Abstract:
Disclosed is a system and method for generating an ultrasound echo signal with reduced linear components. In one embodiment, the system comprises an excitation signal source configured to generate a first excitation signal and a second excitation signal, and a transducer coupled to the excitation signal source, the transducer being configured to emit a first asymmetric ultrasonic pulse in response to the first excitation signal and a second asymmetric ultrasonic pulse in response to the second excitation signal, the first and second asymmetric ultrasonic pulses being emitted into a medium. The system also entails a beamformer coupled to the transducer, the beamformer configured to generate a first echo signal and a second echo signal received by the transducer from the medium in response to the first and second asymmetric ultrasonic pulses, and an envelope detector coupled to the beamformer, the envelope detector configured to generate a first echo envelope from the first echo signal and a second echo envelope from the second echo signal. The present system also includes an acoustic line memory coupled to the envelope detector configured to store the first echo envelope, and an arithmetic junction coupled to the envelope detector, the arithmetic junction configured to subtract the first echo envelope from the second echo envelope, thereby canceling nonlinearities.

Description:
TECHNICAL FIELD 
     The present invention is generally related to the field of ultrasonic imaging, and, more particularly, is related to a system and method for ultrasonic imaging using linear cancellation. 
     BACKGROUND OF THE INVENTION 
     Ultrasonic imaging systems hold the promise of picturing the inner workings of a particular medium such as a human body without invasive surgical procedures, etc. Generally, such systems generate ultrasonic pulses that propagate through the body or other medium and the various structures therein generate echoes that are detected by the ultrasonic system. These echoes are used to generate an image of the interior structure of the medium. 
     In some cases, the medium may exhibit a nonlinear response to the ultrasonic pulses. At times, it is desirable to isolate the non-linearity in the medium to generate images of the medium in question. Among the techniques that have been developed to isolate the non-linearities is harmonic imaging. In harmonic imaging, a narrowband ultrasonic pulse is transmitted into the medium and the echo signals received are filtered to isolate the second harmonic that is believed to contain primarily non-linear echo information. However, this approach provides poor spatial resolution because it is a narrowband technique. 
     Another approach is to use phase inversion techniques to cancel out linearities. According to this approach, a relatively wideband, ultrasonic pulse is first transmitted into the medium. Shortly thereafter, the same pulse with an inverted polarity is transmitted as well. The received echo signals are added to each other, and presumably any linearities are canceled. Unfortunately, this approach suffers due to its susceptibility to motion artifact that results in unsatisfactory imaging. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method for generating an ultrasound echo signal with reduced linear components. In one embodiment, the present system comprises an excitation signal source configured to generate a first excitation signal and a second excitation signal, and a transducer coupled to the excitation signal source, the transducer being configured to emit a first asymmetric ultrasonic pulse in response to the first excitation signal and a second asymmetric ultrasonic pulse in response to the second excitation signal, the first and second asymmetric ultrasonic pulses being emitted into a medium. The system also entails a beamformer coupled to the transducer, the beamformer configured to generate a first echo signal and a second echo signal received by the transducer from the medium in response to the first and second asymmetric ultrasonic pulses, and an envelope detector coupled to the beamformer, the envelope detector configured to generate a first echo envelope from the first echo signal and a second echo envelope from the second echo signal. The present system also includes an acoustic line memory coupled to the envelope detector configured to store the first echo envelope, and an arithmetic junction coupled to the envelope detector, the arithmetic junction configured to subtract the first echo envelope from the second echo envelope, thereby generating a nonlinear echo signal. 
     The present invention can also be viewed as providing a method for generating a nonlinear ultrasound echo signal with reduced linear components. In this regard, the method can be broadly summarized by the following steps: consecutively generating a first asymmetric ultrasonic pulse and a second asymmetric ultrasonic pulse, the first and second asymmetric ultrasonic pulses being emitted into a medium, thereby generating a first and second echo signals, respectively; generating a first envelope signal from the first echo signal and a second envelope signal from the second echo signal; and performing a subtraction between the first envelope signal and the second envelope signal, thereby generating the nonlinear ultrasound echo signal. 
     Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. 
     Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is a functional block diagram of a phase inversion ultrasonic imaging system; 
     FIG. 2A is a graph of a first symmetrical ultrasonic pulse employed in the ultrasonic imaging system of FIG. 1; 
     FIG. 2B is a graph of a second symmetrical ultrasonic pulse employed in the ultrasonic imaging system of FIG. 1; 
     FIG. 2C is a graph of a nonlinear echo signal produced by the ultrasonic imaging system of FIG. 1; 
     FIG. 2D is a graph of a nonlinear echo envelope produced by the ultrasonic imaging system of FIG. 1; 
     FIG. 3 is a functional block diagram of an ultrasonic imaging system according to the present invention; 
     FIG. 4 is a graph of a first asymmetrical ultrasonic pulse employed in the ultrasonic imaging system of FIG. 3; 
     FIG. 5A is a graph of a first echo envelope produced by the ultrasonic imaging system of FIG. 3; 
     FIG. 5B is a graph of a second echo envelope produced by the ultrasonic imaging system of FIG. 3; and 
     FIG. 5C is a graph of a nonlinear echo signal produced by the ultrasonic imaging system of FIG. 3 from the first and second echo envelopes of FIGS.  5 A and  5 B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, shown is a functional block diagram of an ultrasonic imaging system  101  that employs phase inversion techniques. The ultrasonic imaging system  101  may be virtually any type of ultrasound imaging system, for example but not limited to, a brightness-mode (B-mode) system, a Doppler-based imaging system, a color flow imaging system, etc., as well as a custom manufactured ultrasound imaging system. 
     In employing phase inversion techniques, the ultrasonic imaging system  101  includes a transmitter  103  that provides excitation signals to a transducer  106  that generates an ultrasonic pulse  109  in response thereto that is generally symmetrical in form. The ultrasonic pulse  109  is transmitted into a medium  113  that may be, for example, a human body or other medium. As the ultrasonic pulse travels through the medium in question, the various portions of the medium  113  that lie in the path of the ultrasonic pulse  109  create a pulse echo  116  as the ultrasonic pulse travels therethrough. These pulse echoes  116  are detected by the transducer  106  that generates electrical signals that are applied to a receive beamformer  119 . 
     The transducer  106  may be any suitable type of transducer, including a phased array transducer, etc. If a phased array transducer is employed, then several elements in the phase array each transmit the ultrasonic pulse  109  and receive the pulse echoes  116 , generating a corresponding electrical signal therefrom. The receive beamformer  119  receives the signal or signals generated by the transducer  196  due to the pulse echoes  116  and generates a single echo signal therefrom. In particular, the receive beamformer  119  employs various time delays to focus the receiving capability of the transducer  106  on a particular position in the medium  113 . In the usual case, this position follows the propagation of the ultrasonic pulse  109  along a predetermined axis to obtain echo information about the medium along that particular axis. 
     The echo signal generated by the receive beamformer  119  is then provided to both an acoustic line memory  123  and an adder  126 . The output of the acoustic line memory  123  is also applied to the adder  126 . The output of the adder  126  is applied to an envelope detector  129  that generates a resulting echo envelope. The echo envelope is thereafter applied to a scan converter/display  133  that generates a rendering of the image from one or more echo envelopes, etc. 
     As the ultrasonic pulse  109  progresses through the medium  113 , the various regions or structures within the medium  126  cause echoes  116  to scatter back to the transducer  106 . Generally, a single ultrasonic pulse  109  generates echoes  116  that last as long as the pulse propagates through the medium  113 . Note that the medium  13  may be comprised of different matter at various densities and states that may have various linear and non-linear responses. 
     The medium  113  may be, for example, a portion of a human body, etc. In some cases, a contrast agent may be added to, for example, the blood stream in a human body that gives the blood stream a strong non-linear response. Such a contrast agent may be, for example, a microbubble contrast agent in which microscopic air bubbles, or microbubbles, are injected into the blood. Microbubbles tend to have a strong nonlinear response to ultrasonic pulses  109  due in part to the characteristics surrounding the compressibility of air bubbles. As a consequence, blood that is profuse with microbubbles would have a strong nonlinear response when stimulated by the ultrasonic pulse  109 . If this stronger nonlinearity could be isolated from weaker nonlinearities and from relatively linear responses of the tissue and other structures surrounding the blood, then it would be possible to generate a rendering on the display  133  of the blood itself, thereby gaining valuable insight as to the nature of the blood flow at various points in the human body, etc. Even without such contrast agents, the weaker nonlinear propagation sound waves generate echoes that can produce a higher quality image if the linear responses are suppressed. 
     With reference to FIGS. 2A-2D, shown are waveforms that are employed with the ultrasonic imaging system  101  (FIG.  1 ). The waveforms are discussed to illustrate the operation of the ultrasonic imaging system  101  using a phase inversion technique to isolate nonlinearities in the medium  113  (FIG.  1 ). First, in FIG. 2A shown is graph of the pressure of a first ultrasonic pulse  109   a  in terms of time. Note that the first ultrasonic pulse  109   a  is often symmetrical above and below the zero axis. Upon transmitting the first ultrasonic pulse  109   a  into the medium  113 , a first echo signal is generated as previously discussed and stored in the acoustic line memory  123  (FIG.  1 ). 
