Patent Publication Number: US-10324027-B2

Title: Elastic wave receiving apparatus, elastic wave receiving method, photoacoustic apparatus, and program

Description:
RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 13/503,785, filed Apr. 24, 2012. It claims benefit of that application under 35 U.S.C. § 120, and claims benefit under 35 U.S.C. § 119 of Japanese Patent Application No. 2009-284541, filed on Dec. 15, 2009. The entire contents of each of the mentioned prior applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an elastic wave receiving apparatus, an elastic wave receiving method, a photoacoustic apparatus and program. 
     BACKGROUND ART 
     In recent years, an apparatus for receiving an elastic wave generated from the inside of a subject by a probe and imaging the structure of the inside is being studied and developed. As one of applications, a photoacoustic diagnostic apparatus that irradiates the inside of a subject with a laser beam, receives a photoacoustic wave generated from the inside of the subject by an ultrasound probe, and displaying a tissue image of the inside of the subject is proposed (refer to, for example, NPL1). In an example of the photoacoustic diagnostic apparatus, a subject (breast) is compressed by plates and an ultrasound probe and irradiated with a pulse laser beam over the plate. A photoacoustic wave generated on the inside of the subject is received by the ultrasound probe, and a tissue image of the inside of the subject is reconstructed and displayed. A two-dimensional scanning mechanism capable of performing a two-dimensional scan by the probe and a pulse laser generator facing the probe to obtain a tissue image of the entire subject (breast) is provided, and a photoacoustic wave is measured in a plurality of measurement points. 
     An ultrasonic diagnosis apparatus having a mechanism of performing a mechanical scan by a probe and a method for controlling the same are known (refer to, for example, PTL1). In the ultrasonic diagnosis apparatus, the probe is provided with a position detector, a speed signal is generated from a position signal, and the difference between a target position and target speed is fed back to a drive signal of the probe. 
     In those diagnosis apparatuses, at the time of receiving a photoacoustic wave or ultrasound generated in the subject by the probe, when there is a space between the subject and the probe, the picture quality of a tissue image of the part deteriorates markedly for the following reason. The acoustic impedance of air in the space and that of the subject are largely different from each other, and the ultrasonic wave hardly passes through the space. To avoid the issue, a matching agent whose acoustic impedance is close to that of the subject is inserted between the probe and the subject so as not to have a space. To prevent a space from being created at the time of performing a mechanical scan by the probe, the probe, the subject, and the plates have to be sufficiently closely attached to each other so as not to have a space. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Application Laid-Open No. 2004-195088 
       
