Patent Publication Number: US-2011077517-A1

Title: Ultrasonic diagnostic apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ultrasonic diagnostic apparatus, which applies an ultrasonic wave to a human body part and images the inside of the body part based on an echo of the ultrasonic wave. 
     2. Description Related to the Prior Art 
     An ultrasonic diagnostic apparatus is used for examination of a fetus in utero and various internal body parts including lacteal gland, thyroid gland, and the like, because of the advantage of noninvasively imaging the inside of tissue of the body part in real time. In a conventional ultrasonic diagnostic apparatus, ultrasonic transducers apply an ultrasonic wave of a predetermined frequency to the human body part to be imaged. Then, the same ultrasonic transducers receive an echo of the ultrasonic wave, and output reception signals based on the echo. The ultrasonic diagnostic apparatus images a cross section of the body part based on the reception signals. 
     Each ultrasonic transducer has a piezoelectric element that is made of a piezoelectric material such as lead zirconate titanate (PZT), for example, formed into a predetermined shape. Upon application of a pulse voltage to front and bottom surfaces of the piezoelectric element, the piezoelectric element repeats expansion and contraction, and emits the ultrasonic wave. When the echo from the internal body part is incident upon the ultrasonic transducer, on the other hand, the piezoelectric element expands or contracts. The expansion or contraction brings about electric potential difference between the front and bottom surfaces of the piezoelectric element, and produces the reception signal. Since the resonant frequency of the piezoelectric element is determined by the size and shape of the piezoelectric element, the ultrasonic transducer mainly emits the ultrasonic wave of the resonant frequency (hereinafter called fundamental frequency), and outputs the reception signal in which a fundamental frequency component (hereinafter called fundamental component) of the incident echo is mainly reflected. 
     Furthermore, it is known that the echo contains components the frequencies of which are other than the fundamental frequency. This is because dispersion of the ultrasonic wave by a living body is a nonlinear phenomenon, and these components having the frequencies other than the fundamental frequency reflect detailed tissue structure of the internal body part. Thus, a method called harmonic imaging is recently used, in which the component (hereinafter called harmonic component) the frequency of which is an integer multiple of the fundamental frequency is used for production of an ultrasonic image. The harmonic imaging can reduce adverse effects of multiple reflection and side lobe. As a result, the ultrasonic image produced with use of the harmonic component has better lateral, resolution and contrast resolution than those of the ultrasonic image produced only from the fundamental component, and hence the sharper ultrasonic image is obtained (refer to Japanese Patent No. 4192598 and Japanese Patent Laid-Open Publication No. 11-276478). 
     Conventionally, the ultrasonic diagnostic apparatus was large stationary equipment set up in a large hospital. However, the portable ultrasonic diagnostic apparatus, which can be set up in a medical clinic or carried about bedsides of a hospital ward for use, is widely available in recent years. In such a portable ultrasonic diagnostic apparatus, it is desired to reduce power consumption as much as possible, considering that the portable ultrasonic diagnostic apparatus is driven with electric power supply only from an internal battery. However, if the ultrasonic transducers for transmitting and receiving the ultrasonic wave are driven at a low voltage, the echo itself from the internal body part is weakened. This causes a shortage of sensitivity and degradation in image quality of the ultrasonic image. Especially, since the harmonic component of the echo is conspicuously reduced, it becomes difficult to observe the detailed tissue structure and make a correct diagnosis. 
     The ultrasonic diagnostic apparatus is constituted of an ultrasonic probe and an ultrasonic observing device (processor device) that processes the reception signals obtained by the ultrasonic probe and displays the ultrasonic image. A cable for connecting the ultrasonic probe to the ultrasonic observing device sometimes interferes with operation of the ultrasonic probe. The portable ultrasonic diagnostic apparatus, in particular, is small in size and light in weight. Thus, if the cable is thick or rigid, the ultrasonic observing device moves together with movement of the ultrasonic probe, and causes interference with the diagnosis. For this reason, it is desired to reduce the diameter of the cable or eliminate the cable by using wireless communication between the ultrasonic probe and the ultrasonic observing device. 
     SUMMARY OF THE INVENTION 
     A main object of the present invention is to provide an ultrasonic diagnostic apparatus that can sensitively receive a harmonic component at low power. 
     Another object of the present invention is to provide the ultrasonic diagnostic apparatus having an ease-to-operate ultrasonic probe by reducing the diameter of a cable between the ultrasonic probe and an ultrasonic observing device. 