     Next, with reference to FIG. 2B, a second ultrasonic pulse  109   b  is transmitted into the medium  113  that is 180° out of phase with the first ultrasonic pulse  108   a . A second echo signal is generated from the second ultrasonic pulse  109   b . The first echo signal stored in the acoustic line memory  123  and the second echo signal are both applied to the adder  126 , thereby generating a nonlinear signal  143  as shown in FIG.  2 C. In effect, the linear portion of the first echo signal cancels the linear portion of the second echo signal due to the fact that the second ultrasonic pulse  109   b  is 180° out of phase with the first ultrasonic pulse  108   a , resulting in the nonlinear signal  143 . The nonlinear signal  143  has a frequency that is twice that of the ultrasonic pulses  109   a  and  109   b  which accounts for the fact that much of the nonlinear response is found at the second harmonic of the base frequency of the ultrasonic pulses  109   a  and  109   b.    
     Thereafter, as shown in FIG. 2D, this nonlinear signal  143  is applied to the envelope detector  129  (FIG. 1) that generates a nonlinear envelope signal  146  that is applied to the scan conversion and display  133 . 
     The phase inversion technique detailed above is not without significant problems. One such problem relates to the fact that phase inversion techniques process “radio frequency” (RF) waveforms that are generally high frequency in nature. In the context of ultrasound, the term “RF waveforms” generally refers to echo signals as they exist before envelope detection so that they are of a relatively high frequency with high frequency peaks such as is the case with the first and second ultrasonic pulses  109   a  and  109   b . The RF waveforms employed make phase inversion techniques susceptible to adverse effects due to slight movement in the medium  113 , such as is the case with a beating heart in a human body, etc. In these situations, the second echo signal will differ from the first due to the movement and, consequently, the linear cancellation is adversely effected because a slight misalignment results. 
     Attempts to address this misalignment problem are seen in pulse inversion Doppler techniques that use more than two ultrasonic pulses along with elaborate filters, etc. Such techniques attempt to address the misalignment problem but create an additional problem in that more acoustic lines are transmitted and received thereby reducing the frame rate adversely. 
     With reference to FIG. 3, shown is an ultrasound imaging system  200  according to an embodiment of the present invention. The ultrasound imaging system  200  may generally be any type of ultrasound imaging system, for example but not limited to, a brightness-mode (B-mode) system, a Doppler-based imaging system, a color flow imaging system, etc. 
     In one embodiment, the ultrasound imaging system  200  includes an asymmetric transmitter  203  that generates an excitation signal that is applied to a transducer  206 . In response, the transducer  206  generates an asymmetric ultrasonic pulse  209 . Note that the excitation signal generated by the asymmetric transmitter  203  may not actually appear asymmetric, however, the excitation signal generated by the asymmetric transmitter  203  ultimately results in an asymmetric ultrasonic pulse  209  when applied to an appropriate transducer  206 . 
     The asymmetric ultrasonic pulse  209  then propagates through the medium  113 , resulting in echoes  213  that propagate back to the transducer  206 . The transducer  206  generates echo signals from the received echoes that are applied to a receive beamformer  216 . Note that in the transducer  206  is a phased array transducer that generates multiple echo signals from a number of elements in the array, however, other suitable types of transducers other than phased array transducers may be employed as well. The receive beamformer  216  generates a summed echo signal from the echo signals by applying appropriate delays to the various echo signals to focus on echoes from particular points in the medium as desired, such as along the axis of propagation of the asymmetric ultrasonic pulse  209 . The receive beamformer  216  applies the summed echo signal to the envelope detector  219  that generates an echo envelope signal therefrom. The echo envelope signal is applied to both an acoustic line memory  223  and an arithmetic unit acting as a subtractor  226 . The output of the subtractor  226  is applied to a scan conversion and display  229  that generates an ultrasonic image. 
     Note that the various components of the functional block diagram of FIG. 3 may also include other components between and around those shown, where the components shown are to facilitate the explanation of the present invention. Thus, it is understood that the various embodiments of the present invention are not limited to configurations with only those components shown in FIG.  3 . 
     Turning to FIG. 4, shown is a graph of an example of the asymmetric ultrasonic pulse  209 . The asymmetric ultrasonic pulse  209  is generally characterized by at least one extended pressure peak  236  on a first side  243  of the zero pressure axis  239  and at least one shallow pressure peak  246  on a second side  249  of the zero pressure axis  239 , where various numbers of the extended and shallow pressure peaks  236  and  246  may be employed. Thus, it may be possible that the asymmetric ultrasonic pulse  209  include a number of extended pressure peaks  236  on the first side  243  and a number of shallow pressure peaks  246  on the second side  249 . 
