Patent Publication Number: US-2021177453-A1

Title: Ultrasound transducer, ultrasound treatment tool, and manufacturing method of ultrasound transducer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/JP2018/032216, filed on Aug. 30, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an ultrasound transducer, an ultrasound treatment tool, and a manufacturing method of an ultrasound transducer. 
     2. Related Art 
     In the relater art, an ultrasound treatment tool that includes an ultrasound transducer generating ultrasound vibration according to a supplied drive signal and that treats a living tissue by applying the ultrasound vibration to the living tissue is known (see, for example, Japanese Patent No. 6261833). 
     The ultrasound transducer disclosed in Japanese Patent No. 6261833 includes a plurality of piezoelectric elements that are stacked and integrated by a bolt penetrating along a stacking direction of the plurality of piezoelectric elements and that alternately repeat expansion and contraction along the stacking direction according to a drive signal. That is, the ultrasound transducer includes a bolt-tightened Langevin type transducer (BLT). 
     SUMMARY 
     In some embodiments, an ultrasound transducer includes a transducer body configured to generate ultrasound vibration to vibrate a treatment portion that treats a living tissue, the transducer body including a plurality of piezoelectric elements configured to alternately repeat expansion and contraction according to a supplied drive signal, the plurality of piezoelectric elements being integrally fastened in a state where the plurality of piezoelectric elements are stacked along a direction of expansion and contraction, the plurality of piezoelectric elements including a first piezoelectric element configured to expand at a first timing and contract at a second timing, and a second piezoelectric element whose expansion and contraction timings are different from expansion and contraction timings of the first piezoelectric element, the second piezoelectric element being arranged so that a polarization direction of the second piezoelectric element is a same as a polarization direction of an adjacent piezoelectric element. 
     In some embodiments, an ultrasound treatment tool includes a treatment portion configured to treat a living tissue and an ultrasound transducer including a transducer body configured to generate ultrasound vibration to vibrate the treatment portion, the transducer body including a plurality of piezoelectric elements configured to alternately repeat expansion and contraction according to a supplied drive signal, the plurality of piezoelectric elements being integrally fastened in a state where the plurality of piezoelectric elements are stacked along a direction of expansion and contraction, the plurality of piezoelectric elements including a first piezoelectric element configured to expand at a first timing and contract at a second timing, and a second piezoelectric element whose expansion and contraction timings are different from expansion and contraction timings of the first piezoelectric element, the second piezoelectric element being arranged so that a polarization direction of the second piezoelectric element is a same as a polarization direction of an adjacent piezoelectric element. 
     In some embodiments, provided is a manufacturing method of an ultrasound transducer. The ultrasound transducer includes a plurality of piezoelectric elements configured to alternately repeat expansion and contraction according to a supplied drive signal, the plurality of piezoelectric elements being fastened integrally in a state where the plurality of piezoelectric elements are stacked along an expansion and contraction direction, the ultrasound transducer being configured to generate ultrasound vibration by expansion and contraction of the plurality of piezoelectric elements in constant current control. The manufacturing method includes measuring amplitude of the ultrasound transducer in a state of arrangement in a posture in which polarization directions of the plurality of piezoelectric elements are inverted for each piezoelectric element with respect to a normal state of arrangement in a posture in which the polarization directions of the plurality of piezoelectric elements are along a specific direction, and determining a piezoelectric element whose polarization direction is to be inverted, based on the amplitude, from among the plurality of piezoelectric elements. 
     The above and other features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a treatment system according to an embodiment. 
         FIG. 2  is a diagram illustrating an ultrasound transducer. 
         FIG. 3  is a diagram illustrating a first electrode plate. 
         FIG. 4  is a diagram illustrating a second electrode plate. 
         FIG. 5  is a flowchart illustrating a manufacturing method of an ultrasound transducer. 
         FIGS. 6A and 6B  are diagrams for explaining a manufacturing method of an ultrasound transducer. 
         FIG. 7  is a diagram for explaining a manufacturing method of an ultrasound transducer. 
