Patent Publication Number: US-11638574-B2

Title: Ultrasonic characterization of non-linear properties of tissue

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/832,388 to Glen W. McLaughlin et al., titled ULTRASONIC TISSUE CHARACTERIZATION OF THE B/A COEFFICIENT, and filed Apr. 11, 2019, the entire disclosure of which is hereby incorporated herein by this reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to diagnostic sonography and more particularly to identifying non-linear properties of tissue through exponentially swept ultrasound chirp pulses. 
     BACKGROUND OF THE INVENTION 
     Ultrasound imaging is widely used for examining a wide range of materials and objects across a wide array of different applications. Ultrasound imaging provides a fast and easy tool for analyzing materials and objects in a non-invasive manner. As a result, ultrasound imaging is especially common in the practice of medicine as an ailment diagnosis, treatment, and prevention tool. Specifically, because of its relatively non-invasive nature, low cost and fast response time ultrasound imaging is widely used throughout the medical industry to diagnose and prevent ailments. Further, as ultrasound imaging is based on non-ionizing radiation it does not carry the same risks as other diagnosis imaging tools, such as X-ray imaging or other types of imaging systems that use ionizing radiation. 
     Characterization of tissue properties, and in particular in-vivo tissue properties, with ultrasound has been a long-standing area of research for the past several decades. Specifically, efforts have been undertaken to efficiently gather and interpret ultrasound measurements for characterizing both bulk tissue properties and regional tissue properties. However, extracting meaningful and consistent ultrasound measurements and interpreting these ultrasound measurements to identify tissue properties has been challenging endeavor for numerous reasons. 
     SUMMARY 
     According to various embodiments, a method for performing diagnostic sonography includes collecting ultrasound information of a subject region. The ultrasound information can be based on one or more exponentially swept ultrasound chirp pulses transmitted toward the subject region and backscatter of the subject region from the one or more exponentially swept ultrasound chirp pulses. The method can also include separating one or more corresponding harmonic responses and a corresponding fundamental response for each of the one or more exponentially swept ultrasound chirp pulses from the ultrasound information. Further, the method can include identifying one or more non-linear properties of the subject region based on either or both of the one or more corresponding harmonic responses and the corresponding fundamental response for each of the one or more exponentially swept ultrasound chirp pulses. 
     In certain embodiments, a system for performing diagnostic sonography includes an ultrasound transducer and a main processing console. The ultrasound transducer can collect ultrasound information of a subject region. The ultrasound information can be based on one or more exponentially swept ultrasound chirp pulses transmitted toward the subject region and backscatter of the subject region from the one or more exponentially swept ultrasound chirp pulses. The main processing console can separate one or more corresponding harmonic responses and a corresponding fundamental response for each of the one or more exponentially swept ultrasound chirp pulses from the ultrasound information. The main processing console can also identify one or more non-linear properties of the subject region based on either or both of the one or more corresponding harmonic responses and the corresponding fundamental response for each of the one or more exponentially swept ultrasound chirp pulses. 
     In various embodiments, a system for performing diagnostic sonography includes one or more processors and a computer-readable medium providing instructions accessible to the one or more processors to cause the one or more processors to collect ultrasound information of a subject region. The ultrasound information can be based on one or more exponentially swept ultrasound chirp pulses transmitted toward the subject region and backscatter of the subject region from the one or more exponentially swept ultrasound chirp pulses. The instructions can further cause the one or more processors to separate one or more corresponding harmonic responses and a corresponding fundamental response for each of the one or more exponentially swept ultrasound chirp pulses from the ultrasound information. Additionally, the instructions can cause the one or more processors to identify one or more non-linear properties of the subject region based on either or both of the one or more corresponding harmonic responses and the corresponding fundamental response for each of the one or more exponentially swept ultrasound chirp pulses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an example of an ultrasound system. 
         FIG.  2    is a plot of intensities of fundamental and harmonic responses to an exponentially swept ultrasound chirp pulse as a function of time for both linear and non-linear subject regions. 
         FIG.  3 A  is a positive phase image of a subject region created through a Gaussian ultrasound transmit profile. 
         FIG.  3 B  is a negative phase image of the subject region created through the Gaussian ultrasound transmit profile. 
         FIG.  3 C  is a sum image of the images shown in  FIGS.  3 A and  3 B . 
         FIG.  3 D  is a difference image of the images shown in  FIGS.  3 A and  3 B . 
         FIG.  4 A  is an intensity plot of fundamental and harmonic responses to a narrowband Gaussian transmit profile centered at a 10 mm depth. 
         FIG.  4 B  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 20 mm depth. 
         FIG.  4 C  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 30 mm depth. 
         FIG.  4 D  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 40 mm depth. 
         FIG.  4 E  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 50 mm depth. 
         FIG.  4 F  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 60 mm depth. 
         FIG.  5 A  is a positive phase image of a subject region created through an exponentially swept ultrasound chirp signal. 
         FIG.  5 B  is a negative phase image of the subject region created through the exponentially swept ultrasound chirp signal. 
         FIG.  5 C  is a sum image of the images shown in  FIGS.  5 A and  5 B . 
         FIG.  5 D  is a difference image of the images shown in  FIGS.  5 A and  5 B . 
         FIG.  6 A  is an intensity plot of fundamental and harmonic responses to an exponentially swept ultrasound chirp pulse centered at a 10 mm depth. 
         FIG.  6 B  is an intensity plot of the fundamental and harmonic responses to the exponentially swept ultrasound chirp pulse centered at a 20 mm depth. 
         FIG.  6 C  is an intensity plot of the fundamental and harmonic responses to the exponentially swept ultrasound chirp pulse centered at a 30 mm depth. 
         FIG.  6 D  is an intensity plot of the fundamental and harmonic responses to the exponentially swept ultrasound chirp pulse centered at a 40 mm depth. 
         FIG.  6 E  is an intensity plot of the fundamental and harmonic responses to the exponentially swept ultrasound chirp pulse centered at a 50 mm depth. 
         FIG.  6 F  is an intensity plot of the fundamental and harmonic responses to the exponentially swept ultrasound chirp pulse centered at a 60 mm depth. 
         FIG.  7 A  is a plot of sums and differences of the fundamental and harmonic responses in  FIG.  6 A  centered at a 10 mm depth. 
         FIG.  7 B  is a plot of sums and differences of the fundamental and harmonic responses in  FIG.  6 B  centered at a 20 mm depth. 
         FIG.  7 C  is a plot of sums and differences of the fundamental and harmonic responses in  FIG.  6 C  centered at a 30 mm depth. 
         FIG.  7 D  is a plot of sums and differences of the fundamental and harmonic responses in  FIG.  6 D  centered at a 40 mm depth. 
         FIG.  7 E  is a plot of sums and differences of the fundamental and harmonic responses in  FIG.  6 E  centered at a 50 mm depth. 
         FIG.  7 F  is a plot of sums and differences of the fundamental and harmonic responses in  FIG.  6 F  centered at a 60 mm depth. 
         FIG.  8    is a flowchart of an example method for identifying non-linear properties of a subject region through an exponentially swept ultrasound chirp pulse. 
         FIG.  9    is a flowchart of an example method for estimating a B/A parameter of a subject region through an exponential swept ultrasound chirp signal. 
     
