Abstract:
An ultrasonic diagnosis apparatus includes an ultrasonic probe. A transmitter supplies a transmission pulse to the ultrasonic probe to repeatedly transmit an ultrasonic wave to each of a plurality of scanning lines. A receiver receives echoes of the ultrasonic waves through the ultrasonic probe and obtaining a plurality of received signals for each of the plurality of scanning lines. A displacement estimating means estimates a relative change accompanying a tissue motion between received signals associated with each of scanning lines. A displacement correcting means corrects the received signals in accordance with the change detected by the displacement estimating means. A harmonic component extracting means extracts a harmonic component from the received signals corrected by the displacement correcting means. A display means generates an ultrasonic image on the basis of the harmonic component extracted by the harmonic component extracting means. A monitor displays the image generated by the display means.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-108097, filed Apr. 10, 2000, the entire contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to an ultrasonic diagnosis apparatus for extracting harmonic components generated by nonlinear propagation of the living tissue and the nonlinear response of a contrast medium (microbubbles) from a received signal, and generating an ultrasonic image on the basis of the harmonic components.  
           [0003]    Imaging based on harmonic components can substantially narrow a beam as compared with imaging based on fundamental wave components that spread from the center frequency of a transmission ultrasonic wave, and hence can realize high resolution. In addition, a certain sound pressure is required to generate harmonic components, imaging based on harmonic components can reduce side lobes. As a method of imaging harmonic components originating from the tissue, THI (Tissue Harmonic Imaging) is available.  
           [0004]    A filter method is a typical method of extracting nonlinear components from a received signal. As a method of extracting harmonic components in a wider band, a pulse inversion method is available, in which two ultrasonic pulses having opposite polarities are transmitted at two rates, and signals received at the respective rates are added to cancel out a fundamental wave component, thereby extracting harmonic components as disclosed in Iwao Abiru and Tomoo Kamakura, “Nonlinear Propagation of Ultrasonic Pulses” (Technical Report of IEICE, US89-23, p. 53). This method uses the phenomenon that harmonic components are generated in proportion to the square of a fundamental wave. More specifically, since the phases of fundamental wave components maintain the phase of a transmission ultrasonic wave, the first rate fundamental wave component appears in opposite polarity to the second rate fundamental wave component. Therefore, the fundamental wave components cancel out each other. On the other hand, harmonic components are generated in proportion to the square of a fundamental wave, and hence the first rate harmonic component and second rate harmonic component appear in the same polarity. Therefore, the harmonic components are amplified.  
           [0005]    The fundamental wave elimination characteristic of the above pulse inversion method is based on the premise that the tissue serving as a propagation medium is still. If, therefore, a moving organ such as the heart is imaged by the pulse inversion method, the received signal waveform and relative positions (depths) of the received signal changes between the two rates in accordance with the motion of the organ. As a consequence, fundamental wave components are left to cause motion artifacts on an image.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    It is an object of the present invention to eliminate motion artifacts due to the motion of tissue such as the heart, in particular, in an ultrasonic diagnosis apparatus for repeatedly transmitting an ultrasonic wave, extracting harmonic components from a plurality of received signals obtained by the transmission of the ultrasonic wave, and generating images on the basis of the harmonic components.  
           [0007]    An ultrasonic diagnosis apparatus includes an ultrasonic probe, a transmitter configured to supply a transmission pulse to the ultrasonic probe to repeatedly transmit an ultrasonic wave to each of a plurality of scanning lines, and a receiver configured to receive echoes of the ultrasonic waves through the ultrasonic probe and obtaining a plurality of received signals for each of the plurality of scanning lines. A displacement estimating means estimates a relative change accompanying a tissue motion between a plurality of received signals associated with each of the plurality of scanning lines. A displacement correcting means corrects the plurality of received signals in accordance with the change detected by the displacement estimating means. A harmonic component extracting means extracts a harmonic component from the plurality of received signals corrected by the displacement correcting means. A display means generates an ultrasonic image on the basis of the harmonic component extracted by the harmonic component extracting means. A monitor displays the image generated by the display means.  
           [0008]    According to the present invention, harmonic components can be extracted in a broad band even from moving organs such as the heart, in particular, and high-quality images unique to harmonic images can be provided for clinical sites.  