     Regardless of the number of extended and shallow pressure peaks  236  and  246 , it is understood that the extended pressure peak(s)  236  should have a magnitude that is greater than the magnitude of the shallow pressure peak(s)  246 . The greater the difference in these magnitudes, then the resulting nonlinear response will be correspondingly greater and more distinct as will be discussed. For the best results, the extended pressure peak(s)  236  preferably exceed the magnitude of the shallow pressure peak(s)  246  by a factor of at least two, although any ratio may ultimately be used if the results obtained are adequate for the specific application. Since the shape of the asymmetric ultrasonic pulse  209  represents a pressure perturbation from the ambient pressure, the asymmetric ultrasonic pulse  209  is considered to have a zero average value. This generally implies that the extended pressure peak(s)  236  will have a shorter time extent than the shallow pressure peak(s)  246 . 
     With reference to FIG. 5A, shown is a first echo envelope signal  253  generated by the envelope detector  219 . The first echo envelope signal  253  results from a first asymmetric ultrasonic pulse  209   a  (FIG. 4) applied to the transducer  206  (FIG.  3 ). Note the first echo envelope signal  253  is relatively smooth as the portion shown is from a relatively small duration of time, where the echo signals from which the first echo envelope signal  253  may be generated from a portion of the medium  113  that is only a few millimeters thick. However, this portion is shown to clearly illustrate the features of the present invention that follow. 
     In particular, the first echo envelope signal  253  includes linear portions  256  and a nonlinear portion  259 . The nonlinear portion  259  may result, for example, from echoes received from a blood vessel in the medium which contains blood that is profuse with the microbubble contrast agent as discussed previously. During the operation of the ultrasonic imaging system  200  (FIG.  3 ), the first echo envelope signal  253  is generated and then stored in the acoustic line memory  223 . 
     Thereafter, with reference to FIG. 5B, a second echo envelope signal  263  is generated in rapid succession of the first echo envelope signal  253  (FIG.  5 A). Although the second echo envelope signal  263  is shown as have a positive magnitude, the second echo envelope signal  263  is generated from a second asymmetric ultrasonic pulse  209   b  that is an inverted version of the first asymmetric ultrasonic pulse  209   a . The second echo envelope signal  263  detected is generally the absolute value of the second echo signal received. Note that since the polarity of the second asymmetric ultrasonic pulse  209   b  is inverted from the first asymmetric ultrasonic pulse  209   a , the linear portions  266  are approximately equal to the linear portions  256  of the first echo envelope signal  253 . However, since the magnitudes of the first and second asymmetric ultrasonic pulses  209   a  and  209   b  differ substantially due to the fact that the second is an inverted copy of the first, the nonlinear portions  259  and  269  will differ substantially. 
     With reference to FIG. 5C, once the second echo envelope signal  263  (FIG. 5B) is generated, both the first and second envelope signals  253  (FIG. 5A) and  263  are applied to the subtractor  226  (FIG.  3 ). The resulting output of the subtractor  226  is a nonlinear echo signal  273  due to the cancellation of linear portions. The nonlinear echo signal  273  is applied to the scan conversion and display  229  (FIG. 3) and an appropriate image generated therefrom. 
     The ultrasonic imaging system  200  provides a significant advantage over the phase inversion imaging systems in that linear cancellation is achieved with much less susceptibility to error due to slight movement in the medium  113 . In particular, since the echo envelopes are taken before linear cancellation, slight differences in the echo signals due to changes in position of the medium  113 , such as, for example the position of a wall of a beating heart, do not obscure the linear cancellation to a great degree. In the case of a beating heart, it is preferable that the second asymmetric ultrasonic pulse  209   b  follows the first asymmetric ultrasonic pulse  209   a  as closely as possible to provide for minimum movement of the heart walls therebetween. Nonetheless, slight movement is practically impossible to avoid due to the maximum speed of sound, etc. Advantageously, the present invention overcomes the problem of degradation of linear cancellation due to such movement. 
     Turning back to FIG. 3, the various functions of several of the blocks indicated may be accomplished via a hardware implementation using a dedicated logical circuit of various digital components, via a software program using a high speed processor or multiple high speed processors, or via an optimized combination of dedicated logical circuits and processor circuits executing various modules of a software program. It is understood that all such permutations are included within the scope of the present invention. Note that an appropriate display device may be employed in the scan conversion and display  229  such as a cathode ray tube (CRT) or other suitable device. 
     In addition, any software program of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the preferred embodiment(s), the program is implemented in software or firmware that is stored in a memory and that is executed by a suitable instruction execution system. If implemented in hardware, the present invention may comprise, for example, a dedicated logical circuit on an application specific integrated circuit (ASIC) or a combination of discrete logic components, etc. 
     Also, any software program, which comprises an ordered listing of executable instructions for implementing logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a nonexhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM or Flash memory) (magnetic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
     Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention.