Patent Publication Number: US-2023148999-A1

Title: Ultrasound diagnostic apparatus and operation method of ultrasound diagnostic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of copending application Ser. No. 16/428,687, filed on May 31, 2019, which claims priority under 35 U.S.C. § 119(a) to Application No. 2018-124170, filed in Japan on Jun. 29, 2018, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ultrasound diagnostic apparatus, which performs polarization processing on a plurality of ultrasound transducers that an ultrasound endoscope comprises, and an operation method of an ultrasound diagnostic apparatus. 
     2. Description of the Related Art 
     An ultrasound diagnostic apparatus that acquires an ultrasound image of the inside of a subject by transmitting and receiving ultrasound waves by driving a plurality of ultrasound transducers inside the subject is already known. In the ultrasound diagnostic apparatus, the plurality of ultrasound transducers are, for example, single crystal transducers that are piezoelectric elements, and are usually used in a polarized state. The ultrasound transducer that is a single crystal transducer can receive ultrasound waves with high sensitivity, but a depolarization phenomenon in which the degree of polarization decreases as the driving time increases may occur. In a case where a depolarization phenomenon occurs, the reception sensitivity of the ultrasound transducer decreases, which may affect the image quality of the ultrasound image. 
     In particular, in the case of transmitting and receiving ultrasound waves by driving each ultrasound transducer inside the subject, since it is necessary to set the frequency of the ultrasound wave to a high frequency band of 7 MHz to 8 MHz level, a transducer having a relatively small thickness is used. However, as the thickness of the transducer decreases, the risk of occurrence of a depolarization phenomenon increases. 
     For this reason, techniques for countermeasures against depolarization in the ultrasound diagnostic apparatus have been developed so far. For example, an ultrasound diagnostic apparatus (referred to as a “piezoelectric sensor apparatus” in JP2013-005137A) described in JP2013-005137A has a piezoelectric element having a piezoelectric body and a pair of electrodes interposing the piezoelectric body therebetween, a detection circuit for performing detection processing for detecting a detection signal output from the piezoelectric element, and a polarization processing circuit for performing polarization processing by applying a polarization voltage to the piezoelectric element. In the ultrasound diagnostic apparatus described in JP2013-005137A having such a configuration, for example, polarization processing is performed at a timing at which the electric power is supplied, a timing at which a request signal for performing detection processing is input (each reception timing), or a timing at which a predetermined standby transition time has passed after the end of detection processing, and the polarization processing is ended in a case where it is determined that the polarization processing has ended by counting the processing time with a timer. Therefore, even in a case where a depolarization phenomenon occurs in the piezoelectric element, the piezoelectric element can be polarized again. As a result, it is possible to maintain the reception sensitivity of the piezoelectric element. 
     As another example, an ultrasound diagnostic apparatus (referred to as an “ultrasound sensor” in JP2017-143353A) described in JP2017-143353A has a piezoelectric element and a driving circuit for driving the piezoelectric element. The driving circuit drives the piezoelectric element with a driving waveform having first to sixth steps. The first step is a step of maintaining the polarization of the piezoelectric element with a first potential V1. The second step is a step of transmitting an ultrasound wave to the piezoelectric element after the first step. The third step is a step of causing the piezoelectric element to stand by at a second potential V2 after the second step. The fourth step is a step of increasing the second potential V2 to a third potential V3 after the third step. The fifth step is a step of maintaining the third potential V3 while the piezoelectric element receives an ultrasound wave after the fourth step. The sixth step is a step of returning the third potential V3 to the first potential V1 after the fifth step. In the ultrasound diagnostic apparatus described in JP2017-143353A having such a configuration, it is possible to drive the piezoelectric element while maintaining the polarization of the piezoelectric element by driving the piezoelectric element with the driving waveform having the first to sixth steps described above. 
     The ultrasound diagnostic apparatus described in JP2012-139460A has an ultrasound probe including a piezoelectric element, a storage unit that stores a threshold value of a physical quantity that changes with the degree of depolarization of the piezoelectric element, a recording unit that records the cumulative use time of the ultrasound probe, a detection unit that detects a physical quantity in the ultrasound probe, and a high voltage application unit that applies a high voltage for repolarizing the piezoelectric element to an electrode pair of the piezoelectric element. In the ultrasound diagnostic apparatus described in JP2012-139460A having such a configuration, a physical quantity (for example, a voltage value of a reception signal) is detected in a case where the cumulative use time of the ultrasound probe reaches a predetermined time, and a high voltage is applied to the electrode pair to perform repolarization processing in a case where it is determined that the detection result of the physical quantity is equal to or less than the threshold value. In this manner, it is possible to cope with degradation of the polarization characteristic of the piezoelectric element of the ultrasound probe at an appropriate timing. 
     An ultrasound diagnostic apparatus (referred to as an “ultrasound apparatus” in JP6158017B) described in JP6158017B has an ultrasound transducer that transmits and receives ultrasound waves to and from a subject and a controller that performs control to apply a polarization voltage to the ultrasound transducer. In the ultrasound diagnostic apparatus described in JP6158017B having such a configuration, a polarization voltage set to be a voltage having a magnitude used to transmit ultrasound waves for acquiring an ultrasound image is applied to the ultrasound transducer in a state in which the ultrasound transducer is excited and heated. By heating the ultrasound transducer, the polarization voltage can be made lower than that at the room temperature. Therefore, it is possible to perform repolarization processing using a circuit that performs transmission beam forming. 
     SUMMARY OF THE INVENTION 
     As described above, in the ultrasound diagnostic apparatus described in each of JP2013-005137A, JP2017-143353A, JP2012-139460A, and P6158017B, it is possible to restore or maintain the polarization of the piezoelectric element. 
     However, as in the ultrasound diagnostic apparatus described in JP2013-005137A, providing a dedicated circuit for performing repolarization, a depolarization detection mechanism, and the like is a large hardware change factor. Accordingly, it is very difficult to mount those described above in the existing system. The same applies to a case where it is necessary to provide a high voltage application unit that applies a high voltage for repolarizing a piezoelectric element to an electrode pair of the piezoelectric element as in the ultrasound diagnostic apparatus described in JP2012-139460A. 
     On the other hand, JP6158017B describes the ultrasound diagnostic apparatus that performs repolarization processing using a circuit that performs transmission beam forming. However, in the ultrasound diagnostic apparatus described in JP6158017B, it is necessary to output a DC waveform for applying a polarization voltage to the ultrasound transducer in order to perform repolarization processing in addition to pulse waves for exciting the ultrasound transducer to transmit ultrasound waves in order to perform transmission beam forming. Therefore, since the pulse wave and the DC waveform are output in the same circuit, the circuit size becomes large. This may cause an increase in cost. 
     In the ultrasound diagnostic apparatus described in JP2017-143353A, in order to maintain polarization, the pulse length of the driving waveform is increased by inserting a DC component into each driving waveform. Accordingly, the frame rate may be reduced to affect the image quality of the ultrasound image. 
     It is an object of the invention to provide an ultrasound diagnostic apparatus and an operation method of an ultrasound diagnostic apparatus capable of performing polarization processing using an existing circuit without causing a cost increase and without affecting the image quality of an ultrasound image. 
     In order to achieve the aforementioned object, the invention provides an ultrasound diagnostic apparatus acquiring an ultrasound image and an endoscope image. The ultrasound diagnostic apparatus comprises: an ultrasound endoscope comprising an ultrasound observation portion that transmits ultrasound waves using an ultrasound transducer array in which a plurality of ultrasound transducers are arranged, receives reflected waves of the ultrasound waves, and outputs a reception signal; and an ultrasound processor apparatus that generates the ultrasound image by converting the reception signal into an image. The ultrasound processor apparatus comprises: a control circuit that performs polarization processing on the plurality of ultrasound transducers in a non-diagnosis period, during which transmission of the ultrasound waves and reception of the reflected waves for performing ultrasound diagnosis are not performed, in a case where a cumulative driving time of the plurality of ultrasound transducers for performing the ultrasound diagnosis becomes equal to or longer than a specified time; and a transmission circuit that generates a transmission signal for driving the plurality of ultrasound transducers to generate the ultrasound waves using a pulse generation circuit and supplies the transmission signal to the plurality of ultrasound transducers under control of the control circuit. The transmission circuit generates a first transmission signal having a driving voltage for performing the ultrasound diagnosis using the pulse generation circuit in a case of performing the ultrasound diagnosis, and generates a second transmission signal having a polarization voltage for performing the polarization processing using the same pulse generation circuit as in the case of generating the first transmission signal in a case of performing the polarization processing. Reception signals of the plurality of ultrasound transducers in a frequency band of a first ultrasound wave generated by the first transmission signal and reception signals of the plurality of ultrasound transducers in a frequency band of a main lobe of a second ultrasound wave generated by the second transmission signal have different band characteristics. 
     Here, it is preferable that band characteristics of the reception signals of the plurality of ultrasound transducers in the frequency band of the first ultrasound wave generated by the first transmission signal and band characteristics of the reception signals of the plurality of ultrasound transducers in the frequency band of the main lobe of the second ultrasound wave generated by the second transmission signal do not overlap at a level of −20 dB or more. 
     It is preferable that an operation mode includes a first mode in which the polarization processing is not performed during the non-diagnosis period and a second mode in which the polarization processing is performed during the non-diagnosis period. It is preferable that the control circuit shifts the operation mode from the first mode to the second mode in a case where the cumulative driving time becomes equal to or longer than the specified time in the first mode and shifts the operation mode from the second mode to the first mode in a case where a difference obtained by subtracting the cumulative driving time from a cumulative processing time of the plurality of ultrasound transducers for performing the polarization processing becomes equal to or greater than a threshold value in the second mode. 
     It is preferable that the control circuit performs the polarization processing in a case of a freeze mode in which an image of one frame of the ultrasound image is displayed. 
     It is preferable that the control circuit performs the polarization processing in a case where a screen for setting a control parameter of the ultrasound diagnostic apparatus is displayed. 
     It is preferable that the control circuit performs the polarization processing in a case where a screen for inputting information of a patient to be subjected to the ultrasound diagnosis is displayed. 
     It is preferable that the control circuit performs the polarization processing in a case where a screen for designating a part to be subjected to the ultrasound diagnosis is displayed. 
     It is preferable that the control circuit performs the polarization processing in a case where a screen for displaying an ultrasound image generated in a past is displayed. 
     It is preferable that the control circuit performs the polarization processing in a case where only the endoscope image is displayed. 
     It is preferable that the ultrasound processor apparatus further comprises a notification circuit that notifies a user that the polarization processing is being performed. It is preferable that the control circuit controls the notification circuit to notify the user that the polarization processing is being performed in a case where the ultrasound image is displayed so as to be smaller than the endoscope image by picture in picture and sets an operation mode to a freeze mode in which an image of one frame of the ultrasound image is displayed to perform the polarization processing. 
