Patent Publication Number: US-2019187278-A1

Title: Ultrasound probe and ultrasound diagnostic apparatus

Description:
The entire disclosure of Japanese patent Application No. 2017-243897, filed on Dec. 20, 2017, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present disclosure relates to an ultrasound probe and an ultrasound diagnostic apparatus. 
     Description of the Related Art 
     A conventional ultrasound diagnostic apparatus has ultrasound transducers disposed in an ultrasound probe. 
     In an ultrasound diagnostic apparatus of this type, the main body of the ultrasound diagnostic apparatus and the ultrasound probe are normally connected by a cable, and the number of ultrasound transducers connected to the main body is restricted by the number of signal lines in the cable and the number of system channels of a transmitting/receiving circuit. 
     In an ultrasound diagnostic apparatus of this type, the ultrasound transducers to be driven are switched and controlled in a time-sharing manner by switching circuits called multiplexers, so that transmission/reception beam deflection control and transmission/reception beam aperture movement are achieved (see JP 2006-288547 A, for example). 
     As for such an ultrasound diagnostic apparatus, there is a demand for higher target-detection sensitivity, higher ultrasound image resolution, and the like (which will be hereinafter referred to as the “acoustic characteristics of ultrasound transducers”). In view of this, reducing the signal degradation between the ultrasound transducers and the transmitting/receiving circuit is an important goal. 
     As a result of intensive studies, the inventors of this application have arrived at the conclusion that the above switching circuits (multiplexers, for example) are the cause of the signal degradation. Specifically, a switching circuit has a different circuit constant from that of the signal line extending from the transmitting/receiving circuit side. Therefore, when a transmission signal is transmitted from the transmitting/receiving circuit to an ultrasound transducer, or when a reception signal is transmitted from an ultrasound transducer to the transmitting/receiving circuit, a signal reflection phenomenon or the like might be caused, and the reflection phenomenon might cause signal degradation. 
     JP 2006-288547 A teaches that there is a difference in signal strength between a circuit including a switching circuit and a circuit not including any switching circuit, and, in view of this, a resistor equivalent to the on-resistance of a switching circuit is inserted into a circuit not including any switching circuit. However, JP 2006-288547 A does not mention any signal reflection phenomenon in switching circuits, and is unable to solve signal degradation caused by such a reflection phenomenon. 
     SUMMARY 
     The present disclosure has been made in view of the above problems, and an object thereof is to provide an ultrasound probe and an ultrasound diagnostic apparatus that are capable of reducing signal degradation caused by a switching circuit that selectively switches targets to be driven among a plurality of ultrasound transducers. 
     To achieve the abovementioned object, according to an aspect of the present invention, an ultrasound probe of an ultrasound diagnostic apparatus reflecting one aspect of the present invention comprises: 
     a plurality of ultrasound transducers that perform mutual conversion between an ultrasound wave and an electrical signal; 
     a switching circuit that selectively switches ultrasound transducers to be electrically connected to a transmitting/receiving circuit among the plurality of ultrasound transducers; and 
     a matching circuit that is provided at one of a preceding stage and a subsequent stage of the switching circuit, is connected to a position connected directly to the switching circuit, and performs impedance matching between the switching circuit and a circuit disposed at one of the preceding stage and the subsequent stage of the switching circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention: 
         FIG. 1  is a diagram showing the exterior of an ultrasound diagnostic apparatus according to a first embodiment; 
         FIG. 2  is a block diagram showing the configuration of the entire ultrasound diagnostic apparatus according to the first embodiment; 
         FIG. 3  is a circuit diagram showing the configuration of an ultrasound probe according to the first embodiment; 
         FIGS. 4A and 4B  are diagrams showing the array structure of ultrasound transducers according to the first embodiment; 
         FIGS. 5A and 5B  are diagrams for explaining switching operations at multiplexers according to the first embodiment; 
         FIGS. 6A and 6B  are diagrams showing equivalent circuits of multiplexers; 
         FIG. 7  shows the results of a simulation conducted to determine the transmission/reception characteristics of the ultrasound probe according to the first embodiment; 
         FIGS. 8A and 8B  are diagrams showing a mounting structure for the circuit components of the ultrasound probe according to the first embodiment; 
         FIGS. 9A and 9B  are diagrams showing a modification of the mounting structure for the circuit components of the ultrasound probe; and 
         FIG. 10  is a circuit diagram showing the configuration of an ultrasound probe according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more preferred embodiments of the present disclosure will be described in detail with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. In this specification and the drawings, components having substantially the same function are denoted by the same reference numeral so that the same explanation will not be repeated. 
     First Embodiment 
     [Configuration of an Entire Ultrasound Diagnostic Apparatus] 
     Referring to  FIGS. 1 and 2 , an example of the configuration of an entire ultrasound diagnostic apparatus  1  according to this embodiment is described below. 
       FIG. 1  is a diagram showing the exterior of the ultrasound diagnostic apparatus  1  according to this embodiment.  FIG. 2  is a block diagram showing the configuration of the entire ultrasound diagnostic apparatus  1  according to this embodiment. 
