrained to a maximum value of 1.9, and many ultrasound systems use an MI of approximately 1.4 to account for transducer and system variability. The MI acts to limit the overall output, thereby limiting the displacement amplitude. Additionally, the transducer is limited by the maximum voltage that can be applied across the piezoelectric element to ensure the safety of the transducer. If the applied voltage is too negative, the transducer may depolarize or lose its ability to convert electrical energy into mechanical energy.

[0005] If limited by the MI or peak voltage, push pulses of longer duration are used to increase displacement. However, the maximum duration of a given transmission event is limited by the physics of elastography and the stiffness of the tissue being imaged. Summary of the invention

[0006] By way of introduction, the preferred embodiments described below include methods, instructions, and systems for asymmetry of acoustic radiation force impulse (ARFI) scanning. MI is based on negative pressures. By increasing the positive pressure more than the negative pressure, the amplitude of the displacement can be increased without exceeding the MI limit. Similarly, negative voltages depolarize, while positive voltages do not depolarize, so using ARFI or push pulses with asymmetric positive-negative peak pressures or voltages allows for the generation of larger amplitude displacements without damaging the transducer.

[0007] In a first aspect, a method for performing acoustic radiation force pulse scanning by an ultrasound system is provided. The ultrasound system transmits a transmit beam as an acoustic radiation force pulse from an ultrasound transducer, wherein the absolute value of the peak positive pressure is at least 1.2 times the absolute value of the peak negative pressure at the focal position of the transmit beam. The ultrasound system tracks the displacement of tissue using the ultrasound transducer, wherein the displacement is responsive to the acoustic radiation force pulse. An image is generated as a function of the tissue displacement.

[0008] In a second aspect, an ultrasound system for acoustic radiation force pulse scanning is provided. An ultrasonic transducer for transmitting an acoustic radiation force pulse in a patient is provided. A transmit beamformer is configured to generate an electrical waveform of an acoustic radiation force pulse. The generated electrical waveform has a plurality of cycles, and for at least about 1 / 2 of the cycle, the ratio of the positive peak voltage to the negative peak voltage is at least 1.5. A receive beamformer is configured to output data representing a spatial position as a function of a received acoustic signal in response to tissue movement due to the acoustic radiation force pulse. A processor is configured to estimate the displacement of tissue in the patient's body over time as a function of the output data. The display is operable to display an image, wherein the image is a function of the displacement.

[0009] In a third aspect, a method for imaging by an ultrasound system is provided. The ultrasound system transmits an acoustic waveform from an element of an ultrasound transducer. The acoustic waveform at the element has a beamformer-generated asymmetry between a peak positive pressure and a peak negative pressure for at least 1 / 3 of a cycle. The acoustic waveform causes tissue displacement. Shear wave imaging is performed based on the displacement.

[0010] The present invention is defined by the appended claims, and nothing in this section should be considered as limiting those claims.Further aspects and advantages of the present invention are discussed below in conjunction with preferred embodiments, and may be claimed later, either individually or in combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the present invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

[0012] Figure 1 is a flow chart of one embodiment of a method for ARFI imaging using asymmetric push pulses;

[0013] Figure 2 and 3 is an example asymmetric push pulse waveform; and

[0014] Figure 4 is a block diagram of one embodiment of an ultrasound system for asymmetric push pulses in elastography. DETAILED DESCRIPTION

[0015] Asymmetric push pulses are used for radiation force-based ultrasound imaging. Low displacement is addressed by increasing acoustic output while maintaining patient safety (i.e., staying within regulatory limits) and transducer hardware reliability and performance (i.e., avoiding depolarization and other degradation issues associated with high drive voltages).

[0016] The push pulse is modified to allow the same peak negative voltage and pressure while increasing the acoustic intensity. The safety and reliability limits for both voltage and pressure are tighter at the negative end of the pulse than at the positive end. Since the peak negative voltage and pressure are related to transducer safety and patient safety, keeping these values ​​within the limits while increasing the acoustic intensity by increasing the positive pressure and voltage allows for more displacement signal. Driving with a larger voltage in the positive direction will not result in depolarization, and may even result in depolarization of previously depolarized elements. When the MI limit or the top voltage limit is reached, the peak negative voltage applied to the transducer is, at least initially, the limiting factor. The intensity of the push pulse is increased by utilizing an asymmetric transmit pulse with a higher absolute positive voltage / pressure than the negative voltage / pressure. An asymmetric transmit pulse can increase the local acoustic intensity, and therefore the displacement signal is higher in radiation force based imaging.

