Quantitative Viscoelastic Ultrasound Imaging

Abstract

The present invention relates to quantitative viscoelastic ultrasound imaging. A viscosity is included in the quantification by an ultrasound imaging system. A logarithm of a spectrum of displacements as a function of time is determined (36, 38) for each of different positions subjected to shear or other waves. Solving (40) using the logarithm as a function of position provides a complex wave number. Various viscoelastic parameters, such as a loss modulus and a storage modulus, are determined (34) based on the complex wave number.

Description

[0001] This application is a divisional application of the patent application with the application number 201610338551.8, the application date of April 15, 2016, and the invention name of "Quantitative Viscoelastic Ultrasound Imaging". Technical Field

[0002] This embodiment relates to ultrasound imaging. Specifically, ultrasound viscoelastic imaging is improved. Background Art

[0003] A number of commercial ultrasound systems provide quantitative values or images of tissue hardness, such as measuring hardness using shear wave imaging. The hardness is estimated by assuming that the tissue is purely elastic (i.e., assuming that viscosity can be ignored). Different methods are used to measure hardness. Even while assuming that the tissue is purely elastic, these different methods can emphasize different frequency bands of the shear wave spectrum. For example, some methods find the peak displacement caused by the shear wave, while others find the peak in the derivative of the displacement. The derivative function changes the measured frequency band. Therefore, even for the same tissue, different ultrasound systems provide different values for hardness or shear wave parameters. In addition, human tissue is viscoelastic, so there is shear wave dispersion. Different shear wave frequencies travel at different speeds. The dispersion is governed by the frequency-dependent storage modulus and loss modulus to manage. Summary of the Invention

[0004] As an introduction, the preferred embodiments described below include methods, instructions, and systems for quantitative viscoelastic ultrasound imaging. Viscosity is included in the quantification by an ultrasound imaging system. For each of different positions subjected to shear or other waves, the logarithm of the spectrum of the displacement as a function of time is determined. Solving using the logarithm as a function of position provides a complex wavenumber. Different viscoelastic parameters, such as loss modulus and storage modulus, are determined based on the complex wavenumber.

[0005] In a first aspect, a method for quantification in viscoelastic ultrasound imaging is provided. An ultrasound system measures the displacements of tissue over time at first and second positions within a patient in response to a pulsed excitation. For each of the first and second positions, a processor applies a Fourier transform in time to the displacement over time. The processor calculates the logarithm of the result of the transform and solves for a complex wavenumber based on the logarithm of the result. The value of a frequency-dependent viscoelastic parameter is determined using the complex wavenumber. The value for the tissue is output to a display.

[0006] In a second aspect, a non-transitory computer-readable storage medium having stored therein data representing instructions executable by a programmed processor for quantification in viscoelastic ultrasound imaging. The storage medium includes instructions for determining tissue displacement as a function of time within a patient, estimating a loss modulus, a storage modulus, or both as a function of frequency based on the tissue displacement, and outputting the loss modulus, the storage modulus, or both.

[0007] In a third aspect, a system for quantification in viscoelastic ultrasound imaging is provided. A transducer is configured to send acoustic pulse excitations into a patient and is configured to scan a region of the patient using ultrasound. A receive beamformer is configured to generate data representing the region at different times after the acoustic pulse excitation. The data is generated based on the scan using ultrasound. A processor is configured to estimate tissue displacement induced by the acoustic pulse excitation and calculate viscoelastic properties of regional amplitude and phase of the tissue displacement from different positions in the region. A display is configured to display an image representing the viscoelastic properties.

[0008] The invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. More aspects and advantages of the invention are described below in connection with the preferred embodiments, and may subsequently be claimed independently or in combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The components and the drawings are not necessarily to scale, but rather emphasis is placed upon illustrating the principles of the invention. Further, in the drawings, like reference numerals designate corresponding parts throughout the different views.

