Patent Publication Number: US-2005124895-A1

Title: Ultrasonic speckle reduction using nonlinear echo combinations

Description:
This invention claims the benefit of Provisional U.S. Patent Application Ser. No. 60/527,538, filed Dec. 5, 2003. 
    
    
      This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which reduce image artifacts in nonlinear imaging.  
      In ultrasonic harmonic imaging, two dimensional (2D) or three dimensional (3D) images are formed by transmitting ultrasound at one frequency (or range of frequencies) and receiving at the higher harmonics of the transmit frequency. These harmonic signals are generated either by scattering from microbubbles of a harmonic contrast agent as described in U.S. Pat. No. 5,833,613 (Averkiou et al.) or by non-linear propagation in tissue (tissue harmonic imaging, or THI) as described in U.S. Pat. No. 5,879,303 (Averkiou et al.) Typically, receive beams are formed predominantly from the second harmonic echo signals, with signals at the transmitted (or “fundamental”) frequency being removed either by filtering or by cancellation techniques such as pulse inversion. See U.S. Pat. No. 5,951,478 (Hwang et al.)  
      Due to the coherent nature of ultrasonic waves, ultrasound images contain an artifact known as speckle. The speckle artifact results from acoustic interaction of differently phased signals within the medium being imaged. The phenomenon occurs in both fundamental frequency imaging and in harmonic imaging. Two techniques have been developed to reduce the speckle artifact. One technique is known as frequency compounding, and is described in U.S. Pat. No. 4,561,019 (Lizzi et al.) With frequency compounding, echo signals from each point in the image field are separated into different frequency bands, either by transmit frequency modulation or receive frequency separation. The separate frequency bands are detected then combined to reduce the speckle artifact, as the different frequency bands will exhibit different speckle characteristics. Combining the detected signals will average out the speckle artifact, reducing its appearance in the image.  
      The other technique for reducing speckle is spatial compounding which is described in U.S. Pat. No. 6,210,328 (Robinson et al.) Each point in the image field is insonified from multiple different look directions. The returning echoes from the different look directions are detected and combined to average out the speckle artifact. This reduction in speckle is due to the differing speckle characteristics of ultrasound which has undergone different transmission paths in the medium.  
      One approach for reducing speckle in harmonic imaging is described in U.S. Pat. No. 6,206,833 (Christopher). In this patent the inventor proposes to form an image which is the sum of both a fundamental frequency image and its corresponding second harmonic image. Since the speckle patterns of the two images are to a certain extent out of phase, the sum image will exhibit reduced speckle. This approach however will contaminate the harmonic image with clutter from the fundamental image, clutter that harmonic imaging eliminates. It would be desirable to be able to reduce speckle in harmonic images without the need for the fundamental signal, which is many dB stronger than the second harmonic signal and is often contaminated with multipath clutter. It would also be desirable to reduce speckle in nonlinear imaging through processing which do not require extensive or complicated bandpass filtering for signal separation.  
      In accordance with the principles of the present invention, echo signals from transmit sequences of differently modulated transmit signals are combined in different ways to produce nonlinear components with different speckle characteristics. The nonlinear components are combined to produce an image with reduced speckle content. Unwanted linear fundamental frequency components are eliminated by signal processing techniques such as pulse inversion and power modulation and their combinations, obviating the need for bandpass filtering. 
    
    
      In the drawings:  
       FIG. 1  illustrates in block diagram form an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention;  
       FIGS. 2   a ,  2   b  and  2   c  illustrate a pulse sequence and combining circuits for producing two nonlinear signals by pulse inversion;  
       FIG. 2   d  illustrates a frequency spectrum of nonlinear signals separated by pulse inversion;  
       FIGS. 3   a ,  3   b  and  3   c  illustrate a pulse sequence and combining circuits for producing two nonlinear signals by power modulation;  
       FIG. 3   d  illustrates a frequency spectrum of nonlinear signals separated by power modulation;  
       FIGS. 4   a ,  4   b , and  4   c  illustrate a pulse sequence and combining circuits for producing two nonlinear signals by a combination of power modulation and pulse inversion;  
       FIG. 4   d  illustrates a frequency spectrum of nonlinear signals separated by a combination of power modulation and pulse inversion; and  
       FIGS. 5   a - 5   f  illustrate a pulse sequence and combining circuits for producing five different nonlinear signals by pulse inversion, power modulation, and a combination of pulse inversion and power modulation. 
    
