Patent Publication Number: US-10768286-B2

Title: Method and system for measuring a characteristic loop sensitivity for an acoustic transducer

Description:
BACKGROUND 
     Technical Field 
     The present invention relates to a method and system for measuring a characteristic loop sensitivity for an acoustic transducer in an acoustic probe. 
     Description of Related Art 
     An acoustic transducer is a key component in an acoustic imaging system. The technologies of acoustic imaging have been frequently employed to non-destructive testing, clinical diagnosis, and under water applications due to such advantages of acoustic imaging as non-invasive, non-ionization, real-time imaging, and cost-effectiveness. For example, acoustic imaging for clinical diagnosis, which is used for assessing the soft tissue structure and blood flow, is currently the most used clinical imaging modality after conventional X-ray radiography. 
       FIGS. 1A ˜ 1 B show a typical structure for an acoustic probe in a prior art. An acoustic probe  113  has a transducer array  117 A which comprises a plurality of acoustic transducer  117 . The number of acoustic transducer  117  in the transducer array  117 A is greater than or equal to one. 
     In the prior art, a sensitivity is used to assess the characteristics of an acoustic transducer  117 .  FIGS. 2A ˜ 2 B show the method of sensitivity measurement for an acoustic transducer in an acoustic probe in a prior art.  FIG. 2A  shows a measuring arrangement for reference signal in a prior art. A sine burst generator  200  is arranged to output a sine burst signal at a specific frequency on an external 50-ohm load as a reference signal V r (t)  204 .  FIG. 2B  shows a measuring arrangement for an acoustic probe  113  in a prior art. The sine burst generator of  200  is electrically coupled to an acoustic probe  113  which is immersed in a water bath  208  with an acoustic mirror  212 . The acoustic probe  113  is driven by the sine burst generator  200  and transmits an acoustic sine burst wave  214  at the specific frequency. The acoustic probe  113  receives the reflected sine burst wave  218  from the acoustic mirror  212  and outputs an echo signal V e (t)  224 . 
       FIG. 3A  shows a reference signal for an acoustic probe in a prior art. The reference signal V r (t)  204  is a sine burst signal with a minimum-run of 15 cycles at a specific frequency; and, a peak-to-peak voltage of reference signal (V ppr ) is obtained.  FIG. 3B  shows an echo signal for an acoustic probe in a prior art. The echo signal V e (t)  224  is a sine burst signal at the specific frequency; and a peak-to-peak voltage of echo signal (V ppe ) is obtained. A loop sensitivity for the acoustic transducer is calculated based upon the peak-to-peak voltage of echo signal (V ppe ) to the peak-to-peak voltage of reference signal (V ppr ). 
     The disadvantage for the prior art is that one specific frequency is adopted for measuring a loop sensitivity of an acoustic transducer  117  in an acoustic probe  113 . In an early stage, traditional acoustic probe responds to narrow band frequency only. However, wideband acoustic probe has been developed due to rapid progress in the acoustic technology development in recent years. Therefore, there is a general need for a method and system for measuring wideband characteristics of an acoustic transducer such as characteristic loop sensitivity (S LC ). 
     SUMMARY 
     The present invention discloses a method and system for measuring a characteristic loop sensitivity (S LC ) for an acoustic transducer in an acoustic probe. 
     A method for measuring a characteristic loop sensitivity (S LC ) for an acoustic transducer in an acoustic probe is introduced according to the present invention. 
     A pulse generator of 50-ohm source impedance, which is used to generate unipolar pulse and/or bipolar pulse, electrically couples to an external 50-ohm load to obtain a wideband reference signal V r (t) on the 50-ohm load and further obtain a function {circumflex over (V)} r (f) that is a Fourier Transform of the wideband reference signal V r (t). An energy of reference signal (E r ) for the wideband reference signal V r (t) is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband reference signal. 
     In a first and a second embodiment, the adopted pulse is a negative-going unipolar pulse and a positive-going unipolar pulse, respectively; and in a third and a fourth embodiment, the adopted pulse is a negative-positive bipolar pulse and a positive-negative bipolar pulse, respectively. 
     Further, obtain an energy spectrum of wideband reference signal based on the function {circumflex over (V)} r (f), and calculate a bandwidth of reference signal (BW r ) for the energy spectrum of wideband reference signal. 
     An energy density of reference signal (D r ) for the wideband reference signal is calculated as the ratio of the energy of reference signal (E r ) for the wideband reference signal to the bandwidth of reference signal (BW r ) for the wideband reference signal. 
     The pulse generator of 50-ohm source impedance electrically couples to an acoustic probe for measuring the wideband characteristics of an acoustic transducer. The acoustic probe is immersed into a water bath with an acoustic mirror. An acoustic transducer in the acoustic probe is driven by the pulse generator of 50-ohm source impedance and transmits a wideband acoustic wave toward the acoustic mirror. The acoustic transducer receives the reflected wideband acoustic wave and outputs a wideband echo signal V e (t); and, a function {circumflex over (V)} e (f) that is a Fourier Transform of the wideband echo signal V e (t) is obtained. An energy of echo signal (E e ) for the wideband echo signal V e (t) is calculated as one of a time-integral of the power of wideband echo signal and a frequency-integral of the energy spectrum density of wideband echo signal. 
     Further, obtain an energy spectrum of the wideband echo signal based on the function {circumflex over (V)} e (f), and calculate a bandwidth of echo signal (BW e ) for the energy spectrum of the wideband echo signal. 
     An energy density of echo signal (D e ) for the wideband echo signal is calculated as the ratio of the energy of echo signal (E e ) for the wideband echo signal to the bandwidth of echo signal (BW e ) for the wideband echo signal. 
     A characteristic loop sensitivity (S LC ) for the acoustic transducer is defined as the ratio of the energy density of echo signal (D e ) for the wideband echo signal to the energy density of reference signal (D r ) for the wideband reference signal in decibel according to the present invention. 