     
    
    
     DETAILED DESCRIPTION 
     Schematic Configuration of Treatment System 
       FIG. 1  is a diagram illustrating a schematic configuration of a treatment system  1  according to the present embodiment. 
     The treatment system  1  treats a site to be treated (hereinafter, referred to as a target site) in a living tissue by applying ultrasound energy to the target site. Here, the treatment means, for example, coagulation and incision of a target site. As illustrated in  FIG. 1 , the treatment system  1  includes an ultrasound treatment tool  2  and a control device  3 . 
     Configuration of Ultrasound Treatment Tool 
     The ultrasound treatment tool  2  is, for example, a medical treatment tool that treats a target site while passing through the abdominal wall. As illustrated in  FIG. 1 , the ultrasound treatment tool  2  includes a handpiece  4  and an ultrasound transducer  5 . 
     As illustrated in  FIG. 1 , the handpiece  4  includes a holding case  41 , an operation knob  42 , a switch  43 , a sheath  44 , a jaw  45 , and an ultrasound probe  46 . 
     The holding case  41  supports the entire ultrasound treatment tool  2 . 
     The operation knob  42  is movably attached to the holding case  41  and accepts opening/closing operation by a surgeon. 
     The switch  43  is provided in a state of being exposed to the outside of the holding case  41 , and accepts output start operation by a surgeon. Then, the switch  43  outputs an operation signal corresponding to the output start operation to the control device  3 . 
     The sheath  44  has a cylindrical shape. Hereinafter, the central axis of the sheath  44  is referred to as a central axis Ax ( FIG. 1 ). Hereinafter, one side along the central axis Ax is referred to as a distal end side Ar 1  ( FIG. 1 ), and the other side will be referred to as a proximal end side Ar 2  ( FIG. 1 ). A part of the proximal end side Ar 2  is inserted from the distal end side Ar 1  of the holding case  41  to the inside of the holding case  41 , so that the sheath  44  is attached to the holding case  41 . 
     The jaw  45  is rotatably attached to the end of the distal end side Ar 1  of the sheath  44  and grips the target site in between the jaw  45  and a portion of the distal end side Ar 1  of the ultrasound probe  46 . The inside of the holding case  41  and the sheath  44  described above is provided with an opening/closing mechanism (not shown) that opens/closes the jaw  45  with respect to the portion of the distal end side Ar 1  of the ultrasound probe  46  in response to an opening/closing operation of the operation knob  42  by a surgeon. 
     The ultrasound probe  46  corresponds to a treatment portion according to the disclosure. This ultrasound probe  46  has a long shape extending linearly along the central axis Ax, and as illustrated in  FIG. 1 , is inserted to the inside of the sheath  44  in a state where a portion of the distal end side Ar 1  projecting outward. The end of the proximal end side Ar 2  of the ultrasound probe  46  is connected to an ultrasound transducer  6  (see (a) of  FIG. 2 ) included in the ultrasound transducer  5 . Then, the ultrasound probe  46  transmits the ultrasound vibration generated by the ultrasound transducer  6  from the end of the proximal end side Ar 2  to the end of the distal end side Ar 1 . 
     As illustrated in  FIG. 1 , the ultrasound transducer  5  includes an ultrasound transducer case  51  and the ultrasound transducer  6  (see (a) of  FIG. 2 ). 
     The ultrasound transducer case  51  supports the ultrasound transducer  6  and is attachably and detachably connected to the holding case  41 . 
       FIG. 2  is a diagram illustrating the ultrasound transducer  6 . 
     The ultrasound transducer  6  generates ultrasound vibration under the control of the control device  3 . In the present embodiment, the ultrasound transducer  6  includes a bolt-tightened Langevin type transducer (BLT). As illustrated in (a) of  FIG. 2 , the ultrasound transducer  6  includes a transducer body  7 , a front mass  8 , and a back mass  9 . 
     As illustrated in (a) of  FIG. 2 , the transducer body  7  includes first and second electrode plates  71 ,  72 , and a plurality of (six in the present embodiment) piezoelectric elements  73 . 
       FIG. 3  is a diagram illustrating the first electrode plate  71 . 
