Abstract:
A nonlinear distortion-based ultrasonic diagnostic imaging system displays a raised-resolution video of tissue inside a body at an increased frame rate. Using a two-pulse technique, a transducer driver supplies narrower-width and wider-width driving pulses to a transducer, which transmits weaker and stronger ultrasonic wave pulses alternately while putting the same intervals between adjacent ultrasonic wave pulses to obtain a weaker echo and a stronger echo. An equalizer equalizes each weaker echo to the stronger echo into an equalized weaker echo. An interpolator calculates an interpolation value between the equalized weaker echo and an equalized previous weaker echo obtained from a previous weaker echo. For each weaker ultrasonic wave pulse, a detector finds a difference between the interpolation value and a stronger echo obtained between the weaker echo and the previous weaker echo. The equalization and interpolation enables high-speed scanning, which has not been achieved with two-pulse technique. Thus, a raised-resolution video signal of the tissue is formed at an increased frame rate on the basis of the difference signal and a scan control signal which is also used in the transducer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an ultrasonic diagnostic imaging system that uses nonlinear distortion for imaging tissue inside a body. 
     2. Description of the Prior Art 
     Ultrasonic diagnostic imaging systems for imaging tissue inside a body by utilizing nonlinear propagation distortion caused by harmonics occurring during ultrasonic wave propagation are well known in the art. In such a system, a transducer is driven alternately by a first and a second drive pulse of A and 2A, respectively, in amplitude. The transducer responsively transmits a first and a second ultrasonic wave, which are reflected by tissue in the body and received by the transducer as a first and a second ultrasonic echo of B and 2B, respectively, in amplitude. The first and second echoes are amplified by a variable gain amplifier with gains of C and C/2 to yield a first and a second signal of B·C and 2B·(C/2), respectively. Since the sidelobes of the first and second echoes are much smaller than the main lobes and accordingly small in distortion, the amplitudes of the sidelobes of the second echo are substantially twice those of the sidelobes of the first echo. Thus calculating the differences between the first and second signals, i.e., B·C−2B·(C/2) enables the detection of the depth of reflection point. Since a pair of pulses is used for each analysis, such systems as described above are called “two-pulse” systems. A first and a second pulse in such a two-pulse system are hereinafter referred to as a “former pulse” and a “latter pulse”. 
     However, in order for the above imaging technique to work satisfactorily, the reflection points or ultrasonic wave transmission directions from which the former and later echoes are obtained must be substantially the same. This restriction prevents high-speed scanning in conventional nonlinear distortion-based ultrasonic diagnostic imaging system. 
     SUMMARY OF THE INVENTION 
     In light of the above, it is an object of the present invention to provide a nonlinear distortion-based ultrasonic diagnostic imaging system which displays a raised-resolution video of tissue inside a body at an increased frame rate. 
     According to an aspect of the invention, a transducer transmits a ultrasonic wave pulse in response to a driving pulse while scanning the transmission direction in response to a scan control signal and receives an echo of the ultrasonic wave pulse to provide an echo signal. A transducer driver supplies the driving pulses and the scan control signal to the transducer such that the transducer transmits weaker and stronger ultrasonic wave pulses alternately while putting the same intervals between adjacent ultrasonic wave pulses to obtain a weaker echo of the weaker ultrasonic wave pulse and a stronger echo of the stronger ultrasonic wave pulse from the transducer. An equalizer equalizes each weaker echo to the stronger echo into an equalized weaker echo. An interpolator calculates an interpolation value between the equalized weaker echo and an equalized previous weaker echo obtained from a previous weaker echo. For each weaker ultrasonic wave pulse, a detector finds a value indicative of a difference between the interpolation value and a stronger echo obtained between the weaker echo and the previous weaker echo. An image processor generates a raised-resolution video signal of the tissue at an increased frame rate on the basis of the values and the scan control signal. 
     In one embodiment, the equalizer calculates a convolution by using each weaker echo as one of two components. 
