Patent Publication Number: US-2013239687-A1

Title: Acoustical wave measuring apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for measuring an acoustical wave, such as an ultrasonic apparatus adapted to run a probe for scanning along a scanning guide. 
     BACKGROUND ART 
     Ultrasonic apparatuses which acquire image information of a test object by running an ultrasonic probe for mechanical scanning have been known. Since an apparatus using ultrasonic waves performs acoustic impedance matching, the apparatus needs to be configured such that there is no gap to admit air between members, between which ultrasonic waves are transmitted. Note that an acoustic impedance match, an acoustic match, acoustic impedance matching in this specification means that the difference between the values of the acoustic impedances of two different substances is not more than about 20%. In the case of mechanical scanning, if the shape of a surface of a test object changes along a direction in which a probe is run for scanning, the distance between the probe and the test object changes. This may form a gap to disable acquisition of acoustic signals. As a unit for solving the problem, PTL 1 discloses an ultrasonic scanner including a matching agent whose shape changes in response to a change in the shape of a test object.  FIG. 8  is a schematic view of the ultrasonic scanner disclosed in PTL 1. In the ultrasonic scanner, a couplant  113  having flexibility is provided as a matching agent between a test object  111  and a probe  112 . The probe  112  is run for scanning by a driving mechanism  114 . At the time of scanning, the flexible couplant is deformed according to rotation or linear scanning of the probe, and an acoustic impedance match between the probe  112  and the couplant  113  is maintained. Additionally, the flexible couplant is deformed to fit in with projections and recesses at a surface of the test object and comes into intimate contact with the test object, and an acoustic impedance match between the couplant and the test object is also maintained. 
     PTL 2 discloses an apparatus which performs acoustic impedance matching by applying a matching oil serving as a liquid matching agent between a compression plate which compresses a test object and a probe.  FIG. 9A  is a perspective view of a probe in PTL 2, and  FIG. 9B  is a sectional view of the probe. The apparatus in PTL 2 includes a sponge  123  which is moistened with a matching oil in order to fill a space between a probe  121  and a compression plate  122  with the matching oil. A cover  125  including spacers  124  between which gaps are formed is provided in order to form a thin film on the compression plate  122  from the matching oil, with which the sponge  123  is moistened. With this configuration, when the probe  121  moves along the compression plate  122 , a thin film of the matching oil is deposited, which enables acoustic impedance matching between the probe and the compression plate. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent No. 3,447,148 
     PTL 2: Japanese Patent Application Laid-Open No. 2003-325523 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, if a probe is fastened to a flexible couplant, like the ultrasonic scanner disclosed in PTL 1, a range within which an ultrasonic image is acquired is limited to a range within which the couplant can change the shape. If scanning is performed while the probe slides on the flexible couplant, the couplant needs to be large enough to cover the image acquisition range, and it is hard to handle. If a variation in a test object is larger than a variation in the shape of the couplant, a gap may be formed between the probe and the test object and it disables acquisition of acoustic signals. 
     In the apparatus disclosed in PTL 2, if the compression plate is deformed when a test object is compressed, the spacers, between which the gaps for forming a thin film are formed, cannot keep the distance between the probe and the compression plate constant. This may form a gap to disable acquisition of acoustic signals. Especially in the case of mechanical scanning, the distance between the probe and the compression plate varies widely. Even if an elastic body of, e.g., rubber is provided, the apparatus can only cover an amount of deformation within a limited range. The process of thickening the compression plate or providing a frame to the compression plate in order to suppress deformation of the compression plate is also conceivable. However, if the process is adopted, signal attenuation may occur or the frame may cause formation of a dead space which prevents propagation of acoustical waves to reduce the image acquisition range. 
     In consideration of the problems, an acoustical wave measuring apparatus according to the present invention includes a holding member which holds a test object, a probe which receives an acoustical wave, and a sealing member, and the acoustical wave is received by running the probe for scanning with respect to the holding member while an acoustic matching agent for performing acoustic impedance matching between the probe and the holding member is injected into between a receiving surface of the probe and the holding member. The sealing member includes a portion with elasticity that is arranged to surround the receiving surface and is biased in a direction which brings the sealing member into contact with the holding member such that the portion with elasticity contacts the holding member to seal a space between the receiving surface and the holding member. 