     The first electrode plate  71  is a portion to which a drive signal is supplied from the control device  3 . As illustrated in (a) of  FIG. 2  or  FIG. 3 , the first electrode plate  71  includes a plurality of (four in the present embodiment) negative electrode plates  711  and a plurality of (three in the present embodiment) negative electrode wiring portions  712 , and a lead wire connection terminal  713 . 
     As illustrated in  FIG. 3 , the plurality of negative electrode plates  711  each have a disk shape having an aperture  711   a  in the center, and are arrayed side by side along the central axis Ax. 
     The plurality of negative electrode wiring portions  712  are portions in which the outer edges of the negative electrode plates  711  adjacent to each other are electrically connect to each other. 
     The lead wire connection terminal  713  is a portion to which a lead wire C 1  ((a) of  FIG. 2 ) included in an electric cable C ( FIG. 1  and (a) of  FIG. 2 ) is connected. The lead wire connection terminal  713  extends from the outer edge of the negative electrode plate  711  located at the most proximal end side Ar 2  among the plurality of negative electrode plates  711  to the proximal end side Ar 2 . 
       FIG. 4  is a diagram illustrating the second electrode plate  72 . 
     The second electrode plate  72  is a portion to which a drive signal is supplied from the control device  3 . As illustrated in (a) of  FIG. 2  or  FIG. 4 , the second electrode plate  72  includes a plurality of (three in the present embodiment) positive electrode plates  721 , a plurality of (two in the present embodiment) positive electrode wiring portions  722 , and a lead wire connection terminal  723 . 
     As illustrated in  FIG. 4 , the plurality of positive electrode plates  721  each have a disk shape having an aperture  721   a  in the center, and are arrayed side by side along the central axis Ax. The positive electrode plate  721  has substantially the same shape as the negative electrode plate  711 . 
     The plurality of positive electrode wiring portions  722  are portions in which the outer edges of the positive electrode plates  721  adjacent to each other are electrically connected to each other. 
     The negative electrode plates  711  and the positive electrode plates  721  are alternately arranged along the central axis Ax as illustrated in (a) of  FIG. 2 . 
     The lead wire connection terminal  723  is a portion to which the lead wire C 1  included in the electric cable C is connected. The lead wire connection terminal  723  extends from the outer edge of the positive electrode plate  721  located at the most proximal end side Ar 2  among the plurality of positive electrode plates  721  to the proximal end side Ar 2 . 
     Each of the plurality of piezoelectric elements  73  has a disk shape having an aperture (not shown) in the center, and is arranged between the negative electrode plate  711  and the positive electrode plate  721 . That is, the plurality of piezoelectric elements  73  are stacked along the central axis Ax. Then, a potential difference is generated in the stacking direction along the central axis Ax in response to the drive signals supplied to the first and second electrode plates  71 ,  72 , so that the plurality of piezoelectric elements  73  alternately repeat expansion and contraction along the stacking direction. As a result, the ultrasound transducer  6  generates ultrasound vibration of longitudinal vibration whose vibration direction is the stacking direction. 
     The front mass  8  has an elongated shape extending linearly along the central axis Ax. As illustrated in (a) of  FIG. 2 , the front mass  8  includes an element mounting portion  81 , a cross-sectional area changing portion  82 , and a probe mounting portion  83 . 
     The element mounting portion  81  is a bolt extending linearly along the central axis Ax, and the element mounting portions  81  are inserted to each aperture  711   a  of the plurality of negative electrode plates  711 , each aperture  721   a  of the plurality of positive electrode plates  721 , and each aperture (not shown) of the plurality of piezoelectric elements  73 . As illustrated in (a) of  FIG. 2 , the back mass  9  formed of a nut is attached to an end of the element mounting portion  81  on the proximal end side Ar 2 . 