    
     Non Patent Literature 
     
         
         NPL 1: Srirang Manohar, et. al: Region-of-interest breast studies using the Twente Photoacoustic Mammoscope (PAM), Proc. of SPIE, Vol. 6437, pp. 1-9, 2007. 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in a conventional photoacoustic diagnosis apparatus, a subject is compressed by plates and a matching agent is applied measurement by measurement. At this time, there is a case that a load at the time of moving the probe over the plate fluctuates due to individual difference of the subject, strain of the plates, scratch, an application state of a matching agent, fluctuations in atmosphere temperature, and the like. Consequently, due to the load fluctuations, there is a case that the position of the probe upon reception of the photoacoustic wave is deviated from the target position, and the precision of the reconstructed tissue image deteriorates. In the case where the load on the probe at the time of movement over the plate becomes large and the position of the probe is markedly deviated, measurement has to be started again, and it requires extra laser beam application to the subject. There is an issue that the stress on the patient increases. Further, to drive the probe accurately to the target position in spite of a large load, an actuator of larger capacity is necessary, and an issue occurs such that cost reduction of the apparatus is hindered. 
     The present invention has been devised in consideration of the situations of the conventional arts as described above. An object of the present invention is to provide an elastic wave receiving apparatus and an elastic wave receiving method realizing increased position precision of a probe without increasing cost even in a situation that the load on the probe at the time of movement over a plate fluctuates. 
     Solution to Problem 
     An elastic wave receiving apparatus according to this invention comprising: 
     a plate-like member that supports a subject in a supporting side as one of faces; 
     a probe that receives an elastic wave generated from the subject by scanning a scanning side as the other side of the supporting side of the plate-like member; 
     a driving unit that drives the probe so as to scan the scanning side of the plate-like member; 
     a drive controlling unit that supplies a drive signal to the driving unit so that the probe moves to a predetermined target position on the scanning side; and 
     an information acquiring unit that preliminarily acquires and stores a physical value corresponding to a load generated in the driving unit when the scanning side is scanned by the probe, 
     wherein the drive controlling unit corrects the drive signal so that the probe moves to the target position regardless of the load by using the physical value stored in the information acquiring unit. 
     An elastic wave receiving method according to this invention for making a probe scan a plate-like member that supports a subject by an instruction signal to move the probe to a predetermined target position, and receiving an elastic wave generated from the subject by the probe, comprising: 
     an information acquiring step of preliminarily acquiring a physical value corresponding to a load necessary to move the probe, over the plate-like member; 
     an instruction signal correcting step of correcting the instruction signal by using the physical value acquired in the information acquiring step; and 
     a driving step of moving the probe to the target position by the instruction signal corrected in the instruction signal correcting step. 
     Advantageous Effects of Invention 
     According to the present invention, by estimating a load at the time of making a probe scan the surface of a plate-like member and changing a driving method so that the probe can move to a target position, the position precision of the probe can be increased and the precision of a tissue image can be improved without increasing cost. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of an elastic wave receiving apparatus according to a first embodiment of the invention. 
         FIG. 2  is a flowchart showing a measurement flow according to the first embodiment of the invention. 
         FIG. 3  is a diagram showing a mode of a scan of a probe according to the first embodiment of the invention. 
         FIG. 4  is a diagram showing the relation between a target position of the probe and time. 
         FIG. 5  is a diagram showing the relation between a measurement position of the probe and time. 
         FIG. 6  is a block diagram of an elastic wave receiving apparatus according to a second embodiment of the invention. 
         FIG. 7  is a flowchart showing a measurement flow according to the second embodiment of the invention. 
         FIG. 8  is a block diagram of an elastic wave receiving apparatus according to a third embodiment of the invention. 
         FIG. 9  is a flowchart showing a measurement flow according to the third embodiment of the invention. 
         FIG. 10  is a diagram showing layout of strain gauges in the third embodiment of the invention. 
         FIGS. 11A and 11B  are diagrams showing example of a state of compression plates in the third embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments for carrying out the present invention will be illustratively and specifically described below with reference to the drawings. 
     Example 1 