     Advantageous Effects of Invention 
     According to the present invention, a solid matching agent is not necessary, and an image acquisition is not limited to a particular range. Additionally, attachment of a matching agent is also unnecessary, which leads to ease of handling. Furthermore, since a sealing member is biased to be movable, even when the distance between a holding member and a probe changes during scanning, an acoustic match between the probe and the holding member can be maintained. 
     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 ] FIG. 1  is a perspective view of a main portion of an acoustical wave measuring apparatus according to a first embodiment. 
       [ FIG. 2A ] FIG. 2A  is a perspective view of a probe unit according to the first embodiment. 
       [ FIG. 2B ] FIG. 2B  is a longitudinal sectional view of the probe unit according to the first embodiment. 
       [ FIGS. 3A and 3B ] FIGS. 3A and 3B  are a front view and a side view, respectively, of the acoustical wave measuring apparatus according to the first embodiment. 
       [ FIG. 4 ] FIG. 4  is a sectional view taken along line A-A in  FIG. 3A . 
       [ FIGS. 5A and 5B ] FIGS. 5A and 5B  are sectional views taken along line A-A in  FIG. 3A  when a living body to be measured is held. 
       [ FIG. 6A ] FIG. 6A  is a schematic view illustrating a probe unit in an acoustical wave measuring apparatus according to a second embodiment. 
       [ FIG. 6B ] FIG. 6B  is a schematic view when the probe unit is run for scanning in the second embodiment. 
       [ FIG. 7A ] FIG. 7A  is a schematic view illustrating a probe unit and a carrier in an acoustical wave measuring apparatus according to a third embodiment. 
       [ FIG. 7B ] FIG. 7B  is a schematic view when the probe unit is run for scanning in the third embodiment. 
       [ FIG. 8 ] FIG. 8  is a schematic view of a conventional ultrasonic scanner. 
       [ FIG. 9A ] FIG. 9A  is a perspective view of a probe in a conventional ultrasonic apparatus. 
       [ FIG. 9B ] FIG. 9B  is a sectional view of the probe in the conventional ultrasonic apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A feature of the present invention lies in inclusion of a sealing member which is biased in a direction bringing the sealing member into contact with a holding member such that an elastic portion arranged at a receiving surface of a probe contacts the holding member to seal a space between the receiving surface and the holding member and acoustical wave coupling by acoustic matching between the probe and the holding member. Based on the concept, an acoustical wave measuring apparatus according to the present invention has a basic configuration as described above. In the present invention, any type of probe (e.g., a transducer using piezoceramic, a Capacitive Micro-Machined Ultrasonic Transducer (CMUT) of a capacitance type, a Magnetic Micro-Machined Ultrasonic Transducer (MMUT) using a magnetic film, or a Piezoelectric Micro-Machined Ultrasonic Transducer (PMUT) using a piezoelectric thin film) can be adopted as a probe serving as an electromechanical transducer. Acoustical waves in this specification include ones called a sound wave, an ultrasonic wave, and a photoacoustic wave. Examples of an acoustical wave include an acoustical wave which is generated inside an object to be measured when light such as a near infrared ray (an electromagnetic wave) is applied into the object to be measured and a reflected acoustical wave which is reflected inside an object to be measured when an acoustical wave is transmitted into the object to be measured. 
     Embodiments of an acoustical wave measuring apparatus according to the present invention will be described below. 