[0017] Figure 1 A method for imaging with an ultrasound system is shown. The ultrasound system uses ARFI scanning to image the response of tissue to displacement. Push pulse ultrasound transmission is used to generate tissue displacement. The push pulse uses an asymmetric transmission waveform with a higher positive peak pressure than the negative peak pressure. This allows for larger displacements while meeting MI and transducer limitations.

[0018] This method is Figure 4 The invention relates to an ultrasound system or a different system. For example, a transmit beamformer is used to generate an element waveform, and a transducer generates an acoustic waveform based on the element waveform. The acoustic waveform constructively interferes in the patient as an ARFI transmit beam. The transmit beamformer and the receive beamformer are used to track the tissue displacement caused by the ARFI transmit beam. The ultrasound system generates an image based on the displacement.

[0019] Additional, different, or fewer actions may be provided. For example, in action 36, the method is performed without generating an image. In yet another example, an action for estimating a tissue characteristic or property based on the displacement is provided. The actions are performed in the order described or shown (e.g., from top to bottom or by numerical value), but may be performed in other orders.

[0020] In act 30, an acoustic radiation force impulse (ARFI) beam is transmitted. The beam has an asymmetry between positive and negative peak pressures. A transmit beamformer generates an electrical waveform for an element of the transducer, wherein the electrical waveform has an asymmetry, or can be electrically or acoustically combined to provide the asymmetry. When applied to the element, the element generates an acoustic waveform at the face of the element. The acoustic waveform has an asymmetry, or can be combined to provide the asymmetry. The acoustic waveforms are constructively summed along a scan line and at a focus so that the transmit beam includes an asymmetry. An array of elements in an ultrasound transducer transmits the ARFI beam converted from the electrical waveform. Acoustic energy having a push pulse asymmetry is transmitted to tissue in a patient.

[0021] The acoustic waveform of the beam is transmitted for generating shear waves, longitudinal waves or other waves as stress to displace tissue. The excitation is an ultrasonic push pulse. The acoustic energy is focused to apply sufficient energy to cause the generation of one or more waves that travel through the tissue from one focal position or multiple focal positions. The acoustic waveform itself can displace tissue.

[0022] One or more shear waves or longitudinal waves are generated at the focal area and propagate from the focal area laterally, axially and / or in other directions. The waves can travel in multiple directions. As the waves travel through the tissue, the waves decrease in amplitude.

[0023] In order to generate waves or displace tissue, high amplitude or power excitation is desired. For example, the excitation has a mechanical index close to but not exceeding 1.9 at the focal position and / or in the field of view. To be conservative and account for probe variations, a mechanical index of 1.4, 1.7 or other levels may be used as an upper limit. Greater (e.g., MI exceeding 1.9) or less power may be used.

[0024] ARFI beams are delivered in waveforms having any number of cycles. In one embodiment, one, most, or all of the waveforms (e.g., electrical waveforms, element-level acoustic waveforms, or beam waveforms) of the delivery event of the push pulse have 100-2000 cycles. For a continuous delivery waveform applied to the elements of the ARFI beam array, the number of cycles is tens, hundreds, thousands, or more. Unlike the 1-5 cycle imaging pulses, ARFI push pulses have a greater number of cycles to generate sufficient stress to cause waves (e.g., shear waves) to displace tissue with amplitudes sufficient to be detected. A push pulse beam of acoustic energy is delivered. ARFI is delivered from the array by applying a continuous delivery waveform to the elements of the array over this period of time.

[0025] The delivery length combined with the waveform amplitude provides acoustic power to the tissue. Since the asymmetry provides a larger amplitude for the positive pressure or voltage, the waveform of the same duration provides greater power. This can allow the push pulse to use fewer cycles while still providing sufficient or even greater power than without the asymmetry. Greater acoustic power can cause greater tissue displacement, which can be measured more consistently and / or more reliably with ultrasound scanning.

[0026] The waveform applied to the element is generated as a continuous waveform. Various waveforms can be used, such as square waves, sine waves, or other bipolar AC waveforms. The waveform does not have any extended periods of zero output, except for the start and end waveforms. The extended period is one or more cycles. There may be a portion of each cycle that is at zero (such as for a unipolar square wave), but another portion of the cycle has a non-zero (positive or negative) output per cycle.

[0027] The electrical waveform generated for each element in the aperture is either a square wave or a sine wave. The generated electrical waveform is synchronously applied to the elements, resulting in the generation of an ARFI transmit beam.