       FIG. 1  is a block diagram showing a schematic configuration of an elastic wave receiving apparatus according to the embodiment. In  FIG. 1 , a configuration other than a light source  104  corresponds to an elastic wave receiving apparatus, and a configuration including the light source  104  corresponds to a photoacoustic apparatus. In  FIG. 1, 101  denotes a breast as a subject. The subject  101  is supported by being sandwiched and compressed by compression plates  102  and  103  each having a plate-like shape as a plate-like member. The compression plate  102  compresses and supports the subject by its supporting side as one of sides. In this state, a laser beam L for making the subject  101  generates a photoacoustic wave is emitted from the pulse laser light source  104 . The pulse laser light source  104  irradiates the subject  101  with the laser beam L while making the laser beam L fluctuate in pulses at predetermined frequency. A photoacoustic wave generated when energy of the emitted laser beam is absorbed by the subject  101  and diffused is received as an elastic wave W by a probe  105  and converted to an electric signal. By being driven by a motor  106  as a driving unit, the probe  105  can two-dimensionally scan the surface of the scanning side as the opposite side of the supporting side of the compression plate  102  in the entire subject  101 . 
     The motor  106  is a pulse motor, which is driven by a pulse signal S 3  generated by a controller  107  as a drive controller, and the driving of the motor  106  is controlled according to the number and frequency of the pulse signals S 3  output from the controller  107 . Concretely, the rotational angle of the motor  106  changes in proportion to the number of pulse signals S 3 . The rotation speed of the motor  106  changes in proportion to the frequency of the pulse signal S 3 . That is, by controlling the number and frequency of the pulses output from the controller  107 , the position, speed, and acceleration of the probe  105  can be controlled. The position information on the compression plate  102  of the probe  105  is acquired by an encoder  108  as a position detecting unit. 
     Position information S 1  of the probe  105  acquired by the encoder  108  is transmitted to a load estimating unit  109  as an information acquiring unit. In the load estimating unit  109 , an estimation value corresponding to a load at the time of making the probe  105  move over the compression plate  102  is calculated based on the position information S 1  of the probe  105  and stored. The load estimating unit  109  includes a microcontroller (or computer) and software (or a program) assembled in the microcontroller. An estimation value S 2  corresponding to the load calculated by the load estimating unit  109  is transmitted to the controller  107 . The controller  107  has the function of changing the number and timing of the drive pulse signals of the motor  106  based on the transmitted estimation value S 2 . A user interface  110  is an interface used by the operator to perform an operation start instruction of the elastic wave receiving apparatus, an end instruction, reception data viewing, and the like. 
       FIG. 2  shows a measurement flow since the subject  101  is supported by being compressed by the compression plates  102  and  103  until an elastic wave is received/recorded by the probe  105  in the case where the elastic wave receiving apparatus according to the embodiment is used. When the measurement flow starts, first, in step S 201 , the subject  101  is compressed/supported by the compression plates  102  and  103 , and completion of setup of the subject  101  is detected. Concretely, the operator performs a series of preparations such as compression of the subject  101 , fixing, and application of the matching agent and, after that, transmits an operation start instruction signal to the controller  107  via the user interface  110 . By detecting the operation start instruction signal by the controller  107 , completion of setup of the subject  101  is detected. 
     In step S 202 , the controller  107  generates a motor drive signal based on a predetermined target position at each time of the probe  105  to start a scan of the probe  105 . The details of the scan of the probe  105  will now be described with reference to  FIGS. 3 and 4 .  FIG. 3  is a diagram when the subject  101  and the compression plate  102  are seen from the direction of the probe  105 . In  FIG. 3, 302  indicates a scan region as a region to be scanned in the subject  101  by the probe  105 . In  FIG. 3 , the left upper corner in the diagram of the scan region  302 , a right-pointing axis in the horizontal direction is defined as X axis, and a down-pointing axis in the perpendicular direction is defined as Y axis. In the embodiment, the controller  107  makes the probe  105  scan the scan region  302  in a pattern indicated by an arrow  301 . For simplicity, the case where the probe  105  performs a scan once in the X axis direction will be assumed and described. 
       FIG. 4  is a diagram showing a target position f(t) at each predetermined time. In the diagram, time at which the operator gives the measurement start instruction is set as “0”, and the horizontal axis indicates time “t”. The vertical axis indicates the horizontal position X of the probe  105 . The target position f(t) of the probe  105  in each time is expressed by a solid line  405 . In f(t), at time of a point  401 , the probe  105  starts moving at time indicated by the point  401  and accelerates at constant acceleration until time indicated by a point  402 . After that, the probe  105  moves at constant speed until time indicated by a point  403  and decelerates at constant acceleration until time indicated by a point  404 . At the time indicated by the point  404 , the probe  105  reaches the right end of the scan region. 
     Description of the measurement flow of  FIG. 2  will be continued. The target position f(t) of the probe  105  at each time is calculated in advance at predetermined time intervals “dt” and stored in an internal memory of the controller  107 . The number N(t) of pulses, which are input from the controller  107  to the motor  106  from time “t” to time “t+dt” in step S 202 , is calculated by the following expression (1). 
     