     First Embodiment 
       FIG. 1  illustrates a main portion of an ultrasonic apparatus as a first embodiment of an acoustical wave measuring apparatus according to the present invention. The ultrasonic apparatus according to the present embodiment is an ultrasonic apparatus of a mechanical scanning type which acquires an image of the inside of a living body using photoacoustic effects. The ultrasonic apparatus according to the present embodiment includes a holding mechanism  2  for holding the position of a living body  1  serving as a test object, a probe unit  3 , a horizontal scanning mechanism  4 , a vertical scanning mechanism  5 , and a light projecting unit  6 . The probe unit  3  is a unit for receiving acoustical waves. The horizontal scanning mechanism  4  and vertical scanning mechanism  5  are mechanisms for running the probe unit  3  for scanning horizontally and vertically with respect to a fixed holding plate  21 . The light projecting unit  6  is a unit for applying light to the living body  1 . The living body  1  is held while sandwiched between the fixed holding plate  21  serving as a holding member and a movable holding plate  22  also serving as a holding member and arranged to face the fixed holding plate  21 . The fixed holding plate  21  is attached to a frame  21   a  which is fixed to a base  23 . The movable holding plate  22  is securely attached to a fixed plate  22   a.  The fixed plate  22   a  is fixed to a linear guide  24  which is provided on a linear guide base  25 . That is, the movable holding plate  22  is movable along the linear guide  24  in a direction toward the fixed holding plate  21 . In the present embodiment, a probe is provided on the fixed holding plate  21  side. However, according to the present invention, a probe may be provided on the movable holding plate  22  side or may be provided for each holding plate. 
     A material which matches well acoustically with the test object  1  (i.e., a material whose acoustic impedance is matched to the acoustic impedance of the test object  1 ) can be used as the material for the fixed holding plate  21 . Polymethylpentene is especially suitable. As illustrated in the perspective view in  FIG. 2A  and the longitudinal sectional view in  FIG. 2B , the probe unit  3  includes a probe  31 , a housing  32 , an oil seal  33  which constitutes a main portion of a sealing member, an oil seal base  34 , and a compression spring  35  serving as a biasing member. Note that the word “biasing” in this specification refers to applying force or pressure and can be interchanged with pressurization. The probe  31  is fixed to the housing  32 . The oil seal  33  is attached to the oil seal base  34 . Although the oil seal  33  including an elastic portion is arranged to surround a receiving surface of the probe  31  to have a hollow square shape in the present embodiment, the shape of the oil seal  33  is not limited to this. For example, a shape open at an upper surface (a surface toward a direction opposite to a gravity direction) may be adopted as long as a matching oil  7  (to be described later) does not leak. An alternate long and short dash line in the oil seal  33  indicates a ridge which is to contact the fixed holding plate  21 . Any material that has elasticity enough to absorb an amount Δt 1  of deformation (illustrated in  FIGS. 5A and 5B ) of the fixed holding plate  21  within a range  33   a  enclosed by the alternate long and short dash line may be used for the oil seal  33 , and silicon rubber can be used, for example. That is, the oil seal  33  only needs to have elasticity enough to achieve a difference not less than the amount Δt 1  of deformation between the distance of a front end of the oil seal  33  when the oil seal  33  is elastically deformed to the maximum and the distance when the oil seal  33  is not elastically deformed. Although the oil seal  33  may be wholly made of an elastic material, it suffices that at least a front end portion of the oil seal  33  is made of a material having the above-described degree of elasticity. The housing  32  and oil seal base  34  have a fitting portion  32   a  at which the housing  32  and oil seal base  34  fit in with each other. The fitting portion  32   a  is configured to enable the oil seal base  34  to move along a normal direction  31   b  of a receiving surface  31   a  of the probe  31 . The fitting portion  32   a  can include a gap within which leakage of the matching oil  7  that affects measurement can be avoided. 
     The movable distance in the normal direction  31   b  of the oil seal base  34  is set to be larger than a total amount Δt 0  of deformation (illustrated in  FIGS. 5A and 5B ) of the fixed holding plate  21  caused by a force generated when the living body  1  is held. The compression spring  35  serving as the biasing member is provided between the housing  32  and the oil seal base  34 , and the oil seal base  34  and oil seal  33  are biased toward the fixed holding plate  21  by a biasing force of the compression spring  35 . That is, the oil seal base  34  and oil seal  33  are biased in a contact direction for contact with the fixed holding plate  21  such that the front end portion of the oil seal  33  comes into intimate contact with the fixed holding plate  21  to seal in the matching oil  7 . In the present embodiment, the oil seal  33  is biased by the compression spring  35  such that the contact direction is parallel to the normal direction  31   b  of the receiving surface of the probe. If the biasing force of the compression spring  35  is weaker than an elastic force of the oil seal  33  when deformed, the oil seal  33  may contact the fixed holding plate  21  only on one side. For this reason, in the present embodiment, the biasing force of the compression spring  35  can be set to be stronger than a force required for the oil seal  33  to be elastically deformed by Δt 1 . 