[0028] Each or at least some of the electrical waveforms for a given push pulse, ARFI beam, or transmit event have a peak positive amplitude absolute value that is at least 1.2 times the peak negative amplitude absolute value. The peak positive amplitude or voltage is greater than the peak negative amplitude or voltage. The absolute value of the positive peak amplitude is greater than the absolute value of the negative peak amplitude. Any ratio of positive to negative amplitude may be used, such as 1.2, 1.5, or 2.0. The ratio is at least 1.2, 1.5, or 2.0. The ratio may be higher, such as a ratio of 10, as long as the overall power limit (e.g., ISPTA) is not exceeded.

[0029] The ratio is provided for each cycle of the electrical waveform, as an average across all cycles, and / or as an average for at least 1 / 3 of the cycles. For example, the ratio is provided for approximately 1 / 2 of the cycles, where the electrical waveform is comprised of a phase-aligned combination or superposition of a fundamental waveform (e.g., a 4 MHz sine wave) and a second harmonic waveform (e.g., an 8 MHz sine wave). The "approximately" for the number of cycles is used to account for one or more cycle ringing or ramping up or down relative to the steady state start and / or end of the amplitude envelope.

[0030] Each electrical waveform is generated separately, but some waveforms can be generated together. A transmission beamformer or waveform generator generates electrical waveforms. In one embodiment, a transmission beamformer having the ability to generate two or more transmission beams simultaneously is used. Before being applied to the transducer or as part of being applied to the transducer, the electrical waveform of one transmission beam is combined with the electrical waveform of another transmission beam. The two beams are collinear. The electrical waveforms are combined or superimposed in a manner having a positive amplitude greater than a negative amplitude. For example, a fundamental waveform at a center frequency is aligned with the zero crossing of another waveform at a second harmonic frequency to provide asymmetry. Other harmonics can be used. In other embodiments, the electrical waveform of the element is generated by a switch and / or a pulse generator. A positive voltage source with a larger amplitude is used together with a negative voltage source with a smaller amplitude. The control switch switches between the positive voltage source and the negative voltage source. In yet other embodiments, a sine wave with asymmetry is generated, such as according to a memory or digital-to-analog conversion. Other electrical waveform generation can be used.

[0031] In other embodiments, different electrical waveforms are generated for different elements.The electrical waveforms may not include asymmetry, but rather be at different frequencies and / or phases such that constructive summation of the acoustic waveforms provides asymmetry for the transmit beam at the focal location.

[0032] The electrical waveform includes both positive and negative components. The amplitude of the positive component is greater than the amplitude of the negative component. The amount of positive power (eg, the integral of the positive voltage) may be greater than the amount of negative power (eg, the integral of the negative voltage) in a ratio.

[0033] The electrical waveform of the transmit aperture is applied to the elements of the transducer. The elements convert the electrical waveform into acoustic energy. At the face of each element, an acoustic waveform is generated to transmit the acoustic waveform into the patient. Even with any nonlinear operation of the elements, the acoustic waveform of the transmit aperture has an asymmetry, such as a ratio of 1.5 for at least about 1 / 2 or all periods, or a ratio of 2.0 for about 1 / 2 periods. Alternatively, the electrical waveform and / or the corresponding element-based acoustic waveform are symmetrical, but constructively combine to provide asymmetry along the transmit beam.

[0034] In one embodiment, the acoustic waveform has a beamformer generated asymmetry between peak positive pressure and peak negative pressure. The transmit beamformer generates an electrical waveform with asymmetry with or without taking into account any nonlinear interactions or propagation of electrical and / or acoustic energy. The asymmetry is intentionally generated and is not a result of transmission.

[0035] An acoustic waveform propagates from the element. The acoustic waveforms from the element interfere constructively along a transmit scan line, such as at a focal position or region. At the focal position, the constructive summation of the acoustic waveforms provides asymmetry to the transmit beam. For example, at the transmit beam focal position, the absolute value of the peak positive pressure is at least 1.2, 1.5, or 2.0 times the absolute value of the peak negative pressure. The ratio may be 5 or greater (e.g., 10). The ratio is provided as an average value for all cycles or for any number of cycles (e.g., 1 / 3 or more or about 1 / 2).

[0036] In one embodiment, the asymmetry at the focal location is based on a transmit beam having fundamental and second harmonic frequency components aligned to form a ratio of the absolute values ​​of the peak positive pressure and the peak negative pressure. Figure 2 An example of an asymmetric transmit pulse is shown, which is a combination of a sine wave and a phase-shifted sine wave at twice the frequency. The peak negative amplitude of the push pulse is slightly larger in absolute value than the waveform at the fundamental frequency or the waveform at the second harmonic frequency. The peak positive amplitude is approximately twice as high as a conventional sine wave (i.e., the fundamental and second harmonic waveforms). "Approximately" when referring to a ratio takes into account waveform generation tolerances and / or propagation distortion or attenuation. In other embodiments, the fundamental and second harmonic have different relative amplitudes. The peak positive and peak negative of the fundamental and harmonic waveforms are the same, but may be unequal in other embodiments.