     The cross-sectional area changing portion  82  corresponds to a horn according to the disclosure. The cross-sectional area changing portion  82  is provided at an end of the element mounting portion  81  on the distal end side Ar 1 , and is a portion in which the amplitude of ultrasound vibration is amplified. As illustrated in (a) of  FIG. 2 , the cross-sectional area changing portion  82  is set so that an end of the cross-sectional area changing portion  82  on the proximal end side Ar 2  has a larger diameter than the element mounting portion  81 , and the cross-sectional area changing portion  82  has a truncated cone shape in which the cross-sectional area of the cross-sectional area changing portion  82  decreases toward the distal end side Ar 1 . That is, the element mounting portion  81  penetrates the plurality of negative electrode plates  711 , the plurality of positive electrode plates  721 , and the plurality of piezoelectric elements  73  along the central axis Ax, and the plurality of negative electrode plates  711 , the plurality of positive electrode plates  721 , and the plurality of piezoelectric elements  73  are sandwiched between the cross-sectional area changing portion  82  and the back mass  9 , so that the plurality of negative electrode plates  711 , the plurality of positive electrode plates  721 , and the plurality of piezoelectric elements  73  are integrally fastened in a state of having a substantially cylindrical shape. In the present embodiment, an insulating plate  84  ((a) of  FIG. 2 ) is interposed between the cross-sectional area changing portion  82  and the piezoelectric element  73 , and between the back mass  9  and the piezoelectric element  73 . 
     As illustrated in (a) of  FIG. 2 , the probe mounting portion  83  is provided at an end of the cross-sectional area changing portion  82  on the distal end side Ar 1 , and extends linearly along the central axis Ax. An end of the ultrasound probe  46  on the proximal end side Ar 2  is connected to an end of the probe mounting portion  83  on the distal end side Ar 1  in a state where the ultrasound transducer  5  is connected to the handpiece  4 . 
     Configuration of Control Device 
     The ultrasound treatment tool  2  is attachably and detachably connected to the control device  3  by the electric cable C ( FIG. 1 ). The control device  3  comprehensively controls the operation of the ultrasound treatment tool  2  by passing through the electric cable C. As illustrated in  FIG. 1 , the control device  3  includes an energy source  31 , a detection circuit  32 , and a processor  33 . 
     The energy source  31  outputs a drive signal, which is AC power, to the ultrasound transducer  6  via the electric cable C (a pair of lead wires C 1 ) under the control of the processor  33 . 
     The detection circuit  32  detects a US signal based on the drive signal output from the energy source  31  to the ultrasound transducer  6 . Then, the detection circuit  32  outputs the detected US signal to the processor  33 . 
     Here, examples of the US signal include a voltage phase signal in the drive signal (hereinafter, referred to as a US voltage phase signal), a current phase signal in the drive signal (hereinafter, referred to as a US current phase signal), a current value in the drive signal (hereinafter, referred to as a US current), a voltage value in the drive signal (hereinafter, referred to as US voltage), and an impedance value calculated from the US current and the US voltage (hereinafter, referred to as US impedance). 
     The processor  33  is, for example, a central processing unit (CPU) or a field-programmable gate array (FPGA), and controls the operation of the entire treatment system  1  according to a program stored in a memory (not shown). 
     The functions of the processor  33  will be described in the “Operation of treatment system” below. 
     Operation of Treatment System 
     Next, the operation of the treatment system  1  described above will be described. 
     A surgeon holds the ultrasound treatment tool  2  by hand and inserts a portion of the distal end side Ar 1  of the ultrasound treatment tool  2  into the abdominal cavity after passing the portion through the abdominal wall using, for example, a trocca. Then, the surgeon performs aperture and closing operation of the operation knob  42 , and grips the target site by the jaw  45  and the portion of the distal end side Ar 1  of the ultrasound probe  46 . Thereafter, the surgeon presses the switch  43 . Then, the processor  33  executes the controls described below in response to the operation signal from the switch  43 . 