     As illustrated in  FIGS. 3A and 3B , the probe unit  3  is attached to a carrier  41  which is provided at the horizontal scanning mechanism  4 . The carrier  41  includes a bearing  42  which fits on a horizontal main shaft  43  serving as a horizontal guide. A horizontal shaft  44  is provided in parallel to the horizontal main shaft  43  to restrict movement in a direction of rotation about the horizontal main shaft  43  of the carrier  41 . The horizontal main shaft  43  and horizontal shaft  44  are fixed to a right side plate  45 R and a left side plate  45 L. A horizontal drive motor  46  which drives the carrier  41  is attached to the right side plate  45 R while a timing pulley  47  is attached to the left side plate  45 L. A horizontal timing belt  48  is coupled to a lower portion of the carrier  41 . The timing belt  48  engages with a timing pinion  46   a  which is provided at the horizontal drive motor  46  and the timing pulley  47 , and power of the horizontal drive motor  46  is transmitted to the carrier  41 . A bearing  49  which fits on a vertical main shaft  51  (to be described later) is provided at the right side plate  45 R. The horizontal scanning mechanism  4  is vertically driven by the vertical scanning mechanism  5 . In the horizontal scanning mechanism  4 , the bearing  49  is fit on the vertical main shaft  51  serving as a vertical scanning guide, and the position in a direction of rotation of the horizontal scanning mechanism  4  is restricted by a detent (not shown) which is coupled to the left side plate  45 L and a vertical shaft  52 . A right vertical timing belt  53 R is coupled to the right side plate  45 R. The right vertical timing belt  53 R engages with a vertical timing pulley  54  which is provided at a top plate  56  and a vertical timing pinion (not shown) which is provided at a vertical drive motor  55 R, and power of the vertical drive motor  55 R is transmitted to the horizontal scanning mechanism  4 . A driving mechanism on the left side is similar to the driving mechanism on the right side. A belt is coupled to the left side plate  45 L, and motor drive is transmitted. With the above-described configuration, the probe unit  3  can be horizontally and vertically run for scanning. 
     The light projecting unit  6  can emit light with a light source (not shown) and an optical system which guides light to the light projecting unit. The light projecting unit  6  can be horizontally and vertically run for scanning by being attached to scanning mechanisms that is similar to the scanning mechanisms for the probe unit  3 .  FIG. 4  is a sectional view taken along line A-A in  FIG. 3A . The oil seal  33  is in intimate contact with the fixed holding plate  21  under the biasing force of the compression spring  35 . The expression “intimate contact” refers to a state in which the varying amount of the matching oil  7  can be kept so as not to affect acoustic coupling during image acquisition. The matching oil  7  serving as an acoustic matching agent which couples acoustical waves between the probe  31  and the fixed holding plate  21  is injected into a space which is formed between the fixed holding plate  21  and the probe  31  by the oil seal  33 . Although castor oil is suitable as the matching oil  7 , the present invention is not limited to this, and any other liquid such as water may be used instead. That is, any substance may be used as long as the substance intervenes between the receiving surface of the probe and the holding plate and can perform acoustic impedance matching between the probe and the acoustic matching agent and acoustic impedance matching between the acoustic matching agent and the holding plate to couple acoustical waves. The matching oil  7  is desirably degassed. In  FIG. 4 , the living body  1  is not held, and the fixed holding plate  21  is not deformed. That is, in the state in  FIG. 4 , the distance between the horizontal main shaft  43  and the fixed holding plate  21  is the longest. 