     The ultrasound diagnostic apparatus  1  according to this embodiment has an ultrasound probe  20  attached to a main body  10  of the ultrasound diagnostic apparatus  1  (this main body will be hereinafter referred to simply as the “main body  10 ”). The main body  10  and the ultrasound probe  20  are electrically connected via a cable C. 
     Note that the ultrasound diagnostic apparatus  1  according to this embodiment may generate any ultrasound image, such as a B-mode image, a color Doppler image, a three-dimensional ultrasound image, or an M-mode image. Likewise, any ultrasound probe, such as a convex probe, a linear probe, a sector probe, or a three-dimensional probe, can be used as the ultrasound probe  20 . 
     The main body  10  of the ultrasound diagnostic apparatus  1  includes a control unit  11 , a transmitting/receiving unit  12 , an image generating unit  13 , a display unit  14 , a storage unit  15 , and an operation unit  16 . Meanwhile, the ultrasound probe  20  includes ultrasound transducers  21 , switching circuits  22 , and matching circuits  23 . 
     The transmitting/receiving unit  12  (hereinafter also referred to as the “transmitting/receiving circuit  12 ”) is a transmitting/receiving circuit that causes the ultrasound transducers  21  of the ultrasound probe  20  to transmit and receive ultrasound waves. The transmitting/receiving unit  12  includes a transmitting circuit that generates a voltage pulse (hereinafter referred to as a “transmission signal”) and transmits the voltage pulse to the ultrasound transducers  21 , and a receiving circuit that performs a reception process (an amplification process and an A-D conversion process, for example) on an electrical signal (hereinafter referred to as a “reception signal) relating to an ultrasound echo generated by the ultrasound transducers  21 . Under the control of the control unit  11 , the transmitting circuit and the receiving circuit each perform an operation to cause the ultrasound transducers  21  to transmit and receive ultrasound waves. 
     It should be noted that the transmitting/receiving unit  12  has system channels, and is capable of operating the ultrasound transducers  21  for each of the system channels. 
     The transmitting/receiving unit  12  is connected to the ultrasound transducers  21  of the ultrasound probe  20  via the cable C. Electrical signals are exchanged between the transmitting/receiving unit  12  and the ultrasound transducers  21  via signal lines contained in the cable C. In the ultrasound probe  20  according to this embodiment, any circuit configuration for performing reception processes and transmission processes is not provided, and signals having analog waveforms flow through the signal lines in the cable C. 
     The image generating unit  13  performs predetermined signal processing (logarithmic compression, wave detection, and FFT analysis, for example) on a reception signal acquired from the transmitting/receiving unit  12 , to generate an ultrasound image (a B-mode image, a color Doppler image, or a three-dimensional ultrasound image, for example). Since the details of the process to be performed for generating an ultrasound image are well known, explanation of them are not made herein. 
     The display unit  14  is a liquid crystal display or the like, for example, and displays an ultrasound image generated by the image generating unit  13 . The storage unit  15  is a memory such as a hard disk, a ROM, or a RAM, for example, and stores a control program and various kinds of data (various kinds of setting data to be set in the transmitting/receiving unit  12 ) to be referred to by the control unit  11 , image data generated by the image generating unit  13 , and the like. The operation unit  16  is a keyboard, a mouse, or the like, and acquires an operation signal that is input by an operator. 
     The control unit  11  communicates with the respective components (the transmitting/receiving unit  12 , the image generating unit  13 , the display unit  14 , the storage unit  15 , and the operation unit  16 ) of the ultrasound diagnostic apparatus  1 , to comprehensively control these components. 
     The control unit  11  includes a transmission/reception control unit  11   a  and a switching control unit  11   b.    
     The transmission/reception control unit  11   a  causes the transmitting/receiving unit  12  to transmit a transmission signal to each ultrasound transducer  21 , and causes the transmitting/receiving unit  12  to perform a reception process on a reception signal from each ultrasound transducer  21 . 
     The switching control unit  11   b  controls the switching circuits  22 , to control switching of the ultrasound transducers  21  to be driven among the ultrasound transducers  21 . In other words, the switching control unit  11   b  controls the switching circuits  22 , to perform time-sharing switching control on the ultrasound transducers  21  connected to the respective system channels of the transmitting/receiving unit  12 , for example. Switching on and off of driven states of the ultrasound transducers  21  may be controlled individually for each ultrasound transducer  21 , or may be controlled on a block-by-block basis. 
     The control unit  11  includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), an input port, an output port, and the like, for example. Each of the functions described above is achieved when the CPU refers to a control program and various kinds of data stored in the ROM and the RAM. However, some or all of the above functions are not necessarily achieved by processing performed by software, and can of course also be achieved with a dedicated hardware circuit or a combination of software and hardware. 
     [Configuration of the Ultrasound Probe] 
     Referring now to  FIGS. 3 through 7 , an example of the configuration of the ultrasound probe  20  according to this embodiment is described. 
       FIG. 3  is a circuit diagram showing the configuration of the ultrasound probe  20  according to this embodiment. 