     In addition, the invention provides an operation method of an ultrasound diagnostic apparatus acquiring an ultrasound image and an endoscope image. The operation method of an ultrasound diagnostic apparatus comprises: a step in which an ultrasound observation portion that an ultrasound endoscope of the ultrasound diagnostic apparatus comprises transmits ultrasound waves using an ultrasound transducer array in which a plurality of ultrasound transducers are arranged, receives reflected waves of the ultrasound waves, and outputs a reception signal; and a step in which an ultrasound processor apparatus of the ultrasound diagnostic apparatus generates the ultrasound image by converting the reception signal into an image. The step of generating the ultrasound image includes: a step in which a control circuit of the ultrasound processor apparatus performs polarization processing on the plurality of ultrasound transducers in a non-diagnosis period, during which transmission of the ultrasound waves and reception of the reflected waves for performing ultrasound diagnosis are not performed, in a case where a cumulative driving time of the plurality of ultrasound transducers for performing the ultrasound diagnosis becomes equal to or longer than a specified time; and a step in which a transmission circuit of the ultrasound processor apparatus generates a transmission signal for driving the plurality of ultrasound transducers to generate the ultrasound waves using a pulse generation circuit and supplies the transmission signal to the plurality of ultrasound transducers under control of the control circuit. The step of generating the transmission signal includes: a step of generating a first transmission signal having a driving voltage for performing the ultrasound diagnosis using the pulse generation circuit in a case of performing the ultrasound diagnosis; and a step of generating a second transmission signal having a polarization voltage for performing the polarization processing using the same pulse generation circuit as in the case of generating the first transmission signal in a case of performing the polarization processing. Reception signals of the plurality of ultrasound transducers in a frequency band of a first ultrasound wave generated by the first transmission signal and reception signals of the plurality of ultrasound transducers in a frequency band of a main lobe of a second ultrasound wave generated by the second transmission signal have different band characteristics. 
     Here, it is preferable that band characteristics of the reception signals of the plurality of ultrasound transducers in the frequency band of the first ultrasound wave generated by the first transmission signal and band characteristics of the reception signals of the plurality of ultrasound transducers in the frequency band of the main lobe of the second ultrasound wave generated by the second transmission signal do not overlap at a level of −20 dB or more. 
     It is preferable that an operation mode includes a first mode in which the polarization processing is not performed during the non-diagnosis period and a second mode in which the polarization processing is performed during the non-diagnosis period. It is preferable that, in the step of performing the polarization processing, the operation mode is shifted from the first mode to the second mode in a case where the cumulative driving time becomes equal to or longer than the specified time in the first mode, and the operation mode is shifted from the second mode to the first mode in a case where a difference obtained by subtracting the cumulative driving time from a cumulative processing time of the plurality of ultrasound transducers for performing the polarization processing becomes equal to or greater than a threshold value in the second mode. 
     In the invention, the polarization processing is performed using the existing pulse generation circuit. In the ultrasound diagnostic apparatus, the second transmission signal in the case of performing polarization processing is a pulse wave, and the pulse generation circuit does not need to output a DC waveform. Therefore, it is possible to perform the polarization processing without significantly changing the existing circuit and accordingly without increasing the cost. 
     In addition, since the polarization processing is performed during the non-diagnosis period, the frame rate is not reduced. Therefore, without reducing the image quality of the ultrasound image, the reception sensitivities of the plurality of ultrasound transducers can always be kept satisfactory. As a result, a high-quality ultrasound image can always be acquired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram showing the schematic configuration of an ultrasound diagnostic apparatus according to an embodiment of the invention. 
         FIG.  2    is a plan view showing a distal end portion of an insertion part of an ultrasound endoscope and its periphery. 
         FIG.  3    is a diagram showing a cross section of the distal end portion of the insertion part of the ultrasound endoscope taken along the line I-I in  FIG.  2   . 
         FIG.  4    is a block diagram showing the configuration of an ultrasound processor apparatus. 
         FIG.  5    is a diagram showing the flow of a diagnostic process using the ultrasound diagnostic apparatus. 
         FIG.  6    is a diagram showing the flow of a diagnostic process in a first mode. 
         FIG.  7    is a diagram showing the flow of a diagnostic process in a second mode. 
         FIG.  8    is a diagram showing the procedure of a diagnostic step in the diagnostic process. 
         FIG.  9    is an explanatory diagram showing the relationship between the cumulative driving time and the polarization processing execution time of an ultrasound transducer and the reception sensitivity of the ultrasound transducer. 
         FIG.  10    is a conceptual diagram of an example showing display modes. 
         FIG.  11 A  is a graph showing an example of a driving waveform of a polarization driving pulse transmitted from a transmission circuit shown in  FIG.  4   . 
         FIG.  11 B  is a graph showing the relationship between the frequency and the sensitivity of the driving waveform of the polarization driving pulse shown in  FIG.  11 A . 
         FIG.  12 A  is a graph showing another example of the driving waveform of the polarization driving pulse transmitted from the transmission circuit shown in  FIG.  4   . 
         FIG.  12 B  is a graph showing the relationship between the frequency and the sensitivity of the driving waveform of the polarization driving pulse shown in  FIG.  11 A  and the relationship between the frequency and the sensitivity of the driving waveform of the polarization driving pulse shown in  FIG.  12 A . 
         FIG.  13 A  is a graph showing another example of the pulse waveform of the polarization driving pulse transmitted from the transmission circuit shown in  FIG.  4   . 
         FIG.  13 B  is a graph showing the relationship between the frequency and the sensitivity of the driving waveform of the polarization driving pulse shown in  FIG.  13 A . 
         FIG.  13 C  is a graph showing another example of the pulse waveform of the polarization driving pulse transmitted from the transmission circuit shown in  FIG.  4   . 
         FIG.  13 D  is a graph showing the relationship between the frequency and the sensitivity of the driving waveform of the polarization driving pulse shown in  FIG.  13 C . 
         FIG.  14 A  is a graph showing another example of the pulse waveform of the diagnostic driving pulse transmitted from the transmission circuit shown in  FIG.  4   . 
         FIG.  14 B  is a graph showing the relationship between the frequency and the sensitivity of the driving waveform of the diagnostic driving pulse shown in  FIG.  14 A . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An ultrasound diagnostic apparatus according to an embodiment (the present embodiment) of the invention will be described in detail below with reference to preferred embodiments shown in the accompanying diagrams. 
     The present embodiment is a representative embodiment of the invention, but is merely an example and does not limit the invention. 
     In addition, in this specification, the numerical range expressed by using “˜” means a range including numerical values described before and after “˜” as a lower limit and an upper limit. 
     &lt;&lt;Outline of Ultrasound Diagnostic Apparatus&gt;&gt; 
     The outline of an ultrasound diagnostic apparatus  10  according to the present embodiment will be described with reference to  FIG.  1   .  FIG.  1    is a diagram showing the schematic configuration of ultrasound diagnostic apparatus  10 . 
     The ultrasound diagnostic apparatus  10  is used to observe (hereinafter, also referred to as ultrasound diagnosis) the state of an observation target part in a body of a patient, who is a subject, using ultrasound waves. Here, the observation target part is a part that is difficult to examine from the body surface side of the patient, for example, a gallbladder or a pancreas. By using the ultrasound diagnostic apparatus  10 , it is possible to perform ultrasound diagnosis of the state of the observation target part and the presence or absence of an abnormality through gastrointestinal tracts such as esophagus, stomach, duodenum, small intestine, and large intestine which are body cavities of the patient. 
     The ultrasound diagnostic apparatus  10  acquires an ultrasound image and an endoscope image, and as shown in  FIG.  1   , has an ultrasound endoscope  12 , an ultrasound processor apparatus  14 , an endoscope processor apparatus  16 , a light source device  18 , a monitor  20 , a water supply tank  21   a,  a suction pump  21   b,  and a console  100 . 
     The ultrasound endoscope  12  is an endoscope, and comprises an insertion part  22  to be inserted into the body cavity of a patient, an operation unit  24  operated by an operator (user), such as a doctor or a technician, and an ultrasound transducer unit  46  attached to a distal end portion  40  of the insertion part  22  (refer to  FIGS.  2  and  3   ). By the function of the ultrasound endoscope  12 , the operator can acquire an endoscope image of the inner wall of the body cavity of the patient and an ultrasound image of the observation target part. 
     Here, the “endoscope image” is an image obtained by imaging the inner wall of the body cavity of the patient using an optical method. The “ultrasound image” is an image obtained by receiving a reflected wave (echo) of an ultrasound wave transmitted from the inside of the body cavity of the patient to the observation target part and converting the reception signal into an image. 
     The ultrasound endoscope  12  will be described in detail later. 
     The ultrasound processor apparatus  14  is connected to the ultrasound endoscope  12  through a universal cord  26  and an ultrasound connector  32   a  provided at an end portion of the universal cord  26 . The ultrasound processor apparatus  14  controls the ultrasound transducer unit  46  of the ultrasound endoscope  12  to transmit the ultrasound wave. In addition, the ultrasound processor apparatus  14  generates an ultrasound image by converting the reception signal in a case where the reflected wave (echo) of the transmitted ultrasound wave is received by the ultrasound transducer unit  46  into an image. 
     The ultrasound processor apparatus  14  will be described in detail later. 
     The endoscope processor apparatus  16  is connected to the ultrasound endoscope  12  through the universal cord  26  and an endoscope connector  32   b  provided at an end portion of the universal cord  26 . The endoscope processor apparatus  16  generates an endoscope image by acquiring image data of an observation target adjacent part imaged by the ultrasound endoscope  12  (more specifically, a solid-state imaging element  86  to be described later) and performing predetermined image processing on the acquired image data. 
     Here, the “observation target adjacent part” is a portion of the inner wall of the body cavity of the patient that is adjacent to the observation target part. 
     In the present embodiment, the ultrasound processor apparatus  14  and the endoscope processor apparatus  16  are formed by two apparatuses (computers) provided separately. However, the invention is not limited thereto, and both the ultrasound processor apparatus  14  and the endoscope processor apparatus  16  may be formed by one apparatus. 
     The light source device  18  is connected to the ultrasound endoscope  12  through the universal cord  26  and a light source connector  32   c  provided at an end portion of the universal cord  26 . The light source device  18  emits white light or specific wavelength light formed of three primary color light components of red light, green light, and blue light at the time of imaging the observation target adjacent part using the ultrasound endoscope  12 . The light emitted from the light source device  18  propagates through the ultrasound endoscope  12  through a light guide (not shown) included in the universal cord  26 , and is emitted from the ultrasound endoscope  12  (more specifically, an illumination window  88  to be described later). As a result, the observation target adjacent part is illuminated with the light from the light source device  18 . 