    
    
     DETAILED DESCRIPTION 
     Characterization of tissue properties, and in particular in-vivo tissue properties, with ultrasound has been a long-standing area of research for the past several decades. Specifically, efforts have been undertaken to efficiently gather and interpret ultrasound measurements for characterizing both bulk tissue properties and regional tissue properties. However, extracting meaningful and consistent ultrasound measurements and interpreting these ultrasound measurements to identify tissue properties has been challenging endeavor for numerous reasons. First, basic ultrasound measurements tend to be operator dependent making it difficult to consistently and accurately identify tissue characteristics from measurements gathered across different operators. Additionally, ultrasound measurements are prone to noise making it difficult to accurately identify tissue characteristics from the measurements. Further, ultrasound can have difficulties penetrating areas of interest at great depths making it difficult to gather measurements for accurately identifying tissue characteristics of the areas of interest. Additionally, correlation across different ultrasound system implementations is challenging, thereby leading to consistency and accuracy issues associated with characterizing tissue from measurements gathered across the different system implementations. 
     With respect to in-vivo tissue properties, movement of the tissue creates problems in gathering meaningful ultrasound measurements and accurately identifying tissue characteristics from the measurements. As a result, measurements for identifying tissue characteristics, e.g. non-linear tissue characteristics, are typically made through excised tissue. However, the process of surgically removing a patient&#39;s tissue complicates the overall process of tissue characterization and presents numerous risks for the patient. 
     The following disclosure describes systems, methods, and computer-readable media for solving these problems/discrepancies. Specifically, the present technology involves system, methods, and computer-readable media for identifying non-linear properties of a subject region through one or more exponentially swept ultrasound chirp pulses transmitted towards the subject region. More specifically, the present technology involves systems, methods, and computer-readable media for identifying non-linear properties of the subject region based on either or both one or more corresponding harmonic responses and a corresponding fundamental response for each of the one or more exponentially swept ultrasound chirp pulses 
     Reference is now made to the figures, where like components are designated by like reference numerals throughout the disclosure. Some of the infrastructure that can be used with embodiments disclosed herein is already available, such as general-purpose computers, computer programming tools and techniques, digital storage media, and communications networks. A computing device may include a processor such as a microprocessor, microcontroller, logic circuitry, or the like. The processor may include a special purpose processing device such as an ASIC, PAL, PLA, PLD, FPGA, or other customized or programmable device. The computing device may also include a computer-readable storage device such as non-volatile memory, static RAM, dynamic RAM, ROM, CD-ROM, disk, tape, magnetic, optical, flash memory, or other non-transitory computer-readable storage medium. 
     Various aspects of certain embodiments may be implemented using hardware, software, firmware, or a combination thereof. As used herein, a software module or component may include any type of computer instruction or computer executable code located within or on a computer-readable storage medium. A software module may, for instance, comprise one or more physical or logical blocks of computer instructions, which may be organized as a routine, program, object, component, data structure, etc., which performs one or more tasks or implements particular abstract data types. 
     In certain embodiments, a particular software module may comprise disparate instructions stored in different locations of a computer-readable storage medium, which together implement the described functionality of the module. Indeed, a module may comprise a single instruction or many instructions, and may be distributed over several different code segments, among different programs, and across several computer-readable storage media. Some embodiments may be practiced in a distributed computing environment where tasks are performed by a remote processing device linked through a communications network. 
     The embodiments of the disclosure will be best understood by reference to the drawings. The components of the disclosed embodiments, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Furthermore, the features, structures, and operations associated with one embodiment may be applicable to or combined with the features, structures, or operations described in conjunction with another embodiment. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of this disclosure. 
     Thus, the following detailed description of the embodiments of the systems and methods of the disclosure is not intended to limit the scope of the disclosure, as claimed, but is merely representative of possible embodiments. In addition, the steps of a method do not necessarily need to be executed in any specific order, or even sequentially, nor need the steps be executed only once. 
       FIG.  1    is a schematic block diagram of one exemplary embodiment of a medical imaging device, such as an ultrasound imaging device  100 . Those skilled in the art will recognize that the principles disclosed herein may be applied to a variety of medical imaging devices, including, without limitation, an X-ray imaging device, a computed tomography (CT) imaging device, a magnetic resonance imaging (MRI) device, and a positron-emission tomography (PET) imaging device. As such, the components of each device may vary from what is illustrated in  FIG.  1   . 
