Patent Publication Number: US-2022218311-A1

Title: Diagnostic assistance device, diagnostic assistance system, and diagnostic assistance method

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/JP2020/036433 filed on Sep. 25, 2020, which claims priority to Japanese Application No. 2019-178979 filed on Sep. 30, 2019, the entire content of both of which is incorporated herein by reference. 
    
    
     TECHNOLOGICAL FIELD 
     The present disclosure generally relates to a diagnostic assistance device, a diagnostic assistance system, and a diagnostic assistance method. 
     BACKGROUND DISCUSSION 
     U.S. Patent Application Publication No. 2010/0215238, U.S. Pat. Nos. 6,385,332, and 6,251,072 disclose a technique of generating a three-dimensional image of a cardiac cavity or a blood vessel using an ultrasound (US) image system. 
     Treatment using intravascular ultrasound (IVUS) is widely performed on a cardiac cavity, a cardiac blood vessel, a lower limb artery region, and the like. The IVUS is a device or a method for providing a two-dimensional image of a plane perpendicular to a long axis (longitudinal axis) of a catheter. 
     At present, an operator needs to perform treatment while reconstructing a three-dimensional structure by stacking two-dimensional images of IVUS in his/her brain, which can be a barrier particularly to young doctors or inexperienced doctors. In order to remove such a barrier, it is conceivable to automatically generate a three-dimensional image representing a structure of a biological tissue such as a cardiac cavity or a blood vessel from the two-dimensional images of IVUS and display the generated three-dimensional image toward the operator. When the three-dimensional image is to be displayed, it is conceivable to add shading to the three-dimensional image to express unevenness and depth in a three-dimensional space. 
     However, when the color tone is the same, it is difficult for the operator to grasp the unevenness and depth. 
     SUMMARY 
     The present disclosure facilitates grasping of unevenness or depth in the three-dimensional space for the operator of a diagnostic assistance device. 
     A diagnostic assistance device according to an aspect of the present disclosure is a diagnostic assistance device configured to generate three-dimensional data of a biological tissue based on tomographic data of the biological tissue, and display the generated three-dimensional data as a three-dimensional image on a display. The diagnostic assistance device includes a control unit configured to adjust a color tone of each voxel of the three-dimensional image according to a distance from a reference point, a reference line, or a reference plane in a three-dimensional space to each point of the three-dimensional data. 
     In one embodiment, the control unit is configured to form, in the three-dimensional data, an opening that exposes an inner wall surface of the biological tissue to an outside of the biological tissue in the three-dimensional image, and adjust a position of the reference point, the reference line, or the reference plane according to a position of the formed opening. 
     In one embodiment, the control unit is configured to arrange the reference point on a straight line in a cross section of the biological tissue that passes through a viewpoint when the three-dimensional image is displayed on the display and a midpoint of a straight line connecting a first end edge of the opening and a second end edge of the opening. 
     In one embodiment, the control unit is configured to arrange the reference point on a straight line drawn perpendicularly to a straight line connecting a first end edge of the opening and a second end edge of the opening from a midpoint of the straight line. 
     In one embodiment, when at least a part of a point group in the three-dimensional data in a range to be displayed on the display as the three-dimensional image has a difference in color tone with respect to a difference in distance from the reference point, the reference line, or the reference plane that does not satisfy a condition, the control unit corrects the color tone of each voxel of the three-dimensional image in accordance with the condition. 
     In one embodiment, the diagnostic assistance device further includes an input unit configured to receive an operation of a user. The control unit is configured to receive, via the input unit, an operation of changing a difference in color tone with respect to a difference in distance from the reference point, the reference line, or the reference plane of at least a part of a point group in the three-dimensional data in a range to be displayed on the display as the three-dimensional image, and change the color tone of each voxel of the three-dimensional image in response to the received operation. 
     In one embodiment, the diagnostic assistance device further includes an input unit configured to receive an operation of a user. The control unit is configured to receive, via the input unit, an operation of changing a position of the reference point, the reference line, or the reference plane, and change the color tone of each voxel of the three-dimensional image in response to the received operation. 
     A diagnostic assistance system according to an aspect of the present disclosure includes the diagnostic assistance device and a sensor configured to acquire the tomographic data while moving in the biological tissue. 
     In one embodiment, the diagnostic assistance system further includes the display. 
     A diagnostic assistance method according to an aspect of the present disclosure is a diagnostic assistance method for generating three-dimensional data of a biological tissue based on tomographic data of the biological tissue, and displaying the generated three-dimensional data as a three-dimensional image on a display. The diagnostic assistance method includes adjusting a color tone of each voxel of the three-dimensional image according to a distance from a reference point, a reference line, or a reference plane in a three-dimensional space to each point of the three-dimensional data. 
     According to the present disclosure, it is easy for a user to grasp unevenness or depth in a three-dimensional space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a diagnostic assistance system according to an embodiment. 
         FIG. 2  is a perspective view of a probe and a drive unit according to the embodiment. 
         FIG. 3  is a block diagram showing a configuration of a diagnostic assistance device according to the embodiment. 
         FIG. 4  is a flowchart showing an operation of the diagnostic assistance system according to the embodiment. 
         FIG. 5  is a diagram showing a positional relationship between a cross section of a biological tissue, an opening, a viewpoint, and a reference point according to the embodiment. 
         FIG. 6  is a diagram showing a ratio of a size of a three-dimensional image to a screen of a display according to the embodiment. 
         FIG. 7A  is a graph showing a relationship between a distance from the reference point in three-dimensional data and an R value in the three-dimensional image according to the embodiment. 
         FIG. 7B  is a graph showing a relationship between the distance from the reference point in the three-dimensional data and a G value in the three-dimensional image according to the embodiment. 
         FIG. 7C  is a graph showing a relationship between the distance from the reference point in the three-dimensional data and a B value in the three-dimensional image according to the embodiment. 
         FIG. 8  is a diagram showing a gradation according to the embodiment. 
         FIG. 9  is a diagram showing a gradation according to the embodiment. 
         FIG. 10  is a diagram showing a gradation according to a modification. 
         FIG. 11  is a diagram showing a gradation according to the modification. 
         FIG. 12  is a diagram showing a gradation according to the modification. 
     
    
    
     DETAILED DESCRIPTION 
     Set forth below with reference to the accompanying drawings is a detailed description of embodiments of a diagnostic assistance device, a diagnostic assistance system, and a diagnostic assistance method representing examples of the inventive diagnostic assistance device, diagnostic assistance system, and diagnostic assistance method. 
     In the drawings, the same or corresponding parts are denoted by the same reference numerals. In the description of the present embodiment, the description of the same or corresponding parts will be omitted or simplified as appropriate. 
     An outline of the present embodiment will be described with reference to  FIGS. 1, 3, and 5 . 