     The monitor  20  is connected to the ultrasound processor apparatus  14 , and the endoscope processor apparatus  16 , and displays an ultrasound image generated by the ultrasound processor apparatus  14  and an endoscope image generated by the endoscope processor apparatus  16 . As a display method of the ultrasound image and the endoscope image, either one of the images may be switched and displayed on the monitor  20 , or both the images may be displayed at the same time. Display modes of the ultrasound image and the endoscope image will be described later. 
     In the present embodiment, the ultrasound image and the endoscope image are displayed on one monitor  20 . However, a monitor for displaying the ultrasound image and a monitor for displaying the endoscope image may be separately provided. In addition, the ultrasound image and the endoscope image may be displayed in a display form other than the monitor  20 . For example, the ultrasound image and the endoscope image may be displayed on a display of a terminal carried by the operator. 
     The console  100  is an apparatus provided for the operator to input information necessary for ultrasound diagnosis or for the operator to instruct the ultrasound processor apparatus  14  to start ultrasound diagnosis. The console  100  is configured to include, for example, a keyboard, a mouse, a trackball, a touch pad, and a touch panel. In a case where the console  100  is operated, a CPU (control circuit)  152  (refer to  FIG.  4   ) of the ultrasound processor apparatus  14  controls each unit of the apparatus (for example, a reception circuit  142  and a transmission circuit  144  to be described later) according to the operation content. 
     Specifically, the operator inputs examination information (for example, examination order information including a date, an order number, and the like and patient information including a patient ID, a patient name, and the like) through the console  100  before starting the ultrasound diagnosis. In a case where the operator gives an instruction to start the ultrasound diagnosis through the console  100  after the input of the examination information is completed, the CPU  152  of the ultrasound processor apparatus  14  controls each unit of the ultrasound processor apparatus  14  so that the ultrasound diagnosis is performed based on the input examination information. 
     The operator can set various control parameters with the console  100  at the time of performing the ultrasound diagnosis. As the control parameters, for example, selection results of a live mode and a freeze mode, set values of the display depth (depth), selection results of an ultrasound image generation mode, and the like can be mentioned. 
     Here, the “live mode” is a mode in which ultrasound images (moving images) obtained at a predetermined frame rate are sequentially displayed (displayed in real time). The “freeze mode” is a mode in which an image (still image) of one frame of the ultrasound images (moving images) generated in the past is read out from a cine memory  150  to be described later and displayed. 
     There are a plurality of ultrasound image generation modes that can be selected in the present embodiment. Specifically, there are a brightness (B) mode, a color flow (CF) mode, and a pulse wave (PW) mode. The B mode is a mode in which a tomographic image is displayed by converting the amplitude of the ultrasound echo into a brightness. The CF mode is a mode in which average blood flow speed, flow fluctuation, strength of flow signal, flow power, and the like are mapped to various colors and displayed so as to be superimposed on a B mode image. The PW mode is a mode in which the speed (for example, blood flow speed) of the ultrasound echo source detected based on the transmission and reception of the pulse wave is displayed. 
     The ultrasound image generation modes described above are merely examples, and modes other than the above-described three kinds of modes, for example, an amplitude (A) mode and a motion (M) mode may be further included. 
     Next, the operation of the ultrasound diagnostic apparatus  10  will be described. The ultrasound diagnostic apparatus  10  performs an input step of inputting examination information, a diagnostic step of performing ultrasound diagnosis, and a standby step of waiting for preparation for ultrasound diagnosis and the like after the electric power is supplied. At the start of the ultrasound diagnostic apparatus  10 , first, an input step is performed. In the input step, the operator operates the console  100  to input the examination information described above. After the end of the input of examination information, the diagnostic step is started in a case where the operator gives an instruction to start ultrasound diagnosis with the console  100 . The standby step is performed until there is an instruction to start ultrasound diagnosis after the end of the input of examination information. 
     In the present embodiment, the operation mode of the ultrasound diagnostic apparatus  10  is set in performing each step after the input step. The operation mode includes a first mode and a second mode. The first mode is a normal mode in which ultrasound diagnosis is performed according to a normal procedure and polarization processing is not performed in a period (hereinafter, referred to as a non-diagnosis period) other than the execution period of ultrasound diagnosis. The second mode is a recovery mode in which ultrasound diagnosis is performed and polarization processing to be described below is performed in the non-diagnosis period. After the input step, the ultrasound diagnostic apparatus  10  operates according to one of the first mode and the second mode. 
     &lt;&lt;Configuration of Ultrasound Endoscope  12 &gt;&gt; 
     Next, the configuration of the ultrasound endoscope  12  will be described with reference to  FIGS.  1  to  4   .  FIG.  2    is an enlarged plan view showing a distal end portion of an insertion part  22  of an ultrasound endoscope  12  and the periphery thereof.  FIG.  3    is a cross-sectional view showing a cross section of the distal end portion  40  of the insertion part  22  of the ultrasound endoscope  12  taken along the line I-I in  FIG.  2   .  FIG.  4    is a block diagram showing the configuration of an ultrasound processor apparatus  14 . 
     As described above the ultrasound endoscope  12  has the insertion part  22  and the operation unit  24 . As shown in  FIG.  1   , the insertion part  22  comprises the distal end portion  40 , a bending portion  42 , and a flexible portion  43  in order from the distal end side (free end side). As shown in  FIG.  2   , an ultrasound observation portion  36  and an endoscope observation portion  38  are provided in the distal end portion  40 . As shown in  FIG.  3   , the ultrasound transducer unit  46  comprising a plurality of ultrasound transducers  48  is disposed in the ultrasound observation portion  36 . 
     As shown in  FIG.  2   , a treatment tool lead-out port  44  is provided in the distal end portion  40 . The treatment tool lead-out port  44  serves as an outlet of a treatment tool (not shown), such as forceps, an insertion needle, or a high frequency scalpel. In addition, the treatment tool lead-out port  44  serves as a suction port in the case of sucking aspirates, such as blood and body waste. 
     The bending portion  42  is a portion continuously provided on the more proximal side (side opposite to the side where the ultrasound transducer unit  46  is provided) than the distal end portion  40 , and can bend freely. The flexible portion  43  is a portion connecting the bending portion  42  and the operation unit  24  to each other, has flexibility, and is provided so as to extend in an elongated state. 
     A plurality of pipe lines for air and water supply and a plurality of pipe lines for suction are formed in the insertion part  22  and the operation unit  24 , respectively. In addition, a treatment tool channel  45  whose one end communicates with the treatment tool lead-out port  44  is formed in each of the insertion part  22  and the operation unit  24 . 
     Next, the ultrasound observation portion  36 , the endoscope observation portion  38 , the water supply tank  21   a,  the suction pump  21   b,  and the operation unit  24  among the components of the ultrasound endoscope  12  will be described in detail. 
     (Ultrasound Observation Portion  36 ) 
     The ultrasound observation portion  36  is a portion provided to acquire an ultrasound image, and is disposed on the distal end side in the distal end portion  40  of the insertion part  22 . As shown in  FIG.  3   , the ultrasound observation portion  36  comprises the ultrasound transducer unit  46 , a plurality of coaxial cables  56 , and a flexible printed circuit (FPC)  60 . 
     The ultrasound transducer unit  46  corresponds to an ultrasound probe (probe), and transmits an ultrasound wave using an ultrasound transducer array  50 , in which a plurality of ultrasound transducers  48  to be described later are arranged, in the body cavity of the patient, receives a reflected wave (echo) of the ultrasound wave reflected by the observation target part, and outputs a reception signal. The ultrasound transducer unit  46  according to the present embodiment is a convex type, and transmits an ultrasound wave radially (in an arc shape). However, the type (model) of the ultrasound transducer unit  46  is not particularly limited, and other types may be used as long as it is possible to transmit and receive ultrasound waves. For example, a sector type, a linear type, and a radial type may be used. 
     As shown in  FIG.  3   , the ultrasound transducer unit  46  is formed by laminating a backing material layer  54 , an ultrasound transducer array  50 , an acoustic matching layer  74 , and an acoustic lens  76 . 
     The ultrasound transducer array  50  includes a plurality of ultrasound transducers  48  (ultrasound transducers) arranged in a one-dimensional array. More specifically, the ultrasound transducer array  50  is formed by arranging N (for example, N=128) ultrasound transducers  48  at equal intervals in a convex bending shape along the axial direction of the distal end portion  40  (longitudinal axis direction of the insertion part  22 ). The ultrasound transducer array  50  may be one in which a plurality of ultrasound transducers  48  are disposed in a two-dimensional array. 
     Each of the N ultrasound transducers  48  is formed by disposing electrodes on both surfaces of a single crystal transducer that is a piezoelectric element. As the single crystal transducer, any of quartz, lithium niobate, lead magnesium niobate (PMN), lead zinc niobate (PZN), lead indium niobate (PIN), lead titanate (PT), lead magnesium niobate-lead titanate (PMN-PT), zinc niobate-lead titanate (PZN-PT), lithium tantalate, langasite, and zinc oxide can be used. 
     The electrodes is an individual electrode (not shown) individually provided for each of the plurality of ultrasound transducers  48  and a transducer ground (not shown) common to the plurality of ultrasound transducers  48 . In addition, the electrodes are electrically connected to the ultrasound processor apparatus  14  through the coaxial cable  56  and the FPC  60 . 
     The ultrasound transducer  48  according to the present embodiment needs to be driven (vibrated) at a relatively high frequency of 7 MHz to 8 MHz level in order to acquire an ultrasound image in the body cavity of the patient. For this reason, the thickness of the piezoelectric element forming the ultrasound transducer  48  is designed to be relatively small. For example, the thickness of the piezoelectric element forming the ultrasound transducer  48  is 75 μm to 125 μm, preferably 90 μm to 110 μm. 
     A diagnostic driving pulse that is a pulsed driving voltage is supplied from the ultrasound processor apparatus  14  to each ultrasound transducer  48 , as an input signal (transmission signal), through the coaxial cable  56 . In a case where the driving voltage is applied to the electrodes of the ultrasound transducer  48 , the piezoelectric element expands and contracts to drive (vibrate) the ultrasound transducer  48 . As a result, a pulsed ultrasound wave is output from the ultrasound transducer  48 . In this case, the amplitude of the ultrasound wave output from the ultrasound transducer  48  has a magnitude corresponding to the intensity (output intensity) in a case where the ultrasound transducer  48  outputs the ultrasound wave. Here, the output intensity is defined as the magnitude of the sound pressure of the ultrasound wave output from the ultrasound transducer  48 . 