[0010] Figure 1 is a flowchart of an embodiment of a method for quantification in viscoelastic ultrasound imaging;

[0011] Figure 2 shows an example graph of viscoelastic parameters in an elastic phantom, Figure 3 shows an example graph of viscoelastic parameters in a viscoelastic body phantom, and Figure 4 shows an example graph of viscoelastic parameters within the liver of a patient;

[0012] Figure 5 is an example of quantification in viscoelastic ultrasound imaging, where a quantity is determined as a function of frequency; and

[0013] Figure 6 is a block diagram of an embodiment of a system for quantification in viscoelastic imaging. DETAILED DESCRIPTION

[0014] In quantified viscoelastic imaging, the shear wave equation is solved in the frequency domain. The viscoelastic wave equation is given by:

[0015] (1)

[0016] (2)

[0017] (3)

[0018] where s(t, x, y, z) is the particle displacement (m), is the shear modulus (kPa), is the shear viscosity (Pa-s), E is the Young's modulus (kPa), v s is the shear wave speed, and is the density (Kg / m³). The shear modulus has a known relationship with the storage modulus, and the shear viscosity has a known relationship with the loss modulus. Equation 1 assumes that the components of stress are linear functions of the components of strain and their first-order time derivatives. The second term in Equation 1 is the viscous term, which is often ignored in ultrasonic hardness measurements.

[0019] Equation 1 can be used to estimate any viscoelastic parameter. By using the spectrum of displacement, the viscous term is not ignored to obtain a stable solution. Estimating the shear storage modulus, shear loss modulus, shear attenuation, and / or phase velocity within the bandwidth of the propagating shear wave can improve the diagnostic ability of ultrasound.

[0020] Figure 1 A method for quantification in viscoelastic ultrasound imaging is shown. The method uses the spectrum of displacement over time. By taking the logarithm of the spectra from different positions, the viscoelastic parameters can be solved as a function of frequency without using second derivatives that may lead to noisy measurements.

[0021] The method is implemented by Figure 6 the system or a different system. An ultrasound system (e.g., a system having a transducer and a beamformer) performs transmission and detection actions 30 and 32. In action 34, a processor of the ultrasound system or a different computer performs the estimation, and the output of action 42 goes to a display, speaker, or other device. Different components can perform any one or more of the actions.

[0022] Additional, different, or fewer actions may be provided. For example, actions 36, 38, and 40 represent one example for estimation, but other actions may be used. In another example, action 30 is not performed, and a source of pressure is provided by the body, manually, using a heavy object, or by other mechanisms. Action 42 is optional. The actions are performed in the order described or shown, but may be performed in other orders.

[0023] In Figure 1In operation 30, an acoustic excitation is sent into the patient. The acoustic excitation acts as a pulsed excitation. For example, a transmit waveform of 400 cycles is sent with a power or peak amplitude level similar to or higher than that used for B-mode transmission for imaging tissue. In one embodiment, the transmit is a sequence of radiation forces applied to the field of view. Any acoustic radiation force imaging (ARFI) sequence can be used.

[0024] The transmit is configured by power, amplitude, timing, or other characteristics so as to cause a stress on the tissue sufficient to displace the tissue at one or more locations. For example, the transmit focus is positioned near the bottom, center of the field of view so as to cause displacements throughout the field of view. The transmit can be repeated for different sub-regions.

[0025] An excitation is sent from the ultrasound transducer. The excitation is acoustic energy. The acoustic energy is focused, resulting in a three-dimensional acoustic beam profile. Phased array and / or mechanical focusing is used to focus the excitation. The excitation can be unfocused in one dimension, such as the elevation dimension. The excitation is sent into the tissue of the patient.

[0026] In operation 32, a displacement profile of the response in the patient is determined. For example, a displacement profile as a function of time of the displacement is determined for each of different positions spaced from the shear wave origin x 0 (i.e., the shear wave origin at the focus). The excitation causes displacement of the tissue. A shear wave is generated and propagates from the focus region. As the shear wave travels through the tissue, the tissue is displaced. Longitudinal waves or other causes of displacement can be used. The tissue is forced to move in the patient.

[0027] The displacement caused by the force or stress is measured. The displacement is measured over time at different positions. The displacement measurement can be started, for example, using different frequencies or coding, before the stress or pulse ends. Alternatively, the displacement measurement is started after the pulse ends. Since the shear wave that causes displacement in tissue spaced from the point or region of stress takes time to travel, the displacement from a relaxed or partially stressed state to maximum displacement and then to a relaxed state can be measured. When the pulse is stopped, the generated shear wave travels from the focus region. As the shear wave passes each position, the displacement rises, reaches a peak, and then falls. Alternatively, the displacement is measured only when the tissue relaxes.

[0028] The amount or magnitude of the displacement is measured. The tissue moves in any direction. The measurement can be along the direction of maximum movement. The magnitude of the motion vector is determined. Alternatively, the measurement is along a given direction, such as perpendicular to the scan line, regardless of whether the tissue is displaced more or less in other directions.