       
         
           
             
               
                 
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     where the amount of movement of the probe  105  performed in response to input of the pulse signal once to the motor  106  is expressed as “p”. 
     In actual elastic wave measurement, the scan is repeated along the pattern  301 , and the entire scan region  302  is scanned with the probe  105 . When the scan of the probe  105  starts, in step S 203 , the position data of the probe  105  is acquired at predetermined time intervals by using the encoder  108 . The position data and data indicative of the time is input to the load estimating unit  109 . 
     During a scan of the probe  105 , the process in step S 203  is continued. After completion of the scan of the probe  105  in step S 204 , in subsequent step S 205 , a load (necessary for movement) at the time of moving the probe  105  over the compression plate  102  is estimated by the load estimating unit  109 . The load estimating unit will be described in detail by using  FIG. 5 . In  FIG. 5 , the solid line  405  indicates the target position f(t) of the probe  105 . A broken line  501  indicates the actual position of the probe  105  measured in step S 203 . In a place where the value of X of the broken line  501  is larger than the value of X at the same time indicated by the solid line  405 , it is determined that the load at the time of moving the probe  105  over the compression plate  102  is smaller than an assumed value. On the other hand, in a place where the value of X of the broken line  501  is smaller than the value of X at the same time indicated by the solid line  405 , it is determined that the load at the time of making the probe  105  scan the surface of the compression plate  102  is larger than an assumed value. 
     It is understood that, for example, around the point  502  shown in  FIG. 5 , the tilt of the actual position  501  of the probe  105  decreases and is deviated from the target position  405 . It shows that the load at the time of moving the probe  105  over the compression plate  102  increases around the point  502  and the probe  105  cannot be accurately moved to a target position at each time by the position control of the probe  15  executed in step S 202 . To address the situation, in the embodiment, a load correction function C(t) is calculated and, by using the load correction function C(t), the position of the probe  105  is corrected. The actual measurement position of the probe  105  at time “t” shown by the broken line  501  is expressed as g(t), “k” is set as a predetermined positive constant, and the load correction function C(t) is defined as the following expression (2). 
     [Math. 2]
 
 C ( t )= k{f ( t )− g ( t )}  (2)
 