     Furthermore, a plurality of characteristic loop sensitivity (S LC ) for each and all acoustic transducers in an acoustic transducer array is obtained by performing the measuring step for calculating the characteristic loop sensitivity (S LC ) sequentially or randomly over each and all acoustic transducers in an acoustic transducer array according to the present invention. 
     The method for measuring the characteristic loop sensitivity (S LC ) for the acoustic transducer is embedded in one of the firmware and the program memory, and the method for measuring the plurality of characteristic loop sensitivity (S LC ) for each and all acoustic transducers in an acoustic transducer array is embedded in one of the firmware and the program memory according to the present invention. 
     A system for measuring a characteristic loop sensitivity for an acoustic transducer in an acoustic probe is introduced according to the present invention. The system comprises a pulse generator, a signal processing unit, a transducer selector, and a control unit. The control unit further comprises a firmware, a program memory, and a storage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ˜ 1 B show a typical structure for an acoustic probe in a prior art. 
         FIG. 2A  shows a measuring arrangement for reference signal in a prior art. 
         FIG. 2B  shows a measuring arrangement for an acoustic probe in a prior art. 
         FIG. 3A  shows a reference signal for an acoustic probe in a prior art. 
         FIG. 3B  shows an echo signal for an acoustic probe in a prior art. 
         FIGS. 4A ˜ 4 B show a negative-going unipolar pulse used as a wideband reference signal and its energy spectrum for a first embodiment according to the present invention. 
         FIGS. 5A ˜ 5 B show a positive-going unipolar pulse used as a wideband reference signal and its energy spectrum for a second embodiment according to the present invention. 
         FIG. 6A  shows a typical energy spectrum of wideband reference signal based on a unipolar pulse signal for the first and second embodiments according to the present invention. 
         FIG. 6B  shows a typical frequency response for an acoustic transducer in the first and second embodiments according to the present invention. 
         FIGS. 7A ˜ 7 B show a negative-positive bipolar pulse used as a wideband reference signal and its energy spectrum for a third embodiment according to the present invention. 
         FIGS. 8A ˜ 8 B show a positive-negative bipolar pulse used as a wideband reference signal and its energy spectrum for a fourth embodiment according to the present invention. 
         FIG. 9A  shows a typical energy spectrum of wideband reference signal based on a bipolar pulse signal for the third and fourth embodiments according to the present invention. 
         FIG. 9B  shows a typical frequency response for an acoustic transducer in the third and fourth embodiments according to the present invention. 
         FIG. 10A  shows a measuring arrangement for a wideband reference signal according to the present invention. 
         FIG. 10B  shows a measuring arrangement for an acoustic probe according to the present invention. 
         FIG. 11A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a negative-going unipolar pulse for a first embodiment. 
         FIG. 11B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the negative-going unipolar pulse for the first embodiment. 
         FIG. 12A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a positive-going unipolar pulse for a second embodiment. 
         FIG. 12B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the positive-going unipolar pulse for the second embodiment. 
         FIG. 13A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a first bipolar pulse for a third embodiment. 
         FIG. 13B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the first bipolar pulse for the third embodiment. 
         FIG. 14A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a second bipolar pulse for a fourth embodiment. 
         FIG. 14B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the second bipolar pulse for the fourth embodiment. 
         FIG. 15A  shows a typical energy spectrum of wideband reference signal according to the present invention based on a unipolar pulse signal for the first and second embodiments. 
         FIG. 15B  shows a typical energy spectrum of wideband echo signal according to the present invention based on a unipolar pulse signal for the first and second embodiments. 
         FIG. 16A  shows a typical energy spectrum of wideband reference signal according to the present invention based on a bipolar pulse signal for a third and fourth embodiments. 
         FIG. 16B  shows a typical energy spectrum of wideband echo signal according to the present invention based on a bipolar pulse reference signal for the third and fourth embodiments. 
         FIG. 17  shows a flow chart for measuring a characteristic loop sensitivity for an acoustic transducer according to the present invention. 
         FIG. 18  shows a system for measuring a characteristic loop sensitivity of an acoustic transducer according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention discloses a method and system for measuring a characteristic loop sensitivity (S LC ) of an acoustic transducer in an acoustic probe. The “loop” means the pulse-echo mode in which an acoustic transducer transmits an acoustic wave out and a corresponding reflected echo wave is received by the same acoustic transducer. 
     A method for measuring a characteristic loop sensitivity for an acoustic transducer in an acoustic probe is introduced according to the present invention. 
     A pulse signal is adopted as a wideband reference signal for measuring wideband characteristics of an acoustic transducer according to the present invention. There are four embodiments of adopted pulse signal used in the present invention, which include a negative-going unipolar pulse  400  for a first embodiment, a positive-going unipolar pulse  500  for a second embodiment, a negative-positive bipolar pulse  700  for a third embodiment, and a positive-negative bipolar pulse  800  for a fourth embodiment, according to the present invention. 
       FIGS. 4A ˜ 4 B show a negative-going unipolar pulse used as a wideband reference signal and its energy spectrum for a first embodiment according to the present invention. The wideband reference signal V r (t) of negative-going unipolar pulse  400  is adopted in the first embodiment, and an energy spectrum of wideband reference signal 
               1   50     ⁢                V   ^     r     ⁡     (   f   )            2           
of negative-going unipolar pulse  404  is obtained, in which the function {circumflex over (V)} r (f) is a Fourier Transform of the wideband reference signal V r (t) of negative-going unipolar pulse  400 .
 