[0029] The displacement is detected using ultrasonic scanning. The ultrasonic is used to scan an area, such as an area of interest, the entire field of view, or a sub-region of interest. For a given time, the ultrasonic is sent to the tissue or region of interest. Any currently known or later developed displacement imaging can be used. For example, a pulse with 1 - 5 cycles and an intensity less than 720mW / cm 2 is used. Pulses with other intensities can be used.

[0030] Echoes or reflections from the transmission are received. The echoes are beamformed, and the beamformed data represents one or more positions. Multi-beam reception can be used (e.g., receiving along 4, 8, 16, 32, or other numbers of lines in response to each measurement transmission). To detect the displacement, ultrasonic energy is sent to the tissue undergoing displacement, and the reflection of the energy is received. Any transmission and reception sequence can be used.

[0031] By performing the transmission and reception multiple times, data representing a one-, two-, or three-dimensional region at different times is received. The transmission and reception are performed multiple times to determine the changes due to displacement. By repeatedly scanning with ultrasonic, the position of the tissue at different times is determined.

[0032] B-mode or Doppler detection is used to detect the echoes. The displacement is detected based on the differences for each spatial position. For example, velocity, variance, movement (e.g., speckle tracking), or other information in the intensity pattern is detected from the received data as displacement.

[0033] In one embodiment using B-mode data, the data from different scans are correlated. For example, the current data set is correlated with a reference data set. Different relative translations and / or rotations are performed between the two data sets. The position of the data subset in the current set that is concentrated at a given position in the reference set is identified.

[0034] The reference is either the first data set or data from another scan. The same reference is used for the entire displacement detection, or the reference data changes in a moving or sliding window.

[0035] The correlation is one-, two-, or three-dimensional. For example, correlation along scan lines away from and towards the transducer is used. For two-dimensional scans, the translation is along two axes, with or without rotation. For three-dimensional scans, the translation is along three axes, with or without rotation about three or fewer axes. The level of similarity or correlation of the data at each of the different offset positions is calculated. The translation and / or rotation with the maximum correlation represents the motion vector or the time offset associated with the current data being compared to the reference.

[0036] Any currently known or later-developed correlation can be used, such as cross-correlation, pattern matching, or the sum of absolute differences. The tissue structure and / or speckles are correlated. Using Doppler detection, a clutter filter allows information related to moving tissue to pass through. The velocity of the tissue is derived from multiple echoes. The velocity is used to determine displacement towards or away from the transducer. Alternatively, the correlation or difference between velocities at different positions can indicate strain or displacement.

[0037] The magnitude of the distance of the motion vector from the reference data over time provides displacement as a function of time. The analysis period is more than about 10 milliseconds, but can be longer or shorter.

[0038] In operation 34, one or more viscoelastic parameters are estimated. For example, the loss modulus, the storage modulus, or both are estimated. The loss modulus and the storage modulus correspond to viscosity and shear modulus respectively. In alternative embodiments, known relationships between the storage modulus and the shear modulus and / or between the loss modulus and the viscosity are used to derive one from the other. In still other embodiments, the shear modulus and / or the viscosity are estimated instead of the storage modulus and the loss modulus.

[0039] The value for the viscoelastic parameter is estimated for a location. For example, the user selects a location on the ultrasound image. In response, the value for the viscoelastic parameter is output. Values for different locations can be estimated, for example, estimated for locations in the region of interest, and an image in which the pixel values are modulated as a function of the value is displayed.

[0040] The viscoelastic parameter is estimated as a function of frequency based on the tissue displacement as a function of time. For each in the location, it is estimated based on the log spectrum of the tissue displacement as a function of time, rather than using spatial derivatives. The value for the viscoelastic parameter is determined based on the log spectra across different locations. For the location, the estimation uses both the phase and the amplitude of the tissue displacement. The complex wave number is estimated by analyzing the amplitude and phase information of the displacement spectrum. Various frequency-dependent viscoelastic parameters can be obtained from the complex wave number. This estimation utilizes the entire available spectrum of the displacement data, not just the amplitude.

[0041] Operations 36, 38, and 40 represent an exemplary embodiment for performing the estimation of operation 34. In other embodiments, additional, different, or fewer operations are provided. For example, operation 36 is performed, but operations 38 and / or 40 are not performed.