     For example, around the point  502 , the load correction function C(t) is a positive value. 
     After completion of the process in step S 205 , subsequently in step S 206 , the load estimating unit  109  changes the driving method (control parameter) on the motor  106  based on the load correction function C(t) and sets it in the controller  107 . In the embodiment, it is assumed that a value obtained by adding the load correction function C(t) to the input number N(t) of pulses every time interval dt which is preliminarily set is set as a corrected pulse input number N′(t). The controller  107  re-calculates the corrected pulse input number N′(t) at time interval dt to update the data in the memory in the controller  107 . 
     After completion of the process in step S 206 , subsequently in step S 207 , control of making a pulse laser beam emitted from the light source  104  repeatedly in predetermined cycles starts. In step S 208  subsequent to the process in step S 207 , the controller  107  starts the driving of the motor  106  (and the probe  105 ) by using the corrected pulse input number N′(t). Simultaneously, the light source  104  is driven so as to be opposed to the probe  105 , and a photoacoustic wave generated in the subject  101  by the pulse laser beam is received by the probe  105 . 
     For example, around the point  502 , the controller  107  makes pulse signals generated more than the first pulse input number N(t) based on the corrected pulse input number N′(t) so that the probe  105  can follow the target position f(t). Consequently, also in the case where the load at the time of movement of the probe  105  increases around the point  502 , the actual position of the probe  105  at each time can be made closer to the first target position f(t). When the scan of the probe  105  starts in step S 208 , in step S 209 , control of receiving a photoacoustic wave generated from the inside of the subject  101  by the probe  105  and converting it to an electric signal is executed. The electric signal is subjected to signal processing and imaging by another routine (not shown), and an image may be displayed in the user interface  110 . Until the probe  105  reaches the right end Xmax of the scan region, the movement of the probe and the reception of the photoacoustic wave are performed repeatedly. When the probe  105  reaches the right end Xmax of the scan region, the scan of the probe is finished in step S 210  and reception and recording of the elastic wave is also finished. 
     In the measurement flow, the processes in step S 202  to S 204  correspond to an information acquiring step. The processes in steps S 205  and S 206  correspond to an instruction signal correcting step. Further, the processes in steps S 208  to S 210  correspond to a driving step. The pulse input number N(t) corresponds to an instruction signal. For example, in the case where coefficient of friction in a part of the compression plate  102  is low and the load is small, the controller  107  generates pulse signals of the number smaller than the initial pulse input number N(t) based on the corrected pulse input number N′(t). In such a manner, the probe  105  can follow the target position f(t). Therefore, also in the case where the load at the movement of the probe  105  is small, the actual position of the probe  105  at each time can be made closer to the initial target position f(t). 
     In the above example, the case where a scan is performed once in the X direction in  FIG. 3  by the probe  105  has been described. However, in reality, the value of the target position f(t) also on each Y coordinate is stored and the corrected pulse input number N′(t) is calculated at each Y coordinate by a similar method. In such a manner, variations in each place of the load at the time of moving the probe  105  over the compression plate  102  can be corrected in the entire scan region  302 . Although the expression in which the difference between the measurement position and the target position is multiplied by a positive constant is used as the load correction function C(t) in the above example, the load correction function C(t) is not always limited to the expression. Another function which changes according to the difference between the measurement position and the target position may be used. In the embodiment, the difference between the measurement position and the target position corresponds to a physical value corresponding to the load generated in the driving unit. 
     In the embodiment, as a corrected pulse input number, an expression of adding a pulse input number before correction and the load correction function C(t) is used. However, the invention is not always limited to the expression. Another function which changes according to the value of the load correction function C(t) may be used. In the embodiment, the method in which the acceleration start time is the same regardless of the load correction function C(t) has been described. However, the acceleration start time may be also changed. For example, in the case where the load is estimated large, a driving method is changed so as to increase the pulse input number and hasten the timing of the pulse input (increase the drive speed and hasten the drive timing (or increase movement speed and hasten moving timing)). In such a manner, the probe can be moved to a target position before generation of a photoacoustic wave. On the other hand, for example, in the case where the load is estimated small, a driving method is changed so as to decrease the pulse input number and delay the timing of the pulse input (decrease the drive speed and delay the drive timing (or decrease the movement speed and delay the moving timing)). In such a manner, the probe can be moved to a target position. 
     Although the embodiment has been described by using the example that the motor  106  is a pulse motor, the invention is not limited to the kind of the motor. A motor of another kind such as a servo motor may be used as long as the position and speed can be controlled from the controller  107 . 
     Example 2 
       FIG. 6  is a block configuration diagram showing a second embodiment of an elastic wave receiving apparatus according to the present invention. In the elastic wave receiving apparatus of the embodiment, to correct variations in the load in association with movement of the probe  105  due to temperature fluctuations, a temperature sensor  601  and a load estimating unit  602  are provided. In  FIG. 6 , the subject  101 , the compression plate  102 , the compression plate  103 , the light source  104 , the probe  105 , the motor  106 , the controller  107 , and the user interface  110  are similar to those in the first embodiment, so that their description will not be repeated.  FIG. 7  shows a measurement flow using the elastic wave receiving apparatus since the subject  101  in the second embodiment of the invention is sandwiched, compressed, and supported by the compression plates  102  and  103  until an elastic wave is received by the probe  105 . 
     In step S 701 , the operator performs a series of preparations such as compression of the subject  101 , fixing, and application of the matching agent and, after that, transmits an operation start instruction signal to the controller  107 . When the operation start instruction signal is detected by the controller  107 , completion of setup of the subject  101  is detected. In step S 702 , the temperature of the compression plate  102  is measured by the temperature sensor  601  as a temperature detecting unit. A thermistor element is adhered onto the compression plate  102 , and the temperature of the compression plate  102  is measured from a resistance value of the thermistor element. Subsequently, in step S 703 , the load estimating unit  602  estimates the load related to movement of the probe  105  based on the target position f(t) indicated by the solid line  405  in  FIG. 4  and temperature T measured in step S 702 . 
     When viscosity at the temperature T of the matching agent is set as u(T), a load D(t) by viscous property of the matching agent when the probe  105  moves over the compression plate  102  is proportional to viscosity and speed. The speed is obtained by time-differentiating f(t). That is, the load D(t) is estimated by the following expression (3) using u(T) and f(t). 
     