[0037] If the transducer bandwidth includes both transmission frequencies, the output pulses are appropriately generated. For example, the fundamental component is at 1-5 MHz, and the second harmonic component is at 2-10 MHz. A single crystal piezoelectric element can provide the bandwidth for transmission of both the fundamental and second harmonic waveforms. Other types of elements may be used.

[0038] Figure 3 A simulation using a breast transducer model with a bandwidth that includes both 5 MHz and 10 MHz within the -6 dB transducer bandwidth is shown. An asymmetric waveform is the acoustic waveform transmitted by the transducer. In this case, the MI is slightly higher than transmitting the waveform for only the fundamental component. Due to the asymmetry from the superposition of the fundamental and second harmonic components, the acoustic intensity, defined as the squared integral of the pressure, doubles. The displacement amplitude will also double, resulting in improved imaging capabilities in ARFI mode.

[0039] exist Figure 1 In act 32, the ultrasound system performs shear wave imaging based on the displacement induced by ARFI. The tissue is displaced due to the ARFI transmit beam. Compared to symmetrical positive and negative pressures or amplitudes given the same MI, the displacement has a larger amplitude due to the asymmetry. Any type of elastic or ARFI imaging can be used.

[0040] For imaging, the ultrasound system tracks the displacement in act 34 and generates an image based on the displacement in act 36. In act 34, the ultrasound system uses a beamformer, a transducer, and an image processor to track the displacement in the tissue caused by the waves generated by the ARFI transmit beam. The transmit beamformer generates a transmit waveform, the transducer converts the waveform into a transmit beam and converts the echo of the transmit beam into a receive signal, the receive beamformer forms a receive beam based on the receive signal, and the image processor correlates the receive beam or data from the receive beam to determine the displacement.

[0041] The tissue response is a function of the waves created by the ARFI beam and the tissue properties. The displacement of the tissue over time can be expressed as the convolution of the waveform and the tissue properties or response. The tissue response reflects the viscoelastic properties of the tissue. To measure the viscoelastic properties, the displacement of the tissue over time in response to the push pulse is measured. The tissue displacement caused by the created wave or the ARFI pulse itself is determined over time. As the wave passes through a given location, the tissue is displaced by an amount or distance that increases to a peak amount and then decreases as the tissue returns to rest.

[0042] The displacement is calculated as a function of time. The tissue is scanned multiple times to determine the displacement, such as scanning an area at least ten times to determine the displacement at nine different times. The tissue is scanned using any imaging modality capable of scanning the displacement during the tissue's response to a push waveform. The scan occurs within the time range in which the waveform (e.g., shear wave) is expected to pass through the tissue.

[0043] For ultrasound scanning, waves are detected at a focal region adjacent to and / or spaced from the ARFI push pulse. Transmissions are made to the region of interest in order to detect tissue response to the waves in that region. These other transmissions are used to detect waves or displacements, rather than to cause them. Transmissions used for detection may have lower power and / or short pulses (e.g., 1-5 carrier cycles), and use the same or different scan lines as the ARFI beam. Transmissions used for detection may have a wider beam profile along at least one dimension, such as transverse, for forming receive samples simultaneously along multiple scan lines.

[0044] The ARFI transmit beam is not used for receiving echoes. The frequencies of the tracking waveform used for transmitting and the frequencies used for receiving are independent of the frequencies used for the ARFI beam. For example, the ARFI has a fundamental and second harmonic of 3 and 6 MHz, while the tracking beam is a B-mode beam with a center transmit frequency of 2 MHz and receive frequencies of 2 MHz or 4 MHz harmonics. Since the ARFI is not used for receiving, the signal from the ARFI has no or limited interference with the receive signal used for tracking. Tracking transmits have no asymmetry in the positive and negative peaks, but can have asymmetry.

[0045] Waves or displacements may be monitored in one, two or more directions. A region of interest is monitored to detect waves. The region of interest may be of any size. Laterally spaced locations are monitored for shear wave imaging. Displacements are tracked at each of a plurality of laterally spaced locations for one or more depths. Alternatively, displacements are tracked for a single location.

[0046] The detection area is monitored by ultrasound. Monitoring is performed for any number of scan lines. For example, four, eight or more receive beams are formed in response to each monitoring transmission. After transmitting the ARFI excitation to generate waves or displacements, B-mode transmissions are repeatedly performed along one or more transmission scan lines, and reception is performed along corresponding receive scan lines. In other embodiments, only a single receive beam or other number of receive beams are formed in response to each transmission. Some ultrasound data (such as at the beginning or end of the repetition) may not respond to waves or displacements.

[0047] The image processor calculates the displacement based on the ultrasound scan data (e.g., beamforming samples or B-mode detection data). The tissue moves between the two scans. The data of one scan is transformed in one, two or three dimensions relative to the data in the other scan. For each possible relative position, the similarity amount is calculated for the data around the position. The similarity amount is determined using correlation, such as cross-correlation. The minimum sum of absolute differences or other functions can be used. The spatial offset with the highest or sufficient correlation indicates the displacement amount and direction of a given position. In other embodiments, the phase offset of the data received from different times is calculated. The phase offset indicates the displacement amount. In yet other embodiments, the data representing lines (e.g., axial) at different times are correlated to determine the displacement of each of the multiple depths along the line.

[0048] The displacement is determined for a given location at different times, the different times being associated with sequential scans. The displacement is determined relative to an initial or reference frame of scan data (i.e., cumulative displacement). Alternatively, the displacement is determined from an immediately preceding frame of scan data, such that the preceding frame is designated as a reference on an ongoing basis (i.e., incremental displacement). The temporal profile of a given location indicates the displacement caused by the wave over time. Alternatively, the displacement at a given time is determined.

[0049] The displacement information, with or without a time profile, is used to determine properties of the tissue. Properties are determined at one location or for each of a plurality of locations. Any property may be determined, such as elasticity, strain, shear velocity, longitudinal wave velocity, modulus, or other viscoelastic properties. The displacement itself, such as the magnitude of the displacement, may be used to represent tissue properties.

[0050] In act 38, an image is generated. The image represents a tissue characteristic or property. The image is a function of one or more displacements. Using the displacement itself or a property derived from the displacement (e.g., shear modulus or velocity), information to be displayed is calculated. For example, a numerical value or textual indication of the property can be displayed. In other embodiments, a graph and / or a fit line and slope value are output. For example, for each of one or more locations, the displacement over time is displayed. The viscoelastic property is conveyed to the user in the image. The image can be a graph, such as a graph of the values ​​as a function of the position.

[0051] The image may include a one-dimensional, two-dimensional, or three-dimensional representation of properties, displacements, or other wave information as a function of space or position. For example, the shear velocity throughout a region is displayed. The shear velocity value modulates the color of the pixels in the region in the grayscale modulated B-mode image. The image may represent displacement information, such as shear or modulus (e.g., shear modulus) at different locations. The display grid may be different from the scanning grid and / or the grid for which the displacement is calculated. The color, brightness, luminosity, hue, or other characteristics of the pixel are modulated as a function of the information derived from the displacement.

[0052] In other embodiments, displacement is used for shear wave velocity imaging. The shear velocity distribution in a two-dimensional or three-dimensional area is determined and mapped to image values. In another embodiment, shear wave velocity point quantization is performed. The shear wave velocity value at a location is displayed as text or a numerical value. Due to the larger displacement caused by asymmetry, the represented area may be larger and / or the displacement is more reliably detected, resulting in better accuracy and / or more diagnostic image information.

[0053] Figure 4 One embodiment of an ultrasound system 10 for ARFI scanning is shown. Ultrasound generates tissue displacement, such as by creating shear waves or longitudinal waves, and scan data of the tissue response to the displacement is used to determine properties. For increased displacement, ARFI is transmitted with asymmetry between positive and negative peaks. Increased displacement can be tracked more easily and / or more extensively.

[0054] System 10 is a medical diagnostic ultrasound imaging system. In alternative embodiments, system 10 is a personal computer, workstation, PACS station, or other arrangement for real-time or post-acquisition imaging at the same location or distributed over a network. Data from ultrasound scans performed by a beamformer may be obtained via a computer network or memory for processing by a computer or other processing device.

[0055] System 10 Implementation Figure 1The system 10 includes a transmit beamformer 12, a transducer 14, a receive beamformer 16, an image processor 18, a display 20, and a memory 22. Additional, different, or fewer components may be provided. For example, a user input is provided for manually or assisted designation of a region of interest for which information is to be obtained, or for entering an application, tissue type, and / or setting a level of asymmetry (e.g., setting a ratio).