       FIGS. 5A ˜ 5 B show a positive-going unipolar pulse used as a wideband reference signal and its energy spectrum for a second embodiment according to the present invention. The wideband reference signal V r (t) of positive-going unipolar pulse  500  is adopted in the second embodiment, and an energy spectrum of wideband reference signal 
               1   50     ⁢                V   ^     r     ⁡     (   f   )            2           
of positive-going unipolar pulse  504  is obtained, in which the function {circumflex over (V)} r (f) is a Fourier Transform of the wideband reference signal V r (t) of positive-going unipolar pulse  500 .
 
       FIG. 6A  shows a typical energy spectrum of wideband reference signal based on a unipolar pulse signal for the first and second embodiments according to the present invention. A maximum energy spectrum density of the energy spectrum of wideband reference signal  404 ,  504  is at 0 Hz (f 0 ). An upper bound frequency (f 4 ) of the energy spectrum of wideband reference signal  404 ,  504  is a frequency where the energy spectrum density drops down to a certain decibel value (e.g., −6 dB) relative to the maximum energy spectrum density at 0 Hz (f 0 ). 
       FIG. 6B  shows a typical frequency response for an acoustic transducer in the first and second embodiments according to the present invention. A maximum frequency response of an acoustic transducer is usually at its central frequency or resonant frequency. The upper bound frequency (f 3 ) and lower bound frequency (f 2 ) for the frequency response of acoustic transducer  600  are frequencies where the frequency response drops down to a certain decibel value (e.g., −6 dB) relative to its maximum response located at between (f 2 ) and (f 3 ), respectively. 
     To assure a good signal-to-noise ratio for the measurement in the first and second embodiments, the requirement is that the upper bound frequency (f 4 ) of the energy spectrum of wideband reference signal  404 ,  504  is greater than the upper bound frequency (f 3 ) of the frequency response of the acoustic transducer  600 , that is, f 4 &gt;f 3 , according to the present invention. 
       FIGS. 7A ˜ 7 B show a negative-positive bipolar pulse used as a wideband reference signal and its energy spectrum for a third embodiment according to the present invention. The wideband reference signal V r (t) of negative-positive bipolar pulse  700  is adopted in the third embodiment, and an energy spectrum of wideband reference signal 
               1   50     ⁢                V   ^     r     ⁡     (   f   )            2           
of negative-positive bipolar pulse  704  is obtained, in which the function {circumflex over (V)} r (f) is a Fourier Transform of the wideband reference signal V r (t) of negative-positive bipolar pulse  700 .
 
       FIGS. 8A ˜ 8 B show a positive-negative bipolar pulse used as a wideband reference signal and its energy spectrum for a fourth embodiment according to the present invention. The wideband reference signal V r (t) of positive-negative bipolar pulse  800  is adopted in the fourth embodiment, and an energy spectrum of wideband reference signal 
               1   50     ⁢                V   ^     r     ⁡     (   f   )            2           
of positive-negative bipolar pulse  804  is obtained, in which the function {circumflex over (V)} r (f) is a Fourier Transform of the wideband reference signal V r (t) of positive-negative bipolar pulse  800 .
 