     In the embodiment, the transducer driver may supply a narrower driving pulse and a wider driving pulse for the weaker and stronger ultrasonic wave pulses, respectively. Alternatively, the transducer driver may supply fewer driving pulse(s) for the weaker ultrasonic wave pulse and supply more driving pulses for the stronger ultrasonic wave pulse. These driving pulses have an identical width. 
     In the embodiment, the interpolator calculates an arithmetic means of said equalized weaker echo and said equalized previous weaker echo. Alternatively, an arithmetic means of the absolute values of the equalized weaker echo and the equalized previous weaker echo may be calculated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The features and advantages of the present invention will be apparent from the following description of an exemplary embodiment of the invention and the accompanying drawings, in which: 
     FIG. 1 is a schematic block diagram showing an arrangement of an ultrasonic diagnostic imaging system according to an illustrative embodiment of the invention; 
     FIG. 2 is a diagram showing waveforms of driving pulses with respective different pulse widths T 1  and T 2 ; 
     FIG. 3 is a graph showing the relationship between the fundamental wave and the second harmonic of an ultrasonic echo; 
     FIG. 4 is a diagram showing the relationship between the azimuth (i.e., the angle with a normal on the transmission surface of transducer  22 ) and the amplitude of the transmitted ultrasonic wave; and 
     FIG. 5 is a diagram showing various signals for illustrating the operation of the ultrasonic diagnostic imaging system of FIG. 1 
     Throughout the drawing, the same elements when shown in more than one figure are designated by the same reference numerals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic block diagram showing an arrangement of an ultrasonic diagnostic imaging system according to an illustrative embodiment of the invention. In FIG. 1 the ultrasonic diagnostic imaging system  1  comprises a transducer driver  10  for alternately providing a former and a latter driving pulse different from each other in spectral intensity and a probe  20 , which includes a transducer  22  for transmitting a ultrasonic wave pulse in response to a driving pulse and receiving an echo of the transmitted ultrasonic wave pulse. The probe  20  has its scan data input  20   a  connected to the controller scan control output  100   c  and its transducer terminal  20   b  connected to the transducer driver  10  output. The system  1  further comprises an analog-to-digital (A/D) converter  30  having its analog input connected to the transducer terminal  20   b  and its control input connected to the controller output  100   b ; an equalizer  40  having its input connected to the output of the A/D converter  30 ; an interpolator  50  having its input connected to the equalizer  40  output; a memory  60  for temporary storing one pulse&#39;s worth of digital echo samples from the A/D converter  30 ; a detector  70  which uses the interpolator  50  output and the A/D converter  30  output being temporarily stored in the memory  60  to detect a signal indicative of the depth of reflection point; an image processor  80  having its data input connected to the detector  70  output and its control input connected to the controller output  100   c ; a display device  90  having its input connected to the image processor  80 ; and a controller  100  which controls the operation of the whole system  1  especially by providing control signals  100   a  through  100   c.    
     Since the driving pulses from the transducer driver  10  typically have a high voltage, the A/D converter  30  is preferably provided with a limiter (not shown). The interpolator  50  is preferably provided with a not-shown memory (or interpolator memory) with a capacity enough to store one pulse&#39;s worth of equalized digital echo samples from the equalizer  40 . The memory  60 , which is shown as an independent memory in FIG. 1, may be a part of random access memory (not shown) included in the controller  100 . The controller  100  may be any suitable microprocessor-based controller. 