     A diagnostic assistance device  11  according to the present embodiment generates three-dimensional data  52  of a biological tissue  60  based on tomographic data  51  of the biological tissue  60 . The diagnostic assistance device  11  displays the generated three-dimensional data  52  as a three-dimensional image  53  on a display  16 . The diagnostic assistance device  11  adjusts a color tone of each voxel of the three-dimensional image  53  according to a distance from a reference point Ps in a three-dimensional space to each point of the three-dimensional data  52 . 
     According to the present embodiment, it is relatively easy for a user to grasp unevenness or depth in the three-dimensional space. For example, when the user is an operator, it can be relatively easy to grasp a shape of the biological tissue  60 , which facilitates treatment. 
     The biological tissue  60  can be, for example, an organ such as a blood vessel or a heart. 
     A configuration of a diagnostic assistance system  10  according to the present embodiment will be described with reference to  FIG. 1 . 
     The diagnostic assistance system  10  includes the diagnostic assistance device  11 , a cable  12 , a drive unit  13 , a keyboard  14 , a mouse  15 , and the display  16 . 
     The diagnostic assistance device  11  can be a dedicated computer specialized for image diagnosing in the present embodiment, but may also be a general-purpose computer such as a personal computer (PC). 
     The cable  12  is used to connect the diagnostic assistance device  11  and the drive unit  13 . 
     The drive unit  13  is a device to be used by connecting to a probe  20  shown in  FIG. 2  to drive the probe  20 . The drive unit  13  is also referred to as a motor drive unit (MDU). The probe  20  can be applied to IVUS. The probe  20  is also referred to as an IVUS catheter or an image diagnostic catheter. 
     The keyboard  14 , the mouse  15 , and the display  16  are connected to the diagnostic assistance device  11  via any cable or wirelessly. The display  16  can be, for example, a liquid crystal display (LCD), an organic electro luminescence (EL) display, or a head-mounted display (HMD). 
     The diagnostic assistance system  10  optionally further includes a connecting terminal  17  and a cart unit  18 . 
     The connecting terminal  17  is used to connect the diagnostic assistance device  11  and an external device. The connecting terminal  17  can be, for example, a universal serial bus (USB) terminal. The external device can be, for example, a recording medium such as a magnetic disc drive, a magneto-optical disc drive, or an optical disc drive. 
     The cart unit  18  can be a cart equipped with casters for movement. The diagnostic assistance device  11 , the cable  12 , and the drive unit  13  are disposed on a cart body of the cart unit  18 . The keyboard  14 , the mouse  15 , and the display  16  are disposed on an uppermost table of the cart unit  18 . 
     A configuration of the probe  20  and the drive unit  13  according to the present embodiment will be described with reference to  FIG. 2 . 
     The probe  20  includes a drive shaft  21 , a hub  22 , a sheath  23 , an outer tube  24 , an ultrasound transducer  25 , and a relay connector  26 . 
     The drive shaft  21  passes through the sheath  23  to be inserted into a body cavity of a living body and the outer tube  24  connected to a proximal end of the sheath  23 , and extends to an inside of the hub  22  provided at a proximal end of the probe  20 . The drive shaft  21  is provided with the ultrasound transducer  25 , which transmits and receives signals, at a distal end of the drive shaft  21 , and is rotatably provided in the sheath  23  and the outer tube  24 . The relay connector  26  connects the sheath  23  and the outer tube  24 . 
     The hub  22 , the drive shaft  21 , and the ultrasound transducer  25  are connected to each other so as to integrally move forward and backward in an axial direction. Therefore, for example, when the hub  22  is pressed toward a distal side, the drive shaft  21  and the ultrasound transducer  25  move inside the sheath  23  toward the distal side. For example, when the hub  22  is pulled toward a proximal side, the drive shaft  21  and the ultrasound transducer  25  move inside the sheath  23  toward the proximal side as indicated by arrows. 
     The drive unit  13  includes a scanner unit  31 , a slide unit  32 , and a bottom cover  33 . 
     The scanner unit  31  is connected to the diagnostic assistance device  11  via the cable  12 . The scanner unit  31  includes a probe connection unit  34  connected to the probe  20 , and a scanner motor  35  which is a drive source for rotating the drive shaft  21 . 
     The probe connection unit  34  is detachably connected to the probe  20  through an insertion port  36  of the hub  22  provided at the proximal end of the probe  20 . Inside the hub  22 , a proximal end of the drive shaft  21  is rotatably supported, and a rotational force of the scanner motor  35  is transmitted to the drive shaft  21 . A signal is transmitted and received between the drive shaft  21  and the diagnostic assistance device  11  via the cable  12 . In the diagnostic assistance device  11 , a tomographic image of a body lumen is generated and image processing is performed based on the signal transmitted from the drive shaft  21 . 
     The slide unit  32  is mounted with the scanner unit  31  in a manner capable of moving forward and backward, and is mechanically and electrically connected to the scanner unit  31 . The slide unit  32  includes a probe clamp unit  37 , a slide motor  38 , and a switch group  39 . 
     The probe clamp unit  37  is disposed coaxially with the probe connection unit  34  on a distal side of the probe connection unit  34 , and supports the probe  20  to be connected to the probe connection unit  34 . 
     The slide motor  38  is a drive source that generates a driving force in the axial direction. The scanner unit  31  moves forward and backward when driven by the slide motor  38 , and the drive shaft  21  moves forward and backward in the axial direction accordingly. The slide motor  38  can be, for example, a servo motor. 
     The switch group  39  can include, for example, a forward switch and a pull-back switch that are pressed when the scanner unit  31  is to be moved forward or backward, and a scan switch that is pressed when image drawing is to be started or ended. Various switches may be included in the switch group  39  as necessary without being limited to the example here. 
     When the forward switch is pressed, the slide motor  38  rotates forward, and the scanner unit  31  moves forward. On the other hand, when the pull-back switch is pressed, the slide motor  38  rotates backward, and the scanner unit  31  moves backward. 
     When the scan switch is pressed, the image drawing is started, the scanner motor  35  is driven, and the slide motor  38  is driven to move the scanner unit  31  backward. A user such as an operator connects the probe  20  to the scanner unit  31  in advance, and rotates and moves the drive shaft  21  toward the proximal side in the axial direction upon the start of the image drawing. When the scan switch is pressed again, the scanner motor  35  and the slide motor  38  are stopped, and the image drawing is ended. 
     The bottom cover  33  covers a bottom and an entire circumference of a side surface on a bottom side of the slide unit  32 , and is capable of moving toward and away from the bottom of the slide unit  32 . 
     A configuration of the diagnostic assistance device  11  according to the present embodiment will be described with reference to  FIG. 3 . 
     The diagnostic assistance device  11  includes a control unit  41 , a storage unit  42 , a communication unit  43 , an input unit  44 , and an output unit  45 . 
     The control unit  41  includes at least one processor, at least one dedicated circuit, or a combination of at least one processor and at least one dedicated circuit. The processor can be a general-purpose processor such as a central processing unit (CPU) or a graphics processing unit (GPU), or a dedicated processor specialized for a specific process. As the dedicated circuit, for example, a field-programmable gate array (FPGA) or an application specific integrated circuit (ASIC) can be used. The control unit  41  executes processing related to an operation of the diagnostic assistance device  11  while controlling each unit of the diagnostic assistance system  10  including the diagnostic assistance device  11 . 
     The storage unit  42  can include at least one semiconductor memory, at least one magnetic memory, at least one optical memory, or a combination of at least two of the at least one semiconductor memory, the at least one magnetic memory, and the at least one optical memory. The semiconductor memory can be, for example, a random access memory (RAM) or a read only memory (ROM). The RAM can be, for example, a static random access memory (SRAM) or a dynamic random access memory (DRAM). The ROM can be, for example, an electrically erasable programmable read only memory (EEPROM). The storage unit  42  functions as, for example, a main storage device, an auxiliary storage device, or a cache memory. The storage unit  42  stores data used for the operation of the diagnostic assistance device  11 , such as the tomographic data  51 , and data obtained by the operation of the diagnostic assistance device  11 , such as the three-dimensional data  52  and the three-dimensional image  53 . 
     The communication unit  43  includes at least one communication interface. The communication interface is a wired local area network (LAN) interface, a wireless LAN interface, or an image diagnostic interface for receiving IVUS signals and performing analog to digital (A/D) conversion on the IVUS signals. The communication unit  43  receives data used for the operation of the diagnostic assistance device  11  and transmits data obtained by the operation of the diagnostic assistance device  11 . In the present embodiment, the drive unit  13  is connected to the image diagnostic interface included in the communication unit  43 . 
     The input unit  44  includes at least one input interface. The input interface is, for example, a USB interface, a high-definition multimedia interface (HDMI®) interface, or an interface compatible with short-range wireless communication such as Bluetooth®. The input unit  44  receives an operation of inputting data used for the operation of the diagnostic assistance device  11 . In the present embodiment, the keyboard  14  and the mouse  15  are connected to the USB interface or the interface corresponding to short-range wireless communication included in the input unit  44 . When a touch screen is provided integrally with the display  16 , the display  16  may be connected to the USB interface or the HDMI interface included in the input unit  44 . 
     The output unit  45  includes at least one output interface. The output interface is, for example, a USB interface, an HDMI interface, or an interface compatible with short-range wireless communication such as Bluetooth. The output unit  45  outputs the data obtained by the operation of the diagnostic assistance device  11 . In the present embodiment, the display  16  is connected to the USB interface or the HDMI interface included in the output unit  45 . 
     A function of the diagnostic assistance device  11  is implemented by executing a diagnostic assistance program according to the present embodiment by the processor included in the control unit  41 . That is, the function of the diagnostic assistance device  11  is implemented by software. The diagnostic assistance program is a program for causing a computer to execute the processing of steps included in the operation of the diagnostic assistance device  11  to implement a function corresponding to the processing of the steps. That is, the diagnostic assistance program is a program for causing the computer to function as the diagnostic assistance device  11 . 
     The program can be recorded in a computer-readable recording medium. The computer-readable recording medium can be, for example, a magnetic recording device, an optical disc, a magneto-optical recording medium, or a semiconductor memory. The program is distributed by, for example, selling, transferring, or lending a portable recording medium such as a digital versatile disc (DVD) or a compact read only memory (CD-ROM) on which the program is recorded. The program may be distributed by storing the program in a storage of a server and transferring the program from the server to another computer via a network. The program may be provided as a program product. 
     For example, the computer temporarily stores the program recorded in the portable recording medium or the program transferred from the server in the main storage device. The computer causes the processor to read the program stored in the main storage device, and causes the processor to execute processing according to the read program. The computer may read the program directly from the portable recording medium and execute the processing according to the program. Each time the program is transferred from the server to the computer, the computer may sequentially execute processing according to the received program. The processing may be executed by a so-called application service provider (ASP) type service in which the function is implemented only by execution instruction and result acquisition without transferring the program from the server to the computer. The program includes information provided for processing by an electronic computer and conforming to the program. For example, data that is not a direct command to the computer but has a property that defines the processing of the computer corresponds to the “information conforming to the program”. 
     The functions of the diagnostic assistance device  11  may be partially or entirely implemented by the dedicated circuit included in the control unit  41 . That is, the functions of the diagnostic assistance device  11  may be partially or entirely implemented by hardware. 
     An operation of the diagnostic assistance system  10  according to the present embodiment will be described with reference to  FIG. 4 . The operation of the diagnostic assistance system  10  corresponds to a diagnostic assistance method according to the present embodiment. 
     Before the start of a flow in  FIG. 4 , the probe  20  is primed by the user. Thereafter, the probe  20  is fitted into the probe connection unit  34  and the probe clamp unit  37  of the drive unit  13 , and is connected and fixed to the drive unit  13 . The probe  20  is inserted to a target site in the biological tissue  60  such as the blood vessel or the heart. 
     In step S 1 , the scan switch included in the switch group  39  is pressed, and a so-called pull-back operation is performed by pressing the pull-back switch included in the switch group  39 . The probe  20  transmits ultrasound inside the biological tissue  60  by the ultrasound transducer  25  that moves backward in the axial direction by the pullback operation. The ultrasound transducer  25  radially transmits the ultrasound while moving inside the biological tissue  60 . The ultrasound transducer  25  receives a reflected wave of the transmitted ultrasound. The probe  20  inputs a signal of the reflected wave received by the ultrasound transducer  25  to the diagnostic assistance device  11 . The control unit  41  of the diagnostic assistance device  11  processes the input signal to sequentially generate cross-sectional images of the biological tissue  60 , thereby acquiring the tomographic data  51 , which includes a plurality of cross-sectional images. 
     Specifically, the probe  20  transmits the ultrasound in a plurality of directions from a rotation center to an outside by the ultrasound transducer  25  while causing the ultrasound transducer  25  to rotate in a circumferential direction and to move in the axial direction inside the biological tissue  60 . The probe  20  receives the reflected wave from a reflecting object existing in each of the plurality of directions inside the biological tissue  60  by the ultrasound transducer  25 . The probe  20  transmits the signal of the received reflected wave to the diagnostic assistance device  11  via the drive unit  13  and the cable  12 . The communication unit  43  of the diagnostic assistance device  11  receives the signal transmitted from the probe  20 . The communication unit  43  performs A/D conversion on the received signal. The communication unit  43  inputs the A/D-converted signal to the control unit  41 . The control unit  41  processes the input signal to calculate an intensity value distribution of the reflected wave from the reflecting object existing in a transmission direction of the ultrasound of the ultrasound transducer  25 . The control unit  41  sequentially generates two-dimensional images having a luminance value distribution corresponding to the calculated intensity value distribution as the cross-sectional images of the biological tissue  60 , thereby acquiring the tomographic data  51  which is a data set of the cross-sectional images. The control unit  41  stores the acquired tomographic data  51  in the storage unit  42 . 