     When an image of the living body  1  is to be acquired, the living body  1  is inserted between the fixed holding plate  21  and the movable holding plate  22 . The movable holding plate  22  is moved toward the fixed holding plate  21  by a pressure holding mechanism (not shown) such as a mechanism using a trapezoidal thread and a bevel gear or an air cylinder mechanism, and the living body  1  is held between the movable holding plate  22  and the fixed holding plate  21  while a brake (not shown) is put on. In order to achieve an acoustic match, gel may be applied or a water bag may be used between the living body  1  and the fixed holding plate  21  such that an air gap is not formed. After that, the horizontal drive motor  46  and vertical drive motor  55 R drive the probe  31  to move to a site of the living body  1  whose image is desired to be acquired. Similarly, the light projecting unit  6  is moved to a position opposed to the probe  31 . The process of emitting light while performing scanning with the positions of the probe unit  3  and light projecting unit  6  synchronized with each other may be adopted as a method for acquiring an image, in addition to the above-described process of emitting light after moving the probe unit  3  to a site whose image is desired to be acquired. When the living body  1  is irradiated with emitted light, an acoustical wave is generated. The acoustical wave is received by the probe  31 , and an acoustic signal based on the acoustical wave is subjected to publicly known image reconstruction. With this process, an image can be acquired. 
     The states of the fixed holding plate  21  and probe unit  3  when the living body  1  is held is as follows.  FIGS. 5A and 5B  are sectional views taken along line A-A in  FIG. 3A  in the state where the living body  1  is held, and the fixed holding plate  21  is deformed. The fixed holding plate  21  is subjected to a force from the living body  1  resulting from a compressive force of the movable holding plate  22  and is deformed, and the distance of the fixed holding plate  21  to the horizontal main shaft  43  varies.  FIG. 5A  illustrates a case where the probe unit  3  is at a position during movement to a site whose image is desired to be acquired.  FIG. 5B  illustrates a case where the probe unit  3  is at a position where the distance of the horizontal main shaft  43  to the fixed holding plate  21  is the shortest. When the probe unit  3  is run for scanning, and the distance of the horizontal main shaft  43  to the fixed holding plate  21  becomes shorter, the oil seal base  34  is subjected to a force via the oil seal  33  and compresses the compression spring  35 , and the position of the oil seal base  34  moves according to the distance to the fixed holding plate  21 . Since the oil seal  33  has elasticity enough to absorb deformation of the fixed holding plate  21  within the range  33   a  and maintain intimate contact with the fixed holding plate  21 , the matching oil  7  does not leak. When the probe unit  3  is at a position nearest to the fixed holding plate  21 , the oil seal base  34  is at a position when the oil seal base  34  is moved toward the fixed holding plate  21  for the longest distance. However, since the movable distance in the normal direction  31   b  of the oil seal base  34  is set to be larger than an amount of deformation of the fixed holding plate  21 , deformation of the fixed holding plate  21  does not cause compression of the oil seal  33  to the limit to apply stress to the probe unit  3 . Similarly, when the horizontal main shaft  43  and fixed holding plate  21  become farther away from each other, the oil seal base  34  moves according to the distance of the horizontal main shaft  43  to the fixed holding plate  21 , by the biasing force of the compression spring  35 . That is, in a configuration without the compression spring  35  the total amount Δt 0  of deformation of the fixed holding plate  21  needs to be kept at or below the amount Δt 1 , by which the oil seal can be deformed. In the present embodiment with the compression spring  35 , however, the fixed holding plate  21  can be deformed by the amount Δt 1  of deformation or more. Note that the oil seal  33  decreases a little due to, e.g., adhesion to the fixed holding plate  21  during scanning when the probe unit  3  moves on the fixed holding plate  21 . A change in the distance between the probe unit  3  and the fixed holding plate  21  causes the volume of a space between the fixed holding plate  21  and the probe  31  which is filled with the oil seal  33  to fluctuate a little. Accordingly, if the space between the fixed holding plate  21  and the receiving surface of the probe  31  is charged with the oil seal  33 , a unit is desirably provided to put the oil seal  33  into and out of the space, maintain a fully charged state of the space, and cause the space to function well to achieve an acoustic match. 
     It is desirable in image reconstruction to provide a unit which detects the amount of movement of the oil seal base  34  and a unit which measures the distance between the probe  31  and the fixed holding plate  21  and reconstruct an image with the thickness of the matching oil  7  varying depending on a scanning position in mind. 
