Patent Publication Number: US-2018028067-A1

Title: Subject information obtaining apparatus and method for obtaining information regarding subject

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of co-pending U.S. patent application Ser. No. 13/741,711, filed Jan. 15, 2013, which claims the benefit of International Patent Application No. PCT/JP2012/050914, filed Jan. 18, 2012, all of which are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a subject information obtaining apparatus and a method for obtaining information regarding a subject that obtain information regarding a subject by detecting a photoacoustic wave generated by radiating light onto the subject. 
     BACKGROUND ART 
     Studies on an optical imaging apparatus that transmits light through a subject, the light being radiated from a light source such as a laser onto the subject, and that obtains information regarding an inside of the subject are eagerly conducted mainly in the medical field. Photoacoustic imaging (PAI) is one of such optical imaging technologies. The photoacoustic imaging is a technology in which pulse light generated from a light source is radiated onto a subject (living body), a photoacoustic wave generated when the light that has propagated through and diffused in the subject is absorbed in the subject is detected, and the detected acoustic wave is subjected to an analysis process in order to visualize information relating to the optical characteristics of the inside of the subject. By this technology, the distribution of optical characteristic values inside the subject, especially the absorption coefficient distribution, the oxygen saturation distribution, and the like, can be obtained. 
     In the photoacoustic imaging, initial sound pressure P 0  of a photoacoustic wave generated from a region of interest inside the subject can be represented by the following expression. 
         P   0 =Γ·μ a ·Φ  Expression (1)
 
     Here, Γ is a Grueneisen coefficient, which is obtained by dividing a product of a volume expansion coefficient β and the square of a sonic speed c by specific heat at constant pressure C P . It is known that once a subject has been determined, Γ indicates a substantially constant value. In addition, μ a  is an absorption coefficient of the region of interest, and Φ is a value of the integrated light intensity in the region of interest. 
     In PTL 1, a technology is described in which changes over time in sound pressure P of a photoacoustic wave that has propagated through a subject are detected by an acoustic wave detector and the distribution of initial sound pressure inside the subject is calculated on the basis of results of the detection. According to PTL 1, by dividing the calculated initial sound pressure by the Grueneisen coefficient Γ, a product of μ a  and Φ, that is, light energy absorption density, can be obtained. In addition, as indicated by the expression (1), the light energy absorption density needs to be divided by the light intensity Φ in order to obtain the absorption coefficient μ a  from the initial sound pressure P 0 . 
     CITATION LIST 
     Patent Literature 
     PTL 1 Japanese Patent Laid-Open No. 2010-88627 
     PTL 2 Japanese Patent Laid-Open No. 2006-51355 
     However, in the photoacoustic imaging described in PTL 1, it has been desired to obtain an optical characteristic value more accurately. 
     Therefore, an object of the present invention is to provide, in photoacoustic imaging, a subject information obtaining apparatus and a method for obtaining information regarding a subject that can obtain an optical characteristic value more accurately. 
     SUMMARY OF INVENTION 
     In view of the above problem, a subject information obtaining apparatus according to an embodiment of the present invention includes a signal processing unit that includes a setting unit that sets certain sensitive regions corresponding to a plurality of acoustic wave detection elements on the basis of sensitivity distribution of the plurality of acoustic wave detection elements, an initial sound pressure obtaining unit that obtains initial sound pressure in a region of interest without using a detection signal corresponding to the region of interest obtained by an acoustic wave detection element whose certain sensitive region does not include the region of interest, a light intensity obtaining unit that obtains a value of integrated light intensity in the region of interest on the basis of detection signals used by the initial sound pressure obtaining unit, and an optical characteristic value obtaining unit that obtains an optical characteristic value in the region of interest using the initial sound pressure obtained by the initial sound pressure obtaining unit and the value of the integrated light intensity obtained by the light intensity obtaining unit. 
     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 schematic diagram of a subject information obtaining apparatus according to a first embodiment. 
         FIG. 2  is a flowchart diagram of a method for obtaining information regarding a subject according to the first embodiment. 
         FIG. 3A  is a schematic diagram of a subject information obtaining apparatus according to a second embodiment. 
         FIG. 3B  is a schematic diagram of the subject information obtaining apparatus according to the second embodiment. 
         FIG. 3C  is a schematic diagram of the subject information obtaining apparatus according to the second embodiment. 
         FIG. 4A  is a schematic diagram of a subject information obtaining apparatus according to a fourth embodiment. 
         FIG. 4B  is a schematic diagram of the subject information obtaining apparatus according to the fourth embodiment. 
         FIG. 4C  is a schematic diagram of the subject information obtaining apparatus according to the fourth embodiment. 
         FIG. 5A  is a schematic diagram of another subject information obtaining apparatus according to the fourth embodiment. 
         FIG. 5B  is a schematic diagram of the other subject information obtaining apparatus according to the fourth embodiment. 
         FIG. 5C  is a schematic diagram of the other subject information obtaining apparatus according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In photoacoustic imaging, a detection signal obtained by detecting a photoacoustic wave includes background noise. Therefore, in the photoacoustic imaging, it is desirable that initial sound pressure in a region of interest is obtained without using a detection signal that includes background noise and whose S/N is low. For example, in PTL 2, it is described that, although this is a case of ultrasonic imaging, an acoustic wave detection element prevents reception of an acoustic wave from a region of interest when an angle between the region of interest and the acoustic wave detection element is smaller than or equal to a certain value (when the region of interest is not included in a certain sensitive region corresponding to the acoustic wave detection element). By using such a method, an ultrasonic wave image is obtained without using a detection signal whose S/N is low. 
     Therefore, the present inventor has applied the technology described in PTL 2 to the photoacoustic imaging. More specifically, the initial sound pressure in the region of interest is obtained by performing reconfiguration through simulation without using a detection signal obtained by an acoustic wave detection element whose certain sensitive region does not include the region of interest. Since the initial sound pressure obtained in such a manner is initial sound pressure reconfigured without using a detection signal whose S/N is low, an error due to noise is small. Next, the present inventor has obtained an absorption coefficient in the region of interest using this initial sound pressure and the method described in PTL 1. However, the value of the absorption coefficient calculated using the above method is different from a value of the absorption coefficient set in the simulation. 
     Therefore, as a result of the present inventor&#39;s sincere examination in view of the above problem, it has been found out that the cause of the problem is that whereas detection signals to be used to obtain the initial sound pressure are selected on the basis of the sensitivities of acoustic wave detection elements, the sensitivities of the acoustic wave detection elements are not considered when a value of the integrated light intensity is to be obtained. 
     Therefore, the present inventor has found out that when the absorption coefficient is to be obtained, the absorption coefficient as an optical characteristic value can be accurately obtained by obtaining a value of the integrated light intensity on the basis of the sensitivities of the acoustic wave detection elements, in addition to selecting detection signals to be used on the basis of the sensitivities of the acoustic wave detection elements. 
     Embodiments of the present invention using simulation will be described hereinafter. 
     First Embodiment 
       FIG. 1  is a schematic diagram of a subject information obtaining apparatus according to the present embodiment. Pulse light emitted from a light source  10  is guided to an optical system  11  and radiated onto a subject  30  as radiation light  12 . A photoacoustic wave  32  generated from a light absorber  31  inside the subject  30  is detected by an acoustic wave detector  20  including acoustic wave detection elements e 1 , e 2 , and e 3 . A plurality of detection signals obtained by the acoustic wave detector  20  are amplified and subjected to digital conversion by a signal collector  47  and stored in a memory of a signal processing apparatus  40 . Next, an initial sound pressure obtaining module  42  as an initial sound pressure obtaining unit included in the signal processing apparatus  40  as a signal processing unit reconfigures an image using the plurality of detection signals to obtain initial sound pressure in a region of interest  33  inside the subject  30 . In addition, a light intensity obtaining module  43  as a light intensity obtaining unit included in the signal processing apparatus  40  obtains a value of the integrated light intensity in the region of interest  33 . Next, an optical characteristic value obtaining module  44  as an optical characteristic value obtaining unit included in the signal processing apparatus  40  obtains an optical characteristic value in the region of interest  33  using the initial sound pressure and the value of the light intensity in the region of interest  33 . The obtained optical characteristic value is then displayed on a display apparatus  50  as a display unit. 
     Here, the region of interest refers to a voxel, which is a minimum unit of a region reconfigured by the initial sound pressure obtaining module  42 . It is to be noted that the initial sound pressure obtaining module  42  can obtain the distribution of initial sound pressure of the entirety of the subject by setting the region of interest all over the subject  30 . In addition, similarly, the light intensity obtaining module  43  and the optical characteristic value obtaining module  44  can obtain the distribution of values of the integrated light intensity and the distribution of absorption coefficients of the entirety of the subject by setting the region of interest all over the subject. 
     Here, the detection signals corresponding to the region of interest  33  obtained by the acoustic wave detection elements e 1 , e 2 , and e 3  illustrated in  FIG. 1  are denoted by P d1 (r T ), P d2 (r T ), and P d3 (r T ), respectively. In addition, in relation to a photoacoustic wave that is incident from a front of a photoacoustic wave detection element, the efficiency of conversion from a photoacoustic wave that is incident from an angle θ relative to the front of the acoustic wave detection element into a detection signal is denoted by A(θ). In addition, if the angles of the acoustic wave detection elements relative to the region of interest  33  are denoted by θ 1 , θ 2 , and θ 3 , respectively, conversion efficiencies according to the directivity of the acoustic wave detection elements can be expressed as A(θ 1 ), A(θ 2 ), and A(θ 3 ), respectively. In addition, values of the light intensity in the region of interest  33  corresponding to the detection signals P d1 (r T ), P d2 (r T ), and P d3 (r T ) are denoted by Φ 1 (r T ), Φ 2 (r T ), and Φ 3 (r T ), respectively. Here, in the present embodiment, the region of interest  33  is set at a position r T  of the light absorber  31 . 
     Here, the distance from an acoustic wave detection element to the region of interest  33  is denoted by r, the transmission speed of the photoacoustic wave in the subject is denoted by c, and the time at which the radiation light  12  is radiated onto the subject  30  is denoted by t=0. In this case, the detection signals corresponding to the region of interest refer to detection signals obtained by the acoustic wave detection elements at a time t=r/c. In addition, the values of the light intensity in the region of interest  33  corresponding to the detection signals corresponding to the region of interest refer to values of the intensity of the radiation light  12  in the region of interest  33  radiated at the time t=0. 
     (Example of Simulation not Using Detection Signal) 
     An example of simulation in which the absorption coefficient is obtained from initial sound pressure obtained without using a detection signal on the basis of the sensitivities of the acoustic wave detection elements will be described hereinafter with reference to  FIG. 1 . In this simulation, the absorption coefficient of the light absorber  31  was set to μ a =0.088/mm. 
     First, the initial sound pressure obtaining module  42  obtains initial sound pressure P 0 (r T ) in the region of interest  33  using the detection signals P d1 (r T ), P d2 (r T ), and P d3 (r T ) and the conversion efficiencies A(θ 1 ), A(θ 2 ), and A(θ 3 ) as represented by an expression (2). 
     