    
      Referring first to  FIG. 1 , an ultrasound system constructed in accordance with the principles of the present invention is shown in block diagram form. This system operates by scanning a two or three dimensional region of the body being imaged with ultrasonic transmit beams. As each beam is transmitted along its steered path through the body, the beam returns echo signals with linear and nonlinear (fundamental and harmonic) components corresponding to the transmitted frequency components. The transmit signals are modulated by the nonlinear effects of the tissue through which the beam passes or the nonlinear response of a contrast agent microbubble encountered by the beam, thereby generating echo signals with harmonic components.  
      The ultrasound system of  FIG. 1  utilizes a transmitter  16  which transmits waves or pulses of a selected modulation characteristic in a desired beam direction for the return of harmonic echo components from scatterers within the body. The transmitter is responsive to a number of control parameters which determine the characteristics of the transmit beams as shown in the drawing, including the frequency components of the transmit beam, their relative intensities or amplitudes, and the phase or polarity of the transmit signals. The transmitter is coupled by a transmit/receive switch  14  to the elements of an array transducer  12  of a scanhead  10 . The array transducer can be a one dimensional array for planar (two dimensional) imaging or a two dimensional array for two dimensional or volumetric (three dimensional) imaging.  
      The transducer array  12  receives echoes from the body containing linear and harmonic (nonlinear) frequency components which are within the transducer passband. These echo signals are coupled by the switch  14  to a beamformer  18  which appropriately delays echo signals from the different transducer elements then combines them to form a sequence of linear and harmonic signals along the beam from shallow to deeper depths. Preferably the beamformer is a digital beamformer operating on digitized echo signals to produce a sequence of discrete coherent digital echo signals from a near field to a far field depth of field. The beamformer may be a multiline beamformer which produces two or more sequences of echo signals along multiple spatially distinct receive scanlines in response to a single transmit beam, which is particularly useful for 3D imaging. The beamformed echo signals are coupled to an ensemble memory  22   
      In accordance with the principles of the present invention, multiple waves or pulses are transmitted in each beam direction using different modulation techniques, resulting in the reception of multiple echoes for each scanned point in the image field. The echoes corresponding to a common spatial location are referred to herein as an ensemble of echoes, and are stored in the ensemble memory  22 , from which they can be retrieved and processed together. The echoes of an ensemble are combined in various ways as described more fully below by the nonlinear signal separator  24  to produce the desired nonlinear or harmonic signals. The separated signals are filtered by a filter  30  to further remove unwanted frequency components, then subjected to B mode or Doppler detection by a detector  32 . The detected signals are coupled to a nonlinear signal combiner  34  to reduce image speckle content, as described more fully below. The signals are then processed for the formation of two dimensional, three dimensional, spectral, parametric, or other desired image in image processor  36 , and the image is then displayed on a display  38 .  
       FIG. 2   a  illustrates a sequence of differently modulated transmit pulses (“P”) which are transmitted along a beam direction. The subscript of each pulse P indicates the position of the pulse in the sequence. These subscripts are only necessary to clarify the following description, as the pulses in a sequence can be transmitted in any order. The parenthetical of each pulse P indicates the relative amplitude and phase or polarity of a given pulse. In the sequence of  FIG. 2   a  the first transmit pulse P 1 (+1) is seen to have an amplitude of “one”, and a positive phase or polarity relative to other pulses in the sequence. The second pulse P 2 (−1) also has an amplitude of one but a phase or polarity which is the inverse of the first pulse. The third pulse in the time sequence is seen to have an amplitude of one and a positive phase or polarity. Thus it is seen that the second pulse is differently modulated (in phase or polarity) relative to the other two pulses in the sequence.  
      Echoes are received along the beam direction in response to each pulse, resulting in an ensemble of three echoes (“E”) at each sample point of the beam. The echoes of the ensembles are combined in different ways by the nonlinear signal separator  24 . In the signal separator circuit  40  of  FIG. 2   a , echo E 1 (+1) from the first pulse is weighted by a weight of 0.5 in weighting circuit W 1  and applied to a summer  42 . Echo E 2 (−1) from the second pulse is weighted by a weight of 0.5 in weighting circuit W 2  and also applied to summer  42 , where the two weighted echoes are combined. Since the two echoes are from pulses of opposite phase or polarity and of equal amplitude, the equally weighted combining of the echoes results in cancellation of fundamental signal components of the echoes returned from stationary targets and reinforcement of the nonlinear (second and higher order even harmonic) signal components, a phenomenon known in the art as pulse inversion. See U.S. Pat. Nos. 5,706,819 (Hwang) and 5,951,478 (Hwang et al.) The resultant nonlinear signals are denoted as PI 1 , indicating that these nonlinear signals were separated by a first pulse inversion combination. In the case of moving scatterers such as contrast agent microbubbles pulse inversion processing also produces signals from the motion of microbubbles that lie mostly in the fundamental frequency band.  