     Each ultrasound transducer  48  vibrates (is driven) upon receiving the reflected wave (echo) of the ultrasound wave, and the piezoelectric element of each ultrasound transducer  48  generates an electric signal. The electric signal is output from each ultrasound transducer  48  to the ultrasound processor apparatus  14  as a reception signal of the ultrasound wave. In this case, the magnitude (voltage value) of the electric signal output from the ultrasound transducer  48  has a magnitude corresponding to the reception sensitivity in a case where the ultrasound transducer  48  receives the ultrasound wave. Here, the reception sensitivity is defined as a ratio of the amplitude of the electric signal, which is output from the ultrasound transducer  48  in response to reception of the ultrasound wave, to the amplitude of the ultrasound wave transmitted by the ultrasound transducer  48 . 
     In the present embodiment, by sequentially driving the N ultrasound transducers  48  with an electronic switch such as a multiplexer  140  (refer to  FIG.  4   ), an ultrasound scan occurs in a scanning range along the curved surface on which the ultrasound transducer array  50  is disposed, for example, in the range of about several tens of mm from the center of curvature of the curved surface. More specifically, in the case of acquiring a B mode image (tomographic image) as an ultrasound image, a driving voltage is supplied to m (for example, m=N/2) ultrasound transducers  48  (hereinafter, referred to as driving target transducers) arranged in series, among the N ultrasound transducers  48 , by opening channel selection of the multiplexer  140 . As a result, the m driving target transducers are driven, and an ultrasound wave is output from each driving target transducer of the opening channel. The ultrasound waves output from the m driving target transducers are immediately synthesized, and the composite wave (ultrasound beam) is transmitted to the observation target part. Thereafter, each of the m driving target transducers receives an ultrasound wave (echo) reflected at the observation target part, and outputs an electric signal (reception signal) corresponding to the reception sensitivity at that point in time. 
     Then, the above-described series of steps (that is, supply of a driving voltage, transmission and reception of ultrasound waves, and output of an electric signal) are repeatedly performed while shifting the position of the driving target transducer, among the N ultrasound transducers  48 , one by one (one ultrasound transducer  48  at a time). Specifically, the above-described series of steps are started from m driving target transducers on both sides of the ultrasound transducer  48  located at one end among the N ultrasound transducers  48 . Then, the above-described series of steps are repeated each time the position of the driving target transducer is shifted due to switching of the opening channel by the multiplexer  140 . Finally, the above-described series of steps are repeatedly performed a total of N times up to m driving target transducers on both sides of the ultrasound transducer  48  located at the other end among the N ultrasound transducers  48 . 
     The backing material layer  54  supports each ultrasound transducer  48  of the ultrasound transducer array  50  from the back surface side. In addition, the backing material layer  54  has a function of attenuating ultrasound waves propagating to the backing material layer  54  side among ultrasound waves emitted from the ultrasound transducer  48  or ultrasound waves (echoes) reflected by the observation target part. The backing material is a material having rigidity, such as hard rubber, and an ultrasound damping material (ferrite, ceramics, and the like) is added as necessary. 
     The acoustic matching layer  74  is superimposed on the ultrasound transducer array  50 , and is provided for acoustic impedance matching between the body of the patient and the ultrasound transducer  48 . Since the acoustic matching layer  74  is provided, it is possible to increase the transmittance of the ultrasound wave. As a material of the acoustic matching layer  74 , it is possible to use various organic materials whose acoustic impedance values are closer to that of the body of the patient than the piezoelectric element of the ultrasound transducer  48 . Specific examples of the material of the acoustic matching layer  74  include epoxy resin, silicone rubber, polyimide, polyethylene, and the like. 
     The acoustic lens  76  superimposed on the acoustic matching layer  74  converges ultrasound waves emitted from the ultrasound transducer array  50  toward the observation target part. The acoustic lens  76  is formed of, for example, silicon resin (millable silicone rubber (HTV rubber), liquid silicone rubber (RTV rubber), and the like), butadiene resin, and polyurethane resin, and powders of titanium oxide, alumina, silica, and the like are mixed as necessary. 
     The FPC  60  is electrically connected to the electrode of each ultrasound transducer  48 . Each of the plurality of coaxial cables  56  is wired to the FPC  60  at one end thereof. Then, in a case where the ultrasound endoscope  12  is connected to the ultrasound processor apparatus  14  through the ultrasound connector  32   a,  each of the plurality of coaxial cables  56  is electrically connected to the ultrasound processor apparatus  14  at the other end (side opposite to the FPC  60 ). 
     In the present embodiment, the ultrasound endoscope  12  comprises an endoscope side memory  58  (refer to  FIG.  4   ). The endoscope side memory  58  stores driving times of the plurality of ultrasound transducers  48  at the time of ultrasound diagnosis. Strictly speaking, in the endoscope side memory  58 , the cumulative driving time of the driving target transducer after the operation mode of the ultrasound diagnostic apparatus  10  becomes the first mode, among the plurality of ultrasound transducers  48 , is stored. 
     In the present embodiment, an execution period of ultrasound diagnosis, that is, a period from the start of acquisition of an ultrasound image (moving image) to the end thereof (more specifically, a time during which ultrasound diagnosis is performed in the live mode), is set as the cumulative driving time. However, the invention is not limited thereto, and the time for which the driving voltage is supplied to the driving target transducer may be set as the cumulative driving time. 
     In a state in which the ultrasound endoscope  12  is connected to the ultrasound processor apparatus  14 , the CPU  152  of the ultrasound processor apparatus  14  can access the endoscope side memory  58  to read the cumulative driving time stored in the endoscope side memory  58 . In addition, the CPU  152  of the ultrasound processor apparatus  14  rewrites the cumulative driving time stored in the endoscope side memory  58  to a default value, or updates the stored cumulative driving time to a new cumulative driving time in a case where the cumulative driving time changes with the execution of ultrasound diagnosis. 
     (Endoscope Observation Portion  38 ) 
     The endoscope observation portion  38  is a portion provided to acquire an endoscope image, and is disposed on the more proximal side than ultrasound observation portion  36  in the distal end portion  40  of the insertion part  22 . As shown in  FIGS.  2  and  3   , the endoscope observation portion  38  includes the observation window  82 , an objective lens  84 , the solid-state imaging element  86 , the illumination window  88 , the cleaning nozzle  90 , a wiring cable  92 , and the like. 
     The observation window  82  is attached so as to be inclined with respect to the axial direction (longitudinal axis direction of the insertion part  22 ) at the distal end portion  40  of the insertion part  22 . Light incident through the observation window  82  and reflected at the observation target adjacent part is focused on the imaging surface of the solid-state imaging element  86  by the objective lens  84 . 
     The solid-state imaging element  86  photoelectrically converts the reflected light of the observation target adjacent part, which is focused on the imaging surface after being transmitted through the observation window  82  and the objective lens  84 , and outputs an imaging signal. As the solid-state imaging element  86 , it is possible to use a charge coupled device (CCD), a complementary metal oxide semiconductor (CMOS), and the like. The captured image signal output from the solid-state imaging element  86  is transmitted to the endoscope processor apparatus  16  by the universal cord  26  through the wiring cable  92  extending from the insertion part  22  to the operation unit  24 . 
     The illumination window  88  is provided at both side positions of the observation window  82 . An exit end of a light guide (not shown) is connected to the illumination window  88 . The light guide extends from the insertion part  22  to the operation unit  24 , and its incidence end is connected to the light source device  18  connected through the universal cord  26 . The illumination light emitted from the light source device  18  is transmitted through the light guide and is emitted from the illumination window  88  toward the observation target adjacent part. 
     The cleaning nozzle  90  is an ejection hole formed at the distal end portion  40  of the insertion part  22  in order to clean the surfaces of the observation window  82  and the illumination window  88 . From the cleaning nozzle  90 , air or cleaning liquid is ejected toward the observation window  82  and the illumination window  88 . In the present embodiment, the cleaning liquid ejected from the cleaning nozzle  90  is water, in particular, degassed water. However, the cleaning liquid is not particularly limited, and other liquids, for example, normal water (water that is not degassed) may be used. 
     (Water Supply Tank  21   a  and Suction Pump  21   b ) 
     The water supply tank  21   a  is a tank that stores degassed water, and is connected to the light source connector  32   c  by an air and water supply tube  34   a.  Degassed water is used as a cleaning liquid ejected from the cleaning nozzle  90 . 
     The suction pump  21   b  sucks aspirates (including degassed water supplied for cleaning) inside the body cavity through the treatment tool lead-out port  44 . The suction pump  21   b  is connected to the light source connector  32   c  by a suction tube  34   b.  The ultrasound diagnostic apparatus  10  may comprise an air supply pump for supplying air to a predetermined air supply destination and the like. 
     In the insertion part  22  and the operation unit  24 , the treatment tool channel  45  and an air and water supply pipe line (not shown)  62  are provided. 
     The treatment tool channel  45  communicates between a treatment tool insertion port  30  and the treatment tool lead-out port  44  provided in the operation unit  24 . The treatment tool channel  45  is connected to a suction button  28   b  provided in the operation unit  24 . The suction button  28   b  is connected to the suction pump  21   b  in addition to the treatment tool channel  45 . 
     The air and water supply pipe line  62  communicates with the cleaning nozzle  90  at one end side, and is connected to an air and water supply button  28   a  provided in the operation unit  24  at the other end side. The air and water supply button  28   a  is connected to the water supply tank  21   a  in addition to the air and water supply pipe line. 
     (Operation Unit  24 ) 
     The operation unit  24  is a unit operated by the operator at the start of ultrasound diagnosis, during diagnosis, at the end of diagnostic, and one end of the universal cord  26  is connected to one end of the operation unit  24 . As shown in  FIG.  1   , the operation unit  24  has the air and water supply button  28   a,  the suction button  28   b,  a pair of angle knobs  29 , and a treatment tool insertion port (forceps port)  30 . 
     In a case where each of the pair of angle knobs  29  is rotated, the bending portion  42  is remotely operated to be bent and deformed. By this deformation operation, the distal end portion  40  of the insertion part  22  in which the ultrasound observation portion  36  and the endoscope observation portion  38  are provided can be directed in a desired direction. 
     The treatment tool insertion port  30  is a hole formed to insert a treatment tool (not shown), such as forceps, and communicates with the treatment tool lead-out port  44  through the treatment tool channel  45 . The treatment tool inserted into the treatment tool insertion port  30  is introduced into the body cavity from the treatment tool lead-out port  44  after passing through the treatment tool channel  45 . 