       
         
           
             
               
                 
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     The load correction function C(t) is calculated by the following expression (4). “k” denotes a constant which is pre-stored internally. 
     [Math. 4]
 
 C ( t )= kD ( t )  (4)
 
     Generally, the lower the temperature T is, the larger the load D(t) by the viscous property of the matching agent becomes. When the load D(t) by the viscous property of the matching agent is large, the load correction function C(t) also becomes a large value. 
     In step S 704 , the load estimating unit  109  changes the target position based on the load correction function C(t) and sets it in the controller  107 . In the embodiment, a value obtained by adding the load correction function C(t) to the predetermined pulse input number N(t) is set as a corrected pulse input number N′(t). The controller  107  re-calculates the corrected pulse input number N′(t) at time intervals dt and updates the internal memory. Subsequently, in step S 705 , control of repeatedly emitting a pulse laser from the light source  104  in predetermined cycles is started. In the following step S 706 , the controller  107  starts driving the motor  106  and the probe  105  by using the corrected pulse input number N′(t). Concurrently, the light source  104  is also driven so as to face the probe  105  so that a photoacoustic wave generated on the inside of the subject  101  by the pulse laser beam can be received by the probe  105 . 
     For example, in the case where it is expected that the temperature of the compression plate  102  is low and the load related to movement of the probe  105  is large, the controller  107  generates pulse signals more than the initial pulse input number N(t) based on the corrected pulse input number N′(t) so that the probe  105  can follow the target position f(t). Consequently, also in the case where the load related to movement of the probe  105  increases due to low temperature, the position and time of the probe  105  can be made closer to the target position f(t). For example, in the case where it is expected that the temperature of the compression plate  102  is high and the load related to movement of the probe  105  is small, the controller  107  generates pulse signals smaller than the first pulse input number N(t) based on the corrected pulse input number N′(t) so that the probe  105  can follow the target position f(t). Consequently, also in the case where the load related to movement of the probe  105  decreases due to high temperature, the position and time of the probe  105  can be made closer to the target position f(t). 
     Subsequently, in step S 707 , control of receiving a photoacoustic wave generated from the inside of the subject  101  by the probe  105  and converting it to an electric signal is executed. The electric signal may be subjected to signal processing and imaging on the outside and an image may be displayed in the user interface  110 . In step S 708 , the scan with the probe  105  and reception of the elastic wave is finished. In the measurement flow, the process in step S 702  corresponds to an information acquiring step. The processes in steps S 703  and S 704  correspond to an instruction signal correcting step. Further, the processes in steps S 706  to S 708  correspond to a driving step. The pulse input number N(t) corresponds to an instruction signal. 
     As described above, according to the second embodiment of the invention, fluctuations in the load related to movement of the probe  105  accompanying temperature fluctuations can be estimated by a simple method without making the probe  105  scan before measurement. Consequently, without increasing the measurement time, the load fluctuations accompanying temperature fluctuations are corrected, and the position precision of the probe  105  can be improved. Although the expression in which a product of viscosity and speed is multiplied by a positive constant is used as the load correction function C(t) in the above example, the load correction function C(t) is not always limited to the expression. Another function which changes according to temperature may be used. Although the expression in which the pulse input number N(t) before correction and the load correction function C(t) are added is used as the corrected pulse input number N′(t) in the embodiment, the invention is not always limited to the expression. Another function which changes according to the value of the load correction function C(t) may be used. In the embodiment, the temperature of the compression plate  102  corresponds to the physical value corresponding to the load generated in the driving unit. 