       FIG. 9A  shows a typical energy spectrum of wideband reference signal based on a bipolar pulse signal for the third and fourth embodiments according to the present invention. The lower bound frequency (f 1 ) and upper bound frequency (f 4 ) of the energy spectrum of wideband reference signal  704 ,  804  are frequencies where the energy spectrum density drops down to a certain decibel value (e.g., −6 dB) relative to its maximum located at between (f 1 ) and (f 4 ), respectively. 
       FIG. 9B  shows a typical frequency response for an acoustic transducer in the third and fourth embodiments according to the present invention. A maximum frequency response for the acoustic transducer is usually at its central frequency or resonant frequency. The upper bound frequency (f 3 ) and lower bound frequency (f 2 ) for the frequency response of acoustic transducer  900  are frequencies where the frequency response drops down to a certain decibel value (e.g., −6 dB) relative to its maximum response located at between (f 2 ) and (f 3 ), respectively. 
     To assure a good signal-to-noise ratio for the measurement in the third and fourth embodiments, the requirement is that the upper bound frequency (f 4 ) of the energy spectrum of wideband reference signal  704 ,  804  is greater than the upper bound frequency (f 3 ) of the frequency response of the acoustic transducer  900  and the lower bound frequency (f 1 ) of the energy spectrum of wideband reference signal  704 ,  804  is smaller than the lower bound frequency (f 2 ) of the frequency response of the acoustic transducer  900 ; that is, f 4 &gt;f 3 &gt;f 2 &gt;f 1 , according to the present invention. 
       FIG. 10A  shows a measuring arrangement for a wideband reference signal according to the present invention. An external 50-ohm load is electrically coupled to a pulse generator of 50-ohm source impedance  1000  that generates unipolar pulse and/or bipolar pulse to obtain a wideband reference signal V r (t)  400 ,  500 ,  700 ,  800  on the 50-ohm load. 
       FIG. 10B  shows a measuring arrangement for an acoustic probe according to the present invention. The pulse generator of 50-ohm source impedance  1000  electrically couples to an acoustic probe  113  for measuring the wideband characteristics of an acoustic transducer  117 . The acoustic probe  113  is immersed into a water bath  208  with an acoustic mirror  212 . The acoustic probe  113  is aligned so that the acoustic wave is normally incident to and reflected from the acoustic mirror  212 . An acoustic transducer  117  in the acoustic probe  113  is driven by the pulse generator of 50-ohm source impedance  1000  and transmits a wideband acoustic wave toward the acoustic mirror  212 . The transmitted wideband acoustic wave  1004  travels and reaches the acoustic mirror  212  in the water bath  208  and is reflected backward to the acoustic transducer  117 . The acoustic transducer  117  receives the reflected wideband acoustic wave  1008  and outputs a wideband echo signal V e (t)  1100 ,  1200 ,  1300 ,  1400 . 
       FIG. 11A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a negative-going unipolar pulse for a first embodiment. The wideband reference signal V r (t) of negative-going unipolar pulse  400  is adopted in the first embodiment and a function {circumflex over (V)} r (f), that is a Fourier Transform of the wideband reference signal V r (t) of negative-going unipolar pulse  400 , is obtained. Meanwhile, an energy of reference signal (E r ) for wideband reference signal V r (t) of negative-going unipolar pulse  400  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband reference signal; that is, 
     
       
         
           
             
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       FIG. 11B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the negative-going unipolar pulse for the first embodiment. A wideband echo signal V e (t) based on negative-going unipolar pulse  1100  is obtained in the first embodiment and a function {circumflex over (V)} r (f), that is a Fourier Transform of the wideband echo signal V e (t) based on negative-going unipolar pulse  1100 , is further obtained. Meanwhile, an energy of echo signal (E e ) for wideband echo signal V e (t) based on negative-going unipolar pulse  1100  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband echo signal; that is, 
     
       
         
           
             
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       FIG. 12A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a positive-going unipolar pulse for a second embodiment. The wideband reference signal V r (t) of positive-going unipolar pulse  500  is adopted in the second embodiment and a function {circumflex over (V)} r (f), that is a Fourier Transform of the wideband reference signal V r (t) of positive-going unipolar pulse  500 , is obtained. Meanwhile, an energy of reference signal (E r ) for wideband reference signal V r (t) of positive-going unipolar pulse  500  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband reference signal; that is, 
     
       
         
           
             
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       FIG. 12B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the positive-going unipolar pulse for the second embodiment. A wideband echo signal V e (t) based on positive-going unipolar pulse  1200  is obtained in the second embodiment and a function {circumflex over (V)} e (f), that is a Fourier Transform of the wideband echo signal V e (t) based on positive-going unipolar pulse  1200 , is further obtained. Meanwhile, an energy of echo signal (E e ) for wideband echo signal V e (t) based on positive-going unipolar pulse  1200  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband echo signal; that is, 
     
       
         
           
             