           [0009]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0010]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
         [0011]    [0011]FIG. 1 is a block diagram showing the arrangement of an ultrasonic diagnosis apparatus according to a preferred embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a block diagram showing the arrangement of a harmonic processor in FIG. 1;  
         [0013]    [0013]FIGS. 3A and 3B are graphs showing examples of two types of ultrasonic pulses which have opposite polarities and are transmitted in accordance with the pulse inversion method in this embodiment;  
         [0014]    [0014]FIG. 4A is a graph showing how a fundamental wave is canceled by addition according to the pulse inversion method in this embodiment;  
         [0015]    [0015]FIG. 4B is a graph showing how harmonic components is amplified by addition according to the pulse inversion method in this embodiment;  
         [0016]    [0016]FIG. 4C is a graph showing a state where a fundamental wave is not canceled in the present of a tissue displacement in this embodiment;  
         [0017]    [0017]FIG. 4D is a graph showing extracted harmonic components in this embodiment;  
         [0018]    [0018]FIG. 5 is a flow chart showing a procedure for extracting harmonic components without any motion artifact in this embodiment;  
         [0019]    [0019]FIG. 6 is a view showing complex signals at the same depth between two rates in this embodiment;  
         [0020]    [0020]FIG. 7 is a view for a supplementary explanation of moving average processing in this embodiment;  
         [0021]    [0021]FIG. 8 is a view for a supplementary explanation of processing for the calculation of a displacement at each depth with the use of a wavelength table in this embodiment;  
         [0022]    [0022]FIGS. 9A and 9B are graphs for a supplementary explanation of filtering processing for the extraction of low-frequency components in a narrow band which is performed before phase estimation in this embodiment;  
         [0023]    [0023]FIGS. 10A and 10B are views showing cross-correction between two rates in this embodiment;  
         [0024]    [0024]FIG. 11 is a view for a supplementary explanation of processing of recording a complex value corresponding to displacement correction in an other rate memory in this embodiment;  
         [0025]    [0025]FIG. 12 is a view for a supplementary explanation of processing of recording a complex value when an estimated displacement is smaller than a sampling interval;  
         [0026]    [0026]FIG. 13 is a view for a supplementary explanation of processing of realizing correction of a displacement smaller than a sampling interval by using an interpolation method in this embodiment;  
         [0027]    [0027]FIG. 14 is a supplementary explanation of processing of correcting a displacement by multiplying a complex value representing a phase term at each depth in this embodiment;  
         [0028]    [0028]FIGS. 15A to  15 C are graphs showing the difference between the displacement and product of phase terms on the frequency axis in this embodiment;  
         [0029]    [0029]FIG. 16 is a view showing a displacement corresponding to a sampling interval unit and a displacement smaller than the unit;  
         [0030]    [0030]FIG. 17 is a graph showing the pulse inversion method in which displacement correction is performed in this embodiment;  
         [0031]    [0031]FIGS. 18A to  18 C are flow charts showing variations of quadrature phase detection processing, motion correction processing, and addition processing in this embodiment;  
         [0032]    [0032]FIG. 19 is a flow chart showing variations of quadrature phase detection processing, motion correction processing, and addition processing in this embodiment;  
         [0033]    [0033]FIGS. 20A to  20 C are graphs showing signal components based on rate differences in this embodiment;  
         [0034]    [0034]FIG. 21 is a graph showing a rate difference after displacement correction in this embodiment;  
         [0035]    [0035]FIG. 22 is a graph showing the pulse inversion method in which displacement correction is performed when a permanent echo and motion echo from the tissue are present in this embodiment;  
         [0036]    [0036]FIG. 23 is a flow chart showing a procedure for extracting harmonic components while eliminating permanent echoes and motion echoes by using three rates in this embodiment;  
         [0037]    [0037]FIGS. 24A to  24 C are graphs showing received signal components in the three-rate pulse inversion method in this embodiment;  
         [0038]    [0038]FIGS. 25A and 25B are graphs showing the signal components of an addition signal in this embodiment;  
         [0039]    [0039]FIG. 26 is a graph for a supplementary explanation of filtering processing for the elimination of harmonic components in this embodiment;  
         [0040]    [0040]FIGS. 27A and 27B are graphs for a supplementary explanation of processing of performing displacement correction for addition signals and further adding them in this embodiment;  
         [0041]    [0041]FIGS. 28A to  28 C are graphs showing signal components based on rate differences in a three-rate difference method in this embodiment;  
         [0042]    [0042]FIGS. 29A and 29B graphs showing difference signal components in this embodiment;  
         [0043]    [0043]FIG. 30 is a graph for a supplementary explanation of filtering processing of eliminating harmonic components in this embodiment;  
         [0044]    [0044]FIG. 31 is a graph for a supplementary explanation of processing of performing displacement correction for difference signals and further subtracting them from each other in this embodiment; and  
         [0045]    [0045]FIGS. 32A to  32 C are graphs showing a plurality of rates in this embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0046]    An ultrasonic diagnosis apparatus according to a preferred embodiment of the present invention will be described below with reference to the views of the accompanying drawing.  
         [0047]    [0047]FIG. 1 shows an example of the arrangement of an ultrasonic diagnosis apparatus according to this embodiment. This ultrasonic diagnosis apparatus is comprised of an apparatus body  2  as a main component, an ultrasonic probe  1 , and a monitor  3 . In the FIG. 1, each processing of sections  5 - 11  is realized by a special purpose hardware or a software which can be carried out with a computer. In the apparatus body  2 , a transmitting pulse generator  13  applies a pulse voltage to a transmitting/receiving circuit  12 . The transmitting/receiving circuit  12  applies a driving voltage to the ultrasonic probe  1  having a plurality of vibrators arranged one- or two-dimensionally. The transmitting pulse generator  13  controls a transmission pulse voltage applied to a plurality of vibrators of the ultrasonic probe  1  to two- or three-dimensionally control the direction and convergence of an ultrasonic beam. This operation will be referred to as transmission beam forming hereinafter.  
         [0048]    An ultrasonic signal applied from the ultrasonic probe  1  connected to the apparatus body  2  into the body and reflected by an acoustic impedance boundary in the body tissue or an ultrasonic signal backscattered by a small scatterer is received by the same ultrasonic probe  1 . This received signal contains an echo from a harmonic component originating from the nonlinearity of vital propagation and an echo from a contrast medium. The received signal is sent to processors  6 ,  7 , and  9  via a preamplifier  4  and reception delay circuit  5 . The reception delay circuit  5  performs beam forming in receiving operation to control the directions and convergence of beams, and is comprised of a plurality of circuit sets BF 1 , BF 2 , . . . , BF n  to form a plurality of beams and perform parallel, concurrent reception.  