       
         
           
             
               
                 
                   
                     
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     Here, the detection signals obtained by the simulation and the conversion efficiencies set in the simulation were as follows: 
         P   d1 ( r   T )=132 Pa 
         P   d2 ( r   T )=231 Pa 
         P   d3 ( r   T )=198 Pa 
         A (θ1)=0.4
 
         A (θ2)=0.7
 
         A (θ3)=0.6
 
     Accordingly, the initial sound pressure calculated from the expression (2) using these parameters is P 0 (r T )=990. 
     In addition, in  FIG. 1 , regions (certain sensitive regions) in which the conversion efficiencies of the acoustic wave detection elements are larger than a certain value are represented by triangular regions indicated by broken lines. Here, a conversion efficiency A(θ)=0.5 is set as the certain value. 
     Here, in the present embodiment, the region of interest  33  is not included in the triangular region (the certain sensitive region) corresponding to the acoustic wave detection element e 1 . Therefore, the initial sound pressure obtaining module  42  obtains initial sound pressure P 0 ′(r T ) in the region of interest  33  represented by an expression (3) without using the detection signal P d1 (r T ) corresponding to the region of interest  33  obtained by the acoustic wave detection element e 1 . 
     
       
         
           
             
               
                 
                   
                     
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     Accordingly, the initial sound pressure in the region of interest  33  calculated from the expression (3) using the above parameters was P 0 ′(r T )=660. 
     Next, the light intensity obtaining module  43  obtains a value of the integrated light intensity in the subject from a background optical coefficient of the subject or the like using a light propagation Monte Carlo method, a transport equation, a light diffusion equation, or the like. 
     For example, the light intensity obtaining module  43  obtains the values of the light intensity Φ 1 (r T ), Φ 2 (r T ), and Φ 3 (r T ) in the region of interest  33  corresponding to the detection signals P d1 (r T ), P d2 (r T ), and P d3 (r T ), respectively. 
     The light intensity obtaining module  43  then obtains the value of the integrated light intensity Φ(r T ) in the region of interest  33  represented by an expression (4) using these values. 
       Φ( r   T )=Φ 1 ( r   T )+Φ 2 ( r   T )+Φ 3 ( r   T )  Expression (4)
 
     Here, the values of the light intensity in the region of interest obtained by the simulation were as follows: 
       Φ 1 ( r   T )=3,750 mJ/m 2  
 
       Φ 2 ( r   T )=3,750 mJ/m 2  
 
       Φ 3 ( r   T )=3,750 mJ/m 2  
 
     Accordingly, the value of the integrated light intensity in the region of interest from the expression (4) using these parameters is Φ(r T )=11,250 mJ/m 2 . 
     Next, the optical characteristic value obtaining module  44  obtains the absorption coefficient μ a (r T ) in the region of interest  33  represented by an expression (5) using the initial sound pressure P 0 ′(r T ) in the region of interest  33  represented by the expression (3) and the value of the integrated light intensity Φ(r T ) in the region of interest  33  represented by the expression (4). 
     Here, a Grueneisen coefficient Γ=1. 
     