     To achieve the above and other objects, an ultrasonic diagnostic apparatus according to the present invention includes an ultrasonic probe and an ultrasonic observing device. The ultrasonic probe includes an ultrasonic transducer array, a reception circuit, a detector, a parallel-to-serial converter, a switching device, and a controller. The ultrasonic transducer array has a plurality of channels arranged in a line. Each of the channels has a pair of a first ultrasonic transducer for transmitting and receiving an ultrasonic wave of a fundamental frequency and a second ultrasonic transducer for receiving a harmonic wave having a frequency of an integer multiple of the fundamental frequency. The reception circuit amplifies a first reception signal from the first ultrasonic transducer and a second reception signal from the second ultrasonic transducer, and applies analog-to-digital conversion to the first and second reception signals. The detector detects an output signal from the reception circuit with use of a reference signal with a predetermined angular frequency. The parallel-to-serial converter converts an output signal from the detector into a serial signal. The switching device switches between a first mode and a second mode. In the first mode, the first reception signal from the first ultrasonic transducer and the second reception signal from the second ultrasonic transducer are added on a pair basis, and inputted to the reception circuit. In the second mode, only the second reception signal from the second ultrasonic transducer is inputted to the reception circuit. The controller changes the angular frequency of the reference signal in accordance with a state of the switching device. The ultrasonic observing device produces an ultrasonic image from the serial signal transmitted from the ultrasonic probe. 
     The ultrasonic diagnostic apparatus may further include a resonant circuit having a variable resonant frequency disposed between the second ultrasonic transducer and the reception circuit. The controller determines the angular frequency in accordance with the resonant frequency. 
     The controller may set the resonant frequency at the fundamental frequency in the first mode, and set the resonant frequency at a frequency of the harmonic wave in the second mode. 
     In the first mode, the resonant frequency may be changed in accordance with reception timing of the ultrasonic wave. 
     The resonant circuit may include an inductor and a variable capacitance capacitor connected in parallel. The resonant frequency is adjusted by varying a capacitance of the variable capacitance capacitor. The variable capacitance capacitor may be a varicap. 
     It is preferable that the first ultrasonic transducer has a piezoelectric element made of an inorganic material, and the second ultrasonic transducer has a piezoelectric element made of an organic material. 
     The pair of the first ultrasonic transducer and the second ultrasonic transducer may be stacked. 
     It is preferable that the ultrasonic probe and the ultrasonic observing device are portable. 
     The ultrasonic diagnostic apparatus may further include a cable for transmitting the serial signal from the ultrasonic probe to the ultrasonic observing device. The cable adheres to one of standards of USB3.0, sATAgen2, and 10 GbaseT. 
     The serial signal may be transmitted by wireless communication from the ultrasonic probe to the ultrasonic observing device. 
     According to the present invention, the ultrasonic probe can sensitively receive the harmonic component, even if driven at a low voltage. The present invention can reduce the diameter of the cable between the ultrasonic probe and the ultrasonic observing device, and facilitates handling of the ultrasonic probe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For more complete understanding of the present invention, and the advantage thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a portable ultrasonic diagnostic apparatus; 
         FIG. 2  is a block diagram of the ultrasonic diagnostic apparatus; 
         FIG. 3  is a circuit diagram in a normal mode; 
         FIG. 4  is a circuit diagram in a tissue harmonic imaging (THI) mode; 
         FIG. 5  is a timing chart in an operation state of the ultrasonic diagnostic apparatus; 
         FIG. 6  is a graph showing the sensitivity of an ultrasonic transducer; and 
         FIG. 7  is a timing chart in another operation state of the ultrasonic diagnostic apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 1 , a portable ultrasonic diagnostic apparatus  10  is constituted of an ultrasonic observing device (processor device)  11  and an ultrasonic probe  12 . The ultrasonic observing device  11  is composed of a main body  13  and a cover  14 . On a top surface of the main body  13 , an operation unit  16  having a plurality of buttons and a trackball for inputting various operation commands is provided. Inside of the cover  14 , there is provided a monitor  17  (for example, liquid crystal display) for displaying an ultrasonic image and various operation screens. 
     The cover  14  is hinged on the main body  13  with a hinge  18 , and is rotatable between an open position in which the operation unit  16  and the monitor  17  are exposed, and a closed position in which the top surface of the main body  13  is faced to an inner surface of the cover  14  to cover both of the operation unit  16  and the monitor  17  for protection. A grip (not illustrated) is attached to a side surface of the main body  13  to make the ultrasonic observing device  11  convenient to carry about in a state of closing the main body  13  and the cover  14 . In the other opposite side surface of the main body  13 , there is provided a probe connection portion  19  to which the ultrasonic probe  12  is detachably connected. 