         [0049]    The echo processor  6  generates a two- or three-dimensional space distribution image corresponding to a signal amplitude intensity with a reference frequency after envelope detection. This image contains the two- or three-dimensional form information of an object to be examined. If a contrast medium is used for the object, the image contains form information and contrast medium information. The doppler processor  7  generates a two- or three-dimensional space distribution image of velocity, power, or variance by measuring a change in phase between received signals over time. This image contains blood flow rate information on the heart and its neighboring portion. The harmonic processor  9  extracts a fundamental wave harmonic component from a received signal. The signal processed by the respective processors is sent to a displaying unit  11  to be logarithmically compressed and scanned/converted. The resultant signal is displayed as an image on the monitor  3 .  
         [0050]    The above processors other than the harmonic processor  9  are commercially available products that have known arrangements and are based on known techniques. Since they are not directly relevant to the present invention, a detailed description thereof will be omitted.  
         [0051]    The harmonic processor  9  that is relevant to the present invention will be described with reference to FIG. 2. The received signal that is beam-formed and output from the reception delay circuit  5  is converted into a digital signal by an ADC  17 . This signal is subjected to quadrature phase detection in a quadrature phase detector  15  with the frequency of a fundamental wave. With this operation, a complex signal is obtained. This complex signal is stored in a buffer memory  18 . A displacement estimating unit  19 , displacement correcting unit  20 , and harmonic extracting unit  21  extract harmonic components from a plurality of ultrasonic signals with respect to a plurality of scanning lines which are stored in the buffer memory  18 . In actual signal processing, correction of an estimated displacement and signal processing for harmonic extraction may be simultaneously performed. The displacement correcting unit  20  and harmonic extracting unit  21  may therefore be integrated into one unit.  
         [0052]    If a coded ultrasonic or pulse compression technique is to be used, quadrature phase detection is not always required. The quadrature phase detector  15  may be bypassed or a signal may be passed therethrough without any processing. If the apparatus is exclusively designed for this technique, the quadrature phase detector may be omitted.  
         [0053]    (Signal Processing)  
         [0054]    The principle of actual extraction of harmonic components by nonlinear propagation of tissue without any motion artifact with the above arrangement will be described in detail below by taking general two-rate ultrasonic pulse transmission as an example. An ultrasonic pulse  32  in FIG. 3B, as one of pulses based on two rates, has an opposite polarity to an ultrasonic pulse  31  in FIG. 3A. In other words, the two types of pulses are 180° out of phase (opposite phases). As described above, the two types of ultrasonic pulses  31  and  32  are also used in the conventional pulse inversion method.  
         [0055]    [0055]FIGS. 4A to  4 D show echo signals at a depth of interest, among received signals that have returned to the ultrasonic probe after the respective ultrasonic pulses  31  and  32  were transmitted into the living body at the respective rates, propagated nonlinearly, and were reflected by acoustic impedance boundaries in the tissue or backscattered by small scatterers. If the motion (displacement) of the living body in rate intervals at the depth of interest is zero or negligible, there is no displacement (motion) between the received signals at the two rates. That is, no motion artifact occurs. FIG. 4A shows examples of echo signals each consisting of only the real part of the complex data of a fundamental wave component contained in a received signal. FIG. 4D is a graph in a frequency space. In this case, if the two signals are added as in the prior art, the fundamental wave components are completely eliminated. FIG. 4B shows only the real parts of harmonic components, which are extracted by adding, and the resultant signal amplitude is enhanced twice.  
         [0056]    When the heart is to be directly scanned or the liver or the like influenced by the movement of the heart is to be scanned, the tissue is displaced during a rate interval (the reciprocal of a pulse repeating frequency PRF). In this case, the two signals have the relationship shown in FIG. 4C. In this case, reference symbol Ad denotes the displacement of an echo signal near the depth of interest. This displacement corresponds to a value twice the distance displaced by the tissue in the rate interval.  
         [0057]    Even if these signals are simply added as in the prior art, the fundamental wave components are left as shown in FIG. 4C. In a frame or image portion in which the tissue moves, a fundamental wave component having a higher signal intensity than a harmonic component appears as a motion artifact, and it looks as if it were flashing on the image. According to an apex cordis approach, a motion artifact is noticeable at a basis cordis portion or cardiac apex portion which actively moves especially at the timing of systole or diastole.  
         [0058]    (Elimination of Motion Artifact)  
         [0059]    A processing method for the extraction of harmonic components instead of motion artifacts, which is the gist of the present invention, will be described next. FIG. 5 is a flow chart showing a signal processing procedure in outline.  
         [0060]    (Estimating Displacement  51 )  
         [0061]    First of all, the displacement estimating unit  19  measures (estimates) a relative displacement d at each depth between two rate signals. As will be described below, a plurality of methods are available as methods for this estimation, and any one of them can be used.  
         [0062]    According to the first method, as shown in FIG. 6, a complex value  61  of one rate signal at each depth d is multiplied by a complex conjugate value obtained by advancing the phase of a complex value  62  of the other rate signal at the corresponding depth by π (inverting a code consisting of only an imaginary number), and a phase argument θ of a multiplication result C. The reason why the phase is advanced by π is that these signals are received signals of pulses having opposite polarities.  
         [0063]    A method of calculating a complex vector product by performing similar processing between a plurality of rates is an auto-correlation method. This case can be regarded as a special case of the auto-correction method, in which the number of data is two.  