       
         
           
             
               
                 
                   
                     
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     Here, the absorption coefficient in the region of interest  33  set at the position r T  of the light absorber, the absorption coefficient being calculated by the expression (5) using the above parameters, is μ a =0.059/mm. On the other hand, the absorption coefficient of the light absorber  31  set in the simulation is μ a =0.088/mm. Therefore, it can be seen that the absorption coefficient obtained by the expression (5) is smaller than the set value. That is, when the absorption coefficient is to be obtained using the initial sound pressured obtained using the above method, a further improvement is needed to obtain the value of the integrated light intensity. 
     (Example of Simulation not Using Detection Signal and Value of Light Intensity) 
     Therefore, a method for obtaining information regarding a subject according to the present embodiment found by the present inventor will be described hereinafter with reference to a flowchart of  FIG. 2 . The following numbers agree with processing numbers illustrated in  FIG. 2 . 
     (S 100 : Step of Setting Certain Sensitive Regions on Basis of Sensitivity Distribution of Acoustic Wave Detection Elements) 
     In this step, certain sensitive regions corresponding to the plurality of acoustic wave detection elements are set on the basis of the sensitivity distribution of the plurality of acoustic wave detection elements. Tables of the certain sensitive regions corresponding to the acoustic wave detection elements are stored in the memory of the signal processing apparatus  40 . 
     Here, a setting module  41  as a setting unit included in the signal processing apparatus  40  may set regions in which the sensitivities of the acoustic wave detection elements are higher than a certain value as the certain sensitive regions. It is to be noted that the certain value may be automatically set by the setting module  41  on the basis of system noise. In addition, the certain value may be determined by an operator by displaying the sensitivities of the acoustic wave detection elements on the display apparatus  50  as a histogram and by selecting the certain value on the basis of the histogram. At this time, the certain value is preferably selected in consideration of the system noise. 
     Here, the sensitivities of the acoustic wave detection elements are determined from, for example, the conversion efficiencies of the acoustic wave detection elements, attenuation rates, which indicate attenuation caused by diffusion and scattering of the photoacoustic wave from the region of interest to the acoustic wave detection elements, and the like. It is to be noted that the conversion efficiencies are determined from angles at which the photoacoustic wave is incident on the acoustic wave detection elements and the like. In addition, the attenuation rates are determined from the distances between the region of interest and the acoustic wave detection elements and the like. 
     For example, in the case of the example of simulation described above, the setting module  41  set the conversion efficiency A(θ)=0.5 as the certain value. In addition, with respect to each of the acoustic wave detection elements e 1 , e 2 , and e 3 , a region in which the conversion efficiency A(θ) is higher than 0.5 was indicated by a triangular region. As a result, the region of interest  33  was not included in the region (the certain sensitive region) in which the conversion efficiency A(θ) of the acoustic wave detection element e 1  is higher than 0.5. 
     In addition, the certain sensitive regions can be set on the basis of arbitrary regions selected in an image of the sensitivity distribution of the acoustic wave detection elements. 
     For example, first, the display apparatus  50  is caused to display image data regarding the sensitivity distribution of the acoustic wave detection elements stored in the memory of the signal processing apparatus  40 . The operator then selects arbitrary regions using an input device of a PC in the displayed image of the sensitivity distribution. Therefore, the setting module  41  can set the selected arbitrary regions as the certain sensitive regions. At this time, for example, each arbitrary region can be selected by connecting a start point and an end point using recognition by a mouse or a recognition method by a sensor on a touch panel while the image of the sensitive region is being displayed. 
     It is to be noted that the setting module  41  may set the certain sensitive regions on the basis of the sensitivity distribution of the selected arbitrary regions, instead. For example, the certain sensitive regions may be set using the lowest sensitivity as a reference. 
     In addition, a certain sensitive region may be individually set for each of the acoustic wave detection elements or a certain sensitive region may be set for a single acoustic wave detection element and the same sensitive region as the certain sensitive region may be set for each of the other acoustic wave detection elements. 
     (S 200 : Step of Obtaining Initial Sound Pressure in Region of Interest without Using Detection Signal Obtained by Acoustic Wave Detection Element Whose Certain Sensitive Region does not Include Region of Interest) 
     In this step, the initial sound pressure in the region of interest is obtained without using a detection signal corresponding to the region of interest obtained by an acoustic wave detection element whose certain sensitive region set in S 100  does not include the region of interest. Thereafter, data regarding the initial sound pressure is stored in the memory of the signal processing apparatus  40 . 
     For example, in the case of the example of simulation described above, the region of interest  33  was not included in the certain sensitive region corresponding to the acoustic wave detection element e 1 . Therefore, the initial sound pressure obtaining module  42  obtains the initial sound pressure P 0 ′(r T ) represented by the expression (4) by reconfiguring an image without using, among the detection signals P d1 (r T ), P d2 (r T ), and P d3 (r T ), the detection signal P d1 (r T ) corresponding to the region of interest obtained by the acoustic wave detection element e 1 . 
     At this time, as an image reconfiguration algorithm executed by the initial sound pressure obtaining module  42 , for example, reverse projection in a time domain or a Fourier domain normally used in a tomography technology or the like may be used. 
     It is to be noted that, in an embodiment of the present invention, if a certain sensitive region is included in at least a part of the region of interest, it can be said that the region of interest is included in the certain sensitive region. 
     In addition, in an embodiment of the present invention, not using a detection signal is a concept including not using a detection signal at all and not using a detection signal essentially when the initial sound pressure is obtained. 
     (S 300 : Step of Obtaining Value of Integrated Light Intensity in Region of Interest without Using Value of Light Intensity Corresponding to Detection Signal not Used to Obtain Initial Sound Pressure) 
     In this step, the value of the integrated light intensity in the region of interest is obtained without using the value of the light intensity in the region of interest corresponding to the detection signal that has not been used in S 200 . Thereafter, data regarding the value of the integrated light intensity is stored in the memory of the signal processing apparatus  40 . 
     For example, the light intensity obtaining module  43  obtains a value of the integrated light intensity Φ′(r T ) in the region of interest represented by an expression (6) without using, among the values of the light intensity Φ 1 (r T ), Φ 2 (r T ), and Φ 3 (r T ), the value of the light intensity Φ 1 (r T ) in the region of interest corresponding to the detection signal P d1 (r T ) that has not been used by the initial sound pressure obtaining module  42 . 
       Φ′( r   T )=Φ 2 ( r   T )+Φ 3 ( r   T )  Expression (6)
 