     In the present embodiment, the signal of the reflected wave received by the ultrasound transducer  25  corresponds to raw data of the tomographic data  51 , and the cross-sectional images generated by processing the signal of the reflected wave by the diagnostic assistance device  11  correspond to processed data of the tomographic data  51 . 
     In a modification of the present embodiment, the control unit  41  of the diagnostic assistance device  11  may store the signal input from the probe  20  as it is in the storage unit  42  as the tomographic data  51 . Alternatively, the control unit  41  may store data indicating the intensity value distribution of the reflected wave calculated by processing the signal input from the probe  20  in the storage unit  42  as the tomographic data  51 . That is, the tomographic data  51  is not limited to the data set of the cross-sectional images of the biological tissue  60 , and may be data representing a cross section of the biological tissue  60  at each moving position of the ultrasound transducer  25  in some format. 
     In a modification of the present embodiment, an ultrasound transducer that transmits ultrasound in a plurality of directions without rotating may be used instead of the ultrasound transducer  25  that transmits ultrasound in the plurality of directions while rotating in the circumferential direction. 
     In a modification of the present embodiment, the tomographic data  51  may be acquired using optical frequency domain imaging (OFDI) or optical coherence tomography (OCT) instead of being acquired by using the IVUS. When OFDI or OCT is used, as a sensor that acquires the tomographic data  51  while moving in the biological tissue  60 , a sensor that acquires the tomographic data  51  by emitting light in the biological tissue  60  is used instead of the ultrasound transducer  25  that acquires the tomographic data  51  by transmitting the ultrasound in the biological tissue  60 . 
     In a modification of the present embodiment, instead of the diagnostic assistance device  11  generating the data set of the cross-sectional images of the biological tissue  60 , another device may generate the same data set, and the diagnostic assistance device  11  may acquire the data set from the other device. That is, instead of the control unit  41  of the diagnostic assistance device  11  processing the IVUS signal to generate the cross-sectional image of the biological tissue  60 , another device may process the IVUS signal to generate the cross-sectional image of the biological tissue  60  and input the generated cross-sectional image to the diagnostic assistance device  11 . 
     In step S 2 , the control unit  41  of the diagnostic assistance device  11  generates the three-dimensional data  52  of the biological tissue  60  based on the tomographic data  51  acquired in step S 1 . 
     Specifically, the control unit  41  of the diagnostic assistance device  11  generates the three-dimensional data  52  of the biological tissue  60  by stacking the cross-sectional images of the biological tissue  60  included in the tomographic data  51  stored in the storage unit  42 , and converting the same into three-dimensional data. As a method of three-dimensional conversion, any process among a rendering method such as surface rendering or volume rendering, texture mapping accompanying the rendering method, such as environment mapping and bump mapping, or the like is used. The control unit  41  stores the generated three-dimensional data  52  in the storage unit  42 . 
     In step S 3 , the control unit  41  of the diagnostic assistance device  11  displays the three-dimensional data  52  generated in step S 2  on the display  16  as the three-dimensional image  53 . At this point, the control unit  41  may arrange, at any position, a viewpoint when displaying the three-dimensional image  53  on the display  16  and virtual light sources  72 . The “viewpoint” is a position of a virtual camera  71  as shown in  FIG. 5 , which is arranged in the three-dimensional space. The number and the relative positions of the light sources  72  are not limited to those illustrated in the drawings, and can be changed as appropriate. 
     In accordance with an exemplary embodiment, the control unit  41  of the diagnostic assistance device  11  generates the three-dimensional image  53  from the three-dimensional data  52  stored in the storage unit  42 . The control unit  41  displays the generated three-dimensional image  53  on the display  16  via the output unit  45 . 
     In step S 4 , when operated by the user, the processing from step S 5  to step S 8  is performed. When not operated by the user, the processing from step S 5  to step S 8  is skipped (or omitted). 
     In step S 5 , the control unit  41  of the diagnostic assistance device  11  receives, via the input unit  44 , an operation of setting a position of the opening  62  as shown in  FIG. 5 . The position of the opening  62  is set to a position at which an inner wall surface  61  of the biological tissue  60  is exposed to the outside of the biological tissue  60  through the opening  62  in the three-dimensional image  53  displayed in step S 3 . 
     Specifically, the control unit  41  of the diagnostic assistance device  11  receives, via the input unit  44 , an operation of the user cutting off a portion of the biological tissue  60  using the keyboard  14 , the mouse  15 , or the touch screen provided integrally with the display  16  in the three-dimensional image  53  displayed on the display  16 . In the example of  FIG. 5 , the control unit  41  receives an operation of cutting off a portion of the biological tissue  60  so that the inner wall surface  61  of the biological tissue  60  has an opened shape in the cross section of the biological tissue  60 . The term “cross section of the biological tissue  60 ” refers to, for example, a tomographic cross section having two end edges of the opening  62  facing each other and the inner wall surface  61  of the biological tissue  60  facing the opening  62 , but is not limited to this tomographic cross section, and may be a transverse cross section of the biological tissue  60 , a longitudinal cross section of the biological tissue  60 , or another cross section of the biological tissue  60 . The term “transverse cross section of the biological tissue  60 ” refers to a cross section obtained by cutting the biological tissue  60  perpendicularly to a direction in which the ultrasound transducer  25  moves in the biological tissue  60 . The term “longitudinal cross section of the biological tissue  60 ” refers to a cross section obtained by cutting the biological tissue  60  along a direction in which the ultrasound transducer  25  moves in the biological tissue  60 . The term “another cross section of the biological tissue  60 ” refers to a cross section obtained by cutting the biological tissue  60  obliquely with respect to a direction in which the ultrasound transducer  25  moves in the biological tissue  60 . The term “opened shape” refers to, for example, a substantially C shape, a substantially U shape, a substantially “3” shape, or a shape in which any of these shapes is partially missing due to a hole originally opened in the biological tissue  60 , such as a bifurcated portion of the blood vessel or pulmonary vein ostia. In the example of  FIG. 5 , a shape of the inner wall surface  61  of the biological tissue  60  is a substantially C shape, and a portion facing the opening  62  is missing. 
     In step S 6 , the control unit  41  of the diagnostic assistance device  11  determines the position set by the operation received in step S 5  as the position of the opening  62 . 
     In accordance with an embodiment, the control unit  41  of the diagnostic assistance device  11  specifies, as three-dimensional coordinates of an edge of the opening  62 , three-dimensional coordinates of a boundary of a portion of the biological tissue  60  cut off by the operation of the user in the three-dimensional data  52  stored in the storage unit  42 . The control unit  41  stores the specified three-dimensional coordinates in the storage unit  42 . 