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       FIG. 13A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a first bipolar pulse for a third embodiment. The wideband reference signal V r (t) of negative-positive bipolar pulse  700  is adopted in the third embodiment and a function {circumflex over (V)} r (f), that is a Fourier Transform of the wideband reference signal V r (t) of negative-positive bipolar pulse  700 , is obtained. Meanwhile, an energy of reference signal (E r ) for wideband reference signal V r (t) of negative-positive bipolar pulse  700  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband reference signal; that is, 
     
       
         
           
             
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       FIG. 13B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the first bipolar pulse for the third embodiment. A wideband echo signal V e (t) based on negative-positive bipolar pulse  1300  is obtained in the third embodiment and a function {circumflex over (V)} e (f), that is a Fourier Transform of the wideband echo signal V e (t) based on negative-positive bipolar pulse  1300 , is further obtained. Meanwhile, an energy of echo signal (E e ) for wideband echo signal V e (t) based on negative-positive bipolar pulse  1300  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband echo signal; that is, 
     
       
         
           
             
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       FIG. 14A  shows an electrical waveform of a wideband reference signal and its Fourier Transform according to the present invention based on a second bipolar pulse for a fourth embodiment. The wideband reference signal V r (t) of positive-negative bipolar pulse  800  is adopted in the fourth embodiment and a function {circumflex over (V)} r (f) , that is a Fourier Transform of the wideband reference signal V r (t) of positive-negative bipolar pulse  800 , is obtained. Meanwhile, an energy of reference signal (E r ) for wideband reference signal V r (t) of positive-negative bipolar pulse  800  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband reference signal; that is, 
     
       
         
           
             
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       FIG. 14B  shows an electrical waveform of a wideband echo signal and its Fourier Transform according to the present invention based on the second bipolar pulse for the fourth embodiment. A wideband echo signal V e (t) based on positive-negative bipolar pulse  1400  is obtained in the fourth embodiment and a function {circumflex over (V)} e (f), that is a Fourier Transform of the wideband echo signal V e (t) based on positive-negative bipolar pulse  1400 , is further obtained. Meanwhile, an energy of echo signal (E e ) for wideband echo signal V e (t) based on positive-negative bipolar pulse  1400  is calculated as one of a time-integral of the power of wideband reference signal and a frequency-integral of the energy spectrum density of wideband echo signal; that is, 
     
       
         
           
             
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                       df 
                       . 
                     
                   
                 
               
             
           
         
       
     
       FIG. 15A  shows a typical energy spectrum of wideband reference signal according to the present invention based on a unipolar pulse signal for the first and second embodiments. The maximum energy spectrum density of the energy spectrum of wideband reference signal  404 ,  504  is at 0 Hz (f 0 ). The upper bound frequency (f 4 ) of the energy spectrum of wideband reference signal  404 ,  504  is a frequency where the energy spectrum density drops down to a certain decibel value (e.g., −6 dB) relative to the maximum energy spectrum density at 0 Hz (f 0 ). A frequency bandwidth of reference signal (BW r ) for the energy spectrum of wideband reference signal  404 ,  504  is obtained as the frequency difference between the upper bound frequency (f 4 ) and 0 Hz (f 0 ); that is, BW r =f 4 −f 0 . 
       FIG. 15B  shows a typical energy spectrum of wideband echo signal according to the present invention based on a unipolar pulse signal for the first and second embodiments. A maximum energy spectrum density of the energy spectrum of wideband echo signal  1504  is usually at resonant or central frequency of an acoustic transducer. The lower bound frequency (f 2 ) and upper bound frequency (f 3 ) of the energy spectrum of wideband echo signal  1504  are frequencies where the energy spectrum density drops down to a certain decibel value (e.g., −6 dB) relative to the maximum energy spectrum density located at between (f 2 ) and (f 3 ), respectively. A frequency bandwidth of echo signal (BW e ) for the energy spectrum of wideband echo signal  1504  is obtained as the frequency difference between the upper bound frequency (f 3 ) and lower bound frequency (f 2 ); that is, BW e =f 3 −f 2 . 
       FIG. 16A  shows a typical energy spectrum of wideband reference signal according to the present invention based on a bipolar pulse signal for a third and fourth embodiments. The lower bound frequency (f 1 ) and upper bound frequency (f 4 ) of the energy spectrum of wideband reference signal  704 ,  804  are frequencies where the energy spectrum density drops down to a certain decibel value (e.g., −6 dB) relative to its maximum located at between (f 1 ) and (f 4 ), respectively. A frequency bandwidth of reference signal (BW r ) for the energy spectrum of wideband reference signal  704 ,  804  is obtained as the frequency difference between the upper bound frequency (f 4 ) and lower bound frequency (f 1 ); that is, BW r =f 4 −f 1 . 
       FIG. 16B  shows a typical energy spectrum of wideband echo signal according to the present invention based on a bipolar pulse reference signal for the third and fourth embodiments. A maximum energy spectrum density of the energy spectrum of wideband echo signal  1604  is usually at resonant or central frequency of an acoustic transducer. The lower bound frequency (f 2 ) and upper bound frequency (f 3 ) of the energy spectrum of wideband echo signal  1604  are frequencies where the energy spectrum density drops down to a certain decibel value (e.g., −6 dB) relative to the maximum energy spectrum density located at between (f 2 ) and (f 3 ), respectively. A frequency bandwidth of echo signal (BW e ) for the energy spectrum of wideband echo signal  1604  is obtained as the frequency difference between the upper bound frequency (f 3 ) and lower bound frequency (f 2 ); that is, BW e =f 3 −f 2 . 
     For the wideband reference signal V r (t), an energy density of reference signal (D r ) is calculated as the ratio of the energy of reference signal (E r ) to the frequency bandwidth of reference signal (BW r ); that is, 
                 D   r     =       E   r       BW   r         ,         
according to the present invention.
 