[0042] In operation 36, the processor applies a Fourier transform in time. Any transform to the frequency domain can be used, such as the fast Fourier transform (FFT). The displacement as a function of time for a given location is transformed. The displacement profile is transformed into a profile as a function of frequency. The processor calculates the spectrum of the displacement over time.

[0043] A separate spectrum is calculated for a separate spatial position. For each spatial position, the displacement as a function of time is Fourier-transformed. The Fourier transform is applied to each position independent of the displacement from other positions. The transform provides a set of spectra for the corresponding set of positions. Any number of positions can be used, such as two or more. In an alternative embodiment, a given spectrum is calculated based on displacements at more than one position.

[0044] The complex wave number and the resulting viscoelastic parameters can be calculated based on the displacement in the frequency domain. To estimate the frequency-dependent viscoelastic parameters, the viscoelastic wave equation in the frequency domain is given by:

[0045] (4)

[0046] where is the spectrum of the displacement at the lateral position , is the angular frequency, and h is the complex wave number. The complex wave number is given by:

[0047] (5)

[0048] where is the density of the tissue, i is the imaginary component, and and are the storage modulus and the loss modulus, respectively. The density can be assumed or considered constant. Any density can be used, such as 1000 kg / m³. Equation (4) is the generalization of Equation (1) in the frequency domain (i.e., the components of stress are linear functions of the components of strain and their first and / or higher-order time derivatives). This generalization results in frequency-dependent storage and loss moduli.

[0049] Equation 5 is a second-order differential equation, and its solution is given by:

[0050] (6)

[0051] where is the spectrum of the displacement at the lateral position x 0 (i.e., the starting point of the shear wave), is the wave number, and is the attenuation coefficient. The parameters and are given by:

[0052] (7)

[0053] (8)

[0054] wherein, is the real part of the complex wave number, and is the imaginary part.

[0055] In operation 38, the processor calculates the logarithm of the result of the transform. A logarithm is determined for each spectrum. For each position, the logarithm of the frequency response of the displacement as a function of time is calculated.

[0056] Any logarithm can be used. In one embodiment, the natural logarithm is used. To determine the complex wave number, the natural logarithm of Equation (6) is expressed as:

[0057] (9)

[0058] Equation 9 defines a line for each frequency as a function of position x.

[0059] In operation 40, the processor solves for the complex wave number. The logarithm of the spectrum as a function of position defines the slope. For the complex wave number, h solving Equation 9 provides:

[0060] (10)

[0061] Any solution for finding the slope for a given frequency can be used. The slope between the first and second positions of the logarithm of the spectrum represents the complex wave number. The slope has an imaginary component.

[0062] In one embodiment, the processor applies a linear least squares fit to the logarithm. The linear least squares fit of the logarithm of the spectrum as a function of position represents the slope or the complex wave number. In other embodiments, a spatial derivative is used to calculate the complex wave number. Other slope determinations can be used.

[0063] Returning to operation 34, values for one or more frequency-dependent viscoelastic parameters are estimated based on the complex wave number. Since the parameters are frequency-dependent, values for the parameters at different frequencies are determined, or values for a desired or representative frequency are determined.

[0064] Any viscoelastic parameter can be estimated based on the complex wave number, such as storage modulus, loss modulus, shear modulus, viscosity, phase velocity (i.e., velocity at a frequency), attenuation, or a combination thereof. By using Equation 5, the processor estimates the frequency-dependent storage modulus and loss modulus as:

[0065] (11)

[0066] (12)

[0067] Using equations 7 and 8, the processor estimates the frequency-dependent phase velocity and shear wave attenuation as:

[0068] (13)

[0069] (14)

[0070] Other calculations can be used to derive values of any of the parameters as a function of frequency. Equations 11 - 14 show that the frequency-dependent viscoelastic parameters are determined solely by the complex wave number h generated in equation 10.

[0071] Figures 2 - 4 Examples of the estimation of viscoelastic parameters are shown. Figure 2 Parameters calculated according to the elastic phantom are shown. Figure 3 Parameters calculated according to the viscoelastic phantom are shown. Figure 4 Parameters calculated according to the patient's liver are shown.

[0072] Figures 2 - 4 Each shows examples of the shear modulus (e.g., storage modulus) (Equation 11), the shear viscosity (e.g., loss modulus) (Equation 12), the phase velocity (Equation 13), and the shear wave attenuation (Equation 14). The plot of the phase velocity shows the phase velocity considered as viscoelastic. The values of the parameters are shown as a function of frequency, which itself can aid in diagnosis.