       FIG. 2   c  illustrates a second signal separator circuit  44  which also separates nonlinear signals from fundamental frequency components by the pulse inversion technique. The echo E 1 (+1) is weighted by a weighting factor of 0.25 in weighting circuit W 1  and applied to a summer or combiner  46 . Echo E 2 (−1) is weighted by a weighting factor of 0.5 in weighting circuit W 2  and also applied to summer  46 . Echo E 3 (+1) from the third pulse is weighted by a weighting factor of 0.25 in weighting circuit W 3  and also applied to summer  46 . Like the weights of the signal separator circuit  40 , the weights of this signal separator circuit are also normalized to a sum of one. The one-quarter weightings of the echoes from the positive phase or polarity pulses when combined with the one-half weighting of the echo from the negative phase or polarity pulse P 2 (−1) results in pulse inversion separation of nonlinear signals PI 2  with suppression of the fundamental frequency components of the echoes from stationary targets and reinforcement of the nonlinear (second harmonic) signal components. Thus it is seen that the two signal separator circuits both produce nonlinear signal components and flow components from a given point in an image field but by different pulse inversion signal combinations. The different receive weights cause PI 1  and PI 2  to detect different velocities of moving scatterers.  
       FIG. 2   d  illustrates a typical frequency spectrum of the signals separated by pulse inversion (PI 1  or PI 2 ). This frequency spectrum is seen to be dominated by a major peak response  48  at the second harmonic and a lesser peak  49  at the fourth harmonic.  
       FIG. 3   a  illustrates another pulse sequence in which the pulses are differently modulated in amplitude. The first and third pulses P 1 (+0.5) and P 3 (+0.5) are each seen to have an amplitude of one-half relative to the amplitude of the second pulse P 2 (+1). All of the pulses are seen to exhibit the same (positive) phase or polarity. Each three-echo ensemble is then processed as shown by the signal separator circuits  50  and  54 . Circuit  50  weights an echo E 1 (+0.5) from the first pulse by a weight of 2 in weighting circuit W 1  and applies the weighted echo to the summer  52 . The echo E 2 (+1) from the second pulse is weighted by a weight of −1 in weighting circuit W 2  and applied to the summer  52 . The combined weightings of the echoes from the differently amplitude modulated (power modulated) pulses results in separation of nonlinear signal components PM 1  by the power modulation technique. See U.S. Pat. No. 5,577,505 (Brock Fisher et al.)  
      In  FIG. 3   c  nonlinear components are again separated by the power modulation technique, but this time using three echo signals. The echo E 1 (+0.5) is weighted by a weight of 1 in weighting circuit W 1  and applied to summer  56 . The echo E 2 (+1) is weighted by a weight of −1 and applied to the summer  56 . The echo E 3 (+0.5) is weighted by a weight of 1 in weighting circuit W 3  and applied to the summer  56 . The combination of the three weighted, differently amplitude modulated signals results in another nonlinear signal PM 2  separated by a different power modulation combination of echoes. The two nonlinear signals PM 1  and PM 2  will exhibit a frequency spectrum such as that illustrated in  FIG. 3   d , which is seen to be characterized by a major response peak  58  at the second harmonic and lesser peaks  59  at the fundamental (first) and third and fourth harmonics.  
       FIG. 4   a  illustrates a sequence of transmit pulses for a given beam direction which are differently modulated in both amplitude and phase or polarity. The first and third pulses P 1 (+0.5) and P 3 (+0.5) are both seen to have a positive phase or polarity and a relative amplitude of one-half. The second pulse P 1 (−1) is seen to exhibit an inverse phase or polarity and an amplitude of one, which is twice that of the first and third pulses. Various echo combinations can be formed to separate nonlinear or harmonic components by the combined technique referred to herein as power modulation/pulse inversion (PMPI).  FIG. 4   b  shows a signal separator circuit  60  in which echo E 1 (+0.5) is weighted by a weight of 2 and combined in summer  62  with echo E 2 (−1) which is weighted by a weight of 1. The amplitude difference of the two echoes is equalized by the weighting factors and the differently phased echoes combine to produce a first nonlinear signal PMPI 1  by a combination of pulse inversion and power modulation. In  FIG. 4   c  the three echoes of an ensemble are each weighted by a weight of one and combined by summer  66  of signal separator circuit  64  to produce a second nonlinear signal PMPI 2 . The signals produced by the different summations of PMPI modulated signals will produce a frequency spectrum such as that shown in  FIG. 4   d , which is seen to be characterized by a major response peak  68  at the third harmonic and a lesser peak  69  at the fundamental (first) harmonic.  