[0056] The transmit beamformer 12 is an ultrasound transmitter, a memory, a pulse generator, a waveform generator, an analog circuit, a digital circuit, or a combination thereof. The transmit beamformer 12 is configured to generate a waveform of multiple channels with different or relative amplitudes, delays, and / or phasing. The waveform includes asymmetry or a frequency and amplitude combination to provide asymmetry at the focal area. For a given ARFI (e.g., 100-1000 cycles), asymmetry is provided for approximately 1 / 3, 1 / 2, all, or other number of cycles. The asymmetry is performed for all channels or a subset of channels of the transmit beamformer 12.

[0057] In one embodiment, the transmit beamformer 12 includes different voltage sources, multiple simultaneous transmit beam capabilities (e.g., two channels per element), pulse generators, switches, memory, digital-to-analog converters, or other waveform generators for generating asymmetric waveforms or waveforms that can be combined to form asymmetry. The combination occurs electrically before or at the transducer, or acoustically after the transducer.

[0058] The transmit beamformer 12 is configured to generate an electrical waveform having or for inducing an acoustic waveform having a peak positive amplitude greater than a negative amplitude. For example, for at least about 1 / 2 of a cycle, the positive peak voltage to negative peak voltage ratio is at least 1.5. In one embodiment, the ratio is at least 2.0, such as 2.0-10.0. In another embodiment, the ratio is about 2.0, wherein fundamental and second harmonic electrical waveforms having equal amplitudes (i.e., equal positive and negative peak amplitudes or voltages within and between the fundamental and harmonic waveforms) are combined. The electrical waveforms are aligned in phase so that the superposition causes a positive peak greater than a negative peak in the combination. The combination is asymmetric and is applied to the transducer 14. The relative amplitudes between the waveforms can be set to provide a desired asymmetry ratio in terms of positive peak to negative peak.

[0059] The transmit beamformer 12 is connected to the transducer 14, such as by a transmit / receive switch. When acoustic waves are transmitted from the transducer 14 in response to the generated waveform, one or more beams are formed during a given transmit event. At least one beam is an ARFI pulse having asymmetry in terms of positive and negative pressures. In order to scan tissue displacement, a sequence of other transmit beams is generated after the ARFI is transmitted. The other transmit beams are symmetrical in terms of peak pressure, but may be asymmetric in terms of peak pressure. The sequence of transmit beams scans a one-dimensional, two-dimensional, or three-dimensional area. Sector, Vector®, linear, or other scanning formats may be used. The same area is scanned multiple times. Scanning by the transmit beamformer 12 occurs after the ARFI pulse is transmitted. The same elements of the transducer 14 are used for both scanning and for displacing tissue, but different elements, transducers, and / or beamformers may be used.

[0060] The transducer 14 is a 1, 1.25, 1.5, 1.75 or 2 dimensional array of piezoelectric (PZT) or capacitive film elements. In one embodiment, the element is a single crystal PZT element. The transducer 14 includes a plurality of elements for transducing between acoustic energy and electrical energy. For example, the transducer 14 is a one-dimensional PZT array having approximately 64-256 elements.

[0061] The transducer 14 is connected to the transmit beamformer 12 for converting the electrical waveform into an acoustic waveform and to the receive beamformer 16 for converting the acoustic echo into an electrical receive signal. The transducer 14 transmits the ARFI. The transmit beam of the ARFI is focused on a tissue region or location of interest in the patient. In response to applying the electrical waveform to the transducer elements, an acoustic waveform is generated. The ARFI causes tissue displacement directly or by generating waves (e.g., shear waves).

[0062] To scan with ultrasound to detect displacement (tracking), the transducer 14 transmits acoustic energy and receives echoes based on a further waveform from the transmit beamformer 12. The receive signals are generated by the receive beamformer 16 in response to the ultrasound energy (echoes) impinging on the elements of the transducer 14.

[0063] The receive beamformer 16 includes multiple channels with amplifiers, delays and / or phase rotators, and one or more summers. Each channel is connected to one or more transducer elements. The receive beamformer 16 applies relative delays, phases, and / or apodization to form one or more receive beams in response to each transmission for detection. Dynamic focusing of reception can be provided. The receive beamformer 16 outputs data representing one or more spatial locations using the received acoustic signal. The relative delay and / or phasing and summing of the signals from different elements provide beamforming. In an alternative embodiment, the receive beamformer 16 is a processor for generating samples using Fourier or other transforms.