     Similar method as the horizontal scanning described above can be applied to vertical scanning. Leakage of the matching oil  7  can also be prevented even if the fixed holding plate  21  is vertically deformed. 
     As described above, a solid matching agent is not necessary in the present embodiment. Therefore, an image acquisition range is not limited to a particular one, and an image can be acquired within a scannable range for the probe unit  3 . Since the sealing member is biased to be movable, even when the distance between the holding plate and the scanning guide changes during scanning, an acoustic match can be maintained. Accordingly, a permissible amount of deformation of the holding plate increases, and attenuation of acoustical waves can be suppressed by reducing the thickness of the holding plate. Even if a frame for suppressing deformation of the holding plate is provided, the size of the frame can be reduced, and a dead space formed by the frame can be reduced. 
     Second Embodiment 
     A second embodiment is a modification of the first embodiment and is different in the configuration of a probe unit. Components other than a probe unit in the second embodiment are the same as the components in the first embodiment, and a description of the components will be omitted. As illustrated in  FIG. 6A  that is a schematic view of a probe unit  8  in the present embodiment, the probe unit  8  includes a probe  81 , a housing  82 , an oil seal  83 , a linear motion base  84 , a rotation base  85 , and a compression spring  86 . The probe  81  is fixed to the housing  82 . The rotation base  85  is attached to the linear motion base  84  so as to rotate about X. Since the oil seal  83  is attached to the rotation base  85 , the oil seal  83  is also rotatable. The oil seal  83  is made of an elastic body which enables the oil seal  83  to follow inclination of a fixed holding plate  21  and absorb deformation of the fixed holding plate  21  within a range  83   a  where the probe  81  contacts the fixed holding plate  21  and to come into intimate contact with the fixed holding plate  21 . An inner surface of the housing  82  and an outer surface of the linear motion base  84  have a fitting portion  84   a  at which the inner surface and outer surface fit in with each other. The fitting portion  84   a  is configured to enable the linear motion base  84  to move in a normal direction  31   b  of a receiving surface  31   a  of the probe  31 . The movable distance in the normal direction  31   b  of the linear motion base  84  is set to be larger than an amount of deformation of the fixed holding plate  21  caused by a force generated when a living body  1  is held. The compression spring  86  is provided between the housing  82  and the linear motion base  84 , and the linear motion base  84 , rotation base  85 , and oil seal  83  are biased toward the fixed holding plate  21  by a biasing force of the compression spring  86 . The probe unit  8  is also sealed with an elastic body (not shown) so as to prevent leakage of a matching oil  7  caused by displacements of the linear motion base  84  and rotation base  85 . 
     Action of the probe unit  8  when the fixed holding plate  21  is deformed is as follows.  FIG. 6B  is a schematic view of a case where the probe unit  8  is run for scanning along the deformed fixed holding plate  21 . The oil seal  83  is biased toward the fixed holding plate  21  via the linear motion base  84  and rotation base  85  by the biasing force of the compression spring  86 . The biasing force rotates the rotation base  85  in a direction which brings the whole oil seal  83  into contact with the fixed holding plate  21  with respect to the linear motion base  84 . Namely, the rotation base  85  rotates towards a direction such that contact direction  83   b  of the oil seal  83  to the fixed holding plate  21  coincides with a normal direction  21   a  of the fixed holding plate  21  within a range where the oil seal  83  is in contact with the fixed holding plate  21 . Thus, when the probe unit  8  is run for scanning, the orientation of the oil seal  83  follows the inclination of the fixed holding plate  21  in response to deformation of the fixed holding plate  21 . Accordingly, in the present embodiment, the oil seal  83  can rotate such that the contact direction of the oil seal  83  follows the normal direction of the fixed holding plate  21  and is biased by the compression spring  86 . When the rotation base  85  has an inclination of a, the oil seal  83  absorbs deformation of the fixed holding plate  21  within the range  83   a  that is in a normal direction of a direction of rotation of the rotation base  85 , and intimate contact between the fixed holding plate  21  and the oil seal  83  is maintained. Note that since the probe  81  does not rotate, the receiving surface of the probe  81  is inclined with respect to the fixed holding plate  21 . Although  FIG. 6B  illustrates only rotation about one axis, the probe unit  8  can cope with horizontal deformation and vertical deformation of the fixed holding plate  21  by providing a mechanism for rotation about two axes. 