     In one embodiment, the ultrasound imaging device  100  may include an array focusing unit, referred to herein as a beam former  102 , by which image formation can be performed on a scanline-by-scanline basis. The device may be controlled by a master controller  104 , implemented by a microprocessor or the like, which accepts operator inputs through an operator interface and in turn controls the various subsystems of the device  100 . 
     For each scanline, a transmitter  106  generates a radio-frequency (RF) excitation voltage pulse waveform and applies it with appropriate timing across a transmit aperture (defined, in one embodiment, by a sub-array of active elements) to generate a focused acoustic beam along the scanline. 
     RF echoes received by one or more receive apertures or receiver  108  are amplified, filtered, and then fed into the beam former  102 , which may perform dynamic receive focusing, i.e., realignment of the RF signals that originate from the same locations along various scan lines. Collectively, the transmitter  106  and receiver  108  may be components of a transducer  110 . Various types of transducers  110  are known in the ultrasound imaging art, such as linear probes, curvilinear probes, and phased array probes. 
     An image processor  112  may perform processing tasks specific to various active imaging mode(s) including 2D scan conversion that transforms the image data from an acoustic line grid into an X-Y pixel image for display. For other modes, such as a spectral Doppler mode, the image processor  112  may perform wall filtering followed by spectral analysis of Doppler-shifted signal samples using typically a sliding FFT-window. The image processor  112  may also generate a stereo audio signal output corresponding to forward and reverse flow signals. In cooperation with the master controller  104 , the image processor  112  may also format images from two or more active imaging modes, including display annotation, graphics overlays and replay of cine loops and recorded timeline data. 
     A cine memory  114  provides resident digital image storage to enable single image or multiple image loop review, and acts as a buffer for transfer of images to digital archival devices, such as hard disk drives or optical storage. In some systems, the video images at the end of the data processing path may be stored to the cine memory. In state-of-the-art systems, amplitude-detected, beamformed data may also be stored in cine memory  114 . For spectral Doppler mode, wall-filtered, baseband Doppler 1/Q data for a user-selected range gate may be stored in cine memory  114 . Subsequently, a display  116 , such as a computer monitor, may display ultrasound images created by the image processor  112  and/or images using data stored in the cine memory  114 . 
     The beam former  102 , the master controller  104 , the image processor  112 , the cine memory  114 , and the display  116  can be included as part of a main processing console  118  of the ultrasound imaging device  100 , which may include more or fewer components or subsystems than are illustrated. The ultrasound transducer  110  may be incorporated into an apparatus that is separate from the main processing console  118 , e.g. in a separate apparatus that is wired or wirelessly connected to the main processing console  118 . This allows for easier manipulation of the ultrasound transducer  110  when performing specific ultrasound procedures on a patient. Further, the transducer  110  can be an array transducer that includes an array of transmitting and receiving elements for transmitting and receiving ultrasound waves. 
     Those skilled in the art will recognize that a wide variety of ultrasound imaging devices are available on the market, and additional details relating to how images are generated is unnecessary for a thorough understanding of the principles disclosed herein. Specifically, the systems, methods, and computer-readable media described herein can be applied through an applicable ultrasound imaging device of the wide variety of ultrasound imaging devices available on the market. 
     Exponentially swept ultrasound chirps can be applied to characterize non-linearity of a subject region. Specifically, exponentially swept ultrasound chirps can be applied to identify non-linear properties of tissue. More specifically, exponentially swept ultrasound chirps can be applied to identify non-linear properties of in-vivo tissue. Characterizing tissue that remains in-vivo is advantageous as it reduces the complexity associated with surgically removing the tissue to ultimately characterize the tissue. Further, characterizing tissue that remains in-vivo is advantageous as it eliminates patient risks associated with the surgical procedure(s) of removing the tissue. 
     Non-linear properties of a subject region can include applicable non-linear properties of a subject region capable of being identified through ultrasound. Specifically, non-linear properties of a subject region can include applicable acoustic non-linear properties of tissue capable of being identified through ultrasound. For example, non-linear properties of a subject region can include values of an acoustic non-linearity parameter B/A for tissue. 
     Exponentially swept chirp pulses can be represented as Equations 1 and 2 shown below. 
                     x   ⁡   (   t   )     =     e     i   ⁢   2   ⁢   π   ⁢       f   1     a     ⁢     (       e   at     -   1     )                 Equation   ⁢         1                             θ   ⁡   (   f   )     =       A   ⁡   (       f   ⁢           log   ⁡   (   f   )       -   f     )     -       (       A   ⁢         log   ⁢     f   1       -     T   start       )     ⁢   f               Equation   ⁢         2               
In equation 1,
 