     In operation, the transducer driver  10  alternately outputs a former and a latter driving pulse different from each other in duty cycle in response to a transmission control signal from the controller  100  output terminal  100   a  as shown in FIG.  5 A. FIG. 2 shows the former driving pulse pa(t) and the latter driving pulse pb(t), which means that the former and latter driving pulses are expressed by respective functions of time t, i.e., pa(t) and pb(t). The pulses preferably have three values, i.e., 0 and positive and negative levels of a predetermined amplitude. The pulses have respective pulse widths T 1  and T 2 . FIG. 5 shows various signals for illustrating the operation of the ultrasonic diagnostic imaging system  1  of FIG.  1 . In FIG. 5, a letter “j” is used to indicate the sequence of pulses (i.e., “j” is a serial number assigned to each pair of a former and a latter pulse in order of generation). For example, in FIG. 5A, the current former driving pulse is denoted by pa(t, j) and the previous former driving pulse is denoted by pa(t, j−1). In the same way, ultrasonic echoes of the ultrasonic wave pulses transmitted in response to the driving pulses pa(t, j) and pa(t, j−1) are denoted by ra(r, j) and ra(t, j−1), respectively. However, if there is no need of differentiating the sequence of the pulses, we will use simplified expressions like pa(t), ra(t), etc., omitting the sequence ID terms in the following. 
     The transducer  22  alternately transmits former ultrasonic waves ga(t) and latter ultrasonic waves gb(t)that are in a fundamental frequency band and correspond to the former pa(t) and latter pb(t) driving pulses. Preferably, the probe  20  is so arranged as to automatically scan the direction of ultrasonic wave transmission in response to the scan control data from the controller output  100   c . Since the transducer  22  has a narrower frequency band width as compared with the driving pulses, changing the spectral intensity of the driving pulse (i.e., changing the pulse width of the driving pulse with its amplitude kept constant) enables the control of the amplitude of the transmitted ultrasonic waves. For this reason, the former pa(t) and latter pb(t) driving pulses with respective pulse widths of T 1  and T 2  cause the transducer  22  to transmit the former ga(t) and latter gb(t) ultrasonic waves of respective amplitudes responsive to T 1  and T 2 . 
     FIG. 3 shows the relationship between the fundamental wave and the latter harmonic in an echo of a transmitted ultrasonic wave pulse. As seen from FIG. 3, the ultrasonic wave pulses ga(t) and gb(t) transmitted from the transducer  22  increase in nonlinear distortion as they travel a longer path within the body. The larger the amplitude of the ultrasonic waves is, the harder the nonlinear distortion is. Since the nonlinear distortion is due to harmonics, especially, due to the latter harmonic, the fundamental wave component decreases in amplitude as the latter harmonic increases. For this reason, the peak portion of the main lobe, in which the amplitude of the beam of ultrasonic wave pulse is relatively large, is subjected to larger nonlinear distortion, while the sidelobes, in which the amplitude is relative small, are subjected to smaller nonlinear distortion. 
     The former ga(t, j) and latter gb(t, j) ultrasonic wave pulses transmitted from transducer  22  in response to the driving pulses pa(t, j) and pb(t) is reflected by tissue within the body, and returned to and received by transducer  22  as a former and a latter ultrasonic echo ra(t, j) and rb(t, j), respectively, as shown in FIG.  5 B. Each of former ra(t, j) and latter rb(t, j) echo pulses is sampled and converted by A/D converter  30  into a series of digital echo samples (or signals), ra(k, j) and rb(k, j), as shown in FIG. 5C, where k=1, 2, . . . , N, where N is the number of digital echo samples for one driving or echo pulse. 
     In order to facilitate the understanding of the invention, it is now assumed that the transducer driver  10  has just supplied a j-th former driving pulse pa(t, j) and, accordingly, now is just the time to analyze echo pulses ra(t, j−1), rb(t, j−1) and ra(t, j) to get the j−1)th result. At the time of transmission of a j-th former ultrasonic wave pulse ga(t, j) from the transducer  22 , the digital samples of the j−1)th former echo pulse, i.e., ra(1, j−1), ra(2, j−1), . . . , ra(N, j−1) (hereinafter, denoted as {ra(k, j−1)|k=1˜N}) have been stored in memory of either interpolator  50  or controller  100  (not shown), and the digital samples of the j−1)th latter echo pulse, i.e., rb(1, j−1), rb( 2 , j−1), . . . , rb(N, j−1) (hereinafter, denoted as {rb(k, j−1)|k=1˜N}) have been stored in memory  60  as shown in FIG.  1 . Then, each of the digital samples of the j-th former echo pulse ra(t, j) which are supplied from A/D converter  30  is processed on a sample by sample basis. In the following, we will discuss how the k-th sample ra(k, j) of the j-th former echo pulse ra(t, j) is processed along the circuit path following A/D converter  30 . 
     Specifically, the k-th former echo digital sample ra(k, j) is equalized by equalizer  40  into an equalized digital sample rb′(k, j) as detailed later. Interpolator  50  uses the just equalized signal rb′(k, j) for interpolation together with the corresponding one rb′(k, j−1) of the equalized digital samples of the preceding former echo rb′(1, j−1), rb′(2, j−1), . . . , rb′(N, j−1). For this purpose, interpolator  50  preferably retains the recent N equalized samples: 
     