     The ultrasound probe  20  according to this embodiment includes the ultrasound transducers  21 , the switching circuits  22 , the matching circuits  23 , a cable connector  20 C, signal lines La (384 signal lines La- 001  through La- 384  in this example) on the side of the ultrasound transducers  21 , and signal lines Lb (192 signal lines Lb- 001  through Lb- 192  in this example) on the system channel side. 
     In the ultrasound probe  20  according to this embodiment, the switching circuits  22 , the matching circuits  23 , and the cable connector  20 C are connected in this order from the side of the ultrasound transducers  21  (hereinafter, the side of the ultrasound transducers  21  will be also referred to as the “preceding-stage side”, and the side of the cable connector  20 C will be also referred to as the “subsequent-stage side”). 
     The transmitting/receiving circuit  12  according to this embodiment has 192 channels (Ch- 001  through Ch- 192 ) as the system channels for driving the ultrasound transducers  21 . The respective system channels Ch- 001  through Ch- 192  as the 192 channels are connected to switching circuits  22 -M 001  through  22 -M 192  that are different from one another, via the signal lines Lb- 001  through Lb- 192  connected thereto. The respective system channels Ch- 001  through Ch- 192  as the 192 channels exchange electrical signals with ultrasound transducers  21 -T 001  through  21 -T 384  selected by the switching circuits  22 -M 001  through  22 -M 192  connected thereto. 
     In the ultrasound probe  20  according to this embodiment, the 384 ultrasound transducers  21 -T 001  through  21 -T 384 , the 192 switching circuits  22 -M 001  through  22 -M 192  individually connected to the respective ultrasound transducers  21 -T 001  through  21 -T 384 , and the 192 matching circuits  23 -N 001  through  23 -N 192  individually connected to the respective switching circuits  22 -M 001  through  22 -M 192  are arranged adjacent to one another. 
     The 384 signal lines La- 001  through La- 384  are disposed between the ultrasound transducers  21 -T 001  through  21 -T 384  and the switching circuits  22 -M 001  through  22 -M 192 , being connected to the respective transducers and the switching circuits. Meanwhile, the 192 signal lines Lb- 001  through Lb- 192  are disposed between the switching circuits  22 -M 001  through  22 -M 192  and the system channels Ch- 001  through Ch- 192  of the transmitting/receiving circuit  12 , being connected to the respective switching circuits and the respective system channels. 
     The cable C contains the same number of signal lines Lb as the number of system channels (the 192 channels in this example) for driving the ultrasound transducers  21 , and control lines (not shown) for performing switching control on the switching circuits  22 . Electrical signals are exchanged between the ultrasound probe  20  and the main body  10  via the cable connector  20 C on the side of the ultrasound probe  20 , the cable C, and a cable connector  10 C on the side of the main body  10 . 
     In the description below, the configurations of the respective ultrasound transducers  21 -T 001  through  21 -T 384  are assumed to be the same, and are referred to simply as the ultrasound transducers  21  in a case where there is no need to specifically distinguish them from one another. The configurations of the switching circuits  22 -M 001  through  22 -M 192  are also assumed to be the same, and are referred to simply as the switching circuits  22  in a case where there is no need to specifically distinguish them from one another. The configurations of the matching circuits  23 -N 001  through  23 -N 192  are also assumed to be the same, and are referred to simply as the matching circuits  23  in a case where there is no need to specifically distinguish them from one another. The signal lines La- 001  through La- 384  are also referred to simply as the signal lines La in a case where there is no need to specifically distinguish them from one another. The signal lines Lb- 001  through Lb- 192  are referred to simply as the signal lines Lb in a case where there is no need to specifically distinguish them from one another. 
     Configuration of the Ultrasound Transducers  21   
     Each ultrasound transducer  21  is a piezoelectric element that performs mutual conversion between ultrasound waves and an electrical signal. The ultrasound transducer  21  converts a transmission signal transmitted from the transmitting/receiving circuit  12  into ultrasound waves, transmits the ultrasound waves into the subject, converts the ultrasound echo reflected within the subject into an electrical signal, and transmits the electrical signal to the transmitting/receiving circuit  12 . The ultrasound transducer  21  includes a piezoelectric body such as a piezoceramic material, a polymer piezoelectric material, or a piezoelectric single-crystal material, a signal electrode disposed on one side surface of the piezoelectric body, and a ground electrode disposed on the other side surface of the piezoelectric body, for example. A signal line La is connected to the signal electrode of the ultrasound transducer  21 , and a grounding conductor is connected to the ground electrode. 
     The ultrasound transducers  21  are individually connected to the respective switching circuits  22  via the respective signal lines La, and electrical connection to the transmitting/receiving circuit  12  are selectively switched on and off by switching at the switching circuits  22 . Of these ultrasound transducers  21 , the ultrasound transducer  21  electrically connected to the transmitting/receiving circuit  12  is the ultrasound transducer  21  to be driven, and exchanges electric signals (a transmission signal and a reception signal) with the transmitting/receiving circuit  12 . 
     In this configuration, under the control of the control unit  11  (the switching control unit  11   b ), the ultrasound transducer  21  to be driven is selected in a time-sharing manner from among the ultrasound transducers  21 , and transmission/reception beam deflection control and electronic scanning such as aperture movement of the transmission/reception beam are performed. 
     The switching on and off of the electrically connected state (that is, switching between a driven state and a non-driven state) between the ultrasound transducers  21  and the transmitting/receiving circuit  12  may be individually controlled for each ultrasound transducer  21 , but may also be controlled for each block of the ultrasound transducers  21 . Alternatively, switching control may be performed so that each ultrasound transducer  21  that performs an ultrasound wave transmitting operation and each ultrasound transducer  21  that performs an ultrasound echo receiving operation are separated from each other among the ultrasound transducers  21 . 
       FIGS. 4A and 4B  are diagrams showing an example of the array structure of the ultrasound transducers  21 .  FIG. 4A  shows the array structure of the ultrasound transducers  21  according to this embodiment. The 384 ultrasound transducers  21  according to this embodiment are arranged in a row in an azimuth direction in the following order: the ultrasound transducer  21 -T 001 , the ultrasound transducer  21 -T 002 , the ultrasound transducer  21 -T 003 , . . . , and the ultrasound transducer  21 -T 384 . 
       FIG. 4B  shows another form of the array structure of the ultrasound transducers  21 . In  FIG. 4B , the ultrasound transducers  21  are arranged in an elevating direction as well as the azimuth direction, and 384×3 ultrasound transducers are arranged in the azimuth direction and the elevating direction to form a matrix-like array in a two-dimensional plane. In the array structure of the ultrasound transducers  21  shown in  FIG. 4B , 384 ultrasound transducers  21 -T 001  through  21 -T 384  are sequentially arranged in the azimuth direction in the first row, 384 ultrasound transducers  21 -T 385  through  21 -T 768  are sequentially arranged in the azimuth direction in the second row, and 384 ultrasound transducers  21 -T 769  through  21 -T 1152  are sequentially arranged in the azimuth direction in the third row. 
     Configuration of the Switching Circuits  22   
     The switching circuits  22  are switching circuits that select ultrasound transducers  21  to be connected to the transmitting/receiving circuit  12  among the ultrasound transducers  21 . The switching circuits  22  are typically formed with multiplexer ICs (each of the switching circuits  22 -M 001  through  22 -M 192  in  FIG. 3  is equivalent to a multiplexer IC, and will be hereinafter also referred to as a “multiplexer  22 ” or a “MUX  22 ”). A control line (not shown) to which a control signal from the control unit  11  (the switching control unit  11   b ) is input is connected to each multiplexer  22 , and, in accordance with the control signal, each multiplexer  22  selects a ultrasound transducer  21  to be driven from among the ultrasound transducers  21 . 
     A signal line Lb is connected to the system channel side of each of the 192 multiplexers  22 -M 001  through  22 -M 192  according to this embodiment, and signal lines La (two signal lines La in this example) are connected in parallel to each multiplexer on the side of the ultrasound transducers  21 . The respective signal lines Lb- 001  through Lb- 192  are connected to the 192 multiplexers  22 -M 001  through  22 -M 192  on the system channel side, and the respective signal lines La- 001  through La- 384  are connected to the multiplexers  22 -M 001  through  22 -M 192  on the side of the ultrasound transducers  21 . 
     Meanwhile, a control signal from the control unit  11  (the switching control unit  11   b ) is input to each of the multiplexers  22 -M 001  through  22 -M 192 . In accordance with the control signal, each of the multiplexers  22 -M 001  through  22 -M 192 , which are independent of one another, selectively connects one of the channels on the side of the ultrasound transducers  21  to one system channel on the side of the transmitting/receiving circuit  12  in an electrical manner. In other words, the 192 multiplexers  22 -M 001  through  22 -M 192  are designed to correspond to the system channels Ch- 001  through Ch- 192 , respectively, and independently switch ultrasound transducers  21  to be driven, for each of the system channels. 
       FIGS. 5A and 5B  are diagrams for explaining switching operations at the multiplexers  22  according to this embodiment. 
       FIG. 5A  shows electrically connected states of the respective multiplexers  22 -M 001  through  22 -M 192  at respective timings in chronological order.  FIG. 5B  is a diagram schematically showing the ultrasound transducers  21 -T 001  through  21 -T 384  to be driven in  FIG. 5A . 
     The left column in  FIG. 5A  shows the identification numbers of the multiplexers  22 -M 001  through  22 -M 192 . On the assumption that time elapses in the order of t=0, t=1, t=2, t=3, . . . , the respective rows in  FIG. 5A  show the electrically connected states of the multiplexers  22 -M 001  through  22 -M 192  corresponding to the identification numbers shown in the left column at the respective timings. 
     In this example, the multiplexer “ 22 -M 001 ” has a signal line La connected to the ultrasound transducer “ 21 -T 001 ” and the ultrasound transducer “ 21 -T 193 ”, and electrically connects one of the two ultrasound transducers to the system channel Ch- 001 , in accordance with a control signal from the main body  10  (the switching control unit  11   b ). Likewise, the multiplexer “ 22 -M 002 ” has a signal line La connected to the ultrasound transducer “ 21 -T 002 ” and the ultrasound transducer “ 21 -T 194 ”, and electrically connects one of the two ultrasound transducers to the system channel Ch- 002 , in accordance with a control signal from the main body  10  (the switching control unit  11   b ). 
     The control unit  11  (the switching control unit  11   b ) determines the ultrasound transducer  21  to be driven among a plurality of ultrasound transducers  21  in a time-sharing manner. 
     At t=0, the control unit  11  (the switching control unit  11   b ) sets the electrically connected states of the multiplexers “ 22 -M 001 ” through “ 22 -M 192 ” to A-side connection, so that the ultrasound transducers “ 21 -T 001 ” through “ 21 -T 192 ” are driven. 
     At t=1, the control unit  11  (the switching control unit  11   b ) sets the electrically connected state of the multiplexer “ 22 -M 001 ” to B-side connection, and sets the electrically connected states of the multiplexers “ 22 -M 002 ” through “ 22 -M 192 ” to A-side connection, so that the ultrasound transducers “ 21 -T 002 ” through “ 21 -T 193 ” are driven. 
     At t=2, the control unit  11  (the switching control unit  11   b ) sets the electrically connected states of the multiplexers “ 22 -M 001 ” and “ 22 -M 002 ” to B-side connection, and sets the electrically connected states of the multiplexers “ 22 -M 003 ” through “ 22 -M 192 ” to A-side connection, so that the ultrasound transducers “ 21 -T 003 ” through “ 21 -T 194 ” are driven. 
     At t=3, the control unit  11  (the switching control unit  11   b ) sets the electrically connected states of the multiplexers “ 22 -M 001 ” through “ 22 -M 003 ” to B-side connection, and sets the electrically connected states of the multiplexers “ 22 -M 004 ” through “ 22 -M 192 ” to A-side connection, so that the ultrasound transducers “ 21 -T 004 ” through “ 21 -T 195 ” are driven. 
     In this manner, the respective multiplexers  22  sequentially select the ultrasound transducers  21  to be connected to the system channels Ch- 001  through Ch- 192  from among the ultrasound transducers  21 -T 001  through  21 -T 384 , in accordance with a control signal from the switching control unit  11   b . As a result, the scanning block formed with the ultrasound transducers  21  in a driven state sequentially slide with time. 
     For the respective multiplexers  22 -M 001  through  22 -M 192  according to this embodiment, connection targets among the ultrasound transducers  21 -T 001  through  21 -T 384  are set, so that the number of connected ultrasound transducers  21  becomes constant (“A-side” or “B-side” in this example) for each one system channel when the scanning block slides. Specifically, of the ultrasound transducers  21 -T 001  through  21 -T 384 , the same number of adjacent ultrasound transducers  21  as the number of system channels (192 channels in this example) are connected to different multiplexers  22  from each other. 
     In this manner, the circuit parameters in the circuits between the transmitting/receiving circuit  12  and the ultrasound transducers  21  are kept constant, and the same matching condition for impedance matching in the matching circuit  23  is always maintained. 
     Configuration of the Matching Circuits  23   
     The matching circuits  23  are connected to positions directly connected to the multiplexer  22 , and perform impedance matching between the multiplexers  22  and the circuits connected to the multiplexers  22 . In other words, the matching circuits  23  compensate for the characteristic impedance mismatch caused between circuits due to insertion of the multiplexers  22 , and thus, reduces signal degradation. 
     It should be noted that such a phenomenon of signal reflection often occurs at the boundaries between the multiplexers  22  and the circuits at the subsequent stage of the multiplexers  22 , when a transmission signal is transmitted from the transmitting/receiving circuit  12  to the ultrasound transducers  21 , or when a reception signal is transmitted from the ultrasound transducers  21  to the transmitting/receiving circuit  12 . 
     In view of this, the matching circuits  23  according to this embodiment are connected to positions connected directly to the multiplexers  22  on the side of the transmitting/receiving circuit  12 . 
     The matching circuits  23  according to this embodiment are also disposed separately from one another in the respective signal lines Lb- 001  through Lb- 192 . In other words, the matching circuits  23  according to this embodiment are formed with the individual matching circuits  23 -N 001  through  23 -N 192  connected in series to the subsequent stages of the respective multiplexers  22 -M 001  through  22 -M 192 . 
     The matching circuits  23 -N 001  through  23 -N 192  typically include inductor elements connected in series, so as to be connected directly to the respective multiplexers  22 -M 001  through  22 -M 192 . 
       FIGS. 6A and 6B  are diagrams showing equivalent circuits of multiplexers  22 . Note that the equivalent circuits shown in  FIGS. 6A and 6B  are known from the following literature, for example: “The Acoustic and Thermal Effects Using Multiplexers in Small Invasive Probes”, L. J. Busse, C. G. Oakley, M. J. Fife, J. V. Ranalletta, R. D. Morgan, and D. R. Dietz, IEEE Ultrasonics Symposium, 1997, 1721-1724. 
     For one of the system channels, each of the multiplexers  22  according to this embodiment switches on an electrically connected state of one of the N channels (two channels in  FIG. 3 ) of the ultrasound transducers  21  connected thereto via the signal line La. Accordingly, as shown in  FIG. 6A , an equivalent circuit of each multiplexer  22  can be expressed as a configuration in which switch portions (the transistors constituting the multiplexer  22 , for example) are connected in parallel between the input and the output. 
     Since each of the multiplexers  22  according to this embodiment is constantly in an electrically connected state on one of “A-side” and “B-side”, and is constantly in an electrically disconnected state on the other side. Accordingly, the impedance formed by the multiplexer  22  always has the same value. 
       FIG. 6B  is an equivalent circuit showing the circuit parameters formed by each multiplexer  22 , on the basis of the circuit model shown in  FIG. 6A . 
     As shown in  FIG. 6B , the impedance formed by each multiplexer  22  can be expressed as a combination of an on-resistance Ron connected in series between the input and the output in an on-state switch portion, a parasitic capacitance Con/2 connected between the signal line La and the ground in the on-state switch portion, a parasitic capacitance Con/2 connected between the signal line Lb and the ground in the on-state switch portion, and a parasitic capacitance Coff connected between the signal line La and the ground in an off-state switch portion. 
     The impedance of each multiplexer  22  is normally a value including capacitive components as described above. Further, as the impedance of each multiplexer  22  is a combination of capacitive components connected in parallel, the capacitive components easily become larger, and characteristic impedance mismatch is often caused, particularly at the boundaries with the signal lines connected to the multiplexers  22  on the system channel side. 
     In view of this, the circuit constant of each matching circuit  23  is set so that the reactance component that is the sum of the impedances of the multiplexer  22  and the matching circuit  23  becomes zero, for example. For example, the circuit constant of each matching circuit  23  is set so that the resistive component that is the sum of the impedances of the multiplexer  22  and the matching circuit  23  becomes close to the characteristic impedance (50Ω, for example) of the signal line Lb connected to the matching circuit  23 . 
     Alternatively, the circuit constant of each matching circuit  23  may be set so that the electrical impedance of the ultrasound probe  20  is electrically matched with the input/output impedance of the transmitting/receiving unit  12 , for example. In that case, however, while the output impedance of the transmitting portion of the transmitting/receiving unit  12  is several tens of Ω, the input impedance of the receiving portion is several hundreds of Ω. Therefore, the circuit constant of each matching circuit  23  may be set so that electrical matching can be achieved between the transmitting portion and the receiving portion. 
     In the above manner, reflection phenomena at the boundaries between the multiplexers  22  and the circuits on the subsequent-stage side of the multiplexers  22  can be reduced, both when a transmission signal is transmitted from the transmitting/receiving circuit  12  to the ultrasound transducers  21 , and when a reception signal is transmitted from the ultrasound transducers  21  to the transmitting/receiving circuit  12 . 
     A configuration having an inductance value is typically selected as the configuration of each matching circuit  23 , because the impedance of each multiplexer  22  is inclined toward the capacitive component side. The configuration of each matching circuit  23  typically includes inductor elements that are connected in series, as described above. 
     However, the matching circuits  23 -N 001  through  23 -N 192  do not necessarily include inductor elements connected in series, and may of course have a configuration that performs impedance matching depending on the line length of a transmission line (λ/4 line, for example), or a configuration that performs impedance matching using stubs or the like. 
     It is also possible to set the circuit constant of each matching circuit  23 , taking into account not only the impedance of the multiplexer  22 , but also the impedance of the signal line La on the side of the ultrasound transducer  21 , the impedance of the ultrasound transducer  21 , the impedance of the signal line Lb on the side of the transmitting/receiving circuit  12  (including the area leading to the transmission/reception circuit  12 ), and the like. In a case where the condition for impedance matching differs between transmission of a transmission signal from the transmitting/receiving circuit  12  to the ultrasound transducers  21  and transmission of a reception signal from the ultrasound transducers  21  to the transmitting/receiving circuit  12 , on the other hand, the circuit constant of each matching circuit  23  may be set depending on the case where reflection phenomena are to be reduced. 
     In such a case, the circuit constant of each matching circuit  23  can be designed by the same technique as a known technique for impedance matching. For example, it is possible to use a technique for setting a circuit constant so that the impedance on the side of the transmitting/receiving circuit  12  has a complex conjugate relationship with the impedance on the side of the ultrasound transducer  21 , with the reference point being the connecting point on the subsequent-stage side of the matching circuit  23 , and the impedance on the side of the transmitting/receiving circuit  12  has a complex conjugate relationship with the impedance on the side of the ultrasound transducer  21 , with the reference point being the connecting point on the preceding-stage side of the matching circuit  23 . 
       FIG. 7  shows the results of a simulation conducted to determine the transmission/reception characteristics of the ultrasound probe  20  according to this embodiment. 
     In this simulation, predetermined circuit parameters are set for the respective components in the circuit configuration of the ultrasound probe  20  shown in  FIG. 3 , and transmission/reception characteristics are calculated. In this simulation, transmission/reception characteristics are calculated in a series of processes: a transmission signal is transmitted from the transmitting/receiving circuit  12  to the ultrasound transducers  21 , the ultrasound transducers  21  are made to transmit ultrasound waves, the ultrasound echo returned from a predetermined target is converted into a reception signal at the ultrasound transducers  21 , and the reception signal returned to the transmitting/receiving circuit  12  is acquired. 
     The transmission/reception characteristics calculated in this simulation indicate the degree of decrease in the signal strength of the reception signal returned to the transmitting/receiving circuit  12  with respect to the signal strength of the transmission signal transmitted from the transmitting/receiving circuit  12  to the ultrasound transducers  21 . In this simulation, a degree of decrease in signal strength is also calculated at each frequency of transmission signals. 
     In the graph shown in  FIG. 7 , the solid line indicates the transmission/reception characteristics in the circuit configuration shown in  FIG. 3  (which includes the multiplexers  22  and the matching circuits  23 ). The dot-and-dash line indicates the transmission/reception characteristics in a mode that has the circuit configuration shown in  FIG. 3  but do not include the multiplexers  22  and the matching circuits  23 . The dotted line indicates the transmission/reception characteristics in a mode that has the circuit configuration shown in  FIG. 3  but do not include the matching circuits  23 . 
     In  FIG. 7 , the abscissa axis indicates transmission signal frequency, and the ordinate axis indicates the degree of decrease in signal strength when the ultrasound transducers  21  perform transmission and reception (=the ratio of the power of the transmission signal transmitted from the transmitting/receiving circuit  12  to the power of the reception signal acquired by the transmitting/receiving circuit  12 ). The circuit constants of the respective components shown in  FIG. 3  are the same in the respective modes. 
     As can be seen from  FIG. 7 , in the case where the multiplexers  22  are included (the dotted line), the decrease in signal strength is smaller than that in the mode without the multiplexers  22  and the matching circuits  23  (the dot-and-dash line). In the case where the matching circuits  23  are included (the solid line) as in the ultrasound probe  20  according to this embodiment, the degree of decrease in signal strength is smaller than that in the case where the matching circuits  23  are not included (the dotted line). 
     Furthermore, in the ultrasound probe  20  according to this embodiment (the solid line), the degree of decrease in signal strength is substantially the same as that in the mode without the multiplexers  22  and the matching circuits  23  (the dot-and-dash line). This proves that the matching circuits  23  can substantially compensate for signal degradation caused by the multiplexers  22 . 
     [Mounting Structure for the Ultrasound Probe] 
       FIGS. 8A and 8B  are diagrams showing a mounting structure for the circuit components of the ultrasound probe  20  according to this embodiment.  FIG. 8A  is a plan view of the mounting structure, and  FIG. 8B  is a side view of the mounting structure. It should be noted that, in  FIG. 8B , a pair of mounting structures, each of which is the same as the mounting structure shown in  FIG. 8A , are mounted. 
     The ultrasound probe  20  according to this embodiment includes a first circuit board Pa 1  in which the multiplexers  22  are mounted, a second circuit board Pa 2  in which the matching circuits  23  are mounted, a first flexible wiring board Pb 1  that connects the ultrasound transducers  21  and the first circuit board Pa 1  with wires, and a second flexible wiring board Pb 2  that connects the first circuit board Pa 1  and the second circuit board Pa 2  with wires. 
     In this mode, the switching circuits  22  are formed with individual multiplexer ICs. A multilayer wiring board is used as the first circuit board Pa 1  in which the multiplexers  22  are mounted. With this arrangement, the signal lines La- 001  through La- 384  corresponding to the large number of channels of the ultrasound transducers  21 -T 001  through  21 -T 384  can be formed in the substrate of the first circuit board Pa 1 . 
     In each of the first flexible wiring board Pb 1  and the second flexible wiring board Pb 2 , a wiring part is formed with an anisotropic conductive film, for example. A connector-connector connection Pc is used as the connecting structure between the wiring part of the second circuit board Pa 2  and the wiring part of the second flexible wiring board Pb 2 . 
     As described above, in the ultrasound probe  20  according to this embodiment, the multiplexers  22  and the matching circuits  23  are mounted in the different circuit boards Pa 1  and Pa 2 , and the circuit boards Pa 1  and Pa 2  are connected by the first flexible wiring board Pb 1 . With this arrangement, an increase in the size of the circuit boards is prevented, and the ultrasound probe  20  is made smaller in size. In other words, this structure increases the degree of freedom in the layout of the components in the ultrasound probe  20 . Thus, it is possible to insert the matching circuits  23  without affecting the exterior of the ultrasound probe  20 . 
     To achieve a smaller size, two mounting structures, each of which is the same as that shown in  FIG. 8A , are preferably mounted as a pair in the ultrasound probe  20 , as shown in  FIG. 8B . Specifically, two mounting structures, each of which is the same as that shown in  FIG. 8A , are preferably provided as a pair so that the respective board surfaces of the first circuit board Pa 1  and the second circuit board Pa 2  face each other. With this arrangement, it is possible to form the signal lines La- 001  through La- 384  corresponding to the large number of channels of the ultrasound transducers  21 -T 001  through  21 -T 384 . 
       FIGS. 9A and 9B  show a modification of the mounting structure shown in  FIGS. 8A and 8B .  FIG. 9A  is a plan view of a mounting structure according to this modification, and  FIG. 9B  is a side view of the mounting structure according to this modification. 
     The mounting structure shown in  FIGS. 9A and 9B  differs from the mounting structure shown in  FIGS. 8A  and  8 B only in that some of the multiplexers  22  are mounted in the first circuit board Pa 1 , and the other multiplexers  22  are mounted in the second circuit board Pa 2 . As the multiplexers  22  are disposed on both the first circuit board Pa 1  and the second circuit board Pa 2 , an increase in the size of the first circuit board Pa 1  can be more effectively prevented. 
     [Effects] 
     As described above, in the ultrasound probe  20  of the ultrasound diagnostic apparatus  1  according to this embodiment, the matching circuits  23  that perform impedance matching between the multiplexers  22  (equivalent to switching circuits) and the circuits on the subsequent-stage side connected to the multiplexers  22  are connected to the positions connected directly to the multiplexers  22  on the side of the transmitting/receiving circuit  12 . 
     With this configuration, reflection phenomena at the connecting positions between the multiplexers  22  and the circuits in the subsequent stage can be reduced, when a transmission signal is transmitted from the transmitting/receiving circuit  12  to the ultrasound transducers  21 , or when a reception signal is transmitted from the ultrasound transducers  21  to the transmitting/receiving circuit  12 . In other words, this configuration can reduce signal degradation due to reflection phenomena, and improve the acoustic characteristics of the ultrasound transducers  21 . 
     Further, in the ultrasound probe  20  according to this embodiment, it is possible to overcome the difficulty in reducing the circuit size while providing the matching circuits  23  between the multiplexers  22  and the respective ultrasound transducers  21  so as to perform impedance matching at the positions of connection to the subsequent stage of the multiplexers  22 . 
     Furthermore, in the ultrasound probe  20  according to this embodiment, the number of ultrasound transducers  21  to be connected to the transmitting/receiving circuit  12  by the multiplexers  22  is maintained constant at each timing during electronic scanning. Thus, the same matching condition for impedance matching at the matching circuits  23  can be always maintained. 
     Second Embodiment 
     Next, an example of the configuration of an ultrasound probe  20  according to a second embodiment is described, with reference to  FIG. 10 . 
       FIG. 10  is a circuit diagram showing the configuration of the ultrasound probe  20  according to this embodiment. The ultrasound probe  20  according to this embodiment differs from the ultrasound probe  20  according to the first embodiment in further including matching circuits  24  on the preceding-stage side of positions connected directly to the multiplexers  22 . For clear distinction, the matching circuits  23  on the subsequent-stage side of the multiplexers  22  will be hereinafter referred to as the “first matching circuits  23 ”, and the matching circuits  24  on the preceding-stage side of the multiplexers  22  will be hereinafter referred to as the “second matching circuits  24 ”. 
     The second matching circuits  24  are connected in series to the respective ultrasound transducers  21 -T 001  through  21 -T 384  on the preceding-stage side of the multiplexers  22 -M 001  through  22 -M 192 . That is, the ultrasound probe  20  according to this embodiment includes 384 second matching circuits  24 -W 001  through  24 -W 384 . 
     Like the first matching circuits  23 , the second matching circuits  24  are typically formed with series-connected inductor elements or the like. 
     As described above, in the ultrasound probe  20  of the ultrasound diagnostic apparatus  1  according to this embodiment, the second matching circuits  24  that perform impedance matching between the multiplexers  22  (equivalent to switching circuits) and the circuits on the preceding-stage side connected to the multiplexers  22  are connected to the positions connected directly to the multiplexers  22  on the side of the ultrasound transducers  21 . With this configuration, reflection phenomena can be even more effectively reduced. 
     Other Embodiments 
     The present invention is not limited to the above embodiments, and various modifications can be made to them. 
     In the above embodiments, as an example of the switching circuits  22 , the individual multiplexers  22 -M 001  through  22 -M 192  are provided for the respective system channels Ch- 001  through Ch- 192  of the transmitting/receiving circuit  12 . However, the switching circuit  22  may be formed with any structures that are capable of selectively connecting the ultrasound transducers  21  to be driven to the transmitting/receiving circuit  12 . 
     Further, in an example of the ultrasound probe  20  in the above embodiments, the ultrasound transducers  21  have the same structures, the switching circuits  22  have the same structures, and the matching circuits  23  have the same structures. However, some of these components may of course be different. 
     Also, in an example of the ultrasound probe  20  in the above embodiments, all the transmitting circuits that transmit transmission signals to the ultrasound transducers  21 , and all the receiving circuits that acquire reception signals from the ultrasound transducers  21  and perform a reception process on the reception signals are disposed in the main body  10  of the ultrasound diagnostic apparatus  1 . However, some or all of these components may be disposed in the housing of the ultrasound probe  20 . 
     With an ultrasound probe according to an embodiment of the present disclosure, it is possible to reduce degradation of acoustic characteristics due to switching circuits that selectively switch ultrasound transducers to be driven among a plurality of ultrasound transducers. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims. The inventions disclosed in the claims include technologies achieved by making various changes and modifications to the above specific embodiments.