     For the wideband echo signal V e (t), an energy density of echo signal (D e ) is calculated as the ratio of the energy of echo signal (E e ) to the frequency bandwidth of echo signal (BW r ); that is, 
                 D   e     =       E   e       BW   e         ,         
according to the present invention.
 
     A characteristic loop sensitivity (S LC ) for the acoustic transducer is defined as the ratio of the energy density of echo signal (D e ) for the wideband echo signal to the energy density of reference signal (D r ) for the wideband reference signal in decibel; that is, 
                 S   LC     ⁢     =   def     ⁢     10   ⁢           ⁢     log   ⁡     (       D   e       D   r       )           ,         
according to the present invention.
 
       FIG. 17  shows a flow chart for measuring a characteristic loop sensitivity for an acoustic transducer according to the present invention. 
     The measuring step for obtaining an energy density of reference signal (D r ) for a wideband reference signal comprises:
         preparing a pulse generator and a signal processing unit;   generating a pulse to create a wideband signal as a reference signal;   obtaining a wideband reference signal V r (t);   obtaining a function {circumflex over (V)} r (f) that is a Fourier Transform of the wideband reference signal V r (t);   obtaining an energy spectrum of wideband reference signal based on the function {circumflex over (V)} r (f);   calculating a bandwidth of reference signal (BW r ) at a designated decibel for the energy spectrum of the wideband reference signal;   calculating an energy of reference signal (E r ) for the wideband reference signal V r (t);   calculating an energy density of reference signal (D r ) for the wideband reference signal as follows:       

                 D   r     =       E   r       BW   r         ;         
and
         storing the energy density of reference signal (D r ) in one of a firmware and a program memory.       

     The measuring step for obtaining an energy density of echo signal (D e ) for a wideband echo signal comprises:
         coupling the pulse generator and the signal processing unit to an acoustic transducer;   generating a wideband acoustic wave from the acoustic transducer;   obtaining a wideband echo signal V e (t) after the wideband acoustic wave being reflected from an acoustic mirror;   obtaining a function {circumflex over (V)} e (f) that is a Fourier Transform of the wideband echo signal V e (t);   obtaining an energy spectrum of wideband echo signal based on the function {circumflex over (V)} e (f);   calculating a bandwidth of echo signal (BW e ) at a designated decibel for the energy spectrum of the wideband echo signal;   calculating an energy of echo signal (E e ) for the wideband echo signal V e (t);   calculating an energy density of echo signal (D e ) for the wideband echo signal as follows:       

                 D   e     =       E   e       BW   e         ;         
and
         storing the energy density of echo signal (D e ) in the program memory.       

     The measuring step for obtaining a characteristic loop sensitivity (S LC ) for the acoustic transducer comprises:
         obtaining an energy density of reference signal (D r ) for a wideband reference signal;   obtaining an energy density of echo signal (D e ) for a wideband echo signal;   defining a characteristic loop sensitivity (S LC ) for the acoustic transducer as follows:       

     
       
         
           
             
               
                 S 
                 LC 
               
               ⁢ 
               
                 = 
                 def 
               
               ⁢ 
               
                 10 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   log 
                   ⁡ 
                   
                     ( 
                     
                       
                         D 
                         e 
                       
                       
                         D 
                         r 
                       
                     
                     ) 
                   
                 
               
             
             ; 
           
         
       
         
         
           
             storing the characteristic loop sensitivity (S LC ) in a storage device; and 
             outputting data stored in the storage device. 
           
         
       
    
     Furthermore, a plurality of characteristic loop sensitivity (S LC ) for each and all acoustic transducers in an acoustic transducer array can be obtained; the measuring step for which comprises:
         performing the measuring step for calculating the characteristic loop sensitivity (S LC ) sequentially or randomly over each and all acoustic transducers in an acoustic transducer array;   obtaining a plurality of characteristic loop sensitivity (S LC );   storing the plurality of characteristic loop sensitivity (S LC ) in the storage device; and   outputting data stored in the storage device.       