     In step S 7 , the control unit  41  of the diagnostic assistance device  11  forms, in the three-dimensional data  52 , the opening  62  that exposes the inner wall surface  61  of the biological tissue  60  to the outside of the biological tissue  60  in the three-dimensional image  53 . 
     In accordance with an embodiment, the control unit  41  of the diagnostic assistance device  11  sets a portion in the three-dimensional data  52  stored in the storage unit  42  that is specified by the three-dimensional coordinates stored in the storage unit  42  to be hidden or transparent when the three-dimensional image  53  is to be displayed on the display  16 . 
     In step S 8 , the control unit  41  of the diagnostic assistance device  11  adjusts the viewpoint when displaying the three-dimensional image  53  on the display  16  according to the position of the opening  62  formed in step S 7 . In the present embodiment, the control unit  41  arranges the viewpoint on a straight line extending from the inner wall surface  61  of the biological tissue  60  to the outside of the biological tissue  60  through the opening  62 . Therefore, the user can virtually observe the inner wall surface  61  of the biological tissue  60  by looking into the biological tissue  60  through the opening  62 . 
     Specifically, the control unit  41  of the diagnostic assistance device  11  arranges the virtual camera  71  at a position where the inner wall surface  61  of the biological tissue  60  can be seen through the portion set to be hidden or transparent in the three-dimensional image  53  displayed on the display  16 . In the example of  FIG. 5 , the control unit  41  arranges the virtual camera  71  in a region AF sandwiched between a first straight line L 1  and a second straight line L 2  in the cross section of the biological tissue  60 . The first straight line L 1  extends from the inner wall surface  61  of the biological tissue  60  to the outside of the biological tissue  60  through a first end edge E 1  of the opening  62 . The second straight line L 2  extends from the inner wall surface  61  of the biological tissue  60  to the outside of the biological tissue  60  through a second end edge E 2  of the opening  62 . A point at which the first straight line L 1  intersects the inner wall surface  61  of the biological tissue  60  is a point Pt identical to a point at which the second straight line L 2  intersects the inner wall surface  61  of the biological tissue  60 . Therefore, the user can observe the point Pt on the inner wall surface  61  of the biological tissue  60  regardless of a position of the virtual camera  71  in the region AF. 
     In the example of  FIG. 5 , the point Pt is identical to a point at which a fourth straight line L 4  intersects the inner wall surface  61  of the biological tissue  60 . The fourth straight line L 4  is drawn perpendicularly to a third straight line L 3  from a midpoint Pc of the third straight line L 3 . The third straight line L 3  connects the first end edge E 1  of the opening  62  and the second end edge E 2  of the opening  62 . Therefore, the user can rather easily observe the point Pt on the inner wall surface  61  of the biological tissue  60  through the opening  62 . In particular, as shown in  FIG. 5 , when the virtual camera  71  is arranged on an extension line of the fourth straight line L 4 , the user can rather easily observe the point Pt on the inner wall surface  61  of the biological tissue  60 . 
     The position of the virtual camera  71  may be any position at which the inner wall surface  61  of the biological tissue  60  can be observed through the opening  62 , and is within a range facing the opening  62  in the present embodiment. The position of the virtual camera  71 , for example, is preferably set to an intermediate position facing a central portion of the opening  62 . 
     In the example of  FIG. 6 , a minimum value Smin and a maximum value Smax are set for a ratio S of a distance Un from a center to one end of the three-dimensional image  53  displayed on a screen  80  of the display  16  to a distance Um from a center to one end of the screen  80  such that the centers of the screen  80  and the three-dimensional image  53  overlap with each other. For example, Smin can be set to ⅓, and Smax can be set to 1. In the example of  FIG. 5 , a minimum distance from the point Pt to the position of the camera  71  may be set according to the minimum value Sm in, and a maximum distance from the point Pt to the position of the virtual camera  71  may be set according to the maximum value Smax. Alternatively, the minimum distance from the point Pt to the position of the camera  71  may be set to such a distance that the camera  71  is not closer to the point Pt than the opening  62  regardless of the minimum value Sm in. The maximum distance from the point Pt to the position of the virtual camera  71  may be set to such a distance that the camera  71  is not away from the point Pt more than such a distance that the user cannot observe the inner wall surface  61  of the biological tissue  60  regardless of the maximum value Smax. 
     Furthermore, in step S 8 , the control unit  41  of the diagnostic assistance device  11  adjusts the color tone of each voxel of the three-dimensional image  53  according to the distance from the reference point Ps in the three-dimensional space to each point of the three-dimensional data  52 . The control unit  41  may arrange the reference point Ps at any position, but in the present embodiment, the control unit  41  adjusts a position of the reference point Ps according to the position of the opening  62  formed in step S 7 . 
     Specifically, the control unit  41  of the diagnostic assistance device  11  calculates the distance from the reference point Ps in the three-dimensional space to each point of the three-dimensional data  52 . The control unit  41  stores, in the storage unit  42 , the calculated distance for each point of the three-dimensional data  52 . The control unit  41  converts the distance stored in the storage unit  42  into a color tone for each point of the three-dimensional data  52  by using a conversion formula or a conversion table set in advance. The control unit  41  stores, in the storage unit  42 , the color tone calculated using the conversion formula or the conversion table for each point of the three-dimensional data  52 . The control unit  41  sets a color tone of the corresponding voxel of the three-dimensional image  53  to the color tone stored in the storage unit  42  for each point of the three-dimensional data  52 . In the example of  FIG. 5 , the control unit  41  can arrange the reference point Ps on a straight line in the cross section of the biological tissue  60  that passes through the position of the virtual camera  71  and the midpoint Pc of the third straight line L 3 . The position of the virtual camera  71  is the viewpoint when the three-dimensional image  53  is displayed on the display  16 . The third straight line L 3  connects the first end edge E 1  of the opening  62  and the second end edge E 2  of the opening  62 . Therefore, since the positions of the viewpoint, the reference point Ps, and the midpoint Pc in a left-right direction are aligned, when the user observes the inner wall surface  61  of the biological tissue  60  through the opening  62 , the unevenness and depth can be rather easily grasped by the difference in color tone of the three-dimensional image  53 . In particular, as shown in  FIG. 5 , when the reference point Ps is arranged on the fourth straight line L 4 , the user can rather easily grasp the point Pt of the inner wall surface  61  of the biological tissue  60  and the unevenness and depth around the point Pt. In the example of  FIG. 5 , the reference point Ps is arranged at the same position as the midpoint Pc. 
     In step S 9 , when the tomographic data  51  is updated, the processing of step S 1  and the subsequent steps are performed again. When the tomographic data  51  is not updated, the step of whether operated by the user is confirmed again in step S 4 . 