     The air and water supply button  28   a  and the suction button  28   b  are two-stage switching type push buttons, and are operated to switch the opening and closing of the pipe line provided inside each of the insertion part  22  and the operation unit  24 . 
     &lt;&lt;Configuration of Ultrasound Processor Apparatus  14 &gt;&gt; 
     The ultrasound processor apparatus  14  causes the ultrasound transducer unit  46  to transmit and receive ultrasound waves, and generates an ultrasound image by converting the reception signal, which is output from the ultrasound transducer  48  (specifically, a driving target element) at the time of ultrasound wave reception, into an image. In addition, the ultrasound processor apparatus  14  displays the generated ultrasound image on the monitor  20 . 
     In the present embodiment, the ultrasound processor apparatus  14  supplies a polarization voltage to a polarization target transducer, among the N ultrasound transducers  48 , to polarize the polarization target transducer. By performing the polarization processing, the depolarized ultrasound transducer  48  can be polarized again by repeating the ultrasound diagnosis. As a result, it is possible to restore the reception sensitivity of the ultrasound transducer  48  with respect to ultrasound waves to a satisfactory level. 
     As shown in  FIG.  4   , the ultrasound processor apparatus  14  has the multiplexer  140 , the reception circuit  142 , the transmission circuit  144 , an A/D converter  146 , an application specific integrated circuit (ASIC)  148 , the cine memory  150 , a notification circuit  156 , a central processing unit (CPU)  152 , and a digital scan converter (DSC)  154 . 
     The reception circuit  142  and the transmission circuit  144  are electrically connected to the ultrasound transducer array  50  of the ultrasound endoscope  12 . The multiplexer  140  selects a maximum of m driving target transducers from the N ultrasound transducers  48 , and opens their channels. 
     The transmission circuit  144  is configured to include a field programmable gate array (FPGA), a pulser (pulse generation circuit  158 ), a switch (SW), and the like, and is connected to the multiplexer  140  (MUX). Instead of the FPGA, an application specific integrated circuit (ASIC) may be used. 
     The transmission circuit  144  is a circuit that supplies a driving voltage for ultrasound wave transmission to the driving target transducers selected by the multiplexer  140 , according to the control signal transmitted from the CPU  152 , in order to transmit ultrasound waves from the ultrasound transducer unit  46 . The driving voltage is a pulsed voltage signal (transmission signal), and is applied to the electrodes of the driving target transducers through the universal cord  26  and the coaxial cable  56 . 
     The transmission circuit  144  has a pulse generation circuit  158  that generates a transmission signal based on a control signal. Under the control of the CPU  152 , a transmission signal for driving a plurality of ultrasound transducers  48  to generate ultrasound waves is generated using the pulse generation circuit  158 , and the generated transmission signal is supplied to the plurality of ultrasound transducers  48 . 
     In addition, under the control of the CPU  152 , in the case of performing ultrasound diagnosis, the transmission circuit  144  generates a first transmission signal (diagnostic driving pulse) having a driving voltage for performing ultrasound diagnosis using the pulse generation circuit  158 . In addition, under the control of the CPU  152 , in the case of performing polarization processing, a second transmission signal (polarization driving pulse) having a polarization voltage for performing polarization processing is generated using the same pulse generation circuit  158  as in the case of generating the first transmission signal. The first and second transmission signals are pulse waves. 
     In the invention, the polarization driving pulse is generated by the pulse generation circuit  158  of the transmission circuit  144  that generates a diagnostic driving pulse for acquiring an ultrasound image. That is, the transmission circuit  144  has the same circuit configuration as an existing transmission circuit that does not have a new circuit configuration for generating the polarization driving pulse. Therefore, the polarization driving pulse (second transmission signal) applied to the ultrasound transducer  48  at the time of polarization processing is generated using the diagnostic driving pulse (first transmission signal) applied to the ultrasound transducer  48  at the time of acquisition of an ultrasound image. 
     Here, although the voltage ranges of the first transmission signal and the second transmission signal generated by the same pulse generation circuit  158  are the same, the output voltage (driving voltage) of the first transmission signal and the output voltage (polarization voltage) of the second transmission signal can be different voltage values within the adjustable range of the output voltage. For example, the output voltages of the first transmission signal and the second transmission signal can be the same voltage, or the output voltage of the second transmission signal can be larger than the output voltage of the first transmission signal. 
     Although the details will be described later, the polarization driving pulse (main lobe) is a driving pulse in a frequency band different from the probe frequency band of the diagnostic driving pulse. More specifically, reception signals of the plurality of ultrasound transducers  48  within the frequency band of the first ultrasound wave generated by the first transmission signal (diagnostic driving pulse) and reception signals of the plurality of ultrasound transducers  48  within the frequency band of the main lobe of the second ultrasound wave generated by the second transmission signal (polarization driving pulse) have different band characteristics. For example, as shown in  FIG.  11 B , it is preferable that the band characteristics of the reception signals of the plurality of ultrasound transducers  48  within the frequency band of the first ultrasound wave generated by the first transmission signal and the band characteristics of the reception signals of the plurality of ultrasound transducers  48  within the frequency band of the main lobe of the second ultrasound wave generated by the second transmission signal do not overlap at a level of −20 dB or more. 
     From the above, the invention has an existing transmission circuit configuration. Using the transmission circuit  144  for the same driving pulse output as acquisition of an ultrasound image, a polarization driving pulse in a frequency band different from the probe frequency band of the diagnostic driving pulse is output, and polarization processing of the ultrasound transducer  48  of the ultrasound endoscope  12  is performed at a time different from the time for acquiring an ultrasound image. 
     The magnitude (voltage value or potential) and the supply time of the polarization voltage of the polarization driving pulse are set to appropriate values, which satisfy the conditions for obtaining the repolarization effect, by the CPU  152  in accordance with the specification of the ultrasound transducer  48  (specifically, the thickness and the material of the ultrasound transducer  48 ) provided in the ultrasound endoscope  12  connected to the ultrasound processor apparatus  14 . Thereafter, the CPU  152  performs polarization processing based on the set values described above. 
     That is, in the invention, in the case of acquiring an ultrasound image, the CPU (control circuit)  152  controls the transmission circuit  144  (pulse generation circuit  158 ) to generate a diagnostic driving pulse (first transmission signal) to be applied to each of the plurality of ultrasound transducers  48  that generate ultrasound waves for acquisition of an ultrasound image. 
     On the other hand, in the case of performing polarization processing, in order to perform polarization processing of the plurality of ultrasound transducers  48 , the CPU (control circuit)  152  controls the transmission circuit  144  (pulse generation circuit  158 ) to generate a polarization driving pulse (second transmission signal) having a frequency different from the probe frequency band as an ultrasound probe (ultrasound transducer unit  46 ) for acquiring an ultrasound image. 
     As a result, in the invention, in the case of performing polarization processing, the polarization driving pulse is applied to the plurality of ultrasound transducers  48 , and the polarization processing of the plurality of ultrasound transducers  48  is performed by the polarization driving pulse. 
     Then, the reception circuit  142  is a circuit that receives an electric signal output from the driving target transducer that has received an ultrasound wave (echo), that is, a reception signal. In addition, according to the control signal transmitted from the CPU  152 , the reception circuit  142  amplifies the reception signal received from the ultrasound transducer  48  and transmits the amplified signal to the A/D converter  146 . The A/D converter  146  is connected to the reception circuit  142 , and converts the reception signal received from the reception circuit  142  from an analog signal to a digital signal and outputs the converted digital signal to the ASIC  148 . 
     The ASIC  148  is connected to the A/D converter  146 . As shown in  FIG.  4   , the ASIC  148  forms a phase matching unit  160 , a B mode image generation unit  162 , a PW mode image generation unit  164 , a CF mode image generation unit  166 , and a memory controller  151 . 
     In the present embodiment, the above-described functions (specifically, the phase matching unit  160 , the B mode image generation unit  162 , the PW mode image generation unit  164 , the CF mode image generation unit  166 , and the memory controller  151 ) are realized by a hardware circuit, such as the ASIC  148 . However, the invention is not limited thereto. The above-described functions may be realized by making the central processing unit (CPU) and software (computer program) for executing various kinds of data processing cooperate with each other. 
     The phase matching unit  160  performs processing for phasing addition (addition after matching the phases of reception data) by giving a delay time to the reception signal (reception data) digitized by the A/D converter  146 . By the phasing addition processing, a sound ray signal with narrowed focus of the ultrasound echo is generated. 
     The B mode image generation unit  162 , the PW mode image generation unit  164 , and the CF mode image generation unit  166  generate an ultrasound image based on the electric signal (strictly speaking, the sound ray signal generated by phasing and adding the reception data) that is output from the driving target transducer among the plurality of ultrasound transducers  48  in a case where the ultrasound transducer unit  46  receives the ultrasound wave. 
     The B mode image generation unit  162  is an image generation unit that generates a B mode image that is a tomographic image of the inside of the patient (inside of the body cavity). For the sequentially generated sound ray signals, the B mode image generation unit  162  corrects the attenuation due to the propagation distance according to the depth of the reflection position of the ultrasound wave by sensitivity time control (STC). The B mode image generation unit  162  performs envelope detection processing and logarithm (Log) compression processing on the corrected sound ray signal, thereby generating a B mode image (image signal). 
     The PW mode image generation unit  164  is an image generation unit that generates an image showing the speed of blood flow in a predetermined direction. The PW mode image generation unit  164  extracts a frequency component by applying a fast Fourier transform to a plurality of sound ray signals in the same direction among the sound ray signals sequentially generated by the phase matching unit  160 . Thereafter, the PW mode image generation unit  164  calculates the speed of blood flow from the extracted frequency component, and generates a PW mode image (image signal) showing the calculated speed of blood flow. 
     The CF mode image generation unit  166  is an image generation unit that generates an image showing blood flow information in a predetermined direction. The CF mode image generation unit  166  generates an image signal indicating the blood flow information by calculating the autocorrelation between a plurality of sound ray signals in the same direction among the sound ray signals sequentially generated by the phase matching unit  160 . Thereafter, based on the image signal described above, the CF mode image generation unit  166  generates a CF mode image (image signal) as a color image in which the blood flow information is superimposed on the B mode image signal generated by the B mode image generation unit  162 . 
     The memory controller  151  stores the image signal generated by the B mode image generation unit  162 , the PW mode image generation unit  164 , or the CF mode image generation unit  166  in the cine memory  150 . 
     The DSC  154  is connected to the ASIC  148 , and converts (raster conversion) the signal of the image generated by the B mode image generation unit  162 , the PW mode image generation unit  164 , or the CF mode image generation unit  166  into an image signal according to a normal television signal scanning method, performs various kinds of required image processing, such as gradation processing, on the image signal, and then outputs an obtained signal to the monitor  20 . 