     That is, the light intensity obtaining module  43  obtains the value of the integrated light intensity in the region of interest using the values of the light intensity in the region of interest corresponding to detection signals used when the initial sound pressure obtaining module  42  has calculated the initial sound pressure. 
     Here, the value of the integrated light intensity in the region of interest  33  obtained by the expression (6) using the above parameters is Φ′(r T )=7,500 mJ/m 2 . 
     It is to be noted that, in an embodiment of the present invention, not using a value of the light intensity is a concept including not using a value of the light intensity at all and not using a value of the light intensity essentially when the value of the integrated light intensity is obtained. 
     In addition, in the present embodiment, since radiation conditions of the radiation light  12  are constant, the light intensity radiated onto the region of interest  33  remains the same. Therefore, the values of the light intensity corresponding to the plurality of detection signals obtained by the plurality of acoustic wave detection elements also remain the same. In such a case, the light intensity obtaining module  43  may obtain a value obtained by multiplying the number of detection signals used when the initial sound pressure obtaining module  42  has obtained the initial sound pressure by the light intensity that has reached the region of interest  33  as the value of the integrated light intensity in the region of interest  33 . In an embodiment of the present invention, the value of the integrated light intensity obtained in such a manner is treated as the value of the integrated light intensity obtained without using a value of the light intensity. 
     (S 400 : Step of Obtaining Optical Characteristic Value in Region of Interest Using Initial Sound Pressure and Value of Integrated Light Intensity in Region of Interest) 
     In this step, the absorption coefficient as an optical characteristic value in the region of interest is obtained using the initial sound pressure in the region of interest obtained in S 200  and the value of the integrated light intensity in the region of interest obtained in S 300 . 
     For example, in the case of the example of simulation described above, the optical characteristic value obtaining module  44  applies the initial sound pressure P 0 ′(r T ) represented by the expression (3) and the value of the integrated light intensity Φ′(r T ) represented by the expression (6) to the expression (1). In doing so, an absorption coefficient μ a (r T ) in the region of interest  33  represented by the expression (7) is obtained. Here, the Grueneisen coefficient Γ=1. 
     