     An example of measuring a characteristic loop sensitivity of for an acoustic transducer in an acoustic probe was performed according to the present invention. The measured characteristic loop sensitivity of the given acoustic transducer is −49 dB. 
     The acoustic transducer under test in the example is in a transducer array of a commercial acoustic probe containing one hundred and ninety-two (192) acoustic transducers. The central frequency and bandwidth of the transducer are 7.3 MHz and 80%, respectively. In the measurement, a negative-going unipolar pulse with an amplitude of −75 volts and an upper bound frequency of 55 MHz was adopted as a wideband reference signal. The distance between the acoustic transducer and acoustic mirror is 20 mm. And, the material of the acoustic mirror is stainless-steel with an acoustic reflection coefficient of 0.93 in a water bath. 
       FIG. 18  shows a system for measuring a characteristic loop sensitivity of an acoustic transducer according to the present invention. The system  1800  comprises a pulse generator  1801 , a signal processing unit  1802 , a transducer selector  1804 , and a control unit  1806 . The control unit  1806  further comprises a firmware  1807 , a program memory  1808 , and a storage  1809 . 
     The control unit  1806  is electrically coupled to the pulse generator  1801 , to the signal processing unit  1802 , and to external output devices  1830 . 
     The pulse generator  1801  is electrically coupled to an acoustic transducer through the transducer selector  1804  for generating a pulse to create a wideband acoustic wave from the acoustic transducer. The pulse is one of a unipolar pulse and a bipolar pulse. The unipolar pulse is one of a negative-going pulse  400  and a positive-going pulse  500 . The bipolar pulse is one of a negative-positive bipolar pulse  700  and a positive-negative bipolar pulse  800 . 
     The reflected wideband echo wave is received by the acoustic transducer through the transducer selector  1804  to the signal processing unit  1802  for further processing. The transducer selector  1804  sequentially or randomly selects one transducer of a transducer array in an acoustic probe  113 . 
     The measuring method for obtaining an energy density of reference signal (D r ) for a wideband reference signal is embedded in one of the firmware  1807  and the program memory  1808  according to the present invention. 
     The measuring method for obtaining an energy density of echo signal (D e ) for a wideband echo signal is embedded in one of the firmware  1807  and the program memory  1808  according to the present invention. 
     The method for measuring a characteristic loop sensitivity (S LC ) for the acoustic transducer is embedded in one of the firmware  1807  and the program memory  1808  according to the present invention. 
     The method for measuring the plurality of characteristic loop sensitivity (S LC ) for each and all acoustic transducers in an acoustic transducer array is embedded in one of the firmware  1807  and the program memory  1808  according to the present invention. 
     All data of measurement are stored in the storage  1809  and output to the output devices  1830  according to the present invention. 
     While several embodiments have been described by way of example, it will be apparent to those skilled in the art that various modifications may be configured without departing from the spirit of the present invention. Such modifications are all within the scope of the present invention, as defined by the appended claims. 
     Numerical System 
     
       
         
           
               
             
               
                   
               
             
            
               
                 113 acoustic probe 
               
               
                 117A transducer array 
               
               
                 117 acoustic transducer 
               
               
                 200 sine burst generator 
               
               
                 204 reference signal 
               
               
                 208 water bath 
               
               
                 212 acoustic mirror 
               
               
                 214 transmitted acoustic sine burst wave 
               
               
                 218 reflected sine burst wave 
               
               
                 224 echo signal 
               
               
                 400 wideband reference signal of negative-going unipolar pulse 
               
               
                 404 energy spectrum of wideband reference signal of negative-going 
               
               
                 unipolar pulse 
               
               
                 500 wideband reference signal of positive-going unipolar pulse 
               
               
                 504 energy spectrum of wideband reference signal of positive-going 
               
               
                 unipolar pulse 
               
               
                 600 frequency response of acoustic transducer 
               
               
                 700 wideband reference signal of negative-positive bipolar pulse 
               
               
                 704 energy spectrum of wideband reference signal of negative-positive 
               
               
                 bipolar pulse 
               
               
                 800 wideband reference signal of positive-negative bipolar pulse 
               
               
                 804 energy spectrum of wideband reference signal of positive-negative 
               
               
                 bipolar pulse 
               
               
                 900 frequency response of acoustic transducer 
               
               
                 1000 pulse generator 
               
               
                 1004 transmitted wideband acoustic wave 
               
               
                 1008 reflected wideband acoustic wave 
               
               
                 1100 wideband echo signal based on negative-going unipolar pulse 
               
               
                 1200 wideband echo signal based on positive-going unipolar pulse 
               
               
                 1300 wideband echo signal based on negative-positive bipolar pulse 
               
               
                 1400 wideband echo signal based on positive-negative bipolar pulse 
               
               
                 1504 energy spectrum of wideband reference signal 
               
               
                 1604 energy spectrum of wideband reference signal 
               
               
                 1800 system 
               
               
                 1801 pulse generator 
               
               
                 1802 signal processing unit 
               
               
                 1804 transducer selector 
               
               
                 1806 control unit 
               
               
                 1807 firmware 
               
               
                 1808 program memory 
               
               
                 1809 storage 
               
               
                 1830 output devices 
               
               
                   