         [0064]    A displacement ΔdD can be obtained by normalizing the phase argument with 2π and calculating the product of the argument and the wavelength of a barycentric frequency component representing a fundamental wave component. The actual displacement in the living body is ½ the displacement obtained in this case. This is because a reflected signal is used as a received signal. Reference symbol A 1  denotes the amplitude of a first rate received signal; A 2 , the amplitude of a second rate received signal, λ, the wavelength of a barycentric frequency component.  
             RATE                 1               r   1          (   d   )       =         A   1          (   d   )            e     i                     θ   1          (   d   )                         RATE                 2               r   2          (   d   )              A   2          (   d   )            e     i                     θ   2          (   d   )                                 C        (   d   )       =         A   1          (   d   )            e     i                     θ   1          (   d   )           ×       A   2          (   d   )            e     -     i   (                    θ   2          (   d   )       +   π     )                       =         -       A   1          (   d   )         ·       A   2          (   d   )              e       i                     θ   1          (   d   )         -     i                     θ   2          (   d   )                             θ                   (   d   )       =     arg        {     C        (   d   )       }                     Δ                   d        (   d   )         =       θ     2      π       ·   λ                                 
 
         [0065]    If a displacement is obtained after the calculated phase argument is spatially averaged near the depth of interest, variations due to noise and speckle can be reduced. If a spatial vector average is obtained with a complex vector before the acquisition of the phase argument, a probability average based on signal intensity can be obtained. This makes it possible to calculate a phase argument more accurately. This vector average is given by  
           c        (   d   )       _     =         ∑   N              r   1          (   d   )       ×       (       r   2   *          (   d   )       )       θ   →     θ   +   π             N                           
 
         [0066]    That is, cross-correlation operation with π=0 is equivalent to this.  
         [0067]    As shown in FIG. 7, however, calculating a moving average after obtaining a complex vector at each point allows high-speed operation by interchanging the first and last values of data in moving operation.  
         [0068]    As another method of obtaining phases, a method using cross spectra is available. However, this method is equivalent to the cross-correlation method, and hence a description thereof will be omitted.  
         [0069]    In addition, spatial averaging may be performed across different scanning lines. After phases are calculated with scanning lines, the phases may be averaged across a plurality of scanning lines at the same depth. The barycentric frequency or the like of a fundamental wave component changes in the direction of depth owing to the influence of attenuation, a displacement may be calculated by using tables  81 ,  82 , and  83  of the wavelengths of barycentric frequency components at different depths which are measured in advance. This makes it possible to accurately measure a displacement without measuring a barycentric frequency for every operation.  
         [0070]    In the above method of obtaining phases, if a reflector or scatterer is displaced by ¼ the wavelength of a fundamental wave component between rates, aliasing occurs and a phase may not be accurately measured. In most organs, no problem arises because no tissue moves very fast. In circulatory organs, however, valves may present problems. A displacement that causes aliasing is determined by the wavelength of a frequency component of a fundamental wave component; a decrease in frequency will reduce the tendency to cause aliasing.  
         [0071]    As shown in FIGS. 9A and 9B, if a phase is estimated from a signal as a low-frequency component extracted by filtering each rate signal, the precision improves, and the occurrence of aliasing can be prevented. If, for example, the speed of sound is 1,500 m/sec at a barycentric frequency of 3 MHz, the aliasing speed is 60 cm/sec. If, however, the barycentric frequency is decreased to 1.5 MHz by a filter or the like, the aliasing speed is doubled to 120 cm/sec.  
         [0072]    The second method is a method of obtaining a displacement by a cross-correlation R near a depth of interest. As shown in FIG. 10, calculations are made while a cross-correlation coefficient between one signal with a length n near the depth and the other signal is relatively shifted so as to detect a shift width Δd with which the coefficient exhibits its peak. This value may be converted into a displacement. According to this calculation, calculations must be made after one of the codes is inverted owing to the polarity relationship between rates.  
         R        (     d   ,     Δ                 d       )       =       ∑     d   -     n   z         d   +     n   z                  r   1          (   d   )       ×     {     -       r   2   *          (     d   +     Δ                 d       )         }                               
 
         [0073]    In general, it is easy to set a shift width in units of sampling intervals. However, a distance shorter than the sampling interval may be set as a shift width by interpolation.  
         [0074]    The third method uses the least squares method, in which a add total E of values near the squares of the differences between one signal near the depth of interest and the other signal is calculated in the same manner as described above while one of the rate signals is shifted, as shown in FIG. 10, thereby obtaining a shift width exhibiting the minimum value as a displacement. As in the above case, owing to the polarity relationship between the rates, calculations must be made after one of the codes is inverted.  
         [0075]    This operation can be expressed by the following equation; Ad that minimizes E may be found near the depth d.  
           E ( d, Δd )=Σ| r   1 ( d )−{− r   2 ( d+Δd )}| 2    
         [0076]    Since it is expected that much time is required for the cross-correction or least squares method, a technique of increasing the processing speed by, for example, thinning out signals or interpolating signals may be used. Method of thinning out signals include a method of thinning out signals at predetermined intervals and a method of making a calculation by using only components with strong signal intensity. In these methods, calculations may be performed for a plurality of scanning lines with respect to ultrasonic waves scanned three-dimensionally to three-dimensionally obtain a displacement, and the obtained displacement may be corrected. In addition, the relative positions of a pair of signals which are added by the pulse inversion method to be described later can also be obtained three-dimensionally.  