     The cine memory  150  has a capacity for storing an image signal for one frame or several frames. The image signal generated by the ASIC  148  is output to the DSC  154 , and is also stored in the cine memory  150  by the memory controller  151 . In the freeze mode, the memory controller  151  reads the image signal stored in the cine memory  150  and outputs the read image signal to the DSC  154 . As a result, an ultrasound image (still image) based on the image signal read from the cine memory  150  is displayed on the monitor  20 . 
     The notification circuit  156  is connected to the CPU  152 . In the second mode, under the control of the CPU  152 , the notification circuit  156  notifies the user that polarization processing is being performed in the case of the third display mode to be described later. The notification method is not particularly limited. For example, a message indicating that polarization processing is being performed may be displayed on the monitor  20 , or notification may be provided using a sound, or it may be notified that polarization processing is being performed using a display lamp or the like. 
     The CPU  152  functions as a controller that controls each unit of the ultrasound processor apparatus  14 . The CPU  152  is connected to the reception circuit  142 , the transmission circuit  144 , the A/D converter  146 , and the ASIC  148  to control these devices. Specifically, the CPU  152  is connected to the console  100 , and controls each unit of the ultrasound processor apparatus  14  according to examination information, control parameters, and the like input through the console  100 . 
     The CPU  152  automatically recognizes the ultrasound endoscope  12  based on a method, such as Plug and Play (PnP), in a case where the ultrasound endoscope  12  is connected to the ultrasound processor apparatus  14  through the ultrasound connector  32   a.    
     Thereafter, the CPU  152  accesses the endoscope side memory  58  of the ultrasound endoscope  12  to read the cumulative driving time stored in the endoscope side memory  58 . In addition, the CPU  152  accesses the endoscope side memory  58  at the end of the ultrasound diagnosis, and updates the cumulative driving time stored in the endoscope side memory  58  to a value obtained by adding the time required for the ultrasound diagnosis performed immediately before to the cumulative driving time stored in the endoscope side memory  58 . 
     In the present embodiment, the cumulative driving time is stored on the ultrasound endoscope  12  side. However, the invention is not limited thereto, and the cumulative driving time may be stored on the ultrasound processor apparatus  14  side for each ultrasound endoscope  12 . 
     In addition, while the operation mode of the ultrasound diagnostic apparatus  10  is the second mode, the CPU  152  controls the transmission circuit  144  to perform polarization processing using the non-diagnosis period. More specifically, in a case where the cumulative driving time of the plurality of ultrasound transducers  48  for performing ultrasound diagnosis, in which a driving voltage is supplied to the driving target transducer, becomes equal to or longer than a specified time, the CPU  152  controls the transmission circuit  144  (pulse generation circuit  158 ) to perform polarization processing on the plurality of ultrasound transducers  48  in a period other than the execution period of ultrasound diagnosis, that is, in a non-diagnosis period during which transmission of ultrasound waves and reception of reflected waves for performing ultrasound diagnosis are not performed. 
     The specified time is a time set in advance, and is recorded on the ultrasound processor apparatus  14  side. The specified time is any time, and may be on the order of several hours or on the order of several frame times. The specified time may be different for each ultrasound endoscope  12 , or may be a value common to the ultrasound endoscopes  12 . A time of a default value may be set as the specified time, or the operator may set any specified time through the console  100 . 
     &lt;&lt;Operation Example of Ultrasound Diagnostic Apparatus  10 &gt;&gt; 
     Next, as an operation example of the ultrasound diagnostic apparatus  10 , a flow of a series of processes relevant to ultrasound diagnosis (hereinafter, also referred to as diagnostic process) will be described with reference to  FIGS.  5  to  8   .  FIG.  5    is a diagram showing the flow of the diagnostic process using the ultrasound diagnostic apparatus  10 .  FIG.  6    is a diagram showing the flow of the diagnostic process in the first mode, and  FIG.  7    is a diagram showing the flow of diagnostic process in the second mode.  FIG.  8    is a diagram showing the procedure of a diagnostic step in the diagnostic process. 
     In a case where each unit of the ultrasound diagnostic apparatus  10  is powered on in a state in which the ultrasound endoscope  12  is connected to the ultrasound processor apparatus  14 , the endoscope processor apparatus  16 , and the light source device  18 , the diagnostic process starts with the power-ON as a trigger. In the diagnostic process, as shown in  FIG.  5   , an input step is performed first (S 001 ). In the input step, the operator inputs examination information, control parameters, and the like through the console  100 . In a case where the input step is completed, a standby step is performed until there is an instruction to start diagnosis (S 002 ). Using the standby step, the CPU  152  of the ultrasound processor apparatus  14  reads a cumulative driving time from the endoscope side memory  58  of the ultrasound endoscope  12  (S 003 ). 
     Thereafter, the CPU  152  determines whether or not the read cumulative driving time is equal to or longer than the specified time (S 004 ). 
     In a case where it is determined that the cumulative driving time is less than the specified time (No in S 004 ), the CPU  152  sets the operation mode of the ultrasound diagnostic apparatus  10  to the first mode (S 005 ). In the present embodiment, it is assumed that the operation mode at the initial setting stage is set to the first mode. 
     In a case where the operation mode is set to the first mode, normal steps in the case of performing ultrasound diagnosis are performed according to steps shown in  FIG.  6   . Specifically, first, it is determined whether or not there is a diagnosis start instruction from the operator (S 011 ). In a case where there is no diagnosis start instruction from the operator (No in S 011 ), the process returns to step S 011  to repeat the operation described above. In a case where there is a diagnosis start instruction from the operator (Yes in S 011 ), the CPU  152  controls each unit of the ultrasound processor apparatus  14  to perform a diagnostic step (S 012 ). 
     The diagnostic step is performed according to steps shown in  FIG.  8   . That is, in a case where the designated image generation mode is the B mode (Yes in S 031 ), each unit of the ultrasound processor apparatus  14  is controlled so as to generate a B mode image (S 032 ). In a case where the designated image generation mode is not the B mode (No in S 031 ) but the CF mode (Yes in S 033 ), each unit of the ultrasound processor apparatus  14  is controlled so as to generate a CF mode image (S 034 ). In a case where the designated image generation mode is not the CF mode (No in S 033 ) but the PW mode (Yes in S 035 ), each unit of the ultrasound processor apparatus  14  is controlled so as to generate a PW mode image (S 036 ). In a case where the designated image generation mode is not the PW mode (No in S 035 ), the process proceeds to step S 037 . 
     Then, the CPU  152  determines whether or not the ultrasound diagnosis has ended (S 037 ). In a case where the ultrasound diagnosis has not ended (No in S 037 ), the process returns to step S 031 , and the generation of an ultrasound image in each image generation mode is repeatedly performed until the diagnosis end conditions are satisfied. As the diagnosis end conditions, for example, the operator gives an instruction to end the diagnosis through the console  100 . 
     On the other hand, in a case where the diagnosis end conditions are satisfied (Yes in S 037 ), the CPU  152  adds the time required for the ultrasound diagnosis performed so far to the cumulative driving time read out from the endoscope side memory  58  in step S 003 , and updates the cumulative driving time stored in the endoscope side memory  58  to the cumulative driving time after the addition (S 038 ). The diagnostic step ends at a point in time at which the series of steps (steps S 031  to S 038 ) in the diagnostic step end. 
     Then, returning to  FIG.  5   , it is determined whether or not each unit of the ultrasound diagnostic apparatus  10  is powered off (S 007 ). In a case where each unit of the ultrasound diagnostic apparatus  10  is powered off (Yes in S 007 ), the diagnostic process ends. On the other hand, in a case where the power of each unit of the ultrasound diagnostic apparatus  10  is maintained in the ON state (No in S 007 ), the process returns to step S 001 , and each step of the diagnostic process described above is repeated. 
     On the other hand, in a case where it is determined that the cumulative driving time read from the endoscope side memory  58  is equal to or longer than the specified time in step S 004 , that is, in a case where the cumulative driving time of the plurality of ultrasound transducers for performing ultrasound diagnosis becomes equal to or longer than the specified time in the first mode (Yes in S 004 ), the CPU  152  shifts the operation mode of the ultrasound diagnostic apparatus  10  from the first mode to the second mode (S 006 ). While the operation mode is the second mode, ultrasound diagnosis is performed, and polarization processing is performed during the non-diagnosis period as described above. That is, in the present embodiment, the polarization voltage is supplied to the polarization target transducer only in a case where the operation mode is the second mode. 
     The reason why such a configuration is adopted will be described below with reference to  FIG.  9   .  FIG.  9    is an explanatory diagram showing the relationship between the cumulative driving time and the polarization processing execution period of the ultrasound transducer  48  and the reception sensitivity of the ultrasound transducer  48 . The symbol S in the diagram indicates an execution period of diagnostic step, the symbol Q in the diagram indicates an execution period of standby step, and the symbol R in the diagram indicates an execution period of polarization processing. 
     The ultrasound transducer  48  is polarized up to a predetermined level at an initial time (for example, at the time of factory shipment), and can transmit and receive ultrasound waves with a reception sensitivity (hereinafter, initial sensitivity Pi) according to the degree of polarization. On the other hand, in a case where the ultrasound transducer  48  is driven to transmit and receive ultrasound waves for performing ultrasound diagnosis, depolarization progresses as the cumulative driving time increases, and the reception sensitivity also decreases accordingly. Such a tendency becomes noticeable in a case where the ultrasound transducer  48  is a single crystal transducer. Therefore, in a case where the cumulative driving time of the plurality of ultrasound transducers  48  for ultrasound diagnosis becomes equal to or longer than the specified time, it is necessary to generate a trigger and perform polarization processing. 
     Here, the cumulative driving time Ta of the ultrasound transducer (driving target transducer)  48  is expressed as the total time of required times (ta 1 , ta 2 , . . . , and tan in  FIG.  9   ) for each ultrasound diagnosis. However, in a case where the cumulative driving time Ta exceeds the specified time, the reception sensitivity of the ultrasound transducer  48  falls below the lower limit sensitivity P 1 , as shown in  FIG.  9   . The lower limit sensitivity P 1  corresponds to the lower limit level of the sensitivity to be satisfied in maintaining the image quality of the ultrasound image. In other words, the specified time is set to a value corresponding to the lower limit sensitivity P 1 . 
     The CPU  152  cannot directly detect whether or not the reception sensitivity of the ultrasound transducer  48  falls below the lower limit sensitivity P 1 . Therefore, in a case where the cumulative driving time Ta exceeds the above-described specified time, the CPU  152  determines that the reception sensitivity falls below the lower limit sensitivity P 1 . 