     The processor  33  controls the operation of the energy source  31 , sweeps the frequency in the drive signal output from the energy source  31 , and searches for the resonance frequency of the ultrasound transducer  6  based on the US signal output from the detection circuit  32  (for example, the phase difference of the US current, the US current phase signal, and the US voltage phase signal). Then, the processor  33  controls the operation of the energy source  31 , and performs PLL control of locking the frequency in the drive signal output from the energy source  31  to the resonance frequency that has been found by the search, based on the US signal (for example, the phase difference among the US current, the US current phase signal, and the US voltage phase signal). In the present embodiment, the processor  33  performs constant current control of keeping the US current constant when performing PLL control. As a result, the ultrasound transducer  6  generates ultrasound vibration (longitudinal vibration). Here, the end of the cross-sectional area changing portion  82  on the proximal end side Ar 2  serves as a node P 1  of longitudinal vibration ((b) of  FIG. 2 ). The end of the probe mounting portion  83  on the distal end side Ar 1  and the end of the back mass  9  on the proximal end side Ar 2  are antinodes P 2  and P 3  ((b) of  FIG. 2 ) of longitudinal vibration, respectively. That is, the total length of the ultrasound transducer  6  is about half the wavelength of longitudinal vibration. 
     Then, the longitudinal vibration generated in the ultrasound transducer  6  is transmitted to the ultrasound probe  46 . As a result, a portion of the ultrasound probe  46  on the distal end side Ar 1  vibrates with a desired amplitude due to the longitudinal vibration. That is, ultrasound vibration is applied to the target site gripped between the jaw  45  and the portion of the ultrasound probe  46  on the distal end side Ar 1 , from the portion of the distal end side Ar 1 . In other words, ultrasound energy is applied to the target site from the portion of the ultrasound probe  46  on the distal end side Ar 1 . As a result, frictional heat is generated between the portion of the distal end side Ar 1  and the target site. Then, the target site is incised while coagulating. 
     Manufacturing Method of Ultrasound Transducer 
     Next, a manufacturing method of the ultrasound transducer  6  will be described. 
       FIG. 5  is a flowchart illustrating a manufacturing method of the ultrasound transducer  6 .  FIGS. 6A, 6B and 7  are diagrams for explaining a manufacturing method of the ultrasound transducer  6 . Specifically,  FIG. 6A  illustrates a configuration in which a plurality of piezoelectric elements  73  are arranged in a normal state.  FIG. 6B  illustrates a configuration of arrangement in a posture in which the polarization direction of a piezoelectric element  73   e  of the plurality of piezoelectric elements  73  is reversed from the normal state. 
     In  FIGS. 6A and 6B , the mounting positions of the plurality of piezoelectric elements  73  are set to the mounting positions “1” to “6” in order from the side close to the cross-sectional area changing portion  82  (node P 1 ) toward the proximal end side Ar 2 . The piezoelectric elements  73  at the mounting positions “1” to “6” are the piezoelectric elements  73   a  to  73   f , respectively. The positive charge inside the piezoelectric element  73  is represented by “+”, the negative charge is represented by “−”, and the polarization direction is represented by an arrow. The positions of the negative electrode plate  711  and the positive electrode plate  721  are represented by the alternate long and short dash line. 
     Here, the normal state illustrated in  FIG. 6A  means a state where the plurality of piezoelectric elements  73  are arranged in a posture in which the negative charge “−” side faces the negative electrode plate  711  and the positive charge “+” side faces the positive electrode plate  721 . 
     First, an operator performs Step S 1  described below. 
     The operator sets the plurality of piezoelectric elements  73  to the normal state illustrated in  FIG. 6A . The operator operates the control device  3  and causes the control device  3  to perform constant current control of the ultrasound transducer  6 . As a result, the ultrasound transducer  6  generates ultrasound vibration (longitudinal vibration). In this case, since the plurality of piezoelectric elements  73  are set to the normal state, the plurality of piezoelectric elements  73  expand at the same first timing and contract at the same second timing. Then, the operator measures the amplitude of the longitudinal vibration by using a vibrometer or the like. 
     After Step S 1 , the operator determines whether the amplitude measured in Step S 1  is the desired amplitude (Step S 2 ). 
     When it is determined that the amplitude is the desired amplitude (Step S 2 : Yes), the operator decides to use the ultrasound transducer  6  in the normal state (Step S 3 ). After Step S 3 , the operator finishes the manufacturing method of the ultrasound transducer  6 . 
     On the other hand, when it is determined that the amplitude is not the desired amplitude (Step S 2 : No), the operator performs Step S 4  described below. 