     The ultrasonic probe  12  is constituted of a scan head  21 , which a doctor holds and presses against a human body part to be imaged, a connector  22  connected to the probe connection portion  19 , and a cable  23  for connecting the scan head  21  to the connector  22 . The scan head  21  contains an ultrasonic transducer array  24  at its distal end. In the ultrasonic transducer array  24 , ultrasonic transducers for composing a plurality of channels are aligned in an azimuth (AZ) direction. 
     Viewing a cross section of the ultrasonic transducer array  24  in an elevation (EL) direction, as shown in  FIG. 2 , the ultrasonic transducer array  24  has such structure that a backing material  31 , a first electrode  32 , a first piezoelectric element  33 , a common electrode  34 , a second piezoelectric element  36 , a second electrode  37 , an acoustic impedance matching layer  38 , and an acoustic lens  39  are stacked in this order on a plate-shaped mount support (not illustrated) made of a glass-epoxy resin or the like. The first electrode  32 , the first piezoelectric element  33 , and the common electrode  34  compose a first ultrasonic transducer  41 . The common electrode  34 , the second piezoelectric element  36 , and the second electrode  37  compose a second ultrasonic transducer  42 . Thus, in the ultrasonic transducer array  24 , the single first ultrasonic transducer  41  and the single second ultrasonic transducer  42  are stacked in a single channel. Each of the first and second ultrasonic transducers  41  and  42  has the shape of a block long in the EL direction. A lot of pairs of the stacked first and second ultrasonic transducers  41  and  42  are aligned in the AZ direction at regular intervals via a filling material therebetween. 
     The backing material  31  is made of an epoxy resin, a silicone resin, or the like, and absorbs ultrasonic wave that is emitted from the first ultrasonic transducer  41  in the direction of the mount support. The backing material  31  is in a gentle dome shape at a top surface, and has a convex cross-section in the AZ direction, which is orthogonal to the EL direction. 
     The first electrode  32  is so disposed as to sandwich the first piezoelectric element  33  together with the common electrode  34 . To the first electrode  32 , a drive pulse (Tx) is inputted to drive the first piezoelectric element  33 . In addition, upon reception of echo from the internal body part by the first piezoelectric element  33 , a reception signal is obtained through the first electrode  32 . 
     The first piezoelectric element  33  is made of an inorganic material such as lead zirconate titanate (PZT), for example. Each first piezoelectric element  33  has the shape of a block long in the EL direction. Upon input of the drive pulse to the first electrode  32 , the first piezoelectric element  33  corresponding to the first electrode  32  expands and contracts in response to the drive pulse, and generates the ultrasonic wave of a frequency (fundamental frequency) f 1 , which is determined by the shape of the first piezoelectric element  33 . Upon reception of the echo from the internal body part, on the other hand, the first piezoelectric element  33  generates electric potential difference between the first electrode  32  and the common electrode  34  in accordance with the echo. The electric potential difference produces a first reception signal, which is obtained through the first electrode  32 . Since the resonant frequency of the first piezoelectric element  33  depends on the shape of the first piezoelectric element  33 , the first reception signal obtained from the first piezoelectric element  33  is sensitive to the fundamental frequency f 1  and the vicinity thereof. The first ultrasonic transducer  41  is used for both of transmission and reception. 
     The common electrode  34  is disposed between the first piezoelectric element  33  and the second piezoelectric element  36 , and establishes a ground connection on a package of the scan head  21 . The common electrode  34  also functions as an acoustic impedance matching layer that relieves the difference in acoustic impedance between the first piezoelectric element  33  and the second piezoelectric element  36 . 
     The second piezoelectric element  36  is made of an organic material such as polyvinylidene fluoride (PVDF), for example. Each second piezoelectric element  36 , as with the first piezoelectric element  33 , has the shape of a block long in the EL direction. Although the second piezoelectric element does not clearly have a resonance characteristic because of being made of the organic material, the thickness of the second piezoelectric element  36  is so designed as to mainly resonate with the second harmonic wave (frequency of 2 f 1 ) of the echo. Upon reception of the echo, the second piezoelectric element  36  generates the electric potential difference between the common electrode  34  and the second electrode  37  in accordance with the echo. From the electric potential difference, a signal is produced in which wide frequency components including a second harmonic component are reflected. At the same time, the second piezoelectric element  36  functions as the acoustic impedance matching layer, and relieves the difference in the acoustic impedance from an ambient structure. The second piezoelectric element  36  does not generate the ultrasonic wave, and thus is used only for the reception. 