     In steps S 5  to S 8  for the second and subsequent times, when the position of the opening  62  is changed from a first position to a second position, the control unit  41  of the diagnostic assistance device  11  moves the viewpoint from a third position corresponding to the first position to a fourth position corresponding to the second position. The control unit  41  moves the virtual light sources  72  when the three-dimensional image  53  is to be displayed on the display  16 , in accordance with movement of the viewpoint from the third position to the fourth position. 
     The control unit  41  moves the virtual light sources  72  by using a rotation matrix used for moving the virtual camera  71  when changing the position of the opening  62  in the circumferential direction in the cross section of the biological tissue  60 . 
     The control unit  41  may instantly switch the viewpoint from the third position to the fourth position when changing the position of the opening  62  from the first position to the second position, but in the present embodiment, a video in which the viewpoint gradually moves from the third position to the fourth position is displayed on the display  16  as the three-dimensional image  53 . Therefore, the movement of the viewpoint can be easily introduced to the user. 
     In a modification of the present embodiment, in step S 5 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of setting a position of a target point that the user wants to see and an operation of setting the position of the opening  62 . 
     Specifically, in the three-dimensional image  53  displayed on the display  16 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of designating the position of the target point using the keyboard  14 , the mouse  15 , or the touch screen provided integrally with the display  16  by the user. In the example of  FIG. 5 , the control unit  41  may receive, via the input unit  44 , an operation of setting a position of the point Pt as a position of the point at which the first straight line L 1  and the second straight line L 2  intersect the inner wall surface  61  of the biological tissue  60 . 
     In a modification of the present embodiment, in step S 5 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of setting the position of the target point that the user wants to see, instead of the operation of setting the position of the opening  62 . In step S 6 , the control unit  41  may determine the position of the opening  62  according to the position set by the operation received in step S 5 . 
     Specifically, in the three-dimensional image  53  displayed on the display  16 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , the operation of designating the position of the target point using the keyboard  14 , the mouse  15 , or the touch screen provided integrally with the display  16  by the user. The control unit  41  may determine the position of the opening  62  according to the position of the target point. In the example of  FIG. 5 , the control unit  41  may receive, via the input unit  44 , an operation of setting the position of the point Pt as the position of the point at which the first straight line L 1  and the second straight line L 2  intersect the inner wall surface  61  of the biological tissue  60 . In the cross section of the biological tissue  60 , the control unit  41  may determine, as the region AF, a fan-shaped region centered on the point Pt and having a central angle that can be preset or that is an angle specified by the user. The control unit  41  may determine a position in the biological tissue  60  that overlaps with the region AF as the position of the opening  62 . The control unit  41  may determine a normal line of the inner wall surface  61  of the biological tissue  60  perpendicular to a tangent line passing through the point Pt as the fourth straight line L 4 . 
     The region AF may be set to be narrower than a width of the opening  62 . That is, the region AF may be set so as not to include at least one of the first end edge E 1  of the opening  62  and the second end edge E 2  of the opening  62 . 
     In a modification of the present embodiment, the point at which the first straight line L 1  intersects the inner wall surface  61  of the biological tissue  60  may not be identical to the point at which the second straight line L 2  intersects the inner wall surface  61  of the biological tissue  60 . For example, a point P 1  at which the first straight line L 1  intersects the inner wall surface  61  of the biological tissue  60  and a point P 2  at which the second straight line L 2  intersects the inner wall surface  61  of the biological tissue  60  may be on a circumference centered on the point Pt. That is, the point P 1  and the point P 2  may be substantially equidistant from the point Pt. 
     In a modification of the present embodiment, in step S 8 , when at least a part of a point group in the three-dimensional data  52  in a range to be displayed on the display  16  as the three-dimensional image  53  has a difference in color tone with respect to a difference in a distance from the reference point Ps that does not satisfy a condition, the control unit  41  of the diagnostic assistance device  11  may correct the color tone of each voxel of the three-dimensional image  53  in accordance with the condition. 
     In accordance with an embodiment, the control unit  41  of the diagnostic assistance device  11  analyzes a relationship between the distance from the reference point Ps stored in the storage unit  42  and the color tone calculated using the conversion formula or the conversion table stored in the storage unit  42 , for a point group in a range that can be observed through the opening  62  among a point group of the three-dimensional data  52 . The control unit  41  specifies a relationship between the distance and RGB values as shown in  FIGS. 7A, 7B, and 7C  as an analysis result. For example, as shown in  FIGS. 5 and 7A to 7C , it can be assumed that a ratio of the difference in color tone to the difference between a distance D1 from the reference point Ps to a point Pn and a distance D2 from the reference point Ps to the point Pt is less than a ratio Rt set as a condition. Alternatively, as shown in  FIGS. 5 and 7A to 7C , it is assumed that a ratio of the difference in color tone to the difference between the distance D2 from the reference point Ps to the point Pt and a distance D3 from the reference point Ps to a point Pf is less than the ratio Rt set as a condition. In this case, the control unit  41  corrects the color tone of the point group in the range that can be observed through the opening  62  among the point group of the three-dimensional data  52  such that a slope of a graph as shown in each of  FIGS. 7A to 7C  increases. In the example of  FIG. 5 , the point Pt is located at a center inside a concave portion of the inner wall surface  61  of the biological tissue  60  facing the opening  62 . The point Pn is located outside the concave portion of the inner wall surface  61  of the biological tissue  60 . The point Pf is on an inner wall surface of an expanding portion  63  on a side opposite to the opening  62  with respect to the concave portion of the biological tissue  60 . The expanding portion  63  is formed by expanding a portion of the biological tissue  60  in a long axis direction (axial direction) that does not include the point Pt. Therefore, even from the position of the virtual camera  71 , the inner wall surface of the expanding portion  63  can be observed diagonally through the opening  62  so as to avoid the point Pt of the inner wall surface  61  of the biological tissue  60  and the periphery of the point Pt. The ratio Rt can be, for example, preferably set such that the difference in color tone with respect to the difference between the distance D1 and the distance D2 is a difference in color tone that can be recognized by humans. The ratio Rt can be, for example, more preferably set such that the difference in color tone with respect to the difference between the distance D2 and the distance D3 is a difference in color tone that can be recognized by humans. As the analysis result, the control unit  41  may specify a relationship between the distance and a value related to the other color tones, such as a relationship between the distance and the brightness, or a relationship between the distance and the saturation, instead of specifying the relationship between the distance and the RGB value. 
     In a modification of the present embodiment, in step S 5 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of changing a difference in color tone with respect to a difference in a distance from the reference point Ps of at least a part of the point group in the three-dimensional data  52  in the range to be displayed on the display  16  as the three-dimensional image  53 . In step S 8 , the control unit  41  may change the color tone of each voxel of the three-dimensional image  53  in response to the operation received in step S 5 . 