[0073] Group values can be determined. A group value is a parameter for a range of frequencies. For example, the mean value over a range of frequencies is calculated. As another example, the derivative over a range of frequencies is calculated. Other functions can be calculated based on the values at different frequencies, such as integrals, differences, variances, or other statistics. The viscoelastic parameters can be determined at specific frequencies and / or frequency ranges.

[0074] Both the amplitude and phase of the displacement over time are used to determine the viscoelastic parameters. The assumption that the shear modulus and shear viscosity are independent of frequency is not used. Model fitting is not used. The viscoelastic parameters are solved for as a function of frequency. A single pulse excitation is all that is required, and thus the values are estimated without information in response to additional pulse excitations. A single ARFI push pulse is sufficient to estimate the viscoelastic parameters as a function of frequency. In an alternative embodiment, information in response to more than one pulse excitation is used to estimate the values of one or more viscoelastic parameters.

[0075] The calculation of the value from the measurement of displacement to the output of the value is performed without spatial derivatives (e.g., without second-order spatial derivatives). All viscoelastic parameters can be calculated without second-order spatial derivatives, resulting in a more stable solution in the low signal-to-noise environment of shear wave imaging. In an alternative embodiment, spatial derivatives are used.

[0076] In operation 42, one or more values are output to a display. Values for loss modulus, storage modulus, shear modulus, viscosity, phase velocity, attenuation, or a combination thereof are output. The one or more values are for a given frequency. Multiple values can be output for a given parameter at different frequencies. Group values can be output, such as a combination of values of parameters from different frequencies.

[0077] The output can be text, e.g., text on or adjacent to an ultrasound image. The text can be alphanumeric. Figure 5 An exemplary ultrasound image is shown. In response to a user placing a gate at a location, a graph of phase velocity, loss modulus, storage modulus, and attenuation as a function of frequency is provided for that gate location. The graph is a chart or spreadsheet of viscoelastic parameter values at different frequencies. Additional, different, or fewer information can be provided.

[0078] In another embodiment, one or more curves are output. For example, output Figure 4 one or more of the curves shown in. The curves can cover any frequency range, e.g., frequencies within the bandwidth of the transducer.

[0079] In other embodiments, an image is generated based on one or more values. For example, values are calculated for each of a plurality of locations. The solution for the value at a given location is based on the spectrum in a kernel centered at that location. The kernel defines a spatial (e.g., one-dimensional) window around the location of interest. By adjusting the kernel to other locations, values for parameters are calculated for different locations. Combinations of parameters or any one of them can be used. Any given frequency or group of values can be used. The spatial distribution of the values is mapped to pixel values. The pixels are modulated at least in part by the viscoelastic parameter values.

[0080] Other outputs can be used. By outputting values for the tissue of a patient, diagnostically useful information can be output. By measuring displacement with ultrasound, viscoelastic information about the tissue of interest of the patient can be measured and output. Compared to shear wave imaging that takes elastic tissue operations, viscoelastic imaging provides more information about the mechanical properties of the tissue.

[0081] Output values for viscoelastic parameters, either alone or in combination with other information. For example, also output a B-mode image. Shear velocity of tissue hardness and / or other elastographic imaging may be output together with the viscoelastic parameter values.

[0082] Figure 6 An embodiment of a system 10 for quantification in viscoelastic ultrasound imaging is shown. System 10 implements Figure 1 the method or other methods. 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, user input is provided for user interaction with the system. As another example, a separate processor, such as a general or control processor, is provided for deriving displacements and calculating viscoelastic parameters.

[0083] System 10 is a medical diagnostic ultrasound imaging system. In alternative embodiments, the image processor 18, the display 20, and / or the memory 22 are part of a personal computer, a workstation, a PACS workstation, or other arrangement located at the same location or distributed via a network for real-time or post-acquisition imaging with an ultrasound scanner.

[0084] The transmit beamformer 12 is an ultrasound transmitter, a memory, a pulse generator, analog circuitry, digital circuitry, or a combination thereof. The transmit beamformer 12 is configured to generate waveforms with different or related amplitudes, delays, and / or phases for multiple channels. When acoustic waves are transmitted from the transducer 14 in response to the generated waveforms, one or more beams are formed. A sequence of transmit beams is generated to scan a two-dimensional or three-dimensional region. Sector, Vector®, linear, or other scan formats may be used. The same region is scanned multiple times. For flow or Doppler imaging and for shear imaging, a sequence of scans is used. In Doppler imaging, the sequence may include multiple beams along the same scan line before scanning adjacent scan lines. For shear imaging, scanning or frame interleaving (i.e., scanning the entire region before scanning again) may be used. In alternative embodiments, the transmit beamformer 12 generates plane waves or diverging waves for faster scanning.