     In the present embodiment as well, it is desirable in image reconstruction to provide a unit which detects the amount of movement of the oil seal base and a unit which measures the distance between the probe and the fixed holding plate  21  and reconstruct an image with the thickness of the matching oil varying depending on a scanning position in mind. 
     According to the present embodiment, rotation of the rotation base  85 , to which the oil seal  83  is attached, can cause the orientation of the oil seal  83  to follow the inclination of the fixed holding plate  21  when deformed. Since an amount of deformation of the oil seal  83  that needs to absorb deformation of the fixed holding plate  21  is reduced, conditions concerning the material for and the shape of the oil seal can be relaxed, in addition to the advantageous effects of the first embodiment. Further, the need to set the biasing force of the compression spring  86  to be stronger than an elastic force of the oil seal  83  is eliminated. 
     Third Embodiment 
       FIG. 7A  is a schematic view of a probe unit  9  and a carrier  41  according to a third embodiment. In the present embodiment, the probe unit  9  is provided to be rotatable about Y with respect to the carrier  41 . The probe unit  9  includes a probe  91 , a housing  92 , an oil seal  93 , an oil seal base  94 , and a compression spring  95 . The probe  91  is coupled to the housing  92 . The oil seal  93  is coupled to the oil seal base  94 . The oil seal base  94  and housing  92  have a fitting portion  92   a . With this configuration, the oil seal base  94  is movable in a normal direction of a receiving surface of the probe  91  with respect to the housing  92  while the oil seal base  94  is biased toward a fixed holding plate  21  by a biasing force of the compression spring  95 . The movable distance of the oil seal  93  is set to be larger than an amount of deformation of the fixed holding plate  21  caused by a force generated when a living body  1  is held. 
       FIG. 7B  is a sectional view of a state of the probe unit  9  with respect to the deformed fixed holding plate  21  and illustrates a difference in the state of the probe unit  9  caused by a difference in position. The probe unit  9  according to the present embodiment is provided with the compression spring  95 , which biases the oil seal base  94  attached to the carrier  41  so as to rotate together with the probe  91 . For this reason, the orientation of the probe unit  9  follows the normal direction of the surface of the fixed holding plate  21  by cooperation of a contact force between the oil seal  93  and the fixed holding plate  21  and the biasing force of the compression spring  95 . That is, action of the biasing force of the compression spring  95  moves the oil seal base  94  in response to a change in the distance of the fixed holding plate  21 . Simultaneously, the probe unit  9  rotates by reaction resulting from the contact of the oil seal  93  with the fixed holding plate  21  to cause the orientation of the receiving surface to follow the normal direction. Accordingly, in the present embodiment, a direction of contact of the sealing member with the biased holding member follows not only the normal direction of the receiving surface of the probe but also a normal direction of a surface of the holding member. In the above-described manner, intimate contact of the oil seal  93  with the fixed holding plate  21  is maintained. 
     In the present embodiment as well, it is desirable in image reconstruction to provide a unit which detects the amount of movement of the oil seal base and a unit which measures the distance between the probe and the fixed holding plate  21  and reconstruct an image with the thickness of the matching oil varying depending on a scanning position in mind. In the present embodiment, the receiving surface of the probe is not inclined with respect to the surface of the fixed holding plate  21  and is kept substantially parallel, which makes the process of performing image reconstruction easier with the thickness of the matching oil in mind. 
     The configuration of the present embodiment can also achieve the same advantageous effects as the advantageous effects in the first and second embodiments. In the present embodiment, conditions concerning the material for and the shape of the oil seal can be relaxed, and the need to set the biasing force of the compression spring  95  to be stronger than an elastic force of the oil seal  93  is eliminated, as in the second embodiment. 
     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. 
     This application claims the benefit of Japanese Patent Application No. 2010-265843, filed Nov. 30, 2010, which is hereby incorporated by reference herein in its entirety.