     The second electrode  37  is so disposed as to sandwich the second piezoelectric element  36  together with the common electrode  34 . As described above, the electric potential difference generated between the common electrode  34  and the second electrode  37  by the second piezoelectric element  36  in response to the reception of the echo yields a second reception signal outputted from the second electrode  37 . 
     The acoustic impedance matching layer  38  relieves the difference in the acoustic impedance between the ultrasonic transducer array  24  and a human body. The acoustic lens  39  is made of a silicone resin or the like, and has a convex cross section in the EL direction. Therefore, the acoustic lens  39  focuses the ultrasonic wave generated from the first ultrasonic transducer  41  on the internal body part to be imaged in the EL direction. 
     The ultrasonic probe  12  is provided with multiplexers (MUXs)  51  and  52 , a transmission circuit  53 , a resonant circuit  54 , a reception circuit  56 , a quadrature detector  57 , a parallel-to-serial converter  58 , a communication interface  61 , and a controller  62 , in addition to the ultrasonic transducer array  24  having above structure. 
     The MUX  51  successively connects the transmission circuit  53  to the single first ultrasonic transducer  41  selected out of the plurality of first ultrasonic transducers  41 . Upon reception of the echo, the MUX  51  successively connects one of the plurality of first ultrasonic transducers  41  to the reception circuit  56  through a mode change switch (hereinafter simply called switch)  55 . The MUX  52  also connects the single second ultrasonic transducer  42  selected out of the plurality of the second ultrasonic transducers  42  to the reception circuit  56 . The first and second ultrasonic transducers  41  and  42  are grouped by the MUXs  51  and  52 , and driven from group to group. In the adjoining groups, the ultrasonic transducers  41 ,  42  are partly overlapped. 
     The transmission circuit  53  inputs the drive pulse to the first ultrasonic transducer  41  connected through the MUX  51 . The transmission circuit  53  successively inputs the drive pulse to each of the first ultrasonic transducers  41  belonging to the same group with predetermined time delay. Thus, the ultrasonic transducer array  24  scans the internal body part with an ultrasonic beam, which is focused at a predetermined depth in the AZ direction. 
     The resonant circuit  54  is connected in parallel with the second ultrasonic transducer  42  in the vicinity of the second ultrasonic transducer  42 . The resonant frequency of the resonant circuit  54  is variable, so that the resonant circuit  54  can adjust the frequency of the second reception signal inputted from the second ultrasonic transducer  42  to the reception circuit  56 . If the switch  55  is turned off, the second reception signal is inputted by itself from the second ultrasonic transducer  42  to the reception circuit  56 . At this time, adjusting the resonant frequency of the resonant circuit  54  can select the frequency of the reception signal (the second reception signal) inputted to the reception circuit  56 . If the switch  55  is turned on, on the other hand, both of the first reception signal from the first ultrasonic transducer  41  and the second reception signal from the second ultrasonic transducer  42  travel the same signal output line, and are inputted to the reception circuit  56  as a composite reception signal added from pair to pair. At this time, the resonant circuit  54  acts only on a second reception signal component out of the composite reception signal of each pair. Thus, if the switch  55  is turned on, the composite reception signal, into which the first reception signal and the second reception signal having the frequency selected by the resonant circuit  54  are added from pair to pair, is inputted to the reception circuit  56 . 
     The reception circuit  56  includes a plurality of sets of amplifiers  63 , low-pass filters (LPFs)  64 , and analog-to-digital converters (A/Ds)  66 . To the reception circuit  56 , depending on a state of the switch  55  as described above, the analog composite reception signal into which the first reception signal and the second reception signal are added is inputted if the switch  55  is turned on, and the analog second reception signal obtained from the second ultrasonic transducer  42  is inputted if the switch  55  is turned off. In the reception circuit  56 , the amplifier  63  amplifies the inputted reception signal, and the LPF  64  removes noise of high frequencies. Then, the A/D  66  converts the analog reception signal into the digital reception signal, which is then inputted to the quadrature detector  57 . The number of sets of the amplifiers  63 , the LPFs  64 , and the A/Ds  66  corresponds with the number of the first and second ultrasonic transducers  41  and  42  belonging to the single group, which are selected by the MUXs  51  and  52  on an occasion of reception of the echo. Thus, the reception circuit  56  simultaneously applies above processing to the plurality of reception signals inputted on the single occasion, and inputs to the quadrature detector  57  the processed reception signals in parallel with one another. 