               
            
           
         
       
     
     Notation 
     Reference Signal 
                                    (V ppr )   peak-to-peak voltage of reference signal               (E r )   energy of reference signal; 
         E   r     =         1   50     ⁢     ∫           V   r     ⁡     (   t   )       2     ⁢   dt         =       1   50     ⁢     ∫                  V   ^     r     ⁡     (   𝒻   )            2     ⁢   df               
               (BW r )   bandwidth of reference signal;               (D r )   energy density of reference signal; 
         D   r     =       E   r       BW   r           
               V r (t)   wideband reference signal;       {circumflex over (V)} r (ƒ)   Fourier Transform of the wideband reference signal V r (t);                         1   50     ⁢                V   ^     r     ⁡     (   𝒻   )            2             energy spectrum of wideband reference signal;                    
Echo Signal
 
                                    (V ppe )   peak-to-peak voltage of echo signal;               (E e )   energy of echo signal; 
         E   e     =         1   50     ⁢     ∫           V   e     ⁡     (   t   )       2     ⁢   dt         =       1   50     ⁢     ∫                  V   ^     e     ⁡     (   𝒻   )            2     ⁢   df               
               (BW e )   bandwidth of echo signal;               (D e )   energy density of echo signal; 
         D   e     =       E   e       BW   e           
               V e (t)   wideband echo signal;       {circumflex over (V)} e (ƒ)   Fourier Transform of the wideband echo signal V e (t);                         1   50     ⁢                V   ^     e     ⁡     (   𝒻   )            2             energy spectrum of wideband echo signal;                    
Definition
 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 {circumflex over (X)}(ƒ) 
                 normalized loop frequency response 
             X   ^     ⁡     (   𝒻   )       ⁢     =   def     ⁢           V   ^     e     ⁡     (   𝒻   )             V   ^     r     ⁡     (   𝒻   )           ;         X   ^     ⁡     (   𝒻   )       ⁢     =   def           
 
               
               
                   
               
               
                 {circumflex over (V)} e (ƒ)/V r (ƒ); 
                   
               
               
                 X(t) 
                 normalized loop time response; Inverse Fourier Transform of the {circumflex over (X)}(ƒ) 
               
               
                   
               
               
                   
                 
                   
                     
                       
                         
                           X 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         ⁢ 
                         
                           = 
                           def 
                         
                         ⁢ 
                         
                           Inverse 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Fourier 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Transform 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               X 
                               ^ 
                             
                             ⁡ 
                             
                               ( 
                               𝒻 
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 S L (ƒ) 
                 wideband loop sensitivity is defined as an absolute square of the {circumflex over (X)}(ƒ) in decibel; 
               
               
                   
               
               
                   
                 
                   
                     
                       
                         
                           
                             S 
                             L 
                           
                           ⁡ 
                           
                             ( 
                             𝒻 
                             ) 
                           
                         
                         ⁢ 
                         
                           = 
                           def 
                         
                         ⁢ 
                         
                           10 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           log 
                           ⁢ 
                           
                             
                                
                               
                                 
                                   X 
                                   ^ 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   𝒻 
                                   ) 
                                 
                               
                                
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 (S LC ) 
                 characterisitic loop sensitivity 
         S   LC     ⁢     =   def     ⁢     10   ⁢           ⁢     log   ⁡     (       D   e       D   r       )             
 
               
               
                   
               
               
                 G(t) 
                 
                   
                     
                       
                         
                           Inverse 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Fourier 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Transform 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             
                               
                                 X 
                                 ^ 
                               
                               ⁡ 
                               
                                 ( 
                                 𝒻 
                                 ) 
                               
                             
                           
                         
                         ; 
                       
                     
                   
                   
                     
                       
                         
                             
                         
                         ⁢ 
                         
                           
                             G 
                             ⁡ 
                             
                               ( 
                               t 
                               ) 
                             
                           
                           = 
                           
                             Inverse 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             Fourier 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             Transform 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             of 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             the 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 
                                   X 
                                   ^ 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   𝒻 
                                   ) 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                   
                 self-deconvolution of the X(t); G(t) = Self-deconvolution of the X(t) 
               
               
                 B(t) 
                 an optimum drive signal on energy efficiency basis for the acoustic transducer; 
               
               
                   
               
               
                   
                             B   ⁡     (   t   )       ⁢     =   def     ⁢     α   *     G   ⁡     (   t   )           ,       
wherein a coefficient α is determined to multiply the function G(t).