     Therefore, in the present embodiment, in a case where the cumulative driving time Ta becomes equal to or longer than the specified time, that is, in a case where the reception sensitivity of the ultrasound transducer  48  becomes equal to or less than the lower limit sensitivity P 1 , the operation mode is shifted from the first mode to the second mode, and polarization processing is appropriately performed in the second mode. As a result, the depolarized ultrasound transducer  48  can be repolarized to restore the reception sensitivity of the ultrasound transducer  48 . 
     Returning to the description of the diagnostic process, in a case where the operation mode is set to the second mode, ultrasound diagnosis and polarization processing are performed according to the steps shown in  FIG.  7   . Specifically, it is determined whether or not there is a diagnosis start instruction from the operator (S 021 ). In a case where there is a diagnosis start instruction from the operator (Yes in S 021 ), the CPU  152  controls each unit of the ultrasound processor apparatus  14  to perform a diagnostic step shown in  FIG.  8    (S 022 ). Thereafter, the process returns to step S 007  in  FIG.  5    to repeat the above-described operation. 
     In a case where there is no diagnosis start instruction from the operator in step S 021  (No in S 021 ), it is then determined whether or not this is a non-diagnosis period (S 023 ). In a case where it is determined that this is not a non-diagnosis period (No in S 023 ), the process returns to step S 021  to repeat the operation described above. 
     In a case where it is determined that this is a non-diagnosis period in step S 023  (Yes in S 023 ), the CPU  152  performs polarization processing during the non-diagnosis period (S 024 ). Specifically, in the polarization processing, a polarization voltage is supplied to the polarization target transducer for a predetermined time. In one polarization processing, all the N ultrasound transducers  48  are used as the polarization target transducers. More specifically, in one polarization processing, first, a polarization voltage is supplied to half (m) of the N ultrasound transducers  48 , and then a polarization voltage is supplied to the remaining half (m) of the ultrasound transducers  48 . 
     In the present embodiment, while the operation mode is the second mode, as shown in  FIG.  9   , polarization processing is repeatedly performed each time a non-diagnosis period comes. 
     After the execution of the polarization processing, the CPU  152  determines whether or not a difference (Tr−Ta) obtained by subtracting the cumulative driving time Ta of the plurality of ultrasound transducers  48  for performing ultrasound diagnosis from the cumulative processing time Tr of the plurality of ultrasound transducers  48  for performing polarization processing is equal to or greater than a threshold value (S 025 ). 
     The cumulative processing time Tr is expressed as the total time of required times (tr 1 , tr 2 , . . . , and trn in  FIG.  9   ) for each polarization processing. The threshold value is set to an appropriate value for restoring the reception sensitivity of the ultrasound transducer  48  to the initial sensitivity Pi, and is recorded on the ultrasound processor apparatus  14  side. 
     The threshold value may be different for each ultrasound endoscope  12 , or may be a value common to the ultrasound endoscopes  12 . In addition, a default value may be set as the threshold value, or the operator may change the threshold value through the console  100 . 
     From the cumulative driving time (transmission time) Ta, it can be seen how much the reception sensitivity of the ultrasound transducer  48  has decreased. In the example shown in  FIG.  9   , it can be calculated from ta 1 +ta 2  to what extent the reception sensitivity of the ultrasound transducer  48  has decreased exceeding the lower limit sensitivity P 1 . In addition, from the cumulative processing time (recovery time) Tr, it can be seen how much the reception sensitivity of the ultrasound transducer  48  is restored. 
     In the example shown in  FIG.  9   , assuming that the sensitivity reduction rate and the sensitivity recovery rate according to time are the same, ta 1 +ta 2 +ta 3 +ta 4 =tr 1 +tr 2 +tr 3 . In practice, sensitivity recovery is possible in a short time, and the sensitivity recovery rate is higher than the sensitivity reduction rate. Therefore, ta 1 +ta 2 +ta 3 +ta 4 =α(tr 1 +tr 2 +tr 3 ) (α&gt;1), and the relationship of Ta=αTr is satisfied. 
     For example, a threshold value can be determined based on the above-described relationship. Alternatively, a correspondence table showing the relationship between the cumulative driving time Ta and a threshold value required to restore the reception sensitivity of the ultrasound transducer  48 , which has been lowered according to the cumulative driving time Ta, up to the initial sensitivity Pi can be created in advance, and a threshold value can be calculated and used from the cumulative driving time Ta using the correspondence table. 
     As a result, in the second mode, in a case where the difference obtained by subtracting the cumulative driving time Ta from the cumulative processing time Tr is less than the threshold value (No in S 025 ), the process returns to step S 021  to repeat the above-described operation. 
     On the other hand, in a case where the difference obtained by subtracting the cumulative driving time Ta from the cumulative processing time Tr is equal to or greater than the threshold value (Yes in S 025 ), the CPU  152  accesses the endoscope side memory  58  and clears the cumulative driving time Ta stored in the endoscope side memory  58  so as to be rewritten to the initial value (zero) (S 026 ). Step S 026  in which the cumulative driving time Ta is cleared may be performed after the operation mode is returned from the second mode to the first mode in the next step S 027 . 
     Then, the CPU  152  returns the operation mode of the ultrasound diagnostic apparatus  10  from the second mode to the first mode (S 027 ). Thereafter, the process returns to step S 007  in  FIG.  5    to repeat the above-described operation. That is, in the present embodiment, in a case where the difference obtained by subtracting the cumulative driving time Ta from the cumulative processing time Tr is equal to or greater than the threshold value in the second mode, the operation mode is shifted from the second mode to the first mode. This is because it is thought that the depolarized ultrasound transducer  48  is already sufficiently polarized at this time. 
     This will be specifically described below with reference to  FIG.  9   . In a case where the operation mode shifts to the second mode, polarization processing is performed during the non-diagnosis period as described above. As a result, as shown in  FIG.  9   , the polarization level and the reception sensitivity of each ultrasound transducer  48  are restored by an amount corresponding to the cumulative processing time Tr (tr 1 , tr 2 , and tr 3  in  FIG.  9   ) of the plurality of ultrasound transducers  48  for performing polarization processing. On the other hand, even while the operation mode is the second mode, ultrasound diagnosis is performed in a case where there is an instruction to start diagnosis. For this reason, even while the operation mode is the second mode, the cumulative driving time Ta of the plurality of ultrasound transducers  48  for performing ultrasound diagnosis increases by the time required for each ultrasound diagnosis (ta 3  and ta 4  in  FIG.  9   ), and the polarization level and the reception sensitivity of each ultrasound transducer  48  decrease with an increase in the cumulative driving time Ta. 
     As described above, while the operation mode is the second mode, as shown in  FIG.  9   , the polarization of the ultrasound transducer  48  by polarization processing and the depolarization of the ultrasound transducer  48  by ultrasound diagnosis coexist. Then, in a case where the polarization processing and the ultrasound diagnosis are repeatedly performed, the cumulative processing time Tr (=tr 1 +tr 2 +tr 3 ) becomes larger than the cumulative driving time Ta (=ta 1 +ta 2 +ta 3 +ta 4 ) soon while the operation mode is the second mode, and finally, the difference (Tr−Ta) obtained by subtracting the cumulative driving time Ta from the cumulative processing time Tr becomes equal to or greater than the threshold value. At this point in time, as is apparent from  FIG.  9   , each ultrasound transducer  48  is polarized up to a level at which the reception sensitivity becomes the initial sensitivity Pi. In such a state, it is no longer necessary to perform the polarization processing. Therefore, in the present embodiment, the operation mode is returned from the second mode to the first mode in a case where the above-described conditions are satisfied. 
     Next, a specific example of the non-diagnosis period will be described. 
     For example, in a freeze mode in which an image (still image) of one frame of an ultrasound image (moving images) is displayed on the monitor  20 , the CPU  152  can perform polarization processing. In the freeze mode, no ultrasound image is acquired. Since the freeze mode is a period other than the execution period of ultrasound diagnosis, that is, a non-diagnosis period during which transmission of ultrasound waves and reception of reflected waves for performing ultrasound diagnosis are not performed, it is possible to appropriately perform the polarization processing. 
     The CPU  152  can perform polarization processing in a case where a screen for setting control parameters of the ultrasound diagnostic apparatus, a screen for inputting information of a patient (patient&#39;s name, ID, and the like) to be subjected to ultrasound diagnosis, a screen for designating a part to be subjected to ultrasound diagnosis, and the like are displayed on the monitor  20 . In a case where these screens are displayed, the user can input corresponding data. Accordingly, since this is a non-diagnosis period similarly, it is possible to appropriately perform the polarization processing. 
     In addition, the CPU  152  can perform polarization processing in a case where a screen, on which an ultrasound image generated (acquired) in the past and stored in the cine memory  150  is read out and displayed, is displayed on the monitor  20 . In a case where the screen displaying an ultrasound image generated in the past is displayed, the user views the ultrasound image generated in the past. Therefore, since this is a non-diagnosis period similarly, it is possible to appropriately perform the polarization processing. 
     The ultrasound diagnostic apparatus  10  can acquire an ultrasound image and an endoscope image and display the ultrasound image and the endoscope image on the monitor  20  in various display modes. 
     As shown in  FIG.  10   , the display modes include a first display mode in which only an ultrasound image is displayed, a second display mode in which an ultrasound image is displayed so as to be larger than an endoscope image by using picture in picture (PinP), a third display mode in which an ultrasound image is displayed so as to be smaller than an endoscope image by using the PinP similarly, and a fourth display mode in which only an endoscope image is displayed. The first to fourth display modes can be freely switched and displayed according to the user&#39;s instruction. 
     In the fourth display mode, that is, in a case where only the endoscope image is displayed on the monitor  20 , the CPU  152  can perform polarization processing. In the fourth display mode, no ultrasound image is acquired. Accordingly, since this is a non-diagnosis period similarly, it is possible to appropriately perform the polarization processing. 
     In addition, the CPU  152  can perform polarization processing in the third display mode, that is, in a case where the ultrasound image is displayed on the monitor  20  so as to be smaller than the endoscope image by picture in picture. In the third display mode, ultrasound diagnosis is actually performed, and this is not a non-diagnosis period but the ultrasound image is displayed so as to be smaller than the endoscope image. Therefore, it is possible to appropriately perform the polarization processing regardless of the image quality. 
     In this case, the CPU  152  controls the notification circuit  156  to notify the user that the polarization processing is being performed. In response to this, the notification circuit  156  notifies the user that the polarization processing is being performed. That is, in the third display mode, the user can know that the polarization processing is being performed. Thereafter, the CPU  152  sets the freeze mode, that is, forcibly sets the non-diagnosis period and performs polarization processing. 