         [0077]    The above displacement estimation is performed at each depth. For the sake of convenience, a table of displacements estimated in accordance with depths, like the table  83  in FIG. 8, may be formed in a memory to be looked up for correction.  
         [0078]    (Technique  52  of Correcting Displacement)  
         [0079]    A method of correcting the displacement obtained by “estimating displacement  51 ” by using the displacement correcting unit  20 . Assume that in the first correcting method, two rate signals are stored in a memory, as shown in FIG. 11. The signal value of a signal  112 , stored in the memory, which corresponds to a signal  111  at a given depth d is written in a new memory. This operation is performed at each depth, and signal processing may be performed by using the signal  111  and a signal  113  in the new memory.  
         [0080]    In the second correcting method, as shown in FIG. 12, if the displacement calculated from cross-correlation operation using phases and interpolation is smaller than a sampling interval Δt, a displacement can be realized by interpolation. FIG. 13 shows an example of interpolation, which is realized by a sinc interpolation method of convoluting a rate signal to a sinc function, and data between sampling intervals is interpolated, thereby realizing a displacement. Obviously, this interpolation is not limited to “sinc”. The interpolated signal may be written in a new memory  113 .  
         [0081]    As the third correcting method, a method of multiplying one signal by a phase term corresponding to the phase argument obtained above is available, although this is an approximation method.  
         [0082]    The following are signal models of a received frequency component of a fundamental wave and its harmonic component before quadrature phase detection in a case where a reflector or scatterer is displaced by Ad between rate signals, and the displacement corresponds to T for each rate signal. Note that reference symbol F denotes the envelope of the fundamental wave component; and H, the envelope of the harmonic component.  
         T     [   sec   ]       =             2   ·   Δ                     d        [   m   ]           1500              [     m   /   sec     ]                     t     =       2   ·   d     1500                             
 
           RATE  1 : r   1 ( t )= F   1 ( t ) e   i(ωt+θ     1     )   +H   2 ( t ) e   i(2ωt+θ     2     )    
           RATE  2 : r   2 ( t )= F   2 ( t )e i{ω(t−T)+θ     1     }   +H   2 ( t )e i{2ω(t−T)+θ     2     }   
         [0083]    The phase estimated by the above method is a change amount ωT of the fundamental wave component. The phase change amount of the harmonic component is 2ωT. Even if, therefore, the phase term of ωT is multiplied as follows for correction, a phase difference corresponding to ωT remains in the harmonic component and becomes an error factor. Reference symbol r′ denotes a received signal after correction.  
           r   2 ′( t )= r   2 ( t )· e   iωT   =F   2 ( t ) e   i(ωt+θ     1     )   +H   2 ( t ) e   i{ 2ω t−ωT+θ 2} 
         [0084]    Consider operation on the frequency axis. Displacing a received signal near a given depth d amounts to changing, for example, a straight line  151 , which represents the phase distribution of frequencies, to a straight line  152  in proportion to the frequency, as shown in FIG. 15A. In addition, as shown in FIG. 15B, multiplying a phase term is equivalent to adding/subtracting phases independent of frequencies like changing the straight line  151  to a straight line  153 . When the processing results are overlaid as shown in FIG. 15C, a difference e obviously appears, which is an error factor. However, such an error can be neglected if the displacement is small, and hence a sufficient correcting effect can be expected even by such simple approximation processing.  
         [0085]    As the fourth correcting method, a method of making a displacement in units of sampling intervals in a memory, and realizing the displacement amount obtained by subtracting the displacement from the original displacement by phase rotation may be used. Referring to FIG. 16, a displacement is made in the memory at a portion s, and a portion p is approximated by phase rotation. It can be expected that a displacement corresponding to the phase rotation amount used in this case is small with respect to the wavelength of a harmonic component as well. Therefore, sufficiently high precision can be expected. Obviously, correction can be performed with respect to any of the rates.  
         [0086]    (Harmonic Extraction (Adding Processing)  53 )  
         [0087]    By adding two signals having undergone displacement correction, fundamental wave components can be eliminated without any motion artifacts, and the harmonic components are amplified. FIG. 17 schematically shows a fundamental wave and harmonic component of a received signal from one reflector at a given depth in a case where a fundamental wave of a signal before phase detection can be expressed by a waveform corresponding to one period in “sin”.  
         [0088]    In adding operation, in correspondence with each depth of one rate signal, the corresponding depth value may be read out from the other rate signal by looking up the displacement table FIG. 8 instead of forming any new corrected memory as in the above case. In addition, adding can be performed after at least one signal is multiplied by a coefficient. By multiplying one signal by a positive number smaller than 1 and adding the resultant signals (multiplying different coefficients between rates and adding the resultant signals) at a near distance where a sufficient harmonic component is not produced or a deep portion where a harmonic component is greatly attenuated, a fundamental wave component remains and can be used for the generation of an image.  
         [0089]    (Quadrature Phase Detection  54 )  
         [0090]    The adding result is stored in a memory for rate signals. Thereafter, to generate an image by using a harmonic component, quadrature phase detection is performed by quadrature phase detector  15  again by using the frequency of the harmonic component. The above processing is performed on all scanning lines necessary for the generation of a harmonic image under systematic management by a CPU  10 . Thereafter, logarithmic compression, scanning/conversion, and the like are performed by the displaying unit  11  to display the resultant image on the monitor  3 .  