     The quadrature detector  57  applies quadrature detection processing to each of the reception signals inputted from the reception circuit  56  to produce an I-phase signal and a Q-phase signal, and sampling processing at a predetermined sampling frequency to produce a complex baseband signal. The quadrature detector  57 , as described later on, carries out the quadrature detection processing with use of a reference signal, which depends on the resonant frequency of the resonant circuit  54 . The quadrature detector  57 , as described above, simultaneously applies the quadrature detection processing to the plurality of reception signals inputted from the reception circuit  56  at the same time to produce the complex base band signals, and inputs the complex base band signals to the parallel-to-serial converter  58 . 
     The parallel-to-serial converter  58  converts the plurality of complex baseband signals inputted in parallel from the quadrature detector  57  into a serial reception signal. The serial reception signal is transferred to the ultrasonic diagnostic apparatus  11  with a predetermined protocol through a communication interface  61 , which includes the connector  22 , the cable  23 , and the like. Information and the like inputted from the operation unit  16  is inputted to the controller  62  of the ultrasonic probe  12  through the communication interface  61 . 
     The controller  62  is connected to each part inside the ultrasonic probe  12 , to overall control the ultrasonic probe  12 . For example, the controller  62  controls the transmission circuit  53  so that the predetermined ultrasonic beam is emitted from the ultrasonic transducer array  24 , as described above. The controller  62  switches operation modes of the ultrasonic diagnostic apparatus  10  by switching a turn on or off of the switch  55  in response to input from the operation unit  16 . In accordance with a state of the switch  55  and the like, the controller  62  adjusts the resonant frequency of the resonant circuit  54 , and controls the quadrature detector  57  to carry out the quadrature detection processing with use of the reference signal corresponding to the adjusted resonant frequency of the resonant circuit  54 . 
     The ultrasonic observing device  11  is provided with an image generator  71 , a controller  72 , a battery  76 , and the like. The image generator  71  generates the ultrasonic image from the reception signal transmitted from the ultrasonic probe  12 . At this time, the image generator  71  first converts the reception signal obtained through the communication interface  73  back into original parallel data. The image generator  71  applies reception focusing processing to the parallel data by phase addition, and produces acoustic ray data along predetermined scanning lines. Then, the image generator  71  produces a B-mode image or an M-mode image of the ultrasonic image from the acoustic ray data of a single frame in accordance with setting, and displays the B-mode or M-mode image on the monitor  17 . 
     The controller  72  overall controls each part of the ultrasonic observing device  11  in accordance with input from the operation unit  16 , and inputs a control signal to the controller  62  of the ultrasonic probe  12  through the communication interface  73  to control operation of the ultrasonic probe  12 . 
     The battery  76  supplies electric power to each part of the ultrasonic observing device  11 , and supplies electric power to each part of the ultrasonic probe  12  through the probe connection portion  19 , the connector  22 , the cable  23 , and the like (refer to  FIG. 1 ). 
     The ultrasonic diagnostic apparatus  10  having above structure is switchable by turning on or off of the switch  55  between two modes, that is, a normal mode in which the ultrasonic image is produced from a fundamental component of the echo, and a tissue harmonic imaging (THI) mode in which the ultrasonic image is produced from a harmonic component of the echo. In either of the two operation modes, upon input of the drive pulses from the transmission circuit  53  to the first ultrasonic transducers  41 , the ultrasonic beam is applied from the ultrasonic transducer array  24  to the body part to be imaged. In either of the normal mode and the THI mode, the ultrasonic diagnostic apparatus  10  drives the ultrasonic transducer array  24  at a low voltage, in order to reduce power consumption of the battery  76 . 
     When the ultrasonic diagnostic apparatus  10  is in the normal mode, as shown in  FIG. 3 , the switch  55  is turned on, and a signal output line of the first ultrasonic transducer  41  is connected to a signal output line of the second ultrasonic transducer  42 . Thus, in the normal mode, the composite reception signal, into which the first reception signal from the first ultrasonic transducer  41  and the second reception signal from the second ultrasonic transducer  42  are added, is inputted to the reception circuit  56 . 
     The first ultrasonic transducer  41  is regarded as a capacitor having a capacitance of C a , and the second ultrasonic transducer  42  is regarded as a capacitor having a capacitance of C b . The resonant circuit  54  is composed of an inductor (hereinafter called inductor L) having an inductance of L and a capacitor (hereinafter called variable capacitance capacitor C v ,) having a capacitance of C v  that are connected in parallel. The resonant circuit  54  is connected to the signal output line via a damping resistance R. As the variable capacitance capacitor C v , a variable capacitance diode (so-called varicap) is used, in which the thickness of a depletion layer is actively variable in accordance with the magnitude of an applied direct-current voltage. 
     In the normal mode, the controller  62  of the ultrasonic probe  12  sets the variable capacitance capacitor C v  at C 1  satisfying 
     