       
         
           
             
               
                 
                   
                     
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     For example, the absorption coefficient in the region of interest  33  obtained by the expression (7) using the above parameters is μ a =0.088/mm. On the other hand, the absorption coefficient in the region of interest  33  obtained by the expression (5) is μ a =0.059/mm. In addition, the absorption coefficient of the light absorber  31  set in the simulation is μ a =0.088/mm. That is, according to the expression (7), the absorption coefficient can be obtained accurately compared to the expression (5). 
     As described above, an absorption coefficient in which an error due to noise is small and whose quantativity is high can be obtained by, with respect to an acoustic wave detection element whose certain sensitive region does not include the region of interest, not using a detection signal corresponding to the region of interest obtained by the acoustic wave detection element and a value of the light intensity in the region of interest corresponding to the detection signal. 
     It is to be noted that by using the above steps for a plurality of wavelengths, an absorption coefficient for each wavelength may be obtained, instead. In addition, using these absorption coefficients, oxygen saturation as an optical characteristic value may be obtained. 
     Alternatively, a program including the above steps may be executed by the signal processing apparatus  40  as a computer. The program may be instructions encoded on a non-transitory computer readable medium. 
     Second Embodiment 
       FIGS. 3A to 3C  are schematic diagrams of a subject information obtaining apparatus according to the present embodiment. 
     The subject information obtaining apparatus according to the present embodiment includes an acoustic wave detector  20  including a single acoustic wave detection element. In addition, a detector moving mechanism  21  for moving the subject  30  and the acoustic wave detector  20  relatively to each other is included. In the present embodiment, the detector moving mechanism  21  can detect the photoacoustic wave at a plurality of positions by moving the acoustic wave detector  20  including the single acoustic wave detection element in a right direction of the paper. Here, the acoustic wave detection element at positions illustrated in  FIGS. 3A, 3B, and 3C  is denoted by e 1 , e 2 , and e 3 , respectively. In addition, triangular regions indicated by lines represent a certain sensitive region corresponding to the acoustic wave detection element. 
     In an embodiment of the present invention, a plurality of acoustic wave detection elements refer to an acoustic wave detection element that can detect the photoacoustic wave at a plurality of positions. That is, as in the present embodiment, an acoustic wave detection element that can detect the photoacoustic wave at a plurality of positions by moving the acoustic wave detector  20  is also referred to as a plurality of acoustic wave detection elements. 
     In addition, the subject information obtaining apparatus according to the present embodiment is provided with an optical moving mechanism  13  that moves the optical system  11  in order to move the radiation light  12 . In addition, in the present embodiment, the acoustic wave detector  20  and the radiation light  12  are moved in synchronization with each other. Thus, by moving the acoustic wave detector  20  and the radiation light  12  in synchronization with each other, the radiation light  12  is constantly radiated onto the certain sensitive region (triangular region) corresponding to the acoustic wave detection element, thereby making it possible to constantly obtain a detection signal whose S/N is high. 
     In the subject information obtaining apparatus according to the present embodiment, too, as in the first embodiment, the region of interest  33  is not included in the certain sensitive region corresponding to the acoustic wave detection element e 1 . Therefore, the initial sound pressure obtaining module  42  obtains the initial sound pressure in the region of interest  33  without using a detection signal corresponding to the region of interest  33  obtained by the acoustic wave detection element e 1 . Thereafter, the light intensity obtaining module  43  obtains the value of the integrated light intensity in the region of interest  33  without using a value of the light intensity in the region of interest  33  corresponding to the detection signal that has not been used to obtain the initial sound pressure. Thereafter, the optical characteristic value obtaining module  44  obtains the absorption coefficient in the region of interest  33  represented by the expression (7) using the initial sound pressure and the value of the integrated light intensity. By obtaining the absorption coefficient in such a manner, in the present embodiment, too, the absorption coefficient can be accurately obtained. 
     Third Embodiment 
     In the first embodiment and the second embodiment, the absorption coefficient is obtained without using, with respect to an acoustic wave detection element whose certain sensitive region does not include the region of interest, a detection signal corresponding to the region of interest obtained by the acoustic wave detection element and a value of the light intensity in the region of interest corresponding to the detection signal. On the other hand, in the present embodiment, the absorption coefficient is obtained while reducing the detection signal and the value of the light intensity in the region of interest corresponding to the detection signal. 
     A method for obtaining information regarding a subject according to the present embodiment will be described hereinafter using the subject information obtaining apparatus illustrated in  FIG. 1 . 
     In the present embodiment, the initial sound pressure obtaining module  42  multiplies the detection signal corresponding to the region of interest  33  obtained by the acoustic wave detection element e 1  whose certain sensitive region does not include the region of interest  33  by a first reduction coefficient. The initial sound pressure obtaining module  42  then obtains the initial sound pressure in the region of interest  33  using the detection signal multiplied by the first reduction coefficient, too. 