             a   =       ln   ⁡   (       f   1       f   2       )     T           
and in equation 2,
 
             =       T     ln   ⁢       f   2       f   1           .           f   1             
is the start frequency of the exponentially swept chirp pulse and f 2  is the stop frequency of the exponentially swept chirp pulse. T is the pulse duration of the exponentially swept chip pulse and T start  is the pulse start time.
 
     Exponentially swept chirp signals have the characteristic that the group delay between fundamental and harmonic responses to the chirp signals is a function of the N th  order harmonic. More specifically, the group delay between the fundamental and harmonic responses to the chirp signals is not a function of the frequency of the harmonic responses, e.g. the instantaneous frequency of the chirp signals corresponding to the harmonic responses. Accordingly, the fundamental and the N th  order harmonic responses for an exponentially swept chirp signal sum to form their own respective impulse responses offset by the difference in corresponding group delays between the corresponding harmonic responses and the corresponding fundamental response, what is otherwise referred to as the time delay between the corresponding harmonic responses and fundamental response. 
     These response characteristics of exponentially swept chirp signals are further illustrated by the following equations. Equation 3 is the instantaneous frequency of the exponentially swept chirp signal. 
                       f   instant     (   t   )     =       1     2   ⁢   π       ⁢       ∂       θ   time     (   t   )         ∂   t                 Equation   ⁢         3               
Equation 4 is the production time for the fundamental response τ groupDelay (f) of the exponentially swept chirp signal.
 
                       τ   groupDelay     (   f   )     =       -     1     2   ⁢   π         ⁢       ∂       θ   freq     (   f   )         ∂   f                 Equation   ⁢         4               
As follows, the production time for the fundamental response τ groupDelay (f), as shown in Equation 5, can be represented as a function of the start frequency f 1  and the stop frequency f 2  of the exponentially swept chirp pulse.
 
                       τ   groupDelay     (   f   )     =     T   ⁢       ln   ⁡   (     f     f   1       )       ln   ⁡   (       f   2       f   1       )                 Equation   ⁢         5               
Further, the production time for the N th  order harmonic {right arrow over (τ)} groupDelay (f) of the exponentially swept chirp signal can be represented as shown in Equation 6.
 
                         τ   ^     groupDelay     (   f   )     =     T   ⁢       ln   ⁡   (     f     Nf   1       )       ln   ⁡   (       f   2       f   1       )                 Equation   ⁢         6               
Accordingly, the time delay Δ t  between the corresponding N th  order harmonic responses and the fundamental response for the exponentially swept chirp signal can be expressed by Equation 7.
 