     Specifically, the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , the operation of changing the difference in color tone with respect to the difference in the distance from the reference point Ps using the keyboard  14 , the mouse  15 , or the touch screen provided integrally with the display  16  by the user. In the example of  FIGS. 7A to 7C , the control unit  41  may display graphs of  FIGS. 7A to 7C  on the display  16  and receive an operation of changing slopes of the graphs via the input unit  44 . The control unit  41  changes the color tone of the corresponding voxel of the three-dimensional image  53  for each point of the three-dimensional data  52  such that the slopes of the graphs shown in  FIGS. 7A to 7C  have a magnitude corresponding to the received operation. 
     In a modification of the present embodiment, in step S 5 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of changing the position of the reference point Ps. In step S 8 , the control unit  41  may change the color tone of each voxel of the three-dimensional image  53  in response to the operation received in step S 5 . 
     In accordance with an exemplary embodiment, in the three-dimensional image  53  displayed on the display  16 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , the operation of changing the position of the reference point Ps using the keyboard  14 , the mouse  15 , or the touch screen provided integrally with the display  16  by the user. In that case, the control unit  41  calculates a distance from the changed reference point Ps to each point of the three-dimensional data  52 . The control unit  41  stores, in the storage unit  42 , the calculated distance for each point of the three-dimensional data  52 . The control unit  41  converts the distance stored in the storage unit  42  into the color tone for each point of the three-dimensional data  52  by using the conversion formula or the conversion table set in advance. The control unit  41  stores, in the storage unit  42 , the color tone calculated using the conversion formula or the conversion table for each point of the three-dimensional data  52 . The control unit  41  changes the color tone of the corresponding voxel of the three-dimensional image  53  to the color tone stored in the storage unit  42  for each point of the three-dimensional data  52 . 
     In the present embodiment, the control unit  41  of the diagnostic assistance device  11  can adjust the color tone of each voxel of the three-dimensional image  53  according to the distance from the reference point Ps in the three-dimensional space to each point of the three-dimensional data  52 , thereby forming a gradation that changes radially in a three-dimensional manner from the reference point Ps in the three-dimensional image  53  as shown in  FIGS. 8 and 9 . 
     In the example of  FIGS. 8 and 9 , G C  is a center point of the gradation, G D  is a distance from G C  to a gradation edge, G NEAR  is a color at G C , and G FAR  is a color at a position separated by G D  from G C . The “gradation edge” is an outer edge of a range applied with the gradation. V P  is any visible voxel information in the three-dimensional data  52 . The “visible voxel information” refers to voxel information in which the biological tissue  60  is present in the three-dimensional data  52 . Dist is a distance between V P  and G C , and Color P  is a color of V P . G NEAR , G FAR , and Color P  are RGB values. G NEAR  and G FAR  in this example are specifically shown below, with maximum values of red, green, and blue in RGB values each being 1. 
         G   NEAR =(1.00,0.95,0.48) 
         G   FAR =(0.20,0.47,1.00) 
     The control unit  41  sets the color tone of each voxel of the three-dimensional image  53  by the following calculation. 
       Dist=distance( V   P   ,G   c ) 
         Clr   F =clamp(Dist/ G   D ,0,1) 
       Color P   =G   NEAR ×(1− Clr   F )+ G   FAR   ×Clr   F  
 
     Here, distance( ) is a function for calculating the distance, and clamp( ) is a clamp function. 
     In the example of  FIGS. 5, 7A to 7C, and 8 , the reference point Ps corresponds to G C , the point Pn, the point Pt, and the point Pf correspond to V P , and the distance D1, the distance D2, and the distance D3 correspond to Dist. 
     As a specific example, it is assumed that G C , V P , and G D  are set as follows. 
         G   C =(0.5,0.5,0.5) 
         V   P =(0.4,0.4,0.25) 
         G   D =0.3 
     In this case, Dist, Clr F , and Color P  can be obtained as follows. 
       Dist=distance( V   P   ,G   C )=√((0.5−0.4) 2 +(0.5-0.4) 2 +(0.5-0.25) 2 )=0.29
 
         Clr   F =clamp(Dist/ G   D ,0, 1)=0.29/0.3=0.96 
       Color P   =G   NEAR ×(1− Clr   F )+ G   FAR   ×Clr   F =(1.00,0.95,0.48)×0.04+(0.20,0.47,1.00)×0.96=(0.23,0.49,0.98)
 
     In a modification of the present embodiment, the control unit  41  of the diagnostic assistance device  11  may adjust the color tone of each voxel of the three-dimensional image  53  according to a distance from a reference plane in the three-dimensional space to each point of the three-dimensional data  52 , thereby forming a gradation that changes in layers from the reference plane in the three-dimensional image  53  as shown in  FIGS. 10 and 11 . 
     In the example of  FIGS. 10 and 11 , GN is a gradation direction, G P  is a gradation plane position, G D  is a distance from G P  to a gradation edge, G NEAR  is a color at G P , and G FAR  is a color at a position separated by G D  from G P  to GN. GN is a direction perpendicular to G P . V P  is any visible voxel information in the three-dimensional data  52 . vDist is a distance between V P  and G P , and Color P  is the color of V P . G NEAR , G FAR , and Color P  are RGB values. G NEAR  and G FAR  in this example are the same as those in  FIGS. 8 and 9 . 
     The control unit  41  sets the color tone of each voxel of the three-dimensional image  53  by the following calculation. 
         v Dist= v distance( V   P   ,G   P ) 
         Clr   F =clamp( v Dist/ G   D ,0,1) 
       Color P   =G   NEAR ×(1− Clr   F )+ G   FAR   ×Clr   F  
 
     Here, vdistance( ) is a function that calculates a length of a perpendicular line drawn from V P  to G P  as a distance. In the three-dimensional space, when GN is in a Y axis direction, G P  is a plane parallel to an XZ plane, and vDist is an absolute value of a difference between y components of V P  and G P . 
     As a specific example, it is assumed that the reference plane is a plane parallel to a Y axis, and G P , V P , and G D  are set as follows. 
         G   P =(0.5) 
         V   P =(0.35,0.7,0.0) 
         G   D =0.3 
     In this case, vDist, Clr F , and Color P  can be obtained as follows. 
         v Dist= v distance( V   P   ,G   P )=|0.5−0.7|=0.2
 
         Clr   F =clamp( v Dist/ G   D ,0,1)=0.2/0.3=0.67 
       Color P   =G   NEAR ×(1− Clr   F )+ G   FAR   ×Clr   F =(1.00,0.95,0.48)×0.33+(0.20,0.47,1.00)×0.67=(0.46,0.63,0.83)
 
     In step S 8 , the control unit  41  of the diagnostic assistance device  11  may adjust a position of the reference plane according to the position of the opening  62  formed in step S 7 . 
     In step S 8 , when at least a part of the point group in the three-dimensional data  52  in the range to be displayed on the display  16  as the three-dimensional image  53  has a difference in color tone with respect to a difference in a distance from the reference plane that does not satisfy a condition, the control unit  41  of the diagnostic assistance device  11  may correct the color tone of each voxel of the three-dimensional image  53  in accordance with the condition. 