     Thus, by multiplying, with respect to an acoustic wave detection element whose certain sensitive region does not include the region of interest, a detection signal corresponding to the region of interest obtained by the acoustic wave detection element by the first reduction coefficient, the initial sound pressure can be obtained while reducing the detection signal, whose S/N is low. Therefore, initial sound pressure in which an error due to noise is small can be obtained. 
     Next, the light intensity obtaining module  43  multiplies, by a second reduction coefficient, the value of the light intensity in the region of interest  33  corresponding to the detection signal multiplied by the first reduction coefficient. The light intensity obtaining module  43  then obtains the value of the integrated light intensity in the region of interest  33  using the value of the light intensity multiplied by the second reduction coefficient, too. 
     Thereafter, the optical characteristic value obtaining module  44  obtains the absorption coefficient in the region of interest  33  using the initial sound pressure obtained by the initial sound pressure obtaining module  42  and the value of the integrated light intensity obtained by the light intensity obtaining module  43 . 
     Thus, by multiplying the value of the light intensity corresponding to the detection signal by the second reduction coefficient in addition to multiplying the detection signal by the first reduction coefficient, the absorption coefficient can be accurately obtained. 
     It is to be noted that the first reduction coefficient and the second reduction coefficient are values smaller than 1. Alternatively, another reduction coefficient may be set in accordance with the region of interest. In addition, it is preferable that the first reduction coefficient and the second reduction coefficient are the same value. Here, the same value is a concept including the completely same value and values that become essentially the same when the absorption coefficient is obtained. 
     Fourth Embodiment 
     An embodiment of the present invention can be applied to a subject information obtaining apparatus illustrated in  FIGS. 4A to 4C  and a subject information obtaining apparatus illustrated in  FIGS. 5A to 5C . The subject information obtaining apparatus illustrated in  FIGS. 4A to 4C  can detect the photoacoustic wave at a plurality of positions by rotatively moving the acoustic wave detector  20  around the subject  30  using the detector moving mechanism  21 . In addition, in order to establish acoustic impedance matching between the subject  30  and the acoustic wave detector  20 , the subject  30  is immersed in water  80  that fills a water tank  81 . In addition, a subject moving mechanism  34  that moves the subject  30  is included. By using such a configuration, a portion whose shape cannot be defined by a holding board or the like can be measured. In addition, because detection elements can be set in a lot of directions relative to the subject, data whose amount of information is large can be obtained. 
     In the case of the subject information obtaining apparatus illustrated in  FIGS. 4A to 4C , a state illustrated in  FIG. 4A  changes to a state illustrated in  FIG. 4B  by moving the subject in a lower direction of the paper using the subject moving mechanism  34 . In addition, the state illustrated in  FIG. 4B  changes to a state illustrated in  FIG. 4C  by moving the acoustic wave detector  20  using the detector moving mechanism  21 . Here, the acoustic wave detection element in the states illustrated in  FIGS. 4A, 4B , and  4 C is denoted by e 1 , e 2 , and e 3 , respectively. In addition, triangular regions indicated by broken lines represent a certain sensitive region corresponding to the acoustic wave detection element. 
     In addition, the subject information obtaining apparatus illustrated in  FIGS. 5A to 5C  is provided with the acoustic wave detector  20  and the optical system  11  stored in a single housing  70 . In addition, the housing  70  is provided with a hand-held mechanism  71 , and the operator can move the housing  70  by grasping the hand-held mechanism  71 . By moving the housing  70  in such a manner, the acoustic wave detection element can detect the photoacoustic wave at a plurality of positions. In  FIGS. 5A to 5C , the acoustic wave detection element detects the photoacoustic wave while the operator is moving the housing  70  in a right direction of the paper by grasping the hand-held mechanism  71 . Here, the acoustic wave detection element in states illustrated in  FIGS. 5A, 5B, and 5C  is denoted by e 1 , e 2 , and e 3 , respectively. In addition, triangular regions indicated by broken lines represent a certain sensitive region corresponding to the acoustic wave detection element. 
     However, in the present embodiment, unlike the other embodiments, the acoustic wave detector  20  is moved not mechanically but the housing  70  is arbitrarily moved by the operator by grasping the hand-held mechanism  71 . Therefore, the positional relationship between the acoustic wave detector  20  and the region of interest  33  when the photoacoustic wave  32  has been detected cannot be identified. However, in order to extract a detection signal corresponding to the region of interest from a detection signal obtained by the acoustic wave detector  20 , it is necessary to identify the positional relationship between the acoustic wave detector  20  and the region of interest  33 . Therefore, in the present embodiment, it is preferable that the housing  70  includes a position detector  72  for detecting the position of the housing  70 , that is, the positions of the acoustic wave detector  20  and the optical system  11  stored in the housing  70 . 