[0064] The receive beamformer 16 may include a filter, such as a filter for isolating information at the second harmonic or other frequency band relative to the transmit frequency band. Such information may be more likely to include desired tissue, contrast agent, and / or flow information. In another embodiment, the receive beamformer 16 includes a memory or buffer and a filter or adder. Two or more receive beams are combined to isolate information at a desired frequency band, such as the second harmonic, the third fundamental, or another frequency band.

[0065] The receive beamformer 16 outputs beam summation data representing one or more spatial locations. Data for a single location, a location along a line, a location for a region, or a location for a volume is output. The data can be used for different purposes. For example, a different scan is performed for B-mode or tissue data than for shear wave detection. Alternatively, the scan for B-mode imaging is used to determine tissue displacement. The receive beamformer 16 outputs data representing a spatial location, wherein the data is a function of the received acoustic signal in response to the movement of the tissue due to the ARFI. When a direct echo from the ARFI impinges on the transducer 14, the receive beamformer 16 does not operate, and therefore the receive beamformer 16 is configured to output data without an acoustic echo from the ARFI.

[0066] The processor 18 is a B-mode detector, a Doppler detector, a pulsed wave Doppler detector, a correlation processor, a Fourier transform processor, an application specific integrated circuit, a general purpose processor, a control processor, an image processor, a field programmable gate array, a digital signal processor, an analog circuit, a digital circuit, a combination thereof, or other devices now known or later developed for detecting and processing information from beamformed ultrasound samples. In one embodiment, the processor 18 includes one or more detectors and a separate processor. The separate processor is a control processor, a general purpose processor, a digital signal processor, a graphics processing unit, an application specific integrated circuit, a field programmable gate array, a network, a server, a processor group, a data path, a combination thereof, or other devices now known or later developed for determining displacement and / or calculating tissue properties. The processor 18 is configured by software, firmware, and / or hardware to perform actions.

[0067] In one embodiment, the processor 18 estimates tissue displacement over time as a function of output data from the receive beamformer 16. The displacement is estimated as a profile or data representing a curve of displacement amplitude as a function of time. The displacement profile may be obtained by correlating reference data and data obtained to represent tissue at different times or otherwise determining a level of similarity between reference data and data obtained to represent tissue at different times. The displacement profile is determined for each location. In an alternative embodiment, the displacement at a given time is determined for each of one or more locations.

[0068] The processor 18 is configured to calculate a tissue property based on the displacement of the tissue over time or based on the displacement at a time. For example, shear velocity is calculated based on the displacement over time. The amount of displacement identifies the time when the shear wave occurs. The distance from the origin of the shear wave (e.g., the ARFI focal position) divided by the time provides the velocity. The relative phasing of the displacement profiles over different positions can be used to determine the shear velocity.

[0069] In one embodiment, processor 18 calculates viscosity and / or modulus. Processor 18 may calculate other properties, such as strain or elasticity. In yet other embodiments, processor 18 determines maximum displacement or other characteristics of displacement or displacement profile as a characteristic.

[0070] Processor 18 generates and outputs an image or display value mapped from the property to display 20. For example, a shear modulus or other value is determined. A textual or numerical indication of the property is displayed to the user. A graph of the property over time may be displayed.

[0071] In one embodiment, a property (e.g., shear wave velocity) is displayed as a function of position. In response to an ARFI pulse, displacements for multiple locations are available. A one-dimensional, two-dimensional, or three-dimensional distribution of those locations provides a corresponding spatial distribution of the estimated velocity or property. For representation of tissue, the amplitude of the tissue property modulates the color, hue, brightness, and / or other display characteristics of different pixels representing tissue regions. The processor 18 determines a pixel value (e.g., RGB) or a scalar value converted to a pixel value. An image is generated as a scalar or pixel value. The image can be output to a video processor, a lookup table, a color map, or directly to a display 20.

[0072] The processor 18 and the transmit beamformer 12 operate according to instructions stored in the memory 22 or another memory. The instructions configure the processor 18 and / or the transmit beamformer 12 for operation by being loaded into the controller, by causing the loading of a value table (e.g., a beamformer control table), and / or by being executed. The transmit beamformer 12 is configured by the instructions to cause the generation of an ARFI beam having asymmetry in terms of positive peak amplitude and negative peak amplitude. The processor 18 is programmed to measure tissue displacement and generate images.

[0073] The memory 22 is a non-temporary computer-readable storage medium. Instructions for implementing the processes, methods and / or techniques discussed herein are provided on a computer-readable storage medium or memory, such as a cache, a buffer, a RAM, a removable medium, a hard drive or other computer-readable storage medium. Computer-readable storage media include various types of volatile and non-volatile storage media. 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 figures or described herein are executed. Functions, actions or tasks are independent of a 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 may include multi-processing, multi-tasking, parallel processing, etc. In one embodiment, instructions are stored on a removable media device for reading by a local or remote system. In other embodiments, instructions are stored in a remote location for transmission over a computer network or over a telephone line. In yet other embodiments, instructions are stored in a given computer, CPU, GPU or system.