     In the embodiment, the method in which the acceleration start time of the probe  105  is the same regardless of the load correction function C(t) has been described. However, the acceleration start time may be also changed together with a change in the number of input pulses. For example, in the case where the load related to movement of the probe  105  is estimated large, a driving method is changed so as to increase the pulse input number and hasten the timing of the pulse input (increase the drive speed and hasten the drive timing (or increase the movement speed and hasten the moving timing)). In such a manner, the probe  105  can be moved to a target position well in advance of generation of a photoacoustic wave. On the other hand, for example, in the case where the load related to movement of the probe  105  is estimated small, a driving method is changed so as to increase the pulse input number and delay the timing of the pulse input (decrease the drive speed and delay the drive timing (or decrease the movement speed and delay the moving timing)). In such a manner, the probe  105  can be moved to a target position. 
     Example 3 
       FIG. 8  is a block configuration diagram showing a third embodiment of an elastic wave receiving apparatus according to the present invention. In the elastic wave receiving apparatus of the embodiment, to correct variations in a friction load caused by a strain in the compression plate  102  which occurs when the subject  101  is fixed to the compression plate  102 , a strain sensor  801  as a strain detecting unit and a load estimating unit  802  as an information acquiring unit are provided. In  FIG. 8 , the subject  101 , the compression plate  102 , the compression plate  103 , the light source  104 , the probe  105 , the motor  106 , the controller  107 , and the user interface  110  are similar to those in the first embodiment, so that their description will not be repeated. 
       FIG. 9  shows a measurement flow using the elastic wave receiving apparatus since the subject  101  in the third embodiment of the invention is fixed by the compression plates  102  and  103  until an elastic wave is received by the probe  105 . In step S 901 , in a manner similar to the first and second embodiments, completion of setup of the subject  101  is detected. Next, in step S 902 , a strain in the compression plate  102  is measured by the strain sensor  801 . The details of layout of the strain sensors  801  are shown in  FIG. 10 . Strain gauges  1001  and  1002  are adhered at both ends of a face on the probe  105  side, of the compression plate  102 . Strain gauges  1003  and  1004  are adhered to both ends of a face on the subject  101  side. In step S 902 , the resistance value of each of the strain gauges is detected. In  FIG. 10 , this side of the drawing sheet is the face on the probe  105  side, and the depth side is the face on the subject  101  side. 
     Subsequently, in step S 903 , the load estimating unit  802  estimates friction load at each position based on the resistance value of each of the strain gauges measured in step S 902 . The basic concept is as follows. Specifically, when the resistance values of the strain gauges  1001  and  1002  increase and the resistance values of the strain gauges  1003  and  1004  decrease, it is considered that the compression plate  102  is distorted so as to project to the probe side as shown in  FIG. 11A . In this case, the frictional force between the probe  105  and the compression plate  102  is large around the center part of the compression plate  102  and is small around ends. On the other hand, when the resistance values of the strain gauges  1001  and  1002  decrease and the resistance values of the strain gauges  1003  and  1004  increase, it is considered that the compression plate  102  is distorted so that the probe  105  side is recessed as shown in  FIG. 11B . In this case, the frictional force between the probe  105  and the compression plate  102  is small in a center part and becomes large around ends. 
     Next, a concrete method for estimating the friction load will be described. In the embodiment, as the load correction function C(t), the following quadratic function (5) is employed. 
     