     For example, as illustrated in  FIG. 6B , the operator performs arrangement in a posture in which the polarization direction of one piezoelectric element  73  at any of the mounting positions “1” to “6” (the piezoelectric element  73   e  in the example of  FIG. 6B ) is inverted from the normal state. As similar to Step S 1 , the operator operates the control device  3  and causes the control device  3  to perform constant current control of the ultrasound transducer  6 . As a result, the ultrasound transducer  6  generates ultrasound vibration (longitudinal vibration). In this case, for example, in the state of  FIG. 6B , of the plurality of piezoelectric elements  73 , the piezoelectric elements  73   a  to  73   d ,  73   f  expand at the same first timing and contract at the same second timing. That is, the piezoelectric elements  73   a  to  73   d ,  73   f  correspond to the first piezoelectric element according to the disclosure. On the other hand, since the polarization direction of the piezoelectric element  73   e  is inverted from the normal state, the piezoelectric element  73   e  contracts at the first timing and expands at the second timing, contrary to the piezoelectric elements  73   a  to  73   d ,  73   f . That is, the piezoelectric element  73   e  corresponds to the second piezoelectric element according to the disclosure. Then, the operator measures the amplitude of the longitudinal vibration by using a vibrometer or the like. 
     After Step S 4 , the operator determines whether or not Step S 4  has been performed for all the piezoelectric elements  73  at the mounting positions “1” to “6” (Step S 5 ). 
     When it is determined that Step S 4  has not been performed for all the piezoelectric elements  73  (Step S 5 : No), the operator returns the process to Step S 4 . 
     On the other hand, when it is determined that Step S 4  has been performed for all the piezoelectric elements  73  (Step S 5 : Yes), the operator performs Step S 6  described below. 
     The operator refers to the amplitude measured in Step S 4 , and decides a mounting position at which an amplitude close to a desired amplitude is obtained by inverting the polarization direction, from among the mounting positions “1” to “6”. Then, the operator decides to use the ultrasound transducer  6  in a posture in which the polarization direction of the piezoelectric element  73  at the decided mounting position is inverted from the normal state. In the present embodiment, as illustrated in  FIG. 6B , the mounting position “5” is decided as the mounting position for inverting the polarization direction. 
     The relationship between the increase rate of each amplitude measured in Step S 4  with respect to the amplitude in the normal state measured in Step S 1  (hereinafter, referred to as the amplitude increase rate) and mounting positions “1” to “6” of the piezoelectric element  73  of which polarization direction has been inverted is as illustrated in  FIG. 7 . 
     The amplitude increase rate is a positive value regardless of which of the polarization direction of any of the piezoelectric elements  73   a  to  73   f  at the mounting positions “1” to “6” is inverted from the normal state. In other words, the amplitude is higher than the amplitude in the normal state. The reasons as below can be considered for this. 
     When the polarization direction of any of the piezoelectric elements  73   a  to  73   f  is reversed from the normal state, the vibration efficiency of the ultrasound transducer  6  becomes worse than that in the normal state. When the vibration efficiency deteriorates, the voltage value applied to each of the piezoelectric elements  73   a  to  73   f  in the constant current control becomes higher than in the normal state. That is, when the polarization direction of any of the piezoelectric elements  73   a  to  73   f  is inverted from the normal state, the applied voltage value becomes higher than the normal state, so that the amplitude becomes higher than the amplitude in the normal state. 
     The amplitude increase rate is highest when the polarization direction of the piezoelectric element  73   a  at the mounting position “1” is inverted from the normal state, and the amplitude decreases as the mounting position of the piezoelectric element  73  at which the polarization direction is inverted changes from “2” to “6” (as the mounting position changes so as to be away from the node P 1 ). The reasons as below can be considered for this. 
     Of the plurality of piezoelectric elements  73 , the piezoelectric element  73   a  mounted at a position close to the node P 1  has the highest vibration contribution rate that contributes to the vibration of the ultrasound transducer  6 , and the vibration contribution rate is smaller in the order of the piezoelectric elements  73   a  to  73   f  mounted at the mounting positions “1” to “6”. That is, when the polarization direction of the piezoelectric element  73  having a high vibration contribution rate is inverted from the normal state, the amplitude increase rate becomes high, and when the polarization direction of the piezoelectric element  73  having a low vibration contribution rate is inverted from the normal state, the amplitude increase rate becomes low. 