[0074] The display 20 is a CRT, LCD, projector, plasma or other display for displaying a two-dimensional image or a three-dimensional representation. The display 20 displays one or more images representing tissue characteristics or other information derived from a displacement or multiple displacements (i.e., an image representing the response of the tissue to ARFI). As an example, a two-dimensional image or a three-dimensional representation of the displacement or tissue characteristics as a function of position is displayed. Alternatively or additionally, the image is a chart, a number or a textual representation of a value or chart. For example, shear velocity, shear modulus, strain, elasticity or other values ​​are displayed as annotations on the image or B-mode image.

[0075] 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 present invention. Therefore, it is intended that the foregoing detailed description be regarded as illustrative rather than restrictive, and it will be appreciated that it is intended that the appended claims (including all equivalents) define the spirit and scope of the present invention.

Claims

1. A method for performing acoustic radiation force pulse scanning by an ultrasonic system, the method comprising: transmitting (30) by the ultrasound system and from the ultrasound transducer (14) as an acoustic radiation force pulse a transmit beam, wherein at a focal position of the transmit beam an absolute value of a peak positive pressure is at least 1.2 times an absolute value of a peak negative pressure; tracking (34) displacement of tissue by the ultrasound system using the ultrasound transducer (14), the displacement being responsive to the acoustic radiation force pulse; as well as An image is generated (36) that is a function of the displacement of the tissue.

2. The method of claim 1, wherein transmitting (30) comprises transmitting (30) the transmit beam wherein an absolute value of a peak positive pressure is at least 2.0 times greater than an absolute value of a peak negative pressure.

3. The method of claim 1, wherein transmitting (30) comprises generating (36) a transmit beam having an element waveform continuous over time, the element waveform having a peak positive amplitude absolute value that is at least 1.2 times the peak negative amplitude absolute value.

4. The method of claim 1 , wherein transmitting (30) comprises transmitting (30) a transmission beam having a fundamental frequency and a second harmonic frequency component, wherein the fundamental frequency and the second harmonic frequency components are aligned to form a peak positive pressure absolute value at the focal position that is at least 1.2 times the peak negative pressure absolute value.

5. An ultrasonic system for acoustic radiation force pulse scanning, the system comprising: an ultrasonic transducer (14) for transmitting (30) an acoustic radiation force pulse within a patient; a transmit beamformer (12) configured to generate an electrical waveform of the acoustic radiation force pulse, the generated electrical waveform having a plurality of cycles and having a positive peak voltage to negative peak voltage ratio of at least 1.5 for at least 1 / 2 of the cycles; a receive beamformer (16) configured to output data representing a spatial position as a function of a received acoustic signal responsive to movement of tissue due to the acoustic radiation force pulse; a processor (18) configured to estimate displacement of the tissue within the patient over time as a function of the output data; as well as A display (20) is operable to display an image, the image being a function of the displacement.

6. The ultrasound system according to claim 5, wherein: The transmit beamformer (12) is configured to generate the electrical waveform having a ratio of at least 2.

0.

7. The ultrasound system according to claim 5, wherein: The transmit beamformer (12) is configured to generate one of the electrical waveforms for each element of the ultrasound transducer (14), each of the electrical waveforms being a superposition of a fundamental wave and harmonic waves aligned in phase to form the ratio.

8. A method for imaging by an ultrasound system, the method comprising: an acoustic waveform transmitting (30) by the ultrasound system from an element of an ultrasound transducer (14) an acoustic radiation force pulse, the acoustic waveform at the element having a beamformer generated asymmetry between peak positive pressure and peak negative pressure for at least 1 / 2 of a cycle, the acoustic waveform causing displacement of tissue; and performing shear wave imaging (32) based on the displacement, wherein transmitting (30) comprises transmitting (30) using the acoustic waveform, the acoustic waveform having a peak positive pressure to peak negative pressure ratio of at least 1.5 for at least 1 / 2 of the cycle.

9. The method according to claim 8, wherein: Transmitting (30) includes transmitting (30) using the acoustic waveform, the acoustic waveform having a peak positive pressure to peak negative pressure ratio of at least 2 for a period of 1 / 2, the asymmetry generated by the beamformer being formed by a combination of phase-aligned fundamental and second harmonic electrical waves.

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