       
         
           
             
               
                 f 
                 1 
               
               = 
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                     
                       L 
                       × 
                       
                         C 
                         1 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     so that the resonant frequency of the resonant circuit  54  becomes the fundamental frequency f 1 . The resonant circuit  54  functions as a circuit that has almost infinite impedance to a signal of the fundamental frequency f 1 , and has lower impedance than that of the reception circuit  56  to signals other than the fundamental frequency f 1 . Therefore, components of the second reception signal having frequencies other than the fundamental frequency f 1  are absorbed to ground through the resonant circuit  54 . On the other hand, a component of the second reception signal having the fundamental frequency f 1  is transmitted to the reception circuit  56  through the signal output line. As a result, the composite reception signal inputted to the reception circuit  56  is an addition of the first reception signal, which is outputted from the first ultrasonic transducer  41  and almost only has the component of the fundamental frequency f 1 , and the fundamental component of the second reception signal outputted from the second ultrasonic transducer  42 , on a pair basis. The controller  62  also set an angular frequency ω of the reference signal used in the quadrature detector  57  at ω 1  satisfying 
     
       
         
           
             
               ω 
               1 
             
             = 
             
               
                 
                   f 
                   1 
                 
                 
                   2 
                    
                   π 
                 
               
               . 
             
           
         
       
     
     The quadrature detector  57  divides each reception signal outputted from the reception circuit  56  in two. One of the divided reception signal is multiplied by a reference signal cos ω 1 t, and is passed through a low-pass filter (LPF)  81  to produce the I-phase signal. Then, since a sampling circuit  82  applies sampling processing to the I-phase signal with a predetermined sampling frequency, the baseband I-phase reception signal is inputted to the parallel-to-serial converter  58 . The other one of the divided reception signal is multiplied by a reference signal sin ω 1 t, and is passed through a low-pass filter (LPF)  83  to produce the Q-phase signal. Then, a sampling circuit  84  applies sampling processing to the Q-phase signal, as with the sampling circuit  82 , so that the baseband Q-phase reception signal is inputted to the parallel-to-serial converter  58 . 
     The parallel-to-serial converter  58  coverts the reception signals processed as described above into serial data, and transfers the serial data to the ultrasonic observing device  11 . The ultrasonic observing device  11  produces the ultrasonic image from the reception signals obtained as described above, and display the ultrasonic image on the monitor  17 . Thus, the tomographic image displayed on the monitor  17  in the normal mode visualizes the internal body part with the fundamental component of the echo. 
     When the ultrasonic diagnostic apparatus  10  is in the THI mode, as shown in  FIG. 4 , the switch  55  is turned off, and the signal output line connected to the first ultrasonic transducer  41  is cut off from the reception circuit  56 . Thus, only the second reception signal from the second ultrasonic transducer  42  is inputted to the reception circuit  56 . 
     In the THI mode, the controller  62  of the ultrasonic probe  12  sets the variable capacitance capacitor C v  at C 2  satisfying 
     
       
         
           
             
               
                 2 
                  
                 
                   f 
                   1 
                 
               
               = 
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                     
                       L 
                       × 
                       
                         C 
                         2 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     so that the resonant frequency of the resonant circuit  54  becomes the second harmonic frequency f 2 . Thus, the resonant circuit  54  functions as a circuit that has almost infinite impedance to a signal of the frequency 2 f 1 , and has lower impedance than that of the reception circuit  56  to signals having frequencies of other than the frequency 2 f 1 . The reception signal inputted to the reception circuit  56  contains only the second harmonic component of the second reception signal outputted from the second ultrasonic transducer  42 . Thus, the ultrasonic diagnostic apparatus  10  is sensitive to the second harmonic component, even if driven at the low voltage. The controller  62  also set an angular frequency ω of the reference signal used in the quadrature detector  57  at ω 2  satisfying 
     
       
         
           
             
               ω 
               2 
             
             = 
             
               
                 
                   2 
                    
                   
                     f 
                     1 
                   
                 
                 
                   2 
                    
                   π 
                 
               
               . 
             
           
         
       
     