[0085] The same transmit beamformer 12 generates a pulse excitation or an electrical waveform to produce acoustic energy to cause displacements. In alternative embodiments, a different transmit beamformer is provided to generate the pulse excitation. The transmit beamformer 12 causes the transducer 14 to produce a high-intensity focused ultrasonic waveform.

[0086] The transducer 14 is an array for generating acoustic energy from an electrical waveform. For the array, the acoustic energy is focused with relative delays. Given the delays, a given transmit event corresponds to the transmission of acoustic energy by different elements at approximately the same time. The transmit event provides a pulse of ultrasonic energy for tissue displacement. The pulse is a pulsed excitation. The pulsed excitation includes a waveform having a plurality of cycles (e.g., 500 cycles), but it occurs in a relatively short time to cause tissue displacement over a longer time.

[0087] The transducer 14 is a 1, 1.25, 1.5, 1.75, or 2-dimensional array of piezoelectric or capacitive membrane elements. The transducer 14 includes a plurality of elements for converting between acoustic and electrical energy. A received signal is generated in response to ultrasonic energy (echoes) impinging on the elements of the transducer 14. The elements are connected to the channels of the transmit and receive beamformers 12, 16. Alternatively, a single element with mechanical focusing is used.

[0088] The receive beamformer 16 includes a plurality of channels having amplifiers, delayers, and / or phase rotators, and one or more adders. Each channel is connected to one or more transducer elements. The receive beamformer 16 is configured by hardware or software to apply relative delays, phases, and / or apodization in response to each imaging transmit to form one or more receive beams. For echoes from the pulsed excitation used to displace tissue, the receive operation may not occur. The receive beamformer 16 uses the received signals to output data representing spatial locations. The relative delays and / or phases and summation of 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.

[0089] The receive beamformer 16 may include filters, such as filters for isolating information in a second harmonic or other frequency band relative to the transmit frequency band. Such information may more likely include desired tissue, contrast agent, and / or flow information. In another embodiment, the receive beamformer 16 includes a memory or buffer and filters or adders. Two or more receive beams are combined to isolate information in a desired frequency band (e.g., second harmonic, third harmonic, or other frequency band).

[0090] In coordination with the transmit beamformer 12, the receive beamformer 16 generates data representing regions at different times. After the acoustic pulse excitation, the receive beamformer 16 generates beams representing different lines or positions over time. By scanning the region of interest with ultrasound, data (e.g., beamformed samples) are generated.

[0091] The receive beamformer 16 outputs beam sum data representing spatial locations. Data for a single location, locations along a line, locations of a region, or locations of a volume are output. Dynamic focusing may be provided. The data may be used for different purposes. For example, different scans are performed for B-mode or tissue data compared to displacement. Alternatively, B-mode data is also used to determine displacement. As another example, data for viscoelastic parameter calculation and shear imaging is performed with a series of shared scans, and B-mode or Doppler scans are performed separately or using some of the same data.

[0092] The image processor 18 is a B-mode detector, Doppler detector, pulsed wave Doppler detector, correlation processor, Fourier transform processor, application specific integrated circuit, general purpose processor, control processor, image processor, field programmable gate array, digital signal processor, analog circuit, digital circuit, combinations thereof, or other now known or later developed devices for detecting and processing information from beamformed ultrasound samples for display. In one embodiment, the processor 18 includes one or more detectors and separate processors. The separate processors are control processors, general purpose processors, digital signal processors, application specific integrated circuits, field programmable gate arrays, networks, servers, processor groups, data paths, combinations thereof or other now known or later developed devices for determining displacement and calculating viscoelastic properties. For example, the separate processors are configured by hardware and / or software to perform Figure 1 any combination of one or more of the actions 34-42 shown in

[0093] The processor 18 is configured to estimate tissue displacement induced by acoustic pulse excitation. Correlation, tracking, motion detection, or other displacement measurements are used to estimate the amount of movement of the tissue's location. The estimation is performed multiple times over a period, e.g., from before the tissue moves due to the pulse to after the tissue has mostly or fully returned to a relaxed state (e.g., recovered from the stress caused by the pulse excitation). For each of multiple locations, the processor 18 estimates tissue displacement as a function of time.