     
       
         
           
             
               
                 
                   
                     Δ 
                     t 
                   
                   = 
                   
                     
                       
                         
                           τ 
                           groupDelay 
                         
                         ( 
                         f 
                         ) 
                       
                       - 
                       
                         
                           
                             τ 
                             ^ 
                           
                           groupDelay 
                         
                         ( 
                         f 
                         ) 
                       
                     
                     = 
                     
                       T 
                       ⁢ 
                       
                         
                           ln 
                           ⁡ 
                           ( 
                           N 
                           ) 
                         
                         
                           ln 
                           ⁡ 
                           ( 
                           
                             
                               f 
                               2 
                             
                             
                               f 
                               1 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   7 
                 
               
             
           
         
       
     
     As shown in Equation 7, Δ t  is a function of N but not a function of ƒ, e.g. the instantaneous frequency of the chirp signal corresponding to the harmonic responses. Therefore, across frequency, all N th  order harmonics have the same group delay and will thus sum to form a “harmonic” impulse response. Specifically, all frequencies arising from a particular harmonic order can arrive at the same time, creating a harmonic impulse response that is offset from a corresponding fundamental impulse response by the time delay Δ t . 
     As the harmonic responses for an N th  order harmonic can be summed to effectively form a single harmonic impulse response, an exponentially swept ultrasound chirp pulse can effectively create two impulse responses, a fundamental response and a corresponding N th  harmonic response. Specifically, backscatter from a subject region created in response to an exponentially swept ultrasound chirp pulse interacting with the subject region can include a fundamental response and one or more harmonic responses, e.g. corresponding to each N harmonic. These responses, as shown in Equation 7, are displaced in time by the time delay Δ t . This time delay can be a known amount, e.g. based on N and the start and stop frequencies f 1  and f 2  of the exponentially swept ultrasound chirp pulse. 
     This time delay between the harmonic response(s) and the fundamental response to the exponentially swept chirp can be dependent on the presence of non-linearity in the subject region. Specifically, this time delay between the fundamental response and the harmonic response(s) can be created when the subject region interacting with the exponentially swept chirp includes non-linearity. Conversely, when the subject region lacks non-linearity, then the created fundamental response and the harmonic response(s) can lack this time shift. In turn, the time delay created in response to non-linearity in the subject region can serve as a basis for identifying non-linear properties of the subject region. Specifically, the time delay between the fundamental response and the harmonic response(s) can form the basis for the superposition of two or more images created from the ultrasound backscatter. In turn, the amount of displacement present between the two or more superposition images, e.g. based at least in part on the time delay between the fundamental response and the harmonic response(s), can be analyzed to identify non-linear properties of the subject region. 
       FIG.  2    shows the time offset between fundamental and harmonic responses that is created when the subject region has non-linearity. Specifically,  FIG.  2    is a plot  200  of intensities of fundamental and harmonic responses to an exponentially swept ultrasound chirp pulse as a function of time for both linear and non-linear subject regions. As shown in  FIG.  2   , both the linear subject region and the non-linear subject region have a fundamental impulse response at time 0.0 uS,  202 . Additionally, and as shown in  FIG.  2   , the non-linear subject region has a time shifted harmonic response  204  at time −2.0 uS,  206 . However, the linear subject region does not show corresponding time shifted harmonic response(s), e.g. due to the lack of non-linearity of the linear subject region. 
       FIG.  3 A  is a positive phase image  300  of a subject region created through a Gaussian ultrasound transmit profile.  FIG.  3 B  is a negative phase image  302  of the subject region created through the Gaussian ultrasound transmit profile.  FIG.  3 C  is a sum image  304  of the images shown in  FIGS.  3 A and  3 B .  FIG.  3 D  is a difference image  306  of the images shown in  FIGS.  3 A and  3 B . 
     The Gaussian ultrasound transmit profile is typically used in forming images through ultrasound. Specifically, phase inversion or a positive oriented pulse and a negative oriented pulse through a Gaussian ultrasound transmit profile used to generate two separate images through harmonic mode imaging. In turn, the fundamental component can cancel out when the two images are combined to leave only the harmonic component(s). The corresponding positive phase image  300  shown in  FIG.  3 A  is created through a positive narrowband Gaussian transmit profile while the corresponding negative phase image  302  is created through a negative narrowband Gaussian transmit profile. 
     As shown in the positive and negative phase images  300  and  302 , points in the imaging field are offset by 10 mm. For example, the positive phase image  300  has a point  308  at 60 mm that is offset from adjacent points, e.g. the point at 50 mm, by around 10 mm. The same can be seen in the negative phase image  302 , where points, e.g. the point at 60 mm  310 , are offset from adjacent points by around 10 mm. Further and as shown in the positive and negative phase images  300  and  302 , the positive phase and negative phase images  300  and  302 , the harmonic signal(s) are not offset from the fundamental signal(s). Specifically, corresponding harmonic signal(s) and fundamental signal(s) form single corresponding points, e.g. point  308 , and not two separate points, as will be shown in greater detail later. This shows the lack of separation, e.g. due to time delay, that is created through application of a typical ultrasound transmit profile, e.g. a Gaussian ultrasound transmit profile. 