     As in the case of the normal mode, the quadrature detector  57  divides each reception signal outputted from the reception circuit  56  in two. The divided reception signals are multiplied by reference signals (cos ω 2 t and sin ω 2 t) of the angular frequency ω 2  determined as above, and are passed through the LPFs  81  and to produce the I-phase signal and the Q-phase signal, respectively. Then, since the sampling circuits  82  and  84  apply the sampling processing to the I-phase signal and the Q-phase signal, respectively, the baseband I-phase reception signal and the baseband Q-phase reception signal are inputted to the parallel-to-serial converter  58 . 
     After that, as in the case of the normal mode, the ultrasonic observing device  11  produces the ultrasonic image, and displays the ultrasonic image on the monitor  17 . The ultrasonic image generated from the second harmonic component is displayed in the THI mode, though the ultrasonic image generated from the fundamental component is displayed in the normal mode. Consequently, although the ultrasonic transducer array  24  is driven at the low voltage in both of the normal mode and the THI mode, the ultrasonic image obtained in the THI mode has higher definition than that of the ultrasonic image obtained in the normal mode. 
     The ultrasonic diagnostic apparatus  10  can be switched between the normal mode and the THI mode at almost arbitrary timing with the operation from the operation unit  16 . Now, a case is considered, as shown in  FIG. 5 , where after the drive pulse Tx for transmitting the ultrasonic wave from the ultrasonic transducer array  24  is inputted at T 1 , the operation for switching from the normal mode to the THI mode is carried out from the operation unit  16 , before the next drive pulse Tx for transmitting the next ultrasonic wave is inputted at T 2 . 
     In this case, the controller  62  receives the control signal that commands mode switching from the controller  72  of the ultrasonic observing device  11 , but the controller  62  keeps the switch  55  turned on between T 1  and T 2 . Also, the variable capacitance capacitor C v  is kept at C 1 , and the angular frequency ω of the reference signal is kept at ω 1  in the quadrature detector  57 , to drive the ultrasonic probe  12  in the normal mode. Accordingly, the reception signal Rx inputted to the reception circuit  56  has the fundamental frequency f 1  between T 1  and T 2 . 
     Then, the controller  62  turns the switch  55  off at T 2 . The controller  62  also changes the variable capacitance capacitor C v  to C 2 , and changes the angular frequency ω of the reference signal to ω 2  in the quadrature detector  57 , to drive the ultrasonic probe in the THI mode. Accordingly, after T 2 , the reception signal Rx inputted to the reception circuit  56  contains only the second harmonic component of the second reception signal outputted from the second ultrasonic transducer  42 . 
     The ultrasonic diagnostic apparatus  10 , as described above, is flexibly switchable between the normal mode and the THI mode, and the angular frequency ω of the reference signal used in the quadrature detector  57  is changed in accordance with the selected operation mode. Thus, even if the ultrasonic transducer array  24  is driven at the low voltage to reduce the power consumption, and the reception sensitivity is lowered with reduction in the transmission power, the quadrature detector  57  can enhance the frequency component that is necessary in each operation mode relative to the other frequency components by application of the quadrature detection processing. Especially in the THI mode, the second harmonic component is sensitively received, even if the ultrasonic transducer array  24  is driven at the lower voltage. 
     As described above, since the second ultrasonic transducer  42  is so designed as to mainly resonate with the second harmonic wave (frequency 2 f 1 ), the second ultrasonic transducer  42  can sensitively receive the second harmonic component without the resonant circuit  54 . However, providing the resonant circuit  54  for the ultrasonic transducer  42  further enhances the reception sensitivity to the second harmonic component. Therefore, even if the ultrasonic transducer array  24  is driven at the low voltage, the ultrasonic image with the higher definition can be easily obtained in the THI mode. 
     In the ultrasonic diagnostic apparatus  10 , the reception signals from the ultrasonic transducer array  24  are detected in the ultrasonic probe  12 , and the reception signals are transmitted to the ultrasonic observing device  11  after serialization. Thus, it is possible to reduce the diameter of the cable  23  (or eliminate the cable  23 ) between the ultrasonic observing device  11  and the ultrasonic probe  12 , and the ultrasonic probe  12  becomes easier to use. 
     In the above embodiment, the capacitance of the variable capacitance capacitor C v  is set at C 1 , when the ultrasonic diagnostic apparatus  10  is in the normal mode. In the case of observing a deep view (deep area) in the normal mode, the ultrasonic probe  12  is preferably driven with varying the capacitance of the variable capacitance capacitor C v  as follows. 
       FIG. 6  shows the sensitivity characteristics of the first ultrasonic transducer  41  (PZT) and the second ultrasonic transducer  42  (PVDF). The sensitivity of the first ultrasonic transducer  41  is high at a low frequency band including the fundamental frequency f 1 , and is gradually reduced with increase in the frequency above a certain frequency. On the other hand, the sensitivity of the second ultrasonic transducer  42  is approximately constant in a range of frequencies where the first ultrasonic transducer  41  is sensible, though the second ultrasonic transducer  42  is so designed as to resonate with the frequency 2 f 1  of the second harmonic wave. Accordingly, when f H  denotes the frequency at which the sensitivity of the first ultrasonic transducer  41  becomes almost zero, and f L  denotes the frequency at which the sensitivity of the first ultrasonic transducer  41  and the sensitivity of the second ultrasonic transducer  42  intersect in a graph, the second ultrasonic transducer  42  is more sensitive than the first ultrasonic transducer  41  to a signal in a frequency band between the frequencies f L  and f H . 
     It is known that the ultrasonic wave attenuates in accordance with the magnitude of the frequency, concurrently with propagation distance. Especially inside the living body, it is known that the ultrasonic wave attenuates in proportion to the frequency. The echo from a deep point of the internal body part tends to lose a high frequency component, in comparison with the echo from a shallow point of the internal body part. Thus, even if the echo from the deep point has a center frequency of the fundamental frequency f 1  at the time of occurrence of the echo, the echo loses almost all signals in the frequency band (f L  to f H ), in which the sensitivities of the first and second ultrasonic transducers  41  and  42  are reversed, at the time of reception by the ultrasonic transducer array,  24 . As a result, it&#39;becomes impossible to produce the ultrasonic image with high definition with which tissue structure of the internal body part is observable. 
     Thus, as shown in  FIG. 7 , when the echo from a shallow point A is received in the normal mode, the reception signal is obtained with setting the variable capacitance capacitor C v  at C 1  and setting the angular frequency ω of the reference signal used in the quadrature detector  57  at ω 1 , as described in the above embodiment. On the contrary, if the echo from a deep point B is received with setting the variable capacitance capacitor C v  at C 1  and setting the angular frequency ω of the reference signal at ω 1 , the reception signal Rx, as schematically shown by a chain double-dashed line, is useless for production of the ultrasonic image due to attenuation of high frequency components. 
     For this reason, in receiving the echo from the deep point B, the capacitance of the variable capacitance capacitor C v  is proportionally increased from C L  to C H  with time. The capacitance C L  is determined by 
     
       
         
           
             
               
                 f 
                 L 
               
               = 
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                     
                       L 
                       × 
                       
                         C 
                         L 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     and the capacitance C H  is determined by 
     
       
         
           
             
               f 
               H 
             
             = 
             
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                     
                       L 
                       × 
                       
                         C 
                         H 
                       
                     
                   
                 
               
               . 
             