[0094] The processor 18 is configured to calculate viscoelastic properties. The amplitudes and phases of tissue displacements from different locations within a region are used. By calculating the log-spectrum of tissue displacement over time for a location, the processor 18 determines the complex wave number. One or more values for one or more viscoelastic parameters are calculated based on the complex wave number. The complex wave number represents a shear wave as a function of frequency, thereby allowing the viscoelastic parameters to be determined as a function of frequency.

[0095] Processor 18 operates in accordance with instructions stored in memory 22 or another memory for quantification in viscoelastic ultrasound imaging. Memory 22 is a non-transitory 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, buffer, RAM, removable media, hard drive, or other computer-readable storage medium). Computer-readable storage media include various types of volatile and non-volatile storage media. The functions, acts, or tasks shown in the figures and described herein are performed in response to one or more sets of instructions stored in or on the computer-readable storage medium. The functions, acts, or tasks are independent of a particular type of instruction set, storage medium, processor, or processing strategy and may be performed by software, hardware, integrated circuits, firmware, microcode, etc., operating alone or in combination. Similarly, processing strategies may include multiprocessing, multitasking, 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 transmitted via a computer network or over a telephone line. In still other embodiments, the instructions are stored within a given computer, CPU, GPU, or system.

[0096] Display 20 is a CRT, LCD, projector, plasma, or other display for displaying two-dimensional images or three-dimensional representations. Display 20 is configured by a signal input to be displayed as an image by processor 18 or another device. Display 20 displays an image representing the viscoelastic properties at one or more locations. The image represents the viscoelastic properties in any manner, such as a test, graph, or modulation of pixels in the region of interest or the entire image.

[0097] Although the invention has been described above with reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. Accordingly, the foregoing detailed description is to be regarded as illustrative rather than restrictive, and it is intended that the following claims (including all equivalents) define the spirit and scope of the invention.

Claims

1. A method for quantification in viscoelastic ultrasound imaging, the method comprises: measuring (30, 32) the displacement over time of tissue within a patient at a first location in response to a pulsed excitation using an ultrasound system; measuring (30, 32) the displacement over time of tissue within a patient at a second location in response to a pulsed excitation using an ultrasound system; performing a temporal Fourier transform (36) over time on the displacement for each of the first and second locations by a processor (18), wherein the transform provides a set of spectra for a corresponding set of locations; calculating (38) the logarithm of the result of the transform (36) by the processor (18); solving (40) for a complex wavenumber by the processor (18) based on the logarithm of the result; determining (34) the value of a frequency-dependent viscoelastic parameter using the complex wavenumber, wherein the frequency-dependent viscoelastic parameter is a storage modulus, a loss modulus, a shear modulus, a viscosity, or a combination thereof; and outputting (42) the value for the tissue to a display, wherein the solving (40) comprises determining the slope between a first and a second location of the logarithm of the spectra as the complex wavenumber, the slope being the imaginary component.

2. The method of claim 1, further comprises: sending (30) an acoustic excitation into the patient, the pulsed excitation including the acoustic excitation; wherein measuring (30, 32) the displacement comprises repeatedly scanning the first and second locations using ultrasound.

3. The method of claim 1, wherein, measuring (30, 32) the displacement at the first and second locations comprises: sending (30) ultrasound into the tissue and receiving reflections from the sending (30), performing the sending (30) and receiving of ultrasound multiple times, and detecting (32) the displacement from the multiple receptions based on the reflections.

4. The method of claim 1, wherein measuring (30, 32) comprises measuring (30, 32) the displacement at the first and second locations caused by shear waves generated by the pulsed excitation.

5. The method of claim 1, wherein, measuring (30, 32) the displacement over time at the first and second locations comprises measuring (30, 32) the displacement after the pulsed excitation.

6. The method of claim 1, wherein, determining (34) the value for the frequency-dependent viscoelastic parameter comprises determining (34) as a function of a series of different frequencies.

7. The method of claim 1, wherein, determining (34) comprises determining (34) using both the amplitude and phase of the displacement over time.

8. A non-transitory computer-readable storage medium having data stored therein representing instructions executable by a programmed processor (18) for quantification in viscoelastic ultrasound imaging, the storage medium comprising instructions for causing execution of the method of any one of claims 1 - 7.

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