     Although the specific examples of the non-diagnosis period have been described above, the polarization processing may be performed in any non-diagnosis period other than in the specific examples described above. 
     Next, a pulse waveform and a driving waveform (transmission waveform) of a polarization driving pulse (transmission wave for polarization) of the second transmission signal transmitted from the transmission circuit  144  to the ultrasound transducer  48  in the invention will be described. 
       FIGS.  11 A and  11 B  are graphs of an example of a driving waveform of a polarization driving pulse transmitted from the transmission circuit shown in  FIG.  4   , and are graphs showing the relationship between the sensitivity and the frequency of the driving waveform. The driving waveform shown in  FIG.  11 A  is a waveform of one unipolar wave having a frequency of 1.25 MHz. 
     In the invention, the driving waveform of the polarization driving pulse is not particularly limited but has a unipolar waveform shown in  FIG.  11 A , and it is preferable to perform polarization processing of the ultrasound transducer  48  using a polarization driving pulse having a driving waveform having a frequency characteristic shown by the solid line in  FIG.  11 B . In the example shown in  FIG.  11 B , for example, at a sensitivity level of −20 dB or more, the probe frequency band for acquiring an ultrasound image is about 2.7 MHz to about 11.7 MHz as shown by the broken line, while the band of the main lobe of the driving waveform of the polarization driving pulse shown by the solid line is about 2.3 MHz or less. That is, the band characteristic of the frequency of the polarization driving pulse and the band characteristic of the frequency of the diagnostic driving pulse do not overlap each other at a sensitivity level of −20 dB or more. 
     That is, in the invention, as shown in  FIG.  11 B , in the driving waveform of the polarization driving pulse, it is preferable that the frequency band of the main lobe and the probe frequency band shown by the broken line do not overlap each other at a sensitivity level of −20 dB or more. In addition, it is preferable that the frequency band of the main lobe is lower than the probe frequency band at a sensitivity level of −20 dB or more. The reason is that, in the polarization processing, it is necessary to reduce an influence on the ultrasound image by preventing excessive ultrasound wave output and to reduce an influence on the body cavity of the subject due to temperature rise by preventing the temperature rise. In particular, the upper limit temperature of the distal end portion of the ultrasound endoscope  12  inserted into the body cavity of the subject is strictly limited so as not to affect the body cavity and the like, and it is necessary to prevent the temperature rise. 
     In the invention, since the polarization driving pulse (main lobe) is transmitted outside the probe frequency band, the energy input to the ultrasound transducer  48  is reduced. Therefore, the temperature rise can be suppressed. In addition, since the outside of the probe frequency band is the outside of a resonance band in which the ultrasound transducer  48  resonates. Accordingly, the output sound pressure is reduced even though the polarization driving pulse (main lobe) is applied to the ultrasound transducer  48 . 
     In the driving waveform of the polarization driving pulse shown in  FIG.  11 B , it can be seen that, in addition to the main lobe, within the probe frequency band, one or more side lobes similarly shown by the solid line (in the example shown in  FIG.  11 B , four side lobes) are generated. As shown in  FIG.  11 B , it is preferable that all the maximum sensitivities of the side lobes within the probe frequency band are equal to or less than −10 dB and the average sensitivity of the side lobes is equal to or less than −20 dB. The reason is as follows. 
     In general, the specification of the frequency characteristic of the probe is expressed in the −20 dB band of the transmission and reception sensitivity. This is because the signal of 1/10 or less from the peak of the sensitivity hardly affects an image. On the other hand, the band of the transmission wave is different from that in the case of the probe. Since only a transmission portion is taken into consideration, the level of 20 dB/2=10 dB is the threshold value. For this reason, −10 dB is more preferable in a case where a transmission component is considered. 
     In the invention, the driving waveform of the polarization driving pulse is not particularly limited, and may be a bipolar waveform shown in  FIG.  12 A . However, the driving waveform of the polarization driving pulse is preferably a unipolar waveform as shown in  FIG.  11 A . The reason is that, as in the frequency characteristic of the driving waveform shown in  FIG.  12 B , the sensitivity of the main lobe does not change whether the driving waveform is a unipolar waveform shown by the solid line or a bipolar waveform shown by the one-dot chain line, but the sensitivities of all of the four side lobes in the case of the unipolar waveform are lower than those in the case of the bipolar waveform. 
     Therefore, by forming the transmission waveform as a unipolar waveform as shown in  FIG.  11 A , not only the main lobe but harmonic components can be suppressed. As a result, higher effects can be expected. 
     As shown in  FIG.  13 A , a plurality of unipolar waveforms may be transmitted as the polarization driving pulses. In the example shown in  FIG.  13 A , two pulse waves may be transmitted. The polarization driving pulse shown in  FIG.  13 A  has a driving waveform including two pulse waves as a driving waveform of the polarization driving pulse shown in  FIG.  11 A . The frequency characteristic of the driving waveform of the polarization driving pulse shown in  FIG.  13 A  is shown in  FIG.  13 B . The frequency characteristic of the driving waveform shown in  FIG.  13 B  is different from the frequency characteristic of the driving waveform shown in  FIG.  11 B  in terms of the waveform of the main lobe, but the waveform of the side lobe does not change much. 
     In addition, as shown in  FIG.  13 C , it is preferable to transmit a polarization driving pulse in which a plurality of pulse waveforms are connected to each other with the time of the minimum number of clocks between driving waveforms of the polarization driving pulse as unipolar waveforms. That is, in the invention, it is preferable that the transmission circuit  144  outputs a plurality of unipolar waveforms as polarization driving pulses with the time of the minimum number of clocks defined in the ultrasound processor apparatus  14  as an interval between the unipolar waveforms. 
     The reason is that it is optimal to apply a DC voltage for polarization processing, but the DC voltage cannot be transmitted in a case where the transmission circuit  144  having an existing transmission circuit configuration is used as in the invention. 
     The minimum and maximum time widths are determined depending on the type of pulser (pulse generation circuit  158 ) of the transmission circuit  144  of the ultrasound processor apparatus  14  used in the ultrasound diagnostic apparatus  10 . Therefore, by using the time of the minimum number of clocks defined in the transmission circuit  144  as the minimum time width, a high repolarization effect can be expected by putting the minimum time width between a plurality of unipolar waveforms so that a polarization processing waveform close to a DC voltage is obtained. The minimum time width between two unipolar pulse waveforms, that is, the minimum pulse width is determined by the specification of the pulser (pulse generation circuit  158 ) of the transmission circuit  144 . Control to comply with this specification is provided from the above-described FPGA in the transmission circuit  144 . 
     As shown by a two-dot chain line in  FIG.  13 D , by using a combination of a plurality of unipolar waveforms shown in  FIG.  13 C  as the driving waveform of the polarization driving pulse, it is possible to reduce the maximum sensitivity of the side lobe more than in the case of the driving waveform of the polarization driving pulse including one unipolar waveform shown by the solid line in  FIG.  13 D . 
     On the other hand,  FIGS.  14 A and  14 B  are graphs of an example of a driving waveform of a diagnostic driving pulse transmitted from the transmission circuit shown in  FIG.  4   , and are graphs showing the relationship between the sensitivity and the frequency of the driving waveform. The driving waveform shown in  FIG.  14 A  is a waveform of one bipolar wave having a center frequency of 6 MHz. The frequency characteristic of the driving waveform of the diagnostic driving pulse is shown in  FIG.  14 B . 
     &lt;&lt;Effectiveness of Ultrasound Diagnostic Apparatus  10  of the Invention&gt;&gt; 
     The ultrasound diagnostic apparatus  10  performs polarization processing using the existing transmission circuit  144 , more specifically, the pulse generation circuit  158 . In the ultrasound diagnostic apparatus  10 , the second transmission signal in the case of performing polarization processing is a pulse wave, and the pulse generation circuit  158  does not need to output a DC waveform. Therefore, it is possible to perform the polarization processing without significantly changing the existing circuit and accordingly without increasing the cost. 
     In addition, since the polarization processing is performed during the non-diagnosis period, the frame rate is not reduced. Therefore, without reducing the image quality of the ultrasound image, the reception sensitivities of the plurality of ultrasound transducers  48  can always be kept satisfactory. As a result, a high-quality ultrasound image can always be acquired. 
     The total number of ultrasound transducers  48  and the number of opening channels may be changed to any number. For example, in a case where the number of opening channels is the same as the total number of ultrasound transducers  48 , the  128  ultrasound transducers  48  can be simultaneously subjected to polarization processing instead of performing the polarization processing in two steps. Alternatively, in a case where the number of opening channels is ¼ of the total number of ultrasound transducers  48 , the  32  ultrasound transducers  48  can be simultaneously subjected to polarization processing in each of four steps. The characteristics of the above respective embodiments may be implemented in combination. 
     While the invention has been described in detail, the invention is not limited to the above-described embodiment, and various improvements and modifications may be made without departing from the scope and spirit of the invention. 
     EXPLANATION OF REFERENCES 
       10 : ultrasound diagnostic apparatus 
       12 : ultrasound endoscope 
       14 : ultrasound processor apparatus 
       16 : endoscope processor apparatus 
       18 : light source device 
       20 : monitor 
       21   a:  water supply tank 
       21   b:  suction pump 
       22 : insertion part 
       24 : operation unit 
       26 : universal cord 
       28   a:  air and water supply button 
       28   b:  suction button 
       29 : angle knob 
       30 : treatment tool insertion port 
       32   a:  ultrasound connector 
       32   b:  endoscope connector 
       32   c:  light source connector 
       34   a:  air and water supply tube 
       34   b:  suction tube 
       36 : ultrasound observation portion 
       38 : endoscope observation portion 
       40 : distal end portion 
       42 : bending portion 
       43 : flexible portion 
       44 : treatment tool lead-out port 
       45 : treatment tool channel 
       46 : ultrasound transducer unit 
       48 : ultrasound transducer 
       50 : ultrasound transducer array 
       54 : backing material layer 
       56 : coaxial cable 
       58 : endoscope side memory 
       60 : FPC 
       74 : acoustic matching layer 
       76 : acoustic lens 
       82 : observation window 
       84 : objective lens 
       86 : solid-state imaging element 
       88 : illumination window 
       90 : cleaning nozzle 
       92 : wiring cable 
       100 : console 
       140 : multiplexer 
       142 : reception circuit 
       144 : transmission circuit 
       146 : A/D converter 
       148 : ASIC 
       150 : cine memory 
       151 : memory controller 
       152 : CPU 
       154 : DSC 
       156 : notification circuit 
       158 : pulse generation circuit 
       160 : phase matching unit 
       162 : B mode image generation unit 
       164 : PW mode image generation unit 
       166 : CF mode image generation unit