     According to the present embodiment described above, the effects as below are obtained. 
     In the ultrasound transducer  6  according to the present embodiment, the plurality of piezoelectric elements  73  includes the piezoelectric elements  73   a  to  73   d ,  73   f  that expand at the first timing and contract at the second timing, and the piezoelectric element  73   e  that contracts at the first timing and expands at the second timing. That is, a desired amplitude can be obtained only by inverting the polarization direction of the piezoelectric element  73   e  among the plurality of piezoelectric elements  73  from the normal state. At this time, even when the polarization direction of the piezoelectric element  73   e  is inverted from the normal state, the capacitance of the entire piezoelectric element  73  does not change from the capacitance of the entire piezoelectric element  73  in the normal state. Therefore, even when the polarization direction of the piezoelectric element  73   e  is inverted from the normal state, it is not necessary to change the control structure from the design manufacturing structure in order to control the ultrasound transducer  6 . That is, there is no need to make major design changes. 
     From the above, according to the ultrasound transducer  6  according to the present embodiment, a desired amplitude can be easily obtained without making a significant design change. 
     In the ultrasound transducer  6  according to the present embodiment, the plurality of piezoelectric elements  73  are stacked along the central axis Ax at the proximal end side Ar 2  relative to the node P 1 . In other words, the ultrasound transducer  6  has only one longitudinal vibration node P 1 , and its total length is about half the wavelength of the longitudinal vibration. Therefore, it is possible to reduce the size of the ultrasound transducer  6  and, further, the size of the ultrasound treatment tool  2 . 
     Other Embodiments 
     The embodiment for carrying out the disclosure has been described, but the disclosure should not be limited only by the above-described embodiment. 
     In the above-described embodiment, the plurality of piezoelectric elements  73  include six elements, but may include elements of other numbers. Further, of the six piezoelectric elements  73 , the fifth piezoelectric element  73   e  from the node P 1  i used as the second piezoelectric element according to the disclosure, but the disclosure is not limited to this, and the piezoelectric element  73  at other positions may be used as the second piezoelectric element according to the disclosure. The number of the second piezoelectric elements according to the disclosure is not limited to one, and may be any other number. 
     In the above-described embodiment, the piezoelectric element  73   e , which is the second piezoelectric element according to the disclosure contracts at the first timing and expands at the second timing, contrary to the piezoelectric elements  73   a  to  73   d ,  73   f  which are the first piezoelectric elements according to the disclosure. As the second piezoelectric element according to the disclosure, the expansion and contraction timings may be other timings as long as the expansion and contraction timings are different from those of the first piezoelectric element according to the disclosure. That is, if the timing of expansion and contraction of the second piezoelectric element according to the disclosure is different from that of the first piezoelectric element according to the disclosure, it is considered that the amplitude is higher than the amplitude in the normal state since the ultrasound transducer  6  has a worse vibration efficiency than that in the normal state. 
     In the above-described embodiment, the ultrasound transducer  6  has a total length of about half the wavelength of the ultrasound vibration, but the disclosure is not limited to this, and the ultrasound transducer  6  may have other total lengths, for example, a total length substantially the same as the wavelength. 
     In the above-described embodiment, the ultrasound treatment tool  2  employs a configuration in which ultrasound energy is applied to the target site, but the disclosure is not limited to this, and the ultrasound treatment tool  2  may employ a configuration in which ultrasound energy and at least one of high frequency and thermal energy is applied to the target site. Here, “applying high frequency energy to the target site” means passing a high-frequency current to the target site. Further, “applying heat energy to the target site” means transferring heat from a heater or the like to the target site. 
     According to an ultrasound transducer, an ultrasound treatment tool, and a manufacturing method of an ultrasound transducer according to the disclosure, a desired amplitude can be easily obtained. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the disclosure in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.