       
         rb′(k, j−1), rb′(k+1, j−1), . . . , rb′(N, j−1), rb′(1, j), rb′(2, j), . . . , rb′(k−1, j). 
       
     
     Then, interpolator  50  has only to use the just equalized signal rb′(k, j) and the oldest one of the stored signals, rb′(k, j−1) to calculate and output an interpolation value si(k, j−1). 
     It is noted that as shown in FIG. 1 the recent N equalized samples are actually stored in the following order: 
     
       
         rb′(1, j), rb′(2, j), . . . , rb′(k−1, j), rb′(k, j−1), rb′(k+1, j−1), . . . , rb′(N, j−1).  (data 1) 
       
     
     This is because, on completing the calculation of interpolation value si(k, j−1), interpolator  50  writes the newest (or just used) equalized sample rb′(k, j) over the oldest (or just used) one rb′(k, j−1) of the equalized digital samples (data 1) stored in the interpolation  50  memory. 
     The detector  70  calculates the difference between the interpolator  50  output ri(k, j−1) and the corresponding one rb(k, j−1) of the digital samples of the preceding (i.e., j−1)th) latter which are stored in memory  60  as follows: 
     
       
         Δr(k, j−1)=ri(k, j−1)−rb(k, j−1). 
       
     
     The image processor  80  processes thus obtained differences Δr(k, j−1) for k=1˜N for each of j=1, 2, . . . together with the scan data from the controller output terminal  100   c  to provide video images of tissue inside the body. The video images are displayed on the display device  90 . 
     The principles of the invention, especially, the operation of equalizer  40  and interpolator  50  will be detailed in the following. The Fourier transforms for a former pa(t) and a latter pb(t) driving pulse are denoted by Pa(ω) and Pb(ω), where ω is the angular frequency of the former and latter driving pulses. Similarly, the Fourier transforms for a former ga(t) and a latter gb(t) ultrasonic wave pulse are denoted by Ga(ω) and Gb(ω). Also, assuming the impulse response of the transducer  22  to be h(t), then the Fourier transform for the impulse response h(t) is denoted by H(ω). 
     Then, since a transmitted ultrasonic wave pulse ga(t) is expressed by the convolution of the impulse response h(t) and the driving pulse pa(t), it follows: 
     
       
         ga(t)=h(t)*pa(t)  (1) 
       
     
     where X*Y indicates the convolution of X and Y. This means 
     
       
         Ga(ω)=H(ω)×Pa(ω).  (2) 
       
     
     Multiplying the both sides of equation (2) by Pb(ω)/Pa(ω), we obtain                        Ga        (   ω   )       ×     (         Pb        (   ω   )       /   P                     a        (   ω   )         )       =       H        (   ω   )       ×   P                   a        (   ω   )       ×     (         Pb        (   ω   )       /   P                     a        (   ω   )         )                   =       H        (   ω   )       ×   P                   b        (   ω   )                     =       Gb        (   ω   )       .                   (   3   )                                
     Expressing the equation (3) in the time domain yields 
     
       
         gb(t)=ga(t)*invf(Pb(ω)/Pa(ω))  (4) 
       
     
     where the function invf(F(ω)) indicates the inverse Fourier transform for the function F(ω). The equation means that calculating the convolution between the former ultrasonic wave function ga(t) of the time when transducer  22  is a driven by a driving pulse pa(t) and the function invf(Pb(ω)/Pa(ω)) yields the latter ultrasonic wave function gb(t) of the time when transducer  22  is driven by a driving pulse pb(t). 
     Assuming that a returned echo of a transmitted ultrasonic wave is expressed by a linear combination of the transmitted ultrasonic wave, then the equation (4) can be written, for j-th former and latter echoes, as: 
     
       
         rb(t, j)=ra(t, j)*invf(Pb(ω, j)/Pa(ω, j)).  (5) 
       
     
     From this equation, it is seen that if equalizer  40  calculates the convolution of a j-th former echo ra(t, j) and the function invf(Pb(ω, j)/Pa(ω), j)), then equalizer  40  must provide a j-th latter echo rb(t, j). However, since the ultrasonic echoes ga(t) and gb(t) differ in amplitude, the nonlinear distortions in the ultrasonic echoes ga(t) and gb(t) also differ in degree. Taking this difference into account, the equation (5) should be written as: 
     
       
         rb(t, j)=ra(t, j)*invf(Pb(ω, j)/Pa(ω, j))+Δr(t, j).  (6) 
       