     In step S 5 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of changing the difference in color tone with respect to the difference in the distance from the reference plane of at least a part of the point group in the three-dimensional data  52  in the range to be displayed on the display  16  as the three-dimensional image  53 . In step S 8 , the control unit  41  may change the color tone of each voxel of the three-dimensional image  53  in response to the operation received in step S 5 . 
     In step S 5 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of changing the position of the reference plane. In step S 8 , the control unit  41  may change the color tone of each voxel of the three-dimensional image  53  in response to the operation received in step S 5 . 
     In a modification of the present embodiment, the control unit  41  of the diagnostic assistance device  11  adjusts the color tone of each voxel of the three-dimensional image  53  according to a distance from a reference line in the three-dimensional space to each point of the three-dimensional data  52 , thereby forming a gradation that changes radially in a two-dimensional manner from the reference line in the three-dimensional image  53  as shown in  FIG. 12 . 
     In the example of  FIG. 12 , G CL  is a centerline of a gradation, G D  is a distance from G CL  to a gradation edge, G NEAR  is a color at G CL , and G FAR  is a color at a position separated by G D  from G CL  in a perpendicular direction. V P  is any visible voxel information in the three-dimensional data  52 . vDist is a distance between V P  and G CL , and Color P  is the color of V P . G NEAR , G FAR , and Color P  are RGB values. G NEAR  and G FAR  in this example are the same as those in  FIGS. 8 and 9 , and  FIGS. 10 and 11 . 
     The control unit  41  sets the color tone of each voxel of the three-dimensional image  53  by the following calculation. 
         v Dist= v distance( V   P   ,G   CL ) 
         Clr   F =clamp( v Dist/ G   D ,0,1) 
       Color P   =G   NEAR ×(1− Clr   F )+ G   FAR   ×Clr   F  
 
     Here, vdistance( ) is a function for calculating a length of a perpendicular line drawn from V P  to G CL  as a distance. G CL  can be set as any line. For example, G CL  may be a straight line parallel to the Z axis in the three-dimensional space, may be a line connecting a center of gravity of the biological tissue  60  along an extending direction of the tubular biological tissue  60 , or may be an axis line of the probe  20 . 
     As a specific example, it is assumed that the reference line is a straight line parallel to the Z axis, and G CL , V P , and G D  are set as follows. 
         G   P =(0.5,0.5) 
         V   P =(0.5,0.55,0.5) 
         G   D =0.3 
     In this case, vDist, Clr F , and Color P  can be obtained as follows. 
         v Dist= v distance( V   P   ,G   CL )=√((0.5−0.5) 2 +(0.5−0.55) 2 )=0.05
 
         Clr   F =clamp( v Dist/ G   D ,0,1)=0.05/0.3=0.17 
       Color P   =G   NEAR ×(1− Clr   P )+ G   FAR   ×Clr   P =(1.00,0.95,0.48)×0.83+(0.20,0.47,1.00)×0.17=(0.86,0.86,0.57)
 
     In step S 8 , the control unit  41  of the diagnostic assistance device  11  may adjust a position of the reference line according to the position of the opening  62  formed in step S 7 . 
     In step S 8 , when at least a part of the point group in the three-dimensional data  52  in the range to be displayed on the display  16  as the three-dimensional image  53  has a difference in color tone with respect to a difference in a distance from the reference line that does not satisfy a condition, the control unit  41  of the diagnostic assistance device  11  may correct the color tone of each voxel of the three-dimensional image  53  in accordance with the condition. 
     In step S 5 , the control unit  41  of the diagnostic assistance device  11  may receive, via the input unit  44 , an operation of changing the difference in color tone with respect to the difference in the distance from the reference line of at least a part of the point group in the three-dimensional data  52  in the range to be displayed on the display  16  as the three-dimensional image  53 . In step S 8 , the control unit  41  may change the color tone of each voxel of the three-dimensional image  53  in response to the operation received in step S 5 . 
     In step S 5 , the control unit  41  of the diagnostic assistance device  11  receives, via the input unit  44 , an operation of changing the position of the reference line. In step S 8 , the control unit  41  may change the color tone of each voxel of the three-dimensional image  53  in response to the operation received in step S 5 . 
     As described above, in the present embodiment, the control unit  41  of the diagnostic assistance device  11  generates the three-dimensional data  52  of the biological tissue  60  based on the tomographic data  51  of the biological tissue  60 . The control unit  41  displays the generated three-dimensional data  52  as the three-dimensional image  53  on the display  16 . The control unit  41  adjusts the color tone of each voxel of the three-dimensional image  53  according to the distance from the reference point Ps in the three-dimensional space to each point of the three-dimensional data  52 . 
     According to the present embodiment, it can be rather easy for a user to grasp unevenness or depth in the three-dimensional space. For example, when the user is an operator, it can be rather easy to grasp the shape of the biological tissue  60 , which facilitates treatment. 
     In the present embodiment, once the position of the opening  62  is determined, the positions of the camera  71  and the light sources  72  move such that the inside of the biological tissue  60  can be seen from the opening  62 . Therefore, when the position of the opening  62  is changed to another position, it is possible to avoid a situation that only the outer wall surface of the biological tissue  60  can be seen and an object of interest cannot be confirmed. 
     According to the present embodiment, the unevenness in the three-dimensional space can be expressed. The depth in the three-dimensional space can be expressed. When the position of the opening  62  is changed and the positions of the camera  71  and the light sources  72  are changed, the color tone is also changed in accordance with the change, and thus the unevenness can be rather easily expressed. 
     In the present embodiment, since the color tone is changed according to the distance from the reference point Ps, it is relatively easy to grasp the unevenness or the depth. 
     In the present embodiment, the reference point Ps is automatically changed every time the position of the opening  62  is changed, so that the usefulness of the diagnostic assistance system  10  is improved. 
     The present disclosure is not limited to the above-described embodiment. For example, a plurality of blocks described in a block diagram may be integrated, or one block may be divided. Instead of executing a plurality of steps described in a flowchart in time series according to the description, the steps may be executed in parallel or in a different order according to the processing capability of the device that executes each step or as necessary. In addition, modifications can be made without departing from a gist of the present disclosure. 
     For example, a method for setting the color tone of each voxel of the three-dimensional image  53  in the control unit  41  may be not only calculation based on the RGB values described above, but also calculation based on another index for expressing color, such as an ARGB value including transparency in the RGB values. 
     The detailed description above describes embodiments of a diagnostic assistance device, a diagnostic assistance system, and a diagnostic assistance method. The invention is not limited, however, to the precise embodiments and variations described. Various changes, modifications and equivalents may occur to one skilled in the art without departing from the spirit and scope of the invention as defined in the accompanying claims. It is expressly intended that all such changes, modifications and equivalents which fall within the scope of the claims are embraced by the claims.