     This lack of separation between corresponding harmonic signal(s) and fundamental signal(s) is further illustrated in the sum image  304  and the difference image  306  shown in  FIGS.  3 C and  3 D . The sum image  304  of the positive oriented pulse and the negative oriented pulse applied to create the corresponding positive and negative phase images  300  and  302  represents the fundamental signal(s). Further, the difference image  306  of the positive oriented pulse and the negative oriented pulse applied to create the corresponding positive and negative phase images  300  and  302  represents the harmonic signal(s). As shown, in the sum image  304 , the fundamental signal at 60 mm is a single point  312 . Similarly, in the difference image  306 , the harmonic signal is also a single point  314  that is located at 60 mm. The points  312  and  314  are both at 60 mm and do not show a separation of depth corresponding to a lack of time delay between the fundamental and harmonic signals. 
       FIG.  4 A  is an intensity plot of fundamental and harmonic responses to a narrowband Gaussian transmit profile centered at a 10 mm depth.  FIG.  4 B  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 20 mm depth.  FIG.  4 C  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 30 mm depth.  FIG.  4 D  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 40 mm depth.  FIG.  4 E  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 50 mm depth.  FIG.  4 F  is an intensity plot of the fundamental and harmonic responses to the narrowband Gaussian transmit profile centered at a 60 mm depth. 
     As can be seen in  FIG.  4 C , the response from the fundamental signal  412  and the response from the harmonic component  414  are both collocated in space at approximately 30 mm of depth. The same collocation of the fundamental and harmonic signal can bee see from the other intensity plots  FIGS.  4 A,  4 B,  4 D,  4 E, and  4 F . This further shows that a narrowband Gaussian transmit profile, that is the typical transmit used in ultrasound systems, does not produce an offset in distance between the fundamental and harmonic components. 
       FIG.  5 A  is a positive phase image  500  of a subject region created through an exponentially swept ultrasound chirp signal.  FIG.  5 B  is a negative phase image  502  of the subject region created through the exponentially swept ultrasound chirp signal.  FIG.  5 C  is a sum image  504  of the images shown in  FIGS.  5 A and  5 B .  FIG.  5 D  is a difference image  506  of the images shown in  FIGS.  5 A and  5 B . 
       FIGS.  5 A-D  show the time delay created between fundamental and harmonic responses to an exponentially swept ultrasound chirp signal. In the positive phase image  500 , the fundamental signal is located at the point  508  at 30 mm. Further, the harmonic response in the positive phase image  500  is at point  510  before 30 mm, which is offset from the point  508  of the fundamental response at 30 mm. This offset corresponds to the time delay between the fundamental and harmonic responses created through application of the exponentially swept ultrasound chirp signal. This spatial offset is also shown in the negative phase image  502  where the fundamental response is located at point  512  at 30 mm, while the harmonic response is located at point  514 , which is offset from the point  512  of the fundamental response at  512 . 
     This time separation between corresponding harmonic signal(s) and fundamental signal(s) is further illustrated in the sum image  504  and the difference image  506  shown in  FIGS.  5 C and  5 D . The sum image  504  of the positive oriented exponentially swept ultrasound chirp pulse and the negative oriented exponentially swept ultrasound chirp pulse, corresponding to  FIGS.  5 A and  5 B , represents the fundamental signal(s). Further, the difference image  506  of the positive oriented exponentially swept ultrasound chirp pulse and the negative oriented exponentially swept ultrasound chirp pulse, corresponding to  FIGS.  5 A and  5 B , represents the harmonic signal(s). As shown in  FIG.  5 C , there is a fundamental response at point  516  at 30 mm. Further and as shown in  FIG.  5 D , there a harmonic response at point  518 . The harmonic response at point  518  is shifted from the fundamental response at point  516 . Specifically, the fundamental response at point  516  is at 30 mm, while the harmonic response at point  518  is shifted closer to 26 mm. This spatial shift corresponds to the time shift between the fundamental response and the harmonic response to the exponentially swept ultrasound chirp pulses. 
       FIGS.  6 A-F  are intensity plots of fundamental and harmonic responses of a subject region to an exponentially swept ultrasound chirp signal centered at varying depths of the subject region. As shown in  FIG.  6 C , the fundamental response, e.g. at point  600 , is not collocated at a depth of 30 mm with the harmonic response, e.g. at point  602 . The same non-collocation of the fundamental and harmonic signals can be seen from the other intensity plots  FIGS.  6 A , B, D, E, and F with respect to range at the points located at other increments of 10 mm offsets. As such, the exponentially swept ultrasound chirp signal is able to separate in time/distance the harmonic and fundamental responses. 
       FIGS.  7 A-F  are plots of sums and differences of the fundamental and harmonic responses shown in  FIGS.  6 A-F  at the varying depths of the subject region. As can be seen from the target at 30 mm the response from the fundamental signal, at point  700 , is located at the expected range, while that of the harmonic signal is located closer to the 26 mm range, at point  702 . A similar offset in time/range is also observed from the points throughout the entire graph. This clearly demonstrates that not only are the harmonic and fundamental responses separated in time/distance, but also the signal response that is associated with the fundamental and that with the harmonic can also be separated for analysis, in particular for identifying non-linear properties of the subject region. 