     
     Since the first term of the right side of equation (6) can be calculated by equalizer  40  as: 
     
       
         rb′(t, j)=ra(t, j)*invf(Pb(ω, j)/Pa(ω, j)).  (7) 
       
     
     The calculation of equation (7) by equalizer  40  can be realized by, for example, a digital filter etc. 
     Using rb′(t, j) in equation (6) yields 
     
       
         rb(t, j)=rb′(t, j)+Δr(t, j).  (8) 
       
     
     Since the signals in a circuit path which follows A/D converter  30  are digital samples, equation (8) can be expressed as: 
     
       
         rb(k, j)=rb′(k, j)+Δr(k, j).  (9) 
       
     
     However, since the scanning directions or positions (i.e., reflection points of transmitted latter gb(t) and former ga(t) ultrasonic wave pulses) that caused the ultrasonic echoes rb(t) and ra(t) (i.e., rb′(k, j)), respectively, are actually different from each other as seen from FIG. 5C, equation (9) is not valid as it is. In order to make the signals rb′(k, j) or ra(k, j) uniform in the scanning direction, the value rb′(k, j) is replaced, in interpolator  50 , with:                ri        (     k   ,   j     )       =             rb   ′          (     k   ,   j     )       +       rb   ′          (     k   ,     j   +   1       )         2     .             (   10   )                                
     By doing this, the difference Δr(k, j) in equation (9) is given, in detector  70 , by: 
     
       
         Δr(k, j)=rb(k, j)−ri(k, j).  (11) 
       
     
     Considering that the pulse numbers j and j−1indicate the current pulse and the preceding pulse, respectively, in actual operation, FIGS. 1 and 5C are drawn such that interpolator  50  calculates:                  ri        (     k   ,     j   -   1       )       =           rb   ′          (     k   ,     j   -   1       )       +       rb   ′          (     k   ,   j     )         2       ,   and           (     10   ′     )                                
     detector  70  calculates 
     
       
         Δr(k, j−1)=rb(k, j−1)−ri(k, j−1).  (11′) 
       
     
     The difference ri(k, j) is regarded as a value caused by the peak portion of the main lobe in the latter or larger-amplitude ultrasonic echo rb(t, j) and indicates the depth of reflection point. 
     According to the present invention, as seen from FIG. 5C, the depth of reflection point (or tissue inside the body) in the scanning direction of a weaker and stronger ultrasonic wave pulse pair is detected by using three successive scanning points including one used for the preceding pair. Since such three successive scanning points are permitted to be specially apart from one other, this enables a high-speed scanning, i.e., displaying an increased number of frames per unit time, permitting the motion of tissue to be displayed smoothly. 
     However, it is noted that it is preferable to place the same intervals between adjacent driving signals. 
     Also, since the difference Δr(k, j−1) includes substantially no sidelobe components, high-resolution images are obtained. 
     Modification 
     Interpolator  50  may calculate                ri        (     k   ,     j   -   1       )       =                rb   ′          (     k   ,     j   -   1       )               +               rb   ′          (     k   ,   j     )           2             (   12   )                                
     instead of equation (10′). 
     Detector  70  may calculate 
     
       
         Δr(k, j−1)=|rb(k, j−1)|−ri(k, j−1)|  (13) 
       
     
     instead of (11′). 
     If equation (12) or (13) is used, then the use of absolute value eliminates phase components, causing only amplitude information to be used. This frees the difference Δr(k, j−1) from becoming too large due to variation in phases of received echoes. 
     In the above illustrative embodiment, driving pulses of different pulse widths are used for driving pulse pairs. Pulse pairs may be realized by changing the number of pulses of a narrow pulse width. 
     A filter for compensating the spectral difference between the former and the latter driving pulses may be used for equalizer  40 . 
     In the above illustrative embodiment, the weaker ultrasonic echoes have been equalized to the stronger ultrasonic echoes. Alternatively, the stronger ultrasonic echoes may be equalized to the weaker ultrasonic echoes. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.