         [0091]    The above processing is performed according to the procedure shown in FIG. 18A. However, adding (C) may be performed after quadrature phase detection (D), as shown in FIG. 18B. Alternatively, as shown in FIG. 18C, quadrature phase detection (A′) may be performed first with a frequency twice a reference frequency f 0 . In addition, as shown in FIG. 19, a motion (displacement) may be estimated (E) on the basis of the result obtained by quadrature phase detection (A) with f 0 , and motion correcting (B) may be performed by using this displacement with respect to the result obtained by performing quadrature phase detection (A′) with 2·f 0 .  
         [0092]    The method of extracting a nonlinear component of tissue propagation at two rates while eliminating motion artifacts has been described above. However, the present invention can be effectively applied to a case where a nonlinear component is extracted from a contrast echo as well as the case of tissue propagation. If a nonlinear component is visualized by this method at the time of application of a contrast medium, a fundamental wave component can be eliminated without any motion artifacts, and both a contrast echo and a tissue harmonic component can be visualized.  
         [0093]    A method of extracting a contrast echo by eliminating a tissue harmonic component without any motion artifacts according to this embodiment will be described next.  
         [0094]    (Rate Difference)  
         [0095]    As ultrasonic pulses at two rates, pulses having the same waveform are used. If there is no tissue motion, the displacement between the two rates at each depth is  0 . FIGS. 20A and 20B show a combination of a fundamental wave component and tissue harmonic component of a signal at a given depth and a contrast medium harmonic component. By calculating the difference between these signals, the fundamental wave component and tissue harmonic component are canceled out, and only a change in the nonlinear response of microbubbles as a contrast medium between the rates over time is extracted. A change over time is caused by a change in scatterer distribution, e.g., a change in scattering intensity due to collapse, segmentation, and coagulation of bubbles, a change in diameter, and the like. As in the pulse inversion method, if there is a tissue motion, a displacement occurs between the reception rates at each depth, and the fundamental wave component and tissue harmonic component are not eliminated, as shown in FIG. 20C.  
         [0096]    If a phase and displacement are estimated by the same method as in the case of pulse inversion and corrected and the resultant data is finally subjected to subtraction by the same method as described above, only a component corresponding to a change in contrast medium over time can be extracted. FIG. 21 shows how a displacement between a fundamental wave component and a tissue harmonic component in signals at two rates is corrected and eliminated.  
         [0097]    (Problem Associated with Motion Artifact and Permanent Echo)  
         [0098]    The above description has been made about a case where signal processing is performed by using two rates. As a method of detecting a displacement, a method of correcting a displacement, and a signal processing method, general methods can be used, and the present invention can be equally applied to a case where more signals are used. Motion artifact elimination based on addition/subtraction of pulses between the two rates described above is effective when a motion component of a fundamental wave echo contains only one type. That is, the effect of this method can be expected in eliminating motion artifacts due to, for example, heart beats in an abdominal region.  
         [0099]    With regards to ultrasonic echoes from the heart, however, an echo from the moving heart tissue and a permanent echo such as a multiple echo at the pleurapophysis or an echo from the lung are often mixed in a received signal of a fundamental wave. Even if a displacement is estimated for such a received signal, both echoes may not be perfectly eliminated for the following reason. It is expected that displacement estimation will be greatly influenced by one of a permanent echo and a tissue echo which exhibits a higher signal intensity. Even if displacement correction and harmonic extraction are processed on the basis of this displacement estimation, one of the echoes may remain in amount that cannot be neglected for a harmonic component.  
         [0100]    [0100]FIG. 22 schematically shows a fundamental wave component signal before quadrature phase detection. More specifically, FIG. 22 shows how a motion echo is left after the received signal is corrected with a displacement of  0  in accordance with a permanent echo having a high signal intensity.  
         [0101]    A method of eliminating both the moving tissue echo and permanent echo as fundamental wave components will be described with reference to the flow chart of FIG. 23.  
         [0102]    (Third Rate Harmonic)  
         [0103]    In transmitting operation, two types of ultrasonic pulses having opposite polarities are transmitted to each of a plurality of scanning lines three times, and the resultant signals are received. In this case, transmission/reception is performed in the order of positive polarity, negative polarity, and positive polarity. Obviously, however, this operation can be performed in the order of negative polarity, positive polarity, and negative polarity.  
         [0104]    [0104]FIGS. 24A to  24 C are graphs each showing a received signal at a given depth before quadrature phase detection, and more specifically, a motion echo, permanent echo, and tissue harmonic component, as fundamental wave components, separately. Note that “Re{A}” indicates the real part of A.  
           RATE  1 : r   1 ( t )= Re   {Ae   iωt   +Be   iωt   +He   i·2ωt}   
           RATE  2 : r   2 ( t )= Re   {Ae   iωt   −Be   iω(t−T)   +He   i·2ω(t−T) } 
           RATE  3 : r   3 ( t )= Re   {Ae   iωt   +Be   iω(t−2T)   +He   i·2ω(t−2T) } 
         [0105]    In step  231 , a first rate received signal and second rate received signal are added to produce a new signal R 12 . Likewise, the second rate received signal and a third rate received signal are added to produce a new signal R 23 . That is, simple pulse inversion is performed twice. FIGS. 25A and 25B show the signals obtained by this addition, which are expressed by the following equations. Each of FIGS. 25A and 25B separately shows signal components before addition; the actual amplitude is the add of the amplitudes of these signal components.  