     In the cases of the subject information obtaining apparatuses illustrated in  FIGS. 4A to 4C  and  FIGS. 5A to 5C , the certain sensitive region corresponding to the acoustic wave detection element e 1  does not include the region of interest  33 . Therefore, the signal processing apparatus  40  can obtain the absorption coefficient in the region of interest  33  using the method for obtaining information regarding a subject described in the first and second embodiments or the method for obtaining information regarding a subject described in the third embodiment. By obtaining the absorption coefficient in such a manner, in the present embodiment, too, the absorption coefficient can be accurately obtained. 
     A principal configuration will be described hereinafter. 
     (Light Source  10 ) 
     The light source  10  includes a light source that can generate pulse light of 5 nanoseconds to 50 nanoseconds. As the light source, a laser from which a large output can be obtained is preferable, but a light-emitting diode may be used instead of the laser. As the laser, one of various lasers such as a solid-state laser, a gas laser, a dye laser, and a semiconductor laser may be used. Ideally, a Ti:Sa laser excited by Nd:YAG or an alexandrite laser that has a large output and whose wavelength can be continuously changed is used. A plurality of single-wavelength lasers having different wavelengths may be included, instead. 
     (Optical System  11 ) 
     The pulse light emitted from the light source  10  is guided to the subject while being processed in such a way as to have a desired optical distribution shape typically by optical components such as lenses and mirrors, but it is also possible to transmit the pulse light using an optical waveguide such as an optical fiber. The optical system  11  is, for example, a mirror that reflects light, a lens that collects or spreads light or changes the shape of light, a diffusion plate that diffuses light, and the like. Such optical components may be any components insofar as the pulse light emitted from the light source is radiated onto the subject in a desired shape. It is to be noted that it is preferable that the light is spread to a certain area compared to that the light is collected by a lens, in terms of the security of the subject and an increase in a diagnostic range. It is to be noted that an optical moving mechanism may be provided in the optical system  11  in order to move the radiation light. 
     (Acoustic Wave Detector  20 ) 
     The acoustic wave detector  20 , which is a detector that detects a photoacoustic wave generated on a surface of and an inside of the subject using light, detects an acoustic wave and converts the acoustic wave into an electrical signal that is an analog signal. The acoustic wave detector  20  will also be referred to simply as a probe or a transducer hereinafter. Any type of acoustic wave detector such as a transducer using a piezoelectric phenomenon, a transducer using optical resonance, or a transducer using changes in capacitance may be used insofar as the transducer can detect photoacoustic wave signals. 
     In addition, the acoustic wave detector  20  includes a plurality of acoustic wave detection elements. By arranging the plurality of acoustic wave detection elements in one dimension or two dimensions as an array, a photoacoustic wave can be detected at a plurality of positions. By using such multidimensionally arranged elements, an acoustic wave can be simultaneously detected at a plurality of positions, thereby reducing detection time and an effect such as vibration of the subject. 
     It is to be noted that, in order to enable detection of the photoacoustic wave at a plurality of positions, the acoustic wave detector  20  may be configured such that the acoustic wave detector  20  can be mechanically moved by the detector moving mechanism  21 . In addition, a hand-held mechanism that is grasped by the operator in order to arbitrarily move the acoustic wave detector  20  may also be included. 
     (Signal Collector  47 ) 
     It is preferable that the signal collector  47  that amplifies an electrical signal obtained by the acoustic wave detector  20  and that converts the electrical signal from an analog signal to a digital signal is included. The signal collector  47  is typically configured by an amplifier, an A/D convertor, an FPGA (Field Programmable Gate Array) chip, and the like. When a plurality of detection signals are obtained by the acoustic wave detector, it is desirable that the plurality of signals can be simultaneously processed. Accordingly, the time until an image is formed can be reduced. It is to be noted that the “detection signal” herein is a concept including both an analog signal output from the acoustic wave detector  20  and a digital signal obtained by AD conversion by the signal collector  47 . 
     (Signal Processing Apparatus  40 ) 
     The signal processing apparatus  40  obtains an optical characteristic value inside the subject by performing reconfiguration of an image or the like. As the signal processing apparatus  40 , a workstation or the like is typically used, and a process for reconfiguring an image or the like is typically performed by software that has been programmed in advance. For example, the software used in the workstation includes the setting module  41 , the initial sound pressure obtaining module  42 , the light intensity obtaining module  43 , the optical characteristic value obtaining module  44 , and the like. 
     It is to be noted that each module may be provided as a separate piece of hardware. In this case, the pieces of hardware as a whole may be used as the signal processing apparatus  40 . 
     In addition, the signal collector  47  and the signal processing apparatus  40  may be integrated with each other depending on the case. In this case, an optical characteristic value of the subject can be generated not by a software process such as one performed in the workstation but by a hardware process. 
     The present invention is not limited to the above embodiments and can be changed and modified in various ways without deviating from the spirit and the scope thereof. Therefore, in order to make public the scope of the present invention, the following claims are attached. 
     According to an embodiment of the present invention, a subject information obtaining apparatus and a method for obtaining information regarding a subject that can obtain an optical characteristic value more accurately can be provided. 
     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.