      In accordance with the principles of the present invention, echoes returned from microbubbles which have been differently processed by the PI, PM and PMPI techniques described above to yield signals with differing spectra such as those shown in  FIGS. 2   d ,  3   d , and  4   d  are combined to reduce the speckle content of an ultrasonic contrast image.  FIGS. 5   a - 5   c  illustrate an embodiment of the present invention in which a transmit sequence of five pulses is employed in each beam direction, resulting in ensembles of five echoes for each sample point in the image field. As  FIG. 5   a  illustrates, the first and fifth pulses P 1 (+0.5) and P 5 (+0.5) both exhibit the same phase or polarity as well as the same amplitude. The other three pulses P 2 (−1), P 3 (+1) and P 4 (−1) all have an amplitude which is twice that of the first and last pulses. The third pulse exhibits the same phase or polarity as the first and last pulses and the second and fourth pulses are of an inverse (opposite) phase or polarity.  
      In  FIGS. 5   b - 5   f , echoes from the resulting five-echo ensembles are combined for harmonic separation in five different ways, using pulse inversion (PI), power modulation (PM), and power modulation/pulse inversion (PMPI). In the signal separator circuit  70  of  FIG. 5   b  echoes from the second, third, and fourth pulses are weighted by respective weights of 1, 2, and 1 and combined to separate nonlinear signal components PI 1  by pulse inversion. These signal components include second and fourth harmonic components of the transmitted fundamental frequencies, of which the second harmonic is the dominant signal (see  FIG. 2   d ). In the signal separator circuit  72  of  FIG. 5   c  echoes from the second and third pulses are weighted equally and combined, again separating nonlinear signal components PI 2  by pulse inversion. These signal components also include second and fourth harmonic components of the transmitted fundamental frequencies, of which the second harmonic is the dominant signal. In the signal separator circuit  74  of  FIG. 5   d  echoes from the first, third and fifth pulses are combined with respective weights of 1, −1, and 1, resulting in the production of nonlinear signal components PM by power modulation. The separated signal components include the first, second, third and fourth harmonics, of which the second harmonic is the greatest contributor and the first, third and fourth harmonics are lesser contributors (see  FIG. 3   d ). In  FIG. 5   e  echoes from the first, second and fifth pulses are equally weighted and combined by a signal separator circuit  76  to produce nonlinear signal components PMPI 1  by the combined PMPI technique. These signal components include the first through the fourth harmonics, of which the first and third harmonics are the major contributors (see  FIG. 4   d ). In  FIG. 5   f  a signal separator circuit  78  operates on echoes from all five pulses. Echoes from the first and last pulses, which exhibit the lesser amplitudes, are weighted by −8. Echoes from the second, third, and fourth pulses are weighted by weights of −1, 6, and −1, respectively. This combination will result in nonlinear signal components PMPI 2  by the combined technique, and include the first, second, and third harmonics, of which the first and third are predominant.  
      It is seen from the preceding examples that the various separated nonlinear signals are dominated by varying frequency components. Thus, the signals have differing frequency content. As a consequence, when these five signals are combined by the nonlinear signal combiner  34 , speckle reduction will occur by a frequency compounding effect.  
      In a constructed embodiment of the present invention it is often preferable to combine the echo signals, not with dedicated hardware separator circuits, but mathematically in a matrix operation. Using the previous five-pulse embodiment as an example, the transmit matrix would be of the form  
       [           0.5   ,             -   1     ,           1   ,             -   1     ,         0.5         ]       
 
 and the receive matrix would be of the form  
         [           0   ,           1   ,           2   ,           1   ,         0             0   ,           1   ,           1   ,           0   ,         0             1   ,           0   ,             -   1     ,           0   ,         1             1   ,           1   ,           0   ,           0   ,         1             -   8           -   1         6         -   1           -   8           ]             
 
 The desired signals are produced by multiplication of matrices of this form. Since the different combining techniques extract different nonlinear components, the combination of their results will produce a frequency compounded image with reduced image speckle. 
 
      It will be understood that weights other than 0.5 and 1 may be used, and phases other than 0 and p may be used. The specific transmit sequence used will be determined at least in part by the desired harmonic content to be obtained. The relative content of the different harmonics introduced according to the receive processing may be scaled so that different effects are emphasized. For the matrix representation above a different scaling may be applied to various rows of the matrix. If for example it is desired to emphasize the relative effect of pulse inversion by a factor of two, then the above matrix would become  
         [           0   ,           2   ,           4   ,           2   ,         0             0   ,           2   ,           2   ,           0   ,         0             1   ,           0   ,             -   1     ,           0   ,         1             1   ,           1   ,           0   ,           0   ,         1             -   8           -   1         6         -   1           -   8           ]