       
         
           
             
               
                 
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     When A≥0, B=0 is defined. “h” denotes length (height) from the lower end to the upper end of the compression plate  102  in  FIGS. 11A and 11B . 
     A is defined by expression (7) and is a coefficient which becomes negative when the compression plate  102  is distorted so as to project to the probe side and becomes positive when the compression plate  102  is distorted so as to recess to the probe side. k denotes a positive constant which is determined at the time of shipping of the apparatus. The value of A is determined according to resistance values R 1001 , R 1002 , R 1003 , and R 1004  of the strain gauges  1001 ,  1002 ,  1003 , and  1004 , respectively, detected in step S 902  and is stored.
 
 A=k {( R 1003· R 1004)/( R 1002· R 1001)−1}  (7)
 
     For example, in the case where the compression plate  102  is distorted so as to project to the probe side as shown in  FIG. 11A , (R 1002 ·R 1001 )&gt;(R 1003 ·R 1004 ) is satisfied and, as a result, “A” becomes a negative value. In this case, C(t)=0 at ends (f(t)=0 and f(t)=h) of the compression plate  102 , and the friction load is estimated small. C(t)=−A(h 2 /4) in the center (f(t)=h/2) of the compression plate  102 , and the friction load is estimated large. For example, in the case where the compression plate  102  is distorted so as to recess as shown in  FIG. 11B , (R 1002 ·R 1001 )&lt;(R 1003 ·R 1004 ) is satisfied and A becomes a positive value. In this case, C(t)=A(h 2 /4) at ends (f(t)=0 and f(t)=h) of the compression plate  102 , and the friction load is estimated large. C(t)=0 in the center (f(t)=h/2) of the compression plate  102 , and the friction load is estimated small. In the case where the compression plate  102  is not distorted, (R 1002 ·R 1001 )=(R 1003 ·R 1004 ) and, as a result, A=B=0. In this case, C(t) is always equal to zero (C(t)=0). 
     Next, in step S 904 , the load estimating unit  802  changes the target position based on the load correction function C(t) and sets it in the controller  107 . In the embodiment, a value obtained by adding the load correction function C(t) to the predetermined pulse input number N(t) is set as a corrected pulse input number N′(t). The controller  107  re-calculates the corrected pulse input number N′(t) at predetermined time intervals and updates the internal memory. Subsequently, in step S 905 , control of repeatedly emitting a pulse laser from the light source  104  in predetermined cycles is started. In step S 906 , the controller  107  starts driving the motor  106  and the probe  105  by using the corrected pulse input number N′(t). Concurrently, the light source  104  is also driven so as to face the probe  105  so that a photoacoustic wave generated on the inside of the subject  101  by the pulse laser beam can be received by the probe  105 . 
     The operation of the probe  105  in the case where the compression plate  102  is distorted so as to project to the probe  105  side and the friction load is expected to be large around the center will now be considered. In this case, the controller  107  generates pulse signals more than the initial pulse input number N(t) around the center of the compression plate  102  based on the corrected pulse input number N′(t) so that the probe  105  can follow the target position f(t). On the other hand, in the case where it is expected that the compression plate  102  is distorted so as to recess and the friction load is large around the ends, the controller  107  generates pulse signals more than the initial pulse input number N(t) around the ends of the compression plate  102  based on the corrected pulse input number N′(t) so that the probe  105  can follow the target position f(t). Consequently, also in the case where the friction load changes in some place due to distortion of the compression plate  102 , the position at each time, of the probe  105  can be made closer to the target position f(t). 
     Subsequently, in step S 907 , control of receiving a photoacoustic wave generated from the inside of the subject  101  by using the probe  105  and converting it to an electric signal is executed. The electric signal may be subjected to signal processing and imaging by another routine and an image may be displayed via the user interface  110 . In step S 908 , the scan and measurement of an elastic wave of the probe  105  is finished. According to the third embodiment of the invention, fluctuations in the friction load accompanying distortion of the compression plate  102  can be corrected by a simple method without making the probe  105  scan before measurement. Consequently, without increasing the measurement time, the load fluctuations accompanying distortion of the compression plate  102  are corrected, and the position precision of the probe  105  can be improved. In the measurement flow, the process in step S 902  corresponds to the information acquiring step. The processes in steps S 903  and S 904  correspond to the instruction signal correcting step. Further, the processes in steps S 906  to S 908  correspond to the driving step. The pulse input number N(t) corresponds to an instruction signal. 
     Although the quadratic function expressed by the expression (5) is used as the load correction function C(t) in the above example, the load correction function C(t) is not always limited to the expression. Another function which changes according to a measurement value of a strain gauge may be used. Although the expression in which the pulse input number N(t) before correction and the load correction function C(t) are added is used as the corrected pulse input number N′(t) in the embodiment, the invention is not always limited to the expression. Another function which changes according to the value of the load correction function C(t) may be used. In the embodiment, the method in which the acceleration start time is the same regardless of the load correction function C(t) has been described. However, the acceleration start time may be also changed together with a change in the number of input pulses. For example, in the case where the load is estimated large, a driving method is changed so as to increase the pulse input number and hasten the timing (increase the drive speed and hasten the drive timing (or increase the movement speed and hasten the moving timing)). In such a manner, the probe  105  can be moved to a target position well in advance of generation of a photoacoustic wave. On the other hand, for example, in the case where the load is estimated small, a driving method is changed so as to decrease the pulse input number and delay the timing (decrease the drive speed and delay the drive timing (or decrease the movement speed and delay the moving timing)). In such a manner, the probe  105  can be moved to a target position. In the embodiment, the distortion of the compression plate  102  corresponds to a physical value corresponding to the load generated in the driving unit. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.