       FIG.  8    is a flowchart  800  of an example method for identifying non-linear properties of a subject region through an exponentially swept ultrasound chirp pulse. The example method shown in  FIG.  8   , and other methods and techniques for ultrasound imaging described herein, can be performed by an applicable ultrasound imaging system, such as the ultrasound system  100  shown in  FIG.  1   . For example, the techniques for ultrasound imaging described herein can be implemented using either or both the ultrasound transducer  110  and the main processing console  118 , e.g. the image processor  112 , of the ultrasound system  100 . 
     At step  802 , ultrasound information of a subject region is collected. The ultrasound information can include ultrasound information of one or more exponentially swept ultrasound chirp purses transmitted towards a subject region. For example, the ultrasound information can include a transmit profile of one or more exponentially swept ultrasound chirp pulses transmitted towards the subject region. Further, the ultrasound information can include backscatter information of both fundamental and harmonic responses generated by the subject region in response to one or more exponentially swept ultrasound chirp pulses transmitted towards the subject region. 
     As the exponentially swept ultrasound chirp pulse produces time shifted harmonic and fundamental responses, the backscatter information can include information related to the time shifted harmonic and fundamental responses. For example, the ultrasound information can include the harmonic and fundamental responses time shifted with respect to each other as a result of non-linearity in the subject region. In turn, the harmonic and fundamental responses can be analyzed, e.g. based on the time shift, to identify non-linear properties of the subject region. 
     The ultrasound information can include a plurality of ultrasound images, or otherwise data used to generate the plurality ultrasound images, of the subject region based on the fundamental and harmonic signals. The plurality of ultrasound images of the subject region can be superimposed with respect to each other based on one or more spatial offsets. For example, a feature in the subject region can be displaced by a spatial offset in the plurality of ultrasound images. The one or more spatial offsets can correspond to the fundamental and harmonic signals. Specifically, the one or more spatial offsets can correspond to one or more time delays between the harmonic and fundamental signals. In turn and as will be discussed in greater detail later, the spatial offset(s) in the superimposed ultrasound images can serve as a basis for identifying non-linear properties of the subject region from the ultrasound information. 
     At step  804 , one or more corresponding harmonic responses and a corresponding fundamental response can be separated from the ultrasound information for each of the one or more exponentially swept ultrasound chirp pulses. The corresponding harmonic response(s) and the fundamental response can be separated from each other in the ultrasound information to identify one or more non-linear properties of the subject region. Specifically, the corresponding harmonic response(s) and the fundamental response can be separated from each other to identify a time offset between the harmonic response(s) and the fundamental response. More specifically, the corresponding harmonic response(s) and the fundamental responses can be separated from each other to identify a spatial offset between one or more ultrasound images generated from the harmonic response(s) and the fundamental response. 
     In turn and at step  806 , one or more non-linear properties of the subject region can be identified based on either or both the corresponding harmonic response(s) and the fundamental response for each of the one or more exponentially swept ultrasound chirp pulses. Specifically and as will be discussed in greater detail later, the non-linear properties of the subject region can be identified based on the time offset and/or the spatial offset corresponding to the harmonic response(s) and the fundamental response for each of the applied exponentially swept ultrasound chirp pulses. 
     The fundamental response and the harmonic response(s) can be filtered from the ultrasound information in order to separate the harmonic response(s) and the fundamental response. Specifically, the fundamental response and the harmonic response(s) can be separately filtered from the ultrasound information to separate the fundamental response and the harmonic response(s). For example, the fundamental response and the harmonic response(s) can be filtered from the ultrasound information based on time, e.g. to separate the fundamental response and the harmonic response(s) based on one or more time delays between the fundamental response and the harmonic response(s). 
     Additionally, the fundamental response and the harmonic response(s) can be separated by canceling out the fundamental response from the ultrasound information. Specifically, the ultrasound information can be processed to cancel out the fundamental response, effectively isolating the harmonic response(s) in the ultrasound information. In turn, non-linear properties of the subject region can be identified based on the remaining harmonic response(s) and potentially in combination with the fundamental response. The fundamental response can be canceled out from the ultrasound information through an applicable signal processing technique. For example, pulse inversion can be applied to cancel out the fundamental response from the ultrasound information. 
     The fundamental response and the harmonic response(s) can be correlated with each other from the ultrasound information to generate correlated harmonic and fundamental response information. In turn, the non-linear properties of the subject region can be identified from the correlated harmonic and fundamental response information. The harmonic response(s) and the fundamental response can be correlated with each other based on time offset(s) created between the harmonic response(s) and the fundamental response. The time offset(s), as discussed previously can be created as a result of application of the one or more exponentially swept ultrasound chirp pulses to the subject region when the subject region includes non-linear properties. This time offset(s) can be known, based on the characteristics of the one or more exponentially swept ultrasound chirp pulses, e.g. the start and stop frequencies of the pulses. Accordingly, the non-linear properties of the subject region can be identified based on the known time offset(s). 
     In identifying the non-linear properties of the subject region based on the fundamental and harmonic response(s), one or more corresponding spatial offsets in the subject region can be identified from the correlated harmonic and fundamental response information. In turn, the non-linear properties of the subject region can be identified from the spatial offsets in the subject region. The spatial offsets can correspond to the time delay between the fundamental and harmonic response(s). Further, the spatial offsets can be represented as spatial offsets in a result of processing the ultrasound information including the fundamental and harmonic response(s). Specifically, the spatial offsets can correspond to a spatial offset, e.g. of features, in a plurality of ultrasound images created based on the fundamental and harmonic response(s). 
     Further, in identifying the non-linear properties of the subject region based on the fundamental and harmonic response(s), the fundamental and harmonic response(s) can be averaged to generate averaged harmonic and fundamental response information. In turn, the non-linear properties of the subject region can be identified based on the averaged harmonic and fundamental response information. The fundamental and harmonic response(s) can be averaged through an applicable averaging technique applied to the ultrasound information including the harmonic and fundamental responses. For example, coherent data averaging can be applied to the ultrasound information to generate the averaged harmonic and fundamental response information. 
     The non-linear properties identified based on either or both the fundamental and harmonic response(s) for each exponentially swept ultrasound chirp signal can be identified as part of bulk non-linear property estimates for the subject region. For example, identified estimates of a non-linear property can be averaged across the subject region to identify a bulk non-linear property estimate for the specific non-linear property across the subject region. Further, the non-linear properties can be identified on a sub-region basis for different sub-regions of the subject region. For example, values of a non-linear property can be identified or estimated for different portions of the subject region. As follows, a map of non-linear properties of the subject region can be generated. The map of non-linear properties of the subject region can be generated based on the non-linear properties identified on a sub-region basis for the subject region. For example, a map showing different values of a non-linear property across different sub-regions of the subject region can be generated. 
     The subject region can be a volume region. Specifically, the subject region can be a volume of tissue, e.g. in-vivo tissue. For example, the subject region can include a volume of fatty liver tissue, Cirrhotic liver tissue, Thyroid cancer tissue, Prostate cancer tissue, or Breast cancer tissue. The non-linear properties can be volume non-linear properties of the volume region. Specifically, non-linear properties for the volume region can be bulk non-linear property estimates for the volume region. The ultrasound information for the volume region can be gathered by an applicable ultrasound system for gathering volume ultrasound information. For example, the ultrasound information for the volume region can be gathered by an ultrasound system that incorporates one or more volumetric-based ultrasound transducers. 
       FIG.  9    is a flowchart  900  of an example method for estimating a B/A parameter of a subject region through an exponential swept ultrasound chirp signal. At step  902 , an exponential swept ultrasound chirp signal is formed and at step  904  the signal is transmitted towards a subject region. At step  906 , the backscatter from the transmit signal is received and processed into one or more images. The desired harmonic signal, typically the second, is than correlated, at step  908 , against the fundamental signal, e.g. based on the image(s), to determine the spatial offset in the subject region. At step  910 , a B/A parameter for the subject region is estimated based on the correlation between the fundamental and harmonic signals. 
     The techniques described herein can be applied in an applicable ultrasound imaging mode, such as B-Mode, contrast-enhanced ultrasound (‘CEUS’), CD-Mode, 2D/3D/4D, and the like. Specifically, the techniques described herein are not limited to B-Mode but can also be applied to other modes where improved temporal resolution within a region of interest has substantial clinical benefits, such as CEUS. 
     This disclosure has been made with reference to various exemplary embodiments including the best mode. However, those skilled in the art will recognize that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present disclosure. For example, various operational steps, as well as components for carrying out operational steps, may be implemented in alternate ways depending upon the particular application or in consideration of any number of cost functions associated with the operation of the system, e.g., one or more of the steps may be deleted, modified, or combined with other steps. 
     While the principles of this disclosure have been shown in various embodiments, many modifications of structure, arrangements, proportions, elements, materials, and components, which are particularly adapted for a specific environment and operating requirements, may be used without departing from the principles and scope of this disclosure. These and other changes or modifications are intended to be included within the scope of the present disclosure. 
     The foregoing specification has been described with reference to various embodiments. However, one of ordinary skill in the art will appreciate that various modifications and changes can be made without departing from the scope of the present disclosure. Accordingly, this disclosure is to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope thereof. Likewise, benefits, other advantages, and solutions to problems have been described above with regard to various embodiments. However, benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, a required, or an essential feature or element. As used herein, the terms “comprises,” “comprising,” and any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, a method, an article, or an apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, system, article, or apparatus. Also, as used herein, the terms “coupled,” “coupling,” and any other variation thereof are intended to cover a physical connection, an electrical connection, a magnetic connection, an optical connection, a communicative connection, a functional connection, and/or any other connection. 
     Those having skill in the art will appreciate that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.