           
         
       
     
     Similarly, in receiving the echo from the deep point B, the angular frequency ω of the reference signal used in the quadrature detector  57  is proportionally reduced from ω L  to ω H  with time. The angular frequency ω L  is determined by 
     
       
         
           
             
               
                 ω 
                 L 
               
               = 
               
                 
                   f 
                   L 
                 
                 
                   2 
                    
                   π 
                 
               
             
             , 
           
         
       
     
     and the angular frequency ω H  is determined by 
     
       
         
           
             
               ω 
               H 
             
             = 
             
               
                 
                   f 
                   H 
                 
                 
                   2 
                    
                   π 
                 
               
               . 
             
           
         
       
     
     By setting the capacitance C v  and the angular frequency ω as described above, the reception signal Rx produced from the echo from the deep point B majorly contains the second reception signal outputted from the second ultrasonic transducer  42 . 
     As described above, in receiving the echo from the deep point B, if the attenuation of the high frequency component is small, setting the capacitance C v  and the angular frequency ω as above within a frequency range (fundamental frequency range) originally receivable by the first ultrasonic transducer  41  is effective at obtaining the ultrasonic image with high definition that is available for diagnosis of the deep point B. 
     The depth of a border at which the capacitance C v  is changed from C 1  to C L  (the angular frequency ω is changed from ω 1  to ω L ) is variable in accordance with input of the control signal from the operation unit  16 . It is preferable that the depth at which the capacitance C v  starts being changed from C 1  and the angular frequency ω starts being changed from ω 1  in the normal mode be automatically variable in accordance with the depth of a focus of the ultrasonic beam, the sound pressure of the ultrasonic beam, material properties of the body part to be imaged, and the like. 
     In the above embodiment, the ultrasonic probe  12  and the ultrasonic observing device  11  are connected via the cable  23 . In the ultrasonic diagnostic apparatus  10 , however, the reception signal is digitized and serialized in the ultrasonic probe  12 , and then is transmitted to the ultrasonic observing device  11 . Thus, a small-diameter cable for transmission of digital data is usable as the cable  23 . It is preferable that the cable  23  used in the ultrasonic diagnostic apparatus  10  adhere to any standard of USB3.0, sATAgen2, and 10 GbaseT, for example. Use of such a small-diameter cable significantly facilitates handling of the ultrasonic probe  12 . 
     The ultrasonic probe  12  and the ultrasonic observing device  11  are connected with the cable  23  in the above embodiment, but transmission and reception of data between the ultrasonic probe  12  and the ultrasonic observing device  11  may be carried out by wireless communication. In this case, the communication interfaces  61  and  73  are compliant with a wireless communication interface. 
     In the above embodiment, the first ultrasonic transducer  41  and the second ultrasonic transducer  42  are vertically stacked. However, the first ultrasonic transducers  41  and the second ultrasonic transducers  42  may be arranged in such a way that alternate arrangement in the AZ direction, parallel (two lines) arrangement, or the like. 
     The varicap is used as the variable capacitance capacitor C v  in the above embodiment, but anything is available as the variable capacitance capacitor C v  as long as the capacitance is variable. 
     In the above embodiment, the second ultrasonic transducer  42  is amenable to the reception of the second harmonic wave, but may be amenable to the third or more harmonic wave. 
     In the above embodiment, the first piezoelectric element  33  is made of PZT, and the second piezoelectric element  36  is made of PVDF. However, the first piezoelectric element  33  may be made of any piezoelectric material as long as the piezoelectric element can transmit and receive the ultrasonic wave of the fundamental frequency f 1 , and the second piezoelectric element  36  may be made of any piezoelectric material as long as the piezoelectric material can receive the harmonic wave. However, if the first ultrasonic transducer  41  and the second ultrasonic transducer  42  are stacked as described above, it is preferable that the first piezoelectric element  33  for transmission and reception of the fundamental wave be made of the inorganic material such as PZT, and the second piezoelectric element  36  for reception of the harmonic wave be made of the organic material such as PVDF. 
     The present invention is applied to the portable ultrasonic diagnostic apparatus  10  in the above embodiment, but may be applicable to a stationary type of ultrasonic diagnostic apparatus. 
     In the above embodiment, the ultrasonic diagnostic apparatus  10  is switched from the normal mode to the THI mode in synchronization with input of the drive pulse Tx to the ultrasonic transducer array  24 . However, the operation mode may be switched after output of the ultrasonic image of a frame, after the control signal for mode switching is inputted from the operation unit  16 , for example. The same goes for switching from the THI mode to the normal mode. 
     Although the present invention has been fully described by the way of the preferred embodiment thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.