           R 12( t )= r   1 ( t )+ r   2 ( t ) = Re   {Be   iωt (1 +e   −(ωT+π) )+ He   i·2ωt (1 +e   i·2ωT )} 
           R 23( t )= r   2 ( t )+ r   3 ( t ) = Re   {Be   i(ωt−T) (1 +e   −i(ωT+π) )+He 1·2ω(t−T) (1 +e   i·2ωT )} 
         [0106]    Although the tissue echo component is left upon this operation, the permanent echo can be eliminated. Each of the signals R 12  and R 23  contains a tissue echo of a fundamental wave and a harmonic component.  
         [0107]    In step  232 , since both the fundamental wave component and the harmonic component may have the same signal level, filtering is performed to extract only the fundamental wave in order to improve the precision in measuring a phase with the fundamental wave in the following operation. FIG. 26 is a graph showing a change in frequency component accompanying addition in simple pulse inversion and how a harmonic component is eliminated by a filter.  
         [0108]    In step  233 , whether the tissue echo of the fundamental wave is sufficiently canceled by simple pulse inversion after filtering is determined for each sample point. At a sampling point where it is determined that a tissue echo is sufficiently canceled, the processing is stopped. That the tissue echo is sufficiently canceled indicates that the tissue echo is obtained from the tissue at rest and is canceled concurrently with the permanent echo. Therefore, there is no need to continue the processing.  
         [0109]    Even if the following processing is continued in spite of sufficient cancellation of fundamental wave components, since the signal amplitude is small and the S/N ratio is low, the reliability of the result is low. As a criterion for determining whether fundamental wave components are sufficiently canceled, the power value of a signal is preferably used. A power value is calculated at each sampling point, and the calculated value is compared with a preset value. If the calculated value is smaller than the preset value, the processing may be interrupted. As this value, noise level or the like can be suitably used.  
         [0110]    In step  234 , a phase or displacement is detected at each depth with respect to the signals R 12  and R 23  and corrected as in the case of two rates.  
         [0111]    In step  236 , if addition is performed in the same manner as described above, a motion echo of a fundamental wave component is canceled. At this time, tissue harmonic components are added and extracted. FIG. 27 shows how signals are corrected and added. An extracted signal H 123  can be expressed by  
           H   123   =R 12( t )+ R 23( t+T ) = Re{ {4 He   i·2ωt (1 +e   i·2ωT )} 
         [0112]    In the above manner, after conventional pulse inversion is performed twice by using ultrasonic pulses at three rates whose polarities alternately change, displacement correction is performed, and the resultant signals are added. This makes it possible to extract a harmonic component while eliminating both a permanent echo contained in a fundamental wave component and a tissue echo that causes motion artifacts.  
         [0113]    The above description has been made about a tissue harmonic component without any contrast medium. If the above method is applied to a contrast echo, both a tissue harmonic component and a nonlinear response component based on the contrast medium can be extracted. If a fundamental wave component contains both a permanent echo and a tissue motion echo, as in the case of two rates, a nonlinear component based on the contrast medium can be separated from a tissue harmonic component and extracted by the following processing.  
         [0114]    (Three Rate Contrast medium)  
         [0115]    In transmitting operation, the same waveform is transmitted three times with respect to each of a plurality of scanning lines, and the resultant signals are received. Each of FIGS. 28A to  28 C separately shows fundamental wave components of a received signal, i.e., a motion echo, permanent echo, and tissue harmonic component. Note that reference symbol A denotes the envelope of the permanent echo; B, the envelope of the motion echo; and C, the envelope of the harmonic component.  
           r   1 ( t )= Re   {A ( t ) iωt   +B ( t ) iωt   +H ( t ) i·2ωt   }+C   1   e   i·2ωt    
           r   2 ( t )= Re   {+Ae   iωt +Be iω(t−T)   +He   i·2ω(t−T)   }+C   2   e   i·2ω(t−T)    
           r   3 ( t )= Re   {Ae   iωt   +Be   iω(t−2T)   +He   i·2ω(t−2T)   }+C   3   e   i·2ω(t−2T)    
         [0116]    First of all, the first rate received signal and second rate received signal are subtracted from each other to produce a new signal CR 12 . The second rate received signal and third rate received signal are subtracted from each other to produce a signal CR 23 . That is, simple rate subtractions are performed. FIGS. 29A and 29B show the two signals obtained by the subtractions.  
             C     R12        (   t   )             =           r     1        (   t   )         +     r     2        (   t   )                               =                   Re          {         Be   iωt          (     1   -     e     -     (       ϖ                 T     +   π                )           )       +                                                 He       i   ·   2        ϖt            (     1   -     e       i   ·   2        ϖT         )       }         -       C   2          (   t   )              e       i   ·   2        ϖ             +     C   1     (   T   )              e       1   ·   2        ϖt                             C     R23        (   t   )             =           r     2        (   t   )         +     r     3        (   t   )                               =                   Re          {       +       Be     i        (     ωt   -   T     )              (     1   -     e     -     (       ϖ                 T     +   π                )           )         +                                                 He       i   ·   2          ϖ        (     t   -   T     )                (     1   -     e       i   ·   2        ϖT         )       }         -       C   3          (   t   )              e       i   ·   2        ϖ             +       C   2          (   t   )              e       i   ·   2        ϖ                                     
 
         [0117]    Each of FIGS. 29A and 29B separately shows signal components before a subtraction; the actual amplitude is the difference between them. With this operation, although a tissue echo component that causes a motion artifact is left, a permanent echo can be eliminated. The signals CR 12  and CR 23  contain tissue echoes of the fundamental wave and harmonic components.  
         [0118]    As shown in FIG. 30, after a simple rate subtraction, since both a fundamental wave and a harmonic component may have similar signal levels, filtering is performed to extract only the fundamental wave in order to improve the precision in measuring a phase with the fundamental wave.  
         [0119]    After the filtering processing, whether the tissue echo of the fundamental wave is sufficiently canceled by one simple rate subtraction is determined for each sample point. At a sampling point where it is determined that a tissue echo is sufficiently canceled, the processing is stopped. That the tissue echo is sufficiently canceled indicates that the tissue echo is obtained from the tissue at rest and is canceled concurrently with the permanent echo.  
         [0120]    In addition, at this time, a tissue harmonic component of the harmonic components is an echo at rest between rate intervals and canceled. only a change in nonlinear response of the contrast medium between the rates is a residual signal component, and hence only a nonlinear component based on bubbles is extracted.  
         [0121]    Therefore, there is no need to continue the processing. Even if the following processing is continued in spite of sufficient cancellation of fundamental wave components, since the signal amplitude is small and the S/N ratio is low, the reliability of the result is low.  
         [0122]    As a criterion for determining whether fundamental wave components are sufficiently canceled, the power value of a signal is preferably used. A power value is calculated at each sampling point, and the calculated value is compared with a preset value. If the calculated value is smaller than the preset value, the processing may be interrupted. As this value, noise level or the like can be suitably used.  
         [0123]    In steps  234  and  235 , as in the case of two rates, a phase or displacement is detected and corrected at each depth. If a subtraction  236  is performed in the same manner, a motion echo of a fundamental wave component is canceled, together with a tissue harmonic component. In this case, only a component corresponding to a change in nonlinear response of the contrast medium over time between the rates is extracted. FIG. 31 shows how signal components are corrected and subtracted. An extracted signal CH 123  can be expressed by the equation shown in FIG. 31.  
         [0124]    As described above, after two conventional rate subtractions are performed by using ultrasonic pulses at three rates which have the same waveform, displacement correction is performed, and the resultant signals are subtracted from each other. This makes it possible to extract harmonic components based on the contrast medium while eliminating three components, i.e., a permanent echo contained in a fundamental wave component, a tissue echo that cases a motion artifact, and a tissue harmonic component.  
         [0125]    In the above displacement detection and correction processing with two and three rates, an error that cannot be neglected may occur in detection and correction of a displacement due to system noise, a quantization error, or overlapping of an original echo and a multiple echo (from the heart and pleurapophysis/lungs). FIG. 32A is a graph showing fundamental wave components in a case where ultrasonic pulses which are alternately inverted are transmitted at a plurality of rates, and an error is caused in processing for echoes from a single scatterer. Even if the signals containing this error are simply added, the fundamental wave is left. To eliminate the influence of this error and prevent the fundamental wave from being left, signal processing may be performed after correction is performed and different coefficients are multiplied for the respective rates. That is, filtering may be performed in the rate direction (the time direction of data at the same position). In this case, the rate period becomes equal to the sampling period, and the reciprocal of the rate period becomes a sampling frequency f. With regard to fundamental wave components whose polarities are alternately inverted, if no error occurs, a phase rotates through π for each rate. If, therefore, frequency analysis is performed in the rate direction, a spectrum appears at f/2. A second harmonic component is distributed at zero as shown in FIG. 32B because the phase does not rotate. If an error occurs, second harmonic components are distributed near f/2 and zero, as shown in FIG. 32C. To extract the second harmonic component while eliminating fundamental wave components, a low-pass filter may be formed to eliminate the spectrum near f/2 and leave the spectrum near zero. Characteristics such as a cutoff characteristic of this filter may be designed in accordance with the occurrence state of an error. In addition, the filter coefficient may be changed at each depth. Although some rate may be cut upon filtering, the remaining rates may be used to generate an image. In addition, when the filter method is used, a fundamental wave can be eliminated with an odd number of rates unlike the conventional pulse inversion method. As described above in association with the second harmonic component and fundamental wave, of other harmonic components, some components are distributed at f/2, and the other components are distributed at zero. Secondary harmonic components are distributed at zero.  
         [0126]    The present invention is not limited to the above embodiment and can be variously modified. For example, according to the above description, in the processing for the extraction of harmonic components according to the present invention, ultrasonic pulses at two and three rates are used. However, the present invention is not limited and can be practiced in various modifications. When a displacement is to be detected and corrected at each depth with four or more rates, displacements between all the adjacent rates may be detected and corrected, or an average displacement throughout all the rates may be detected and may be corrected at each rate. When a contrast medium is used, a displacement may not be accurately detected due to the influences of the contrast medium in which the first rate exists in the fundamental wave band and changes randomly. In this case, a displacement may be detected between the subsequent rates, and the first rate may be corrected by approximately using the detected value. Alternatively, displacements obtained between a plurality of rates may be simply averaged to reduce the influences of variations.  
         [0127]    In addition, as a transmission ultrasonic wave, ultrasonic pulses that have been used in the prior art need not always be used. Obviously, the above technique of correcting displacements between pulses is a general method and hence can be applied to any other signal processes using more rates. Furthermore, displacement correction need not always be performed between adjacent rates. Moreover, pulse inversion and rate subtraction may be combined.  
         [0128]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.